Distinct activities of the related protein kinases Cdk1 and Ime2

In budding yeast, commitment to DNA replication during the normal cell cycle requires degradation of the cyclin-dependent kinase (CDK) inhibitor Sic1. The G1 cyclin–CDK complexes Cln1–Cdk1 and Cln2–Cdk1 initiate the process of Sic1 removal by directly catalyzing Sic1 phosphorylation at multiple sites. Commitment to DNA replication during meiosis also appears to require Sic1 degradation, but the G1 cyclin–CDK complexes are not involved. It has been proposed that the meiosis-specific protein kinase Ime2 functionally replaces the G1 cyclin–CDK complexes to promote Sic1 destruction. To investigate this possibility, we compared Cln2–Cdk1 and Ime2 protein kinase activities in vitro. Both enzyme preparations were capable of catalyzing phosphorylation of a GST–Sic1 fusion protein, but the phosphoisomers generated by the two activities had significantly different electrophoretic mobilities. Furthermore, mutation of consensus CDK phosphorylation sites in Sic1 affected Cln2–Cdk1- but not Ime2-dependent phosphorylation. Phosphoamino acid analysis and phosphopeptide mapping provided additional evidence that Cln2–Cdk1 and Ime2 targeted different residues within Sic1. Examination of other substrates both in vitro and in vivo also revealed differing specificities. These results indicate that Ime2 does not simply replace G1 cyclin–CDK complexes in promoting Sic1 degradation during meiosis.