Mammalian glycogen synthesis is initiated by the self-glucosylation of glycogenin, which has a single tyrosine (Tyr-194) for covalent saccharide attachment. Glglp and Glg2p, Saccharomyccs Cercvisiae homologues of glycogenin, were recently identified. They both contain a tyrosine residue (Tyr-232) in correspondence with Tyr-194. Using antibodies raised against recocnbinant Glg2p, we detected endogenous Glg2p released from purified yeast glycogen by ct-amylase treatment. To determine the site(s) important for Glg2p self-glucosylation, we performed site-directed mutagenesis based on sequence homology between glycogenin and the Gig proteins. Our results indicate that there is more than one glucosylation site in Glg2p. The single mutants Y232F or Y230F can each self-glucosylate and serve as substrate for glycogen synthase after self-priming as well as wild type protein. Glg2p with both Tyr-230 and Tyr-232 mutated gives no detectable self-glucosylation in vitro. However, after transforming these mutants into a glglg!g2 strain which does not accumulate glycogen, we found that the Glg2p double mutant could sustain glycogen synthesis, albeit to a much reduced level. By introducing a third COOH-terminal mutation (Y367F) in GLG2, we generated a triple mutant that did not support glycogen accumulation in glglglg2 cells. All the Glg2p mutants were active in transglucosylation as judged by analysis of the transfer of glucose to n-dodecyl-β-D-maltoside as acceptor even though this activity is significantly reduced in the Tyr-232 and Tyr-230 double mutated Glg2p. Therefore we conclude that multiple tyrosine residues are involved in the self-glucosylation of Glg2p.
|Original language||English (US)|
|State||Published - Dec 1 1996|
ASJC Scopus subject areas
- Molecular Biology