P40SDB25, a putative CDK inhibitor, has a role in the M/G1 transition in Saccharomyces cerevisiae

The Saccharomyces cerevisiae protein kinase Dbf2 carries out an essential function in late mitosis, and its kinase activity is cell-cycle regulated around anaphase/telophase. We have isolated SDB25, a high copy suppressor of temperature-sensitive dbf2 mutants, and genetic analysis 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 phosphoprotein, and Sdb25 immunoprecipitates have a histone H1 kinase activity that is CDC28-dependent. Remarkably, Sdb25 transcript levels, protein levels, and associated kinase activity are precisely cell-cycle regulated, sharing a common peak in late mitosis. Moreover, Sdb25 protein levels and associated kinase activity are sharply up-regulated at the peak of Dbf2 kinase activity in cells released from a dbf2 ts block. The Sdb25 protein then disappears around Start in the next cell cycle. This indicates that SDB25 function is confined to M/G1, and morphological analysis of sdb25 delta cells supports this conclusion. Our data suggest that Sdb25 functions in a pathway in late mitosis leading to the down-regulation of Cdc28 kinase activity as cells traverse the M/G1 boundary.