Conserved Asp-137 imparts flexibility to tropomyosin and affects function

Functional and Structural Properties of Cytoplasmic Tropomyosin Isoforms Tpm1.8 and Tpm1.9

The actin cytoskeleton is one of the most important players in cell motility, adhesion, division, and functioning. The regulation of specific microfilament formation largely determines cellular functions. The main actin-binding protein in animal cells is tropomyosin (Tpm). The unique structural and functional diversity of microfilaments is achieved through the diversity of Tpm isoforms. In our work, we studied the properties of the cytoplasmic isoforms Tpm1.8 and Tpm1.9. The results showed that these isoforms are highly thermostable and differ in the stability of their central and C-terminal fragments. The properties of these isoforms were largely determined by the 6th exons. Thus, the strength of the end-to-end interactions, as well as the affinity of the Tpm molecule for F-actin, differed between the Tpm1.8 and Tpm1.9 isoforms. They were determined by whether an alternative internal exon, 6a or 6b, was included in the Tpm isoform structure. The strong interactions of the Tpm1.8 and Tpm1.9 isoforms with F-actin led to the formation of rigid actin filaments, the stiffness of which was measured using an optical trap. It is quite possible that the structural and functional features of the Tpm isoforms largely determine the appearance of these isoforms in the rigid actin structures of the cell cortex.

Glutamate 139 of tropomyosin is critical for cardiac thin filament blocked-state stabilization

The cardiac thin filament proteins troponin and tropomyosin control actomyosin formation and thus cardiac contractility. Calcium binding to troponin changes tropomyosin position along the thin filament, allowing myosin head binding to actin required for heart muscle contraction. The thin filament regulatory proteins are hot spots for genetic mutations causing heart muscle dysfunction. While much of the thin filament structure has been characterized, critical regions of troponin and tropomyosin involved in triggering conformational changes remain unresolved. A poorly resolved region, helix-4 (H4) of troponin I, is thought to stabilize tropomyosin in a position on actin that blocks actomyosin interactions at low calcium concentrations during muscle relaxation. We have proposed that contact between glutamate 139 on tropomyosin and positively charged residues on H4 leads to blocking-state stabilization. In this study, we attempted to disrupt these interactions by replacing E139 with lysine (E139K) to define the importance of this residue in thin filament regulation. Comparison of mutant and wild-type tropomyosin was carried out using in-vitro motility assays, actin co-sedimentation, and molecular dynamics simulations to determine perturbations in troponin-tropomyosin function caused by the tropomyosin mutation. Motility assays revealed that mutant thin filaments moved at higher velocity at low calcium with increased calcium sensitivity demonstrating that tropomyosin residue 139 is vital for proper tropomyosin-mediated inhibition during relaxation. Similarly, molecular dynamic simulations revealed a mutation-induced decrease in interaction energy between tropomyosin-E139K and troponin I (R170 and K174). These results suggest that salt-bridge stabilization of tropomyosin position by troponin IH4 is essential to prevent actomyosin interactions during cardiac muscle relaxation.

Structural and Functional Properties of Kappa Tropomyosin

In the myocardium, the TPM1 gene expresses two isoforms of tropomyosin (Tpm), alpha (αTpm; Tpm 1.1) and kappa (κTpm; Tpm 1.2). κTpm is the result of alternative splicing of the TPM1 gene. We studied the structural features of κTpm and its regulatory function in the atrial and ventricular myocardium using an in vitro motility assay. We tested the possibility of Tpm heterodimer formation from α- and κ-chains. Our result shows that the formation of ακTpm heterodimer is thermodynamically favorable, and in the myocardium, κTpm most likely exists as ακTpm heterodimer. Using circular dichroism, we compared the thermal unfolding of ααTpm, ακTpm, and κκTpm. κκTpm had the lowest stability, while the ακTpm was more stable than ααTpm. The differential scanning calorimetry results indicated that the thermal stability of the N-terminal part of κκTpm is much lower than that of ααTpm. The affinity of ααTpm and κκTpm to F-actin did not differ, and ακTpm interacted with F-actin significantly worse. The troponin T1 fragment enhanced the κκTpm and ακTpm affinity to F-actin. κκTpm differently affected the calcium regulation of the interaction of pig and rat ventricular myosin with the thin filament. With rat myosin, calcium sensitivity of thin filaments containing κκTpm was significantly lower than that with ααTpm and with pig myosin, and the sensitivity did not differ. Thin filaments containing κκTpm and ακTpm were better activated by pig atrial myosin than those containing ααTpm.

Computational and biophysical determination of pathogenicity of variants of unknown significance in cardiac thin filament

Point mutations within sarcomeric proteins have been associated with altered function and cardiomyopathy development. Difficulties remain, however, in establishing the pathogenic potential of individual mutations, often limiting the use of genotype in management of affected families. To directly address this challenge, we utilized our all-atom computational model of the human full cardiac thin filament (CTF) to predict how sequence substitutions in CTF proteins might affect structure and dynamics on an atomistic level. Utilizing molecular dynamics calculations, we simulated 21 well-defined genetic pathogenic cardiac troponin T and tropomyosin variants to establish a baseline of pathogenic changes induced in computational observables. Computational results were verified via differential scanning calorimetry on a subset of variants to develop an experimental correlation. Calculations were performed on 9 independent variants of unknown significance (VUS), and results were compared with pathogenic variants to identify high-resolution pathogenic signatures. Results for VUS were compared with the baseline set to determine induced structural and dynamic changes, and potential variant reclassifications were proposed. This unbiased, high-resolution computational methodology can provide unique structural and dynamic information that can be incorporated into existing analyses to facilitate classification both for de novo variants and those where established approaches have provided conflicting information.

Further Investigation into the Biochemical Effects of Phosphorylation of Tropomyosin Tpm1.1(α). Serine-283 Is in Communication with the Midregion

The phosphorylated and unphosphorylated forms of tropomyosin Tpm1.1(α) are prepared from adult rabbit heart and compared biochemically. Electrophoresis confirms the high level of enrichment of the chromatography fractions and is consistent with a single site of phosphorylation. Covalently bound phosphate groups at position 283 of Tpm1.1(α) increase the rate of digestion at Leu-169, suggestive of a conformational rearrangement that extends to the midregion. Such a rearrangement, which is supported by ellipticity measurements between 25 and 42 °C, is consistent with a phosphorylation-mediated tightening of the interaction between various myofilament components. In a nonradioactive, co-sedimentation assay [30 mM KCl, 1 mM Mg(II), and 4 °C], phosphorylated Tpm1.1(α) displays a higher affinity for F-actin compared to that of the unphosphorylated control (K_d, 0.16 μM vs 0.26 μM). Phosphorylation decreases the concentration of thin filaments (pCa 4 plus ATP) required to attain a half-maximal rate of release of product from a pre-power stroke complex [myosin-S1-2-deoxy-3-O-(N-methylanthraniloyl)ADP-P_i], as investigated by double-mixing stopped-flow fluorescence, suggestive of a change in the proportion of active (turned on) and inactive (turned off) conformers, but similar maximum rates of product release are observed with either type of reconstituted thin filament. Phosphorylated thin filaments (pCa 4 and 8) display a higher affinity for myosin-S1(ADP) versus the control scenario without affecting isotherm steepness. Specific activities of ATP and Tpm1.1(α) are determined during an in vitro incubation of rat cardiac tissue [12 day-old, 50% phosphorylated Tpm1.1(α)] with [³²P]orthophosphate. The incorporation of an isotope into tropomyosin lags behind that of ATP by a factor of approximately 10, indicating that transfer is a comparatively slow process.

