[Frontiers In Bioscience, Landmark, 22, 1845-1866, June 1, 2017]

Rerouting the traffic from a virus perspective

Linda Cruz1, Nicholas J. Buchkovich1

1Department of Microbiology and Immunology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania, USA

TABLE OF CONTENTS

1. Abstract
2. Introduction
3. Viruses, the Golgi Apparatus and Arfs
3.1. Altering trafficking by modulating the Arf pathway
3.2. Viruses and Golgi morphology
4. Exiting the endoplasmic reticulum
4.1. Viral hijacking of COPII vesicles
5. Virus alterations to organelles
5.1. Altering the ER
5.2. Displacing the TGN
5.3. Endosomes and lysosomes
6. Virus-induced compartments
7. Conclusions
8. Acknowledgments
9. References

1. ABSTRACT

Viruses are important human and animal pathogens causing disease that affect global health and the economy. One outcome of many virus infections is the regulation of cellular trafficking machinery. Viral proteins recruit and interact with cellular trafficking proteins to divert the normal trafficking of key proteins or to induce the formation of novel membrane structures in the host cell. These alterations often increase replication efficiency by mislocalizing immune regulators or restriction factors ot by creating platforms for replication and assembly of new virus particles. Our knowledge of how viruses interact with the cellular trafficking machinery is still limited and furthering this understanding will be important for the future development of prophylactic and therapeutic treatments. This review provides a glimpse of the types of interplay between viral and cellular factors that result in a disruption of cellular trafficking or modifications to cellular membranes.

2. INTRODUCTION

The membrane system of eukaryotic cells is a complex, highly developed network of distinct compartments consisting of unique repertoires of lipid and protein composition. The diverse membrane composition and shape of organelles provide distinct identities and allow for execution of specific functions. The sophisticated network of transport machinery in eukaryotic cells ensures a balanced flow of membrane and membrane cargo between these compartments. The regulated and organized flux among compartments is essential for maintaining organelle identity and membrane homeostasis. Viruses, many of which require interactions with membranes at multiple steps during their replication cycle, often alter the membrane profiles of cells. These alterations have several benefits for the virus and may be essential for immune evasion strategies or to create a novel milieu optimal for replication. These changes may be a direct result of viral proteins targeting cellular machinery, or an indirect effect associated with viral replication. Whether direct or indirect, these changes are remarkable. In this review, we address both viral-mediated regulation of trafficking events and the morphological alterations to the membranes of cellular organelles. Recognizing the large amount of quality research in this area, we focus on post-entry events, namely alterations associated with genome replication and virion assembly and consider only a subset of the mechanisms of viral-mediated regulation of membrane transport and organelle morphology.

3. VIRUSES, THE GOLGI APPARATUS AND ARFS

ADP ribosylation factors (Arfs) constitute a family of GTP binding proteins that regulate membrane trafficking pathways. There are three classes of mammalian Arf proteins, distinguished by size and homology (1). As a generalization, class I and II Arfs mainly localize to the trans-Golgi network (TGN), endoplasmic reticulum-Golgi intermediate complex (ERGIC) and Golgi apparatus to mediate membrane trafficking between these compartments, whereas the class III Arf6 localizes to the plasma membrane and is involved in endocytosis (2-4). Like other GTPases, Arf proteins cycle between the active GTP-bound and inactive GDP-bound states. Upon activation, a myristoylated N-terminus is exposed to promote membrane binding and activity. Upon inactivation, Arf dissociates from membranes and returns to the cytosol. This cycle is mediated by guanine nucleotide exchange factors (GEFs) and guanine nucleotide activating proteins (GAPs) known as Arf GEFs and Arf GAPs. A key feature of Arf GEFs is the presence of a 200 amino acid Sec7 domain which catalyzes release of GDP (5). GBF1 is an Arf GEF that acts on both class I and class II Arfs, whereas BIG1 and BIG2 preferentially activate class I Arfs (6-8). GBF1, BIG1 and BIG2 are mostly at the Golgi and are sensitive to the trafficking inhibitor brefeldin A. Many of the initial observations linking the Arfs to virus replication are based on the observation that replication is sensitive to brefeldin A. It should be noted that some Arf GEFs, like ARNO and EFA6, are resistant to brefeldin A treatment.

Activated Arfs have several effector proteins. The outcome of Arf activation often depends on the Arf GEF and where activation occurs. For example, GBF1 is localized to both the ERGIC and cis-Golgi and is associated with recruitment of the COPI coat. COPI, consisting of seven subunits, mediates trafficking from the cis-Golgi to the endoplasmic reticulum. Arf recruitment of COPI was the first to be described and is well studied. In contrast, BIG1 and BIG2 localize to the TGN and recycling endosomes, and these Arf GEFs are associated with recruitment of adaptor protein complex 1 (AP1) and Golgi-localized gamma-ear-containing Arf-binding (GGA) protein complexes. Both complexes are involved in clathrin-dependent trafficking. In addition to these coats, Arf proteins also stimulate effectors that modulate lipid composition. These include phospholipase D and phosphatidylinositol 4-phosphate 5-kinase (9-11). Thus Arf proteins modulate a number of different cellular processes. Many of the processes intersect with important stages of viral replication and as such, many viruses modulate Arf proteins to optimize replication conditions (summarized in Figure 1).

3.1. Altering trafficking by modulating the Arf pathway

One outcome of infection with poliovirus, a member of the picornavirus family, is the rapid inhibition of protein secretion (12). This block in secretion promotes immune evasion by decreasing MHC class I and TNF receptors at the cell surface and blocking secretion of interleukins 6, 8 and beta-interferon (13-15). The block occurs in trafficking from the ER or ERGIC to the Golgi apparatus, and is replicated by expression of protein 3A from either poliovirus or the related coxsackievirus B (CVB) (12, 16-18). This inhibition is a result of the direct binding of 3A to GBF1, a guanine nucleotide exchange factor for Arf1 (19, 20). Overexpression of either GBF1 or Arf1 rescues the 3A-mediated block in protein secretion (19). Furthermore while virus replication is normally inhibited by brefeldin A, a resistant form of GBF1 containing a single amino acid substitution is sufficient to support viral replication in the presence of the drug (21). Thus, the 3A-mediated regulation of GBF1 is an important step for blocking protein secretion during coxsackievirus and poliovirus infection.

Other picornaviruses utilize different strategies to modulate protein secretion. Expression of 3A from human rhinovirus 14 (HRV14), foot-and-mouth disease virus (FMDV), enterovirus 71 (EV71), hepatitis A, Theiler’s virus and encephalomyocarditis virus (EMCV), does not inhibit protein secretion (20, 22). Analysis of the 3A protein from HRV, which is closely related to that of CVB and poliovirus, reveals a reduced ability of this 3A protein to bind GBF1, which likely accounts for the inability of HRV 3A to block protein secretion. In another case, the EV71 3A interacts with GBF1, which together with Arf1 and Arf3, is essential for replication of the virus (23). This result suggests that EV71 3A regulation of GBF1 is essential for viral replication, even though it doesn’t block protein secretion as it does for the related CVB and poliovirus. EV71, and the other viruses mentioned above, have 3A-independent strategies to block protein secretion. At least for FMDV, the activity is associated with the nonstructural protein 2BC, as the coexpression of both the 2B and 2C proteins together, but not expression of the 3A protein from FMDV, was responsible for perturbing protein secretion (24, 25). Thus, picornaviruses have multiple strategies for blocking protein secretion.

In addition to GBF1, other Arf GEFS are regulated during enterovirus infection. Whereas 3A regulates GBF1, the viral protein 3CD, through regulation of BIG1 and BIG2, promotes the membrane-association of Arfs (26). The GBF1 recruitment by 3A brings COPI to membranes. In contrast, 3CD helps recruit the GGA3 coat to membranes (26). Mutations in 3CD that abrogate Arf activation also impair virus production. Thus, 3CD activation of Arfs through BIG1 and BIG2 is not redundant with the 3A-mediated activation of GBF1, and each Arf GEF may mediate a different activity during virus replication. Thus, by regulating various Arf effectors different enterovirus proteins can direct alterations in the infected cell that are essential for replication.

Other virus families also have developed mechanisms to block protein secretion. One effector is the non-structural protein precursor NS4A/B of Hepatatis C virus (HCV), a flavivirus (27). A link between the NS4A/B-mediated block in secretion and the Arf pathway has not yet been demonstrated, although GBF1, Arf1 and COPI components all are critical for HCV replication (28-30). HCV proteins also alter membrane morphology, producing membrane buds at the rough ER and forming a membranous web that is tightly associated with vesicles (31). GBF1 does not appear to be required for membranous web formation, suggesting that alternative trafficking pathways deliver membranes to the HCV replication compartment (29). In contrast, the Arf1 pathway is required for HCV proteins to localize to this compartment. In the absence of functional Arf1, NS3 an NS5A, two multifunctional viral proteins that both play a role in RNA replication, are redirected from replication compartments to the periphery of lipid droplets, resulting in reduced replication of viral RNA (28). In addition, Arfs are required for delivering cellular factors to sites of replication. During HCV replication, PI4KIIIbeta is delivered to replication membranes in an Arf-dependent manner to generate a PI4P enriched environment (32). In this case, HCV redirects transport to ensure important cargo is delivered to its replication membranes. Of note, HCV is also dependent on COPI and requires the secretory pathway for virion maturation and exit. Thus, HCV relies heavily on Arf-mediated events for its replication.

HCV is not the only flavivirus that regulates Arfs during infection. GB virus C (GBV-C) is a flavivirus that infects humans. Clinically, even though the virus has not been associated with its own disease, GBV-C appears to delay progression of AIDS in HIV-infected patients (34). Among the different mechanisms that have been proposed to explain this block in disease progression by GBV-C, one is that HIV gag is unable to be delivered to the plasma membrane due to the altered trafficking associated with GBV-C E2 regulation of Arf (33). The E2 protein of GBV-C decreases Arf1 levels by promoting its degradation, resulting in disrupted Golgi morphology and impaired vesicle trafficking to and from the Golgi (33). In this context, the inhibition of protein trafficking by GBV-C may have unintended and beneficial clinical consequences that are unrelated to GBV-C replication. Thus, global inhibition of protein secretion is a feature shared by several virus families and not only may help viruses avoid an immune response but also may produce additional unappreciated outcomes.

Dengue virus infection relies on a unique form of Arf-mediated trafficking. In infected cells, the Dengue C protein accumulates around lipid droplets (35). Transport of C from the ER to lipid droplets uses the GBF-Arf1-COPI pathway (36). Delivery of cellular proteins to the surface of lipid droplets also is COPI-dependent (37). However, because lipid droplets contain a phosopholipid monolayer, as opposed to most transport vesicles that contain a lipid bilayer, transport to lipid droplets is likely to be different from canonical vesicle trafficking. More work is needed to elucidate this trafficking pathway in both uninfected and infected cells and Dengue virus infection could provide a useful model.

A number of other viruses require a functional Arf pathway for replication. Proper processing of the G protein of the vesicular stomatitis virus (VSV), a negative-strand RNA virus, requires an intact secretory system. Its topology and processing were first investigated several decades ago, and since then it has become a membrane protein among the most well-studied by both virologists and cell biologists (38). A recent human genome-wide siRNA screen revealed that COPI subunits are required for a productive infection by VSV, as are Arf1 and GBF1 (39). Unexpectedly though, COPI, Arf1 and GBF1 are required for an early step in replication, viral gene expression. Furthermore, the block in gene expression is independent of the entry and uncoating of the virus that requires endosomal transport, since gene expression is also blocked in the absence of COPI when the genome is delivered by transfection (39). Similar observations were made for two other negative-strand RNA viruses, the arenavirus lymphocytic choriomenigitis (LCMV) and the paramyxovirus parainfluenza virus type 3 (HPIV3). As reported for VSV, knockdown of Arf1 and COPI subunits inhibits LCMV gene expression. In contrast, for HPIV3, COPI subunit knockdown, but not Arf1 (at least to the level reported), prevents HPIV3 gene expression (39). How the Arf pathway contributes to the gene expression of these viruses and the differential requirements for pathway components remains an open question. These findings highlight the important contribution that cellular trafficking events have on multiple stages of infection.

