Adaptor protein (AP) complexes select cargo for inclusion into coated vesicles in the late secretory and endocytic pathways. AP complexes are heterotetramers. AP-3 is associated with endosomes and, to a less extent, the trans-Golgi network and appears to function independently of clathrin. Drosophila AP-3 genes were first linked to defects in the biogenesis of visual pigment granules. (Adapted from FBrf0129983 and PMID:15066634).