The size, shape and aggregation of tropomyosin particles

Tropomyosin Structure, Function, and Interactions: A Dynamic Regulator

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

Equilibrium self-association of tropomyosin

It has recently been reported that tropomyosin exists exclusively as a dimer in physiological salt conditions. It is shown in the present work using analytical ultracentrifugation that, on the contrary, tropomyosin is in equilibrium between monomer, dimer and tetramer with a weak tendency to dimerize and tetramerize. Such a finding has consequences for the assembly of the tropomyosin-actin complex.

Tropomyosin is a tetramer under physiological salt conditions

Tropomyosin (TM) is a coiled-coil dimer of alpha-helical peptides, which self associates in a head- to-tail fashion along actin polymers, conferring stability to the microfilaments and serving a regulatory function in acto-myosin driven force generation. While the major amount of TM is associated with filaments also in non-muscle cells, it was recently reported that there are isoform-specific pools of TM multimers (not associated with F-actin), which appear to be utilized during actin polymerization and reformed during depolymerization. To determine the size of these multimers, skeletal muscle TM was studied under different salt conditions using gel-filtration and sucrose gradient sedimentation, and compared with purified non-muscle TM 1 and 5, as well as with TM present in non-muscle cell extracts and skeletal muscle TM added to such extracts. Under physiological salt conditions TM appears as a single homogenous peak with the Stokes radius 8.2 nm and the molecular weight (mw) 130,000. The corresponding values for TM 5 are 7.7 nm and 104,000, respectively. This equals four peptides, implying that native TM is a tetramer in physiological salt. It is therefore concluded that the TM multimers are tetramers.

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.

Vertebrate tropomyosin: distribution, properties and function

Tropomyosin (TM) is widely distributed in all cell types associated with actin as a fibrous molecule composed of two alpha-helical chains arranged as a coiled-coil. It is localised, polymerised end to end, along each of the two grooves of the F-actin filament providing structural stability and modulating the filament function. To accommodate the wide range of functions associated with actin filaments that occur in eucaryote cells TM exists in a large number isoforms, over 20 of which have been identified. These isoforms which are expressed by alternative promoters and alternative RNA processing of four genes, TPM1, 2, 3 and 4, all conform to a general pattern of structure. Their amino acid sequences consist of an integral number, six or seven in vertebrates, of quasiequivalent regions of about 40 residues that are considered to represent the actin-binding regions of the molecule. In addition to the variable regions a large part of the polypeptide chains of the TM isoforms, mainly centrally located and expressed by five exons, is invariant. Many of the isoforms are tissue and filament specific in their distribution implying that the exons expressed in them and the regions of the molecule they represent are of significance for the function of the filament system with which they are associated. In the case of muscle there is clear evidence that the TM moves its position on the F-actin filament during contraction and it is therefore considered to play an important part in the regulation of the process. It is uncertain how the role of TM in muscle compares to that in non-muscle systems and if its function in the former tissue is unique to muscle.

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

Tropomyosin is a coiled-coil protein that polymerizes by head-to-tail interactions in an ionic strength-dependent manner. We produced a recombinant full-length chicken alpha-tropomyosin containing a 5-hydroxytryptophan residue at position 269 (formerly an alanine), 15 residues from the C terminus, and show that its fluorescence intensity specifically reports tropomyosin head-to-tail interactions. We used this property to quantitatively study the monomer-polymer equilibrium in tropomyosin and to calculate the equilibrium constant of the head-to-tail interaction as a function of ionic strength. Our results show that the affinity constant changes by almost 2 orders of magnitude over an ionic strength range of 50 mm (between I = 0.045 and 0.095). We were also able to calculate the average polymer length as a function of concentration and ionic strength, which is an important parameter in the interpretation of binding isotherms of tropomyosin with other thin filament proteins such as actin and troponin.

