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Mol Cell Biol, February 1998, p. 753-761, Vol. 18, No. 2
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Functional Inactivation of the Retinoblastoma Protein Requires Sequential Modification by at Least Two Distinct Cyclin-cdk Complexes

Ante S. Lundberg1,2,* and Robert A. Weinberg1,3

The Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 021421; Dana-Farber Cancer Institute, Boston, Massachusetts 021152; and Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 021393

Received 5 June 1997/Returned for modification 9 July 1997/Accepted 31 October 1997

The retinoblastoma protein (pRb) acts to constrain the G1-S transition in mammalian cells. Phosphorylation of pRb in G1 inactivates its growth-inhibitory function, allowing for cell cycle progression. Although several cyclins and associated cyclin-dependent kinases (cdks) have been implicated in pRb phosphorylation, the precise mechanism by which pRb is phosphorylated in vivo remains unclear. By inhibiting selectively either cdk4/6 or cdk2, we show that endogenous D-type cyclins, acting with cdk4/6, are able to phosphorylate pRb only partially, a process that is likely to be completed by cyclin E-cdk2 complexes. Furthermore, cyclin E-cdk2 is unable to phosphorylate pRb in the absence of prior phosphorylation by cyclin D-cdk4/6 complexes. Complete phosphorylation of pRb, inactivation of E2F binding, and activation of E2F transcription occur only after sequential action of at least two distinct G1 cyclin kinase complexes.

* Corresponding author. Mailing address: The Whitehead Institute for Biomedical Research, 9 Cambridge Center, Cambridge, MA 02142. Phone: (617) 258-5176. Fax: (617) 258-5213. E-mail: Lundberg{at}wi.mit.edu.




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