Troponin regulatory function and dynamics revealed by H/D exchange-mass spectrometry

Arg92Leu-cTnT Alters the cTnC-cTnI Interface Disrupting PKA-Mediated Relaxation

BACKGROUND

Impaired left ventricular relaxation, high filling pressures, and dysregulation of Ca2+ homeostasis are common findings contributing to diastolic dysfunction in hypertrophic cardiomyopathy (HCM). Studies have shown that impaired relaxation is an early observation in the sarcomere-gene-positive preclinical HCM cohort, which suggests the potential involvement of myofilament regulators in relaxation. A molecular-level understanding of mechanism(s) at the level of the myofilament is lacking. We hypothesized that mutation-specific, allosterically mediated, changes to the cTnC (cardiac troponin C)-cTnI (cardiac troponin I) interface can account for the development of early-onset diastolic dysfunction via decreased PKA accessibility to cTnI.

METHODS

HCM mutations R92L-cTnT (cardiac troponin T; Arg92Leu) and Δ160E-cTnT (Glu160 deletion) were studied in vivo, in vitro, and in silico via 2-dimensional echocardiography, Western blotting, ex vivo hemodynamics, stopped-flow kinetics, time-resolved fluorescence resonance energy transfer, and molecular dynamics simulations.

RESULTS

The HCM-causative mutations R92L-cTnT and Δ160E-cTnT result in different time-of-onset diastolic dysfunction. R92L-cTnT demonstrated early-onset diastolic dysfunction accompanied by a localized decrease in phosphorylation of cTnI. Constitutive phosphorylation of cTnI (cTnI-D23D24) was sufficient to recover diastolic function to non-Tg levels only for R92L-cTnT. Mutation-specific changes in Ca2+ dissociation rates associated with R92L-cTnT reconstituted with cTnI-D23D24 led us to investigate potential involvement of structural changes in the cTnC-cTnI interface as an explanation for these observations. We probed the interface via time-resolved fluorescence resonance energy transfer revealing a repositioning of the N-terminus of cTnI, closer to cTnC, and concomitant decreases in distance distributions at sites flanking the PKA consensus sequence. Implementing time-resolved fluorescence resonance energy transfer distances as constraints into our atomistic model identified additional electrostatic interactions at the consensus sequence.

CONCLUSIONS

These data show that the early diastolic dysfunction observed in a subset of HCM is attributable to allosterically mediated structural changes at the cTnC-cTnI interface that impair accessibility of PKA, thereby blunting β-adrenergic responsiveness and identifying a potential molecular target for therapeutic intervention.

The HCM - Linked Mutation Arg92Leu in TNNT2 Allosterically Alters the cTnC - cTnI Interface and Disrupts the PKA-mediated Regulation of Myofilament Relaxation

Background

Impaired left ventricular relaxation, high filling pressures, and dysregulation of Ca 2+ homeostasis are common findings contributing to diastolic dysfunction in hypertrophic cardiomyopathy (HCM). Studies have shown that impaired relaxation is an early observation in the sarcomere-gene-positive preclinical HCM cohort which suggests potential involvement of myofilament regulators of relaxation. Yet, a molecular level understanding of mechanism(s) at the level of the myofilament is lacking. We hypothesized that mutation-specific, allosterically mediated, changes to the cardiac troponin C-cardiac troponin I (cTnC-cTnI) interface can account for the development of early-onset diastolic dysfunction via decreased PKA accessibility to cTnI.

Methods

HCM mutations R92L-cTnT (Arg92Leu) and Δ160E-cTnT (Glu160 deletion) were studied in vivo , in vitro, and in silico via 2D echocardiography, western blotting, ex vivo hemodynamics, stopped-flow kinetics, time resolved fluorescence resonance energy transfer (TR-FRET), and molecular dynamics simulations.

