Troponin Structural Dynamics in the Native Cardiac Thin Filament Revealed by Cryo Electron Microscopy

Cardiac muscle contraction occurs due to repetitive interactions between myosin thick and actin thin filaments (TF) regulated by Ca2+ levels, active cross-bridges, and cardiac myosin-binding protein C (cMyBP-C). The cardiac TF (cTF) has two nonequivalent strands, each comprised of actin, tropomyosin (Tm), and troponin (Tn). Tn shifts Tm away from myosin-binding sites on actin at elevated Ca2+ levels to allow formation of force-producing actomyosin cross-bridges. The Tn complex is comprised of three distinct polypeptides - Ca2+-binding TnC, inhibitory TnI, and Tm-binding TnT. The molecular mechanism of their collective action is unresolved due to lack of comprehensive structural information on Tn region of cTF. C1 domain of cMyBP-C activates cTF in the absence of Ca2+ to the same extent as rigor myosin. Here we used cryo-EM of native cTFs to show that cTF Tn core adopts multiple structural conformations at high and low Ca2+ levels and that the two strands are structurally distinct. At high Ca2+ levels, cTF is not entirely activated by Ca2+ but exists in either partially or fully activated state. Complete dissociation of TnI C-terminus is required for full activation. In presence of cMyBP-C C1 domain, Tn core adopts a fully activated conformation, even in absence of Ca2+. Our data provide a structural description for the requirement of myosin to fully activate cTFs and explain increased affinity of TnC to Ca2+ in presence of active cross-bridges. We suggest that allosteric coupling between Tn subunits and Tm is required to control actomyosin interactions.

Bringing into focus the central domains C3-C6 of myosin binding protein C

Myosin binding protein C (MyBPC) is a multi-domain protein with each region having a distinct functional role in muscle contraction. The central domains of MyBPC have often been overlooked due to their unclear roles. However, recent research shows promise in understanding their potential structural and regulatory functions. Understanding the central region of MyBPC is important because it may have specialized function that can be used as drug targets or for disease-specific therapies. In this review, we provide a brief overview of the evolution of our understanding of the central domains of MyBPC in regard to its domain structures, arrangement and dynamics, interaction partners, hypothesized functions, disease-causing mutations, and post-translational modifications. We highlight key research studies that have helped advance our understanding of the central region. Lastly, we discuss gaps in our current understanding and potential avenues to further research and discovery.

Regulating Striated Muscle Contraction: Through Thick and Thin

Force generation in striated muscle is primarily controlled by structural changes in the actin-containing thin filaments triggered by an increase in intracellular calcium concentration. However, recent studies have elucidated a new class of regulatory mechanisms, based on the myosin-containing thick filament, that control the strength and speed of contraction by modulating the availability of myosin motors for the interaction with actin. This review summarizes the mechanisms of thin and thick filament activation that regulate the contractility of skeletal and cardiac muscle. A novel dual-filament paradigm of muscle regulation is emerging, in which the dynamics of force generation depends on the coordinated activation of thin and thick filaments. We highlight the interfilament signaling pathways based on titin and myosin-binding protein-C that couple thin and thick filament regulatory mechanisms. This dual-filament regulation mediates the length-dependent activation of cardiac muscle that underlies the control of the cardiac output in each heartbeat.

Activation of skeletal muscle is controlled by a dual-filament mechano-sensing mechanism

