Conformational change in the stator of the bacterial flagellar motor

Colicin biology

Colicins are proteins produced by and toxic for some strains of Escherichia coli. They are produced by strains of E. coli carrying a colicinogenic plasmid that bears the genetic determinants for colicin synthesis, immunity, and release. Insights gained into each fundamental aspect of their biology are presented: their synthesis, which is under SOS regulation; their release into the extracellular medium, which involves the colicin lysis protein; and their uptake mechanisms and modes of action. Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The structures of some colicins are known at the atomic level and are discussed. Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or the TonB system. The components of each system are known, and their implication in the functioning of the system is described. Colicins then reach their lethal target and act either by forming a voltage-dependent channel into the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA. The mechanisms of inhibition by specific and cognate immunity proteins are presented. Finally, the use of colicins as laboratory or biotechnological tools and their mode of evolution are discussed.

Mutational Analyses Define Helix Organization and Key Residues of a Bacterial Membrane Energy-transducing Complex

In Gram-negative bacteria, many biological processes are coupled to inner membrane ion gradients. Ions transit at the interface of helices of integral membrane proteins, generating mechanical energy to drive energetic processes. To better understand how ions transit through these channels, we used a model system involved in two different processes, one of which depends on inner membrane energy. The Tol machinery of the Escherichia coli cell envelope is dedicated to maintaining outer membrane stability, a process driven by the proton-motive force. The Tol system is parasitized by bacterial toxins called colicins, which are imported through the outer membrane using an energy-independent process. Herein, we mutated TolQ and TolR transmembrane residues, and we analyzed the mutants for outer membrane stability, colicin import and protein complex formation. We identified residues involved in the assembly of the complex, and a new class of discriminative mutations that conferred outer membrane destabilization identical to a tol deletion mutant, but which remained fully sensitive to colicins. Further genetic approaches revealed transmembrane helix interactions and organization in the bilayer, and suggested that most of the discriminative residues are located in a putative aqueous ion channel. We discuss a model for the function of related bacterial molecular motors.

TonB-dependent energy transduction between outer and cytoplasmic membranes

The TonB system of Escherichia coli (and most other Gram-negative bacteria) is distinguished by its importance to iron acquisition, its contribution to bacterial pathogenesis, and a unique and mysterious mechanism of action. This system somehow gathers the potential energy of the cytoplasmic membrane (CM) proton gradient and delivers it to active transporters in the outer membrane (OM). Our understanding of this system is confounded by the challenge of reconciling often contradictory in vivo and in vitro studies that are presented in this review.

The Escherichia coli MotAB Proton Channel Unplugged

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.

Mutational analysis of the flagellar protein FliG: sites of interaction with FliM and implications for organization of the switch complex

The switch complex at the base of the bacterial flagellum is essential for flagellar assembly, rotation, and switching. In Escherichia coli and Salmonella, the complex contains about 26 copies of FliG, 34 copies of FliM, and more then 100 copies of FliN, together forming the basal body C ring. FliG is involved most directly in motor rotation and is located in the upper (membrane-proximal) part of the C ring. A crystal structure of the middle and C-terminal parts of FliG shows two globular domains connected by an alpha-helix and a short extended segment. The middle domain of FliG has a conserved surface patch formed by the residues EHPQ(125-128) and R(160) (the EHPQR motif), and the C-terminal domain has a conserved surface hydrophobic patch. To examine the functional importance of these and other surface features of FliG, we made mutations in residues distributed over the protein surface and measured the effects on flagellar assembly and function. Mutations preventing flagellar assembly occurred mainly in the vicinity of the EHPQR motif and the hydrophobic patch. Mutations causing aberrant clockwise or counterclockwise motor bias occurred in these same regions and in the waist between the upper and lower parts of the C-terminal domain. Pull-down assays with glutathione S-transferase-FliM showed that FliG interacts with FliM through both the EHPQR motif and the hydrophobic patch. We propose a model for the organization of FliG and FliM subunits that accounts for the FliG-FliM interactions identified here and for the different copy numbers of FliG and FliM in the flagellum.

