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

Abstract

Glycogenin-2 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 α-amylase. Similarly, purifed high M(r) glycogen was only detected after release by α-amylase treatment. Based on analysis by polymerase chain reaction, the predominant isoform in liver was glycogenin- 2β. Glycogenin-2 was found in Ewing's sarcoma RD-ES cells where, however, it was not associated with high M(r) carbohydrate. Both human liver and human RD-ES cell extracts also contained glycogenin-1. Glycogenin-1 and glycogenin- 2 interact with one another, based on in vitro interactions and co- immunoprecipitation from liver and cell extracts. Mutation of Tyr-196 in glycogenin-2 to a Phe residue abolished the ability of glycogenin-2 to self- glucosylate but not to interact with glycogenin-1. Stable overexpression of glycogenin-2α in Rat-1 fibroblast cells resulted in a 5-fold increase in the level of glycogen 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 potential to control glycogen synthesis.