Catalyzes the covalent attachment of ubiquitin to other proteins. Functions in degradation of misfolded or regulated proteins localized in the endoplasmic reticulum (ER) lumen or membrane via the ubiquitin-proteasome system. Cognate E2 conjugating enzyme for the DOA10 ubiquitin ligase complex, which is part of the ERAD-C pathway responsible for the rapid degradation of membrane proteins with misfolded cytoplasmic domains, and of the HRD1 ubiquitin ligase complex, which is part of the ERAD-L and ERAD-M pathways responsible for the rapid degradation of soluble lumenal and membrane proteins with misfolded lumenal domains (ERAD-L), or ER-membrane proteins with misfolded transmembrane domains (ERAD-M). Involved in resistance to cadmium poisoning. S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine + [E2 ubiquitin-conjugating enzyme]-L-cysteine = [E1 ubiquitin-activating enzyme]-L-cysteine + S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Forms a heterodimer with CUE1. Interacts with SSM4/DOA10 and HDR1. Interacts with UFD4. Anchored via the membrane protein CUE1 to the endoplasmic reticulum membrane. By cadmium. Autoubiquitinated at Cys-89; undergoes 'Lys-48'-linked polyubiquitination, which leads to proteasome-dependent protein degradation. Degradation is autoregulated when its levels exceed that of its binding partner CUE1. Present with 2100 molecules/cell in log phase SD medium. Belongs to the ubiquitin-conjugating enzyme family.