Dynamic electron microscopy of ATP-induced myosin head movement in living muscle thick filaments

Liquid cell electrochemical TEM: Unveiling the real-time interfacial reactions of advanced Li-metal batteries

Li metal batteries (LMBs) reveal great application prospect in next-generation energy storage, because of their high energy density and low electrochemical potential, especially when paired with elemental sulfur and oxygen cathodes. Complex interfacial reactions have long been a big concern because of the elusive formation/dissolution of Li metal at the solid-electrolyte interface (SEI) layer, which leads to battery degradation under practical operating conditions. To precisely track the reactions at the electrode/electrolyte interfaces, in the past ten years, high spatio-temporal resolution, in situ electrochemical transmission electron microscopy (EC-TEM) has been developed. A preliminary understanding of the structural and chemical variation of Li metal during nucleation/growth and SEI layer formation has been obtained. In this perspective, we give a brief introduction of liquid cell development. Then, we comparably discuss the different configurations of EC-TEM based on open-cell and liquid-cell, and focus on the recent advances of liquid-cell EC-TEM and its investigation in the electrodes, electrolytes, and SEI. Finally, we present a perspective of liquid-cell EC-TEM for future LMB research.

Visualizing Dynamic Environmental Processes in Liquid at Nanoscale via Liquid-Phase Electron Microscopy

Visualizing the structure and processes in liquids at the nanoscale is essential for understanding the fundamental mechanisms and underlying processes of environmental research. Cutting-edge progress of in situ liquid-phase (scanning) transmission electron microscopy (LP-S/TEM) and inferred possible applications are highlighted as a more and more indispensable tool for visualization of dynamic environmental processes in this Perspective. Advancements in nanofabrication technology, high-speed imaging, comprehensive detectors, and spectroscopy analysis have made it increasingly convenient to use LP S/TEM, thus providing an approach for visualization of direct and insightful scientific information with the exciting possibility of solving an increasing number of tricky environmental problems. This includes evaluating the transformation fate and path of contamination, assessing toxicology of nanomaterials, simulating solid surface corrosion processes in the environment, and observing water pollution control processes. Distinct nanoscale or even atomic understanding of the reaction would provide dependable and precise identification and quantification of contaminants in dynamic processes, thus facilitating trouble-tracing of environmental problems with amplifying complexity.

Nanoscale Chemical and Structural Analysis during In Situ Scanning/Transmission Electron Microscopy in Liquids

Liquid-cell scanning/transmission electron microscopy (S/TEM) has impacted our understanding of multiple areas of science, most notably nanostructure nucleation and growth and electrochemistry and corrosion. In the case of electrochemistry, the incorporation of electrodes requires the use of silicon nitride membranes to confine the liquid. The combined thickness of the liquid layer and the confining membranes prevents routine atomic-resolution characterization. Here, we show that by performing electrochemical water splitting in situ to generate a gas bubble, we can reduce the thickness of the liquid to a film approximately 30 nm thick that remains covering the sample. The reduced thickness of the liquid allows the acquisition of atomic-scale S/TEM images with chemical and valence analysis through electron energy loss spectroscopy (EELS) and structural analysis through selected area electron diffraction (SAED). This contrasts with a specimen cell entirely filled with liquid, where the broad plasmon peak from the liquid obscures the EELS signal from the sample and induces beam incoherence that impedes SAED analysis. The gas bubble generation is fully reversible, which allows alternating between a full cell and thin-film condition to obtain optimal experimental and analytical conditions, respectively. The methodology developed here can be applied to other scientific techniques, such as X-ray scattering, Raman spectroscopy, and X-ray photoelectron spectroscopy, allowing for a multi-modal, nanoscale understanding of solid-state samples in liquid media.

Liquid-Phase Electron Microscopy for Soft Matter Science and Biology

Innovations in liquid-phase electron microscopy (LP-EM) have made it possible to perform experiments at the optimized conditions needed to examine soft matter. The main obstacle is conducting experiments in such a way that electron beam radiation can be used to obtain answers for scientific questions without changing the structure and (bio)chemical processes in the sample due to the influence of the radiation. By overcoming these experimental difficulties at least partially, LP-EM has evolved into a new microscopy method with nanometer spatial resolution and sub-second temporal resolution for analysis of soft matter in materials science and biology. Both experimental design and applications of LP-EM for soft matter materials science and biological research are reviewed, and a perspective of possible future directions is given.

