SCFMet30-mediated control of the transcriptional activator Met4 is required for the G1-S transition

Abstract

Progression through the cell cycle requires the coordination of basal metabolism with the cell cycle and growth machinery. Repression of the sulfur gene network is mediated by the ubiquitin ligase SCFMet30, which targets the transcription factor Met4p for degradation. Met30p is an essential protein in yeast. We have found that a met4Δmet30Δ double mutant is viable, suggesting that the essential function of Met30p is to control Met4p. In support of this hypothesis, a Met4p mutant unable to activate transcription does not cause inviability in a met30Δ strain. Also, overexpression of an unregulated Met4p mutant is lethal in wild‐type cells. Under non‐permissive conditions, conditional met30Δ strains arrest as large, unbudded cells with 1N DNA content, at or shortly after the pheromone arrest point. met30Δ conditional mutants fail to accumulate CLN1 and CLN2, but not CLN3 mRNAs, even when CLN1 and CLN2 are expressed from strong heterologous promoters. One or more genes under the regulation of Met4p may delay the progression from G1 into S phase through specific regulation of critical G1 phase mRNAs.

The Yeast Ubiquitin Ligase SCFMet30Regulates Heavy Metal Response

“…The low level of CLN2 expression could possibly explain the cell cycle entry delay observed in cadmium-treated samples (Figure 2A). Both activation of Met4/Met32 and Mec1/Rad53 have been demonstrated to prevent accumulation of CLN2 transcripts (Sidorova and Breeden, 1997;Patton et al, 2000). Cadmium induced both the Met4/Met32 and the Mec1/Rad53 pathways (Figures 3 and 4) and could therefore be responsible for the cell cycle delay after cadmium treatment ( Figure 2B).…”

“…Second, inactivation of SCF Met30 is a physiological response to cadmium exposure to achieve Met4 activation. Full activation of Met4 also induces a cell cycle arrest (Kaiser et al, 1998;Patton et al, 2000;Flick et al, 2004). The activity of the mutant Met30-6 protein is partially compromised; therefore, cadmium exposure would lead to further SCF Met30 inactivation.…”

“…Deletion of MET4 or of its cofactor MET32 eliminate the cell cycle requirement for SCF Met30 (Kaiser et al, 2000;Patton et al, 2000;Flick et al, 2004). We therefore tested cadmium sensitivity of met30 met4 and met30 met32 double mutants ( Figure 2B).…”

“…Accordingly, deletion of MET4 suppresses the essential function of Met30 and met4⌬ met30⌬ double mutants are viable (Kaiser et al, 2000;Patton et al, 2000). However, like ⌬met4 mutants, met4⌬ met30⌬ double mutants are methionine auxotroph because of the importance of Met4 as a master regulator of the sulfur amino acid biosynthesis pathway .…”

“…In addition, full activation of Met4, for example, by blocking its ubiquitination, induces a cell cycle arrest that might be important to safeguard cellular integrity (Kaiser et al, 1998(Kaiser et al, , 2000Flick et al, 2004). How SAM regulates Met4 ubiquitination is not known, but it is clear that the ubiquitin ligase SCF Met30 together with the ubiquitin-conjugating enzyme Cdc34 catalyze Met4 ubiquitination (Kaiser et al, 2000;Patton et al, 2000). SCF Met30 and specifically Met30 are therefore the key regulators of Met4.…”