May play a role in vesicle-mediated protein trafficking to lysosomal compartments and in membrane docking/fusion reactions of late endosomes/lysosomes. Mediates phagolysosomal fusion in macrophages. Proposed to be involved in endosomal maturation implicating vipas39. In epithelial cells, the vps33b:vipas39 complex may play a role in the apical recycling pathway and in the maintenance of the apical-basolateral polarity (By similarity). Plays a role in bile duct development. Interacts with vipas39. Cytoplasmic, peripheral membrane protein associated with late endosomes/lysosomes. Widely expressed from 4 hours post-fertilization (hpf) to 24 hpf. At 48 hpf, localized to brain, retina, ear, liver and proximal intestine. This expression pattern is more pronounced at 72 hpf and persists through 5 days post-fertilization (dpf). At 3 dpf and 4 dpf, expression in the liver is predominantly in developing biliary epithelial cells. No expression detected in kidney or spinal cord. Belongs to the STXBP/unc-18/SEC1 family.