Liquid cell transmission electron microscopy and its applications

Transmission electron microscopy (TEM) has long been an essential tool for understanding the structure of materials. Over the past couple of decades, this venerable technique has undergone a number of revolutions, such as the development of aberration correction for atomic level imaging, the realization of cryogenic TEM for imaging biological specimens, and new instrumentation permitting the observation of dynamic systems in situ. Research in the latter has rapidly accelerated in recent years, based on a silicon-chip architecture that permits a versatile array of experiments to be performed under the high vacuum of the TEM. Of particular interest is using these silicon chips to enclose fluids safely inside the TEM, allowing us to observe liquid dynamics at the nanoscale. In situ imaging of liquid phase reactions under TEM can greatly enhance our understanding of fundamental processes in fields from electrochemistry to cell biology. Here, we review how in situ TEM experiments of liquids can be performed, with a particular focus on microchip-encapsulated liquid cell TEM. We will cover the basics of the technique, and its strengths and weaknesses with respect to related in situ TEM methods for characterizing liquid systems. We will show how this technique has provided unique insights into nanomaterial synthesis and manipulation, battery science and biological cells. A discussion on the main challenges of the technique, and potential means to mitigate and overcome them, will also be presented.

Liquid electron microscopy: then, now and future

Contemporary microscopic imaging at near-atomic resolution of diverse embodiments in liquid environment has gained keen interest. In particular, Electron Microscopy (EM) can provide comprehensive framework on the structural and functional characterization of samples in liquid phase. In the past few decades, liquid based electron microscopic modalities have developed tremendously to provide insights into various backgrounds like biological, chemical, nanoparticle and material researches. It serves to be a promising analytical tool in deciphering unique insights from solvated systems. Here, the basics of liquid electron microscopy with few examples of its applications are summarized in brief. The technical developments made so far and its preference over other approaches is shortly presented. Finally, the experimental limitations and an outlook on the future technical advancement for liquid EM have been discussed.

High flow rate nanofluidics for in-liquid electron microscopy and diffraction

We introduce a nanofluidic platform that can be used to carry out femtosecond electron diffraction (FED) and transmission electron microscopy (TEM) measurements in liquid samples or in-liquid specimens, respectively. The nanofluidic cell (NFC) system presented herein has been designed to withstand high sample refreshing rates (over one kilohertz), a prerequisite to succeed with FED experiments in our lab. Short beam paths, below 1 μm, in combination with ultrathin membranes (less than 100 nm thick) are necessary conditions for in-liquid FED and TEM studies due to the strongly interacting nature of electrons. Depending on the application, the beam path in our NFC can be tuned between 50 nm and 10 μm with ultrathin stoichiometric silicon nitride (Si₃N₄) windows as thin as 20 nm. Stoichiometric Si₃N₄ has been selected to reduce membrane bulging owing to its higher tensile stress and transparency in the UV-vis-NIR region to allow for laser excitation in FED experiments. Key design parameters and improvements made over previous NFC systems are discussed, and some preliminary electron images obtained by 200 kV scanning TEM are presented.

Basic properties of ATP-induced myosin head movement in hydrated myosin filaments, studied using the gas environmental chamber

Although more than 50 years have passed since the monumental discovery of Huxley and Hanson that muscle contraction results from relative sliding between actin and myosin filaments, coupled with ATP hydrolysis, the mechanism underlying the filament sliding still remains to be a mystery. It is generally believed that the myofilament sliding is caused by cyclic attachment-detachment between myosin heads in myosin filaments and myosin-binding sites in actin filaments. Attempts to prove the myosin head movement using techniques of X-ray diffraction and chemical probes attached to myosin heads have failed to obtain clear results because of the asynchronous nature of myosin head movement. Using the gas environmental chamber (EC) attached to an electron microscope, we succeeded in recording myosin head movement in hydrated myosin filaments, coupled with ATP hydrolysis with the following results: (1)In the absence of actin filaments, myosin heads fluctuate around a definite neutral position, so that their time-averaged position remains unchanged; (2) On ATP application, myosin heads bind with ATP to be in the charged-up state, M-ADP-Pi, and perform a recovery stroke in the direction away from the myosin filament central bare zone and stay in the post-recovery stroke position; (3) In the actin-myosin filament mixture, myosin heads form rigor linkages with actin, and bind with applied ATP to be in the charged-up state, M-ADP-Pi, and perform a power stroke in the direction towards the myosin filament bare zone, while releasing ADP and Pi to stay in the post-power stroke position; (4) In both recovery and power strokes, myosin heads in the non charged-up state return to the neutral position. These results indicate that the charged-up myosin heads decide their direction of movement without being guided by actin filaments.