The HIV multifunctional protein Nef is a known regulator of intracellular trafficking. Nef prevents the plasma membrane localization of a number of key immune regulatory proteins, including MHC-I. Nef binds MHC-I early in the secretory process and reroutes it from the TGN to lysosomal compartments for degradation (40). Nef accomplishes this rerouting by promoting a direct interaction between the mu subunit of the clathrin coat adaptor protein AP1 and MHC-I. Arf1 activates AP1 at the TGN and recent structural studies of the AP1:Arf1 multimer promoted by Nef reveal a previously unappreciated organization to the inner layer of the AP1-clathrin coat (41). This is an example of how studying a viral-mediated trafficking event can provide clues to the normal function of these factors. In addition to rerouting traffic early in the secretory pathway Nef1 also interacts with trafficking machinery at the plasma membrane to change its protein composition. A direct interaction between Nef1 and the plasma membrane localized AP2 complex is required for the Nef-mediated downregulation of CD4, which is important for HIV infection (42). Thus, Nef has evolved distinct strategies to modulate protein trafficking at different locations in the cell.

Infection with other clinically important viruses also involves Arf-mediated trafficking. Ebolavirus virion production requires Rab1a-dependent activation of GBF1 (43). Influenza virus requires COPI indirectly for entry and perhaps more directly for protein production and assembly (44). The Kaposi’s sarcoma-associated herpesvirus (KSHV) regulates Arf1 during infection with clinically important implications. The KSHV protein kaposin A protein binds the Arf-GEF cytohesin-1, resulting in activation of Arf1 and regulation of integrin-mediated cell adhesion (45). This regulation is important for KSHV-mediated cellular transformation and disease, as a mutant cytohesin-1 that is unable to catalyze guanine nucleotide exchange does not transform cells.

3.2. Viruses and Golgi morphology

Virus infection can result in gross morphological changes in organelles. Alterations of picornavirus-infected cells were first observed over a half-century ago (46, 47). Disappearance of the Golgi apparatus is accompanied by the appearance of an extensive membrane network used as a platform for replication. Although the loss of the Golgi might be linked to the block in protein secretion, several lines of evidence indicate that the two processes are separate. First, synthesis of viral proteins has differing effects on the two processes. Protein 2C disrupts the Golgi but has no apparent effect on protein secretion (12, 48). Conversely, protein 2B inhibits protein secretion without noticeably altering Golgi morphology (12), unless high levels of 2B are produced (49). Second, after infection with a mutant virus with a single amino acid insertion in protein 3A that reduces the ability to inhibit protein secretion, dispersion of the Golgi resembles that produced by wildtype virus, once again demonstrating the uncoupling of the inhibition of protein secretion from Golgi dispersion (18). Thus, viral-induced membrane alterations can result directly from viral regulation and are not merely a by-product of the block in trafficking. Tomographic analysis of infected cells can trace membrane rearrangements throughout infection. Initial formation of single membrane branching tubules early in infection gradually transform into double-membrane structures and ultimately into the double-membrane vesicles present during the late stages of infection (50). Elucidating the mechanism behind this extensive membrane rearrangement, including the source of the membrane, has been the subject of extensive study as well as some controversy. Mechanistically, production of the viral proteins 2C or 2BC leads to membrane rearrangements that include the formation of vesicles and the disappearance of Golgi stacks (48). The cellular factors involved in this 2C and 2BC-mediated mechanism remained to be elucidated.

A number of other viruses also induce morphological alterations to the Golgi. The ORF 3a protein of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is multifunctional. One of its functions is to induce Golgi fragmentation, which is restored by overexpression of Arf1, suggesting that 3a directly regulates Arf1 or an upstream factor such as GBF1 (51). Like other positive-strand viruses, SARS-CoV infection modifies intracellular membranes, forming double membrane vesicles, convoluted membranes and vesicle packets (52, 53). Consistent with the role of ORF 3a in regulating Arf1 and modulating Golgi morphology, double membrane vesicles do not form in its absence (51). However, replication of the viral genome is unaffected. Because the double membrane vesicles are not required for genome replication, they could be merely a consequence of the regulation of Arf1 and protein trafficking by ORF 3a rather than a requirement for productive infection.

In mouse hepatitis virus (MHV)-infected cells, inhibition of the Arf1 pathway reduces the number of double-membrane vesicle replication compartments. This reduction appears to be important for infection, because expression of a dominant-negative mutant of Arf1 reduces infection by about 75%, while expression of a constitutively active Arf1 mutant results in wildtype levels of virus (54). Arf1 activation during MHV infection is associated with GBF1, but not BIG1 or BIG2. Arf1 does not associate directly with the replication compartments (54). Thus, it does not act at replication sites and may instead facilitate the delivery of key components, such as phospholipids, necessary for generating the compartments.

In summary, viruses often alter cellular trafficking, in many cases by targeting the Arf pathway. In some cases this modulation may morphologically alter the membrane landscape, while in more subtle cases it may simply be to direct and concentrate cargo to a new location important for viral replication.

4. EXITING THE ENDOPLASMIC RETICULUM

The Arf-related GTPase Sar1 regulates coat protein complex II (COPII). COPII consists of Sec23, Sec24, Sec13 and Sec31, which together form a complex capable of forming vesicles from membranes (55, 56). COPII acts on the cytosolic face of the ER by inducing membrane curvature, concentrating cargo, and releasing budding vesicles. The complex is formed in a sequential manner, beginning with the activation and recruitment of the Arf-related GTPase Sar1 by the ER-resident GEF Sec12 (57). The N-terminal amphipathic helix of Sar1 is inserted in to the ER membrane and the ER-bound Sar1-GTP recruits the Sec23/24 heterodimer by binding to Sec23 (56, 58). Sec24 is the main adaptor protein of the COPII coat and interacts directly with cargos and cargo-bound receptors (59, 60). Sec23 recruits another heterodimer, Sec13/Sec31, by binding to Sec31 (61). Sec13/31 forms the outer coat of the forming vesicle and its cage-like formation drives bending and curvature formation of the membrane (62-64). Sec23 also is the GAP for Sar1 and with Sec31 promotes GTP hydrolysis and ultimately the release of vesicles from ER exit sites (65, 66). This GTPase activity is opposed by Sec16, preventing premature vesicle scission. Smaller vesicles are generated in the absence of this Sec16 regulation (67). Thus, the formation of COPII vesicles requires the concerted action of a number of factors regulated spatially and temporally, providing multiple points for viral intervention.

4.1. Viral hijacking of COPII vesicles

Poliovirus infection, as it does for the Arf pathway, also alters SarI and COPII-mediated trafficking. Vesicles that form the poliovirus replication complex are associated with COPII. The nonstructural proteins 2B and 2BC are sufficient to generate these vesicles (68). Furthermore, Sec16, which interferes with COPII GTPase activity increases early in infection (67, 69, 70). The increase in Sec16 occurs simultaneously with an increase in COPII-derived vesicles, and this transient increase in vesicles may increase the pool of membranes that are available to form replication compartments (70). Thus, poliovirus exemplifies how a virus can target multiple aspects of the cellular trafficking system, the Arf pathway and formation of COPII vesicles, to generate an environment optimal for viral replication.

Unlike poliovirus and other enterovirus family members, FMDV replication is resistant to brefeldin A. Further, dominant-negative versions of Arf1 or Rab1a that completely disrupt Golgi morphology actually enhance FMDV replication (71). Instead, replication is sensitive to inhibition of Sar1a function, indicating that FMDV requires COPII-mediated trafficking. A dominant-active form of Sar1a that completely disrupts both the ERGIC and the secretory pathway by stabilizing COPII coats also supports infection (71). This result suggests that FMDV utilizes early secretory membranes for replication and also creates a replication compartment like that of other virus family members. However, unlike related viruses that target the Arf pathway, FMDV does so by targeting formation of COPII vesicles.

Norwalk virus, a single-stranded, positive-sense RNA virus, also disrupts protein secretion. The mechanism is unknown, however the viral protein p22 is sufficient to both block protein secretion and disrupt Golgi morphology (72). p22 contains an ER-export mimic sequence that allows it to incorporate into COPII vesicles (72). One hypothesis is that p22 reroutes the COPII vesicles from their normal Golgi destination. Disrupting the flow of incoming vesicles to the Golgi would inhibit protein secretion and disassemble the Golgi apparatus. Important, unanswered questions include whether the block in protein secretion is essential for a productive infection. Investigations of the p22 homologues of non-human noroviruses may be informative. Whereas p22 from human noroviruses both blocks protein secretion and disassembles the Golgi, the murine homologue, p18, disassembles the Golgi but reduces protein secretion only modestly (73). The feline calicivirus homologue, p30, does not block protein secretion or disrupt the Golgi. Thus, these two activities do not appear to be essential conserved features of norovirus p22 homologues, and at least in these species are not required for productive infection.

5. VIRUS ALTERATIONS TO ORGANELLES

In addition to regulating the transport between organelles, viruses often directly modify organelles to generate replication platforms. These modifications often result in gross alterations to the organelle morphology. For some viruses, cell death and lysis is the end game, and whether the organelles remain functional or not is inconsequential to the replication strategy. However, some viruses have protracted replication periods and must maintain at least some semblance of normal cell function. For organelles, maintenance of function may limit the kinds of changes in structure and morphology that can be tolerated. In the sections below, we will discuss some viral-induced morphological alterations to organelles (Figure 2), and describe the viral and cellular proteins responsible for these modifications.

5.1. Altering the ER

Many viruses utilize ER membranes as a platform for genome replication or as the site of envelopment. Two notable modifications to ER structure are the formation of replication spherules and the regulated rupture of collapsed ER membranes. Brome Mosaic Virus (BMV), a small, positive-strand RNA plant virus, generates small spherules that bud into the ER. Expression of a single viral protein, 1a, is sufficient to form these 50-70 nm unscissioned vesicles, which protrude inward into the ER (74). Several cellular proteins also contribute to spherule formation. BMV is notable because it can infect yeast, and the ease in genetically manipulating yeast cells facilitates the identification and analysis of cellular factors that participate in BMV infection. Screening of yeast gene collections has led to the identification of at least 123 cellular genes that either enhance or inhibit BMV replication (75, 76). One outcome is the discovery of a role for both ESCRT and reticulon proteins in the proper formation of ER spherules (77, 78). Reticulons, which normally contribute to ER morphology, may help establish the spherules by reducing the curvature of vesicles lined with the viral protein 1a. The ESCRTs probably function at the neck of the bud, similar to their role in intraluminal vesicle formation, to maintain the spherule opening. Because ESCRT-mediated reactions usually lead to membrane scission, to stabilize the ER spherules BMV must somehow stall the progression of the ESCRT-mediated deformation event at the open bud. Thus BMV spherule formation is example of how a virus modulates host machinery to take advantage of some of its functions while preventing others.

African Swine Fever Virus (ASFV), a double-stranded DNA virus member of the nucleocytoplasmic large DNA viruses (NCLDV), also modifies ER membranes. ASFV virion membranes may be derived from open membrane precursors that originate from ruptured ER (79). Although the mechanism of this rupture is not known, the defined diameter of the membrane curls suggests that the process is carefully regulated and involves scission of the ER at regular intervals. The viral protein p54/J13Lp is necessary for the appearance of membrane precursors at virus factories (80). p54 is sufficient to induce collapse of ER cisternae, which occurs by two separate interactions mediated by distinct domains of p54. The cytoplasmic domains between p54 proteins located on neighboring cisternae form antiparallel interactions, while the luminal domains of proteins on opposite membranes of the same cisternae form disulphide bonds (81). The p54-mediated collapse of the ER cisternae may be a prerequisite for ER rupture and membrane curl formation.

