Cell cycle-mediated structural and functional alteration of P85^gag-mos protein kinase activity

We investigated cell cycle-dependent regulation of protein kinase activity encoded by the viral mos gene by using a normal rat kidney cell line (NRK-6m2) chronically infected with a temperature-sensitive mutant (ts110) of Moloney murine sarcoma virus, which produces the P85^gag-mos transforming protein. In elutriation experiments, in which cells in various phases of the cell cycle are separated based upon size, a twofold increase in the specific activity of the P85^gag-mos protein kinase was observed as cells progressed from G₀/G₁ through S and G₂/M. A three- to fourfold increase in gas-mos protein kinase specific activity relative to unsynchronized cells was observed in mitotic NRK-6m2 cells synchronized by treatment with thymidine followed by colcemid or with nocodazole alone. Interestingly, the gag-mos protein was structurally altered in mitotic cells generating a protein species moving slower than P85^gag-mos in SDS-polyacrylamide gels. Our findings indicate that viral mos protein kinase activity is regulated during the cell cycle via phosphorylation. We propose that the mos transforming protein functions in a pleiotropic manner, affecting both cytoplasmic and nuclear targets.