Glucose-6-P Control of Glycogen Synthase Phosphorylation in Yeast

Abstract

The SNF1 gene encodes a protein kinase necessary for expression of glucose-repressible genes and for the synthesis of the storage polysaccharide glycogen. From a genetic screen, we have found that mutation of the PFK2 gene, which encodes the ␤-subunit of 6-phosphofructo-1-kinase, restores glycogen accumulation in snf1 cells. Loss of PFK2 causes elevated levels of metabolites such as glucose-6-P, hyperaccumulation of glycogen, and activation of glycogen synthase, whereas glucose-6-P is reduced in snf1 cells. Other mutations that increase glucose-6-P, deletion of PFK1, which codes for the ␣-subunit of 6-phosphofructo-1-kinase, or of PGI1, the phosphoglucoisomerase gene, had similar effects on glycogen metabolism as did pfk2 mutants. We propose that elevated glucose-6-P mediates the effects of these mutations on glycogen storage. Glycogen synthase kinase activity was reduced in extracts from pfk2 cells but was restored to that of wild type if the extract was gel-filtered to remove small molecules. Also, added glucose-6-P inhibited the glycogen synthase kinase activity in extracts from wild-type cells, half-maximally at ϳ2 mM. We suggest that glucose-6-P controls glycogen synthase activity by two separate mechanisms. First, glucose-6-P is a direct activator of glycogen synthase, and second, it controls the phosphorylation state of glycogen synthase by inhibiting a glycogen synthase kinase.

Cyclin Partners Determine Pho85 Protein Kinase Substrate Specificity In Vitro and In Vivo: Control of Glycogen Biosynthesis by Pcl8 and Pcl10

“…Several Pcl's still have unassigned functions, including Pcl6 and Pcl7 from the Pho80 subfamily. Deletion of PCL6 and PCL7, alone or together, did not cause a significant change in glycogen accumulation (35), but if the novel pathway is not limiting in wild-type cells, this result does not exclude the involvement of Pcl6 or Pcl7.…”

“…For assay of glycogen synthase kinase activity in cell extracts, the direct phosphorylation of added, purified recombinant Gsy2p was determined by analyzing the incorporation of 32 P into Gsy2p from [␥-32 P]ATP (36). The cells were resuspended in a homogenization buffer containing 50 mM Tris-HCl (pH 7.4), FIG.…”

Pho85p, a Cyclin-Dependent Protein Kinase, and the Snf1p Protein Kinase Act Antagonistically To Control Glycogen Accumulation in Saccharomyces cerevisiae

“…Mutations that restore glycogen accumulation in snf1 cells therefore should include lossof-function mutations in the kinase. Furthermore, we know that a single mutation at any of the three phosphorylation sites is sufficient to increase the glycogen synthase activity ratio and to suppress the glycogen defect in snf1 mutants (31). We therefore mutagenized an snf1 strain with ethylmethane sulfonate, as described in Materials and Methods, and isolated cells with restored glycogen accumulation.…”

Effect of AICAR Treatment on Glycogen Metabolism in Skeletal Muscle

“…G-6-P may increase glycogen synthase activity via allosteric stimulation and also by covalently binding to the enzyme and causing a conformational change in the protein structure that makes the regulatory serine residues more susceptible to dephosphorylation by phosphatases (41). In yeast, both G-6-P and fructose-6-phosphate (F-6-P) also promote dephosphorylation of glycogen synthase via inhibition of glycogen synthase kinase (42). It is unknown whether this occurs in mammalian cells, but could represent another potential mechanism to promote glycogen synthesis.…”