Roles of charged residues of rotor and stator in flagellar rotation: comparative study using H+-driven and Na+-driven motors in Escherichia coli

In Escherichia coli, rotation of the flagellar motor has been shown to depend upon electrostatic interactions between charged residues of the stator protein MotA and the rotor protein FliG. These charged residues are conserved in the Na+-driven polar flagellum of Vibrio alginolyticus, but mutational studies in V. alginolyticus suggested that they are relatively unimportant for motor rotation. The electrostatic interactions detected in E. coli therefore might not be a general feature of flagellar motors, or, alternatively, the V. alginolyticus motor might rely on similar interactions but incorporate additional features that make it more robust against mutation. Here, we have carried out a comparative study of chimeric motors that were resident in E. coli but engineered to use V. alginolyticus stator components, rotor components, or both. Charged residues in the V. alginolyticus rotor and stator proteins were found to be essential for motor rotation when the proteins functioned in the setting of the E. coli motor. Patterns of synergism and suppression in rotor/stator double mutants indicate that the V. alginolyticus proteins interact in essentially the same way as their counterparts in E. coli. The robustness of the rotor-stator interface in V. alginolyticus is in part due to the presence of additional charged residues in PomA but appears mainly due to other factors, because an E. coli motor using both rotor and stator components from V. alginolyticus remained sensitive to mutation. Motor function in V. alginolyticus may be enhanced by the proteins MotX and MotY.

Torque-speed relationship of the bacterial flagellar motor

Many swimming bacteria are propelled by flagellar filaments driven by a rotary motor. Each of these tiny motors can generate an impressive torque. The motor torque vs. speed relationship is considered one of the most important measurable characteristics of the motor and therefore is a major criterion for judging models proposed for the working mechanism. Here we give an explicit explanation for this torque-speed curve. The same physics also can explain certain puzzling properties of other motors.

Assembly of motor proteins, PomA and PomB, in the Na+-driven stator of the flagellar motor

PomA and PomB are transmembrane proteins that form the stator complex in the sodium-driven flagellar motor of Vibrio alginolyticus and are believed to surround the rotor part of the flagellar motor. We constructed and observed green fluorescent protein (GFP) fusions of the stator proteins PomA and PomB in living cells to clarify how stator proteins are assembled and installed into the flagellar motor. We were able to demonstrate that GFP-PomA and GFP-PomB localized to a cell pole dependent on the presence of the polar flagellum. Localization of the GFP-fused stator proteins required their partner subunit, PomA or PomB, and the C-terminal domain of PomB, which has a peptidoglycan-binding motif. Each of the GFP-fused stator proteins was co-isolated with its partner subunit from detergent-solubilized membrane. From these lines of evidence, we have demonstrated that the stator proteins are incorporated into the flagellar motor as a PomA/PomB complex and are fixed to the cell wall via the C-terminal domain of PomB.

FliG subunit arrangement in the flagellar rotor probed by targeted cross-linking

FliG is a component of the switch complex on the rotor of the bacterial flagellum. Each flagellar motor contains about 25 FliG molecules. The protein of Escherichia coli has 331 amino acid residues and comprises at least two discrete domains. A C-terminal domain of about 100 residues functions in rotation and includes charged residues that interact with the stator protein MotA. Other parts of the FliG protein are essential for flagellar assembly and interact with the MS ring protein FliF and the switch complex protein FliM. The crystal structure of the middle and C-terminal parts of FliG shows two globular domains joined by an alpha-helix and a short extended segment that contains two well-conserved glycine residues. Here, we describe targeted cross-linking studies of FliG that reveal features of its organization in the flagellum. Cys residues were introduced at various positions, singly or in pairs, and cross-linking by a maleimide or disulfide-inducing oxidant was examined. FliG molecules with pairs of Cys residues at certain positions in the middle domain formed disulfide-linked dimers and larger multimers with a high yield, showing that the middle domains of adjacent subunits are in fairly close proximity and putting constraints on the relative orientation of the domains. Certain proteins with single Cys replacements in the C-terminal domain formed dimers with moderate yields but not larger multimers. On the basis of the cross-linking results and the data available from mutational and electron microscopic studies, we propose a model for the organization of FliG subunits in the flagellum.

