Bulk delivery of a preassembled apical surface initiates epithelial lumen formation

During de novo lumenogenesis, epithelial cells establish luminal identity by directing apical cargo to an apical membrane initiation site (AMIS). Although this process has been widely studied, the mechanisms governing AMIS formation and its progression into a luminal precursor remain poorly understood. Here we combine quantitative light and electron microscopy with proximity proteomics to demonstrate that apical cargo is delivered to the AMIS in large apical precursor organelles, termed vacuolar apical compartments (VACs). VACs possess a microvilli-rich cortex and undergo exocytic fusion at the AMIS to generate a nascent lumen. Lumen initiation is tightly coordinated with the assembly and rearrangement of apical cell junctions and requires the Crumbs complex protein PatJ. Together, our results show that PatJ is a key structural determinant of the apical-lateral interface and indicate that VACs act as specialized transport organelles that deliver a preassembled apical surface to the AMIS, enabling rapid and efficient lumen initiation.