Contraction of skeletal muscle is triggered by a transient rise in intracellular calcium concentration leading to a structural change in the actin-containing thin filaments that allows binding of myosin motors from the thick filaments. Most myosin motors are unavailable for actin binding in resting muscle because they are folded back against the thick filament backbone. Release of the folded motors is triggered by thick filament stress, implying a positive feedback loop in the thick filaments. However, it was unclear how thin and thick filament activation mechanisms are coordinated, partly because most previous studies of the thin filament regulation were conducted at low temperatures where the thick filament mechanisms are inhibited. Here, we use probes on both troponin in the thin filaments and myosin in the thick filaments to monitor the activation states of both filaments in near-physiological conditions. We characterize those activation states both in the steady state, using conventional titrations with calcium buffers, and during activation on the physiological timescale, using calcium jumps produced by photolysis of caged calcium. The results reveal three activation states of the thin filament in the intact filament lattice of a muscle cell that are analogous to those proposed previously from studies on isolated proteins. We characterize the rates of the transitions between these states in relation to thick filament mechano-sensing and show how thin- and thick-filament-based mechanisms are coupled by two positive feedback loops that switch on both filaments to achieve rapid cooperative activation of skeletal muscle.

Titin activates myosin filaments in skeletal muscle by switching from an extensible spring to a mechanical rectifier

Titin is a molecular spring in parallel with myosin motors in each muscle half-sarcomere, responsible for passive force development at sarcomere length (SL) above the physiological range (>2.7 μm). The role of titin at physiological SL is unclear and is investigated here in single intact muscle cells of the frog (Rana esculenta), by combining half-sarcomere mechanics and synchrotron X-ray diffraction in the presence of 20 μM para-nitro-blebbistatin, which abolishes the activity of myosin motors and maintains them in the resting state even during activation of the cell by electrical stimulation. We show that, during cell activation at physiological SL, titin in the I-band switches from an SL-dependent extensible spring (OFF-state) to an SL-independent rectifier (ON-state) that allows free shortening while resisting stretch with an effective stiffness of ~3 pN nm^-1 per half-thick filament. In this way, I-band titin efficiently transmits any load increase to the myosin filament in the A-band. Small-angle X-ray diffraction signals reveal that, with I-band titin ON, the periodic interactions of A-band titin with myosin motors alter their resting disposition in a load-dependent manner, biasing the azimuthal orientation of the motors toward actin. This work sets the stage for future investigations on scaffold and mechanosensing-based signaling functions of titin in health and disease.

Activation of the myosin motors in fast-twitch muscle of the mouse is controlled by mechano-sensing in the myosin filaments

Myosin motors in resting muscle are inactivated by folding against the backbone of the myosin filament in an ordered helical array and must be released from that conformation to engage in force generation. Time-resolved X-ray diffraction from single fibres of amphibian muscle showed that myosin filament activation could be inhibited by imposing unloaded shortening at the start of stimulation, suggesting that filaments were activated by mechanical stress. Here we improved the signal-to-noise ratio of that approach using whole extensor digitorum longus muscles of the mouse contracting tetanically at 28°C. Changes in X-ray signals associated with myosin filament activation, including the decrease in the first-order myosin layer line associated with the helical motor array, increase in the spacing of a myosin-based reflection associated with packing of myosin tails in the filament backbone, and increase in the ratio of the 1,1 and 1,0 equatorial reflections associated with movement of motors away from the backbone, were delayed by imposing 10-ms unloaded shortening at the start of stimulation. These results show that myosin filaments are predominantly activated by filament stress, as in amphibian muscle. However, a small component of filament activation at zero load was detected, implying an independent mechanism of partial filament activation. X-ray interference measurements indicated a switch-like change in myosin motor conformation at the start of force development, accompanied by transient disordering of motors in the regions of the myosin filament near its midpoint, suggesting that filament zonal dynamics also play a role in its activation. KEY POINTS: Activation of myosin filaments in extensor digitorum longus muscles of the mouse is delayed by imposing rapid shortening from the start of stimulation. Stress is the major mechanism of myosin filament activation in these muscles, but there is a small component of filament activation during electrical stimulation at zero stress. Myosin motors switch rapidly from the folded inhibited conformation to the actin-attached force-generating conformation early in force development.

