Glycogen hyperaccumulation in Saccharomyces cerevisiae ras2 mutant. A biochemical study.

The mechanism by which yeast ras2 mutant hyperaccumulates glycogen has been investigated. Total glycogen synthase activity was between 2.5 and 1.3 times higher in the ras2 mutant than in an isogenic strain. In addition, while in the normal strain the glycogen synthase activation state decreased along the exponential phase, in the mutant strain the opposite behaviour was observed: glycogen synthase activation state rose continuously reaching full activation at the beginning of the stationary phase. Glycogen phosphorylase a activity was up to 40 times higher in the mutant than in the normal strain. Glucose 6-phosphate and fructose 2,6-bisphosphate levels were slightly more elevated in the mutants. The increase in total glycogen synthase and, particularly, the full activation of this enzyme may explain glycogen hyperaccumulation in the ras2 mutant even in the presence of elevated levels of glycogen phosphorylase a.