The phospholipid PI(3,4)P is an apical identity determinant.

Apical-basal polarization is essential for epithelial tissue formation, segregating cortical domains to perform distinct physiological functions. Cortical lipid asymmetry has emerged as a determinant of cell polarization. We report a network of phosphatidylinositol phosphate (PIP)-modifying enzymes, some of which are transcriptionally induced upon embedding epithelial cells in extracellular matrix, and that are essential for apical-basal polarization. Unexpectedly, we find that PI(3,4)P localization and function is distinct from the basolateral determinant PI(3,4,5)P. PI(3,4)P localizes to the apical surface, and Rab11a-positive apical recycling endosomes. PI(3,4)P is produced by the 5-phosphatase SHIP1 and Class-II PI3-Kinases to recruit the endocytic regulatory protein SNX9 to basolateral domains that are being remodeled into apical surfaces. Perturbing PI(3,4)P levels results in defective polarization through subcortical retention of apically destined vesicles at apical membrane initiation sites. We conclude that PI(3,4)P is a determinant of apical membrane identity.