Each MotB monomer progressively engages with each MotA monomer to form an ion channel through which one proton can pass to drive the rotation of the MotA pentamer ring.īrownian ratchet models have attracted physicists' attention for modeling molecular motors since Richard Feynman popularized the idea more than half a century ago ( Feynman et al., 1966 Parrondo et al., 1998 Astumian, 2010 Golubeva et al., 2012). The position of the MotB dimer, which is bound to the peptidoglycan layer in the rigid cell wall, remains fixed, while the MotA pentamer forms a ring that can rotate around the MotB dimer. Both studies report that a stator unit consists of a MotB dimer surrounded by a MotA pentamer ring with a diameter of ~ 5 − 7.5 nm. The molecular mechanism of torque generation by the stator units has been suggested by two recent Cryo-EM studies of the stator structure ( Deme et al., 2020 Santiveri et al., 2020). The membrane-bound stators, which consist of the MotA and MotB proteins, are powered by the PMF to drive the rotation of the rotor via direct interaction between MotA and FliG ( Chun and Parkinson, 1988 Blair and Berg, 1990 Kojima and Blair, 2004 Roujeinikova, 2008). The C-ring is responsible for coupling the rotor to the stator units and for switching rotational directions. The cytoplasmic face of the MS-ring is attached to the C-ring, which comprises the FliG, FliM, and FliN proteins. FliF forms the MS-ring embedded within the cytoplasmic membrane ( Asai et al., 1997 Sato and Homma, 2000 Yorimitsu et al., 2004 Morimoto and Minamino, 2014 Baker et al., 2016). The rotor consists of FliF, FliG, FliM, and FliN. The bacterial flagellar motor consists of a rotor and a number of stators that can vary from 0 to 11 depending on gene expression and mechanical load ( Blair and Berg, 1988 Lele et al., 2013 Tipping et al., 2013 Nord et al., 2017 Wadhwa et al., 2019, 2021). coli cell changes its orientation randomly without translational movement in an event called a “tumble.” The CCW rotation of the BFMs causes their flagellar filaments to form a coherent bundle and propels the swimming cells in a roughly straight line that is called a “run.” However, the binding of CheY-P to BFM causes it to rotate in the clockwise (CW) direction, which disrupts the filament bundle. Without binding to the phosphorylated response regulator CheY protein (CheY-P), the BFM rotates in a counter-clockwise (CCW, viewed from the filament to the motor) direction. coli are fueled by electrochemical potential difference of protons ( H +) across the cytoplasmic membrane, i.e., the proton motive force (PMF) ( Larsen et al., 1974 Hirota et al., 1981 Berg, 2003). The swimming motion of many bacteria is powered by the rotary bacterial flagellar motor (BFM). Possible approaches to verify and improve the model to further understand the molecular mechanism for torque generation in BFM are also discussed. More importantly, the model predicts distinctive rotor and stator dynamics and their load dependence, which may be tested by future experiments. Preliminary results from the structure-informed model are consistent with the observed torque-speed relation. Specifically, translocation of protons through the stator complex drives rotation of the MotA pentamer ring, which in turn drives rotation of the FliG ring in the rotor via interactions between the MotA ring of the stator and the FliG ring of the rotor. The BFM is modeled as two rotating nano-rings that interact with each other. The structure suggested that the stator also rotates. In this article, we develop a mathematical model for the rotary bacterial flagellar motor (BFM) based on the recently discovered structure of the stator complex (MotA 5MotB 2). Watson Research Center, New York, NY, United States 2Yuanpei College, Center for Quantitative Biology, Peking University, Beijing, China.1Department of Physics, University of California, San Diego, San Diego, CA, United States.Yuansheng Cao 1 † Tairan Li 2 † Yuhai Tu 3 *
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