The Chemoreceptor Sensory Adaptation System Produces Coordinated Reversals of the Flagellar Motors on an Escherichia coli Cell

In isotropic environments, an Escherichia coli cell exhibits coordinated rotational switching of its flagellar motors, produced by fluctuations in the intracellular concentration of phosphorylated CheY (CheY-P) emanating from chemoreceptor signaling arrays. In this study, we show that these CheY-P fluctuations arise through modifications of chemoreceptors by two sensory adaptation enzymes: the methyltransferase CheR and the methylesterase CheB. A cell containing CheR, CheB, and the serine chemoreceptor Tsr exhibited motor synchrony, whereas a cell lacking CheR and CheB or containing enzymatically inactive forms did not. Tsr variants with different combinations of methylation-mimicking Q residues at the adaptation sites also failed to show coordinated motor switching in cells lacking CheR and CheB. Cells containing CheR, CheB, and Tsr [NDND], a variant in which the adaptation site residues are not substrates for CheR or CheB modifications, also lacked motor synchrony. TsrΔNWETF, which lacks a C-terminal pentapeptide-binding site for CheR and CheB, and the ribose-galactose receptor Trg, which natively lacks this motif, failed to produce coordinated motor switching, despite the presence of CheR and CheB. However, addition of the NWETF sequence to Trg enabled Trg-NWETF to produce motor synchrony, as the sole receptor type in cells containing CheR and CheB. Finally, CheBc, the catalytic domain of CheB, supported motor coordination in combination with CheR and Tsr. These results indicate that the coordination of motor switching requires CheR/CheB-mediated changes in receptor modification state. We conclude that the opposing receptor substrate-site preferences of CheR and CheB produce spontaneous blinking of the chemoreceptor array's output activity. IMPORTANCE Under steady-state conditions with no external stimuli, an Escherichia coli cell coordinately switches the rotational direction of its flagellar motors. Here, we demonstrate that the CheR and CheB enzymes of the chemoreceptor sensory adaptation system mediate this coordination. Stochastic fluctuations in receptor adaptation states trigger changes in signal output from the receptor array, and this array blinking generates fluctuations in CheY-P concentration that coordinate directional switching of the flagellar motors. Thus, in the absence of chemoeffector gradients, the sensory adaptation system controls run-tumble swimming of the cell, its optimal foraging strategy.

Stator Dynamics Depending on Sodium Concentration in Sodium-Driven Bacterial Flagellar Motors

Bacterial flagellar motor (BFM) is a large membrane-spanning molecular rotary machine for swimming motility. Torque is generated by the interaction between the rotor and multiple stator units powered by ion-motive force (IMF). The number of bound stator units is dynamically changed in response to the external load and the IMF. However, the detailed dynamics of stator unit exchange process remains unclear. Here, we directly measured the speed changes of sodium-driven chimeric BFMs under fast perfusion of different sodium concentration conditions using computer-controlled, high-throughput microfluidic devices. We found the sodium-driven chimeric BFMs maintained constant speed over a wide range of sodium concentrations by adjusting stator units in compensation to the sodium-motive force (SMF) changes. The BFM has the maximum number of stator units and is most stable at 5 mM sodium concentration rather than higher sodium concentration. Upon rapid exchange from high to low sodium concentration, the number of functional stator units shows a rapidly excessive reduction and then resurrection that is different from predictions of simple absorption model. This may imply the existence of a metastable hidden state of the stator unit during the sudden loss of sodium ions.

Selective intracellular vaporisation of antibody-conjugated phase-change nano-droplets in vitro

While chemotherapy is a major mode of cancer therapeutics, its efficacy is limited by systemic toxicities and drug resistance. Recent advances in nanomedicine provide the opportunity to reduce systemic toxicities. However, drug resistance remains a major challenge in cancer treatment research. Here we developed a nanomedicine composed of a phase-change nano-droplet (PCND) and an anti-cancer antibody (9E5), proposing the concept of ultrasound cancer therapy with intracellular vaporisation. PCND is a liquid perfluorocarbon nanoparticle with a liquid–gas phase that is transformable upon exposure to ultrasound. 9E5 is a monoclonal antibody targeting epiregulin (EREG). We found that 9E5-conjugated PCNDs are selectively internalised into targeted cancer cells and kill the cells dynamically by ultrasound-induced intracellular vaporisation. In vitro experiments show that 9E5-conjugated PCND targets 97.8% of high-EREG-expressing cancer cells and kills 57% of those targeted upon exposure to ultrasound. Furthermore, direct observation of the intracellular vaporisation process revealed the significant morphological alterations of cells and the release of intracellular contents.