A chimeric N-terminal Escherichia coli--C-terminal Rhodobacter sphaeroides FliG rotor protein supports bidirectional E. coli flagellar rotation and chemotaxis

Flagellate bacteria such as Escherichia coli and Salmonella enterica serovar Typhimurium typically express 5 to 12 flagellar filaments over their cell surface that rotate in clockwise (CW) and counterclockwise directions. These bacteria modulate their swimming direction towards favorable environments by biasing the direction of flagellar rotation in response to various stimuli. In contrast, Rhodobacter sphaeroides expresses a single subpolar flagellum that rotates only CW and responds tactically by a series of biased stops and starts. Rotor protein FliG transiently links the MotAB stators to the rotor, to power rotation and also has an essential function in flagellar export. In this study, we sought to determine whether the FliG protein confers directionality on flagellar motors by testing the functional properties of R. sphaeroides FliG and a chimeric FliG protein, EcRsFliG (N-terminal and central domains of E. coli FliG fused to an R. sphaeroides FliG C terminus), in an E. coli FliG null background. The EcRsFliG chimera supported flagellar synthesis and bidirectional rotation; bacteria swam and tumbled in a manner qualitatively similar to that of the wild type and showed chemotaxis to amino acids. Thus, the FliG C terminus alone does not confer the unidirectional stop-start character of the R. sphaeroides flagellar motor, and its conformation continues to support tactic, switch-protein interactions in a bidirectional motor, despite its evolutionary history in a bacterium with a unidirectional motor.

Deletion analysis of the carboxyl-terminal region of the PomB component of the vibrio alginolyticus polar flagellar motor

The stator of the sodium-driven flagellar motor of Vibrio alginolyticus is a membrane protein complex composed of four PomA and two PomB subunits. PomB has a peptidoglycan-binding motif in the C-terminal region. In this study, four kinds of PomB deletions in the C terminus were constructed. None of the deletion proteins restored motility of the DeltapomB strain. The PomA protein was coisolated with all of the PomB derivatives under detergent-solubilized conditions. Homotypic disulfide cross-linking of all of the deletion derivatives through naturally occurring Cys residues was detected. We conclude that the C-terminal region of PomB is essential for motor function but not for oligomerization of PomB with itself or PomA.

Evidence for two flagellar stators and their role in the motility of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a ubiquitous bacterium capable of twitching, swimming, and swarming motility. In this study, we present evidence that P. aeruginosa has two flagellar stators, conserved in all pseudomonads as well as some other gram-negative bacteria. Either stator is sufficient for swimming, but both are necessary for swarming motility under most of the conditions tested, suggesting that these two stators may have different roles in these two types of motility.

Concerted effects of amino acid substitutions in conserved charged residues and other residues in the cytoplasmic domain of PomA, a stator component of Na+-driven flagella

PomA is a membrane protein that is one of the essential components of the sodium-driven flagellar motor in Vibrio species. The cytoplasmic charged residues of Escherichia coli MotA, which is a PomA homolog, are believed to be required for the interaction of MotA with the C-terminal region of FliG. It was previously shown that a PomA variant with neutral substitutions in the conserved charged residues (R88A, K89A, E96Q, E97Q, and E99Q; AAQQQ) was functional. In the present study, five other conserved charged residues were replaced with neutral amino acids in the AAQQQ PomA protein. These additional substitutions did not affect the function of PomA. However, strains expressing the AAQQQ PomA variant with either an L131F or a T132M substitution, neither of which affected motor function alone, exhibited a temperature-sensitive (TS) motility phenotype. The double substitutions R88A or E96Q together with L131F were sufficient for the TS phenotype. The motility of the PomA TS mutants immediately ceased upon a temperature shift from 20 to 42 degrees C and was restored to the original level approximately 10 min after the temperature was returned to 20 degrees C. It is believed that PomA forms a channel complex with PomB. The complex formation of TS PomA and PomB did not seem to be affected by temperature. Suppressor mutations of the TS phenotype were mapped in the cytoplasmic boundaries of the transmembrane segments of PomA. We suggest that the cytoplasmic surface of PomA is changed by the amino acid substitutions and that the interaction of this surface with the FliG C-terminal region is temperature sensitive.

