Functional alpha-tropomyosin produced in Escherichia coli. A dipeptide extension can substitute the amino-terminal acetyl group

Myopathy-causing mutation R91P in the TPM3 gene drastically impairs structural and functional properties of slow skeletal muscle tropomyosin γβ-heterodimer

Tropomyosin (Tpm) is a regulatory actin-binding protein involved in Ca2+ activation of contraction of striated muscle. In human slow skeletal muscles, two distinct Tpm isoforms, γ and β, are present. They interact to form three types of dimeric Tpm molecules: γγ-homodimers, γβ-heterodimers, or ββ-homodimers, and a majority of the molecules are present as γβ-Tpm heterodimers. Point mutation R91P within the TPM3 gene encoding γ-Tpm is linked to the condition known as congenital fiber-type disproportion (CFTD), which is characterized by severe muscle weakness. Here, we investigated the influence of the R91P mutation in the γ-chain on the properties of the γβ-Tpm heterodimer. We found that the R91P mutation impairs the functional properties of γβ-Tpm heterodimer more severely than those of earlier studied γγ-Tpm homodimer carrying this mutation in both γ-chains. Since a significant part of Tpm molecules in slow skeletal muscle is present as γβ-heterodimers, our results explain why this mutation leads to muscle weakness in CFTD.

Troponin and a Myopathy-Linked Mutation in TPM3 Inhibit Cofilin-2-Induced Thin Filament Depolymerization

Uniform actin filament length is required for synchronized contraction of skeletal muscle. In myopathies linked to mutations in tropomyosin (Tpm) genes, irregular thin filaments are a common feature, which may result from defects in length maintenance mechanisms. The current work investigated the effects of the myopathy-causing p.R91C variant in Tpm3.12, a tropomyosin isoform expressed in slow-twitch muscle fibers, on the regulation of actin severing and depolymerization by cofilin-2. The affinity of cofilin-2 for F-actin was not significantly changed by either Tpm3.12 or Tpm3.12-R91C, though it increased two-fold in the presence of troponin (without Ca2+). Saturation of the filament with cofilin-2 removed both Tpm variants from the filament, although Tpm3.12-R91C was more resistant. In the presence of troponin (±Ca2+), Tpm remained on the filament, even at high cofilin-2 concentrations. Both Tpm3.12 variants inhibited filament severing and depolymerization by cofilin-2. However, the inhibition was more efficient in the presence of Tpm3.12-R91C, indicating that the pathogenic variant impaired cofilin-2-dependent actin filament turnover. Troponin (±Ca2+) further inhibited but did not completely stop cofilin-2-dependent actin severing and depolymerization.

A Novel Variant in TPM3 Causing Muscle Weakness and Concomitant Hypercontractile Phenotype

A novel variant of unknown significance c.8A > G (p.Glu3Gly) in TPM3 was detected in two unrelated families. TPM3 encodes the transcript variant Tpm3.12 (NM_152263.4), the tropomyosin isoform specifically expressed in slow skeletal muscle fibers. The patients presented with slowly progressive muscle weakness associated with Achilles tendon contractures of early childhood onset. Histopathology revealed features consistent with a nemaline rod myopathy. Biochemical in vitro assays performed with reconstituted thin filaments revealed defects in the assembly of the thin filament and regulation of actin-myosin interactions. The substitution p.Glu3Gly increased polymerization of Tpm3.12, but did not significantly change its affinity to actin alone. Affinity of Tpm3.12 to actin in the presence of troponin ± Ca2+ was decreased by the mutation, which was due to reduced interactions with troponin. Altered molecular interactions affected Ca2+-dependent regulation of the thin filament interactions with myosin, resulting in increased Ca2+ sensitivity and decreased relaxation of the actin-activated myosin ATPase activity. The hypercontractile molecular phenotype probably explains the distal joint contractions observed in the patients, but additional research is needed to explain the relatively mild severity of the contractures. The slowly progressive muscle weakness is most likely caused by the lack of relaxation and prolonged contractions which cause muscle wasting. This work provides evidence for the pathogenicity of the TPM3 c.8A > G variant, which allows for its classification as (likely) pathogenic.

Alternative splicing of a single exon causes a major impact on the affinity of Caenorhabditis elegans tropomyosin isoforms for actin filaments

Tropomyosin is generally known as an actin-binding protein that regulates actomyosin interaction and actin filament stability. In metazoans, multiple tropomyosin isoforms are expressed, and some of them are involved in generating subpopulations of actin cytoskeleton in an isoform-specific manner. However, functions of many tropomyosin isoforms remain unknown. Here, we report identification of a novel alternative exon in the Caenorhabditis elegans tropomyosin gene and characterization of the effects of alternative splicing on the properties of tropomyosin isoforms. Previous studies have reported six tropomyosin isoforms encoded by the C. elegans lev-11 tropomyosin gene. We identified a seventh isoform, LEV-11U, that contained a novel alternative exon, exon 7c (E7c). LEV-11U is a low-molecular-weight tropomyosin isoform that differs from LEV-11T only at the exon 7-encoded region. In silico analyses indicated that the E7c-encoded peptide sequence was unfavorable for coiled-coil formation and distinct from other tropomyosin isoforms in the pattern of electrostatic surface potentials. In vitro, LEV-11U bound poorly to actin filaments, whereas LEV-11T bound to actin filaments in a saturable manner. When these isoforms were transgenically expressed in the C. elegans striated muscle, LEV-11U was present in the diffuse cytoplasm with tendency to form aggregates, whereas LEV-11T co-localized with sarcomeric actin filaments. Worms with a mutation in E7c showed reduced motility and brood size, suggesting that this exon is important for the optimal health. These results indicate that alternative splicing of a single exon can produce biochemically diverged tropomyosin isoforms and suggest that a tropomyosin isoform with poor actin affinity has a novel biological function.

Novel Mutation Glu98Lys in Cardiac Tropomyosin Alters Its Structure and Impairs Myocardial Relaxation

We characterized a novel genetic variant c.292G > A (p.E98K) in the TPM1 gene encoding cardiac tropomyosin 1.1 isoform (Tpm1.1), found in a proband with a phenotype of complex cardiomyopathy with conduction dysfunction and slow progressive neuromuscular involvement. To understand the molecular mechanism by which this mutation impairs cardiac function, we produced recombinant Tpm1.1 carrying an E98K substitution and studied how this substitution affects the structure of the Tpm1.1 molecule and its functional properties. The results showed that the E98K substitution in the N-terminal part of the Tpm molecule significantly destabilizes the C-terminal part of Tpm, thus indicating a long-distance destabilizing effect of the substitution on the Tpm coiled-coil structure. The E98K substitution did not noticeably affect Tpm's affinity for F-actin but significantly impaired Tpm's regulatory properties. It increased the Ca2+ sensitivity of the sliding velocity of regulated thin filaments over cardiac myosin in an in vitro motility assay and caused an incomplete block of the thin filament sliding at low Ca2+ concentrations. The incomplete motility block in the absence of Ca2+ can be explained by the loosening of the Tpm interaction with troponin I (TnI), thus increasing Tpm mobility on the surface of an actin filament that partially unlocks the myosin binding sites. This hypothesis is supported by the molecular dynamics (MD) simulation that showed that the E98 Tpm residue is involved in hydrogen bonding with the C-terminal part of TnI. Thus, the results allowed us to explain the mechanism by which the E98K Tpm mutation impairs sarcomeric function and myocardial relaxation.