Results

The HCM-causative mutations R92L-cTnT and Δ160E-cTnT result in different time-of-onset of diastolic dysfunction. R92L-cTnT demonstrated early-onset diastolic dysfunction accompanied by a localized decrease in phosphorylation of cTnI. Constitutive phosphorylation of cTnI (cTnI-D 23 D 24 ) was sufficient to recover diastolic function to Non-Tg levels only for R92L-cTnT. Mutation-specific changes in Ca 2+ dissociation rates associated with R92L-cTnT reconstituted with cTnI-D 23 D 24 led us to investigate potential involvement of structural changes in the cTnC-cTnI interface as an explanation for these observations. We probed the interface via TR-FRET revealing a repositioning of the N-terminus of cTnI, closer to cTnC, and concomitant decreases in distance distributions at sites flanking the PKA consensus sequence. Implementing TR-FRET distances as constraints into our atomistic model identified additional electrostatic interactions at the consensus sequence.

Conclusion

These data indicate that the early diastolic dysfunction observed in a subset of HCM is likely attributable to structural changes at the cTnC-cTnI interface that impair accessibility of PKA thereby blunting β-adrenergic responsiveness and identifying a potential molecular target for therapeutic intervention.

Dynamics of proteins in solution

The dynamics of proteins in solution includes a variety of processes, such as backbone and side-chain fluctuations, interdomain motions, as well as global rotational and translational (i.e. center of mass) diffusion. Since protein dynamics is related to protein function and essential transport processes, a detailed mechanistic understanding and monitoring of protein dynamics in solution is highly desirable. The hierarchical character of protein dynamics requires experimental tools addressing a broad range of time- and length scales. We discuss how different techniques contribute to a comprehensive picture of protein dynamics, and focus in particular on results from neutron spectroscopy. We outline the underlying principles and review available instrumentation as well as related analysis frameworks.

The missing links within troponin

The cardiac contraction-relaxation cycle is controlled by a sophisticated set of machinery. Of particular interest, is the revelation that allosteric networks transmit effects of binding at one site to influence troponin complex dynamics and structural-mediated signaling in often distal, functional sites in the myofilament. Our recent observations provide compelling evidence that allostery can explain the function of large-scale macromolecular events. Here we elaborate on our recent findings of interdomain communication within troponin C, using cutting-edge structural biology approaches, and highlight the importance of unveiling the unknown, distant communication networks within this system to obtain more comprehensive knowledge of how allostery impacts cardiac physiology and disease.

Role of the C-terminus mobile domain of cardiac troponin I in the regulation of thin filament activation in skinned papillary muscle strips

The C-terminus mobile domain of cTnI (cTnI-MD) is a highly conserved region which stabilizes the actin-cTnI interaction during the diastole. Upon Ca²⁺-binding to cTnC, cTnI-MD participates in a regulatory switching that involves cTnI to switch from interacting with actin toward interacting with the Ca²⁺-regulatory domain of cTnC. Despite many studies targeting the cTnI-MD, the role of this region in the length-dependent activation of cardiac contractility is yet to be determined. The present study investigated the functional consequences of losing the entire cTnI-MD in cTnI(1-167) truncation mutant, as it was exchanged for endogenous cTnI in skinned rat papillary muscle fibers. The influence of cTnI-MD truncation on the extent of the N-domain of cTnC hydrophobic cleft opening and the steady-state force as a function of sarcomere length (SL), cross-bridge state, and [Ca²⁺] was assessed using the simultaneous in situ time-resolved FRET and force measurements at short (1.8 μm) and long (2.2 μm) SLs. Our results show the significant role of cTnI-MD in the length dependent thin filament activation and the coupling between thin and thick filament regulations affected by SL. Our results also suggest that cTnI-MD transmits the effects of SL change to the core of troponin complex.

Troponin through the looking-glass: emerging roles beyond regulation of striated muscle contraction

Troponin is a heterotrimeric Ca²⁺-binding protein that has a well-established role in regulating striated muscle contraction. However, mounting evidence points to novel cellular functions of troponin, with profound implications in cancer, cardiomyopathy pathogenesis and skeletal muscle aging. Here, we highlight the non-canonical roles and aberrant expression patterns of troponin beyond the sarcomeric milieu. Utilizing bioinformatics tools and online databases, we also provide pathway, subcellular localization, and protein-protein/DNA interaction analyses that support a role for troponin in multiple subcellular compartments. This emerging knowledge challenges the conventional view of troponin as a sarcomere-specific protein exclusively involved in muscle contraction and may transform the way we think about sarcomeric proteins, particularly in the context of human disease and aging.

