Adaptors are heterotetrameric complexes that mediate the incorporation of cargo into transport vesicles by interacting with sorting signals present in the cytosolic domain of transmembrane proteins. Four adaptors, AP-1 (beta 1, gamma, mu 1A or mu 1B, sigma 1), AP-2 (beta 2, alpha, mu 2, sigma 2), AP-3 (beta 3 , delta, mu 3, sigma 3) or AP-4 (beta 4, epsilon, mu 4, sigma 4), have been characterized. AP-1 and AP-3 mediate sorting events at the level of the TGN and/or endosomes, whereas AP-2 functions in endocytic clathrin coated vesicle formation; no function is known so far for AP-4. Here, we show that AP-4 can bind different types of cytosolic signals known to mediate basolateral transport in epithelial cells. Furthermore, in MDCK cells with depleted mu 4 protein levels, several basolateral proteins are mis-sorted to the apical surface, showing that AP-4 participates in basolateral sorting in epithelial cells.