Participates in the endoplasmic reticulum unfolded protein response (UPR) by inducing ERN1/IRE1 activity. Involved in cellular calcium homeostasis regulation. Homodimer, and heterodimer with VAPA (PubMed:20940299). Interacts with VAMP1 and VAMP2 (PubMed:9920726, PubMed:20940299). Interacts (via MSP domain) with ZFYVE27 (PubMed:21976701, PubMed:19289470). Interacts with RMDN3 (PubMed:22131369). Interacts with KIF5A in a ZFYVE27-dependent manner (PubMed:21976701). Interacts with STARD3 (via FFAT motif) (PubMed:24105263). Interacts with STARD3NL (via FFAT motif) (PubMed:24105263). Interacts with CERT1 (PubMed:16895911). Interacts with PLEKHA3 and SACM1L to form a ternary complex (PubMed:30659099). Interacts with VPS13A (via FFAT motif) (PubMed:30741634). (Microbial infection) Interacts (via MSP domain) with hepatitis C virus (HCV) non-structural protein 5A (via disordered domain D3) (PubMed:16227268, PubMed:22720086). Interacts with HCV RNA-directed RNA polymerase (PubMed:16227268). Present in mitochondria-associated membranes that are endoplasmic reticulum membrane regions closely apposed to the outer mitochondrial membrane. Ubiquitous. Isoform 1 predominates. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the VAMP-associated protein (VAP) (TC 9.B.17) family. ALS genetic mutations db Leiden Open Variation Database (LOVD)