Electron microscopy of the actin-myosin head complex in the presence of ATP

Assessment of the Enzymatic Dephosphorylation Kinetics in the Assemblies of a Phosphopentapeptide that Forms Intranuclear Nanoribbons

Although the formation of peptide assemblies catalyzed by alkaline phosphatase (ALP) has received increasing attention in inhibiting cancer cells, the detailed enzyme kinetics of the dephosphorylation of the corresponding phosphopeptide assemblies have yet to be determined. We recently discovered that assemblies from a phosphopentapeptide can form intracellular nanoribbons that kill induced pluripotent stem cells or osteosarcoma cells, but the kinetics of enzymatic dephosphorylation remain unknown. Thus, we chose to examine the enzyme kinetics of the dephosphorylation of the phosphopentapeptide [NBD-LLLLpY (1)] from concentrations below to above its critical micelle concentration (CMC). Our results show that the phosphopeptide exhibits a CMC of 75 μM in phosphate saline buffer, and the apparent Vmax and Km values of alkaline phosphatase catalyzed dephosphorylation are approximately 0.24 μM/s and 5.67 mM, respectively. Despite dephosphorylation remaining incomplete at 60 min in all the concentrations tested, dephosphorylation of the phosphopeptide at concentrations of 200 μM or above mainly results in nanoribbons, dephosphorylation at concentrations of CMC largely produces nanofibers, and dephosphorylation below the CMC largely generates nanoparticles. Moreover, the formation of nanoribbons correlates with the intranuclear accumulation of the pentapeptide. By providing the first examination of the enzymatic kinetics of phosphopeptide assemblies, this work further supports the notion that the assemblies of phosphopentapeptides can act as a new functional entity for controlling cell fates.

Enzymatic Delivery of Magnetic Nanoparticles into Mitochondria of Live Cells

Delivering magnetic nanoparticles (MNPs) into mitochondria provide a facile approach to manipulate cell life because mitochondria play essential roles in cell survival and death. Here we report the use of enzyme-responsive peptide assemblies to deliver MNPs into mitochondria of live cells. The mitochondria-targeting peptide (Mito-Flag), as the substrate of enterokinase (ENTK), assembles with MNPs in solution. The MNPs that are encapsulated by Mito-Flag peptides selectively accumulate to the mitochondria of cancer cells, rather than normal cells. The mitochondrial localization of MNPs reduces the viability of the cancer cells, but hardly affects the survival of the normal cell. This work demonstrates a new and facile strategy to specifically transport MNPs to the mitochondria in cancer cells for exploring the applications of MNPs as the targeted drug for biomedicine and cancer therapy.

Enzymatically Forming Cell Compatible Supramolecular Assemblies of Tryptophan-Rich Short Peptides

Here we report a new type of tryptophan-rich short peptides, which act as hydrogelators, form supramolecular assemblies via enzymatic dephosphorylation, and exhibit cell compatibility. The facile synthesis of the peptides starts with the production of phosphotyrosine, then uses solid phase peptide synthesis (SPPS) to build the phosphopeptides that contain multiple tryptophan residues. Besides exhibiting excellent solubility, these phosphopeptides, unlike the previously reported cytotoxic phenylalanine-rich phosphopeptides, are largely compatible toward mammalian cells. Our preliminary mechanistic study suggests that the tryptophan-rich peptides, instead of forming pericellular assemblies, largely accumulate in lysosomes. Such lysosomal localization may account for their cell compatibility. Moreover, these tryptophan-rich peptides are able to transiently reduce the cytotoxicity of phenylalanine-rich peptide assemblies. This rather unexpected result implies that tryptophan may act as a useful aromatic building block for developing cell compatible supramolecular assemblies for soft materials and find applications for protecting cells from cytotoxic peptide assemblies.

GTP-dependent formation of straight tubulin oligomers leads to microtubule nucleation

Nucleation of microtubules (MTs) is essential for cellular activities, but its mechanism is unknown because of the difficulty involved in capturing rare stochastic events in the early stage of polymerization. Here, combining rapid flush negative stain electron microscopy (EM) and kinetic analysis, we demonstrate that the formation of straight oligomers of critical size is essential for nucleation. Both GDP and GTP tubulin form single-stranded oligomers with a broad range of curvatures, but upon nucleation, the curvature distribution of GTP oligomers is shifted to produce a minor population of straight oligomers. With tubulin having the Y222F mutation in the β subunit, the proportion of straight oligomers increases and nucleation accelerates. Our results support a model in which GTP binding generates a minor population of straight oligomers compatible with lateral association and further growth to MTs. This study suggests that cellular factors involved in nucleation promote it via stabilization of straight oligomers.

Long-Range and Directional Allostery of Actin Filaments Plays Important Roles in Various Cellular Activities

A wide variety of uniquely localized actin-binding proteins (ABPs) are involved in various cellular activities, such as cytokinesis, migration, adhesion, morphogenesis, and intracellular transport. In a micrometer-scale space such as the inside of cells, protein molecules diffuse throughout the cell interior within seconds. In this condition, how can ABPs selectively bind to particular actin filaments when there is an abundance of actin filaments in the cytoplasm? In recent years, several ABPs have been reported to induce cooperative conformational changes to actin filaments allowing structural changes to propagate along the filament cables uni- or bidirectionally, thereby regulating the subsequent binding of ABPs. Such propagation of ABP-induced cooperative conformational changes in actin filaments may be advantageous for the elaborate regulation of cellular activities driven by actin-based machineries in the intracellular space, which is dominated by diffusion. In this review, we focus on long-range allosteric regulation driven by cooperative conformational changes of actin filaments that are evoked by binding of ABPs, and discuss roles of allostery of actin filaments in narrow intracellular spaces.

Direct observation shows superposition and large scale flexibility within cytoplasmic dynein motors moving along microtubules

Cytoplasmic dynein is a dimeric AAA(+) motor protein that performs critical roles in eukaryotic cells by moving along microtubules using ATP. Here using cryo-electron microscopy we directly observe the structure of Dictyostelium discoideum dynein dimers on microtubules at near-physiological ATP concentrations. They display remarkable flexibility at a hinge close to the microtubule binding domain (the stalkhead) producing a wide range of head positions. About half the molecules have the two heads separated from one another, with both leading and trailing motors attached to the microtubule. The other half have the two heads and stalks closely superposed in a front-to-back arrangement of the AAA(+) rings, suggesting specific contact between the heads. All stalks point towards the microtubule minus end. Mean stalk angles depend on the separation between their stalkheads, which allows estimation of inter-head tension. These findings provide a structural framework for understanding dynein's directionality and unusual stepping behaviour.

