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1.A.30 The H+- or Na+-translocating Bacterial Flagellar Motor/ExbBD Outer Membrane Transport Energizer (Mot-Exb) Superfamily

The Mot-Exb Superfamily consists of two distant families, each with a distinct function. The Mot family energizes bacterial flagellar rotation while the Exb family energizes accumulation of large molecules (i.e. iron-siderophores, vitamin B12, DNA from phage and colicins) from the external medium across the outer Gram-negative bacterial membrane into the periplasm. The pmf (or smf) is the driving force in both cases. MotAB and (PomAB) are homologous to ExbBD and TolQR. MotAB, the stator, is known to form a proton channel. This stator is composed of MotA and MotB proteins, which form a hetero-hexameric complex with a stoichiometry of four MotA and two MotB molecules. MotA can form a tetramer in the absence of MotB (Takekawa et al. 2016).

Yonekura et al. (2011) presented the first three-dimensional structure of the PomAB torque-generating stator unit analyzed by electron microscopy. The structure of PomAB revealed two arm domains, which contain the PG-binding site, connected to a large base made of the TM and cytoplasmic domains. The arms lean downward to the membrane surface, likely representing a 'plugged' conformation, which would prevent ions leaking through the channel. They propose a model for how PomAB units are placed around the flagellar basal body to function as torque generators. 

Leu46 of MotB acts as the gate for hydronium ion permeation, which induces the formation of a water wire that may mediate the proton transfer to Asp32 on MotB. The free energy barrier for H3O+ permeation is consistent with the proton transfer rate deduced from the flagellar rotational speed and number of protons per rotation, suggesting that gating is the rate-limiting step (Nishihara and Kitao 2015). Structure and dynamics of the MotA/B with nonprotonated and protonated Asp32 suggested a  size-dependent ion selectivity. In MotA/B with the nonprotonated Asp32, the A3 segment in MotA maintained a kink whereas protonation induced a straighter shape. Assuming that the cytoplasmic domain not included in the atomic model moves as a rigid body, the protonation/deprotonation of Asp32 is inferred to induce a ratchet motion of the cytoplasmic domain correlated with the motion of the flagellar rotor (Nishihara and Kitao 2015).

References associated with 1.A.30 family:

Bulathsinghala, C.M., B. Jana, K.R. Baker, and K. Postle. (2013). ExbB cytoplasmic loop deletions cause immediate, proton motive force-independent growth arrest. J. Bacteriol. 195: 4580-4591. 23913327
Castillo, D.J., S. Nakamura, Y.V. Morimoto, Y.S. Che, N. Kami-Ike, S. Kudo, T. Minamino, and K. Namba. (2013). The C-terminal periplasmic domain of MotB is responsible for load-dependent control of the number of stators of the bacterial flagellar motor. Biophysics (Nagoya-shi) 9: 173-181. 27493556
Hosking, E.R., C. Vogt, E.P. Bakker, and M.D. Manson. (2006). The Escherichia coli MotAB proton channel unplugged. J. Mol. Biol. 364: 921-937. 17052729
Ito, M., D.B. Hicks, T.M. Henkin, A.A. Guffanti, B.D. Powers, L. Zvi, K. Uematsu, and T.A. Krulwich. (2004). MotPS is the stator-force generator for motility of alkaliphilic Bacillus, and its homologue is a second functional Mot in Bacillus subtilis. Mol. Microbiol. 53: 1035-1049. 15306009
Kitao, A. and Y. Nishihara. (2017). Structure of the MotA/B Proton Channel. Methods Mol Biol 1593: 133-145. 28389950
Klebba, P.E. (2016). ROSET Model of TonB Action in Gram-Negative Bacterial Iron Acquisition. J. Bacteriol. 198: 1013-1021. 26787763
Koerdt, A., A. Paulick, M. Mock, K. Jost, and K.M. Thormann. (2009). MotX and MotY are required for flagellar rotation in Shewanella oneidensis MR-1. J. Bacteriol. 191: 5085-5093. 19502394
Kojima, S., K. Imada, M. Sakuma, Y. Sudo, C. Kojima, T. Minamino, M. Homma, and K. Namba. (2009). Stator assembly and activation mechanism of the flagellar motor by the periplasmic region of MotB. Mol. Microbiol. 73: 710-718. 19627504
Lo, C.J., Y. Sowa, T. Pilizota, and R.M. Berry. (2013). Mechanism and kinetics of a sodium-driven bacterial flagellar motor. Proc. Natl. Acad. Sci. USA 110: E2544-2551. 23788659
Nan, B., J. Chen, J.C. Neu, R.M. Berry, G. Oster, and D.R. Zusman. (2011). Myxobacteria gliding motility requires cytoskeleton rotation powered by proton motive force. Proc. Natl. Acad. Sci. USA 108: 2498-2503. 21248229
Nishihara Y. and Kitao A. (2015). Gate-controlled proton diffusion and protonation-induced ratchet motion in the stator of the bacterial flagellar motor. Proc Natl Acad Sci U S A. 112(25):7737-42. 26056313
O'Neill, J., M. Xie, M. Hijnen, and A. Roujeinikova. (2011). Role of the MotB linker in the assembly and activation of the bacterial flagellar motor. Acta Crystallogr D Biol Crystallogr 67: 1009-1016. 22120737
O'Neill, J., M. Xie, M. Hijnen, and A. Roujeinikova. (2011). Role of the MotB linker in the assembly and activation of the bacterial flagellar motor. Acta Crystallogr D Biol Crystallogr 67: 1009-1016. 18540076
Okabe, M., T. Yakushi, and M. Homma. (2005). Interactions of MotX with MotY and with the PomA/PomB sodium ion channel complex of the Vibrio alginolyticus polar flagellum. J. Biol. Chem. 280: 25659-25664. 15866878
Takekawa, N., N. Terahara, T. Kato, M. Gohara, K. Mayanagi, A. Hijikata, Y. Onoue, S. Kojima, T. Shirai, K. Namba, and M. Homma. (2016). The tetrameric MotA complex as the core of the flagellar motor stator from hyperthermophilic bacterium. Sci Rep 6: 31526. 27531865
Wille T., Wagner C., Mittelstadt W., Blank K., Sommer E., Malengo G., Dohler D., Lange A., Sourjik V., Hensel M. and Gerlach RG. (2014). SiiA and SiiB are novel type I secretion system subunits controlling SPI4-mediated adhesion of Salmonella enterica. Cell Microbiol. 16(2):161-78. 24119191
Yonekura, K., S. Maki-Yonekura, and M. Homma. (2011). Structure of the flagellar motor protein complex PomAB: implications for the torque-generating conformation. J. Bacteriol. 193: 3863-3870. 21642461
Zhu S., Homma M. and Kojima S. (2012). Intragenic suppressor of a plug deletion nonmotility mutation in PotB, a chimeric stator protein of sodium-driven flagella. J Bacteriol. 194(24):6728-35. 23024347