Role of the Head-to-Tail Overlap Region in Smooth and Skeletal Muscle β-Tropomyosin

M8R tropomyosin mutation disrupts actin binding and filament regulation: The beginning affects the middle and end

Dilated cardiomyopathy (DCM) is associated with mutations in cardiomyocyte sarcomeric proteins, including α-tropomyosin. In conjunction with troponin, tropomyosin shifts to regulate actomyosin interactions. Tropomyosin molecules overlap via tropomyosin-tropomyosin head-to-tail associations, forming a continuous strand along the thin filament. These associations are critical for propagation of tropomyosin's reconfiguration along the thin filament and key for the cooperative switching between heart muscle contraction and relaxation. Here, we tested perturbations in tropomyosin structure, biochemistry, and function caused by the DCM-linked mutation, M8R, which is located at the overlap junction. Localized and nonlocalized structural effects of the mutation were found in tropomyosin that ultimately perturb its thin filament regulatory function. Comparison of mutant and WT α-tropomyosin was carried out using in vitro motility assays, CD, actin co-sedimentation, and molecular dynamics simulations. Regulated thin filament velocity measurements showed that the presence of M8R tropomyosin decreased calcium sensitivity and thin filament cooperativity. The co-sedimentation of actin and tropomyosin showed weakening of actin-mutant tropomyosin binding. The binding of troponin T's N terminus to the actin-mutant tropomyosin complex was also weakened. CD and molecular dynamics indicate that the M8R mutation disrupts the four-helix bundle at the head-to-tail junction, leading to weaker tropomyosin-tropomyosin binding and weaker tropomyosin-actin binding. Molecular dynamics revealed that altered end-to-end bond formation has effects extending toward the central region of the tropomyosin molecule, which alter the azimuthal position of tropomyosin, likely disrupting the mutant thin filament response to calcium. These results demonstrate that mutation-induced alterations in tropomyosin-thin filament interactions underlie the altered regulatory phenotype and ultimately the pathogenesis of DCM.

Cryo-EM and Molecular Docking Shows Myosin Loop 4 Contacts Actin and Tropomyosin on Thin Filaments

The motor protein myosin drives muscle and nonmuscle motility by binding to and moving along actin of thin filaments. Myosin binding to actin also modulates interactions of the regulatory protein, tropomyosin, on thin filaments, and conversely tropomyosin affects myosin binding to actin. Insight into this reciprocity will facilitate a molecular level elucidation of tropomyosin regulation of myosin interaction with actin in muscle contraction, and in turn, promote better understanding of nonmuscle cell motility. Indeed, experimental approaches such as fiber diffraction, cryoelectron microscopy, and three-dimensional reconstruction have long been used to define regulatory interaction of tropomyosin and myosin on actin at a structural level. However, their limited resolution has not proven sufficient to determine tropomyosin and myosin contacts at an atomic-level and thus to fully substantiate possible functional contributions. To overcome this deficiency, we have followed a hybrid approach by performing new cryogenic electron microscopy reconstruction of myosin-S1-decorated F-actin-tropomyosin together with atomic scale protein-protein docking of tropomyosin to the EM models. Here, cryo-EM data were derived from filaments reconstituted with α1-actin, cardiac αα-tropomyosin, and masseter muscle β-myosin complexes; masseter myosin, which shares sequence identity with β-cardiac myosin-heavy chain, was used because of its stability in vitro. The data were used to build an atomic model of the tropomyosin cable that fits onto the actin filament between the tip of the myosin head and a cleft on the innermost edge of actin subunits. The docking and atomic scale fitting showed multiple discrete interactions of myosin loop 4 and acidic residues on successive 39-42 residue-long tropomyosin pseudorepeats. The contacts between S1 and tropomyosin on actin appear to compete with and displace ones normally found between actin and tropomyosin on myosin-free thin filaments in relaxed muscle, thus restructuring the filament during myosin-induced activation.

Cardiomyopathy Mutation Alters End-to-End Junction of Tropomyosin and Reduces Calcium Sensitivity

