Calcium binds cooperatively to the regulatory sites of the cardiac thin filament

Effect of the N-terminal extension in myosin essential light chain A1 on the mechanism of actomyosin ATP hydrolysis

Myosin essential light chains A1 and A2 are identical isoforms except for an extension of ∼40 amino acids at the N terminus of A1 that binds F-actin. The extension has no bearing on the burst hydrolysis rate (M-ATP → M-ADP-Pi) as determined by chemical quench flow (100 μM isoenzyme). Whereas actomyosin-S1A2 steady state MgATPase (low ionic strength, 20 °C) is hyperbolically dependent on concentration: Vmax 7.6 s-1, Kapp 6.4 μM (F-actin) and Vmax 10.1 s-1, Kapp 5.5 μM (native thin filaments, pCa 4), the relationship for myosin-S1A1 is bimodal; an initial rise at low concentration followed by a decline to one-third the Vmax of S1A2, indicative of more than one rate-limiting step and A1-enforced flux through the slower actomyosin-limited hydrolysis pathway. In double-mixing stopped-flow with an indicator, Ca(II)-mediated activation of Pi dissociation (regulatedAM-ADP-Pi → regulatedAM-ADP + Pi) is attenuated by A1 attachment to thin filaments (pCa 4). The maximum accelerated rates of Pi dissociation are: 81 s-1 (S1A1, Kapp 8.9 μM) versus 129 s-1 (S1A2, Kapp 58 μM). To investigate apomyosin-S1-mediated activation, thin filaments (EGTA) are premixed with a given isomyosin-S1 and double-mixing is repeated with myosin-S1A1 in the first mix. Similar maximum rates of Pi dissociation are observed, 44.5 s-1 (S1A1) and 47.1 s-1 (S1A2), which are lower than for Ca(II) activation. Overall, these results biochemically demonstrate how the longer light chain A1 can contribute to slower contraction and higher force and the shorter version A2 to faster contraction and lower force, consistent with their distribution in different types of striated muscle.

Myosin essential light chain 1sa decelerates actin and thin filament gliding on β-myosin molecules

The β-myosin heavy chain expressed in ventricular myocardium and the myosin heavy chain (MyHC) in slow-twitch skeletal Musculus soleus (M. soleus) type-I fibers are both encoded by MYH7. Thus, these myosin molecules are deemed equivalent. However, some reports suggested variations in the light chain composition between M. soleus and ventricular myosin, which could influence functional parameters, such as maximum velocity of shortening. To test for functional differences of the actin gliding velocity on immobilized myosin molecules, we made use of in vitro motility assays. We found that ventricular myosin moved actin filaments with ∼0.9 µm/s significantly faster than M. soleus myosin (0.3 µm/s). Filaments prepared from isolated actin are not the native interaction partner of myosin and are believed to slow down movement. Yet, using native thin filaments purified from M. soleus or ventricular tissue, the gliding velocity of M. soleus and ventricular myosin remained significantly different. When comparing the light chain composition of ventricular and M. soleus β-myosin, a difference became evident. M. soleus myosin contains not only the "ventricular" essential light chain (ELC) MLC1sb/v, but also an additional longer and more positively charged MLC1sa. Moreover, we revealed that on a single muscle fiber level, a higher relative content of MLC1sa was associated with significantly slower actin gliding. We conclude that the ELC MLC1sa decelerates gliding velocity presumably by a decreased dissociation rate from actin associated with a higher actin affinity compared to MLC1sb/v. Such ELC/actin interactions might also be relevant in vivo as differences between M. soleus and ventricular myosin persisted when native thin filaments were used.

Troponin Revealed: Uncovering the Structure of the Thin Filament On-Off Switch in Striated Muscle

Recently, our understanding of the structural basis of troponin-tropomyosin's Ca2+-triggered regulation of striated muscle contraction has advanced greatly, particularly via cryo-electron microscopy data. Compelling atomic models of troponin-tropomyosin-actin were published for both apo- and Ca2+-saturated states of the cardiac thin filament. Subsequent electron microscopy and computational analyses have supported and further elaborated the findings. Per cryo-electron microscopy, each troponin is highly extended and contacts both tropomyosin strands, which lie on opposite sides of the actin filament. In the apo-state characteristic of relaxed muscle, troponin and tropomyosin hinder strong myosin-actin binding in several different ways, apparently barricading the actin more substantially than does tropomyosin alone. The troponin core domain, the C-terminal third of TnI, and tropomyosin under the influence of a 64-residue helix of TnT located at the overlap of adjacent tropomyosins are all in positions that would hinder strong myosin binding to actin. In the Ca2+-saturated state, the TnI C-terminus dissociates from actin and binds in part to TnC; the core domain pivots significantly; the N-lobe of TnC binds specifically to actin and tropomyosin; and tropomyosin rotates partially away from myosin's binding site on actin. At the overlap domain, Ca2+ causes much less tropomyosin movement, so a more inhibitory orientation persists. In the myosin-saturated state of the thin filament, there is a large additional shift in tropomyosin, with molecular interactions now identified between tropomyosin and both actin and myosin. A new era has arrived for investigation of the thin filament and for functional understandings that increasingly accommodate the recent structural results.

Mechanical contribution to muscle thin filament activation

Vertebrate striated muscle thin filaments are thought to be thermodynamically activated in response to an increase in Ca²⁺ concentration. We tested this hypothesis by measuring time intervals for gliding runs and pauses of individual skeletal muscle thin filaments in cycling myosin motility assays. A classic thermodynamic mechanism predicts that if chemical potential is constant, transitions between runs and pauses of gliding thin filaments will occur at constant rate as given by a Poisson distribution. In this scenario, rate is given by the odds of a pause, and hence, run times between pauses fit an exponential distribution that slopes negatively for all observable run times. However, we determined that relative density of observed run times fits an exponential only at low Ca²⁺ levels that activate filament gliding. Further titration with Ca²⁺, or adding excess regulatory proteins tropomyosin and troponin, shifted the relative density of short run times to fit the positive slope of a gamma distribution, which derives from waiting times between Poisson events. Events that arise during a run and prevent the chance of ending a run for a random interval of time account for the observed run time distributions, suggesting that the events originate with cycling myosin. We propose that regulatory proteins of the thin filament require the mechanical force of cycling myosin to achieve the transition state for activation. During activation, combinations of cycling myosin that contribute insufficient activation energy delay deactivation.

Clinically Divergent Mutation Effects on the Structure and Function of the Human Cardiac Tropomyosin Overlap

The progression of genetically inherited cardiomyopathies from an altered protein structure to clinical presentation of disease is not well understood. One of the main roadblocks to mechanistic insight remains a lack of high-resolution structural information about multiprotein complexes within the cardiac sarcomere. One example is the tropomyosin (Tm) overlap region of the thin filament that is crucial for the function of the cardiac sarcomere. To address this central question, we devised coupled experimental and computational modalities to characterize the baseline function and structure of the Tm overlap, as well as the effects of mutations causing divergent patterns of ventricular remodeling on both structure and function. Because the Tm overlap contributes to the cooperativity of myofilament activation, we hypothesized that mutations that enhance the interactions between overlap proteins result in more cooperativity, and conversely, those that weaken interaction between these elements lower cooperativity. Our results suggest that the Tm overlap region is affected differentially by dilated cardiomyopathy-associated Tm D230N and hypertrophic cardiomyopathy-associated human cardiac troponin T (cTnT) R92L. The Tm D230N mutation compacts the Tm overlap region, increasing the cooperativity of the Tm filament, contributing to a dilated cardiomyopathy phenotype. The cTnT R92L mutation causes weakened interactions closer to the N-terminal end of the overlap, resulting in decreased cooperativity. These studies demonstrate that mutations with differential phenotypes exert opposite effects on the Tm-Tn overlap, and that these effects can be directly correlated to a molecular level understanding of the structure and dynamics of the component proteins.