Local heating of molecular motors using single carbon nanotubes

Temperature globally affects all chemical processes and biomolecules in living cells. Elevating the temperature of an entire cell accelerates so many biomolecular reactions simultaneously that it is difficult to distinguish the various mechanisms involved. The ability to localize temperature changes to the nanometer range within a cell could provide a powerful new tool for regulating biomolecular activity at the level of individual molecules. The search for a nanoheater for biological research has prompted experiments with carbon nanotubes (CNTs), which have the highest conductivity of any known material. The adsorption of skeletal muscle myosin molecules along the length of single multi-walled CNTs (~10 μm) has allowed researchers to observe the ATP-driven sliding of fluorescently labeled actin filaments. In one study, red-laser irradiation focused on one end of a myosin-coated CNT was used to heat myosin motors locally without directly heating the surrounding water; this laser irradiation instantly accelerated the actin-filament sliding speeds from ~6 to ~12 μm/s in a reversible manner, indicating a local, real-time heating of myosin motors by approximately Δ12 K. Calculation of heat transfer using the finite element method, based on the estimated temperature along a single CNT with a diameter of 170 nm, indicated a high thermal conductivity of ~1540 Wm^-1K^-1 in solution, consistent with values measured in vacuum in earlier studies. Temperature distribution indicated by half-decrease distances was ~3660 nm along the length of the CNT and ~250 nm perpendicular to the length. These results suggest that single-CNT-based heating at the nanometer- or micrometer-range could be used to regulate various biomolecules in many areas of biological, physical, and chemical research.

Torque-induced precession of bacterial flagella

The bacterial flagellar motor is an ion-driven rotary machine in the cell envelope of bacteria. Using a gold nanoparticle as a probe, we observed the precession of flagella during rotation. Since the mechanism of flagella precession was unknown, we investigated it using a combination of full simulations, theory, and experiments. The results show that the mechanism can be well explained by fluid mechanics. The validity of our theory was confirmed by our full simulation, which was utilized to predict both the filament tilt angle and motor torque from experimental flagellar precession data. The knowledge obtained is important in understanding mechanical properties of the bacterial motor and hook.

Single carbon nanotube-based reversible regulation of biological motor activity

Because of their small size and high thermal conductivity, carbon nanotubes (CNTs) are excellent candidates for exploring heat transfer at the level of individual molecules in biological research. With a view toward examining the thermal regulation of single biomolecules, we here developed single CNTs as a new platform for observing the motile activity of myosin motors. On multiwall CNTs (diameter ∼170 nm; length ∼10 μm) coated with skeletal-muscle myosin, the ATP-driven sliding of single actin filaments was clearly observable. The normal sliding speed was ∼6 μm/s. Locally irradiating one end of the CNT with a red laser (642 nm), without directly irradiating the active myosin motors, accelerated the sliding speed to ∼12 μm/s, indicating the reversible activation of protein function on a single CNT in real time. The temperature along the CNT, which was estimated from the temperature-dependence of the sliding speed, decreased with the distance from the irradiated spot. Using these results with the finite element method, we calculated a first estimation of the thermal conductivity of multiwall CNTs in solution, as 1540 ± 260 (Wm(-1) K(-1)), which is consistent with the value estimated from the width dependency of multiwall CNTs and the length dependency of single-wall CNTs in a vacuum or air. The temporal regulation of local temperature through individual CNTs should be broadly applicable to the selective activation of various biomolecules in vitro and in vivo.

Effects of lipid composition and solution conditions on the mechanical properties of membrane vesicles

The mechanical properties of cell-sized giant unilamellar liposomes were studied by manipulating polystyrene beads encapsulated within the liposomes using double-beam laser tweezers. Mechanical forces were applied to the liposomes from within by moving the beads away from each other, which caused the liposomes to elongate. Subsequently, a tubular membrane projection was generated in the tip at either end of the liposome, or the bead moved out from the laser trap. The force required for liposome transformation reached maximum strength just before formation of the projection or the moving out of the bead. By employing this manipulation system, we investigated the effects of membrane lipid compositions and environment solutions on the mechanical properties. With increasing content of acidic phospholipids, such as phosphatidylglycerol or phosphatidic acid, a larger strength of force was required for the liposome transformation. Liposomes prepared with a synthetic dimyristoylphosphatidylcholine, which has uniform hydrocarbon chains, were transformed easily compared with liposomes prepared using natural phosphatidylcholine. Surprisingly, bovine serum albumin or fetuin (soluble proteins that do not bind to membranes) decreased liposomal membrane rigidity, whereas the same concentration of sucrose showed no particular effect. These results show that the mechanical properties of liposomes depend on their lipid composition and environment.

Temperature changes in brown adipocytes detected with a bimaterial microcantilever

Mammalian cells must produce heat to maintain body temperature and support other biological activities. Methods to measure a cell's thermogenic ability by inserting a thermometer into the cell or measuring the rate of oxygen consumption in a closed vessel can disturb its natural state. Here, we developed a noninvasive system for measuring a cell's heat production with a bimaterial microcantilever. This method is suitable for investigating the heat-generating properties of cells in their native state, because changes in cell temperature can be measured from the bending of the microcantilever, without damaging the cell and restricting its supply of dissolved oxygen. Thus, we were able to measure increases in cell temperature of <1 K in a small number of murine brown adipocytes (n = 4-7 cells) stimulated with norepinephrine, and observed a slow increase in temperature over several hours. This long-term heat production suggests that, in addition to converting fatty acids into heat energy, brown adipocytes may also adjust protein expression to raise their own temperature, to generate more heat. We expect this bimaterial microcantilever system to prove useful for determining a cell's state by measuring thermal characteristics.

