Structure of the regulatory N-domain of human cardiac troponin C in complex with human cardiac troponin I147-163 and bepridil

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.

Molecular characterization and immunohistochemical localization of a novel troponin C during silkworm development

We have cloned and sequenced a novel Bombyx mori gene that encodes a protein having a high degree of homology with other known troponin C (TnC) proteins. The amino acid sequence, DX[DN]X[DSG]X(6)E, a highly conserved putative Ca(2+) -binding motif found in loops within the globular domains of previously identified TnC proteins, is also present in BmTnC. We have expressed and purified to homogeneity a His-tagged BmTnC fusion protein having a molecular weight of approximately 21.6 kDa. We have used this purified fusion protein to produce polyclonal antibodies against BmTnC for Western blot analyses. These analyses have revealed that BmTnC is expressed in the larval head, the Malpighian tubule, the epidermis, the testis, and the gut, as has been confirmed by immunohistochemistry. In addition, real-time reverse transcription/polymerase chain reaction has shown that BmTnC mRNA levels differ substantially among these tissues. Our findings indicate that BmTnC is selectively expressed in the muscular tissues of the silkworm, including portions of the head, the Malpighian tubule, the body wall, and the gut.

Calcium structural transition of troponin in the complexes, on the thin filament, and in muscle fibres, as studied by site-directed spin-labelling EPR

We have measured the intersite distance, side-chain mobility and orientation of specific site(s) of troponin (Tn) complex on the thin filaments or in muscle fibres as well as in solution by means of site-directed spin labeling electron paramagnetic resonance (SDSL-EPR). We have examined the Ca(2+)-induced movement of the B and C helices relative to the D helix in a human cardiac (hc)TnC monomer state and hcTnC-hcTnI binary complex. An interspin distance between G42C (B helix) and C84 (D helix) was 18.4 angstroms in the absence of Ca2+. The distance between Q58C (C helix) and C84 (D helix) was 18.3 angstroms. Distance changes were observed by the addition of Ca2+ and by the formation of a complex with TnI. Both Ca2+ and TnI are essential for the full opening -3 angstroms of the N-domain in cardiac TnC. We have determined the in situ distances between C35 and C84 by measuring pulsed electron-electron double resonance (PELDOR) spectroscopy. The distances were 26.0 and 27.2 A in the monomer state and in reconstituted fibres, respectively. The addition of Ca2+ decreased the distance to 23.2 angstroms in fibres but only slightly in the monomer state, indicating that Ca2+ binding to the N-lobe of hcTnC induced a larger structural change in muscle fibres than in the monomer state. We also succeeded in synthesizing a new bifunctional spin labels that is firmly fixed on a central E-helix (94C-101C) of skeletal(sk)TnC to examine its orientation in reconstituted muscle fibres. EPR spectrum showed that this helix is disordered with respect to the filament axis. We have studied the calcium structural transition in skTnI and tropomyosin on the filament by SDSL-EPR. The spin label at a TnI switch segment (C133) showed three motional states depending on Ca2+ and actin. The data suggested that the TnI switch segment binds to TnC N-lobe in +Ca2+ state, and that in -Ca2+ state it is free in TnC-I-T complex alone while fixed to actin in the reconstituted thin filaments. In contrast, the side chain spin labels along the entire tropomyosin molecule showed no Ca(2+)-induced mobility changes.

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

With the recent advances in structure determination of the troponin complex, it becomes even more important to understand the dynamics of its components and how they are affected by the presence or absence of Ca(2+). We used NMR techniques to study the backbone dynamics of skeletal troponin C (TnC) in the complex. Transverse relaxation-optimized spectroscopy pulse sequences and deuteration of TnC were essential to assign most of the TnC residues in the complex. Backbone amide (15)N relaxation times were measured in the presence of Ca(2+) or EGTA/Mg(2+). T(1) relaxation times could not be interpreted precisely, because for a molecule of this size, the longitudinal backbone amide (15)N relaxation rate due to chemical shift anisotropy and dipole-dipole interactions becomes too small, and other relaxation mechanisms become relevant. T(2) relaxation times were of the expected magnitude for a complex of this size, and most of the variation of T(2) times in the presence of Ca(2+) could be explained by the anisotropy of the complex, suggesting a relatively rigid molecule. The only exception was EF-hand site III and helix F immediately after, which are more flexible than the rest of the molecule. In the presence of EGTA/Mg(2+), relaxation times for residues in the C-domain of TnC are very similar to values in the presence of Ca(2+), whereas the N-domain becomes more flexible. Taken together with the high flexibility of the linker between the two domains, we concluded that in the absence of Ca(2+), the N-domain of TnC moves independently from the rest of the complex.

