Yeast Ran-binding protein Yrb1p is required for efficient proteolysis of cell cycle regulatory proteins Pds1p and Sic1p

Ubiquitin dependent proteolysis of specific target proteins is required for several important steps during the cell cycle, Degradation of such protein s is strictly cell cycle-regulated and triggered by two large ubiquitin lig ases, termed anaphase-promoting complex (APC) and Skp1/Cullin/F-box complex (SCF), Here we show that yeast Ran-binding protein 1 (Yrb1p), a predominan tly cytoplasmic protein implicated in nucleocytoplasmic transport, is requi red for cell cycle regulated protein degradation. Depletion of Yrb1p result s in the accumulation of unbudded G(1) cells and of cells arrested in mitos is implying a function of Yrb1p in the G(1)/S transition and in the progres sion through mitosis, Temperature-sensitive yrb1-51 mutants are defective i n APC-mediated degradation of the anaphase inhibitor protein Pds1p and in d egradation of the cyclin-dependent kinase inhibitor Sic1p, a target of SCF. Thus, Yrb1p is crucial for efficient APC- and SCF-mediated proteolysis of important cell cycle regulatory proteins. We have identified the UBS1 gene as a multicopy suppressor of yrb1-51 mutants. Ubs1p is a nuclear protein, a nd its deletion is synthetic lethal with a yrb1-51 mutation. Interestingly, UBS1 was previously identified as a multicopy suppressor of cdc34-2 mutant s, which are defective in SCF activity. We suggest that Ubs1p may represent a link between nucleocytoplasmic transport and ubiquitin ligase activity.