CDC34 AND THE F-BOX PROTEIN MET30 ARE REQUIRED FOR DEGRADATION OF THECDK-INHIBITORY KINASE SWE1

Ubiquitin-mediated proteolysis controls the abundance of many cell cyc le regulatory proteins. Recent work in Saccharomyces cerevisiae sugges ts that a complex consisting of Cdc53, Skp1, and a third component kno wn as an F-box protein (termed SCF) in combination with Cdc34 specific ally targets regulatory proteins for degradation, and that substrate s pecificity is likely to be mediated by the F-box subunit. A screen for genetic interactions with a cdc34 mutation yielded MET30, which encod es an F-box protein. MET30 is an essential gene required for cell cycl e progression and met30 mutations interact genetically with mutations in SCF components. Furthermore, physical interactions between Met30, C dc53, Cdc34, and Skp1 in vivo provide evidence for an SCEMet30 complex . We demonstrate the involvement of Met30 in the degradation of the Cd k-inhibitory kinase Swe1. Swe1 is stabilized in met30 mutants and GST- Met30 pull-down experiments reveal that Met30 specifically binds Swe1 in vivo. Furthermore, extracts prepared from cdc34 or met30 mutants ar e defective in polyubiquitination of Swe1. Taken together, these data suggest that SCF-mediated proteolysis may contribute to the regulation of entry into mitosis. Our data, in combination with previously publi shed results, also provide evidence for distinct SCE complexes in vivo and support the idea that their F-box subunits mediate SCE substrate specificity.