Three-dimensional reconstruction of the Z disk of sectioned bee flight muscle

The cryo-EM 3D image reconstruction of isolated Lethocerus indicus Z-discs

The Z-disk of striated muscle defines the ends of the sarcomere, which repeats many times within the muscle fiber. Here we report application of cryoelectron tomography and subtomogram averaging to Z-disks isolated from the flight muscles of the large waterbug Lethocerus indicus. We use high salt solutions to remove the myosin containing filaments and use gelsolin to remove the actin filaments of the A- and I-bands leaving only the thin filaments within the Z-disk which were then frozen for cryoelectron microscopy. The Lethocerus Z-disk structure is similar in many ways to the previously studied Z-disk of the honeybee Apis mellifera. At the corners of the unit cell are positioned trimers of paired antiparallel F-actins defining a large solvent channel, whereas at the trigonal positions are positioned F-actin trimers converging slowly towards their (+) ends defining a small solvent channel through the Z-disk. These near parallel F-actins terminate at different Z-heights within the Z-disk. The two types of solvent channel in Lethocerus are similar in size compared to those of Apis which are very different in size. Two types of α-actinin crosslinks were observed between oppositely oriented actin filaments. In one of these, the α-actinin long axis is almost parallel to the F-actins it crosslinks. In the other, the α-actinins are at a small but distinctive angle with respect to the crosslinked actin filaments. The utility of isolated Z-disks for structure determination is discussed.

The insect perspective on Z-disc structure and biology

Myofibrils are the intracellular structures formed by actin and myosin filaments. They are paracrystalline contractile cables with unusually well-defined dimensions. The sliding of actin past myosin filaments powers contractions, and the entire system is held in place by a structure called the Z-disc, which anchors the actin filaments. Myosin filaments, in turn, are anchored to another structure called the M-line. Most of the complex architecture of myofibrils can be reduced to studying the Z-disc, and recently, important advances regarding the arrangement and function of Z-discs in insects have been published. On a very small scale, we have detailed protein structure information. At the medium scale, we have cryo-electron microscopy maps, super-resolution microscopy and protein-protein interaction networks, while at the functional scale, phenotypic data are available from precise genetic manipulations. All these data aim to answer how the Z-disc works and how it is assembled. Here, we summarize recent data from insects and explore how it fits into our view of the Z-disc, myofibrils and, ultimately, muscles.

Nanoscopy reveals the layered organization of the sarcomeric H-zone and I-band complexes

Sarcomeres are extremely highly ordered macromolecular assemblies where structural organization is intimately linked to their functionality as contractile units. Although the structural basis of actin and Myosin interaction is revealed at a quasiatomic resolution, much less is known about the molecular organization of the I-band and H-zone. We report the development of a powerful nanoscopic approach, combined with a structure-averaging algorithm, that allowed us to determine the position of 27 sarcomeric proteins in Drosophila melanogaster flight muscles with a quasimolecular, ∼5- to 10-nm localization precision. With this protein localization atlas and template-based protein structure modeling, we have assembled refined I-band and H-zone models with unparalleled scope and resolution. In addition, we found that actin regulatory proteins of the H-zone are organized into two distinct layers, suggesting that the major place of thin filament assembly is an M-line-centered narrow domain where short actin oligomers can form and subsequently anneal to the pointed end.

Structure of isolated Z-disks from honeybee flight muscle

The Z-disk is a complex structure comprising some 40 proteins that are involved in the transmission of force developed during muscle contraction and in important signalling pathways that govern muscle homeostasis. In the Z-disk the ends of antiparallel thin filaments from adjacent sarcomeres are crosslinked by α-actinin. The structure of the Z-disk lattice varies greatly throughout the animal kingdom. In vertebrates the thin filaments form a tetragonal lattice, whereas invertebrate flight muscle has a hexagonal lattice. The width of the Z-disk varies considerably and correlates with the number of α-actinin bridges. A detailed description at a high resolution of the Z-disk lattice is needed in order to better understand muscle function and disease. The molecular architecture of the Z-disk lattice in honeybee (Apis mellifera) is known from plastic embedded thin sections to a resolution of 7 nm, which is not sufficient to dock component protein crystal structures. It has been shown that sectioning is a damaging process that leads to the loss of finer details present in biological specimens. However, the Apis Z-disk is a thin structure (120 nm) suitable for cryo EM. We have isolated intact honeybee Z-disks from indirect flight muscle, thus obviating the need of plastic sectioning. We have employed cryo electron tomography and image processing to investigate the arrangement of proteins within the hexagonal lattice of the Apis Z-disk. The resolution obtained, ~6 nm, was probably limited by damage caused by the harshness of the conditions used to extract the myofibrils and isolate the Z-disks.

