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1.
MotA and MotB are integral membrane proteins of Escherichia coli that form the stator of the proton-fueled flagellar rotary motor. The motor contains several MotA/MotB complexes, which function independently to conduct protons across the cytoplasmic membrane and couple proton flow to rotation. MotB contains a conserved aspartic acid residue, Asp32, that is critical for rotation. We have proposed that the protons energizing the motor interact with Asp32 of MotB to induce conformational changes in the stator that drive movement of the rotor. To test for conformational changes, we examined the protease susceptibility of MotA in membrane-bound complexes with either wild-type MotB or MotB mutated at residue 32. Small, uncharged replacements of Asp32 in MotB (D32N, D32A, D32G, D32S, or D32C) caused a significant change in the conformation of MotA, as evidenced by a change in the pattern of proteolytic fragments. The conformational change does not require any flagellar proteins besides MotA and MotB, as it was still seen in a strain that expresses no other flagellar genes. It affects a cytoplasmic domain of MotA that contains residues known to interact with the rotor, consistent with a role in the generation of torque. Influences of key residues of MotA on conformation were also examined. Pro173 of MotA, known to be important for rotation, is a significant determinant of conformation: Dominant Pro173 mutations, but not recessive ones, altered the proteolysis pattern of MotA and also prevented the conformational change induced by Asp32 replacements. Arg90 and Glu98, residues of MotA that engage in electrostatic interactions with the rotor, appear not to be strong determinants of conformation of the MotA/MotB complex in membranes. We note sequence similarity between MotA and ExbB, a cytoplasmic-membrane protein that energizes outer-membrane transport in Gram-negative bacteria. ExbB and associated proteins might also employ a mechanism involving proton-driven conformational change. 相似文献
2.
MotA and MotB are membrane proteins that form the stator of the bacterial flagellar motor. Each motor contains several MotA 4MotB 2 complexes, which function independently to conduct protons across the membrane and couple proton flow to rotation. The mechanism of rotation is not understood in detail but is thought to involve conformational changes in the stator complexes driven by proton association/dissociation at a critical Asp residue of MotB (Asp 32 in the protein of Escherichia coli). MotA has four membrane segments and MotB has one. Previous studies using targeted disulfide cross-linking showed that the membrane segments of the two MotB subunits are together at the center of the complex, surrounded by the TM3 and TM4 segments of the four MotA subunits. Here, the cross-linking studies are extended to TM1 and TM2 of MotA, using Cys residues introduced in several positions in the segments. The observed patterns of disulfide cross-linking indicate that the TM2 segment is positioned between segments TM3 and TM4 of the same subunit, where it could contribute to the proton-channel-forming part of the structure. TM1 is at the interface between TM4 of its own subunit and the TM3 segment of another subunit, where it could stabilize the complex. A structural model based on the cross-linking results shows unobstructed pathways reaching from the periplasm to the Asp 32 residues near the inner ends of the MotB segments. The model indicates a close proximity for certain conserved, functionally important residues. The results are used to develop an explicit model for the proton-induced conformational change in the stator. 相似文献
3.
Bacterial flagella are turned by rotary motors that obtain energy from the membrane gradient of protons or sodium ions. The stator of the flagellar motor is formed from the membrane proteins MotA and MotB, which associate in complexes that contain multiple copies of each protein. The complexes conduct ions across the membrane, and couple ion flow to motor rotation by a mechanism that appears to involve conformational changes [Kojima, S., and Blair, D. F. (2001) Biochemistry 40, 13041-13050]. Structural information on the MotA/MotB complex is very limited. MotA has four membrane-spanning segments, and MotB has one. We have begun a targeted disulfide-cross-linking study to probe the arrangement of membrane segments in the MotA/MotB complex, beginning with the single membrane segment of MotB. Cys residues were introduced in 21 consecutive positions in the segment, and disulfide cross-linking was studied in MotA/MotB complexes either in membranes or detergent solution. Most of the Cys-substituted MotB proteins formed disulfide-linked dimers in significant yield upon oxidation. The yield of dimer varied regularly with the position of the Cys substitution, following the pattern expected for a parallel, symmetric dimer of alpha-helices. In a structural model based on the cross-linking experiments, critical Asp32 residues that are believed to facilitate proton movement are positioned on separate surfaces of the MotB dimer and so probably function within two distinct proton channels. Regions accessible to solvent were mapped by measuring the reactivity of introduced Cys residues toward N-ethyl maleimide and a charged methanethiosulfonate reagent. Positions near the middle of the segment were inaccessible to sulhydryl reagents. Positions within 6-8 residues of either end, which includes residues around Asp32, were accessible. 相似文献
4.
