Structural Biology of Sensory Rhodopsin

描述（由申请人提供）：感觉受体视网膜结合蛋白是光信号转导中的主要介质。最具特征的跨膜感觉受体是视紫红质，G蛋白偶联受体超家族的一个范例，和嗜盐古细菌的感觉视紫红质II。视紫红质利用光来触发级联反应中的初始步骤，该级联反应在视觉中达到高潮，而感觉视紫红质II利用光来启动磷酸化级联反应，该磷酸化级联反应调节细胞的鞭毛马达以控制趋光性。我们最近阐明了SRII的晶体结构，分辨率为2.1 A，使我们现在能够阐明光信号转导的机制。此外，在并行计算工作中，我们确定了调节颜色调谐的物理决定因素。然而，SRII在基态的这种结构仅代表沿着复杂的反应途径的单个步骤，该反应途径从光激发开始，随后是受体的构象变化，以及光信号向同源蛋白的传输。显然，为了充分理解SRII介导的感觉信号传导机制，我们需要深入了解该信号传导途径中沿着的中间体的原子结构，以及SRII-HtrII复合物的结构。该项目解决了一个基本问题：视网膜光异构化如何触发蛋白质结构变化，导致换能器激活，从而启动感觉反应？阐明这一问题对于阐明感觉受体介导的信号转导的分子机制至关重要。该项目的目的是通过捕获和解决1）K-和2）M-中间体的结构，鉴定SRII中最终形成受体信号状态的结构运动，3）通过解决基态SRII-HtrII复合物的X射线结构，阐明SRII识别和结合其同源转导蛋白的机制，和4）通过解析其X-射线结构来鉴定感觉视紫红质I的结构决定因素，所述感觉视紫红质I可以充当光吸引剂和光排斥剂。实现这些特定的目标应该使我们最终能够阐明，在高分辨率的感觉视紫红质介导的信号转导的分子机制。这些预期类似于视紫红质，因此与健康和疾病中视觉机制的分子理解相关。

DESCRIPTION (provided by applicant): Sensory receptor retinal-binding proteins are major mediators in light signal transduction. The best characterized transmembrane sensory receptors are rhodopsin, a paradigm of the G protein-coupled receptor superfamily, and sensory rhodopsin II from halophilic archeabacteria. Rhodopsin uses light to trigger the initial steps in a cascade that culminates in vision, while sensory rhodopsin II uses light to initiate a phosphorylation cascade that regulates the cell's flagellar motors in order to control phototaxis. We very recently elucidated the crystal structure of SRII to 2.1 A resolution, allowing us now to elucidate the mechanisms of light signal transduction. Moreover, in a parallel computational work, we identified the physical determinants that regulate color tuning. However, this structure of SRII in the ground state represents only a single step along a complex reaction pathway, which starts with photoexcitation, followed by conformational changes of the receptor, and transmission of the light signal to cognate proteins. Clearly, to fully understand the mechanism of sensory signaling mediated by SRII we require insight into the atomic structures of the intermediates along this signaling pathway, as well as the structure of the SRII-HtrII complex. This project addresses a fundamental question: How does retinal photoisomerization trigger protein structural changes that lead to activation of the transducer, thereby initiating the sensory response?Answering this question is essential for elucidating the molecular mechanisms of signal transduction mediated by sensory receptors. The aims of this project are to identify the structural movements in SRII that culminate in the formation of the signaling state of the receptor, by trapping and solving the structures of the 1) K- and 2) M-intermediates, 3) elucidate the mechanism by which SRII recognizes and binds its cognate transducer protein, by solving the X-ray structure of the ground-state SRII-HtrII complex, and 4) identify the structural determinants of sensory rhodopsin I, which can act as a photoattractant and photorepellent, by solving its X-ray structure. Achieving these Specific Aims should allow us finally to elucidate, at high resolution the molecular mechanisms of sensory rhodopsin-mediated signaling. These are expected to be similar to that of visual rhodopsin, and so relevant to a molecular understanding of the mechanism of vision in health and disease.