GRR1 FUNCTIONS IN THE UBIQUITIN PATHWAY IN SACCHAROMYCES-CEREVISIAE THROUGH ASSOCIATION WITH SKP1

Cdc34, a ubiquitin-conjugating enzyme in Saccharomyces cerevisiae, is required for cell cycle progression. Sic1, an S-phase cyclin-dependent kinase (CDK) inhibitor, is a critical target of Cdc34-mediated ubiqui tination. Other essential target protein(s) could be defined since cdc 34 sic1 double mutants still arrest in G2 phase. To identify proteins which function in the Cdc34-dependent ubiquitin pathway, a series of e xtragenic suppressors of the cdc34-1 sic1 double mutations was isolate d. One of them was found to be defective in GRR1, which is involved no t only in glucose repression but also in G1 cyclin destabilization. Ho wever, neither lack of glucose repression nor stabilization of G1 cycl in caused the suppression of cdc34-1 sic1. Conversely, Grr1 overproduc tion in cdc34-1 sic1 cells impaired colony formation, even at the perm issive temperature. A multicopy suppressor, MGO1, which rescued the gr owth defect associated with Grr1 overproduction was isolated, and foun d to be identical to SKP1. Furthermore, Grr1 bound Skp1 directly in vi tro. These results strongly suggest that Grr1 functions in the ubiquit in pathway through association with Skp1.