To understand muscle you must take it apart

Striated muscle is an elegant system for study at many levels. Much has been learned about the mechanism of contraction from studying the mechanical properties of intact and permeabilized (or skinned) muscle fibers. Structural studies using electron microscopy, X-ray diffraction or spectroscopic probes attached to various contractile proteins were possible because of the highly ordered sarcomeric arrangement of actin and myosin. However, to understand the mechanism of force generation at a molecular level, it is necessary to take the system apart and study the interaction of myosin with actin using in vitro assays. This reductionist approach has lead to many fundamental insights into how myosin powers muscle contraction. In addition, nature has provided scientists with an array of muscles with different mechanical properties and with a superfamily of myosin molecules. Taking advantage of this diversity in myosin structure and function has lead to additional insights into common properties of force generation. This review will highlight the development of the major assays and methods that have allowed this combined reductionist and comparative approach to be so fruitful. This review highlights the history of biochemical and biophysical studies of myosin and demonstrates how a broad comparative approach combined with reductionist studies have led to a detailed understanding of how myosin interacts with actin and uses chemical energy to generate force and movement in muscle contraction and motility in general.

A naphthalene-containing amino acid enables hydrogelation of a conjugate of nucleobase-saccharide-amino acids

Here we report the first example of a hydrogelator made of a conjugate of nucleobase-saccharide-amino acids by incorporating L-3-(2-naphthyl)-alanine to the conjugate, which illustrates a facile and effective method for generating bioactive and functional hydrogelators from the basic biological building blocks.

Imaging self-assembly dependent spatial distribution of small molecules in a cellular environment

Self-assembly of small molecules, as a more common phenomenon than one previously thought, can be either beneficial or detrimental to cells. Despite its profound biological implications, how the self-assembly of small molecules behave in a cellular environment is largely unknown and barely explored. This work studies four fluorescent molecules that consist of the same peptidic backbone (e.g., Phe-Phe-Lys) and enzyme trigger (e.g., a phosphotyrosine residue), but bear different fluorophores on the side chain of the lysine residue of the peptidic motif. These molecules, however, exhibit a different ability of self-assembly before and after enzymatic transformation (e.g., dephosphorylation). Fluorescent imaging reveals that self-assembly directly affects the distribution of these small molecules in a cellular environment. Moreover, cell viability tests suggest that the states and the locations of the molecular assemblies in the cellular environment control the phenotypes of the cells. For example, the molecular nanofibers of one of the small molecules apparently stabilize actin filaments and alleviate the insult of an F-actin toxin (e.g., latrunculin A). Combining fluorescent imaging and enzyme-instructed self-assembly of small peptidic molecules, this work demonstrates self-assembly as a key factor for dictating the spatial distribution of small molecules in a cellular environment. In addition, it illustrates a useful approach, based on enzyme-instructed self-assembly of small molecules, to modulate spatiotemporal profiles of small molecules in a cellular environment, which allows the use of the emergent properties of small molecules to control the fate of cells.

Probing nanoscale self-assembly of nonfluorescent small molecules inside live mammalian cells

Like cellular proteins that form fibrillar nanostructures, small hydrogelator molecules self-assemble in water to generate molecular nanofibers. In contrast to the well-defined (dys)functions of endogenous protein filaments, the fate of intracellular assembly of small molecules remains largely unknown. Here we demonstrate the imaging of enzyme-triggered self-assembly of nonfluorescent small molecules by doping the molecular assemblies with a fluorescent hydrogelator. The cell fractionation experiments, fluorescent imaging, and electron microscopy indicate that the hydrogelators self-assemble and localize to the endoplasmic reticulum (ER) and are likely processed via the cellular secretory pathway (i.e., ER-Golgi-lysosomes/secretion). This work, as the first example of the use of correlative light and electron microscopy for probing the self-assembly of nonfluorescent small molecules inside live mammalian cells, not only establishes a general strategy to provide the spatiotemporal profile of the assemblies of small molecules inside cells but may lead to a new paradigm for regulating cellular functions based on the interactions between the assemblies of small molecules (e.g., molecular nanofibers) and subcellular organelles.

Catalytic dephosphorylation of adenosine monophosphate (AMP) to form supramolecular nanofibers/hydrogels

The use of enzyme to instruct the self-assembly of the nucleoside of adenosine in water provides a new class of molecular nanofibers/hydrogels as functional soft materials.

Imaging enzyme-triggered self-assembly of small molecules inside live cells

Self-assembly of small molecules in water to form nanofibres, besides generating sophisticated biomaterials, promises a simple system inside cells for regulating cellular processes. But lack of a convenient approach for studying the self-assembly of small molecules inside cells hinders the development of such systems. Here we report a method to image enzyme-triggered self-assembly of small molecules inside live cells. After linking a fluorophore to a self-assembly motif to make a precursor, we confirmed by (31)P NMR and rheology that enzyme-triggered conversion of the precursor to a hydrogelator results in the formation of a hydrogel via self-assembly. The imaging contrast conferred by the nanofibres of the hydrogelators allowed the evaluation of intracellular self-assembly, the dynamics and the localization of the nanofibres of the hydrogelators in live cells. This approach explores supramolecular chemistry inside cells and may lead to new insights, processes or materials at the interface of chemistry and biology.

Supramolecular hydrogelators of N-terminated dipeptides selectively inhibit cancer cells

Consisting of N-terminated diphenylalanine, a new type of supramolecular hydrogelators forms hydrogels within a narrow pH window (pH 5.0 to 6.0) and selectively inhibits growth of HeLa cells, which provides important and useful insights for designing molecular nanofibers as potential nanomedicines.

Multifunctional, biocompatible supramolecular hydrogelators consist only of nucleobase, amino acid, and glycoside

The integration of nucleobase, amino acid, and glycoside into a single molecule results in a novel class of supramolecular hydrogelators, which not only exhibit biocompatibility and biostability but also facilitate the entry of nucleic acids into cytosol and nuclei of cells. This work illustrates a simple way to generate an unprecedented molecular architecture from the basic biological building blocks for the development of sophisticated soft nanomaterials, including supramolecular hydrogels.

