Subunit of non-clathrin- and clathrin-associated adaptor protein complex 3 (AP-3) that plays a role in protein sorting in the late-Golgi/trans-Golgi network (TGN) and/or endosomes. The AP complexes mediate both the recruitment of clathrin to membranes and the recognition of sorting signals within the cytosolic tails of transmembrane cargo molecules. AP-3 appears to be involved in the sorting of a subset of transmembrane proteins targeted to lysosomes and lysosome-related organelles. 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. AP-3 associates with the BLOC-1 complex (By similarity). 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). Component of the coat surrounding the cytoplasmic face of coated vesicles located at the Golgi complex. Isoform 1 expression is specific to nervous system. Expressed in nerve terminal and cell body, and is associated with nerve-terminal vesicles. Expression seen in Purkinje cells, cortical neurons, neuroectodermal tumors and graded in cerebral cortex (deeper layers exhibit stronger expression) (PubMed:1851215). Isoform 2 is expressed at high levels in brain and testis (PubMed:17453999). Expressed from early in development through adulthood. The disease may be caused by variants affecting the gene represented in this entry. Belongs to the adaptor complexes large subunit family.