Calcium-dependent Changes in the Flexibility of the Regulatory Domain of Troponin C in the Troponin Complex

Constructing a structural model of troponin using site-directed spin labeling: EPR and PRE-NMR

The relative ease of introducing a paramagnetic species onto a protein, and advances in electron paramagnetic resonance (EPR) over the past two decades, have established spin labeling as a vital structural biology technique for revealing the functional workings of the troponin muscle regulatory complex-an ~80 kDa heterotrimeric protein switch for turning on striated muscle contraction. Through the site-directed spin labeling (SDSL) of cysteine residues at key sites in troponin, a molecular-level understanding of the troponin muscle regulatory system across all levels of structural hierarchy has been achieved. Through the application of EPR, mobility and accessibility trends in the EPR signals of the spin labels attached to consecutive residues can reveal the secondary structure of troponin elements and also help map the interaction between subunits. Distance restraints calculated from the interspin interactions between spin label pairs have helped with building a structural model of the troponin complex. Further, when SDSL is paired with NMR, paramagnetic relaxation enhancement (PRE)-NMR has been used to obtain high-resolution structural detail for both intra- and interdomain interactions in troponin and revealed details of protein conformational changes and dynamics accompanying troponin function. In this review, we provide an overview of the SDSL labeling methodology and its application towards building a dynamic structural model of the multi-subunit troponin complex which details the calcium-induced conformational changes intimately linked to muscle regulation. We also describe how the SDSL method, in conjunction with EPR or NMR, can be used to obtain insights into structural perturbations to troponin caused by disease-causing mutations.

Through thick and thin: dual regulation of insect flight muscle and cardiac muscle compared

Both insect flight muscle and cardiac muscle contract rhythmically, but the way in which repetitive contractions are controlled is different in the two types of muscle. We have compared the flight muscle of the water bug, Lethocerus, with cardiac muscle. Both have relatively high resting elasticity and are activated by an increase in sarcomere length or a quick stretch. The larger response of flight muscle is attributed to the highly ordered lattice of thick and thin filaments and to an isoform of troponin C that has no exchangeable Ca²⁺-binding site. The Ca²⁺ sensitivity of cardiac muscle and flight muscle can be manipulated so that cardiac muscle responds to Ca²⁺ like flight muscle, and flight muscle responds like cardiac muscle, showing the malleability of regulation. The interactions of the subunits in flight muscle troponin are described; a model of the complex, using the structure of cardiac troponin as a template, shows an overall similarity of cardiac and flight muscle troponin complexes. The dual regulation by thick and thin filaments in skeletal and cardiac muscle is thought to operate in flight muscle. The structure of inhibited myosin heads folded back on the thick filament in relaxed Lethocerus fibres has not been seen in other species and may be an adaptation to the rapid contractions of flight muscle. A scheme for regulation by thick and thin filaments during oscillatory contraction is described. Cardiac and flight muscle have much in common, but the differing mechanical requirements mean that regulation by both thick and thin filaments is adapted to the particular muscle.

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.

Nascent β Structure in the Elongated Hydrophobic Region of a Gerstmann-Sträussler-Scheinker PrP Allele

Prion diseases are neurodegenerative disorders caused by the misfolding of the cellular prion protein (PrP^C). Gerstmann-Sträussler-Scheinker syndrome is an inherited prion disease with one early-onset allele (HRdup) containing an eight-amino-acid insertion; this LGGLGGYV insert is positioned after valine 129 (human PrP^C sequence) in a hydrophobic tract in the natively disordered region. Here we have characterized the structure and explored the molecular motions and dynamics of HRdup PrP and a control allele. High-resolution NMR data suggest that the core of HRdup has a canonical PrP^C structure, yet a nascent β-structure is observed in the flexible elongated hydrophobic region of HRdup. In addition, using mouse PrP^C sequence, we observed that a methionine/valine polymorphism at codon 128 (equivalent of methionine/valine 129 in human sequence) and oligomerization caused by high protein concentration affects conformational exchange dynamics at residue G130. We hypothesize that with the β-structure at the N-terminus, the hydrophobic region of HRdup can adopt a fully extended configuration and fold back to form an extended β-sheet with the existing β-sheet. We propose that these structures are early chemical events in disease pathogenesis.