Temperature dependences of torque generation and membrane voltage in the bacterial flagellar motor

In their natural habitats bacteria are frequently exposed to sudden changes in temperature that have been shown to affect their swimming. With our believed to be new methods of rapid temperature control for single-molecule microscopy, we measured here the thermal response of the Na(+)-driven chimeric motor expressed in Escherichia coli cells. Motor torque at low load (0.35 μm bead) increased linearly with temperature, twofold between 15°C and 40°C, and torque at high load (1.0 μm bead) was independent of temperature, as reported for the H(+)-driven motor. Single cell membrane voltages were measured by fluorescence imaging and these were almost constant (∼120 mV) over the same temperature range. When the motor was heated above 40°C for 1-2 min the torque at high load dropped reversibly, recovering upon cooling below 40°C. This response was repeatable over as many as 10 heating cycles. Both increases and decreases in torque showed stepwise torque changes with unitary size ∼150 pN nm, close to the torque of a single stator at room temperature (∼180 pN nm), indicating that dynamic stator dissociation occurs at high temperature, with rebinding upon cooling. Our results suggest that the temperature-dependent assembly of stators is a general feature of flagellar motors.

Micrometer-size vesicle formation triggered by UV light

Vesicle formation is a fundamental kinetic process related to the vesicle budding and endocytosis in a cell. In the vesicle formation by artificial means, transformation of lamellar lipid aggregates into spherical architectures is a key process and known to be prompted by e.g. heat, infrared irradiation, and alternating electric field induction. Here we report UV-light-driven formation of vesicles from particles consisting of crumpled phospholipid multilayer membranes involving a photoactive amphiphilic compound composed of 1,4-bis(4-phenylethynyl)benzene (BPEB) units. The particles can readily be prepared from a mixture of these components, which is casted on the glass surface followed by addition of water under ultrasonic radiation. Interestingly, upon irradiation with UV light, micrometer-size vesicles were generated from the particles. Neither infrared light irradiation nor heating prompted the vesicle formation. Taking advantage of the benefits of light, we successfully demonstrated micrometer-scale spatiotemporal control of single vesicle formation. It is also revealed that the BPEB units in the amphiphile are essential for this phenomenon.

Morphological and topological transformation of membrane vesicles

Liposomes are micro-compartments made of lipid bilayer membranes withcharacteristics quite similar to those of biological membranes. To formartificial cell-like structures, we generated liposomes that containedsubunit proteins of cytoskeletons: tubulin or actin. Spherical liposomeswere transformed into bipolar or cell-like shapes by mechanical forcesgenerated by the polymerization of encapsulated subunits of microtubules.Disk- or dumbbell-shaped liposomes were developed by the polymerizationof encapsulated actin. Dynamic processes of morphological transformationsof liposomes were visualized by high intensity dark-field lightmicroscopy.Topological changes, such as fusion and division of membrane vesicles,play an essential role in cellular activities. To investigate themechanism of these processes, we visualized in real time the liposomesundergoing topological transformation. A variety of novel topologicaltransformations were found, including the opening-up of liposomes and thedirect expulsion of inner vesicles.

Velocity-dependent actomyosin ATPase cycle revealed by in vitro motility assay with kinetic analysis

The actomyosin interaction plays a key role in a number of cellular functions. Single-molecule measurement techniques have been developed to study the mechanism of the actomyosin contractile system. However, the behavior of isolated single molecules does not always reflect that of molecules in a complex system such as a muscle fiber. Here, we developed a simple method for studying the kinetic parameters of the actomyosin interaction using small numbers of molecules. This approach does not require the specialized equipment needed for single-molecule measurements, and permits us to observe behavior that is more similar to that of a complex system. Using an in vitro motility assay, we examined the duration of continuous sliding of actin filaments on a sparsely distributed heavy meromyosin-coated surface. To estimate the association rate constant of the actomyosin motile system, we compared the distribution of experimentally obtained duration times with a computationally simulated distribution. We found that the association rate constant depends on the sliding velocity of the actin filaments. This technique may be used to reveal new aspects of the kinetics of various motor proteins in complex systems.

