Interactions of the AP-1 Golgi adaptor with the polymeric immunoglobulin receptor and their possible role in mediating brefeldin A-sensitive basolateral targeting from the trans-Golgi network

We provide morphological, biochemical, and functional evidence suggesting that the AP-1 clathrin adaptor complex of the trans-Golgi network interacts with the polymeric immunoglobulin receptor in transfected Madin- Darby canine kidney cells. Our results indicate that immunofluorescently labeled γ-adaptin subunit of the adaptor complex and the polymeric immunoglobulin receptor partially co-localize in polarized and semi-polarized cells. γ-Adaptin is co-immunoisolated with membranes expressing the wild- type receptor. The entire AP-1 adaptor complex could be chemically cross- linked to the receptor in filter-grown cells. γ-Adaptin could be co- immunoprecipitated with the wild-type receptor, with reduced efficiency with receptor mutant whose basolateral sorting motif has been deleted, and not with receptor lacking its cytoplasmic tail. Co-immunoprecipitation of γ- adaptin was inhibited by brefeldin A. Mutation of cytoplasmic serine 726 inhibited receptor interactions with AP-1 but did not abrogate the fidelity of its basolateral targeting from the trans-Golgi network. However, the kinetics of receptor delivery to the basolateral cell surface were slowed by the mutation. Although surface delivery of the wild-type receptor was inhibited by brefeldin A, the delivery of the mutant receptor was insensitive to the drug. Our results are consistent with a working model in which phosphorylated cytoplasmic serine modulates the recruitment of the polymeric immunoglobulin receptor into AP-1/clathrin-coated areas in the trans-Golgi network. This process may regulate the efficiency of receptor targeting from the trans-Golgi network.