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Structural characterization of the molecular platform for type III secretion system assembly

机译：III型分泌系统组装分子平台的结构表征

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Type III secretion systems ( TTSSs) are multi-protein macromolecular 'machines' that have a central function in the virulence of many Gram-negative pathogens by directly mediating the secretion and translocation of bacterial proteins ( termed effectors) into the cytoplasm of eukaryotic cells(1). Most of the 20 unique structural components constituting this secretion apparatus are highly conserved among animal and plant pathogens and are also evolutionarily related to proteins in the flagellar-specific export system. Recent electron microscopy experiments have revealed the gross 'needle-shaped' morphology of the TTSS2-4, yet a detailed understanding of the structural characteristics and organization of these protein components within the bacterial membranes is lacking. Here we report the 1.8-angstrom crystal structure of EscJ from enteropathogenic Escherichia coli (EPEC), a member of the YscJ/ PrgK family whose oligomerization represents one of the earliest events in TTSS assembly(5). Crystal packing analysis and molecular modelling indicate that EscJ could form a large 24-subunit 'ring' superstructure with extensive grooves, ridges and electrostatic features. Electron microscopy, labelling and mass spectrometry studies on the orthologous Salmonella typhimurium PrgK within the context of the assembled TTSS support the stoichiometry, membrane association and surface accessibility of the modelled ring. We propose that the YscJ/ PrgK protein family functions as an essential molecular platform for TTSS assembly.
机译：III型分泌系统（TTSS）是多蛋白大分子“机器”，通过直接介导细菌蛋白（称为效应子）的分泌和转运进入真核细胞的细胞质，在许多革兰氏阴性病原体的毒力中具有核心功能。 1）。构成该分泌装置的20种独特结构中的大多数在动植物病原体中高度保守，并且在鞭毛特异性输出系统中也与蛋白质进化相关。最近的电子显微镜实验已经揭示了TTSS2-4的总体“针状”形态，但仍缺乏对细菌膜内这些蛋白质成分的结构特征和组织的详细了解。在这里，我们报道了来自肠道致病性大肠杆菌（EPEC）的EscJ的1.8埃晶体结构，这是YscJ / PrgK家族的成员，其低聚代表了TTSS组装中最早的事件之一（5）。晶体堆积分析和分子建模表明，EscJ可以形成具有宽大的沟槽，凸脊和静电特征的大型24个亚基“环”上层结构。在组装的TTSS范围内对直系同源鼠伤寒沙门氏菌PrgK的电子显微镜，标记和质谱研究支持了建模环的化学计量，膜缔合和表面可及性。我们建议YscJ / PrgK蛋白家族起着TTSS组装必不可少的分子平台的作用。

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