Myosin-based regulation of twitch and tetanic contractions in mammalian skeletal muscle

Time-resolved X-ray diffraction of isolated fast-twitch muscles of mice was used to show how structural changes in the myosin-containing thick filaments contribute to the regulation of muscle contraction, extending the previous focus on regulation by the actin-containing thin filaments. This study shows that muscle activation involves the following sequence of structural changes: thin filament activation, disruption of the helical array of myosin motors characteristic of resting muscle, release of myosin motor domains from the folded conformation on the filament backbone, and actin attachment. Physiological force generation in the 'twitch' response of skeletal muscle to single action potential stimulation is limited by incomplete activation of the thick filament and the rapid inactivation of both filaments. Muscle relaxation after repetitive stimulation is accompanied by a complete recovery of the folded motor conformation on the filament backbone but by incomplete reformation of the helical array, revealing a structural basis for post-tetanic potentiation in isolated muscles.

Stress-dependent activation of myosin in the heart requires thin filament activation and thick filament mechanosensing

The efficiency of the heart as a pump depends on an autoregulatory mechanism, the Frank–Starling law of the heart, that potentiates the strength of contraction in response to an increase in ventricular filling. Disruption of this mechanism compromises the ability of the heart to pump blood, potentially leading to heart failure. We used fluorescent probes on myosin in heart muscle cells to investigate the molecular basis of the Frank–Starling mechanism. Our results show that the stronger contraction of heart muscle at longer lengths is due to a calcium-dependent interfilament signaling pathway that links stress sensing in the myosin-containing filaments with calcium activation of the actin-containing filaments. This pathway can potentially be targeted for treating heart failure.

Myosin-based regulation in the heart muscle modulates the number of myosin motors available for interaction with calcium-regulated thin filaments, but the signaling pathways mediating the stronger contraction triggered by stretch between heartbeats or by phosphorylation of the myosin regulatory light chain (RLC) remain unclear. Here, we used RLC probes in demembranated cardiac trabeculae to investigate the molecular structural basis of these regulatory pathways. We show that in relaxed trabeculae at near-physiological temperature and filament lattice spacing, the RLC-lobe orientations are consistent with a subset of myosin motors being folded onto the filament surface in the interacting-heads motif seen in isolated filaments. The folded conformation of myosin is disrupted by cooling relaxed trabeculae, similar to the effect induced by maximal calcium activation. Stretch or increased RLC phosphorylation in the physiological range have almost no effect on RLC conformation at a calcium concentration corresponding to that between beats. These results indicate that in near-physiological conditions, the folded myosin motors are not directly switched on by RLC phosphorylation or by the titin-based passive tension at longer sarcomere lengths in the absence of thin filament activation. However, at the higher calcium concentrations that activate the thin filaments, stretch produces a delayed activation of folded myosin motors and force increase that is potentiated by RLC phosphorylation. We conclude that the increased contractility of the heart induced by RLC phosphorylation and stretch can be explained by a calcium-dependent interfilament signaling pathway involving both thin filament sensitization and thick filament mechanosensing.

Myosin filament-based regulation of the dynamics of contraction in heart muscle

Cardiovascular disease continues to be the leading cause of death worldwide, and is frequently associated with heart failure. Efforts to develop better therapeutics for heart failure have been held back by limited understanding of the normal control of contraction on the timescale of the heartbeat. We used synchrotron X-ray diffraction to determine the dynamic structural changes in the myosin motors that drive contraction in the heart muscle, and show that myosin filament-based control mechanisms determine the time course and strength of contraction, allowing those mechanisms to be targeted for developing new therapies for heart disease.

