Part of the AP-3 complex, an adaptor-related complex which is not clathrin-associated. The complex is associated with the Golgi region as well as more peripheral structures. It facilitates the budding of vesicles from the Golgi membrane and may be directly involved in trafficking to lysosomes (By similarity). Involved in process of CD8+ T-cell and NK cell degranulation (By similarity). In concert with the BLOC-1 complex, AP-3 is required to target cargos into vesicles assembled at cell bodies for delivery into neurites and nerve terminals (PubMed:21998198). Adaptor protein complex 3 (AP-3) is a heterotetramer composed of two large adaptins (delta-type subunit AP3D1 and beta-type subunit AP3B1 or AP3B2), a medium adaptin (mu-type subunit AP3M1 or AP3M2) and a small adaptin (sigma-type subunit APS1 or AP3S2) (By similarity). AP-3 associates with the BLOC-1 complex. Interacts with SLC30A2. Interacts with CLN3 (via dileucine motif); this interaction facilitates lysosomal targeting (By similarity). Belongs to the adaptor complexes large subunit family.