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 t hat the AP-1 clathrin adaptor complex of the trans-Golgi network interacts with the polymeric immunoglobulin receptor in transfected Madin-Darby canin e kidney cells. Our results indicate that immunofluorescently labeled gamma -adaptin subunit of the adaptor complex and the polymeric immunoglobulin re ceptor partially co-localize in polarized and semi-polarized cells. gamma-A daptin is co-immunoisolated with membranes expressing the wild-type recepto r. The entire AP-1 adaptor complex could be chemically cross-linked to the receptor in filter-grown cells. gamma-Adaptin could be co-immunoprecipitate d with the wild-type receptor, with reduced efficiency with receptor mutant whose basolateral sorting motif has been deleted, and not with receptor la cking its cytoplasmic tail. Co-immunoprecipitation of gamma-adaptin was inh ibited by brefeldin A. Mutation of cytoplasmic serine 726 inhibited recepto r interactions with AP-1 but did not abrogate the fidelity of its basolater al targeting from the trans-Golgi network. However, the kinetics of recepto r delivery to the basolateral cell surface were slowed by the mutation. Alt hough surface delivery of the wild-type receptor was inhibited by brefeldin A, the delivery of the mutant receptor was insensitive to the drug. Our re sults are consistent with a working model in which phosphorylated cytoplasm ic serine modulates the recruitment of the polymeric immunoglobulin recepto r into AP-1/clathrin-coated areas in the trans-Golgi network. This process may regulate the efficiency of receptor targeting from the trans-Golgi netw ork.