Plexins form a conserved family of transmembrane receptors for semaphorins and may be the ancestors of semaphorins. Ligand binding activates signal transduction pathways controlling axon guidance in the nervous system and other developmental processes, including cell migration and morphogenesis, immune function, and tumor progression. Plexins are divided into four types (A-D) according to sequence similarity. In vertebrates, type A Plexins serve as the co-receptors for neuropilins to mediate the signaling of class 3 semaphorins except Sema3E, which signals through Plexin D1. Plexins serve as direct receptors for several other members of the semaphorin family: class 6 semaphorins signal through type A plexins and class 4 semaphorins through type B. Plexin C1 serves as the receptor of Sema7A and plays regulation roles in both immune and nervous systems. Plexins contain a C-terminal RasGAP domain, which functions as an enhancer of the hydrolysis of GTP that is bound to Ras-GTPases. Plexins display GAP activity towards the Ras homolog Rap. Other proteins having a RasGAP domain include p120GAP, IQGAP, Rab5-activating protein 6, and Neurofibromin. Although the Rho (Ras homolog) GTPases are most closely related to members of the Ras family, RhoGAP and RasGAP show no sequence homology at their amino acid level. RasGTPases function as molecular switches in a large number of of signaling pathways. When bound to GTP they are in the on state and when bound to GDP they are in the off state. The RasGAP domain speeds up the hydrolysis of GTP in Ras-like proteins acting as a negative regulator.
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