Part of the AP-3 complex, an adaptor-related complex which seems to be 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 the vacuole. It also function in maintaining the identity of lytic vacuoles and in regulating the transition between storage and lytic vacuoles. 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). Component of the coat surrounding the cytoplasmic face of coated vesicles located at the Golgi complex. A number of isoforms are produced. According to EST sequences. No obvious phenotype except defective lytic vacuoles with altered morphology and accumulation of proteins. Belongs to the adaptor complexes large subunit family.