Synthesis of novel acceptor substrates for the dolichyl phosphate mannose synthase from yeast

DOLICHOL PHOSPHATE MANNOSE SYNTHASE1 mediates the biogenesis of isoprenyl-linked glycans and influences development, stress response, and ammonium hypersensitivity in Arabidopsis

The most abundant posttranslational modification in nature is the attachment of preassembled high-mannose-type glycans, which determines the fate and localization of the modified protein and modulates the biological functions of glycosylphosphatidylinositol-anchored and N-glycosylated proteins. In eukaryotes, all mannose residues attached to glycoproteins from the luminal side of the endoplasmic reticulum (ER) derive from the polyprenyl monosaccharide carrier, dolichol P-mannose (Dol-P-Man), which is flipped across the ER membrane to the lumen. We show that in plants, Dol-P-Man is synthesized when Dol-P-Man synthase1 (DPMS1), the catalytic core, interacts with two binding proteins, DPMS2 and DPMS3, that may serve as membrane anchors for DPMS1 or provide catalytic assistance. This configuration is reminiscent of that observed in mammals but is distinct from the single DPMS protein catalyzing Dol-P-Man biosynthesis in bakers' yeast and protozoan parasites. Overexpression of DPMS1 in Arabidopsis thaliana results in disorganized stem morphology and vascular bundle arrangements, wrinkled seed coat, and constitutive ER stress response. Loss-of-function mutations and RNA interference-mediated reduction of DPMS1 expression in Arabidopsis also caused a wrinkled seed coat phenotype and most remarkably enhanced hypersensitivity to ammonium that was manifested by extensive chlorosis and a strong reduction of root growth. Collectively, these data reveal a previously unsuspected role of the prenyl-linked carrier pathway for plant development and physiology that may help integrate several aspects of candidate susceptibility genes to ammonium stress.

Synthesis and modifications of carbohydrates, using biotransformations

Enzymes continue to be used as important catalysts, for the generation of rare and 'unnatural' monosaccharides and for the selective formation of glycosidic linkages. Multi-enzyme systems have been employed in one-pot strategies for multistep reaction sequences and for co-factor regeneration. The efficiency of glycosidases for glycosylation reactions has been dramatically increased by active-site mutagenesis to generate glycosynthases. First reports have detailed the expansion and optimization of glycosynthase substrate specificity by directed evolution. Novel glycosyltransferases are being identified from genomic databases and have been shown to glycosylate complex metabolites, such as glycopeptide antibiotics, with exquisite selectivity and in good yields. An emerging field is the application of glycosynthases and glycosyltransferases to reactions on solid support, generating potential applications in microarrays.