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The bacterial flagellar export apparatus is required for the construction of the bacterial flagella beyond the cytoplasmic membrane. The membrane-embedded part of the export apparatus, which consists of FlhA, FlhB, FliO, FliP, FliQ and FliR, is located in the central pore of the MS ring formed by 26 copies of FliF. The C-terminal cytoplasmic domain of FlhA is located in the centre of the cavity within the C ring made of FliG, FliM and FliN. FlhA interacts with FliF, but its assembly mechanism remains unclear. Here, we fused yellow fluorescent protein ( YFP) and cyan fluorescent protein ( CFP) to the C-termini of FliF and FlhA and investigated their subcellular localization by fluorescence microscopy. The punctate pattern of FliF- YFP localization required FliG but neither FliM, FliN, FlhA, FlhB, FliO, FliP, FliQ nor FliR. In contrast, FlhA- CFP localization required FliF, FliG, FliO, FliP, FliQ and FliR. The number of FlhA- YFP molecules associated with the MS ring was estimated to be about nine. We suggest that FlhA assembles into the export gate along with other membrane components during the MS ring complex formation in a co-ordinated manner. [ABSTRACT FROM AUTHOR]

MotA and MotB form a transmembrane proton channel that acts as the stator of the bacterial flagellar motor to couple proton flow with torque generation. The C-terminal periplasmic domain of MotB plays a role in anchoring the stators to the motor. However, it remains unclear where their initial binding sites are. Here, we constructed Salmonella strains expressing GFP-MotB and MotA-mCherry and investigated their subcellular localization by fluorescence microscopy. Neither the D33N and D33A mutations in MotB, which abolish the proton flow, nor depletion of proton motive force affected the assembly of GFP-MotB into the motor, indicating that the proton translocation activity is not required for stator assembly. Overexpression of MotA markedly inhibited wild-type motility, and it was due to the reduction in the number of functional stators. Consistently, MotA-mCherry was observed to colocalize with GFP-FliG even in the absence of MotB. These results suggest that MotA alone can be installed into the motor. The R90E and E98K mutations in the cytoplasmic loop of MotA (MotA), which has been shown to abolish the interaction with FliG, significantly affected stator assembly, suggesting that the electrostatic interaction of MotA with FliG is required for the efficient assembly of the stators around the rotor. [ABSTRACT FROM AUTHOR]

The bacterial flagellar motor accommodates ten stator units around the rotor to produce large torque at high load. But when external load is low, some previous studies showed that a single stator unit can spin the rotor at the maximum speed, suggesting that the maximum speed does not depend on the number of active stator units, whereas others reported that the speed is also dependent on the stator number. To clarify these two controversial observations, much more precise measurements of motor rotation would be required at external load as close to zero as possible. Here, we constructed a Salmonella filament‐less mutant that produces a rigid, straight, twice longer hook to efficiently label a 60 nm gold particle and analyzed flagellar motor dynamics at low load close to zero. The maximum motor speed was about 400 Hz. Large speed fluctuations and long pausing events were frequently observed, and they were suppressed by either over‐expression of the MotAB stator complex or increase in the external load, suggesting that the number of active stator units in the motor largely fluctuates near zero load. We conclude that the lifetime of the active stator unit becomes much shorter when the motor operates near zero load. [ABSTRACT FROM AUTHOR]

The Salmonella flagellar motor consists of a rotor and about a dozen stator elements. Each stator element, consisting of MotA and MotB, acts as a proton channel to couple proton flow with torque generation. A highly conserved Asp33 residue of MotB is directly involved in the energy coupling mechanism, but it remains unknown how it carries out this function. Here, we show that the MotB( D33 E) mutation dramatically alters motor performance in response to changes in external load. Rotation speeds of the MotA/ B( D33 E) and MotA( V35 F)/ B( D33 E) motors were markedly slower than the wild-type motor and fluctuated considerably at low load but not at high load, whereas the rotation rate of the wild-type motor was stable at any load. At low load, pausing events were frequently observed in both mutant motors. The proton conductivities of these mutant stator channels in their 'unplugged' forms were only half of the conductivity of the wild-type channel. These results suggest that the D33 E mutation induces a load-dependent inactivation of the MotA/ B complex. We propose that the stator element is a load-sensitive proton channel that efficiently couples proton translocation with torque generation and that Asp33 of MotB is critical for this co-ordinated proton translocation. [ABSTRACT FROM AUTHOR]

Most bacterial flagellar proteins are exported by the flagellar type III protein export apparatus for their self-assembly. FliI ATPase forms a complex with its regulator FliH and facilitates initial entry of export substrates to the export gate composed of six integral membrane proteins. The FliH–FliI complex also binds to the C ring of the basal body through a FliH–FliN interaction for efficient export. However, it remains unclear how these reactions proceed within the cell. Here, we analysed subcellular localization of FliI–YFP by fluorescence microscopy. FliI–YFP was localized to the flagellar base, and its localization required both FliH and the C ring. The ATPase activity of FliI was not required for its localization. FliI–YFP formed a complex with FliHΔ1 (missing residues 2–10) but the complex did not show any localization. FliHΔ1 did not interact with FliN, and alanine-scanning mutagenesis revealed that only Trp-7 and Trp-10 of FliH are essential for the interaction with FliN. Overproduction of the FliH–FliI complex improved the export activity of the fliN mutant whereas neither of the FliH(W7A)-FliI nor FliH(W10A)-FliI complexes did, suggesting that Trp-7 and Trp-10 of FliH are also required for efficient localization of the FliH–FliI complex to the export gate. [ABSTRACT FROM AUTHOR]