Structure of co-crystals of tropomyosin and troponin

Cardiac thin filament regulation

Myocardial contraction is initiated upon the release of calcium into the cytosol from the sarcoplasmic reticulum following membrane depolarization. The fundamental physiological role of the heart is to pump an amount blood that is determined by the prevailing requirements of the body. The physiological control systems employed to accomplish this task include regulation of heart rate, the amount of calcium release, and the response of the cardiac myofilaments to activator calcium ions. Thin filament activation and relaxation dynamics has emerged as a pivotal regulatory system tuning myofilament function to the beat-to-beat regulation of cardiac output. Maladaptation of thin filament dynamics, in addition to dysfunctional calcium cycling, is now recognized as an important cellular mechanism causing reduced cardiac pump function in a variety of cardiac diseases. Here, we review current knowledge regarding protein-protein interactions involved in the dynamics of thin filament activation and relaxation and the regulation of these processes by protein kinase-mediated phosphorylation.

Structural basis for tropomyosin overlap in thin (actin) filaments and the generation of a molecular swivel by troponin-T

Head-to-tail polymerization of tropomyosin is crucial for its actin binding, function in actin filament assembly, and the regulation of actin-myosin contraction. Here, we describe the 2.1 A resolution structure of crystals containing overlapping tropomyosin N and C termini (TM-N and TM-C) and the 2.9 A resolution structure of crystals containing TM-N and TM-C together with a fragment of troponin-T (TnT). At each junction, the N-terminal helices of TM-N were splayed, with only one of them packing against TM-C. In the C-terminal region of TM-C, a crucial water in the coiled-coil core broke the local 2-fold symmetry and helps generate a kink on one helix. In the presence of a TnT fragment, the asymmetry in TM-C facilitates formation of a 4-helix bundle containing two TM-C chains and one chain each of TM-N and TnT. Mutating the residues that generate the asymmetry in TM-C caused a marked decrease in the affinity of troponin for actin-tropomyosin filaments. The highly conserved region of TnT, in which most cardiomyopathy mutations reside, is crucial for interacting with tropomyosin. The structure of the ternary complex also explains why the skeletal- and cardiac-muscle specific C-terminal region is required to bind TnT and why tropomyosin homodimers bind only a single TnT. On actin filaments, the head-to-tail junction can function as a molecular swivel to accommodate irregularities in the coiled-coil path between successive tropomyosins enabling each to interact equivalently with the actin helix.

Fluorescence resonance energy transfer between residues on troponin and tropomyosin in the reconstituted thin filament: modeling the troponin-tropomyosin complex

Troponin (Tn), in association with tropomyosin (Tm), plays a central role in the calcium regulation of striated muscle contraction. Fluorescence resonance energy transfer (FRET) between probes attached to the Tn subunits (TnC, TnI, TnT) and to Tm was measured to study the spatial relationship between Tn and Tm on the thin filament. We generated single-cysteine mutants of rabbit skeletal muscle alpha-Tm, TnI and the beta-TnT 25-kDa fragment. The energy donor was attached to a single-cysteine residue at position 60, 73, 127, 159, 200 or 250 on TnT, at 98 on TnC and at 1, 9, 133 or 181 on TnI, while the energy acceptor was located at 13, 146, 160, 174, 190, 209, 230, 271 or 279 on Tm. FRET analysis showed a distinct Ca(2+)-induced conformational change of the Tm-Tn complex and revealed that TnT60 and TnT73 were closer to Tm13 than Tm279, indicating that the elongated N-terminal region of TnT extends beyond the beginning of the next Tm molecule on the actin filament. Using the atomic coordinates of the crystal structures of Tm and the Tn core domain, we searched for the disposition and orientation of these structures by minimizing the deviations of the calculated FRET efficiencies from the observed FRET efficiencies in order to construct atomic models of the Tn-Tm complex with and without bound Ca(2+). In the best-fit models, the Tn core domain is located on residues 160-200 of Tm, with the arrowhead-shaped I-T arm tilting toward the C-terminus of Tm. The angle between the Tm axis and the long axis of TnC is approximately 75 degrees and approximately 85 degrees with and without bound Ca(2+), respectively. The models indicate that the long axis of TnC is perpendicular to the thin filament without bound Ca(2+), and that TnC and the I-T arm tilt toward the filament axis and rotate around the Tm axis by approximately 20 degrees upon Ca(2+) binding.

Some binding properties of Omp T digested muscle tropomyosin

Cleavage of vertebrate muscle tropomyosin by bacterial Omp T produces an amino-terminally truncated product (residues 7-284). The proteolysed protein, which is resolved from the parent by electrophoresis in the presence of sodium dodecylsulphate, can be generated from a variety of striated and smooth muscle tropomyosins, including ones from mammal, bird and fish. Edman-based sequencing and mass analysis confirm that the main site of chain hydrolysis is the peptide bond between Lys 6 and Lys 7. Loss of the hexapeptide, together with the blocking group, from tropomyosin weakens its affinity for troponin. Compared to wild type, the shortened forms of rabbit skeletal tropomyosin and Atlantic salmon fast skeletal tropomyosin, as well as the unacetylated (full-length) version of the latter, all display reduced affinity for both troponin and the amino-terminal fragment of troponin-T (residues 1-158), as judged by affinity chromatography. This is consistent with the view that the amino terminal region is required for full interaction with troponin-T. Truncated tropomyosin fails to bind to F-actin at micromolar concentration, as expected. Interestingly, binding is restored by troponin in the presence of either added Ca(2+) or EGTA. Digestion of muscle tropomyosin by Omp T, which can be carried out on quantitative amounts of protein, is concluded to yield a product that has useful biochemical applications.

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

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

Molecular and immunohistochemical analyses of cardiac troponin T during cardiac development in the Mexican axolotl,Ambystoma mexicanum

The Mexican axolotl, Ambystoma mexicanum, is an excellent animal model for studying heart development because it carries a naturally occurring recessive genetic mutation, designated gene c, for cardiac nonfunction. The double recessive mutants (c/c) fail to form organized myofibrils in the cardiac myoblasts resulting in hearts that fail to beat. Tropomyosin expression patterns have been studied in detail and show dramatically decreased expression in the hearts of homozygous mutant embryos. Because of the direct interaction between tropomyosin and troponin T (TnT), and the crucial functions of TnT in the regulation of striated muscle contraction, we have expanded our studies on this animal model to characterize the expression of the TnT gene in cardiac muscle throughout normal axolotl development as well as in mutant axolotls. In addition, we have succeeded in cloning the full-length cardiac troponin T (cTnT) cDNA from axolotl hearts. Confocal microscopy has shown a substantial, but reduced, expression of TnT protein in the mutant hearts when compared to normal during embryonic development.

