Structural basis for the regulation of muscle contraction by troponin and tropomyosin

Sarcomere control mechanisms and the dynamics of the cardiac cycle

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

Why does troponin I have so many phosphorylation sites? Fact and fancy

We discuss a current controversy regarding the relative role of phosphorylation sites on cardiac troponin I (cTnI) (Fig. 1) in physiological and patho-physiological cardiac function. Studies with mouse models and in vitro studies indicate that multi-site phosphorylations are involved in both control of maximum tension and sarcomeric responsiveness to Ca(2+). Thus one hypothesis is that cardiac function reflects a balance of cTnI phosphorylations and a tilt in this balance may be maladaptive in acquired and genetic disorders of the heart. Studies on human heart samples taken mainly at end-stage heart failure, and in depth proteomic analysis of human and rat heart samples demonstrate that Ser23/Ser24 are the major and perhaps the only sites likely to be relevant to control cardiac function. Thus functional significance of Ser23/Ser24 phosphorylation is taken as fact, whereas the function of some other sites is treated as fancy. Maybe the extremes will meet: in any case we both agree that further work needs to be carried out with relatively large mammals and with determination of the time course of changes in phosphorylation to identify transient modifications that may be relevant at a beat-to-beat basis. Moreover, we agree that the changes and effects of cTnI phosphorylation need to be fully integrated into the effects of other phosphorylations in the cardiac myocyte.

X-ray fiber diffraction modeling of structural changes of the thin filament upon activation of live vertebrate skeletal muscles

In order to clarify the structural changes of the thin filaments related to the regulation mechanism in skeletal muscle contraction, the intensities of thin filament-based reflections in the X-ray fiber diffraction patterns from live frog skeletal muscles at non-filament overlap length were investigated in the relaxed state and upon activation. Modeling the structural changes of the whole thin filament due to Ca²⁺-activation was systematically performed using the crystallographic data of constituent molecules (actin, tropomyosin and troponin core domain) as starting points in order to determine the structural changes of the regulatory proteins and actin. The results showed that the globular core domain of troponin moved toward the filament axis by ∼6 Å and rotated by ∼16° anticlockwise (viewed from the pointed end) around the filament axis by Ca²⁺-binding to troponin C, and that tropomyosin together with the tail of troponin T moved azimuthally toward the inner domains of actin by ∼12° and radially by ∼7 Å from the relaxed position possibly to partially open the myosin binding region of actin. The domain structure of the actin molecule in F-actin we obtained for frog muscle thin filament was slightly different from that of the Holmes F-actin model in the relaxed state, and upon activation, all subdomains of actin moved in the direction to closing the nucleotide-binding pocket, making the actin molecule more compact. We suggest that the troponin movements and the structural changes within actin molecule upon activation are also crucial components of the regulation mechanism in addition to the steric blocking movement of tropomyosin.

Regulation of oscillatory contraction in insect flight muscle by troponin

Insect indirect flight muscle is activated by sinusoidal length change, which enables the muscle to work at high frequencies, and contracts isometrically in response to Ca(2+). Indirect flight muscle has two TnC isoforms: F1 binding a single Ca(2+) in the C-domain, and F2 binding Ca(2+) in the N- and C-domains. Fibres substituted with F1 produce delayed force in response to a single rapid stretch, and those with F2 produce isometric force in response to Ca(2+). We have studied the effect of TnC isoforms on oscillatory work. In native Lethocerus indicus fibres, oscillatory work was superimposed on a level of isometric force that depended on Ca(2+) concentration. Maximum work was produced at pCa 6.1; at higher concentrations, work decreased as isometric force increased. In fibres substituted with F1 alone, work continued to rise as Ca(2+) was increased up to pCa 4.7. Fibres substituted with various F1:F2 ratios produced maximal work at a ratio of 100:1 or 50:1; a higher proportion of F2 increased isometric force at the expense of oscillatory work. The F1:F2 ratio was 9.8:1 in native fibres, as measured by immunofluorescence, using isoform-specific antibodies. The small amount of F2 needed to restore work to levels obtained for the native fibre is likely to be due to the relative affinity of F1 and F2 for TnH, the Lethocerus homologue of TnI. Affinity of TnC isoforms for a TnI fragment of TnH was measured by isothermal titration calorimetry. The K(d) was 1.01 muM for F1 binding and 22.7 nM for F2. The higher affinity of F2 can be attributed to two TnH binding sites on F2 and a single site on F1. Stretch may be sensed by an extended C-terminal domain of TnH, resulting in reversible dissociation of the inhibitory sequence from actin during the oscillatory cycle.

