G1 CYCLIN CDK-INDEPENDENT PHOSPHORYLATION AND ACCUMULATION OF P130 DURING THE TRANSITION FROM G1 TO G0 LEAD TO ITS ASSOCIATION WITH E2F-4/

During the transition from G1 to G0, p130 undergoes a specific phospho rylation event - leading to p130-form 2 - that is mediated by a kinase /s other than the known G1, S and G2/M cyclin/CDKs. Changes in the pho sphorylation status of p130 during this transition are responsible, at least in part, for the concomitant formation of p130/E2F4 complexes, which are characteristic of GO. These complexes remain abundant during early G1 upon restimulation, but not after mitosis, and are dissociat ed in mid G1 when p130 is abruptly hyperphosphorylated to form 3. Subs equently, p130 forms 1 and 2 are no longer detected during the remaind er of the cell cycle. Consistently, phosphorylation to form 3 and diss ociation from E2F-4 complexes is reproduced by a cyclin/CDK holoenzyme in vitro. TGF-beta-induced G1 arrest abrogates cyclin/CDK phosphoryla tion of p130 but not phosphorylation to form 2. The cell cycle-depende nt phosphorylation pattern of p130 is thus shown to comprise two disti nct steps that are catalyzed by different kinases. The differential re gulation of p130 and pRB phosphorylation during the transition from G1 to G0 may explain the fact that p130 and E2F-4 are the major componen ts of E2F complexes in quiescent cells. Moreover, the newly described phosphorylation of p130 at the transition from G1 to G0 defines a nove l mechanism of cell cycle exit regulation.