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

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

Characterisation of fast, slow and cardiac muscle tropomyosins from salmonid fish

Tropomyosin (TM) has been isolated from the cardiac muscle, and fast and slow trunk (myotomal) muscles of the mature salmonid fish Atlantic salmon (Salmo salar) and rainbow trout (Salmo gairdneri). When examined electrophoretically, isoforms of TM were detected which were specific, and exclusive, to each type of muscle. Cardiac and fast muscles contained single and distinct isoforms, while slow muscle contained two distinct isoforms, closely related in terms of apparent M(r), and pI. There was no detectable difference between the same TM type from either salmon or trout. On a variety of gel systems, the cardiac and slow isoforms migrated in close proximity to each other and to rabbit alpha-TM. The fast isoform comigrated with rabbit beta-TM. In developing salmon fry, a more acidic (unphosphorylated) variant of TM was present in addition to, and of similar M(r) to, the fast adult isoform. This TM declined in steady-state level during maturation and was virtually undetected in adult muscle. All of the isolated TMs contained little or no covalently bound phosphate and were blocked at the N-terminus. The amino acids released by carboxypeptidase A, when ordered to give maximal similarity to other muscle TMs, were consistent with the following sequences: fast (LDNALNDMTSI) and cardiac (LDHALNDMTSL). The C-terminal region of the slow TM contained His but was heterogeneous. In viscosity measurements, performed as a function of increasing protein concentration, at low ionic strength (t = 5 degrees C, pH 7.00), fast TM exhibited the highest relative viscosity values. Lower and equivalent levels of polymerisation occurred with the cardiac and slow TMs. Polymerisation of all three isoforms was temperature-dependent, with cardiac TM being least sensitive and fast TM being most sensitive. Determination of the complete coding sequence of adult fast TM confirmed the findings of the carboxypeptidase analysis, but the remainder of the sequence more closely resembled alpha-type TMs than beta-type TMs. Overall, salmon fast TM contains 20 (mostly conservative) substitutions compared to rabbit striated muscle alpha-TM and 40 (mostly conservative) substitutions compared to rabbit striated muscle beta-TM. This demonstrates that electrophoretic mobility is not, in all instances, a suitable method to assess the isomorphic nature of striated muscle TMs.

Relationship between alternatively spliced exons and functional domains in tropomyosin

Smooth and striated muscle alpha-tropomyosins differ as a consequence of alternative splicing of exons 2 and 9 encoding amino acid residues 39-80 and 258-284, respectively [Ruiz-Opazo, N., Weinberger, J. & Nadal-Ginard, B. (1985) Nature (London) 315, 67-70]. To understand the relationship between alternatively spliced exons and functional domains in tropomyosin, recombinant unacetylated striated muscle, smooth muscle, and chimeric rat alpha-tropomyosins (+H3N-tropomyosins) expressed in and purified from Escherichia coli were analyzed. The functional differences between the isoforms can be primarily ascribed to exon 9. +H3N-Tropomyosins with the smooth muscle exon 9 bound to skeletal muscle filamentous actin with at least a 5-fold higher affinity than +H3N-tropomyosins with the striated muscle exon 9. On the other hand, in the presence of Ca2+, troponin increased the affinity of +H3N-tropomyosins with the striated muscle exon 9 at least 50-fold, whereas it had little effect on +H3N-tropomyosins with the smooth muscle exon 9. The unique striated muscle alpha-tropomyosin exon 9 seems to be specialized for Ca(2+)-insensitive interaction with troponin on the thin filament. The unique smooth muscle alpha-tropomyosin exon 2 was associated with a slightly lower actin affinity than the striated muscle exon 2. Although the regions encoded by exons 2 and 9 correspond to functional domains, they are not recognizable as independent units or structural domains in the extended coiled-coil structure of this fibrous actin binding protein.