Biological cryo-electron microscopy in China

Cryo‐electron microscopy (cryo‐EM) plays an increasingly more important role in structural biology. With the construction of an arm of the Chinese National Protein Science Facility at Tsinghua University, biological cryo‐EM has entered a phase of rapid development in China. This article briefly reviews the history of biological cryo‐EM in China, describes its current status, comments on its impact on the various biological research fields, and presents future outlook.

Some distinctive features of zebrafish myogenesis based on unexpected distributions of the muscle cytoskeletal proteins actin, myosin, desmin, alpha-actinin, troponin and titin

The current myofibrillogenesis model is based mostly on in vitro cell cultures and on avian and mammalian embryos in situ. We followed the expression of actin, myosin, desmin, alpha-actinin, titin, and troponin using immunofluorescence microscopy of zebrafish (Danio rerio) embryos. We could see young mononucleated myoblasts with sharp striations. The striations were positive for all the sarcomeric proteins. Desmin distribution during muscle maturation changes from dispersed aggregates to a perinuclear concentration to striated afterwards. We could not observe desmin-positive, myofibrillar-proteins-negative cells, and we could not find any non-striated distribution of sarcomeric proteins, such as stress fiber-like structures. Some steps, like fusion before striation, seem to be different in the zebrafish when compared with the previously described myogenesis sequences.

Connecting filaments: a historical prospective

This short review covers the development of the extensible filament research from the very beginning until the most recent results. This work emphasizes the milestones of discovery, which led us from initial observations that were solely ultrastructural to the molecular understanding of the extensible process of these filaments.

Links in the chain: the contribution of kettin to the elasticity of insect muscles

Asynchronous flight muscle fibers are activated by periodic stretches and need to be stiff for strain to be transmitted to the contractile system. Kettin associated with thin filaments and projectin with thick filaments contribute to fiber stiffness. Kettin extends along thin filaments with the N-terminus in the Z-disc and the C-terminus outside. C filaments connecting thick filaments to the Z-disc contain projectin but not kettin. Insect flight myofibrils have a titin PEVK epitope which is only exposed on stretch, suggesting it is short and inaccessible. It is concluded that kettin stiffens thin filaments near the Z-disc and projectin and titin provide elasticity to C filaments.

Three-dimensional structure of a vertebrate muscle Z-band: implications for titin and alpha-actinin binding

The Z-band in vertebrate striated muscles, mainly comprising actin filaments, alpha-actinin, and titin, serves to organise the antiparallel actin filament arrays in adjacent sarcomeres and to transmit tension between sarcomeres during activation. Different Z-band thicknesses, formed from different numbers of zigzag crosslinking layers and found in different fibre types, are thought to be associated with the number of repetitive N-terminal sequence domains of titin. In order to understand myofibril formation it is necessary to correlate the ultrastructures and sequences of the actin filaments, titin, and alpha-actinin in characteristic Z-bands. Here electron micrographs of the intermediate width, basketweave Z-band of plaice fin muscle have been subject to a novel 3D reconstruction process. The reconstruction shows that antiparallel actin filaments overlap in the Z-band by about 22-25 nm. There are three levels of Z-links (probably alpha-actinin) in which at each level two nearly diametrically opposed links join an actin filament to two of its antiparallel neighbours. One set of links is centrally located in the Z-band and there are flanking levels orthogonal to this. A 3D model of the observed structure shows how Z-bands of different widths may be formed and it provides insights into the structural arrangements of titin and alpha-actinin in the Z-band. The model shows that the two observed symmetries in different Z-bands, c2 and p12(1), may be attributed respectively to whether the number of Z-link levels is odd or even.

