Shown are the semaphorin homologous extracellular region of plexin-B1 (dark blue), R-Ras GAP homologous regions (orange), Rac1/Rnd1 binding domains (violet), Rac1/Rnd1/RhoD GTPase (red). A dimeric extracellular region is altered by binding of the dimeric ligand Semaphorin4D (gray), and the interaction of the RBD with active Rho GTPases facilitates conformational change on the cytoplasmic side. Inside and outside signals cooperate in a destabilization of the inactive, dimeric form of the receptor.