Quantitative analysis of tropomyosin linear polymerization equilibrium as a function of ionic strength

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.

Competition between Tropomyosin, Fimbrin, and ADF/Cofilin drives their sorting to distinct actin filament networks

The fission yeast actin cytoskeleton is an ideal, simplified system to investigate fundamental mechanisms behind cellular self-organization. By focusing on the stabilizing protein tropomyosin Cdc8, bundling protein fimbrin Fim1, and severing protein coffin Adf1, we examined how their pairwise and collective interactions with actin filaments regulate their activity and segregation to functionally diverse F-actin networks. Utilizing multi-color TIRF microscopy of in vitro reconstituted F-actin networks, we observed and characterized two distinct Cdc8 cables loading and spreading cooperatively on individual actin filaments. Furthermore, Cdc8, Fim1, and Adf1 all compete for association with F-actin by different mechanisms, and their cooperative association with actin filaments affects their ability to compete. Finally, competition between Fim1 and Adf1 for F-actin synergizes their activities, promoting rapid displacement of Cdc8 from a dense F-actin network. Our findings reveal that competitive and cooperative interactions between actin binding proteins help define their associations with different F-actin networks.

DOI:http://dx.doi.org/10.7554/eLife.23152.001

Cells use a protein called actin to provide shape, to generate the forces needed for cells to divide, and for many other essential processes. Inside a cell, individual actin proteins join up to form long filaments. These actin filaments are organized in different ways to make networks that have distinct properties, each tailored for a specific process. For instance, bundles of straight actin filaments help a cell to divide, whereas a network of branched actin filaments allows cells to move.

The different proteins that bind to actin filaments influence how quickly actin filaments are assembled and organized into networks. Therefore, many of the properties of an actin filament network are due to the actin binding proteins that are associated with it. Two actin binding proteins called fimbrin and cofilin associate with a type of actin filament network known as the actin patch. A third actin binding protein called tropomyosin associates with a different network that forms a ring. It is not known how particular actin binding proteins choose to associate with one actin network instead of another.

Christensen et al. used a fluorescence microscopy technique to study how fimbrin, cofilin and tropomyosin associate with different actin networks in a single-celled organism called fission yeast. This technique involved incubating actin and actin binding proteins together in a microscope chamber. The experiments show that some actin binding proteins, like tropomyosin, cooperate to bind to actin. Individual tropomyosin molecules find it difficult to bind actin filaments on their own, but once one tropomyosin molecule is attached to the filament, others rapidly join to coat the filament.

On the other hand, some actin-binding proteins compete for binding to filaments. For example, the binding of fimbrin to actin filaments causes tropomyosin to be removed from the actin network. Further experiments revealed that fimbrin and cofilin work with each other to rapidly generate a dense actin network and displace tropomyosin. Together, the findings of Christensen et al. suggest that competitions between actin binding proteins determine which actin binding proteins are associated with an actin network.

The next challenge is to understand how the most competitive actin-binding proteins are kept off actin networks where they do not belong. Further studies will shed light on how these interactions cause large changes in how the cell is organized.

DOI:http://dx.doi.org/10.7554/eLife.23152.002

Isolation and characterisation of tropomyosin from shrimp (Penaeus vannamei Boone) and its association property at high ionic strength

Shrimps are important and highly demanded seafood, but they have been reported as a cause of food hypersensitive reaction. The major allergen of shrimp is tropomyosin (TM). However, so far, there has been few report on such purification procedure. In this study, we developed a strategy for the purification of TM from shrimp (Penaeus vannamei Boone). Subsequently, we demonstrated that the apparent MW of this protein is about 66 kDa, and this protein naturally contains two subunits (38.5 and 36.6 kDa) with a ratio of 1 to 1. Interestingly, different from other known TMs from vertebrates, shrimp TM can self-assemble into nanofibres at high ionic strength induced by ATP. These findings help to understand the structure and polymerisation property of TM from shrimps.

Mg2+ ions bind at the C-terminal region of skeletal muscle alpha-tropomyosin

Tropomyosin (Tm) is a dimeric coiled-coil protein that polymerizes through head-to-tail interactions. These polymers bind along actin filaments and play an important role in the regulation of muscle contraction. Analysis of its primary structure shows that Tm is rich in acidic residues, which are clustered along the molecule and may form sites for divalent cation binding. In a previous study, we showed that the Mg(2+)-induced increase in stability of the C-terminal half of Tm is sensitive to mutations near the C-terminus. In the present report, we study the interaction between Mg(2+) and full-length Tm and smaller fragments corresponding to the last 65 and 26 Tm residues. Although the smaller Tm peptide (Tm(259-284(W269))) is flexible and to large extent unstructured, the larger Tm(220-284(W269)) fragment forms a coiled coil in solution whose stability increases significantly in the presence of Mg(2+). NMR analysis shows that Mg(2+) induces chemical shift perturbations in both Tm(220-284(W269)) and Tm(259-284(W269)) in the vicinity of His276, in which are located several negatively charged residues.

