Interaction of yeast repressor-activator protein Ume6p with glycogen synthase kinase 3 homolog Rim11p

Meiosis and expression of early meiotic genes in the budding yeast Saccharomyces cerevisiae depend upon Rim11p, Ume6p, and Ime1p. Rim11p (also called Mds1p and SeGSK3) is a protein kinase related to glycogen synthase kinase 3 (GSK3); Ume6p is an architectural transcription factor; and Ime1p is a Ume6p-binding protein that provides a transcriptional activation domain, Rim11p is required for Ime1p-Ume6p interaction, and prior studies have shown that Rim11p binds to and phosphorylates Ime1p. We show here that Rim11p binds to and phosphorylates Ume6p, as well. Amino acid substitutions in Ume6p that alter a consensus GSK3 site reduce or abolish Rim11p-Ume6p interaction and Rim11p-dependent phosphorylation, and they cause defects in interaction between Ume6p and Imelp and in meiotic gene expression. Therefore, interaction between Rim11p and Ume6p, resulting in phosphorylation of Ume6p, is required for Ime1p-Ume6p complex formation, Rim11p, like metazoan GSK3 beta, phosphorylates both interacting subunits of a target protein complex.