Structural determinants of muscle thin filament cooperativity

End-to-end connections between adjacent tropomyosin molecules along the muscle thin filament allow long-range conformational rearrangement of the multicomponent filament structure. This process is influenced by Ca(2+) and the troponin regulatory complexes, as well as by myosin crossbridge heads that bind to and activate the filament. Access of myosin crossbridges onto actin is gated by tropomyosin, and in the case of striated muscle filaments, troponin acts as a gatekeeper. The resulting tropomyosin-troponin-myosin on-off switching mechanism that controls muscle contractility is a complex cooperative and dynamic system with highly nonlinear behavior. Here, we review key information that leads us to view tropomyosin as central to the communication pathway that coordinates the multifaceted effectors that modulate and tune striated muscle contraction. We posit that an understanding of this communication pathway provides a framework for more in-depth mechanistic characterization of myopathy-associated mutational perturbations currently under investigation by many research groups.

Integration of troponin I phosphorylation with cardiac regulatory networks

We focus here on the modulation of thin filament activity by cardiac troponin I phosphorylation as an integral and adaptive mechanism in cardiac homeostasis and as a mechanism vulnerable to maladaptive response to stress. We discuss a current concept of cardiac troponin I function in the A-band region of the sarcomere and potential signaling to cardiac troponin I in a network involving the ends of the thin filaments at the Z-disk and the M-band regions. The cardiac sarcomere represents a remarkable set of interacting proteins that functions not only as a molecular machine generating the heartbeat but also as a hub of signaling. We review how phosphorylation signaling to cardiac troponin I is integrated, with parallel signals controlling excitation-contraction coupling, hypertrophy, and metabolism.

An analysis of deformation-dependent electromechanical coupling in the mouse heart

To investigate the effects of the coupling between excitation and contraction on whole-organ function, we have developed a novel biophysically based multiscale electromechanical model of the murine heart. Through comparison with a comprehensive in vivo experimental data set, we show good agreement with pressure and volume measurements at both physiological temperatures and physiological pacing frequencies. This whole-organ model was used to investigate the effects of material and haemodynamic properties introduced at the tissue level, as well as emergent function of our novel cell contraction model. Through a comprehensive sensitivity analysis at both the cellular and whole organ level, we demonstrate the sensitivity of the model's results to its parameters and the constraining effect of experimental data. These results demonstrate the fundamental importance of length- and velocity-dependent feedback to the cellular scale for whole-organ function, and we show that a strong velocity dependence of tension is essential for explaining the differences between measured single cell tension and whole-organ pressure transients.

Dual regulatory functions of the thin filament revealed by replacement of the troponin I inhibitory peptide with a linker

Striated muscles are relaxed under low Ca(2+) concentration conditions due to actions of the thin filament protein troponin. To investigate this regulatory mechanism, an 11-residue segment of cardiac troponin I previously termed the inhibitory peptide region was studied by mutagenesis. Several mutant troponin complexes were characterized in which specific effects of the inhibitory peptide region were abrogated by replacements of 4-10 residues with Gly-Ala linkers. The mutations greatly impaired two of troponin's actions under low Ca(2+) concentration conditions: inhibition of myosin subfragment 1 (S1)-thin filament MgATPase activity and cooperative suppression of myosin S1-ADP binding to thin filaments with low myosin saturation. Inhibitory peptide replacement diminished but did not abolish the Ca(2+) dependence of the ATPase rate; ATPase rates were at least 2-fold greater when Ca(2+) rather than EGTA was present. This residual regulation was highly cooperative as a function of Ca(2+) concentration, similar to the degree of cooperativity observed with WT troponin present. Other effects of the mutations included 2-fold or less increases in the apparent affinity of the thin filament regulatory Ca(2+) sites, similar decreases in the affinity of troponin for actin-tropomyosin regardless of Ca(2+), and increases in myosin S1-thin filament ATPase rates in the presence of saturating Ca(2+). The overall results indicate that cooperative myosin binding to Ca(2+)-free thin filaments depends upon the inhibitory peptide region but that a cooperatively activating effect of Ca(2+) binding does not. The findings suggest that these two processes are separable and involve different conformational changes in the thin filament.

Effects of actin-myosin kinetics on the calcium sensitivity of regulated thin filaments

Activation of thin filaments in striated muscle occurs when tropomyosin exposes myosin binding sites on actin either through calcium-troponin (Ca-Tn) binding or by actin-myosin (A-M) strong binding. However, the extent to which these binding events contributes to thin filament activation remains unclear. Here we propose a simple analytical model in which strong A-M binding and Ca-Tn binding independently activates the rate of A-M weak-to-strong binding. The model predicts how the level of activation varies with pCa as well as A-M attachment, N·k(att), and detachment, k(det), kinetics. To test the model, we use an in vitro motility assay to measure the myosin-based sliding velocities of thin filaments at different pCa, N·k(att), and k(det) values. We observe that the combined effects of varying pCa, N·k(att), and k(det) are accurately fit by the analytical model. The model and supporting data imply that changes in attachment and detachment kinetics predictably affect the calcium sensitivity of striated muscle mechanics, providing a novel A-M kinetic-based interpretation for perturbations (e.g. disease-related mutations) that alter calcium sensitivity.

Investigation of a transgenic mouse model of familial dilated cardiomyopathy

We have investigated a transgenic mouse model of inherited dilated cardiomyopathy that stably expresses the ACTC E361G mutation at around 50% of total actin in the heart. F-actin isolated from ACTC E361G mouse hearts was incorporated into thin filaments with native human tropomyosin and troponin and compared with NTG mouse actin by in vitro motility assay. There was no significant difference in sliding speed, fraction of filaments motile or Ca(2+)-sensitivity (ratio EC(50) E361G/NTG=0.95+/-0.08). The Ca(2+)-sensitivity of force in skinned trabeculae from ACTC E361G mice was slightly higher than NTG (EC(50) E361G/NTG=0.78+/-0.04). The molecular phenotype was revealed when troponin was dephosphorylated; Ca(2+)-sensitivity of E361G-containing thin filaments was now lower than NTG (EC(50) E361G(dPTn)/NTG(dPTn)=2.15+/-0.09). We demonstrated that this was due to uncoupling of Ca(2+)-sensitivity from troponin I phosphorylation by comparing Ca(2+)-sensitivity of phosphorylated and dephosphorylated thin filaments. For NTG actin-containing thin filaments EC(50) native/dPTn=3.0+/-0.3 but for E361G-containing thin filaments EC(50) native/dPTn=1.04+/-0.07.We studied contractility in isolated myocytes and found no significant differences under basal conditions. We measured cardiac performance by cine-MRI, echocardiography and with a conductance catheter over a period of 4 to 18 months and found minimal systematic differences between NTG and ACTC E361G mice under basal conditions. However, the increase in septal thickening, ejection fraction, heart rate and cardiac output following dobutamine treatment was significantly less in ACTC E361G mice compared with NTG. We propose that the ACTC E361G mutation uncouples myofilament Ca(2+)-sensitivity from Troponin I phosphorylation and blunts the response to adrenergic stimulation, leading to a reduced cardiac reserve with consequent contractile dysfunction under stress, leading to dilated cardiomyopathy.