Structural and Functional Properties of Tropomyosin Isoforms Tpm4.1 and Tpm2.1

Tropomyosin (Tpm) is one of the most important partners of actin filament that largely determines its properties. In animal organisms, there are different isoforms of Tpm, which are believed to be involved in the regulation of various cellular functions. However, molecular mechanisms by which various Tpm cytoplasmic regulate of the functioning of actin filaments are still poorly understood. Here, we investigated the properties of Tpm2.1 and Tpm4.1 isoforms and compared them to each other and to more extensively studied Tpm isoforms. Tpm2.1 and Tpm4.1 were very similar in their affinity to F-actin, thermal stability, and resistance to limited proteolysis by trypsin, but differed markedly in the viscosity of their solutions and thermal stability of their complexes with F-actin. The main difference of Tpm2.1 and Tpm4.1 from other Tpm isoforms (e.g., Tpm1.6 and Tpm1.7) was their extremely low thermal stability as measured by the CD and DSC methods. We suggested the possible causes of this instability based on comparing the amino acid sequences of Tpm4.1 and Tpm2.1 with the sequences of Tpm1.6 and Tpm1.7 isoforms, respectively, that have similar exon structure.

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.

De Novo Asp219Val Mutation in Cardiac Tropomyosin Associated with Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM), caused by mutations in thin filament proteins, manifests as moderate cardiac hypertrophy and is associated with sudden cardiac death (SCD). We identified a new de novo variant, c.656A>T (p.D219V), in the TPM1 gene encoding cardiac tropomyosin 1.1 (Tpm) in a young SCD victim with post-mortem-diagnosed HCM. We produced recombinant D219V Tpm1.1 and studied its structural and functional properties using various biochemical and biophysical methods. The D219V mutation did not affect the Tpm affinity for F-actin but increased the thermal stability of the Tpm molecule and Tpm-F-actin complex. The D219V mutation significantly increased the Ca2+ sensitivity of the sliding velocity of thin filaments over cardiac myosin in an in vitro motility assay and impaired the inhibition of the filament sliding at low Ca2+ concentration. The molecular dynamics (MD) simulation provided insight into a possible molecular mechanism of the effect of the mutation that is most likely a cause of the weakening of the Tpm interaction with actin in the "closed" state and so makes it an easier transition to the “open” state. The changes in the Ca2+ regulation of the actin-myosin interaction characteristic of genetic HCM suggest that the mutation is likely pathogenic.

Impact of Troponin in Cardiomyopathy Development Caused by Mutations in Tropomyosin

Tropomyosin (Tpm) mutations cause inherited cardiac diseases such as hypertrophic and dilated cardiomyopathies. We applied various approaches to investigate the role of cardiac troponin (Tn) and especially the troponin T (TnT) in the pathogenic effects of Tpm cardiomyopathy-associated mutations M8R, K15N, A277V, M281T, and I284V located in the overlap junction of neighboring Tpm dimers. Using co-sedimentation assay and viscosity measurements, we showed that TnT1 (fragment of TnT) stabilizes the overlap junction of Tpm WT and all Tpm mutants studied except Tpm M8R. However, isothermal titration calorimetry (ITC) indicated that TnT1 binds Tpm WT and all Tpm mutants similarly. By using ITC, we measured the direct K_D of the Tpm overlap region, N-end, and C-end binding to TnT1. The ITC data revealed that the Tpm C-end binds to TnT1 independently from the N-end, while N-end does not bind. Therefore, we suppose that Tpm M8R binds to TnT1 without forming the overlap junction. We also demonstrated the possible role of Tn isoform composition in the cardiomyopathy development caused by M8R mutation. TnT1 dose-dependently reduced the velocity of F-actin-Tpm filaments containing Tpm WT, Tpm A277V, and Tpm M281T mutants in an in vitro motility assay. All mutations impaired the calcium regulation of the actin-myosin interaction. The M281T and I284V mutations increased the calcium sensitivity, while the K15N and A277V mutations reduced it. The Tpm M8R, M281T, and I284V mutations under-inhibited the velocity at low calcium concentrations. Our results demonstrate that Tpm mutations likely implement their pathogenic effects through Tpm interaction with Tn, cardiac myosin, or other protein partners.

Distinct actin–tropomyosin cofilament populations drive the functional diversification of cytoskeletal myosin motor complexes

The effects of N-terminal acetylation of the high molecular weight tropomyosin isoforms Tpm1.6 and Tpm2.1 and the low molecular weight isoforms Tpm1.12, Tpm3.1, and Tpm4.2 on the actin affinity and the thermal stability of actin-tropomyosin cofilaments are described. Furthermore, we show how the exchange of cytoskeletal tropomyosin isoforms and their N-terminal acetylation affects the kinetic and chemomechanical properties of cytoskeletal actin-tropomyosin-myosin complexes. Our results reveal the extent to which the different actin-tropomyosin-myosin complexes differ in their kinetic and functional properties. The maximum sliding velocity of the actin filament as well as the optimal motor density for continuous unidirectional movement, parameters that were previously considered to be unique and invariant properties of each myosin isoform, are shown to be influenced by the exchange of the tropomyosin isoform and the N-terminal acetylation of tropomyosin.

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Tpm diversity is largely determined by sequences contributing to the overlap region

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Global sequence differences are of greater importance than variable exon 6 usage

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Tpm isoforms confer distinctly altered properties to cytoskeletal myosin motors

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Cytoskeletal myosins are differentially affected by N-terminal acetylation of Tpm

The effects of the tropomyosin cardiomyopathy mutations on the calcium regulation of actin-myosin interaction in the atrium and ventricle differ

The molecular mechanisms of pathogenesis of atrial myopathy associated with hypertrophic (HCM) and dilated (DCM) mutations of sarcomeric proteins are still poorly understood. For this, one needs to investigate the effects of the mutations on actin-myosin interaction in the atria separately from ventricles. We compared the impact of the HCM and DCM mutations of tropomyosin (Tpm) on the calcium regulation of the thin filament interaction with atrial and ventricular myosin using an in vitro motility assay. We found that the mutations differently affect the calcium regulation of actin-myosin interaction in the atria and ventricles. The DCM E40K Tpm mutation significantly reduced the maximum sliding velocity of thin filaments with ventricular myosin and its Ca²⁺-sensitivity. With atrial myosin, its effects were less pronounced. The HCM I172T mutation reduced the Ca²⁺-sensitivity of the sliding velocity of filaments with ventricular myosin but increased it with the atrial one. The HCM L185R mutation did not affect actin-myosin interaction in the atria. The results indicate that the difference in the effects of Tpm mutations on the actin-myosin interaction in the atria and ventricles may be responsible for the difference in pathological changes in the atrial and ventricular myocardium.

