CDC53 IS A SCAFFOLD PROTEIN FOR MULTIPLE CDC34 SKP1/F-BOX PROTEIN COMPLEXES THAT REGULATE CELL-DIVISION AND METHIONINE BIOSYNTHESIS IN YEAST/

In budding yeast, ubiquitination of the cyclin-dependent kinase (Cdk) inhibitor Sic1 is catalyzed by the E2 ubiquitin conjugating enzyme Cdc 34 in conjunction with an E3 ubiquitin ligase complex composed of Skp1 , Cdc53 and the F-box protein, Cdc4 (the SCECdc4 complex). Skp1 binds a motif called the F-box and in turn E-box proteins appear to recruit specific substrates for ubiquitination. We find that Skp1 interacts wi th Cdc53 in vivo, and that Skp1 bridges Cdc53 to three different F-box proteins, Cdc4, Met30, and Grr1. Cdc53 contains independent binding s ites for Cdc34 and Skp1 suggesting it functions as a scaffold protein within an E2/E3 core complex. E-box proteins show remarkable functiona l specificity in vivo: Cdc4 is specific for degradation of Sic1, Grr1 is specific for degradation of the G(1) cyclin Cln2, and Met30 is spec ific for repression of methionine biosynthesis genes. In contrast, the Cdc34-Cdc53-Skp1 E2/E3 core complex is required for all three functio ns. Combinatorial control of SCE complexes may provide a basis for the regulation of diverse cellular processes.