Binding of myosin subfragment 1 to glycerinated insect flight muscle in the rigor state

Insights into Actin-Myosin Interactions within Muscle from 3D Electron Microscopy

Much has been learned about the interaction between myosin and actin through biochemistry, in vitro motility assays and cryo-electron microscopy (cryoEM) of F-actin, decorated with myosin heads. Comparatively less is known about actin-myosin interactions within the filament lattice of muscle, where myosin heads function as independent force generators and thus most measurements report an average signal from multiple biochemical and mechanical states. All of the 3D imaging by electron microscopy (EM) that has revealed the interplay of the regular array of actin subunits and myosin heads within the filament lattice has been accomplished using the flight muscle of the large water bug Lethocerus sp. The Lethocerus flight muscle possesses a particularly favorable filament arrangement that enables all the myosin cross-bridges contacting the actin filament to be visualized in a thin section. This review covers the history of this effort and the progress toward visualizing the complex set of conformational changes that myosin heads make when binding to actin in several static states, as well as the fast frozen actively contracting muscle. The efforts have revealed a consistent pattern of changes to the myosin head structures as determined by X-ray crystallography needed to explain the structure of the different actomyosin interactions observed in situ.

Time-Resolved Structural Studies on Insect Flight Muscle after Photolysis of Caged-ATP

The time course of structural changes occurring on ATP-induced relaxation of glycerinated insect flight muscle from the rigor state has been investigated using synchrotron radiation as a source of high intensity x rays and photolysis of caged-ATP to produce a rapid rise in ATP concentration. Temporal resolutions of 1 ms for the strongest equatorial reflections and 5 ms for the 14.5 nm meridional reflection are attainable from single events (i.e., without averaging over several cycles). The equatorial intensity changes completely, the meridional intensity partially, towards their respective relaxed values on a much faster time scale than relaxation of tension. The results suggest that actively cycling bridges present shortly after ATP-release are either too few in number to be detected in the equatorial diffraction pattern or that their structure is different from that of rigor bridges.

Evidence for unique structural change of thin filaments upon calcium activation of insect flight muscle

Upon activation of living or skinned vertebrate skeletal muscle fibers, the sixth X-ray layer-line reflection from actin (6th ALL) is known to intensify, without a shift of its peak position along the layer line. Since myosin attachment to actin is expected to shift the peak towards the meridian, this intensification is considered to reflect the structural change of individual actin monomers in the thin filament. Here, we show that the 6th ALL of skinned insect flight muscles (IFMs) is rather weakened upon isometric calcium activation, and its peak shifts away from the meridian. This suggests that the actin monomers in the two types of muscles change their structures in substantially different manners. The changes that occurred in the 6th ALL of IFM were not diminished by lowering the temperature from 20 to 5 degrees C, while active force was greatly reduced. The inclusion of 100 microM blebbistatin (a myosin inhibitor) did not affect the changes either. This suggests that calcium binding to troponin C, rather than myosin binding to actin, causes the structural change of IFM actin.

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.

Diversity of structural behavior in vertebrate conventional myosins complexed with actin

Low-resolution three-dimensional structures of acto-myosin subfragment-1 (S1) complexes were retrieved from X-ray fiber diffraction patterns, recorded either in the presence or absence of ADP. The S1 was obtained from various myosin-II isoforms from vertebrates, including rabbit fast-skeletal and cardiac, chicken smooth and human non-muscle IIA and IIB species, and was diffused into an array of overstretched, skinned skeletal muscle fibers. The S1 attached to the exposed actin filaments according to their helical symmetry. Upon addition of ADP, the diffraction patterns from acto-S1 showed an increasing magnitude of response in the order as listed above, with features of a lateral compression of the whole diffraction pattern (indicative of increased radius of the acto-S1 complex) and an enhancement of the fifth layer-line reflection. The structure retrieval indicates that these changes are mainly due to the swing of the light chain (LC) domain in the direction consistent with the cryo-electron microscopic results. In the non-muscle isoforms, the swing is large enough to affect the manner of quasi-crystal packing of the S1-decorated actin filaments and their lattice dimension, with a small change in the twist of actin filaments. Variations also exist in the behavior of the 50K-cleft, which apparently opens upon addition of ADP to the non-muscle isoforms but not to other isoforms. The fast-skeletal S1 remains as the only isoform that does not clearly exhibit either of the structural changes. The results indicate that the "conventional" myosin-II isoforms exhibit a wide variety of structural behavior, possibly depending on their functions and/or the history of molecular evolution.