Novel human cell expression method reveals the role and prevalence of posttranslational modification in nonmuscle tropomyosins

Biochemical studies require large quantities of proteins, which are typically obtained using bacterial overexpression. However, the folding machinery in bacteria is inadequate for expressing many mammalian proteins, which additionally undergo posttranslational modifications (PTMs) that bacteria, yeast, or insect cells cannot perform. Many proteins also require native N- and C-termini and cannot tolerate extra tag amino acids for proper function. Tropomyosin (Tpm), a coiled coil protein that decorates most actin filaments in cells, requires both native N- and C-termini and PTMs, specifically N-terminal acetylation (Nt-acetylation), to polymerize along actin filaments. Here, we describe a new method that combines native protein expression in human cells with an intein-based purification tag that can be precisely removed after purification. Using this method, we expressed several nonmuscle Tpm isoforms (Tpm1.6, Tpm1.7, Tpm2.1, Tpm3.1, Tpm3.2, and Tpm4.2) and the muscle isoform Tpm1.1. Proteomics analysis revealed that human-cell-expressed Tpms present various PTMs, including Nt-acetylation, Ser/Thr phosphorylation, Tyr phosphorylation, and Lys acetylation. Depending on the Tpm isoform (humans express up to 40 Tpm isoforms), Nt-acetylation occurs on either the initiator methionine or on the second residue after removal of the initiator methionine. Human-cell-expressed Tpms bind F-actin differently than their Escherichia coli-expressed counterparts, with or without N-terminal extensions intended to mimic Nt-acetylation, and they can form heterodimers in cells and in vitro. The expression method described here reveals previously unknown features of nonmuscle Tpms and can be used in future structural and biochemical studies with Tpms and other proteins, as shown here for α-synuclein.

Single-molecule imaging reveals the concerted release of myosin from regulated thin filaments

Regulated thin filaments (RTFs) tightly control striated muscle contraction through calcium binding to troponin, which enables tropomyosin to expose myosin-binding sites on actin. Myosin binding holds tropomyosin in an open position, exposing more myosin-binding sites on actin, leading to cooperative activation. At lower calcium levels, troponin and tropomyosin turn off the thin filament; however, this is antagonised by the high local concentration of myosin, questioning how the thin filament relaxes. To provide molecular details of deactivation, we used single-molecule imaging of green fluorescent protein (GFP)-tagged myosin-S1 (S1-GFP) to follow the activation of RTF tightropes. In sub-maximal activation conditions, RTFs are not fully active, enabling direct observation of deactivation in real time. We observed that myosin binding occurs in a stochastic step-wise fashion; however, an unexpectedly large probability of multiple contemporaneous detachments is observed. This suggests that deactivation of the thin filament is a coordinated active process.

Comparative structural and functional studies of low molecular weight tropomyosin isoforms, the TPM3 gene products

Tropomyosin (Tpm) is an actin-associated protein and key regulator of actin filament structure and dynamics in muscle and non-muscle cells where it participates in many vital processes. Human non-muscle cells produce many Tpm isoforms; however, little is known yet about their structural and functional properties. In the present work, we have applied various methods to investigate the properties of five low molecular weight Tpm isoforms (Tpm3.1, Tpm3.2, Tpm3.4, Tpm3.5, and Tpm3.7), the products of TPM3 gene, which significantly differ by alternatively spliced internal exon 6 (6a or 6b) and C-terminal exon 9 (9a, 9c or 9d). Our results clearly demonstrate that the properties of these Tpm isoforms are quite different depending on sequence variations in alternatively spliced regions of their molecules. These differences can be important in further studies to explain why these Tpm isoforms play a key role in organization and dynamics of the cytoskeleton.

Allosteric modulation of cardiac myosin mechanics and kinetics by the conjugated omega-7,9 trans-fat rumenic acid

KEY POINTS

Direct binding of rumenic acid to the cardiac myosin-2 motor domain increases the release rate for orthophosphate and increases the Ca²⁺ responsiveness of cardiac muscle at low load. Physiological cellular concentrations of rumenic acid affect the ATP turnover rates of the super-relaxed and disordered relaxed states of β-cardiac myosin, leading to a net increase in myocardial metabolic load. In Ca²⁺ -activated trabeculae, rumenic acid exerts a direct inhibitory effect on the force-generating mechanism without affecting the number of force-generating motors. In the presence of saturating actin concentrations rumenic acid binds to the β-cardiac myosin-2 motor domain with an EC₅₀ of 200 nM. Molecular docking studies provide information about the binding site, the mode of binding, and associated allosteric communication pathways. Free rumenic acid may exceed thresholds in cardiomyocytes above which contractile efficiency is reduced and interference with small molecule therapeutics, targeting cardiac myosin, occurs.

ABSTRACT

Based on experiments using purified myosin motor domains, reconstituted actomyosin complexes and rat heart ventricular trabeculae, we demonstrate direct binding of rumenic acid, the cis-delta-9-trans-delta-11 isomer of conjugated linoleic acid, to an allosteric site located in motor domain of mammalian cardiac myosin-2 isoforms. In the case of porcine β-cardiac myosin, the EC₅₀ for rumenic acid varies from 10.5 μM in the absence of actin to 200 nM in the presence of saturating concentrations of actin. Saturating concentrations of rumenic acid increase the maximum turnover of basal and actin-activated ATPase activity of β-cardiac myosin approximately 2-fold but decrease the force output per motor by 23% during isometric contraction. The increase in ATP turnover is linked to an acceleration of the release of the hydrolysis product orthophosphate. In the presence of 5 μM rumenic acid, the difference in the rate of ATP turnover by the super-relaxed and disordered relaxed states of cardiac myosin increases from 4-fold to 20-fold. The equilibrium between the two functional myosin states is not affected by rumenic acid. Calcium responsiveness is increased under zero-load conditions but unchanged under load. Molecular docking studies provide information about the rumenic acid binding site, the mode of binding, and associated allosteric communication pathways. They show how the isoform-specific replacement of residues in the binding cleft induces a different mode of rumenic acid binding in the case of non-muscle myosin-2C and blocks binding to skeletal muscle and smooth muscle myosin-2 isoforms.

Structural and Functional Peculiarities of Cytoplasmic Tropomyosin Isoforms, the Products of TPM1 and TPM4 Genes

Tropomyosin (Tpm) is one of the major protein partners of actin. Tpm molecules are α-helical coiled-coil protein dimers forming a continuous head-to-tail polymer along the actin filament. Human cells produce a large number of Tpm isoforms that are thought to play a significant role in determining actin cytoskeletal functions. Even though the role of these Tpm isoforms in different non-muscle cells is more or less studied in many laboratories, little is known about their structural and functional properties. In the present work, we have applied various methods to investigate the properties of five cytoplasmic Tpm isoforms (Tpm1.5, Tpm 1.6, Tpm1.7, Tpm1.12, and Tpm 4.2), which are the products of two different genes, TPM1 and TPM4, and also significantly differ by alternatively spliced exons: N-terminal exons 1a2b or 1b, internal exons 6a or 6b, and C-terminal exons 9a, 9c or 9d. Our results demonstrate that structural and functional properties of these Tpm isoforms are quite different depending on sequence variations in alternatively spliced regions of their molecules. The revealed differences can be important in further studies to explain why various Tpm isoforms interact uniquely with actin filaments, thus playing an important role in the organization and dynamics of the cytoskeleton.