5.2. Displacing the TGN

In addition to rupturing the endoplasmic reticulum to form membrane curls, ASFV also disperses the TGN. Resident proteins, TGN46, p230, sialyltransferase and AP1 relocalize to vesicles at the periphery of the ASFV assembly compartment, also referred to as the virus factory (82, 83). The fates of the different TGN proteins are not uniform, as TGN46 and p230 appear to redistribute to distinct vesicles (82). Mechanistically, dispersion of the TGN markers requires an intact microtubule network and may involve an interaction between the viral protein CD2v and the adaptor complex AP1 (82, 84). Functionally, TGN dispersion slows down trafficking to the plasma membrane and lysosomes, an outcome that likely contributes to immune evasion. There are many remaining questions such as how the virus directs different TGN proteins to different compartments, and whether this is functionally important for virus production.

Kunjin virus, the Australian strain of West Nile virus, also morphologically alters the TGN. Among the several distinct membrane alterations is the appearance of vesicle packets that co-localize with TGN markers (85). These vesicle packets may arise from repurposed TGN membranes to serve as the site of viral RNA synthesis (86). Two other membrane structures, paracrystalline arrays and convoluted membranes, arise during Kunjin virus infection from ERGIC membranes in close association with or perhaps continuous with the rough ER (85, 87). Virions assemble at these rough ER membranes, enter the ER lumen and then transit through the secretory pathway for release (88). Which viral proteins direct formation of these distinct membrane structures, each commissioned for its unique function? An NS4A-NS4B cassette containing the viral protease (NS2B-3pro) is sufficient to produce the membrane rearrangements characteristic of viral infection (89). This result suggests that cleavage of the NS4A-4B poly-protein is a key event. How the cleavage products cause the dramatic alteration of cellular membranes and which cellular proteins are required remain to be elucidated.

Other flaviviruses replicate in specialized viral-induced membrane structures that vary in composition and originate from different organelles than the related Kunjin virus. Rather than TGN, the New York 99 strain of West Nile virus uses ER-derived membranes for replication (90). The NS4B protein of this strain associates with these compartments and is involved in initiating the formation of the viral-induced membrane structures, unlike the Kunjin homologue that does not alone induce the membrane rearrangements (89, 90). Another flavivirus, DENV, also induces the formation of vesicle packets and convoluted membranes that appear to originate from the ER (91). In this case, the NS4A protein produces membrane alterations resembling those during infection (92). Thus, among viruses of the same family, strategies that utilize different membrane origins and require distinct viral proteins lead to similar outcomes.

5.3. Endosomes and lysosomes

Endosomal and lysosomal membranes are also sites of viral modifications. Rubella virus, a togavirus, replicates in a “cytopathic vacuole” derived from modified endosomes and lysosomes (93, 94). These vacuoles consist of vesicles of varying sizes at the periphery and an internal rigid membrane sheet that is packed with replicase proteins (95). These vacuoles are in contact with rough endoplasmic reticulum, the Golgi and mitochondria (94). Electron tomography shows that these factories are not only in close proximity to these organelles, but at least in the case of rough ER and Golgi also appear to form contacts with the cytopathic vacuole. These contacts include protein bridges, closely apposed membranes, and what is described as “fuzzy material” (95). Interestingly, the formation of these vacuoles in close association with other organelles does not affect endo-lysosomal trafficking and the ability to receive incoming material from the plasma membrane (95). Thus, despite drastic morphological alterations to accommodate viral replication, these organelles maintain their normal functionality.

Two alphaviruses, Semliki Forest virus and Sindbus virus, also utilize modified lysosomes and endosomes for replication. First observed nearly 50 years ago and historically referred to as type 1 cytopathic vacuoles, these compartments resemble those formed by rubella virus in that they have membrane invaginations or spherules of approximately 50 nanometers spaced around the limiting membrane of the vacuole (96). Their formation and endosomal association of replication proteins requires an intact polyprotein containing the viral non-structural proteins 1 and 3. The individual non-structural proteins are not sufficient (97). Although associated with lysosomes, in vertebrate cells these replication spherules appear to originate at the plasma membrane and are subsequently internalized and delivered to endosomal and lysosomal membranes (98, 99). Migration of the replication spherules depends on endocytosis that requires phosphatidylinositol-3-kinase, actin and myosin, followed by long-range transport on the microtubule network (100). With the exception of dynamin and to a lesser extent nocadazole (which may have other effects on replication), endocytosis inhibitors that prevent the migration of vesicles do not profoundly reduce viral replication, suggesting that the virus can replicate in spherules that remain located at the plasma membrane (99). Notably, in mosquito cells the distribution of alphavirus replication spherules between the plasma membrane and endosomal/lysosomal membranes is different from distribution in vertebrate (99). This may simply reflect a difference in endocytosis dynamics, or may be a more direct consequence of the actions of the particular viral proteins that direct compartment formation in each kind of cell.

Replication of infectious bursal disease virus (IBDV) of the Birnaviridae family also requires the endocytic compartment for replication. The virus causes immunosuppression in chickens and thus its control is economically important to the poultry industry. IBDV replicates on modified membranes of endocytic compartments that label with EEA1, Rab5, LAMP-1 and LAMP-2 (101). Unlike the membranes of the replication compartments of the viruses discussed above, those of IBDV compartments are not grossly altered. Rather, IBDV encodes a 46 amino acid peptide, pep46, which induces small pores of less than 10 nm in endosomal membranes. These pores allow the exchange of molecules that initiate replication (102). One model is that replication factors exit the endosome through the pores, allowing the viral protein VP3, which localizes to endosomes, to direct the association of these proteins with the limiting membrane. These replication factors would then remain associated with endosomal membranes as the endosomes traverse the microtubule network to the Golgi complex, where viral assembly is completed (101). In this model, viral modification of the organelle membrane allows entry, replication and assembly to be coordinated in a well-integrated, dedicated subcellular space.

6. VIRUS-INDUCED COMPARTMENTS

In addition to modifying existing organelles, viruses often create their own environment or “organelle” that accumulates the viral and cellular proteins, lipids, and other factors required for optimal replication. This process often involves gross rearrangements and/or mixing of existing organelles. Replication of the aforementioned NCLDVs occurs in this type of viral “factory.” The distinctive cytoplasmic virus factory of the NCLDV member vaccinia virus, a model poxvirus, transitions through several different states during infection. After the onset of viral DNA synthesis, the virus factory becomes completely enwrapped by ER membrane, a process that requires the viral E8R protein (103). This form of the viral DNA-containing membrane-bound compartment resembles a mini-nucleus. As virus assembly begins, DNA replication declines rapidly and the ER membrane dissociates from the factory (103). Subsequently, membrane crescents that will ultimately form the viral envelope associate with the compartment. These crescents are derived in the cytoplasm from small patches of pre-existing intracellular membrane (104). Unlike most viruses that acquire an envelope by budding through the plasma membrane or the limiting membrane of an organelle, poxviruses derive their primary envelope from coalescence of these crescents.

The viral proteins required for the proper formation, delivery and assembly of poxvirus membranes have been investigated by genetic analysis. Because many of these proteins are essential, conditional expression systems must be used to grow mutant viruses. Some of these systems are not without limitations, as inducible gene schemes and temperature-sensitive mutants can be “leaky” under supposed null-production conditions. An alternative is to grow null viral mutants in complementing cell lines. A potential advantage of using both approaches is exemplified by genetic analysis of the vaccine virus H7 gene. Instead of the crescents formed in the presence of wild type virus, cells infected under non-inducing conditions by a virus with an inducible form of H7 accumulate small membrane arcs coated with spicules in association with dense inclusions that likely represent the viroplasm that is observed in wild type virus infection (105). In contrast, these membrane arcs are not observed in cells infected with viruses completely lacking H7, produced in a complementing cell line (106). These results show a concentration-dependent effect for H7 on membrane alterations and reveal a membrane intermediate under conditions where H7 is limiting. In addition to H7, similar genetic approaches have implicated vaccinia proteins D13, A14, A17, A6, A11, L2 and A30.5. in membrane precursor formation and envelopment (105, 107-119). Together with H7, the latter four proteins make up a group referred to as viral membrane assembly proteins, or VMAPs (109). The viral kinase F10 has been implicated in orchestrating the formation of membrane crescents, likely by phosphorylting A14, A17 and/or other candidate membrane-associated proteins (114, 120, 121). The properties of these proteins and how they contribute to formation of crescents and the different classes of vaccinia virions has been recently reviewed in detail (122).

The origin of the patches that give rise to the membrane crescents has been investigated for over half a century. It was originally proposed that the crescents were synthesized “de novo” because they preferentially incorporated newly synthesized phospholipid and had a different composition than host cell membranes (104). It was then proposed that the membranes originate from the ERGIC complex because some vaccinia proteins were found to associate with ERGIC membranes (123, 124). However, formation of the immature virions does not require transport between the ER and ERGIC or Golgi, suggesting that the virus directly trafficks from the ER to the sites of immature virion formation (125). A number of recent studies provide evidence that the membrane crescents form directly from ruptured ER, capturing spicule-coated structures trapped in the lumens of partially ruptured ER structures (109, 126, 127). EM tomography supports the hypothesis that the crescents consist of a single membrane and are formed by rupturing a pre-existing membrane (111). The mechanistic events of this ER rupture are emerging, however it is not currently known how or even whether viral proteins induce breaks in the ER. The uniformity of the membrane crescents suggests a highly regulated process. A17 and D13 are important in regulating the size and shape of the growing crescents. A17 is a reticulon-like protein with membrane remodeling capability that promotes extensive tubulation of the ER upon expression (128). The reticulon-like property of A17 helps shape the growing membrane crescents in combination with the D13 scaffold, which forms a lattice that supports the growing membrane crescent (111, 112). In summary, the unique events of vaccinia virus membrane acquisition provide opportunities for investigating novel protein-membrane interactions and their effects on membrane integrity, shape and size.

The virus factory of another NCLDV, ASFV, contains partially and fully assembled virions and is the site of virus assembly (Figure 3). This compartment excludes obvious cellular organelle markers. These factories, which form at the microtubule organizing center (MTOC), have several characteristics of aggresomes and may utilize similar features for their formation. For example, like aggresomes, ASFV viral factories are dependent on microtubules and dynein and are susceptible to disruption of the dynynein/dynactin complex by overexpression of p50 dynamitin (129, 130). Additionally, viral factories are surrounded by a collapsed vimentin cage, another trait shared with aggresomes (129, 131). Early in infection, vimentin forms an aster at the future site of the virus factory next to the MTOC, which eventually converts into a cage around the factory in a process dependent on calmodulin-dependent protein kinase II (132). Similarly, in cells infected by an unrelated virus, a positive stranded RNA picornavirus, enterovirus 71, vimentin is phosphorylated by CaMK-II and rearranged around replication centers (133). This rearrangement, characteristic of aggresome formation, may simply be part of the cellular response to the accumulation of viral proteins at the factory. Alternatively, viruses may actually coordinate the vimentin rearrangement to generate both a structural component in virus factory formation and later a cage to maintain a high local concentration of viral replication components. Along this line, vimentin not only surrounds the replication compartment of vaccinia virus, but also associates with the assembling immature virions (124). Thus at least in this case, vimentin is likely to play a role in the virus life cycle that extends beyond the cellular aggresome response.

