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

Myosin-actin crossbridge independent sarcomere length induced Ca2+ sensitivity changes in skinned myocardial fibers: Role of myosin heads

Sarcomere length-dependent activation (LDA) is essential to engaging the Frank-Starling mechanism in the beat-to-beat regulation of cardiac output. Through LDA, the heart increases the Ca2+ sensitivity of myocardial contraction at a longer sarcomere length, leading to an enhanced maximal force at the same level of Ca2+. Despite its importance in both normal and pathological states, the molecular mechanism underlying LDA, especially the origin of the sarcomere length (SL) induced increase in myofilament Ca2+sensitivity, remains elusive. The aim of this study is to interrogate the role of changes in the state of myosin heads during diastole as well as effects of strong force-generating cross-bridges (XB) as determinants of SL-induced Ca2+ sensitivity of troponin in membrane-free (skinned) rat myocardial fibers. Skinned myocardial fibers were reconstituted with troponin complex containing a fluorophore-modified cardiac troponin C, cTnC(13C/51C)AEDANS-DDPM, and recombinant cardiac troponin I (cTnI) mutant, ΔSP-cTnI, in which the switch peptide (Sp) of cTnI was replaced by a non-functional peptide link to partially block the force-generating reaction of myosin with actin. We used the reconstituted myocardial fibers as a platform to investigate how Ca2+ sensitivity of troponin within skinned myocardial fibers responds to sarcomere stretch with variations in the status of myosin-actin XBs. Muscle mechanics and fluorescence measurements clearly showed similar SL-induced increases in troponin Ca2+ sensitivity in either the presence or the absence of strong XBs, suggesting that the SL-induced Ca2+ sensitivity change is independent of reactions of force generating XB with the thin filament. The presence of mavacamten, a selective myosin-motor inhibitor known to promote transition of myosin heads from the weakly actin-bound state (ON or disordered relaxed (DRX) state) to the ordered off state (OFF or super-relaxed (SRX) state), blunted the observed SL-induced increases in Ca2+ sensitivity of troponin regardless of the presence of XBs, suggesting that the presence of the myosin heads in the weakly actin bound state, is essential for Ca2+-troponin to sense the sarcomere stretch. Results from skinned myocardial fibers reconstituted with troponin containing engineered TEV digestible mutant cTnI and cTnT suggest that the observed SL effect on Ca2+ sensitivity may involve potential interactions of weakly bound myosin heads with troponin in the actin/Tm cluster region interacting with cTnT-T1 and residues 182-229 of cTnT-T2. The mechanical stretch effects may then be subsequently transmitted to the N-cTnC via the IT arm of troponin and the N-terminus of cTnI. Our findings strongly indicate that SL-induced potential myosin-troponin interaction in diastole, rather than strong myosin-actin XBs, may be an essential molecular mechanism underlying LDA of myofilament.

Velcro-binding by cardiac troponin-I traps tropomyosin on actin in a low-energy relaxed state

During muscle relaxation at low sarcoplasmic Ca2+-concentration, the 40-nm long tropomyosin coiled coil is attracted by the C-terminal regulatory domain of troponin subunit-I to a "steric-blocking" B-state position on actin subunits of cardiac and skeletal muscle thin filaments. Tropomyosin located in this B-state position obstructs myosin-binding sites on actin, limiting access of myosin-crossbridge heads on actin. In turn, the steric-hindrance imposed on myosin-binding diminishes actomyosin ATPase, crossbridge movement along actin, and contractility, thus causing relaxation. In contrast, during muscle activation, at high sarcoplasmic Ca2+ levels, the troponin-induced tropomyosin interference is relieved, the tropomyosin coiled coil returns to its default C-state position on actin, and contractility proceeds. In the current study, we examined the energetics associated with tropomyosin's shift in position from its C-state to its B-state on actin and the influence of troponin-I on this relaxed state transition. Control studies showed that in the absence of troponin, the free energy difference between B- and C-state positions of tropomyosin on actin is negligible, i.e. neither B- nor C-state is obviously preferred on troponin-free actin. In contrast, widely separated sites along the C-terminal regulatory domain of troponin-I are responsible for a favorable free energy change of about -0.75 kcal/mol, driving the tropomyosin C-state to B-state shift. Corresponding truncation and point mutations along C-terminal region of TnI lead to a less favorable regulatory transition and are linked to cardiac muscle dysfunction.

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

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

Myofilament-associated proteins with intrinsic disorder (MAPIDs) and their resolution by computational modeling

The cardiac sarcomere is a cellular structure in the heart that enables muscle cells to contract. Dozens of proteins belong to the cardiac sarcomere, which work in tandem to generate force and adapt to demands on cardiac output. Intriguingly, the majority of these proteins have significant intrinsic disorder that contributes to their functions, yet the biophysics of these intrinsically disordered regions (IDRs) have been characterized in limited detail. In this review, we first enumerate these myofilament-associated proteins with intrinsic disorder (MAPIDs) and recent biophysical studies to characterize their IDRs. We secondly summarize the biophysics governing IDR properties and the state-of-the-art in computational tools toward MAPID identification and characterization of their conformation ensembles. We conclude with an overview of future computational approaches toward broadening the understanding of intrinsic disorder in the cardiac sarcomere.

Intrinsically disordered proteins: Ensembles at the limits of Anfinsen's dogma

Intrinsically disordered proteins (IDPs) are proteins that lack rigid 3D structure. Hence, they are often misconceived to present a challenge to Anfinsen's dogma. However, IDPs exist as ensembles that sample a quasi-continuum of rapidly interconverting conformations and, as such, may represent proteins at the extreme limit of the Anfinsen postulate. IDPs play important biological roles and are key components of the cellular protein interaction network (PIN). Many IDPs can interconvert between disordered and ordered states as they bind to appropriate partners. Conformational dynamics of IDPs contribute to conformational noise in the cell. Thus, the dysregulation of IDPs contributes to increased noise and "promiscuous" interactions. This leads to PIN rewiring to output an appropriate response underscoring the critical role of IDPs in cellular decision making. Nonetheless, IDPs are not easily tractable experimentally. Furthermore, in the absence of a reference conformation, discerning the energy landscape representation of the weakly funneled IDPs in terms of reaction coordinates is challenging. To understand conformational dynamics in real time and decipher how IDPs recognize multiple binding partners with high specificity, several sophisticated knowledge-based and physics-based in silico sampling techniques have been developed. Here, using specific examples, we highlight recent advances in energy landscape visualization and molecular dynamics simulations to discern conformational dynamics and discuss how the conformational preferences of IDPs modulate their function, especially in phenotypic switching. Finally, we discuss recent progress in identifying small molecules targeting IDPs underscoring the potential therapeutic value of IDPs. Understanding structure and function of IDPs can not only provide new insight on cellular decision making but may also help to refine and extend Anfinsen's structure/function paradigm.