Supramolecular hydrogels based on the epitope of potassium ion channels

Imparting aromatic-aromatic interactions to the potassium binding epitope affords a supramolecular hydrogelator that responds to the K(+) concentration by self-assembly into nanofibers of different widths and crosslinking patterns, which illustrates a simple approach to generate biomimic materials based on tunable, hierarchical self-assembly of small molecules.

Electron tomography of cryofixed, isometrically contracting insect flight muscle reveals novel actin-myosin interactions

Background

Isometric muscle contraction, where force is generated without muscle shortening, is a molecular traffic jam in which the number of actin-attached motors is maximized and all states of motor action are trapped with consequently high heterogeneity. This heterogeneity is a major limitation to deciphering myosin conformational changes in situ.

Methodology

We used multivariate data analysis to group repeat segments in electron tomograms of isometrically contracting insect flight muscle, mechanically monitored, rapidly frozen, freeze substituted, and thin sectioned. Improved resolution reveals the helical arrangement of F-actin subunits in the thin filament enabling an atomic model to be built into the thin filament density independent of the myosin. Actin-myosin attachments can now be assigned as weak or strong by their motor domain orientation relative to actin. Myosin attachments were quantified everywhere along the thin filament including troponin. Strong binding myosin attachments are found on only four F-actin subunits, the “target zone”, situated exactly midway between successive troponin complexes. They show an axial lever arm range of 77°/12.9 nm. The lever arm azimuthal range of strong binding attachments has a highly skewed, 127° range compared with X-ray crystallographic structures. Two types of weak actin attachments are described. One type, found exclusively in the target zone, appears to represent pre-working-stroke intermediates. The other, which contacts tropomyosin rather than actin, is positioned M-ward of the target zone, i.e. the position toward which thin filaments slide during shortening.

Conclusion

We present a model for the weak to strong transition in the myosin ATPase cycle that incorporates azimuthal movements of the motor domain on actin. Stress/strain in the S2 domain may explain azimuthal lever arm changes in the strong binding attachments. The results support previous conclusions that the weak attachments preceding force generation are very different from strong binding attachments.

Novel mode of cooperative binding between myosin and Mg2+ -actin filaments in the presence of low concentrations of ATP

Cooperative interaction between myosin and actin filaments has been detected by a number of different methods, and has been suggested to have some role in force generation by the actomyosin motor. In this study, we observed the binding of myosin to actin filaments directly using fluorescence microscopy to analyze the mechanism of the cooperative interaction in more detail. For this purpose, we prepared fluorescently labeled heavy meromyosin (HMM) of rabbit skeletal muscle myosin and Dictyostelium myosin II. Both types of HMMs formed fluorescent clusters along actin filaments when added at substoichiometric amounts. Quantitative analysis of the fluorescence intensity of the HMM clusters revealed that there are two distinct types of cooperative binding. The stronger form was observed along Ca(2+)-actin filaments with substoichiometric amounts of bound phalloidin, in which the density of HMM molecules in the clusters was comparable to full decoration. The novel, weaker form was observed along Mg(2+)-actin filaments with and without stoichiometric amounts of phalloidin. HMM density in the clusters of the weaker form was several-fold lower than full decoration. The weak cooperative binding required sub-micromolar ATP, and did not occur in the absence of nucleotides or in the presence of ADP and ADP-Vi. The G680V mutant of Dictyostelium HMM, which over-occupies the ADP-Pi bound state in the presence of actin filaments and ATP, also formed clusters along Mg(2+)-actin filaments, suggesting that the weak cooperative binding of HMM to actin filaments occurs or initiates at an intermediate state of the actomyosin-ADP-Pi complex other than that attained by adding ADP-Vi.

Dynein pulls microtubules without rotating its stalk

Dynein is a microtubule motor that powers motility of cilia and flagella. There is evidence that the relative sliding of the doublet microtubules is due to a conformational change in the motor domain that moves a microtubule bound to the end of an extension known as the stalk. A predominant model for the movement involves a rotation of the head domain, with its stalk, toward the microtubule plus end. However, stalks bound to microtubules have been difficult to observe. Here, we present the clearest views so far of stalks in action, by observing sea urchin, outer arm dynein molecules bound to microtubules, with a new method, "cryo-positive stain" electron microscopy. The dynein molecules in the complex were shown to be active in in vitro motility assays. Analysis of the electron micrographs shows that the stalk angles relative to microtubules do not change significantly between the ADP.vanadate and no-nucleotide states, but the heads, together with their stalks, shift with respect to their A-tubule attachments. Our results disagree with models in which the stalk acts as a lever arm to amplify structural changes. The observed movement of the head and stalk relative to the tail indicates a new plausible mechanism, in which dynein uses its stalk as a grappling hook, catching a tubulin subunit 8 nm ahead and pulling on it by retracting a part of the tail (linker).

Cross-bridge number, position, and angle in target zones of cryofixed isometrically active insect flight muscle

Electron micrographic tomograms of isometrically active insect flight muscle, freeze substituted after rapid freezing, show binding of single myosin heads at varying angles that is largely restricted to actin target zones every 38.7 nm. To quantify the parameters that govern this pattern, we measured the number and position of attached myosin heads by tracing cross-bridges through the three-dimensional tomogram from their origins on 14.5-nm-spaced shelves along the thick filament to their thin filament attachments in the target zones. The relationship between the probability of cross-bridge formation and axial offset between the shelf and target zone center was well fitted by a Gaussian distribution. One head of each myosin whose origin is close to an actin target zone forms a cross-bridge most of the time. The probability of cross-bridge formation remains high for myosin heads originating within 8 nm axially of the target zone center and is low outside 12 nm. We infer that most target zone cross-bridges are nearly perpendicular to the filaments (60% within 11 degrees ). The results suggest that in isometric contraction, most cross-bridges maintain tension near the beginning of their working stroke at angles near perpendicular to the filament axis. Moreover, in the absence of filament sliding, cross-bridges cannot change tilt angle while attached nor reach other target zones while detached, so may cycle repeatedly on and off the same actin target monomer.

