SUPPRESSION OF CELL-TRANSFORMATION BY THE CYCLIN-DEPENDENT KINASE INHIBITOR P57(KIP2) REQUIRES BINDING TO PROLIFERATING CELL NUCLEAR ANTIGEN

Proper control of the mammalian cell cycle requires the function of cy clin-dependent kinase (CDK) inhibitors. The p21 family currently inclu des three distinct genes, p21, p27(Kip1), and p57(Kip2), that share a common N-terminal domain for binding to and inhibiting the kinase acti vity of CDK-cyclin complexes, The p21 protein also binds to proliferat ing cell nuclear antigen (PCNA) through a separate C-terminal domain a ffecting DNA replication and repair, The p27 and p57 proteins also eac h contain unique C-terminal domains whose functions are unknown, Here we show that the human p57 protein, like p21, contains a PCNA-binding domain within its C terminus that, when separated from its N-terminal CDK-cyclin binding domain, can prevent DNA replication in vitro and S phase entry in vivo. Disruption of either CDK/cyclin or PCNA binding p artially reduced p57's ability to suppress myc/RAS-mediated transforma tion in primary cells, while loss of both inhibitory functions complet ely eliminated p57's suppressive activity, Thus, control of cell cycle and suppression of cell transformation by p57 require both CDK and PC NA inhibitory activity, and disruption of either or both functions may lead to uncontrolled cell growth.