Rap1 Negatively Regulates the Hippo Pathway to Polarize Directional Protrusions in Collective Cell Migration

In collective cell migration, directional protrusions orient cells in response to external cues, which requires coordinated polarity among the migrating cohort. However, the molecular mechanism has not been well defined. Drosophila border cells (BCs) migrate collectively and invade via the confined space between nurse cells, offering an in vivo model to examine how group polarity is organized. Here, we show that the front/back polarity of BCs requires Rap1, hyperactivation of which disrupts cluster polarity and induces misoriented protrusions and loss of asymmetry in the actin network. Conversely, hypoactive Rap1 causes fewer protrusions and cluster spinning during migration. A forward genetic screen revealed that downregulation of the Hippo (Hpo) pathway core components hpo or mats enhances the Rap1 ^V12 -induced migration defect and misdirected protrusions. Mechanistically, association of Rap1 ^V12 with the kinase domain of Hpo suppresses its activity, which releases Hpo signaling-mediated suppression of F-actin elongation, promoting cellular protrusions in collective cell migration. In collective cell movement, coordinated polarity among the migrating cohort is required for directional protrusions in response to external cues. Chang et al. show that such group polarity requires Rap1 activity, which promotes cellular protrusions and gives cells an advantage in competing to lead the cluster.