Human cytomegalovirus (HCMV) infection also extensively alters the cellular membrane landscape (Figure 3). The cytoplasmic viral assembly compartment (cVAC) of HCMV is in many ways different from the factories discussed above. One major functional difference is that because viral DNA is replicated and packaged into capsids in the nucleus, DNA replication and packaging do not occur in the cVAC. Rather, it is the site of tegument acquisition and envelopment. Morphologically, the compartment consists of nested cylinders of organelle specific vesicles derived from the Golgi, TGN and early and recycling endosomes (134, 135). Like other virus factories, the cVAC forms at the perinuclear MTOC and requires an intact microtubule network and the molecular motor dynein, but unlike the other factories, the HCMV cVAC is not surrounded by a vimentin cage (136, 137). siRNA knockdown of candidate viral genes reveals three essential genes for cVAC formation: UL48, UL94 and UL103 (138). Their roles in cVAC formation have not yet been elucidated. A number of cellular proteins, many of which are involved in trafficking, are associated with the formation and/or maintenance of the cVAC,. The proteins include Rab11, Bicaudal D1, FIP4, BiP and, as mentioned previously, dynein (137, 139-141). Other cellular candidates include VAMP3, RAB5C, RAB11A, SNAP23 and CDC42, which are targeted by virus-encoded microRNAs (142). Ongoing studies of HCMV assembly should identify any additional cellular proteins and elucidate how each component participates in the redirection and reshaping of cellular organelles to form the cVAC.

What are the functional consequences of this drastic reorganization of the cellular membrane system? The HCMV life cycle can extend from four days to more than a week, depending on the cell type, and must maintain cell viability for most of this period. Viral glycoproteins, of which HCMV encodes no less than 65, must traverse the cellular secretory system for proper processing and localization, thus ruling out global inhibition of protein secretion as described above for other viruses. On the other hand, HCMV encodes a number of proteins dedicated to immune evasion by preventing the trafficking of specific cellular proteins that are required for recognition of the infected cell by the immune system. Recent reviews provide a comprehensive summary of these viral proteins and their immune evasion strategies (143, 144). Briefly, the HCMV proteins US3, US10, UL16, UL82, UL141 and UL142 block the progression of certain cellular proteins through the secretory pathway enroute to the plasma membrane (145-154). US18 and US20 direct MICA, a ligand that binds to immune cells expressing the NKG2D receptor, to the lysosomes for degradation (155). US2 and US11 target MHC class I molecules in the ER and promote their ER dislocation and subsequent proteasomal degradation (156-161). US10 can act in a similar manner to target HLA-G for degradation (162). UL20 contains an immunoglobulin-like ectodomain and is rapidly transported to the lysosome for degradation after synthesis, so that it never reaches the plasma membrane (163). Why would HCMV encode such as short-lived protein? Perhaps UL20 binds to and chaperones particular cellular proteins to the lysosome for degradation, which would add yet another layer of viral regulation of trafficking. Thus, although HCMV may not induce the global block in protein trafficking observed in other viruses, it has evolved a more targeted approach for deterring the trafficking of a number of proteins, primarily those involved in immune recognition that would compromise survival of the infected cell.

7. CONCLUSION

Viruses exhibit remarkable diversity in structure, genomes, replication and assembly strategies. Yet they face similar challenges by having to interact with the host cell environment to produce infectious progeny. One aspect of the host system often targeted by viral proteins is the machinery involved in the transport of proteins and membrane. Viral products recruit cellular trafficking components to generate specialized compartments for optimal viral replication and assembly. They may inhibit trafficking components to prevent the proper localization of cellular proteins, a process particularly important for immune evasion. In many instances, virus infection alters the structure of organelles or disperses them altogether. In a few cases, the mechanism of action is known. In far more instances, very little is known. Increasing our understanding of how viruses interact with the cellular trafficking machinery will not only expand our knowledge of these fascinating entities, but also contribute to development of better therapeutics and vaccines.

8. ACKNOWLEDGEMENTS

The authors would like to thank David Spector and Rebecca Craven for critically reviewing the manuscript and providing helpful suggestions and important insight.

9. REFERENCES

1. M. Tsuchiya, S. R. Price, S. C. Tsai, J. Moss and M. Vaughan: Molecular identification of ADP-ribosylation factor mRNAs and their expression in mammalian cells. J Biol Chem 266(5), 2772-7 (1991)

2. P. J. Peters, V. W. Hsu, C. E. Ooi, D. Finazzi, S. B. Teal, V. Oorschot, J. G. Donaldson and R. D. Klausner: Overexpression of wild-type and mutant ARF1 and ARF6: distinct perturbations of nonoverlapping membrane compartments. J Cell Biol 128(6), 1003-17 (1995)
DOI: 10.1083/jcb.128.6.1003
PMid:7896867

3. W. E. Balch, R. A. Kahn and R. Schwaninger: ADP-ribosylation factor is required for vesicular trafficking between the endoplasmic reticulum and the cis-Golgi compartment. J Biol Chem 267(18), 13053-61 (1992)

4. T. C. Taylor, R. A. Kahn and P. Melançon: Two distinct members of the ADP-ribosylation factor family of GTP-binding proteins regulate cell-free intra-Golgi transport. Cell 70(1), 69-79 (1992)
DOI: 10.1016/0092-8674(92)90534-J

5. P. Chardin, S. Paris, B. Antonny, S. Robineau, S. Béraud-Dufour, C. L. Jackson and M. Chabre: A human exchange factor for ARF contains Sec7- and pleckstrin-homology domains. Nature 384(6608), 481-4 (1996)
DOI: 10.1038/384481a0
PMid:8945478

6. A. Claude, B. P. Zhao, C. E. Kuziemsky, S. Dahan, S. J. Berger, J. P. Yan, A. D. Armold, E. M. Sullivan and P. Melançon: GBF1: A novel Golgi-associated BFA-resistant guanine nucleotide exchange factor that displays specificity for ADP-ribosylation factor 5. J Cell Biol 146(1), 71-84 (1999)
DOI: 10.1083/jcb.146.1.71
PMid:10402461 PMCid:PMC2199737

7. A. Togawa, N. Morinaga, M. Ogasawara, J. Moss and M. Vaughan: Purification and cloning of a brefeldin A-inhibited guanine nucleotide-exchange protein for ADP-ribosylation factors. J Biol Chem 274(18), 12308-15 (1999)
DOI: 10.1074/jbc.274.18.12308
PMid:10212200

8. H. D. Jones, J. Moss and M. Vaughan: BIG1 and BIG2, brefeldin A-inhibited guanine nucleotide-exchange factors for ADP-ribosylation factors. Methods Enzymol 404, 174-84 (2005)
DOI: 10.1016/S0076-6879(05)04017-6

9. S. Cockcroft, G. M. Thomas, A. Fensome, B. Geny, E. Cunningham, I. Gout, I. Hiles, N. F. Totty, O. Truong and J. J. Hsuan: Phospholipase D: a downstream effector of ARF in granulocytes. Science 263(5146), 523-6 (1994)
DOI: 10.1126/science.8290961
PMid:8290961

10. H. A. Brown, S. Gutowski, C. R. Moomaw, C. Slaughter and P. C. Sternweis: ADP-ribosylation factor, a small GTP-dependent regulatory protein, stimulates phospholipase D activity. Cell 75(6), 1137-44 (1993)
DOI: 10.1016/0092-8674(93)90323-I

11. A. Honda, M. Nogami, T. Yokozeki, M. Yamazaki, H. Nakamura, H. Watanabe, K. Kawamoto, K. Nakayama, A. J. Morris, M. A. Frohman and Y. Kanaho: Phosphatidylinositol 4-phosphate 5-kinase alpha is a downstream effector of the small G protein ARF6 in membrane ruffle formation. Cell 99(5), 521-32 (1999)
DOI: 10.1016/S0092-8674(00)81540-8

12. J. R. Doedens and K. Kirkegaard: Inhibition of cellular protein secretion by poliovirus proteins 2B and 3A. EMBO J 14(5), 894-907 (1995)

13. S. B. Deitz, D. A. Dodd, S. Cooper, P. Parham and K. Kirkegaard: MHC I-dependent antigen presentation is inhibited by poliovirus protein 3A. Proc Natl Acad Sci U S A 97(25), 13790-5 (2000)
DOI: 10.1073/pnas.250483097
PMid:11095746 PMCid:PMC17654

14. D. A. Dodd, T. H. Giddings and K. Kirkegaard: Poliovirus 3A protein limits interleukin-6 (IL-6), IL-8, and beta interferon secretion during viral infection. J Virol 75(17), 8158-65 (2001)
DOI: 10.1128/JVI.75.17.8158-8165.2001
PMid:11483761 PMCid:PMC115060

15. N. Neznanov, A. Kondratova, K. M. Chumakov, B. Angres, B. Zhumabayeva, V. I. Agol and A. V. Gudkov: Poliovirus protein 3A inhibits tumor necrosis factor (TNF)-induced apoptosis by eliminating the TNF receptor from the cell surface. J Virol 75(21), 10409-20 (2001)
DOI: 10.1128/JVI.75.21.10409-10420.2001
PMid:11581409 PMCid:PMC114615

16. J. R. Doedens, T. H. Giddings and K. Kirkegaard: Inhibition of endoplasmic reticulum-to-Golgi traffic by poliovirus protein 3A: genetic and ultrastructural analysis. J Virol 71(12), 9054-64 (1997)

17. E. Wessels, D. Duijsings, R. A. Notebaart, W. J. Melchers and F. J. van Kuppeveld: A proline-rich region in the coxsackievirus 3A protein is required for the protein to inhibit endoplasmic reticulum-to-golgi transport. J Virol 79(8), 5163-73 (2005)
DOI: 10.1128/JVI.79.8.5163-5173.2005
PMid:15795300 PMCid:PMC1069528

18. O. Beske, M. Reichelt, M. P. Taylor, K. Kirkegaard and R. Andino: Poliovirus infection blocks ERGIC-to-Golgi trafficking and induces microtubule-dependent disruption of the Golgi complex. J Cell Sci 120(Pt 18), 3207-18 (2007)

19. E. Wessels, D. Duijsings, T. K. Niu, S. Neumann, V. M. Oorschot, F. de Lange, K. H. Lanke, J. Klumperman, A. Henke, C. L. Jackson, W. J. Melchers and F. J. van Kuppeveld: A viral protein that blocks Arf1-mediated COP-I assembly by inhibiting the guanine nucleotide exchange factor GBF1. Dev Cell 11(2), 191-201 (2006)
DOI: 10.1016/j.devcel.2006.06.005
PMid:16890159

20. E. Wessels, D. Duijsings, K. H. Lanke, S. H. van Dooren, C. L. Jackson, W. J. Melchers and F. J. van Kuppeveld: Effects of picornavirus 3A Proteins on Protein Transport and GBF1-dependent COP-I recruitment. J Virol 80(23), 11852-60 (2006)

21. G. A. Belov, Q. Feng, K. Nikovics, C. L. Jackson and E. Ehrenfeld: A critical role of a cellular membrane traffic protein in poliovirus RNA replication. PLoS Pathog 4(11), e1000216 (2008)

22. S. S. Choe, D. A. Dodd and K. Kirkegaard: Inhibition of cellular protein secretion by picornaviral 3A proteins. Virology 337(1), 18-29 (2005)
DOI: 10.1016/j.virol.2005.03.036
PMid:15914217

23. J. Wang, J. Du and Q. Jin: Class I ADP-ribosylation factors are involved in enterovirus 71 replication. PLoS One 9(6), e99768 (2014)

24. K. Moffat, G. Howell, C. Knox, G. J. Belsham, P. Monaghan, M. D. Ryan and T. Wileman: Effects of foot-and-mouth disease virus nonstructural proteins on the structure and function of the early secretory pathway: 2BC but not 3A blocks endoplasmic reticulum-to-Golgi transport. J Virol 79(7), 4382-95 (2005)
DOI: 10.1128/JVI.79.7.4382-4395.2005
PMid:15767438 PMCid:PMC1061540