Muscle contraction is governed by tropomyosin (Tpm) shifting azimuthally between three states on F-actin (B-, C-, and M-states) in response to calcium binding to troponin and actomyosin cross-bridge formation. The Tpm coiled coil polymerizes head to tail along the long-pitch helix of F-actin to form continuous superhelical cables that wrap around the actin filaments. The end-to-end bonds formed between the N- and C-terminus of adjacent Tpm molecules define Tpm continuity and play a critical role in the ability of Tpm to cooperatively bind to actin, thus facilitating Tpm conformational switching to cooperatively propagate along F-actin. We expect that a missense mutation in this critical overlap region associated with dilated cardiomyopathy, A277V, will alter Tpm binding and thin filament activation by altering the overlap structure. Here, we used cosedimentation assays and in vitro motility assays to determine how the mutation alters Tpm binding to actin and its ability to regulate actomyosin interactions. Analytical viscometry coupled with molecular dynamics simulations showed that the A277V mutation results in enhanced Tpm end-to-end bond strength and a reduced curvature of the Tpm overlap domain. The mutant Tpm exhibited enhanced actin-Tpm binding affinity, consistent with overlap stabilization. The observed A277V-induced decrease in cooperative activation observed with regulated thin filament motility indicates that increased overlap stabilization is not correlated with Tpm-Tpm overlap binding strength or mechanical rigidity as is often assumed. Instead, A277V-induced structural changes result in local and delocalized increases in Tpm flexibility and prominent coiled-coil twisting in pseudorepeat 4. An A277V-induced decrease in Ca²⁺ sensitivity, consistent with a mutation-induced bolstering of the B-state Tpm-actin electrostatic contacts and an increased Tpm troponin T1 binding affinity, was also observed.

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.

Direct observation of tropomyosin binding to actin filaments

Tropomyosin is an elongated α-helical coiled coil that binds to seven consecutive actin subunits along the long-pitch helix of actin filaments. Once bound, tropomyosin polymerizes end-to-end and both stabilizes F-actin and regulates access of various actin-binding proteins including myosin in muscle and nonmuscle cells. Single tropomyosin molecules bind weakly to F-actin with millimolar Kd , whereas the end-to-end linked tropomyosin associates with about a 1000-fold greater affinity. Despite years of study, the assembly mechanism of tropomyosin onto actin filaments remains unclear. In this study, we used total internal reflection fluorescence microscopy to directly monitor the cooperative binding of fluorescently labeled tropomyosin molecules to phalloidin-stabilized actin filaments. We find that tropomyosin molecules assemble from multiple growth sites after random low-affinity binding of single molecules to actin. As the length of the tropomyosin chain increases, the probability of detachment decreases, which leads to further chain growth. Tropomyosin chain extension is linearly dependent on the concentration of tropomyosin, occurring at approximately 100 monomers/(μM*s). The random tropomyosin binding to F-actin leads to discontinuous end-to-end association where gaps in the chain continuity smaller than the required seven sequential actin monomers are available. Direct observation of tropomyosin detachment revealed the number of gaps in actin-bound tropomyosin, the time course of gap annealing, and the eventual filament saturation process.

Phosphorylation of Ser283 enhances the stiffness of the tropomyosin head-to-tail overlap domain

The ends of coiled-coil tropomyosin molecules are joined together by nine to ten residue-long head-to-tail "overlapping domains". These short four-chained interconnections ensure formation of continuous tropomyosin cables that wrap around actin filaments. Molecular Dynamics simulations indicate that the curvature and bending flexibility at the overlap is 10-20% greater than over the rest of the molecule, which might affect head-to-tail filament assembly on F-actin. Since the penultimate residue of striated muscle tropomyosin, Ser283, is a natural target of phosphorylating enzymes, we have assessed here if phosphorylation adjusts the mechanical properties of the tropomyosin overlap domain. MD simulations show that phosphorylation straightens the overlap to match the curvature of the remainder of tropomyosin while stiffening it to equal or exceed the rigidity of canonical coiled-coil regions. Corresponding EM data on phosphomimetic tropomyosin S283D corroborate these findings. The phosphorylation-induced change in mechanical properties of tropomyosin likely results from electrostatic interactions between C-terminal phosphoSer283 and N-terminal Lys12 in the four-chain overlap bundle, while promoting stronger interactions among surrounding residues and thus facilitating tropomyosin cable assembly. The stiffening effect of D283-tropomyosin noted correlates with previously observed enhanced actin-tropomyosin activation of myosin S1-ATPase, suggesting a role for the tropomyosin phosphorylation in potentiating muscle contraction.

