Component of the endoplasmic reticulum quality control (ERQC) system involved in ubiquitin-dependent degradation of missfolded endoplasmic reticulum proteins. Component 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). ERAD-L substrates are ubiquitinated through HRD1 in conjunction with the E2 ubiquitin-conjugating enzymes UBC1 and UBC7-CUE1. Ubiquitinated substrates are then removed to the cytosol via the action of the UFD1-NPL4-CDC48/p97 (UNC) AAA ATPase complex and targeted to the proteasome. ERAD-M substrates are processed by the same HRD1-HRD3 core complex, but only a subset of the other components is required for ERAD-M. Stabilizes the HRD1 ubiquitin-protein ligase. Has also a function in recruiting misfolded protein substrates (By similarity). Interacts with HRD1. Belongs to the sel-1 family.