Interaction of PomB with the third transmembrane segment of PomA in the Na+-driven polar flagellum of Vibrio alginolyticus

The marine bacterium Vibrio alginolyticus has four motor components, PomA, PomB, MotX, and MotY, responsible for its Na(+)-driven flagellar rotation. PomA and PomB are integral inner membrane proteins having four and one transmembrane segments (TMs), respectively, which are thought to form an ion channel complex. First, site-directed Cys mutagenesis was systematically performed from Asp-24 to Glu-41 of PomB, and the resulting mutant proteins were examined for susceptibility to a sulfhydryl reagent. Secondly, the Cys substitutions at the periplasmic boundaries of the PomB TM (Ser-38) and PomA TMs (Gly-23, Ser-34, Asp-170, and Ala-178) were combined. Cross-linked products were detected for the combination of PomB-S38C and PomA-D170C mutant proteins. The Cys substitutions in the periplasmic boundaries of PomA TM3 (from Met-169 to Asp-171) and the PomB TM (from Leu-37 to Ser-40) were combined to construct a series of double mutants. Most double mutations reduced the motility, whereas each single Cys substitution slightly affected it. Although the motility of the strain carrying PomA-D170C and PomB-S38C was significantly inhibited, it was recovered by reducing reagent. The strain with this combination showed a lower affinity for Na(+) than the wild-type combination. PomA-D148C and PomB-P16C, which are located at the cytoplasmic boundaries of PomA TM3 and the PomB TM, also formed the cross-linked product. From these lines of evidence, we infer that TM3 of PomA and the TM of PomB are in close proximity over their entire length and that cooperation between these two TMs is required for coupling of Na(+) conduction to flagellar rotation.

Point mutations in transmembrane helices 2 and 3 of ExbB and TolQ affect their activities in Escherichia coli K-12

Replacement of glutamate 176, the only charged amino acid in the third transmembrane helix of ExbB, with alanine (E176A) abolished ExbB activity in all determined ExbB-dependent functions of Escherichia coli. Combination of the mutations T148A in the second transmembrane helix and T181A in the third transmembrane helix, proposed to form part of a proton pathway through ExbB, also resulted in inactive ExbB. E176 and T148 are strictly conserved in ExbB and TolQ proteins, and T181 is almost strictly conserved in ExbB, TolQ, and MotA.

Isolation of Vibrio alginolyticus sodium-driven flagellar motor complex composed of PomA and PomB solubilized by sucrose monocaprate

The polar flagella ofVibrio alginolyticushave sodium-driven motors, and four membrane proteins, PomA, PomB, MotX and MotY, are essential for torque generation of the motor. PomA and PomB are believed to form a sodium-conducting channel. This paper reports the purification of the motor complex by using sucrose monocaprate, a non-ionic detergent, to solubilize the complex. Plasmid pKJ301, which encodes intact PomA, and PomB tagged with a C-terminal hexahistidine that does not interfere with PomB function, was constructed. The membrane fraction of cells transformed with pKJ301 was solubilized with sucrose monocaprate, and the solubilized materials were applied to a Ni-NTA column. The imidazole eluate contained both PomA and PomB, which were further purified by anion-exchange chromatography. Gel-filtration chromatography was used to investigate the apparent molecular size of the complex; the PomA/PomB complex was eluted as approx. 900 kDa and PomB alone was eluted as approx. 260 kDa. These findings suggest that the motor complex may have a larger structure than previously assumed.

How bacteria assemble flagella

The bacterial flagellum is both a motor organelle and a protein export/assembly apparatus. It extends from the cytoplasm to the cell exterior. All the protein subunits of the external elements have to be exported. Export employs a type III pathway, also utilized for secretion of virulence factors. Six of the components of the export apparatus are integral membrane proteins and are believed to be located within the flagellar basal body. Three others are soluble: the ATPase that drives export, a regulator of the ATPase, and a general chaperone. Exported substrates diffuse down a narrow channel in the growing structure and assemble at the distal end, often with the help of a capping structure.

The bacterial flagellar motor: structure and function of a complex molecular machine

The bacterial flagellar motor harnesses ion flow to drive rotary motion, at speeds reaching 100000 rpm and with apparently tight coupling. The functional properties of the motor are quite well understood, but its molecular mechanism remains unknown. Studies of motor physiology, together with mutational and biochemical studies of the components, place 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 aspartate residue in the stator protein MotB, and the resulting forces are applied to the rotor protein FliG. The bacterial flagellum is a complex structure built from about two dozen proteins. Its construction requires an apparatus at the base that exports many flagellar components to their sites of installation by way of an axial channel through the structure. The sequence of events in assembly is understood in general terms, but not yet at the molecular level. A fuller understanding of motor rotation and flagellar assembly will require more data on the structures and organization of the constituent proteins.