A comparison of muscle thin filament models obtained from electron microscopy reconstructions and low-angle X-ray fibre diagrams from non-overlap muscle

The regulation of striated muscle contraction involves changes in the interactions of troponin and tropomyosin with actin thin filaments. In resting muscle, myosin-binding sites on actin are thought to be blocked by the coiled-coil protein tropomyosin. During muscle activation, Ca2+ binding to troponin alters the tropomyosin position on actin, resulting in cyclic actin-myosin interactions that accompany muscle contraction. Evidence for this steric regulation by troponin-tropomyosin comes from X-ray data [Haselgrove, J.C., 1972. X-ray evidence for a conformational change in the actin-containing filaments of verterbrate striated muscle. Cold Spring Habor Symp. Quant. Biol. 37, 341-352; Huxley, H.E., 1972. Structural changes in actin and myosin-containing filaments during contraction. Cold Spring Habor Symp. Quant. Biol. 37, 361-376; Parry, D.A., Squire, J.M., 1973. Structural role of tropomyosin in muscle regulation: analysis of the X-ray diffraction patterns from relaxed and contracting muscles. J. Mol. Biol. 75, 33-55] and electron microscope (EM) data [Spudich, J.A., Huxley, H.E., Finch, J., 1972. Regulation of skeletal muscle contraction. II. Structural studies of the interaction of the tropomyosin-troponin complex with actin. J. Mol. Biol. 72, 619-632; O'Brien, E.J., Gillis, J.M., Couch, J., 1975. Symmetry and molecular arrangement in paracrystals of reconstituted muscle thin filaments. J. Mol. Biol. 99, 461-475; Lehman, W., Craig, R., Vibert, P., 1994. Ca2+-induced tropomyosin movement in Limulus thin filaments revealed by three-dimensional reconstruction. Nature 368, 65-67] each with its own particular strengths and limitations. Here we bring together some of the latest information from EM analysis of single thin filaments from Pirani et al. [Pirani, A., Xu, C., Hatch, V., Craig, R., Tobacman, L.S., Lehman, W. (2005). Single particle analysis of relaxed and activated muscle thin filaments. J. Mol. Biol. 346, 761-772], with synchrotron X-ray data from non-overlapped muscle fibres to refine the models of the striated muscle thin filament. This was done by incorporating current atomic-resolution structures of actin, tropomyosin, troponin and myosin subfragment-1. Fitting these atomic coordinates to EM reconstructions, we present atomic models of the thin filament that are entirely consistent with a steric regulatory mechanism. Furthermore, fitting the atomic models against diffraction data from skinned muscle fibres, stretched to non-overlap to preclude crossbridge binding, produced very similar results, including a large Ca2+-induced shift in tropomyosin azimuthal location but little change in the actin structure or apparent alteration in troponin position.

Oxidative modification of tropomyosin and myocardial dysfunction following coronary microembolization

AIMS

We addressed a potential mechanism of myocardial dysfunction following coronary microembolization at the level of myofibrillar proteins.

METHODS AND RESULTS

Anaesthetized pigs underwent intracoronary infusion of microspheres. After 6 h, the microembolized areas (MEA) had decreased systolic wall thickening to 38 +/- 7% of baseline and a 2.62 +/- 0.40-fold increase in the formation of disulphide cross-bridges (DCB) in tropomyosin relative to that in remote areas. The impairment in contractile function correlated inversely with DCB formation (r = -0.68; P = 0.015) and was associated with increased TNF-alpha content. DCB formation was reflected by increased tropomyosin immunoreactivity and abolished in vitro by dithiothreitol. Ascorbic acid prevented contractile dysfunction as well as increased DCB and TNF-alpha. In anaesthetized dogs, 8 h after intracoronary microspheres infusion, contractile function was reduced to 8+/-10% of baseline and DCB in MEA was 1.48+/-0.12 higher than that in remote areas. In conscious dogs, 6 days after intracoronary microspheres infusion, myocardial function had returned to baseline and DCB was no longer different between remote and MEA. Again contractile function correlated inversely with DCB formation (r = -0.83; P = 0.005).

CONCLUSION

Myofibrillar protein oxidation may represent a mechanistic link between inflammation and contractile dysfunction following coronary microembolization.

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

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

Tissue expression of four troponin I genes and their molecular interactions with two troponin C isoforms in Caenorhabditis elegans

Gene duplication is a major genetic event that can produce multiple protein isoforms. Comparative sequence and functional analysis of related gene products can provide insights into protein family evolution. To characterize the Caenorhabditis elegans troponin I family, we analyzed gene structures, tissue expression patterns and RNAi phenotypes of four troponin I isoforms. Tissue expression patterns were determined using lacZ/gfp/rfp reporter gene assays. The tni-1, tni-2/unc-27 and tni-3 genes, each encoding a troponin I isoform, are uniquely expressed in body wall, vulval and anal muscles but at different levels; tni-4 was expressed solely in the pharynx. Expressing tni-1 and -2 gene RNAi caused motility defects similar to unc-27 (e155) mutant, a tni-2 null allele. The tni-3 RNAi expression produced egg laying defects while the tni-4 RNAi caused arrest at gastrulation. Overlay analyses were used to assay interactions between the troponin I and two troponin C isoforms. The three body wall troponin I isoforms interacted with body wall and pharyngeal troponin C isoforms; TNI-4 interacted only with pharyngeal troponin C. Our results suggest the body wall genes have evolved following duplication of the pharynx gene and provide important data about gene duplication and functional differentiation of nematode troponin I isoforms.

Mini-thin filaments regulated by troponin-tropomyosin

Striated muscle thin filaments contain hundreds of actin monomers and scores of troponins and tropomyosins. To study the cooperative mechanism of thin filaments, "mini-thin filaments" were generated by isolating particles nearly matching the minimal structural repeat of thin filaments: a double helix of actin subunits with each strand approximately seven actins long and spanned by a troponin-tropomyosin complex. One end of the particles was capped by a gelsolin (segment 1-3)-TnT fusion protein (substituting for normal TnT), and the other end was capped by tropomodulin. EM showed that the particles were 46 +/- 9 nm long, with a knob-like mass attributable to gelsolin at one end. Average actin, tropomyosin, and gelsolin-troponin composition indicated one troponin-tropomyosin attached to each strand of the two-stranded actin filament. The minifilaments thus nearly represent single regulatory units of thin filaments. The myosin S1 MgATPase rate stimulated by the minifilaments was Ca2+-sensitive, indicating that single regulatory length particles are sufficient for regulation. Ca2+ bound cooperatively to cardiac TnC in conventional thin filaments but noncooperatively to cardiac TnC in minifilaments in the absence of myosin. This suggests that thin filament Ca2+-binding cooperativity reflects indirect troponin-troponin interactions along the long axis of conventional filaments, which do not occur in minifilaments. Despite noncooperative Ca2+ binding to minifilaments in the absence of myosin, Ca2+ cooperatively activated the myosin S1-particle ATPase rate. Two-stranded single regulatory units therefore may be sufficient for myosin-mediated Ca2+-binding cooperativity. Functional mini-thin filaments are well suited for biochemical and structural analysis of thin-filament regulation.