25. K. Moffat, C. Knox, G. Howell, S. J. Clark, H. Yang, G. J. Belsham, M. Ryan and T. Wileman: Inhibition of the secretory pathway by foot-and-mouth disease virus 2BC protein is reproduced by coexpression of 2B with 2C, and the site of inhibition is determined by the subcellular location of 2C. J Virol 81(3), 1129-39 (2007)
DOI: 10.1128/JVI.00393-06
PMid:17121791 PMCid:PMC1797538

26. G. A. Belov, C. Habbersett, D. Franco and E. Ehrenfeld: Activation of cellular Arf GTPases by poliovirus protein 3CD correlates with virus replication. J Virol 81(17), 9259-67 (2007)
DOI: 10.1128/JVI.00840-07
PMid:17567696 PMCid:PMC1951455

27. K. V. Konan, T. H. Giddings, M. Ikeda, K. Li, S. M. Lemon and K. Kirkegaard: Nonstructural protein precursor NS4A/B from hepatitis C virus alters function and ultrastructure of host secretory apparatus. J Virol 77(14), 7843-55 (2003)
DOI: 10.1128/JVI.77.14.7843-7855.2003
PMid:12829824 PMCid:PMC161946

28. M. Matto, E. H. Sklan, N. David, N. Melamed-Book, J. E. Casanova, J. S. Glenn and B. Aroeti: Role for ADP ribosylation factor 1 in the regulation of hepatitis C virus replication. J Virol 85(2), 946-56 (2011)
DOI: 10.1128/JVI.00753-10
PMid:21068255 PMCid:PMC3020004

29. L. Goueslain, K. Alsaleh, P. Horellou, P. Roingeard, V. Descamps, G. Duverlie, Y. Ciczora, C. Wychowski, J. Dubuisson and Y. Rouillé: Identification of GBF1 as a cellular factor required for hepatitis C virus RNA replication. J Virol 84(2), 773-87 (2010)
DOI: 10.1128/JVI.01190-09
PMid:19906930 PMCid:PMC2798365

30. A. W. Tai, Y. Benita, L. F. Peng, S. S. Kim, N. Sakamoto, R. J. Xavier and R. T. Chung: A functional genomic screen identifies cellular cofactors of hepatitis C virus replication. Cell Host Microbe 5(3), 298-307 (2009)
DOI: 10.1016/j.chom.2009.02.001
PMid:19286138 PMCid:PMC2756022

31. D. Egger, B. Wölk, R. Gosert, L. Bianchi, H. E. Blum, D. Moradpour and K. Bienz: Expression of hepatitis C virus proteins induces distinct membrane alterations including a candidate viral replication complex. J Virol 76(12), 5974-84 (2002)
DOI: 10.1128/JVI.76.12.5974-5984.2002
PMid:12021330 PMCid:PMC136238

32. H. Li, X. Yang, G. Yang, Z. Hong, L. Zhou, P. Yin, Y. Xiao, L. Chen, R. T. Chung and L. Zhang: Hepatitis C virus NS5A hijacks ARFGAP1 to maintain a phosphatidylinositol 4-phosphate-enriched microenvironment. J Virol 88(11), 5956-66 (2014)
DOI: 10.1128/JVI.03738-13
PMid:24623438 PMCid:PMC4093857

33. C. Wang, C. L. Timmons, Q. Shao, B. L. Kinlock, T. M. Turner, A. Iwamoto, H. Zhang, H. Liu and B. Liu: GB virus type C E2 protein inhibits human immunodeficiency virus type 1 Gag assembly by downregulating human ADP-ribosylation factor 1. Oncotarget 6(41), 43293-309 (2015)

34. N. Bhattarai and J. T. Stapleton: GB virus C: the good boy virus? Trends Microbiol 20(3), 124-30 (2012)

35. M. M. Samsa, J. A. Mondotte, N. G. Iglesias, I. Assunção-Miranda, G. Barbosa-Lima, A. T. Da Poian, P. T. Bozza and A. V. Gamarnik: Dengue virus capsid protein usurps lipid droplets for viral particle formation. PLoS Pathog 5(10), e1000632 (2009)

36. N. G. Iglesias, J. A. Mondotte, L. A. Byk, F. A. De Maio, M. M. Samsa, C. Alvarez and A. V. Gamarnik: Dengue Virus Uses a Non-Canonical Function of the Host GBF1-Arf-COPI System for Capsid Protein Accumulation on Lipid Droplets. Traffic 16(9), 962-77 (2015)
DOI: 10.1111/tra.12305
PMid:26031340 PMCid:PMC4543523

37. K. G. Soni, G. A. Mardones, R. Sougrat, E. Smirnova, C. L. Jackson and J. S. Bonifacino: Coatomer-dependent protein delivery to lipid droplets. J Cell Sci 122(Pt 11), 1834-41 (2009)

38. F. N. Katz and H. F. Lodish: Transmembrane biogenesis of the vesicular stomatitis virus glycoprotein. J Cell Biol 80(2), 416-26 (1979)
DOI: 10.1083/jcb.80.2.416
PMid:222771

39. D. Panda, A. Das, P. X. Dinh, S. Subramaniam, D. Nayak, N. J. Barrows, J. L. Pearson, J. Thompson, D. L. Kelly, I. Ladunga and A. K. Pattnaik: RNAi screening reveals requirement for host cell secretory pathway in infection by diverse families of negative-strand RNA viruses. Proc Natl Acad Sci U S A 108(47), 19036-41 (2011)
DOI: 10.1073/pnas.1113643108
PMid:22065774 PMCid:PMC3223448

40. J. F. Roeth, M. Williams, M. R. Kasper, T. M. Filzen and K. L. Collins: HIV-1 Nef disrupts MHC-I trafficking by recruiting AP-1 to the MHC-I cytoplasmic tail. J Cell Biol 167(5), 903-13 (2004)
DOI: 10.1083/jcb.200407031
PMid:15569716 PMCid:PMC2172469

41. Q. T. Shen, X. Ren, R. Zhang, I. H. Lee and J. H. Hurley: HIV-1 Nef hijacks clathrin coats by stabilizing AP-1:Arf1 polygons. Science 350(6259), aac5137 (2015)

42. R. Chaudhuri, O. W. Lindwasser, W. J. Smith, J. H. Hurley and J. S. Bonifacino: Downregulation of CD4 by human immunodeficiency virus type 1 Nef is dependent on clathrin and involves direct interaction of Nef with the AP2 clathrin adaptor. J Virol 81(8), 3877-90 (2007)
DOI: 10.1128/JVI.02725-06
PMid:17267500 PMCid:PMC1866153

43. S. Yamayoshi, G. Neumann and Y. Kawaoka: Role of the GTPase Rab1b in ebolavirus particle formation. J Virol 84(9), 4816-20 (2010)
DOI: 10.1128/JVI.00010-10
PMid:20164217 PMCid:PMC2863720

44. E. Sun, J. He and X. Zhuang: Dissecting the role of COPI complexes in influenza virus infection. J Virol 87(5), 2673-85 (2013)
DOI: 10.1128/JVI.02277-12
PMid:23255804 PMCid:PMC3571408

45. S. Kliche, W. Nagel, E. Kremmer, C. Atzler, A. Ege, T. Knorr, U. Koszinowski, W. Kolanus and J. Haas: Signaling by human herpesvirus 8 kaposin A through direct membrane recruitment of cytohesin-1. Mol Cell 7(4), 833-43 (2001)
DOI: 10.1016/S1097-2765(01)00227-1

46. S. Dales, H. J. Eggers, i. Tamm and G. E. Palade: electron microscopic study of the formation of poliovirus. Virology 26, 379-89 (1965)
DOI: 10.1016/0042-6822(65)90001-2

47. D. Stuart and J. Fogh: Micromorphology of Fl cells infected with polio and coxsackie viruses. Virology 13(2), 177-& (1961)

48. M. W. Cho, N. Teterina, D. Egger, K. Bienz and E. Ehrenfeld: Membrane rearrangement and vesicle induction by recombinant poliovirus 2C and 2BC in human cells. Virology 202(1), 129-45 (1994)
DOI: 10.1006/viro.1994.1329
PMid:8009827

49. I. V. Sandoval and L. Carrasco: Poliovirus infection and expression of the poliovirus protein 2B provoke the disassembly of the Golgi complex, the organelle target for the antipoliovirus drug Ro-090179. J Virol 71(6), 4679-93 (1997)

50. G. A. Belov, V. Nair, B. T. Hansen, F. H. Hoyt, E. R. Fischer and E. Ehrenfeld: Complex dynamic development of poliovirus membranous replication complexes. J Virol 86(1), 302-12 (2012)
DOI: 10.1128/JVI.05937-11
PMid:22072780 PMCid:PMC3255921

51. E. C. Freundt, L. Yu, C. S. Goldsmith, S. Welsh, A. Cheng, B. Yount, W. Liu, M. B. Frieman, U. J. Buchholz, G. R. Screaton, J. Lippincott-Schwartz, S. R. Zaki, X. N. Xu, R. S. Baric, K. Subbarao and M. J. Lenardo: The open reading frame 3a protein of severe acute respiratory syndrome-associated coronavirus promotes membrane rearrangement and cell death. J Virol 84(2), 1097-109 (2010)
DOI: 10.1128/JVI.01662-09
PMid:19889773 PMCid:PMC2798367

52. K. Knoops, M. Kikkert, S. H. Worm, J. C. Zevenhoven-Dobbe, Y. van der Meer, A. J. Koster, A. M. Mommaas and E. J. Snijder: SARS-coronavirus replication is supported by a reticulovesicular network of modified endoplasmic reticulum. PLoS Biol 6(9), e226 (2008)

53. E. J. Snijder, Y. van der Meer, J. Zevenhoven-Dobbe, J. J. Onderwater, J. van der Meulen, H. K. Koerten and A. M. Mommaas: Ultrastructure and origin of membrane vesicles associated with the severe acute respiratory syndrome coronavirus replication complex. J Virol 80(12), 5927-40 (2006)
DOI: 10.1128/JVI.02501-05
PMid:16731931 PMCid:PMC1472606

54. M. H. Verheije, M. Raaben, M. Mari, E. G. Te Lintelo, F. Reggiori, F. J. van Kuppeveld, P. J. Rottier and C. A. de Haan: Mouse hepatitis coronavirus RNA replication depends on GBF1-mediated ARF1 activation. PLoS Pathog 4(6), e1000088 (2008)

55. C. Barlowe, L. Orci, T. Yeung, M. Hosobuchi, S. Hamamoto, N. Salama, M. F. Rexach, M. Ravazzola, M. Amherdt and R. Schekman: COPII: a membrane coat formed by Sec proteins that drive vesicle budding from the endoplasmic reticulum. Cell 77(6), 895-907 (1994)
DOI: 10.1016/0092-8674(94)90138-4

56. K. Matsuoka, L. Orci, M. Amherdt, S. Y. Bednarek, S. Hamamoto, R. Schekman and T. Yeung: COPII-coated vesicle formation reconstituted with purified coat proteins and chemically defined liposomes. Cell 93(2), 263-75 (1998)
DOI: 10.1016/S0092-8674(00)81577-9

57. C. Barlowe and R. Schekman: SEC12 encodes a guanine-nucleotide-exchange factor essential for transport vesicle budding from the ER. Nature 365(6444), 347-9 (1993)
DOI: 10.1038/365347a0
PMid:8377826

58. X. Bi, R. A. Corpina and J. Goldberg: Structure of the Sec23/24-Sar1 pre-budding complex of the COPII vesicle coat. Nature 419(6904), 271-7 (2002)
DOI: 10.1038/nature01040
PMid:12239560

