Glycogenin is a self-glucosylating protein involved in the initiation phase of glycogen biosynthesis. A single mammalian gene had been reported to account for glycogen biogenesis in liver and muscle, the two major repositories of glycogen. We describe the characterization of novel forms of glycogenin, designated glycogenin-2 (GN-2), encoded by a second gene that is expressed preferentially in certain tissues, including liver, heart and pancreas. Cloning of cDNAs encoding glycogenin-2 indicated the existence of multiple species, including three liver forms, GN-2ct, GN-2β and GN-2Y, generated in part by alternative splicing. Overall, GN-2 has 40-45% idenitity to muscle glycogenin but is 72% identical over a 200 residue segment thought to contain the catalytic domain. GN-2 expressed in Escherichia coli or COS cells is active in self-glucosylation assays. Self-glucosylated GN-2 can be elongated by glycogen synthase. Antibodies raised against GN-2 produced in E. coli recognized proteins of Mr -66,000 present in extracts of rat liver and in cultured H4IIEC3 hepatoma cells. In H4IIEC3 cells, most of the GN-2 was present as a free protein but some was covalently associated with glycogen fractions and was only released by treatment with a-amylase. H4IIEC3 cells also expressed the muscle form of glycogenin (glycogenin-1) which was attached to chromatographically separable glycogen fractions. The different isoforms may therefore be associated with different populations of glycogen molecules. Supported by NIH grant DK 27221.
|Original language||English (US)|
|State||Published - Dec 1 1997|
ASJC Scopus subject areas
- Molecular Biology