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SummaryClathrin and the epithelial-specific clathrin adaptor AP-1B mediate basolateral trafficking in epithelia. However, several epithelia lack AP-1B, and mice knocked out for AP-1B are viable, suggesting the existence of additional mechanisms that control basolateral polarity. Here, we demonstrate a distinct role of the ubiquitous clathrin adaptor AP-1A in basolateral protein sorting. Knockdown of AP-1A causes missorting of basolateral proteins in MDCK cells, but only after knockdown of AP-1B, suggesting that AP-1B can compensate for lack of AP-1A. AP-1A localizes predominantly to the TGN, and its knockdown promotes spillover of basolateral proteins into common recycling endosomes, the site of function of AP-1B, suggesting complementary roles of both adaptors in basolateral sorting. Yeast two-hybrid assays detect interactions between the basolateral signal of transferrin receptor and the medium subunits of both AP-1A and AP-1B. The basolateral sorting function of AP-1A reported here establishes AP-1 as a major regulator of epithelial polarity.