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.

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.

Coarse-Grained Modeling of Peptide Docking Associated with Large Conformation Transitions of the Binding Protein: Troponin I Fragment-Troponin C System

Most of the current docking procedures are focused on fine conformational adjustments of assembled complexes and fail to reproduce large-scale protein motion. In this paper, we test a new modeling approach developed to address this problem. CABS-dock is a versatile and efficient tool for modeling the structure, dynamics and interactions of protein complexes. The docking protocol employs a coarse-grained representation of proteins, a simplified model of interactions and advanced protocols for conformational sampling. CABS-dock is one of the very few tools that allow unrestrained docking with large conformational freedom of the receptor. In an example application we modeled the process of complex assembly between two proteins: Troponin C (TnC) and the N-terminal helix of Troponin I (TnI N-helix), which occurs in vivo during muscle contraction. Docking simulations illustrated how the TnC molecule undergoes significant conformational transition on complex formation, a phenomenon that can be modeled only when protein flexibility is properly accounted for. This way our procedure opens up a new possibility for studying mechanisms of protein complex assembly, which may be a supporting tool for rational drug design.

Versatile cardiac troponin chimera for muscle protein structural biology and drug discovery

Investigation of the molecular interactions within and between subunits of the heterotrimeric troponin complex, and with other proteins in the sarcomere, has revealed salient structural elements involved in regulation of muscle contraction. The discovery of new cardiotonic drugs and structural studies utilizing intact troponin, or regulatory complexes formed between the key regions identified in troponin C and troponin I, face intrinsic and technical difficulties associated with weak protein-protein interactions and with solubility, aggregation, stability of the overall architecture, isotope labeling, and size, respectively. We have designed and characterized a chimeric troponin C-troponin I hybrid protein with a cleavable linker that is useful for producing isotopically labeled troponin peptides, stabilizes their interaction, and has proven to be a faithful representation of the original complex in the systolic state, but lacking its disadvantages, making it particularly suitable for drug screening and structural studies.

Regulatory domain of troponin moves dynamically during activation of cardiac muscle

Heart muscle is activated by Ca²⁺ to generate force and shortening, and the signaling pathway involves allosteric mechanisms in the thin filament. Knowledge about the structure-function relationship among proteins in the thin filament is critical in understanding the physiology and pathology of the cardiac function, but remains obscure. We investigate the conformation of the cardiac troponin (Tn) on the thin filament and its response to Ca²⁺ activation and propose a molecular mechanism for the regulation of cardiac muscle contraction by Tn based uniquely on information from in situ protein domain orientation. Polarized fluorescence from bifunctional rhodamine is used to determine the orientation of the major component of Tn core domain on the thin filaments of cardiac muscle. We show that the C-terminal lobe of TnC (CTnC) does not move during activation, suggesting that CTnC, together with the coiled coil formed by the TnI and TnT chains (IT arm), acts as a scaffold that holds N-terminal lobe of TnC (NTnC) and the actin binding regions of troponin I. The NTnC, on the other hand, exhibits multiple orientations during both diastole and systole. By combining the in situ orientation data with published in vitro measurements of intermolecular distances, we construct a model for the in situ structure of the thin filament. The conformational dynamics of NTnC plays an important role in the regulation of cardiac muscle contraction by moving the C-terminal region of TnI from its actin-binding inhibitory location and enhancing the movement of tropomyosin away from its inhibitory position.

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In situ conformational changes of troponin in myocardium were investigated.

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A model for the cardiac thin filament was constructed based on the in situ data.

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The IT arm of cardiac troponin acts as a scaffold that holds the regulatory domain.

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The regulatory domain of cardiac troponin moves dynamically during activation.