Construction, expression and unexpected regulatory properties of a tropomyosin mutant with a 31-residue deletion at the C-terminus (exon 9)

The cDNA coding for human skeletal muscle beta-tropomyosin was expressed in Escherichia coli to produce an unacetylated beta-tropomyosin. This cDNA was deleted from the sequence corresponding to the exon 9 and expressed in E. coli to produce an unacetylated beta-tropomyosin mutant lacking the C-terminal residues 254-284. The main structural and functional properties of the two isolated proteins, designated tropomyosin-1 and des-(254-284)-tropomyosin, respectively, were characterized in comparison with those of the genuine rabbit skeletal muscle alpha beta-tropomyosin. The folding and thermal stability of the three tropomyosins were indistinguishable. Tropomyosin-1, but not des-(254-284)-tropomyosin, was polymerized in the presence of troponin and did bind to actin in the presence of the troponin complex. Despite its weak binding to actin, des-(254-284)-tropomyosin displayed a regulatory function in the presence of troponin with a marked activation of the actomyosin subfragment-1 ATPase in the presence of Ca2+ and low concentrations of subfragment-1. The data were interpreted in the light of the allosteric models of regulation and suggest the involvement of the sequence coded by exon 9 in the stabilization by tropomyosin of the off state of the thin filament.

Tropomyosin from human erythrocyte membrane polymerizes poorly but binds F-actin effectively in the presence and absence of spectrin

Actin in the human erythrocyte forms short protofilaments which are only long enough to accommodate tropomyosin monomers (Shen, B.W., Josephs, R. and Steck, T.L. (1986) J. Cell Biol. 102, 997-1006). This interaction between actin and tropomyosin monomers is predicted to be weak, since tropomyosin polymerization parallels its affinity for F-actin. We examine the binding of human erythrocyte tropomyosin to actin in the presence and absence of spectrin and its ability to polymerize. The binding of human erythrocyte tropomyosin to F-actin is not affected appreciably by the present of spectrin. Saturating F-actin with erythrocyte tropomyosin, however, weakens the binding of spectrin dimers to actin. Although tropomyosin from human erythrocyte and rabbit cardiac muscle have similar affinity for F-actin, the polymerizability of erythrocyte tropomyosin as determined by viscosity measurements is much reduced relative to muscle tropomyosin. This unusual property of erythrocyte tropomyosin is likely due to differences in its primary structure from other known tropomyosin at the amino and carboxyl terminal regions which are responsible for its head-to-tail polymerization and cooperative binding to F-actin. Analysis of the distribution of tyrosine by 2-dimensional tryptic mapping of 125I-labelled erythrocyte tropomyosin shows that tyrosine at positions 162, 214, 221, 261 and 267 in rabbit cardiac tropomyosin are conserved in human erythrocyte tropomyosin but Tyr-60 is absent. This observation suggests that erythrocyte tropomyosin has a carboxyl terminal region similar to its muscle counterparts but its amino terminal region resembles that of platelet tropomyosin which also lacks Tyr-60.

Universal calibration of gel permeation chromatography and determination of molecular shape in solution

Gel permeation chromatography (GPC) has become a routine technique for both biology and polymer chemistry. By comparison our theoretical perception of the separation principle of GPC is still immature and conflicting and so is the assessment of the analytical informational content of this method. In order to discriminate between the various parameters that might influence GPC and thus to decide among the numerous propositions of calibration, several odd biopolymers (tropomyosin, spectrin, DNA, tobacco mosaic virus, alpha-actinin, ovomucoid) were selected. They were characterized by analytical ultracentrifugation as well as quasielastic light scattering, and they were compared to globular proteins including icosahedral viruses (tomato bushy stunt virus, turnip yellow mosaic virus, Q beta, MS2) on several different HPLC column matrices. The results demonstrate that the universal calibration principle of GPC is the viscosity radius, i.e., the molecular volume times a shape function which is defined by the intrinsic viscosity. Alternate propositions such as molecular weight, second virial coefficient, diffusion coefficient (Stokes radius), radius of gyration, mean linear projected length, contour length, and related measures seem to be excluded on the basis of the evidence presented. These results help to focus the physical picture which seems to govern GPC. Finally it is demonstrated that GPC is a versatile and unique tool with which to characterize molecular shape and dynamics in solution.