The rotary motor of bacterial flagella

Flagellated bacteria, such as Escherichia coli, swim by rotating thin helical filaments, each driven at its base by a reversible rotary motor, powered by an ion flux. A motor is about 45 nm in diameter and is assembled from about 20 different kinds of parts. It develops maximum torque at stall but can spin several hundred Hz. Its direction of rotation is controlled by a sensory system that enables cells to accumulate in regions deemed more favorable. We know a great deal about motor structure, genetics, assembly, and function, but we do not really understand how it works. We need more crystal structures. All of this is reviewed, but the emphasis is on function.

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

The Conserved Charged Residues of the C-terminal Region of FliG, a Rotor Component of the Na+-driven Flagellar Motor

FliG is an essential component of the flagellar motor and functions in flagellar assembly, torque generation and regulation of the direction of flagellar rotation. The five charged residues important for the rotation of the flagellar motor were identified in Escherichiacoli FliG (FliG(E)). These residues are clustered in the C terminus and are all conserved in FliG(V) of the Na(+)-driven motor of Vibrioalginolyticus (Lys284, Arg301, Asp308, Asp309 and Arg317). To investigate the roles of these charged residues in the Na(+)-driven motor, we cloned the VibriofliG gene and introduced single or multiple substitutions into the corresponding positions in FliG(V). FliG(V) with double Ala replacements in all possible combinations at these five conserved positions still retained significant motile ability, although some of the mutations completely eliminated the function of FliG(E). All of the triple mutants constructed in this study also remained motile. These results suggest that the important charged residues may be located in different places and the conserved charged residues are not so important for the Na(+)-driven flagellar motor of Vibrio. The chimeric FliG protein (FliG(VE)), composed of the N-terminal domain from V.alginolyticus and the C-terminal domain from E.coli, functions in Vibrio cells. The mutations of the charge residues of the C-terminal region in FliG(VE) affected swarming ability as in E.coli. Both the FliG(V) and the FliG(VE) proteins with the triple mutation were more susceptible to proteolysis than proteins without the mutation, suggesting that their conformations were altered.

Molecular modeling of the bacterial outer membrane receptor energizer, ExbBD/TonB, based on homology with the flagellar motor, MotAB

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.

Helix rotation model of the flagellar rotary motor

A new model of the flagellar motor is proposed that is based on established dynamics of the KcsA potassium ion channel and on known genetic, biochemical, and biophysical facts, which accounts for the mechanics of torque generation, force transmission, and reversals of motor rotation. It predicts that proton (or in some species sodium ion) flow generates short, reversible helix rotations of the MotA-MotB channel complex (the stator) that are transmitted by Coulomb forces to the FliG segments at the rotor surface. Channels are arranged as symmetric pairs, S and T, that swing back and forth in synchrony. S and T alternate in attaching to the rotor, so that force transmission proceeds in steps. The sense of motor rotation can be readily reversed by conformationally switching the position of charged groups on the rotor so that they interact with the stator during the reverse rather than forward strokes. An elastic device accounts for the observed smoothness of rotation and a prolonged attachment of the torque generators to the rotor, i.e., a high duty ratio of each torque-generating unit.

The speed of the flagellar rotary motor of Escherichia coli varies linearly with protonmotive force

A protonmotive force (pmf) across the cell's inner membrane powers the flagellar rotary motor of Escherichia coli. Speed is known to be proportional to pmf when viscous loads are heavy. Here we show that speed also is proportional to pmf when viscous loads are light. Two motors on the same bacterium were monitored as the cell was slowly deenergized. The first motor rotated the entire cell body (a heavy load), while the second motor rotated a small latex bead (a light load). The first motor rotated slowly and provided a measure of the cell's pmf. The second motor rotated rapidly and was compared with the first, to give the speed-pmf relation for light loads. Experiments were done at 24.0 degrees C and 16.2 degrees C, with initial speeds indicating operation well into the high-speed, low-torque regime. Speed was found to be proportional to pmf over the entire (accessible) dynamic range (0-270 Hz). If the passage of a fixed number of protons carries the motor through each revolution, i.e., if the motor is tightly coupled, a linear speed-pmf relation is expected close to stall, where the work done against the viscous load matches the energy dissipated in proton flow. A linear relation is expected at high speeds if proton translocation is rate-limiting and involves multiple steps, a model that also applies to simple proton channels. The present work shows that a linear relation is true more generally, providing an additional constraint on possible motor mechanisms.