A high-throughput fluorescence lifetime-based assay to detect binding of myosin-binding protein C to F-actin

Binding properties of actin-binding proteins are typically evaluated by cosedimentation assays. However, this method is time-consuming, involves multiple steps, and has a limited throughput. These shortcomings preclude its use in screening for drugs that modulate actin-binding proteins relevant to human disease. To develop a simple, quantitative, and scalable F-actin-binding assay, we attached fluorescent probes to actin's Cys-374 and assessed changes in fluorescence lifetime upon binding to the N-terminal region (domains C0-C2) of human cardiac myosin-binding protein C (cMyBP-C). The lifetime of all five probes tested decreased upon incubation with cMyBP-C C0-C2, as measured by time-resolved fluorescence (TR-F), with IAEDANS being the most sensitive probe that yielded the smallest errors. The TR-F assay was compared with cosedimentation to evaluate in vitro changes in binding to actin and actin-tropomyosin arising from cMyBP-C mutations associated with hypertrophic cardiomyopathy (HCM) and tropomyosin binding. Lifetime changes of labeled actin with added C0-C2 were consistent with cosedimentation results. The HCM mutation L352P was confirmed to enhance actin binding, whereas PKA phosphorylation reduced binding. The HCM mutation R282W, predicted to disrupt a PKA recognition sequence, led to deficits in C0-C2 phosphorylation and altered binding. Lastly, C0-C2 binding was found to be enhanced by tropomyosin and binding capacity to be altered by mutations in a tropomyosin-binding region. These findings suggest that the TR-F assay is suitable for rapidly and accurately determining quantitative binding and for screening physiological conditions and compounds that affect cMyBP-C binding to F-actin for therapeutic discovery.

Acidosis modifies effects of phosphorylated tropomyosin on the actin-myosin interaction in the myocardium

Phosphorylation of α-tropomyosin (Tpm1.1), a predominant Tpm isoform in the myocardium, is one of the regulatory mechanisms of the heart contractility. The Tpm 1.1 molecule has one site of phosphorylation, Ser283. The degree of the Tpm phosphorylation decreases with age and also changes in heart pathologies. Myocardial pathologies, in particular ischemia, are usually accompanied by pH lowering in the cardiomyocyte cytosol. We studied the effects of acidosis on the structural and functional properties of the pseudo-phosphorylated form of Tpm1.1 with the S283D substitution. We found that in acidosis, the interaction of the N- and C-ends of the S283D Tpm molecules decreases, whereas that of WT Tpm does not change. The pH lowering increased thermostability of the complex of F-actin with S283D Tpm to a greater extent than with WT Tpm. Using an in vitro motility assay with NEM- modified myosin as a load, we assessed the effect of the Tpm pseudo-phosphorylation on the force of the actin-myosin interaction. In acidosis, the force generated by myosin in the interaction with thin filaments containing S283D Tpm was higher than with those containing WT Tpm. Also, the pseudo-phosphorylation increased the myosin ability to resist a load. We conclude that ischemia changes the effect of the phosphorylated Tpm on the contractile function of the myocardium.

Effects of myopathy-causing mutations R91P and R245G in the TPM3 gene on structural and functional properties of slow skeletal muscle tropomyosin

Tropomyosin (Tpm) is an actin-binding protein that plays a crucial role in the regulation of muscle contraction. Numerous point mutations in the TPM3 gene encoding Tpm of slow skeletal muscles (Tpm 3.12 or γ-Tpm) are associated with the genesis of various congenital myopathies. Two of these mutations, R91P and R245G, are associated with congenital fiber-type disproportion (CFTD) characterized by hypotonia and generalized muscle weakness. We applied various methods to investigate how these mutations affect the structural and functional properties of γγ-Tpm homodimers. The results show that both these mutations lead to strong structural changes in the γγ-Tpm molecule and significantly impaired its functional properties. These changes in the Tpm properties caused by R91P and R245G mutations give insight into the molecular mechanism of the CFTD development and the weakness of slow skeletal muscles observed in this inherited disease.

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.

Tropomyosin pseudo-phosphorylation can rescue the effects of cardiomyopathy-associated mutations

We applied various methods to investigate how mutations S283D and S61D that mimic phosphorylation of tropomyosin (Tpm) affect structural and functional properties of cardiac Tpm carrying cardiomyopathy-associated mutations in different parts of its molecule. Using differential scanning calorimetry and molecular dynamics, we have shown that the S61D mutation (but not the S283 mutation) causes significant destabilization of the N-terminal part of the Tpm molecule independently of the absence or presence of cardiomyopathy-associated mutations. Our results obtained by cosedimentation of Tpm with F-actin demonstrated that both S283D and S61D mutations can reduce or even eliminate undesirable changes in Tpm affinity for F-actin caused by some cardiomyopathy-associated mutations. The results indicate that Tpm pseudo-phosphorylation by mutations S283D or S61D can rescue the effects of mutations in the TPM1 gene encoding a cardiac isoform of Tpm that lead to the development of such severe inherited heart diseases as hypertrophic or dilated cardiomyopathies.

Dynamics of Tpm1.8 domains on actin filaments with single-molecule resolution

Tropomyosins regulate the dynamics and functions of the actin cytoskeleton by forming long chains along the two strands of actin filaments that act as gatekeepers for the binding of other actin-binding proteins. The fundamental molecular interactions underlying the binding of tropomyosin to actin are still poorly understood. Using microfluidics and fluorescence microscopy, we observed the binding of the fluorescently labeled tropomyosin isoform Tpm1.8 to unlabeled actin filaments in real time. This approach, in conjunction with mathematical modeling, enabled us to quantify the nucleation, assembly, and disassembly kinetics of Tpm1.8 on single filaments and at the single-molecule level. Our analysis suggests that Tpm1.8 decorates the two strands of the actin filament independently. Nucleation of a growing tropomyosin domain proceeds with high probability as soon as the first Tpm1.8 molecule is stabilized by the addition of a second molecule, ultimately leading to full decoration of the actin filament. In addition, Tpm1.8 domains are asymmetrical, with enhanced dynamics at the edge oriented toward the barbed end of the actin filament. The complete description of Tpm1.8 kinetics on actin filaments presented here provides molecular insight into actin-tropomyosin filament formation and the role of tropomyosins in regulating actin filament dynamics.

