E3 ubiquitin-protein ligase involved in protein quality control (PQC) under proteotoxic stress. Is essential to plant survival under proteotoxic stress. Functions by removing damaged proteins before they form cytotoxic aggregates. Recognizes misfolded proteins selectively and tethers polyubiquitin chains to the proteins directly for subsequent degradation by the 26S proteasome pathway. Targets misfolded proteins independently of cytoplasmic chaperones. Associates with the 26S proteasome and sustains the structural integrity of the proteasome complex at the initial stage of proteotoxic stress. Under normal conditions, MPSR1 becomes highly unstable by its autoubiquitination activity and is stabilized during proteotoxic stress by conjugating ubiquitins on misfolded proteins. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Induced by arsenite and the proline toxic analog azetidine-2-carboxylate. Autoubiquitinated.