Quantitative self-organizing maps for clustering electron tomograms

Tomography emerges as a powerful methodology for determining the complex architectures of biological specimens that are better regarded from the structural point of view as singular entities. However, once the structure of a sufficiently large number of singular specimens is solved, quite possibly structural patterns start to emerge. This latter situation is addressed here, where the clustering of a set of 3D reconstructions using a novel quantitative approach is presented. In general terms, we propose a new variant of a self-organizing neural network for the unsupervised classification of 3D reconstructions. The novelty of the algorithm lies in its rigorous mathematical formulation that, starting from a large set of noisy input data, finds a set of "representative" items, organized onto an ordered output map, such that the probability density of this set of representative items resembles at its possible best the probability density of the input data. In this study, we evaluate the feasibility of application of the proposed neural approach to the problem of identifying similar 3D motifs within tomograms of insect flight muscle. Our experimental results prove that this technique is suitable for this type of problem, providing the electron microscopy community with a new tool for exploring large sets of tomogram data to find complex patterns.

Molecular modeling of averaged rigor crossbridges from tomograms of insect flight muscle

Electron tomography, correspondence analysis, molecular model building, and real-space refinement provide detailed 3-D structures for in situ myosin crossbridges in the nucleotide-free state (rigor), thought to represent the end of the power stroke. Unaveraged tomograms from a 25-nm longitudinal section of insect flight muscle preserved native structural variation. Recurring crossbridge motifs that repeat every 38.7 nm along the actin filament were extracted from the tomogram and classified by correspondence analysis into 25 class averages, which improved the signal to noise ratio. Models based on the atomic structures of actin and of myosin subfragment 1 were rebuilt to fit 11 class averages. A real-space refinement procedure was applied to quantitatively fit the reconstructions and to minimize steric clashes between domains introduced during the fitting. These combined procedures show that no single myosin head structure can fit all the in situ crossbridges. The validity of the approach is supported by agreement of these atomic models with fluorescent probe data from vertebrate muscle as well as with data from regulatory light chain crosslinking between heads of smooth muscle heavy meromyosin when bound to actin.

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

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

Interplay between passive tension and strong and weak binding cross-bridges in insect indirect flight muscle. A functional dissection by gelsolin-mediated thin filament removal

The interplay between passive and active mechanical properties of indirect flight muscle of the waterbug (Lethocerus) was investigated. A functional dissection of the relative contribution of cross-bridges, actin filaments, and C filaments to tension and stiffness of passive, activated, and rigor fibers was carried out by comparing mechanical properties at different ionic strengths of sarcomeres with and without thin filaments. Selective thin filament removal was accomplished by treatment with the actin-severving protein gelsolin. Thin filament, removal had no effect on passive tension, indicating that the C filament and the actin filament are mechanically independent and that passive tension is developed by the C filament in response to sarcomere stretch. Passive tension increased steeply with sarcomere length until an elastic limit was reached at only 6-7% sarcomere extension, which corresponds to an extension of 350% of the C filament. The passive tension-length relation of insect flight muscle was analyzed using a segmental extension model of passive tension development (Wang, K, R. McCarter, J. Wright, B. Jennate, and R Ramirez-Mitchell. 1991. Proc. Natl. Acad. Sci. USA. 88:7101-7109). Thin filament removal greatly depressed high frequency passive stiffness (2.2 kHz) and eliminated the ionic strength sensitivity of passive stiffness. It is likely that the passive stiffness component that is removed by gelsolin is derived from weak-binding cross-bridges, while the component that remains is derived from the C filament. Our results indicate that a significant number of weak-binding cross-bridges exist in passive insect muscle at room temperature and at an ionic strength of 195 mM. Analysis of rigor muscle indicated that while rigor tension is entirely actin based, rigor stiffness contains a component that resists gelsolin treatment and is therefore likely to be C filament based. Active tension and active stiffness of unextracted fibers were directly proportional to passive tension before activation. Similarly, passive stiffness due to weak bridges also increased linearly with passive tension, up to a limit. These correlations lead us to propose a stress-activation model for insect flight muscle in which passive tension is a prerequisite for the formation of both weak-binding and strong-binding cross-bridges.

