Scaffold protein of the endoplasmic reticulum-associated degradation (ERAD) (also known as endoplasmic reticulum quality control, ERQC) pathway involved in ubiquitin-dependent degradation of misfolded 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). Has multiple functions in ERAD including recruitment of DER1 to the HRD1 ubiquitin ligase, and regulation of HRD1 activity. Involved in oligomerization of HRD1 and in HRD1 autoubiquitination and degradation. Component of the HRD1 ubiquitin ligase complex which contains the E3 ligase HRD1, its cofactors HRD3, USA1 and DER1, substrate recruiting factor YOS9 and CDC48-binding protein UBX2 (PubMed:16873066). Within the complex, interacts directly with HRD1 (via N-terminus) and DER1 (via C-terminus) and indirectly with HRD3 (PubMed:20005842, PubMed:19898607, PubMed:21074049, PubMed:32327568). In ERAD-L, HRD3 and YOS9 jointly bind misfolded glycoproteins in the endoplasmic reticulum (ER) lumen (PubMed:32327568). Movement of ERAD-L substrates through the ER membrane is facilitated by HRD1 and DER1 which have lateral gates facing each other and which distort the membrane region between the lateral gates, making it much thinner than a normal phospholipid bilayer (PubMed:32327568). Substrates insert into the membrane as a hairpin loop with one strand interacting with DER1 and the other with HRD1 (PubMed:32327568). The HRD1 complex interacts with the heterotrimeric CDC48-NPL4-UFD1 ATPase complex which is recruited by UBX2 via its interaction with CDC48 and which moves ubiquitinated substrates to the cytosol for targeting to the proteasome (PubMed:16873066). The ubiquitin-like domain is required for HRD1 trans-ubiquitination and degradation. Reported to be involved in ERAD-M but not ERAD-L function (PubMed:20005842). However, a contradictory report states that it is not required for either ERAD-L or ERAD-M function (PubMed:19940128). Reported to be required for HRD1 oligomer formation (PubMed:19940128). However, a contradictory report does not observe any defects in oligomerization following deletion of the domain (PubMed:21074049). Impaired degradation of proteins with misfolded lumenal domains such as CPY*, a mutant, misfolded form of carboxypeptidase Y which is a known ERAD-L substrate. Impaired degradation of proteins with misfolded intramembrane domains. Impaired trans-ubiquitination and degradation of the HRD1 ligase. Degradation of proteins with misfolded cytosolic domains is not affected. Interaction of substrate with HRD1 is reduced; in USA1 and YOS9 double mutants this interaction is completely abolished. Present with 1890 molecules/cell in log phase SD medium.