Impact of A134 and E218 Amino Acid Residues of Tropomyosin on Its Flexibility and Function

Tropomyosin (Tpm) is one of the major actin-binding proteins that play a crucial role in the regulation of muscle contraction. The flexibility of the Tpm molecule is believed to be vital for its functioning, although its role and significance are under discussion. We choose two sites of the Tpm molecule that presumably have high flexibility and stabilized them with the A134L or E218L substitutions. Applying differential scanning calorimetry (DSC), molecular dynamics (MD), co-sedimentation, trypsin digestion, and in vitro motility assay, we characterized the properties of Tpm molecules with these substitutions. The A134L mutation prevented proteolysis of Tpm molecule by trypsin, and both substitutions increased the thermal stability of Tpm and its bending stiffness estimated from MD simulation. None of these mutations affected the primary binding of Tpm to F-actin; still, both of them increased the thermal stability of the actin-Tpm complex and maximal sliding velocity of regulated thin filaments in vitro at a saturating Ca²⁺ concentration. However, the mutations differently affected the Ca²⁺ sensitivity of the sliding velocity and pulling force produced by myosin heads. The data suggest that both regions of instability are essential for correct regulation and fine-tuning of Ca²⁺-dependent interaction of myosin heads with F-actin.

Threonine-77 Is a Determinant of the Low-Temperature Conditioning of the Most Abundant Isoform of Tropomyosin in Atlantic Salmon

The Atlantic salmon Salmo salar survives below 10 °C. The main skeletal muscle is composed of a single isoform of tropomyosin (classified as Tpm1 α-fast) that is >92% identical to the mammalian homologue. How salmon Tpm1 maintains flexibility is investigated by reversing the only full charge substitution; threonine-77(g) in salmon and lysine in other vertebrates. The mutation (Thr-77 to Lys), which falls within a known destabilizing alanine cluster, (i) yields a useful electrophoretic shift in the absence and presence of an anionic detergent, (ii) increases the T_ms of both cooperative transitions (calorimetry, 0.1 M salt, pH 7) [35 °C (minor) and 44 °C (major); ΔT_m1 = 5 °C, ΔT_m2 = 3.5 °C], (iii) increases the T_m of CN1A (residues 11-127) to 53 °C (ΔT_m = 13 °C), a value similar to that of mammalian CN1A, (iv) markedly reduces the rate of proteolysis at Leu-169, and (v) weakens the affinity of salmon Tpm1 for troponin-Sepharose. Glu-82(e), the interstrand ionic partner of Lys-77(g), is conserved. The change in ionic interactions at this locus is postulated to be "sensed" in actin period 5 (residues 166-207) and likely beyond. Wild type (acetylated) salmon Tmp1 binds more tightly to F-actin at 4 °C than at 22 °C, which is the opposite of the long-known relationship displayed by the mammalian homologue. All of the evidence indicates that the presence of a neutral 77th amino acid destabilizes a sizable portion of salmon Tpm1 that includes the midregion. Threonine-77 is a key factor in rescuing the thin filament from the peril of cold-induced rigidity.

A new twist on tropomyosin binding to actin filaments: perspectives on thin filament function, assembly and biomechanics

Tropomyosin, best known for its role in the steric regulation of muscle contraction, polymerizes head-to-tail to form cables localized along the length of both muscle and non-muscle actin-based thin filaments. In skeletal and cardiac muscles, tropomyosin, under the control of troponin and myosin, moves in a cooperative manner between blocked, closed and open positions on filaments, thereby masking and exposing actin-binding sites necessary for myosin crossbridge head interactions. While the coiled-coil signature of tropomyosin appears to be simple, closer inspection reveals surprising structural complexity required to perform its role in steric regulation. For example, component α-helices of coiled coils are typically zippered together along a continuous core hydrophobic stripe. Tropomyosin, however, contains a number of anomalous, functionally controversial, core amino acid residues. We argue that the atypical residues at this interface, including clusters of alanines and a charged aspartate, are required for preshaping tropomyosin to readily fit to the surface of the actin filament, but do so without compromising tropomyosin rigidity once the filament is assembled. Indeed, persistence length measurements of tropomyosin are characteristic of a semi-rigid cable, in this case conducive to cooperative movement on thin filaments. In addition, we also maintain that tropomyosin displays largely unrecognized and residue-specific torsional variance, which is involved in optimizing contacts between actin and tropomyosin on the assembled thin filament. Corresponding twist-induced stiffness may also enhance cooperative translocation of tropomyosin across actin filaments. We conclude that anomalous core residues of tropomyosin facilitate thin filament regulatory behavior in a multifaceted way.

The Effect of Tropomyosin Mutations on Actin-Tropomyosin Binding: In Search of Lost Time

The initial binding of tropomyosin onto actin filaments and then its polymerization into continuous cables on the filament surface must be precisely tuned to overall thin-filament structure, function, and performance. Low-affinity interaction of tropomyosin with actin has to be sufficiently strong to localize the tropomyosin on actin, yet not so tight that regulatory movement on filaments is curtailed. Likewise, head-to-tail association of tropomyosin molecules must be favorable enough to promote tropomyosin cable formation but not so tenacious that polymerization precedes filament binding. Arguably, little molecular detail on early tropomyosin binding steps has been revealed since Wegner's seminal studies on filament assembly almost 40 years ago. Thus, interpretation of mutation-based actin-tropomyosin binding anomalies leading to cardiomyopathies cannot be described fully. In vitro, tropomyosin binding is masked by explosive tropomyosin polymerization once cable formation is initiated on actin filaments. In contrast, in silico analysis, characterizing molecular dynamics simulations of single wild-type and mutant tropomyosin molecules on F-actin, is not complicated by tropomyosin polymerization at all. In fact, molecular dynamics performed here demonstrates that a midpiece tropomyosin domain is essential for normal actin-tropomyosin interaction and that this interaction is strictly conserved in a number of tropomyosin mutant species. Elsewhere along these mutant molecules, twisting and bending corrupts the tropomyosin superhelices as they "lose their grip" on F-actin. We propose that residual interactions displayed by these mutant tropomyosin structures with actin mimic ones that occur in early stages of thin-filament generation, as if the mutants are recapitulating the assembly process but in reverse. We conclude therefore that an initial binding step in tropomyosin assembly onto actin involves interaction of the essential centrally located domain.

Proof of Principle that Molecular Modeling Followed by a Biophysical Experiment Can Develop Small Molecules that Restore Function to the Cardiac Thin Filament in the Presence of Cardiomyopathic Mutations

This article reports a coupled computational experimental approach to design small molecules aimed at targeting genetic cardiomyopathies. We begin with a fully atomistic model of the cardiac thin filament. To this we dock molecules using accepted computational drug binding methodologies. The candidates are screened for their ability to repair alterations in biophysical properties caused by mutation. Hypertrophic and dilated cardiomyopathies caused by mutation are initially biophysical in nature, and the approach we take is to correct the biophysical insult prior to irreversible cardiac damage. Candidate molecules are then tested experimentally for both binding and biophysical properties. This is a proof of concept study-eventually candidate molecules will be tested in transgenic animal models of genetic (sarcomeric) cardiomyopathies.

