Biochemical and mutational analysis of intracellular regions of the Plexin‐B1 guidance receptor as a R‐RasGAP

Abstract

Plexins are single transmembrane receptors for semaphorin guidance cues, which regulate the cell motility through changes in the cells actin cytoskeleton. Since the actin cytoskeleton is regulated by small GTPases of the Rho family, interactions with small GTPases are of particular importance in the signaling mechanism of plexins. We mapped the interactions of small Rho GTPases, Rnd1 and active Rac1, with the plexin‐B1 RBD (Rho GTPase Binding Domain) to a common region of the structure [1]. In our recent study [2], the crystal structure of the intracellular region of human plexin‐B1 shows high homology to p120Ras GTPase‐activating proteins (GAPs). Down‐regulation of Ras GTPases appears to be a key mechanism for the plexin‐mediated regulation of cell motility. Molecular modeling and ITC measurements demonstrated that Ras GTPases can bind to the plexin GAP region. Furthermore, we showed that the intracellular plexin‐B1 binds R‐Ras but not H‐Ras. Recently we have for the first time been able to measure the GAP activity of plexin‐B1 with R‐Ras substrate using a biochemical assay. This assay now allows us to elucidate the role of the interactions between the intracellular regions and Rho GTPases for plexin function.