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. Interacts with RMDN3, VAMP1 and VAMP2 (By similarity). Interacts (via MSP domain) with ZFYVE27 (PubMed:24251978). Interacts with KIF5A in a ZFYVE27-dependent manner (By similarity). Interacts with STARD3 (via FFAT motif) (By similarity). Interacts with STARD3NL (via FFAT motif) (By similarity). Interacts with CERT1 (By similarity). Interacts with PLEKHA3 and SACM1L to form a ternary complex (By similarity). Interacts with VPS13A (via FFAT motif) (By similarity). Belongs to the VAMP-associated protein (VAP) (TC 9.B.17) family.