Potential impacts of the cardiac troponin I mobile domain on myofilament activation and relaxation

The cardiac thin filament is regulated in a Ca²⁺-dependent manner through conformational changes of troponin and tropomyosin (Tm). It has been generally understood that under conditions of low Ca²⁺ the inhibitory peptide domain (IP) of troponin I (TnI) binds to actin and holds Tm over the myosin binding sites on actin to prevent crossbridge formation. More recently, evidence that the C-terminal mobile domain (MD) of TnI also binds actin has made for a more complex scenario. This study uses a computational model to investigate the consequences of assuming that TnI regulates Tm movement via two actin-binding domains rather than one. First, a 16-state model of the cardiac thin filament regulatory unit was created with TnI-IP as the sole regulatory domain. Expansion of this to include TnI-MD formed a 24-state model. Comparison of these models showed that assumption of a second actin-binding site allows the individual domains to have a lower affinity for actin than would be required for IP acting alone. Indeed, setting actin affinities of the IP and MD to 25% of that assumed for the IP in the single-site model was sufficient to achieve precisely the same degree of Ca²⁺ regulation. We also tested the 24-state model's ability to represent steady-state experimental data in the case of disruption of either the IP or MD. We were able to capture qualitative changes in several properties that matched what was seen in the experimental data. Lastly, simulations were run to examine the effect of disruption of the IP or MD on twitch dynamics. Our results suggest that both domains are required to keep diastolic cross-bridge activity to a minimum and accelerate myofilament relaxation. Overall, our analyses support a paradigm in which two domains of TnI bind with moderate affinity to actin, working in tandem to complete Ca²⁺-dependent regulation of the thin filament.

Non-symmetrical structural behavior of a symmetric protein: the case of homo-trimeric TRAF2 (tumor necrosis factor-receptor associated factor 2)

The oligomeric state of TRAF2 (tumor necrosis factor-receptor associated factor 2), a TNF (tumor necrosis factor) receptor-associated factor, is crucial for membrane binding and probably plays a fundamental role in regulating the protein function in vivo. In this study we have combined molecular dynamics with the protein contact network approach to characterize the interaction of the three identical subunits of TRAF2. The average structure obtained after a 225 ns simulation reveals that two clusters of different size are formed, one of which includes almost completely two subunits, while the third monomer appears to be more independent. This picture is also confirmed by the estimated average number of inter-subunit contacts and by the comparison of side chains mobility in each monomer. The analysis of equilibrium pressure-induced dissociation measurements supports such findings, indicating that the dimeric-monomeric (2 + 1) might be prevalent with respect to the trimeric configuration, especially in the case of more diluted samples. These findings suggest that the formation of monomeric species, which is crucial for the formation of intra-luminal vesicles, might depend on preferential asymmetric interactions among the three subunits.Communicated by Ramaswamy H. Sarma.

Spontaneous Switching among Conformational Ensembles in Intrinsically Disordered Proteins

The common conception of intrinsically disordered proteins (IDPs) is that they stochastically sample all possible configurations driven by thermal fluctuations. This is certainly true for many IDPs, which behave as swollen random coils that can be described using polymer models developed for homopolymers. However, the variability in interaction energy between different amino acid sequences provides the possibility that some configurations may be strongly preferred while others are forbidden. In compact globular IDPs, core hydration and packing density can vary between segments of the polypeptide chain leading to complex conformational dynamics. Here, we describe a growing number of proteins that appear intrinsically disordered by biochemical and bioinformatic characterization but switch between restricted regions of conformational space. In some cases, spontaneous switching between conformational ensembles was directly observed, but few methods can identify when an IDP is acting as a restricted chain. Such switching between disparate corners of conformational space could bias ligand binding and regulate the volume of IDPs acting as structural or entropic elements. Thus, mapping the accessible energy landscape and capturing dynamics across a wide range of timescales are essential to recognize when an IDP is acting as such a switch.

Sarcomere length-dependent effects on Ca2+-troponin regulation in myocardium expressing compliant titin

Increases in sarcomere length cause enhanced force generation in cardiomyocytes by an unknown mechanism. Li et al. reveal that titin-based passive tension contributes to length-dependent activation of myofilaments and that tightly bound myosin–actin cross-bridges are associated with this effect.

Cardiac performance is tightly regulated at the cardiomyocyte level by sarcomere length, such that increases in sarcomere length lead to sharply enhanced force generation at the same Ca²⁺ concentration. Length-dependent activation of myofilaments involves dynamic and complex interactions between a multitude of thick- and thin-filament components. Among these components, troponin, myosin, and the giant protein titin are likely to be key players, but the mechanism by which these proteins are functionally linked has been elusive. Here, we investigate this link in the mouse myocardium using in situ FRET techniques. Our objective was to monitor how length-dependent Ca²⁺-induced conformational changes in the N domain of cardiac troponin C (cTnC) are modulated by myosin–actin cross-bridge (XB) interactions and increased titin compliance. We reconstitute FRET donor- and acceptor-modified cTnC(13C/51C)AEDANS-DDPM into chemically skinned myocardial fibers from wild-type and RBM20-deletion mice. The Ca²⁺-induced conformational changes in cTnC are quantified and characterized using time-resolved FRET measurements as XB state and sarcomere length are varied. The RBM20-deficient mouse expresses a more compliant N2BA titin isoform, leading to reduced passive tension in the myocardium. This provides a molecular tool to investigate how altered titin-based passive tension affects Ca²⁺-troponin regulation in response to mechanical stretch. In wild-type myocardium, we observe a direct association of sarcomere length–dependent enhancement of troponin regulation with both Ca²⁺ activation and strongly bound XB states. In comparison, measurements from titin RBM20-deficient animals show blunted sarcomere length–dependent effects. These results suggest that titin-based passive tension contributes to sarcomere length–dependent Ca²⁺-troponin regulation. We also conclude that strong XB binding plays an important role in linking the modulatory effect of titin compliance to Ca²⁺-troponin regulation of the myocardium.

