title: Mutations of an alpha1,6 mannosyltransferase inhibit endoplasmic reticulum-associated degradation of defective brassinosteroid receptors in Arabidopsis.
abstract: Asn-linked glycans, or the glycan code, carry crucial information for protein folding, transport, sorting, and degradation. The biochemical pathway for generating such a code is highly conserved in eukaryotic organisms and consists of ordered assembly of a lipid-linked tetradeccasaccharide. Most of our current knowledge on glycan biosynthesis was obtained from studies of yeast asparagine-linked glycosylation (alg) mutants. By contrast, little is known about biosynthesis and biological functions of N-glycans in plants. Here, we show that loss-of-function mutations in the Arabidopsis thaliana homolog of the yeast ALG12 result in transfer of incompletely assembled glycans to polypeptides. This metabolic defect significantly compromises the endoplasmic reticulum-associated degradation of bri1-9 and bri1-5, two defective transmembrane receptors for brassinosteroids. Consequently, overaccumulated bri1-9 or bri1-5 proteins saturate the quality control systems that retain the two mutated receptors in the endoplasmic reticulum and can thus leak out of the folding compartment, resulting in phenotypic suppression of the two bri1 mutants. Our results strongly suggest that the complete assembly of the lipid-linked glycans is essential for successful quality control of defective glycoproteins in Arabidopsis.
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, University of Michigan, An Arbor, Michigan 48109-1048, USA.
authors.forename: Zhi
authors.initials: Z
authors.lastname: Hong
authors.forename: Hua
authors.initials: H
authors.lastname: Jin
authors.forename: Anne-Catherine
authors.initials: AC
authors.lastname: Fitchette
authors.forename: Yang
authors.initials: Y
authors.lastname: Xia
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Monk
authors.forename: Loïc
authors.initials: L
authors.lastname: Faye
authors.forename: Jianming
authors.initials: J
authors.lastname: Li
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D018076:DNA, Complementary / Q000235:genetics; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant; D005816:Genetic Complementation Test; D006023:Glycoproteins / Q000096:biosynthesis*; D006031:Glycosylation; D008364:Mannosyltransferases / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D011987:Receptors, Steroid / Q000096:biosynthesis*; D016415:Sequence Alignment
chemical_list: D029681:Arabidopsis Proteins; D018076:DNA, Complementary; D006023:Glycoproteins; D011987:Receptors, Steroid; D008364:Mannosyltransferases; C058478:alpha 1,6-mannosyltransferase