Effects of an Interchain Disulfide Bond on Tropomyosin Structure: A Molecular Dynamics Study

Tropomyosin (Tpm) is a coiled-coil actin-binding dimer protein that participates in the regulation of muscle contraction. Both Tpm chains contain Cys190 residues which are normally in the reduced state, but form an interchain disulfide bond in failing heart. Changes in structural and functional properties of Tpm and its complexes with actin upon disulfide cross-linking were studied using various experimental methods. To understand the molecular mechanism underlying these changes and to reveal the possible mechanism of the involvement of the cross-linking in heart failure, molecular dynamics (MD) simulations of the middle part of Tpm were performed in cross-linked and reduced states. The cross-linking increased bending stiffness of Tpm assessed from MD trajectories at 27 °C in agreement with previous experimental observations. However, at 40 °C, the cross-linking caused a decrease in Tpm stiffness and a significant reduction in the number of main chain hydrogen bonds in the vicinity of residues 133 and 134. These data are in line with observations showing enhanced thermal unfolding of the least stable part of Tpm at 30–40 °C and accelerated trypsin cleavage at residue 133 at 40 °C (but not at 27 °C) upon cross-linking. These results allow us to speculate about the possible mechanism of involvement of Tpm cross-linking to heart failure pathogenesis.

Precise Binding of Tropomyosin on Actin Involves Sequence-Dependent Variance in Coiled-Coil Twisting

Often considered an archetypal dimeric coiled coil, tropomyosin nonetheless exhibits distinctive "noncanonical" core residues located at the hydrophobic interface between its component α-helices. Notably, a charged aspartate, D137, takes the place of nonpolar residues otherwise present. Much speculation has been offered to rationalize potential local coiled-coil instability stemming from D137 and its effect on regulatory transitions of tropomyosin over actin filaments. Although experimental approaches such as electron cryomicroscopy reconstruction are optimal for defining average tropomyosin positions on actin filaments, to date, these methods have not captured the dynamics of tropomyosin residues clustered around position 137 or elsewhere. In contrast, computational biochemistry, involving molecular dynamics simulation, is a compelling choice to extend the understanding of local and global tropomyosin behavior on actin filaments at high resolution. Here, we report on molecular dynamics simulation of actin-free and actin-associated tropomyosin, showing noncanonical residue D137 as a locus for tropomyosin twist variation, with marked effects on actin-tropomyosin interactions. We conclude that D137-sponsored coiled-coil twisting is likely to optimize electrostatic side-chain contacts between tropomyosin and actin on the assembled thin filament, while offsetting disparities between tropomyosin pseudorepeat and actin subunit periodicities. We find that D137 has only minor local effects on tropomyosin coiled-coil flexibility, (i.e., on its flexural mobility). Indeed, D137-associated overtwisting may actually augment tropomyosin stiffness on actin filaments. Accordingly, such twisting-induced stiffness of tropomyosin is expected to enhance cooperative regulatory translocation of the tropomyosin cable over actin.

Structural and Dynamic Properties of Allergen and Non-Allergen Forms of Tropomyosin

To what extent do structural and biophysical features of food allergen proteins distinguish them from other proteins in our diet? Invertebrate tropomyosins (Tpms) as a class are considered "pan-allergens," inducing food allergy to shellfish and respiratory allergy to dust mites. Vertebrate Tpms are not known to elicit allergy or cross-reactivity, despite their high structural similarity and sequence identity to invertebrate homologs. We expect allergens are sufficiently stable against gastrointestinal proteases to survive for immune sensitization in the intestines, and that proteolytic stability will correlate with thermodynamic stability. Thermal denaturation of shrimp Tpm shows that it is more stable than non-allergen vertebrate Tpm. Shrimp Tpm is also more resistant to digestion. Molecular dynamics uncover local dynamics that select epitopes and global differences in flexibility between shrimp and pig Tpm that discriminate allergens from non-allergens. Molecular determinants of allergenicity depend not only on sequence but on contributions of protein structure and dynamics.

HCM and DCM cardiomyopathy-linked α-tropomyosin mutations influence off-state stability and crossbridge interaction on thin filaments

Calcium regulation of cardiac muscle contraction is controlled by the thin-filament proteins troponin and tropomyosin bound to actin. In the absence of calcium, troponin-tropomyosin inhibits myosin-interactions on actin and induces muscle relaxation, whereas the addition of calcium relieves the inhibitory constraint to initiate contraction. Many mutations in thin filament proteins linked to cardiomyopathy appear to disrupt this regulatory switching. Here, we tested perturbations caused by mutant tropomyosins (E40K, DCM; and E62Q, HCM) on intra-filament interactions affecting acto-myosin interactions including those induced further by myosin association. Comparison of wild-type and mutant human α-tropomyosin (Tpm1.1) behavior was carried out using in vitro motility assays and molecular dynamics simulations. Our results show that E62Q tropomyosin destabilizes thin filament off-state function by increasing calcium-sensitivity, but without apparent affect on global tropomyosin structure by modifying coiled-coil rigidity. In contrast, the E40K mutant tropomyosin appears to stabilize the off-state, demonstrates increased tropomyosin flexibility, while also decreasing calcium-sensitivity. In addition, the E40K mutation reduces thin filament velocity at low myosin concentration while the E62Q mutant tropomyosin increases velocity. Corresponding molecular dynamics simulations indicate specific residue interactions that are likely to redefine underlying molecular regulatory mechanisms, which we propose explain the altered contractility evoked by the disease-causing mutations.

Functional role of the core gap in the middle part of tropomyosin

Tropomyosin (Tpm) is an α-helical coiled-coil actin-binding protein playing an essential role in the regulation of muscle contraction. The middle part of the Tpm molecule has some specific features, such as the presence of noncanonical residues as well as a substantial gap at the interhelical interface, which are believed to destabilize a coiled-coil and impart structural flexibility to this part of the molecule. To study how the gap affects structural and functional properties of α-striated Tpm (the Tpm1.1 isoform that is expressed in cardiac and skeletal muscles) we replaced large conserved apolar core residues located at both sides of the gap with smaller ones by mutations M127A/I130A and M141A/Q144A. We found that in contrast with the stabilizing substitutions D137L and G126R studied earlier, these substitutions have no appreciable influence on thermal unfolding and domain structure of the Tpm molecule. They also do not affect actin-binding properties of Tpm. However, they strongly increase sliding velocity of regulated actin filaments in an in vitro motility assay and cause an oversensitivity of the velocity to Ca²⁺ similar to the stabilizing substitutions D137L and G126R. Molecular dynamics shows that the substitutions studied here increase bending stiffness of the coiled-coil structure of Tpm, like that of G126R/D137L, probably due to closure of the interhelical gap in the area of the substitutions. Our results clearly indicate that the conserved middle part of Tpm is important for the fine tuning of the Ca²⁺ regulation of actin-myosin interaction in muscle.