Current X-ray diffraction experiments using a synchrotron radiation source

A Fuji imaging plate and synchrotron radiation are the most distinct innovations of the last twenty years in the X-ray diffraction experiments on biological materials. Here we present results of recent experiments on skeletal muscles made at Photon Factory, Tsukuba. It is now possible to record a two-dimensional X-ray diffraction pattern from a rabbit or frog single skinned fiber with a 30-sec exposure. Although weaker compared with those from whole muscles, it shows layer-lines up to 5.1 nm. When the fiber is activated by Ca2+, the pattern changes in a way similar to that observed when a live muscle is electrically stimulated. Use of single fibers makes various types of structural experiments much easier than using whole muscles or fiber bundles. Not only suitable for physiological experiments, better diffusion makes it also suitable for biochemical experiments using various kinds of labels. Time-resolved experiments with imaging plates are possible by using an imaging-plate exchanger devised by Dr. Y. Amemiya. By combining this and a fast-acting mechanical shutter, it is possible to record a two-dimensional diffraction pattern from a frog whole muscle shortening at the maximum speed. The pattern thus obtained shows weakening of the 5.1 and 5.9-nm actin layer-lines and the third (14.3 nm) and the sixth (7.2 nm) myosin meridional reflections, compared with the pattern from isometrically contracting muscles. On the other hand, the second meridional reflection from the thick filament is intensified. These results suggest very different arrangement of myosin heads during active shortening from that during isometric contraction.

Insect crossbridges, relaxed by spin-labeled nucleotide, show well-ordered 90 degrees state by X-ray diffraction and electron microscopy, but spectra of electron paramagnetic resonance probes report disorder

The structure of glycerinated Lethocerus insect flight muscle fibers, relaxed by spin-labeled ATP and vanadate (Vi), was examined using X-ray diffraction, electron microscopy and electron paramagnetic resonance (e.p.r.) spectra. We obtained excellent relaxation of MgATP quality as determined by mechanical criteria, using vanadate trapping of 2' spin-labeled 3' deoxyATP at 3 degree C. In rigor fibers, when the diphosphate analog is bound in the absence of Vi, the probes on myosin heads are well-ordered, in agreement with electron microscopic and X-ray patterns showing that myosin heads are ordered when attached strongly to actin. In relaxed muscle, however, e.p.r. spectra report orientational disorder of bound (Vi-trapped) spin-labeled nucleotide, while electron microscopic and X-ray patterns both show well-ordered bridges at a uniform 90 degrees angle to the filament axis. The spin-labeled nucleotide orientation is highly disordered, but not completely isotropic; the slight anisotropy observed in probe spectra is consistent with a shift of approximately 10% of probes from angles close to 0 degrees to angles close to 90 degrees. Measurements of probe mobility suggest that the interaction between probe and protein remains as tight in relaxed fibers as in rigor, and thus that the disorder in relaxed fibers arises from disorders of (or within) the protein and not from disorder of the probe relative to the protein. Fixation of the relaxed fibers with glutaraldehyde did not alter any aspect of the spectrum of the Vi-trapped analog, including the slight order observed, showing that the extensive inter- and intra-molecular cross-linking of the first step of sample preparation for electron microscopy had not altered relaxed crossbridge orientations. Two models that may reconcile the apparently disparate results obtained on relaxed fibers are presented: (1) a rigid myosin head could possess considerable disorder in the regular array about the thick filament; or (2) the nucleotide site could be on a disordered, probably distal, domain of myosin, while a more proximal region is well ordered on the thick filament backbone. Our findings suggest that when e.p.r. probes signal disorder of a local site or domain, this is complementary, not contradictory, to signals of general order. The e.p.r. spectra show that a portion of the myosin molecule can be disordered at the same time as the X-ray diffraction and electron microscopy show the bulk of myosin head mass to be uniformly oriented and regularly arrayed.

