Graduate School of Frontier Biosciences, Osaka University, Suita, Osaka, Japan.
Department of Ophthalmology and Visual Sciences, Kyoto University Graduate School of Medicine, Kyoto, Japan.
Commun Biol. 2021 May 31;4(1):646. doi: 10.1038/s42003-021-02177-z.
The flagellar protein export apparatus switches substrate specificity from hook-type to filament-type upon hook assembly completion, thereby initiating filament assembly at the hook tip. The C-terminal cytoplasmic domain of FlhA (FlhA) serves as a docking platform for flagellar chaperones in complex with their cognate filament-type substrates. Interactions of the flexible linker of FlhA (FlhA) with its nearest FlhA subunit in the FlhA ring is required for the substrate specificity switching. To address how FlhA brings the order to flagellar assembly, we analyzed the flhA(E351A/W354A/D356A) ΔflgM mutant and found that this triple mutation in FlhA increased the secretion level of hook protein by 5-fold, thereby increasing hook length. The crystal structure of FlhA(E351A/D356A) showed that FlhA bound to the chaperone-binding site of its neighboring subunit. We propose that the interaction of FlhA with the chaperon-binding site of FlhA suppresses filament-type protein export and facilitates hook-type protein export during hook assembly.
鞭毛蛋白输出装置在完成钩装配后，从钩型到丝型的底物特异性转换，从而在钩尖开始丝装配。FlhA（FlhA）的 C 端胞质结构域作为鞭毛伴侣与它们同源的丝型底物复合物的 docking 平台。FlhA（FlhA）的柔性接头与 FlhA 环中最近的 FlhA 亚基的相互作用对于底物特异性转换是必需的。为了解析 FlhA 如何为鞭毛组装带来秩序，我们分析了 flhA(E351A/W354A/D356A)ΔflgM 突变体，并发现 FlhA 中的这三个突变增加了 5 倍的钩蛋白分泌水平，从而增加了钩的长度。FlhA(E351A/D356A)的晶体结构表明，FlhA 结合到其相邻亚基的伴侣结合位点。我们提出，FlhA 与 FlhA 的伴侣结合位点的相互作用抑制了丝型蛋白的输出，并在钩装配过程中促进了钩型蛋白的输出。