Interaction of tropomyosin and troponin T: a proton nuclear magnetic resonance study

Proton nuclear magnetic resonance (1H NMR) has been used to study the nature of the interaction between tropomyosin (TM) and troponin T (Tn-T). Resonances corresponding to the histidine residues in fragments of both TM and Tn-T can be resolved and assigned in the 1H NMR spectrum. Changes in the pH titration profiles of these resonances when the various fragments are mixed provide probes of the interaction sites between the proteins. Fragment T1 (residues 1-158) of Tn-T appears to interact weakly but specifically with fragments of TM in which the NH2-terminus (residues 1-10) is intact. While fragment CB2 (residues 71-151) of Tn-T interacts weakly (dissociation constant of 0.1-0.2 mM) with NH2-terminal fragments of TM, this appears to be nonspecific since the absence of residues 1-10 and 128-189 of TM does not affect the observed perturbations of the titration profiles of His-79 of CB2. Although a strong interaction between T1 and the COOH-terminal Cy2 fragment (residues 190-284) of TM has been previously demonstrated, no perturbation of His-276 of Cy2 or of His-7, -23, -29, or -36 of T1 was observed in a mixture of T1/Cy2. The pKa of His-276 was also not affected in a mixture of Cy1/Cy2 (where Cy1 is residues 1-189 of TM) but was significantly decreased in the ternary complex T1/Cy1/Cy2. The importance of residues 1-70 of Tn-T in its binding to TM is illustrated by the specificity it confers on the T1/Cy1 interaction and by the absence of His-276 perturbation in the mixture CB2/Cy1/Cy2.

Structural and functional properties of the non-muscle tropomyosins

The non-muscle tropomyosins (TMs), isolated from such tissues as platelets, brain and thyroid, are structurally very similar to the muscle TMs, being composed of two highly alpha-helical subunits wound around each other to form a rod-like molecule. The non-muscle TMs are shorter than the muscle TMs; sequence analysis demonstrates that each subunit of equine platelet TM consists of 247 amino acids, 37 fewer than for skeletal muscle TM. The major differences in sequence between platelet and skeletal muscle TM are found near the amino and carboxyl terminal ends of the proteins. Probably as the result of such alterations, the non-muscle TMs aggregate in a linear end-to-end manner much more weakly than do the muscle TMs. Since end-to-end interactions are responsible for the highly cooperative manner in which TM binds to actin, the non-muscle TMs have a lower affinity for actin filaments than do the muscle TMs. However, the attachment of other proteins to actin (e.g. the Tn-I subunit of skeletal muscle troponin or the S-1 subfragment of skeletal muscle myosin) can increase the affinity of actin filaments for non-muscle TM. The non-muscle TMs interact functionally with the Tn-I component of skeletal muscle troponin to inhibit the ATPase activity of muscle actomyosin and with whole troponin to regulate the muscle actomyosin ATPase in a Ca++-dependent manner, even though one of the binding sites for troponin on skeletal TM is missing in non-muscle TM. A novel actomyosin regulatory system can be produced using Tn-I, calmodulin and non-muscle TM; in this case inhibition is released when the non-muscle TM detaches from the actin filament in the presence of Ca++. Although it has not yet been demonstrated that the non-muscle TMs participate in a Ca++-dependent contractile regulatory system in vivo it does appear that they are associated with actin filaments in vivo.

Nuclear magnetic resonance evidence for a flexible region at the C-terminus of alpha-tropomyosin

The 1H nuclear magnetic resonance spectrum of alpha-tropomyosin contains a number of sharp peaks indicative of the presence of small regions of high flexibility in the molecule. Removal of 9 to 11 residues from the C-terminus by digestion with carboxypeptidase A causes a marked decline in the intensity of these peaks. The difference is consistent with at least the C-terminal four residues of the sequence (-Met-Thr-Ser-Ile) being highly mobile. The conformation of the C-terminus is thus radically different from the alpha-helical coiled-coil from which the bulk of the molecule is constructed.

