New Insight Into the Role of the Cdc34 Ubiquitin-Conjugating Enzyme in Cell Cycle Regulation via Ace2 and Sic1

Abstract

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 Cdc34 tm 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 SCF Grr1 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 CDC34 tm cells. Levels of Swi5, Ace2, and the SCF Grr1 targets Cln1 and Cln2 are elevated in Cdc34 tm cells, and loss of Grr1 causes an increase in Ace2 levels. Sic1 levels are similar in CDC34 tm 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 CDC34 tm uncovered novel regulators of Sic1, including genes encoding the polyubiquitin chain receptors Rad23 and Rpn10.

A Genetic Screen for High Copy Number Suppressors of the Synthetic Lethality Betweenelg1Δandsrs2Δin Yeast

“…Interestingly, Ubp14 collaborates with the Gid complex (Regelmann et al 2003) and mutations in this gene are synthetic lethal with mutations in Cdc34 (Cocklin et al 2011). …”

Sic1 as a timer of Clb cyclin waves in the yeast cell cycle – design principle of not just an inhibitor

“…Phosphorylated Sic1 is recognized by Cdc4, a subunit of the E3 ligase SCF [57,59–61,65–68] that polyubiquitinates Sic1 on the N‐terminal residues together with the E2 enzyme Cdc34 [33,56,58,69–74], and the process can occur when Sic1 is either free or bound to Cdk1/Clb5 [26,57]. Sic1 directly recognizes and co‐precipitates with Cdc4 but not with the other components of the SCF(Cdc4) ubiquitin ligase complex [26,57].…”

Nutrient Sensing Kinases PKA and Sch9 Phosphorylate the Catalytic Domain of the Ubiquitin-Conjugating Enzyme Cdc34

“…Furthermore, over-expression of CLN3 or loss of SIC1 makes yeast sensitive to otherwise tolerable levels of the TORC1 inhibitor rapamycin, which mimics nutrient deprivation [36]. Cdc34 tm increases the rate of Sic1 degradation while decreasing the rate of Cln1 degradation [15]. Therefore, we hypothesized that the highly conserved S73/S97/loop motif of Cdc34 contributes to the ability of cells to survive low dose rapamycin treatment or nutrient depletion.…”