New X-ray diffraction observations on vertebrate muscle: organisation of C-protein (MyBP-C) and troponin and evidence for unknown structures in the vertebrate A-band

Structure of mavacamten-free human cardiac thick filaments within the sarcomere by cryoelectron tomography

Heart muscle has the unique property that it can never rest; all cardiomyocytes contract with each heartbeat which requires a complex control mechanism to regulate cardiac output to physiological requirements. Changes in calcium concentration regulate the thin filament activation. A separate but linked mechanism regulates the thick filament activation, which frees sufficient myosin heads to bind the thin filament, thereby producing the required force. Thick filaments contain additional nonmyosin proteins, myosin-binding protein C and titin, the latter being the protein that transmits applied tension to the thick filament. How these three proteins interact to control thick filament activation is poorly understood. Here, we show using 3-D image reconstruction of frozen-hydrated human cardiac muscle myofibrils lacking exogenous drugs that the thick filament is structured to provide three levels of myosin activation corresponding to the three crowns of myosin heads in each 429Å repeat. In one crown, the myosin heads are almost completely activated and disordered. In another crown, many myosin heads are inactive, ordered into a structure called the interacting heads motif. At the third crown, the myosin heads are ordered into the interacting heads motif, but the stability of that motif is affected by myosin-binding protein C. We think that this hierarchy of control explains many of the effects of length-dependent activation as well as stretch activation in cardiac muscle control.

Interacting-Heads Motif Explains the X-Ray Diffraction Pattern of Relaxed Vertebrate Skeletal Muscle

Electron microscopy (EM) shows that myosin heads in thick filaments isolated from striated muscles interact with each other and with the myosin tail under relaxing conditions. This "interacting-heads motif" (IHM) is highly conserved across the animal kingdom and is thought to be the basis of the super-relaxed state. However, a recent X-ray modeling study concludes, contrary to expectation, that the IHM is not present in relaxed intact muscle. We propose that this conclusion results from modeling with a thick filament 3D reconstruction in which the myosin heads have radially collapsed onto the thick filament backbone, not from absence of the IHM. Such radial collapse, by about 3-4 nm, is well established in EM studies of negatively stained myosin filaments, on which the reconstruction was based. We have tested this idea by carrying out similar X-ray modeling and determining the effect of the radial position of the heads on the goodness of fit to the X-ray pattern. We find that, when the IHM is modeled into a thick filament at a radius 3-4 nm greater than that modeled in the recent study, there is good agreement with the X-ray pattern. When the original (collapsed) radial position is used, the fit is poor, in agreement with that study. We show that modeling of the low-angle region of the X-ray pattern is relatively insensitive to the conformation of the myosin heads but very sensitive to their radial distance from the filament axis. We conclude that the IHM is sufficient to explain the X-ray diffraction pattern of intact muscle when placed at the appropriate radius.

The molecular basis for sarcomere organization in vertebrate skeletal muscle

Sarcomeres are force-generating and load-bearing devices of muscles. A precise molecular picture of how sarcomeres are built underpins understanding their role in health and disease. Here, we determine the molecular architecture of native vertebrate skeletal sarcomeres by electron cryo-tomography. Our reconstruction reveals molecular details of the three-dimensional organization and interaction of actin and myosin in the A-band, I-band, and Z-disc and demonstrates that α-actinin cross-links antiparallel actin filaments by forming doublets with 6-nm spacing. Structures of myosin, tropomyosin, and actin at ~10 Å further reveal two conformations of the "double-head" myosin, where the flexible orientation of the lever arm and light chains enable myosin not only to interact with the same actin filament, but also to split between two actin filaments. Our results provide unexpected insights into the fundamental organization of vertebrate skeletal muscle and serve as a strong foundation for future investigations of muscle diseases.

Lattice arrangement of myosin filaments correlates with fiber type in rat skeletal muscle

Ma et al. studied the 3-D arrangement of thick filaments in skeletal muscle by x-ray diffraction and electron microscopy and found a correlation between thick filament lattice type (simple or superlattice) and fiber type (fast/slow). This suggests that lattice organization contributes to muscle functional properties.

