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 (PubMed:18474358). 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 (PubMed:20190753). Seems to be involved in the sorting of specific cargos from the trans-Golgi network to alpha-granule-destined multivesicular bodies (MVBs) promoting MVBs maturation in megakaryocytes (By similarity). Interacts with RAB11A and VIPAS39. Associates with adaptor protein complex 3 (AP-3), clathrin:AP-3 and clathrin:HGS complexes (PubMed:21411634). (Microbial infection) Interacts with M.tuberculosis PtpA. Colocalizes in clusters with VIPAS39 at cytoplasmic organelles (PubMed:19109425). Colocalizes with RAB11A and VIPAS39 on recycling endosomes (PubMed:22753090). Colocalizes with AP-3, clathrin, Rab5 and Rab7b (PubMed:21411634). Colocalizes with M.tuberculosis PtpA in the cytosol of tuberculosis-infected macrophages and associates with phagosomes (PubMed:18474358). Ubiquitous; highly expressed in testis and low expression in the lung. Phosphorylated on tyrosine residues. (Microbial infection) Dephosphorylated by M.tuberculosis PtpA, which induces the reduction of host phagolysosome fusion in M.tuberculosis-infected macrophages. The disease is caused by variants affecting the gene represented in this entry. Belongs to the STXBP/unc-18/SEC1 family. According to PubMed:18474358, it is autophosphorylated. However, it is not related with protein kinases, suggesting it is phosphorylated by another protein.