Evolutionarily conserved glycan signal to degrade aberrant brassinosteroid receptors in Arabidopsis.
Asparagine-linked glycans (N-glycans) are crucial signals for protein folding, quality control, and endoplasmic reticulum (ER)-associated degradation (ERAD) in yeast and mammals. Although similar ERAD processes were reported in plants, little is known about their biochemical mechanisms, especially their relationships with N-glycans. Here, we show that a missense mutation in the Arabidopsis EMS-mutagenized bri1 suppressor 3 (EBS3) gene suppresses a dwarf mutant, bri1-9, the phenotypes of which are caused by ER retention and ERAD of a brassinosteroid receptor, BRASSINOSTEROID-INSENSITIVE 1 (BR1). EBS3 encodes the Arabidopsis ortholog of the yeast asparagine-linked glycosylation 9 (ALG9), which catalyzes the ER luminal addition of two terminal α1,2 mannose (Man) residues in assembling the three-branched N-glycan precursor [glucose(Glc)](3)(Man)(9)[N-acetylglucosamine(GlcNAc)](2). Consistent with recent discoveries revealing the importance of the Glc(3)Man(9)GlcNAc(2) C-branch in generating an ERAD signal, the ebs3-1 mutation prevents the Glc(3)Man(9)GlcNAc(2) assembly and inhibits the ERAD of bri1-9. By contrast, overexpression of EBS4 in ebs3-1 bri1-9, which encodes the Arabidopsis ortholog of the yeast ALG12 catalyzing the ER luminal α1,6 Man addition, adds an α1,6 Man to the truncated N-glycan precursor accumulated in ebs3-1 bri1-9, promotes the bri1-9 ERAD, and neutralizes the ebs3-1 suppressor phenotype. Furthermore, a transfer (T)-DNA insertional alg3-T2 mutation, which causes accumulation of an even smaller N-glycan precursor carrying a different exposed α1,6 Man, promotes the ERAD of bri1-9 and enhances its dwarfism. Taken together, our results strongly suggest that the glycan signal to mark an ERAD client in Arabidopsis is likely conserved to be an α1,6 Man-exposed N-glycan.
Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI 48109, USA. zhihong@nju.edu.cn
Zhi
Z
Hong
Hiroyuki
H
Kajiura
Wei
W
Su
Hua
H
Jin
Akihisa
A
Kimura
Kazuhito
K
Fujiyama
Jianming
J
Li
D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism / Q000502:physiology*; D060406:Brassinosteroids / Q000378:metabolism*; D003001:Cloning, Molecular; D004721:Endoplasmic Reticulum / Q000378:metabolism; D019143:Evolution, Molecular; D005816:Genetic Complementation Test; D005964:Glucosyltransferases / Q000235:genetics* / Q000502:physiology*; D006031:Glycosylation; D009154:Mutation; D020125:Mutation, Missense; D010641:Phenotype; D010940:Plant Proteins / Q000378:metabolism; D030821:Plants, Genetically Modified; D011134:Polysaccharides / Q000737:chemistry / Q000378:metabolism; D011494:Protein Kinases / Q000235:genetics* / Q000502:physiology*
D029681:Arabidopsis Proteins; D060406:Brassinosteroids; D010940:Plant Proteins; D011134:Polysaccharides; D005964:Glucosyltransferases; C072423:mannosylglycoprotein 1,3-glucosyltransferase; D011494:Protein Kinases; C108270:BRI1 protein, Arabidopsis