Sarcomere control mechanisms and the dynamics of the cardiac cycle

This review focuses on recent developments in the molecular mechanisms by which Ca activates cardiac sarcomeres and how these mechanisms play out in the cardiac cycle. I emphasize the role of mechanisms intrinsic to the sarcomeres as significant determinants of systolic elastance and ventricular stiffening during ejection. Data are presented supporting the idea that processes intrinsic to the thin filaments may promote cooperative activation of the sarcomeres and be an important factor in maintaining and modifying systolic elastance. Application of these ideas to translational medicine and rationale drug design forms an important rationale for detailed understanding of these processes.

Effect of calcium-sensitizing mutations on calcium binding and exchange with troponin C in increasingly complex biochemical systems

The calcium-dependent interactions between troponin C (TnC) and other thin and thick filament proteins play a key role in the regulation of cardiac muscle contraction. Five hydrophobic residues (Phe(20), Val(44), Met(45), Leu(48), and Met(81)) in the regulatory domain of TnC were individually substituted with polar Gln, to examine the effect of these mutations that sensitized isolated TnC to calcium on (1) the calcium binding and exchange with TnC in increasingly complex biochemical systems and (2) the calcium sensitivity of actomyosin ATPase. The hydrophobic residue mutations drastically affected calcium binding and exchange with TnC in increasingly complex biochemical systems, indicating that side chain intra- and intermolecular interactions of these residues play a crucial role in determining how TnC responds to calcium. However, the mutations that sensitized isolated TnC to calcium did not necessarily increase the calcium sensitivity of the troponin (Tn) complex or reconstituted thin filaments with or without myosin S1. Furthermore, the calcium sensitivity of reconstituted thin filaments (in the absence of myosin S1) was a better predictor of the calcium dependence of actomyosin ATPase activity than that of TnC or the Tn complex. Thus, both the intrinsic properties of TnC and its interactions with the other contractile proteins play a crucial role in modulating the binding of calcium to TnC in increasingly complex biochemical systems.

Phosphorylation of cardiac troponin I at protein kinase C site threonine 144 depresses cooperative activation of thin filaments

There is evidence for PKC-dependent multisite phosphorylation of cardiac troponin I (cTnI) at Ser-23 and Ser-24 (also PKA sites) in the cardiac-specific N-terminal extension and at Thr-144, a unique residue in the inhibitory region. The functional effect of these phosphorylations in combination is of interest in view of data indicating intramolecular interaction between the N-terminal extension and the inhibitory region of cTnI. To determine the role of PKC-dependent phosphorylation of cTnI on sarcomeric function, we measured contractile regulation at multiple levels of complexity. Ca(2+) binding to thin filaments reconstituted with either cTnI(wild-type) or pseudo-phosphorylated cTnI(S23D/S24D), cTnI(T144E), and cTnI(S23D/S24D/T144E) was determined. Compared with controls regulated by cTnI(wild-type), thin filaments with cTnI(S23D/S24D) and cTnI(S23D/S24D/T144E) exhibited decreased Ca(2+) sensitivity. In contrast, there was no significant difference between Ca(2+) binding to thin filaments with cTnI(wild-type) and with cTnI(T144E). Studies of the pCa-force relations in skinned papillary fibers regulated by these forms of cTnI yielded similar results. However, in both the Ca(2+) binding measurements and the skinned fiber tension measurements, the presence of cTnI(S23D/S24D/T144E) induced a much lower Hill coefficient than either wild type, S23D/S24D, or T144E. These data highlight the importance of thin filament-based cooperative mechanisms in cardiac regulation, with implications for mechanisms of control of function in normal and pathological hearts.

The molecular basis of the steep force-calcium relation in heart muscle

Contraction of heart muscle is regulated by binding of Ca²⁺ ions to troponin in the muscle thin filaments, causing a change in filament structure that allows myosin binding and force generation. The steady-state relationship between force and Ca²⁺ concentration in demembranated ventricular trabeculae is well described by the Hill equation, with parameters EC₅₀, the Ca²⁺ concentration that gives half the maximum force, and n_H, the Hill coefficient describing the steepness of the Ca²⁺ dependence. Although each troponin molecule has a single regulatory Ca²⁺ site, n_H is typically around 3, indicating co-operativity in the regulatory mechanism. This review focuses on the molecular basis of this co-operativity, and in particular on the popular hypothesis that force-generating myosin cross-bridges are responsible for the effect. Although cross-bridges can switch on thin filaments at low MgATP concentrations, we argue that the evidence from contracting heart muscle cells shows that this mechanism does not operate in more physiological conditions, and would not play a significant role in the intact heart. Interventions that alter maximum force and EC₅₀ do not in general produce a significant change in n_H. Complete abolition of force generation by myosin inhibitors does not affect the n_H values for either Ca²⁺ binding to the thin filaments or changes in troponin structure, and both values match that for force generation in the absence of inhibitors. These results provide strong evidence that the co-operative mechanism underlying the high value of n_H is not due to force-generating cross-bridges but is rather an intrinsic property of the thin filaments.

Ala scanning of the inhibitory region of cardiac troponin I

In skeletal and cardiac muscles, troponin (Tn), which resides on the thin filament, senses a change in intracellular Ca(2+) concentration. Tn is composed of TnC, TnI, and TnT. Ca(2+) binding to the regulatory domain of TnC removes the inhibitory effect by TnI on the contraction. The inhibitory region of cardiac TnI spans from residue 138 to 149. Upon Ca(2+) activation, the inhibitory region is believed to be released from actin, thus triggering actin-activation of myosin ATPase. In this study, we created a series of Ala-substitution mutants of cTnI to delineate the functional contribution of each amino acid in the inhibitory region to myofilament regulation. We found that most of the point mutations in the inhibitory region reduced the ATPase activity in the presence of Ca(2+), which suggests the same region also acts as an activator of the ATPase. The thin filaments can also be activated by strong myosin head (S1)-actin interactions. The binding of N-ethylmaleimide-treated myosin subfragment 1 (NEM-S1) to actin filaments mimics such strong interactions. Interestingly, in the absence of Ca(2+) NEM-S1-induced activation of S1 ATPase was significantly less with the thin filaments containing TnI(T144A) than that with the wild-type TnI. However, in the presence of Ca(2+), there was little difference in the activation of ATPase activity between these preparations.

