Component of the endoplasmic reticulum-associated degradation (ERAD) pathway. Specifically required for the ERAD-L pathway which mediates the degradation of proteins with misfolded lumenal domains within the endoplasmic reticulum (ER). Facilitates retrotranslocation of misfolded proteins from the ER lumen through the ER membrane in conjunction with HRD1 (PubMed:32327568). Both proteins have lateral gates facing each other and 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). 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, PubMed:32327568). Within the complex, interacts with USA1 (via C-terminus) (PubMed:20005842, 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). Up-regulated by the unfolded protein response (UPR). N-terminally acetylated by acetyltransferase NatB which enhances DER1 stability and is required for ERAD-L function. 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. Degradation of proteins with misfolded intramembrane domains is not affected. Present with 238 molecules/cell in log phase SD medium. Belongs to the derlin family.