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. Interacts (via MSP domain) with ZFYVE27. Interacts with KIF5A in a ZFYVE27-dependent manner. 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.