Destabilization of Binding to Cofactors and SCFMet30 Is the Rate-Limiting Regulatory Step in Degradation of Polyubiquitinated Met4

Summary The Met4 transcriptional activator of methionine biosynthesis is negatively regulated by the SCF Met30 ubiquitin ligase in response to accumulation of methionine. This mechanism requires polyubiquitination, but not proteolysis. We report that a previously unappreciated mechanism involving growth control regulates Met4. Unless methionine is present in the growth medium, polyubiquitinated Met4 is stabilized in late exponential cultures, correlating with transcriptional repression. Polyubiquitinated Met4 becomes destabilized in a proteasome-dependent manner upon reentry into exponential growth, correlating with transcriptional activation. Met4 stabilization is regulated at the level of SCF Met30 binding and requires transcriptional cofactors. These lock Met4 and SCF Met30 into a tight complex active in ubiquitination but incapable of binding the proteasome. Release of polyubiquitinated Met4 from SCF Met30 is sufficient for degradation, and specific sulfur amino acids can promote the degradation by destabilizing Met4 binding to cofactors and SCF Met30 . Thus, destabilization of cofactors and SCF Met30 binding is the rate-limiting regulatory step in Met4 proteolysis.