The Cdc34 ubiquitin-conjugating enzyme plays a central role in progression of the cell cycle. Through analysis of the phenotype of a mutant missing a highly conserved sequence motif within the catalytic domain of Cdc34, we discovered previously unrecognized levels of regulation of the Ace2 transcription factor and the cyclin-dependent protein kinase inhibitor Sic1. In cells carrying the Cdc34tm mutation, which alters the conserved sequence, the cyclin-dependent protein kinase inhibitor Sic1, an SCF Cdc4 substrate, has a shorter half-life, while the cyclin Cln1, an SCFGrr1 substrate, has a longer half-life than in wild-type cells. Expression of the SIC1 gene cluster, which is regulated by Swi5 and Ace2 transcription factors, is induced in CDC34tm cells. Levels of Swi5, Ace2, and the SCFGrr1 targets Cln1 and Cln2 are elevated in Cdc34tm cells, and loss of Grr1 causes an increase in Ace2 levels. Sic1 levels are similar in CDC34tm ace2D and wild-type cells, explaining a paradoxical increase in the steady-state level of Sic1 protein despite its reduced half-life. A screen for mutations that interact with CDC34tm uncovered novel regulators of Sic1, including genes encoding the polyubiquitin chain receptors Rad23 and Rpn10.
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