Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka, 565-0871, Japan.
Mol Microbiol. 2013 Dec;90(6):1249-61. doi: 10.1111/mmi.12430. Epub 2013 Oct 30.
Assembly of the bacterial flagellar filament is strictly sequential; the junction proteins, FlgK and FlgL, are assembled at the distal end of the hook prior to the FliD cap, which supports assembly of as many as 30 000 FliC molecules into the filament. Export of these proteins requires assistance of flagellar chaperones: FlgN for FlgK and FlgL, FliT for FliD and FliS for FliC. The C-terminal cytoplasmic domain of FlhA (FlhAC ), a membrane component of the export apparatus, provides a binding-site for these chaperone-substrate complexes but it remains unknown how it co-ordinates flagellar protein export. Here, we report that the highly conserved hydrophobic dimple of FlhAC is involved in the export of FlgK, FlgL, FliD and FliC but not in proteins responsible for the structure and assembly of the hook, and that the binding affinity of FlhAC for the FlgN/FlgK complex is slightly higher than that for the FliT/FliD complex and about 14-fold higher than that for the FliS/FliC complex, leading to the proposal that the different binding affinities of FlhAC for these chaperone/substrate complexes may confer an advantage for the efficient formation of the junction and cap structures at the tip of the hook prior to filament formation.
细菌鞭毛丝的组装是严格顺序进行的；连接蛋白 FlgK 和 FlgL 在钩的远端组装在 FliD 盖帽之前，该盖帽支持多达 30000 个 FliC 分子组装到丝中。这些蛋白质的输出需要鞭毛伴侣的协助：FlgN 用于 FlgK 和 FlgL，FliT 用于 FliD，FliS 用于 FliC。鞭毛输出装置的膜成分 FlhA（FlhAC）的 C 端细胞质结构域为这些伴侣-底物复合物提供了结合位点，但它如何协调鞭毛蛋白输出仍然未知。在这里，我们报告说，FlhAC 高度保守的疏水凹陷参与了 FlgK、FlgL、FliD 和 FliC 的输出，但不参与钩的结构和组装的蛋白质，并且 FlhAC 与 FlgN/FlgK 复合物的结合亲和力略高于与 FliT/FliD 复合物的结合亲和力，约为与 FliS/FliC 复合物的 14 倍，这表明 FlhAC 对这些伴侣/底物复合物的不同结合亲和力可能为在丝形成之前在钩的尖端形成连接和盖帽结构提供优势。