Looking for Targets to Restore the Contractile Function in Congenital Myopathy Caused by Gln147Pro Tropomyosin

We have used the technique of polarized microfluorimetry to obtain new insight into the pathogenesis of skeletal muscle disease caused by the Gln¹⁴⁷Pro substitution in β-tropomyosin (Tpm2.2). The spatial rearrangements of actin, myosin and tropomyosin in the single muscle fiber containing reconstituted thin filaments were studied during simulation of several stages of ATP hydrolysis cycle. The angular orientation of the fluorescence probes bound to tropomyosin was found to be changed by the substitution and was characteristic for a shift of tropomyosin strands closer to the inner actin domains. It was observed both in the absence and in the presence of troponin, Ca²⁺ and myosin heads at all simulated stages of the ATPase cycle. The mutant showed higher flexibility. Moreover, the Gln¹⁴⁷Pro substitution disrupted the myosin-induced displacement of tropomyosin over actin. The irregular positioning of the mutant tropomyosin caused premature activation of actin monomers and a tendency to increase the number of myosin cross-bridges in a state of strong binding with actin at low Ca²⁺.

Mechanisms of disturbance of the contractile function of slow skeletal muscles induced by myopathic mutations in the tropomyosin TPM3 gene

Several congenital myopathies of slow skeletal muscles are associated with mutations in the tropomyosin (Tpm) TPM3 gene. Tropomyosin is an actin-binding protein that plays a crucial role in the regulation of muscle contraction. Two Tpm isoforms, γ (Tpm3.12) and β (Tpm2.2) are expressed in human slow skeletal muscles forming γγ-homodimers and γβ-heterodimers of Tpm molecules. We applied various methods to investigate how myopathy-causing mutations M9R, E151A, and K169E in the Tpm γ-chain modify the structure-functional properties of Tpm dimers, and how this affects the muscle functioning. The results show that the features of γγ-Tpm and γβ-Tpm with substitutions in the Tpm γ-chain vary significantly. The characteristics of the γγ-Tpm depend on whether these mutations located in only one or both γ-chains. The mechanism of the development of nemaline myopathy associated with the M9R mutation was revealed. At the molecular level, a cause-and-effect relationship has been established for the development of myopathy by the K169E mutation. Also, we described the structure-functional properties of the Tpm dimers with the E151A mutation, which explain muscle weakness linked to this substitution. The results demonstrate a diversity of the molecular mechanisms of myopathy pathogenesis induced by studied Tpm mutations.

Regulation of Actin Filament Length by Muscle Isoforms of Tropomyosin and Cofilin

In striated muscle the extent of the overlap between actin and myosin filaments contributes to the development of force. In slow twitch muscle fibers actin filaments are longer than in fast twitch fibers, but the mechanism which determines this difference is not well understood. We hypothesized that tropomyosin isoforms Tpm1.1 and Tpm3.12, the actin regulatory proteins, which are specific respectively for fast and slow muscle fibers, differently stabilize actin filaments and regulate severing of the filaments by cofilin-2. Using in vitro assays, we showed that Tpm3.12 bound to F-actin with almost 2-fold higher apparent binding constant (K_app) than Tpm1.1. Cofilin2 reduced K_app of both tropomyosin isoforms. In the presence of Tpm1.1 and Tpm3.12 the filaments were longer than unregulated F-actin by 25% and 40%, respectively. None of the tropomyosins affected the affinity of cofilin-2 for F-actin, but according to the linear lattice model both isoforms increased cofilin-2 binding to an isolated site and reduced binding cooperativity. The filaments decorated with Tpm1.1 and Tpm3.12 were severed by cofilin-2 more often than unregulated filaments, but depolymerization of the severed filaments was inhibited. The stabilization of the filaments by Tpm3.12 was more efficient, which can be attributed to lower dynamics of Tpm3.12 binding to actin.

Unique functional properties of slow skeletal muscle tropomyosin

Tropomyosin (Tpm) is an α-helical coiled-coil actin-binding protein playing an essential role in the regulation of muscle contraction. The α- (Tpm 1.1) and γ- (Tpm 3.12) Tpm isoforms are expressed in fast and slow human skeletal muscles, respectively, while β-Tpm (Tpm 2.2) is expressed in both muscle types. This results in the formation of Tpm αα- and γγ-homodimers as well as αβ- and γβ-heterodimers. The properties of αα-homodimer are well studied, whereas very little is known about the functional properties of γγ-homodimer and γβ-heterodimer. We investigated interaction characteristics of Tpm γγ-homodimer and γβ-heterodimer with actin filaments and Ca²⁺-regulation of actin-myosin interaction on myosin from fast and slow skeletal muscles. The results showed that complexes formed by γγ-Tpm and γβ-Tpm with F-actin are more stable than those with αα-Tpm and αβ-Tpm. The maximum sliding speed of regulated thin filaments with either γγ-Tpm or γβ-Tpm moving over skeletal myosin was significantly less than that of the filaments with αα-Tpm or αβ-Tpm. The results indicate that isoforms of Tpm along with isoforms of myosin determine of functional properties of skeletal muscles and support an idea on the combined expression of myosin and Tpm isoforms.

Cardiac muscle thin filament structures reveal calcium regulatory mechanism

Contraction of striated muscles is driven by cyclic interactions of myosin head projecting from the thick filament with actin filament and is regulated by Ca2+ released from sarcoplasmic reticulum. Muscle thin filament consists of actin, tropomyosin and troponin, and Ca2+ binding to troponin triggers conformational changes of troponin and tropomyosin to allow actin-myosin interactions. However, the structural changes involved in this regulatory mechanism remain unknown. Here we report the structures of human cardiac muscle thin filament in the absence and presence of Ca2+ by electron cryomicroscopy. Molecular models in the two states built based on available crystal structures reveal the structures of a C-terminal region of troponin I and an N-terminal region of troponin T in complex with the head-to-tail junction of tropomyosin together with the troponin core on actin filament. Structural changes of the thin filament upon Ca2+ binding now reveal the mechanism of Ca2+ regulation of muscle contraction.

Molecular integration of the anti-tropomyosin compound ATM-3507 into the coiled coil overlap region of the cancer-associated Tpm3.1

Tropomyosins (Tpm) determine the functional capacity of actin filaments in an isoform-specific manner. The primary isoform in cancer cells is Tpm3.1 and compounds that target Tpm3.1 show promising results as anti-cancer agents both in vivo and in vitro. We have determined the molecular mechanism of interaction of the lead compound ATM-3507 with Tpm3.1-containing actin filaments. When present during co-polymerization of Tpm3.1 with actin, ³H-ATM-3507 is incorporated into the filaments and saturates at approximately one molecule per Tpm3.1 dimer and with an apparent binding affinity of approximately 2 µM. In contrast, ³H-ATM-3507 is poorly incorporated into preformed Tpm3.1/actin co-polymers. CD spectroscopy and thermal melts using Tpm3.1 peptides containing the C-terminus, the N-terminus, and a combination of the two forming the overlap junction at the interface of adjacent Tpm3.1 dimers, show that ATM-3507 shifts the melting temperature of the C-terminus and the overlap junction, but not the N-terminus. Molecular dynamic simulation (MDS) analysis predicts that ATM-3507 integrates into the 4-helix coiled coil overlap junction and in doing so, likely changes the lateral movement of Tpm3.1 across the actin surface resulting in an alteration of filament interactions with actin binding proteins and myosin motors, consistent with the cellular impact of ATM-3507.