Solution Structure of the Chicken Skeletal Muscle Troponin Complex Via Small-angle Neutron and X-ray Scattering

Troponin is a Ca2+-sensitive switch that regulates the contraction of vertebrate striated muscle by participating in a series of conformational events within the actin-based thin filament. Troponin is a heterotrimeric complex consisting of a Ca2+-binding subunit (TnC), an inhibitory subunit (TnI), and a tropomyosin-binding subunit (TnT). Ternary troponin complexes have been produced by assembling recombinant chicken skeletal muscle TnC, TnI and the C-terminal portion of TnT known as TnT2. A full set of small-angle neutron scattering data has been collected from TnC-TnI-TnT2 ternary complexes, in which all possible combinations of the subunits have been deuterated, in both the +Ca2+ and -Ca2+ states. Small-angle X-ray scattering data were also collected from the same troponin TnC-TnI-TnT2 complex. Guinier analysis shows that the complex is monomeric in solution and that there is a large change in the radius of gyration of TnI when it goes from the +Ca2+ to the -Ca2+ state. Starting with a model based on the human cardiac troponin crystal structure, a rigid-body Monte Carlo optimization procedure was used to yield models of chicken skeletal muscle troponin, in solution, in the presence and in the absence of regulatory calcium. The optimization was carried out simultaneously against all of the scattering data sets. The optimized models show significant differences when compared to the cardiac troponin crystal structure in the +Ca2+ state and provide a structural model for the switch between +Ca2+ and -Ca2+ states. A key feature is that TnC adopts a dumbbell conformation in both the +Ca2+ and -Ca2+ states. More importantly, the data for the -Ca2+ state suggest a long extension of the troponin IT arm, consisting mainly of TnI. Thus, the troponin complex undergoes a large structural change triggered by Ca2+ binding.

The rates of switching movement of troponin T between three states of skeletal muscle thin filaments determined by fluorescence resonance energy transfer

Troponin (Tn) plays the key roles in the regulation of striated muscle contraction. Tn consists of three subunits (TnT, TnC, and TnI). In combination with the stopped-flow method, fluorescence resonance energy transfer between probes attached to Cys-60 or Cys-250 of TnT and Cys-374 of actin was measured to determine the rates of switching movement of the troponin tail domain (Cys-60) and of the TnT-TnI coiled-coil C terminus (Cys-250) between three states (relaxed, closed, and open) of the thin filament. When the free Ca(2+) concentration was rapidly changed, these domains moved with rates of approximately 450 and approximately 85 s(-1) at pH 7.0 on Ca(2+) up and down, respectively. When myosin subfragment 1 (S1) was dissociated from thin filaments by rapid mixing with ATP, these domains moved with a single rate constant of approximately 400 s(-1) in the presence and absence of Ca(2+). The light scattering measurements showed that ATP-induced S1 dissociation occurred with a rate constant >800 s(-1). When S1 was rapidly mixed with the thin filament, these domains moved with almost the same or slightly faster rates than those of S1 binding measured by light scattering. In most but not all aspects, the rates of movement of the troponin tail domain and of the TnT-TnI coiled-coil C terminus were very similar to those of certain TnI sites (N terminus, Cys-133, and C terminus) previously characterized (Shitaka, Y., Kimura, C., Iio, T., and Miki, M. (2004) Biochemistry 43, 10739-10747), suggesting that a series of conformational changes in the Tn complex during switching on or off process occurs synchronously.

cTnT1, a cardiac troponin T isoform, decreases myofilament tension and affects the left ventricular pressure waveform

Four isoforms of cardiac troponin T (cTnT), a protein essential for calcium-dependent myocardial force development, are expressed in the human; they differ in charge and length. Their expression is regulated developmentally and is affected by disease states. Human cTnT (hcTnT) isoform effects have been examined in reconstituted myofilaments. In this study, we evaluated the modulatory effects of overexpressing one cTnT isoform on in vitro and in vivo myocardial function. A hcTnT isoform, hcTnT(1), expressed during development and in heart disease but not in the normal adult heart, was expressed in transgenic (TG) mice (1-30% of total cTnT). Maximal active tension measured in skinned myocardium decreased as a function of relative hcTnT(1) expression. The pCa at half-maximal force development, Hill coefficient, and rate of redevelopment of force did not change significantly with hcTnT(1) expression. In vivo maximum rates of rise and fall of left ventricular pressure decreased, and the half-time of isovolumic relaxation increased, with hcTnT(1) expression. Substituting total cTnT charge for hcTnT(1) expression resulted in similar conclusions. Morphometric analysis and electron microscopy revealed no differences between wild-type (non-TG) and TG myocardium. No differences in isoform expression of tropomyosin, myosin heavy chain, essential and regulatory myosin light chains (MLC), TnI, or in posttranslational modifications of mouse cTnT, cTnI, or regulatory MLC were observed. These results support the hypothesis that cTnT isoform amino-terminal differences affect myofilament function and suggest that hcTnT(1) expression levels present during human development and in human heart disease can affect in vivo ventricular function.

Charged residue alterations in the inner-core domain and carboxy-terminus of alpha-tropomyosin differentially affect mouse cardiac muscle contractility

Two important charge differences between the alpha- and beta-tropomyosin (TM) isoforms are the exchange of a serine residue in the inner-core region at position 229, and a histidine residue at the carboxy-terminal end at position 276, with glutamic acid and asparagine, respectively. We have recently shown that altering these two residues in alpha-TM to their beta-TM counterparts in transgenic (TG) mouse hearts causes a depression in both +dP/dt and -dP/dt and a decrease in calcium sensitivity. In this study, we address whether independent charge changes at these two residues in alpha-TM modulate cardiac function differentially. To test this hypothesis we generated two TG lines: alpha-TMSer229Glu and alpha-TMHis276Asn. Molecular analyses show that 98% of native alpha-TM is replaced by mutated protein in alpha-TM229 hearts whereas alpha-TM276 hearts show 82% replacement with the mutated protein. Isolated working heart data show that alpha-TM229 TG hearts exhibit a significant decrease in both +dP/dt (7%) and -dP/dt (8%) compared with nontransgenics (NTGs) and time to peak pressure (TPP) is also reduced in alpha-TM229 hearts. alpha-TM276 hearts show a decrease only in -dP/dt (14%) and TPP is increased. pCa(2+)-tension relationships in skinned fibre preparations indicate decreased calcium sensitivity in alpha-TM229 but no change in alpha-TM276 preparations. Force-[Ca(2+)](IC) measurements from intact papillary fibres indicate that alpha-TM276 fibres produce more force per given [Ca(2+)](IC) when compared to NTG fibres, while alpha-TM229 fibres produce less force per given [Ca(2+)](IC). These data demonstrate that changing charged residues at either the inner-core domain or the carboxyl end of TM alters sarcomeric performance differently, suggesting that the function of TM is compartmentalized along its length.

Ca2+-induced Rolling of Tropomyosin in Muscle Thin Filaments

Tropomyosin is a filamentous coiled-coil protein directly involved in the regulation of the actomyosin interaction responsible for muscle contraction: it transmits the local calcium-induced conformational change in troponin to the helical array of myosin-binding sites on the surface of the actin filament. McLachlan and Stewart (McLachlan, A. D., and Stewart, M. (1976) J. Mol. Biol. 103, 271-298) proposed that the tropomyosin coiled-coil structure can be divided into 14 alternating 19- to 20-residue "alpha- and beta-bands," which could act as alternate 7-fold sets of sites for specific binding to actin in the different conformational states of the regulated thin filament. Here we present the first direct experimental evidence in support of the alpha- and beta-band hypothesis: we analyze the acrylamide quenching of the fluorescence of mutant tropomyosins containing 5-hydroxytryptophan residues at different positions along the coiled-coil structure under a variety of conditions (alone, complexed with actin, and complexed with actin and troponin with or without Ca(2+)). We show that fluorescent probes placed in the alpha-bands become less solvent-exposed in the absence of calcium, whereas those in the beta-bands become less solvent-exposed in the presence of calcium. A model in which the tropomyosin coiled-coil rolls across the actin surface in response to Ca(2+)-binding to troponin most easily explains these observations.

