Abstract
The bacterial flagellum is a locomotive organelle that propels the bacterial cell body in liquid environments. The flagellum is a supramolecular complex composed of about 30 different proteins and consists of at least three parts: a rotary motor, a universal joint, and a helical filament. The flagellar motor of Escherichia coli and Salmonella enterica is powered by an inward-directed electrochemical potential difference of protons across the cytoplasmic membrane. The flagellar motor consists of a rotor made of FliF, FliG, FliM and FliN and a dozen stators consisting of MotA and MotB. FliG, FliM and FliN also act as a molecular switch, enabling the motor to spin in both counterclockwise and clockwise directions. Each stator is anchored to the peptidoglycan layer through the C-terminal periplasmic domain of MotB and acts as a proton channel to couple the proton flow through the channel with torque generation. Highly conserved charged residues at the rotor–stator interface are required not only for torque generation but also for stator assembly around the rotor. In this review, we will summarize our current understanding of the structure and function of the proton-driven bacterial flagellar motor.
Highlights
Many bacteria swim in liquid media by rotating bacterial flagella
As torque generation by the motor applies an equal and opposite force on the PG layer through MotBC, MotBC is proposed to act as a load-sensitive structural switch to regulate the assembly and disassembly cycle of the stators in response to the load changes and that an appropriate length of a linker connecting the peptidoglycan binding (PGB) domain to MotB-transmembrane spans (TMs) may stabilize this structural switch during the torque generation cycle [100]
Since the rotation rate of the flagellar motor is not limited by the rate of the mechanochemical cycle of the motor at low speed near stall [103], it is suggested that the D33E mutation misaligns MotAC relative to FliG at the rotor–stator interface, causing the 50% reduction in the energy coupling efficiency of the motor and that the suppressor mutations readjusts the alignment of MotAC relative to FliG, thereby restoring the coupling efficiency to the wild-type level [102]
Summary
Many bacteria swim in liquid media by rotating bacterial flagella. The bacterial flagellum is a supramolecular complex made of about 30 different proteins with copy numbers ranging from a few to a few tens of thousands (Figure 1). CheY-P binds to FliM and FliN, inducing cooperative conformational changes of the FliG ring to allow the motor to spin in the CW direction [41,42]. Mutations located in and around helixMC, which connects FliGM and FliGC, generate a diversity of phenotype, including motors that are strongly CW biased, infrequent switchers, rapid switchers, and transiently or permanently paused This suggests that helixMC is involved in switching of the direction of flagellar motor rotation [51]. This mutant motor remains in the CW rotation even in the absence of CheY-P, indicating that the motor is locked in the CW state [37] This suggests that the PAA deletion induces a conformational change of FliG at the rotor–stator interface in a way similar to the binding of CheY-P to FliM and FliN. The number of FliM subunits in the C ring increases in response to a reduction in the concentration of CheY-P, suggesting that the flagellar motor adapts to changes in the steady-state level of CheY-P by adjusting the number of FliM molecules to which CheY-P binds [58,59]
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