Evolutionary conserved glycan signal to degrade aberrant brassinosteroid receptors in Arabidopsis

摘要

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 bril suppressor 3 (EBS3) gene suppresses a dwarf mutant, brii-9, the phenotypes of which are caused by ER retention and ERAD of a brassinosteroid receptor, BRASSINO-STEROID-INSENSITIVE 1 (BR1). EBS3 encodes the Arabidopsis ortho-log 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_3Man_9GlcNAc_2 C-branch in generating an ERAD signal, the ebs3-1 mutation prevents the Glc_3Man_9GlcNAc_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 a1,6 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 phe-notype. 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.