Mechanical and kinetic effects of shortened tropomyosin reconstituted into myofibrils

The effects of tropomyosin on muscle mechanics and kinetics were examined in skeletal myofibrils using a novel method to remove tropomyosin (Tm) and troponin (Tn) and then replace these proteins with altered versions. Extraction employed a low ionic strength rigor solution, followed by sequential reconstitution at physiological ionic strength with Tm then Tn. SDS-PAGE analysis was consistent with full reconstitution, and fluorescence imaging after reconstitution using Oregon-green-labeled Tm indicated the expected localization. Myofibrils remained mechanically viable: maximum isometric forces of myofibrils after sTm/sTn reconstitution (control) were comparable (~84%) to the forces generated by non-reconstituted preparations, and the reconstitution minimally affected the rate of isometric activation (k_act), calcium sensitivity (pCa₅₀), and cooperativity (n_H). Reconstitutions using various combinations of cardiac and skeletal Tm and Tn indicated that isoforms of both Tm and Tn influence calcium sensitivity of force development in opposite directions, but the isoforms do not otherwise alter cross-bridge kinetics. Myofibrils reconstituted with Δ23Tm, a deletion mutant lacking the second and third of Tm’s seven quasi-repeats, exhibited greatly depressed maximal force, moderately slower k_act rates and reduced n_H. Δ23Tm similarly decreased the cooperativity of calcium binding to the troponin regulatory sites of isolated thin filaments in solution. The mechanisms behind these effects of Δ23Tm also were investigated using P_i and ADP jumps. P_i and ADP kinetics were indistinguishable in Δ23Tm myofibrils compared to controls. The results suggest that the deleted region of tropomyosin is important for cooperative thin filament activation by calcium.

Calcium- and myosin-dependent changes in troponin structure during activation of heart muscle

Each heartbeat is triggered by a pulse of intracellular calcium ions which bind to troponin on the actin-containing thin filaments of heart muscle cells, initiating a change in filament structure that allows myosin to bind and generate force. We investigated the molecular mechanism of calcium regulation in demembranated trabeculae from rat ventricle using polarized fluorescence from probes on troponin C (TnC). Native TnC was replaced by double-cysteine mutants of human cardiac TnC with bifunctional rhodamine attached along either the C helix, adjacent to the regulatory Ca(2+)-binding site, or the E helix in the IT arm of the troponin complex. Changes in the orientation of both troponin helices had the same steep Ca(2+) dependence as active force production, with a Hill coefficient (n(H)) close to 3, consistent with a single co-operative transition controlled by Ca(2+) binding. Complete inhibition of active force by 25 microM blebbistatin had very little effect on the Ca(2+)-dependent structural changes and in particular did not significantly reduce the value of n(H). Binding of rigor myosin heads to thin filaments following MgATP depletion in the absence of Ca(2+) also changed the orientation of the C and E helices, and addition of Ca(2+) in rigor produced further changes characterized by increased Ca(2+) affinity but with n(H) close to 1. These results show that, although myosin binding can switch on thin filaments in rigor conditions, it does not contribute significantly under physiological conditions. The physiological mechanism of co-operative Ca(2+) regulation of cardiac contractility must therefore be intrinsic to the thin filaments.

Effects of thin and thick filament proteins on calcium binding and exchange with cardiac troponin C

Understanding the effects of thin and thick filament proteins on the kinetics of Ca(2+) exchange with cardiac troponin C is essential to elucidating the Ca(2+)-dependent mechanisms controlling cardiac muscle contraction and relaxation. Unlike labeling of the endogenous Cys-84, labeling of cardiac troponin C at a novel engineered Cys-53 with 2-(4'-iodoacetamidoanilo)napthalene-6-sulfonic acid allowed us to accurately measure the rate of calcium dissociation from the regulatory domain of troponin C upon incorporation into the troponin complex. Neither tropomyosin nor actin alone affected the Ca(2+) binding properties of the troponin complex. However, addition of actin-tropomyosin to the troponin complex decreased the Ca(2+) sensitivity ( approximately 7.4-fold) and accelerated the rate of Ca(2+) dissociation from the regulatory domain of troponin C ( approximately 2.5-fold). Subsequent addition of myosin S1 to the reconstituted thin filaments (actin-tropomyosin-troponin) increased the Ca(2+) sensitivity ( approximately 6.2-fold) and decreased the rate of Ca(2+) dissociation from the regulatory domain of troponin C ( approximately 8.1-fold), which was completely reversed by ATP. Consistent with physiological data, replacement of cardiac troponin I with slow skeletal troponin I led to higher Ca(2+) sensitivities and slower Ca(2+) dissociation rates from troponin C in all the systems studied. Thus, both thin and thick filament proteins influence the ability of cardiac troponin C to sense and respond to Ca(2+). These results imply that both cross-bridge kinetics and Ca(2+) dissociation from troponin C work together to modulate the rate of cardiac muscle relaxation.

Myofibrillar troponin exists in three states and there is signal transduction along skeletal myofibrillar thin filaments

Activation of striated muscle contraction is a highly cooperative signal transduction process converting calcium binding by troponin C (TnC) into interactions between thin and thick filaments. Once calcium is bound, transduction involves changes in protein interactions along the thin filament. The process is thought to involve three different states of actin-tropomyosin (Tm) resulting from changes in troponin's (Tn) interaction with actin-Tm: a blocked (B) state preventing myosin interaction, a closed (C) state allowing weak myosin interactions and favored by calcium binding to Tn, and an open or M state allowing strong myosin interactions. This was tested by measuring the apparent rate of Tn dissociation from rigor skeletal myofibrils using labeled Tn exchange. The location and rate of exchange of Tn or its subunits were measured by high-resolution fluorescence microscopy and image analysis. Three different rates of Tn exchange were observed that were dependent on calcium concentration and strong cross-bridge binding that strongly support the three-state model. The rate of Tn dissociation in the non-overlap region was 200-fold faster at pCa 4 (C-state region) than at pCa 9 (B-state region). When Tn contained engineered TnC mutants with weakened regulatory TnI interactions, the apparent exchange rate at pCa 4 in the non-overlap region increased proportionately with TnI-TnC regulatory affinity. This suggests that the mechanism of calcium enhancement of the rate of Tn dissociation is by favoring a TnI-TnC interaction over a TnI-actin-Tm interaction. At pCa 9, the rate of Tn dissociation in the overlap region (M-state region) was 100-fold faster than the non-overlap region (B-state region) suggesting that strong cross-bridges increase the rate of Tn dissociation. At pCa 4, the rate of Tn dissociation was twofold faster in the non-overlap region (C-state region) than the overlap region (M-state region) that likely involved a strong cross-bridge influence on TnT's interaction with actin-Tm. At sub-maximal calcium (pCa 6.2-5.8), there was a long-range influence of the strong cross-bridge on Tn to enhance its dissociation rate, tens of nanometers from the strong cross-bridge. These observations suggest that the three different states of actin-Tm are associated with three different states of Tn. They also support a model in which strong cross-bridges shift the regulatory equilibrium from a TnI-actin-Tm interaction to a TnC-TnI interaction that likely enhances calcium binding by TnC.

