A novel clathrin adaptor complex mediates basolateral targeting in polarized epithelial cells

Although polarized epithelial cells are well known to maintain distinct apical and basolateral plasma membrane domains, the mechanisms responsible for targeting membrane proteins to the apical or basolateral surfaces have remained elusive. We have identified a novel form of the AP-1 clathrin adaptor complex that contains as one of its subunits mu 1B, an epithelial cell-specific homolog of the ubiquitously expressed mu 1A. LLC-PK1 kidney epithelial cells do not express mu 1B and missort many basolateral proteins to the apical surface. Stable expression of mu 1B selectively restored basolateral targeting, improved the overall organization of LLC-PK1 monolayers, and had no effect on apical targeting. We conclude that basolateral sorting is mediated by an epithelial cell-specific version of the AP-1 complex containing mu 1B.