The stator of the bacterial flagellar motor is formed from the membrane proteins MotA and MotB, which associate in complexes with stoichiometry MotA(4)MotB(2) (Kojima, S., and Blair, D. F., preceding paper in this issue). The MotA/MotB complexes conduct ions across the membrane, and couple ion flow to flagellar rotation by a mechanism that appears to involve conformational changes within the complex. MotA has four membrane-crossing segments, termed A1-A4, and MotB has one, termed B. We are studying the organization of the 18 membrane segments in the MotA(4)MotB(2) complex by using targeted disulfide cross-linking. A previous cross-linking study showed that the two B segments in the complex (one from each MotB subunit) are arranged as a symmetrical dimer of alpha-helices. Here, we extend the cross-linking study to segments A3 and A4. Single Cys residues were introduced by mutation in several consecutive positions in segments A3 and A4, and double mutants were made by pairwise combination of subsets of the Cys replacements in segments A3, A4, and B. Disulfide cross-linking of the single- and double-Cys proteins was studied in whole cells, in membranes, and in detergent solution. Several combinations of Cys residues in segments A3 and B gave a high yield of disulfide-linked MotA/MotB heterodimer upon oxidation with iodine. Positions of efficient cross-linking identify a helix face on segment A3 that is in proximity to segment(s) B. Some combinations of Cys residues in segments A4 and B also gave a significant yield of disulfide-linked heterodimer, indicating that segment A4 is also near segment(s) B. Certain combinations of Cys residues in segments A3 and A4 cross-linked to form MotA tetramers in high yield upon oxidation. The high-yield positions identify faces on A3 and A4 that are at an interface between MotA subunits. Taken together with mutational studies and patterns of amino acid conservation, the cross-linking results delineate the overall arrangement of 10 membrane segments in the MotA/MotB complex, and identify helix faces likely to line the proton channels. 相似文献
5.
Function of Proline Residues of MotA in Torque Generation by the Flagellar Motor of Escherichia coli
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Timothy F. Braun Susan Poulson Jonathan B. Gully J. Courtney Empey Susan Van Way Anglica Putnam David F. Blair 《Journal of bacteriology》1999,181(11):3542-3551
Bacterial flagellar motors obtain energy for rotation from the membrane gradient of protons or, in some species, sodium ions. The molecular mechanism of flagellar rotation is not understood. MotA and MotB are integral membrane proteins that function in proton conduction and are believed to form the stator of the motor. Previous mutational studies identified two conserved proline residues in MotA (Pro 173 and Pro 222 in the protein from Escherichia coli) and a conserved aspartic acid residue in MotB (Asp 32) that are important for function. Asp 32 of MotB probably forms part of the proton path through the motor. To learn more about the roles of the conserved proline residues of MotA, we examined motor function in Pro 173 and Pro 222 mutants, making measurements of torque at high load, speed at low and intermediate loads, and solvent-isotope effects (D2O versus H2O). Proton conduction by wild-type and mutant MotA-MotB channels was also assayed, by a growth defect that occurs upon overexpression. Several different mutations of Pro 173 reduced the torque of the motor under high load, and a few prevented motor rotation but still allowed proton flow through the MotA-MotB channels. These and other properties of the mutants suggest that Pro 173 has a pivotal role in coupling proton flow to motor rotation and is positioned in the channel near Asp 32 of MotB. Replacements of Pro 222 abolished function in all assays and were strongly dominant. Certain Pro 222 mutant proteins prevented swimming almost completely when expressed at moderate levels in wild-type cells. This dominance might be caused by rotor-stator jamming, because it was weaker when FliG carried a mutation believed to increase rotor-stator clearance. We propose a mechanism for torque generation, in which specific functions are suggested for the proline residues of MotA and Asp32 of MotB. 相似文献
6.