Modulation of the picosecond dynamics of troponin by the cardiomyopathy-causing mutation K247R of troponin T observed by quasielastic neutron scattering

Troponin (Tn), consisting of three subunits (TnC, TnI, and TnT), regulates cardiac muscle contraction in a Ca²⁺-dependent manner. Various point mutations of human cardiac Tn are known to cause familial hypertrophic cardiomyopathy due to aberration of the regulatory function. In this study, we investigated the effects of one of these mutations, K247R of TnT, on the picosecond dynamics of the Tn core domain (Tn-CD), consisting of TnC, TnI and TnT2 (183-288 residues of TnT), by carrying out the quasielastic neutron scattering measurements on the reconstituted Tn-CD containing either the wild-type TnT2 (wtTn-CD) or the mutant TnT2 (K247R-Tn-CD) in the absence and presence of Ca²⁺. It was found that Ca²⁺-binding to the wtTn-CD decreases the residence time of atomic motions in the Tn-CD with slight changes in amplitudes, suggesting that the regulatory function mainly requires modulation of frequency of atomic motions. On the other hand, the K247R-Tn-CD shows different dynamic behavior from that of the wtTn-CD both in the absence and presence of Ca²⁺. In particular, the K247R-Tn-CD exhibits a larger amplitude than the wtTn-CD in the presence of Ca²⁺, suggesting that the mutant can explore larger conformational space than the wild-type. This increased flexibility should be relevant to the functional aberration of this mutant.

Troponins, intrinsic disorder, and cardiomyopathy

Cardiac troponin is a dynamic complex of troponin C, troponin I, and troponin T (TnC, TnI, and TnT, respectively) found in the myocyte thin filament where it plays an essential role in cardiac muscle contraction. Mutations in troponin subunits are found in inherited cardiomyopathies, such as hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). The highly dynamic nature of human cardiac troponin and presence of numerous flexible linkers in its subunits suggest that understanding of structural and functional properties of this important complex can benefit from the consideration of the protein intrinsic disorder phenomenon. We show here that mutations causing decrease in the disorder score in TnI and TnT are significantly more abundant in HCM and DCM than mutations leading to the increase in the disorder score. Identification and annotation of intrinsically disordered regions in each of the troponin subunits conducted in this study can help in better understanding of the roles of intrinsic disorder in regulation of interactomes and posttranslational modifications of these proteins. These observations suggest that disease-causing mutations leading to a decrease in the local flexibility of troponins can trigger a whole plethora of functional changes in the heart.

Cardiac Troponin and Tropomyosin: Structural and Cellular Perspectives to Unveil the Hypertrophic Cardiomyopathy Phenotype

Inherited myopathies affect both skeletal and cardiac muscle and are commonly associated with genetic dysfunctions, leading to the production of anomalous proteins. In cardiomyopathies, mutations frequently occur in sarcomeric genes, but the cause-effect scenario between genetic alterations and pathological processes remains elusive. Hypertrophic cardiomyopathy (HCM) was the first cardiac disease associated with a genetic background. Since the discovery of the first mutation in the β-myosin heavy chain, more than 1400 new mutations in 11 sarcomeric genes have been reported, awarding HCM the title of the “disease of the sarcomere.” The most common macroscopic phenotypes are left ventricle and interventricular septal thickening, but because the clinical profile of this disease is quite heterogeneous, these phenotypes are not suitable for an accurate diagnosis. The development of genomic approaches for clinical investigation allows for diagnostic progress and understanding at the molecular level. Meanwhile, the lack of accurate in vivo models to better comprehend the cellular events triggered by this pathology has become a challenge. Notwithstanding, the imbalance of Ca²⁺ concentrations, altered signaling pathways, induction of apoptotic factors, and heart remodeling leading to abnormal anatomy have already been reported. Of note, a misbalance of signaling biomolecules, such as kinases and tumor suppressors (e.g., Akt and p53), seems to participate in apoptotic and fibrotic events. In HCM, structural and cellular information about defective sarcomeric proteins and their altered interactome is emerging but still represents a bottleneck for developing new concepts in basic research and for future therapeutic interventions. This review focuses on the structural and cellular alterations triggered by HCM-causing mutations in troponin and tropomyosin proteins and how structural biology can aid in the discovery of new platforms for therapeutics. We highlight the importance of a better understanding of allosteric communications within these thin-filament proteins to decipher the HCM pathological state.