Functional and ultrastructural effects of a missense mutation in the indirect flight muscle-specific actin gene of Drosophila melanogaster

A single-site mutation of the flight-muscle-specific actin gene of Drosophila melanogaster causes a substitution of glutamic acid 93 by lysine in all the actin encoded in the indirect flight muscle (IFM). In these Act88FE93K mutants, myofibrillar bundles of thick and thin filaments are present but lack Z-discs and all sarcomeric repeats. Dense filament bundles, which are probably aberrant Z-discs, are seen in myofibrils of pupal flies, but early in adult life these move to the periphery of the fibrils and are not seen in skinned adult fibres. Consistent with this observation, alpha-actinin and other high molecular weight proteins, possibly associated with Z-discs, are not detected on SDS/polyacrylamide gels or Western blots of skinned adult IFM. The mutation lies at the beginning of a loop in the small domain of actin, near the myosin binding region. However, that the mutant actin binds myosin heads is shown by (1) rigor crossbridges in electron micrographs, (2) the appropriate rise in stiffness when ATP is withdrawn in mechanical experiments, and (3) equal protection against tryptic digestion provided by rigor binding between actin and myosin in both wild-type and mutant fibres. Reversal of rigor chevron angle along some thin filaments reflects reversal of thin-filament polarity due to lattice disorder. The absence of Z-discs, alpha-actinin and two high molecular weight proteins, and binding studies by others, suggest that the substitution at residue 93 affects the binding of the mutant actin to a protein, possibly alpha-actinin, which is necessary for Z-disc assembly or maintenance.

X-ray studies of order-disorder transitions in the myosin heads of skinned rabbit psoas muscles

Using x-rays from a laboratory source and an area detector, myosin layer lines and the diffuse scattering between them in the moderate angle region have been recorded. At full overlap, incubation of rigor muscles with S-1 greatly reduces the diffuse scattering. Also, three of the four actin-based layer lines lying close to the meridian (Huxley, H. E., and W. Brown, 1967. J. Mol. Biol. 30:384-434; Haselgrove, J. C. 1975. J. Mol. Biol. 92:113-143) increase, suggesting fuller labeling of the actin filaments. These results are consistent with the idea (Poulsen, F. R., and J. Lowy, 1983. Nature [Lond.]. 303:146-152) that some of the diffuse scattering in rigor muscles is due to a random mixture of actin monomers with and without attached myosin heads (substitution disorder). In relaxed muscles, regardless of overlap, lowering the temperature from 24 to 4 degrees C practically abolishes the myosin layer lines (a result first obtained by Wray, J.S. 1987. J. Muscle Res. Cell Motil. 8:62 (a). Abstr.), whilst the diffuse scattering between these layer lines increases appreciably. Similar changes occur in the passage from rest to peak tetanic tension in live frog muscle (Lowy, J., and F.R. Poulsen. 1990. Biophys. J. 57:977-985). Cooling the psoas demonstrates that the intensity relation between the layer lines and the diffuse scattering is of an inverse nature, and that the transition occurs over a narrow temperature range (12-14 degrees C) with a sigmoidal function. From these results it would appear that the helical arrangement of the myosin heads is very temperature sensitive, and that the disordering effect does not depend on the presence of actin. Measurements along the meridian reveal that the intensity of the diffuse scattering increases relatively little and does so in a nearly linear manner: evidently the axial order of the myosin heads is much less temperature sensitive. The combined data support the view (Poulsen, F. R., and J. Lowy. 1983. Nature [Lond.]. 303:146-152) that in relaxed muscles a significant part of the diffuse scattering originates from disordered myosin heads. The observation that the extent of the diffuse scattering is greater in the equatorial than in the meridional direction suggests that the disordered myosin heads have an orientation which is on average more parallel to the filament axis.