Purification and characterization of an alpha-actinin-binding PDZ-LIM protein that is up-regulated during muscle differentiation

alpha-Actinin is required for the organization and function of the contractile machinery of muscle. In order to understand more precisely the molecular mechanisms by which alpha-actinin might contribute to the formation and maintenance of the contractile apparatus within muscle cells, we performed a screen to identify novel alpha-actinin binding partners present in chicken smooth muscle cells. In this paper, we report the identification, purification, and characterization of a 36-kDa smooth muscle protein (p36) that interacts with alpha-actinin. Using a variety of in vitro binding assays, we demonstrate that the association between alpha-actinin and p36 is direct, specific, and saturable and exhibits a moderate affinity. Furthermore, native co-immunoprecipitation reveals that the two proteins are complexed in vivo. p36 is expressed in cardiac muscle and tissues enriched in smooth muscle. Interestingly, in skeletal muscle, a closely related protein of 40 kDa (p40) is detected. The expression of p36 and p40 is dramatically up-regulated during smooth and skeletal muscle differentiation, respectively, and p40 colocalizes with alpha-actinin at the Z-lines of differentiated myotubes. We have established the relationship between p36 and p40 by molecular cloning of cDNAs that encode both proteins and have determined that they are the products of a single gene. Both proteins display an identical N-terminal PDZ domain and an identical C-terminal LIM domain; an internal 63-amino acid sequence present in p36 is replaced by a unique 111-amino acid sequence in p40. Analysis of the sequences of p36 and p40 suggest that they are the avian forms of the actinin-associated LIM proteins (ALPs) recently described in rat (Xia, H., Winokur, S. T., Kuo, W.-L., Altherr, M. R., and Bredt, D. S. (1997) J. Cell Biol. 139, 507-515). The expression of the human ALP gene has been postulated to be affected by mutations that cause facioscapulohumeral muscular dystrophy; thus, the characterization of ALP function may ultimately provide insight into the mechanism of this disease.

Titin: a molecular control freak

Recent studies of the giant protein titin have shed light on its roles in muscle assembly and elasticity and include the surprising findings described here. We now know that the titin kinase domain, which has long been a puzzle, has a novel regulation mechanism. A substrate, telethonin, has been identified that is located over one micron away from the kinase domain in mature muscle. Single-molecule studies have demonstrated the fascinating process of reversible mechanical unfolding of titin. Lastly, and most surprisingly, it has been claimed that titin controls assembly and elasticity in chromosomes.

Association of kettin with actin in the Z-disc of insect flight muscle

The Z-discs of insect muscle contain kettin, a modular protein of 500-700 kDa. The Drosophila protein is made up of a chain of immunoglobulin (Ig) domains separated by linker sequences. Kettin differs from other modular muscle proteins of the Ig superfamily in binding to thin filaments rather than thick filaments. Kettin isolated from Lethocerus (waterbug) muscle is an elongated molecule 180 nm long, which binds to F-actin with high affinity (Kd=1.2 nM) and a stoichiometry of one Ig domain per actin protomer. Competition between kettin and tropomyosin for binding to actin excludes tropomyosin from the Z-disc. In contrast, kettin and alpha-actinin bind simultaneously to actin, which would reinforce the Z-disc lattice. In vitro, kettin promotes the antiparallel association of actin filaments, and a similar process may occur in the developing sarcomere: actin filaments interdigitate in an antiparallel fashion in the Z-disc with the N terminus of kettin within the Z-disc, and the C terminus some way outside. We propose a model for the association of kettin with actin in which the molecule follows the genetic helix of actin and Ig domains, separated by linker sequences, bind to each actin protomer.

Three-dimensional structure of the Z band in a normal mammalian skeletal muscle

The three-dimensional structure of the vertebrate skeletal muscle Z band reflects its function as the muscle component essential for tension transmission between successive sarcomeres. We have investigated this structure as well as that of the nearby I band in a normal, unstimulated mammalian skeletal muscle by tomographic three-dimensional reconstruction from electron micrograph tilt series of sectioned tissue. The three-dimensional Z band structure consists of interdigitating axial filaments from opposite sarcomeres connected every 18 +/- 12 nm (mean +/- SD) to one to four cross-connecting Z-filaments are observed to meet the axial filaments in a fourfold symmetric arrangement. The substantial variation in the spacing between cross-connecting Z-filament to axial filament connection points suggests that the structure of the Z band is not determined solely by the arrangement of alpha-actinin to actin-binding sites along the axial filament. The cross-connecting filaments bind to or form a "relaxed interconnecting body" halfway between the axial filaments. This filamentous body is parallel to the Z band axial filaments and is observed to play an essential role in generating the small square lattice pattern seen in electron micrographs of unstimulated muscle cross sections. This structure is absent in cross section of the Z band from muscles fixed in rigor or in tetanus, suggesting that the Z band lattice must undergo dynamic rearrangement concomitant with crossbridge binding in the A band.