Phosphorylation of tropomyosin extends cooperative binding of myosin beyond a single regulatory unit

Tropomyosin (Tm) is one of the major phosphoproteins comprising the thin filament of muscle. However, the specific role of Tm phosphorylation in modulating the mechanics of actomyosin interaction has not been determined. Here we show that Tm phosphorylation is necessary for long-range cooperative activation of myosin binding. We used a novel optical trapping assay to measure the isometric stall force of an ensemble of myosin molecules moving actin filaments reconstituted with either natively phosphorylated or dephosphorylated Tm. The data show that the thin filament is cooperatively activated by myosin across regulatory units when Tm is phosphorylated. When Tm is dephosphorylated, this "long-range" cooperative activation is lost and the filament behaves identically to bare actin filaments. However, these effects are not due to dissociation of dephosphorylated Tm from the reconstituted thin filament. The data suggest that end-to-end interactions of adjacent Tm molecules are strengthened when Tm is phosphorylated, and that phosphorylation is thus essential for long range cooperative activation along the thin filament.

Deciphering the role of the electrostatic interactions in the alpha-tropomyosin head-to-tail complex

Skeletal alpha-tropomyosin (Tm) is a dimeric coiled-coil protein that forms linear assemblies under low ionic strength conditions in vitro through head-to-tail interactions. A previously published NMR structure of the Tm head-to-tail complex revealed that it is formed by the insertion of the N-terminal coiled-coil of one molecule into a cleft formed by the separation of the helices at the C-terminus of a second molecule. To evaluate the contribution of charged residues to complex stability, we employed single and double-mutant Tm fragments in which specific charged residues were changed to alanine in head-to-tail binding assays, and the effects of the mutations were analyzed by thermodynamic double-mutant cycles and protein-protein docking. The results show that residues K5, K7, and D280 are essential to the stability of the complex. Though D2, K6, D275, and H276 are exposed to the solvent and do not participate in intermolecular contacts in the NMR structure, they may contribute to head-to-tail complex stability by modulating the stability of the helices at the Tm termini.

Helminths and allergy: the example of tropomyosin

Parasitic worms contain potent allergens, but epidemiological and experimental studies suggest that infections with certain helminths are negatively associated with the prevalence of allergic diseases. This seeming contradiction can be addressed by using filarial tropomyosin as an example. This protein shares structural features and crossreacting B-cell epitopes with other highly allergenic invertebrate tropomyosins. Nevertheless, it usually does not provoke allergic disease in infected individuals. In addition, it is one of the most prominent candidates for an anti-nematode vaccine. Recent data suggest mechanisms that might prevent hosts from developing allergic reactions against allergens of their parasites, such as filarial tropomyosin.

Cooperative binding of tropomyosin to actin

Tropomyosin molecules attach to the thin filament conjointly rather than separately, in a pattern indicating very high cooperativity. The equilibrium process drawing tropomyosins together on the actin filament can be measured by application ofa linear lattice model to bindingisotherm data and hypotheses on the mechanism of cooperativity can be tested. Each end of tropomyosin overlaps and attaches to the end ofa neighboring tropomyosin, facilitating the formation of continuous tropomyosin strands, without gaps between neighboring molecules along the thin filament. Interestingly, the overlap complexes vary greatly in size and composition among tropomyosin isoforms, despite consistently cooperative binding to actin. Also, the tendency of tropomyosin to bind to actin cooperatively rather than randomly does not correlate with the strength ofend-to-end binding.By implication, tropomyosin's actin-binding cooperativity likely involves effects on the actin filament, as well as direct interactions between adjacent tropomyosins.

