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.
Corresponding Author：Tel: (+8625)84530931; E-mail: zhihong@nju.edu.cn

Abstract
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