Three-dimensional reconstruction of thin filaments decorated with a Ca2+-regulated myosin

High-speed AFM reveals subsecond dynamics of cardiac thin filaments upon Ca2+ activation and heavy meromyosin binding

The advent of high-speed atomic force microscopy (HS-AFM) changed the field of biology considerably. HS-AFM is the only method where in situ dynamics of biological samples and imaging can be coupled with a spatial resolution of 1 to 5 nm in the horizontal direction. Unlike electron or cryo-electron microscopy, HS-AFM does not require fixation or freezing of the samples, and has the ability to derive kinetic parameters by recording the live movements of single-molecule dynamics. In this paper, we used HS-AFM to investigate directly the mechanisms of cardiac muscle activation. We visualized the muscle regulatory tropomyosin–troponin complex movements during activation by calcium or myosin (motor that drives contraction), and the structural transitions that happen during these events.

High-speed atomic force microscopy (HS-AFM) can be used to study dynamic processes with real-time imaging of molecules within 1- to 5-nm spatial resolution. In the current study, we evaluated the 3-state model of activation of cardiac thin filaments (cTFs) isolated as a complex and deposited on a mica-supported lipid bilayer. We studied this complex for dynamic conformational changes 1) at low and high [Ca²⁺] (pCa 9.0 and 4.5), and 2) upon myosin binding to the cTF in the nucleotide-free state or in the presence of ATP. HS-AFM was used to directly visualize the tropomyosin–troponin complex and Ca²⁺-induced tropomyosin movements accompanied by structural transitions of actin monomers within cTFs. Our data show that cTFs at relaxing or activating conditions are not ultimately in a blocked or activated state, respectively, but rather the combination of states with a prevalence that is dependent on the [Ca²⁺] and the presence of weakly or strongly bound myosin. The weakly and strongly bound myosin induce similar changes in the structure of cTFs as confirmed by the local dynamical displacement of individual tropomyosin strands in the center of a regulatory unit of cTF at the relaxed and activation conditions. The displacement of tropomyosin at the relaxed conditions had never been visualized directly and explains the ability of myosin binding to TF at the relaxed conditions. Based on the ratios of nonactivated and activated segments within cTFs, we proposed a mechanism of tropomyosin switching from different states that includes both weakly and strongly bound myosin.

Electron Microscopy: From 2D to 3D Images with Special Reference to Muscle

This is a brief and necessarily very sketchy presentation of the evolution in electron microscopy (EM) imaging that was driven by the necessity of extracting 3-D views from the essentially 2-D images produced by the electron beam. The lens design of standard transmission electron microscope has not been greatly altered since its inception. However, technical advances in specimen preparation, image collection and analysis gradually induced an astounding progression over a period of about 50 years. From the early images that redefined tissues, cell and cell organelles at the sub-micron level, to the current nano-resolution reconstructions of organelles and proteins the step is very large. The review is written by an investigator who has followed the field for many years, but often from the sidelines, and with great wonder. Her interest in muscle ultrastructure colors the writing. More specific detailed reviews are presented in this issue.

Isolation, electron microscopy and 3D reconstruction of invertebrate muscle myofilaments

Understanding the molecular mechanism of muscle contraction and its regulation has been greatly influenced and aided by studies of myofilament structure in invertebrate muscles. Invertebrates are easily obtained and cover a broad spectrum of species and functional specializations. The thick (myosin-containing) filaments from some invertebrates are especially stable and simple in structure and thus much more amenable to structural analysis than those of vertebrates. Comparative studies of invertebrate filaments by electron microscopy and image processing have provided important generalizations of muscle molecular structure and function. This article reviews methods for preparing thick and thin filaments from invertebrate muscle, for imaging filaments by electron microscopy, and for determining their three dimensional structure by image processing. It also highlights some of the key insights into filament function that have come from these studies.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

An actin subdomain 2 mutation that impairs thin filament regulation by troponin and tropomyosin

Striated muscle thin filaments adopt different quaternary structures, depending upon calcium binding to troponin and myosin binding to actin. Modification of actin subdomain 2 alters troponin-tropomyosin-mediated regulation, suggesting that this region of actin may contain important protein-protein interaction sites. We used yeast actin mutant D56A/E57A to examine this issue. The mutation increased the affinity of tropomyosin for actin 3-fold. The addition of Ca(2+) to mutant actin filaments containing troponin-tropomyosin produced little increase in the thin filament-myosin S1 MgATPase rate. Despite this, three-dimensional reconstruction of electron microscope images of filaments in the presence of troponin and Ca(2+) showed tropomyosin to be in a position similar to that found for muscle actin filaments, where most of the myosin binding site is exposed. Troponin-tropomyosin bound with comparable affinity to mutant and wild type actin in the absence and presence of calcium, and in the presence of myosin S1, tropomyosin bound very tightly to both types of actin. The mutation decreased actin-myosin S1 affinity 13-fold in the presence of troponin-tropomyosin and 2.6-fold in the absence of the regulatory proteins. The results suggest the importance of negatively charged actin subdomain 2 residues 56 and 57 for myosin binding to actin, for tropomyosin-actin interactions, and for regulatory conformational changes in the actin-troponin-tropomyosin complex.