Increased Ca2+ affinity of cardiac thin filaments reconstituted with cardiomyopathy-related mutant cardiac troponin I

To understand the molecular mechanisms whereby cardiomyopathy-related cardiac troponin I (cTnI) mutations affect myofilament activity, we have investigated the Ca2+ binding properties of various assemblies of the regulatory components that contain one of the cardiomyopahty-related mutant cTnI. Acto-S1 ATPase activities in reconstituted systems were also determined. We investigated R145G and R145W mutations from the inhibitory region and D190H and R192H mutations from the second actin-tropomyosin-binding site. Each of the four mutations sensitized the acto-S1 ATPase to Ca2+. Whereas the mutations from the inhibitory region increased the basal level of ATPase activity, those from the second actin-tropomyosin-binding site did not. The effects on the Ca2+ binding properties of the troponin ternary complex and the troponin-tropomyosin complex with one of four mutations were either desensitization or no effect compared with those with wild-type cTnI. All of the mutations, however, affected the Ca2+ sensitivities of the reconstituted thin filaments in the same direction as the acto-S1 ATPase activity. Also the thin filaments with one of the mutant cTnIs bound Ca2+ with less cooperativity compared with those with wild-type cTnI. These data indicate that the mutations found in the inhibitory region and those from the second actin-tropomyosin site shift the equilibrium of the states of the thin filaments differently. Moreover, the increased Ca2+ bound to myofilaments containing the mutant cTnIs may be an important factor in triggered arrhythmias associated with the cardiomyopathy.

A quantitative analysis of cardiac myocyte relaxation: a simulation study

The determinants of relaxation in cardiac muscle are poorly understood, yet compromised relaxation accompanies various pathologies and impaired pump function. In this study, we develop a model of active contraction to elucidate the relative importance of the [Ca2+]i transient magnitude, the unbinding of Ca2+ from troponin C (TnC), and the length-dependence of tension and Ca2+ sensitivity on relaxation. Using the framework proposed by one of our researchers, we extensively reviewed experimental literature, to quantitatively characterize the binding of Ca2+ to TnC, the kinetics of tropomyosin, the availability of binding sites, and the kinetics of crossbridge binding after perturbations in sarcomere length. Model parameters were determined from multiple experimental results and modalities (skinned and intact preparations) and model results were validated against data from length step, caged Ca2+, isometric twitches, and the half-time to relaxation with increasing sarcomere length experiments. A factorial analysis found that the [Ca2+]i transient and the unbinding of Ca2+ from TnC were the primary determinants of relaxation, with a fivefold greater effect than that of length-dependent maximum tension and twice the effect of tension-dependent binding of Ca2+ to TnC and length-dependent Ca2+ sensitivity. The affects of the [Ca2+]i transient and the unbinding rate of Ca2+ from TnC were tightly coupled with the effect of increasing either factor, depending on the reference [Ca2+]i transient and unbinding rate.

Kinetics of cardiac thin-filament activation probed by fluorescence polarization of rhodamine-labeled troponin C in skinned guinea pig trabeculae

A genetically engineered cardiac TnC mutant labeled at Cys-84 with tetramethylrhodamine-5-iodoacetamide dihydroiodide was passively exchanged for the endogenous form in skinned guinea pig trabeculae. The extent of exchange averaged nearly 70%, quantified by protein microarray of individual trabeculae. The uniformity of its distribution was verified by confocal microscopy. Fluorescence polarization, giving probe angle and its dispersion relative to the fiber long axis, was monitored simultaneously with isometric tension. Probe angle reflects underlying cTnC orientation. In steady-state experiments, rigor cross-bridges and Ca2+ with vanadate to inhibit cross-bridge formation produce a similar change in probe orientation as that observed with cycling cross-bridges (no Vi). Changes in probe angle were found at [Ca2+] well below those required to generate tension. Cross-bridges increased the Ca2+ dependence of angle change (cooperativity). Strong cross-bridge formation enhanced Ca2+ sensitivity and was required for full change in probe position. At submaximal [Ca2+], the thin filament regulatory system may act in a coordinated fashion, with the probe orientation of Ca2+-bound cTnC significantly affected by Ca2+ binding at neighboring regulatory units. The time course of the probe angle change and tension after photolytic release [Ca2+] by laser photolysis of NP-EGTA was Ca2+ sensitive and biphasic: a rapid component approximately 10 times faster than that of tension and a slower rate similar to that of tension. The fast component likely represents steps closely associated with Ca2+ binding to site II of cTnC, whereas the slow component may arise from cross-bridge feedback. These results suggest that the thin filament activation rate does not limit the tension time course in cardiac muscle.

Mini-thin filaments regulated by troponin-tropomyosin

Striated muscle thin filaments contain hundreds of actin monomers and scores of troponins and tropomyosins. To study the cooperative mechanism of thin filaments, "mini-thin filaments" were generated by isolating particles nearly matching the minimal structural repeat of thin filaments: a double helix of actin subunits with each strand approximately seven actins long and spanned by a troponin-tropomyosin complex. One end of the particles was capped by a gelsolin (segment 1-3)-TnT fusion protein (substituting for normal TnT), and the other end was capped by tropomodulin. EM showed that the particles were 46 +/- 9 nm long, with a knob-like mass attributable to gelsolin at one end. Average actin, tropomyosin, and gelsolin-troponin composition indicated one troponin-tropomyosin attached to each strand of the two-stranded actin filament. The minifilaments thus nearly represent single regulatory units of thin filaments. The myosin S1 MgATPase rate stimulated by the minifilaments was Ca2+-sensitive, indicating that single regulatory length particles are sufficient for regulation. Ca2+ bound cooperatively to cardiac TnC in conventional thin filaments but noncooperatively to cardiac TnC in minifilaments in the absence of myosin. This suggests that thin filament Ca2+-binding cooperativity reflects indirect troponin-troponin interactions along the long axis of conventional filaments, which do not occur in minifilaments. Despite noncooperative Ca2+ binding to minifilaments in the absence of myosin, Ca2+ cooperatively activated the myosin S1-particle ATPase rate. Two-stranded single regulatory units therefore may be sufficient for myosin-mediated Ca2+-binding cooperativity. Functional mini-thin filaments are well suited for biochemical and structural analysis of thin-filament regulation.

Myosin Crossbridge Activation of Cardiac Thin Filaments

At the level of the myofibrillar proteins, activation of myocardial contraction is thought to involve switch-like regulation of crossbridge binding to the thin filaments. A central feature of this view of regulation is that Ca2+ binding to the low-affinity (approximately 3 micromol/L) site on troponin C alters the interactions of proteins in the thin filament regulatory strand, which leads to movement of tropomyosin from its blocking position on the thin filament and binding of crossbridges to actin. Although Ca2+ binding is a critical step in initiating contraction, this event alone does not account for the activation dependence of contractile properties of myocardium. Instead, activation is a highly cooperative process in which initial crossbridge binding to the thin filaments recruits additional crossbridge binding to actin as well as increased Ca2+ binding to troponin C. This review addresses possible roles of thin filament cooperativity in myocardium as a process that modulates the activation dependence of force and the rate of force development and also possible mechanisms by which cooperative signals are transmitted along the thick filament. Emerging evidence suggests that such mechanisms could contribute to the regulation of fundamental mechanical properties of myocardium and alterations in regulation that underlie contractile disorders in diseases such as cardiomyopathies.

