Component of the adaptor complexes which link clathrin to receptors in coated vesicles. Clathrin-associated protein complexes are believed to interact with the cytoplasmic tails of membrane proteins, leading to their selection and concentration. Ap47 is a subunit of the plasma membrane adaptor. 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 (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). AP-3 associates with the BLOC-1 complex (By similarity). Component of the coat surrounding the cytoplasmic face of coated vesicles located at the Golgi complex. Belongs to the adaptor complexes medium subunit family.