Conformational changes due to calcium-induced calmodulin dissociation in brush border myosin I-decorated F-actin revealed by cryoelectron microscopy and image analysis

Brush border myosin I (BBMI) is a single-headed molecular motor. Its catalytic domain exhibits extensive sequence homology to the catalytic domain of myosin II, while its tail lacks the coiled-coil nature of myosin II. The BBMI tail domain contains at least three IQ motifs and binds calmodulin. Addition of calcium removes one of these calmodulin light chains, with effects on ATPase activity and motility in in vitro assays. Using the techniques of cryoelectron microscopy and helical image analysis we have calculated three-dimensional (3D) maps of BBMI-decorated actin filaments prepared in the presence and absence of calcium. The 3D maps describe a BBMI catalytic domain that is strikingly similar to the catalytic domain of myosin II subfragment 1 (S1), with the exception of a short amino-terminal region of the heavy chain, which is absent from BBMI. The tail domains of BBMI and S1 are highly divergent in structure, continuing on from their respective motor domains with very different geometries. Addition of calcium to BBMI, and the concomitant loss of a calmodulin light chain, results in an extensive reorganization of mass in the tail domain.

Kinetics of association of myosin subfragment-1 to unlabeled and pyrenyl-labeled actin

The kinetics of reaction of myosin subfragment-1 (S1) with F-actin have been monitored by the changes in light scattering and in pyrenyl-actin fluorescence at 20 degrees C, pH 7.5, and physiological ionic strength. The association rate constant of S1 to F-actin decreases about 10-fold as the molar ratio of bound S1 increases from 0 to 1. This decrease in k+ is most likely due to the steric hindrance of available binding sites by initially bound S1. The apparent rate constant for association of S1 to bare filaments is 9 microM-1 s-1, a value 1 order of magnitude higher than the one previously estimated from experiments in which S1 was in excess over F-actin. The anticooperative binding kinetics of S1 to F-actin are consistent with the negative cooperativity displayed in the equilibrium binding curves of S1 to pyrenyl-F-actin. Fluorescence titration curves of partially labeled pyrenyl-F-actin by S1 are sigmoidal, consistent with a 4-fold higher affinity of S1 for unlabeled than for labeled action. This conclusion is strengthened by kinetic data of S1 binding to partially labeled F-actin, which exhibit a biphasic behavior due to the slower dissociation of S1 from unlabeled than from labeled actin.

PHOELIX: a package for semi-automated helical reconstruction

We describe a set of procedures and algorithms which have been developed to provide an efficient and reliable method for reconstructing a three-dimensional density map from specimens with helical symmetry. These procedures build on the original MRC helical processing suite, with extensions principally developed using the SUPRIM image processing package. Actomyosin is used as a model specimen to demonstrate the utility of this repackaged and expanded set of routines. The time required to complete a three-dimensional map has been reduced from several weeks using traditional manual techniques to a few days. The increased signal/noise provided has allowed for the extraction of additional layer lines not previously identified by manual techniques.

Proposal of alignment-independent classification of electron microscopic images with helical symmetry and its application to reconstituted thin filaments os skeletal muscle

A new method to classify noisy images of filaments with helical symmetry was developed and applied to cryo-electron microscopic images of skeletal muscle thin filaments. In this method, images are transformed to minimum data sets which are independent of the relative view parameters, such as rotation of a three-dimensional object around the axis of helical symmetry, translation along an axis and operation to make a filament upside down, and classified by a principal component analysis. By this method, atypical data can be identified without aligning procedures, and small structural changes and polymorphic variation are expected to be detected objectively.

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.

Determination of the alpha-actinin-binding site on actin filaments by cryoelectron microscopy and image analysis

The three-dimensional structure of actin filaments decorated with the actin-binding domain of chick smooth muscle alpha-actinin (alpha A1-2) has been determined to 21-A resolution. The shape and location of alpha A1-2 was determined by subtracting maps of F-actin from the reconstruction of decorated filaments. alpha A1-2 resembles a bell that measures approximately 38 A at its base and extends 42 A from its base to its tip. In decorated filaments, the base of alpha A1-2 is centered about the outer face of subdomain 2 of actin and contacts subdomain 1 of two neighboring monomers along the long-pitch (two-start) helical strands. Using the atomic model of F-actin (Lorenz, M., D. Popp, and K. C. Holmes. 1993. J. Mol. Biol. 234:826-836.), we have been able to test directly the likelihood that specific actin residues, which have been previously identified by others, interact with alpha A1-2. Our results indicate that residues 86-117 and 350-375 comprise distinct binding sites for alpha-actinin on adjacent actin monomers.

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.

A functional recombinant myosin II lacking a regulatory light chain-binding site

Myosin II, which converts the energy of adenosine triphosphate hydrolysis into the movement of actin filaments, is a hexamer of two heavy chains, two essential light chains, and two regulatory light chains (RLCs). Dictyostelium myosin II is known to be regulated in vitro by phosphorylation of the RLC. Cells in which the wild-type myosin II heavy chain was replaced with a recombinant form that lacks the binding site for RLC carried out cytokinesis and almost normal development, processes known to be dependent on functional myosin II. Characterization of the purified recombinant protein suggests that a complex of RLC and the RLC binding site of the heavy chain plays an inhibitory role for adenosine triphosphatase activity and a structural role for the movement of myosin along actin.

