Molecular structure of the bacterial chemotaxis apparatus

细菌趋化性是研究最多的信号转导途径之一，几十年来对信号转导的结构、生化、遗传和生理成分的分析有助于对整个信号转导过程的广泛理解。在过去的一年里，我们在完整细菌细胞的结构成像和预测计算模型的开发方面都取得了重大进展，这些模型可以捕获细胞对其环境变化的反应，从而为现代细胞生物学的核心目标做出贡献。细菌通过将配体与趋化性受体家族结合来感知其局部化学环境中的许多变化，这反过来又触发了最终调节鞭毛马达旋转的信号通路的激活。一个详细的了解的空间和时间架构的细菌的趋化性是一个问题的根本利益，因为它将提供一个框架，整合广泛的遗传，生理，生化和结构分析这个过程。随着先进的成像方法的出现，允许特定的蛋白质复合物在细胞内的空间定位，前景的发展在分子水平上的完整的细菌细胞的结构的理解是潜在的范围内。趋化性是一种特别容易处理的信号传导途径，因为涉及的组分数量少，并且知道信号转导级联的前端的空间定位。在过去的几年里，我们已经采取了系统的步骤，桥梁的差距差距从结构到生理学，因为它涉及到推导一个定量模型的趋化性，考虑到空间和分子组织的蛋白质组分的信号通路。我们首先确定了将受体定位在细菌细胞中的可行性，然后确定了当受体蛋白仍在完整细菌中时获得其3D结构的可能性。 这反过来又导致了在膜平面上发现受体的部分有序六边形排列。 我们现在已经扩展了这些基础，将受体结构、空间分布和生长介质丰富度的信息转化为趋化性信号的可测试、预测计算模型。过去一年的主要进展包括：（i）在多种革兰氏阴性细菌中发现了化学感受器阵列的部分有序排列;（ii）在分离的受体组件中发现了二聚体的三聚体结构的明确证据;（iii）将冷冻电子断层扫描研究扩展到类核，以建立类核结构与受体结构之间的联系;（iv）开发计算模型，其正确地预测受体排列和密度随培养基中营养条件变化的变化，以及（v）向将EM/西姆斯组合成像应用于完整细菌细胞的进展。

Bacterial chemotaxis is among the best-studied signal transduction pathways, where decades of analysis of structural, biochemical, genetic and physiological components of signaling have contributed to a broad understanding of the overall signaling process. We have taken significant steps over the last year both in structural imaging of intact bacterial cells and in the development of predictive computational models that capture cellular responses to changes in their environment, thus contributing to a central goal of modern cellular biology. Bacteria sense many of the changes in their local chemical environment by the binding of ligands to a family of chemotaxis receptors, which in turn trigger the activation of a signaling pathway that ultimately regulates the rotation of the flagellar motor. A detailed understanding of the spatial and temporal architecture of the bacterial apparatus for chemotaxis is a problem of fundamental interest because it will provide a framework to integrate the extensive genetic, physiological, biochemical and structural analyses of this process. With the advent of advanced imaging methods that allow spatial localization of specific protein complexes within the cell, the prospect of developing an integrated structural understanding of whole bacterial cells at the molecular level is potentially within range. Chemotaxis is a particularly tractable signaling pathway given the small number of components involved and the knowledge of the spatial localization of the front end of the signal transduction cascade. Over the last few years, we have taken systematic steps to bridge the gap from structure to physiology as it relates to deriving a quantitative model for chemotaxis that takes into account the spatial and molecular organization of the protein components in the signaling pathway. We began with first establishing the feasibility of localizing the receptors in the bacterial cell, and moved on to establishing the possibility of obtaining 3D structures of receptor proteins when they are still in an intact bacterium. This in turn, led to the discovery of the partially ordered hexagonal arrangement of receptors in the plane of the membrane. We have now extended these foundations to translating the information on the structure of the receptors, their spatial distribution, and richness of the growth medium into a testable, predictive computational model for chemotaxis signaling. Highlights of progress over the last year include (i) discovery of the partially ordered arrangement of chemoreceptor arrays in multiple gram-negative bacteria; (ii) definitive evidence of the trimer-of-dimer organization in isolated receptor assemblies; (iii) extension of cryo-electron tomographic studies to the nucleoid to establish the connection between nucleoid structure and receptor organization; (iv) development of computational models that correctly predict the changes in receptor arrangement and density with changing nutrient conditions in the medium and (v) progress towards applying combined EM/SIMS imaging to intact bacterial cells.