Hyperactive glycogen synthase mutants of Saccharomyces cerevisiae suppressthe glc7-1 protein phosphatase mutant

A yeast glc7-1 mutant expressing a variant of protein phosphatase type 1 fa ils to accumulate glycogen. This defect is associated with hyperphosphoryla ted and inactive glycogen synthase, consistent with Glc7p acting directly t o dephosphorylate and activate glycogen synthase. To characterize the glyco gen synthesis defect of this mutant in more detail, we isolated 26 pseudore vertants of the glc7-1 mutant. All pseudoreversion events were due to misse nse mutations in GSY2, the gene encoding the major isoform of glycogen synt hase. A majority of the mutations responsible for the suppression were in t he 3' end of the gene, corresponding to the phosphorylated COOH terminus of Gsy2p. Phosphorylation of the mutant proteins was reduced, suggesting that they are poor substrates for glycogen synthase kinases. Suppressor mutatio ns outside this domain did not decrease the phosphorylation of the resultin g proteins, indicating that these proteins are immune to the regulatory eff ects of phosphorylation. Since no growth defect has been observed for strai ns with altered glycogen levels, the relative levels of fitness of GSY2 mut ants that fail to accumulate glycogen and that hyperaccumulate glycogen wer e assayed by cocultivation experiments, A wild-type strain outcompeted both hypo- and hyperaccumulating strains, suggesting that glycogen levels contr ibute substantially to the fitness of yeast.