Time-resolved X-ray diffraction studies on stretch-activated insect flight muscle

The specific feature of stretch activation of the indirect flight muscle of the tropical waterbug Lethocerus was used to correlate mechanical and structural aspects of muscle contraction. The time courses of the changes in intensities of the strongest equatorial reflections, the (10) and (20) and of the first meridional reflection at 14.5 nm-1 were monitored using synchrotron radiation as a high intensity X-ray source. The ratio of the intensities of the equatorial reflections, (I20/I10), which reflects the mass distribution within the filament lattice array, increases by about 10% relative to the Ca(2+)-activated level when a rapid stretch is imposed, compared with a 200% change seen when fibres change from the relaxed to the rigor state, while the spacing of the lattice planes decreases by about 1%. The intensity of the first meridional reflection at 14.5 nm-1 decreases by about 35% during stretch activation with a slightly faster time course than the delayed tension increase. The results suggest that the average structure of cycling crossbridges is different from that present in the rigor state.

Binding of ADP and adenosine 5'-[beta, gamma-imido]triphosphate to insect flight muscle fibrils

We have studied the binding of ADP and adenosine 5'-[beta, gamma-imido]triphosphate (AdoPP[NH]P) to insect flight muscle fibrils. We find that 25% of the myosin heads, presumably those which do not interact with actin, bind AdoPP[NH]P with a binding constant greater than 3 X 10(6) M-1, similar to the binding constant of the same compound to the rabbit myosin heads which do not overlap with actin. The remaining heads in insect myofibrils bind AdoPP[NH]P with an association constant of 8 X 10(3) M-1, which is eight times stronger than the affinity of this compound for rabbit myosin heads in overlap with actin. Therefore, in contrast to the situation with rabbit myofibrils where AdoPP[NH]P binds much more weakly than ADP, with insect myofibrils these two adenosine phosphates bind with almost equal affinity. This is consistent with the numerous structural studies on insect flight muscle which were interpreted on the basis that most of the actomyosin sites were saturated with nucleotide at an AdoPP[NH]P concentration of 1 mM.

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.

Thin filaments of rabbit skeletal muscle are in helical register

Thin sections of rapidly frozen and freeze-substituted rabbit glycerinated muscle fibres loaded with myosin subfragment-1 were used to examine a three-dimensional arrangement of thin filaments in vertebrate skeletal muscle. Clearer images of the "arrowhead" structure were obtained when specimens were freeze-substituted first in a tannic acid solution and then in an OsO4 solution. The images obtained showed that the arrowheads were aligned laterally. This indicates that all the thin filaments have the same rotational orientation in a half sarcomere of rabbit skeletal muscle in the rigor state.

Troponin of asynchronous flight muscle

Troponin has been prepared from the asynchronous flight muscle of Lethocerus (water bug) taking special care to prevent proteolysis. The regulatory complex contained tropomyosin and troponin components. The troponin components were Tn-C (18,000 Mr), Tn-T (apparent Mr 53,000) and a heavy component, Tn-H (apparent Mr 80,000). The troponin was tightly bound to tropomyosin and could not be dissociated from it in non-denaturing conditions. A complex of Tn-T, Tn-H and tropomyosin inhibited actomyosin ATPase activity and the inhibition was relieved by Tn-C from vertebrate striated muscle in the presence of Ca2+. However, unlike vertebrate Tn-I, Tn-H by itself was not inhibitory. Monoclonal antibodies were obtained to Tn-T and Tn-H. Antibody to Tn-T was used to screen an expression library of Drosophila cDNA cloned in lambda phage. The sequence of cDNA coding for the protein was determined and hence the amino acid sequence. The Drosophila protein has a sequence similar to that of vertebrate skeletal and cardiac Tn-T. The sequence extends beyond the carboxyl end of the vertebrate sequences, and the last 40 residues are acidic. Part of the sequence of Drosophila Tn-T is homologous to the carboxyl end of the Drosophila myosin light chain MLC-2 and one anti-Tn-T antibody cross-reacted with the light chain. Lethocerus Tn-H is related to the large tropomyosins of Drosophila flight muscle, for which the amino acid sequence is known, since antibodies that recognize this component also recognize the large tropomyosins. Tn-H is easily digested by calpain, suggesting that part of the molecule has an extended configuration. Electron micrographs of negatively stained specimens showed that Lethocerus thin filaments have projections at about 39 nm intervals, which are not seen on thin filaments from vertebrate striated muscle and are probably due to the relatively large troponin complex. Decoration of the thin filaments with myosin subfragment-1 in rigor conditions appeared not to be affected by the troponin. The troponin of asynchronous flight muscle lacks the Tn-I component of vertebrate striated muscle. Tn-H occurs only in the flight muscle and may be involved in the activation of this muscle by stretch.