Ca2+ and Myosin Cycle States Work as Allosteric Effectors of Troponin Activation

In cardiac muscle, troponin (Tn) and tropomyosin inhibit actin and myosin interactions through the steric blocking of myosin binding to F-actin. Ca²⁺ binding to Tn C modulates this inhibition. Thin filaments become activated upon Ca²⁺ binding, which enables strong binding of myosin with a concomitant release of ATP hydrolysis products and level arm swinging responsible for force generation. Despite this level of description, the current cross-bridge cycle model does not fully define the structural events that take place within Tn during combinatorial myosin and Ca²⁺ interventions. Here, we studied conformational changes within Tn bound to F-actin and tropomyosin by fluorescence lifetime imaging combined with Förster resonance energy transfer. Fluorescent dye molecules covalently bound to the Tn C C-lobe and Tn I C-terminal domain report Ca²⁺- and myosin-induced activation of Tn. Reconstituted thin filaments were deposited on a myosin-coated surface similar to an in vitro motility assay setup without filament sliding involved. Under all the tested conditions, Ca²⁺ was responsible for the most significant changes in Tn activation. Rigor myosin activated Tn at subsaturated Ca²⁺ conditions but not to the degree seen in thin filaments with Ca²⁺. ATP-γ-S did not affect Tn activation significantly; however, blebbistatin induced significant activation at subsaturating Ca²⁺ levels. The relation between the extent of Tn activation and its conformational flexibility suggests that active/inactive Tn states coexist in different proportions that depend on the combination of effectors. These results satisfy an allosteric activation model of the thin filament as a function of Ca²⁺ and the myosin catalytic cycle state.

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.

Functional significance of C-terminal mobile domain of cardiac troponin I

Ca²⁺-regulation of cardiac contractility is mediated through the troponin complex, which comprises three subunits: cTnC, cTnI, and cTnT. As intracellular [Ca²⁺] increases, cTnI reduces its binding interactions with actin to primarily interact with cTnC, thereby enabling contraction. A portion of this regulatory switching involves the mobile domain of cTnI (cTnI-MD), the role of which in muscle contractility is still elusive. To study the functional significance of cTnI-MD, we engineered two cTnI constructs in which the MD was truncated to various extents: cTnI(1-167) and cTnI(1-193). These truncations were exchanged for endogenous cTnI in skinned rat papillary muscle fibers, and their influence on Ca²⁺-activated contraction and cross-bridge cycling kinetics was assessed at short (1.9 μm) and long (2.2 μm) sarcomere lengths (SLs). Our results show that the cTnI(1-167) truncation diminished the SL-induced increase in Ca²⁺-sensitivity of contraction, but not the SL-dependent increase in maximal tension, suggesting an uncoupling between the thin and thick filament contributions to length dependent activation. Compared to cTnI(WT), both truncations displayed greater Ca²⁺-sensitivity and faster cross-bridge attachment rates at both SLs. Furthermore, cTnI(1-167) slowed MgADP release rate and enhanced cross-bridge binding. Our findings imply that cTnI-MD truncations affect the blocked-to closed-state transition(s) and destabilize the closed-state position of tropomyosin.

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.

Heart Failure-Related Hyperphosphorylation in the Cardiac Troponin I C Terminus Has Divergent Effects on Cardiac Function In Vivo

BACKGROUND

In human heart failure, Ser199 (equivalent to Ser200 in mouse) of cTnI (cardiac troponin I) is significantly hyperphosphorylated, and in vitro studies suggest that it enhances myofilament calcium sensitivity and alters calpain-mediated cTnI proteolysis. However, how its hyperphosphorylation affects cardiac function in vivo remains unknown.

METHODS AND RESULTS

To address the question, 2 transgenic mouse models were generated: a phospho-mimetic cTnIS200D and a phospho-silenced cTnIS200A, each driven by the cardiomyocyte-specific α-myosin heavy chain promoter. Cardiac structure assessed by echocardiography and histology was normal in both transgenic models compared with littermate controls (n=5). Baseline in vivo hemodynamics and isolated muscle studies showed that cTnIS200D significantly prolonged relaxation and lowered left ventricular peak filling rate, whereas ejection fraction and force development were normal (n=5). However, with increased heart rate or β-adrenergic stimulation, cTnIS200D mice had less enhanced ejection fraction or force development versus controls, whereas relaxation improved similarly to controls (n=5). By contrast, cTnIS200A was functionally normal both at baseline and under the physiological stresses. To test whether either mutation impacted cardiac response to ischemic stress, isolated hearts were subjected to ischemia/reperfusion. cTnIS200D were protected, recovering 88±8% of contractile function versus 35±15% in littermate controls and 28±8% in cTnIS200A (n=5). This was associated with less cTnI proteolysis in cTnIS200D hearts.

CONCLUSIONS

Hyperphosphorylation of this serine in cTnI C terminus impacts heart function by depressing diastolic function at baseline and limiting systolic reserve under physiological stresses. However, paradoxically, it preserves heart function after ischemia/reperfusion injury, potentially by decreasing proteolysis of cTnI.

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.

The Importance of Intrinsically Disordered Segments of Cardiac Troponin in Modulating Function by Phosphorylation and Disease-Causing Mutations

Troponin plays a central role in regulation of muscle contraction. It is the Ca²⁺ switch of striated muscles including the heart and in the cardiac muscle it is physiologically modulated by PKA-dependent phosphorylation at Ser22 and 23. Many cardiomyopathy-related mutations affect Ca²⁺ regulation and/or disrupt the relationship between Ca²⁺ binding and phosphorylation. Unlike the mechanism of heart activation, the modulation of Ca²⁺-sensitivity by phosphorylation of the cardiac specific N-terminal segment of TnI (1–30) is structurally subtle and has proven hard to investigate. The crystal structure of cardiac troponin describes only the relatively stable core of the molecule and the crucial mobile parts of the molecule are missing including TnI C-terminal region, TnI (1–30), TnI (134–149) (“inhibitory” peptide) and the C-terminal 28 amino acids of TnT that are intrinsically disordered. Recent studies have been performed to answer this matter by building structural models of cardiac troponin in phosphorylated and dephosphorylated states based on peptide NMR studies. Now these have been updated by more recent concepts derived from molecular dynamic simulations treating troponin as a dynamic structure. The emerging model confirms the stable core structure of troponin and the mobile structure of the intrinsically disordered segments. We will discuss how we can describe these segments in terms of dynamic transitions between a small number of states, with the probability distributions being altered by phosphorylation and by HCM or DCM-related mutations that can explain how Ca²⁺-sensitivity is modulated by phosphorylation and the effects of mutations.

Toward Understanding the Molecular Bases of Stretch Activation: A STRUCTURAL COMPARISON OF THE TWO TROPONIN C ISOFORMS OF LETHOCERUS

Muscles are usually activated by calcium binding to the calcium sensory protein troponin-C, which is one of the three components of the troponin complex. However, in cardiac and insect flight muscle activation is also produced by mechanical stress. Little is known about the molecular bases of this calcium-independent activation. In Lethocerus, a giant water bug often used as a model system because of its large muscle fibers, there are two troponin-C isoforms, called F1 and F2, that have distinct roles in activating the muscle. It has been suggested that this can be explained either by differences in structural features or by differences in the interactions with other proteins. Here we have compared the structural and dynamic properties of the two proteins and shown how they differ. We have also mapped the interactions of the F2 isoform with peptides spanning the sequence of its natural partner, troponin-I. Our data have allowed us to build a model of the troponin complex and may eventually help in understanding the specialized function of the F1 and F2 isoforms and the molecular mechanism of stretch activation.