Instrumentation for in situ flow electrochemical Scanning Transmission X-ray Microscopy (STXM)

We report the design and performance of a 3-electrode device for real time in situ scanning transmission X-ray microscopy studies of electrochemical processes under both static (sealed, non-flow) conditions and with a continuous flow of electrolytes. The device was made using a combination of silicon microfabrication and 3D printing technologies. The performance is illustrated by results of a study of copper deposition and stripping at a gold working electrode. X-ray absorption spectromicroscopy at the Cu 2p edge was used to follow the evolution as a function of potential and time of the spatial distributions of Cu(0) and Cu(i) species electro-deposited from an aqueous solution of copper sulphate. The results are interpreted in terms of competing mechanisms for the reduction of Cu(ii).

Electron Microscopic Recording of the Power and Recovery Strokes of Individual Myosin Heads Coupled with ATP Hydrolysis: Facts and Implications

The most straightforward way to get information on the performance of individual myosin heads producing muscle contraction may be to record their movement, coupled with ATP hydrolysis, electron-microscopically using the gas environmental chamber (EC). The EC enables us to visualize and record ATP-induced myosin head movement in hydrated skeletal muscle myosin filaments. When actin filaments are absent, myosin heads fluctuate around a definite neutral position, so that their time-averaged mean position remains unchanged. On application of ATP, myosin heads are found to move away from, but not towards, the bare region, indicating that myosin heads perform a recovery stroke (average amplitude, 6 nm). After exhaustion of ATP, myosin heads return to their neutral position. In the actin⁻myosin filament mixture, myosin heads form rigor actin myosin linkages, and on application of ATP, they perform a power stroke by stretching adjacent elastic structures because of a limited amount of applied ATP ≤ 10 µM. The average amplitude of the power stroke is 3.3 nm and 2.5 nm at the distal and the proximal regions of the myosin head catalytic domain (CAD), respectively. The power stroke amplitude increases appreciably at low ionic strength, which is known to enhance Ca²⁺-activated force in muscle. In both the power and recovery strokes, myosin heads return to their neutral position after exhaustion of ATP.

In Situ Environmental TEM in Imaging Gas and Liquid Phase Chemical Reactions for Materials Research

Gas and liquid phase chemical reactions cover a broad range of research areas in materials science and engineering, including the synthesis of nanomaterials and application of nanomaterials, for example, in the areas of sensing, energy storage and conversion, catalysis, and bio-related applications. Environmental transmission electron microscopy (ETEM) provides a unique opportunity for monitoring gas and liquid phase reactions because it enables the observation of those reactions at the ultra-high spatial resolution, which is not achievable through other techniques. Here, the fundamental science and technology developments of gas and liquid phase TEM that facilitate the mechanistic study of the gas and liquid phase chemical reactions are discussed. Combined with other characterization tools integrated in TEM, unprecedented material behaviors and reaction mechanisms are observed through the use of the in situ gas and liquid phase TEM. These observations and also the recent applications in this emerging area are described. The current challenges in the imaging process are also discussed, including the imaging speed, imaging resolution, and data management.

