Studies on the in Vitro Phosphorylation of HSSB-p34 and -p107 by Cyclin-dependent Kinases

doi:10.1074/jbc.271.37.22847

Studies on the in Vitro Phosphorylation of HSSB-p34 and -p107 by Cyclin-dependent Kinases

CYCLIN-SUBSTRATE INTERACTIONS DICTATE THE EFFICIENCY OF PHOSPHORYLATION*

From the Graduate Program in Molecular Biology, Memorial Sloan-Kettering Cancer Center, New York, New York 10021

^§ Professor of the American Cancer Society, To whom correspondence should be addressed:
Graduate Program in Molecular Biology, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue/Box 97, New York, NY 10021
.
^‡ Present address:
Derald H. Ruttenberg Cancer Center, The Mount Sinai Medical Center, One Gustave L. Levy Place, New York, NY 10029
.

Abstract

Cyclin-dependent kinases (Cdks) are required for cell cycle progression. Two potentially significant Cdk substrates in human cells are the human single-stranded binding protein (HSSB or RPA), which plays an essential role in DNA replication, repair, and recombination, and the tumor suppressor p107 which acts to negatively regulate cell growth.

In this report we describe the in vitro phosphorylation of these two proteins by Cdks in an attempt to understand how cyclin-substrate interactions direct phosphorylation efficiencies. We show that cyclin A-Cdk2 efficiently phosphorylates the p34 subunit of HSSB (HSSB-p34) alone or as a part of the heterotrimeric complex. In contrast, cyclin E-Cdk2 that is active in phosphorylating histone H1, does not support the phosphorylation of the p34 subunit of HSSB. We provide evidence that this differential phosphorylation results from a specific interaction between HSSB-p34 and cyclin A, but not cyclin E. Thus the observed cell cycle-dependent phosphorylation of HSSB-p34 at the G₁ to S transition is most likely catalyzed by cyclin A-Cdk2 initiated by the direct interaction between cyclin A and the HSSB-p34 subunit.

These studies are consistent with our previous observation that p107, which directly binds cyclin A, is efficiently phosphorylated by cyclin A-Cdk2 but not cyclin B-associated kinases. Here we further demonstrate that cyclin A only complexes with p107 in its unphosphorylated form. These data suggest a catalytic mechanism by which Cdk acts: substrate targeting by a cyclin-substrate interaction followed by dissociation of the Cdk upon phosphate incorporation allowing the Cdk to become available for the next cycle of phosphorylation.

Footnotes

↵* This work was supported by National Institutes of Health Grant GM38559 (to J. H.). The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵¹ The abbreviations used are:

Cdk

cyclin-dependent kinase

DTT

dithiothreitol

PMSF

phenylmethylsulfonyl fluoride

PAGE

polyacrylamide gel electrophoresis

HSSB

human single-stranded DNA binding protein (also called RP-A)

IPTG

isopropyl-1-thio-β-D-galactopyranoside

pRB

retinoblastoma gene product

GST

glutathione S-transferase

DNA-PK

DNA-dependent kinase

Cdc

cell division cycle.
- Received April 29, 1996.