title: A novel pathway for phosphorylated oligosaccharide biosynthesis. Identification of an oligosaccharide-specific phosphate methyltransferase in dictyostelium discoideum.
abstract: The N-linked oligosaccharides on three lysosomal enzymes in Dictyostelium discoideum were found to contain mannose 6-phosphomethyl residues. We have identified and partially characterized a novel S-adenosylmethionine-dependent methyltransferase that is probably responsible for the synthesis of this unusual diester from Man-6-P. The enzyme selectively methylates the phosphate group of Man-6-P (Km 4.3 mM). Glucose-6-P and fructose-1-P are relatively poor acceptors; however, the enzyme is inactive against a broad array of other phosphorylated compounds. Using model di-, tri-, and pentasaccharide acceptors that include portions of the three different branches of high mannose-type oligosaccharides, we found that the enzyme prefers terminal alpha 1----2-linked Man-6-P residues (Km 0.15-1.25 mM) found on the known phosphorylated branches. The enzyme is membrane bound, has a neutral pH optimum and cofractionates on sucrose gradients with GlcNAc-1-P transferase, which resembles its mammalian counterpart, and is, presumably, the first enzyme in the phosphorylation pathway. Based on the substrate specificity and colocalization with GlcNAc-1-P transferase, the phosphate methyltransferase is likely to be responsible for the generation of mannose-6-phosphomethyldiester on Dictyostelium oligosaccharides.
authors.affiliation: La Jolla Cancer Research Foundation, California 92037.
authors.forename: H H
authors.initials: HH
authors.lastname: Freeze
authors.forename: O
authors.initials: O
authors.lastname: Hindsgaul
authors.forename: M
authors.initials: M
authors.lastname: Ichikawa
mesh_terms: D000818:Animals; D002458:Cell Fractionation; D002462:Cell Membrane / Q000201:enzymology; D002850:Chromatography, Gel; D004023:Dictyostelium / Q000201:enzymology*; D005964:Glucosyltransferases / Q000378:metabolism; D006026:Glycoside Hydrolases / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008247:Lysosomes / Q000201:enzymology; D008745:Methylation; D008780:Methyltransferases / Q000378:metabolism*; D017351:N-Acetylglucosaminyltransferases; D009844:Oligosaccharides / Q000096:biosynthesis* / Q000378:metabolism; D010766:Phosphorylation; D011233:Precipitin Tests; D013696:Temperature
chemical_list: D009844:Oligosaccharides; D008780:Methyltransferases; C073145:S-adenosylmethionine-dependent phosphate methyltransferase; D005964:Glucosyltransferases; D017351:N-Acetylglucosaminyltransferases; C024850:alpha-1,3-mannosyl-glycoprotein beta-1,2-N-acetylglucosaminyltransferase I; D006026:Glycoside Hydrolases; C020668:glycanase