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 fiveUXS 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 inEscherichia 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.

Functional Characterization of UDP-apiose Synthases from Bryophytes and Green Algae Provides Insight into the Appearance of Apiose-containing Glycans during Plant Evolution

“…Plants, many animals, fungi, Bacteria, and Archaea produce enzymes (UXS) that convert UDP-GlcA to UDP-Xyl (27,(37)(38)(39)(40), whereas UAS forms both UDP-Api and UDP-Xyl in a ratio of ϳ2:1. It is not known whether UXS or UAS is the ancestral gene.…”

“…UAS, UXS, RsU4kpxs, and ArnA are all decarboxylases that contain a conserved N-terminal Gly-X-X-Gly-X-X-Gly motif (supplemental Fig. S1) that is proposed to be involved in NAD ϩ binding as well as the conserved Tyr-X-X-X-Lys motif with an upstream Ser that forms the catalytic site of the short-chain dehydrogenase/ reductase family (25)(26)(27).…”

The physiological, transcriptional and genetic responses of an ozone- sensitive and an ozone tolerant poplar and selected extremes of their F 2 progeny

“…These genes code for UDP-xyl synthase 5 (USX5), a tonoplast-intrinsic protein, and arabinogalactan protein 15. UDP-xyl synthase is involved in the production of many cell wall products as well as the upstream inhibition of enzymes (Harper and Bar-Peled, 2002). The tonoplast-intrinsic protein is a putative aquaporin, but of unknown function.…”

The inositol oxygenase gene family of Arabidopsis is involved in the biosynthesis of nucleotide sugar precursors for cell-wall matrix polysaccharides

“…This precursor is converted by an epimerase into UDP-galacturonic acid (Usadel et al 2004a;Mølhøj et al 2004), being an important precursor for pectic polymers. Another branch leads from UDP-GlcA after decarboxylation to UDP-xylose (Harper and Bar-Peled 2002;Mølhøj et al 2003), which can be subsequently epimerized to UDP-arabinose (Burget et al 2003).…”