Myosin-based mechanisms are increasingly recognized as supplementing their better-known actin-based counterparts to control the strength and time course of contraction in both skeletal and heart muscle. Here we use synchrotron small-angle X-ray diffraction to determine the structural dynamics of local domains of the myosin filament during contraction of heart muscle. We show that, although myosin motors throughout the filament contribute to force development, only about 10% of the motors in each filament bear the peak force, and these are confined to the filament domain containing myosin binding protein-C, the “C-zone.” Myosin motors in domains further from the filament midpoint are likely to be activated and inactivated first in each contraction. Inactivated myosin motors are folded against the filament core, and a subset of folded motors lie on the helical tracks described previously. These helically ordered motors are also likely to be confined to the C-zone, and the associated motor conformation reforms only slowly during relaxation. Myosin filament stress-sensing determines the strength and time course of contraction in conjunction with actin-based regulation. These results establish the fundamental roles of myosin filament domains and the associated motor conformations in controlling the strength and dynamics of contraction in heart muscle, enabling those structures to be targeted to develop new therapies for heart disease.

Distortion of the Actin A-Triad Results in Contractile Disinhibition and Cardiomyopathy

Striated muscle contraction is regulated by the movement of tropomyosin over the thin filament surface, which blocks or exposes myosin binding sites on actin. Findings suggest that electrostatic contacts, particularly those between K326, K328, and R147 on actin and tropomyosin, establish an energetically favorable F-actin-tropomyosin configuration, with tropomyosin positioned in a location that impedes actomyosin associations and promotes relaxation. Here, we provide data that directly support a vital role for these actin residues, termed the A-triad, in tropomyosin positioning in intact functioning muscle. By examining the effects of an A295S α-cardiac actin hypertrophic cardiomyopathy-causing mutation, over a range of increasingly complex in silico, in vitro, and in vivo Drosophila muscle models, we propose that subtle A-triad-tropomyosin perturbation can destabilize thin filament regulation, which leads to hypercontractility and triggers disease. Our efforts increase understanding of basic thin filament biology and help unravel the mechanistic basis of a complex cardiac disorder.

Why Is there a Limit to the Changes in Myofilament Ca2+-Sensitivity Associated with Myopathy Causing Mutations?

Mutations in striated muscle contractile proteins have been found to be the cause of a number of inherited muscle diseases; in most cases the mechanism proposed for causing the disease is derangement of the thin filament-based Ca²⁺-regulatory system of the muscle. When considering the results of experiments reported over the last 15 years, one feature has been frequently noted, but rarely discussed: the magnitude of changes in myofilament Ca²⁺-sensitivity due to myopathy-causing mutations in skeletal or heart muscle seems to be always in the range 1.5-3x EC₅₀. Such consistency suggests it may be related to a fundamental property of muscle regulation; in this article we will investigate whether this observation is true and consider why this should be so. A literature search found 71 independent measurements of HCM mutation-induced change of EC₅₀ ranging from 1.15 to 3.8-fold with a mean of 1.87 ± 0.07 (sem). We also found 11 independent measurements of increased Ca²⁺-sensitivity due to mutations in skeletal muscle proteins ranging from 1.19 to 2.7-fold with a mean of 2.00 ± 0.16. Investigation of dilated cardiomyopathy-related mutations found 42 independent determinations with a range of EC₅₀ wt/mutant from 0.3 to 2.3. In addition we found 14 measurements of Ca²⁺-sensitivity changes due skeletal muscle myopathy mutations ranging from 0.39 to 0.63. Thus, our extensive literature search, although not necessarily complete, found that, indeed, the changes in myofilament Ca²⁺-sensitivity due to disease-causing mutations have a bimodal distribution and that the overall changes in Ca²⁺-sensitivity are quite small and do not extend beyond a three-fold increase or decrease in Ca²⁺-sensitivity. We discuss two mechanism that are not necessarily mutually exclusive. Firstly, it could be that the limit is set by the capabilities of the excitation-contraction machinery that supplies activating Ca²⁺ and that striated muscle cannot work in a way compatible with life outside these limits; or it may be due to a fundamental property of the troponin system and the permitted conformational transitions compatible with efficient regulation.

