A conserved glycan signal is required to degrade abberent brassinosteroid receptor in Arabidopsis
Zhi Hong 1,*, Hiroyuki Kajiura2, Wei Su3, Hua Jin3, Kazuhito Fujiyama2, Jianming Li3
1College of Life Sciences, Nanjing University, Nanjing 210093.
2The International Center for Biotechnology, Osaka University, Japan.
3Department of Molecular, Cellular and Developmental Biology, University of Michigan, USA.
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Asparagine-linked glycans (N-glycans) are crucial signals for protein folding, quality control, and endoplasmic reticulum (ER)-associated degradation (ERAD) in yeast and mammals. 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 (BRI1). EBS3 encodes the Arabidopsis ortholog of the yeast asparagine-linked glycosylation 9 (ALG9). The ebs3-1 mutation prevents the three-branched N-glycan precursor 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 an α1,6 Man-exposed N-glycan signal is likely conserved to mark an ERAD client in Arabidopsis.
Key Words: N-glycans; ER retention; ER-associated degradation; BR receptor