The endoplasmic reticulum-associated degradation(ERAD)is a highly conserved mechanism to remove misfolded membrane/secretory proteins from the endoplasmic reticulum(ER).While many of the individual components of the ERAD machinery are well characterized in yeast and mammals,our knowledge of a plant ERAD process is rather limited.Here,we report a functional study of an Arabidopsis homolog(AtOS9)of an ER luminal lectin Yos9(OS-9 in mammals)that recognizes a unique asparagine-linked glycan on misfolded proteins.We discovered that AtOS9 is an ER-localized glycoprotein that is co-expressed with many known/predicted ER chaperones.AT-DNA insertional atos9-t mutation blocks the degradation of a structurally imperfect yet biochemically competent brassinosteroid(BR)receptor bri1-9,causing its increased accumulation in the ER and its consequent leakage to the cell surface responsible for restoring the BR sensitivity and suppressing the dwarfism of the bri1-9 mutant.In addition,we identified a missense mutation in AtOS9 in a recently discovered ERAD mutant ems-mutagenized bri1 suppressor 6(ebs6-1).Moreover,we showed that atos9-t also inhibits the ERAD of bri1-5,another ER-retained BR receptor,and a misfolded EFR,a BRI1-like receptor for the bacterial translation elongation factor EF-Tu.Furthermore,we found that AtOS9 interacted biochemically and genetically with EBS5,an Arabidopsis homolog of the yeast Hrd3/mammalian Sel1L known to collaborate with Yos9/OS-9 to select ERAD clients.Taken together,our results demonstrated a functional role of AtOS9 in a plant ERAD process that degrades misfolded receptor-like kinases.