Dynamics of cross-bridge cycling, ATP hydrolysis, force generation, and deformation in cardiac muscle

Despite extensive study over the past six decades the coupling of chemical reaction and mechanical processes in muscle dynamics is not well understood. We lack a theoretical description of how chemical processes (metabolite binding, ATP hydrolysis) influence and are influenced by mechanical processes (deformation and force generation). To address this need, a mathematical model of the muscle cross-bridge (XB) cycle based on Huxley's sliding filament theory is developed that explicitly accounts for the chemical transformation events and the influence of strain on state transitions. The model is identified based on elastic and viscous moduli data from mouse and rat myocardial strips over a range of perturbation frequencies, and MgATP and inorganic phosphate (Pi) concentrations. Simulations of the identified model reproduce the observed effects of MgATP and MgADP on the rate of force development. Furthermore, simulations reveal that the rate of force re-development measured in slack-restretch experiments is not directly proportional to the rate of XB cycling. For these experiments, the model predicts that the observed increase in the rate of force generation with increased Pi concentration is due to inhibition of cycle turnover by Pi. Finally, the model captures the observed phenomena of force yielding suggesting that it is a result of rapid detachment of stretched attached myosin heads.

Electron microscopic recording of myosin head power stroke in hydrated myosin filaments

Muscle contraction results from cyclic attachment and detachment between myosin heads and actin filaments, coupled with ATP hydrolysis. Despite extensive studies, however, the amplitude of myosin head power stroke still remains to be a mystery. Using the gas environmental chamber, we have succeeded in recording the power stroke of position-marked myosin heads in hydrated mixture of actin and myosin filaments in a nearly isometric condition, in which myosin heads do not produce gross myofilament sliding, but only stretch adjacent elastic structures. On application of ATP, individual myosin heads move by ~3.3 nm at the distal region, and by ~2.5 nm at the proximal region of myosin head catalytic domain. After exhaustion of applied ATP, individual myosin heads return towards their initial position. At low ionic strength, the amplitude of myosin head power stroke increases to >4 nm at both distal and proximal regions of myosin heads catalytic domain, being consistent with the report that the force generated by individual myosin heads in muscle fibers is enhanced at low ionic strength. The advantages of the present study over other in vitro motility assay systems, using myosin heads detached from myosin filaments, are discussed.

Definite differences between in vitro actin-myosin sliding and muscle contraction as revealed using antibodies to myosin head

Muscle contraction results from attachment-detachment cycles between myosin heads extending from myosin filaments and actin filaments. It is generally believed that a myosin head first attaches to actin, undergoes conformational changes to produce force and motion in muscle, and then detaches from actin. Despite extensive studies, the molecular mechanism of myosin head conformational changes still remains to be a matter for debate and speculation. The myosin head consists of catalytic (CAD), converter (CVD) and lever arm (LD) domains. To give information about the role of these domains in the myosin head performance, we have examined the effect of three site-directed antibodies to the myosin head on in vitro ATP-dependent actin-myosin sliding and Ca2+-activated contraction of muscle fibers. Antibody 1, attaching to junctional peptide between 50K and 20K heavy chain segments in the CAD, exhibited appreciable effects neither on in vitro actin-myosin sliding nor muscle fiber contraction. Since antibody 1 covers actin-binding sites of the CAD, one interpretation of this result is that rigor actin-myosin linkage is absent or at most a transient intermediate in physiological actin-myosin cycling. Antibody 2, attaching to reactive lysine residue in the CVD, showed a marked inhibitory effect on in vitro actin-myosin sliding without changing actin-activated myosin head (S1) ATPase activity, while it showed no appreciable effect on muscle contraction. Antibody 3, attaching to two peptides of regulatory light chains in the LD, had no significant effect on in vitro actin-myosin sliding, while it reduced force development in muscle fibers without changing MgATPase activity. The above definite differences in the effect of antibodies 2 and 3 between in vitro actin-myosin sliding and muscle contraction can be explained by difference in experimental conditions; in the former, myosin heads are randomly oriented on a glass surface, while in the latter myosin heads are regularly arranged within filament-lattice structures.

