Abstract
Author SummaryThe bacterial flagellum is a rotating organelle that governs cell motility. At the base of each flagellum is a motor powered by the electrochemical potential difference of specific ions across the cytoplasmic membrane. In response to environmental stimuli, rotation of the motor switches between counterclockwise and clockwise, with a corresponding effect on the swimming direction of the cell. Switching is triggered by the binding of the signaling protein phospho-CheY to FliM and FliN, and achieved by conformational changes in the rotor protein FliG. The actual switching mechanism, however, remains unclear. In this study, we characterized a fliG mutant of Salmonella that shows an extreme clockwise-biased rotation, and determined the structure of a fragment of FliG (FliGMC) of the equivalent mutant variant of Thermotoga maritima. FliGMC is composed of two domains and covers the regions essential for torque generation and FliM binding. We showed that the mutant structure has a conformational change in the helix connecting the two domains, leading to a domain orientation distinct from that of the wild-type FliG. On the basis of this structure, we propose a new model for the arrangement of FliG subunits in the rotor that is consistent with the previous mutational studies and explains how cooperative switching occurs in the motor.
Highlights
Bacteria such as Escherichia coli and Salmonella enterica swim by rotating multiple flagella, which arise randomly over the cell surface
We characterized a fliG mutant of Salmonella that shows an extreme clockwise-biased rotation, and determined the structure of a fragment of FliG (FliGMC) of the equivalent mutant variant of Thermotoga maritima
We showed that the mutant structure has a conformational change in the helix connecting the two domains, leading to a domain orientation distinct from that of the wild-type FliG
Summary
Bacteria such as Escherichia coli and Salmonella enterica swim by rotating multiple flagella, which arise randomly over the cell surface. Each flagellum is a huge protein complex made up of about 30 different proteins and can be divided into three distinct parts: the basal body, the hook, and the filament. The hook and the filament extend outwards in the cell exterior. The filament is a helical propeller that propels the cell body. The hook connects the basal body with the filament and functions as a universal joint to transmit torque produced by the motor to the filament. CCW rotation causes the cell to swim smoothly in what is termed a run, whereas brief CW rotation of one or more flagella causes a tumble. The direction of motor rotation is controlled by environmental signals that are processed by a sensory signal transduction pathway to generate chemotaxis behavior [1,2,3]
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