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The dynamics of regulatory domain is important in cardiac muscle regulation.

Interdomain orientation of cardiac troponin C characterized by paramagnetic relaxation enhancement NMR reveals a compact state

Cardiac troponin C (cTnC) is the calcium binding subunit of the troponin complex that triggers the thin filament response to calcium influx into the sarcomere. cTnC consists of two globular EF-hand domains (termed the N- and C-domains) connected by a flexible linker. While the conformation of each domain of cTnC has been thoroughly characterized through NMR studies involving either the isolated N-domain (N-cTnC) or C-domain (C-cTnC), little attention has been paid to the range of interdomain orientations possible in full-length cTnC that arises as a consequence of the flexibility of the domain linker. Flexibility in the domain linker of cTnC is essential for effective regulatory function of troponin. We have therefore utilized paramagnetic relaxation enhancement (PRE) NMR to assess the interdomain orientation of cTnC. Ensemble fitting of our interdomain PRE measurements reveals that isolated cTnC has considerable interdomain flexibility and preferentially adopts a bent conformation in solution, with a defined range of relative domain orientations.

Invited review: probing the structures of muscle regulatory proteins using small-angle solution scattering

Small-angle X-ray and neutron scattering with contrast variation have made important contributions in advancing our understanding of muscle regulatory protein structures in the context of the dynamic molecular processes governing muscle action. The contributions of the scattering investigations have depended upon the results of key crystallographic, NMR, and electron microscopy experiments that have provided detailed structural information that has aided in the interpretation of the scattering data. This review will cover the advances made using small-angle scattering techniques, in combination with the results from these complementary techniques, in probing the structures of troponin and myosin binding protein C. A focus of the troponin work has been to understand the isoform differences between the skeletal and cardiac isoforms of this major calcium receptor in muscle. In the case of myosin binding protein C, significant data are accumulating, indicating that this protein may act to modulate the primary calcium signals from troponin, and interest in its biological role has grown because of linkages between gene mutations in the cardiac isoform and serious heart disease.

Switch action of troponin on muscle thin filament as revealed by spin labeling and pulsed EPR

We have used pulsed electron-electron double resonance (PELDOR) spectroscopy to measure the distance between spin labels at Cys(133) of the regulatory region of TnI (TnI133) and a native or genetically substituted cysteine of TnC (TnC44, TnC61, or TnC98). In the +Ca(2+) state, the TnC44-TnI133-T distance was 42 A, with a narrow distribution (half-width of 9 A), suggesting that the regulatory region binds the N-lobe of TnC. Distances for TnC61-TnI133 and TnC98-TnI133 were also determined to be 38 A (width of 12 A) and 22 A (width of 3.4 A), respectively. These values were all consistent with recently published crystal structure (Vinogradova, M. V., Stone, D. B., Malanina, G. G., Karatzaferi, C., Cooke, R., Mendelson, R. A., and Fletterick, R. J. (2005) Proc. Natl Acad. Sci. U.S.A. 102, 5038-5043). Similar distances were obtained with the same spin pairs on a reconstituted thin filament in the +Ca(2+) state. In the -Ca(2+) state, the distances displayed broad distributions, suggesting that the regulatory region of TnI was physically released from the N-lobe of TnC and consequently fluctuated over a variety of distances on a large scale (20-80 A). The interspin distance appeared longer on the filament than on troponin alone, consistent with the ability of the region to bind actin. These results support a concept that the regulatory region of TnI, as a molecular switch, binds to the exposed hydrophobic patch of TnC and traps the inhibitory region of TnI away from actin in Ca(2+) activation of muscle.

Mutations in Troponin that cause HCM, DCM AND RCM: what can we learn about thin filament function?