Enhancement of force generated by individual myosin heads in skinned rabbit psoas muscle fibers at low ionic strength

Although evidence has been presented that, at low ionic strength, myosin heads in relaxed skeletal muscle fibers form linkages with actin filaments, the effect of low ionic strength on contraction characteristics of Ca²⁺-activated muscle fibers has not yet been studied in detail. To give information about the mechanism of muscle contraction, we have examined the effect of low ionic strength on the mechanical properties and the contraction characteristics of skinned rabbit psoas muscle fibers in both relaxed and maximally Ca²⁺-activated states. By progressively decreasing KCl concentration from 125 mM to 0 mM (corresponding to a decrease in ionic strength μ from 170 mM to 50 mM), relaxed fibers showed changes in mechanical response to sinusoidal length changes and ramp stretches, which are consistent with the idea of actin-myosin linkage formation at low ionic strength. In maximally Ca²⁺-activated fibers, on the other hand, the maximum isometric force increased about twofold by reducing KCl concentration from 125 to 0 mM. Unexpectedly, determination of the force-velocity curves indicated that, the maximum unloaded shortening velocity V_max, remained unchanged at low ionic strength. This finding indicates that the actin-myosin linkages, which has been detected in relaxed fibers at low ionic strength, are broken quickly on Ca²⁺ activation, so that the linkages in relaxed fibers no longer provide any internal resistance against fiber shortening. The force-velocity curves, obtained at various levels of steady Ca²⁺-activated isometric force, were found to be identical if they are normalized with respect to the maximum isometric force. The MgATPase activity of muscle fibers during isometric force generation was found not to change appreciably at low ionic strength despite the two-fold increase in Ca²⁺-activated isometric force. These results can be explained in terms of enhancement of force generated by individual myosin heads, but not by any changes in kinetic properties of cyclic actin-myosin interaction.

Electron microscopy of specimens in liquid

Imaging samples in liquids with electron microscopy can provide unique insights into biological systems, such as cells containing labelled proteins, and into processes of importance in materials science, such as nanoparticle synthesis and electrochemical deposition. Here we review recent progress in the use of electron microscopy in liquids and its applications. We examine the experimental challenges involved and the resolution that can be achieved with different forms of the technique. We conclude by assessing the potential role that electron microscopy of liquid samples can play in areas such as energy storage and bioimaging.

Electron microscopic evidence for the myosin head lever arm mechanism in hydrated myosin filaments using the gas environmental chamber

Muscle contraction results from an attachment-detachment cycle between the myosin heads extending from myosin filaments and the sites on actin filaments. The myosin head first attaches to actin together with the products of ATP hydrolysis, performs a power stroke associated with release of hydrolysis products, and detaches from actin upon binding with new ATP. The detached myosin head then hydrolyses ATP, and performs a recovery stroke to restore its initial position. The strokes have been suggested to result from rotation of the lever arm domain around the converter domain, while the catalytic domain remains rigid. To ascertain the validity of the lever arm hypothesis in muscle, we recorded ATP-induced movement at different regions within individual myosin heads in hydrated myosin filaments, using the gas environmental chamber attached to the electron microscope. The myosin head were position-marked with gold particles using three different site-directed antibodies. The amplitude of ATP-induced movement at the actin binding site in the catalytic domain was similar to that at the boundary between the catalytic and converter domains, but was definitely larger than that at the regulatory light chain in the lever arm domain. These results are consistent with the myosin head lever arm mechanism in muscle contraction if some assumptions are made.

Electron microscopic visualization of the cross-bridge movement coupled with ATP hydrolysis in muscle thick filaments in aqueous solution, reminiscences and future prospects

Although it has been well established that muscle contraction results from cyclic attachment-detachment between the cross-bridges extending from the thick filaments and the sites on the thin filaments, the movement of the cross-bridges coupled with ATP hydrolysis still remains to be a matter for debate and speculation. The most straightforward way to elucidate this mystery is to record individual cross-bridge movement in response to ATP. Using a gas environmental chamber (EC, or hydration chamber), with which biological specimens retaining their physiological function can be observed under an electron microscope, my coworkers and I succeeded in recording the ATP-induced individual cross-bridge movement in two different kinds of synthetic thick filaments in 1997 and 2008. In the synthetic bipolar filaments consisting of rabbit skeletal muscle myosin, the amplitude of cross-bridge movement exhibits a peak at 5-7.5 nm, and the direction of cross-bridge movement is away from, but not towards, the filament bare region in the absence of thin filaments. After exhaustion of ATP, the cross-bridges return towards their initial position, indicating that the initial cross-bridge state may be analogous to that after completion of power stroke. These results constitute the first visualization of the cross-bridge recovery stroke, indicating that the EC is a powerful tool to open new horizons in the research fields of life sciences.

