Multiple isoforms of UDP-glucose pyrophosphorylase in rice

Uridine diphosphate (UDP)-glucose pyrophosphorylases (UGPases, EC 2.7.7.9) are key enzymes in plant carbohydrate metabolism and cell-wall biosynthesis, catalyzing the reversible production of glucose-1-phosphate and uridine triphosphate from UDP-glucose and pyrophosphate. In the study presented here, two-dimensional gel electrophoresis followed by peptide sequencing analysis using nanospray electrospray ionization tandem mass spectrometry showed that rice (Oryza sativa L.) UGPase undergoes N-terminal acetylation, which may be a conserved modification of plant UGPases. We also obtained indications, using two-dimensional gel electrophoresis in combination with western blot analysis, that multiple isoforms of UGPase are present in rice in vivo and are regulated tissue-specifically. The rice genome contains two homologous UGPase genes, OsUgp1 and OsUgp2. We present evidence that both OsUgp1 and OsUgp2 are ubiquitously expressed throughout rice development, and that OsUgp1 is expressed at much higher levels than OsUgp2. In accordance with the gene expression patterns, the UGPase isoform derived from the OsUgp1 gene predominated in various rice tissues and exhibited qualitative variations (position shifts and presence/absence) between rice varieties B5 and Taichung native 1 (TN1). Our results demonstrate that these qualitative variations are attributable to a single amino acid substitution of Asp-462 in B5 by His in TN1, corresponding to the allelic difference in the OsUgp1 gene between B5 and TN1.