Division of Biomedical Convergence, College of Biomedical Science, Kangwon National University, Chuncheon, 24341, Republic of Korea.
Laboratory of Microbiology and Immunology, College of Pharmacy, Kangwon National University, Chuncheon, 24341, Republic of Korea.
Sci Rep. 2018 Sep 24;8(1):14307. doi: 10.1038/s41598-018-32460-9.
Bacteria move toward attractants and away from repellants by rotating their flagellum. The bacterial flagellum assembles through the ordered organization of more than 30 different proteins. Among the diverse flagellar proteins, FlgL forms the junction between the hook and the filament in the flagellum together with FlgK and provides a structural base where flagellin, a filament-forming protein, is inserted for the initiation of filament elongation. However, the functional and structural information available for FlgL is highly limited. To provide structural insights into the cross-linkage between the FlgL junction and the flagellin filament, we determined the crystal structures of FlgL from gram-positive Bacillus cereus (bcFlgL) and gram-negative Xanthomonas campestris (xcFlgL). bcFlgL contains one domain (D1), whereas xcFlgL adopts a two-domain structure that consists of the D1 and D2 domains. The constant D1 domain of FlgL adopts a rod structure that is generated by four longitudinal segments. This four-segment structure is recapitulated in filament and junction proteins but not in hook and rod proteins, allowing us to propose a junction-filament assembly mechanism based on a quasi-homotypic interaction. The D2 domain of xcFlgL resembles that of another junction protein, FlgK, suggesting the structural and functional relatedness of FlgL and FlgK.
细菌通过旋转鞭毛来向吸引物移动，远离排斥物。细菌鞭毛的组装是通过 30 多种不同蛋白质的有序组织实现的。在各种鞭毛蛋白中，FlgL 与 FlgK 一起形成鞭毛钩和鞭毛丝之间的连接，并提供一个结构基础，鞭毛丝形成蛋白插入其中，开始进行丝的延伸。然而，FlgL 的功能和结构信息非常有限。为了提供 FlgL 连接点与鞭毛丝之间交联的结构见解，我们测定了来自革兰氏阳性的蜡状芽孢杆菌（bcFlgL）和革兰氏阴性的野油菜黄单胞菌（xcFlgL）的 FlgL 的晶体结构。bcFlgL 含有一个结构域（D1），而 xcFlgL 采用由 D1 和 D2 结构域组成的二结构域结构。FlgL 的恒定 D1 结构域采用由四个纵向片段产生的杆状结构。这种四段结构在丝和连接蛋白中得到了重现，但在钩和杆蛋白中没有，这使我们能够基于准同源相互作用提出连接-丝组装机制。xcFlgL 的 D2 结构域类似于另一个连接蛋白 FlgK，表明 FlgL 和 FlgK 的结构和功能相关性。