The Relaxation Properties of Myofibrils Are Compromised by Amino Acids that Stabilize α-Tropomyosin

We investigated the functional impact of α-tropomyosin (Tm) substituted with one (D137L) or two (D137L/G126R) stabilizing amino acid substitutions on the mechanical behavior of rabbit psoas skeletal myofibrils by replacing endogenous Tm and troponin (Tn) with recombinant Tm mutants and purified skeletal Tn. Force recordings from myofibrils (15°C) at saturating [Ca²⁺] showed that Tm-stabilizing substitutions did not significantly affect the maximal isometric tension and the rates of force activation (k_ACT) and redevelopment (k_TR). However, a clear effect was observed on force relaxation: myofibrils with D137L/G126R or D137L Tm showed prolonged durations of the slow phase of relaxation and decreased rates of the fast phase. Both Tm-stabilizing substitutions strongly decreased the slack sarcomere length (SL) at submaximal activating [Ca²⁺] and increased the steepness of the SL-passive tension relation. These effects were reversed by addition of 10 mM 2,3-butanedione 2-monoxime. Myofibrils also showed an apparent increase in Ca²⁺ sensitivity. Measurements of myofibrillar ATPase activity in the absence of Ca²⁺ showed a significant increase in the presence of these Tms, indicating that single and double stabilizing substitutions compromise the full inhibition of contraction in the relaxed state. These data can be understood with the three-state (blocked-closed-open) theory of muscle regulation, according to which the mutations increase the contribution of the active open state in the absence of Ca²⁺ (M^-). Force measurements on myofibrils substituted with C-terminal truncated TnI showed similar compromised relaxation effects, indicating the importance of TnI-Tm interactions in maintaining the blocked state. It appears that reducing the flexibility of native Tm coiled-coil structure decreases the optimum interactions of the central part of Tm with the C-terminal region of TnI. This results in a shift away from the blocked state, allowing myosin binding and activity in the absence of Ca²⁺. This work provides a basis for understanding the effects of disease-producing mutations in muscle proteins.

Clinically Divergent Mutation Effects on the Structure and Function of the Human Cardiac Tropomyosin Overlap

The progression of genetically inherited cardiomyopathies from an altered protein structure to clinical presentation of disease is not well understood. One of the main roadblocks to mechanistic insight remains a lack of high-resolution structural information about multiprotein complexes within the cardiac sarcomere. One example is the tropomyosin (Tm) overlap region of the thin filament that is crucial for the function of the cardiac sarcomere. To address this central question, we devised coupled experimental and computational modalities to characterize the baseline function and structure of the Tm overlap, as well as the effects of mutations causing divergent patterns of ventricular remodeling on both structure and function. Because the Tm overlap contributes to the cooperativity of myofilament activation, we hypothesized that mutations that enhance the interactions between overlap proteins result in more cooperativity, and conversely, those that weaken interaction between these elements lower cooperativity. Our results suggest that the Tm overlap region is affected differentially by dilated cardiomyopathy-associated Tm D230N and hypertrophic cardiomyopathy-associated human cardiac troponin T (cTnT) R92L. The Tm D230N mutation compacts the Tm overlap region, increasing the cooperativity of the Tm filament, contributing to a dilated cardiomyopathy phenotype. The cTnT R92L mutation causes weakened interactions closer to the N-terminal end of the overlap, resulting in decreased cooperativity. These studies demonstrate that mutations with differential phenotypes exert opposite effects on the Tm-Tn overlap, and that these effects can be directly correlated to a molecular level understanding of the structure and dynamics of the component proteins.

The structural basis of alpha-tropomyosin linked (Asp230Asn) familial dilated cardiomyopathy

Recently, linkage analysis of two large unrelated multigenerational families identified a novel dilated cardiomyopathy (DCM)-linked mutation in the gene coding for alpha-tropomyosin (TPM1) resulting in the substitution of an aspartic acid for an asparagine (at residue 230). To determine how a single amino acid mutation in α-tropomyosin (Tm) can lead to a highly penetrant DCM we generated a novel transgenic mouse model carrying the D230N mutation. The resultant mouse model strongly phenocopied the early onset of cardiomyopathic remodeling observed in patients as significant systolic dysfunction was observed by 2months of age. To determine the precise cellular mechanism(s) leading to the observed cardiac pathology we examined the effect of the mutation on Ca²⁺ handling in isolated myocytes and myofilament activation in vitro. D230N-Tm filaments exhibited a reduced Ca²⁺ sensitivity of sliding velocity. This decrease in sensitivity was coupled to increase in the peak amplitude of Ca²⁺ transients. While significant, and consistent with other DCMs, these measurements are comprised of complex inputs and did not provide sufficient experimental resolution. We then assessed the primary structural effects of D230N-Tm. Measurements of the thermal unfolding of D230N-Tm vs WT-Tm revealed an increase in stability primarily affecting the C-terminus of the Tm coiled-coil. We conclude that the D230N-Tm mutation induces a decrease in flexibility of the C-terminus via propagation through the helical structure of the protein, thus decreasing the flexibility of the Tm overlap and impairing its ability to regulate contraction. Understanding this unique structural mechanism could provide novel targets for eventual therapeutic interventions in patients with Tm-linked cardiomyopathies.

Cooperativity of myosin interaction with thin filaments is enhanced by stabilizing substitutions in tropomyosin

Muscle contraction is powered by myosin interaction with actin-based thin filaments containing Ca²⁺-regulatory proteins, tropomyosin and troponin. Coiled-coil tropomyosin molecules form a long helical strand that winds around actin filament and either shields actin from myosin binding or opens it. Non-canonical residues G126 and D137 in the central part of tropomyosin destabilize its coiled-coil structure. Their substitutions for canonical ones, G126R and D137L, increase structural stability and the velocity of sliding of reconstructed thin filaments along myosin coated surface. The effect of these stabilizing mutations on force of the actin-myosin interaction is unknown. It also remains unclear whether the stabilization affects single actin-myosin interactions or it modifies the cooperativity of the binding of myosin molecules to actin. We used an optical trap to measure the effects of the stabilization on step size, unitary force and duration of the interactions at low and high load and compared the results with those obtained in an in vitro motility assay. We found that significant prolongation of lifetime of the actin-myosin complex under high load observed at high extent of tropomyosin stabilization, i.e. with double mutant, G126R/D137L, correlates with higher force in the motility assay. Also, the higher the extent of stabilization of tropomyosin, the fewer myosin molecules are needed to propel the thin filaments. The data suggest that the effects of the stabilizing mutations in tropomyosin on the myosin interaction with regulated thin filaments are mainly realized via cooperative mechanisms by increasing the size of cooperative unit.

Tropomyosin Structure, Function, and Interactions: A Dynamic Regulator

Tropomyosin is the archetypal-coiled coil, yet studies of its structure and function have proven it to be a dynamic regulator of actin filament function in muscle and non-muscle cells. Here we review aspects of its structure that deviate from canonical leucine zipper coiled coils that allow tropomyosin to bind to actin, regulate myosin, and interact directly and indirectly with actin-binding proteins. Four genes encode tropomyosins in vertebrates, with additional diversity that results from alternate promoters and alternatively spliced exons. At the same time that periodic motifs for binding actin and regulating myosin are conserved, isoform-specific domains allow for specific interaction with myosins and actin filament regulatory proteins, including troponin. Tropomyosin can be viewed as a universal regulator of the actin cytoskeleton that specifies actin filaments for cellular and intracellular functions.