Ca(2+)-regulated structural changes in troponin

Troponin senses Ca2+ to regulate contraction in striated muscle. Structures of skeletal muscle troponin composed of TnC (the sensor), TnI (the regulator), and TnT (the link to the muscle thin filament) have been determined. The structure of troponin in the Ca(2+)-activated state features a nearly twofold symmetrical assembly of TnI and TnT subunits penetrated asymmetrically by the dumbbell-shaped TnC subunit. Ca ions are thought to regulate contraction by controlling the presentation to and withdrawal of the TnI inhibitory segment from the thin filament. Here, we show that the rigid central helix of the sensor binds the inhibitory segment of TnI in the Ca(2+)-activated state. Comparison of crystal structures of troponin in the Ca(2+)-activated state at 3.0 angstroms resolution and in the Ca(2+)-free state at 7.0 angstroms resolution shows that the long framework helices of TnI and TnT, presumed to be a Ca(2+)-independent structural domain of troponin are unchanged. Loss of Ca ions causes the rigid central helix of the sensor to collapse and to release the inhibitory segment of TnI. The inhibitory segment of TnI changes conformation from an extended loop in the presence of Ca2+ to a short alpha-helix in its absence. We also show that Anapoe, a detergent molecule, increases the contractile force of muscle fibers and binds specifically, together with the TnI switch helix, in a hydrophobic pocket of TnC upon activation by Ca ions.

Calcium Structural Transition of Human Cardiac Troponin C in Reconstituted Muscle Fibres as Studied by Site-directed Spin Labelling

The in situ structure of human cardiac troponin C (hcTnC) has been studied with site-directed, spin labelling, electron paramagnetic resonance (SDSL-EPR). Analysis of the in situ structures of hcTnC is essential for elucidating the molecular mechanism behind its Ca(2+)-sensitive regulation. We prepared two hcTnC mutants (C35S and C84S) containing one native cysteine residue (84 and 35, respectively) for spin labelling. The mutants were labelled with a methane thiosulfonate spin label (MTSSL) and the TnC was reconstituted into permeabilized muscle fibres. The mobility of Cys84-MTSSL changed markedly after addition of Ca2+, while that of the Cys35 residue did not change in the monomer state or in fibres. The rotational correlation time of Cys84-MTSSL decreased from 32ns to 13ns upon Ca(2+)-binding in the monomer state, whereas in fibres the spectrum of Cys84-MTSSL was resolved into mobile (16ns) and immobile (35ns) components and the addition of Ca2+ increased the immobile component. Moreover, the accessibility of Cys84-MTSSL to molecular oxygen increased slightly in the presence of Ca2+. These data suggest that Cys35 remains in the same location regardless of the addition of Ca2+, whereas Cys84 is located at the position that interacts with B and C helices of hcTnC and interacts with troponin I (TnI) at high concentrations of Ca2+. We determined the distances between Cys35 and Cys84 by measuring pulsed electron-electron double resonance spectra. The distances were 26.0 angstroms and 27.2 angstroms in the monomer state and in fibres, respectively, and the addition of Ca2+ decreased the distance to 23.2 angstroms in fibres but only slightly in the monomer state, showing that Ca2+ binding to the N-domain of hcTnC induced a larger structural change in muscle fibres than in the monomer state.

Structural based insights into the role of troponin in cardiac muscle pathophysiology

Troponin is a molecular switch, directly regulating the Ca2+-dependent activation of myofilament in striated muscle contraction. Cardiac troponin is subject to covalent and noncovalent modifications; phosphorylation modulates myofilament physiology, mutations are linked to familial hypertrophic cardiomyopathy, intracellular acidification causes myocardial infarction, and cardiotonic drugs modify myofilament response to Ca2+. The structure of troponin provides insights into the mechanism of this molecular switch and an understanding of the effects of protein modification under pathophysiological conditions. Although the structure of troponin C has been solved in various Ca2+-bound states for some time, structural information on troponin I and troponin T has only emerged recently. This review summarizes recent advances on the structure of complexes of troponin subunits with the aim of assessing how these proteins interact with each other to execute its role as a molecular switch and how covalent and noncovalent modifications affect the structure of troponin and the switch mechanism. We focus on pinpointing the specific amino acid residues involved in phosphorylation and mutation and the pH sensitive regions in the structure of troponin. We also present recent structural work that have identified the docking sites of several cardiotonic drugs on cardiac troponin C and discuss their relevance in the direction of troponin based drug design in the therapy of heart disease.

Conformational Changes of Troponin C Within the Thin Filaments Detected by Neutron Scattering

Regulation of skeletal and cardiac muscle contraction is associated with structural changes of the thin filament-based proteins, troponin consisting of three subunits (TnC, TnI, and TnT), tropomyosin, and actin, triggered by Ca2+-binding to TnC. Knowledge of in situ structures of these proteins is indispensable for elucidating the molecular mechanism of this Ca2+-sensitive regulation. Here, the in situ structure of TnC within the thin filaments was investigated with neutron scattering, combined with selective deuteration and the contrast matching technique. Deuterated TnC (dTnC) was first prepared, this dTnC was then reconstituted into the native thin filaments, and finally neutron scattering patterns of these reconstituted thin filaments containing dTnC were measured under the condition where non-deuterated components were rendered "invisible" to neutrons. The obtained scattering curves arising only from dTnC showed distinct difference in the absence and presence of Ca2+. These curves were analyzed by model calculations using the Monte Carlo method, in which inter-dTnC interference was explicitly taken into consideration. The model calculation showed that in situ radius of gyration of TnC was 23 A (99% confidence limits between 22 A and 23 A) and 24 A (99% confidence limits between 23 A and 25 A) in the absence and presence of Ca2+, respectively, indicating that TnC within the thin filaments assumes a conformation consistent with the extended dumbbell structure, which is different from the structures found in the crystals of various Tn complexes. Elongation of TnC by binding of Ca2+ was also suggested. Furthermore, the radial position of TnC within the thin filament was estimated to be 53 A (99% confidence limits between 49 A and 57 A) and 49 A (99% confidence limits between 44 A and 53 A) in the absence and presence of Ca2+, respectively, suggesting that this radial movement of TnC by 4A is associated with large conformational changes of the entire Tn molecule by binding of Ca2+.

