CELL CYCLE-DEPENDENT PHOSPHORYLATION OF THE RETINOBLASTOMA-RELATED PROTEIN P130

The retinoblastoma-related protein p130 is a putative negative regulat or of cell proliferation in mammalian cells. In this study, p130 is ex ist in multiple phosphorylated forms in human cells. In glioblastoma T 98G cells synchronized by serum deprivation, specific phosphorylated f orms of p130 are found at different times after serum re-stimulation. Two phosphorylated forms of p130 only found in serum-arrested T98G cel ls and in early G(1) phase associate with the adenovirus oncoprotein E 1A in vitro. One of these two forms corresponds to the in vivo E1A-ass ociated p130 in 293 cells, which express endogenous E1A protein. Moreo ver, p130 undergoes an abrupt shift to a unique phosphorylated form in mid G(1) which is the only p130 form found during the remaining phase s of the cell cycle. This phosphorylated form possesses an associated histone H1 kinase activity that is most active in late S phase and G(2 )/M. The cell cycle-dependent expression pattern of cyclins in T98G ce lls is compatible with cyclin D1/CDK complexes driving the shift to th is phosphorylated p130 form in mid G(1). These results suggest that th e putative growth inhibitory function of p130 is regulated by phosphor ylation of this protein. They also suggest that differential phosphory lation of p130 during the cell cycle plays distinct roles in the regul ation of p130 function.