Heat shock-induced zinc efflux repurposes Arabidopsis E3 ligase EMR as a molecular chaperone.

EMR, an endoplasmic reticulum (ER)-associated degradation-mediating RING-finger E3 ligase initially identified as an ER-resident protein, was unexpectedly detected in the cytoplasm under ER stress conditions. This unanticipated subcellular distribution prompted us to explore the uncharacterized functions of cytoplasmic EMR. Using an integrated approach that combines bioinformatics, biochemical, and physiological analyses, we show that EMR also acts as a specific heat-responsive regulator. Heat exposure causes Zn2+ to dissociate from the RING-finger domain of EMR, triggering a conformational change from monomeric to oligomeric form driven by increased hydrophobicity. This structural reorganization, mediated by Zn2+ coordination, transforms EMR's function from an E3 ligase to a molecular chaperone. Importantly, this functional switch contributes significantly to plant thermotolerance, as EMR's chaperone activity effectively prevents heat-induced protein unfolding and aggregation of vital cellular proteins. Consequently, transgenic Arabidopsis lines overexpressing EMR in an EMR-null-mutant (emr) background, 35S:EMR, showed significantly improved thermotolerance compared to wild-type and emr plants. The critical role of EMR's chaperone activity in conferring heat-stress resistance was further substantiated by the observation that 35S:EMR(C/S) plants, in which EMR(C/S) primarily retains chaperone function, exhibited heat-stress tolerance comparable to that of 35S:EMR plants. Proteomic profiling after heat shock identified candidate client substrates of EMR's chaperone activity, which are mainly involved in critical cellular processes such as protein translation and energy metabolism, highlighting EMR's broad cytoprotective role. Taken together, these results demonstrate that the bifunctional properties of EMR are essential for environmental-stress recognition and adaptive signaling, thereby driving thermotolerance in plants.