Phosphorylation and calcium antagonistically tune myosin-binding protein C's structure and function

During each heartbeat, cardiac contractility results from calcium-activated sliding of actin thin filaments toward the centers of myosin thick filaments to shorten cellular length. Cardiac myosin-binding protein C (cMyBP-C) is a component of the thick filament that appears to tune these mechanochemical interactions by its N-terminal domains transiently interacting with actin and/or the myosin S2 domain, sensitizing thin filaments to calcium and governing maximal sliding velocity. Both functional mechanisms are potentially further tunable by phosphorylation of an intrinsically disordered, extensible region of cMyBP-C's N terminus, the M-domain. Using atomic force spectroscopy, electron microscopy, and mutant protein expression, we demonstrate that phosphorylation reduced the M-domain's extensibility and shifted the conformation of the N-terminal domain from an extended structure to a compact configuration. In combination with motility assay data, these structural effects of M-domain phosphorylation suggest a mechanism for diminishing the functional potency of individual cMyBP-C molecules. Interestingly, we found that calcium levels necessary to maximally activate the thin filament mitigated the structural effects of phosphorylation by increasing M-domain extensibility and shifting the phosphorylated N-terminal fragments back to the extended state, as if unphosphorylated. Functionally, the addition of calcium to the motility assays ablated the impact of phosphorylation on maximal sliding velocities, fully restoring cMyBP-C's inhibitory capacity. We conclude that M-domain phosphorylation may have its greatest effect on tuning cMyBP-C's calcium-sensitization of thin filaments at the low calcium levels between contractions. Importantly, calcium levels at the peak of contraction would allow cMyBP-C to remain a potent contractile modulator, regardless of cMyBP-C's phosphorylation state.

C0 and C1 N-terminal Ig domains of myosin binding protein C exert different effects on thin filament activation

Mutations in genes encoding myosin, the molecular motor that powers cardiac muscle contraction, and its accessory protein, cardiac myosin binding protein C (cMyBP-C), are the two most common causes of hypertrophic cardiomyopathy (HCM). Recent studies established that the N-terminal domains (NTDs) of cMyBP-C (e.g., C0, C1, M, and C2) can bind to and activate or inhibit the thin filament (TF). However, the molecular mechanism(s) by which NTDs modulate interaction of myosin with the TF remains unknown and the contribution of each individual NTD to TF activation/inhibition is unclear. Here we used an integrated structure-function approach using cryoelectron microscopy, biochemical kinetics, and force measurements to reveal how the first two Ig-like domains of cMyPB-C (C0 and C1) interact with the TF. Results demonstrate that despite being structural homologs, C0 and C1 exhibit different patterns of binding on the surface of F-actin. Importantly, C1 but not C0 binds in a position to activate the TF by shifting tropomyosin (Tm) to the "open" structural state. We further show that C1 directly interacts with Tm and traps Tm in the open position on the surface of F-actin. Both C0 and C1 compete with myosin subfragment 1 for binding to F-actin and effectively inhibit actomyosin interactions when present at high ratios of NTDs to F-actin. Finally, we show that in contracting sarcomeres, the activating effect of C1 is apparent only once low levels of Ca(2+) have been achieved. We suggest that Ca(2+) modulates the interaction of cMyBP-C with the TF in the sarcomere.

Myosin-binding protein C corrects an intrinsic inhomogeneity in cardiac excitation-contraction coupling

The beating heart exhibits remarkable contractile fidelity over a lifetime, which reflects the tight coupling of electrical, chemical, and mechanical elements within the sarcomere, the elementary contractile unit. On a beat-to-beat basis, calcium is released from the ends of the sarcomere and must diffuse toward the sarcomere center to fully activate the myosin- and actin-based contractile proteins. The resultant spatial and temporal gradient in free calcium across the sarcomere should lead to nonuniform and inefficient activation of contraction. We show that myosin-binding protein C (MyBP-C), through its positioning on the myosin thick filaments, corrects this nonuniformity in calcium activation by exquisitely sensitizing the contractile apparatus to calcium in a manner that precisely counterbalances the calcium gradient. Thus, the presence and correct localization of MyBP-C within the sarcomere is critically important for normal cardiac function, and any disturbance of MyBP-C localization or function will contribute to the consequent cardiac pathologies.

