The yeast ubiquitin ligase SCFMet30: connecting environmental and intracellular conditions to cell division

Abstract

Ubiquitination regulates a host of cellular processes and is well known for its role in progression through the cell division cycle. In budding yeast, cadmium and arsenic stress, the availability of sulfur containing amino acids, and the intracellular concentration of S-adenosylmethionine are linked to cell cycle regulation through the ubiquitin ligase SCFMet30. Regulation is achieved by ubiquitination of the transcription factor Met4. Met4 activity is controlled by a regulatory K48-linked ubiquitin chain that is synthesized by Cdc34/SCFMet30. A ubiquitin-interacting-motif (UIM) present in Met4 prevents degradation of ubiquitinated Met4 allowing the ubiquitin chain to function as a reversible switch of Met4 activity. Here we discuss mechanisms of Met4 and SCFMet30 regulation in response to intracellular and environmental conditions, and describe the integration of these signals with cell cycle control.

Figure 1

Regulation of the ubiquitin ligase SCF^Met30 and its connection to cell cycle control. SCF^Met30 ubiquitinates the transcriptional activator Met4 on lysine 163 to maintain it in an inactive state. Degradation of poly-ubiquitinated Met4 is prevented by an ubiquitin-interacting motif (UIM) in Met4. The sulfur containing compounds methionine, cysteine, and S-adenosylmethionine stimulate Met4 ubiquitination, but when their intracellular concentrations are low, reduced ubiquitination combined with Met4 deubiquitination by a so far unidentified deubiquitinating enzyme(s) (Ubp) leads to Met4 activation. Cadmium and arsenic stress inhibit Met4 ubiquitination by inactivation of the ubiquitin ligase SCF^Met30. The resulting activation of Met4 raises cellular glutathione levels due to induced expression of Gsh1. Active Met4 induces expression of a number of genes involved in the sulfur amino acid synthesis pathway (MET-genes) as well as GSH1. MET-gene expression depends on either one of the two homologous zinc-finger proteins Met31 and Met32. Met4 activation also inhibits a number of cell cycle events and can lead to cell cycle arrest. The Met4-dependent cell cycle arrest requires Met32 but cannot be mediated by Met31.

Figure 2

Met4 domain structure. TA: transactivating domain. IR: inhibitory region required for repression of Met4 activity by methionine. AUX: auxiliary domain required to fully relieve IR-mediated repression under activating conditions. INT: protein-protein interaction domain that binds Met31 and Met32. bZIP: basic leucine zipper domain that mediates Met4 dimerization and binding to Met28 as well as Cbf1. UIM: ubiquitin-interacting motif that prevents degradation of poly-ubiquitinated Met4. K163: ubiquitin acceptor site.

Figure 3

Cadmium-induced regulation of SCF^Met30. Cadmium leads to dissociation of Met30 from the SCF-core by preventing the Met30/Skp1 interaction and blocks Met4 ubiquitination. The resulting activation of Met4 induces synthesis of sulfur amino acids and glutathione as well as a process called isozyme switching, which is part of the sulfur sparing response. Cadmium also leads to activation of three independent cell cycle arrest pathways. The Met4/Met32-dependend and the Mec1/Rad53-dependent pathways, as well as one so far unidentified pathway, which was indicated by genetic results.