Coordinated regulation of multiple flagellar motors by the Escherichia coli chemotaxis system

Escherichia coli cells swim toward a favorable environment by chemotaxis. The chemotaxis system regulates the swimming behavior of the bacteria by controlling the rotational direction of their flagellar motors. Extracellular stimuli sensed by chemoreceptors are transduced to an intracellular signal molecule, phosphorylated CheY (CheY-P), that switches the rotational direction of the flagellar motors from counterclockwise (CCW) to clockwise (CW) or from CW to CCW. Many studies have focused on identifying the proteins involved in the chemotaxis system, and findings on the structures and intracellular localizations of these proteins have largely elucidated the molecular pathway. On the other hand, quantitative evaluations of the chemotaxis system, including the process of intracellular signaling by the propagation of CheY-P and the rotational switching of flagellar motor by binding of CheY-P molecules, are still uncertain. For instance, scientific consensus has held that the flagellar motors of an E. coli cell switch rotational direction asynchronously. However, recent work shows that the rotational switching of any two different motors on a single E. coli cell is highly coordinated; a sub-second switching delay between motors is clearly correlated with the relative distance of each motor from the chemoreceptor patch located at one pole of the cell. In this review of previous studies and our recent findings, we discuss the regulatory mechanism of the multiple flagellar motors on an individual E. coli cell and the intracellular signaling process that can be inferred from this coordinated switching.

Coordinated reversal of flagellar motors on a single Escherichia coli cell

An Escherichia coli cell transduces extracellular stimuli sensed by chemoreceptors to the state of an intracellular signal molecule, which regulates the switching of the rotational direction of the flagellar motors from counterclockwise (CCW) to clockwise (CW) and from CW back to CCW. Here, we performed high-speed imaging of flagellar motor rotation and show that the switching of two different motors on a cell is controlled coordinatedly by an intracellular signal protein, phosphorylated CheY (CheY-P). The switching is highly coordinated with a subsecond delay between motors in clear correlation with the distance of each motor from the chemoreceptor patch localized at a cell pole, which would be explained by the diffusive motion of CheY-P molecules in the cell. The coordinated switching becomes disordered by the expression of a constitutively active CheY mutant that mimics the CW-rotation stimulating function. The coordinated switching requires CheZ, which is the phosphatase for CheY-P. Our results suggest that a transient increase and decrease in the concentration of CheY-P caused by a spontaneous burst of its production by the chemoreceptor patch followed by its dephosphorylation by CheZ, which is probably a wavelike propagation in a subsecond timescale, triggers and regulates the coordinated switching of flagellar motors.

Single-molecule measurements using microneedles

Myosin is both an enzyme and a molecular motor that hydrolyzes ATP and interacts with actin filaments for force generation. Manipulation techniques with microneedles and laser traps have recently been developed to capture and manipulate the actomyosin interaction for the purpose of revealing the mechanics of this system. Combined with single-molecule imaging techniques, the coupling between chemical processes (ATP hydrolysis) and mechanical processes (myosin force generation) has been directly determined. In this chapter, we describe these two manipulation techniques, especially microneedle method, in detail.

Verification of Single-Molecule Imaging and Single-Molecule Measurements

Single-molecule imaging and single-molecule measurements constitute an integral part of life science researches. The research method, which allows direct measurements of individual motions and events of biomolecules, has contributed much to the development of life science research by providing us with numerous findings. It is left up to the judgment of researchers, however, whether the measurements really represent single-molecule events, which make it harder for more researchers to enter into this field. This paper deals with how single-molecule measurements were presented in past research outcomes and illustrates statistical-mechanical methods as well.

Visualization of Functional Rotor Proteins of the Bacterial Flagellar Motor in the Cell Membrane

The bacterial flagellar motor is a rotary motor driven by the electrochemical potentials of specific ions across the cell membrane. Direct interactions between the rotor protein FliG and the stator protein MotA are thought to generate the rotational torque. Here, we used total internal reflection fluorescent microscopy to observe the localization of green fluorescent protein (GFP)-fused FliG in Escherichia coli cells. We identified three types of fluorescent punctate signals: immobile dots, mobile dots that exhibited simple diffusion, and mobile dots that exhibited restricted diffusion. When GFP-FliG was expressed in a DeltafliG background, most of the cells were not mobile. When the cells were tethered to a glass side, however, rotating cells were commonly observed and a single fluorescent dot was always observed at the rotational center of the tethered cell. These fluorescent dots were likely positions at which functional GFP-FliG had been incorporated into a flagellar motor. Our results suggest that flagellar basal bodies diffuse in the cytoplasmic membrane until the axial structure and/or other structures assemble.

Direct observation of steps in rotation of the bacterial flagellar motor

The bacterial flagellar motor is a rotary molecular machine that rotates the helical filaments that propel many species of swimming bacteria. The rotor is a set of rings up to 45 nm in diameter in the cytoplasmic membrane; the stator contains about ten torque-generating units anchored to the cell wall at the perimeter of the rotor. The free-energy source for the motor is an inward-directed electrochemical gradient of ions across the cytoplasmic membrane, the protonmotive force or sodium-motive force for H+-driven and Na+-driven motors, respectively. Here we demonstrate a stepping motion of a Na+-driven chimaeric flagellar motor in Escherichia coli at low sodium-motive force and with controlled expression of a small number of torque-generating units. We observe 26 steps per revolution, which is consistent with the periodicity of the ring of FliG protein, the proposed site of torque generation on the rotor. Backwards steps despite the absence of the flagellar switching protein CheY indicate a small change in free energy per step, similar to that of a single ion transit.

