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 in part VIPAS39 (By similarity). In epithelial cells, the VPS33B:VIPAS39 complex may play a role in the apical RAB11A-dependentrecycling 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 (PubMed:25947942). Interacts with RAB11A and VIPAS39 (PubMed:20190753). Associates with adaptor protein complex 3 (AP-3), clathrin:AP-3 and clathrin:HGS complexes (PubMed:21411634). Colocalizes in clusters with VIPAS39 at cytoplasmic organelles. Colocalizes with RAB11A and VIPAS39 on recycling endosomes. Colocalizes with AP-3, clathrin, Rab5 and Rab7b. Phosphorylated on tyrosine residues. Increased platelet account and deficiency in platelet alpha-granules associated with bleeding diathesis and decreases aggregate formation; reduction of mature ype-II multivesicular bodies (MVB II) in megakaryocytes. Vps33b-deficient inner medullary collecting duct cells display abnormal expression of membrane proteins such as Ceacam5, structural and functional tight junction defects and reduced E-cadherin expression. Belongs to the STXBP/unc-18/SEC1 family.