PHO85P, A CYCLIN-DEPENDENT PROTEIN-KINASE, AND THE SNF1P PROTEIN-KINASE ACT ANTAGONISTICALLY TO CONTROL GLYCOGEN ACCUMULATION IN SACCHAROMYCES-CEREVISIAE

In Saccharomyces cerevisiae, nutrient levels control multiple cellular processes. Cells lacking the SNF1 gene cannot express glucose-repress ible genes and do not accumulate the storage polysaccharide glycogen. The impaired glycogen synthesis is due to maintenance of glycogen synt hase in a hyperphosphorylated, inactive state. In a screen for second site suppressors of the glycogen storage defect of snf1 cells, we iden tified a mutant gene that restored glycogen accumulation and which was allelic with PH085, which encodes a member of the cyclin-dependent ki nase family. In cells with disrupted PH085 genes, we observed hyperacc umulation of glycogen, activation of glycogen synthase, and impaired g lycogen synthase kinase activity. In snf1 cells, glycogen synthase kin ase activity was elevated. Partial purification of glycogen synthase k inase activity from yeast extracts resulted in the separation of two f ractions by phenyl-Sepharose chromatography, both of which phosphoryla ted and inactivated glycogen synthase. The activity of one of these, G PK2, was inhibited by olomoucine, which potently inhibits cyclin-depen dent protein kinases, and contained an similar to 36-kDa species that reacted with antibodies to Pho85p. Analysis of Ser-to-Ala mutations at the three potential Gsy2p phosphorylation sites in pko85 cells implic ated Ser-654 and/or Thr-667 in PH085 control of glycogen synthase. We propose that Pho85p is a physiological glycogen synthase kinase, possi bly acting downstream of Snf1p.