The Neck Domain of Myosin II Primarily Regulates the Actomyosin Kinetics, not the Stepsize

In order to study the role of the neck domain of myosin in muscle contraction, we measured the steps of individual myosin II molecules engineered to have no neck domain (light chain-binding domain) by optical trapping nanometry. The actin filament and myosin cofilaments interacted on a glass surface to minimize the angle between them, and to minimize the interaction between myosin and the glass surface. The results showed that the average myosin stepsize did not change much when the neck domain was removed, but the sliding velocity decreased approximately fivefold. Furthermore, the duration of steps for neckless myosin was several times longer at saturated ATP concentration, indicating that the slower velocity was due to a slower dissociation rate of myosin heads from actin. From these data, we conclude that the neck domain of myosin-II primarily regulates the actomyosin kinetics, not the mechanics.

Formation and Maintenance of Tubular Membrane Projections Require Mechanical Force, but their Elongation and Shortening do not Require Additional Force

Living cells develop their own characteristic shapes depending on their physiological functions, and their morphologies are based on the mechanical characteristics of the cytoskeleton and of membranes. To investigate the role of lipid membranes in morphogenesis, we constructed a simple system that can manipulate liposomes and measure the forces required to transform their shapes. Two polystyrene beads (1 microm in diameter) were encapsulated in giant liposomes and were manipulated using double-beam laser tweezers. Without any specific interaction between the lipid membrane and beads, mechanical forces could be applied to the liposome membrane from the inside. Spherical liposomes transformed into a lemon shape with increasing tension, and tubular membrane projections were subsequently generated in the tips at either end. This process is similar to the liposomal transformation caused by elongation of encapsulated cytoskeletons. In the elongation stage of lemon-shaped liposomes, the force required for the transformation became larger as the end-to-end length increased. Just before the tubular membrane was generated, the force reached the maximum strength (approximately 11 pN). However, immediately after the tubular membrane developed, the force suddenly decreased and was maintained at a constant strength (approximately 4 pN) that was independent of further tube elongation or shortening, even though there was no excess membrane reservoir as occurs in living cells. When the tube length was shortened to approximately 2 microm, the liposome reversed to a lemon shape and the force temporarily increased (to approximately 7 pN). These results indicate that the simple application of mechanical force is sufficient to form a protrusion in a membrane, that a critical force and length is needed to form and to maintain the protrusion, and suggest that the lipid bilayer itself has the ability to buffer the membrane tension.

Mechanical analyses of morphological and topological transformation of liposomes

Liposomes are micro-compartments made of lipid bilayer membranes possessing the characteristics quite similar to those of biological membranes. To form artificial cell-like structures, we made liposomes that contained subunit proteins of cytoskeletons: tubulin or actin. Spherical liposomes were transformed into bipolar or cell-like shapes by mechanical forces generated by the polymerization of encapsulated subunits of microtubules. On the other hand, disk- or dumbbell-shaped liposomes were developed by the polymerization of encapsulated actin. Dynamic processes of morphological transformations of liposomes were visualized by high intensity dark-field light microscopy. Topological changes, such as fusion and division of membrane vesicles, play an essential role in cellular activities. To investigate the mechanism of these processes, we visualized the liposomes undergoing topological transformation in real time. A variety of novel topological transformations were found, including the opening-up of liposomes and the direct expulsion of inner vesicles.

A kinetic mechanism for the fast movement of Chara myosin

Endoplasmic streaming of characean cells of Nitella or Chara is known to be in the range 30-100 microm/second. The Chara myosin extracted from the cells and fixed onto a glass surface was found to move muscle actin filaments at a velocity of 60 microm/second. This is ten times faster than that of skeletal muscle myosin (myosin II). In this study, the displacement caused by single Chara myosin molecules was measured using optical trapping nanometry. The step size of Chara myosin was approximately 19nm. This step size is longer than that of skeletal muscle myosin but shorter than that of myosin V. The dwell time of the steps was relatively long, and this most likely resulted from two rate-limiting steps, the dissociation of ADP and the binding of ATP. The rate of ADP release from Chara myosin after the completion of the force-generation step was similar to that of myosin V, but was considerably slower than that of skeletal muscle myosin. The 19nm step size and the dwell time obtained could not explain the fast movement. The fast movement could be explained by the load-dependent release of ADP. As the load imposed on the myosin decreased, the rate of ADP release increased. We propose that the interaction of Chara myosin with an actin filament resulted in a negative load being imposed on other myosin molecules interacting with the same actin filament. This resulted in an accelerated release of ADP and the fast sliding movement.