Role of cardiac troponin I carboxy terminal mobile domain and linker sequence in regulating cardiac contraction

Inhibition of striated muscle contraction at resting Ca(2+) depends on the C-terminal half of troponin I (TnI) in thin filaments. Much focus has been on a short inhibitory peptide (Ip) sequence within TnI, but structural studies and identification of disease-associated mutations broadened emphasis to include a larger mobile domain (Md) sequence at the C-terminus of TnI. For Md to function effectively in muscle relaxation, tight mechanical coupling to troponin's core-and thus tropomyosin-is presumably needed. We generated recombinant, human cardiac troponins containing one of two TnI constructs: either an 8-amino acid linker between Md and the rest of troponin (cTnILink8), or an Md deletion (cTnI1-163). Motility assays revealed that Ca(2+)-sensitivity of reconstituted thin filament sliding was markedly increased with cTnILink8 (∼0.9 pCa unit leftward shift of speed-pCa relation compared to WT), and increased further when Md was missing entirely (∼1.4 pCa unit shift). Cardiac Tn's ability to turn off filament sliding at diastolic Ca(2+) was mostly (61%), but not completely eliminated with cTnI1-163. TnI's Md is required for full inhibition of unloaded filament sliding, although other portions of troponin-presumably including Ip-are also necessary. We also confirm that TnI's Md is not responsible for superactivation of actomyosin cycling by troponin.

Effect of N-Terminal Extension of Cardiac Troponin I on the Ca(2+) Regulation of ATP Binding and ADP Dissociation of Myosin II in Native Cardiac Myofibrils

Cardiac troponin I (cTnI) has a unique N-terminal extension that plays a role in modifying the calcium regulation of cardiac muscle contraction. Restrictive cleavage of the N-terminal extension of cTnI occurs under stress conditions as a physiological adaptation. Recent studies have shown that in comparison with controls, transgenic mouse cardiac myofibrils containing cTnI lacking the N-terminal extension (cTnI-ND) had a lower sensitivity to calcium activation of ATPase, resulting in enhanced ventricular relaxation and cardiac function. To investigate which step(s) of the ATPase cycle is regulated by the N-terminal extension of cTnI, here we studied the calcium dependence of cardiac myosin II ATPase kinetics in isolated cardiac myofibrils. ATP binding and ADP dissociation rates were measured by using stopped-flow spectrofluorimetry with mant-dATP and mant-dADP, respectively. We found that the second-order mant-dATP binding rate of cTnI-ND mouse cardiac myofibrils was 3-fold faster than that of wild-type myofibrils at low Ca(2+) concentrations. The ADP dissociation rate of cTnI-ND myofibrils was positively dependent on calcium concentration, while the wild-type controls were not significantly affected. These data from experiments using native cardiac myofibrils under physiological conditions indicate that modification of the N-terminal extension of cTnI plays a role in the calcium regulation of the kinetics of actomyosin ATPase.

