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The flagellar motor of Vibrio alginolyticus undergoes major structural remodeling during rotational switching.

ABSTRACT: The bacterial flagellar motor switches rotational direction between counterclockwise (CCW) and clockwise (CW) to direct the migration of the cell. The cytoplasmic ring (C-ring) of the motor, which is composed of FliG, FliM, and FliN, is known for controlling the rotational sense of the flagellum. However, the mechanism underlying rotational switching remains elusive. Here, we deployed cryo-electron tomography to visualize the C-ring in two rotational biased mutants in Vibrio alginolyticus. We determined the C-ring molecular architectures, providing novel insights into the mechanism of rotational switching. We report that the C-ring maintained 34-fold symmetry in both rotational senses, and the protein composition remained constant. The two structures show FliG conformational changes elicit a large conformational rearrangement of the rotor complex that coincides with rotational switching of the flagellum. FliM and FliN form a stable spiral-shaped base of the C-ring, likely stabilizing the C-ring during the conformational remodeling.

SUBMITTER: Carroll BL

PROVIDER: S-EPMC7505661 | biostudies-literature | 2020 Sep

REPOSITORIES: biostudies-literature

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The flagellar motor of <i>Vibrio alginolyticus</i> undergoes major structural remodeling during rotational switching.

Carroll Brittany L BL Nishikino Tatsuro T Guo Wangbiao W Zhu Shiwei S Kojima Seiji S Homma Michio M Liu Jun J

eLife 20200907

The bacterial flagellar motor switches rotational direction between counterclockwise (CCW) and clockwise (CW) to direct the migration of the cell. The cytoplasmic ring (C-ring) of the motor, which is composed of FliG, FliM, and FliN, is known for controlling the rotational sense of the flagellum. However, the mechanism underlying rotational switching remains elusive. Here, we deployed cryo-electron tomography to visualize the C-ring in two rotational biased mutants in <i>Vibrio alginolyticus</i> ...[more]

PMID: 32893817

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Project description:The bacterial flagellar motor can rotate either clockwise (CW) or counterclockwise (CCW). Three flagellar proteins, FliG, FliM, and FliN, are required for rapid switching between the CW and CCW directions. Switching is achieved by a conformational change in FliG induced by the binding of a chemotaxis signaling protein, phospho-CheY, to FliM and FliN. FliG consists of three domains, FliG(N), FliG(M), and FliG(C), and forms a ring on the cytoplasmic face of the MS ring of the flagellar basal body. Crystal structures have been reported for the FliG(MC) domains of Thermotoga maritima, which consist of the FliG(M) and FliG(C) domains and a helix E that connects these two domains, and full-length FliG of Aquifex aeolicus. However, the basis for the switching mechanism is based only on previously obtained genetic data and is hence rather indirect. We characterized a CW-biased mutant (fliG(?PAA)) of Salmonella enterica by direct observation of rotation of a single motor at high temporal and spatial resolution. We also determined the crystal structure of the FliG(MC) domains of an equivalent deletion mutant variant of T. maritima (fliG(?PEV)). The FliG(?PAA) motor produced torque at wild-type levels under a wide range of external load conditions. The wild-type motors rotated exclusively in the CCW direction under our experimental conditions, whereas the mutant motors rotated only in the CW direction. This result suggests that wild-type FliG is more stable in the CCW state than in the CW state, whereas FliG(?PAA) is more stable in the CW state than in the CCW state. The structure of the TM-FliG(MC)(?PEV) revealed that extremely CW-biased rotation was caused by a conformational change in helix E. Although the arrangement of FliG(C) relative to FliG(M) in a single molecule was different among the three crystals, a conserved FliG(M)-FliG(C) unit was observed in all three of them. We suggest that the conserved FliG(M)-FliG(C) unit is the basic functional element in the rotor ring and that the PAA deletion induces a conformational change in a hinge-loop between FliG(M) and helix E to achieve the CW state of the FliG ring. We also propose a novel model for the arrangement of FliG subunits within the motor. The model is in agreement with the previous mutational and cross-linking experiments and explains the cooperative switching mechanism of the flagellar motor.

Dataset Information

The flagellar motor of Vibrio alginolyticus undergoes major structural remodeling during rotational switching.

Publications

The flagellar motor of <i>Vibrio alginolyticus</i> undergoes major structural remodeling during rotational switching.

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