Tropomyosin exon 6b is troponin-specific and required for correct acto-myosin regulation

The specificity of tropomyosin (Tm) exon 6b for interaction with and functioning of troponin (Tn) has been studied using recombinant fibroblast Tm isoforms 5a and 5b. These isoforms differ internally by exons 6a/6b and possess non-muscle exons 1b/9d at the termini, hence they lack the primary TnT(1)-tropomyosin interaction, allowing study of exon 6 exchange in isolation from this. Using kinetic techniques to measure regulation of myosin S1 binding to actin and fluorescently labeled Tm to directly measure Tn binding, we show that binding of Tn to both isoforms is similar (0.1-0.5 microm) and both produce well regulated systems. Calcium has little effect on Tn binding to the actin.Tm complex and both exons produce a 3-fold reduction in the S1 binding rate to actin.Tm.Tn in its absence. This confirms previous results that show exon 6 has little influence on Tn affinity to actin.Tm or its ability to fully inhibit the acto-myosin interaction. Thin filaments reconstituted with Tn and Tm5a or skeletal Tm (containing exon 6b) show nearly identical calcium dependence of acto-myosin regulation. However, Tm5b produces a dramatic increase in calcium sensitivity, shifting the activation mid-point by almost an order of magnitude. This shows that exon 6 sequence and, hence, Tm structure in this region have a significant effect upon the calcium regulation of Tn. This finding supports evidence that familial hypertrophic cardiomyopathy mutations occurring adjacent to this region can effect calcium regulation.

Structure of the core domain of human cardiac troponin in the Ca(2+)-saturated form

Troponin is essential in Ca(2+) regulation of skeletal and cardiac muscle contraction. It consists of three subunits (TnT, TnC and TnI) and, together with tropomyosin, is located on the actin filament. Here we present crystal structures of the core domains (relative molecular mass of 46,000 and 52,000) of human cardiac troponin in the Ca(2+)-saturated form. Analysis of the four-molecule structures reveals that the core domain is further divided into structurally distinct subdomains that are connected by flexible linkers, making the entire molecule highly flexible. The alpha-helical coiled-coil formed between TnT and TnI is integrated in a rigid and asymmetric structure (about 80 angstrom long), the IT arm, which bridges putative tropomyosin-anchoring regions. The structures of the troponin ternary complex imply that Ca(2+) binding to the regulatory site of TnC removes the carboxy-terminal portion of TnI from actin, thereby altering the mobility and/or flexibility of troponin and tropomyosin on the actin filament.

Differential expression of mutually exclusive exons of the fast skeletal muscle troponin T gene in the chicken wing and leg muscles

To determine the physiological significance of developmentally regulated and muscle tissue-specific expression of troponin T (TnT) isoforms and also to elucidate mechanisms of alternative splicing, we investigated splicing patterns of fast skeletal muscle TnT by sequencing cDNAs obtained from the chicken wing and leg muscles. The chicken fast skeletal muscle TnT gene has 27 exons, and produces potentially 2(15) transcripts if alternative exons are randomly spliced. For the fourteen alternative exons in the 5'-region of the gene, we found 7 and 12 splice variants in the wing biceps brachii and leg gastrocnemius, respectively. Out of 19, 12 variants were newly found in this study. Twenty two different splice variants were reported by the previous studies, but this number has now increased to 34 in the chicken TnT gene, suggesting that alternative splicing is not random, but strictly regulated. We also investigated splicing patterns of mutually exclusive exons 16 and 17 in the 3'-region of the gene. Approximately equal amounts of cDNAs containing exons 16 and 17 were detected in proximal regions of adult biceps brachii and gastrocnemius, respectively, whereas cDNA with exon 17 dominated in distal regions. Only cDNA containing exon 17 was found in both proximal and distal regions of 5-day-old gastrocnemius. The inclusion of exon 16 into cDNAs was detected in the proximal region of 15-day-old gastrocnemius and increased during development.

A novel TPM1 mutation in a family with hypertrophic cardiomyopathy and sudden cardiac death in childhood

We sought to define the pathogenic mutation in a family with hypertrophic cardiomyopathy (HC) and a markedly arrhythmogenic phenotype. The proband was an 8-year-old female with a sentinel event of sudden death. Screening echocardiograms revealed HC in 2 of her 3 siblings and her father. Her youngest male sibling was diagnosed with HC at age 2 years and died suddenly at age 6 years from ventricular fibrillation despite an implanted cardioverter defibrillator. Using DNA extracted from peripheral lymphocytes, linkage exclusion was performed by haplotype analysis of polymorphic markers for the HC genes. Genes not excluded by linkage were analyzed for mutations using denaturing high-performance liquid chromatography (DHPLC) and direct DNA sequencing. Using this strategy, a 610 T>G nucleotide substitution in the alpha-tropomyosin gene (TPM1) was identified resulting in a novel L185R (Leucine [L] to Arginine [R]) missense mutation. This mutation was a spontaneous germ-line mutation originating in the proband's father. L185R-TPM1 cosegregated with family members having clinical evidence of HC, including the proband as confirmed by molecular autopsy. The mutation was not present in 400 reference alleles. Thus, a novel missense mutation in TPM1 was discovered in a family with HC and sudden death in childhood. Unlike previously defined mutations that may disrupt the interactions between alpha-tropomyosin monomers, the L185R mutation may affect troponin-T binding. Defining the pathogenic mutation enabled definitive molecular diagnosis of 2 surviving children.

Specific sequences determine the stability and cooperativity of folding of the C-terminal half of tropomyosin

Tropomyosin is a flexible 410 A coiled-coil protein in which the relative stabilities of specific regions may be important for its proper function in the control of muscle contraction. In addition, tropomyosin can be used as a simple model of natural occurrence to understand the inter- and intramolecular interactions that govern the stability of coiled-coils. We have produced eight recombinant tropomyosin fragments (Tm(143-284(5OHW),) Tm(189-284(5OHW)), Tm(189-284), Tm(220-284(5OHW)), Tm(220-284), Tm(143-235), Tm(167-260), and Tm(143-260)) and one synthetic peptide (Ac-Tm(215-235)) to investigate the relative conformational stability of different regions derived from the C-terminal region of the protein, which is known to interact with the troponin complex. Analytical ultracentrifugation experiments show that the fragments that include the last 24 residues of the molecule (Tm(143-284(5OHW)), Tm(189-284(5OHW)), Tm(220-284(5OHW)), Tm(220-284)) are completely dimerized at 10 microm dimer (50 mm phosphate, 100 mm NaCl, 1.0 mm dithiothreitol, and 0.5 mm EDTA, 10 degrees C), whereas fragments that lack the native C terminus (Tm(143-235),Tm(167-260), and Tm(143-260)) are in a monomer-dimer equilibrium under these conditions. The presence of trifluoroethanol resulted in a reduction in the [theta](222)/[theta](208) circular dichroism ratio in all of the fragments and induced stable trimer formation only in those containing residues 261-284. Urea denaturation monitored by circular dichroism and fluorescence revealed that residues 261-284 of tropomyosin are very important for the stability of the C-terminal half of the molecule as a whole. Furthermore, the absence of this region greatly increases the cooperativity of urea-induced unfolding. Temperature and urea denaturation experiments show that Tm(143-235) is less stable than other fragments of the same size. We have identified a number of factors that may contribute to this particular instability, including an interhelix repulsion between g and e' positions of the heptad repeat, a charged residue at the hydrophobic coiled-coil interface, and a greater fraction of beta-branched residues located at d positions.

