Two glycogen synthase isoforms in Saccharomyces cerevisiae are coded by distinct genes that are differentially controlled

In previous work, we identified a Saccharomyces cerevisiae glycogen synthase gene, GSY1, which codes for an 85-kDa polypeptide present in purified yeast glycogen synthase (Farkas, I., Hardy, T.A., DePaoli-Roach, A.A., and Roach, P.J. (1990) J. Biol. Chem. 265, 20879-20886). We have now cloned another gene, GSY2, which encodes a second S. cerevisiae glycogen synthase. The GSY2 sequence predicts a protein of 704 residues, molecular weight 79,963, with 80% identity to the protein encoded by GSY1. Amino acid sequences obtained from a second polypeptide of 77 kDa present in yeast glycogen synthase preparations matched those predicted by GSY2. GSY1 resides on chromosome VI, and GSY2 is located on chromosome XII. Disruption of the GSY1 gene produced a strain retaining about 85% of wild type glycogen synthase activity at stationary phase, while disruption of the GSY2 gene yielded a strain with only about 10% of wild type enzyme activity. The level of glycogen synthase activity in yeast cells disrupted for GSY1 increased in stationary phase, whereas the activity remained at a constant low level in cells disrupted for GSY2. Disruption of both genes resulted in a viable haploid that totally lacked glycogen synthase activity and was defective in glycogen deposition. In conclusion, yeast expresses two forms of glycogen synthase with activity levels that behave differently in the growth cycle. The GSY2 gene product appears to be the predominant glycogen synthase with activity linked to nutrient depletion.