A deficiency screen of the <i>X</i> chromosome for Rap1 GTPase dominant interacting genes in <i>Drosophila</i> border cell migration

Collective cell migration is critical to embryonic development, wound healing, and the immune response, but also drives tumor dissemination. Understanding how cell collectives coordinate migration in vivo has been a challenge, with potential therapeutic benefits that range from addressing developmental defects to designing targeted cancer treatments. The small GTPase Rap1 has emerged as a regulator of both embryogenesis and cancer cell migration. How active Rap1 coordinates downstream signaling functions required for coordinated collective migration is poorly understood. Drosophila border cells undergo a stereotyped and genetically tractable in vivo migration within the developing egg chamber of the ovary. This group of 6-8 cells migrates through a densely packed tissue microenvironment and serves as an excellent model for collective cell migration during development and disease. Rap1, like all small GTPases, has distinct activity state switches that link extracellular signals to organized cell behaviors. Proper regulation of Rap1 activity is essential for successful border cell migration yet the signaling partners and other downstream effectors are poorly characterized. Using the known requirement for Rap1 in border cell migration, we conducted a dominant suppressor screen for genes whose heterozygous loss modifies the migration defects observed upon constitutively active Rap1V12 expression. Here, we identified 7 genomic regions on the X chromosome that interact with Rap1V12. We mapped three genomic regions to single Rap1-interacting genes, frizzled 4, Ubiquitin-specific protease 16/45, and strawberry notch. Thus, this unbiased screening approach identified multiple new candidate regulators of Rap1 activity with roles in collective border cell migration.