A 13-A map of the actin-scruin filament from the limulus acrosomal process

We have determined the structure of the actin-scruin filament to 13-A resolution using a combination of low-dose EM and image analysis. The three-dimensional map reveals four actin-actin contacts: two within each strand and two between strands. The conformation of the actin subunit is different from that in the Holmes et al. (1990) model as refined by Lorenz et al. (1993). In particular, subdomain II is tilted in a similar way to that seen by Orlova and Egelman (1993) in F-Mg2(+)-ADP actin filaments in the absence of Ca2+. Scruin appears to consist of two domains of approximately equal volume. Each scruin subunit cross-links the two strands in the actin filament. Domain I of scruin contacts subdomain I of actin and makes a second contact at the junction of subdomains III and IV. Domain II of scruin contacts actin at subdomains I and II of a neighboring actin subunit. The two scruin domains thus bind differently to actin.

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.

In vitro production of enzymatically active myosin heavy chain

In order to initiate studies on the structural and functional relationships of the myosin heavy chain, we constructed a full-length complementary DNA encoding the isoform that is found in the fast white muscle of the embryonic chicken. The complementary DNA contained 108 basepairs of its 3'-untranslated region and was preceded by a leader sequence derived from the alfalfa mosaic virus. Similarly, a complementary DNA encoding 963 amino acids which encompass the subfragment-1 of myosin and part of the subfragment-2 was also constructed. Each was inserted into the expression vector pMT2 and transiently transfected into COS-1 cells. Both constructs directed the expression of the respective proteins, each of which was immunogenic. The full-length and subfragment-1 proteins interacted with actin and demonstrated high levels of a K(+)-activated, EDTA-resistant ATPase activity, which is characteristic of myosin.

Gross structural features of myosin head during sliding movement of actin as studied by quick-freeze deep-etch electron microscopy

With quick-freeze deep-etch electron microscopy coupled with mica-flake technique, I showed previously that myosin subfragment-1 (S1) attached to F-actin in the presence of ATP is short and rounded, in contrast to its elongated and tilted appearance under rigor condition [J. Biochem. 106, 751-770 (1989)]. I further indicated that each head of heavy meromyosin (HMM) changes its configuration in a likely manner as above by the addition of various nucleotides, i.e. heads were pear-shaped in the absence of nucleotide, in a ball-on-a-stick appearance when complexed with ADP and strongly kinked to the particular direction in the presence of ATP or ADP.Vi [J. Muscle Res. Cell Motility 12, 313 (1991)]. Such morphological data not only corroborates the independent biophysical evidences suggesting gross conformational changes of myosin head upon binding ATP or ADP.Vi, but also provide strong evidence for the distinct polarity in the structure of each myosin head. Negatively stained image of chemically cross-linked acto-S1 also included cross-bridges sharply kinked to the same direction, confirming the above observation. Attempts were made to examine if such conformational change of myosin cross-bridge occurs during actomyosin superprecipitation. Samples were quick-frozen during rapid turbidity-increasing phase where actin filaments actively slide past myosin heads. The resultant image included actin-attached myosin heads all in a kinked configuration with the same polarity as observed for HMM. Several heads associated with a single actin filament were bent to the same direction suggesting that myosin heads might be in a kinked configuration with distinct polarity during contraction.

Structure of the myosin head in solution and the effect of light chain 2 removal

Structural properties of rabbit skeletal myosin head (S1) and the influence of the DTNB light chain (LC2) on the size and shape of myosin heads in solution were investigated by small angle x-ray scattering. The LC2 deficient myosin head, S1 (-LC2), and the S1 containing LC2 light chain, S1 (+LC2) were studied in parallel. The respective values of the radius of gyration were found to be (40.2 +/- 0.5) A and (46.7 +/- 1) A, while the maximum dimension was (190 +/- 15) A for both species. The large difference between the two Rg values suggest that LC2 is located close to one extremity of the myosin head, in agreement with most electron microscopy observations. All models derived from the x-ray scattering pattern of the native myosin head share a common overall morphology, showing two main regions, an asymmetric globular portion which tapers smoothly into a thinner domain of roughly equivalent length making an angle of approximately 60 degrees, with a contour length of approximately 210 A.

Shape and length of myosin heads

There is controversy concerning the shape and length of myosin heads. In the present paper we try to analyse the data and to draw clear conclusions in this field. When the myosin heads are isolated (S1) from the rest of the molecule, their length is approximately 12 nm and their shape is close to that of a prolate ellipsoid with an axial ratio approximately 2.3 (in solution) or close to that of a comma when attached to F-actin (with a length of 12-13 nm). When the myosin heads are observed on a whole molecule, their length is approximately 19 nm and they are pear-shaped. Here we suggest that all these observations are compatible. We believe that, for a whole myosin molecule, a large part of the head-rod joint (S1/S2 joint) is measured with the head, owing to a particularly heavy staining or shadowing of this joint. On the other hand, S1 is probably built up of a head part plus the S1/S2 joint, which is not revealed by the usual techniques (hydrodynamics, X-ray and neutron scattering). Finally, the comma shape would be related to a flexible part in the head region of S1, which is significantly bent when S1 is attached to F-actin, but which would be less bent for S1 in solution. A similar bending also occurs in crystalline S1.