Order-Disorder Transitions in the Cardiac Troponin Complex

The troponin complex is a molecular switch that ties shifting intracellular calcium concentration to association and dissociation of actin and myosin, effectively allowing excitation-contraction coupling in striated muscle. Although there is a long history of muscle biophysics and structural biology, many of the mechanistic details that enable troponin's function remain incompletely understood. This review summarizes the current structural understanding of the troponin complex on the muscle thin filament, focusing on conformational changes in flexible regions of the troponin I subunit. In particular, we focus on order-disorder transitions in the C-terminal domain of troponin I, which have important implications in cardiac disease and could also have potential as a model system for the study of coupled binding and folding.

Sarcomere length dependent effects on the interaction between cTnC and cTnI in skinned papillary muscle strips

Sarcomere length dependent activation (LDA) of myocardial force development is the cellular basis underlying the Frank-Starling law of the heart, but it is still elusive how the sarcomeres detect the length changes and convert them into altered activation of thin filament. In this study we investigated how the C-domain of cardiac troponin I (cTnI) functionally and structurally responds to the comprehensive effects of the Ca(2+), crossbridge, and sarcomere length of chemically skinned myocardial preparations. Using our in situ technique which allows for simultaneous measurements of time-resolved FRET and mechanical force of the skinned myocardial preparations, we measured changes in the FRET distance between cTnI(167C) and cTnC(89C), labeled with FRET donor and acceptor, respectively, as a function of [Ca(2+)], crossbridge state and sarcomere length of the skinned muscle preparations. Our results show that [Ca(2+)], cross-bridge feedback and sarcomere length have different effects on the structural transition of the C-domain cTnI. In particular, the interplay between crossbridges and sarcomere length has significant impacts on the functional structural change of the C-domain of cTnI in the relaxed state. These novel observations suggest the importance of the C-domain of cTnI and the dynamic and complex interplay between various components of myofilament in the LDA mechanism.

Conformation and Dynamics of the Troponin I C-Terminal Domain: Combining Single-Molecule and Computational Approaches for a Disordered Protein Region

In recent years, single-molecule Förster resonance energy transfer (smFRET) has emerged as a critical and flexible tool in structural biology, particularly in the study of highly dynamic regions and molecular assemblies. The usefulness of smFRET can be further extended by combining it with computational approaches, marrying the coarse-grained experimental data with higher-resolution in silico calculations. Here we use smFRET to determine six pairwise distances within the intrinsically disordered C-terminal domain of the troponin I subunit (TnIC) of the cardiac troponin complex. We used published conflicting structures of TnIC as starting models for molecular dynamics simulations, which were validated through successful comparison with smFRET measurements before extracting information on conformational dynamics. We find that pairwise distances between residues fluctuate widely in silico, but simulations are generally in good agreement with longer time scale smFRET measurements after averaging across time. Finally, Monte Carlo simulations establish that the lower-energy conformers of TnIC are indeed varied, but that the highest-sampled clusters resemble the published, conflicting models. In this way, we find that the controversial structures are simply stabilized local minima of this dynamic region, and a population including all three would still be consistent with spectroscopic measurements. Taken together, the combined approaches described here allow us to critically evaluate existing models of TnIC, giving insight into the conformation and dynamics of TnIC's disordered state prior to its probable disorder-order transition. Moreover, they provide a framework for combining computational and experimental methods with different time scales for the study of disordered and dynamic protein states.

In situ time-resolved FRET reveals effects of sarcomere length on cardiac thin-filament activation

During cardiac thin-filament activation, the N-domain of cardiac troponin C (N-cTnC) binds to Ca(2+) and interacts with the actomyosin inhibitory troponin I (cTnI). The interaction between N-cTnC and cTnI stabilizes the Ca(2+)-induced opening of N-cTnC and is presumed to also destabilize cTnI-actin interactions that work together with steric effects of tropomyosin to inhibit force generation. Recently, our in situ steady-state FRET measurements based on N-cTnC opening suggested that at long sarcomere length, strongly bound cross-bridges indirectly stabilize this Ca(2+)-sensitizing N-cTnC-cTnI interaction through structural effects on tropomyosin and cTnI. However, the method previously used was unable to determine whether N-cTnC opening depends on sarcomere length. In this study, we used time-resolved FRET to monitor the effects of cross-bridge state and sarcomere length on the Ca(2+)-dependent conformational behavior of N-cTnC in skinned cardiac muscle fibers. FRET donor (AEDANS) and acceptor (DDPM)-labeled double-cysteine mutant cTnC(T13C/N51C)AEDANS-DDPM was incorporated into skinned muscle fibers to monitor N-cTnC opening. To study the structural effects of sarcomere length on N-cTnC, we monitored N-cTnC opening at relaxing and saturating levels of Ca(2+) and 1.80 and 2.2-μm sarcomere length. Mg(2+)-ADP and orthovanadate were used to examine the structural effects of noncycling strong-binding and weak-binding cross-bridges, respectively. We found that the stabilizing effect of strongly bound cross-bridges on N-cTnC opening (which we interpret as transmitted through related changes in cTnI and tropomyosin) become diminished by decreases in sarcomere length. Additionally, orthovanadate blunted the effect of sarcomere length on N-cTnC conformational behavior such that weak-binding cross-bridges had no effect on N-cTnC opening at any tested [Ca(2+)] or sarcomere length. Based on our findings, we conclude that the observed sarcomere length-dependent positive feedback regulation is a key determinant in the length-dependent Ca(2+) sensitivity of myofilament activation and consequently the mechanism underlying the Frank-Starling law of the heart.