Different effects of trifluoroethanol and glycerol on the stability of tropomyosin helices and the head-to-tail complex

Tropomyosin (Tm) is a dimeric coiled-coil protein, composed of 284 amino acids (410 A), that forms linear homopolymers through head-to-tail interactions at low ionic strength. The head-to-tail complex involves the overlap of approximately nine N-terminal residues of one molecule with nine C-terminal residues of another Tm molecule. In this study, we investigate the influence of 2,2,2-trifluoroethanol (TFE) and glycerol on the stability of recombinant Tm fragments (ASTm1-142, Tm143-284(5OHW269)) and of the dimeric head-to-tail complex formed by the association of these two fragments. The C-terminal fragment (Tm143-284(5OHW269)) contains a 5-hydroxytryptophan (5OHW) probe at position 269 whose fluorescence is sensitive to the head-to-tail interaction and allows us to accompany titrations of Tm143-284(5OHW269) with ASTm1-142 to calculate the dissociation constant (Kd) and the interaction energy at TFE and glycerol concentrations between 0% and 15%. We observe that TFE, but not glycerol, reduces the stability of the head-to-tail complex. Thermal denaturation experiments also showed that the head-to-tail complex increases the overall conformational stability of the Tm fragments. Urea and thermal denaturation assays demonstrated that both TFE and glycerol increase the stability of the isolated N- and C-terminal fragments; however, only TFE caused a significant reduction in the cooperativity of unfolding these fragments. Our results show that these two cosolvents stabilize the structures of individual Tm fragments in different manners and that these differences may be related to their opposing effects on head-to-tail complex formation.

Solution NMR Structure of the Junction between Tropomyosin Molecules: Implications for Actin Binding and Regulation

Tropomyosin is a coiled-coil protein that binds head-to-tail along the length of actin filaments in eukaryotic cells, stabilizing them and providing protection from severing proteins. Tropomyosin cooperatively regulates actin's interaction with myosin and mediates the Ca2+ -dependent regulation of contraction by troponin in striated muscles. The N-terminal and C-terminal ends are critical functional determinants that form an "overlap complex". Here we report the solution NMR structure of an overlap complex formed of model peptides. In the complex, the chains of the C-terminal coiled coil spread apart to allow insertion of 11 residues of the N-terminal coiled coil into the resulting cleft. The plane of the N-terminal coiled coil is rotated 90 degrees relative to the plane of the C terminus. A consequence of the geometry is that the orientation of postulated periodic actin binding sites on the coiled-coil surface is retained from one molecule to the next along the actin filament when the overlap complex is modeled into the X-ray structure of tropomyosin determined at 7 Angstroms. Nuclear relaxation NMR data reveal flexibility of the junction, which may function to optimize binding along the helical actin filament and to allow mobility of tropomyosin on the filament surface as it switches between regulatory states.

Quantitative atomic force microscopy image analysis of unusual filaments formed by the Acanthamoeba castellanii myosin II rod domain

We describe a quantitative analysis of Acanthamoeba castellanii myosin II rod domain images collected from atomic force microscope experiments. These images reveal that the rod domain forms a novel filament structure, most likely requiring unusual head-to-tail interactions. Similar filaments are seen also in negatively stained electron microscopy images. Truncated myosins from Acanthamoeba and other model organisms have been visualized before, revealing laterally associated bipolar minifilaments. In contrast, the filament structures that we observe are dominated by axial rather than lateral polymerization. The unusually small features in this structure (1-5 nm) required the development of quantitative and statistical techniques for filament image analysis. These techniques enhance the extraction of features that hitherto have been difficult to ascertain from more qualitative imaging approaches. The heights of the filaments are observed to have a bimodal distribution consistent with the diameters of a single rod domain and a pair of close-packed rod domains. Further quantitative analysis indicates that in-plane association is limited to at most a pair of rod domains. Taken together, this implies that the filaments contain no more than four rod domains laterally associated with one another, somewhat less than that seen in bipolar minifilaments. Analysis of images of the filaments decorated with an anti-FLAG antibody reveals head-to-tail association with mean distances between the antibodies of 75 +/- 15 nm. We consider a set of molecular models to help interpret possible structures of the filaments.

Folding and stability of a coiled-coil investigated using chemical and physical denaturing agents: comparative analysis of polymerized and non-polymerized forms of alpha-tropomyosin

