AP-3: an adaptor-like protein complex with ubiquitous expression

We have identified two closely related human proteins (σ 3A and σ 3B ) that are homologous to the small chains, σ 1 and σ 2 , of clathrin‐associated adaptor complexes. Northern and Western blot analyses demonstrate that the products of both the σ 3A and σ 3B genes are expressed in a wide variety of tissues and cell lines. σ 3A and σ 3B are components of a large complex, named AP‐3, that also contains proteins of apparent molecular masses of 47, 140 and 160 kDa. In non‐neuronal cells, the 47 kDa protein most likely corresponds to the medium chain homolog p47A, and the 140 kDa protein is a homolog of the neuron‐specific protein β‐NAP. Like other members of the medium‐chain family, the p47A chain is capable of interacting with the tyrosine‐based sorting signal YQRL from TGN38. Immunofluorescence microscopy analyses show that the σ 3 ‐containing complex is present both in the area of the TGN and in peripheral structures, some of which contain the transferrin receptor. These results suggest that the σ 3 chains are components of a novel, ubiquitous adaptor‐like complex involved in the recognition of tyrosine‐based sorting signals.