EM-tomography of section collapse, a non-linear phenomenon

Using back projection for reconstruction and tilt series of Epon or Lowicryl embedded and sectioned material, we demonstrated: (1) a reduction in thickness of 50% for Epon and 80% for Lowicryl sections, and (2) a non-uniform density distribution along the electron-optical axis in sections. The highest density was found at the vacuum exposed side of the section. The formvar side of the section showed a similar increase in density, but not to the same extent. Minimalization of electron exposure, even without pre-exposure, did not affect the reconstructed thickness, nor did it affect the non-uniform density distribution. However, parallax measurements showed that at 150K, collapse of Epon sections does not take place. For EM-tomography of plastic embedded material our findings imply that at the top and bottom portion of the sections the dimensions of the reconstructed structures are distorted, but that in the middle portion the dimensions are reliably retained.

The muscle Z band: lessons in stress management

Two of the most characteristic features of striated muscle are (i) its ability to contract and generate tension when activated and (ii) its ability to return to its original length and form after contraction or stretching ceases. These two properties are to a large extent the primary manifestations of separate sets of filament systems: contractile actin and myosin filaments and viscoelastic titin and intermediate filaments. Z bands function as a common link that mechanically integrates contractile and elastic elements and as such they play a fundamental role in transmission of active and passive forces. Differences in Z band structure have been described for distinct classes of muscle and fibre types. The diversity in Z band architecture has been built around its phylogenetically conserved role as an actin-anchoring structure. Novel proteins are likely to account for structural and functional differences seen across the phyla.

Experiments on rigor crossbridge action and filament sliding in insect flight muscle

We have explored three aspects of rigor crossbridge action: 1. Under rigor conditions, slow stretching (2% per hour) of insect flight muscle (IFM) from Lethocerus causes sarcomere ruptures but never filament sliding. However, in 1 mM AMPPNP, slow stretching (5%/h) causes filament sliding but no sarcomere ruptures, although stiffness equals rigor values. Thus loaded rigor attachments in IFM show no strain relief over several hours, but near-rigor states that allow short-term strain relief indicate different grades of strongly bound bridges, and suggest approaches to annealing the rigor lattice. 2. Sarcomeres of Lethocerus flight muscle, stretched 20-60% and then rigorized, show "hybrid" crossbridge patterns, with overlap zones in rigor, but H-bands relaxed and revealing four-stranded R-hand helical thick filament structure. The sharp boundary exhibits precise phasing between relaxed and rigor arrays along each thick filament. Extrapolating one lattice into the other should allow detailed modeling of the action of each myosin head as it enters rigor. 3. The "A-(bee)-Z problem" exposes a conflict about actin rotational alignment between A-bands and Z-bands of bee IFM, raising the possibility that rigor induction might rotate actins forcefully from one pattern to the other. As Squire noted, 3-D reconstructions of Z-bands in relaxed bee IFM2) imply A-bands where actin target zones form rings rather than helices around thick filaments. However, we confirm Trombitás et al. that rigor crossbridges in bee IFM mark helically arrayed target zones. Moreover, we find that loose crossbridge interactions in relaxed bee IFM mark the same helical pattern. Thus no change of actin rotational alignment by rigor crossbridges seems necessary, but 3-D structure of IFM Z-bands should be re-evaluated regarding the apparent contradiction with A-band symmetry.

Muscle filament lattices and stretch-activation: the match-mismatch model reassessed

A mechanism for the observed enhanced stretch-activation phenomenon in insect asynchronous flight muscles has been postulated and developed in terms of the matched helical structures of the actin and myosin filaments in the asynchronous flight muscles of Lethocerus. It was suggested that at different sarcomere lengths with different filament overlaps there would be a changing probability of myosin crossbridge attachment to actin according to whether there was match or mismatch between the myosin and actin arrays. Evidence is provided here that, when Lethocerus structure is considered in detail, the explanation appears to fail. Results on other insect asynchronous flight muscles of different structure (e.g. Apis) also seem to contradict the match-mismatch model. All striated muscle types considered here (fish, frog, Lethocerus, Apis, blowfly) appear to be designed to give constant probability of crossbridge attachment to actin as the filaments move axially, apart from the well-known effects of changing total filament overlap. Alternative stretch-activation mechanisms are considered, especially in terms of the unusual thin filament regulatory system in some insect asynchronous flight muscles.

