ROLE OF pRB DEPHOSPHORYLATION IN CELL CYCLE REGULATION

Sama Tamrakar ¹, Ethel Rubin ¹ and John W. Ludlow ^1,2

1 Department of Biochemistry and Biophysics, University of Rochester School of Medicine and Dentistry, and ² University of Rochester Cancer Center, Rochester, New York 14642

TABLE OF CONTENTS

1.Abstract

2. Introduction

3.The pRB protein

3.1. Structure
3.2. Phosphorylation
3.3. Relevance to cancer

4. Mediation of growth suppression

4.1. The pRB/E2F interaction
4.2. Other roles for pRB in cell growth

5. Dephosphorylation of pRB

5.1. Introduction
5.2. The pRB phosphatase
5.3. Sequential pRB dephosphorylation
5.4. Phosphorylation of pRB may result in differential regulation of downstream effector pathways
5.5. Dephosphorylation during S and G2
5.6. Intracellular activators of pRB dephosphorylation

6. The PP1-pRB interaction

6.1. Complex formation
6.2. The role of LXSXE in the PP1-pRB interaction

7. Dephosphorylation of other pocket protein family members

8. Perspectives

9. Acknowledgments

10. References

1. ABSTRACT

pRB, the tumor suppressor product of the retinoblastoma susceptibility gene, is regarded as one of the key regulators of the cell cycle. This protein exerts its growth suppressive effect through its ability to bind and interact with a variety of cellular proteins. In turn, pRB binding and interacting ability is governed by its phosphorylation state. In recent years, this negative growth regulatory protein has captured a great deal of attention from investigators around the world due to its ability to modulate the activity of transcription regulatory proteins, enzymes which modify chromatin, and other cellular proteins which contribute to its complex role in mammalian cells. Hypophosphorylated pRB binds and sequesters transcription factors, most notably those of the E2F/DP family, inhibiting the transcription of genes required to traverse the G1 to S phase boundary. This cell cycle inhibitory function is abrogated when pRB undergoes phosphorylation mediated by cyclin/cdk complexes following cell stimulation by mitogens. Removal of these phosphates appears to be carried out by a multimeric complex of protein phosphatase type 1 (PP1) and noncatalytic regulatory subunits at the completion of mitosis. This dephosphorylation returns pRB to its active, growth suppressive state. While the mechanism of pRB phosphorylation has and continues to be extensively studied, dephosphorylation of pRB has received disproportionately less attention. The goal of this review is to revisit the role of pRB dephosphorylation in regulating the cell cycle. Emphasis will be placed on understanding the function and regulation of pRB during the cell cycle as well as our ever-expanding notions of pRB-PP1 interaction and the mechanism of pRB dephosphorylation at mitotic exit.