Integral endoplasmic reticulum membrane protein that coordinates the assembly of the ER-associated protein degradation (ERAD) machinery at the ER membrane. Mediates binding of CDC48 to the E3 ubiquitin ligases SSM4/DOA10 and HRD1, and to ERAD substrates. Component of 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. ERAD-C substrates are ubiquitinated through DOA10 in conjunction with the E2 ubiquitin-conjugating enzymes UBC6 and UBC7-CUE1. Also a 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 CDC48-NPL4-UFD1 ATPase complex and targeted to the proteasome. Component of the DOA10 ubiquitin ligase complex which contains E3 ligase SSM4/DOA10 and CDC48-binding protein UBX2/SEL1 (PubMed:16873066). Component of the HRD1 ubiquitin ligase complex which contains the E3 ligase HRD1, its cofactors HRD3, USA1 and DER1, substrate recruiting factor YOS9 and UBX2 (PubMed:16873066). 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). Both the DOA10 and HRD1 ubiquitin ligase complexes interact 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:15258615, PubMed:16179953, PubMed:16179952, PubMed:16873066). Present with 12600 molecules/cell in log phase SD medium.