Coordinated of Post-Translational Modifications of Yeast Transcriptional Activator Gcn4

Gcn4 is a transcriptional activator that regulates amino acid biosynthetic genes in yeast. Gcn4 is expressed under amino acid starvation conditions, and its degradation depends on phosphorylation, by either Pho85 or Cdk8, which triggers its ubiquitination and subsequent proteolysis by 26S proteasome. Previous studies showed that Gcn4 becomes sumoylated at two Lys residues (K50, 58), specifically after binding to target gene promoters. However, it is not clear how promoter-associated sumoylation of Gcn4 is coordinated with its phosphorylation and ubiquitination as part of a regulatory cascade of PTMs. Our results indicate that Gcn4 sumoylation occurs independently of its phosphorylation or ubiquitination; however, some degree of Cdk8-mediated phosphorylation and ubiquitination is dependent on sumoylation. Also, SUMO modification of promoter-bound Gcn4 triggers its removal by stimulating Cdk8-mediated phosphorylation and subsequent ubiquitination. Together, these results illustrate the detailed mechanism by which a transcriptional activator is regulated after binding DNA through coordinated post-translational modifications.