The impact of tropomyosins on actin filament assembly is isoform specific

Tropomyosin (Tpm) is an α helical coiled-coil dimer that forms a co-polymer along the actin filament. Tpm is involved in the regulation of actin's interaction with binding proteins as well as stabilization of the actin filament and its assembly kinetics. Recent studies show that multiple Tpm isoforms also define the functional properties of distinct actin filament populations within a cell. Subtle structural variations within well conserved Tpm isoforms are the key to their functional specificity. Therefore, we purified and characterized a comprehensive set of 8 Tpm isoforms (Tpm1.1, Tpm1.12, Tpm1.6, Tpm1.7, Tpm1.8, Tpm2.1, Tpm3.1, and Tpm4.2), using well-established actin co-sedimentation and pyrene fluorescence polymerization assays. We observed that the apparent affinity (Kd(app)) to filamentous actin varied in all Tpm isoforms between ∼0.1-5 μM with similar values for both, skeletal and cytoskeletal actin filaments. The data did not indicate any correlation between affinity and size of Tpm molecules, however high molecular weight (HMW) isoforms Tpm1.1, Tpm1.6, Tpm1.7 and Tpm2.1, showed ∼3-fold higher cooperativity compared to low molecular weight (LMW) isoforms Tpm1.12, Tpm1.8, Tpm3.1, and Tpm4.2. The rate of actin filament elongation in the presence of Tpm2.1 increased, while all other isoforms decreased the elongation rate by 27-85 %. Our study shows that the biochemical properties of Tpm isoforms are finely tuned and depend on sequence variations in alternatively spliced regions of Tpm molecules.

Stabilizing the central part of tropomyosin increases the bending stiffness of the thin filament

A two-beam optical trap was used to measure the bending stiffness of F-actin and reconstructed thin filaments. A dumbbell was formed by a filament segment attached to two beads that were held in the two optical traps. One trap was static and held a bead used as a force transducer, whereas an acoustooptical deflector moved the beam holding the second bead, causing stretch of the dumbbell. The distance between the beads was measured using image analysis of micrographs. An exact solution to the problem of bending of an elastic filament attached to two beads and subjected to a stretch was used for data analysis. Substitution of noncanonical residues in the central part of tropomyosin with canonical ones, G126R and D137L, and especially their combination, caused an increase in the bending stiffness of the thin filaments. The data confirm that the effect of these mutations on the regulation of actin-myosin interactions may be caused by an increase in tropomyosin stiffness.

Tropomyosin diffusion over actin subunits facilitates thin filament assembly

Coiled-coil tropomyosin binds to consecutive actin-subunits along actin-containing thin filaments. Tropomyosin molecules then polymerize head-to-tail to form cables that wrap helically around the filaments. Little is known about the assembly process that leads to continuous, gap-free tropomyosin cable formation. We propose that tropomyosin molecules diffuse over the actin-filament surface to connect head-to-tail to partners. This possibility is likely because (1) tropomyosin hovers loosely over the actin-filament, thus binding weakly to F-actin and (2) low energy-barriers provide tropomyosin freedom for 1D axial translation on F-actin. We consider that these unique features of the actin-tropomyosin interaction are the basis of tropomyosin cable formation.

Tropomyosin as a Regulator of Actin Dynamics

Tropomyosin is a major regulatory protein of contractile systems and cytoskeleton, an actin-binding protein that positions laterally along actin filaments and modulates actin-myosin interaction. About 40 tropomyosin isoforms have been found in a variety of cytoskeleton systems, not necessarily connected with actin-myosin interaction and contraction. Involvement of specific tropomyosin isoforms in the regulation of key cell processes was shown, and specific features of tropomyosin genes and protein structure have been investigated with molecular biology and genetics approaches. However, the mechanisms underlying the effects of tropomyosin on cytoskeleton dynamics are still unclear. As tropomyosin is primarily an F-actin-binding protein, it is important to understand how it interacts both with actin and actin-binding proteins functioning in muscles and cytoskeleton to regulate actin dynamics. This review focuses on biochemical data on the effects of tropomyosin on actin assembly and dynamics, as well as on the modulation of these effects by actin-binding proteins. The data indicate that tropomyosin can efficiently regulate actin dynamics via allosteric conformational changes within actin filaments.

Pathogenesis associated with a restrictive cardiomyopathy mutant in cardiac troponin T is due to reduced protein stability and greatly increased myofilament Ca2+ sensitivity

BACKGROUND

Dilated and hypertrophic cardiomyopathy mutations in troponin can blunt effects of protein kinase A (PKA) phosphorylation of cardiac troponin I (cTnI), decreasing myofilament Ca2+-sensitivity; however this effect has never been tested for restrictive cardiomyopathy (RCM) mutants. This study explores whether an RCM cardiac troponin T mutant (cTnT-ΔE96) interferes with convergent PKA regulation and if TnT instability contributes to greatly enhanced Ca2+-sensitivity in skinned fibers.

METHODS

Force of contraction in skinned cardiac porcine fiber and spectroscopic studies were performed.

RESULTS

A decrease of -0.26 and -0.25 pCa units in Ca2+-sensitivity of contraction after PKA incubation was observed for skinned fibers incorporated with WT or cTnT-ΔE96, respectively. To further assess whether cTnT-ΔE96 interferes solely with transmission of cTnI phosphorylation effects, skinned fibers were reconstituted with PKA pseudo-phosphorylated cTnI (cTnI-SS/DD.cTnC). Fibers displaced with cTnT-WT, reconstituted with cTnI-SS/DD.cTnC decreased Ca2+-sensitivity of force (pCa50=5.61) compared to control cTnI-WT.cTnC (pCa50=5.75), similarly affecting cTnT-ΔE96 (pCa50=6.03) compared to control \cTnI-WT.cTnC (pCa50=6.14). Fluorescence studies measuring cTnC(IAANS) Ca2+-affinity changes due to cTnT-ΔE96 indicated that higher complexity (thin filament) better recapitulates skinned fiber Ca2+ sensitive changes. Circular dichroism revealed reduced α-helicity and earlier thermal unfolding for cTnT-ΔE96 compared to WT.

CONCLUSIONS

Although ineffective in decreasing myofilament Ca2+-sensitivity to normal levels, cTnT-ΔE96 does not interfere with PKA cTnI phosphorylation mediated effects; 2) cTnT-ΔE96 requires actin to increase cTnC Ca2+-affinity; and 3) deletion of E96 reduces cTnT stability, likely disrupting crucial thin filament interactions.

GENERAL SIGNIFICANCE

The pathological effect of cTnT-ΔE96 is largely manifested by dramatic myofilament Ca2+-sensitization which still persists even after PKA phosphorylation mediated Ca2+-desensitization.