59. E. Miller, B. Antonny, S. Hamamoto and R. Schekman: Cargo selection into COPII vesicles is driven by the Sec24p subunit. EMBO J 21(22), 6105-13 (2002)
DOI: 10.1093/emboj/cdf605
PMid:12426382 PMCid:PMC137197

60. E. A. Miller, T. H. Beilharz, P. N. Malkus, M. C. Lee, S. Hamamoto, L. Orci and R. Schekman: Multiple cargo binding sites on the COPII subunit Sec24p ensure capture of diverse membrane proteins into transport vesicles. Cell 114(4), 497-509 (2003)
DOI: 10.1016/S0092-8674(03)00609-3

61. X. Bi, J. D. Mancias and J. Goldberg: Insights into COPII coat nucleation from the structure of Sec23.Sar1 complexed with the active fragment of Sec31. Dev Cell 13(5), 635-45 (2007)
DOI: 10.1016/j.devcel.2007.10.006
PMid:17981133 PMCid:PMC2686382

62. S. Fath, J. D. Mancias, X. Bi and J. Goldberg: Structure and organization of coat proteins in the COPII cage. Cell 129(7), 1325-36 (2007)
DOI: 10.1016/j.cell.2007.05.036
PMid:17604721

63. N. Bhattacharya, J. O Donnell and S. M. Stagg: The structure of the Sec13/31 COPII cage bound to Sec23. J Mol Biol 420(4-5), 324-34 (2012)

64. S. M. Stagg, C. Gürkan, D. M. Fowler, P. LaPointe, T. R. Foss, C. S. Potter, B. Carragher and W. E. Balch: Structure of the Sec13/31 COPII coat cage. Nature 439(7073), 234-8 (2006)
DOI: 10.1038/nature04339
PMid:16407955

65. A. Bielli, C. J. Haney, G. Gabreski, S. C. Watkins, S. I. Bannykh and M. Aridor: Regulation of Sar1 NH2 terminus by GTP binding and hydrolysis promotes membrane deformation to control COPII vesicle fission. J Cell Biol 171(6), 919-24 (2005)
DOI: 10.1083/jcb.200509095
PMid:16344311 PMCid:PMC2171319

66. B. Antonny, D. Madden, S. Hamamoto, L. Orci and R. Schekman: Dynamics of the COPII coat with GTP and stable analogues. Nat Cell Biol 3(6), 531-7 (2001)
DOI: 10.1038/35078500
PMid:11389436

67. L. F. Kung, S. Pagant, E. Futai, J. G. D’Arcangelo, R. Buchanan, J. C. Dittmar, R. J. Reid, R. Rothstein, S. Hamamoto, E. L. Snapp, R. Schekman and E. A. Miller: Sec24p and Sec16p cooperate to regulate the GTP cycle of the COPII coat. EMBO J 31(4), 1014-27 (2012)
DOI: 10.1038/emboj.2011.444
PMid:22157747 PMCid:PMC3280547

68. R. C. Rust, L. Landmann, R. Gosert, B. L. Tang, W. Hong, H. P. Hauri, D. Egger and K. Bienz: Cellular COPII proteins are involved in production of the vesicles that form the poliovirus replication complex. J Virol 75(20), 9808-18 (2001)
DOI: 10.1128/JVI.75.20.9808-9818.2001
PMid:11559814 PMCid:PMC114553

69. T. Yorimitsu and K. Sato: Insights into structural and regulatory roles of Sec16 in COPII vesicle formation at ER exit sites. Mol Biol Cell 23(15), 2930-42 (2012)
DOI: 10.1091/mbc.E12-05-0356
PMid:22675024 PMCid:PMC3408419

70. M. Trahey, H. S. Oh, C. E. Cameron and J. C. Hay: Poliovirus infection transiently increases COPII vesicle budding. J Virol 86(18), 9675-82 (2012)
DOI: 10.1128/JVI.01159-12
PMid:22740409 PMCid:PMC3446582

71. R. Midgley, K. Moffat, S. Berryman, P. Hawes, J. Simpson, D. Fullen, D. J. Stephens, A. Burman and T. Jackson: A role for endoplasmic reticulum exit sites in foot-and-mouth disease virus infection. J Gen Virol 94(Pt 12), 2636-46 (2013)

72. T. M. Sharp, S. Guix, K. Katayama, S. E. Crawford and M. K. Estes: Inhibition of cellular protein secretion by norwalk virus nonstructural protein p22 requires a mimic of an endoplasmic reticulum export signal. PLoS One 5(10), e13130 (2010)

73. T. M. Sharp, S. E. Crawford, N. J. Ajami, F. H. Neill, R. L. Atmar, K. Katayama, B. Utama and M. K. Estes: Secretory pathway antagonism by calicivirus homologues of Norwalk virus nonstructural protein p22 is restricted to noroviruses. Virol J 9, 181 (2012)

74. M. Schwartz, J. Chen, M. Janda, M. Sullivan, J. den Boon and P. Ahlquist: A positive-strand RNA virus replication complex parallels form and function of retrovirus capsids. Mol Cell 9(3), 505-14 (2002)
DOI: 10.1016/S1097-2765(02)00474-4

75. B. L. Gancarz, L. Hao, Q. He, M. A. Newton and P. Ahlquist: Systematic identification of novel, essential host genes affecting bromovirus RNA replication. PLoS One 6(8), e23988 (2011)

76. D. B. Kushner, B. D. Lindenbach, V. Z. Grdzelishvili, A. O. Noueiry, S. M. Paul and P. Ahlquist: Systematic, genome-wide identification of host genes affecting replication of a positive-strand RNA virus. Proc Natl Acad Sci U S A 100(26), 15764-9 (2003)
DOI: 10.1073/pnas.2536857100
PMid:14671320 PMCid:PMC307642

77. A. Diaz, J. Zhang, A. Ollwerther, X. Wang and P. Ahlquist: Host ESCRT proteins are required for bromovirus RNA replication compartment assembly and function. PLoS Pathog 11(3), e1004742 (2015)

78. A. Diaz, X. Wang and P. Ahlquist: Membrane-shaping host reticulon proteins play crucial roles in viral RNA replication compartment formation and function. Proc Natl Acad Sci U S A 107(37), 16291-6 (2010)
DOI: 10.1073/pnas.1011105107
PMid:20805477 PMCid:PMC2941330

79. C. Suarez, G. Andres, A. Kolovou, S. Hoppe, M. L. Salas, P. Walther and J. Krijnse Locker: African swine fever virus assembles a single membrane derived from rupture of the endoplasmic reticulum. Cell Microbiol 17(11), 1683-98 (2015)
DOI: 10.1111/cmi.12468
PMid:26096327

80. J. M. Rodríguez, R. García-Escudero, M. L. Salas and G. Andrés: African swine fever virus structural protein p54 is essential for the recruitment of envelope precursors to assembly sites. J Virol 78(8), 4299-1313 (2004)
DOI: 10.1128/JVI.78.8.4299-4313.2004
PMid:15047843 PMCid:PMC374266

81. M. Windsor, P. Hawes, P. Monaghan, E. Snapp, M. L. Salas, J. M. Rodríguez and T. Wileman: Mechanism of collapse of endoplasmic reticulum cisternae during African swine fever virus infection. Traffic 13(1), 30-42 (2012)
DOI: 10.1111/j.1600-0854.2011.01293.x
PMid:21951707 PMCid:PMC3237792

82. C. L. Netherton, M. C. McCrossan, M. Denyer, S. Ponnambalam, J. Armstrong, H. H. Takamatsu and T. E. Wileman: African swine fever virus causes microtubule-dependent dispersal of the trans-golgi network and slows delivery of membrane protein to the plasma membrane. J Virol 80(22), 11385-92 (2006)
DOI: 10.1128/JVI.00439-06
PMid:16956944 PMCid:PMC1642160

83. M. McCrossan, M. Windsor, S. Ponnambalam, J. Armstrong and T. Wileman: The trans Golgi network is lost from cells infected with African swine fever virus. J Virol 75(23), 11755-65 (2001)
DOI: 10.1128/JVI.75.23.11755-11765.2001
PMid:11689656 PMCid:PMC114761

84. D. Pérez-Núñez, E. García-Urdiales, M. Martínez-Bonet, M. L. Nogal, S. Barroso, Y. Revilla and R. Madrid: CD2v Interacts with Adaptor Protein AP-1 during African Swine Fever Infection. PLoS One 10(4), e0123714 (2015)

85. J. M. Mackenzie, M. K. Jones and E. G. Westaway: Markers for trans-Golgi membranes and the intermediate compartment localize to induced membranes with distinct replication functions in flavivirus-infected cells. J Virol 73(11), 9555-67 (1999)

86. P. W. Chu and E. G. Westaway: Molecular and ultrastructural analysis of heavy membrane fractions associated with the replication of Kunjin virus RNA. Arch Virol 125(1-4), 177-91 (1992)

87. E. G. Westaway, J. M. Mackenzie, M. T. Kenney, M. K. Jones and A. A. Khromykh: Ultrastructure of Kunjin virus-infected cells: colocalization of NS1 and NS3 with double-stranded RNA, and of NS2B with NS3, in virus-induced membrane structures. J Virol 71(9), 6650-61 (1997)

88. J. M. Mackenzie and E. G. Westaway: Assembly and maturation of the flavivirus Kunjin virus appear to occur in the rough endoplasmic reticulum and along the secretory pathway, respectively. J Virol 75(22), 10787-99 (2001)
DOI: 10.1128/JVI.75.22.10787-10799.2001
PMid:11602720 PMCid:PMC114660

89. J. Roosendaal, E. G. Westaway, A. Khromykh and J. M. Mackenzie: Regulated cleavages at the West Nile virus NS4A-2K-NS4B junctions play a major role in rearranging cytoplasmic membranes and Golgi trafficking of the NS4A protein. J Virol 80(9), 4623-32 (2006)
DOI: 10.1128/JVI.80.9.4623-4632.2006
PMid:16611922 PMCid:PMC1472005

90. P. H. Kaufusi, J. F. Kelley, R. Yanagihara and V. R. Nerurkar: Induction of endoplasmic reticulum-derived replication-competent membrane structures by West Nile virus non-structural protein 4B. PLoS One 9(1), e84040 (2014)

91. S. Welsch, S. Miller, I. Romero-Brey, A. Merz, C. K. Bleck, P. Walther, S. D. Fuller, C. Antony, J. Krijnse-Locker and R. Bartenschlager: Composition and three-dimensional architecture of the dengue virus replication and assembly sites. Cell Host Microbe 5(4), 365-75 (2009)
DOI: 10.1016/j.chom.2009.03.007
PMid:19380115

92. S. Miller, S. Kastner, J. Krijnse-Locker, S. Bühler and R. Bartenschlager: The non-structural protein 4A of dengue virus is an integral membrane protein inducing membrane alterations in a 2K-regulated manner. J Biol Chem 282(12), 8873-82 (2007)
DOI: 10.1074/jbc.M609919200
PMid:17276984

93. D. Magliano, J. A. Marshall, D. S. Bowden, N. Vardaxis, J. Meanger and J. Y. Lee: Rubella virus replication complexes are virus-modified lysosomes. Virology 240(1), 57-63 (1998)
DOI: 10.1006/viro.1997.8906
PMid:9448689

94. J. Fontana, W. P. Tzeng, G. Calderita, A. Fraile-Ramos, T. K. Frey and C. Risco: Novel replication complex architecture in rubella replicon-transfected cells. Cell Microbiol 9(4), 875-90 (2007)
DOI: 10.1111/j.1462-5822.2006.00837.x
PMid:17087733

95. J. Fontana, C. López-Iglesias, W. P. Tzeng, T. K. Frey, J. J. Fernández and C. Risco: Three-dimensional structure of Rubella virus factories. Virology 405(2), 579-91 (2010)
DOI: 10.1016/j.virol.2010.06.043
PMid:20655079

