Characterization of plant UDP-glucuronic acid 4-epimerases
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Galacturonic acid (GalA) is an important sugar residue found in pectic polysaccharides, xylan sequence 1 structure and the arabinogalactan-proteins (AGPs). These GalA containing glycans are essential components of primary and secondary wall, and are critical for proper plant structure, growth and development. UDP-GalA is a key precursor for the syntheses of GalA containing glycans. In plant, UDP-GalA is synthesized from UDP-glucuronic acid (UDP-GlcA) by UDP-GlcA 4-epimerases (UGlcAE). My research focused on the identification of UGlcAE genes in plants and the characterization of these proteins at the biochemical, cellular and genetic levels. In plants, multiple UGlcAE isoforms exist and each isoform contains two domains: a sub-cellular localization domain at the N-terminus (~120 amino acids long), followed by a highly conserved catalytic domain (300 amino acids long). Based on phylogeny analyses, these isoforms can be further divided into three evolutionary clades: A, B and C. We determined that UGlcAEs from each clade are enzymatically active as dimers, and all have similar enzymatic kinetic properties. Each of the isoforms tested is inhibited by UDP-xylose and UDP but not by UMP, UDP-glucose or UDP-galactose. Interestingly however, the isoforms from different plant species have different properties. For example, the maize UGlcAE is inhibited to a greater extent by UDP-arabinose when compared with rice and Arabidopsis homologs. We found that all identified isoforms are integral membrane proteins and all the isoforms studied (as UGlcAEs- EYFP fusion) are located in the plant Golgi with their catalytic domains in the Golgi lumen. Since each plant cell contains many Golgi apparatus, we could not distinguish the localization of a specific isoform to a specific set of Golgi within one plant cell. The control of UDP-GalA flux for the synthesis of various GalA containing glycans is now more complicated with the recent discovery of another UDP-GalA synthesis pathway, the salvage pathway. While the GalA salvage pathway forms UDP-GalA in the cytoplasm, the inter-conversion pathway we studied here forms UDP-GalA inside the Golgi lumen.