DIFFERENTIAL REGULATION OF THE RETINOBLASTOMA FAMILY OF PROTEINS DURING CELL-PROLIFERATION AND DIFFERENTIATION

In the present study we have analysed the regulation of pocket protein expression and post-transcriptional modifications on cell proliferati on and differentiation, both in vivo and in vitro. There are marked ch anges in pocket protein levels during these transitions, the most stri king differences being observed between p130 and p107. The mechanisms responsible for regulating pocket protein levels seem to be dependent on both cell type and pocket protein, in addition to their dependence on the cell growth status. Changes in retinoblastoma protein and p107 levels are independent of their state of phosphorylation. However, whe reas p130 phosphorylation to forms characteristic of quiescent/differe ntiated cells results in the accumulation of p130 protein, phosphoryla tion of p130 to one or more forms characteristic of cycling cells is a ccompanied by down-regulation of its protein levels. We also show here that the phosphorylation status and protein levels of p130 and p107 a re regulated in vivo as in cultured cells. In vivo, changes in p130 fo rms are correlated with changes in E2F complexes. Moreover, the modula tion of p130 and p107 status during cell differentiation in vitro is c onsistent with the patterns of protein expression and phosphorylation status found in mouse tissues. Thus in addition to the direct disrupti on of pocket protein/E2F complexes induced by cyclin/cyclin-dependent kinase, the results we report here indicate that the differential modu lation of pocket protein levels constitutes a major mechanism that reg ulates the pool of each pocket protein that is accessible to E2F and/o r other transcription factors.