The structure of insect flight muscle in the presence of AMPPNP

Glycerinated insect (Lethocerus) flight muscle in the presence of the non-hydrolysable ATP-analogue AMPPNP (1 mM at 4 degrees C) has been prepared for electron microscopy using X-ray diffraction monitoring during fixation and embedding. Superior preservation of the original structure has been achieved through use of a fixative which included tannic acid and excess Mg2+. New features have been recognized in single filament layers (myac and actin) and 15 nm cross sections. As previously shown, some aspects of relaxed structure (14.5 nm shelves along thick filaments) and of rigor (38.7 nm angled bridges along thin filaments) are retained in a modified form. New observations include: (1) In 15 nm cross sections that show single 14.5 nm levels: (a) The flared X structure characteristic of rigor is replaced by a straight-X figure in which the crossbridge density is aligned along the myosin-actin plane, rather than skewed across it as in rigor. (b) In AMPPNP, each crossbridge appears to have a separate origin from the thick filament, rather than bifurcating two from one stem as in the flared X of rigor. The separation of crossbridge origins is also evidenced by the loss of 'ladder rungs' in actin layers. (2) A halving (19.3 nm) of the 38.7 nm axial repeat along actin, rather than a thirding (12.9 nm) as in rigor, indicates redistribution of bridge attachments in cold AMPPNP. (3) In AMPPNP, the 14.5 nm shelves of density around the thick filament shaft are thicker but extend to smaller radius than similar shelves in ATP-relaxed muscle. This is shown by a lack of 14.5 nm periodicity and diffraction in actin layers of AMPPNP, in contrast to ATP-relaxed actin layers, in which the 14.5 nm period is present. Our results suggest that attached crossbridges are modified by AMPPNP and that ordered features of the analogue state are not accounted for solely by detached myosin heads or by a mixture of relaxed and rigor crossbridges. A two-domain model for the crossbridge is proposed. Domain 1 binds to the thin filament, and while bound, maintains a constant stereospecificity to actin at low resolution, independent of the type or presence of nucleotide at the myosin active site. Domain 2 is proximal to the thick filament and can exist in two characteristic states. In the absence of nucleotide (rigor), Domain 2 adopts a variable relationship to the thick filament, to accommodate the actin-bound end of the bridge.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural relationships of actin, myosin, and tropomyosin revealed by cryo-electron microscopy

We have calculated three-dimensional maps from images of myosin subfragment-1 (S1)-decorated thin filaments and S1-decorated actin filaments preserved in frozen solution. By averaging many data sets we obtained highly reproducible maps that can be interpreted simply to provide a model for the native structure of decorated filaments. From our results we have made the following conclusions. The bulk of the actin monomer is approximately 65 X 40 X 40 A and is composed of two domains. In the filaments the monomers are strongly connected along the genetic helix with weaker connections following the long pitch helix. The long axis of the monomer lies roughly perpendicular to the filament axis. The myosin head (S1) approaches the actin filament tangentially and binds to a single actin, the major interaction being with the outermost domain of actin. In the map the longest chord of S1 is approximately 130 A. The region of S1 closest to actin is of high density, whereas the part furthest away is poorly defined and may be disordered. By comparing maps from decorated thin filaments with those from decorated actin, we demonstrate that tropomyosin is bound to the inner domain of actin just in front of the myosin binding site at a radius of approximately 40 A. A small change in the azimuthal position of tropomyosin, as has been suggested by others to occur during Ca2+-mediated regulation in vertebrate striated muscle, appears to be insufficient to eclipse totally the major site of interaction between actin and myosin.