Interaction between the heavy and the regulatory light chains in smooth muscle myosin subfragment 1

The interaction between the heavy and the regulatory light chains within chicken gizzard myosin heads was investigated by using a zero-length chemical cross-linker, 1-ethyl-3-[3-(dimethylamino)-propyl]carbodiimide (EDC). The chicken gizzard subfragment 1 (S-1) used was treated with papain so that the heavy chain was partly cleaved into the NH2-terminal 72K and the COOH-terminal 24K fragments and the regulatory light chain into the 16K fragment. S-1 was reacted with EDC either alone or in the presence of ATP or F-actin. In all cases, the 16K fragment of the regulatory light chain formed a covalent cross-link with the 24K heavy chain fragment but not with the 72K fragment. The 38K cross-linked peptide, which was the product of cross-linking between the 16K light chain and the 24K heavy chain fragments, was isolated and further cleaved with cyanogen bromide and arginylendopeptidase. Smaller cross-linked peptides were purified by reverse-phase HPLC and then characterized by amino acid analysis and sequencing. The results indicated that cross-linking occurred between Lys-845 in the heavy chain and Asp-168, Asp-170, or Asp-171 in the regulatory light chain. The position of the cross-linked lysine was only three amino acid residues away from the invariant proline residue mapped as the S-1-rod hinge by McLachlan and Karn [McLachlan, A. D., & Karn, J. (1982) Nature (London) 299, 226-231]. We propose that the COOH-terminal region of the regulatory light chain is located in the neck region of myosin and that this region and the phosphorylation site of the regulatory light chain together may play a role in the phosphorylation-induced conformational change of gizzard myosin.

Cryo-electron microscopy of vitrified muscle samples

A great deal of information on the 3-dimensional structure of the protein assemblies involved in muscle contraction has been obtained using conventional transmission electron microscopy. In recent years, developments in cryo-electron microscopy have facilitated work with fully hydrated, non-chemically fixed specimens. It is shown how this technique can be used to visualize muscle sarcomere filaments in quasi-native conditions, to access hitherto inaccessible states of the crossbridge cycle, and to obtain new high resolution structural information on their 3-dimensional protein structure. A short introduction to the crossbridge cycle and its biochemically accessible states illustrates the problems amenable to studies using the electron microscope, as well as the possibilities offered by cryo-microscopy on vitrified samples. Work on vitrified cryo-sections and myosin filament suspensions demonstrates the accessibility of crossbridge states and gives implications on the gross structural features of myosin filaments. Recent studies on actin filaments and myosin (S1) decorated actin filaments provide the first high resolution data on vitrified samples. The use of photolabile nucleotide precursors allows the trapping of short lived states in the millisecond time range, thereby visualizing intermediate states of the crossbridge cycle.

The structural basis for the intrinsic disorder of the actin filament: the "lateral slipping" model

Three-dimensional (3-D) helical reconstructions computed from electron micrographs of negatively stained dispersed F-actin filaments invariably revealed two uninterrupted columns of mass forming the "backbone" of the double-helical filament. The contact between neighboring subunits along the thus defined two long-pitch helical strands was spatially conserved and of high mass density, while the intersubunit contact between them was of lower mass density and varied among reconstructions. In contrast, phalloidinstabilized F-actin filaments displayed higher and spatially more conserved mass density between the two long-pitch helical strands, suggesting that this bicyclic hepta-peptide toxin strengthens the intersubunit contact between the two strands. Consistent with this distinct intersubunit bonding pattern, the two long-pitch helical strands of unstabilized filaments were sometimes observed separated from each other over a distance of two to six subunits, suggesting that the intrastrand intersubunit contact is also physically stronger than the interstrand contact. The resolution of the filament reconstructions, extending to 2.5 nm axially and radially, enabled us to reproducibly "cut out" the F-actin subunit which measured 5.5 nm axially by 6.0 nm tangentially by 3.2 nm radially. The subunit is distinctly polar with a massive "base" pointing towards the "barbed" end of the filament, and a slender "tip" defining its "pointed" end (i.e., relative to the "arrowhead" pattern revealed after stoichiometric decoration of the filaments with myosin subfragment 1). Concavities running approximately parallel to the filament axis both on the inner and outer face of the subunit define a distinct cleft separating the subunit into two domains of similar size: an inner domain confined to radii less than or equal to 2.5-nm forms the uninterrupted backbone of the two long-pitch helical strands, and an outer domain placed at radii of 2-5-nm protrudes radially and thus predominantly contributes to the outer part of the massive base. Quantitative evaluation of successive crossover spacings along individual F-actin filaments revealed the deviations from the mean repeat to be compensatory, i.e., short crossovers frequently followed long ones and vice versa. The variable crossover spacings and diameter of the F-actin filament together with the local unraveling of the two long-pitch helical strands are explained in terms of varying amounts of compensatory "lateral slipping" of the two strands past each other roughly perpendicular to the filament axis. This intrinsic disorder of the actin filament may enable the actin moiety to play a more active role in actin-myosin-based force generation than merely act as a rigid passive cable as has hitherto been assumed.

Orientation of spin-labeled light chain-2 exchanged onto myosin cross-bridges in glycerinated muscle fibers

