Degradation of the transcription factor Gcn4 requires the kinase Pho85 andthe SCFCDC4 ubiquitin-ligase complex

Gcn4, a yeast transcriptional activator that promotes the expression of ami no acid and purine biosynthesis genes, is rapidly degraded in rich medium. Here we report that SCFCDC4, a recently characterized protein complex that acts in conjunction with the ubiquitin-conjugating enzyme Cdc34 to degrade cell cycle regulators, is also necessary for the degradation of the transcr iption factor Gcn4. Degradation of Gcn4 occurs throughout the cell cycle, w hereas degradation of the known cell cycle substrates of Cdc34/SCFCDC4 is c ell cycle regulated. Gcn4 ubiquitination and degradation are regulated by s tarvation for amino acids, whereas the degradation of the cell cycle substr ates of Cdc34/SCFCDC4 is unaffected by starvation. We further show that unl ike the cell cycle substrates of Cdc34/SCFCDC4, which require phosphorylati on by the kinase Cdc28, Gcn4 degradation requires the kinase Pho85. We iden tify the critical target site of Pho85 on Gcn4; a mutation of this site sta bilizes the protein. A specific Pho85-Pcl complex that is able to phosphory late Gcn4 that site is inactive under conditions under which Gcn4 is stable . Thus, Cdc34/SCFCDC4 activity is constitutive, and regulation of the stabi lity of its various substrates occurs at the level of their phosphorylation .