Kinetics of cardiac sarcomeric processes and rate-limiting steps in contraction and relaxation

The sarcomere is the core structure responsible for active mechanical heart function. It is formed primarily by myosin, actin, and titin filaments. Cyclic interactions occur between the cross-bridges of the myosin filaments and the actin filaments. The forces generated by these cyclic interactions provide the molecular basis for cardiac pressure, while the motion produced by these interactions provides the basis for ejection. The cross-bridge cycle is controlled by upstream mechanisms located in the membrane and by downstream mechanisms inside the sarcomere itself. These downstream mechanisms involve the Ca(2+)-controlled conformational change of the regulatory proteins troponin and tropomyosin and strong cooperative interactions between neighboring troponin-tropomyosin units along the actin filament. The kinetics of upstream and downstream processes have been measured in intact and demembranated myocardial preparations. This review outlines a conceptual model of the timing of these processes during the individual mechanical heart phases. Particular focus is given to kinetic data from studies on contraction-relaxation cycles under mechanical loads. Evidence is discussed that the dynamics of cardiac contraction and relaxation are determined mainly by sarcomeric downstream mechanisms, in particular by the kinetics of the cross-bridge cycle. The rate and extent of ventricular pressure development is essentially subjected to the mechanistic principles of cross-bridge action and its upstream and downstream regulation. Sarcomere relengthening during myocardial relaxation plays a key role in the rapid decay of ventricular pressure and in early diastolic filling.

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.

Striated muscle regulation of isometric tension by multiple equilibria

Cooperative activation of striated muscle by calcium is based on the movement of tropomyosin described by the steric blocking theory of muscle contraction. Presently, the Hill model stands alone in reproducing both myosin binding data and a sigmoidal-shaped curve characteristic of calcium activation (Hill TL (1983) Two elementary models for the regulation of skeletal muscle contraction by calcium. Biophys J 44: 383–396.). However, the free myosin is assumed to be fixed by the muscle lattice and the cooperative mechanism is based on calcium-dependent interactions between nearest neighbor tropomyosin subunits, which has yet to be validated. As a result, no comprehensive model has been shown capable of fitting actual tension data from striated muscle. We show how variable free myosin is a selective advantage for activating the muscle and describe a mechanism by which a conformational change in tropomyosin propagates free myosin given constant total myosin. This mechanism requires actin, tropomyosin, and filamentous myosin but is independent of troponin. Hence, it will work equally well with striated, smooth and non-muscle contractile systems. Results of simulations with and without data are consistent with a strand of tropomyosin composed of ∼20 subunits being moved by the concerted action of 3–5 myosin heads, which compares favorably with the predicted length of tropomyosin in the overlap region of thick and thin filaments. We demonstrate that our model fits both equilibrium myosin binding data and steady-state calcium-dependent tension data and show how both the steepness of the response and the sensitivity to calcium can be regulated by the actin-troponin interaction. The model simulates non-cooperative calcium binding both in the presence and absence of strong binding myosin as has been observed. Thus, a comprehensive model based on three well-described interactions with actin, namely, actin-troponin, actin-tropomyosin, and actin-myosin can explain the cooperative calcium activation of striated muscle.

Docking of calcium ions in proteins with flexible side chains and deformable backbones

A method of docking Ca(2+) ions in proteins with flexible side chains and deformable backbones is proposed. The energy was calculated with the AMBER force field, implicit solvent, and solvent exposure-dependent and distance-dependent dielectric function. Starting structures were generated with Ca(2+) coordinates and side-chain torsions sampled in 1000 A(3) cubes centered at the experimental Ca(2+) positions. The energy was Monte Carlo-minimized. The method was tested on fourteen Ca(2+)-binding sites. For twelve Ca(2+)-binding sites the root mean square (RMS) deviation of the apparent global minimum from the experimental structure was below 1.3 and 1.7 A for Ca(2+) ions and side-chain heavy atoms, respectively. Energies of multiple local minima correlate with the RMS deviations from the X-ray structures. Two Ca(2+)-binding sites at the surface of proteinase K were not predicted, because of underestimation of Ca(2+) hydration energy by the implicit-solvent method.