Yong‐Suk Che Shuichi Nakamura Yusuke V. Morimoto Nobunori Kami‐ike Keiichi Namba Tohru Minamino 《Molecular microbiology》2014,91(1):175-184
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(D33E) mutation dramatically alters motor performance in response to changes in external load. Rotation speeds of the MotA/B(D33E) and MotA(V35F)/B(D33E) 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 D33E 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. 相似文献
7.
Function of Protonatable Residues in the Flagellar Motor of Escherichia coli: a Critical Role for Asp 32 of MotB 总被引:3,自引:0,他引:3
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Jiadong Zhou Leslie L. Sharp H. Lucy Tang Scott A. Lloyd Stephanie Billings Timothy F. Braun David F. Blair 《Journal of bacteriology》1998,180(10):2729-2735
Rotation of the bacterial flagellar motor is powered by a transmembrane gradient of protons or, in some species, sodium ions. The molecular mechanism of coupling between ion flow and motor rotation is not understood. The proteins most closely involved in motor rotation are MotA, MotB, and FliG. MotA and MotB are transmembrane proteins that function in transmembrane proton conduction and that are believed to form the stator. FliG is a soluble protein located on the cytoplasmic face of the rotor. Two other proteins, FliM and FliN, are known to bind to FliG and have also been suggested to be involved to some extent in torque generation. Proton (or sodium)-binding sites in the motor are likely to be important to its function and might be formed from the side chains of acidic residues. To investigate the role of acidic residues in the function of the flagellar motor, we mutated each of the conserved acidic residues in the five proteins that have been suggested to be involved in torque generation and measured the effects on motility. None of the conserved acidic residues of MotA, FliG, FliM, or FliN proved essential for torque generation. An acidic residue at position 32 of MotB did prove essential. Of 15 different substitutions studied at this position, only the conservative-replacement D32E mutant retained any function. Previous studies, together with additional data presented here, indicate that the proteins involved in motor rotation do not contain any conserved basic residues that are critical for motor rotation per se. We propose that Asp 32 of MotB functions as a proton-binding site in the bacterial flagellar motor and that no other conserved, protonatable residues function in this capacity. 相似文献
8.
The MotA and MotB proteins of Escherichia coli serve two functions. The MotA4MotB2 complex attaches to the cell wall via MotB to form the stator of the flagellar motor. The complex also couples the flow of hydrogen ions across the cell membrane to movement of the rotor. The TM3 and TM4 transmembrane helices of MotA and the single TM of MotB comprise the proton channel, which is inactive until the complex assembles into a motor. Here, we identify a segment of the MotB protein that acts as a plug to prevent premature proton flow. The plug is in the periplasm just C-terminal to the MotB TM. It consists of an amphipathic alpha helix flanked by Pro52 and Pro65. When MotA is over-expressed with MotB deleted for residues 51-70, a massive influx of protons acidifies the cytoplasm without significantly depleting the proton motive force. Either that acidification or some sequela thereof, such as potassium or water efflux from the cells, inhibits growth. The Pro residues and Ile58, Tyr61, and Phe62 are essential for plug function. Cys-substituted MotB proteins form a disulfide bond between the two plugs that hold the channels open, and the plugs function intrans within the MotA4MotB2 complex. We present a model in which the MotA4MotB2 complex forms in the bulk membrane. Before association with a motor, we propose the plugs insert into the cell membrane parallel with its periplasmic face and interfere with channel formation. When a complex incorporates into a motor, the plugs leave the membrane and associate with each other via their hydrophobic faces to hold the proton channel open. 相似文献
9.