Electron paramagnetic resonance (EPR) spectroscopy has been used to study the angular distribution of a spin label attached to rabbit skeletal muscle myosin light chain 2. A cysteine reactive spin label, 3-(5-fluoro-2,4-dinitroanilino)-2,2,5,5- tetramethyl-1-pyrrolidinyloxy (FDNA-SL) was bound to purified LC2. The labeled LC2 was exchanged into glycerinated muscle fibers and into myosin and its subfragments. Analysis of the spectra of labeled fibers in rigor showed that the probe was oriented with respect to the fiber axis, but that it was also undergoing restricted rotations. The motion of the probe could be modeled assuming rapid rotational diffusion (rotational correlation time faster than 5 ns) within a "cone" whose full width was 70 degrees. Very different spectra of rigor fibers were obtained with the fiber oriented parallel and perpendicular to the magnetic field, showing that the centroid of each cone had the same orientation for all myosin heads, making an angle of approximately 74 degrees to the fiber axis. Binding of light chains or labeled myosin subfragment-1 to ion exchange heads immobilized the probes, showing that most of the motion of the probe arose from protein mobility and not from mobility of the probe relative to the protein. Relaxed labeled fibers produced EPR spectra with a highly disordered angular distribution, consistent with myosin heads being detached from the thin filament and undergoing large angular motions. Addition of pyrophosphate, ADP, or an ATP analogue (AMPPNP), in low ionic strength buffer where these ligands do not dissociate cross-bridges from actin, failed to perturb the rigor spectrum. Applying static strains as high as 0.16 N/mm2 to the labeled rigor fibers also failed to change the orientation of the spin label. Labeled light chain was exchanged into myosin subfragment-1 (S1) and the labeled S1 was diffused into fibers. EPR spectra of these fibers had a component similar to that seen in the spectra of fibers into which labeled LC2 had been exchanged directly. However, the fraction of disordered probes was greater than seen in fibers. In summary, the above data indicate that the region of the myosin head proximal to the thick filament is ordered in rigor, and disordered in relaxation.

The three-dimensional structure of the nemaline rod Z-band

In nemaline myopathy and some cardiac muscles, the Z-band becomes greatly enlarged and contains multiple layers of a zigzag structure similar to that seen in normal muscle. Because of the additional periodicity in the direction of the filament axis, these structures are particularly favorable for three-dimensional analysis since it becomes possible to average the data in all three dimensions and thus improve the reliability of the reconstruction. Individual views of the structure corresponding to tilted longitudinal and transverse sections were combined by matching the phases of common reflections. Examination of the tilted views strongly suggested that to the available resolution, the structure possesses fourfold screw symmetry along the actin filament axes. This symmetry could be used both in establishing the correct alignment for the combination of individual tilted views and to generate additional views not readily accessible in a single tilt series. The reconstruction shows actin filaments from one sarcomere surrounded by an array of four actin filaments with opposite polarity from the adjacent sacormere. The actin filaments show a right-handed twist and are connected by a structure that links adjacent filaments with the same polarity at the same axial level, then runs parallel to the filaments, and finally forms a link between two actin filaments whose polarity is opposite to that of the first pair. The connecting structure is probably composed of alpha-actinin which is located in Z-bands and cross-links actin filaments. The connecting structure may consist of two alpha-actinin molecules linking actin filaments of opposite polarity.

Molecular structure of F-actin and location of surface binding sites

Comparisons of three-dimensional maps of vertebrate muscle thin filaments obtained by cryo-electron microscopy and image analysis, reveal the molecular structure of F-actin, the location of the C terminus of the monomer and the positions of the binding sites of tropomyosin, the myosin head and the N-terminal portion of the myosin A1 light chain on the filament. These data provide strong constraints for evaluating models built from the atomic structure of the monomer and the subsequent identification of molecular contacts.

Cryoenzymic studies on actomyosin ATPase. Evidence that the degree of saturation of actin with myosin subfragment 1 affects the kinetics of the binding of ATP

The initial steps of actomyosin subfragment 1 (acto-S1) ATPase (dissociation and binding of ATP) were studied at -15 degrees C with 40% ethylene glycol as antifreeze. The dissociation kinetics were followed by light scattering in a stopped-flow apparatus, and the binding of ATP was followed by the ATP chase method in a rapid-flow quench apparatus. The data from the chase experiments were fitted to E + ATP in equilibrium (K1) E.ATP----(k2) E*ATP, where E is acto-S1 or S1. The kinetics of the binding of ATP to acto-S1 were sensitive to the degree of saturation of the actin with S1. There was a sharp transition with actin nearly saturated with S1: when the S1 to actin ratio was low, the kinetics were fast (K1 greater than 300 microM, k2 greater than 40 s-1); when it was high, they were slow (K1 = 14 microM, k2 = 2 s-1). With S1 alone K1 = 12 microM and k2 = 0.07 S-1. With acto heavy meromyosin (acto-HMM) the binding kinetics were the same as with saturated acto-S1, regardless of the HMM to actin ratio. The dissociation kinetics were independent of the S1 to actin ratio. Saturation kinetics were obtained with Kd = 460 microM and kd = 75 S-1. The data for the saturated acto-S1 could be fitted to a reaction scheme, but for lack of structural information the abrupt dependence of the ATP binding kinetics upon the S1 to actin ratio is difficult to explain.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular packing in profilin: actin crystals and its implications

Analysis of profilin: actin crystals reveals an extensive intermolecular network, rather than a discrete "monomeric complex", comprising stacked actin ribbons held in place by columns of profilin molecules, wedged in between neighboring actin subunits and running perpendicular to the ribbons. Comparison with data from electron microscopy, X-ray diffraction, spectroscopy, and biochemistry of actin suggests that a simple transformation relates the ribbon to f-actin. The crystals exhibit unusual polymorphic properties, which strengthens the view that movements within the actin monomer are important for force generation.