Coarse-Grained Structural Modeling of Molecular Motors Using Multibody Dynamics

Experimental and computational approaches are needed to uncover the mechanisms by which molecular motors convert chemical energy into mechanical work. In this article, we describe methods and software to generate structurally realistic models of molecular motor conformations compatible with experimental data from different sources. Coarse-grained models of molecular structures are constructed by combining groups of atoms into a system of rigid bodies connected by joints. Contacts between rigid bodies enforce excluded volume constraints, and spring potentials model system elasticity. This simplified representation allows the conformations of complex molecular motors to be simulated interactively, providing a tool for hypothesis building and quantitative comparisons between models and experiments. In an example calculation, we have used the software to construct atomically detailed models of the myosin V molecular motor bound to its actin track. The software is available at www.simtk.org.

Direct demonstration of the cross-bridge recovery stroke in muscle thick filaments in aqueous solution by using the hydration chamber

Despite >50 years of research work since the discovery of sliding filament mechanism in muscle contraction, structural details of the coupling of cyclic cross-bridge movement to ATP hydrolysis are not yet fully understood. An example would be whether lever arm tilting on the myosin filament backbone will occur in the absence of actin. The most direct way to elucidate such movement is to record ATP-induced cross-bridge movement in hydrated thick filaments. Using the hydration chamber, with which biological specimens can be kept in an aqueous environment in an electron microscope, we have succeeded in recording ATP-induced cross-bridge movement in hydrated thick filaments consisting of rabbit skeletal muscle myosin, with gold position markers attached to the cross-bridges. The position of individual cross-bridges did not change appreciably with time in the absence of ATP, indicating stability of time-averaged cross-bridge mean position. On application of ATP, individual cross-bridges moved nearly parallel to the filament long axis. The amplitude of the ATP-induced cross-bridge movement showed a peak at 5-7.5 nm. At both sides of the filament bare region, across which the cross-bridge polarity was reversed, the cross-bridges were found to move away from, but not toward, the bare region. Application of ADP produced no appreciable cross-bridge movement. Because ATP reacts rapidly with the cross-bridges (M) to form complex (M x ADP x Pi) with an average lifetime >10 s, the observed cross-bridge movement is associated with reaction, M + ATP --> M x ADP x Pi. The cross-bridges were observed to return to their initial position after exhaustion of ATP. These results constitute direct demonstration of the cross-bridge recovery stroke.

Dynamic microscopy of nanoscale cluster growth at the solid-liquid interface

Dynamic processes at the solid-liquid interface are of key importance across broad areas of science and technology. Electrochemical deposition of copper, for example, is used for metallization in integrated circuits, and a detailed understanding of nucleation, growth and coalescence is essential in optimizing the final microstructure. Our understanding of processes at the solid-vapour interface has advanced tremendously over the past decade due to the routine availability of real-time, high-resolution imaging techniques yielding data that can be compared quantitatively with theory. However, the difficulty of studying the solid-liquid interface leaves our understanding of processes there less complete. Here we analyse dynamic observations--recorded in situ using a novel transmission electron microscopy technique--of the nucleation and growth of nanoscale copper clusters during electrodeposition. We follow in real time the evolution of individual clusters, and compare their development with simulations incorporating the basic physics of electrodeposition during the early stages of growth. The experimental technique developed here is applicable to a broad range of dynamic phenomena at the solid-liquid interface.

Structure and nucleotide-dependent changes of thick filaments in relaxed and rigor plaice fin muscle

The myosin crossbridge array, positions of non-crossbridge densities on the backbone, and the A-band "end filaments" have been compared in chemically skinned, unfixed, uncryoprotected relaxed, and rigor plaice fin muscles using the freeze-fracture, deep-etch, rotary-shadowing technique. The images provide a direct demonstration of the helical packing of the myosin heads in situ in relaxed muscle and show rearrangements of the myosin heads, and possibly of other myosin filament proteins, when the heads lose ATP on going into rigor. In the H-zone these changes are consistent with crossbridge changes previously shown by others using freeze-substitution. In addition, new evidence is presented of protein rearrangements in the M-region (bare zone), associated with the transition from the relaxed to the rigor state, including a 27-nm increase in the apparent width of the M-region. This is interpreted as being mostly due to loss or rearrangement of a nonmyosin (M9) protein component at the M-region edge. The structure and titin periodicity of the end-filaments are described, as are suggestions of titin structure on the myosin filament backbone.