Troponin (Tn) is a critical regulator of muscle contraction in cardiac muscle. Mutations in Tn subunits are associated with hypertrophic, dilated and restrictive cardiomyopathies. Improved diagnosis of cardiomyopathies as well as intensive investigation of new mouse cardiomyopathy models has significantly enhanced this field of research. Recent investigations have showed that the physiological effects of Tn mutations associated with hypertrophic, dilated and restrictive cardiomyopathies are different. Impaired relaxation is a universal finding of most transgenic models of HCM, predicted directly from the significant changes in Ca(2+) sensitivity of force production. Mutations associated with HCM and RCM show increased Ca(2+) sensitivity of force production while mutations associated with DCM demonstrate decreased Ca(2+) sensitivity of force production. This review spotlights recent advances in our understanding on the role of Tn mutations on ATPase activity, maximal force development and heart function as well as the correlation between the locations of these Tn mutations within the thin filament and myofilament function.

Fast pressure jumps can perturb calcium and magnesium binding to troponin C F29W

We have used rapid pressure jump and stopped-flow fluorometry to investigate calcium and magnesium binding to F29W chicken skeletal troponin C. Increased pressure perturbed calcium binding to the N-terminal sites in the presence and absence of magnesium and provided an estimate for the volume change upon calcium binding (-12 mL/mol). We observed a biphasic response to a pressure change which was characterized by fast and slow reciprocal relaxation times of the order 1000/s and 100/s. Between pCa 8-5.4 and at troponin C concentrations of 8-28 muM, the slow relaxation times were invariant, indicating that a protein isomerization was rate-limiting. The fast event was only detected over a very narrow pCa range (5.6-5.4). We have devised a model based on a Monod-Wyman-Changeux cooperative mechanism with volume changes of -9 and +6 mL/mol for the calcium binding to the regulatory sites and closed to open protein isomerization steps, respectively. In the absence of magnesium, we discovered that calcium binding to the C-terminal sites could be detected, despite their position distal to the calcium-sensitive tryptophan, with a volume change of +25 mL/mol. We used this novel observation to measure competitive magnesium binding to the C-terminal sites and deduced an affinity in the range 200-300 muM (and a volume change of +35 mL/mol). This affinity is an order of magnitude tighter than equilibrium fluorescence data suggest based on a model of direct competitive binding. Magnesium thus indirectly modulates binding to the N-terminal sites, which may act as a fine-tuning mechanism in vivo.

NMR studies of the dynamics of a bifunctional rhodamine probe attached to troponin C

Fluorescence polarization measurements of bifunctional rhodamine (BR) probes provide a powerful approach to determine the in situ orientation of proteins within ordered complexes such as muscle fibers. For accurate interpretation of fluorescence measurements, it is important to understand the probe dynamics relative to the protein to which it is attached. We previously determined the structure of the N-domain of chicken skeletal troponin C, BR-labeled on the C helix, in complex with the switch region of troponin I, and demonstrated that the probe does not perturb the structure or dynamics of the protein. In this study, the motion of the fluorescence label relative to the protein has been characterized using NMR relaxation measurements of 13C-labeled methyl groups on the BR probe and 15N-labeled backbone amides of the protein. Probe dynamics were monitored using off-resonance 13C-R(1rho), 13C-R(1) and {1H}-13C NOE at magnetic field strengths of 500, 600, and 800 MHz. Relaxation data were interpreted in terms of the overall rotational correlation time of the protein and a two-time scale model for internal motion of the BR methyl groups, using a numerical optimization with Monte Carlo parameter error estimation. The analysis yields a 1.5 +/- 0.4 ps correlation time for rotation around the three-fold methyl symmetry axis, and a 0.8 +/- 0.4 ns rotational correlation time for reorientation of the 13C-14N bond with an associated S2s of 0.79 +/- 0.03. Order parameters of the backbone NH vectors in the helix to which the probe is attached average S2 approximately 0.85, implying that the amplitude of independent reorientation of the BR probe is small in magnitude, consistent with results from fluorescence polarization measurements in reconstituted muscle fibers.