The thick (myosin-containing) filaments of vertebrate skeletal muscle are arranged in a hexagonal lattice, interleaved with an array of thin (actin-containing) filaments with which they interact to produce contraction. X-ray diffraction and EM have shown that there are two types of thick filament lattice. In the simple lattice, all filaments have the same orientation about their long axis, while in the superlattice, nearest neighbors have rotations differing by 0° or 60°. Tetrapods (amphibians, reptiles, birds, and mammals) typically have only a superlattice, while the simple lattice is confined to fish. We have performed x-ray diffraction and electron microscopy of the soleus (SOL) and extensor digitorum longus (EDL) muscles of the rat and found that while the EDL has a superlattice as expected, the SOL has a simple lattice. The EDL and SOL of the rat are unusual in being essentially pure fast and slow muscles, respectively. The mixed fiber content of most tetrapod muscles and/or lattice disorder may explain why the simple lattice has not been apparent in these vertebrates before. This is supported by only weak simple lattice diffraction in the x-ray pattern of mouse SOL, which has a greater mix of fiber types than rat SOL. We conclude that the simple lattice might be common in tetrapods. The correlation between fiber type and filament lattice arrangement suggests that the lattice arrangement may contribute to the functional properties of a muscle.

Titin and Nebulin in Thick and Thin Filament Length Regulation

In this review we discuss the history and the current state of ideas related to the mechanism of size regulation of the thick (myosin) and thin (actin) filaments in vertebrate striated muscles. Various hypotheses have been considered during of more than half century of research, recently mostly involving titin and nebulin acting as templates or 'molecular rulers', terminating exact assembly. These two giant, single-polypeptide, filamentous proteins are bound in situ along the thick and thin filaments, respectively, with an almost perfect match in the respective lengths and structural periodicities. However, evidence still questions the possibility that the proteins function as templates, or scaffolds, on which the thin and thick filaments could be assembled. In addition, the progress in muscle research during the last decades highlighted a number of other factors that could potentially be involved in the mechanism of length regulation: molecular chaperones that may guide folding and assembly of actin and myosin; capping proteins that can influence the rates of assembly-disassembly of the myofilaments; Ca²⁺ transients that can activate or deactivate protein interactions, etc. The entire mechanism of sarcomere assembly appears complex and highly dynamic. This mechanism is also capable of producing filaments of about the correct size without titin and nebulin. What then is the role of these proteins? Evidence points to titin and nebulin stabilizing structures of the respective filaments. This stabilizing effect, based on linear proteins of a fixed size, implies that titin and nebulin are indeed molecular rulers of the filaments. Although the proteins may not function as templates in the assembly of the filaments, they measure and stabilize exactly the same size of the functionally important for the muscles segments in each of the respective filaments.

Orientation of myosin binding protein C in the cardiac muscle sarcomere determined by domain-specific immuno-EM

Myosin binding protein C is a thick filament protein of vertebrate striated muscle. The cardiac isoform [cardiac myosin binding protein C (cMyBP-C)] is essential for normal cardiac function, and mutations in cMyBP-C cause cardiac muscle disease. The rod-shaped molecule is composed primarily of 11 immunoglobulin- or fibronectin-like domains and is located at nine sites, 43nm apart, in each half of the A-band. To understand how cMyBP-C functions, it is important to know its structural organization in the sarcomere, as this will affect its ability to interact with other sarcomeric proteins. Several models, in which cMyBP-C wraps around, extends radially from, or runs axially along the thick filament, have been proposed. Our goal was to define cMyBP-C orientation by determining the relative axial positions of different cMyBP-C domains. Immuno-electron microscopy was performed using mouse cardiac myofibrils labeled with antibodies specific to the N- and C-terminal domains and to the middle of cMyBP-C. Antibodies to all regions of the molecule, except the C-terminus, labeled at the same nine axial positions in each half A-band, consistent with a circumferential and/or radial rather than an axial orientation of the bulk of the molecule. The C-terminal antibody stripes were slightly displaced axially, demonstrating an axial orientation of the C-terminal three domains, with the C-terminus closer to the M-line. These results, combined with previous studies, suggest that the C-terminal domains of cMyBP-C run along the thick filament surface, while the N-terminus extends toward neighboring thin filaments. This organization provides a structural framework for understanding cMyBP-C's modulation of cardiac muscle contraction.

