Component of the endoplasmic reticulum quality control (ERQC) system also called ER-associated degradation (ERAD) involved in ubiquitin-dependent degradation of misfolded endoplasmic reticulum proteins (PubMed:16289116, PubMed:28827405). Could enhance presenilin-mediated amyloid-beta protein 40 generation. Binds to ubiquilins and this interaction is required for efficient degradation of CD3D via the ERAD pathway (PubMed:18307982). Interacts with PSEN1 and PSEN2 (PubMed:11799129). Interacts with UBXN6 (PubMed:18656546). Interacts with UBQLN1, UBQLN2 and UBQLN4 (PubMed:18307982). Component of the HRD1 complex, which comprises at least SYNV1/HRD1, FAM8A1, HERPUD1/HERP, OS9, SEL1L and UBE2J1. FAM8A1 binding to SYNV1 may promote recruitment of HERPUD1 to the HRD1 complex (PubMed:16289116, PubMed:28827405). Experimental confirmation may be lacking for some isoforms. Widely expressed; in the brain, expression seems to be restricted to neurons and vascular smooth muscle cells. Present in activated microglia in senile plaques in the brain of patients with Alzheimer disease. Up-regulated by endoplasmic reticulum stress and CREB3. Although the precise topology is not known, experimental data suggest that both the N- and C-termini face the cytosol.