Glycogen, a branched polymer of glucose, is a storage molecule whose accumulation is under rigorous nutritional control in many cells (2). In S. cerevisiae, glycogen biosynthesis involves three processes: nucleation, elongation, and ramification, or branching (4). GLG1 and GLG2 encode self-glucosylating glycogenin glucosyltransferases (EC:2.4.1.186) involved in glycogen nucleation (2). Both Glg1p and Glg2p are able to use UDP-glucose to produce a short alpha (1,4)-glucosyl chain covalently attached to an internal tyrosine residue (1). Glycogen synthase (EC:2.4.1.11, Gsy1p and Gsy2p) is then able to extend the linear alpha (1,4)-chains of glycogen by catalyzing the formation of alpha (1,4)-glucosidic bonds from UDP-glucose at the non-reducing ends (5). Branches can be added into the glycogen molecule by Glc3p, the glycogen branching enzyme (EC:2.4.1.18) in S. cerevisiae (6). No enzyme that releases the glycogen chain from Glg1p or Glg2p has been identified (4).

GLG1 mRNA, and presumably GLG2 mRNA, begins to accumulate when approximately 50% of the environmental glucose is gone, and peaks when environmental glucose is exhausted, similar to other glycogen metabolism genes (7). Glg1p and Glg2p both are important for glycogen nucleation, since both glg1 and glg2 null mutants display normal glycogen accumulation, but a glg1 glg2 double null mutant is unable to accumulate glycogen (2). Glg1p and Glg2p may also be involved in regulating the activity of glycogen synthase (Gsy1p and Gsy2p), because a glg1 glg2 double null mutant displays normal levels of Gsy1p and Gsy2p but reduced activity of these proteins (2). GLG1 and GLG2 display sequence similarity to human glycogenin genes GYG and GYG2 (2).

Last updated: 2005-08-30