Contractile effects of the exchange of cardiac troponin for fast skeletal troponin in rabbit psoas single myofibrils

The effects of the removal of fast skeletal troponin C (fsTnC) and its replacement by cardiac troponin C (cTnC) and the exchange of fast skeletal troponin (fsTn) for cardiac troponin (cTn) were measured in rabbit fast skeletal myofibrils. Electrophoretic analysis of myofibril suspensions indicated that replacement of fsTnC or exchange of fsTn with cTnC or cTn was about 90% complete in the protocols used. Mechanical measurements in single myofibrils, which were maximally activated by fast solution switching, showed that replacement of fsTnC with cTnC reduced the isometric tension, the rate of tension rise following a step increase in Ca2+ (kACT), and the rate of tension redevelopment following a quick release and restretch (kTR), but had no effect on the kinetics of the fall in tension when the concentration of inorganic phosphate (Pi) was abruptly increased (kPi(+)). These data suggest that the chimeric protein produced by cTnC replacement in fsTn alters those steps controlling the weak-to-strong crossbridge attachment transition. Inefficient signalling within the chimeric troponin may cause these changes. However, replacement of fsTn by cTn had no effect on maximal isometric tension, kACT or kTR, suggesting that these mechanics are largely determined by the isoform of the myosin molecule. Replacement of fsTn by cTn, on the other hand, shifted the pCa50 of the pCa-tension relationship from 5.70 to 6.44 and reduced the Hill coefficient from 3.3 to 1.4, suggesting that regulatory protein isoforms primarily alter Ca2+ sensitivity and the cooperativity of the force-generating mechanism.

Cardiomyopathic tropomyosin mutations that increase thin filament Ca2+ sensitivity and tropomyosin N-domain flexibility

The relationship between tropomyosin thermal stability and thin filament activation was explored using two N-domain mutants of alpha-striated muscle tropomyosin, A63V and K70T, each previously implicated in familial hypertrophic cardiomyopathy. Both mutations had prominent effects on tropomyosin thermal stability as monitored by circular dichroism. Wild type tropomyosin unfolded in two transitions, separated by 10 degrees C. The A63V and K70T mutations decreased the melting temperature of the more stable of these transitions by 4 and 10 degrees C, respectively, indicating destabilization of the N-domain in both cases. Global analysis of all three proteins indicated that the tropomyosin N-domain and C-domain fold with a cooperative free energy of 1.0-1.5 kcal/mol. The two mutations increased the apparent affinity of the regulatory Ca2+ binding sites of thin filament in two settings: Ca2+-dependent sliding speed of unloaded thin filaments in vitro (at both pH 7.4 and 6.3), and Ca2+ activation of the thin filament-myosin S1 ATPase rate. Neither mutation had more than small effects on the maximal ATPase rate in the presence of saturating Ca2+ or on the maximal sliding speed. Despite the increased tropomyosin flexibility implied by destabilization of the N-domain, neither the cooperativity of thin filament activation by Ca2+ nor the cooperative binding of myosin S1-ADP to the thin filament was altered by the mutations. The combined results suggest that a more dynamic tropomyosin N-domain influences interactions with actin and/or troponin that modulate Ca2+ sensitivity, but has an unexpectedly small effect on cooperative changes in tropomyosin position on actin.

Activation of striated muscle: nearest-neighbor regulatory-unit and cross-bridge influence on myofilament kinetics

We have formulated a three-compartment model of muscle activation that includes both strong cross-bridge (XB) and Ca(2+)-activated regulatory-unit (RU) mediated nearest-neighbor cooperative influences. The model is based on the tight coupling premise--that XB retain activating Ca(2+) on the thin filament. Using global non-linear least-squares, the model produced excellent fits to experimental steady-state force-pCa and ATPase-pCa data from skinned rat soleus fibers. In terms of the model, nearest-neighbor influences over the range of Ca(2+) required for activation cause the Ca(2+) dissociation rate from regulatory-units (k(off)) to decrease and the cross-bridge association rate (f) to increase each more than ten-fold. Moreover, the rate variations occur in separate Ca(2+) regimes. The energy of activation governing f is strongly influenced by both neighboring RU and XB. In contrast, the energy of activation governing k(off) is less affected by neighboring XB than by neighboring RU. Nearest-neighbor cooperative influences provide both an overall sensitization to Ca(2+) and the well-known steep response of force to free Ca(2+). The apparent sensitivity for Ca(2+)-activation of force and ATPase is a function of cross-bridge kinetic rates. The model and derived parameter set produce simulated behavior in qualitative agreement with steady-state experiments reported in the literature for partial TnC replacement, increased [P(i)], increased [ADP], and MalNEt-S1 addition. The model is an initial attempt to construct a general theory of striated muscle activation-one that can be consistently used to interpret data from various types of muscle manipulation experiments.

Hypertrophic cardiomyopathy caused by a novel alpha-tropomyosin mutation (V95A) is associated with mild cardiac phenotype, abnormal calcium binding to troponin, abnormal myosin cycling, and poor prognosis

BACKGROUND

We report hypertrophic cardiomyopathy (HCM) in a Spanish-American family caused by a novel alpha-tropomyosin (TPM1) mutation and examine the pathogenesis of the clinical disease by characterizing functional defects in the purified mutant protein.

METHODS AND RESULTS

HCM was linked to the TPM1 gene (logarithm of the odds [LOD] score 3.17). Sequencing and restriction digestion analysis demonstrated a TPM1 mutation V95A that cosegregated with HCM. The mutation has been associated with 13 deaths in 26 affected members (11 sudden deaths and 2 related to heart failure), with a cumulative survival rate of 73+/-10% at the age of 40 years. Left ventricular wall thickness (mean 16+/-6 mm) and disease penetrance (53%) were similar to those for the ss-myosin mutations L908V and G256E previously associated with a benign prognosis. Left ventricular hypertrophy was milder than with the ss-myosin mutation R403Q, but the prognosis was similarly poor. With the use of recombinant tropomyosins, we identified several functional alterations at the protein level. The mutation caused a 40% to 50% increase in calcium affinity in regulated thin filament-myosin subfragment-1 (S1) MgATPase assays, a 20% decrease in MgATPase rates in the presence of saturating calcium, a 5% decrease in unloaded shortening velocity in in vitro motility assays, and no change in cooperative myosin S1 binding to regulated thin filaments.

CONCLUSIONS

In contrast to other reported TPM1 mutations, V95A-associated HCM exhibits unusual features of mild phenotype but poor prognosis. Both myosin cycling and calcium binding to troponin are abnormal in the presence of the mutant tropomyosin. The genetic diagnosis afforded by this mutation will be valuable in the management of HCM.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Functional consequences of troponin T mutations found in hypertrophic cardiomyopathy

Missense mutations in the cardiac thin filament protein troponin T (TnT) are a cause of familial hypertrophic cardiomyopathy (FHC). To understand how these mutations produce dysfunction, five TnTs were produced and purified containing FHC mutations found in several regions of TnT. Functional defects were diverse. Mutations F110I, E244D, and COOH-terminal truncation weakened the affinity of troponin for the thin filament. Mutation DeltaE160 resulted in thin filaments with increased calcium affinity at the regulatory site of troponin C. Mutations R92Q and F110I resulted in impaired troponin solubility, suggesting abnormal protein folding. Depending upon the mutation, the in vitro unloaded actin-myosin sliding speed showed small increases, showed small decreases, or was unchanged. COOH-terminal truncation mutation resulted in a decreased thin filament-myosin subfragment 1 MgATPase rate. The results indicate that the mutations cause diverse immediate effects, despite similarities in disease manifestations. Separable but repeatedly observed abnormalities resulting from FHC TnT mutations include increased unloaded sliding speed, increased or decreased Ca(2+) affinity, impairment of folding or sarcomeric integrity, and decreased force. Enhancement as well as impairment of contractile protein function is observed, suggesting that TnT, including the troponin tail region, modulates the regulation of cardiac contraction.

