Distribution and function of AP-1 clathrin adaptor complexes in polarized epithelial cells

Expression of the epithelial cell-specific heterotetrameric adaptor complex AP-1B is required for the polarized distribution of man!: membrane protein s to the basolateral surface of LLC-PK1 kidney cells. AP-1B is distinguishe d from the ubiquitously expressed AP-1A by exchange of its single 50-kD mu subunit, mu 1A, being replaced by the closely related mu 1B. Here we show t hat this substitution is sufficient to couple basolateral plasma membrane p roteins, such as a low-density lipoprotein receptor (LDLR), to the AP-1B co mplex and to clathrin. The interaction between LDLR and AP-1B is likely to occur in the trans-Golgi network (TGN), as was suggested by the localizatio n of functional, epitope-tagged mu1 by immunofluorescence and immunoelectro n microscopy. Tagged AP-1A and AP 1B complexes were found in the perinuclea r region close to the Golgi complex and recycling endosomes, often in clath rin-coated buds and vesicles. Yet, AP-1A and AP-1B localized to different s ubdomains of the TGN, with only AP-1A colocalizing with furin, a membrane p rotein that uses AP-1 to recycle between the TGN and endosomes. We conclude that AP-1B functions by interacting with its cargo molecules and clathrin in the TGN, where it acts to sort basolateral proteins from proteins destin ed for the apical surface and from those selected by AP-1A for transport to endosomes and lysosomes.