Torque-speed relationship of the Na+-driven flagellar motor of Vibrio alginolyticus

The torque-speed relationship of the Na(+)-driven flagellar motor of Vibrio alginolyticus was investigated. The rotation rate of the motor was measured by following the position of a bead, attached to a flagellar filament, using optical nanometry. In the presence of 50mM NaCl, the generated torque was relatively constant ( approximately 3800pNnm) at lower speeds (speeds up to approximately 300Hz) and then decreased steeply, similar to the H(+)-driven flagellar motor of Escherichia coli. When the external NaCl concentration was varied, the generated torque of the flagellar motor was changed over a wide range of speeds. This result could be reproduced using a simple kinetic model, which takes into consideration the association and dissociation of Na(+) onto the motor. These results imply that for a complete understanding of the mechanism of flagellar rotation it is essential to consider both the electrochemical gradient and the absolute concentration of the coupling ion.

The systematic substitutions around the conserved charged residues of the cytoplasmic loop of Na+-driven flagellar motor component PomA

PomA, a homolog of MotA in the H+-driven flagellar motor, is an essential component for torque generation in the Na+-driven flagellar motor. Previous studies suggested that two charged residues, R90 and E98, which are in the single cytoplasmic loop of MotA, are directly involved in this process. These residues are conserved in PomA of Vibrio alginolyticus as R88 and E96, respectively. To explore the role of these charged residues in the Na+-driven motor, we replaced them with other amino acids. However, unlike in the H+-driven motor, both of the single and the double PomA mutants were functional. Several other positively and negatively charged residues near R88 and E96, namely K89, E97 and E99, were neutralized. Motility was retained in a strain producing the R88A/K89A/E96Q/E97Q/E99Q (AAQQQ) PomA protein. The swimming speed of the AAQQQ strain was as fast as that of the wild-type PomA strain, but the direction of motor rotation was abnormally counterclockwise-biased. We could, however, isolate non-motile or poorly motile mutants when certain charged residues in PomA were reversed or neutralized. The charged residues at positions 88-99 of PomA may not be essential for torque generation in the Na+-driven motor and might play a role in motor function different from that of the equivalent residues of the H+-driven motor.

Single molecule nanomanipulation of biomolecules

The development of nanomanipulation techniques has given investigators the ability to manipulate single biomolecules and to record mechanical events of biomolecules at the single molecule level. The techniques were developed to elucidate the mechanism of molecular motors. We can directly monitor the unitary process of the mechanical work and the energy conversion processes by combining these techniques with the single molecule imaging techniques. Our results strongly suggest that the sliding movement of the actomyosin motor is driven by Brownian movement. Other groups have reported data that are more consistent with the lever arm model. These methods and imaging techniques enable us to monitor the behavior of biomolecules at work and will be applied to other molecular machines.

Single molecule nanobioscience

In recent years, the rapid development and progress of single-molecule detection techniques have opened up a new era of biological research. The advantage of single-molecule studies is that data are not obscured by the ensemble-averaged measurements inherent in classical biochemical experiments. These techniques are shedding light on the dynamic and mechanistic properties of molecular machines, both in vivo and in vitro. This review summarizes the single-molecule experiments that have been designed to investigate molecular motors, enzyme reactions, protein dynamics, DNA transcription and cell signaling.

Single-motor mechanics and models of the myosin motor

Recent progress in single-molecule detection techniques is remarkable. These techniques have allowed the accurate determination of myosin-head-induced displacements and how mechanical cycles are coupled to ATP hydrolysis, by measuring individual mechanical events and chemical events of actomyosin directly at the single-molecule level. Here we review our recent work in which we have made detailed measurements of myosin step size and mechanochemical coupling, and propose a model of the myosin motor.

Orientation dependence of displacements by a single one-headed myosin relative to the actin filament

Displacements of single one-headed myosin molecules in a sparse myosin-rod cofilament were measured from bead displacements at various angles relative to an actin filament by dual optical trapping nanometry. The sparse myosin-rod cofilaments, 5-8 micron long, were synthesized by slowly mixing one-headed myosin prepared by papain digestion with myosin rods at molar ratios of 1:400 to 1:1500, so that one to four one-headed myosin molecules were on average scattered along the cofilament. The bead displacement was approximately 10 nm at low loads ( approximately 0.5 pN) and at angles of 5-10 degrees between the actin and myosin filaments (near physiologically correct orientation). The bead displacement decreased with an increase in the angle. The bead displacement at nearly 90 degrees was approximately 0 nm. When the angle was increased to approximately 150 degrees-170 degrees, the bead displacements increased to 5 nm. A native two-headed myosin showed similar size and orientation dependence of bead displacements as a one-headed myosin.

Simultaneous observation of individual ATPase and mechanical events by a single myosin molecule during interaction with actin

We have developed a technique that allows mechanical and ligand-binding events in a single myosin molecule to be monitored simultaneously. We describe how steps in the ATPase reaction are temporally related to mechanical events at the single molecule level. The results show that the force generation does not always coincide with the release of bound nucleotide, presumably ADP. Instead the myosin head produces force several hundreds of milliseconds after bound nucleotide is released. This finding does not support the widely accepted view that force generation is directly coupled to the release of bound ligands. It suggests that myosin has a hysteresis or memory state, which stores chemical energy from ATP hydrolysis.