Atomic resolution probe for allostery in the regulatory thin filament

Calcium binding and dissociation within the cardiac thin filament (CTF) is a fundamental regulator of normal contraction and relaxation. Although the disruption of this complex, allosterically mediated process has long been implicated in human disease, the precise atomic-level mechanisms remain opaque, greatly hampering the development of novel targeted therapies. To address this question, we used a fully atomistic CTF model to test both Ca(2+) binding strength and the energy required to remove Ca(2+) from the N-lobe binding site in WT and mutant troponin complexes that have been linked to genetic cardiomyopathies. This computational approach is combined with measurements of in vitro Ca(2+) dissociation rates in fully reconstituted WT and cardiac troponin T R92L and R92W thin filaments. These human disease mutations represent known substitutions at the same residue, reside at a significant distance from the calcium binding site in cardiac troponin C, and do not affect either the binding pocket affinity or EF-hand structure of the binding domain. Both have been shown to have significantly different effects on cardiac function in vivo. We now show that these mutations independently alter the interaction between the Ca(2+) ion and cardiac troponin I subunit. This interaction is a previously unidentified mechanism, in which mutations in one protein of a complex indirectly affect a third via structural and dynamic changes in a second to yield a pathogenic change in thin filament function that results in mutation-specific disease states. We can now provide atom-level insight that is potentially highly actionable in drug design.

Molecular and functional consequences of mutations in the central helix of cardiac troponin C

The objective of this work was to investigate the role of acidic residues within the exposed middle segment of the central helix of cTnC in (1) cTnC-cTnI interactions, (2) Ca(2+) binding and exchange with the regulatory N-domain of cTnC in increasingly complex biochemical systems, and (3) ability of the cTn complex to regulate actomyosin ATPase. In order to achieve this objective, we introduced the D87A/D88A and E94A/E95A/E96A mutations into the central helix of cTnC. The D87A/D88A and E94A/E95A/E96A mutations decreased affinity of cTnC for the regulatory region of cTnI. The Ca(2+) sensitivity of the regulatory N-domain of isolated cTnC was decreased by the D87A/D88A, but not E94A/E95A/E96A mutation. However, both the D87A/D88A and E94A/E95A/E96A mutations desensitized the cTn complex and reconstituted thin filaments to Ca(2+). Decreases in the Ca(2+) sensitivity of the cTn complex and reconstituted thin filaments were, at least in part, due to faster rates of Ca(2+) dissociation. In addition, the D87A/D88A and E94A/E95A/E96A mutations desensitized actomyosin ATPase to Ca(2+), and decreased maximal actomyosin ATPase activity. Thus, our results indicate that conserved acidic residues within the exposed middle segment of the central helix of cTnC are important for the proper regulatory function of the cTn complex.

Molecular dynamics simulations of the cardiac troponin complex performed with FRET distances as restraints

Cardiac troponin (cTn) is the Ca²⁺-sensitive molecular switch that controls cardiac muscle activation and relaxation. However, the molecular detail of the switching mechanism and how the Ca²⁺ signal received at cardiac troponin C (cTnC) is communicated to cardiac troponin I (cTnI) are still elusive. To unravel the structural details of troponin switching, we performed ensemble Förster resonance energy transfer (FRET) measurements and molecular dynamic (MD) simulations of the cardiac troponin core domain complex. The distance distributions of forty five inter-residue pairs were obtained under Ca²⁺-free and saturating Ca²⁺ conditions from time-resolved FRET measurements. These distances were incorporated as restraints during the MD simulations of the cardiac troponin core domain. Compared to the Ca²⁺-saturated structure, the absence of regulatory Ca²⁺ perturbed the cTnC N-domain hydrophobic pocket which assumed a closed conformation. This event partially unfolded the cTnI regulatory region/switch. The absence of Ca²⁺, induced flexibility to the D/E linker and the cTnI inhibitory region, and rotated the cTnC N-domain with respect to rest of the troponin core domain. In the presence of saturating Ca²⁺ the above said phenomenon were absent. We postulate that the secondary structure perturbations experienced by the cTnI regulatory region held within the cTnC N-domain hydrophobic pocket, coupled with the rotation of the cTnC N-domain would control the cTnI mobile domain interaction with actin. Concomitantly the rotation of the cTnC N-domain and perturbation of the D/E linker rigidity would control the cTnI inhibitory region interaction with actin to effect muscle relaxation.

