The initiation of glycogen biosynthesis in rat heart alpha-1,4 glucans tightly associated with glycogen synthase

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 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-2alpha, GN-2beta, and GN-2gamma) generated in part by alternative splicing. Overall, GN-2 has 40-45% identity 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, and self-glucosylated GN-2 can be elongated by skeletal muscle glycogen synthase. Antibodies raised against GN-2 produced in E. coli recognized proteins of Mr approximately 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 alpha-amylase. H4IIEC3 cells also expressed the muscle form of glycogenin (glycogenin-1), which was attached to a chromatographically separable glycogen fraction.

Characterization of a protein:glucosyltransferase activity in human platelets

Human platelets exhibited significant glucosyltransferase activity, that transfer [14C]glucose from UDP-Glc to an endogenous protein acceptor. The enzyme protein:glucosyltransferase responsible for the catalysis was characterized and compared with glycogen:glucosyltransferase. We describe a partial separation of both activities, the ratio of protein:glucosyltransferase/glycogen:glucosyltransferase varied from 7:1 in a crude homogenate of platelets to 36:1 in the Sephadex G-100 column. This procedure failed to separate the protein:glucosyltransferase from its endogenous acceptor. Glucosylation of protein demonstrated dependence with respect to time and both protein and UDP-Glc concentration, and was saturated by very low concentration of donor and acceptor substrates. It was inhibited 76% by 5 mM Mn2+ concentration and was activated 23 and 11% by 5 mM concentrations of Ca2+ and Mg2+, respectively. With respect to glycogen:glucosyltransferase, when the effect of time, protein, and substrate concentration were determined under identical conditions, it did not show the same dependence. At 5 mM concentration, Mn2+, Ca2+, and Mg2+ were activators of the enzyme 43, 80, and 200%, respectively. On the basis of these characteristics, we conclude that the synthesis of glucoprotein and glycogen are catalyzed by two distinct enzymes. Addition of exogenous glycogen (range 0.002-1%) inhibited the protein:glucosyltransferase, whereas at 0.001-0.007% concentration it was acceptor substrate for glycogen:glucosyltransferase activity. At higher concentrations this activity was strongly inhibited. The concentration of glycogen in platelets could play a regulatory role in forming the glucoprotein and the glycogen molecules.

Glycogen synthesis in the obliquely striated muscle of Ascaris suum

A glycogen synthase, designated GS II, which occurs in a protein/carbohydrate complex has been purified from Ascaris suum muscle. The purified GS-II complex which is eluted from concanavalin-A--Sepharose contains proteins with Mr 140,000 and 66,000 and a glycoprotein with a carbohydrate/protein mass ratio of 3:1. GS II activity was totally dependent on glucose 6-phosphate, but exogenous glycogen was not required for polysaccharide synthesis. The GS-II complex was not phosphorylated by cyclic-AMP-dependent protein kinase, and antibodies to the protein and carbohydrate components of GS II did not cross react with the purified cyclic-AMP-regulated glycogen synthase (GS I) from A. suum muscle. Polysaccharide which was synthesized de novo by the complex was added to the large-molecular-mass glycoprotein in GS II. The glycogen-like character of the newly synthesized polysaccharide was confirmed by the observation that glycogen phosphorylase utilized the polymer as substrate in both the synthesis and degradation reactions. A model is discussed in which a core glycoprotein serves as the substrate for a glycogen synthase which is distinctly different from GS I.

Glycogenin is the priming glucosyltransferase required for the initiation of glycogen biogenesis in rabbit skeletal muscle

Purified preparations of glycogen synthase are a complex of two proteins, the catalytic subunit of glycogen synthase and glycogenin, present in a 1:1 molar ratio [J. Pitcher, C. Smythe, D. G. Campbell & P. Cohen (1987) Eur. J. Biochem. 169, 497-502]. This complex has now been found to contain a further glucosyltransferase activity that catalyses the transfer of glucose residues from UDP-Glc to glucosylated-glycogenin. The glucosyltransferase, which is of critical importance in forming the primer required for de novo glycogen biosynthesis, is distinct from glycogen synthase in several ways. It has an absolute requirement for divalent cations, a 1000-fold lower Km for UDP-Glc and its activity is unaffected by incubation with UDP-pyridoxal or exposure to 2 M LiBr, which inactivate glycogen synthase by 95% and 100%, respectively. The priming glucosyltransferase and glycogen synthase activities coelute on Superose 6, and the rate of glycosylation of glycogenin is independent of enzyme concentration, suggesting that the reaction is catalysed intramolecularly by a subunit of the glycogen synthase complex. This component has been identified as glycogenin, following dissociation of the subunits in 2 M LiBr and their separation on Superose 12. The glycosylation of isolated glycogenin reaches a plateau when five additional glucose residues have been added to the protein, and digestion with alpha-amylase indicates that all the glycogenin molecules contain at least one glucosyl residue prior to autoglucosylation. The priming glucosyltransferase activity of glycogenin is unaffected by either glucose 6-phosphate or by phosphorylation of the catalytic subunit of glycogen synthase. The mechanism of primer formation is discussed in the light of the finding that glycogenin is an enzyme that catalyses its own autoglucosylation.

A 42,000-Da protein in rabbit tissues and in a glycogen synthase preparation cross-reacts with antibodies to glycogenin

Rabbit skeletal muscle glycogen previously has been shown to be covalently bound to a 40,000-Da protein ("glycogenin") via a novel glucosyl-tyrosine linkage [I.R. Rodriguez and W.J. Whelan (1985) Biochem. Biophys. Res. Commun. 132, 829-836]. Antibodies raised against rabbit skeletal muscle glycogenin cross-react with a similar protein present in rabbit heart and liver glycogens, as well as with a 42,000-Da "acceptor protein" present in high-speed supernatants of rabbit muscle, heart, retina, and liver. This 42,000-Da protein incorporates [U-14C]Glc when an ammonium sulfate fraction prepared from the tissue supernatants is incubated with UDP-[U-14C]Glc. The [U-14C]Glc incorporated can be removed quantitatively by treatment with amylolytic enzymes, indicating that the [U-14C]Glc incorporation represents elongation of a preexisting glucan attached to the acceptor protein. Furthermore, a commercial preparation of rabbit skeletal muscle glycogen synthase contains this 42,000-Da protein. We propose that the 42,000-Da protein represents the free form of glycogenin in tissues, with its covalently attached glucan chain(s) providing a "primed" elongation site for glycogen synthesis.

Identification of the 38-kDa subunit of rabbit skeletal muscle glycogen synthase as glycogenin

Glycogen synthase from rabbit skeletal muscle has been shown to be a complex of two types of subunit which have apparent molecular masses of 86 kDa and 38 kDa and are present in a 1:1 molar ratio. The 38-kDa component was separated from the 86-kDa catalytic subunit by gel filtration in the presence of 2 M LiBr, and a number of chymotryptic peptides were sequenced. This demonstrated that the 38-kDa subunit was glycogenin, the protein that is bound covalently to glycogen and believed to be the 'primer' involved in the initiation of de novo glycogen synthesis.