Ca2+ causes release of myosin heads from the thick filament surface on the milliseconds time scale

We have used electron microscopy to study the structural changes induced when myosin filaments are activated by Ca2+. Negative staining reveals that when Ca2+ binds to the heads of relaxed Ca2+ -regulated myosin filaments, the helically ordered myosin heads become disordered and project further from the filament surface. Cryo-electron microscopy of unstained, frozen-hydrated specimens supports this finding, and shows that disordering is reversible on removal of Ca2+. The structural change is thus a result of Ca2+ binding alone and not an artifact of staining. Comparison of the two techniques suggests that negative staining preserves the structure induced by Ca2+ -binding. We therefore used a time-resolved negative staining technique to determine the time scale of the structural change. Full disordering was observed within 30 ms of Ca2+ addition, and had started to occur within 10 ms, showing that the change occurs on the physiological time scale. Comparison with studies of single heavy meromyosin molecules suggests that an increased mobility of myosin heads induced by Ca2+ binding underlies the changes in filament structure that we observe. We conclude that the loosening of the array of myosin heads that occurs on activation is real and physiological; it may function to make activated myosin heads freer to contact actin filaments during muscle contraction.

Capturing time-resolved changes in molecular structure by negative staining

Imaging structural intermediates of biological processes is a key step in understanding biological function. Because intermediates are commonly short-lived, lasting only milliseconds, the main methods used to capture them have been conventional imaging of analog or inhibited states, having extended lifetimes, or rapid (millisecond timescale) freezing of intermediates with subsequent observation by cryo-EM. We have developed a simpler method that fixes structure on the millisecond timescale. The procedure consists of briefly (milliseconds) exposing the macromolecular structure of interest on an EM grid to conditions that initiate the structural change, then immediately fixing with uranyl acetate or tannic acid. Specimens are then observed by negative staining. The key finding that validates this approach is our demonstration that uranyl acetate, and in some cases tannic acid, fixes protein molecular structure on the millisecond timescale. This is demonstrated by our observation that exposure of actin and myosin filaments to these fixatives for as little as 10 ms is sufficient to fully preserve them against changes that normally induce rapid and major alteration in their molecular structure. Fixation appears to stabilize both ionic and hydrophobic bonds. This approach should be of general utility for studying transient molecular changes in many systems.

A model of cross-bridge attachment to actin in the A*M*ATP state based on x-ray diffraction from permeabilized rabbit psoas muscle

A model of cross-bridges binding to actin in the weak binding A*M*ATP state is presented. The modeling was based on the x-ray diffraction patterns from the relaxed skinned rabbit psoas muscle fibers where ATP hydrolysis was inhibited by N-phenylmaleimide treatment (S. Xu, J. Gu, G. Melvin, L. C. Yu. 2002. Biophys. J. 82:2111-2122). Calculations included both the myosin filaments and the actin filaments of the muscle cells, and the binding to actin was assumed to be single headed. To achieve a good fit, considerable flexibility in the orientation of the myosin head and the position of the S1-S2 junction is necessary, such that the myosin head can bind to a nearby actin whereas the tail end was kept in the proximity of the helical track of the myosin filament. Hence, the best-fit model shows that the head binds to actin in a wide range of orientations, and the tail end deviates substantially from its lattice position in the radial direction (approximately 60 A). Surprisingly, the best fit model reveals that the detached head, whose location thus far has remained undetected, seems to be located close to the surface of the myosin filament. Another significant requirement of the best-fit model is that the binding site on actin is near the N terminus of the actin subunit, a position distinct from the putative rigor-binding site. The results support the idea that the essential role played by the weak binding states M*ATP <--> A*M*ATP for force generation lies in its flexibility, because the probability of attachment is greatly increased, compared with the weak binding M*ADP*P(i) <--> A*M*ADP*P(i) states.

Mammalian cardiac muscle thick filaments: their periodicity and interactions with actin

Cardiac muscle has been extensively studied, but little information is available on the detailed macromolecular structure of its thick filament. To elucidate the structure of these filaments I have developed a procedure to isolate the cardiac thick filaments for study by electron microscopy and computer image analysis. This procedure uses chemical skinning with Triton X-100 to avoid contraction of the muscle that occurs using the procedures previously developed for isolation of skeletal muscle thick filaments. The negatively stained isolated filaments appear highly periodic, with a helical repeat every third cross-bridge level (43 nm). Computed Fourier transforms of the filaments show a strong set of layer lines corresponding to a 43-nm near-helical repeat out to the 6th layer line. Additional meridional reflections extend to at least the 12th layer line in averaged transforms of the filaments. The highly periodic structure of the filaments clearly suggests that the weakness of the layer lines in x-ray diffraction patterns of heart muscle is not due to an inherently more disordered cross-bridge arrangement. In addition, the isolated thick filaments are unusual in their strong tendency to remain bound to actin by anti-rigor oriented cross-bridges (state II or state III cross-bridges) under relaxing conditions.

Crossbridge and tropomyosin positions observed in native, interacting thick and thin filaments

Tropomyosin movements on thin filaments are thought to sterically regulate muscle contraction, but have not been visualized during active filament sliding. In addition, although 3-D visualization of myosin crossbridges has been possible in rigor, it has been difficult for thick filaments actively interacting with thin filaments. In the current study, using three-dimensional reconstruction of electron micrographs of interacting filaments, we have been able to resolve not only tropomyosin, but also the docking sites for weak and strongly bound crossbridges on thin filaments. In relaxing conditions, tropomyosin was observed on the outer domain of actin, and thin filament interactions with thick filaments were rare. In contracting conditions, tropomyosin had moved to the inner domain of actin, and extra density, reflecting weakly bound, cycling myosin heads, was also detected, on the extreme periphery of actin. In rigor conditions, tropomyosin had moved further on to the inner domain of actin, and strongly bound myosin heads were now observed over the junction of the inner and outer domains. We conclude (1) that tropomyosin movements consistent with the steric model of muscle contraction occur in interacting thick and thin filaments, (2) that myosin-induced movement of tropomyosin in activated filaments requires strongly bound crossbridges, and (3) that crossbridges are bound to the periphery of actin, at a site distinct from the strong myosin binding site, at an early stage of the crossbridge cycle.