The non-linear elasticity of the muscle sarcomere and the compliance of myosin motors

Force in striated muscle is due to attachment of the heads of the myosin, the molecular motors extending from the myosin filament, to the actin filament in each half-sarcomere, the functional unit where myosin motors act in parallel. Mechanical and X-ray structural evidence indicates that at the plateau of isometric contraction (force T0), less than half of the elastic strain of the half-sarcomere is due to the strain in the array of myosin motors (s), with the remainder being accounted for by the compliance of filaments acting as linear elastic elements in series with the motor array. Early during the development of isometric force, however, the half-sarcomere compliance has been found to be less than that expected from the linear elastic model assumed above, and this non-linearity may affect the estimate of s. This question is investigated here by applying nanometre-microsecond-resolution mechanics to single intact fibres from frog skeletal muscle at 4 °C, to record the mechanical properties of the half-sarcomere throughout the development of force in isometric contraction. The results are interpreted with mechanical models to estimate the compliance of the myosin motors. Our conclusions are as follows: (i) early during the development of an isometric tetanus, an elastic element is present in parallel with the myosin motors, with a compliance of ∼200 nm MPa(-1) (∼20 times larger than the compliance of the motor array at T0); and (ii) during isometric contraction, s is 1.66 ± 0.05 nm, which is not significantly different from the value estimated with the linear elastic model.

Head-head interactions of resting myosin crossbridges in intact frog skeletal muscles, revealed by synchrotron x-ray fiber diffraction

The intensities of the myosin-based layer lines in the x-ray diffraction patterns from live resting frog skeletal muscles with full thick-thin filament overlap from which partial lattice sampling effects had been removed were analyzed to elucidate the configurations of myosin crossbridges around the thick filament backbone to nanometer resolution. The repeat of myosin binding protein C (C-protein) molecules on the thick filaments was determined to be 45.33 nm, slightly longer than that of myosin crossbridges. With the inclusion of structural information for C-proteins and a pre-powerstroke head shape, modeling in terms of a mixed population of regular and perturbed regions of myosin crown repeats along the filament revealed that the myosin filament had azimuthal perturbations of crossbridges in addition to axial perturbations in the perturbed region, producing pseudo-six-fold rotational symmetry in the structure projected down the filament axis. Myosin crossbridges had a different organization about the filament axis in each of the regular and perturbed regions. In the regular region that lacks C-proteins, there were inter-molecular interactions between the myosin heads in axially adjacent crown levels. In the perturbed region that contains C-proteins, in addition to inter-molecular interactions between the myosin heads in the closest adjacent crown levels, there were also intra-molecular interactions between the paired heads on the same crown level. Common features of the interactions in both regions were interactions between a portion of the 50-kDa-domain and part of the converter domain of the myosin heads, similar to those found in the phosphorylation-regulated invertebrate myosin. These interactions are primarily electrostatic and the converter domain is responsible for the head-head interactions. Thus multiple head-head interactions of myosin crossbridges also characterize the switched-off state and have an important role in the regulation or other functions of myosin in thin filament-regulated muscles as well as in the thick filament-regulated muscles.

Cardiac myosin binding protein-C: redefining its structure and function

Mutations of cardiac myosin binding protein-C (cMyBP-C) are inherited by an estimated 60 million people worldwide, and the protein is the target of several kinases. Recent evidence further suggests that cMyBP-C mutations alter Ca(2+) transients, leading to electrophysiological dysfunction. Thus, while the importance of studying this cardiac sarcomere protein is clear, preliminary data in the literature have raised many questions. Therefore, in this article, we propose to review the structure and function of cMyBP-C with particular respect to the role(s) in cardiac contractility and whether its release into the circulatory system is a potential biomarker of myocardial infarction. We also discuss future directions and experimental designs that may lead to expanding the role(s) of cMyBP-C in the heart. In conclusion, we suggest that cMyBP-C is a regulatory protein that could offer a broad clinical utility in maintaining normal cardiac function.

Structure, interactions and function of the N-terminus of cardiac myosin binding protein C (MyBP-C): who does what, with what, and to whom?

