Characterization of human glycogenin-2, a self-glucosylating initiator of liver glycogen metabolism

Glycogenin-a is a recently described self-glucosylating protein potentially involved in the initiation of glycogen biosynthesis (Mu, J., Skurat, A. V. , and Roach, P. J. (1997) J. Biol. Chem. 272, 27589-27597). In human liver extracts, most of the glycogenin-2 was only detectable after treatment with alpha-amylase. Similarly, purifed high M-r glycogen was only detected afte r release by alpha-amylase treatment. Based on analysis by polymerase chain reaction, the predominant isoform in liver was glycogenin-2 beta. Glycogen in-a was found in Ewing's sarcoma RD-ES cells where, however, it was not as sociated with high M-r carbohydrate. Both human liver and human RD-ES cell extracts also contained glycogenin-l. Glycogenin-l and glycogenin-2 interac t with one another, based on in vitro interactions and co-immunoprecipitati on from liver and cell extracts. Mutation of Tyr-196 in glycogenin-a to a P he residue abolished the ability of glycogenin-g to self-glucosylate but no t to interact with glycogenin-l. Stable overexpression of glycogenin-2 alph a in Rat-1 fibroblast cells resulted in a 5-fold increase in the level of g lycogen present in the low speed pellet but little change in the low speed supernatant. This result is important since it indicates that the level of glycogenin-2 can determine glycogen accumulation and hence has the potentia l to control glycogen synthesis.