Insulin signaling, resistance, and the metabolic syndrome: insights from mouse models into disease mechanisms

Insulin resistance is a major underlying mechanism responsible for the 'metabolic syndrome', which is also known as insulin resistance syndrome. The incidence of the metabolic syndrome is increasing at an alarming rate, becoming a major public and clinical problem worldwide. The metabolic syndrome is represented by a group of interrelated disorders, including obesity, hyperglycemia, hyperlipidemia, and hypertension. It is also a significant risk factor for cardiovascular disease and increased morbidity and mortality. Animal studies have demonstrated that insulin and its signaling cascade normally control cell growth, metabolism, and survival through the activation of MAPKs and activation of phosphatidylinositide-3-kinase (PI3K), in which the activation of PI3K associated with insulin receptor substrate 1 (IRS1) and IRS2 and subsequent Akt→Foxo1 phosphorylation cascade has a central role in the control of nutrient homeostasis and organ survival. The inactivation of Akt and activation of Foxo1, through the suppression IRS1 and IRS2 in different organs following hyperinsulinemia, metabolic inflammation, and overnutrition, may act as the underlying mechanisms for the metabolic syndrome in humans. Targeting the IRS→Akt→Foxo1 signaling cascade will probably provide a strategy for therapeutic intervention in the treatment of type 2 diabetes and its complications. This review discusses the basis of insulin signaling, insulin resistance in different mouse models, and how a deficiency of insulin signaling components in different organs contributes to the features of the metabolic syndrome. Emphasis is placed on the role of IRS1, IRS2, and associated signaling pathways that are coupled to Akt and the forkhead/winged helix transcription factor Foxo1.

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.

Significance of troponin dynamics for Ca2+-mediated regulation of contraction and inherited cardiomyopathy

Ca(2+) dissociation from troponin causes cessation of muscle contraction by incompletely understood structural mechanisms. To investigate this process, regulatory site Ca(2+) binding in the NH(2)-lobe of subunit troponin C (TnC) was abolished by mutagenesis, and effects on cardiac troponin dynamics were mapped by hydrogen-deuterium exchange (HDX)-MS. The findings demonstrate the interrelationships among troponin's detailed dynamics, troponin's regulatory actions, and the pathogenesis of cardiomyopathy linked to troponin mutations. Ca(2+) slowed HDX up to 2 orders of magnitude within the NH(2)-lobe and the NH(2)-lobe-associated TnI switch helix, implying that Ca(2+) greatly stabilizes this troponin regulatory region. HDX of the TnI COOH terminus indicated that its known role in regulation involves a partially folded rather than unfolded structure in the absence of Ca(2+) and actin. Ca(2+)-triggered stabilization extended beyond the known direct regulatory regions: to the start of the nearby TnI helix 1 and to the COOH terminus of the TnT-TnI coiled-coil. Ca(2+) destabilized rather than stabilized specific TnI segments within the coiled-coil and destabilized a region not previously implicated in Ca(2+)-mediated regulation: the coiled-coil's NH(2)-terminal base plus the preceding TnI loop with which the base interacts. Cardiomyopathy-linked mutations clustered almost entirely within influentially dynamic regions of troponin, and many sites were Ca(2+)-sensitive. Overall, the findings demonstrate highly selective effects of regulatory site Ca(2+), including opposite changes in protein dynamics at opposite ends of the troponin core domain. Ca(2+) release triggers an intramolecular switching mechanism that propagates extensively within the extended troponin structure, suggests specific movements of the TnI inhibitory regions, and prominently involves troponin's dynamic features.