Structure of the inhibitory region of troponin by site directed spin labeling electron paramagnetic resonance

Site-directed spin labeling EPR (SDSL-EPR) was used to determine the structure of the inhibitory region of TnI in the intact cardiac troponin ternary complex. Maeda and collaborators have modeled the inhibitory region of TnI (skeletal 96-112: the structural motif that communicates the Ca(2+) signal to actin) as a kinked alpha-helix [Vassylyev, D., Takeda, S., Wakatsuki, S., Maeda, K. & Maeda, Y. (1998) Proc. Natl. Acad. Sci. USA 95, 4847-4852), whereas Trewhella and collaborators have proposed the same region to be a flexible beta-hairpin [Tung, C. S., Wall, M. E., Gallagher, S. C. & Trewhella, J. (2000) Protein Sci. 9, 1312-1326]. To distinguish between the two models, residues 129-145 of cardiac TnI were mutated sequentially to cysteines and labeled with the extrinsic spin probe, MTSSL. Sequence-dependent solvent accessibility was measured as a change in power saturation of the spin probe in the presence of the relaxation agent. In the ternary complex, the 129-137 region followed a pattern characteristic of a regular 3.6 residues/turn alpha-helix. The following region, residues 138-145, showed no regular pattern in solvent accessibility. Measurements of 4 intradomain distances within the inhibitory sequence, using dipolar EPR, were consistent with an alpha-helical structure. The difference in side-chain mobility between the ternary (C.I.T) and binary (C.I) complexes revealed a region of interaction of TnT located at the N-terminal end of the inhibitory sequence, residues 130-135. The above findings for the troponin complex in solution do not support either of the computational models of the binary complex; however, they are in very good agreement with a preliminary report of the x-ray structure of the cardiac ternary complex [Takeda, S. Yamashita, A., Maeda, K. & Maeda, Y. (2002) Biophys. J. 82, 832].

The troponin tail domain promotes a conformational state of the thin filament that suppresses myosin activity

In cardiac and skeletal muscles tropomyosin binds to the actin outer domain in the absence of Ca(2+), and in this position tropomyosin inhibits muscle contraction by interfering sterically with myosin-actin binding. The globular domain of troponin is believed to produce this B-state of the thin filament (Lehman, W., Hatch, V., Korman, V. L., Rosol, M., Thomas, L. T., Maytum, R., Geeves, M. A., Van Eyk, J. E., Tobacman, L. S., and Craig, R. (2000) J. Mol. Biol. 302, 593-606) via troponin I-actin interactions that constrain the tropomyosin. The present study shows that the B-state can be promoted independently by the elongated tail region of troponin (the NH(2) terminus (TnT-(1-153)) of cardiac troponin T). In the absence of the troponin globular domain, TnT-(1-153) markedly inhibited both myosin S1-actin-tropomyosin MgATPase activity and (at low S1 concentrations) myosin S1-ADP binding to the thin filament. Similarly, TnT-(1-153) increased the concentration of heavy meromyosin required to support in vitro sliding of thin filaments. Electron microscopy and three-dimensional reconstruction of thin filaments containing TnT-(1-153) and either cardiac or skeletal muscle tropomyosin showed that tropomyosin was in the B-state in the complete absence of troponin I. All of these results indicate that portions of the troponin tail domain, and not only troponin I, contribute to the positioning of tropomyosin on the actin outer domain, thereby inhibiting muscle contraction in the absence of Ca(2+).

The crystal structure of the C-terminal fragment of striated-muscle alpha-tropomyosin reveals a key troponin T recognition site

Contraction in striated and cardiac muscles is regulated by the motions of a Ca(2+)-sensitive tropomyosin/troponin switch. In contrast, troponin is absent in other muscle types and in nonmuscle cells, and actomyosin regulation is myosin-linked. Here we report an unusual crystal structure at 2.7 A of the C-terminal 31 residues of rat striated-muscle alpha-tropomyosin (preceded by a fragment of the GCN4 leucine zipper). The C-terminal 22 residues (263-284) of the structure do not form a two-stranded alpha-helical coiled coil as does the rest of the molecule, but here the alpha-helices splay apart and are stabilized by the formation of a tail-to-tail dimer with a symmetry-related molecule. The site of splaying involves a small group of destabilizing core residues that is present only in striated muscle tropomyosin isoforms. These results reveal a specific recognition site for troponin T and clarify the physical basis for the unique regulatory mechanism of striated muscles.

Divergent abnormal muscle relaxation by hypertrophic cardiomyopathy and nemaline myopathy mutant tropomyosins

Mutations in tropomyosin (Tm) have been linked to distinct inherited diseases of cardiac and skeletal muscle, hypertrophic cardiomyopathy (HCM), and nemaline myopathy (NM). How HCM and NM mutations in nearly identical Tm proteins produce the vastly divergent clinical phenotypes of heightened, prolonged cardiac muscle contraction in HCM and skeletal muscle weakness in NM is currently unknown. We report here a direct comparison of the effects of HCM (A63V) and NM (M9R) mutant Tm on membrane-intact myocyte contractile function as assessed by adenoviral gene transfer to fully differentiated cardiac muscle cells. Wild-type, and mutant HCM, and mutant NM proteins were expressed at similar levels in myocytes and incorporated into sarcomeres. Interestingly, HCM mutant Tm produced significantly longer contractions by slowing relaxation, whereas NM mutant Tm produced the opposite effect of accelerated muscle relaxation. We propose slowed relaxation caused by HCM mutant Tm can directly contribute to diastolic dysfunction seen in HCM even without secondary cardiac remodeling. Conversely, hastening of relaxation by NM mutant Tm may shift the force-frequency relationship in skeletal muscle and contribute to muscle weakness seen in NM. Together, these results implicate divergent, abnormal "turning off" of muscle contraction as a cellular basis for the differential pathogenesis of mutant Tm-associated HCM and NM.

