GLC3 and GHAI of Saccharomyces cerevisiae are allelic and encode the glycogen branching enzyme

In the yeast Saccharomyces cerevisiae, glycogen serves as a major storage carbohydrate. In a previous study, mutants with altered glycogen metabolism were isolated on the basis of the altered iodine-staining properties of colonies. We found that when glycogen produced by strains carrying the glc3-lp (previously called ghal-1) mutation is stained with iodine, the absorption spectrum resembles that of starch rather than that of glycogen, suggesting that this mutation might reduce the level of branching in the glycogen particles. Indeed, glycogen branching activity was undetectable in extracts from a glc3-lp strain but was elevated in strains which expressed GLC3 from a high-copy-number plasmid. These observations suggest that GLC3 encodes the glycogen branching enzyme. In contrast to glc3-lp, the glc34 mutation greatly reduces the ability of yeast to accumulate glycogen. These mutations appear to be allelic despite the striking difference in the phenotypes which they produce. The GLC3 clone complemented both glc3-lp and gkc34. Deletions and transposon insertions in this clone had parallel effects on its ability to complement gk3-lp and glc3-4. Finally, a fragment of the cloned gene was able to direct the repair of both gk3-lp and gkc34. Disruption of GLC3 yielded the glycogen-deficient phenotype, indicating that glycogen deficiency is the null phenotype. The glc3-lp allele appears to encode a partially functional product, since it is dominant over glc34 but recessive to GLC3. These observations suggest that the ability to introduce branches into glycogen greatly increases the ability of the cel