The thick filament protein myosin-binding protein-C shows a highly modular architecture, with the C-terminal region responsible for tethering to the myosin and titin backbone of the thick filament. The N-terminal region shows the most significant differences between cardiac and skeletal muscle isogenes: an entire Ig-domain (C0) is added, together with highly regulated phosphorylation sites between Ig domains C1 and C2. These structural and functional differences at the N-terminus reflect important functions in cardiac muscle regulation in health and disease. Alternative interactions of this part of MyBP-C with the head-tail (S1-S2) junction of myosin or to actin filaments have been proposed, but with conflicting experimental evidence. The regulation of myosin or actin interaction by phosphorylation of the cardiac MyBP-C N-terminus may play an additional role in length-dependent contraction regulation. We discuss here the evidence for these proposed interactions, considering the required properties of MyBP-C, the way in which they may be regulated in muscle contraction and the way they might be related to heart disease. We also attempt to shed some light on experimental pitfalls and future strategies.

Human cardiac myosin binding protein C: structural flexibility within an extended modular architecture

New insights into the modular organization and flexibility of the N-terminal half of human cardiac myosin binding protein C (cMyBP-C) and information on the association state of the full-length protein have been deduced from a combined small-angle X-ray scattering (SAXS) and NMR study. SAXS data show that the first five immunoglobulin domains of cMyBP-C, which include those implicated in interactions with both myosin and actin, remain monodisperse and monomeric in solution and have a highly extended yet distinctively 'bent' modular arrangement that is similar to the giant elastic muscle protein titin. Analyses of the NMR and SAXS data indicate that a proline/alanine-rich linker connecting the cardiac-specific N-terminal C0 domain to the C1 domain provides significant structural flexibility at the N-terminus of the human isoform, while the modular arrangement of domains C1-C2-C3-C4 is relatively fixed. Domain fragments from the C-terminal half of the protein have a propensity to self-associate in vitro, while full-length bacterially expressed cMyBP-C forms flexible extended dimers at micromolar protein concentrations. In summary, our studies reveal that human cMyBP-C combines a distinctive modular architecture with regions of flexibility and that the N-terminal half of the protein is sufficiently extended to span the range of interfilament distances sampled within the dynamic environment of heart muscle. These structural features of cMyBP-C could facilitate its putative role as a molecular switch between actin and myosin and may contribute to modulating the transverse pliancy of the C-zone of the A-band across muscle sarcomeres.

Roles for cardiac MyBP-C in maintaining myofilament lattice rigidity and prolonging myosin cross-bridge lifetime

We investigated the influence of cardiac myosin binding protein-C (cMyBP-C) and its constitutively unphosphorylated status on the radial and longitudinal stiffnesses of the myofilament lattice in chemically skinned myocardial strips of the following mouse models: nontransgenic (NTG), effective null for cMyBP-C (t/t), wild-type cMyBP-C expressed into t/t (WT(t/t)), and constitutively unphosphorylated cMyBP-C (AllP-(t/t)). We found that the absence of cMyBP-C in the t/t and the unphosphorylated cMyBP-C in the AllP-(t/t) resulted in a compressible cardiac myofilament lattice induced by rigor not observed in the NTG and WT(t/t). These results suggest that the presence and phosphorylation of the N-terminus of cMyBP-C provides structural support and radial rigidity to the myofilament lattice. Examination of myofilament longitudinal stiffness under rigor conditions demonstrated a significant reduction in cross-bridge-dependent stiffness in the t/t compared with NTG controls, but not in the AllP-(t/t) compared with WT(t/t) controls. The absence of cMyBP-C in the t/t and the unphosphorylated cMyBP-C in the AllP-(t/t) both resulted in a shorter myosin cross-bridge lifetime when myosin isoform was controlled. These data collectively suggest that cMyBP-C provides radial rigidity to the myofilament lattice through the N-terminus, and that disruption of the phosphorylation of cMyBP-C is sufficient to abolish this structural role of the N-terminus and shorten cross-bridge lifetime. Although the presence of cMyBP-C also provides longitudinal rigidity, phosphorylation of the N-terminus is not necessary to maintain longitudinal rigidity of the lattice, in contrast to radial rigidity.

Roles of titin in the structure and elasticity of the sarcomere

The giant protein titin is thought to play major roles in the assembly and function of muscle sarcomeres. Structural details, such as widths of Z- and M-lines and periodicities in the thick filaments, correlate with the substructure in the respective regions of the titin molecule. Sarcomere rest length, its operating range of lengths, and passive elastic properties are also directly controlled by the properties of titin. Here we review some recent titin data and discuss its implications for sarcomere architecture and elasticity.