The bacterial flagellar motor is a tiny molecular machine that uses a transmembrane flux of H(+) or Na(+) ions to drive flagellar rotation. In proton-driven motors, the membrane proteins MotA and MotB interact via their transmembrane regions to form a proton channel. The sodium-driven motors that power the polar flagellum of Vibrio species contain homologs of MotA and MotB, called PomA and PomB. They require the unique proteins MotX and MotY. In this study, we investigated how ion selectivity is determined in proton and sodium motors. We found that Escherichia coli MotA/B restore motility in DeltapomAB Vibrio alginolyticus. Most hypermotile segregants isolated from this weakly motile strain contain mutations in motB. We constructed proteins in which segments of MotB were fused to complementary portions of PomB. A chimera joining the N terminus of PomB to the periplasmic C terminus of MotB (PotB7(E)) functioned with PomA as the stator of a sodium motor, with or without MotX/Y. This stator (PomA/PotB7(E)) supported sodium-driven motility in motA or motB E.coli cells, and the swimming speed was even higher than with the original stator of E.coli MotA/B. We conclude that the cytoplasmic and transmembrane domains of PomA/B are sufficient for sodium-driven motility. However, MotA expressed with a B subunit containing the N terminus of MotB fused to the periplasmic domain of PomB (MomB7(E)) supported sodium-driven motility in a MotX/Y-dependent fashion. Thus, although the periplasmic domain of PomB is not necessary for sodium-driven motility in a PomA/B motor, it can convert a MotA/B proton motor into a sodium motor. 相似文献
10.
Molecular modeling of the bacterial outer membrane receptor energizer,ExbBD/TonB,based on homology with the flagellar motor,MotAB 总被引:6,自引:0,他引:6
The MotA/MotB proteins serve as the motor that drives bacterial flagellar rotation in response to the proton motive force (pmf). They have been shown to comprise a transmembrane proton pathway. The ExbB/ExbD/TonB protein complex serves to energize transport of iron siderophores and vitamin B12 across the outer membrane of the Gram-negative bacterial cell using the pmf. These two protein complexes have the same topology and are homologous. Based on molecular data for the MotA/MotB proteins, we propose simple three-dimensional channel structures for both MotA/MotB and ExbB/ExbD/TonB using modeling methods. Features of the derived channels are discussed, and two possible proton transfer pathways for the ExbBD/TonB system are proposed. These analyses provide a guide for molecular studies aimed at elucidating the mechanism by which chemiosmotic energy can be transferred either between two adjacent membranes to energize outer membrane transport or to the bacterial flagellum to generate torque. 相似文献
11.
MotA and MotB are integral membrane proteins that form the stator complex of the proton-driven bacterial flagellar motor. The stator complex functions as a proton channel and couples proton flow with torque generation. The stator must be anchored to an appropriate place on the motor, and this is believed to occur through a putative peptidoglycan-binding (PGB) motif within the C-terminal periplasmic domain of MotB. In this study, we constructed and characterized an N-terminally truncated variant of Salmonella enterica serovar Typhimurium MotB consisting of residues 78 through 309 (MotB(C)). MotB(C) significantly inhibited the motility of wild-type cells when exported into the periplasm. Some point mutations in the PGB motif enhanced the motility inhibition, while an in-frame deletion variant, MotB(C)(Delta197-210), showed a significantly reduced inhibitory effect. Wild-type MotB(C) and its point mutant variants formed a stable homodimer, while the deletion variant was monomeric. A small amount of MotB was coisolated only with the secreted form of MotB(C)-His(6) by Ni-nitrilotriacetic acid affinity chromatography, suggesting that the motility inhibition results from MotB-MotB(C) heterodimer formation in the periplasm. However, the monomeric mutant variant MotB(C)(Delta197-210) did not bind to MotB, suggesting that MotB(C) is directly involved in stator assembly. We propose that the MotB(C) dimer domain plays an important role in targeting and stable anchoring of the MotA/MotB complex to putative stator-binding sites of the motor. 相似文献
12.
Yu Feng Zhai 《生物化学与生物物理学报:生物膜》2003,1614(2):201-210
The MotA/MotB proteins serve as the motor that drives bacterial flagellar rotation in response to the proton motive force (pmf). They have been shown to comprise a transmembrane proton pathway. The ExbB/ExbD/TonB protein complex serves to energize transport of iron siderophores and vitamin B12 across the outer membrane of the Gram-negative bacterial cell using the pmf. These two protein complexes have the same topology and are homologous. Based on molecular data for the MotA/MotB proteins, we propose simple three-dimensional channel structures for both MotA/MotB and ExbB/ExbD/TonB using modeling methods. Features of the derived channels are discussed, and two possible proton transfer pathways for the ExbBD/TonB system are proposed. These analyses provide a guide for molecular studies aimed at elucidating the mechanism by which chemiosmotic energy can be transferred either between two adjacent membranes to energize outer membrane transport or to the bacterial flagellum to generate torque. 相似文献
13.
