The regulation of Clb5 kinase activity by mating factor.

Mating factor was found to affect Clb5 kinase activity in Saccharomyces cerevisiae. Mating factor decreased Clb5 kinase activity in a time- and dose-dependent manner. The regulation of Clb5 kinase activity requires functional CLNs (G1 cyclins). Strains without functional CLNs still showed sensitivity to mating factor in the presence of moderately expressing Clb5. This type of mating factor sensitivity is thought to be induced by non-G1 arrest. It is apparent that mating factor treated cells contained inhibitor(s) of Clb5 kinase activity, suggesting that inhibition of Clb5 kinase activity is accompanied by a specific inhibitor. This notion is supported by mixing experiment. Nocodazole treatment showed that the effect of mating factor on Clb5 kinase activity occurred at G1 and connected to mitotic exit. Mating factor regulation of Clb5 kinase activity was found to be dependent on Sic1 protein.