Disease-causing mutations in cardiac troponin T: identification of a critical tropomyosin-binding region

Fifteen percent of the mutations causing familial hypertrophic cardiomyopathy are in the troponin T gene. Most mutations are clustered between residues 79 and 179, a region known to bind to tropomyosin at the C-terminus near the complex between the N- and C-termini. Nine mutations were introduced into a troponin T fragment, Gly-hcTnT(70-170), that is soluble, alpha-helical, binds to tropomyosin, promotes the binding of tropomyosin to actin, and stabilizes an overlap complex of N-terminal and C-terminal tropomyosin peptides. Mutations between residues 92 and 110 (Arg92Leu, Arg92Gln, Arg92Trp, Arg94Leu, Ala104Val, and Phe110Ile) impair tropomyosin-dependent functions of troponin T. Except for Ala104Val, these mutants bound less strongly to a tropomyosin affinity column and were less able to stabilize the TM overlap complex, effects that were correlated with increased stability of the troponin T, measured using circular dichroism. All were less effective in promoting the binding of tropomyosin to actin. Mutations within residues 92-110 may cause disease because of altered interaction with tropomyosin at the overlap region, critical for cooperative actin binding and regulatory function. A model for a five-chained coiled-coil for troponin T in the tropomyosin overlap complex is presented. Mutations outside the region (Ile79Asn, Delta 160Glu, and Glu163Lys) functioned normally and must cause disease by another mechanism.

The third and fourth tropomyosin isoforms of Caenorhabditis elegans are expressed in the pharynx and intestines and are essential for development and morphology

The tropomyosin gene tmy-1/lev-11 of Caenorhabditis elegans spans 14.5 kb and encodes three isoforms by alternative splicing. To identify, characterize and compare the genome and tissue expression of a fourth isoform, the technique of rapid amplification of cDNA ends and microinjection with lacZ and gfp fusion plasmids were employed. We elucidated CeTMIV, a fourth isoform of tmy-1, which encoded a 256 residue polypeptide. CeTMIV isoform had a similar promoter region to CeTMIII isoform, but was alternatively spliced to generate a cDNA that differed in two exons. The tmy-1::lacZ and tmy-1::gfp fusion genes, with 3.2 kb promoter sequence and 1.1 kb of CeTMIV isoform specific exons, were expressed in the pharyngeal and intestinal cells. Further unidirectional deletion of the sequence located the primary promoter region 853 bp upstream from the initial codon. We show within the upstream region, the presence of B and C subelement-like sequences of myo-2, which may be used to stimulate pharyngeal expression. Despite the presence of a ges-1 like sequence, we were unable to locate the two GATA sites required for intestinal expression. Reassessing tissue expression for CeTMIII isoform with newly constructed fusion plasmids, we showed further expression in germ-line tissue and intestinal cells in addition to pharyngeal expression. Finally, to demonstrate that tropomyosin is essential for development, we inactivated the body wall and pharynx-specific isoforms by RNA-mediated interference. In addition to 50-75 % embryonic lethality in both cases, the worms that survived body wall interference had abnormal body morphology and uncoordinated movements, and those that survived pharynx interference had deformed pharynges and gut regions. These results show the function of tropomyosin in normal muscle filament assembly and embryonic development, and illustrate the different expression patterns characteristic of tropomyosin isoforms in C. elegans.

A tropomyosin-2 mutation suppresses a troponin I myopathy in Drosophila

A suppressor mutation, D53, of the held-up(2) allele of the Drosophila melanogaster Troponin I (wupA) gene is described. D53, a missense mutation, S185F, of the tropomyosin-2, Tm2, gene fully suppresses all the phenotypic effects of held-up(2), including the destructive hypercontraction of the indirect flight muscles (IFMs), a lack of jumping, the progressive myopathy of the walking muscles, and reductions in larval crawling and feeding behavior. The suppressor restores normal function of the IFMs, but flight ability decreases with age and correlates with an unusual, progressive structural collapse of the myofibrillar lattice starting at the center. The S185F substitution in Tm2 is close to a troponin T binding site on tropomyosin. Models to explain suppression by D53, derived from current knowledge of the vertebrate troponin-tropomyosin complex structure and functions, are discussed. The effects of S185F are compared with those of two mutations in residues 175 and 180 of human alpha-tropomyosin 1 which cause familial hypertrophic cardiomyopathy (HCM).

High-level production of functional muscle alpha-tropomyosin in Pichia pastoris

Although numerous studies have reported the production of skeletal muscle alpha-tropomyosin in E. coli, the protein needs to be modified at the amino terminus in order to be active. Without these modifications the protein does not bind to actin, does not exhibit head-to-tail polymerization, and does not inhibit the actomyosin Mg(2+)-ATPase in the absence of troponin. On the other hand, the protein produced in insect cells using baculovirus as an expression vector (Urbancikova, M., and Hitchcock-DeGregori, S. E., J. Biol. Chem., 269, 24310-24315, 1994) is only partially acetylated at its amino terminal and therefore is not totally functional. In an attempt to produce an unmodified functional recombinant muscle alpha-tropomyosin for structure-function correlation studies we have expressed the chicken skeletal alpha-tropomyosin cDNA in the yeast Pichia pastoris. Recombinant protein was produced at a high level (20 mg/L) and was similar to the wild type muscle protein in its ability to polymerize, to bind to actin and to regulate the actomyosin S1 Mg(2+)-ATPase.

Ca(2+)-induced switching of troponin and tropomyosin on actin filaments as revealed by electron cryo-microscopy

Muscle contraction is regulated by the intracellular Ca(2+ )concentration. In vertebrate striated muscle, troponin and tropomyosin on actin filaments comprise a Ca(2+)-sensitive switch that controls contraction. Ca(2+ )binds to troponin and triggers a series of changes in actin-containing filaments that lead to cyclic interactions with myosin that generate contraction. However, the precise location of troponin relative to actin and tropomyosin and how its structure changes with Ca(2+ )have been not determined. To understand the regulatory mechanism, we visualized the location of troponin by determining the three-dimensional structure of thin filaments from electron cryo-micrographs without imposing helical symmetry to approximately 35 A resolution. With Ca(2+), the globular domain of troponin was gourd-shaped and was located over the inner domain of actin. Without Ca(2+), the main body of troponin was shifted by approximately 30 A towards the outer domain and bifurcated, with a horizontal branch (troponin arm) covering the N and C-terminal regions of actin. The C-terminal one-third of tropomyosin shifted towards the outer domain of actin by approximately 35 A supporting the steric blocking model, however it is surprising that the N-terminal half of tropomyosin shifted less than approximately 12 A. Therefore tropomyosin shifted differentially without Ca(2+). With Ca(2+), tropomyosin was located entirely over the inner domain thereby allowing greater access of myosin for force generation. The interpretation of three-dimensional maps was facilitated by determining the three-dimensional positions of fluorophores labelled on specific sites of troponin or tropomyosin by applying probabilistic distance geometry to data from fluorescence resonance energy transfer measurements.

Mutations that alter the surface charge of alpha-tropomyosin are associated with dilated cardiomyopathy

Proteins in cardiac myocytes assemble into contractile units known as sarcomeres. Contractile force is generated by interaction between sarcomeric thick and thin filaments. Thin filaments also transmit force within and between myocytes. Mutations in genes encoding the thin filament proteins actin and tropomyosin cause hypertrophic cardiomyopathy. Mutations affecting functionally distinct domains of actin also cause dilated cardiomyopathy (DCM). We used a non-positional candidate gene approach to test further the hypothesis that dysfunction of sarcomeric thin filaments, due to different mutations in the same gene, can lead to either hypertrophic or dilated cardiomyopathy. Mutational analyses of alpha-tropomyosin 1 were performed in patients with idiopathic DCM. We identified two mutations that alter highly conserved residues and that, unlike hypertrophic cardiomyopathy-associated mutations, cause localized charge reversal on the surface of tropomyosin. Therefore, substitution of different amino acid residues in the same thin filament proteins is associated with the distinct phenotypes of cardiac hypertrophy or congestive heart failure.

