Rnd1 is constitutively active and stably associates with Plexin-B1 and regulates the R-Ras GAP activity of the C1 and C2 domains of the Plexin-B1 cytoplasmic tail. These domains interact with each other and in this closed conformation cannot associate with active R-Ras-GTP. Rnd1 binds to the region between C1 and C2 domains and disrupts this interaction, allowing the receptor to associate with GTP-bound R-Ras.