Sizes of actin networks sharing a common environment are determined by the relative rates of assembly

Within the cytoplasm of a single cell, several actin networks can coexist with distinct sizes, geometries, and protein compositions. These actin networks assemble in competition for a limited pool of proteins present in a common cellular environment. To predict how two distinct networks of actin filaments control this balance, the simultaneous assembly of actin-related protein 2/3 (Arp2/3)-branched networks and formin-linear networks of actin filaments around polystyrene microbeads was investigated with a range of actin accessory proteins (profilin, capping protein, actin-depolymerizing factor [ADF]/cofilin, and tropomyosin). Accessory proteins generally affected actin assembly rates for the distinct networks differently. These effects at the scale of individual actin networks were surprisingly not always correlated with corresponding loss-of-function phenotypes in cells. However, our observations agreed with a global interpretation, which compared relative actin assembly rates of individual actin networks. This work supports a general model in which the size of distinct actin networks is determined by their relative capacity to assemble in a common and competing environment.

A biomimetic assay using polystyrene beads compares the rates of actin assembly on linear and branched networks, revealing how the size of rival actin networks in cells is regulated by their relative capacity to assemble in a common environment.

Thermal unfolding of various human non-muscle isoforms of tropomyosin

Tropomyosin (Tpm) is an α-helical coiled-coil protein dimer, which forms a continuous head-to-tail polymer along the actin filament. In striated muscles, Tpm plays an important role in the Ca²⁺-dependent regulation of muscle contraction. However, little is known about functional and especially structural properties of the numerous non-muscle Tpm isoforms. In the present work, we have applied circular dichroism (CD) and differential scanning calorimetry (DSC) to investigate thermal unfolding and domain structure of various non-muscle human Tpm isoforms. These isoforms, the products of two different genes, TPM1 and TPM3, also significantly differ by alternatively spliced exons: N-terminal exons 1a2b or 1b, internal exons 6a or 6b, and C-terminal exons 9a, 9c or 9d. Our results clearly demonstrate that structural properties of various non-muscle Tpm isoforms can be quite different depending on the presence of different alternatively spliced exons in their genes. These data show for the first time a significant difference in the thermal unfolding between muscle and non-muscle Tpm isoforms and indicate that replacement of alternatively spliced exons alters the stability of certain domains in the Tpm molecule.

Actin-tropomyosin distribution in non-muscle cells

The interactions of cytoskeletal actin filaments with myosin family motors are essential for the integrity and function of eukaryotic cells. They support a wide range of force-dependent functions. These include mechano-transduction, directed transcellular transport processes, barrier functions, cytokinesis, and cell migration. Despite the indispensable role of tropomyosins in the generation and maintenance of discrete actomyosin-based structures, the contribution of individual cytoskeletal tropomyosin isoforms to the structural and functional diversification of the actin cytoskeleton remains a work in progress. Here, we review processes that contribute to the dynamic sorting and targeted distribution of tropomyosin isoforms in the formation of discrete actomyosin-based structures in animal cells and their effects on actin-based motility and contractility.

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.

Tropomyosin isoforms differentially tune actin filament length and disassembly

Cellular actin networks exhibit diverse filamentous architectures and turnover dynamics, but how these differences are specified remains poorly understood. Here, we used multicolor total internal reflection fluorescence microscopy to ask how decoration of actin filaments by five biologically prominent Tropomyosin (TPM) isoforms influences disassembly induced by Cofilin alone, or by the collaborative effects of Cofilin, Coronin, and AIP1 (CCA). TPM decoration restricted Cofilin binding to pointed ends, while not interfering with Coronin binding to filament sides. Different isoforms of TPM provided variable levels of protection against disassembly, with the strongest protection by Tpm3.1 and the weakest by Tpm1.6. In biomimetic assays in which filaments were simultaneously assembled by formins and disassembled by CCA, these TPM isoform-specific effects persisted, giving rise to filaments with different lengths and treadmilling behavior. Together, our data reveal that TPM isoforms have quantitatively distinct abilities to tune actin filament length and turnover.

Structural and functional properties of αβ-heterodimers of tropomyosin with myopathic mutations Q147P and K49del in the β-chain

Tropomyosin (Tpm) is an α-helical coiled-coil actin-binding protein that plays a key role in the Ca²⁺-regulated contraction of striated muscles. Two Tpm isoforms, α (Tpm 1.1) and β (Tpm 2.2), are expressed in fast skeletal muscles. These Tpm isoforms can form either αα and ββ homodimers, or αβ heterodimers. However, only αα-Tpm and αβ-Tpm dimers are usually present in most of fast skeletal muscles, because ββ-homodimers are relatively unstable and cannot exist under physiologic conditions. Nevertheless, the most of previous studies of myopathy-causing mutations in the Tpm β-chains were performed on the ββ-homodimers. In the present work, we applied different methods to investigate the effects of two myopathic mutations in the β-chain, Q147P and K49del (i.e. deletion of Lys49), on structural and functional properties of Tpm αβ-heterodimers and to compare them with the properties of ββ-homodimers carrying these mutations in both β-chains. The results show that the properties of αβ-Tpm heterodimers with these mutations in the β-chain differ significantly from the properties of ββ-homodimers with the same substitutions in both β-chains. This indicates that the αβ-heterodimer is a more appropriate model for studying the effects of myopathic mutations in the β-chain of Tpm than the ββ-homodimer which virtually does not exist in human skeletal muscles.

The Primary Causes of Muscle Dysfunction Associated with the Point Mutations in Tpm3.12; Conformational Analysis of Mutant Proteins as a Tool for Classification of Myopathies

Point mutations in genes encoding isoforms of skeletal muscle tropomyosin may cause nemaline myopathy, cap myopathy (Cap), congenital fiber-type disproportion (CFTD), and distal arthrogryposis. The molecular mechanisms of muscle dysfunction in these diseases remain unclear. We studied the effect of the E173A, R90P, E150A, and A155T myopathy-causing substitutions in γ-tropomyosin (Tpm3.12) on the position of tropomyosin in thin filaments, and the conformational state of actin monomers and myosin heads at different stages of the ATPase cycle using polarized fluorescence microscopy. The E173A, R90P, and E150A mutations produced abnormally large displacement of tropomyosin to the inner domains of actin and an increase in the number of myosin heads in strong-binding state at low and high Ca²⁺, which is characteristic of CFTD. On the contrary, the A155T mutation caused a decrease in the amount of such heads at high Ca²⁺ which is typical for mutations associated with Cap. An increase in the number of the myosin heads in strong-binding state at low Ca²⁺ was observed for all mutations associated with high Ca²⁺-sensitivity. Comparison between the typical conformational changes in mutant proteins associated with different myopathies observed with α-, β-, and γ-tropomyosins demonstrated the possibility of using such changes as tests for identifying the diseases.

