GLYCOGENIN-2, A NOVEL SELF-GLUCOSYLATING PROTEIN INVOLVED IN LIVER-GLYCOGEN BIOSYNTHESIS

Glycogenin is a self-glucosylating protein involved in the initiation phase of glycogen biosynthesis. A single mammalian gene had been repor ted to account for glycogen biogenesis in Liver and muscle, the two ma jor repositories of glycogen, We describe the characterization of nove l forms of glycogenin, designated glycogenin-2 (GN-2), encoded by a se cond gene that is expressed preferentially in certain tissues, includi ng liver, heart, and pancreas. Cloning of cDNAs encoding glycogenin-a indicated the existence of multiple species, including three liver for ms (GN-2 alpha, GN-2 beta, and GN-2 gamma) generated in part by altern ative splicing. Overall, GN-2 has 40-45% identity to muscle glycogenin but is 72% identical over a 200-residue segment thought to contain th e catalytic domain. GN-2 expressed in Escherichia coli or COS cells is active in self-glucosylation assays, and self-glucosylated GN-2 can b e elongated by skeletal muscle glycogen synthase. Antibodies raised ag ainst GN-2 produced in E. coli recognized proteins of M-r similar to 6 6,000 present in extracts of rat liver and in cultured H4IIEC3 hepatom a cells. In H4IIEC3 cells, most of the GN-2 was present as a free prot ein but some was covalently associated with glycogen fractions and was only released by treatment with alpha-amylase, H4IIEC3 cells also exp ressed the muscle form of glycogenin (glycogenin-1), which was attache d to a chromatographically separable glycogen fraction.