Functional inactivation of the retinoblastoma protein requires sequential modification by at least two distinct cyclin-cdk complexes.
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.
Lundberg
A S
AS
The Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142, USA. Lundberg@wi.mit.edu
Weinberg
R A
RA
D042846:CDC2-CDC28 Kinases; D002352:Carrier Proteins; D018797:Cell Cycle Proteins / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism; D002478:Cells, Cultured; D056741:Cyclin D; D019927:Cyclin E / Q000378:metabolism*; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000378:metabolism; D050684:E2F Transcription Factors; D016193:G1 Phase; D006801:Humans; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011518:Proto-Oncogene Proteins; D016160:Retinoblastoma Protein / Q000378:metabolism*; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000378:metabolism; D014162:Transfection
D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D056741:Cyclin D; D019927:Cyclin E; D016213:Cyclins; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D011518:Proto-Oncogene Proteins; D016160:Retinoblastoma Protein; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; C495900:CDK4 protein, human; C495911:CDK6 protein, human; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases