PRB PHOSPHORYLATION MUTANTS REVEAL ROLE OF PRB IN REGULATING S-PHASE COMPLETION BY A MECHANISM INDEPENDENT OF E2F

Progression of cells into S phase is controlled by the retinoblastoma protein (pRB) and relies on the functional inactivation of this tumour suppressor in late G1 via protein phosphorylation. We provide evidenc e here that, besides controlling entry of cells into S phase, pRB can operate to inhibit S phase completion. Differential arrays of phosphor ylation appear to regulate these different events, suggesting that cyc le progression at these two stages of the cell cycle may be achieved v ia activation of distinct downstream pRB effector pathways. In agreeme nt with this hypothesis, pRB's ability to prevent S phase entry, but n ot its ability to inhibit S phase completion, correlates with repressi on of E2F-regulated promoters. Furthermore, ectopic expression of E2F or the E2F-regulated cyclin E gene promote S phase entry in cells expr essing phosphorylation-defective pRB but neither is sufficient to trig ger completion of S phase. Our findings raise the possibility that pRB , in addition to its web-established role in controlling a checkpoint in late G1, could be involved in the control of a further checkpoint o perating during S phase and that implementation of this checkpoint rel ies on an as yet unidentified PRE effector distinct from E2F.