A novel phosphorylation site, Serine 199, in the C-terminus of cardiac troponin I regulates calcium sensitivity and susceptibility to calpain-induced proteolysis

Phosphorylation of cardiac troponin I (cTnI) by protein kinase C (PKC) is implicated in cardiac dysfunction. Recently, Serine 199 (Ser199) was identified as a target for PKC phosphorylation and increased Ser199 phosphorylation occurs in end-stage failing compared with non-failing human myocardium. The functional consequences of cTnI-Ser199 phosphorylation in the heart are unknown. Therefore, we investigated the impact of phosphorylation of cTnI-Ser199 on myofilament function in human cardiac tissue and the susceptibility of cTnI to proteolysis. cTnI-Ser199 was replaced by aspartic acid (199D) or alanine (199A) to mimic phosphorylation and dephosphorylation, respectively, with recombinant wild-type (Wt) cTn as a negative control. Force development was measured at various [Ca(2+)] and at sarcomere lengths of 1.8 and 2.2 μm in demembranated cardiomyocytes in which endogenous cTn complex was exchanged with the recombinant human cTn complexes. In idiopathic dilated cardiomyopathy samples, myofilament Ca(2+)-sensitivity (pCa50) at 2.2 μm was significantly higher in 199D (pCa50 = 5.79 ± 0.01) compared to 199A (pCa50 = 5.65 ± 0.01) and Wt (pCa50 = 5.66 ± 0.02) at ~63% cTn exchange. Myofilament Ca(2+)-sensitivity was significantly higher even with only 5.9 ± 2.5% 199D exchange compared to 199A, and saturated at 12.3 ± 2.6% 199D exchange. Ser199 pseudo-phosphorylation decreased cTnI binding to both actin and actin-tropomyosin. Moreover, altered susceptibility of cTnI to proteolysis by calpain I was found when Ser199 was pseudo-phosphorylated. Our data demonstrate that low levels of cTnI-Ser199 pseudo-phosphorylation (~6%) increase myofilament Ca(2+)-sensitivity in human cardiomyocytes, most likely by decreasing the binding affinity of cTnI for actin-tropomyosin. In addition, cTnI-Ser199 pseudo-phosphorylation or mutation regulates calpain I mediated proteolysis of cTnI.

Wrecked regulation of intrinsically disordered proteins in diseases: pathogenicity of deregulated regulators

Biologically active proteins without stable tertiary structure are common in all known proteomes. Functions of these intrinsically disordered proteins (IDPs) are typically related to regulation, signaling, and control. Cellular levels of these important regulators are tightly regulated by a variety mechanisms ranging from firmly controlled expression to precisely targeted degradation. Functions of IDPs are controlled by binding to specific partners, alternative splicing, and posttranslational modifications among other means. In the norm, right amounts of precisely activated IDPs have to be present in right time at right places. Wrecked regulation brings havoc to the ordered world of disordered proteins, leading to protein misfolding, misidentification, and missignaling that give rise to numerous human diseases, such as cancer, cardiovascular disease, neurodegenerative diseases, and diabetes. Among factors inducing pathogenic transformations of IDPs are various cellular mechanisms, such as chromosomal translocations, damaged splicing, altered expression, frustrated posttranslational modifications, aberrant proteolytic degradation, and defective trafficking. This review presents some of the aspects of deregulated regulation of IDPs leading to human diseases.

FRET study of the structural and kinetic effects of PKC phosphomimetic cardiac troponin T mutants on thin filament regulation

FRET was used to investigate the structural and kinetic effects that PKC phosphorylations exert on Ca(2+) and myosin subfragment-1 dependent conformational transitions of the cardiac thin filament. PKC phosphorylations of cTnT were mimicked by glutamate substitution. Ca(2+) and S1-induced distance changes between the central linker of cTnC and the switch region of cTnI (cTnI-Sr) were monitored in reconstituted thin filaments using steady state and time resolved FRET, while kinetics of structural transitions were determined using stopped flow. Thin filament Ca(2+) sensitivity was found to be significantly blunted by the presence of the cTnT(T204E) mutant, whereas pseudo-phosphorylation at additional sites increased the Ca(2+)-sensitivity. The rate of Ca(2+)-dissociation induced structural changes was decreased in the C-terminal end of cTnI-Sr in the presence of pseudo-phosphorylations while remaining unchanged at the N-terminal end of this region. Additionally, the distance between cTnI-Sr and cTnC was decreased significantly for the triple and quadruple phosphomimetic mutants cTnT(T195E/S199E/T204E) and cTnT(T195E/S199E/T204E/T285E), which correlated with the Ca(2+)-sensitivity increase seen in these same mutants. We conclude that significant changes in thin filament Ca(2+)-sensitivity, structure and kinetics are brought about through PKC phosphorylation of cTnT. These changes can either decrease or increase Ca(2+)-sensitivity and likely play an important role in cardiac regulation.

Structural basis for the in situ Ca(2+) sensitization of cardiac troponin C by positive feedback from force-generating myosin cross-bridges

The in situ structural coupling between the cardiac troponin (cTn) Ca(2+)-sensitive regulatory switch (CRS) and strong myosin cross-bridges was investigated using Förster resonance energy transfer (FRET). The double cysteine mutant cTnC(T13C/N51C) was fluorescently labeled with the FRET pair 5-(iodoacetamidoethyl)aminonaphthelene-1-sulfonic acid (IAEDENS) and N-(4-dimethylamino-3,5-dinitrophenyl)maleimide (DDPM) and then incorporated into detergent skinned left ventricular papillary fiber bundles. Ca(2+) titrations of cTnC(T13C/N51C)AEDENS/DDPM-reconstituted fibers showed that the Ca(2+)-dependence of the opening of the N-domain of cTnC (N-cTnC) statistically matched the force-Ca(2+) relationship. N-cTnC opening still occurred steeply during Ca(2+) titrations in the presence of 1mM vanadate, but the maximal extent of ensemble-averaged N-cTnC opening and the Ca(2+)-sensitivity of the CRS were significantly reduced. At nanomolar, resting Ca(2+) levels, treatment with ADP·Mg in the absence of ATP caused a partial opening of N-cTnC. During subsequent Ca(2+) titrations in the presence of ADP·Mg and absence of ATP, further N-cTnC opening was stimulated as the CRS responded to Ca(2+) with increased Ca(2+)-sensitivity and reduced steepness. These findings supported our hypothesis here that strong cross-bridge interactions with the cardiac thin filament exert a Ca(2+)-sensitizing effect on the CRS by stabilizing the interaction between the exposed hydrophobic patch of N-cTnC and the switch region of cTnI.

Structural and kinetic effects of hypertrophic cardiomyopathy related mutations R146G/Q and R163W on the regulatory switching activity of rat cardiac troponin I

Mutations in cardiac troponin I (cTnI) that cause hypertrophic cardiomyopathy (HCM) have been reported to change the contractility of cardiac myofilaments, but the underlying molecular mechanism remains elusive. In this study, Förster resonance energy transfer (FRET) was used to investigate the specific structural and kinetic effects that HCM related rat cTnI mutations R146G/Q and R163W exert on Ca(2+) and myosin S1 dependent conformational transitions in rat cTn structure. Ca(2+)-induced changes in interactions between cTnC and cTnI were individually monitored in reconstituted thin filaments using steady state and time resolved FRET, and kinetics were determined using stopped flow. R146G/Q and R163W all changed the FRET distances between cTnC and cTnI in unique and various ways. However, kinetic rates of conformational transitions induced by Ca(2+)-dissociation were universally slowed when R146G/Q and R163W were present. Interestingly, the kinetic rates of changes in the inhibitory region of cTnI were always slower than that of the regulatory region, suggesting that the fly casting mechanism that normally underlies deactivation is preserved in spite of mutation. In situ rat myocardial fiber studies also revealed that FRET distance changes indicating mutation specific disruption of the cTnIIR-actin interaction were consistent with increased passive tension.