Interactions of tropomyosin Tpm1.1 on a single actin filament: A method for extraction and processing of high resolution TIRF microscopy data

Skeletal muscle tropomyosin (Tpm1.1) is an elongated, rod-shaped, alpha-helical coiled-coil protein that forms continuous head-to-tail polymers along both sides of the actin filament. In this study we use single molecule fluorescence TIRF microscopy combined with a microfluidic device and fluorescently labelled proteins to measure Tpm1.1 association to and dissociation from single actin filaments. Our experimental setup allows us to clearly resolve Tpm1.1 interactions on both sides of the filaments. Here we provide a semi-automated method for the extraction and quantification of kymograph data for individual actin filaments bound at different Tpm1.1 concentrations. We determine boundaries on the kymograph on each side of the actin filament, based on intensity thresholding, performing fine manual editing of the boundaries (if needed) and extracting user defined kinetic properties of the system. Using our analytical tools we can determine (i) nucleation point(s) and rates, (ii) elongation rates of Tpm1.1, (iii) identify meeting points after the saturation of filament, and when dissociation occurs, (iv) initiation point(s), (v) the final dissociation point(s), as well as (vi) dissociation rates.

All of these measurements can be extracted from both sides of the filament, allowing for the determination of possible differences in behaviour on the two sides of the filament, and across concentrations. The robust and repeatable nature of the method allows quantitative, semi-automated analyses to be made of large studies of acto-tropomyosin interactions, as well as for other actin binding proteins or filamentous structures, opening the way for dissection of the dynamics underlying these interactions.

Effects of cardiomyopathy-linked mutations K15N and R21H in tropomyosin on thin-filament regulation and pointed-end dynamics

Missense mutations K15N and R21H in striated muscle tropomyosin are linked to dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM), respectively. Tropomyosin, together with the troponin complex, regulates muscle contraction and, along with tropomodulin and leiomodin, controls the uniform thin-filament lengths crucial for normal sarcomere structure and function. We used Förster resonance energy transfer to study effects of the tropomyosin mutations on the structure and kinetics of the cardiac troponin core domain associated with the Ca²⁺-dependent regulation of cardiac thin filaments. We found that the K15N mutation desensitizes thin filaments to Ca²⁺ and slows the kinetics of structural changes in troponin induced by Ca²⁺ dissociation from troponin, while the R21H mutation has almost no effect on these parameters. Expression of the K15N mutant in cardiomyocytes decreases leiomodin’s thin-filament pointed-end assembly but does not affect tropomodulin’s assembly at the pointed end. Our in vitro assays show that the R21H mutation causes a twofold decrease in tropomyosin’s affinity for F-actin and affects leiomodin’s function. We suggest that the K15N mutation causes DCM by altering Ca²⁺-dependent thin-filament regulation and that one of the possible HCM-causing mechanisms by the R21H mutation is through alteration of leiomodin’s function.

The effects of cardiomyopathy-associated mutations in the head-to-tail overlap junction of α-tropomyosin on its properties and interaction with actin

Tropomyosin (Tpm) plays a crucial role in the regulation of muscle contraction by controlling actin-myosin interaction. Tpm coiled-coil molecules bind each other via overlap junctions of their N- and C-termini and form a semi-rigid strand that binds the helical surface of an actin filament. The high bending stiffness of the strand is essential for high cooperativity of muscle regulation. Point mutations M8R and K15N in the N-terminal part of the junction and the A277V one in the C-terminal part are associated with dilated cardiomyopathy, while the M281T and I284V mutations are related to hypertrophic cardiomyopathy. To reveal molecular mechanism(s) underlying these pathologies, we studied the properties of recombinant Tpm carrying these mutations using several experimental approaches and molecular dynamic simulation of the junction. The M8R and K15N mutations weakened the interaction between the N- and C-termini of Tpm in the overlap junction and reduced the Tpm affinity for actin. These changes possibly led to a reduction in the regulation cooperativity. The C-terminal mutations caused only small and controversial changes in properties of Tpm and its complex with actin. Their involvement in disease phenotype is possibly caused by interaction with other sarcomere proteins.

Myopathic mutations in the β-chain of tropomyosin differently affect the structural and functional properties of ββ- and αβ-dimers

Tropomyosin (Tpm) is an actin-binding protein that plays a vital role in the regulation of muscle contraction. Fast skeletal muscles express 2 Tpm isoforms, α (Tpm 1.1) and β (Tpm 2.2), resulting in the existence of 2 forms of dimeric Tpm molecule: αα-homodimer and αβ-heterodimer. ββ-Homodimer is unstable and absent in the native state, despite which most of the studies of myopathy-relating Tpm mutations have been performed on the ββ-homodimer. Here, we applied different methods to investigate the effects of myopathic mutations R133W and N202K in the β-chain of Tpm on properties of αβ-heterodimers and to compare them with the features of ββ-homodimers with the same mutations. The results show that properties of αβ-Tpm and ββ-Tpm with substitutions in the β-chain differ significantly, and this indicates that the effects of myopathic mutations in the Tpm β-chain should be studied on the Tpm αβ-heterodimer.-Bershitsky, S. Y., Logvinova, D. S., Shchepkin, D. V., Kopylova, G. V., Matyushenko, A. M. Myopathic mutations in the β-chain of tropomyosin differently affect the structural and functional properties of ββ- and αβ-dimers.

The reason for the low Ca2+-sensitivity of thin filaments associated with the Glu41Lys mutation in the TPM2 gene is "freezing" of tropomyosin near the outer domain of actin and inhibition of actin monomer switching off during the ATPase cycle

The E41K mutation in TPM2 gene encoding muscle regulatory protein beta-tropomyosin is associated with nemaline myopathy and cap disease. The mutation results in a reduced Ca²⁺-sensitivity of the thin filaments and in muscle weakness. To elucidate the structural basis of the reduced Ca²⁺-sensitivity of the thin filaments, we studied multistep changes in spatial arrangement of tropomyosin (Tpm), actin and myosin heads during the ATPase cycle in reconstituted fibers, using the polarized fluorescence microscopy. The E41K mutation inhibits troponin's ability to shift Tpm to the closed position at high Ca²⁺, thus restraining the transition of the thin filaments from the "off" to the "on" state. The mutation also inhibits the ability of S1 to shift Tpm to the open position, decreases the amount of the myosin heads bound strongly to actin at high Ca²⁺, but increases the number of such heads at low Ca²⁺. These changes may contribute to the low Ca²⁺-sensitivity and muscle weakness. As the mutation has no effect on troponin's ability to switch actin monomers on at high Ca²⁺ and inhibits their switching off at low Ca²⁺, the use of reagents that increase the Ca²⁺-sensitivity of the troponin complex may not be appropriate to restore muscle function in patients with this mutation.

Tropomyosin isoforms differentially affect muscle contractility in the head and body regions of the nematode Caenorhabditis elegans

Tropomyosin, one of the major actin filament-binding proteins, regulates actin-myosin interaction and actin-filament stability. Multicellular organisms express a number of tropomyosin isoforms, but understanding of isoform-specific tropomyosin functions is incomplete. The nematode Caenorhabditis elegans has a single tropomyosin gene, lev-11, which has been reported to express four isoforms by using two separate promoters and alternative splicing. Here, we report a fifth tropomyosin isoform, LEV-11O, which is produced by alternative splicing that includes a newly identified seventh exon, exon 7a. By visualizing specific splicing events in vivo, we find that exon 7a is predominantly selected in a subset of the body wall muscles in the head, while exon 7b, which is the alternative to exon 7a, is utilized in the rest of the body. Point mutations in exon 7a and exon 7b cause resistance to levamisole--induced muscle contraction specifically in the head and the main body, respectively. Overexpression of LEV-11O, but not LEV-11A, in the main body results in weak levamisole resistance. These results demonstrate that specific tropomyosin isoforms are expressed in the head and body regions of the muscles and contribute differentially to the regulation of muscle contractility.

