Part of the AP-3 complex, an adaptor-related complex which is essential for the compartmentalization of the endocytic pathway. Adaptor protein complex 3 (AP-3) is a heterotetramer composed of two large adaptins (delta-type subunit and beta-type subunit), a medium adaptin (mu-type subunit) and a small adaptin (sigma-type subunit). Belongs to the adaptor complexes large subunit family.