Shape and flexibility in the titin 11-domain super-repeat

Titin is a giant protein of striated muscle with important roles in the assembly, intracellular signalling and passive mechanical properties of sarcomeres. The molecule consists principally of approximately 300 immunoglobulin and fibronectin domains arranged in a chain more than 1 mum long. The isoform-dependent N-terminal part of the molecule forms an elastic connection between the end of the thick filament and the Z-line. The larger, constitutively expressed C-terminal part is bound to the thick filament. Through most of the thick filament part, the immunoglobulin and fibronectin domains are arranged in a repeating pattern of 11 domains termed the 'large super-repeat'. There are 11 contiguous copies of the large super-repeat making up a segment of the molecule nearly 0.5 mum long. We have studied a set of two-domain and three-domain recombinant fragments from the large super-repeat region by electron microscopy, synchrotron X-ray solution scattering and analytical ultracentrifugation, with the goal of reconstructing the overall structure of this part of titin. The data illustrate different average conformations in different domain pairs, which correlate with differences in interdomain linker lengths. They also illustrate interdomain bending and flexibility around average conformations. Overall, the data favour a helical conformation in the super-repeat. They also suggest that this region of titin is dimerized when bound to the thick filament.

Cardiac myosin-binding protein C decorates F-actin: implications for cardiac function

Cardiac myosin-binding protein C (cMyBP-C) is an accessory protein of striated muscle sarcomeres that is vital for maintaining regular heart function. Its 4 N-terminal regulatory domains, C0-C1-m-C2 (C0C2), influence actin and myosin interactions, the basic contractile proteins of muscle. Using neutron contrast variation data, we have determined that C0C2 forms a repeating assembly with filamentous actin, where the C0 and C1 domains of C0C2 attach near the DNase I-binding loop and subdomain 1 of adjacent actin monomers. Direct interactions between the N terminus of cMyBP-C and actin thereby provide a mechanism to modulate the contractile cycle by affecting the regulatory state of the thin filament and its ability to interact with myosin.

Support for a trimeric collar of myosin binding protein C in cardiac and fast skeletal muscle, but not in slow skeletal muscle

Myosin-binding protein C (MyBPC) is proposed to take on a trimeric collar arrangement around the thick filament backbone in cardiac muscle, based on interactions between cardiac MyBPC domains C5 and C8. We have now determined, using yeast two-hybrid and in vitro binding assays, that the C5:C8 interaction is not dependent on the 28-residue cardiac-specific insert in C5. Furthermore, an interaction of similar affinity occurs between domains C5 and C8 of fast skeletal muscle MyBPC, but not between these domains of the slow skeletal muscle protein. These data have implications for the role and quaternary structure of MyBPC in skeletal muscle.

Radial displacement of myosin cross-bridges in mouse myocardium due to ablation of myosin binding protein-C

Myosin binding protein-C (cMyBP-C) is a thick filament accessory protein, which in cardiac muscle functions to regulate the kinetics of cross-bridge interaction with actin; however, the underlying mechanism is not yet understood. To explore the structural basis for cMyBP-C function, we used synchrotron low-angle X-ray diffraction to measure interfilament lattice spacing and the equatorial intensity ratio, I(11)/I(10), in skinned myocardial preparations isolated from wild-type (WT) and cMyBP-C null (cMyBP-C(-/-)). In relaxed myocardium, ablation of cMyBP-C appeared to result in radial displacement of cross-bridges away from the thick filaments, as there was a significant increase ( approximately 30%) in the I(11)/I(10) ratio for cMyBP-C(-/-) (0.37+/-0.03) myocardium as compared to WT (0.28+/-0.01). While lattice spacing tended to be greater in cMyBP-C(-/-) myocardium (44.18+/-0.68 nm) when compared to WT (42.95+/-0.43 nm), the difference was not statistically significant. Furthermore, liquid-like disorder in the myofilament lattice was significantly greater ( approximately 40% greater) in cMyBP-C(-/-) myocardium as compared to WT. These results are consistent with our working hypothesis that cMyBP-C normally acts to tether myosin cross-bridges nearer to the thick filament backbone, thereby reducing the likelihood of cross-bridge binding to actin and limiting cooperative activation of the thin filament.

