ER-resident protein 46 (ERp46) triggers the mannose-trimming activity of ER degradation– enhancing -mannosidase–like protein 3 (EDEM3)

Abstract

Protein folding in the cell is regulated by several quality-control mechanisms. Correct folding of glycoproteins in the endoplasmic reticulum (ER) is tightly monitored by the recognition of glycan signals by lectins in the ER-associated degradation (ERAD) pathway. In mammals, mannose trimming from N-glycans is crucial for disposal of misfolded glycoproteins. The mannosidases responsible for this process are ER mannosidase I and ER degradation– enhancing -mannosidase–like proteins (EDEMs). However, the molecular mechanism of mannose removal by EDEMs remains unclear, partly owing to the difficulty of reconstituting mannosidase activity in vitro. Here, our analysis of EDEM3-mediated mannose-trimming activity on a misfolded glycoprotein revealed that ERp46, an ER-resident oxidoreductase, associates stably with EDEM3. This interaction, which depended on the redox activity of ERp46, involved formation of a disulfide bond between the cysteine residues of the ERp46 redox-active sites and the EDEM3 -mannosidase domain. In a defined in vitro system consisting of recombinant proteins purified from HEK293 cells, the mannose-trimming activity of EDEM3 toward the model misfolded substrate, the glycoprotein T-cell receptor locus (TCR), was reconstituted only when ERp46 had established a covalent interaction with EDEM3. On the basis of these findings, we propose that disposal of misfolded glycoproteins through mannose trimming is tightly connected to redox-mediated regulation in the ER.