Part of the AP-3 complex, an adapter-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). May be a coat protein involved in the formation of specialized structures like pigment granules. Adaptor protein complex 3 (AP-3) is a heterotetramer composed of two large chains (delta and beta3), a medium chain (mu3) and a small chain (sigma3). Component of the coat surrounding the cytoplasmic face of coated vesicles located at the Golgi complex. Belongs to the adaptor complexes large subunit family. Intron retention. This sequence is incomplete at 5'- and 3'-ends and extensively differs from that shown at positions 1-269, 546 and 840-1034. Intron retention. This sequence is incomplete at 5'- and 3'-ends and extensively differs from that shown at positions 1-269, 546 and 840-1034.