X-ray diffraction evidence for the lack of stereospecific protein interactions in highly activated actomyosin complex

The structure of actomyosin complex while hydrolyzing ATP was investigated by recording X-ray diffraction patterns from rabbit skeletal muscle fibers, in which exogenously introduced rabbit skeletal subfragment-1 (S1) was covalently cross-linked to the endogenous actin filaments in rigor by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC). Approximately two-thirds of the introduced S1 was cross-linked. The cross-linking procedure did not affect the profile of the S1-induced enhancement of the actin-based layer line reflections in rigor, indicating that the acto-S1 interactions remained highly stereospecific. In the presence of ATP, the MgATPase of the S1 was highly activated regardless of calcium levels, presumably because the availability of the stereospecific binding sites for both proteins was maximized by the cross-linking. However, the diffraction pattern in the presence of ATP was striking in that the intensity profile of the strong 1/5.9 nm(-1) layer lines was indistinguishable from that from bare actin filaments, despite the fact that the majority of the S1 was still associated with actin. The change of the intensity profiles upon addition of ATP was completely reversible. Model calculations showed that this result can be explained if the S1 is not only swinging around its pivoting point, but the pivoting point itself is also moving on the actin surface in a range of a few nanometers. The results suggest that the stereospecific binding sites, which have been considered important for actomyosin cycling, are paradoxically left unoccupied for most of the time in this highly activated actomyosin complex.

Intrinsic tryptophan fluorescence identifies specific conformational changes at the actomyosin interface upon actin binding and ADP release

The helix-loop-helix (A-site) and myopathy loop (R-site) are located on opposite sides of the cleft that separates the proposed actin-binding interface of myosin. To investigate the structural features of the A- and R-sites, we engineered two mutants of the smooth muscle myosin motor domain with the essential light chain (MDE), containing a single tryptophan located either in the A-site (W546-MDE) or in the R-site (V413W MDE). W546- and V413W-MDE display actin-activated ATPase and actin-binding properties similar to those of wild-type MDE. The steady-state fluorescence properties of W546-MDE [emission peak (lambda(max)) = 344, quantum yield = 0.20, and acrylamide bimolecular quenching constant (k(q)) = 6.4 M(-)(1). ns(-)(1)] and V413W-MDE [lambda(max) = 338, quantum yield = 0.27, and k(q) = 3.6 M(-)(1).ns(-)(1)] demonstrate that Trp-546 and Trp-413 are nearly fully exposed to solvent, in agreement with the crystallographic data on these residues. In the presence of actin, Trp-546 shifts to a more buried environment in both the ADP-bound and nucleotide-free (rigor) actomyosin complexes, as indicated by an average lambda(max) of 337 or 336 nm, respectively, and protection from dimethyl(2-hydroxy-5-nitrobenzyl)sulfonium bromide (DHNBS) oxidation. In contrast, Trp-413 has a single conformation with an average lambda(max) of 338 nm in the ADP-bound complex, but in the rigor complex it is 50% more accessible to DHNBS oxidation and can adopt a range of possible conformations (lambda(max) = 341-347 nm). Our results suggest a structural model in which the A-site remains tightly bound to actin and the R-site adopts a more flexible and solvent-exposed conformation upon ADP release.

Observation of transient disorder during myosin subfragment-1 binding to actin by stopped-flow fluorescence and millisecond time resolution electron cryomicroscopy: evidence that the start of the crossbridge power stroke in muscle has variable geometry

The mechanism of binding of myosin subfragment-1 (S1) to actin in the absence of nucleotides was studied by a combination of stopped-flow fluorescence and ms time resolution electron microscopy. The fluorescence data were obtained by using pyrene-labeled actin and exhibit a lag phase. This demonstrates the presence of a transient intermediate after the collision complex and before the formation of the stable "rigor" complex. The transient intermediate predominates 2-15 ms after mixing, whereas the rigor complex predominates at time >50 ms. Electron microscopy of acto-S1 frozen 10 ms after mixing revealed disordered binding. Acto-S1 frozen at 50 ms or longer showed the "arrowhead" appearance characteristic of rigor. The most likely explanation of the disorder of the transient intermediate is that the binding is through one or more flexible loops on the surfaces of the proteins. The transition from disordered to ordered binding is likely to be part of the force-generating step in muscle.

Microsecond rotational dynamics of spin-labeled myosin regulatory light chain induced by relaxation and contraction of scallop muscle

We have used saturation transfer electron paramagnetic resonance (ST-EPR) to study the rotational dynamics of spin-labeled regulatory light chain (RLC) in scallop (Placopecten magellanicus) muscle fibers. The single cysteine (Cys 51) in isolated clam (Mercenaria) RLC was labeled with an indanedione spin label (InVSL). RLC was completely and specifically extracted from scallop striated muscle fibers, eliminating the Ca sensitivity of ATPase activity and isometric force, which were both completely restored by stoichiometric incorporation of labeled RLC. The EPR spectrum of the isolated RLC revealed nanosecond rotational motions within the RLC, which were completely eliminated when the labeled RLC was bound to myosin heads in myofibrils or fibers in rigor. This is the most strongly immobilized RLC-bound probe reported to date and thus offers the most reliable detection of the overall rotational motion of the LC domain. Conventional EPR spectra of oriented fibers indicated essentially complete probe disorder, independent of ATP and Ca, eliminating orientational dependence and thus making this probe ideal for unambiguous measurement of microsecond rotational motions of the LC domain by ST-EPR. ST-EPR spectra of fibers in rigor indicated an effective rotational correlation time (taureff) of 140 +/- 5 microseconds, similar to that observed for the same spin label bound to the catalytic domain. Relaxation by ATP induced microsecond rotational motion (taureff = 70 +/- 4 microseconds), and this motion was slightly slower upon Ca activation of isometric contraction (taureff = 100 +/- 5 microseconds). These motions in relaxation and contraction are similar to, but slower than, the motions previously reported for the same spin label bound to the catalytic domain. These results support a model for force generation involving rotational motion of the LC domain relative to the catalytic domain and dynamic disorder-to-order transitions in both domains.