Perturbations of Drosophila alpha-actinin cause muscle paralysis, weakness, and atrophy but do not confer obvious nonmuscle phenotypes

We have investigated accumulation of alpha-actinin, the principal cross-linker of actin filaments, in four Drosophila fliA mutants. A single gene is variably spliced to generate one nonmuscle and two muscle isoforms whose primary sequence differences are confined to a peptide spanning the actin binding domain and first central repeat. In fliA3 the synthesis of an adult muscle-specific isoform is blocked in flight and leg muscles, while in fliA4 the synthesis of nonmuscle and both muscle-specific isoforms is severely reduced. Affected muscles are weak or paralyzed, and, in the case of fliA3, atrophic. Their myofibrils, while structurally irregular, are remarkably normal considering that they are nearly devoid of a major contractile protein. Also surprising is that no obvious nonmuscle cell abnormalities can be discerned despite the fact that both the fliA1- and fliA4-associated mutations perturb the nonmuscle isoform. Our observations suggest that alpha-actinin stabilizes and anchors thin filament arrays, rather than orchestrating their assembly, and further imply that alpha-actinin function is redundant in both muscle and nonmuscle cells.

A protocol for 3D image reconstruction from a single image of an oblique section

Oblique section 3D reconstruction can produce a 3D image of a sectioned crystal from a single electron micrograph. We describe here in detail a reconstruction protocol applicable to an electron micrograph of an oblique section through a 3D crystal. The protocol is described in six steps: (1) selection criteria for images, (2) preprocessing steps to correct for image defects, (3) determination of unit cell coordinates, (4) interpolation of strip images with correction for image distortions and crystal disorder, (5) production of a crystallographic serial section reconstruction, (6) correction for skewed sampling to produce an oblique section reconstruction. In addition, we explore Wiener filter deconvolution of the section thickness. We describe a method for determining the section thickness by comparing data from projections of the oblique section reconstruction with corresponding data from a thick longitudinal section. Several schemes for Wiener filter deconvolution are described that differ in the way information on the signal-to-noise ratio is used in the filter.

Cryoultramicrotomy of muscle: improved preservation and resolution of muscle ultrastructure using negatively stained ultrathin cryosections

Ultrathin sections of rapidly frozen, briefly pre-treated muscle tissue are cut and thereafter are thawed and contrasted using a negative staining technique. The method has provided micrographs in which the in-vivo order in the muscle fibres has been preserved well enough to enable both a more complete interpretation of X-ray diffraction evidence from muscle, and also a gain of new ultrastructural information on aspects of myofibril and myofilament architecture in different types of fibre. Examples here are taken from chicken, rabbit and fish muscles and show both the M-band and the bridge region of the A-band in great detail. To enhance the detail in the original images, one-dimensional (1-D) and 2-D averaging techniques (lateral smearing and step averaging, respectively) are used. Although there is major shrinkage in section thickness to about one-third of its original value, demonstrated here for the first time is the fact that the characteristic A-band lattice planes are preserved in these sections in 3-D. This confirms the usefulness of cryosections not just for 1-D and 2-D image processing, but also for 3-D reconstruction. Thus, in combination with techniques of image processing, cryoultramicrotomy can give the muscle morphologist the detailed data that are needed to match the molecular biologists, biochemists and immunologists in the interpretation of their data about physiological and pathophysiological events in muscle fibres at the macromolecular level.

Three-dimensional reconstruction of a simple Z-band in fish muscle

The three-dimensional structure of the Z-band in fish white muscle has been investigated by electron microscopy. This Z-band is described as simple, since in longitudinal sections it has the appearance of a single zigzag pattern connecting the ends of actin filaments of opposite polarity from adjacent sarcomeres. The reconstruction shows two pairs of links, the Z-links, between one actin filament and the facing four actin filaments in the adjacent sarcomere. The members of each pair have nearly diametrically opposed origins. In relation to one actin filament, one pair of links appears to bind along the final 10 nm of the actin filament (proximal site) and the other pair binds along a region extending from 5 to 20 nm from the filament end (distal site). Between one pair and the other, there is a rotation of approximately 80 degrees round the filament axis. A Z-link with a proximal site at the end of one actin filament attaches at a distal site on the oppositely oriented actin filaments of the facing sarcomere and vice versa. The length of each Z-link is consistent with the length of an alpha-actinin molecule. An additional set of links located 10-15 nm from the center of the Z-band occurs between actin filaments of the same polarity. These polar links connect the actin filaments along the same direction on each side of the Z-band. The three-dimensional structure appears to have twofold screw symmetry about the central plane of the Z-band. Only approximate twofold rotational symmetry is observed in directions parallel to the actin filaments. Previous models of the Z-band in which four identical and rotationally symmetrical links emanate from the end of one actin filament and span across to the ends of four actin filaments in the adjacent sarcomere are therefore incorrect.

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.

