Department for Structural Infection Biology, Center for Structural Systems Biology (CSSB) & Helmholtz Centre for Infection Research (HZI), Hamburg, Germany.
Dynamics of Viral Structures, Leibniz Institute for Virology (LIV), Hamburg, Germany.
Protein Sci. 2023 Apr;32(4):e4595. doi: 10.1002/pro.4595.
The type III secretion system (T3SS) is a large, transmembrane protein machinery used by various pathogenic gram-negative bacteria to transport virulence factors into the host cell during infection. Understanding the structure of T3SSs is crucial for future developments of therapeutics that could target this system. However, much of the knowledge about the structure of T3SS is available only for Salmonella, and it is unclear how this large assembly is conserved across species. Here, we combined cryo-electron microscopy, cross-linking mass spectrometry, and integrative modeling to determine the structure of the T3SS needle complex from Shigella flexneri. We show that the Shigella T3SS exhibits unique features distinguishing it from other structurally characterized T3SSs. The secretin pore complex adopts a new fold of its C-terminal S domain and the pilotin MxiM[SctG] locates around the outer surface of the pore. The export apparatus structure exhibits a conserved pseudohelical arrangement but includes the N-terminal domain of the SpaS[SctU] subunit, which was not present in any of the previously published virulence-related T3SS structures. Similar to other T3SSs, however, the apparatus is anchored within the needle complex by a network of flexible linkers that either adjust conformation to connect to equivalent patches on the secretin oligomer or bind distinct surface patches at the same height of the export apparatus. The conserved and unique features delineated by our analysis highlight the necessity to analyze T3SS in a species-specific manner, in order to fully understand the underlying molecular mechanisms of these systems. The structure of the type III secretion system from Shigella flexneri delineates conserved and unique features, which could be used for the development of broad-range therapeutics.
III 型分泌系统（T3SS）是一种大型跨膜蛋白机器，各种致病性革兰氏阴性细菌在感染过程中利用该系统将毒力因子输送到宿主细胞内。了解 T3SS 的结构对于开发针对该系统的治疗方法至关重要。然而，关于 T3SS 结构的大部分知识仅可从沙门氏菌中获得，并且尚不清楚这个大型组件在物种间是如何保守的。在这里，我们结合冷冻电子显微镜、交联质谱和综合建模，确定了来自福氏志贺菌的 T3SS 针复合物的结构。我们表明，福氏志贺菌 T3SS 具有独特的特征，将其与其他结构特征明确的 T3SS 区分开来。分泌孔复合物采用其 C 末端 S 结构域的新折叠，而 Pilin MxiM[SctG] 则位于孔的外表面周围。出口装置结构表现出保守的拟螺旋排列，但包括 SpaS[SctU] 亚基的 N 末端结构域，该结构域不存在于任何先前发表的与毒力相关的 T3SS 结构中。然而，与其他 T3SS 一样，该装置通过一系列柔性接头固定在针复合物内，这些接头可以调整构象以连接到分泌孔寡聚体上的等效斑块，或者结合出口装置同一高度上的不同表面斑块。我们的分析所描绘的保守和独特特征突出了以特定物种分析 T3SS 的必要性，以便充分理解这些系统的潜在分子机制。福氏志贺菌 III 型分泌系统的结构描绘了保守和独特的特征，这可为广谱治疗方法的开发提供依据。