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.

Multiple reaction monitoring to identify site-specific troponin I phosphorylated residues in the failing human heart

BACKGROUND

Human cardiac troponin I is known to be phosphorylated at multiple amino acid residues by several kinases. Advances in mass spectrometry allow sensitive detection of known and novel phosphorylation sites and measurement of the level of phosphorylation simultaneously at each site in myocardial samples.

METHODS AND RESULTS

On the basis of in silico prediction and liquid chromatography/mass spectrometry data, 14 phosphorylation sites on cardiac troponin I, including 6 novel residues (S4, S5, Y25, T50, T180, S198), were assessed in explanted hearts from end-stage heart failure transplantation patients with ischemic heart disease or idiopathic dilated cardiomyopathy and compared with samples obtained from nonfailing donor hearts (n=10 per group). Thirty mass spectrometry-based multiple reaction monitoring quantitative tryptic peptide assays were developed for each phosphorylatable and corresponding nonphosphorylated site. The results show that in heart failure there is a decrease in the extent of phosphorylation of the known protein kinase A sites (S22, S23) and other newly discovered phosphorylation sites located in the N-terminal extension of cardiac troponin I (S4, S5, Y25), an increase in phosphorylation of the protein kinase C sites (S41, S43, T142), and an increase in phosphorylation of the IT-arm domain residues (S76, T77) and C-terminal domain novel phosphorylation sites of cardiac troponin I (S165, T180, S198). In a canine dyssynchronous heart failure model, enhanced phosphorylation at 3 novel sites was found to decline toward control after resynchronization therapy.

CONCLUSIONS

Selective, functionally significant phosphorylation alterations occurred on individual residues of cardiac troponin I in heart failure, likely reflecting an imbalance in kinase/phosphatase activity. Such changes can be reversed by cardiac resynchronization.

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.

Structural dynamics of C-domain of cardiac troponin I protein in reconstituted thin filament

The regulatory function of cardiac troponin I (cTnI) involves three important contiguous regions within its C-domain: the inhibitory region (IR), the regulatory region (RR), and the mobile domain (MD). Within these regions, the dynamics of regional structure and kinetics of transitions in dynamic state are believed to facilitate regulatory signaling. This study was designed to use fluorescence anisotropy techniques to acquire steady-state and kinetic information on the dynamic state of the C-domain of cTnI in the reconstituted thin filament. A series of single cysteine cTnI mutants was generated, labeled with the fluorophore tetramethylrhodamine, and subjected to various anisotropy experiments at the thin filament level. The structure of the IR was found to be less dynamic than that of the RR and the MD, and Ca(2+) binding induced minimal changes in IR dynamics: the flexibility of the RR decreased, whereas the MD became more flexible. Anisotropy stopped-flow experiments showed that the kinetics describing the transition of the MD and RR from the Ca(2+)-bound to the Ca(2+)-free dynamic states were significantly faster (53.2-116.8 s(-1)) than that of the IR (14.1 s(-1)). Our results support the fly casting mechanism, implying that an unstructured MD with rapid dynamics and kinetics plays a critical role to initiate relaxation upon Ca(2+) dissociation by rapidly interacting with actin to promote the dissociation of the RR from the N-domain of cTnC. In contrast, the IR responds to Ca(2+) signals with slow structural dynamics and transition kinetics. The collective findings suggested a fourth state of activation.

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.

A free-energy landscape for coupled folding and binding of an intrinsically disordered protein in explicit solvent from detailed all-atom computations

The N-terminal repressor domain of neural restrictive silencer factor (NRSF) is an intrinsically disordered protein (IDP) that binds to the paired amphipathic helix (PAH) domain of mSin3. An NMR experiment revealed that the minimal binding unit of NRSF is a 15-residue segment that adopts a helical structure upon binding to a cleft of mSin3. We computed a free-energy landscape of this system by an enhanced conformational sampling method, all-atom multicanonical molecular dynamics. The simulation started from a configuration where the NRSF segment was fully disordered and distant from mSin3 in explicit solvent. In the absence of mSin3, the disordered NRSF segment thermally fluctuated between hairpins, helices, and bent structures. In the presence of mSin3, the segment bound to mSin3 by adopting the structures involved in the isolated state, and non-native and native complexes were formed. The free-energy landscape comprised three superclusters, and free-energy barriers separated the superclusters. The native complex was located at the center of the lowest free-energy cluster. When NRSF landed in the largest supercluster, the generated non-native complex moved on the landscape to fold into the native complex, by increasing the interfacial hydrophobic contacts and the helix content. When NRSF landed in other superclusters, the non-native complex overcame the free-energy barriers between the various segment orientations in the binding cleft of mSin3. Both population-shift and induced-fit (or induced-folding) mechanisms work cooperatively in the coupled folding and binding. The diverse structural adaptability of NRSF may be related to the hub properties of the IDP.

Thin filament mutations: developing an integrative approach to a complex disorder

Sixteen years ago, mutations in cardiac troponin (Tn)T and α-tropomyosin were linked to familial hypertrophic cardiomyopathy, thus transforming the disorder from a disease of the β-myosin heavy chain to a disease of the cardiac sarcomere. From the outset, studies suggested that mutations in the regulatory thin filament caused a complex, heterogeneous pattern of ventricular remodeling with wide variations in clinical expression. To date, the clinical heterogeneity is well matched by an extensive array of nearly 100 independent mutations in all components of the cardiac thin filament. Significant advances in our understanding of the biophysics of myofilament activation, coupled to the emerging evidence that thin filament linked cardiomyopathies are progressive, suggests that a renewed focus on the most proximal events in both the molecular and clinical pathogenesis of the disease will be necessary to achieve the central goal of using genotype information to manage affected patients. In this review, we examine the existing biophysical and clinical evidence in support of a more proximal definition of thin filament cardiomyopathies. In addition, new high-resolution, integrated approaches are presented to help define the way forward as the field works toward developing a more robust link between genotype and phenotype in this complex disorder.

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.

The C terminus of cardiac troponin I stabilizes the Ca2+-activated state of tropomyosin on actin filaments

RATIONALE

Ca(2+) control of troponin-tropomyosin position on actin regulates cardiac muscle contraction. The inhibitory subunit of troponin, cardiac troponin (cTn)I is primarily responsible for maintaining a tropomyosin conformation that prevents crossbridge cycling. Despite extensive characterization of cTnI, the precise role of its C-terminal domain (residues 193 to 210) is unclear. Mutations within this region are associated with restrictive cardiomyopathy, and C-terminal deletion of cTnI, in some species, has been associated with myocardial stunning.

