title: Control of glycogen synthase by insulin and isoproterenol in rat adipocytes. Changes in the distribution of phosphate in the synthase subunit in response to insulin and beta-adrenergic receptor activation.
abstract: Rat adipocytes were incubated with [32P]phosphate to label glycogen synthase, which was rapidly immunoprecipitated from cellular extracts and cleaved using either CNBr or trypsin. All of the [32P]phosphate in synthase was recovered in two CNBr fragments, denoted CB-1 and CB-2. Isoproterenol (1 microM) rapidly decreased the synthase activity ratio (-glucose-6-P/+glucose-6-P) and stimulated the phosphorylation of both CB-1 and CB-2 by approximately 30%. Insulin opposed the decrease in activity ratio and blocked the stimulation of phosphorylation by isoproterenol. Incubating cells with insulin alone changed the 32P content of neither CB-1 nor CB-2. Trypsin fragments were separated by reverse phase liquid chromatography and divided into peak fractions, denoted F-I-F-VII in order of increasing hydrophobicity. F-V contained almost half of the [32P]phosphate and was phosphorylated when synthase was immunoprecipitated from unlabeled fat cells and incubated with [gamma-32P]ATP and the cAMP-independent protein kinase, FA/GSK-3. That F-V also had the same retention time as the skeletal muscle synthase fragment containing sites 3(a + b + c) suggests that it contains sites 3. Muscle sites 1a, 5, 1b, and 2 eluted with F-I, F-II, F-VI, and F-VII, respectively. F-V was increased approximately 25% by isoproterenol, but the largest relative increases were observed in F-I (4-fold), F-III (4-fold), and F-VI (2-fold). These results indicate that beta-adrenergic receptor activation results in increased phosphorylation of multiple sites on glycogen synthase. Insulin plus glucose decreased the overall 32P content of synthase by approximately 30%, with the largest decrease (40%) occurring in F-V. Without glucose, insulin decreased the [32P]phosphate in F-V by 17%, an effect which was balanced by increases in F-I, F-II, and F-III so that no net change in the total 32P contents of the fractions was observed. Thus, activation of glycogen synthase by the glucose transport-independent pathway seems to involve a redistribution of phosphate in the synthase subunit.
mesh_terms: D000273:Adipose Tissue / Q000201:enzymology*; D000818:Animals; D003488:Cyanogen Bromide / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D004837:Epinephrine / Q000494:pharmacology; D006006:Glycogen Synthase / Q000378:metabolism*; D007328:Insulin / Q000494:pharmacology*; D007545:Isoproterenol / Q000494:pharmacology*; D046911:Macromolecular Substances; D008729:Methoxamine / Q000494:pharmacology; D010710:Phosphates / Q000378:metabolism*; D051381:Rats; D011943:Receptors, Adrenergic, beta / Q000378:metabolism*; D014357:Trypsin / Q000378:metabolism
chemical_list: D007328:Insulin; D046911:Macromolecular Substances; D010710:Phosphates; D011943:Receptors, Adrenergic, beta; D006006:Glycogen Synthase; D014357:Trypsin; D008729:Methoxamine; D007545:Isoproterenol; D003488:Cyanogen Bromide; D004837:Epinephrine