A new model of cooperative myosin-thin filament binding

Cooperative myosin binding to the thin filament is critical to regulation of cardiac and skeletal muscle contraction. This report delineates and fits to experimental data a new model of this process, in which specific tropomyosin-actin interactions are important, the tropomyosin-tropomyosin polymer is continuous rather than disjointed, and tropomyosin affects myosin-actin binding by shifting among three positions as in recent structural studies. A myosin- and tropomyosin-induced conformational change in actin is proposed, rationalizing the approximately 10,000-fold strengthening effect of myosin on tropomyosin-actin binding. Also, myosin S1 binding to regulated filaments containing mutant tropomyosins with internal deletions exhibited exaggerated cooperativity, implying an allosteric effect of tropomyosin on actin and allowing the effect's measurement. Comparisons among the mutants suggest the change in actin is promoted much more strongly by the middle of tropomyosin than by its ends. Regardless of calcium binding to troponin, this change in actin facilitates the shift in tropomyosin position to the actin inner domain, which is required for tight myosin-actin association. It also increases myosin-actin affinity 7-fold compared with the absence of troponin-tropomyosin. Finally, initiation of a shift in tropomyosin position is 100-fold more difficult than is its extension from one actin to the next, producing the myosin binding cooperativity that underlies cooperative activation of muscle contraction.

Mapping the domain of troponin T responsible for the activation of actomyosin ATPase activity. Identification of residues involved in binding to actin

The in vitro Ca(2+) regulation of the actomyosin Mg(2+)-ATPase at physiological ratios of actin, tropomyosin, and troponin occurs only in the presence of troponin T. We have previously demonstrated that a polypeptide corresponding to the first 191 amino acids of troponin T (TnT-(1-191)) activates the actomyosin Mg(2+)-ATPase in the presence of tropomyosin. In order to further characterize this activation domain, we constructed troponin T fragments corresponding to residues 1-157 (TnT-(1-157)), 1-76 (TnT-(1-76)), 77-157 (TnT-(77-157)), 77-191 (TnT-(77-191)), and 158-191 (TnT-(158-191)). Assays using these fragments demonstrated the following: (a) residues 1-76 do not bind to tropomyosin or actin; (b) residues 158-191 bind to actin cooperatively but not to tropomyosin; (c) the sequence 77-157 is necessary for troponin interaction with residue 263 of tropomyosin; (d) TnT-(77-191) on its own activates the actomyosin ATPase activity as described previously for TnT-(1-191). TnT-(1-157), TnT-(1-76), TnT-(77-157), TnT-(158-191), and combinations of TnT-(158-191) with TnT-(1-157) or TnT-(77-157) showed no effect on the ATPase activity. We conclude that the activation of actomyosin ATPase activity is mediated by a direct interaction between amino acids 77 and 191 of troponin T, tropomyosin, and actin.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Direct, convergent hypersensitivity of calcium-activated force generation produced by hypertrophic cardiomyopathy mutant alpha-tropomyosins in adult cardiac myocytes

Familial hypertrophic cardiomyopathy is a clinically and genetically diverse autosomal dominant disorder characterized by ventricular hypertrophy and myocyte disarray in the absence of known hypertrophic stimuli. It has been linked to many cardiac contractile proteins, including four point mutations in alpha-tropomyosin (Tm). Here we use adenoviral-mediated gene transfer into adult cardiac myocytes in vitro to show that all four hypertrophic cardiomyopathy alpha-Tm proteins can be expressed and incorporated into normal sarcomeric structures in cardiac myocytes at similar levels as normal alpha-Tm proteins after 5-6 days in culture. Isometric force recordings of single cardiac myocytes demonstrated inappropriate increased force output at submaximal calcium concentration with a specific mutant allele hierarchy. These data indicate that the severity of direct calcium-sensitizing effect of mutations in alpha-Tm may predict the clinical severity of mutant alpha-Tm-associated hypertrophic cardiomyopathy.

Functional consequences of troponin T mutations found in hypertrophic cardiomyopathy

Missense mutations in the cardiac thin filament protein troponin T (TnT) are a cause of familial hypertrophic cardiomyopathy (FHC). To understand how these mutations produce dysfunction, five TnTs were produced and purified containing FHC mutations found in several regions of TnT. Functional defects were diverse. Mutations F110I, E244D, and COOH-terminal truncation weakened the affinity of troponin for the thin filament. Mutation DeltaE160 resulted in thin filaments with increased calcium affinity at the regulatory site of troponin C. Mutations R92Q and F110I resulted in impaired troponin solubility, suggesting abnormal protein folding. Depending upon the mutation, the in vitro unloaded actin-myosin sliding speed showed small increases, showed small decreases, or was unchanged. COOH-terminal truncation mutation resulted in a decreased thin filament-myosin subfragment 1 MgATPase rate. The results indicate that the mutations cause diverse immediate effects, despite similarities in disease manifestations. Separable but repeatedly observed abnormalities resulting from FHC TnT mutations include increased unloaded sliding speed, increased or decreased Ca(2+) affinity, impairment of folding or sarcomeric integrity, and decreased force. Enhancement as well as impairment of contractile protein function is observed, suggesting that TnT, including the troponin tail region, modulates the regulation of cardiac contraction.

Regulatory properties of recombinant tropomyosins containing 5-hydroxytryptophan: Ca2+-binding to troponin results in a conformational change in a region of tropomyosin outside the troponin binding site

We have introduced tryptophan codons at different positions of the chicken alpha-tropomyosin cDNA (Monteiro, P. B., Lataro, R. C., Ferro, J. A., and Reinach, F. C. (1994) J. Biol. Chem. 269, 10461-10466) and employed a trp auxotrophic Escherichia coli strain to express the proteins in media containing either normal tryptophan, 5-hydroxytrptophan, or 7-azatryptophan. The fluorescence of these latter two tryptophan analogues is excitable at 312-315 nm at which the natural fluorescence of other thin filament proteins (actin, troponin) is not excited. The recombinant tropomyosins have tryptophans or analogues located at amino acid positions 90, 101, 111, 122, or 185 of the protein, all on the external surface of the tropomyosin coiled-coil (positions "c" or "f" of the hydrophobic heptad repeat). The first four mutations are located within the third actin-binding zone of tropomyosin, a region not expected to interact directly with troponin or with neighboring tropomyosin molecules in muscle thin filaments, while position 185 is located in a region that has been implicated in interactions with the globular domain of troponin. The fluorescence intensity of the mutant containing 5-hydroxytryptophan at position 122 (5OH122W) is sensitive to actin binding and sensitive to Ca2+-binding to thin filaments reconstituted with troponin. Assuming that the globular domain of troponin binds to a site between residues 150 and 190 of tropomyosin, the distance between the troponin-binding site and the fluorescent probes at position 122 can be estimated to be 4.2-10.2 nm. While X-ray diffraction and electron micrograph reconstitution studies have provided evidence of Ca2+-induced changes in tropomyosin's interactions in the thin filament, their resolution was not sufficient to distinguish between changes involving the whole tropomyosin molecule or only that region directly interacting with troponin. Here we provide a clear demonstration that Ca2+-binding to troponin results in a conformational change in a region of tropomyosin outside the troponin binding site which is probably associated with a changed interaction with actin.