96. P. M. Grimley, I. K. Berezesky and R. M. Friedman: Cytoplasmic structures associated with an arbovirus infection: loci of viral ribonucleic acid synthesis. J Virol 2(11), 1326-38 (1968)

97. A. Salonen, L. Vasiljeva, A. Merits, J. Magden, E. Jokitalo and L. Kääriäinen: Properly folded nonstructural polyprotein directs the semliki forest virus replication complex to the endosomal compartment. J Virol 77(3), 1691-702 (2003)
DOI: 10.1128/JVI.77.3.1691-1702.2003
PMid:12525603 PMCid:PMC140886

98. P. Kujala, A. Ikäheimonen, N. Ehsani, H. Vihinen, P. Auvinen and L. Kääriäinen: Biogenesis of the Semliki Forest virus RNA replication complex. J Virol 75(8), 3873-84 (2001)
DOI: 10.1128/JVI.75.8.3873-3884.2001
PMid:11264376 PMCid:PMC114878

99. E. I. Frolova, R. Gorchakov, L. Pereboeva, S. Atasheva and I. Frolov: Functional Sindbis virus replicative complexes are formed at the plasma membrane. J Virol 84(22), 11679-95 (2010)
DOI: 10.1128/JVI.01441-10
PMid:20826696 PMCid:PMC2977861

100. P. Spuul, G. Balistreri, L. Kääriäinen and T. Ahola: Phosphatidylinositol 3-kinase-, actin-, and microtubule-dependent transport of Semliki Forest Virus replication complexes from the plasma membrane to modified lysosomes. J Virol 84(15), 7543-57 (2010)
DOI: 10.1128/JVI.00477-10
PMid:20484502 PMCid:PMC2897599

101. L. R. Delgui, J. F. Rodríguez and M. I. Colombo: The endosomal pathway and the Golgi complex are involved in the infectious bursal disease virus life cycle. J Virol 87(16), 8993-9007 (2013)
DOI: 10.1128/JVI.03152-12
PMid:23741000 PMCid:PMC3754037

102. M. Galloux, S. Libersou, N. Morellet, S. Bouaziz, B. Da Costa, M. Ouldali, J. Lepault and B. Delmas: Infectious bursal disease virus, a non-enveloped virus, possesses a capsid-associated peptide that deforms and perforates biological membranes. J Biol Chem 282(28), 20774-84 (2007)
DOI: 10.1074/jbc.M701048200
PMid:17488723

103. N. Tolonen, L. Doglio, S. Schleich and J. Krijnse Locker: Vaccinia virus DNA replication occurs in endoplasmic reticulum-enclosed cytoplasmic mini-nuclei. Mol Biol Cell 12(7), 2031-46 (2001)
DOI: 10.1091/mbc.12.7.2031
PMid:11452001 PMCid:PMC55651

104. S. Dales and E. H. Mosbach: Vaccinia as a model for membrane biogenesis. Virology 35(4), 564-83 (1968)
DOI: 10.1016/0042-6822(68)90286-9

105. P. S. Satheshkumar, A. Weisberg and B. Moss: Vaccinia virus H7 protein contributes to the formation of crescent membrane precursors of immature virions. J Virol 83(17), 8439-50 (2009)
DOI: 10.1128/JVI.00877-09
PMid:19553304 PMCid:PMC2738178

106. X. Meng, X. Wu, B. Yan, J. Deng and Y. Xiang: Analysis of the role of vaccinia virus H7 in virion membrane biogenesis with an H7-deletion mutant. J Virol 87(14), 8247-53 (2013)
DOI: 10.1128/JVI.00845-13
PPMid:23678177 PMCid:PMC3700178

107. W. Resch, A. S. Weisberg and B. Moss: Vaccinia virus nonstructural protein encoded by the A11R gene is required for formation of the virion membrane. J Virol 79(11), 6598-609 (2005)
DOI: 10.1128/JVI.79.11.6598-6609.2005
PMid:15890898 PMCid:PMC1112135

108. X. Meng, A. Embry, L. Rose, B. Yan, C. Xu and Y. Xiang: Vaccinia virus A6 is essential for virion membrane biogenesis and localization of virion membrane proteins to sites of virion assembly. J Virol 86(10), 5603-13 (2012)
DOI: 10.1128/JVI.00330-12
PMid:22398288 PMCid:PMC3347295

109. L. Maruri-Avidal, A. S. Weisberg and B. Moss: Direct formation of vaccinia virus membranes from the endoplasmic reticulum in the absence of the newly characterized L2-interacting protein A30.5. J Virol 87(22), 12313-26 (2013)
DOI: 10.1128/JVI.02137-13
PMid:24027302 PMCid:PMC380789

110. L. Maruri-Avidal, A. Domi, A. S. Weisberg and B. Moss: Participation of vaccinia virus l2 protein in the formation of crescent membranes and immature virions. J Virol 85(6), 2504-11 (2011)
DOI: 10.1128/JVI.02505-10
PMid:21228235 PMCid:PMC3067936

111. P. Chlanda, M. A. Carbajal, M. Cyrklaff, G. Griffiths and J. Krijnse-Locker: Membrane rupture generates single open membrane sheets during vaccinia virus assembly. Cell Host Microbe 6(1), 81-90 (2009)
DOI: 10.1016/j.chom.2009.05.021
PMid:19616767

112. B. Sodeik, G. Griffiths, M. Ericsson, B. Moss and R. W. Doms: Assembly of vaccinia virus: effects of rifampin on the intracellular distribution of viral protein p65. J Virol 68(2), 1103-14 (1994)

113. P. Traktman, K. Liu, J. DeMasi, R. Rollins, S. Jesty and B. Unger: Elucidating the essential role of the A14 phosphoprotein in vaccinia virus morphogenesis: construction and characterization of a tetracycline-inducible recombinant. J Virol 74(8), 3682-95 (2000)
DOI: 10.1128/JVI.74.8.3682-3695.2000
PMid:10729144 PMCid:PMC111878

114. B. Unger, J. Mercer, K. A. Boyle and P. Traktman: Biogenesis of the vaccinia virus membrane: genetic and ultrastructural analysis of the contributions of the A14 and A17 proteins. J Virol 87(2), 1083-97 (2013)
DOI: 10.1128/JVI.02529-12
PMid:23135725 PMCid:PMC3554067

115. J. R. Rodríguez, C. Risco, J. L. Carrascosa, M. Esteban and D. Rodríguez: Vaccinia virus 15-kilodalton (A14L) protein is essential for assembly and attachment of viral crescents to virosomes. J Virol 72(2), 1287-96 (1998)

116. D. Rodríguez, M. Esteban and J. R. Rodríguez: Vaccinia virus A17L gene product is essential for an early step in virion morphogenesis. J Virol 69(8), 4640-8 (1995)

117. D. Rodríguez, C. Risco, J. R. Rodríguez, J. L. Carrascosa and M. Esteban: Inducible expression of the vaccinia virus A17L gene provides a synchronized system to monitor sorting of viral proteins during morphogenesis. J Virol 70(11), 7641-53 (1996)

118. E. J. Wolffe, D. M. Moore, P. J. Peters and B. Moss: Vaccinia virus A17L open reading frame encodes an essential component of nascent viral membranes that is required to initiate morphogenesis. J Virol 70(5), 2797-808 (1996)

119. H. Bisht, A. S. Weisberg, P. Szajner and B. Moss: Assembly and disassembly of the capsid-like external scaffold of immature virions during vaccinia virus morphogenesis. J Virol 83(18), 9140-50 (2009)
DOI: 10.1128/JVI.00875-09
PMid:19570860 PMCid:PMC2738239

120. T. Betakova, E. J. Wolffe and B. Moss: Regulation of vaccinia virus morphogenesis: phosphorylation of the A14L and A17L membrane proteins and C-terminal truncation of the A17L protein are dependent on the F10L kinase. J Virol 73(5), 3534-43 (1999)

121. P. Traktman, A. Caligiuri, S. A. Jesty, K. Liu and U. Sankar: Temperature-sensitive mutants with lesions in the vaccinia virus F10 kinase undergo arrest at the earliest stage of virion morphogenesis. J Virol 69(10), 6581-7 (1995)

122. B. Moss: Poxvirus membrane biogenesis. Virology 479-480, 619-26 (2015)

123. B. Sodeik, R. W. Doms, M. Ericsson, G. Hiller, C. E. Machamer, W. van ’t Hof, G. van Meer, B. Moss and G. Griffiths: Assembly of vaccinia virus: role of the intermediate compartment between the endoplasmic reticulum and the Golgi stacks. J Cell Biol 121(3), 521-41 (1993)
DOI: 10.1083/jcb.121.3.521
PMid:8486734

124. C. Risco, J. R. Rodríguez, C. López-Iglesias, J. L. Carrascosa, M. Esteban and D. Rodríguez: Endoplasmic reticulum-Golgi intermediate compartment membranes and vimentin filaments participate in vaccinia virus assembly. J Virol 76(4), 1839-55 (2002)
DOI: 10.1128/JVI.76.4.1839-1855.2002
PMid:11799179 PMCid:PMC135913

125. M. Husain and B. Moss: Evidence against an essential role of COPII-mediated cargo transport to the endoplasmic reticulum-Golgi intermediate compartment in the formation of the primary membrane of vaccinia virus. J Virol 77(21), 11754-66 (2003)
DOI: 10.1128/JVI.77.21.11754-11766.2003
PMid:14557660 PMCid:PMC229368

126. L. Maruri-Avidal, A. S. Weisberg, H. Bisht and B. Moss: Analysis of viral membranes formed in cells infected by a vaccinia virus L2-deletion mutant suggests their origin from the endoplasmic reticulum. J Virol 87(3), 1861-71 (2013)
DOI: 10.1128/JVI.02779-12
PMid:23192873 PMCid:PMC3554160

127. L. Maruri-Avidal, A. S. Weisberg and B. Moss: Vaccinia virus L2 protein associates with the endoplasmic reticulum near the growing edge of crescent precursors of immature virions and stabilizes a subset of viral membrane proteins. J Virol 85(23), 12431-41 (2011)
DOI: 10.1128/JVI.05573-11
PMid:21917978 PMCid:PMC3209352

128. K. J. Erlandson, H. Bisht, A. S. Weisberg, S. I. Hyun, B. T. Hansen, E. R. Fischer, J. E. Hinshaw and B. Moss: Poxviruses Encode a Reticulon-Like Protein that Promotes Membrane Curvature. Cell Rep (2016)

129. C. M. Heath, M. Windsor and T. Wileman: Aggresomes resemble sites specialized for virus assembly. J Cell Biol 153(3), 449-55 (2001)
DOI: 10.1083/jcb.153.3.449
PMid:11331297 PMCid:PMC2190574

130. J. A. Johnston, M. E. Illing and R. R. Kopito: Cytoplasmic dynein/dynactin mediates the assembly of aggresomes. Cell Motil Cytoskeleton 53(1), 26-38 (2002)
DOI: 10.1002/cm.10057
PMid:12211113

131. J. A. Johnston, C. L. Ward and R. R. Kopito: Aggresomes: a cellular response to misfolded proteins. J Cell Biol 143(7), 1883-98 (1998)
DOI: 10.1083/jcb.143.7.1883
PMid:9864362 PMCid:PMC2175217

132. S. Stefanovic, M. Windsor, K. I. Nagata, M. Inagaki and T. Wileman: Vimentin rearrangement during African swine fever virus infection involves retrograde transport along microtubules and phosphorylation of vimentin by calcium calmodulin kinase II. J Virol 79(18), 11766-75 (2005)
DOI: 10.1128/JVI.79.18.11766-11775.2005
PMid:16140754 PMCid:PMC1212593

