title: Incorporation of phosphate into glycogen by glycogen synthase.
abstract: The storage polymer glycogen normally contains small amounts of covalently attached phosphate as phosphomonoesters at C2, C3 and C6 atoms of glucose residues. In the absence of the laforin phosphatase, as in the rare childhood epilepsy Lafora disease, the phosphorylation level is elevated and is associated with abnormal glycogen structure that contributes to the pathology. Laforin therefore likely functions in vivo as a glycogen phosphatase. The mechanism of glycogen phosphorylation is less well-understood. We have reported that glycogen synthase incorporates phosphate into glycogen via a rare side reaction in which glucose-phosphate rather than glucose is transferred to a growing polyglucose chain (Tagliabracci et al. (2011) Cell Metab13, 274-282). We proposed a mechanism to account for phosphorylation at C2 and possibly at C3. Our results have since been challenged (Nitschke et al. (2013) Cell Metab17, 756-767). Here we extend the evidence supporting our conclusion, validating the assay used for the detection of glycogen phosphorylation, measurement of the transfer of (32)P from [β-(32)P]UDP-glucose to glycogen by glycogen synthase. The (32)P associated with the glycogen fraction was stable to ethanol precipitation, SDS-PAGE and gel filtration on Sephadex G50. The (32)P-signal was not affected by inclusion of excess unlabeled UDP before analysis or by treatment with a UDPase, arguing against the signal being due to contaminating [β-(32)P]UDP generated in the reaction. Furthermore, [(32)P]UDP did not bind non-covalently to glycogen. The (32)P associated with glycogen was released by laforin treatment, suggesting that it was present as a phosphomonoester. The conclusion is that glycogen synthase can mediate the introduction of phosphate into glycogen, thereby providing a possible mechanism for C2, and perhaps C3, phosphorylation.
authors.affiliation: Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis, IN 46202, USA.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Contreras
authors.affiliation: Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis, IN 46202, USA.
authors.forename: Dyann M
authors.initials: DM
authors.lastname: Segvich
authors.affiliation: Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis, IN 46202, USA.
authors.forename: Krishna
authors.initials: K
authors.lastname: Mahalingan
authors.affiliation: Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis, IN 46202, USA; Dow AgroSciences LLC, 9630 Zionsville Road, Indianapolis, IN 46268, USA.
authors.forename: Vimbai M
authors.initials: VM
authors.lastname: Chikwana
authors.affiliation: Department of Pharmacology and Cell Biophysics, College of Medicine, University of Cincinnati, 231 Albert Sabin Way, Cincinnati, OH 45267, USA.
authors.forename: Terence L
authors.initials: TL
authors.lastname: Kirley
authors.affiliation: Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis, IN 46202, USA.
authors.forename: Thomas D
authors.initials: TD
authors.lastname: Hurley
authors.affiliation: Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis, IN 46202, USA.
authors.forename: Anna A
authors.initials: AA
authors.lastname: DePaoli-Roach
authors.affiliation: Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis, IN 46202, USA. Electronic address: proach@iu.edu.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Roach
mesh_terms: D006003:Glycogen / Q000096:biosynthesis / Q000737:chemistry*; D006006:Glycogen Synthase / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D010710:Phosphates / Q000737:chemistry* / Q000378:metabolism; D054558:Protein Tyrosine Phosphatases, Non-Receptor / Q000737:chemistry / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000201:enzymology*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry* / Q000378:metabolism; D014539:Uridine Diphosphate Sugars / Q000737:chemistry / Q000378:metabolism
chemical_list: D010710:Phosphates; D029701:Saccharomyces cerevisiae Proteins; D014539:Uridine Diphosphate Sugars; D006003:Glycogen; D006006:Glycogen Synthase; D054558:Protein Tyrosine Phosphatases, Non-Receptor; C114990:EPM2A protein, human
keywords: Glycogen; Glycogen synthase; Lafora; Laforin; Phosphorylation