Involved in the targeting and/or fusion of transport vesicles to their target membrane. Interacts with VAPA and VAPB. Isoforms 2 and 3 are found in perinuclear as well as peripheral punctate structures in osteoblasts. Isoforms 2 and 3 are found in perinuclear as well as peripheral punctate structures in osteoblasts. Expressed in brain and spleen (at protein level). Isoform 1 expressed at very high level in brain. Even higher level found in spinal cord. Isoform 3 expressed in kidney, spleen and liver. Isoforms 2 and 3 expressed in osteoblasts of trabecular bone. Also expressed in heart. (Microbial infection) Targeted and hydrolyzed by C.botulinum neurotoxin type D (BoNT/D, botD) which hydrolyzes the 61-Lys-|-Leu-62 bond and inhibits neurotransmitter release (PubMed:8175689). This is a poor substrate for BoNT/D, high concentrations are required to cleave it in vitro (PubMed:8175689). (Microbial infection) Targeted and hydrolyzed by C.botulinum neurotoxin type F (BoNT/F, botF) which hydrolyzes the 60-Gln-|-Lys-61 bond and inhibits neurotransmitter release (PubMed:8505288). Is not targeted by C.botulinum neurotoxin type B (BoNT/B) or by C.tetani toxin (tetX) as their target sequence is not conserved in this protein (PubMed:1331807, PubMed:8175689). Belongs to the synaptobrevin family.