Preformed structural elements feature in partner recognition by intrinsically unstructured proteins

Intrinsically unstructured proteins (IUPs) are devoid of extensive structural order but often display signs of local and limited residual structure. To explain their effective functioning, we reasoned that such residual structure can be crucial in their interactions with their structured partner(s) in a way that preformed structural elements presage their final conformational state. To check this assumption, a database of 24 IUPs with known 3D structures in the bound state has been assembled and the distribution of secondary structure elements and backbone torsion angles have been analysed. The high proportion of residues in coil conformation and with phi, psi angles in the disallowed regions of the Ramachandran map compared to the reference set of globular proteins shows that IUPs are not fully ordered even in their bound form. To probe the effect of partner proteins on IUP folding, inherent conformational preferences of IUP sequences have been assessed by secondary structure predictions using the GOR, ALB and PROF algorithms. The accuracy of predicting secondary structure elements of IUPs is similar to that of their partner proteins and is significantly higher than the corresponding values for random sequences. We propose that strong conformational preferences mark regions in IUPs (mostly helices), which correspond to their final structural state, while regions with weak conformational preferences represent flexible linkers between them. In our interpretation, preformed elements could serve as initial contact points, the binding of which facilitates the reeling of the flexible regions onto the template. This finding implies that IUPs draw a functional advantage from preformed structural elements, as they enable their facile, kinetically and energetically less demanding, interaction with their physiological partner.

Structure and dynamics of the C-domain of human cardiac troponin C in complex with the inhibitory region of human cardiac troponin I

Cardiac troponin C is the Ca2+-dependent switch for heart muscle contraction. Troponin C is associated with various other proteins including troponin I and troponin T. The interaction between the subunits within the troponin complex is of critical importance in understanding contractility. Following a Ca2+ signal to begin contraction, the inhibitory region of troponin I comprising residues Thr128-Arg147 relocates from its binding surface on actin to troponin C, triggering movement of troponin-tropomyosin within the thin filament and thereby freeing actin-binding site(s) for interactions with the myosin ATPase of the thick filament to generate the power stroke. The structure of calcium-saturated cardiac troponin C (C-domain) in complex with the inhibitory region of troponin I was determined using multinuclear and multidimensional nuclear magnetic resonance spectroscopy. The structure of this complex reveals that the inhibitory region adopts a helical conformation spanning residues Leu134-Lys139, with a novel orientation between the E- and H-helices of troponin C, which is largely stabilized by electrostatic interactions. By using isotope labeling, we have studied the dynamics of the protein and peptide in the binary complex. The structure of this inhibited complex provides a framework for understanding into interactions within the troponin complex upon heart contraction.

Single mutation (A162H) in human cardiac troponin I corrects acid pH sensitivity of Ca2+-regulated actomyosin S1 ATPase

In contrast to skeletal muscle, the efficiency of the contractile apparatus of cardiac tissue has long been known to be severely compromised by acid pH as in the ischemia of myocardial infarction and other cardiac myopathies. Recent reports (Westfall, M. V., and Metzger, J. M. (2001) News Physiol. Sci. 16, 278-281; Li, G., Martin, A. F., and Solaro, R. J. (2001) J. Mol. Cell. Cardiol. 33, 1309-1320) have indicated that the reduced Ca(2+) sensitivity of cardiac contractility at low pH (<or=pH 6.5) is attributable to structural difference(s) in the cardiac and skeletal inhibitory components (TnIs) of their troponins. Here, using a reconstituted Ca(2+)-regulated human cardiac troponin-tropomyosin actomyosin S1 ATPase assay, we report that a single TnI mutation, A162H, restores Ca(2+) sensitivity at pH 6.5 to that at pH 7.0. Levels of inhibition (pCa 7.0), activation (pCa 4.0), and cooperativity of ATPase activity were minimally affected. Two other mutations (Q155R and E164V) also previously suggested by us (Pearlstone, J. R., Sykes, B. D., and Smillie, L. B. (1997) Biochemistry 36, 7601-7606) and involving charged residues showed no such effects. With fast skeletal muscle troponin, a single TnI H130A mutation reduced Ca(2+) sensitivity at pH 6.5 to levels approaching the cardiac system at pH 6.5. These observations provide structural insight into long-standing physiological and clinical phenomena and are of potential relevance to therapeutic treatments of heart disease by gene transfer, stem cell, and cell transplantation approaches.