O‐GlcNacylation of Glycogenin Inhibits Glycogen Synthesis

Abstract

It is becoming evident that increased cellular glucose concentration leads to an increase in UDP‐ß‐N‐Acetylglucosamine (UDP‐GlcNac) concentration and O‐GlcNacylation of many proteins. Since glycogenin self‐glycosylates with UDP‐glucose, we investigated whether UDP‐GlcNac serves as its substrate and possible effects of this modification on glycogen synthesis. Incubation of purified glycogen synthase, glycogenin, and apo‐glycogenin with UDPGlcNac led to incorporation of ~0.5 mole of GlcNac per mole of glycogenin. The presence of incorporated GlcNac into glycogenin was confirmed by HPAEC‐PAD and by radiolabeling with galactosyl transferase. The apparent KM for UDP‐GlcNac was 227 μM. Mutating Tyrosine 194 to Phenylalanine eliminated the ability of glycogenin to autoglycosylate with either substrate confirming that GlcNac was added to the same site as glucose. Our results suggest that GlcNac is added to Tyr194 either as a first residue or to the growing oligosaccharide. Furthermore, we observed that recombinant glycogenin modified with O‐GlcNac was still able to autoglucosylate. However, O‐GlcNac modified glycogenin was resistant to the addition of glucose by glycogen synthase. These results suggest that O‐GlcNacylation of glycogenin may inhibit glycogen synthase activity resulting in inhibition of glycogen synthesis. Supported by sabbatical leave to BSK and RISE grant: GM‐071638.