alpha-Tropomyosin (Tm) is a two-stranded alpha-helical coiled-coil protein, which participates in the regulation of muscle contraction. Unlike Tm purified from vertebrate muscle, recombinant Tm expressed in Escherichia coli is not acetylated at the N-terminal residue and loses the capacity to undergo head-to-tail polymerization, to bind actin and to inhibit actomyosin ATPase activity. These functions are restored by fusion of an N-terminal Ala-Ser (AS) dipeptide tail to recombinant Tm. Here, we have employed chemical (guanidine hydrochloride and urea) and physical (elevated hydrostatic pressures and low temperatures) denaturing agents to compare the structural stabilities of polymeric alanine-serine-tropomyosin (ASTm, containing the AS dipeptide) and dimeric "non-fusion" Tm (nfTm, i.e., not containing the AS dipeptide). Binding of the hydrophobic fluorescent dye bis-ANS, circular dichroism and size-exclusion chromatography were used to monitor the stabilities and state of association of both proteins under different solution conditions. Bis-ANS binding was markedly decreased at low concentrations (<1M) of GdnHCl or urea, whereas the secondary structures of both ASTm and nfTm were essentially unaffected in the same range of denaturant concentrations. These results suggest local unfolding of bis-ANS binding domains prior to global unfolding of Tm. In contrast, increased bis-ANS binding was observed when Tm was submitted to high pressures or to low temperatures, implying increased exposure of hydrophobic domains in the protein. Taken together, the different sensitivities of ASTm and nfTm to different denaturing agents support the notion that, at close to physiological conditions, head-to-tail interactions in polymerized ASTm are predominantly stabilized by electrostatic interactions between adjacent Tm dimers, whereas non-polar interactions appear to play a major role in the stability of the coiled-coil structure of individual Tm dimers.

Ca2+ and ionic strength dependencies of S1-ADP binding to actin-tropomyosin-troponin: regulatory implications

Skeletal and cardiac muscle contraction are inhibited by the actin-associated complex of tropomyosin-troponin. Binding of Ca(2+) to troponin or binding of ATP-free myosin to actin reverses this inhibition. Ca(2+) and ATP-free myosin stabilize different tropomyosin-actin structural arrangements. The position of tropomyosin on actin affects the binding of ATP-free myosin to actin but does not greatly affect myosin-ATP binding. Ca(2+) and ATP-free myosin alter both the affinity of ATP-free myosin for actin and the kinetics of that binding. A parallel pathway model of regulation simulated the effects of Ca(2+) and ATP-free myosin binding on both equilibrium binding of myosin-nucleotide complexes to actin and the general features of ATPase activity. That model was recently shown to simulate the kinetics of myosin-S1 binding but the analysis was limited to a single condition because of the limited data available. We have now measured equilibrium binding and binding kinetics of myosin-S1-ADP to actin at a series of ionic strengths and free Ca(2+) concentrations. The parallel pathway model of regulation is consistent with those data. In that model the interaction between adjacent regulatory complexes fully saturated with Ca(2+) was destabilized and the inactive state of actin was stabilized at high ionic strength. These changes explain the previously observed change in binding kinetics with increasing ionic strength.

Nucleation-elongation: a mechanism for cooperative supramolecular polymerization

The kinetic and thermodynamic characteristics of polymerizations following a cooperative, nucleation-elongation mechanism are discussed in comparison to those of non-cooperative, isodesmic polymerizations. Nucleation-elongation polymerization is a relatively unexplored avenue of synthetic polymer chemistry and offers some unique and interesting thermodynamic and kinetic attributes not found in the more classical mechanisms of polymer chemistry.

Ca2+-induced Rolling of Tropomyosin in Muscle Thin Filaments

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

A specific C-terminal deletion in tropomyosin results in a stronger head-to-tail interaction and increased polymerization

Tropomyosin is a 284 residue dimeric coiled-coil protein that interacts in a head-to-tail manner to form linear filaments at low ionic strengths. Polymerization is related to tropomyosin's ability to bind actin, and both properties depend on intact N- and C-termini as well as alpha-amino acetylation of the N-terminus of the muscle protein. Nalpha-acetylation can be mimicked by an N-terminal Ala-Ser fusion in recombinant tropomyosin (ASTm) produced in Escherichia coli. Here we show that a recombinant tropomyosin fragment, corresponding to the protein's first 260 residues plus an Ala-Ser fusion [ASTm(1-260)], polymerizes to a much greater extent than the corresponding full-length recombinant protein, despite the absence of the C-terminal 24 amino acids. This polymerization is sensitive to ionic strength and is greatly reduced by the removal of the N-terminal Ala-Ser fusion [nfTm(1-260)]. CD studies show that nonpolymerizable tropomyosin fragments, which terminate at position 260 [Tm(167-260) and Tm(143-260)], as well as Tm(220-284), are able to interact with ASTm(1-142), a nonpolymerizable N-terminal fragment, and that the head-to-tail interactions observed for these fragment pairs are accompanied by a significant degree of folding of the C-terminal tropomyosin fragment. These results suggest that the new C-terminus, created by the deletion, polymerizes in a manner similar to the full-length protein. Head-to-tail binding for fragments terminating at position 260 may be explained by the presence of a greater concentration of negatively charged residues, while, at the same time, maintaining a conserved pattern of charged and hydrophobic residues found in polymerizable tropomyosins from a variety of sources.