Three-dimensional image reconstruction of insect flight muscle. II. The rigor actin layer

The averaged structure of rigor cross-bridges in insect flight muscle is further revealed by three-dimensional reconstruction from 25-nm sections containing a single layer of thin filaments. These exhibit two thin filament orientations that differ by 60 degrees from each other and from myac layer filaments. Data from multiple tilt views (to +/- 60 degrees) was supplemented by data from thick sections (equivalent to 90 degrees tilts). In combination with the reconstruction from the myac layer (Taylor et al., 1989), the entire unit cell is reconstructed, giving the most complete view of in situ cross-bridges yet obtained. All our reconstructions show two classes of averaged rigor cross-bridges. Lead bridges have a triangular shape with leading edge angled at approximately 45 degrees and trailing edge angled at approximately 90 degrees to the filament axis. We propose that the lead bridge contains two myosin heads of differing conformation bound along one strand of F-actin. The lead bridge is associated with a region of the thin filament that is apparently untwisted. We suggest that the untwisting may reflect the distribution of strain between myosin and actin resulting from two-headed, single filament binding in the lead bridge. Rear bridges are oriented at approximately 90 degrees to the filament axis, and are smaller and more cylindrical, suggesting that they consist of single myosin heads. The rear bridge is associated with a region of apparently normal thin filament twist. We propose that differing myosin head angles and conformations consistently observed in rigor embody different stages of the power stroke which have been trapped by a temporal sequence of rigor cross-bridge formation under the constraints of the intact filament lattice.

Three-dimensional image reconstruction of insect flight muscle. I. The rigor myac layer

We have obtained detailed three-dimensional images of in situ cross-bridge structure in insect flight muscle by electron microscopy of multiple tilt views of single filament layers in ultrathin sections, supplemented with data from thick sections. In this report, we describe the images obtained of the myac layer, a 25-nm longitudinal section containing a single layer of alternating myosin and actin filaments. The reconstruction reveals averaged rigor cross-bridges that clearly separate into two classes constituting lead and rear chevrons within each 38.7-nm axial repeat. These two classes differ in tilt angle, size and shape, density, and slew. This new reconstruction confirms our earlier interpretation of the lead bridge as a two-headed cross-bridge and the rear bridge as a single-headed cross-bridge. The importance of complementing tilt series with additional projections outside the goniometer tilt range is demonstrated by comparison with our earlier myac layer reconstruction. Incorporation of this additional data reveals new details of rigor cross-bridge structure in situ which include clear delineation of (a) a triangular shape for the lead bridge, (b) a smaller size for the rear bridge, and (c) density continuity across the thin filament in the lead bridge. Within actin's regular 38.7-nm helical repeat, local twist variations in the thin filament that correlate with the two cross-bridge classes persist in this new reconstruction. These observations show that in situ rigor cross-bridges are not uniform, and suggest three different myosin head conformations in rigor.

Electron microscopy of negatively stained scallop myosin molecules. Effect of regulatory light chain removal on head structure

The heads of myosin molecules from the striated adductor muscle of scallop have been studied by electron microscopy after negative staining. In common with vertebrate skeletal muscle myosin visualized by this method, the scallop myosin heads were pear-shaped and often showed pronounced curvature. Staining suggestive of two or, more frequently, three domains could often be observed. Removal of regulatory light chains (R-LCs) resulted in a reduction in the length of the heads of about 2.6 nm, with no significant change in maximum width. In desensitized preparations a majority of heads displayed anticlockwise curvature, whereas intact heads were usually seen curved clockwise. Analysis of the head curvature in both intact and desensitized molecules was consistent with an ability of each head to rotate about its long axis. Desensitization resulted in an increased incidence of heads showing two domains. It seems likely that the reduction in length upon removal of the R-LC is due to the two small domains located in the neck region of the head collapsing into one.

Position of Mercenaria regulatory light-chain Cys50 site on the surface of myosin visualized by electron microscopy

Mercenaria regulatory light-chains, specifically labelled at cysteine 50 with N-iodoacetyl-N'-biotinylhexylenediamine, were rebound to regulatory light-chain denuded scallop myosin, and the hybrid myosin formed was decorated with avidin. These hybrid myosins were visualized by rotary-shadowing electron microscopy. Three distinct images of avidin-decorated hybrid myosin molecules were obtained. These comprise singly decorated molecules, where the avidin is bound symmetrically or asymmetrically with respect to the two heads of myosin, in addition to "figures-of-five", where two myosin molecules associate with a centrally placed avidin molecule. Analysis of these images indicates that the Mercenaria regulatory light-chain Cys50 site is located 15 to 35 A from the head-rod junction when the light-chain is bound in situ to myosin. Implications with respect to head topology and probe studies are discussed.

SH-1 modification of rabbit myosin interferes with calcium regulation

The reactive thiol of the myosin head, SH-1, can be selectively labelled in glycerinated rabbit muscle fibres. This residue has been used as an attachment site for either fluorescent or spectroscopic probes which report on head movements and orientations in various functional states of muscle. We have specifically modified SH-1 in vitro, using purified rabbit myosin and conditions similar to those employed in the labelling of muscle fibres (low ionic strength [40 mM NaCl] at 4 degrees C), with stoichiometric amounts of either [14C]-iodoacetamide, 5-(2[iodoacetyl)amino)ethyl) aminonaphthalene-1- sulphonic acid (IAEDANS), or 4-(2-iodoacetamido-2,2,6,6-tetramethyl piperidinooxyl (IASL). The specificity of modification was determined by measuring the well-defined alterations in the high salt ATPase activities of myosin and by localizing both IAAm and IAEDANS to the 20-kDa C-terminal subfragment 1 (S1) which contains SH-1. The low ionic strength actin-activated Mg2+-ATPase of SH-1-modified rabbit myosin was measured in the presence of the thin filament regulatory, complex, troponin-tropomyosin. A significant increase in this activity in the absence of calcium, concomitant with a decrease in activity in the presence of calcium, was observed as the extent of SH-1 modification was incrementally increased from zero to one mole of label bound per mole of SH-1. The elevated myosin Mg2+-ATPase, which results from SH-1 modification, does not account for the increased actin-activated Mg2+-ATPase in resting conditions (i.e. in the absence of calcium).(ABSTRACT TRUNCATED AT 250 WORDS)