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

Muscle contraction is tightly regulated by Ca(2+) binding to the thin filament protein troponin. The mechanism of this regulation was investigated by detailed mapping of the dynamic properties of cardiac troponin using amide hydrogen exchange-mass spectrometry. Results were obtained in the presence of either saturation or non-saturation of the regulatory Ca(2+) binding site in the NH(2) domain of subunit TnC. Troponin was found to be highly dynamic, with 60% of amides exchanging H for D within seconds of exposure to D(2)O. In contrast, portions of the TnT-TnI coiled-coil exhibited high protection from exchange, despite 6 h in D(2)O. The data indicate that the most stable portion of the trimeric troponin complex is the coiled-coil. Regulatory site Ca(2+) binding altered dynamic properties (i.e. H/D exchange protection) locally, near the binding site and in the TnI switch helix that attaches to the Ca(2+)-saturated TnC NH(2) domain. More notably, Ca(2+) also altered the dynamic properties of other parts of troponin: the TnI inhibitory peptide region that binds to actin, the TnT-TnI coiled-coil, and the TnC COOH domain that contains the regulatory Ca(2+) sites in many invertebrate as opposed to vertebrate troponins. Mapping of these affected regions onto the troponin highly extended structure suggests that cardiac troponin switches between alternative sets of intramolecular interactions, similar to previous intermediate resolution x-ray data of skeletal muscle troponin.

Sarcolipin and ubiquitin carboxy-terminal hydrolase 1 mRNAs are over-expressed in skeletal muscles of alpha-tocopherol deficient mice

The transcriptome of ataxic muscles from alpha-tocopherol transfer protein deficient (ATTP-KO), 23-month old, mice was compared with that of their normal littermates. Genes encoding sarcolipin (sln) and ubiquitin carboxyl-terminal hydrolase (uchl1) were over-expressed (> or =10-fold) in ataxic muscles. SLN is a 3.2 kDa membrane protein that binds to sarcoplasmic reticulum calcium ATPase, regulates Ca(+ +) transport and muscle relaxation-contraction cycles. UCHL1 is a 24.8 kDa member of proteosome proteins; it is over-expressed in myofibrillar myopathy and is associated with neurodegenerative diseases. Furthermore, six additional transcripts, three encoding thin-filament proteins and three encoding Ca(+ +) sensing proteins that participate in contraction-relaxation cycle, and eight transcripts that encode members of lysosomal proteins were also over-expressed in ataxic muscles. These observations suggest that chronic alpha-tocopherol (AT) deficiency activates critical genes of muscle contractility and protein degradation pathways, simultaneously. The magnitude of induction of sln and uchl1 was lower in asymptomatic, 8-month old, ATTP-KO mice and in 8-month old mice fed an AT-depleted diet. These studies suggest sln and uchl1 genes as novel targets of AT deficiency and may offer molecular correlates of well documented descriptions of neuromuscular dysfunctions in AT-deficient rodents. Since the neuromuscular deficits of ATTP-KO mice appear to be similar to those of patients with ATTP mutations, it is suggested that over-expression of sln and uchl1 may also contribute to AT-sensitive ataxia in humans.

Human actin mutations associated with hypertrophic and dilated cardiomyopathies demonstrate distinct thin filament regulatory properties in vitro

Two cardiomyopathic mutations were expressed in human cardiac actin, using a Baculovirus/insect cell system; E99K is associated with hypertrophic cardiomyopathy whereas R312H is associated with dilated cardiomyopathy. The hypothesis that the divergent phenotypes of these two cardiomyopathies are associated with fundamental differences in the molecular mechanics and thin filament regulation of the underlying actin mutation was tested using the in vitro motility and laser trap assays. In the presence of troponin (Tn) and tropomyosin (Tm), beta-cardiac myosin moved both E99K and R312H thin filaments at significantly (p<0.05) slower velocities than wild type (WT) at maximal Ca(++). At submaximal Ca(++), R312H thin filaments demonstrated significantly increased Ca(++) sensitivity (pCa(50)) when compared to WT. Velocity as a function of ATP concentration revealed similar ATP binding rates but slowed ADP release rates for the two actin mutants compared to WT. Single molecule laser trap experiments performed using both unregulated (i.e. actin) and regulated thin filaments in the absence of Ca(++) revealed that neither actin mutation significantly affected the myosin's unitary step size (d) or duration of strong actin binding (t(on)) at 20 microM ATP. However, the frequency of individual strong-binding events in the presence of Tn and Tm, was significantly lower for E99K than WT at comparable myosin surface concentrations. The cooperativity of a second myosin head binding to the thin filament was also impaired by E99K. In conclusion, E99K inhibits the activation of the thin filament by myosin strong-binding whereas R312H demonstrates enhanced calcium activation.