Axial dispositions and conformations of myosin crossbridges along thick filaments in relaxed and contracting states of vertebrate striated muscles by X-ray fiber diffraction

X-ray diffraction patterns from live vertebrate striated muscles were analyzed to elucidate the detailed structural models of the myosin crown arrangement and the axial disposition of two-headed myosin crossbridges along the thick filaments in the relaxed and contracting states. The modeling studies were based upon the previous notion that individual myosin filaments had a mixed structure with two regions, a "regular" and a "perturbed". In the relaxed state the distributions and sizes of the regular and perturbed regions on myosin filaments, each having its own axial periodicity for the arrangement of crossbridge crowns within the basic period, were similar to those reported previously. A new finding was that in the contracting state, this mixed structure was maintained but the length of each region, the periodicities of the crowns and the axial disposition of two heads of a crossbridge were altered. The perturbed regions of the crossbridge repeat shifted towards the Z-bands in the sarcomere without changing the lengths found in the relaxed state, but in which the intervals between three successive crowns within the basic period became closer to the regular 14.5-nm repeat in the contracting state. In high resolution modeling for a myosin head, the two heads of a crossbridge were axially tilted in opposite directions along the three-fold helical tracks of myosin filaments and their axial orientations were different from each other in perturbed and regular regions in both states. Under relaxing conditions, one head of a double-headed crossbridge pair appeared to be in close proximity to another head in a pair at the adjacent crown level in the axial direction in the regular region. In the perturbed region this contact between heads occurred only on the narrower inter-crown levels. During contraction, one head of a crossbridge oriented more perpendicular to the fiber axis and the partner head flared axially. Several factors that significantly influence the intensities of the myosin based-meridional reflections and their relative contributions are discussed.

Geometrical correspondence identified and a new interaction unit suggested in striated muscle

It has long been believed that the periodic structure of the myosin helix is a consequence only of compressing the actin-myosin interaction sites. Here, we identify a length correspondence between the smallest helical unit on the thick filament and the helical pitch of the actin filaments in two different contractile muscles. This suggests a rotation/swing of the filaments that creates a new interaction unit in addition to the single interaction between an actin filament and a myosin head. Numerical characteristics of the single interaction are estimated from discussion about an in vivo interaction utilizing the new unit. The estimated twisted angle of the actin filaments is consistent with that calculated from its torsion rigidity and the evaluated step sizes per cross-bridge can be performed by a single bend of a myosin head. By comparing our evaluated step sizes with experimental results, we conclude that the most plausible mechanism at the force-recovery stage involves swings or rotations of both filaments in the same direction (clockwise).

Single particle analysis: a new approach to solving the 3D structure of myosin filaments

Knowledge of the structure of muscle myosin filaments is essential for a proper understanding of sarcomere structure and how myosin heads interact with the actin filaments to produce force and movement. Two principal methods have been used to define the myosin head arrays in filaments in the relaxed state, namely modelling from low-angle X-ray diffraction data and image processing of electron micrographs of isolated filaments. Analysis of filament images by 3D helical reconstruction, which imposes total helical symmetry on the structure, is very effective in some cases, but it relies on the existence of very highly ordered preparations of straight filaments. Resolutions achieved to date are about 70 angstroms. Modelling of X-ray diffraction data recorded from whole relaxed fish or insect muscles has also been used as an independent method. Although the resolution of the diffraction data is often also about 70 angstroms, the effective resolution of the modelling is very much higher than this because additional very high resolution data (e.g. from protein crystallography) is included in the analysis. However, the X-ray diffraction method has to date provided only limited data on non-myosin thick filament proteins such as C-protein and titin and it cannot provide the polarity of the myosin head arrangement. Both the helical reconstruction and X-ray diffraction techniques have advantages and disadvantages, but their disadvantages are avoided in the new approach of single particle analysis of electron micrograph data. Even using the same micrographs as for helical reconstruction, the resolution can be extended by this method to about 50 angstroms or better. In addition, it is not necessary to assume that the myosin filaments are helical; a significant advantage in the case of vertebrate myosin filaments where there is a known crossbridge perturbation. Here we describe the principles of all these approaches, but particularly that of single particle analysis. We outline the application of single particle analysis to myosin filaments from vertebrate skeletal and insect flight (IFM) muscle myosin filaments.