P40(SDB25), A PUTATIVE CDK INHIBITOR, HAS A ROLE IN THE M G(1) TRANSITION IN SACCHAROMYCES-CEREVISIAE/

The Saccharomyces cerevisiae protein kinase Dbf2 carries out an essent ial function in late mitosis, and its kinase activity is cell-cycle re gulated around anaphase/telophase. We have isolated SDB25, a high copy suppressor of temperature-sensitive dbf2 mutants, and genetic analysi s suggests that the two proteins may function in parallel pathways in late mitosis. SDB25 encodes p40, a previously characterized substrate and potent inhibitor of Cdc28 kinase activity. Sdb25 is a phosphoprote in, and Sdb25 immunoprecipitates have a histone H1 kinase activity tha t is CDC28-dependent. Remarkably, Sdb25 transcript levels, protein lev els, and associated kinase activity are precisely cell-cycle regulated , sharing a common peak in late mitosis. Moreover, Sdb25 protein level s and associated kinase activity are sharply up-regulated at the peak of Dbf2 kinase activity in cells released from a dbf2 ts block. The Sd b25 protein then disappears around Start in the next eel cycle. This i ndicates that SDB25 function is confined to M/G(1), and morphological analysis of sdb25 Delta cells supports this conclusion. Our data sugge st that Sdb25 functions in a pathway in late mitosis leading to the do wn-regulation of Cdc28 kinase activity as cells traverse the M/G(1) bo undary.