Identification and functional characterization of cardiac troponin I as a novel disease gene in autosomal dominant dilated cardiomyopathy

RATIONALE

Idiopathic dilated cardiomyopathy (DCM) is inherited in approximately one third of cases, usually as an autosomal dominant trait. More than 30 loci have been identified, several of which encode sarcomeric proteins which can also be mutated to cause hypertrophic cardiomyopathy. One contractile protein gene well known as a hypertrophic cardiomyopathy disease gene, but with no reported mutation in autosomal dominant DCM, is TNNI3 which encodes cardiac troponin I.

OBJECTIVE

To test TNNI3 as a candidate gene, a panel of 96 probands with DCM was analyzed.

METHODS AND RESULTS

Genomic DNA was isolated and TNNI3 exons screened by heteroduplex analysis. Exons with aberrant profiles were sequenced and variants evaluated by segregation analysis and study of normal controls. We report 2 novel TNNI3 missense mutations, Lys36Gln and Asn185Lys, each associated with severe and early onset familial DCM. Of the 5 mutation carriers, cardiac transplantation was required in 3, at ages 6, 15, and 24 years. Analysis of Ca(2+) regulation of actin-tropomyosin-activated myosin ATPase by troponin revealed that troponin reconstituted with either mutant troponin I gave lower maximum ATPase rates and lower Ca(2+) sensitivity than wild type. Furthermore, mutant thin filaments had reduced Ca(2+) affinity compared with normal.

CONCLUSIONS

The functional alterations mirror closely a consistent phenotype found in proven DCM mutations in other thin filament proteins, thus supporting the interpretation that these mutations are disease-causing. These are the first reported autosomal dominant DCM-causing mutations in TNNI3, and so the findings expand the spectrum of disease-causing genes that lead to either hypertrophic cardiomyopathy or DCM depending on the specific mutation.

Ala scanning of the inhibitory region of cardiac troponin I

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

An internal domain of beta-tropomyosin increases myofilament Ca(2+) sensitivity

Tropomyosin (TM) is involved in Ca(2+)-mediated muscle contraction and relaxation in the heart. Striated muscle alpha-TM is the major isoform expressed in the heart. The expression of striated muscle beta-TM in the murine myocardium results in a decreased rate of relaxation and increased myofilament Ca(2+) sensitivity. Replacing the carboxyl terminus (amino acids 258-284) of alpha-TM with beta-TM (a troponin T-binding region) results in decreased rates of contraction and relaxation in the heart and decreased myofilament Ca(2+) sensitivity. We hypothesized that the putative internal troponin T-binding domain (amino acids 175-190) of beta-TM may be responsible for the increased myofilament Ca(2+) sensitivity observed when the entire beta-TM is expressed in the heart. To test this hypothesis, we generated transgenic mice that expressed chimeric TM containing beta-TM amino acids 175-190 in the backbone of alpha-TM (amino acids 1-174 and 191-284). These mice expressed 16-57% chimeric TM and did not develop cardiac hypertrophy or any other morphological changes. Physiological analysis showed that these hearts exhibited decreased rates of contraction and relaxation and a positive response to isoproterenol. Skinned fiber bundle analyses showed a significant increase in myofilament Ca(2+) sensitivity. Biophysical experiments demonstrated that the exchanged amino acids did not influence the flexibility of the TM. This is the first study to demonstrate that a specific domain within TM can increase the Ca(2+) sensitivity of the thin filament and affect sarcomeric performance. Furthermore, these results enhance the understanding of why TM mutations associated with familial hypertrophic cardiomyopathy demonstrate increased myofilament sensitivity to Ca(2+).

