title: R3F, a novel membrane-associated glycogen targeting subunit of protein phosphatase 1 regulates glycogen synthase in astrocytoma cells in response to glucose and extracellular signals.
abstract: Abnormal regulation of brain glycogen metabolism is believed to underlie insulin-induced hypoglycaemia, which may be serious or fatal in diabetic patients on insulin therapy. A key regulator of glycogen levels is glycogen targeted protein phosphatase 1 (PP1), which dephosphorylates and activates glycogen synthase (GS) leading to an increase in glycogen synthesis. In this study, we show that the gene PPP1R3F expresses a glycogen-binding protein (R3F) of 82.8 kDa, present at the high levels in rodent brain. R3F binds to PP1 through a classical 'RVxF' binding motif and substitution of Phe39 for Ala in this motif abrogates PP1 binding. A hydrophobic domain at the carboxy-terminus of R3F has similarities to the putative membrane binding domain near the carboxy-terminus of striated muscle glycogen targeting subunit G(M)/R(GL), and R3F is shown to bind not only to glycogen but also to membranes. GS interacts with PP1-R3F and is hyperphosphorylated at glycogen synthase kinase-3 sites (Ser640 and Ser644) when bound to R3F(Phe39Ala). Deprivation of glucose or stimulation with adenosine or noradrenaline leads to an increased phosphorylation of PP1-R3F bound GS at Ser640 and Ser644 curtailing glycogen synthesis and facilitating glycogen degradation to provide glucose in astrocytoma cells. Adenosine stimulation also modulates phosphorylation of R3F at Ser14/Ser18.
authors.affiliation: Medical Research Council Protein Phosphorylation Unit, College of Life Sciences, University of Dundee, Dundee, Scotland, UK.
authors.forename: Ian R
authors.initials: IR
authors.lastname: Kelsall
authors.forename: Martin
authors.initials: M
authors.lastname: Voss
authors.forename: Shonagh
authors.initials: S
authors.lastname: Munro
authors.forename: Daniel J R
authors.initials: DJ
authors.lastname: Cuthbertson
authors.forename: Patricia T W
authors.initials: PT
authors.lastname: Cohen
mesh_terms: D000241:Adenosine / Q000494:pharmacology; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000595:Amino Acid Sequence; D000818:Animals; D001254:Astrocytoma / Q000201:enzymology* / Q000235:genetics; D001921:Brain / Q000187:drug effects / Q000201:enzymology; D001932:Brain Neoplasms / Q000201:enzymology* / Q000235:genetics; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D045744:Cell Line, Tumor; D004247:DNA / Q000096:biosynthesis / Q000235:genetics; D005110:Extracellular Space / Q000502:physiology*; D005947:Glucose / Q000494:pharmacology*; D006003:Glycogen / Q000378:metabolism; D006006:Glycogen Synthase / Q000096:biosynthesis*; D006801:Humans; D007150:Immunohistochemistry; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009638:Norepinephrine / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000235:genetics / Q000502:physiology*; D010766:Phosphorylation; D054645:Protein Phosphatase 1 / Q000235:genetics / Q000502:physiology*; D012313:RNA / Q000096:biosynthesis / Q000235:genetics; D015398:Signal Transduction / Q000187:drug effects*; D013347:Subcellular Fractions / Q000187:drug effects / Q000378:metabolism
chemical_list: D000316:Adrenergic alpha-Agonists; D002352:Carrier Proteins; D012313:RNA; D006003:Glycogen; D004247:DNA; D006006:Glycogen Synthase; C561048:PPP1R3F protein, rat; D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1; D005947:Glucose; D000241:Adenosine; D009638:Norepinephrine