Genetic Study of Endoplasmic Reticulum-mediated Protein Quality Control in Arabidopsis.

Abstract

Endoplasmic reticulum (ER) quality control (ERQC) is an essential cellular process that permits trafficking of only correctly folded proteins to their final destinations, allows additional folding cycles of incompletely-folded proteins, repairs misfolded ones, and removes terminally-misfolded ones via ER-associate degradation (ERAD). Most of our current knowledge on ERQC and ERAD came from genetic and biochemical investigations in yeast and mammalian cells. Recent studies in the reference plant Arabidopsis thaliana uncovered homologous components and similar mechanisms in plants for monitoring protein folding and for retaining, repairing, and removing misfolded proteins. Among these, one important chaperone-like lectin Calreticulin (CRT), which participates in the glycoprotein folding cycle regulation, has three homologs in Arabidopsis named AtCRT1, AtCRT2 and AtCRT3. Recent studies revealed a crucial role for the CRT3 but not the other CRTs in ER retention of a mutant brassinosteroid receptor, brassinosteroid-insensitive 1-9 (bri1-9) and in complete folding of a plant immunity receptor EF-Tu Receptor (EFR). In this study, the specificity of CRT3 over CRT1/2 in retaining misfolded proteins inside ER was investigated and discussed. I identified a CRT3-specific C-terminal positively charged tetrapeptide that is indispensable for its chaperone function. In addition to the study on this conserved ERQC member, this thesis also reported a novel and plant-specific ERAD component that participates in the degradation of bri1-9 and bri1-5, two ER-retained mutant variants of the cell surface receptor for brassinosteroids (BRs). Loss-of-function ebs7 mutations prevent ERAD of bri1-9 and bri1-5, causing their accumulation in the ER and consequential leakage to the plasma membrane, which is responsible for restoration of BR sensitivity and phenotypic suppression of the bri1-9 and bri1-5 mutants, respectively. EBS7 localizes on ER membrane and it accumulates under ER stress. Mechanistically, EBS7 interacts with the ER membrane-anchored ubiquitin ligase AtHrd1, one of the central components of the Arabidopsis ERAD machinery, and an ebs7 mutation destabilizes AtHrd1 to reduce polyubiquitination of bri1-9. Taken together, this thesis project not only investigated the function of conserved ERQC members through the study in Arabidopsis, but also shed light on mechanisms of plant-specific regulation of ERAD.