Abstract
The rotary bacterial flagellar motor is remarkable in biochemistry for its highly synchronized operation and amplification during switching of rotation sense. The motor is part of the flagellar basal body, a complex multi-protein assembly. Sensory and energy transduction depends on a core of six proteins that are adapted in different species to adjust torque and produce diverse switches. Motor response to chemotactic and environmental stimuli is driven by interactions of the core with small signal proteins. The initial protein interactions are propagated across a multi-subunit cytoplasmic ring to switch torque. Torque reversal triggers structural transitions in the flagellar filament to change motile behavior. Subtle variations in the core components invert or block switch operation. The mechanics of the flagellar switch have been studied with multiple approaches, from protein dynamics to single molecule and cell biophysics. The architecture, driven by recent advances in electron cryo-microscopy, is available for several species. Computational methods have correlated structure with genetic and biochemical databases. The design principles underlying the basis of switch ultra-sensitivity and its dependence on motor torque remain elusive, but tantalizing clues have emerged. This review aims to consolidate recent knowledge into a unified platform that can inspire new research strategies.
Highlights
Bacterial motility has been a long-standing example of motion on a microscopic scale [1]
Advances in bacterial flagellar switch function and structure in the second half of these thirty years were based on the development of high-throughput genetic screens, sophisticated motor rotation assays, isolation and biochemical characterization of the intact switch and sub-complexes together with atomic structures as summarized (Figure 1)
Structures that were morphologically identical to the C ring formed upon overproduction of the switch complex proteins together with the FliF MS ring [18]. 3D reconstructions in ice of the S. enterica basal body [15] combined developments in cryo-electron microscopy with single-particle image analysis to resolve C ring periodicity [21] and position individual domains, with FliG an early example [22]
Summary
Motor dynamics are the direct outcome of the architectural dynamics of the molecular machinery. Filament-associated bead rotation measurements in the phospho-mimetic E. coli CheY13DK mutant strain first revealed gamma distributions for both CW and CCW intervals [59] to challenge the view of switch transitions as a single Poisson process based on tethered cell data. The comprehensive analysis of the rotation of different-sized beads in the double mutant phospho-mimetic strain CheY13DK106YW showed this was the case Both exponential and peaked distributions were obtained, depending on the different load, proton potential and torque [64], to rule out speculation that the gamma distribution was an artefact. FiguFirgeu2r.e A2.dAvdavnacnecseisninswswitcithchpphhyysisoiolologgyy.. ((AA)) TTiimmee--rreessoollvveeddmmoototrorrortoattaiotino:nS:chSecmheamticatoifcaofmaotmorotor rotartoiotantiaosnsaasysawyiwthitahnaannaonsopshpehreerecoconnjujuggaatteeddttoo tthhee hhooookkccoonnnneecctotor rcocnotnigtiugouuosuws iwthitthhethroedroadndabnadsablasal body (reproduced from [54] with permission). (B) CW and CCW interval distributions: (i) Idealized time series of a motor alternating between CW and CCW rotation. (ii) Interval (τCW, τCCW) distributions measured under low and high torque (reproduced from [75] with permission). (C) Coupled energization and switching of rotation: (i) Torque velocity curves for CW and CCW rotation [63]. (ii) body (reproduced from [54] with permission). (B) CW and CCW interval distributions: (i) Idealized time series of a motor alternating between CW and CCW rotation. (ii) Interval (τCW, τCCW) distributions measured under low and high torque (reproduced from [75] with permission). (C) Coupled energization and switching of rotation: (i) Torque velocity curves for CW and CCW rotation [63]. (ii) Free energy diagram of CW CCW transitions and the mechanical work (blue arrow) contribution (reproduced from [72] with permission). (D) Models for non-Poisson interval distributions: (i) Conformational spread seeded from multiple CheY binding events (reproduced from [59] with permission). (ii) Breakdown of detailed balance from motor energy dissipation (reproduced from [64] with permission)
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