Abstract
SummarySwitching of flagellar motor rotation sense dictates bacterial chemotaxis. Multi-subunit FliM-FliG rotor rings couple signal protein binding in FliM with reversal of a distant FliG C-terminal (FliGC) helix involved in stator contacts. Subunit dynamics were examined in conformer ensembles generated by molecular simulations from the X-ray structures. Principal component analysis extracted collective motions. Interfacial loop immobilization by complex formation coupled elastic fluctuations of the FliM middle (FliMM) and FliG middle (FliGM) domains. Coevolved mutations captured interfacial dynamics as well as contacts. FliGM rotation was amplified via two central hinges to the FliGC helix. Intrinsic flexibility, reported by the FliGMC ensembles, reconciled conformers with opposite FliGC helix orientations. FliG domain stacking deformed the inter-domain linker and reduced flexibility; but conformational changes were not triggered by engineered linker deletions that cause a rotation-locked phenotype. These facts suggest that binary rotation states arise from conformational selection by stacking interactions.
Highlights
The switching of bacterial flagellar rotation provides a remarkable example of a cooperative switch in a large, biomolecular assembly (Bray and Duke, 2004)
We examined the ensemble from the FliMMFliGMC complex
Anharmonic collective motions were identified by principal component analysis (PCA) of residue Ca position fluctuations and the principal components (PCs) mapped onto the structure
Summary
The switching of bacterial flagellar rotation provides a remarkable example of a cooperative switch in a large, biomolecular assembly (Bray and Duke, 2004). The assembly, the rotor of the bacterial flagellar motor within the basal body, is composed of about 200 subunits of the component proteins (FliG, FliM, and FliN). These proteins attach to the membrane scaffold formed by FliF subunits forming the C and MS rings (Lux et al, 2000). The chemotactic motor output is a changed clockwise (CW)/counter-clockwise (CCW) rotation bias. Activated CheY elicits an ‘‘ultra-sensitive’’ (H = 21) change in CW/CCW bias (Yuan and Berg, 2013), but its binding to motors in situ or rotor assemblies in vitro is not cooperative (Sagi et al, 2003; Sourjik and Berg, 2002). Cooperativity must arise from mechanical amplification within the rotor
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