Regulatory properties of recombinant tropomyosins containing 5-hydroxytryptophan: Ca2+-binding to troponin results in a conformational change in a region of tropomyosin outside the troponin binding site

We have introduced tryptophan codons at different positions of the chicken alpha-tropomyosin cDNA (Monteiro, P. B., Lataro, R. C., Ferro, J. A., and Reinach, F. C. (1994) J. Biol. Chem. 269, 10461-10466) and employed a trp auxotrophic Escherichia coli strain to express the proteins in media containing either normal tryptophan, 5-hydroxytrptophan, or 7-azatryptophan. The fluorescence of these latter two tryptophan analogues is excitable at 312-315 nm at which the natural fluorescence of other thin filament proteins (actin, troponin) is not excited. The recombinant tropomyosins have tryptophans or analogues located at amino acid positions 90, 101, 111, 122, or 185 of the protein, all on the external surface of the tropomyosin coiled-coil (positions "c" or "f" of the hydrophobic heptad repeat). The first four mutations are located within the third actin-binding zone of tropomyosin, a region not expected to interact directly with troponin or with neighboring tropomyosin molecules in muscle thin filaments, while position 185 is located in a region that has been implicated in interactions with the globular domain of troponin. The fluorescence intensity of the mutant containing 5-hydroxytryptophan at position 122 (5OH122W) is sensitive to actin binding and sensitive to Ca2+-binding to thin filaments reconstituted with troponin. Assuming that the globular domain of troponin binds to a site between residues 150 and 190 of tropomyosin, the distance between the troponin-binding site and the fluorescent probes at position 122 can be estimated to be 4.2-10.2 nm. While X-ray diffraction and electron micrograph reconstitution studies have provided evidence of Ca2+-induced changes in tropomyosin's interactions in the thin filament, their resolution was not sufficient to distinguish between changes involving the whole tropomyosin molecule or only that region directly interacting with troponin. Here we provide a clear demonstration that Ca2+-binding to troponin results in a conformational change in a region of tropomyosin outside the troponin binding site which is probably associated with a changed interaction with actin.

Mutations in actin subdomain 3 that impair thin filament regulation by troponin and tropomyosin

Thin filament-mediated regulation of striated muscle contraction involves conformational switching among a few quaternary structures, with transitions induced by binding of Ca(2+) and myosin. We establish and exploit Saccharomyces cerevisiae actin as a model system to investigate this process. Ca(2+)-sensitive troponin-tropomyosin binding affinities for wild type yeast actin are seen to closely resemble those for muscle actin, and these hybrid thin filaments produce Ca(2+)-sensitive regulation of the myosin S-1 MgATPase rate. Yeast actin filament inner domain mutant K315A/E316A depresses Ca(2+) activation of the MgATPase rate, producing a 4-fold weakening of the apparent Ca(2+) affinity and a 50% decrease in the MgATPase rate at saturating Ca(2+) concentration. Observed destabilization of troponin-tropomyosin binding to actin in the presence of Ca(2+), a 1.4-fold effect, provides a partial explanation. Despite the decrease in apparent MgATPase Ca(2+) affinity, there was no detectable change in the true Ca(2+) affinity of the thin filament, measured using fluorophore-labeled troponin. Another inner domain mutant, E311A/R312A, decreased the MgATPase rate but did not change the apparent Ca(2+) affinity. These results suggest that charged residues on the surface of the actin inner domain are important in Ca(2+)- and myosin-induced thin filament activation.

Roles for the troponin tail domain in thin filament assembly and regulation. A deletional study of cardiac troponin T

Striated muscle contraction is regulated by Ca2+ binding to troponin, which has a globular domain and an elongated tail attributable to the NH2-terminal portion of the bovine cardiac troponin T (TnT) subunit. Truncation of the bovine cardiac troponin tail was investigated using recombinant TnT fragments and subunits TnI and TnC. Progressive truncation of the troponin tail caused progressively weaker binding of troponin-tropomyosin to actin and of troponin to actin-tropomyosin. A sharp drop-off in affinity occurred with NH2-terminal deletion of 119 rather than 94 residues. Deletion of 94 residues had no effect on Ca2+-activation of the myosin subfragment 1-thin filament MgATPase rate and did not eliminate cooperative effects of Ca2+ binding. Troponin tail peptide TnT1-153 strongly promoted tropomyosin binding to actin in the absence of TnI or TnC. The results show that the anchoring function of the troponin tail involves interactions with actin as well as with tropomyosin and has comparable importance in the presence or absence of Ca2+. Residues 95-153 are particularly important for anchoring, and residues 95-119 are crucial for function or local folding. Because striated muscle regulation involves switching among the conformational states of the thin filament, regulatory significance for the troponin tail may arise from its prominent contribution to the protein-protein interactions within these conformations.

Ca2+-dependent muscle dysfunction caused by mutation of the Caenorhabditis elegans troponin T-1 gene

We have investigated the functions of troponin T (CeTnT-1) in Caenorhabditis elegans embryonic body wall muscle. TnT tethers troponin I (TnI) and troponin C (TnC) to the thin filament via tropomyosin (Tm), and TnT/Tm regulates the activation and inhibition of myosin-actin interaction in response to changes in intracellular [Ca2+]. Loss of CeTnT-1 function causes aberrant muscle trembling and tearing of muscle cells from their exoskeletal attachment sites (Myers, C.D., P.-Y. Goh, T. StC. Allen, E.A. Bucher, and T. Bogaert. 1996. J. Cell Biol. 132:1061-1077). We hypothesized that muscle tearing is a consequence of excessive force generation resulting from defective tethering of Tn complex proteins. Biochemical studies suggest that such defective tethering would result in either (a) Ca2+-independent activation, due to lack of Tn complex binding and consequent lack of inhibition, or (b) delayed reestablishment of TnI/TnC binding to the thin filament after Ca2+ activation and consequent abnormal duration of force. Analyses of animals doubly mutant for CeTnT-1 and for genes required for Ca2+ signaling support that CeTnT-1 phenotypes are dependent on Ca2+ signaling, thus supporting the second model and providing new in vivo evidence that full inhibition of thin filaments in low [Ca2+] does not require TnT.

