Interactions between cAMP-dependent and SNF1 protein kinases in the control of glycogen accumulation in Saccharomyces cerevisiae.

The synthesis of glycogen in Saccharomyces cerevisiae is stimulated by nutrient limitation and requires both glycogen synthase and the glycogen branching enzyme. Of the two glycogen synthase genes present in yeast, GSY2 appears to be more important for the accumulation of glycogen upon entry into stationary phase. In cells grown on glucose, GSY2 mRNA levels increased approximately 10-fold during the transition from logarithmic to stationary phase. Growth of cells in glycerol, however, resulted in constitutive expression of GSY2 mRNA and the corresponding protein, GS-2, suggestive of glucose repression of GSY2. Mutants defective in the SNF1 gene, which encodes a protein kinase important in glucose repression mechanisms, are known not to accumulate glycogen. A modest 2-4-fold decrease in total GS-2 level was observed, and upon entry into stationary phase, the enzyme was blocked in the inactive, phosphorylated state in snf1 strains. The GS-2 protein is thought to be regulated by covalent phosphorylation of three COOH-terminal sites (Hardy, T.A., and Roach, P.J. (1993) J. Biol. Chem. 268, 23799-23805), removal of which results in constitutively active glycogen synthase that bypasses phosphorylation controls. Expression of COOH-terminally truncated GS-2 in snf1 cells restored glycogen accumulation, and so we propose that the SNF1 kinase controls the phosphorylation state of GS-2. Cyclic AMP pathways also exert control over glycogen accumulation. In bcy1 cells, which have constitutively active cyclic AMP-dependent protein kinase, greatly reduced levels of both GS-2 message and protein were observed. With wild type GSY2 placed under control of the ADH1 promoter, bcy1 cells did not accumulate glycogen despite increased GS-2. Overexpression of truncated GS-2, however, resulted in definite though reduced glycogen accumulation; the glycogen synthesized was structurally distinct from wild type with properties characteristic of less branched polysaccharide. We conclude that the cAMP pathway controls both the expression and the phosphorylation state of GS-2. Furthermore, other factor(s) necessary for glycogen biosynthesis, such as the branching enzyme GLC3, must also be under negative control by the cAMP pathway. The results demonstrate interactive controls of GS-2 by the cAMP-dependent and SNF1 protein kinases.