Multiple Mechanism–Mediated Retention of a Defective Brassinosteroid Receptor in the Endoplasmic Reticulum of <i>Arabidopsis</i>

Endoplasmic reticulum-mediated quality control (ERQC) is a well-studied process in yeast and mammals that retains and disposes misfolded/unassembled polypeptides. By contrast, how plants exert quality control over their secretory proteins is less clear. Here, we report that a mutated brassinosteroid receptor, bri1-5, that carries a Cys69Tyr mutation, is retained in the ER by an overvigilant ERQC system involving three different retention mechanisms. We demonstrate that bri1-5 interacts with two ER chaperones, calnexin and binding protein (BiP), and is degraded by a proteasome-independent endoplasmic reticulum-associated degradation (ERAD). Mutations in components of the calnexin/calreticulin cycle had little effect on the fidelity of the Arabidopsis thaliana ERQC for bri1-5 retention. By contrast, overexpression of bri1-5, treatment with an ERAD inhibitor, RNA interference-mediated BiP silencing, or simultaneous mutations of Cys-69 and its partner Cys-62 can mitigate this quality control, resulting in significant suppression of the bri1-5 phenotype. Thus, bri1-5 is an excellent model protein to investigate plant ERQC/ERAD in a model organism.