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Identification of the Gene Encoding the 1,3-Mannosyltransferase (ALG3) in Arabidopsis and Characterization of Downstream N-Glycan Processing^[W]

^a Laboratory of Plant Physiology, Wageningen University, 6703 BD Wageningen, The Netherlands
^b Lehrstuhl für Zellbiologie und Pflanzenphysiologie, Universität Regensburg, 93053 Regensburg, Germany
^c Business Unit Bioscience, Plant Research International, Wageningen University and Research Centre, 6708 PB Wageningen, The Netherlands
^d Membrane Enzymology, Department of Chemistry, Utrecht University, 3584 CH Utrecht, The Netherlands

² Address correspondence to dirk.bosch{at}wur.nl.

Glycosyltransferases are involved in the biosynthesis of lipid-linked N-glycans. Here, we identify and characterize a mannosyltransferase gene from Arabidopsis thaliana, which is the functional homolog of the ALG3 (Dol-P-Man:Man₅GlcNAc₂-PP-Dol 1,3-mannosyl transferase) gene in yeast. The At ALG3 protein can complement a alg3 yeast mutant and is localized to the endoplasmic reticulum in yeast and in plants. A homozygous T-DNA insertion mutant, alg3-2, was identified in Arabidopsis with residual levels of wild-type ALG3, derived from incidental splicing of the 11th intron carrying the T-DNAs. N-glycan analysis of alg3-2 and alg3-2 in the complex-glycan-less mutant background, which lacks N-acetylglucosaminyl-transferase I activity, reveals that when ALG3 activity is strongly reduced, almost all N-glycans transferred to proteins are aberrant, indicating that the Arabidopsis oligosaccharide transferase complex is remarkably substrate tolerant. In alg3-2 plants, the aberrant glycans on glycoproteins are recognized by endogenous mannosidase I and N-acetylglucosaminyltransferase I and efficiently processed into complex-type glycans. Although no high-mannose-type glycoproteins are detected in alg3-2 plants, these plants do not show a growth phenotype under normal growth conditions. However, the glycosylation abnormalities result in activation of marker genes diagnostic of the unfolded protein response.

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