Structural implications of conserved aspartate residues located in tropomyosin's coiled-coil core

Polar residues lying between adjacent α-helical chains of coiled-coils often contribute to coiled-coil curvature and flexibility, while more typical core hydrophobic residues anneal the chains together. In tropomyosins, ranging from smooth and skeletal muscle to cytoplasmic isoforms, a highly conserved Asp at residue 137 places negative charges within the tropomyosin coiled-coil core in a position which may affect the conformation needed for tropomyosin binding and regulatory movements on actin. Proteolytic susceptibility suggested that substituting a canonical Leu for the naturally occurring Asp at residue 137 increases inter-chain rigidity by stabilizing the tropomyosin coiled-coil. Using molecular dynamics, we now directly assess changes in coiled-coil curvature and flexibility caused by such mutants. Although the coiled-coil flexibility is modestly diminished near the residue 137 mutation site, as expected, a delocalized increase in flexibility along the overall coiled-coil is observed. Even though the average shape of the D137L tropomyosin is straighter than that of wild-type tropomyosin, it is still capable of binding actin due to this increase in flexibility. We conclude that the conserved, non-canonical Asp-137 destabilizes the local structure resulting in a local flexible region in the middle of tropomyosin that normally is important for tropomyosin steady-state equilibrium position on actin.

cDNA clone and expression analysis of α-Tropomyosin during Japanese flounder (Paralichthys olivaceus) metamorphosis

Tropomyosin (TM) plays a critical role in skeletal and cardiac muscle development and function. To assess the functional significance of α-TM in Japanese flounder (Paralichthys olivaceus) development and metamorphosis, cDNA from Japanese flounder was cloned and α-TM mRNA measured during development and metamorphosis. The full-length cDNA is 1 191 bp, including a 5'-untranslated region of 114 bp, a 3'-UTR of 222 bp, and an open reading frame of 855 bp encoding a polypeptide of 284 amino acids. Real-time quantitative PCR revealed that α-TM mRNA is initially expressed in unfertilized ovum, indicating the α-TM gene is maternal. Relatively low mRNA levels were observed in different embryonic stages. A higher level of α-TM mRNA was detected 3 days post hatching (dph), while the highest level was measured at 29 dph (metamorphic climax) after which it declined towards the end of metamorphosis. The expression of α-TM mRNA was up-regulated in thyroid hormone-treated larvae at 36 dph, but there was no marked difference at other stages when compared to control animals. After thiourea treatment, the expression of α-TM mRNA declined slightly. These data provide basic information that can be utilized in further studies into the role of α-TM in P. olivaceus development and metamorphosis.

New insights into the allergenicity of tropomyosin: a bioinformatics approach

The invertebrate panallergen tropomyosin is a protein with an extremely simple folding. This makes it a perfect target for investigating structural differences between invertebrate and vertebrate tropomyosins, which are not considered allergenic. Phylogenetic and sequence analyses were conducted in order to explore the differences in primary structure between several tropomyosins and to promote an experimental development in the field of food allergy, based on the study of tropomyosin. The phylogenetic analyses showed that tropomyosin is a useful evolutionary marker. The phylogenetic trees obtained with tropomyosin were not always phylogenetically correct, but they might be useful for allergen avoidance by tropomyosin allergic individuals. Sequence analyses revealed that the probability of alpha helix folding in invertebrate tropomyosins was lower than in all the studied vertebrate ones, except for the Atlantic bluefin tuna Thunnus thynnus tropomyosin. This suggested that the lack of alpha helix folding may be involved in the immunogenicity of tropomyosins. More specifically, the regions adjacent to the positions 133-135 and 201 of the invertebrate tropomyosins, presented lower probability of alpha helix folding than those of vertebrates and are candidates to be responsible for their allergenicity.

Instability in the central region of tropomyosin modulates the function of its overlapping ends

The causal link between disparate tropomyosin (Tm) functions and the structural instability in Tm is unknown. To test the hypothesis that the structural instability in the central region of Tm modulates the function of the overlapping ends of contiguous Tm dimers, we used transgenic mice (Tm(DM)) that expressed a mutant α-Tm in the heart; S229E and H276N substitutions induce structural instability in the central region and the overlapping ends of Tm, respectively. In addition, two mouse cardiac troponin T mutants (TnT(1-44Δ) and TnT(45-74Δ)) that have a divergent effect on the overlapping ends of Tm were employed. The S229E-induced instability in the central region of Tm(DM) altered the overlapping ends of Tm(DM), thereby it negated the attenuating effect of H276N on Ca(2+)-activated maximal tension. The rate of cross-bridge detachment (g) decreased in Tm(DM)+TnT(WT) and Tm(H276N)+TnT(WT) fibers but increased in Tm(DM)+TnT(45-74Δ) fibers; however, TnT(45-74Δ) did not alter g, demonstrating that S229E in Tm(DM) had divergent effects on g. The S229E substitution in Tm(DM) ablated the H276N-induced desensitization of myofilament Ca(2+) sensitivity in Tm(DM)+TnT(1-44Δ) fibers. To our knowledge, novel findings from this study show that the structural instability in the central region of Tm modifies cardiac contractile function via its effect on the overlapping ends of contiguous Tm.

Probing the flexibility of tropomyosin and its binding to filamentous actin using molecular dynamics simulations

Tropomyosin (Tm) is a coiled-coil protein that binds to filamentous actin (F-actin) and regulates its interactions with actin-binding proteins like myosin by moving between three positions on F-actin (the blocked, closed, and open positions). To elucidate the molecular details of Tm flexibility in relation to its binding to F-actin, we conducted extensive molecular dynamics simulations for both Tm alone and Tm-F-actin complex in the presence of explicit solvent (total simulation time >400 ns). Based on the simulations, we systematically analyzed the local flexibility of the Tm coiled coil using multiple parameters. We found a good correlation between the regions with high local flexibility and a number of destabilizing regions in Tm, including six clusters of core alanines. Despite the stabilization by F-actin binding, the distribution of local flexibility in Tm is largely unchanged in the absence and presence of F-actin. Our simulations showed variable fluctuations of individual Tm periods from the closed position toward the open position. In addition, we performed Tm-F-actin binding calculations based on the simulation trajectories, which support the importance of Tm flexibility to Tm-F-actin binding. We identified key residues of Tm involved in its dynamic interactions with F-actin, many of which have been found in recent mutational studies to be functionally important, and the rest of which will make promising targets for future mutational experiments.

Structural and functional effects of two stabilizing substitutions, D137L and G126R, in the middle part of α-tropomyosin molecule

Tropomyosin (Tm) is an α-helical coiled-coil protein that binds along the length of actin filament and plays an essential role in the regulation of muscle contraction. There are two highly conserved non-canonical residues in the middle part of the Tm molecule, Asp137 and Gly126, which are thought to impart conformational instability (flexibility) to this region of Tm which is considered crucial for its regulatory functions. It was shown previously that replacement of these residues by canonical ones (Leu substitution for Asp137 and Arg substitution for Gly126) results in stabilization of the coiled-coil in the middle of Tm and affects its regulatory function. Here we employed various methods to compare structural and functional features of Tm mutants carrying stabilizing substitutions Arg137Leu and Gly126Arg. Moreover, we for the first time analyzed the properties of Tm carrying both these substitutions within the same molecule. The results show that both substitutions similarly stabilize the Tm coiled-coil structure, and their combined action leads to further significant stabilization of the Tm molecule. This stabilization not only enhances maximal sliding velocity of regulated actin filaments in the in vitro motility assay at high Ca(2+) concentrations but also increases Ca(2+) sensitivity of the actin-myosin interaction underlying this sliding. We propose that the effects of these substitutions on the Ca(2+)-regulated actin-myosin interaction can be accounted for not only by decreased flexibility of actin-bound Tm but also by their influence on the interactions between the middle part of Tm and certain sites of the myosin head.