Modification of the bi-directional sliding movement of actin filaments along native thick filaments isolated from a clam

The properties of bi-directional sliding of F-actin prepared from rabbit skeletal muscle moving along clam thick filaments have been characterized in the presence of agents known to modify unloaded shortening velocity in muscle to determine if the sliding characteristics of actin are similar in the two directions of movement. Actin filaments moved at a fast velocity towards the central bare zone (11.1 +/- 0.2 microns s-1) and at a slower velocity away from the bare zone (3.9 +/- 0.3 microns s-1). Movement of filaments at the slow sliding velocity is thought to be sustained by a change in orientation of the myosin head. The Michaelis Menten constant (Km values) of approximately 0.3 mM in the presence of MgATP concentrations of 0.01-2.0 mM at an ionic strength of 43.5 mM were reduced to approximately 0.1 mM at low ionic strength (18.5 mM) although the Km values at the fast and slow sliding velocities at each ionic strength were similar. In the presence of constant concentrations of MgATP, increasing the MgADP concentrations from 0.5 to 2mM, decreased the bi-directional sliding velocity of actin. The data were well fitted with an equation described by Michaelis Menten kinetics yielding mean absolute Km and Ki values of 0.41 +/- 0.01 and 0.44 +/- 0.05 mM for the fast velocity and 0.29 +/- 0.07 and 0.45 +/- 0.02 mM for the slow velocity of sliding, respectively. The Km and Ki values were not significantly different from each other at either the fast or slow sliding velocities. The actin filament sliding velocity appeared to be controlled through the thick filament as actin was devoid of regulatory proteins and the presence of Ca2+ modified the MgATP dependent movement of actin. The pCa value for half maximal sliding velocity was 7.0 for both fast and slow velocities. The Km and Ki values and the Ca2+ sensitivity of the actin movement at the fast and slow sliding velocity are similar suggesting that no major biochemical changes have occurred in the myosin head as a result of a change in orientation.

Torsional rigidity of single actin filaments and actin-actin bond breaking force under torsion measured directly by in vitro micromanipulation

Knowledge of the elastic properties of actin filaments is crucial for considering its role in muscle contraction, cellular motile events, and formation of cell shape. The stiffness of actin filaments in the directions of stretching and bending has been determined. In this study, we have directly determined the torsional rigidity and breaking force of single actin filaments by measuring the rotational Brownian motion and tensile strength using optical tweezers and microneedles, respectively. Rotational angular fluctuations of filaments supplied the torsional rigidity as (8.0 +/- 1.2) x 10(-26) Nm2. This value is similar to that deduced from the longitudinal rigidity, assuming the actin filament to be a homogeneous rod. The breaking force of the actin-actin bond was measured while twisting a filament through various angles using microneedles. The breaking force decreased greatly under twist, e.g., from 600-320 pN when filaments were turned through 90 degrees, independent of the rotational direction. Our results indicate that an actin filament exhibits comparable flexibility in the rotational and longitudinal directions, but breaks more easily under torsional load.

Multiple- and single-molecule analysis of the actomyosin motor by nanometer-piconewton manipulation with a microneedle: unitary steps and forces

We have developed a new technique for measurements of piconewton forces and nanometer displacements in the millisecond time range caused by actin-myosin interaction in vitro by manipulating single actin filaments with a glass microneedle. Here, we describe in full the details of this method. Using this method, the elementary events in energy transduction by the actomyosin motor, driven by ATP hydrolysis, were directly recorded from multiple and single molecules. We found that not only the velocity but also the force greatly depended on the orientations of myosin relative to the actin filament axis. Therefore, to avoid the effects of random orientation of myosin and association of myosin with an artificial substrate in the surface motility assay, we measured forces and displacements by myosin molecules correctly oriented in single synthetic myosin rod cofilaments. At a high myosin-to-rod ratio, large force fluctuations were observed when the actin filament interacted in the correct orientation with a cofilament. The noise analysis of the force fluctuations caused by a small number of heads showed that the myosin head generated a force of 5.9 +/- 0.8 pN at peak and 2.1 +/- 0.4 pN on average over the whole ATPase cycle. The rate constants for transitions into (k+) and out of (k-) the force generation state and the duty ratio were 12 +/- 2 s-1, and 22 +/- 4 s-1, and 0.36 +/- 0.07, respectively. The stiffness was 0.14 pN nm-1 head-1 for slow length change (100 Hz), which would be approximately 0.28 pN nm-1 head-1 for rapid length change or in rigor. At a very low myosin-to-rod ratio, distinct actomyosin attachment, force generation (the power stroke), and detachment events were directly detected. At high load, one power stroke generated a force spike with a peak value of 5-6 pN and a duration of 50 ms (k(-)-1), which were compatible with those of individual myosin heads deduced from the force fluctuations. As the load was reduced, the force of the power stroke decreased and the needle displacement increased. At near zero load, the mean size of single displacement spikes, i.e., the unitary steps caused by correctly oriented myosin, which were corrected for the stiffness of the needle-to-myosin linkage and the randomizing effect by the thermal vibration of the needle, was approximately 20 nm.