Differential effects of phosphorylation of regions of troponin I in modifying cooperative activation of cardiac thin filaments

Ischemia and heart failure are associated with protein kinase C (PKC) dependent phosphorylation of cardiac troponin I (cTnI). We investigated the effect of phosphorylation of cTnI PKC sites S43, S45 and T144 under normal (pH 7.0) and acidic (pH 6.5) conditions on tension in skinned fiber bundles from a mouse heart. To mimic the PKC phosphorylation, we exchanged troponin (cTn) in these fiber bundles with cTn complex containing either cTnI-(S43E/S45E) or cTnI-(T144E). We determined how pseudo-phosphorylation and acidic pH affect activation of thin filaments by strongly bound crossbridges by use of n-ethyl maleimide (NEM-S1) to mimic rigor. We hypothesized that PKC phosphorylation of cTnI amplifies the effect of ischemic/hypoxic conditions to depress myofilament force and Ca(2+)-responsiveness by reducing the ability of rigor crossbridge to activate force. Pseudo-phosphorylation of cTnI at S43/S45 exacerbated the effect of acidic pH to induce a rightward shift in the Ca(2+)-tension relation. Under acidic conditions, fibers regulated by cTnI-(S43E/S45E) demonstrated a significant reduction in the ability of NEM-S1 to recruit cycling crossbridges, when compared to controls regulated by cTnI. Similar effects of pseudo-phosphorylation of cTnI-(T144) occurred, but to a lesser extent that those of pseudo-phosphorylation of S43/S45. We conclude that under acidic conditions PKC phosphorylation of cTnI residues at S43/S45 and at T144 is likely to have differential, but significant effects in depressing the ability of both Ca(2+) and rigor crossbridges to activate force generation. Although these effects of PKC dependent phosphorylation may be maladaptive in heart failure, they may also spare ATP consumption and be cardio-protective in ischemia.

Ca2+-dependent photocrosslinking of tropomyosin residue 146 to residues 157-163 in the C-terminal domain of troponin I in reconstituted skeletal muscle thin filaments

The Ca(2+)-dependent interaction of troponin I (TnI) with actin.tropomyosin (Tm) in muscle thin filaments is a critical step in the regulation of muscle contraction. Previous studies have suggested that, in the absence of Ca(2+), TnI interacts with Tm and actin in reconstituted muscle thin filaments, maintaining Tm at the outer domain of actin and blocking myosin-actin interaction. To obtain direct evidence for this Tm-TnI interaction, we performed photochemical crosslinking studies using Tm labeled with 4-maleimidobenzophenone at position 146 or 174 (Tm*146 or Tm*174, respectively), reconstituted with actin and troponin [composed of TnI, troponin T (TnT), and troponin C] or with actin and TnI. After near-UV irradiation, SDS gels of the Tm*146-containing thin filament showed three new high-molecular-weight bands determined to be crosslinked products Tm*146-TnI, Tm*146-troponin C, and Tm*146-TnT using fluorescence-labeled TnI, mass spectrometry, and Western blot analysis. While Tm*146-TnI was produced only in the absence of Ca(2+), the production of other crosslinked species did not show Ca(2+) dependence. Tm*174 mainly crosslinked to TnT. In the absence of actin, a similar crosslinking pattern was obtained with a much lower yield. A tryptic peptide from Tm*146-TnI with a molecular mass of 2601.2 Da that was not present in the tryptic peptides of Tm*146 or TnI was identified using HPLC and matrix-assisted laser desorption/ionization time-of-flight. This was shown, using absorption and fluorescence spectroscopy, to be the 4-maleimidobenzophenone-labeled peptide from Tm crosslinked to TnI peptide 157-163. These data, which show that a region in the C-terminal domain of TnI interacts with Tm in the absence of Ca(2+), support the hypothesis that a TnI-Tm interaction maintains Tm at the outer domain of actin and will help efforts to localize troponin in actin.Tm muscle thin filaments.