Mechanistic heterogeneity in contractile properties of α-tropomyosin (TPM1) mutants associated with inherited cardiomyopathies

The most frequent known causes of primary cardiomyopathies are mutations in the genes encoding sarcomeric proteins. Among those are 30 single-residue mutations in TPM1, the gene encoding α-tropomyosin. We examined seven mutant tropomyosins, E62Q, D84N, I172T, L185R, S215L, D230N, and M281T, that were chosen based on their clinical severity and locations along the molecule. The goal of our study was to determine how the biochemical characteristics of each of these mutant proteins are altered, which in turn could provide a structural rationale for treatment of the cardiomyopathies they produce. Measurements of Ca(2+) sensitivity of human β-cardiac myosin ATPase activity are consistent with the hypothesis that hypertrophic cardiomyopathies are hypersensitive to Ca(2+) activation, and dilated cardiomyopathies are hyposensitive. We also report correlations between ATPase activity at maximum Ca(2+) concentrations and conformational changes in TnC measured using a fluorescent probe, which provide evidence that different substitutions perturb the structure of the regulatory complex in different ways. Moreover, we observed changes in protein stability and protein-protein interactions in these mutants. Our results suggest multiple mechanistic pathways to hypertrophic and dilated cardiomyopathies. Finally, we examined a computationally designed mutant, E181K, that is hypersensitive, confirming predictions derived from in silico structural analysis.

Energy landscapes reveal the myopathic effects of tropomyosin mutations

Striated muscle contraction is regulated by an interaction network connecting the effects of troponin, Ca(2+), and myosin-heads to the azimuthal positioning of tropomyosin along thin filaments. Many missense mutations, located at the actin-tropomyosin interface, however, reset the regulatory switching mechanism either by weakening or strengthening residue-specific interactions, leading to hyper- or hypo-contractile pathologies. Here, we compute energy landscapes for the actin-tropomyosin interface and quantify contributions of single amino acid residues to actin-tropomyosin binding. The method is a useful tool to assess effects of actin and tropomyosin mutations, potentially relating initial stages of myopathy to alterations in thin filament stability and regulation. Landscapes for mutant filaments linked to hyper-contractility provide a simple picture that describes a decrease in actin-tropomyosin interaction energy. Destabilizing the blocked (relaxed)-state parallels previously noted enhanced Ca(2+)-sensitivity conferred by these mutants. Energy landscapes also identify post-translational modifications that can rescue regulatory imbalances. For example, cardiomyopathy-associated E62Q tropomyosin mutation weakens actin-tropomyosin interaction, but phosphorylation of neighboring S61 rescues the binding-deficit, results confirmed experimentally by in vitro motility assays. Unlike results on hyper-contractility-related mutants, landscapes for tropomyosin mutants tied to hypo-contractility do not present a straightforward picture. These mutations may affect other components of the regulatory network, e.g., troponin-tropomyosin signaling.

Molecular mechanical differences between isoforms of contractile actin in the presence of isoforms of smooth muscle tropomyosin

The proteins involved in smooth muscle's molecular contractile mechanism - the anti-parallel motion of actin and myosin filaments driven by myosin heads interacting with actin - are found as different isoforms. While their expression levels are altered in disease states, their relevance to the mechanical interaction of myosin with actin is not sufficiently understood. Here, we analyzed in vitro actin filament propulsion by smooth muscle myosin for [Formula: see text]-actin ([Formula: see text]A), [Formula: see text]-actin-tropomyosin-[Formula: see text] ([Formula: see text]A-Tm[Formula: see text]), [Formula: see text]-actin-tropomyosin-[Formula: see text] ([Formula: see text]A-Tm[Formula: see text]), [Formula: see text]-actin ([Formula: see text]A), [Formula: see text]-actin-tropomyosin-[Formula: see text] ([Formula: see text]A-Tm[Formula: see text]), and [Formula: see text]-actin-tropomoysin-[Formula: see text] ([Formula: see text]A-Tm[Formula: see text]). Actin sliding analysis with our specifically developed video analysis software followed by statistical assessment (Bootstrapped Principal Component Analysis) indicated that the in vitro motility of [Formula: see text]A, [Formula: see text]A, and [Formula: see text]A-Tm[Formula: see text] is not distinguishable. Compared to these three 'baseline conditions', statistically significant differences ([Formula: see text]) were: [Formula: see text]A-Tm[Formula: see text] - actin sliding velocity increased 1.12-fold, [Formula: see text]A-Tm[Formula: see text] - motile fraction decreased to 0.96-fold, stop time elevated 1.6-fold, [Formula: see text]A-Tm[Formula: see text] - run time elevated 1.7-fold. We constructed a mathematical model, simulated actin sliding data, and adjusted the kinetic parameters so as to mimic the experimentally observed differences: [Formula: see text]A-Tm[Formula: see text] - myosin binding to actin, the main, and the secondary myosin power stroke are accelerated, [Formula: see text]A-Tm[Formula: see text] - mechanical coupling between myosins is stronger, [Formula: see text]A-Tm[Formula: see text] - the secondary power stroke is decelerated and mechanical coupling between myosins is weaker. In summary, our results explain the different regulatory effects that specific combinations of actin and smooth muscle tropomyosin have on smooth muscle actin-myosin interaction kinetics.

