Howard Hughes Medical Institute, Baylor College of Medicine, Houston, Texas 77030, USA.
We have identified the yeast and human homologs of the SKP1 gene as a suppressor of cdc4 mutants and as a cyclin F-binding protein. Skp1p indirectly binds cyclin A/Cdk2 through Skp2p, and directly binds Skp2p, cyclin F, and Cdc4p through a novel structural motif called the F-box. SKP1 is required for ubiquitin-mediated proteolysis of Cin2p, Clb5p, and the Cdk inhibitor Sic1p, and provides a link between these molecules and the proteolysis machinery. A large number of proteins contain the F-box motif and are thereby implicated in the ubiquitin pathway. Different skp1 mutants arrest cells in either G1 or G2, suggesting a connection between regulation of proteolysis in different stages of the cycle.