133. C. Haolong, N. Du, T. Hongchao, Y. Yang, Z. Wei, Z. Hua, Z. Wenliang, S. Lei and T. Po: Enterovirus 71 VP1 activates calmodulin-dependent protein kinase II and results in the rearrangement of vimentin in human astrocyte cells. PLoS One 8(9), e73900 (2013)

134. S. Das, A. Vasanji and P. E. Pellett: Three-dimensional structure of the human cytomegalovirus cytoplasmic virion assembly complex includes a reoriented secretory apparatus. J Virol 81(21), 11861-9 (2007)
DOI: 10.1128/JVI.01077-07
PMid:17715239 PMCid:PMC2168812

135. S. Das and P. E. Pellett: Spatial relationships between markers for secretory and endosomal machinery in human cytomegalovirus-infected cells versus those in uninfected cells. J Virol 85(12), 5864-79 (2011)
DOI: 10.1128/JVI.00155-11
PMid:21471245 PMCid:PMC3126327

136. V. Sanchez, K. D. Greis, E. Sztul and W. J. Britt: Accumulation of virion tegument and envelope proteins in a stable cytoplasmic compartment during human cytomegalovirus replication: characterization of a potential site of virus assembly. J Virol 74(2), 975-86 (2000)
DOI: 10.1128/JVI.74.2.975-986.2000
PMid:10623760 PMCid:PMC111618

137. N. J. Buchkovich, T. G. Maguire and J. C. Alwine: Role of the endoplasmic reticulum chaperone BiP, SUN domain proteins, and dynein in altering nuclear morphology during human cytomegalovirus infection. J Virol 84(14), 7005-17 (2010)
DOI: 10.1128/JVI.00719-10
PMid:20484513 PMCid:PMC2898220

138. S. Das, D. A. Ortiz, S. J. Gurczynski, F. Khan and P. E. Pellett: Identification of human cytomegalovirus genes important for biogenesis of the cytoplasmic virion assembly complex. J Virol 88(16), 9086-99 (2014)
DOI: 10.1128/JVI.01141-14
PMid:24899189 PMCid:PMC4136295

139. N. J. Buchkovich, T. G. Maguire, A. W. Paton, J. C. Paton and J. C. Alwine: The endoplasmic reticulum chaperone BiP/GRP78 is important in the structure and function of the human cytomegalovirus assembly compartment. J Virol 83(22), 11421-8 (2009)
DOI: 10.1128/JVI.00762-09
PMid:19741001 PMCid:PMC2772683

140. S. V. Indran, M. E. Ballestas and W. J. Britt: Bicaudal D1-dependent trafficking of human cytomegalovirus tegument protein pp150 in virus-infected cells. J Virol 84(7), 3162-77 (2010)
DOI: 10.1128/JVI.01776-09
PMid:20089649 PMCid:PMC2838089

141. M. A. Krzyzaniak, M. Mach and W. J. Britt: HCMV-encoded glycoprotein M (UL100) interacts with Rab11 effector protein FIP4. Traffic 10(10), 1439-57 (2009)
DOI: 10.1111/j.1600-0854.2009.00967.x
PMid:19761540 PMCid:PMC4118585

142. L. M. Hook, F. Grey, R. Grabski, R. Tirabassi, T. Doyle, M. Hancock, I. Landais, S. Jeng, S. McWeeney, W. Britt and J. A. Nelson: Cytomegalovirus miRNAs target secretory pathway genes to facilitate formation of the virion assembly compartment and reduce cytokine secretion. Cell Host Microbe 15(3), 363-73 (2014)
DOI: 10.1016/j.chom.2014.02.004
PMid:24629342 PMCid:PMC4029511

143. P. Lučin, H. Mahmutefendić, G. Blagojević Zagorac and M. Ilić Tomaš: Cytomegalovirus immune evasion by perturbation of endosomal trafficking. Cell Mol Immunol 12(2), 154-69 (2015)
DOI: 10.1038/cmi.2014.85
PMid:25263490 PMCid:PMC4654299

144. A. Halenius, C. Gerke and H. Hengel: Classical and non-classical MHC I molecule manipulation by human cytomegalovirus: so many targets—but how many arrows in the quiver? Cell Mol Immunol 12(2), 139-53 (2015)

145. T. R. Jones, E. J. Wiertz, L. Sun, K. N. Fish, J. A. Nelson and H. L. Ploegh: Human cytomegalovirus US3 impairs transport and maturation of major histocompatibility complex class I heavy chains. Proc Natl Acad Sci U S A 93(21), 11327-33 (1996)
DOI: 10.1073/pnas.93.21.11327
PMid:8876135 PMCid:PMC38057

146. K. Ahn, A. Angulo, P. Ghazal, P. A. Peterson, Y. Yang and K. Früh: Human cytomegalovirus inhibits antigen presentation by a sequential multistep process. Proc Natl Acad Sci U S A 93(20), 10990-5 (1996)
DOI: 10.1073/pnas.93.20.10990
PMid:8855296 PMCid:PMC38271

147. W. Smith, P. Tomasec, R. Aicheler, A. Loewendorf, I. Nemčovičová, E. C. Wang, R. J. Stanton, M. Macauley, P. Norris, L. Willen, E. Ruckova, A. Nomoto, P. Schneider, G. Hahn, D. M. Zajonc, C. F. Ware, G. W. Wilkinson and C. A. Benedict: Human cytomegalovirus glycoprotein UL141 targets the TRAIL death receptors to thwart host innate antiviral defenses. Cell Host Microbe 13(3), 324-35 (2013)
DOI: 10.1016/j.chom.2013.02.003
PMid:23498957 PMCid:PMC3601332

148. P. Tomasec, E. C. Wang, A. J. Davison, B. Vojtesek, M. Armstrong, C. Griffin, B. P. McSharry, R. J. Morris, S. Llewellyn-Lacey, C. Rickards, A. Nomoto, C. Sinzger and G. W. Wilkinson: Downregulation of natural killer cell-activating ligand CD155 by human cytomegalovirus UL141. Nat Immunol 6(2), 181-8 (2005)
DOI: 10.1038/ni1156
PMid:15640804 PMCid:PMC2844263

149. N. J. Bennett, O. Ashiru, F. J. Morgan, Y. Pang, G. Okecha, R. A. Eagle, J. Trowsdale, J. G. Sissons and M. R. Wills: Intracellular sequestration of the NKG2D ligand ULBP3 by human cytomegalovirus. J Immunol 185(2), 1093-102 (2010)
DOI: 10.4049/jimmunol.1000789
PMid:20530255

150. O. Ashiru, N. J. Bennett, L. H. Boyle, M. Thomas, J. Trowsdale and M. R. Wills: NKG2D ligand MICA is retained in the cis-Golgi apparatus by human cytomegalovirus protein UL142. J Virol 83(23), 12345-54 (2009)
DOI: 10.1128/JVI.01175-09
PMid:19793804 PMCid:PMC2786768

151. J. Wu, N. J. Chalupny, T. J. Manley, S. R. Riddell, D. Cosman and T. Spies: Intracellular retention of the MHC class I-related chain B ligand of NKG2D by the human cytomegalovirus UL16 glycoprotein. J Immunol 170(8), 4196-200 (2003)
DOI: 10.4049/jimmunol.170.8.4196
PMid:12682252

152. M. H. Furman, N. Dey, D. Tortorella and H. L. Ploegh: The human cytomegalovirus US10 gene product delays trafficking of major histocompatibility complex class I molecules. J Virol 76(22), 11753-6 (2002)
DOI: 10.1128/JVI.76.22.11753-11756.2002
PMid:12388737 PMCid:PMC136774

153. J. Trgovcich, C. Cebulla, P. Zimmerman and D. D. Sedmak: Human cytomegalovirus protein pp71 disrupts major histocompatibility complex class I cell surface expression. J Virol 80(2), 951-63 (2006)
DOI: 10.1128/JVI.80.2.951-963.2006
PMid:16378997 PMCid:PMC1346885

154. S. A. Welte, C. Sinzger, S. Z. Lutz, H. Singh-Jasuja, K. L. Sampaio, U. Eknigk, H. G. Rammensee and A. Steinle: Selective intracellular retention of virally induced NKG2D ligands by the human cytomegalovirus UL16 glycoprotein. Eur J Immunol 33(1), 194-203 (2003)
DOI: 10.1002/immu.200390022
PMid:12594848

155. C. A. Fielding, R. Aicheler, R. J. Stanton, E. C. Wang, S. Han, S. Seirafian, J. Davies, B. P. McSharry, M. P. Weekes, P. R. Antrobus, V. Prod’homme, F. P. Blanchet, D. Sugrue, S. Cuff, D. Roberts, A. J. Davison, P. J. Lehner, G. W. Wilkinson and P. Tomasec: Two novel human cytomegalovirus NK cell evasion functions target MICA for lysosomal degradation. PLoS Pathog 10(5), e1004058 (2014)

156. T. R. Jones and L. Sun: Human cytomegalovirus US2 destabilizes major histocompatibility complex class I heavy chains. J Virol 71(4), 2970-9 (1997)

157. B. E. Gewurz, E. W. Wang, D. Tortorella, D. J. Schust and H. L. Ploegh: Human cytomegalovirus US2 endoplasmic reticulum-lumenal domain dictates association with major histocompatibility complex class I in a locus-specific manner. J Virol 75(11), 5197-204 (2001 )
DOI: 10.1128/JVI.75.11.5197-5204.2001
PMid:11333901 PMCid:PMC114925

158. K. Oresic and D. Tortorella: Endoplasmic reticulum chaperones participate in human cytomegalovirus US2-mediated degradation of class I major histocompatibility complex molecules. J Gen Virol 89(Pt 5), 1122-30 (2008)

159. T. R. Jones, L. K. Hanson, L. Sun, J. S. Slater, R. M. Stenberg and A. E. Campbell: Multiple independent loci within the human cytomegalovirus unique short region down-regulate expression of major histocompatibility complex class I heavy chains. J Virol 69(8), 4830-41 (1995)

160. C. E. Shamu, D. Flierman, H. L. Ploegh, T. A. Rapoport and V. Chau: Polyubiquitination is required for US11-dependent movement of MHC class I heavy chain from endoplasmic reticulum into cytosol. Mol Biol Cell 12(8), 2546-55 (2001)
DOI: 10.1091/mbc.12.8.2546
PMid:11514634 PMCid:PMC58612

161. E. J. Wiertz, T. R. Jones, L. Sun, M. Bogyo, H. J. Geuze and H. L. Ploegh: The human cytomegalovirus US11 gene product dislocates MHC class I heavy chains from the endoplasmic reticulum to the cytosol. Cell 84(5), 769-79 (1996)
DOI: 10.1016/S0092-8674(00)81054-5

162. B. Park, E. Spooner, B. L. Houser, J. L. Strominger and H. L. Ploegh: The HCMV membrane glycoprotein US10 selectively targets HLA-G for degradation. J Exp Med 207(9), 2033-41 (2010)
DOI: 10.1084/jem.20091793
PMid:20713594 PMCid:PMC2931171

163. I. Jelcic, J. Reichel, C. Schlude, E. Treutler, C. Sinzger and A. Steinle: The polymorphic HCMV glycoprotein UL20 is targeted for lysosomal degradation by multiple cytoplasmic dileucine motifs. Traffic 12(10), 1444-56 (2011)
DOI: 10.1111/j.1600-0854.2011.01236.x
PMid:21689255

Key Words: Virus, Trafficking, Membrane, Arf1, Organelle Morphology, Transport, Assembly Compartments, Review

Send correspondence to: Nicholas J. Buchkovich, 500 University Drive, Mailcode: H107, Hershey, PA, 17033, Tel: 717-531-0003 x287026, Fax: 717-531-6522, E-mail: nbuchkovich@hmc.psu.edu