- The protein serine/threonine kinases--members of protein kinase C (PKC) family--are important components of the major signaling pathways regulating cell proliferation and differentiation. Recent studies implicate PKC in cell cycle control at two sites--during G1 to S progression and at G2 to M transition. Activation of PKC during G1 progression modulates the activity of the specific cyclin-dependent kinases (CDKs), which phosphorylate the retinoblastoma susceptibility gene product (RB). Phosphorylation of RB is a pivotal event in cell cycle progression leading to G1/S transition. PKC mediated enhancement or inhibition of CDK's activity and the RB phosphorylation state appear to be dependent on the precise timing of PKC activation during G1 and on the particular cell type. At G2/M transition, recent evidence suggests that PKC is involved in the regulation of CDC2 activity, although it is mostly implicated as a regulator of lamin B phosphorylation and the nuclear lamina disassembly.