Rigor crossbridge structure in tilted single filament layers and flared-X formations from insect flight muscle

The averaged structure of rigor crossbridges in insect flight muscle has been studied in filtered images. Their three-dimensional structure has been deduced by relating tilt views of single filament layers in 25 nm longitudinal sections (myac layers and actin layers) to the flared-X appearance in 15 nm cross-sections showing single crossbridge levels. Tilting myac or actin layers around the filament axis makes crossbridges show one of two patterns. Beadlike densities appear either singly over thin filaments ("center-beading") or doubled and flanking thin filaments ("straddle-beading"). These express two different projections from the crossbridge-actin complexes as seen end-on in flared-X formations. Tannic acid/glutaraldehyde fixation gave improved actin preservation, showing, in 15 nm cross-sections, the long-pitch helical strands as "two-dot" profiles of consistent azimuth in the gaps between double chevrons. The azimuth in the flared-X arms was then inferred from lattice relationships, since it was not seen directly. The tangential attachment of comma-shaped crossbridges to the inferred actin dyad fits the binding geometry in recent actin-subfragment 1 complex reconstructions. However, averaged crossbridge structure differs between lead and rear members of double chevrons, unlike the uniform heads on decorated actin. In filtered images of myac layers, the lead bridges are dense and steeply angled; the rear chevron is seen as a dense bead over the thin filament with faint, less angled bars extending laterally. Actin layer images also suggest that rear and lead bridges differ in angle. Left and right flared-X arms are end-on views of lead and rear chevron bridges, respectively, and differ in shape. Improved fixation with tannic acid/glutaraldehyde allows us to distinguish three crossbridge domains in flared-X arms: (1) a dense bulb-like head merged into the thin filament; (2) a dense but thinner neck tangential to actin; and (3) a faint thin stem joining the necks to myosin filaments. Shape differences in lead and rear members between the head-neck-actin complexes are indicated by the names "L sigmoid" and "R dogleg". Within crossbridges, internal angles between the head-neck axis and the head-actin-head axis differ between sigmoid and dogleg by about 30 degrees, implying a flexible junction between bridge-head and bridge-neck. Lead and rear bridges are axially at least 13 nm apart on actin; the expected 60 degrees difference in azimuth is expressed by head-neck portions, but the head-actin-head axis rotates by only 30 degrees.(ABSTRACT TRUNCATED AT 400 WORDS)

Decreased sodium-potassium pump activity in isolated hypertrophied feline ventricular myocytes

The activity of the Na-K pump was assessed in normal and hypertrophied isolated feline myocytes by measuring ouabain-sensitive 42-K uptake. Right ventricular hypertrophy was produced in feline myocardium by placing a constricting band around the pulmonary artery of adult cats. High yields of calcium tolerant myocytes were isolated from the right and left ventricle of banded and sham operated animals. Intracellular sodium (Na) and potassium (K) concentrations (mM) were not significantly different (P greater than 0.5) in normal (Na: 13.2; K: 133.4) and hypertrophied (Na: 12.3; K: 127.5) myocytes. Morphometric analysis demonstrated a 26% increase in width and a 42% increase in volume of hypertrophied myocytes, however, the sarcomere length (1.9 mu) was not different in both cell types. The rate constant (k, min-1) describing 42-K uptake and the calculated total K influx (I, pmol/cm2/sec) were not significantly different (P greater than 0.5) in normal (k = 0.059; I = 15.9) and hypertrophied (k = 0.062; I = 15.3) cells. Ouabain-sensitive (active) K influx, a measure of Na-K pump activity, was maximally inhibited at 10(-4)M ouabain in both cell types. At this concentration, ouabain-sensitive K uptake was decreased 23.5% in hypertrophied myocytes compared to control. The decrease in active K influx may be due to a decrease in the activity of the Na-K ATPase and/or to a reduction in the passive movement of sodium and potassium down their electrochemical gradients.