Structural basis for the activation of muscle contraction by troponin and tropomyosin

The molecular regulation of striated muscle contraction couples the binding and dissociation of Ca(2+) on troponin (Tn) to the movement of tropomyosin on actin filaments. In turn, this process exposes or blocks myosin binding sites on actin, thereby controlling myosin crossbridge dynamics and consequently muscle contraction. Using 3D electron microscopy, we recently provided structural evidence that a C-terminal extension of TnI is anchored on actin at low Ca(2+) and competes with tropomyosin for a common site to drive tropomyosin to the B-state location, a constrained, relaxing position on actin that inhibits myosin-crossbridge association. Here, we show that release of this constraint at high Ca(2+) allows a second segment of troponin, probably representing parts of TnT or the troponin core domain, to promote tropomyosin movement on actin to the Ca(2+)-induced C-state location. With tropomyosin stabilized in this position, myosin binding interactions can begin. Tropomyosin appears to oscillate to a higher degree between respective B- and C-state positions on troponin-free filaments than on fully regulated filaments, suggesting that tropomyosin positioning in both states is troponin-dependent. By biasing tropomyosin to either of these two positions, troponin appears to have two distinct structural functions; in relaxed muscles at low Ca(2+), troponin operates as an inhibitor, while in activated muscles at high Ca(2+), it acts as a promoter to initiate contraction.

Structure and orientation of troponin in the thin filament

The troponin complex on the thin filament plays a crucial role in the regulation of muscle contraction. However, the precise location of troponin relative to actin and tropomyosin remains uncertain. We have developed a method of reconstructing thin filaments using single particle analysis that does not impose the helical symmetry of actin and is independent of a starting model. We present a single particle three-dimensional reconstruction of the thin filament. Atomic models of the F-actin filament were fitted into the electron density maps and troponin and tropomyosin located. The structure provides evidence that the globular head region of troponin labels the two strands of actin with a 27.5-Å axial stagger. The density attributed to troponin appears tapered with the widest point toward the barbed end. This leads us to interpret the polarity of the troponin complex in the thin filament as reversed with respect to the widely accepted model.

The myosin-binding protein C motif binds to F-actin in a phosphorylation-sensitive manner

Cardiac myosin-binding protein C (cMyBP-C) is a regulatory protein expressed in cardiac sarcomeres that is known to interact with myosin, titin, and actin. cMyBP-C modulates actomyosin interactions in a phosphorylation-dependent way, but it is unclear whether interactions with myosin, titin, or actin are required for these effects. Here we show using cosedimentation binding assays, that the 4 N-terminal domains of murine cMyBP-C (i.e. C0-C1-m-C2) bind to F-actin with a dissociation constant (K(d)) of approximately 10 microm and a molar binding ratio (B(max)) near 1.0, indicating 1:1 (mol/mol) binding to actin. Electron microscopy and light scattering analyses show that these domains cross-link F-actin filaments, implying multiple sites of interaction with actin. Phosphorylation of the MyBP-C regulatory motif, or m-domain, reduced binding to actin (reduced B(max)) and eliminated actin cross-linking. These results suggest that the N terminus of cMyBP-C interacts with F-actin through multiple distinct binding sites and that binding at one or more sites is reduced by phosphorylation. Reversible interactions with actin could contribute to effects of cMyBP-C to increase cross-bridge cycling.

Mechanical and kinetic effects of shortened tropomyosin reconstituted into myofibrils

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

Dilated cardiomyopathy mutations in alpha-tropomyosin inhibit its movement during the ATPase cycle

The Glu40Lys and Glu54Lys mutations in alpha-tropomyosin cause dilated cardiomyopathy (DCM). Functional analysis has demonstrated that both mutations decrease thin filament Ca2+-sensitivity and that Glu40Lys reduces maximum activation. To understand the molecular mechanism underlying these changes, we labeled wild type alpha-tropomyosin and both mutants at Cys190 with 5-iodoacetamide-fluorescein and incorporated the labeled proteins into ghost muscle fibers. Using the polarized fluorimetry, the position of the labeled tropomyosins on the thin filament and their affinity for actin were measured and the change in these parameters at different stages of the ATPase cycle determined. Both DCM mutations were found to shift tropomyosin towards the periphery of thin filament and to change the affinity of tropomyosin for actin; during the ATPase cycle the amplitude of tropomyosin movement was reduced and at some stages of the cycle even reversed. The correlation of these structural changes with the observed function effects is discussed.

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.