Neonatal alloimmune thrombocytopenia involving HPA-5b (Br(a)): a rare Japanese case

Neonatal alloimmune thrombocytopenia (NAIT) is caused by platelet antigen incompatibility between the mother and fetus. The frequency of NAIT varies among ethnic groups. In Caucasians, HPA-5b (Br(a)) is the antigen that is second most frequently implicated. In Japan, NAIT due to anti-HPA-5b antibody is quite rare. The present case is the second case of Br(a)-NAIT in Japanese and the first case serologically confirmed by monoclonal antibody-specific immobilization of platelet antigens.

Forces and steps generated by single myosin molecules

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Direct measurement of stiffness of single actin filaments with and without tropomyosin by in vitro nanomanipulation

In order to explain the molecular mechanism of muscle contraction, it is crucial to know the distribution of the sarcomere compliance of active muscle. Here, we directly measure the stiffness of single actin filaments with and without tropomyosin, using a recently developed technique for nanomanipulation of single actin filaments with microneedles. The results show that the stiffness for 1-micron-long actin filaments with and without tropomyosin is 65.3 +/- 6.3 and 43.7 +/- 4.6 pN/nm, respectively. When the distribution of crossbridge forces along the actin filament is taken into account, the elongation of a 1-micron-long thin filament during development of isometric contraction is calculated to be approximately 0.23%. The time constant of force in response to a sudden length change is < 0.2 ms, indicating that the viscoelasticity is negligible in the millisecond time range. These results suggest that approximately 50% of the sarcomere compliance of active muscle is due to extensibility of the thin filaments.

Single-molecule analysis of the actomyosin motor using nano-manipulation

The elementary events in energy transduction by the actomyosin motor, driven by ATP hydrolysis, were directly recorded from multiple and single molecules using a recently developed technique for nano-manipulation of single actin filaments by a microneedle. In order to avoid the effects of random orientation of myosin and association of myosin with an artificial substrate in the surface motility assay, we used single myosin-rod cofilaments with various ratios. Distinct actomyosin attachment, force generation (the power stroke) and detachment events were detected at a very low myosin: rod ratio. At high load, one power stroke generated 5-6 pN peak force and 2.3 pN force averaged over the cycle, which were compatible with those deduced from noise analysis of force fluctuations caused by multiple molecules. As the load was reduced, the length of the power stroke increased. At near zero load, the length of a power stroke was approximately 17 nm. The results suggested that an ATPase cycle produces one power stroke at high load and many ones at low load.

Force-generating domain of myosin motor

To understand the underlying mechanism of force generation by myosin motor, it is crucial to know which part of the molecule is essential for the process. Recent structure determination of myosin motor domain at atomic resolution has revealed that the domain comprises two smaller domains, the "ATPase domain" consisting of only an N-terminal segment of the heavy chain and the "neck domain" consisting of a long alpha-helix of the heavy chain and two light chains. This atomic structure begs the question of whether both domains are required for force generation. To answer it, we genetically truncated the head to generate a recombinant fragment composed of the "ATPase domain" alone. The truncated head drove sliding movement of actin filaments and generated force in a novel in vitro assay system, which allows us to hold a specific site of the head on a glass surface. These results indicate that the compact ATPase domain functions as a force-generating machinery of the myosin motor.

Nano-manipulation of actomyosin molecular motors in vitro: a new working principle

Techniques have been recently developed that allow the direct observation of single actin filaments and their manipulation, using glass microneedles, in the nanometer range. Further development of these techniques has made possible the detection of subpiconewton-level forces of individual myosin heads. This in vitro motility model is sensitive in the submillisecond range and has allowed us to determine the force generation of an actomyosin motor directly at the molecular level. The results have led to a new conceptual framework for chemo-mechanical energy transduction in the molecular motor.

Charge-reversion mutagenesis of Dictyostelium actin to map the surface recognized by myosin during ATP-driven sliding motion

Amino acid residues D24/D25, E99/E100, E360/E361, and D363/E364 in subdomain 1 of Dictyostelium actin were replaced with histidine residues by site-directed mutagenesis. Mutant actins were expressed in Dictyostelium cells and purified to homogeneity. The sliding movement of mutant actin filaments on heavy meromyosin attached to a glass surface was measured to assess the effect of the mutation on the motility of actin. For two C-terminal mutants, force generated by a single actin filament and myosin was also measured. These measurements indicated that both D24/D25 and E99/E100 are involved in ATP-driven sliding, whereas E360/E361/D363/E364 are not essential for ATP-driven sliding and force generation.