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University of Cambridge, Department of Clinical Biochemistry, Cambridge CB2 2QR, United Kingdom
We have recently shown that two proteins
related to two of the adaptor subunits of clathrincoated vesicles, p47 (µ3) and 
-NAP (3B), 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 sequenced a ubiquitously expressed homologue of
-NAP, 3A, as well as homologues of the 
/
and 
adaptor subunits, 
and 3, which are also ubiquitously
expressed. Antibodies raised against recombinant and
3 show that they are the other two subunits of the
adaptor-like complex. We are calling this complex
AP-3, a name that has also been used for the neuronalspecific phosphoprotein AP180, but we feel that it is a
more appropriate designation for an adaptor-related
heterotetramer. Immunofluorescence using anti- antibodies reveals that the AP-3 complex is associated with
the Golgi region of the cell as well as with more peripheral structures. These peripheral structures show only
limited colocalization with endosomal markers and
may correspond to a postTGN biosynthetic compartment. The subunit is closely related to the protein
product of the Drosophila garnet gene, which when mutated results in reduced pigmentation of the eyes and
other tissues. Because pigment granules are believed to
be similar to lysosomes, this suggests either that the
AP-3 complex may be directly involved in trafficking to
lysosomes or alternatively that it may be involved in another pathway, but that missorting in that pathway may
indirectly lead to defects in pigment granules.
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