title: Biosynthesis of UDP-xylose. Cloning and characterization of a novel Arabidopsis gene family, UXS, encoding soluble and putative membrane-bound UDP-glucuronic acid decarboxylase isoforms.
abstract: UDP-xylose (Xyl) is an important sugar donor for the synthesis of glycoproteins, polysaccharides, various metabolites, and oligosaccharides in animals, plants, fungi, and bacteria. UDP-Xyl also feedback inhibits upstream enzymes (UDP-glucose [Glc] dehydrogenase, UDP-Glc pyrophosphorylase, and UDP-GlcA decarboxylase) and is involved in its own synthesis and the synthesis of UDP-arabinose. In plants, biosynthesis of UDP-Xyl is catalyzed by different membrane-bound and soluble UDP-GlcA decarboxylase (UDP-GlcA-DC) isozymes, all of which convert UDP-GlcA to UDP-Xyl. Because synthesis of UDP-Xyl occurs both in the cytosol and in membranes, it is not known which source of UDP-Xyl the different Golgi-localized xylosyltransferases are utilizing. Here, we describe the identification of several distinct Arabidopsis genes (named AtUXS for UDP-Xyl synthase) that encode functional UDP-GlcA-DC isoforms. The Arabidopsis genome contains five UXS genes and their protein products can be subdivided into three isozyme classes (A-C), one soluble and two distinct putative membrane bound. AtUxs from each class, when expressed in Escherichia coli, generate active UDP-GlcA-DC that converts UDP-GlcA to UDP-Xyl. Members of this gene family have a large conserved C-terminal catalytic domain (approximately 300 amino acids long) and an N-terminal variable domain differing in sequence and size (30-120 amino acids long). Isoforms of class A and B appear to encode putative type II membrane proteins with their catalytic domains facing the lumen (like Golgi-glycosyltransferases) and their N-terminal variable domain facing the cytosol. Uxs class C is likely a cytosolic isoform. The characteristics of the plant Uxs support the hypothesis that unique UDP-GlcA-DCs with distinct subcellular localizations are required for specific xylosylation events.
authors.affiliation: Complex Carbohydrate Research Center and Department of Plant Biology, University of Georgia, 220 Riverbend Road, Athens, Georgia 30602-4712, USA.
authors.forename: April D
authors.initials: AD
authors.lastname: Harper
authors.forename: Maor
authors.initials: M
authors.lastname: Bar-Peled
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics*; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism; D002262:Carboxy-Lyases / Q000235:genetics* / Q000378:metabolism; D003001:Cloning, Molecular; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D005810:Multigene Family / Q000235:genetics; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014532:Uridine Diphosphate Glucose / Q000737:chemistry / Q000378:metabolism; D014540:Uridine Diphosphate Xylose / Q000096:biosynthesis* / Q000737:chemistry
chemical_list: D029681:Arabidopsis Proteins; D007527:Isoenzymes; D008565:Membrane Proteins; D014540:Uridine Diphosphate Xylose; D002262:Carboxy-Lyases; C023257:UDPglucuronate decarboxylase; D014532:Uridine Diphosphate Glucose