Tropomyosin dynamics

Tropomyosin is a two chained α-helical coiled coil protein that binds actin filaments and interacts with various actin binding proteins. Tropomyosin function depends on its ability to move to distinct locations on the surface of actin in response to the binding of different thin filament effectors. Tropomyosin dynamics plays an important role in these fluctuating interactions with actin and is thought to be fundamental to many of its biological activities. For example tropomyosin concerted movement on the surface of actin triggered by Ca(2+) binding to troponin or myosin head binding to actin has been argued to be key to the cooperative allosteric regulation of muscle contraction. These large-scale motions are affected by tropomyosin internal dynamics and mechanical properties. Tropomyosin internal dynamics corresponding to smaller and more localised structural fluctuations are increasingly recognised to play an important role in its function. A thorough understanding of the coupling between local and global structural fluctuations in tropomyosin is required to understand how time dependent structural fluctuations in tropomyosin contribute to the overall thin filament dynamics and dictate their various biological activities.

Maladaptive modifications in myofilament proteins and triggers in the progression to heart failure and sudden death

In this review, we address the following question: Are modifications at the level of sarcomeric proteins in acquired heart failure early inducers of altered cardiac dynamics and signaling leading to remodeling and progression to decompensation? There is no doubt that most inherited cardiomyopathies are caused by mutations in proteins of the sarcomere. We think this linkage indicates that early changes at the level of the sarcomeres in acquired cardiac disorders may be significant in triggering the progression to failure. We consider evidence that there are rate-limiting mechanisms downstream of the trigger event of Ca(2+) binding to troponin C, which control cardiac dynamics. We discuss new perspectives on how modifications in these mechanisms may be of relevance to redox signaling in diastolic heart failure, to angiotensin II signaling via β-arrestin, and to remodeling related to altered structural rigidity of tropomyosin. We think that these new perspectives provide a rationale for future studies directed at a more thorough understanding of the question driving our review.

Conserved Asp-137 is important for both structure and regulatory functions of cardiac α-tropomyosin (α-TM) in a novel transgenic mouse model expressing α-TM-D137L

α-Tropomyosin (α-TM) has a conserved, charged Asp-137 residue located in the hydrophobic core of its coiled-coil structure, which is unusual in that the residue is found at a position typically occupied by a hydrophobic residue. Asp-137 is thought to destabilize the coiled-coil and so impart structural flexibility to the molecule, which is believed to be crucial for its function in the heart. A previous in vitro study indicated that the conversion of Asp-137 to a more typical canonical Leu alters flexibility of TM and affects its in vitro regulatory functions. However, the physiological importance of the residue Asp-137 and altered TM flexibility is unknown. In this study, we further analyzed structural properties of the α-TM-D137L variant and addressed the physiological importance of TM flexibility in cardiac function in studies with a novel transgenic mouse model expressing α-TM-D137L in the heart. Our NMR spectroscopy data indicated that the presence of D137L introduced long range rearrangements in TM structure. Differential scanning calorimetry measurements demonstrated that α-TM-D137L has higher thermal stability compared with α-TM, which correlated with decreased flexibility. Hearts of transgenic mice expressing α-TM-D137L showed systolic and diastolic dysfunction with decreased myofilament Ca(2+) sensitivity and cardiomyocyte contractility without changes in intracellular Ca(2+) transients or post-translational modifications of major myofilament proteins. We conclude that conversion of the highly conserved Asp-137 to Leu results in loss of flexibility of TM that is important for its regulatory functions in mouse hearts. Thus, our results provide insight into the link between flexibility of TM and its function in ejecting hearts.

Integration of troponin I phosphorylation with cardiac regulatory networks

We focus here on the modulation of thin filament activity by cardiac troponin I phosphorylation as an integral and adaptive mechanism in cardiac homeostasis and as a mechanism vulnerable to maladaptive response to stress. We discuss a current concept of cardiac troponin I function in the A-band region of the sarcomere and potential signaling to cardiac troponin I in a network involving the ends of the thin filaments at the Z-disk and the M-band regions. The cardiac sarcomere represents a remarkable set of interacting proteins that functions not only as a molecular machine generating the heartbeat but also as a hub of signaling. We review how phosphorylation signaling to cardiac troponin I is integrated, with parallel signals controlling excitation-contraction coupling, hypertrophy, and metabolism.

A periodic pattern of evolutionarily conserved basic and acidic residues constitutes the binding interface of actin-tropomyosin

Actin filament cytoskeletal and muscle functions are regulated by actin binding proteins using a variety of mechanisms. A universal actin filament regulator is the protein tropomyosin, which binds end-to-end along the length of the filament. The actin-tropomyosin filament structure is unknown, but there are atomic models in different regulatory states based on electron microscopy reconstructions, computational modeling of actin-tropomyosin, and docking of atomic resolution structures of tropomyosin to actin filament models. Here, we have tested models of the actin-tropomyosin interface in the "closed state" where tropomyosin binds to actin in the absence of myosin or troponin. Using mutagenesis coupled with functional analyses, we determined residues of actin and tropomyosin required for complex formation. The sites of mutations in tropomyosin were based on an evolutionary analysis and revealed a pattern of basic and acidic residues in the first halves of the periodic repeats (periods) in tropomyosin. In periods P1, P4, and P6, basic residues are most important for actin affinity, in contrast to periods P2, P3, P5, and P7, where both basic and acidic residues or predominantly acidic residues contribute to actin affinity. Hydrophobic interactions were found to be relatively less important for actin binding. We mutated actin residues in subdomains 1 and 3 (Asp(25)-Glu(334)-Lys(326)-Lys(328)) that are poised to make electrostatic interactions with the residues in the repeating motif on tropomyosin in the models. Tropomyosin failed to bind mutant actin filaments. Our mutagenesis studies provide the first experimental support for the atomic models of the actin-tropomyosin interface.

Structural and immunological characterization of recombinant Pan b 1, a major allergen of northern shrimp, Pandalus borealis

BACKGROUND

Shellfish allergy is one of the major causes of life-threatening allergic reactions to food. The shrimp species Pandalus borealis is the commercially most important coldwater shrimp species, and its protein extract is commonly used in shrimp allergy diagnostics. However, the DNA sequence of its major allergen, tropomyosin, designated Pan b 1, was not previously described. Our aim was to identify the cDNA sequence of Pan b 1 and to generate a recombinant protein with similar structure and allergenicity as the natural protein.

METHODS

P. borealis shrimps were caught in the Oslofjord (Norway). cDNA from Pan b 1 was generated, an N-terminal histidine tag was added, and the protein was expressed in Escherichia coli. The recombinant Pan b 1 was characterized by structural and IgE-binding studies and investigated further with basophil activation tests (BATs) and skin prick tests (SPTs) on Norwegian shrimp-allergic individuals.