Shape of the myosin head in the rigor complex. Three-dimensional image reconstruction of the actin-tropomyosin-heavy meromyosin complex

The structure of the actin-tropomyosin-heavy meromyosin rigor complex was studied by image analysis of electron micrographs. The arrowhead of the rigor complex has a whisker-like structure with a dense turning point at the "barb" of the arrowhead. The neck region of the myosin head in the reconstructed three-dimensional image is present in the area corresponding to the dense point. It is concluded that at least one extra-thin area contributes to the neck region, and that the two heads in the heavy meromyosin molecule join a double helical rope beyond the end of the large head (G in this study). (This is different from previous interpretations). It is also concluded that the heavy meromyosin has a short bent part near the head/rod junction in the rigor complex.

Temperature-dependent conformational transition in the head-rod junctional region of the myosin molecule

The effects of temperature, Mg2+, ATP, and actin on the conformation of the neck region of the myosin head were studied by limited proteolysis of heavy meromyosin (HMM) and subfragment 1 (S1) preparations obtained by papain digestion of myosin in the presence of Mg2+ (Mg-S1) or EDTA (EDTA-S1). The preparations were fluorescently labelled at the SH1 thiol group to enable identification of the COOH-terminal fragments of the head portion of the heavy chain where this group is located. The results indicate that the head-rod junctional region of the myosin heavy chain contains at least three different sites readily susceptible to trypsin at 25 degrees C if the light chain LC2 or its LC2' fragment are absent. The susceptibility of one of these sites dramatically decreases when the temperature is lowered to 0 degree C, indicating a temperature-dependent conformational transition in the head-rod junction. With the method used, this transition is detectable only in LC2/LC'2-deficient preparations since all three sites are protected, although to different extents, by LC2 and its LC'2 derivative. It is, however, most probable that the effect of the light chain is confined to steric hindrance of trypsin access and that the temperature-dependent structural transition in the head-rod junction can occur in the presence of intact LC2 as well and may contribute to the temperature sensitivity of force generation in muscle.

Computer image processing of electron micrographs of biological structures with helical symmetry

Methods are described for the analysis of electron micrographs of biological objects with helical symmetry and for the production of three-dimensional models of these structures using computer image reconstruction methods. Fourier-based processing of one- and two-dimensionally ordered planar arrays is described by way of introduction, before analysing the special properties of helices and their transforms. Conceiving helical objects as a sum of helical waves (analogous to the sum of planar waves used to describe a planar crystal) is shown to facilitate analysis and enable three-dimensional models to be produced, often from a single view of the object. The corresponding Fourier transform of such a sum of helical waves consists of a sum of Bessel function terms along layer lines. Special problems deriving from the overlapping along layer lines of terms of different Bessel order are discussed, and methods to separate these terms, based on analysing a number of different azimuthal views of the object by least squares, are described. Corrections to alleviate many technical and specimen-related problems are discussed in conjunction with a consideration of the computer methods used to actually process an image. A range of examples of helical objects, including viruses, microtubules, flagella, actin, and myosin filaments, are discussed to illustrate the range of problems that can be addressed by computer reconstruction methods.

Cross-linking of the skeletal myosin subfragment 1 heavy chain to the N-terminal actin segment of residues 40-113

Glutaraldehyde (GA) and N-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline (EEDQ), a hydrophobic, carboxyl group directed, zero-length protein cross-linker, were employed for the chemical cross-linking of the rigor complex between F-actin and the skeletal myosin S-1. The enzymatic properties and structure of the new covalent complexes obtained with both reagents were determined and compared to those known for the EDC-acto-S-1 complex. The GA- or EEDQ-catalyzed covalent attachment of F-actin to the S-1 heavy chain induced an elevated Mg2+-ATPase activity. The turnover rates of the isolated cross-linked complexes were similar to those for EDC-acto-S-1 (30 s-1). The solution stability of the new complexes is also comparable to that exhibited by EDC-acto-S-1. The proteolytic digestion of the isolated AEDANS-labeled covalent complexes and direct cross-linking experiments between actin and various preformed proteolytic S-1 derivatives indicated that, as observed with EDC, the COOH-terminal 20K and the central 50K heavy chain fragments are involved in the cross-linking reactions of GA and EEDQ. KI-depolymerized acto-S-1 complexes cross-linked by EDC, GA, or EEDQ were digested by thrombin which cuts only actin, releasing S-1 heavy chain-actin peptide cross-linked complexes migrating on acrylamide gels with Mr 100K (EDC), 110K and 105K (GA), and 102K (EEDQ); these were fluorescent only when fluorescent S-1 was used. They were identified by immunostaining with specific antibodies directed against selected parts of he NH2-terminal actin segment of residues 1-113.(ABSTRACT TRUNCATED AT 250 WORDS)

Domain structure of the myosin head in correlation-averaged images of shadowed molecules

Electron microscope images of rotary shadowed myosin heads and subfragment-1 (S1) have been computationally aligned and averaged using correlation methods. Average images show reproducible detail within the 'pear-shaped' envelope of the head; the major features are invariant in S1 and in intact heads, in two mirror-related views of the head, and in the presence and absence of ATP. The averages support the view that the head contains two main structural domains separated by a cleft, and that the region of the neck close to the head-rod junction is flexible. They also reveal the inadequacy of the conventional method of correcting the measured dimensions of shadowed particles for the supposed thickness of the 'metal coat'.

