Plexin‑Bs enhance their GAP activity with a novel activation switch loop generating a cooperative enzyme.

Plexins receive guidance cues from semaphorin ligands and transmit their signal through the plasma membrane. This family of proteins is unique amongst single-pass transmembrane receptors as their intracellular regions interact directly with\r\nseveral small GTPases, which regulate cytoskeletal dynamics and cell adhesion. Here, we characterize the GTPase Activating Protein (GAP) function of Plexin-B1 and fnd that a cooperative GAP activity towards the substrate GTPase, Rap1b,\r\nis associated with the N-terminal Juxtamembrane region of Plexin-B1. Importantly, we unveil an activation mechanism of\r\nPlexin-B1 by identifying a novel functional loop which partially blocks Rap1b entry into the plexin GAP domain. Consistent\r\nwith the concept of allokairy developed for other systems, Plexin-B activity is increased by an apparent substrate-mediated\r\ncooperative efect. Simulations and mutagenesis suggest the repositioned JM conformation is stabilized by the new activation switch loop when the active site is occupied, giving rise to faster enzymatic turnover and cooperative behavior. The\r\nbiological implications, essentially those of a threshold behavior for cell migration, are discussed