Molecular effects of familial hypertrophic cardiomyopathy-related mutations in the TNT1 domain of cTnT

Familial hypertrophic cardiomyopathy (FHC) is one of the most common genetic causes of heart disease. Approximately 15% of FHC-related mutations are found in cTnT [cardiac troponin (cTn) T]. Most of the cTnT FHC-related mutations are in or flanking the N-tail TNT1 domain that directly interacts with overlapping tropomyosin (Tm). We investigate two sets of cTnT mutations at opposite ends of TNT1, mutations in residue 92 in the Tm-Tm overlap region of TNT1 and mutations in residues 160 and 163 in the C-terminal portion of TNT1 adjacent to the cTnT H1-H2 linker. Though all the mutations are located within TNT1, they have widely different phenotypes clinically and biophysically. Using a complete atomistic model of the cTn-Tm complex, we identify mechanisms by which the effects of TNT1 mutations propagate to the cTn core and site II of cTnC, where calcium binding and dissociation occurs. We find that mutations in TNT1 alter the flexibility of TNT1, which is inversely proportional to the cooperativity of calcium activation of the thin filament. Further, we identify a pathway of propagation of structural and dynamic changes from TNT1 to site II of cTnC, including TNT1, cTnT linker, I-T arm, regulatory domain of cTnI, the D-E linker of cTnC, and site II cTnC. Mutationally induced changes at site II of cTnC alter calcium coordination that corresponds to biophysical measurements of calcium sensitivity. Finally, we compare this pathway of mutational propagation with that of the calcium activation of the thin filament and find that they are identical but opposite in direction.

The side chain of glutamine 13 is the acyl-donor amino acid modified by type 2 transglutaminase in subunit T of the native rabbit skeletal muscle troponin complex

Subunit T of the native muscle troponin complex is a recognised substrate of transglutaminase both in vitro and in situ with formation of isopeptide bonds. Using a proteomic approach, we have now determined the precise site of in vitro labelling of the protein. A preparation of troponin purified from ether powder from mixed rabbit skeletal muscles was employed as transglutaminase substrate. The only isoform TnT2F present in our preparation was recognised as acyl-substrate by human type 2 transglutaminase which specifically modified glutamine 13 in the N-terminal region. During the reaction, the troponin protein complex was polymerized. Results are discussed in relation to the structure of the troponin T subunit, in the light of the role of troponins in skeletal and cardiac muscle diseases, and to the rules governing glutamine side chain selection by tissue transglutaminase.

A model of calcium activation of the cardiac thin filament

The cardiac thin filament regulates actomyosin interactions through calcium-dependent alterations in the dynamics of cardiac troponin and tropomyosin. Over the past several decades, many details of the structure and function of the cardiac thin filament and its components have been elucidated. We propose a dynamic, complete model of the thin filament that encompasses known structures of cardiac troponin, tropomyosin, and actin and show that it is able to capture key experimental findings. By performing molecular dynamics simulations under two conditions, one with calcium bound and the other without calcium bound to site II of cardiac troponin C (cTnC), we found that subtle changes in structure and protein contacts within cardiac troponin resulted in sweeping changes throughout the complex that alter tropomyosin (Tm) dynamics and cardiac troponin--actin interactions. Significant calcium-dependent changes in dynamics occur throughout the cardiac troponin complex, resulting from the combination of the following: structural changes in the N-lobe of cTnC at and adjacent to sites I and II and the link between them; secondary structural changes of the cardiac troponin I (cTnI) switch peptide, of the mobile domain, and in the vicinity of residue 25 of the N-terminus; secondary structural changes in the cardiac troponin T (cTnT) linker and Tm-binding regions; and small changes in cTnC-cTnI and cTnT-Tm contacts. As a result of these changes, we observe large changes in the dynamics of the following regions: the N-lobe of cTnC, the mobile domain of cTnI, the I-T arm, the cTnT linker, and overlapping Tm. Our model demonstrates a comprehensive mechanism for calcium activation of the cardiac thin filament consistent with previous, independent experimental findings. This model provides a valuable tool for research into the normal physiology of cardiac myofilaments and a template for studying cardiac thin filament mutations that cause human cardiomyopathies.