OBJECTIVE

We sought to investigate the effect of a cTnI deletion-removal of 17 amino acids from the C terminus- on the structure of troponin-regulated tropomyosin bound to actin.

METHODS AND RESULTS

A truncated form of human cTnI (cTnI(1-192)) was expressed and reconstituted with troponin C and troponin T to form a mutant troponin. Using electron microscopy and 3D image reconstruction, we show that the mutant troponin perturbs the positional equilibrium dynamics of tropomyosin in the presence of Ca(2+). Specifically, it biases tropomyosin position toward an "enhanced C-state" that exposes more of the myosin-binding site on actin than found with wild-type troponin.

CONCLUSIONS

In addition to its well-established role of promoting the so-called "blocked-state" or "B-state," cTnI participates in proper stabilization of tropomyosin in the "Ca(2+)-activated state" or "C-state." The last 17 amino acids perform this stabilizing role. The data are consistent with a "fly-casting" model in which the mobile C terminus of cTnI ensures proper conformational switching of troponin-tropomyosin. Loss of actin-sensing function within this domain, by pathological proteolysis or cardiomyopathic mutation, may be sufficient to perturb tropomyosin conformation.

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.

Kinetic advantage of intrinsically disordered proteins in coupled folding-binding process: a critical assessment of the "fly-casting" mechanism

Intrinsically disordered proteins (IDPs) are recognized to play important roles in many biological functions such as transcription and translation regulation, cellular signal transduction, protein phosphorylation, and molecular assemblies. The coupling of folding with binding through a "fly-casting" mechanism has been proposed to account for the fast binding kinetics of IDPs. In this article, experimental data from the literature were collated to verify the kinetic advantages of IDPs, while molecular simulations were performed to clarify the origin of the kinetic advantages. The phosphorylated KID-kinase-inducible domain interacting domain (KIX) complex was used as an example in the simulations. By modifying a coarse-grained model with a native-centric Gō-like potential, we were able to continuously tune the degree of disorder of the phosphorylated KID domain and thus investigate the intrinsic role of chain flexibility in binding kinetics. The simulations show that the "fly-casting" effect is not only due to the greater capture radii of IDPs. The coupling of folding with binding of IDPs leads to a significant reduction in binding free-energy barrier. Such a reduction accelerates the binding process. Although the greater capture radius has been regarded as the main factor in promoting the binding rate of IDPs, we found that this parameter will also lead to the slower translational diffusion of IDPs when compared with ordered proteins. As a result, the capture rate of IDPs was found to be slower than that of ordered proteins. The main origin of the faster binding for IDPs are the fewer encounter times required before the formation of the final binding complex. The roles of the interchain native contacts fraction (Q(b)) and the mass-center distance (DeltaR) as reaction coordinates are also discussed.

Size-exclusion chromatography can identify faster-associating protein complexes and evaluate design strategies

BACKGROUND

We previously developed a set of rationally designed mutant MICA protein ligands for the NKG2D immunoreceptor in which MICA was mutated at residues that do not contact NKG2D. Some of these MICA mutants, predicted by RosettaDesign to be destabilized, bound NKG2D with affinities enhanced by more than an order of magnitude when evaluated by surface plasmon resonance (SPR).

FINDINGS

Small-zone size-exclusion chromatography (SEC) detected persistent high-affinity MICA mutant-NKG2D complexes in solution as early-eluting peaks. The SEC binding assay used standard protein purification instrumentation to evaluate complex stability, qualitatively paralleled the SPR results, and successfully discriminated among complexes that differed only in on-rates. We used the SEC binding assay, along with SPR, to assess the results of a follow-up design strategy targeting the non-interfacial redesigned region. Both SEC and SPR agreed that these mutations did not enhance affinity as much as previous mutants. When the SEC binding assay was run in 1 M urea, only the highest affinity complex was detected.

CONCLUSION

This SEC binding assay provides a correlation with SPR results for protein complex affinities, detecting changes in complex on-rates, and tunable to lower sensitivity with 1 M urea. The SEC binding assay is complementary to other protein design evaluation methods, can be adapted to the undergraduate research laboratory, and may provide additional structural information about changes in hydrodynamic radii from elution times. Our assay allowed us to conclude that further alteration of MICA at non-contacting residues is unlikely to further enhance NKG2D affinity.

A functional and structural study of troponin C mutations related to hypertrophic cardiomyopathy

Recently four new hypertrophic cardiomyopathy mutations in cardiac troponin C (cTnC) (A8V, C84Y, E134D, and D145E) were reported, and their effects on the Ca(2+) sensitivity of force development were evaluated (Landstrom, A. P., Parvatiyar, M. S., Pinto, J. R., Marquardt, M. L., Bos, J. M., Tester, D. J., Ommen, S. R., Potter, J. D., and Ackerman, M. J. (2008) J. Mol. Cell. Cardiol. 45, 281-288). We performed actomyosin ATPase and spectroscopic solution studies to investigate the molecular properties of these mutations. Actomyosin ATPase activity was measured as a function of [Ca(2+)] utilizing reconstituted thin filaments (TFs) with 50% mutant and 50% wild type (WT) and 100% mutant cardiac troponin (cTn) complexes: A8V, C84Y, and D145E increased the Ca(2+) sensitivity with only A8V demonstrating lowered Ca(2+) sensitization at the 50% ratio when compared with 100%; E134D was the same as WT at both ratios. Of these four mutants, only D145E showed increased ATPase activation in the presence of Ca(2+). None of the mutants affected ATPase inhibition or the binding of cTn to the TF measured by co-sedimentation. Only D145E increased the Ca(2+) affinity of site II measured by 2-(4'-(2''-iodoacetamido)phenyl)aminonaphthalene-6-sulfonic acid fluorescence in isolated cTnC or the cTn complex. In the presence of the TF, only A8V was further sensitized to Ca(2+). Circular dichroism measurements in different metal-bound states of the isolated cTnCs showed changes in the secondary structure of A8V, C84Y, and D145E, whereas E134D was the same as WT. PyMol modeling of each cTnC mutant within the cTn complex revealed potential for local changes in the tertiary structure of A8V, C84Y, and D145E. Our results indicate that 1) three of the hypertrophic cardiomyopathy cTnC mutants increased the Ca(2+) sensitivity of the myofilament; 2) the effects of the mutations on the Ca(2+) affinity of isolated cTnC, cTn, and TF are not sufficient to explain the large Ca(2+) sensitivity changes seen in reconstituted and fiber assays; and 3) changes in the secondary structure of the cTnC mutants may contribute to modified protein-protein interactions along the sarcomere lattice disrupting the coupling between the cross-bridge and Ca(2+) binding to cTnC.