Purification of tropomyosin Br-3 and 5NM1 and characterization of their interactions with actin

Tropomyosins were first identified in neuronal systems in 1973. Although numerous isoforms were found and described since then, many aspects of their function and interactions remained unknown. Tropomyosin isoforms show different sorting pattern in neurogenesis. As one example, TM5NM1/2 is present in developing axons, but it is replaced by TMBr-3 in mature neurons, suggesting that these tropomyosin isoforms contribute differently to the establishment of the functional features of the neuronal actin networks. We developed a method for the efficient purification of TMBr-3 and TM5NM1 as recombinant proteins using bacterial expression system and investigated their interactions with actin. We found that both isoforms bind actin filaments, however, the binding of TM5NM1 was much stronger than that of TMBr-3. TMBr-3 and TM5NM1 modestly affected actin assembly kinetics, in an opposite manner. Consistently with the higher affinity of TM5NM1 it inhibited actin filament disassembly more efficiently than TMBr-3. Similarly to other previously studied tropomyosins TM5NM1 inhibited the Arp2/3 complex-mediated actin assembly. Notably, TMBr-3 did not influence the Arp2/3 complex-mediated polymerization. This is a unique feature of TMBr-3, since so far it is the only known tropomyosin supporting the activity of the Arp2/3 complex, indicating that TMBr-3 may colocalize and work simultaneously with Arp2/3 complex in neuronal cells.

Polymorphism in tropomyosin structure and function

Tropomyosins (Tm) in humans are expressed from four distinct genes and by alternate splicing >40 different Tm polypeptide chains can be made. The functional Tm unit is a dimer of two parallel polypeptide chains and these can be assembled from identical (homodimer) or different (heterodimer) polypeptide chains provided both chains are of the same length. Since most cells express multiple isoforms of Tm, the number of different homo and heterodimers that can be assembled becomes very large. We review the mechanism of dimer assembly and how preferential assembly of some heterodimers is driven by thermodynamic stability. We examine how in vitro studies can reveal functional differences between Tm homo and heterodimers (stability, actin affinity, flexibility) and the implication for how there could be selection of Tm isomers in the assembly on to an actin filament. The role of Tm heterodimers becomes more complex when mutations in Tm are considered, such as those associated with cardiomyopathies, since mutations can appear in only one of the chains.

Identification of two new regions in the N-terminus of cardiac troponin T that have divergent effects on cardiac contractile function

Abstract  Cardiac troponin T (cTnT) has a highly acidic extended N-terminus, the physiological role of which remains poorly understood. To decipher the physiological role of this unique region, we deleted specific regions within the N-terminus of mouse cTnT (McTnT) to create McTnT1-44 and McTnT45-74 proteins. Contractile function and dynamic force-length measurements were made after reconstituting the McTnT deletion proteins into detergent-skinned cardiac papillary fibres harvested from non-transgenic mice that expressed α-tropomyosin (Tm). To further understand how the functional effects of the N-terminus of cTnT are modulated by Tm isoforms, McTnT deletion proteins were reconstituted into detergent-skinned cardiac papillary fibres harvested from transgenic mice that expressed both α- and β-Tm. McTnT1-44, but not McTnT45-74, attenuated maximal activation of the thin filament. Myofilament Ca(2+) sensitivity, as measured by pCa50 (-log of [Ca(2+)]free required for half-maximal activation), decreased in McTnT1-44 (α-Tm) fibres. The desensitizing effect of McTnT1-44 on pCa50 was ablated in β-Tm fibres. McTnT45-74 enhanced pCa50 in both α- and β-Tm fibres, with β-Tm having a bigger effect. The Hill coefficient of tension development was significantly attenuated by McTnT45-74, suggesting an effect on thin-filament cooperativity. The rate of cross-bridge (XB) detachment and the strained XB-mediated impact on other XBs were augmented by McTnT1-44 in β-Tm fibres. The magnitude of the length-mediated recruitment of XBs was attenuated by McTnT1-44 in β-Tm fibres. Our data demonstrate that the 1-44 region of McTnT is essential for maximal activation, whereas the cardiac-specific 45-74 region of McTnT is essential for augmenting cooperativity. Moreover, our data show that α- and β-Tm isoforms have divergent effects on McTnT deletion mutant's ability to modulate cardiac thin-filament activation and Ca(2+) sensitivity. Our results not only provide the first explicit evidence for the existence of two distinct functional regions within the N-terminus of cTnT, but also offer mechanistic insights into the divergent physiological roles of these regions in mediating cardiac contractile activation.