Interaction of cardiac troponin with cardiotonic drugs: a structural perspective

Over the 40 years since its discovery, many studies have focused on understanding the role of troponin as a myofilament based molecular switch in regulating the Ca(2+)-dependent activation of striated muscle contraction. Recently, studies have explored the role of cardiac troponin as a target for cardiotonic agents. These drugs are clinically useful for treating heart failure, a condition in which the heart is no longer able to pump enough blood to other organs. These agents act via a mechanism that modulates the Ca(2+)-sensitivity of troponin; such a mode of action is therapeutically desirable because intracellular Ca(2+) concentration is not perturbed, preserving the regulation of other Ca(2+)-based signaling pathways. This review describes molecular details of the interaction of cardiac troponin with a variety of cardiotonic drugs. We present recent structural work that has identified the docking sites of several cardiotonic drugs in the troponin C-troponin I interface and discuss their relevance in the design of troponin based drugs for the treatment of heart disease.

The Structure of Lethocerus Troponin C: Insights into the Mechanism of Stretch Activation in Muscles

To gain a molecular description of how muscles can be activated by mechanical stretch, we have solved the structure of the calcium-loaded F1 isoform of troponin C (TnC) from Lethocerus and characterized its interactions with troponin I (TnI). We show that the presence of only one calcium cation in the fourth EF hand motif is sufficient to induce an open conformation in the C-terminal lobe of F1 TnC, in contrast with what is observed in vertebrate muscle. This lobe interacts in a calcium-independent way both with the N terminus of TnI and, with lower affinity, with a region of TnI equivalent to the switch and inhibitory peptides of vertebrate muscles. Using both synthetic peptides and recombinant proteins, we show that the N lobe of F1 TnC is not engaged in interactions with TnI, excluding a regulatory role of this domain. These findings provide insights into mechanically stimulated muscle contraction.

Structural basis for calcium-regulated relaxation of striated muscles at interaction sites of troponin with actin and tropomyosin

In summary, we have shown that the TnI-TnC-TnT2 ternary complex (-52 kDa) has a mobile actin-binding domain (-6.1 kDa) that tumbles independently of the core domain. By docking the mobile domain and the core domain into the cryo-EM map obtained for thin filaments at low Ca2+, a model for actin-troponin interaction has been obtained. This model shows the atomic details of interactions of actin with the mobile domain and suggests the mechanism by which troponin generates a shift in the azimuthal position of tropomyosin in response to changes in Ca2+ levels. In this model the mobile domain of troponin interacts with three actins and one troponin interacts with four actin molecules. The relationship between myosin and the mobile domain suggests that the latter may work as a fail-safe latch to secure a relaxed state. The model also provides insights into many mutations associated with human cardiomyopathy and has implications for the function of other actin-binding proteins. Coordinates of the mobile domain have been deposited in the Protein Data Bank under accession codes 1VDI (low Ca2+) and 1VDJ (high Ca2+). Chemical shifts of the mobile domain have been deposited in the BMRB under accession ID 18140.

Fluorescence spectroscopic analysis of the proximity changes between the central helix of troponin C and the C-terminus of troponin T from chicken skeletal muscle

Recent structural studies of the troponin (Tn) core complex have shown that the regulatory head containing the N-lobe of TnC is connected to the IT arm by a flexible linker of TnC. The IT arm is a long coiled-coil formed by alpha-helices of TnI and TnT, plus the C-lobe of TnC. The TnT is thought to play a pivotal role in the linking of Ca(2+) -triggered conformational changes in thin filament regulatory proteins to the activation of cross-bridge cycling. However, a functional domain at the C-terminus of TnT is missing from the Tn core complex. In this study, we intended to determine the proximity relationship between the central helix of TnC and the TnT C-terminus in the binary and the ternary complex with and without Ca2+ by using pyrene excimer fluorescence spectroscopy and fluorescence resonance energy transfer. Chicken fast skeletal TnC contains a Cys102 at the E helix, while TnT has a Cys264 at its C-terminus. These two cysteines were specifically labeled with sulfhydryl-reactive fluorescence probes. The measured distance in the binary complex was about 19 Angstroms and slightly increased when they formed the ternary complex with TnI (20 Angstroms). Upon Ca2+ binding the distance was not affected in the binary complex but increased by approximately 4 Angstroms in the ternary complex. These results suggest that TnI plays an essential role in the Ca(2+) -mediated change in the spatial relationship between the C-lobe of TnC and the C-terminus of TnT.