Flagellar movement driven by proton translocation

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.

Ion-coupling determinants of Na+-driven and H+-driven flagellar motors

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.

Rotary protein motors

Three protein motors have been unambiguously identified as rotary engines: the bacterial flagellar motor and the two motors that constitute ATP synthase (F(0)F(1) ATPase). Of these, the bacterial flagellar motor and F(0) motors derive their energy from a transmembrane ion-motive force, whereas the F(1) motor is driven by ATP hydrolysis. Here, we review the current understanding of how these protein motors convert their energy supply into a rotary torque.

Effect of intracellular pH on rotational speed of bacterial flagellar motors

Weak acids such as acetate and benzoate, which partially collapse the transmembrane proton gradient, not only mediate pH taxis but also impair the motility of Escherichia coli and Salmonella at an external pH of 5.5. In this study, we examined in more detail the effect of weak acids on motility at various external pH values. A change of external pH over the range 5.0 to 7.8 hardly affected the swimming speed of E. coli cells in the absence of 34 mM potassium acetate. In contrast, the cells decreased their swimming speed significantly as external pH was shifted from pH 7.0 to 5.0 in the presence of 34 mM acetate. The total proton motive force of E. coli cells was not changed greatly by the presence of acetate. We measured the rotational rate of tethered E. coli cells as a function of external pH. Rotational speed decreased rapidly as the external pH was decreased, and at pH 5.0, the motor stopped completely. When the external pH was returned to 7.0, the motor restarted rotating at almost its original level, indicating that high intracellular proton (H+) concentration does not irreversibly abolish flagellar motor function. Both the swimming speeds and rotation rates of tethered cells of Salmonella also decreased considerably when the external pH was shifted from pH 7.0 to 5.5 in the presence of 20 mM benzoate. We propose that the increase in the intracellular proton concentration interferes with the release of protons from the torque-generating units, resulting in slowing or stopping of the motors.

Iron transport and signaling in Escherichia coli

Bacteria solve the iron supply problem caused by the insolubility of Fe(3+) by synthesizing iron-complexing compounds, called siderophores, and by using iron sources of their hosts, such as heme and iron bound to transferrin and lactoferrin. Escherichia coli, as an example of Gram-negative bacteria, forms sophisticated Fe(3+)-siderophore and heme transport systems across the outer membrane. The crystal structures of three outer membrane transport proteins now allow insights into energy-coupled transport mechanisms. These involve large long-range structural transitions in the transport proteins in response to substrate binding, including substrate gating. Energy is provided by the proton motive force of the cytoplasmic membrane through the activity of a protein complex that is inserted in the cytoplasmic membrane and that contacts the outer membrane transporters. Certain transport proteins also function in siderophore-mediated signaling cascades that start at the cell surface and flow to the cytoplasm to initiate transcription of genes encoding proteins for transport and siderophore biosynthesis.

MotX and MotY, specific components of the sodium-driven flagellar motor, colocalize to the outer membrane in Vibrio alginolyticus

Rotation of the sodium-driven polar flagella of Vibrio alginolyticus requires four motor proteins: PomA, PomB, MotX and MotY. MotX and MotY, which are unique components of the sodium-driven motor of Vibrio, have been believed to be localized in the inner (cytoplasmic) membrane via their N-terminal hydrophobic segments. Here we show that MotX and MotY colocalize to the outer membrane. Both proteins, when expressed together, were detected in the outer membrane fraction separated by sucrose density gradient centrifugation. As mature MotX and MotY proteins do not have N-terminal hydrophobic segments, the N-termini of the primary translation products must have signal sequences that are removed upon translocation across the inner membrane. Moreover, MotX and MotY require each other for efficient localization to the outer membrane. Based on these lines of evidence, we propose that MotX and MotY form a complex in the outer membrane. This is the first case that describes motor proteins function in the outer membrane for flagellar rotation.