HD exchange and PLIMSTEX determine the affinities and order of binding of Ca2+ with troponin C

Troponin C (TnC), present in all striated muscle, is the Ca(2+)-activated trigger that initiates myocyte contraction. The binding of Ca(2+) to TnC initiates a cascade of conformational changes involving the constituent proteins of the thin filament. The functional properties of TnC and its ability to bind Ca(2+) have significant regulatory influence on the contractile reaction of muscle. Changes in TnC may also correlate with cardiac and various other muscle-related diseases. We report here the implementation of the PLIMSTEX strategy (protein ligand interaction by mass spectrometry, titration, and H/D exchange) to elucidate the binding affinity of TnC with Ca(2+) and, more importantly, to determine the order of Ca(2+) binding of the four EF hands of the protein. The four equilibrium constants, K(1) = (5 ± 5) × 10(7) M(-1), K(2) = (1.8 ± 0.8) × 10(7) M(-1), K(3) = (4.2 ± 0.9) × 10(6) M(-1), and K(4) = (1.6 ± 0.6) × 10(6) M(-1), agree well with determinations by other methods and serve to increase our confidence in the PLIMSTEX approach. We determined the order of binding to the four EF hands to be III, IV, II, and I by extracting from the H/DX results the deuterium patterns for each EF hand for each state of the protein (apo through fully Ca(2+) bound). This approach, demonstrated for the first time, may be general for determining binding orders of metal ions and other ligands to proteins.

Complexation and Calcium-Induced Conformational Changes in the Cardiac Troponin Complex Monitored by Hydrogen/Deuterium Exchange and FT-ICR Mass Spectrometry

Cardiac muscle contraction is regulated by the heterotrimeric complex: troponin. We apply solution-phase hydrogen/deuterium exchange monitored by FT-ICR mass spectrometry to study the structural dynamics and the Ca-induced conformational changes of the cardiac isoform of troponin, by comparing H/D exchange rate constants for TnC alone, the binary TnC:TnI complex, and the ternary TnC:TnI:TnT complex for Ca-free and Ca-saturated states. The wide range of exchange rate constants indicates that the complexes possess both highly flexible and very rigid domains. Fast exchange rates were observed for the N-terminal extension of TnI (specific to the cardiac isoform), the DE linker in TnC alone, and the mobile domain of TnI. The slowest rates were for the IT coiled-coil that grants stability and stiffness to the complex. Ca(2+) binding to site II of the N-lobe of TnC induces short-range allosteric effects, mainly protection for the C-lobe of TnC that transmits long-range conformational changes that reach the IT coiled-coil and even TnT1. The present results corroborate prior X-ray crystallography and NMR interpretations and also illuminate domains that were not resolved or truncated in those experiments.

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.

Low temperature dynamic mapping reveals unexpected order and disorder in troponin

Troponin is a pivotal regulatory protein that binds Ca(2+) reversibly to act as the muscle contraction on-off switch. To understand troponin function, the dynamic behavior of the Ca(2+)-saturated cardiac troponin core domain was mapped in detail at 10 °C, using H/D exchange-mass spectrometry. The low temperature conditions of the present study greatly enhanced the dynamic map compared with previous work. Approximately 70% of assessable peptide bond hydrogens were protected from exchange sufficiently for dynamic measurement. This allowed the first characterization by this method of many regions of regulatory importance. Most of the TnI COOH terminus was protected from H/D exchange, implying an intrinsically folded structure. This region is critical to the troponin inhibitory function and has been implicated in thin filament activation. Other new findings include unprotected behavior, suggesting high mobility, for the residues linking the two domains of TnC, as well as for the inhibitory peptide residues preceding the TnI switch helix. These data indicate that, in solution, the regulatory subdomain of cardiac troponin is mobile relative to the remainder of troponin. Relatively dynamic properties were observed for the interacting TnI switch helix and TnC NH(2)-domain, contrasting with stable, highly protected properties for the interacting TnI helix 1 and TnC COOH-domain. Overall, exchange protection via protein folding was relatively weak or for a majority of peptide bond hydrogens. Several regions of TnT and TnI were unfolded even at low temperature, suggesting intrinsic disorder. Finally, change in temperature prominently altered local folding stability, suggesting that troponin is an unusually mobile protein under physiological conditions.