Mechanism of glycogenin self-glucosylation

Glycogenin, the proposed initiator of mammalian glycogen biosynthesis, transfers glucose residues from UDP-glucose to an oligosaccharide chain attached to Tyr-194 in a self-glucosylation reaction. Mutation of Tyr-194 to either Phe or Thr residues results in the loss of this self-glucosylating activity since the site of oligosaccharide attachment has been lost (Y. Cao, A. M. Mahrenholz, A. A. DePaoli-Roach, and P. J. Roach (1993) J. Biol. Chem. 268, 14687-14693). We describe here that Phe-194 and Thr-194 mutants of glycogenin, as well as wild-type protein, were active in transferring glucose to an exogenous acceptor, maltose, a known inhibitor of the self-glucosylation reaction. The reaction product was exclusively maltotriose with no evidence for further elongation to maltotetraose or maltopentaose. The values of Vmax/Km for maltotriose synthesis for the mutant proteins were 1.5-3.5 times greater than that of the wild type. Analysis of crystals of wild-type glycogenin by X-ray diffraction gives a tetragonal unit cell of a = b = 130 A and c = 174 A in space group I4 with four glycogenin molecules in one asymmetric unit. Considerations of the symmetry and the crystal packing indicate the existence of dimers of glycogenin which may further associate to form a tetramer. The existence of oligomeric forms of glycogenin, together with the idea that glucose transfer to an exogenous acceptor is possible, raises the possibility that the intramolecular self-glucosylation of glycogenin could involve an intersubunit transfer of glucose.