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The retinoblastoma susceptibility gene product, Rb, is suspected to supm~~ cell growth. Rb is a 110114 kd nuclear phosphoprotein. We have p~~ iously demonstrated that SV40 T antigen binds only to unphosphorylated Rb, and not p~ 112-114~~, the family of phosphorylated Rb. Here we demonstrate the cell cycle-dependent phosphorylation of Rb. In GO/Gl cells, virtually all the Rb is unphosphorylated. In contrast, during S and 62, it is largely, if not exclusively, phosphorylated. Rb phosphorylation occurs at the Gl/S boundary in several cell types tested. A 14 residue peptide, corresponding to the SV40 1 domain required for transformation, is able to compete effectively with SV40 T for binding to pllORb. We propose a model to explain how Rb may suppress cell growth by acting as a cell cycle regulatory element.