COLLABORATION OF G(1) CYCLINS IN THE FUNCTIONAL INACTIVATION OF THE RETINOBLASTOMA PROTEIN

The retinoblastoma gene product (pRB) constrains cell proliferation by preventing cell-cycle progression from the G(1) to S phase. Its growth-inhibitory effects appear to be reversed by hyperphosphorylation occurring during G(1). This process is thought to involve G(1) cyclins and cyclin-dependent kinases (cdks). Here we report that the cell cycle-dependent phosphorylation of mammalian pRB is faithfully reproduced when it is expressed in Saccharomyces cerevisiae. As is the case in mammalian tells, this phosphorylation requires an intact oncoprotein-binding domain and is inhibited by a negative growth factor, in this case a mating pheromone. Expression of pRB in cln (-) mutants indicates that specific combinations of endogenous G(1) cyclins, Cln3 and either Cln1 or Cln2 are required for pRB hyperphosphorylation in yeast. Moreover, expression of mammalian G(1) cyclins in cln (-) yeast cells indicates that the functions of Cln2 and Cln3 in pRB hyperphosphorylation can be complemented by human cyclin E and cyclin D1, respectively. These observations suggest a functional heterogeneity among G(1) cyclin-cdk complexes and indicate a need for the involvement of multiple G(1) cyclins in promoting pRB hyperphosphorylation and resulting cell-cycle progression.