Structural changes in troponin in response to Ca2+ and myosin binding to thin filaments during activation of skeletal muscle

Contraction of skeletal and cardiac muscle is regulated by Ca2+ -dependent structural changes in troponin that control the interaction between myosin and actin. We measured the orientations of troponin domains in skeletal muscle fibers using polarized fluorescence from bifunctional rhodamine probes on the C and E helices of troponin C. The C helix, in the regulatory head domain, tilts by approximately 30 degrees when muscle is activated in physiological conditions, with a Ca2+ -sensitivity similar to that of active force. Complete inhibition of active force did not affect C-helix orientation, and binding of rigor myosin heads did not affect its orientation at saturating [Ca2+]. The E helix, in the IT arm of troponin, tilted by approximately 10 degrees on activation, and this was reduced to only 3 degrees when active force was inhibited. Binding of rigor myosin heads produced a larger tilt of the E helix. Thus, in situ, the regulatory head acts as a pure Ca2+ -sensor, whereas the IT arm is primarily sensitive to myosin head binding. The polarized fluorescence data from active muscle are consistent with an in vitro structure of the troponin core complex in which the D and E helices of troponin C are collinear. The present data were used to orient this structure in the fiber and suggest that the IT arm is at approximately 30 degrees to the filament axis in active muscle. In relaxed muscle, the IT arm tilts to approximately 40 degrees but the D/E helix linker melts, allowing the regulatory head to tilt through a larger angle.

An interplay between protein disorder and structure confers the Ca2+ regulation of striated muscle

The troponin (Tn) complex regulates the thin filament of striated muscle by transducing [Ca2+] fluctuations into conformational changes. These changes propagate to tropomyosin (Tm), which then assumes a new disposition with respect to actin, reversibly exposing actin's binding sites for the thick filament motor-ATPase (myosin). To date, the structural biology of thin filament regulation has been studied in the context of two equilibrium states corresponding to high (contraction-activated) and low (contraction-inhibited) sarcomeric [Ca2+]. New electron micrographic reconstructions of the thin filament have resolved Tn, actin, and Tm in high and low [Ca2+] states, integrating high-resolution structures of the Tn core, actin, and Tm. The resultant picture of thin filament regulation does not resolve all of the functionally significant portions of troponin I (TnI) or troponin C (TnC). Those regions of Tn have been shown (using NMR relaxation spectroscopy) to undergo conformational fluctuations, rationalizing the absence of these regions from micrograph-based reconstructions. The disordered portions of Tn are, to date, being interpreted within a canonical structure-activity paradigm. Here we present a new mechanism for the regulation of Tn having explicit descriptions of the kinetic pathways of activation and inhibition. Our thesis is that the intrinsic disorder of TnI is mechanistically significant. As the coupling of folding to binding has been shown to confer an inherent kinetic advantage (known as flycasting activity), our thesis accounts for TnI's conformational heterogeneity and known structure-activity relationships in a parsimonious fashion. We integrate recent NMR structures of the C-terminus of TnI and NMR observations of the conformational dynamics of the Tn complex into high-resolution models of the thin filament. Ways of evaluating the mechanism are discussed. The novel conceptual framework presented here prompts new hypotheses regarding the mechanism of pH sensitivity and of pathogenic mutations in troponin.