Some cardiomyopathy-causing troponin I mutations stabilize a functional intermediate actin state

We examined four cardiomyopathy-causing mutations of troponin I that appear to disturb function by altering the distribution of thin filament states. The R193H (mouse) troponin I mutant had greater than normal actin-activated myosin-S1 ATPase activity in both the presence and absence of calcium. The rate of ATPase activity was the same as that of the wild-type at near-saturating concentrations of the activator, N-ethylmaleimide-S1. This mutant appeared to function by stabilizing the active state of thin filaments. Mutations D191H, R146G, and R146W had lower ATPase activities in the presence of calcium, but higher activities in the absence of calcium. These effects were most pronounced with mutations at position 146. For all three mutants the rates were similar to those of the wild-type at near-saturating concentrations of N-ethylmaleimide-S1. These results, combined with previous results, show that any alteration in the normal distribution of actomyosin states is capable of producing cardiomyopathy. The results of the D191H, R146G, and R146W mutations are most readily explained if the intermediate state of regulated actin has a unique function. The intermediate state appears to have an ability to accelerate the rate of ATP hydrolysis by myosin that exceeds that of the inactive state.

Förster resonance energy transfer structural kinetic studies of cardiac thin filament deactivation

Cardiac thin filament deactivation is initiated by Ca2+ dissociation from troponin C (cTnC), followed by multiple structural changes of thin filament proteins. These structural transitions are the molecular basis underlying the thin filament regulation of cardiac relaxation, but the detailed mechanism remains elusive. In this study Förster resonance energy transfer (FRET) was used to investigate the dynamics and kinetics of the Ca2+-induced conformational changes of the cardiac thin filaments, specifically the closing of the cTnC N-domain, the cTnC-cTnI (troponin I) interaction, and the cTnI-actin interaction. The cTnC N-domain conformational change was examined by monitoring FRET between a donor (AEDANS) attached to one cysteine residue and an acceptor (DDPM) attached the other cysteine of the mutant cTnC(L13C/N51C). The cTnC-cTnI interaction was investigated by monitoring the distance changes from residue 89 of cTnC to residues 151 and 167 of cTnI, respectively. The cTnI-actin interaction was investigated by monitoring the distance changes from residues 151 and 167 of cTnI to residue 374 of actin. FRET Ca2+ titrations and stopped-flow kinetic measurements show that different thin filament structural transitions have different Ca2+ sensitivities and Ca2+ dissociation-induced kinetics. The observed structural transitions involving the regulatory region and the mobile domain of cTnI occurred at fast kinetic rates, whereas the kinetics of the structural transitions involving the cTnI inhibitory region was slow. Our results suggest that the thin filament deactivation upon Ca2+ dissociation is a two-step process. One step involves rapid binding of the mobile domain of cTnI to actin, which is kinetically coupled with the conformational change of the N-domain of cTnC and the dissociation of the regulatory region of cTnI from cTnC. The other step involves switching the inhibitory region of cTnI from interacting with cTnC to interacting with actin. The latter processes may play a key role in regulating cross-bridge kinetics.

Isoform-specific variation in the intrinsic disorder of troponin I

Various intrinsic disorder (ID) prediction algorithms were applied to the three tissue isoforms of troponin I (TnI). The results were interpreted in terms of the known structure and dynamics of troponin. In line with previous results, all isoforms of TnI were predicted to have large stretches of ID. The predictions show that the C-termini of all isoforms are extensively disordered as is the N-terminal extension of the cardiac isoform. Cardiac TnI likely belongs to the group of intrinsically disordered signalling hub proteins. For a given portion of the protein sequence, most ID prediction approaches indicate isoform-dependent variations in the probability of disorder. Comparison of machine learning and physically based approaches suggests the ID variations are only partially attributable to local variations in the ratio of charged to hydrophobic residues. The VSL2B algorithm predicts the largest variations in ID across the isoforms, with the cardiac isoform having the highest probability of structured regions, and the fast-skeletal isoform having no intrinsic structure. The region corresponding to residues 57-95 of the fast-skeletal isoform, known to form a coiled coil substructure with troponin T, was highly variable between isoforms. The isoform-specific ID variations may have mechanistic significance, modulating the extent to which conformational fluctuations in tropomyosin are communicated to the troponin complex. We discuss structural mechanisms for this communication. Overall, the results motivate the development of predictors designed to address relative levels of disorder between highly similar proteins.

Structural studies of interactions between cardiac troponin I and actin in regulated thin filament using Förster resonance energy transfer

The Ca(2+)-induced interaction between cardiac troponin I (cTnI) and actin plays a key role in the regulation of cardiac muscle contraction and relaxation. In this report we have investigated changes of this interaction in response to strong cross-bridge formation between myosin S1 and actin and PKA phosphorylation of cTnI within reconstituted thin filament. The interaction was monitored by measuring Förster resonance energy transfer (FRET) between the fluorescent donor 5-(iodoacetamidoethyl)aminonaphthalene-1-sulfonic acid (AEDANS) attached to the residues 131, 151, 160 167, 188, and 210 of cTnI and the nonfluorescent acceptor 4-(dimethylamino)phenylazophenyl-4'-maleimide (DABM) attached to cysteine 374 of actin. The FRET distance measurements showed that bound Ca(2+) induced large increases in the distances from actin to the cTnI sites, indicating a Ca(2+)-triggered separation of cTnI from actin. Strongly bound myosin S1 induced additional increases in these distances in the presence of bound Ca(2+). The two ligand-induced increases were independent of each other. These two-step changes in distances provide a direct link of structural changes at the interface between cTnI and actin to the three-state model of thin filament regulation of muscle contraction and relaxation. When cTnC was inactivated through mutations of key residues within the 12-residue Ca(2+)-binding loop, strongly bound S1 alone induced increases in the distances in spite of the fact that the filaments no longer bound regulatory Ca(2+). These results suggest bound Ca(2+) or strongly bound S1 alone can partially activate thin filament, but full activation requires both bound Ca(2+) and strongly bound S1. The distributions of the FRET distances revealed different structural dynamics associated with different regions of cTnI in different biochemical states. The second actin-binding region appears more rigid than the inhibitory/regulatory region. In the Mg(2+) state, the regulatory region appears more flexible than the inhibitory region, and in the Ca(2+) state the inhibitory region becomes more flexible. PKA phosphorylation of cTnI at Ser23 and Ser24 distance from actin to cTnI residue 131 by 2.2-5.2 A in different biochemical states and narrowed the distributions of the distances from actin to the inhibitory and regulatory regions of cTnI. The observed phosphorylation effects are likely due to an intramolecular interaction of the phosphorylated N-terminal segment and the inhibitory region of cTnI.