Structural dynamics of troponin during activation of skeletal muscle

Time-resolved changes in the conformation of troponin in the thin filaments of skeletal muscle were followed during activation in situ by photolysis of caged calcium using bifunctional fluorescent probes in the regulatory and the coiled-coil (IT arm) domains of troponin. Three sequential steps in the activation mechanism were identified. The fastest step (1,100 s(-1)) matches the rate of Ca(2+) binding to the regulatory domain but also dominates the motion of the IT arm. The second step (120 s(-1)) coincides with the azimuthal motion of tropomyosin around the thin filament. The third step (15 s(-1)) was shown by three independent approaches to track myosin head binding to the thin filament, but is absent in the regulatory head. The results lead to a four-state structural kinetic model that describes the molecular mechanism of muscle activation in the thin filament-myosin head complex under physiological conditions.

Myosin-binding protein C displaces tropomyosin to activate cardiac thin filaments and governs their speed by an independent mechanism

Myosin-binding protein C (MyBP-C) is an accessory protein of striated muscle thick filaments and a modulator of cardiac muscle contraction. Defects in the cardiac isoform, cMyBP-C, cause heart disease. cMyBP-C includes 11 Ig- and fibronectin-like domains and a cMyBP-C-specific motif. In vitro studies show that in addition to binding to the thick filament via its C-terminal region, cMyBP-C can also interact with actin via its N-terminal domains, modulating thin filament motility. Structural observations of F-actin decorated with N-terminal fragments of cMyBP-C suggest that cMyBP-C binds to actin close to the low Ca(2+) binding site of tropomyosin. This suggests that cMyBP-C might modulate thin filament activity by interfering with tropomyosin regulatory movements on actin. To determine directly whether cMyBP-C binding affects tropomyosin position, we have used electron microscopy and in vitro motility assays to study the structural and functional effects of N-terminal fragments binding to thin filaments. 3D reconstructions suggest that under low Ca(2+) conditions, cMyBP-C displaces tropomyosin toward its high Ca(2+) position, and that this movement corresponds to thin filament activation in the motility assay. At high Ca(2+), cMyBP-C had little effect on tropomyosin position and caused slowing of thin filament sliding. Unexpectedly, a shorter N-terminal fragment did not displace tropomyosin or activate the thin filament at low Ca(2+) but slowed thin filament sliding as much as the larger fragments. These results suggest that cMyBP-C may both modulate thin filament activity, by physically displacing tropomyosin from its low Ca(2+) position on actin, and govern contractile speed by an independent molecular mechanism.

Periodicities designed in the tropomyosin sequence and structure define its functions

Tropomyosin is an actin binding protein that regulates actin filament dynamics and its interactions with actin binding proteins such as myosin, tropomodulin, formin, Arp2/3 and ADF-cofilin in most eukaryotic cells. Tropomyosin is the prototypical two-chained, α-helical coiled coil protein that associates end-to-end and binds to both sides of the actin filament. Each tropomyosin molecule spans four to seven actin monomers in the filament, depending on the size of the tropomyosin. Tropomyosins have a periodic heptad repeat sequence that is characteristic of coiled coil proteins as well as additional periodicities required for its interaction with the actin filament, where each periodic repeat interacts with one actin molecule. This review addresses the role of periodic features of the Tm molecule in carrying out its universal functions of binding to the actin filament and its regulation and the specific features that may determine the isoform specificity of tropomyosins.