Dynamics of the C-terminal region of TnI in the troponin complex in solution

The determination of crystal structures of the troponin complex (Takeda et al. 2003. Nature. 424:35-41; Vinogradova et al. 2005. Proc. Natl. Acad. Sci. USA. 102:5038-5043) has advanced knowledge of the regulation of muscle contraction at the molecular level. However, there are domains important for actin binding that are not visualized. We present evidence that the C-terminal region of troponin I (TnI residues 135-182) is flexible in solution and has no stable secondary structure. We use NMR spectroscopy to observe the backbone dynamics of skeletal [2H, 13C, 15N]-TnI in the troponin complex in the presence of Ca2+ or EGTA/Mg2+. Residues in this region give stronger signals than the remainder of TnI, and chemical shift index values indicate little secondary structure, suggesting a very flexible region. This is confirmed by NMR relaxation measurements. Unlike TnC and other regions of TnI in the complex, the C-terminal region of TnI is not affected by Ca2+ binding. Relaxation measurements and reduced spectral density analysis are consistent with the C-terminal region of TnI being a tethered domain connected to the rest of the troponin complex by a flexible linker, residues 137-146, followed by a collapsed region with at most nascent secondary structure.

The role of electrostatics in the interaction of the inhibitory region of troponin I with troponin C

We have addressed the electrostatic interactions occurring between the inhibitory region of cardiac troponin I with the C-lobe of troponin C using scanning glycine mutagenesis of the inhibitory region. We report variations in the electric potentials due to mutation of charged residues within this complex based upon the solved NMR structure (1OZS). These results demonstrate the importance of electrostatics within this complex, and it is proposed that electrostatic interactions are integral to the formation and function of larger ternary troponin complexes. To address this hypothesis, we report (15)N NMR relaxation measurements, which suggest that, within a ternary complex involving the C-lobe and the N-terminal region of troponin I (residues 34-71), the inhibitory region maintains the electrostatic interactions with the E-helix of the C-lobe as observed within the binary complex. These results imply that, in solution, the cardiac troponin complex behaves in a manner consistent with that of the crystal structure of the skeletal isoform (1YTZ). A cardiac troponin complex possessing domain orientations similar to that of the skeletal isoform provides structural insights into altered troponin I activities as observed for the familial hypertrophic cardiomyopathy mutation R144G and phosphorylation of Thr142.

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.

Structural basis for Ca2+-regulated muscle relaxation at interaction sites of troponin with actin and tropomyosin

Troponin and tropomyosin on actin filaments constitute a Ca2+-sensitive switch that regulates the contraction of vertebrate striated muscle through a series of conformational changes within the actin-based thin filament. Troponin consists of three subunits: an inhibitory subunit (TnI), a Ca2+-binding subunit (TnC), and a tropomyosin-binding subunit (TnT). Ca2+-binding to TnC is believed to weaken interactions between troponin and actin, and triggers a large conformational change of the troponin complex. However, the atomic details of the actin-binding sites of troponin have not been determined. Ternary troponin complexes have been reconstituted from recombinant chicken skeletal TnI, TnC, and TnT2 (the C-terminal region of TnT), among which only TnI was uniformly labelled with 15N and/or 13C. By applying NMR spectroscopy, the solution structures of a "mobile" actin-binding domain (approximately 6.1 kDa) in the troponin ternary complex (approximately 52 kDa) were determined. The mobile domain appears to tumble independently of the core domain of troponin. Ca2+-induced changes in the chemical shift and line shape suggested that its tumbling was more restricted at high Ca2+ concentrations. The atomic details of interactions between actin and the mobile domain of troponin were defined by docking the mobile domain into the cryo-electron microscopy (cryo-EM) density map of thin filament at low [Ca2+]. This allowed the determination of the 3D position of residue 133 of TnI, which has been an important landmark to incorporate the available information. This enabled unique docking of the entire globular head region of troponin into the thin filament cryo-EM map at a low Ca2+ concentration. The resultant atomic model suggests that troponin interacted electrostatically with actin and caused the shift of tropomyosin to achieve muscle relaxation. An important feature is that the coiled-coil region of troponin pushed tropomyosin at a low Ca2+ concentration. Moreover, the relationship between myosin and the mobile domain on actin filaments suggests that the latter works as a fail-safe latch.