Structure and function of the EDEM:PDI ERAD checkpoint complex

The ERAD glycoprotein misfolding checkpoint complex de-mannosylates misfolded glycoproteins targeting them to retrotranslocation, ubiquitination, and proteasomal degradation. The complex comprises an Endoplasmic Reticulum-Degradation Enhancing α-Mannosidase (EDEM) and a Protein Disulfide Isomerase (PDI). We solved Cryo-EM structures of the Chaetomium thermophilum (Ct) EDEM:PDI complex, both by itself and in complex with a classic ERAD substrate, α1-antitrypsin (A1AT-NHK). The EDEM catalytic domain nestles within the PDI arc, while A1AT-NHK binds EDEM’s C-terminal flexible domains. Mass spectrometry reveals a disulfide bond between A1AT-NHK and an exposed Cys in the protease-associated domain of EDEM. Non-reducing SDS-PAGE analysis of protein samples from mammalian cells co-transfected with EDEM:PDI and A1AT-NHK show a shift of the EDEM:PDI band to higher molecular weight. Redox chemistry between EDEM and PDI disulfide bonds generates oxidized, demannosylation-competent EDEM and reduced PDI and primes the PDI to function as the ERAD reductase, facilitating client retrotranslocation.