Three-dimensional interactive graphics for displaying and modelling microscopic data

EUCLID is a three-dimensional (3D) general purpose graphics display package for interactive manipulation of vector, surface and solid drawings on Evans and Sutherland PS300 series graphics processors. It is useful for displaying, comparing, measuring and modelling 3D microscopic images in real time. EUCLID can assemble groups of drawings into a composite drawing, while retaining the ability to operate upon the individual drawings within the composite drawing separately. EUCLID is capable of real time geometrical transformations (scaling, translation and rotation in two coordinate frames) and stereo and perspective viewing transformations. Because of its flexibility, EUCLID is especially useful for fitting models into 3D microscopic images.

Molecular genetics of Drosophila alpha-actinin: mutant alleles disrupt Z disc integrity and muscle insertions

We have isolated a Drosophila melanogaster alpha-actinin gene and partially characterized several mutant alleles. The Drosophila protein sequence is very similar (68% identity) to those of chicken alpha-actinin isoforms, but less closely related (30% identity) to Dictyostelium alpha-actinin. The gene is within subdivision 2C of the X chromosome, coincident with 15 lethal (1)2Cb mutations. At least four alleles, l(1)2Cb1, l(1)2Cb2, l(1)2Cb4, and l(1)2Cb5 are interrupted by rearrangement breakpoints and must be null. In all four cases, hemizygous mutants complete embryogenesis and do not die until the second day of larval growth, signifying that either the role of alpha-actinin in nonmuscle cells is redundant or that a distinct and only distantly related gene encodes the non-muscle isoform. Allelic but less severely affected fliA mutants are apparently due to point mutations, and develop into adults having thoracic muscle abnormalities. EM of mutant muscles reveals that Z discs and myofibrillar attachments are disrupted, whereas epithelial "tendon" cells are less affected. We discuss these phenotypes in the light of presumed in vivo alpha-actinin functions.

Genetic approaches to myofibril form and function in Drosophila

Myofibrils, the contractile organelles of muscle, are apt subjects for studies on the formation and function of actomyosin networks. Molecular genetic approaches are advancing our understanding of myofibril structure and assembly, and may offer a novel and useful approach for investigating the crossbridge cycle. We review recent progress in Drosophila.

Computation of a three dimensional image of a periodic specimen from a single view of an oblique section

We describe here a method for computing a three dimensional map of a periodic specimen from a single electron micrograph of an obliquely cut section. Neighbouring areas of such an image display successively the contents of the unit cell of the structure. The reconstruction procedure can be considered in two steps. The first step involves restacking of successive areas to produce an image akin to that produced by serial section reconstruction. The resolution normal to the section would, at this stage, be limited by the thickness of the section, since the micrograph represents a projection of the density in the section. However, because of the periodic nature of the specimen, the image contains redundant information, which can be used in an attempt to deconvolute the section thickness and thus produce improved resolution normal to the section. The computation can be carried out directly with the densities or more conveniently, particularly for three dimensional crystals, by using Fourier transforms. The approach, which is most powerful when the section is thin, is insensitive to the collapse of the section caused by electron irradiation. Striated muscle provides particularly suitable specimens for such analysis and we present, as examples, computed maps of the M-band of fish muscle and of insect flight muscle in rigor.

Arrangement of filaments and cross-links in the bee flight muscle Z disk by image analysis of oblique sections

Information from oblique thin sections and from three-dimensional reconstructions of tilted, transverse thin sections (Cheng, N., and J. F. Deatherage. 1989. J. Cell Biol. 108:1761-1774) has been combined to determine the three-dimensional structure of the honeybee flight muscle Z disk at 70-A resolution. The overall symmetry and structure of the Z disk and its relationship to the rest of the myofibril have been determined by tracing filaments and connecting elements on electron images of oblique sections which have been enhanced by a local crystallographic averaging technique. In the three-dimensional structure, the connecting density between actin filaments can be described as five compact, crystallographically nonequivalent domains. Features C1 and C2 are located on the transverse twofold rotation axes in the central plane of the Z disk. They are associated with the sides of actin filaments of opposite polarity. Features C3, C4, and C5 are present in two symmetry-related sets which are located on opposite sides of the central plane. C3 and C5 are each associated with two filaments of opposite polarity, interacting with the side of one filament and the end of the other filament. C3 and C5 may be involved in stabilizing actin filament ends inside the Z disk. The location of the threefold symmetric connection C4, relative to the thick filament of the adjacent sarcomere, is determined and its possible relationship to the C filament is considered.