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 several small GTPases, which regulate cytoskeletal dynamics and cell adhesion. Here, we characterize the GTPase Activating Protein (GAP) function of Plexin-B1 and find that a cooperative GAP activity towards the substrate GTPase, Rap1b, is associated with the N-terminal Juxtamembrane region of Plexin-B1. Importantly, we unveil an activation mechanism of Plexin-B1 by identifying a novel functional loop which partially blocks Rap1b entry into the plexin GAP domain. Consistent with the concept of allokairy developed for other systems, Plexin-B activity is increased by an apparent substrate-mediated cooperative effect. 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 biological implications, essentially those of a threshold behavior for cell migration, are discussed.

丛蛋白接收来自信号素配体的引导线索，并通过质膜传递它们的信号。这个蛋白质家族在单程跨膜受体中是独一无二的，因为它们的细胞内区域直接与几个调节细胞骨架动力学和细胞粘附的小 GTP 酶相互作用。在这里，我们描述了 Plexin-B1 的 GTP 酶激活蛋白 (GAP) 功能，并发现对底物 GTP 酶 Rap1b 的协同 GAP 活性与 Plexin-B1 的 N 端近膜区域相关。重要的是，我们通过识别部分阻止 Rap1b 进入 plexin GAP 域的新功能环来揭示 Plexin-B1 的激活机制。与为其他系统开发的 allokairy 概念一致，Plexin-B 活性通过明显的底物介导的协同效应增加。模拟和诱变表明，当活性位点被占据时，重新定位的 JM 构象由新的激活开关环稳定，从而产生更快的酶转换和协同行为。讨论了生物学意义，主要是细胞迁移的阈值行为。