Data from: Characterization of C-ring component assembly in flagellar motors from amino acid coevolution

Bacterial flagellar motility, an important virulence factor, is energized by a rotary motor localized within the flagellar basal body. The rotor module consists of a large framework (C-ring), composed of the FliG, FliM and FliN proteins. FliN and FliM contacts the FliG torque ring to control the direction of flagellar rotation. We report that structure-based models constrained only by residue coevolution can recover the binding interface of atomic X-ray dimer complexes with remarkable accuracy (ca. 1 Å RMSD). We propose a model for FliM-FliN heterodimerization, which agrees accurately with homologous interfaces as well as in-situ cross-linking experiments, and hence supports a proposed architecture for the lower portion of the C-ring. Furthermore, this approach allowed the identification of two discrete and interchangeable homodimerization interfaces between FliM middle domains that agree with experimental measurements and might be associated with C-ring directional switching dynamics triggered upon binding of CheY signal protein. Our findings provide structural details of complex formation at the C-ring that have been difficult to obtain with previous methodologies and clarify the architectural principle that underpins the ultra-sensitive allostery exhibited by this ring assembly that controls the clockwise (CW) or counterclockwise (CCW) rotation of flagella.

细菌鞭毛运动作为一类重要的毒力因子，其能量由定位于鞭毛基体内部的旋转马达供给。转子模块（rotor module）由大型支架结构C环（C-ring）构成，该支架由FliG、FliM与FliN三种蛋白组成。FliN与FliM可结合FliG扭矩环，以此调控鞭毛的旋转方向。本研究表明，仅以残基共进化作为约束条件的基于结构的模型，可极高精度地复现原子级X射线衍射二聚体复合物的结合界面，其均方根偏差（Root Mean Square Deviation, RMSD）约为1埃。我们提出了FliM-FliN异源二聚化模型，该模型与同源结合界面及原位交联实验结果高度吻合，从而验证了此前提出的C环下部区域的结构架构。此外，该方法还成功鉴定出FliM中间结构域之间存在两种独立且可互换的同源二聚化界面，这与实验观测结果一致，且可能与CheY信号蛋白结合后触发的C环方向切换动力学过程相关。本研究结果阐明了此前难以通过传统实验方法获取的C环复合物形成的结构细节，并揭示了支撑该环装组装体超敏感变构效应的结构原理——该组装体可调控鞭毛的顺时针（CW）与逆时针（CCW）旋转。