CHARACTERIZATION OF THE ADAPTER-RELATED PROTEIN COMPLEX, AP-3

We have recently shown that two proteins related to two of the adaptor subunits of clathrin-coated vesicles, p47 (mu 3) and beta-NAP (beta 3 B), are part of an adaptor-like complex not associated with clathrin ( Simpson, F., N.A. Bright, M.A. West, L.S. Newman, R.B. Darnell, and M. S. Robinson, 1996. J. Cell Biol. 133:749-760). In the present study we have searched the EST database and have identified, cloned, and seque nced a ubiquitously expressed homologue of beta-NAP, beta 3A, as well as homologues of the alpha/gamma and sigma adaptor subunits, delta and sigma 3, which are also ubiquitously expressed. Antibodies raised aga inst recombinant delta and sigma 3 show that they are the other two su bunits of the adaptor-like complex. We are calling this complex AP-3, a name that has also been used for the neuronal-specific phosphoprotei n AP180, but we feel that it is a more appropriate designation for an adaptor-related heterotetramer. Immunofluorescence using anti-delta an tibodies reveals that the AP-3 complex is associated with the Golgi re gion of the cell as well as with more peripheral structures, These per ipheral structures show only limited colocalization with endosomal mar kers and may correspond to a postTGN biosynthetic compartment. The del ta subunit is closely related to the protein product of the Drosophila garnet gene, which when mutated results in reduced pigmentation of th e eyes and other tissues. Because pigment granules are believed to be similar to lysosomes, this suggests either that the AP-3 complex may b e directly involved in trafficking to lysosomes or alternatively that it may be involved in another pathway, but that missorting in that pat hway may indirectly lead to defects in pigment granules.