SUMMARY PARAGRAPH for GLC3
Glycogen, a branched polymer of glucose, is a storage molecule whose accumulation is under rigorous nutritional control in many cells (5). In S. cerevisiae, glycogen biosynthesis involves three processes: nucleation, elongation, and ramification, or branching (6). GLG1 and GLG2 encode self-glucosylating glycogenin glucosyltransferases (EC:188.8.131.52) involved in glycogen nucleation (5). 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 (7). Glycogen synthase (EC:184.108.40.206, 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 (8). Branches can be added into the glycogen molecule by Glc3p, the glycogen branching enzyme (EC:220.127.116.11) in S. cerevisiae (2). No enzyme that releases the glycogen chain from Glg1p or Glg2p has been identified (6).
GLC3 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 (9). glc3 null mutants are viable, but do not accumulate glycogen, indicating that branch formation is required for glycogen biosynthesis in S. cerevisiae (2). glc3 null mutants do not display phenotypes similar to the Glycogen Storage Disease IV that is associated with mutations in the human glycogen branching enzyme GBE1 (6).
Last updated: 2005-08-30