Planar polarized Rab35 functions as an oscillatory ratchet during cell intercalation in the Drosophila epithelium

The coordination between membrane trafficking and actomyosin networks is essential to the regulation of cell and tissue shape. Here, we examine Rab protein distributions during Drosophila epithelial tissue remodeling and show that Rab35 is dynamically planar polarized. Rab35 compartments are enriched at contractile interfaces of intercalating cells and provide the first evidence of interfacial monopolarity. When Rab35 function is disrupted, apical area oscillations still occur and contractile steps are observed. However, contractions are followed by reversals and interfaces fail to shorten, demonstrating that Rab35 functions as a ratchet ensuring unidirectional movement. Although actomyosin forces have been thought to drive interface contraction, initiation of Rab35 compartments does not require Myosin II function. However, Rab35 compartments do not terminate and continue to grow into large elongated structures following actomyosin disruption. Finally, Rab35 represents a common contractile cell-shaping mechanism, as mesoderm invagination fails in Rab35 compromised embryos and Rab35 localizes to constricting surfaces.Various stages of tissue morphogenesis involve the contraction of epithelial surfaces. Here, the authors identify the Rab GTPase Rab35 as an essential component of this contractile process, which functions as a membrane ratchet to ensure unidirectional movement of intercalating cells.