AP-1B facilitates endocytosis during cell migration

The epithelial cell-specific clathrin adaptor AP-1B has a well-established role in polarized sorting of cargos to the basolateral membrane. Here we demonstrate a novel function for AP-1B during collective cell migration of epithelial sheets. We show that AP-1B colocalized with {beta}1 integrin in focal adhesions during cell migration using confocal microscopy and total internal reflection fluorescence (TIRF) microscopy on fixed specimens. Further, AP-1B labeling in cell protrusion was distinct from labeling for the canonical endocytic adaptor complex AP-2. Using stochastic optical reconstruction microscopy (STORM) and live TIRF imaging we identified numerous AP-1B-coated structures at or close to the plasma membrane in cell protrusions. Importantly, immuno-electron microscopy (EM) showed AP-1B in clathrin-coated pits and budding vesicles at the plasma membrane during cell migration. Our data therefore established a novel function for AP-1B in endocytosis. We further show that {beta}1 integrin was dependent on AP-1B and its co-adaptor, autosomal recessive hypercholesterolemia protein (ARH), for sorting to the basolateral membrane. Notably, we found that expression of AP-1B (and ARH) slowed epithelial-cell migration, and qRT-PCR analysis of human epithelial-derived cell lines revealed a loss of AP-1B expression in highly metastatic cancer cells indicating that AP-1B-facilitated endocytosis during cell migration might be an anti-cancer mechanism.