X-ray diffraction studies on the structural changes of rigor muscles induced by binding of phosphate analogs in the presence of MgADP

To clarify the structure of the ATP hydrolysis intermediates (ADP.Pi bound state) formed by actomyosin crossbridges, the effects of various phosphate analogs in the presence of MgADP on the structures of the thin and thick filaments in glycerinated rabbit psoas muscle fibers in the rigor state have been investigated by X-ray diffraction with a short exposure time using synchrotron radiation. When MgADP and phosphate analogs such as metallofluorides (BeFx = 3,4 and AlF4) and vanadate (VO4(Vi)) were added to rigor fibers in the presence of the ATP-depletion backup system, the intensities of the actin-based layer lines were markedly weakened. The greatest effect (approximately 50% decrease in intensity) was observed in the presence of BeFx among the analogs examined. The intensity distribution of the 5.9 nm actin-based layer line shifted towards that observed in the Ca(2+)-activated fibers, while the first actin layer line at approximately 1/36.7 nm-1 retained a rigor-like profile with an intensity weakened by approximately 50%. The intensity of the equatorial 10 reflection increased while that of the 11 reflection changed little, resulting in only a small increase (approximately 1.7 fold) in the intensity ratio of the 10 to the 11 reflection. No resting-like pattern appeared upon the addition of MgADP and BeFx. These results indicate that a substantial fraction (approximately 40%) of the myosin heads dissociate from actin but the detached heads remain in the vicinity of the actin filaments when MgADP and BeFx bind. The states produced by binding phosphate analogs to a rigor muscle differ from the resting-like state produced by adding them to a contracting muscle (Takemori et al., J. Biochem. (Tokyo) 117 (1995) 603-608). Our conclusion put forward to explain the data is that one of the two heads of a crossbridge is detached and the other retains a rigor-like attachment.

Quick-freeze deep-etch electron microscopy of the actin-heavy meromyosin complex during the in vitro motility assay

Since mica is a substitute for glass in the in vitro actin motility assay, I examined the structure of heavy meromyosin (HMM) crossbridges supporting actin filaments by quick-freeze deep-etch replica electron microscopy. This method was capable of resolving the inter-domain cleft of the monomeric actin molecule. HMM heads that are not bound to actin, when observed by this technique, were straight and elongated in the absence of ATP but strongly kinked upon addition of ATP or ADP.inorganic vanadate to produce the putative long-lived analog of HMM-ADP.inorganic phosphate. The low-magnification image of the ATP-containing acto-HMM preparation showed features characteristic of sliding actin filaments on glass coverslips. At high magnification, all the HMM molecules were found attached to actin by one head with the majority projecting perpendicular to the filament axis, whereas in the absence of ATP, HMM exhibited two-head binding with a preponderance of molecules tilted at 45 degrees. Detailed examination of the shape of HMM heads involved in sliding showed a rounded, and flat appearance of the tip and comparatively thin neck portion as if the heads grasp actin filament, in contrast to rigor crossbridges which have a pear-shaped configuration with more gradual taper. Such configurations of HMM heads were essentially the same as I observed previously on acto-myosin subfragment-1 (S1) by the same technique, except for the presence of an additional neck portion of HMM which makes interpretaion of the images easier. Interestingly, under actively sliding conditions, very few heads were tilted in the rigor configuration. At first glance, the addition of ADP to the rigor-complex gave images rather like those obtained with ATP, but they turned out to be different. The contribution of the structural change of crossbridges to the force development is discussed.

Tomographic three-dimensional reconstruction of insect flight muscle partially relaxed by AMPPNP and ethylene glycol

Rigor insect flight muscle (IFM) can be relaxed without ATP by increasing ethylene glycol concentration in the presence of adenosine 5'-[beta'gamma- imido]triphosphate (AMPPNP). Fibers poised at a critical glycol concentration retain rigor stiffness but support no sustained tension ("glycol-stiff state"). This suggests that many crossbridges are weakly attached to actin, possibly at the beginning of the power stroke. Unaveraged three-dimensional tomograms of "glycol-stiff" sarcomeres show crossbridges large enough to contain only a single myosin head, originating from dense collars every 14.5 nm. Crossbridges with an average 90 degrees axial angle contact actin midway between troponin subunits, which identifies the actin azimuth in each 38.7-nm period, in the same region as the actin target zone of the 45 degrees angled rigor lead bridges. These 90 degrees "target zone" bridges originate from the thick filament and approach actin at azimuthal angles similar to rigor lead bridges. Another class of glycol-PNP crossbridge binds outside the rigor actin target zone. These "nontarget zone" bridges display irregular forms and vary widely in axial and azimuthal attachment angles. Fitting the acto-myosin subfragment 1 atomic structure into the tomogram reveals that 90 degrees target zone bridges share with rigor a similar contact interface with actin, while nontarget crossbridges have variable contact interfaces. This suggests that target zone bridges interact specifically with actin, while nontarget zone bridges may not. Target zone bridges constitute only approximately 25% of the myosin heads, implying that both specific and nonspecific attachments contribute to the high stiffness. The 90 degrees target zone bridges may represent a preforce attachment that produces force by rotation of the motor domain over actin, possibly independent of the regulatory domain movements.

Critical review of the swinging crossbridge theory and of the cardinal active role of water in muscle contraction

A critical analysis is presented of the experimental findings that led to the sliding filament model and to its offspring--the swinging (by rotating or tilting) crossbridge theory of muscle contraction (SCBT). Several principles that have been taken for granted implicitly and explicitly by the creators of these dogmas are discussed. The failure of numerous efforts to verify predictions of the SCBT, particularly the idea that the myosin molecules undergo a major conformational change, is critically reviewed. Analysis of various experimental data suggests that water may play an active role in muscular contraction. Examination of both the experiments that do not fulfill the expectations of the SCBT and the measurements of water liberation during the "contractile" process suggests a new outlook according to which tension development and movement are not due to major conformational changes but rather to restructuring of the hydration shells of actin and myosin.

Cooperative rigor binding of myosin to actin is a function of F-actin structure

Many aspects of cooperative behavior within pure F-actin filaments have now been described. We have used two myosin fragments, heavy meromyosin (HMM) and Subfragment 1 (S1), to look at the rigor binding to different forms of F-actin. With Ca2+ bound at the high-affinity metal binding site in actin, there is a very large cooperativity in the binding of HMM, but no cooperativity for S1. With Mg2+ bound at the high affinity site, or with conditions that stabilize the conformation of subdomain-2 of actin, there is no cooperativity seen with either HMM or S1. These results show that the two heads of HMM can induce structural changes in F-actin that are not observed with the single head of S1. They also support the notion that the binding of myosin to F-actin induces a conformational change in subdomain-2 of actin, and that under certain conditions this conformational change can be cooperatively propagated through an actin filament.