Assembly of smooth muscle myosin minifilaments: effects of phosphorylation and nucleotide binding

Small bipolar filaments, or "minifilaments," are formed when smooth muscle myosin is dialyzed against low ionic strength pyrophosphate or citrate/Tris buffers. Unlike synthetic filaments formed at approximately physiological ionic conditions, minifilaments are homogeneous as indicated by their hypersharp boundary during sedimentation velocity. Electron microscopy and hydrodynamic techniques were used to show that 20-22S smooth muscle myosin minifilaments are 380 nm long and composed of 12-14 molecules. By varying solvents, a continuum of different size polymers in the range of 15-30S could be obtained. Skeletal muscle myosin, in contrast, preferentially forms a stable 32S minifilament (Reisler, E., P. Cheung, and N. Borochov. 1986. Biophys. J. 49:335-342), suggesting underlying differences in the assembly properties of the two myosins. Addition of salt to the smooth muscle myosin minifilaments caused unidirectional growth into a longer "side-polar" type of filament, whereas bipolar filaments were consistently formed by skeletal muscle myosin. As with synthetic filaments, addition of 1 mM MgATP caused dephosphorylated minifilaments to dissociate to a mixture of folded monomers and dimers. Phosphorylation of the regulatory light chain prevented disassembly by nucleotide, even though it had no detectable effect on the structure of the minifilament. These results suggest that differences in filament stability as a result of phosphorylation are due largely to conformational changes occurring in the myosin head, and are not due to differences in filament packing.

Location of the ATPase site of myosin determined by three-dimensional electron microscopy

Both ATP hydrolysis by myosin and the accompanying cyclic association-dissociation of actin and myosin are essential for muscle contraction. It is important for understanding the molecular mechanism of contraction to know the three-dimensional locations of the two major functional sites of myosin: the ATPase site and the actin-binding site. We have determined the position of the ATPase site of myosin using three-dimensional image reconstruction from electron micrographs and site-specific labelling with the avidin-biotin system. The ATPase site is about 5 nm from the tip of the myosin head and is about 4 nm away from the actin-binding site of myosin. This is the first report of the three-dimensional location of an enzyme active site by electron microscopy.

Electron microscopy of cardiac myosin: its shape and properties as determined by the regulatory light chain

Structural properties of dog cardiac myosin and the influence of the regulatory light chain (LC2) on the shape of myosin heads were investigated by electron microscopy. LC2 was reversibly removed using a neutral protease from myopathic hamsters (Margossian, J. Biol. Chem. 260 (1985) 13747-54). The distribution of myosin head length centred around 17 nm with the mean length being 18.9 nm. Statistical analysis suggested that myosin heads became more globular upon removal of LC2. No extensive aggregation of myosin could be detected after LC2 was dissociated, either by sedimentation velocity or by gels run under non-denaturing conditions. The centre-to-centre distance between heads remained constant at about 21 nm, regardless of the presence or absence of LC2. The distribution of length of the globular region reveals two peaks at 7.5 and 9.5 nm, suggesting an extended and a shorter configuration of this region. The decrease in mass at the head/tail junction upon LC2 removal suggests that it is the binding site for the regulatory light chains. A bend at 57 nm from the head/tail joint was sometimes noticed, corresponding to the myosin hinge region. In high resolution micrographs individual particles revealed invaginations along the contours of the head, possibly delineating the boundaries of structural domains within the head. The conformation of arrowheads in actin decorated with either subfragment 1 (S1) or heavy meromyosin (HMM) was investigated in the presence and absence of LC2.

Structural changes accompanying phosphorylation of tarantula muscle myosin filaments

Electron microscopy has been used to study the structural changes that occur in the myosin filaments of tarantula striated muscle when they are phosphorylated. Myosin filaments in muscle homogenates maintained in relaxing conditions (ATP, EGTA) are found to have nonphosphorylated regulatory light chains as shown by urea/glycerol gel electrophoresis and [32P]phosphate autoradiography. Negative staining reveals an ordered, helical arrangement of crossbridges in these filaments, in which the heads from axially neighboring myosin molecules appear to interact with each other. When the free Ca2+ concentration in a homogenate is raised to 10(-4) M, or when a Ca2+-insensitive myosin light chain kinase is added at low Ca2+ (10(-8) M), the regulatory light chains of myosin become rapidly phosphorylated. Phosphorylation is accompanied by potentiation of the actin activation of the myosin Mg-ATPase activity and by loss of order of the helical crossbridge arrangement characteristic of the relaxed filament. We suggest that in the relaxed state, when the regulatory light chains are not phosphorylated, the myosin heads are held down on the filament backbone by head-head interactions or by interactions of the heads with the filament backbone. Phosphorylation of the light chains may alter these interactions so that the crossbridges become more loosely associated with the filament backbone giving rise to the observed changes and facilitating crossbridge interaction with actin.