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. The gene for this protein is duplicated in strains AX3 and AX4. These strains contain a duplication of a segment of 750 kb of chromosome 2 compared to the corresponding sequence in strain AX2.