Structural changes in muscle crossbridges accompanying force generation

We have investigated the structure of the crossbridges in muscles rapidly frozen while relaxed, in rigor, and at various times after activation from rigor by flash photolysis of caged ATP. We used Fourier analysis of images of cross sections to obtain an average view of the muscle structure, and correspondence analysis to extract information about individual crossbridge shapes. The crossbridge structure changes dramatically between relaxed, rigor, and with time after ATP release. In relaxed muscle, most crossbridges are detached. In rigor, all are attached and have a characteristic asymmetric shape that shows strong left-handed curvature when viewed from the M-line towards the Z-line. Immediately after ATP release, before significant force has developed (20 ms) the homogeneous rigor population is replaced by a much more diverse collection of crossbridge shapes. Over the next few hundred milliseconds, the proportion of attached crossbridges changes little, but the distribution of the crossbridges among different structural classes continues to evolve. Some forms of attached crossbridge (presumably weakly attached) increase at early times when tension is low. The proportion of several other attached non-rigor crossbridge shapes increases in parallel with the development of active tension. The results lend strong support to models of muscle contraction that have attributed force generation to structural changes in attached crossbridges.

Orientation and dynamics of myosin heads in aluminum fluoride induced pre-power stroke states: an EPR study

We have determined the orientation and dynamics of the putative pre-power stroke crossbridges in skinned muscle fibers labeled with maleimide spin-label at Cys-707 of myosin. Orientation was measured using electron paramagnetic resonance (EPR) and mobility by saturation transfer EPR. The crossbridges are trapped in the pre-power stroke conformation in the presence of aluminum fluoride, Ca, and ATP. In agreement with data published for unlabeled fibers (Chase et al., 1994), spin-labeled muscle fibers display 42.5% of rigor stiffness, without the generation of force. The trapped crossbridges are as disordered as the relaxed heads, but their microsecond dynamics are significantly restricted. Modeling of the immobile fraction (35%), in terms of attached heads as estimated from stiffness, suggests that the bound heads rotate with a correlation time tau r = 150-400 microseconds, as compared to tau r = 3 microseconds for the heads in relaxed fibers. These "strongly" attached myosin heads, at orientations other than in rigor, are a candidate for the state from which head rotation generates force, as postulated by H. E. Huxley (1969). Ordering of the heads may well be the structural event driving the generation of force.

Unfixed cryosections of striated muscle to study dynamic molecular events

The structures of the actin and myosin filaments of striated muscle have been studied extensively in the past by sectioning of fixed specimens. However, chemical fixation alters molecular details and prevents biochemically induced structural changes. To overcome these problems, we investigate here the potential of cryosectioning unfixed muscle. In cryosections of relaxed, unfixed specimens, individual myosin filaments displayed the characteristic helical organization of detached cross-bridges, but the filament lattice had disintegrated. To preserve both the filament lattice and the molecular structure of the filaments, we decided to section unfixed rigor muscle, stabilized by actomyosin cross-bridges. The best sections showed periodic, angled cross-bridges attached to actin and their Fourier transforms displayed layer lines similar to those in x-ray diffraction patterns of rigor muscle. To preserve relaxed filaments in their original lattice, unfixed sections of rigor muscle were picked up on a grid and relaxed before negative staining. The myosin and actin filaments showed the characteristic helical arrangements of detached cross-bridges and actin subunits, and Fourier transforms were similar to x-ray patterns of relaxed muscle. We conclude that the rigor structure of muscle and the ability of the filament lattice to undergo the rigor-relaxed transformation can be preserved in unfixed cryosections. In the future, it should be possible to carry out dynamic studies of active sacromeres by cryo-electron microscopy.

Rotational dynamics of actin-bound intermediates of the myosin adenosine triphosphatase cycle in myofibrils

We have used saturation transfer electron paramagnetic resonance (ST-EPR) to measure the microsecond rotational motion of actin-bound myosin heads in spin-labeled myofibrils in the presence of the ATP analogs AMPPNP (5'-adenylylimido-diphosphate) and ATP gamma S (adenosine-5'-O-(3-thiotriphosphate)). AMPPNP and ATP gamma S are believed to trap myosin in two major conformational intermediates of the actomyosin ATPase cycle, respectively known as the weakly bound and strongly bound states. Previous ST-EPR experiments with solutions of acto-S1 have demonstrated that actin-bound myosin heads are rotationally mobile on the microsecond time scale in the presence of ATP gamma S, but not in the presence of AMPPNP. However, it is not clear that results obtained with acto-S1 in solution can be extended to actomyosin constrained within the myofibrillar lattice. Therefore, ST-EPR spectra of spin-labeled myofibrils were analyzed explicitly in terms of the actin-bound component of myosin heads in the presence of AMPPNP and ATP gamma S. The fraction of actin-attached myosin heads was determined biochemically in the spin-labeled myofibrils, using the proteolytic rates actomyosin binding assay. At physiological ionic strength (mu = 165 mM), actin-bound myosin heads were found to be rotationally mobile on the microsecond time scale (tau r = 24 +/- 8 microseconds) in the presence of ATP gamma S, but not AMPPNP. Similar results were obtained at low ionic strength, confirming the acto-S1 solution studies. The microsecond rotational motions of actin-attached myosin heads in the presence of ATP gamma S are similar to those observed for spin-labeled myosin heads during the steady-state cycling of the actomyosin ATPase, both in solution and in an active isometric muscle fiber. These results indicate that weakly bound myosin heads, in the pre-force phase of the ATPase cycle, are rotationally mobile, while strongly bound heads, in the force-generating phase, are rotationally immobile. We propose that force generation involves a transition from a dynamically disordered crossbridge to a rigid and stereospecific one.

Electron cryomicroscopy of acto-myosin-S1 during steady-state ATP hydrolysis

The structure of the complex of actin and myosin subfragment-1 (S1) during steady-state ATP hydrolysis has been examined by electron microscopy. This complex is normally dissociated by ATP in vitro but was stabilized here by low ionic strength. Optimal conditions for attachment were established by light-scattering experiments that showed that approximately 70% of S1 could be bound in the presence of ATP. Micrographs of the unstained complex in vitreous water suggest that S1 attaches to actin in a variety of configurations in ATP; this contrasts with the single attached configuration seen in the presence of ADP. The data are therefore compatible with the idea that a change in attached configuration of the myosin cross-bridge is the origin of muscle force. In control experiments where ATP was allowed to hydrolyze completely the binding of the S1 seemed cooperative.