Distinct sites in tropomyosin specify shared and isoform-specific regulation of myosins II and V

Muscle contraction, cytokinesis, cellular movement, and intracellular transport depend on regulated actin-myosin interaction. Most actin filaments bind one or more isoform of tropomyosin, a coiled-coil protein that stabilizes the filaments and regulates interactions with other actin-binding proteins, including myosin. Isoform-specific allosteric regulation of muscle myosin II by actin-tropomyosin is well-established while that of processive myosins, such as myosin V, which transport organelles and macromolecules in the cell periphery, is less certain. Is the regulation by tropomyosin a universal mechanism, the consequence of the conserved periodic structures of tropomyosin, or is it the result of specialized interactions between particular isoforms of myosin and tropomyosin? Here, we show that striated muscle tropomyosin, Tpm1.1, inhibits fast skeletal muscle myosin II but not myosin Va. The non-muscle tropomyosin, Tpm3.1, in contrast, activates both myosins. To decipher the molecular basis of these opposing regulatory effects, we introduced mutations at conserved surface residues within the six periodic repeats (periods) of Tpm3.1, in positions homologous or analogous to those important for regulation of skeletal muscle myosin by Tpm1.1. We identified conserved residues in the internal periods of both tropomyosin isoforms that are important for the function of myosin Va and striated myosin II. Conserved residues in the internal and C-terminal periods that correspond to Tpm3.1-specific exons inhibit myosin Va but not myosin II function. These results suggest that tropomyosins may directly impact myosin function through both general and isoform-specific mechanisms that identify actin tracks for the recruitment and function of particular myosins.

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.

Molecular mechanisms of dysfunction of muscle fibres associated with Glu139 deletion in TPM2 gene

Deletion of Glu139 in β-tropomyosin caused by a point mutation in TPM2 gene is associated with cap myopathy characterized by high myofilament Ca2+-sensitivity and muscle weakness. To reveal the mechanism of these disorders at molecular level, mobility and spatial rearrangements of actin, tropomyosin and the myosin heads at different stages of actomyosin cycle in reconstituted single ghost fibres were investigated by polarized fluorescence microscopy. The mutation did not alter tropomyosin's affinity for actin but increased strongly the flexibility of tropomyosin and kept its strands near the inner domain of actin. The ability of troponin to switch actin monomers "on" and "off" at high and low Ca2+, respectively, was increased, and the movement of tropomyosin towards the blocked position at low Ca2+ was inhibited, presumably causing higher Ca2+-sensitivity. The mutation decreased also the amount of the myosin heads which bound strongly to actin at high Ca2+ and increased the number of these heads at relaxation; this may contribute to contractures and muscle weakness.

Three mammalian tropomyosin isoforms have different regulatory effects on nonmuscle myosin-2B and filamentous β-actin in vitro

The metazoan actin cytoskeleton supports a wide range of contractile and transport processes. Recent studies have shown how the dynamic association with specific tropomyosin isoforms generates actin filament populations with distinct functional properties. However, critical details of the associated molecular interactions remain unclear. Here, we report the properties of actomyosin–tropomyosin complexes containing filamentous β-actin, nonmuscle myosin-2B (NM-2B) constructs, and either tropomyosin isoform Tpm1.8cy (b.–.b.d), Tpm1.12br (b.–.b.c), or Tpm3.1cy (b.–.a.d). Our results show the extent to which the association of filamentous β-actin with these different tropomyosin cofilaments affects the actin-mediated activation of NM-2B and the release of the ATP hydrolysis products ADP and phosphate from the active site. Phosphate release gates a transition from weak to strong F-actin–binding states. The release of ADP has the opposite effect. These changes in dominant rate-limiting steps have a direct effect on the duty ratio, the fraction of time that NM-2B spends in strongly F-actin–bound states during ATP turnover. The duty ratio is increased ∼3-fold in the presence of Tpm1.12 and 5-fold for both Tpm1.8 and Tpm3.1. The presence of Tpm1.12 extends the time required per ATP hydrolysis cycle 3.7-fold, whereas it is shortened by 27 and 63% in the presence of Tpm1.8 and Tpm3.1, respectively. The resulting Tpm isoform–specific changes in the frequency, duration, and efficiency of actomyosin interactions establish a molecular basis for the ability of these complexes to support cellular processes with widely divergent demands in regard to force production, capacity to move processively, and speed of movement.

The reason for a high Ca2+-sensitivity associated with Arg91Gly substitution in TPM2 gene is the abnormal behavior and high flexibility of tropomyosin during the ATPase cycle

Substitution of Arg for Gly residue in 91th position in β-tropomyosin caused by a point mutation in TPM2 gene is associated with distal arthrogryposis, characterized by a high Ca²⁺-sensitivity of myofilament and contracture syndrome. To understand the mechanisms of this defect, we studied multistep changes in mobility and spatial arrangement of tropomyosin, actin and myosin heads during the ATPase cycle in reconstituted ghost fibres, using the polarized fluorescence microscopy. The mutation was shown to markedly decrease the bending stiffness of β-tropomyosin in the thin filaments. In the absence of the myosin heads the mutation did not alter the ability of troponin to shift tropomyosin to the blocked position and to switch actin monomers off at low Ca²⁺. During the ATPase cycle the movement of the mutant tropomyosin is restrained, it is located near the open position, which allows strong binding of the myosin heads to actin even at low Ca²⁺. This may be the reason for both high Ca²⁺-sensitivity and contractures associated with the Arg91Gly mutation. The use of reagents that decrease the Ca²⁺sensitivity of the troponin complex may not be appropriate to restore muscle function in patients with this mutation.

Deviations in conformational rearrangements of thin filaments and myosin caused by the Ala155Thr substitution in hydrophobic core of tropomyosin

Effects of the Ala155Thr substitution in hydrophobic core of tropomyosin Tpm1.1 on conformational rearrangements of the components of the contractile system (Tpm1.1, actin and myosin heads) were studied by polarized fluorimetry technique at different stages of the actomyosin ATPase cycle. The proteins were labelled by fluorescent probes and incorporated into ghost muscle fibres. The substitution violated the blocked and closed states of thin filaments stimulating abnormal displacement of tropomyosin to the inner domains of actin, switching actin on and increasing the relative number of the myosin heads in strong-binding state. Furthermore, the mutant tropomyosin disrupted the major function of troponin to alter the distribution of the different functional states of thin filaments. At low Ca²⁺ troponin did not effectively switch thin filament off and the myosin head lost the ability to drive the spatial arrangement of the mutant tropomyosin. The information about tropomyosin flexibility obtained from the fluorescent probes at Cys190 indicates that this tropomyosin is generally more rigid, that obviously prevents tropomyosin to bend and adopt the appropriate conformation required for proper regulation.