The Adaptor Protein Complex 1 limits E-cadherin endocytosis during epithelial morphogenesis

Intracellular trafficking regulates the distribution of transmembrane proteins including the key determinants of epithelial polarity and adhesion. The Adaptor Protein 1 (AP-1) complex is the key regulator of vesicle sorting, which binds a large number of specific cargos. We examined roles of the AP-1 complex in epithelial morphogenesis, using the Drosophila wing as a paradigm. We found that AP-1 knockdown leads to ectopic folds caused by trafficking defects of integrins. This occurs concurrently with an increase in the apical cell area and induction of cell death due to defects in E-cadherin trafficking. We discovered a distinct pool of AP-1 localizes at the apical Adherens Junctions, where it limits internalization of E-cadherin from the cell surface. Upon AP-1 knockdown, the accompanying hyperinternalization of E-cadherin induces cell death by an uncharacterised mechanism with a potential tumour-suppressive role. Simultaneously, cells increase expression of E-cadherin in a compensatory mechanism to maintain cell-cell adhesion.