Fat2 polarizes Lar and Sema5c to coordinate the motility of collectively migrating epithelial cells.

Migrating epithelial cells globally align their migration machinery to achieve tissue-level movement. Biochemical signaling across leading-trailing cell-cell interfaces can promote this alignment by partitioning migratory behaviors like protrusion and retraction to opposite sides of the interface. However, how signaling proteins become organized at interfaces to accomplish this is poorly understood. The follicular epithelial cells of Drosophila melanogaster have two signaling modules at their leading-trailing interfaces-one composed of the atypical cadherin Fat2 and the receptor tyrosine phosphatase Lar, and one composed of Semaphorin5c and its receptor Plexin A. Here we show that these modules form one interface signaling system with Fat2 at its core. Trailing edge-enriched Fat2 concentrates both Lar and Semaphorin5c at cells' leading edges, but Lar and Semaphorin5c play little role in Fat2's localization. Fat2 is also more stable at interfaces than Lar and Semaphorin5c. Once localized, Lar and Semaphorin5c act in parallel to promote collective migration. We propose that Fat2 serves as the organizer this interface signaling system by coupling and polarizing the distributions of multiple effectors that work together to align the migration machinery of neighboring cells.