RESULTS

The open reading frame encoded 284 amino acids that shared 97-100% identity with other shrimp tropomyosins. Mass spectroscopy of natural Pan b 1 confirmed the protein's molecular mass and indicated the absence of posttranslational modifications. Circular dichroism spectroscopy revealed virtually identical spectra between recombinant and natural Pan b 1, which together with native PAGE and size exclusion chromatography results indicated a similar structure. Furthermore, immunoblot and ELISA studies as well as BATs and SPTs showed equivalent results of recombinant and natural Pan b 1.

CONCLUSION

A recombinant tropomyosin from P. borealis was generated that can be used in diagnostics and further studies on tropomyosin allergenicity and specific immunotherapy.

Cooperative regulation of myosin-S1 binding to actin filaments by a continuous flexible Tm-Tn chain

The regulation of striated muscle contraction involves cooperative interactions between actin filaments, myosin-S1 (S1), tropomyosin (Tm), troponin (Tn), and calcium. These interactions are modeled by treating overlapping tropomyosins as a continuous flexible chain (CFC), weakly confined by electrostatic interactions with actin. The CFC is displaced locally in opposite directions on the actin surface by the binding of either S1 or Troponin I (TnI) to actin. The apparent rate constants for myosin and TnI binding to and detachment from actin are then intrinsically coupled via the CFC model to the presence of neighboring bound S1s and TnIs. Monte Carlo simulations at prescribed values of the CFC stiffness, the CFC’s degree of azimuthal confinement, and the angular displacements caused by the bound proteins were able to predict the stopped-flow transients of S1 binding to regulated F-actin. The transients collected over a large range of calcium concentrations could be well described by adjusting a single calcium-dependent parameter, the rate constant of TnI detachment from actin, k_−I. The resulting equilibrium constant \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ K_{\text{B}} \equiv 1/K_{\text{I}} $$\end{document} varied sigmoidally with the free calcium, increasing from 0.12 at low calcium (pCa >7) to 12 at high calcium (pCa <5.5) with a Hill coefficient of ~2.15. The similarity of the curves for excess-actin and excess-myosin data confirms their allosteric relationship. The spatially explicit calculations confirmed variable sizes for the cooperative units and clustering of bound myosins at low calcium concentrations. Moreover, inclusion of negative cooperativity between myosin units predicted the observed slowing of myosin binding at excess-myosin concentrations.

Long-range effects of familial hypertrophic cardiomyopathy mutations E180G and D175N on the properties of tropomyosin

Cardiac α-tropomyosin (Tm) single-site mutations D175N and E180G cause familial hypertrophic cardiomyopathy (FHC). Previous studies have shown that these mutations increase both Ca(2+) sensitivity and residual contractile activity at low Ca(2+) concentrations, which causes incomplete relaxation during diastole resulting in hypertrophy and sarcomeric disarray. However, the molecular basis for the cause and the difference in the severity of the manifested phenotypes of disease are not known. In this work we have (1) used ATPase studies using reconstituted thin filaments in solution to show that these FHC mutants result in an increase in Ca(2+) sensitivity and an increased residual level of ATPase, (2) shown that both FHC mutants increase the rate of cleavage at R133, ~45 residues N-terminal to the mutations, when free and bound to actin, (3) shown that for Tm-E180G, the increase in the rate of cleavage is greater than that for D175N, and (4) shown that for E180G, cleavage also occurs at a new site 53 residues C-terminal to E180G, in parallel with cleavage at R133. The long-range decreases in dynamic stability due to these two single-site mutations suggest increases in flexibility that may weaken the ability of Tm to inhibit activity at low Ca(2+) concentrations for D175N and to a greater degree for E180G, which may contribute to differences in the severity of FHC.

Tropomyosin: double helix from the protein world

This review concerns the structure and functions of tropomyosin (TM), an actin-binding protein that plays a key role in the regulation of muscle contraction. The TM molecule is a dimer of α-helices, which form a coiled-coil. Recent views on the TM structure are analyzed, and special attention is concentrated on those structural traits of the TM molecule that distinguish it from the other coiled-coil proteins. Modern data are presented on TM functional properties, such as its interaction with actin and ability to move on the surface of actin filaments, which underlies the regulation of the actin-myosin interaction upon contraction of skeletal and cardiac muscles. Also, part of the review is devoted to analysis of the effects of mutations in TM genes associated with muscle diseases (myopathies) on the structure and functions of TM.

Mechanisms of intramolecular communication in a hyperthermophilic acylaminoacyl peptidase: a molecular dynamics investigation

Protein dynamics and the underlying networks of intramolecular interactions and communicating residues within the three-dimensional (3D) structure are known to influence protein function and stability, as well as to modulate conformational changes and allostery. Acylaminoacyl peptidase (AAP) subfamily of enzymes belongs to a unique class of serine proteases, the prolyl oligopeptidase (POP) family, which has not been thoroughly investigated yet. POPs have a characteristic multidomain three-dimensional architecture with the active site at the interface of the C-terminal catalytic domain and a β-propeller domain, whose N-terminal region acts as a bridge to the hydrolase domain. In the present contribution, protein dynamics signatures of a hyperthermophilic acylaminoacyl peptidase (AAP) of the prolyl oligopeptidase (POP) family, as well as of a deletion variant and alanine mutants (I12A, V13A, V16A, L19A, I20A) are reported. In particular, we aimed at identifying crucial residues for long range communications to the catalytic site or promoting the conformational changes to switch from closed to open ApAAP conformations. Our investigation shows that the N-terminal α1-helix mediates structural intramolecular communication to the catalytic site, concurring to the maintenance of a proper functional architecture of the catalytic triad. Main determinants of the effects induced by α1-helix are a subset of hydrophobic residues (V16, L19 and I20). Moreover, a subset of residues characterized by relevant interaction networks or coupled motions have been identified, which are likely to modulate the conformational properties at the interdomain interface.

Conserved noncanonical residue Gly-126 confers instability to the middle part of the tropomyosin molecule

Tropomyosin (Tm) is a two-stranded α-helical coiled-coil protein with a well established role in regulation of actin cytoskeleton and muscle contraction. It is believed that many Tm functions are enabled by its flexibility whose nature has not been completely understood. We hypothesized that the well conserved non-canonical residue Gly-126 causes local destabilization of Tm. To test this, we substituted Gly-126 in skeletal muscle α-Tm either with an Ala residue, which should stabilize the Tm α-helix, or with an Arg residue, which is expected to stabilize both α-helix and coiled-coil structure of Tm. We have shown that both mutations dramatically reduce the rate of Tm proteolysis by trypsin at Asp-133. Differential scanning calorimetry was used for detailed investigation of thermal unfolding of the Tm mutants, both free in solution and bound to F-actin. It was shown that a significant part of wild type Tm unfolds in a non-cooperative manner at low temperature, and both mutations confer cooperativity to this part of the Tm molecule. The size of the flexible middle part of Tm is estimated to be 60-70 amino acid residues, about a quarter of the Tm molecule. Thus, our results show that flexibility is unevenly distributed in the Tm molecule and achieves the highest extent in its middle part. We conclude that the highly conserved Gly-126, acting in concert with the previously identified non-canonical Asp-137, destabilizes the middle part of Tm, resulting in a more flexible region that is important for Tm function.