Catalytic Roles of Yeast GSK3β/Shaggy Homolog Rim11p in Meiotic Activation

In Saccharomyces cerevisiae, many meiotic genes are activated by a heteromeric transcription factor composed of Ime1p and Ume6p. Ime1p-Ume6p complex formation depends upon the protein kinase Rim11p, which interacts with and phosphorylates both Ime1p and Ume6p in vitro. Rim11p may promote complex formation through its phosphorylation of Ime1p and Ume6p or simply through its interaction with both proteins. Here, we characterize mutant Ime1p derivatives that interact with Rim11p but are not phosphorylated in vitro. These mutant proteins are also defective in interaction with Ume6p. These results argue that Ime1p must be phosphorylated to interact with Ume6p. Our genetic observations suggest that Ime1p tyrosine residues are among the Rim11p phosphoacceptors, and we find that Ime1p reacts with an anti-phosphotyrosine antibody. Ime1p and Rim11p have been thought to act only through Ume6p, but we find that Ime1p and Rim11p promote meiosis at a very low level in the absence of Ume6p. A nonphosphorylatable mutant Ime1p derivative promotes sporulation through this Ume6p-independent pathway, as does a mutant Rim11p derivative that fails to interact with Ime1p. Therefore, Ime1p and Rim11p have two genetically separable functions in the sporulation program. However, catalytic activity of Rim11p is required for sporulation in the presence or absence of Ume6p.