Structural coupling of troponin C and actomyosin in muscle fibers

EPR of spin labeled TnC at Cys98 was used to explore the possible structural coupling between TnC in the thin filament and myosin trapped in the intermediate states of ATPase cycle. Weakly attached myosin heads (trapped by low ionic strength, low temperature and ATP) did not induce structural changes in TnC as compared to relaxed muscle, as spin labeled TnC displayed the same narrow orientational distribution [Li, H.-C., and Fajer, P. G. (1994) Biochemistry 33, 14324]. Ca2+-binding alone resulted in disordering of the labeled domain of TnC. Additional conformational changes of TnC occurred upon the attachment of strongly bound, prepower stroke myosin heads (trapped by AlF4-). These changes were not present in ghost fibers which myosin had been removed, excluding direct effects of AlF4- on the orientation of TnC in muscle fibers. The postpower stroke heads (rigor.ADP/Ca2+ and rigor/Ca2+) induced further changes in the orientational distribution of labeled domain of TnC irrespective of the degree of cooperativity in thin filaments. We thus conclude that troponin C in thin filaments detects structural changes in myosin during force generation, implying that there is a structural coupling between actomyosin and TnC.

Cooperative effect of calcium binding to adjacent troponin molecules on the thin filament-myosin subfragment 1 MgATPase rate

The myosin subfragment 1 (S1) MgATPase rate was measured using thin filaments with known extents of Ca2+ binding controlled by varying the ratio of native cardiac troponin versus an inhibitory troponin with a mutation in the sole regulatory Ca2+ binding site of troponin C. Fractional MgATPase activation was less than the fraction of troponins that bound Ca2+, implying a cooperative effect of bound Ca2+ on cross-bridge cycling. Addition of phalloidin did not alter cooperative effects between bound Ca2+ molecules in the presence or absence of myosin S1. When the myosin S1 concentration was raised sufficiently to introduce cooperative myosin-myosin effects, lower Ca2+ concentrations were needed to activate the MgATPase rate. MgATPase activation remained less than Ca2+ binding, implying a true, not just an apparent, increase in Ca2+ affinity. MgATPase activation by Ca2+ was more cooperative than could be explained by cooperativeness of overall Ca2+ binding, the discrepancy between Ca2+ binding and MgATPase activation, or interactions between myosins. The results suggest the thin filament-myosin S1 MgATPase cycle requires calcium binding to adjacent troponin molecules and that this binding is cooperatively promoted by a single cycling cross-bridge. This mechanism is a potential explanation for Ca2+-mediated regulation of cross-bridge kinetics in muscle fibers.

Fluorescent probes attached to Cys 35 or Cys 84 in cardiac troponin C are differentially sensitive to Ca(2+)-dependent events in vitro and in situ

The goal of the current study was to generate recombinant cTnC proteins with single Cys residues as sites for attachment of fluorescent probes that can distinguish between the structural effects of myosin cross bridges and direct Ca2+ binding to cTnC (cardiac and slow skeletal troponin C) in skinned fibers. We anticipated that cTnC proteins which retain the endogenous Cys 35 (cTnC(C35)) or Cys 84 (cTnC(C84)) would provide fluorescent probes with distinct microenvironments, since these residues are on opposite sides of the globular regulatory domain. In vitro experiments that showed IAANS (2-(4'-(iodoacetamido)anilino)naphthalene-6-sulfonic acid) coupled to Cys 35 can induce unwanted structural perturbations as evidenced by a decreased affinity of site II for Ca2+ when IAANS-labeled cTnC(C35) is bound to cTnI. Important structural features involving Cys 35 in the inactive site I are suggested by a Ca(2+)-dependent increase in reactivity of Cys 35 with sulfhydryl specific reagents when cTnC(C35) is associated with cTnI. These characteristics are not seen for cTnC(C84). When incorporated in situ into skinned cardiac muscle fibers, native cTnC with IAANS bound to both Cys 35 and Cys 84 showed a pCa50 of fluorescence which preceded that of force, while the pCa50 values of both force and fluorescence were coincident for IAANS-labeled cTnC(C84). Disruption of force-producing myosin cross bridges had no effect on the pCa50 of fluorescence for IAANS-labeled cTnC(C84), but induced a rightward shift in the pCa50 of fluorescence for IAANS-labeled native cTnC. These data can be interpreted to indicate that cTnC with IAANS bound to both Cys 35 and C84 senses either myosin cross bridges or direct Ca2+ binding and myosin-induced cooperativity, while IAANS bound to Cys 84 alone senses conformations that are tightly coupled with force generation.

Opposite effects of myosin subfragment 1 on binding of cardiac troponin and tropomyosin to the thin filament

To better understand the regulation of striated muscle contraction, the effects of myosin subfragment 1 (S-1) on the actin binding of cardiac troponin and tropomyosin were investigated. Troponin's affinity for actin-tropomyosin was 4-fold stronger in the absence than in the presence of myosin S-1. CaCl2 had no effect on troponin binding to the thin filament in the presence of myosin S-1. The binding curve was weakly cooperative, implying interactions between adjacent troponin molecules. Myosin S-1 increased (40-200-fold) the affinity of tropomyosin for the thin filament, an effect opposite to the effect of myosin on troponin. This effect was highly cooperative and occurred in the presence of ADP or in the absence of nucleotide. Myosin altered the effect of ionic conditions on tropomyosin-actin binding, consistent with tropomyosin binding to a different site on F-actin in the presence of myosin. The results indicate that troponin-tropomyosin and strongly binding myosin cross-bridges do not compete for an F-actin binding site. Although repositioning of troponin-tropomyosin on the actin filament may be sterically required for tight myosin-actin binding, a myosin-induced conformational change in actin provides a better explanation for the complex effects of myosin on thin filament assembly.

Effects of cycling and rigor crossbridges on the conformation of cardiac troponin C

The results of work by several investigators indicate that crossbridge attachment serves as a positive feedback mechanism that transiently increases the Ca2+ affinity of troponin C (TnC) during each normal heartbeat. To monitor structural changes in the cardiac isoform of TnC (cTnC) associated with Ca2+ binding and crossbridge attachment in muscle, we labeled cTnC with the sulfhydryl-specific fluorescent probe 2-(4'-iodoacetamidoanilino)naphthalene-6-sulfonic acid (IAANS). When IAANS-labeled cTnC (cTnCIAANS) was substituted for endogenous TnC, the fluorescence intensity of cardiac and skeletal muscle preparations increased substantially during rigor crossbridge attachment in the absence of Ca2+ (pCa 9.2). In cardiac muscle, the fluorescence signal increased the same amount in rigor and maximal activation, whereas in skeletal muscle, it was higher in rigor (rigor: cardiac and skeletal = 1; pCa 4.0: cardiac = 0.98 +/- 0.13, skeletal = 0.59 +/- 0.05). This indicates that crossbridge attachment alone is capable of influencing the structure of cTnCIAANS. Because the relative fluorescence intensity of cTnCIAANS was more sensitive to Ca2+ than was force in both preparations (cardiac: pCa50 fluorescence = 6.05 +/- 0.05, pCa50 force = 5.51 +/- 0.11; skeletal: pCa50 fluorescence = 5.94 +/- 0.13, pCa50 force = 5.65 +/- 0.14), we measured the Ca2+ sensitivity of the strong crossbridge attachment (sinusoidal stiffness was measured by imposing 1 kHz at 0.1-0.2% muscle length) in rat trabeculae.(ABSTRACT TRUNCATED AT 250 WORDS)