Signaling and Regulation Mechanisms of Plexin

描述（由申请人提供）：丛蛋白是脑信号蛋白的细胞表面受体。丛状蛋白介导的脑信号蛋白信号传导对于诸如神经系统和心血管系统的发育以及免疫应答和骨稳态的调节等过程是必不可少的。丛蛋白的功能障碍与神经系统疾病和癌症有关。了解丛蛋白的功能将为开发针对相关疾病的靶向治疗方法和改善损伤后的神经元再生铺平道路。丛蛋白的胞内区域含有功能所必需的GTP酶激活蛋白（GAP）结构域。在前一时期，我们已经确定了小GTAPRAP作为真实的基板丛蛋白GAP结构域，并确定了差距域是如何激活的semaphorin诱导的二聚化和它如何通过非经典的催化机制灭活RAP的结构基础。目标.研究丛蛋白的其他层次的调控机制，以及丛蛋白与其几个关键结合伙伴之间的相互调控。研究设计。基于我们的新晶体结构，我们将首先分析抑制性二聚体在丛蛋白调节中的作用，这是该领域的一个长期存在的问题。丛状蛋白信号传导不仅需要其RapGAP活性，还需要其在质膜上组装和控制多蛋白信号传导复合物的活性的能力。许多蛋白质与丛蛋白相互作用，但其作用的结构基础在很大程度上是未知的。我们将集中在一些重要的结合伙伴，解决的问题，他们如何结合plexin和发挥相互调节与plexins。在目标1中，我们将测试丛蛋白调节中的抑制性二聚体模型。我们已经确定了两种晶体结构的丛蛋白A4，它采用了一种新的构象，并形成一个紧凑的二聚体与差距活性位点埋在二聚体界面。我们认为，这种二聚体和之前报道的丛蛋白细胞外区的载脂蛋白二聚体结构共同介导了细胞表面全长丛蛋白的自抑制二聚体状态。将进行基于结构的突变分析以检验该假设。在目标2中，我们将研究RND 1/Rac和RhoD对丛蛋白信号传导的相反作用的基础。RhoGTP酶Rac 1和RND 1与丛蛋白相互作用，并促进其结合和激活的信号蛋白。相比之下，RhoD抑制丛蛋白信号传导，尽管它以相同的模式以相似的亲和力结合丛蛋白。我们的结构分析导致了一个假设，解释了这一悖论，这将在这一目标进行测试。在目的3中，我们将分析丛蛋白与FARPs的相互作用和调节。FARP 1和FARP 2是两个相关的鸟嘌呤核苷酸交换因子（GEF），它们已被证明与丛蛋白直接相互作用，并对其信号传导做出重要贡献。我们将追求的丛蛋白/FARP复合物的结构，以阐明其相互作用的基础，并分析这种相互作用如何有助于释放FARPs的自抑制。

DESCRIPTION (provided by applicant): Plexins are the cell surface receptors of semaphorins. Plexin-mediated semaphorin signaling is essential for processes such as the development of the nervous system and the cardiovascular system and regulation of immune responses and bone homeostasis. Malfunction of plexins has been associated with neurological disorder and cancer. Understanding how plexins function will pave the way for developing targeted therapeutics for fighting the associated diseases and improving neuronal regeneration after injury. The plexin intracellular region contains a GTPase Activating Protein (GAP) domain that is essential for function. In the previous period, we have identified the small GTPase Rap as the authentic substrate for the plexin GAP domain, and have determined the structural basis for how the GAP domain is activated by semaphorin-induced dimerization and how it inactivates Rap through a non-canonical catalytic mechanism. Objectives. To study additional layers of regulation mechanisms of plexins, and mutual regulation between plexins and several of their key binding partners. Research Design. Based on a new crystal structure of ours, we will first analyze the role of the inhibitory dimer in plexin regulation, a long-standing question in the fiel. Plexin signaling requires not only its RapGAP activity, but also its ability to assemble and contro the activity of a multi-protein signaling complex at the plasma membrane. Many proteins interact with plexins, but the structural basis of their actions is largely unknown. We will focus on some of the essential binding partners, address the questions how they bind plexin and exert mutual regulation with plexins. In Aim 1 we will test an inhibitory dimer model in plexin regulation. We have determined two crystal structures of PlexinA4, which adopts a new conformation and forms a compact dimer with the GAP active site buried in the dimer interface. We propose that this dimer and the apo dimer structure of the plexin extracellular region reported previously together mediate the autoinhibited dimeric state of full-length plexin on the cell surface. Structure-based mutational analyses will be performed to test this hypothesis. In Aim 2 we will study the basis for the opposite effects of RND1/Rac and RhoD on plexin signaling. The RhoGTPases Rac1 and RND1 interact with plexin and facilitate its binding and activation by semaphorin. In contrast, RhoD inhibits plexin signaling, although it binds plexin in the same mode with similar affinity. Our structure analyses led to a hypothesis that explains this paradox, which will be tested in this aim. In Aim 3 we will analyze the interaction and regulation of FARPs by plexin. FARP1 and FARP2 are two related guanine nucleotide exchange factors (GEFs) that have been shown to interact directly with plexin and make essential contributing to its signaling. We will pursue a structure of the plexin/FARP complex to elucidate the basis for their interaction, and analyze how this interaction helps release the autoinhibition of FARPs.