The N-terminal region of troponin T is essential for the maximal activation of rat cardiac myofilaments

Troponin T (TnT) is an essential protein in the transduction of the Ca2+-binding signal that triggers striated muscle contraction. Functional diversity among various TnT isoforms found in cardiac and skeletal muscles has been correlated with the sequence heterogeneity at the amino (N-) and the carboxyl (C-) terminal regions. The most striking difference between cardiac TnT (cTnT) and skeletal TnT (sTnT) is that cTnT has an extended N-terminus, which is rich in negatively charged amino acids. To investigate the role of this region in cTnT, we deleted the first 76 amino acids in rat cTnT (cTnT77-289) by site-directed mutagenesis. We exchanged the native troponin complex in rat cardiac myofibrillar preparations and detergent skinned cardiac fiber bundles by treatment with excess cTnT or cTnT77-289. After reconstituting the cTnT77-289 containing myofibrils with cardiac troponin I-cardiac troponin C (cTnI-cTnC), the MgATPase activity was 70% of the cTnT treated myofibrils in the relaxed state and 83% of the cTnT treated myofibrils in the maximal Ca2+-activated state. These observations were supported by force measurements in which cTnT and cTnT77-289 were exchanged into skinned fiber bundles. Prior to reconstitution with cTnI-cTnC, the Ca2+-independent maximal force developed by the cTnT77-289 containing fiber was 45% of the force developed by the cTnT containing fiber. After reconstituting with cTnI-cTnC, the Ca2+-activated maximal force of the cTnT77-289 containing fiber was 62% of the force developed by the cTnT containing +cTnI-cTnC reconstituted fiber. In both assays, no significant changes in the normalized Ca2+-activity relation or in co-operativity were observed. Fluorescence experiments using pyrene-labeled Tm demonstrated that the binding of cTnT77-289 to Tm was 3-4 fold stronger than that of cTnT. Our results suggest that strong interactions between cTnT77-289 and Tm stabilize cardiac myofilaments in a sub-maximally activated state. Our findings also indicate that the N-terminus of cTnT is essential for maximal activation of cardiac myofilaments.

Roles for the troponin tail domain in thin filament assembly and regulation. A deletional study of cardiac troponin T

Striated muscle contraction is regulated by Ca2+ binding to troponin, which has a globular domain and an elongated tail attributable to the NH2-terminal portion of the bovine cardiac troponin T (TnT) subunit. Truncation of the bovine cardiac troponin tail was investigated using recombinant TnT fragments and subunits TnI and TnC. Progressive truncation of the troponin tail caused progressively weaker binding of troponin-tropomyosin to actin and of troponin to actin-tropomyosin. A sharp drop-off in affinity occurred with NH2-terminal deletion of 119 rather than 94 residues. Deletion of 94 residues had no effect on Ca2+-activation of the myosin subfragment 1-thin filament MgATPase rate and did not eliminate cooperative effects of Ca2+ binding. Troponin tail peptide TnT1-153 strongly promoted tropomyosin binding to actin in the absence of TnI or TnC. The results show that the anchoring function of the troponin tail involves interactions with actin as well as with tropomyosin and has comparable importance in the presence or absence of Ca2+. Residues 95-153 are particularly important for anchoring, and residues 95-119 are crucial for function or local folding. Because striated muscle regulation involves switching among the conformational states of the thin filament, regulatory significance for the troponin tail may arise from its prominent contribution to the protein-protein interactions within these conformations.

Structure of the insect troponin complex

Isolated troponin-tropomyosin complex from Lethocerus indicus asynchronous flight muscle forms paracrystals on a positively charged lipid monolayer. Single particle analysis was carried out on individual complexes selected from electron micrographs of negatively stained paracrystals. By a combination of correlation and classification techniques, different average projections of the object were obtained. An initial three-dimensional model was calculated by determining the Euler angles for the different views using a common line approach. This starting model was then used as a reference for the further three-dimensional refinement of the model using the original data set. The refined model of the troponin complex has a diameter of approximately 90 A and a volume corresponding with a molecular mass of about 120 kDa for the globular domain. The resolution of the reconstruction was determined to be 32 A using the differential phase residual method and 26 A using the Fourier shell correlation criterion.

Ca2+-dependent muscle dysfunction caused by mutation of the Caenorhabditis elegans troponin T-1 gene

We have investigated the functions of troponin T (CeTnT-1) in Caenorhabditis elegans embryonic body wall muscle. TnT tethers troponin I (TnI) and troponin C (TnC) to the thin filament via tropomyosin (Tm), and TnT/Tm regulates the activation and inhibition of myosin-actin interaction in response to changes in intracellular [Ca2+]. Loss of CeTnT-1 function causes aberrant muscle trembling and tearing of muscle cells from their exoskeletal attachment sites (Myers, C.D., P.-Y. Goh, T. StC. Allen, E.A. Bucher, and T. Bogaert. 1996. J. Cell Biol. 132:1061-1077). We hypothesized that muscle tearing is a consequence of excessive force generation resulting from defective tethering of Tn complex proteins. Biochemical studies suggest that such defective tethering would result in either (a) Ca2+-independent activation, due to lack of Tn complex binding and consequent lack of inhibition, or (b) delayed reestablishment of TnI/TnC binding to the thin filament after Ca2+ activation and consequent abnormal duration of force. Analyses of animals doubly mutant for CeTnT-1 and for genes required for Ca2+ signaling support that CeTnT-1 phenotypes are dependent on Ca2+ signaling, thus supporting the second model and providing new in vivo evidence that full inhibition of thin filaments in low [Ca2+] does not require TnT.

Developmentally regulated, alternative RNA splicing-generated pectoral muscle-specific troponin T isoforms and role of the NH2-terminal hypervariable region in the tolerance to acidosis

The structure-function relationship of the alternative RNA splicing-generated NH2-terminal variable region of troponin T (TnT) is essential for understanding the physiological significance of developmental or muscle-specific TnT isoforms. Representing the hypervariable nature of the NH2-terminal region, a repeating transition metal-binding sequence (H(E/A)EAH)4-7 (Tx) has been found in chicken fast skeletal muscle TnT. In the present study, the developmentally regulated pectoral muscle-specific expression of this novel TnT isoform has been characterized. It was found that the variable amino terminus determined the isoelectric points of the TnT isoforms expressed, and the adult muscle-specific inclusion of the Tx sequence resulted in pectoralis TnTs, which were significantly more acidic in their NH2-terminal segment versus gastrocnemius TnTs. Experiments testing the effect of pH on TnT interaction with troponin I and tropomyosin indicated that although the interaction of acidic TnT isoforms with troponin I was less sensitive to the decrease of pH than the basic TnTs, the binding affinity of acidic TnT isoforms with tropomyosin was minimally affected by the decreased pH in contrast to basic TnT isoforms. Given that the majority of adult skeletal muscles express basic fast TnT isoforms, the switching between acidic and basic TnT isoforms may play a role in the functional adaptation of muscle to acidosis.