Structural modeling of the flagellum MS ring protein FliF reveals similarities to the type III secretion system and sporulation complex.
Structural modeling of the flagellum MS ring protein FliF reveals similarities to the type III secretion system and sporulation complex.
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作者：
Bergeron JR
鞭毛是在许多细菌的表面发现的大的蛋白质细胞器，其主要作用是通过长的细胞外丝的旋转允许运动。它是许多致病物种中的一种重要毒力因子，也是形成耐药性生物膜的启动组分。鞭毛由胞质侧的输出器、跨越细菌膜和周质的基体和转子以及从细菌表面突出的钩丝组成。由多个拷贝的蛋白质FliF组成的基体MS环区的形成是鞭毛组装的初始步骤之一。然而，人们对FliF的精确架构知之甚少。在这里，我报告的FliF序列的生物信息学分析，从各种细菌物种，表明其周质区是由三个球状域。前两个是同源的III型分泌系统注射体蛋白SctJ，和第三个具有类似的倍的孢子形成复合物组分SpoIIIAG。我还描述了衣原体拥有一个不寻常的FliF蛋白，缺乏部分的SctJ同源结构域和SpoIIIAG样结构域，并融合到转子组件FliG在其C-末端。最后，我结合了FliF的序列分析与MS环的EM图，提出了FliF寡聚体的第一个原子模型，表明FliF在结构上类似于两个注射体组分SctJ和SctD的融合。这些结果进一步定义了鞭毛，注射体和孢子形成复合体之间的关系，并将有助于未来的鞭毛基体的结构表征。
The flagellum is a large proteinaceous organelle found at the surface of many bacteria, whose primary role is to allow motility through the rotation of a long extracellular filament. It is an essential virulence factor in many pathogenic species, and is also a priming component in the formation of antibiotic-resistant biofilms. The flagellum consists of the export apparatus on the cytosolic side; the basal body and rotor, spanning the bacterial membrane(s) and periplasm; and the hook-filament, that protrudes away from the bacterial surface. Formation of the basal body MS ring region, constituted of multiple copies of the protein FliF, is one of the initial steps of flagellum assembly. However, the precise architecture of FliF is poorly understood. Here, I report a bioinformatics analysis of the FliF sequence from various bacterial species, suggesting that its periplasmic region is composed of three globular domains. The first two are homologous to that of the type III secretion system injectisome proteins SctJ, and the third possesses a similar fold to that of the sporulation complex component SpoIIIAG. I also describe that Chlamydia possesses an unusual FliF protein, lacking part of the SctJ homology domain and the SpoIIIAG-like domain, and fused to the rotor component FliG at its C-terminus. Finally, I have combined the sequence analysis of FliF with the EM map of the MS ring, to propose the first atomic model for the FliF oligomer, suggesting that FliF is structurally akin to a fusion of the two injectisome components SctJ and SctD. These results further define the relationship between the flagellum, injectisome and sporulation complex, and will facilitate future structural characterization of the flagellum basal body.