Analysis of cooperativity observed in pH titrations of proton nuclear magnetic resonances of histidine residues of rabbit cardiac tropomyosin

We have investigated in detail the cooperativity which we had previously observed in the pH titration profiles of the histidine residues of rabbit tropomyosin [Edwards, B. F. P., & Sykes, B. D. (1978) Biochemistry 17, 684]. Nonpolymerizing tropomyosin was prepared by carboxypeptidase digestion, and the titration profiles of its histidine residues were compared with those of undigested tropomyosin which was fully polymerized (in 0.1 M KCl) throughout the titration. We have concluded that both histidine-153 and histidine-273 have significant cooperativity in their pH titrations only in polymerized tropomyosin, that the cooperativity arises from an intrinsic pH-dependent conformational transition which links the two residues together and not from the known pH dependence of the polymerization, and that the best model for the cooperativity is a biallosteric adaption of the Monod--Wyman--Changeux formalism involving two classes of binding sites for the same ligand (protons). Three other models which postulated either a Hill transition, an interaction with a neighboring residue that also titrates, or a pH-dependent polymerization were also considered.

The Croonian lecture, 1979: Regulation of muscle contraction

In this lecture I review briefly the history of the recognition of calcium ion as the sole regulatory factor of muscle contraction at the molecular level and how this led to the discovery of the troponin-tropomyosin system, which is the regulatory system of striated muscles of almost all deuterostomias and some protostomias. This is followed by a brief comment on the myosin-linked regulation, which plays a dominating role in many protostomian muscles. The regulatory mechanism in vertebrate smooth muscle is then discussed; the view is advanced that the leiotonin-tropomyosin system may be the only regulatory device for this muscle. Ca-binding components of troponin and smooth muscles of vertebrates are compared with modulator protein, an omnipresent Ca-binding protein of very conservative nature throughout evolution. Finally, the modes of action of Ca ion in different kinds of cell motility are discussed from an evolutionary point of view.

Polymerizability of rabbit skeletal tropomyosin: effects of enzymic and chemical modifications

Polymerizability of tropomyosin was unaffected by the removal of the three terminal residues 282, 283, and 284 using carboxypeptidase A. However, when residue 281 was removed, polymerizability was abolished. These results are consistent with a 9-residue molecular head-to-tail overlap in polymerized tropomyosin, in which residue 281 plays a space-filling role at the center of the overlap core. In acetylation studies, loss of polymerizability closely paralleled the extent of acetylation of lysine-7, and this residue was more susceptible to acetylation than any other. The effect of acetylation on polymerizability was probably caused not only by cleavage of salt-bridge between lysine 7 epsilon-NH2 and residue 284 alpha-COOH but also by distortion of the overlap core by the N-acetyl group. Specific modification of methionine in tropomyosin indicated that, in addition to residue 281, methionine-8 is also involved in formation of the overlap core. Modified nonpolymerizable tropomyosins could still bind to F-actin, indicating that the head-to-tail polymerization of tropomyosin is not a prerequisite for actin binding, although the regularity of tropomyosin molecules along the actin helix is presumably disrupted.

Heart contractile proteins

That several proteins of the sarcomere differ from one muscle to the next is well documented, and it is becoming evident that homogeneous muscles, like the heart, are also species specific. 1) Clear-cut evidence is available concerning myosin, and, to date, several types of molecules have been described. a) The myosins of white skeletal, heart, and smooth muscle differ in the activity of their Ca2+ and K+ATPases, as also in the structure of their light subunits. b) The Ca2+ATPases of the various cardiac myosins have been shown to exhibit species differences and correlate with the speed of shortening of the muscle. 2) The structures of tropomyosin, some troponin components, and alpha actinin (but not actin) appear to be unlike in the different types of muscle. 3) These phylogenic modifications may be related to the changes characteristic of the particular muscles under pathological conditions, which are accompanied by substantial increase in protein synthesis.