《Journal of molecular biology》1996,261(2):209-221
Among the many proteins needed for the assembly and function of bacterial flagella, only five have been suggested to be involved in torque generation. These are MotA, MotB, FliG, FliM and FliN. In this study, we have probed binding interactions among these proteins, by using protein fusions to glutathioneS-transferase or to oligo-histidine, in conjunction with co-isolation assays. The results show that FliG, FliM and FliN all bind to each other, and that each also self-associates. MotA and MotB also bind to each other, and MotA interacts, but only weakly, with FliG and FliM. Taken together with previous genetic, physiological and ultrastructural studies, these results provide strong support for the view that FliG, FliM and FliN function together in a complex on the rotor of the flagellar motor, whereas MotA and MotB form a distinct complex that functions as the stator. Torque generation in the flagellar motor is thus likely to involve interactions between these two protein complexes. 相似文献
14.
Flagellar movement driven by proton translocation 总被引:13,自引:0,他引:13
Blair DF 《FEBS letters》2003,545(1):86-95
The bacterial flagellar motor couples ion flow to rotary motion at high speed and with apparently fixed stoichiometry. The functional properties of the motor are quite well understood, but its molecular mechanism remains unknown. Recent studies of motor physiology, coupled with mutational and biochemical studies of the components, put significant constraints on the mechanism. Rotation is probably driven by conformational changes in membrane-protein complexes that form the stator. These conformational changes occur as protons move on and off a critical Asp residue in the stator protein MotB, and the resulting forces are applied to the rotor protein FliG. 相似文献
15.
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(C) ), which has been shown to abolish the interaction with FliG, significantly affected stator assembly, suggesting that the electrostatic interaction of MotA(C) with FliG is required for the efficient assembly of the stators around the rotor. 相似文献
16.
MotA and MotB form the proton-channel complex of the proton-driven bacterial flagellar motor. A plug segment of Escherichia coli MotB suppresses proton leakage through the MotA/B complex when it is not assembled into the motor. Using a ratiometric pH indicator protein, pHluorin, we show that the proton-conductivity of a Salmonella MotA/B complex not incorporated into the motor is two orders of magnitude lower than that of a complex that is incorporated and activated. This leakage is, however, significant enough to change the cytoplasmic pH to a level at which the chemotaxis signal transduction system responds. 相似文献
17.
Che YS Nakamura S Kojima S Kami-ike N Namba K Minamino T 《Journal of bacteriology》2008,190(20):6660-6667
MotA and MotB form the stator of the proton-driven bacterial flagellar motor, which conducts protons and couples proton flow with motor rotation. Asp-33 of Salmonella enterica serovar Typhimurium MotB, which is a putative proton-binding site, is critical for torque generation. However, the mechanism of energy coupling remains unknown. Here, we carried out genetic and motility analysis of a slowly motile motB(D33E) mutant and its pseudorevertants. We first confirmed that the poor motility of the motB(D33E) mutant is due to neither protein instability, mislocalization, nor impaired interaction with MotA. We isolated 17 pseudorevertants and identified the suppressor mutations in the transmembrane helices TM2 and TM3 of MotA and in TM and the periplasmic domain of MotB. The stall torque produced by the motB(D33E) mutant motor was about half of the wild-type level, while those for the pseudorevertants were recovered nearly to the wild-type levels. However, the high-speed rotations of the motors under low-load conditions were still significantly impaired, suggesting that the rate of proton translocation is still severely limited at high speed. These results suggest that the second-site mutations recover a torque generation step involving stator-rotor interactions coupled with protonation/deprotonation of Glu-33 but not maximum proton conductivity. 相似文献
18.