Asymmetric myosin binding to the thin filament as revealed by a fluorescent nanocircuit

The interplay between myosin, actin, and striated muscle regulatory proteins involves complex cooperative interactions that propagate along the thin filament. A repeating unit of the tropomyosin dimer, troponin heterotrimer, and the actin protofilament heptamer is sometimes assumed to be able to bind myosin at any of its seven actins when activated even though the regulatory proteins are asymmetrically positioned along this repeating unit. Analysis of the impact of this asymmetry on actin and myosin interactions by sensitized emission luminescence resonance energy transfer spectroscopy and a unique fluorescent nanocircuit design reveals that the troponin affects the structure and function of myosin heads bound nearby in a different manner than myosin heads bound further away from the troponin. To test this hypothesis, a fluorescent nanocircuit reported the position of the myosin lever arm only when the myosin was bound adjacent to the troponin, or in controls, only when the myosin was bound distant from the troponin. Confirming the hypothesis, the myosin lever arm is predominantly in the pre powerstroke orientation when bound near troponin, but is predominantly in the post powerstroke orientation when bound distant from troponin. These data are consistent with the hypothesis that troponin is responsible for the formation of myosin binding target zones along the thin filament.

Structural basis of the relaxed state of a Ca2+-regulated myosin filament and its evolutionary implications

Myosin filaments of muscle are regulated either by phosphorylation of their regulatory light chains or Ca(2+) binding to the essential light chains, contributing to on-off switching or modulation of contraction. Phosphorylation-regulated filaments in the relaxed state are characterized by an asymmetric interaction between the two myosin heads, inhibiting their actin binding or ATPase activity. Here, we have tested whether a similar interaction switches off activity in myosin filaments regulated by Ca(2+) binding. Cryo-electron microscopy and single-particle image reconstruction of Ca(2+)-regulated (scallop) filaments reveals a helical array of myosin head-pair motifs above the filament surface. Docking of atomic models of scallop myosin head domains into the motifs reveals that the heads interact in a similar way to those in phosphorylation-regulated filaments. The results imply that the two major evolutionary branches of myosin regulation--involving phosphorylation or Ca(2+) binding--share a common structural mechanism for switching off thick-filament activity in relaxed muscle. We suggest that the Ca(2+)-binding mechanism evolved from the more ancient phosphorylation-based system to enable rapid response of myosin-regulated muscles to activation. Although the motifs are similar in both systems, the scallop structure is more tilted and higher above the filament backbone, leading to different intermolecular interactions. The reconstruction reveals how the myosin tail emerges from the motif, connecting the heads to the filament backbone, and shows that the backbone is built from supramolecular assemblies of myosin tails. The reconstruction provides a native structural context for understanding past biochemical and biophysical studies of this model Ca(2+)-regulated myosin.

Electron microscopy and 3D reconstruction of F-actin decorated with cardiac myosin-binding protein C (cMyBP-C)

Myosin-binding protein C (MyBP-C) is an ∼130-kDa rod-shaped protein of the thick (myosin containing) filaments of vertebrate striated muscle. It is composed of 10 or 11 globular 10-kDa domains from the immunoglobulin and fibronectin type III families and an additional MyBP-C-specific motif. The cardiac isoform cMyBP-C plays a key role in the phosphorylation-dependent enhancement of cardiac function that occurs upon β-adrenergic stimulation, and mutations in MyBP-C cause skeletal muscle and heart diseases. In addition to binding to myosin, MyBP-C can also bind to actin via its N-terminal end, potentially modulating contraction in a novel way via this thick-thin filament bridge. To understand the structural basis of actin binding, we have used negative stain electron microscopy and three-dimensional reconstruction to study the structure of F-actin decorated with bacterially expressed N-terminal cMyBP-C fragments. Clear decoration was obtained under a variety of salt conditions varying from 25 to 180 mM KCl concentration. Three-dimensional helical reconstructions, carried out at the 180-mM KCl level to minimize nonspecific binding, showed MyBP-C density over a broad portion of the periphery of subdomain 1 of actin and extending tangentially from its surface in the direction of actin's pointed end. Molecular fitting with an atomic structure of a MyBP-C Ig domain suggested that most of the N-terminal domains may be well ordered on actin. The location of binding was such that it could modulate tropomyosin position and would interfere with myosin head binding to actin.