ATP-induced axial movement of myosin heads in living thick filaments recorded with a gas environmental chamber attached to the electron microscope

Using a gas environmental (hydration) chamber, in which biological specimens can be kept in wet state, we succeeded in recording images of 'living' muscle thick filaments with gold position markers attached to the myosin heads. The position of individual myosin heads did not change appreciably with time in the absence of ATP, indicating stability of the myosin head mean position. On application of ATP, the position of individual myosin heads was found to move by approximately 20 nm along the filament axis, while no appreciable movement of the filaments was detected. The ATP-induced myosin head movement was not observed in filaments in which ATPase activity of the myosin heads was eliminated. Application of ADP produced no appreciable myosin head movement. These results show that the ATP-induced myosin head movement takes place in the absence of the thin filaments. Since ATP reacts rapidly with the myosin head (M) to form the complex (M.ADP.Pi) having average lifetime of > 10 s, the observed myosin head movement may be mostly associated with reaction, M + ATP-->M.ADP.Pi. This work will open a new research field to study dynamic structural changes of individual biomolecules which are kept in 'living' state in an electron microscope.

Design principles and applications of a cooled CCD camera for electron microscopy

Cooled CCD cameras offer a number of advantages in recording electron microscope images with CCDs rather than film which include: immediate availability of the image in a digital format suitable for further computer processing, high dynamic range, excellent linearity and a high detective quantum efficiency for recording electrons. In one important respect however, film has superior properties: the spatial resolution of CCD detectors tested so far (in terms of point spread function or modulation transfer function) are inferior to film and a great deal of our effort has been spent in designing detectors with improved spatial resolution. Various instrumental contributions to spatial resolution have been analysed and in this paper we discuss the contribution of the phosphor-fibre optics system in this measurement. We have evaluated the performance of a number of detector components and parameters, e.g. different phosphors (and a scintillator), optical coupling with lens or fibre optics with various demagnification factors, to improve the detector performance. The camera described in this paper, which is based on this analysis, uses a tapered fibre optics coupling between the phosphor and the CCD and is installed on a Philips CM12 electron microscope equipped to perform cryo-microscopy. The main use of the camera so far has been in recording electron diffraction patterns from two dimensional crystals of bacteriorhodopsin--from wild type and from different trapped states during the photocycle. As one example of the type of data obtained with the CCD camera a two dimensional Fourier projection map from the trapped O-state is also included. With faster computers, it will soon be possible to undertake this type of work on an on-line basis. Also, with improvements in detector size and resolution, CCD detectors, already ideal for diffraction, will be able to compete with film in the recording of high resolution images.

Comparison of unitary displacements and forces between 2 cardiac myosin isoforms by the optical trap technique: molecular basis for cardiac adaptation

To provide information on the mechanism of cardiac adaptation at the molecular level, we compared the unitary displacements and forces between the 2 rat cardiac myosin isoforms, V1 and V3. A fluorescently labeled actin filament, with a polystyrene bead attached, was caught by an optical trap and brought close to a glass surface sparsely coated with either of the 2 isoforms, so that the actin-myosin interaction took place in the presence of a low concentration of ATP (0.5 micromol/L). Discrete displacement events were recorded with a low trap stiffness (0.03 to 0.06 pN/nm). Frequency distribution of the amplitude of the displacements consisted of 2 gaussian curves with peaks at 9 to 10 and 18 to 20 nm for both V1 and V3, suggesting that 9 to 10 nm is the unitary displacement for both isoforms. The duration of the displacement events was longer for V3 than for V1. On the other hand, discrete force transients were recorded with a high trap stiffness (2.1 pN/nm), and their amplitude showed a broad distribution with mean values between 1 and 2 pN for V1 and V3. The durations of the force transients were also longer for V3 than for V1. These results indicate that both the unitary displacements and forces are similar in amplitude but different in duration between the 2 cardiac myosin isoforms, being consistent with the reports that the tension cost is higher in muscles consisting mainly of V1 than those consisting mainly of V3.