Yonekura K Yakushi T Atsumi T Maki-Yonekura S Homma M Namba K 《Journal of molecular biology》2006,357(1):73-81
A motor protein complex of the bacterial flagellum, PomA/B from Vibrio alginolyticus, was reconstituted into liposomes and visualized by electron cryomicroscopy. PomA/B is a sodium channel, composed of two membrane proteins, PomA and PomB, and converts ion flux to the rotation of the flagellar motor. Escherichia coli and Salmonella have a homolog called MotA/B, which utilizes proton instead of sodium ion. PomB and MotB have a peptidoglycan-binding motif in their C-terminal region, and therefore PomA/B and MotA/B are regarded as the stator. Energy filtering electron cryomicroscopy enhanced the image contrast of the proteins reconstituted into liposomes and showed that two extramembrane domains with clearly different sizes stick out of the lipid bilayers on opposite sides. Image analysis combined with gold labeling and deletion of the peptidoglycan-binding motif revealed that the longer one, approximately 70 A long, is likely to correspond to the periplasmic domain, and the other, about half size, to the cytoplasmic domain. 相似文献
19.
Requirements for conversion of the Na(+)-driven flagellar motor of Vibrio cholerae to the H(+)-driven motor of Escherichia coli
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Bacterial flagella are powered by a motor that converts a transmembrane electrochemical potential of either H(+) or Na(+) into mechanical work. In Escherichia coli, the MotA and MotB proteins form the stator and function in proton translocation, whereas the FliG protein is located on the rotor and is involved in flagellar assembly and torque generation. The sodium-driven polar flagella of Vibrio species contain homologs of MotA and MotB, called PomA and PomB, and also contain two other membrane proteins called MotX and MotY, which are essential for motor rotation and that might also function in ion conduction. Deletions in pomA, pomB, motX, or motY in Vibrio cholerae resulted in a nonmotile phenotype, whereas deletion of fliG gave a nonflagellate phenotype. fliG genes on plasmids complemented fliG-null strains of the parent species but not fliG-null strains of the other species. FliG-null strains were complemented by chimeric FliG proteins in which the C-terminal domain came from the other species, however, implying that the C-terminal part of FliG can function in conjunction with the ion-translocating components of either species. A V. cholerae strain deleted of pomA, pomB, motX, and motY became weakly motile when the E. coli motA and motB genes were introduced on a plasmid. Like E. coli, but unlike wild-type V. cholerae, motility of some V. cholerae strains containing the hybrid motor was inhibited by the protonophore carbonyl cyanide m-chlorophenylhydrazone under neutral as well as alkaline conditions but not by the sodium motor-specific inhibitor phenamil. We conclude that the E. coli proton motor components MotA and MotB can function in place of the motor proteins of V. cholerae and that the hybrid motors are driven by the proton motive force. 相似文献
20.
The role of a cytoplasmic loop of MotA in load‐dependent assembly and disassembly dynamics of the MotA/B stator complex in the bacterial flagellar motor
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Seyedeh Noorolhoda Shajari Pourjaberi Naoya Terahara Keiichi Namba Tohru Minamino 《Molecular microbiology》2017,106(4):646-658
The proton‐driven flagellar motor of Salmonella enterica can accommodate a dozen MotA/B stators in a load‐dependent manner. The C‐terminal periplasmic domain of MotB acts as a structural switch to regulate the number of active stators in the motor in response to load change. The cytoplasmic loop termed MotAC is responsible for the interaction with a rotor protein, FliG. Here, to test if MotAC is responsible for stator assembly around the rotor in a load‐dependent manner, we analyzed the effect of MotAC mutations, M76V, L78W, Y83C, Y83H, I126F, R131L, A145E and E155K, on motor performance over a wide range of external load. All these MotAC mutations reduced the maximum speed of the motor near zero load, suggesting that they reduce the rate of conformational dynamics of MotAC coupled with proton translocation through the MotA/B proton channel. Dissociation of the stators from the rotor by decrease in the load was facilitated by the M76V, Y83H and A145E mutations compared to the wild‐type motor. The E155K mutation reduced the number of active stators in the motor from 10 to 6 under extremely high load. We propose that MotAC is responsible for load‐dependent assembly and disassembly dynamics of the MotA/B stator units. 相似文献