REGULATION OF THE RETINOBLASTOMA PROTEIN-RELATED P107 BY G(1) CYCLIN COMPLEXES

The orderly progression through the cell cycle is mediated by the sequ ential activation of several cyclin/cyclin-dependent kinase (cdk) comp lexes. These kinases phosphorylate a number of cellular substrates, am ong which is the product of the retinoblastoma gene, pRb. Phosphorylat ion of pRb in late G(1) causes the release of the transcription factor E2F from pRb, resulting in the transcriptional activation of E2F-resp onsive genes. We show here that phosphorylation of the pRb-related p10 7 is also cell cycle regulated. p107 is first phosphorylated at 8 hr f ollowing serum stimulation of quiescent fibroblasts, which coincides w ith an increase in cyclin D1 protein levels. Consistent with this, we show that a cyclin D1/cdk4 complex, but not a cyclin E/cdk2 complex, c an phosphorylate p107 in vivo. Furthermore, phosphorylation of p107 ca n be abolished by the overexpression of a dominant-negative form of cd k4. Phosphorylation of p107 results in the loss of the ability to asso ciate with E2F-4, a transcription factor with growth-promoting and onc ogenic activity. A p107-induced cell cycle block can be released by cy clin D1/cdk4 but not by cyclin E/cdk2. These data indicate that the ac tivity of p107 is regulated by phosphorylation through D-type cyclins.