Structural change of crossbridges of rabbit skeletal muscle during isometric contraction

Structural changes of crossbridges during isometric contraction have been studied by electron microscopy. Chemically skinned rabbit fibres were rapidly frozen either in activating solution or in ATP-free (rigor) solution, freeze-substituted and embedded. Longitudinal sections of muscle fibres show that the number of crossbridges in active fibres (isometric contraction) is approximately the same as in rigor fibres. Crossbridges of the active and rigor states differ in their shapes, angles and manner of arrangement on the thin filaments. In rigor many crossbridges are wide near the thin filaments and narrow near the thick filament shafts; in active fibres they have more uniform width along their length. The angle of the crossbridges in active fibres is somewhat variable. The average angle is approximately 90 degrees to the filament axis. The crossbridges are arranged on the thin filament retaining the 14.3 nm thick filament periodicity. The crossbridges in rigor are tilted and their arrangement near the thin filament reveals the 36 nm actin periodicity. The variability in the shapes of the crossbridges in active fibres is still higher when we look at them in cross-sections of muscle fibres. The crossbridge shapes in the cross-sections were classified and the relative frequency of different shapes was determined. The shapes that are commonly observed in active fibres are similar in that the majority of the mass of the crossbridges is farther away from the thin filament than the crossbridges in rigor fibres.

Flash and smash: rapid freezing of muscle fibers activated by photolysis of caged ATP

A new approach was used to study transient structural states of cross-bridges during activation of muscle fibers. Rabbit skinned muscle fibers were rapidly and synchronously activated from the rigor state by photolysis of caged ATP in the presence of Ca2+. At several different times during the switch from rigor to fully active tension development, the fibers were rapidly frozen on a liquid helium-cooled metal block, freeze-substituted, and examined in an electron microscope. The limits of structural preservation and resolution with this technique were analyzed. We demonstrate that the resolution of our images is sufficient to draw the following conclusions about cross-bridge structure. Rigor cross-bridges point away from the Z-line and most of them are wider near the thin filaments than near the backbone of the thick filaments. In contrast, cross-bridges in actively contracting fibers stretch between the thick and thin filaments at a variable angle, and are uniformly thin. Diffraction patterns computed from contracting muscle show layer lines both at 38 and 43 nm indicating that active cross-bridges contribute mass to both the actin- and myosin-based helical periodicities. The images obtained from fibers frozen 20 ms after release of ATP show a mixture of rigor and active type cross-bridge configurations. There is little evidence of cross-bridges with the rigor shape by 50 ms, and the difference in configurations between 50 and 300 ms after photolysis is surprisingly subtle.

Time-resolved electron microscopic analysis of the behavior of myosin heads on actin filaments after photolysis of caged ATP

The interaction between myosin subfragment 1 (S1) and actin filaments after the photolysis of P3-1-(2-nitrophenyl)ethyl ester of ATP (caged ATP) was analyzed with a newly developed freezing system using liquid helium. Actin and S1 (100 microM each) formed a ropelike double-helix characteristic of rigor in the presence of 5 mM caged ATP at room temperature. At 15 ms after photolysis, the ropelike double helix was partially disintegrated. The number of S1 attached to actin filaments gradually decreased up to 35 ms after photolysis, and no more changes were detected from 35 to 200 ms. After depletion of ATP, the ropelike double helix was reformed. Taking recent analyses of actomyosin kinetics into consideration, we concluded that most S1 observed on actin filaments at 35-200 ms are so called "weakly bound S1" (S1.ATP or S1.ADP.Pi) and that the weakly bound S1 under a rapid association-dissociation equilibrium with actin filaments can be captured by electron microscopy by means of our newly developed freezing system. This enabled us to directly compare the conformation of weakly and strongly bound S1. Within the resolution of deep-etch replica technique, there were no significant conformational differences between weakly and strongly bound S1, and neither types of S1 showed any positive cooperativity in their binding to actin filaments. Close comparison revealed that the weakly and strongly bound S1 have different angles of attachment to actin filaments. As compared to strongly bound S1, weakly bound S1 showed a significantly broader distribution of attachment angles. These results are discussed with special reference to the molecular mechanism of acto-myosin interaction in the presence of ATP.

Direct visualization by electron microscopy of the weakly bound intermediates in the actomyosin adenosine triphosphatase cycle

We used a novel stopped-flow/rapid-freezing machine to prepare the transient intermediates in the actin-myosin adenosine triphosphatase (ATPase) cycle for direct observation by electron microscopy. We focused on the low affinity complexes of myosin-adenosine triphosphate (ATP) and myosin-adenosine diphosphate (ADP)-Pi with actin filaments since the transition from these states to the high affinity actin-myosin-ADP and actin-myosin states is postulated to generate the molecular motion that drives muscle contraction and other types of cellular movements. After rapid freezing and metal replication of mixtures of myosin subfragment-1, actin filaments, and ATP, the structure of the weakly bound intermediates is indistinguishable from nucleotide-free rigor complexes. In particular, the average angle of attachment of the myosin head to the actin filament is approximately 40 degrees in both cases. At all stages in the ATPase cycle, the configuration of most of the myosin heads bound to actin filaments is similar, and the part of the myosin head preserved in freeze-fracture replicas does not tilt by more than a few degrees during the transition from the low affinity to high affinity states. In contrast, myosin heads chemically cross-linked to actin filaments differ in their attachment angles from ordered at 40 degrees without ATP to nearly random in the presence of ATP when viewed by negative staining (Craig, R., L.E. Greene, and E. Eisenberg. 1985. Proc. Natl. Acad. Sci. USA. 82:3247-3251, and confirmed here), freezing in vitreous ice (Applegate, D., and P. Flicker. 1987. J. Biol. Chem. 262:6856-6863), and in replicas of rapidly frozen samples. This suggests that many of the cross-linked heads in these preparations are dissociated from but tethered to the actin filaments in the presence of ATP. These observations suggest that the molecular motion produced by myosin and actin takes place with the myosin head at a point some distance from the actin binding site or does not involve a large change in the shape of the myosin head.