α-Tropomyosin with a D175N or E180G mutation in only one chain differs from tropomyosin with mutations in both chains

α-Tropomyosin (Tm) carrying hypertrophic cardiomyopathy mutation D175N or E180G was expressed in Escherichia coli. We have assembled dimers of two polypeptide chains in vitro that carry one (αα*) or two (α*α*) copies of the mutation. We found that the presence of the mutation has little effect on dimer assembly, thereby predicting that individuals heterozygous for the Tm mutations are likely to express both αα* and α*α* Tm. Depending on the expression level, the heterodimer may be the predominant form in individuals carrying the mutation. Thus, it is important to define differences in the properties of Tm molecules carrying one or two copies of the mutation. We examined the Tm homo- and heterodimer properties: actin affinity, thermal stability, calcium regulation of myosin subfragment 1 binding, and calcium regulation of myofibril force. We report that the properties of the heterodimer may be similar to those of the wild-type homodimer (actin affinity, thermal stability, D175N αα*), similar to those of the mutant homodimer (calcium sensitivity, D175N αα*), intermediate between the two (actin affinity, E180G αα*), or different from both (thermal stability, E180G αα*). Thus, the properties of the homodimer are not a completely reliable guide to the properties of the heterodimer.

In vitro formation and characterization of the skeletal muscle α·β tropomyosin heterodimers

Tropomyosin (Tm) is a dimer made of two alpha helical chains associated into a parallel coiled-coil. In mammalian skeletal and cardiac muscle, the Tm is expressed from two separate genes to give the α- and β-Tm isoforms. These associate in vivo to form homo- (α(2)) and heterodimers (α·β) with little β(2) normally observed. The proportion of α(2) vs α·β varies across species and across muscle types from almost 100% α(2)- to 50% α·β-Tm. The ratio can also vary during development and in disease. The functional significance of the presence of these two isoforms has not been defined because it is difficult to isolate or purify the α·β dimer for functional studies. Here we report an effective method for purifying bacterially expressed Tm as α·β dimers using a cleavable N-terminal tag on one of the two chains. The same method can be used to isolate Tm dimers in which one chain carries a mutation. We go on to show that the α·β dimers differ in key properties (actin affinity, thermal stability) from either the α(2)- or β(2)-Tm. However, the ability to regulate myosin binding when combined with cardiac troponin appears unaffected.

Pyrene: a probe to study protein conformation and conformational changes

The review focuses on the unique spectral features of pyrene that can be utilized to investigate protein structure and conformation. Pyrene is a fluorescent probe that can be attached covalently to protein side chains, such as sulfhydryl groups. The spectral features of pyrene are exquisitely sensitive to the microenvironment of the probe: it exhibits an ensemble of monomer fluorescence emission peaks that report on the polarity of the probe microenvironment, and an additional band at longer wavelengths, the appearance of which reflects the presence of another pyrene molecule in spatial proximity (~10 Å). Its high extinction coefficient allows us to study labeled proteins in solution at physiologically relevant concentrations. The environmentally- and spatially-sensitive features of pyrene allow monitoring protein conformation, conformational changes, protein folding and unfolding, protein-protein, protein-lipid and protein-membrane interactions.

Echinococcus granulosus tropomyosin isoforms: from gene structure to expression analysis

Tropomyosins (Trps) constitute a family of actin filament-binding proteins found in all eukaryotic cells. In muscle cells, they play a central role in contraction by regulating calcium-sensitive interaction of actin and myosin. In non-muscle cells, tropomyosins regulate actin filament organization and dynamics. Trps genes exhibit extensive cell type-specific isoform diversity generated by alternative splicing. Here, we report the characterization of tropomyosin gene transcribed sequences from the parasitic platyhelminth Echinococcus granulosus. Using RT-PCR approach we isolated three isoforms (egtrpA, egtrpB and egtrpC), which display significant homologies to know tropomyosins of different phylogenetic origin. The corresponding gene, egtrp (5656 bp), contains eight introns and nine exons. Southern blot hybridization studies showed that egtrp is present as single copy locus in E. granulosus. We demonstrated that egtrp expresses three different transcripts which differ in alternatively spliced exon 4 and intron VI. Interestingly, intron VI suffers intron retention and contains an internal stop codon in frame. Three major bands are also detected by Western blot analysis using a specific anti-rEgTrp antiserum. Immune-localization and in situ hybridization studies showed that egtrp transcription and translation is mostly localized at the protoscoleces suckers. This is the first report of alternative splicing in this parasite.