Insights into the kinetics of Ca2+-regulated contraction and relaxation from myofibril studies

Muscle contraction results from force-generating interactions between myosin cross-bridges on the thick filament and actin on the thin filament. The force-generating interactions are regulated by Ca(2+) via specialised proteins of the thin filament. It is controversial how the contractile and regulatory systems dynamically interact to determine the time course of muscle contraction and relaxation. Whereas kinetics of Ca(2+)-induced thin-filament regulation is often investigated with isolated proteins, force kinetics is usually studied in muscle fibres. The gap between studies on isolated proteins and structured fibres is now bridged by recent techniques that analyse the chemical and mechanical kinetics of small components of a muscle fibre, subcellular myofibrils isolated from skeletal and cardiac muscle. Formed of serially arranged repeating units called sarcomeres, myofibrils have a complete fully structured ensemble of contractile and Ca(2+) regulatory proteins. The small diameter of myofibrils (few micrometres) facilitates analysis of the kinetics of sarcomere contraction and relaxation induced by rapid changes of [ATP] or [Ca(2+)]. Among the processes studied on myofibrils are: (1) the Ca(2+)-regulated switch on/off of the troponin complex, (2) the chemical steps in the cross-bridge adenosine triphosphatase cycle, (3) the mechanics of force generation and (4) the length dynamics of individual sarcomeres. These studies give new insights into the kinetics of thin-filament regulation and of cross-bridge turnover, how cross-bridges transform chemical energy into mechanical work, and suggest that the cross-bridge ensembles of each half-sarcomere cooperate with each other across the half-sarcomere borders. Additionally, we now have a better understanding of muscle relaxation and its impairment in certain muscle diseases.

Modulation of the effects of tropomyosin on actin and myosin conformational changes by troponin and Ca2+

The molecular mechanisms by which troponin (TN)-tropomyosin (TM) regulates the myosin ATPase cycle were investigated using fluorescent probes specifically bound to Cys36 of TM, Cys707 of myosin subfragment-1, and Cys374 of actin incorporated into ghost muscle fibers. Intermediate states of actomyosin were simulated by using nucleotides and non-hydrolysable ATP analogs. Multistep changes in mobility and spatial arrangement of SH1 helix of myosin motor domain and actin subdomain-1 during the ATPase cycle were observed. Each intermediate state of actomyosin induced a definite conformational state and specific position of TM strands on the surface of thin filament. TM increased the amplitude of myosin SH1 helix and actin subdomain-1 movements at transition from weak- to strong-binding states shifting to the center of thin filament at strong-binding and to the periphery of thin filament at weak-binding states. TN modulated those movements in a capital ES, Cyrillicsmall a, Cyrillic(2+)-dependent manner. At high-Ca(2+), TN enhanced the effect of TM on SH1 helix and subdomain-1 movements by transferring TM further to the center of thin filament at strong-binding states. In contrast, at low-Ca(2+), TN inhibited the effect of TM movements, "freezing" actin structure in "OFF" state and TM in the position typical for weak-binding states, resulting in disturbing the interplay of actin and myosin.

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

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

Tropomyosin and the steric mechanism of muscle regulation

Contraction in all muscles must be precisely regulated and requisite control systems must be able to adjust to changes in physiological and myopathic stimuli. In this chapter, we outline the structural evidence for a steric mechanism that governs muscle activity. The mechanism involves calcium and myosin induced changes in the position of tropomyosin along actin-based thin filaments. This process either blocks or uncovers myosin crossbridge binding sites on actin and consequently regulates crossbridge cycling on thin filaments, the sliding of thin and thick filaments and muscle shortening and force production.

The ultimate nanoscale mincer: assembly, structure and active sites of the 20S proteasome core

20S proteasomes constitute the proteolytic core of large protease complexes found in all branches of life. Among these, the eukaryotic 26S proteasome ubiquitously poses as a vital final entity in regulated degradation of intracellular proteins. The composition of 20S core particles has been disclosed in detail, facilitated by groundbreaking studies on ancestral prokaryotic 20S proteasomes of low complexity and culminated in the crystal structure determination of the much more complex eukaryotic particles. This article first summarizes insights into the structural organization of the 20S core followed by characterization of its proteolytic activities, which are confined to the central cavity of the particle. In eukaryotes they reside in three different subunit types differing in their preference for cleavage sites in substrates as well as in their importance for the proteasome's cellular function. The second part reviews current knowledge on the biogenesis pathways of 20S core particles, which have to ensure not only the fixed subunit arrangement but also activation of proteolytic subunits in a late assembly state.