The amino terminus of muscle tropomyosin is a major determinant for function

The unique biology of catch muscles: insights into structure, function, and robotics innovations

The Review covers the current state of functions, neurotransmitter innervation, the structure, and development of the contractile apparatus of unique group of catch muscles inherent only to bivalves. In contrast to conventional muscles, during contraction and relaxation, catch muscles possess a unique ability to enter the contraction holding state, referred to as catch state. The latter consists in energy-efficient maintenance of long-lasting tension developed by the muscle without consuming ATP-derived energy and regulated by serotonin and acetylcholine. Despite the molecular mechanism of catch state phenomenon still remains unclear, the combination of experimental data and the resulting assumptions allow one to design new energy-efficient and chemically-driven artificial muscles. The analysis of the structure and function of the catch muscles in this work opens the way to a conceptually new strategy for energy-efficient biomimetic robotics, including underwater robotics.

Moving beyond simple answers to complex disorders in sarcomeric cardiomyopathies: the role of integrated systems

The classic clinical definition of hypertrophic cardiomyopathy (HCM) as originally described by Teare is deceptively simple, "left ventricular hypertrophy in the absence of any identifiable cause." Longitudinal studies, however, including a seminal study performed by Frank and Braunwald in 1968, clearly described the disorder much as we know it today, a complex, progressive, and highly variable cardiomyopathy affecting ~ 1/500 individuals worldwide. Subsequent genetic linkage studies in the early 1990s identified mutations in virtually all of the protein components of the cardiac sarcomere as the primary molecular cause of HCM. In addition, a substantial proportion of inherited dilated cardiomyopathy (DCM) has also been linked to sarcomeric protein mutations. Despite our deep understanding of the overall function of the sarcomere as the primary driver of cardiac contractility, the ability to use genotype in patient management remains elusive. A persistent challenge in the field from both the biophysical and clinical standpoints is how to rigorously link high-resolution protein dynamics and mechanics to the long-term cardiovascular remodeling process that characterizes these complex disorders. In this review, we will explore the depth of the problem from both the standpoint of a multi-subunit, highly conserved and dynamic "machine" to the resultant clinical and structural human phenotype with an emphasis on new, integrative approaches that can be widely applied to identify both novel disease mechanisms and new therapeutic targets for these primary biophysical disorders of the cardiac sarcomere.

Effect of the Aspect Ratio of Coiled-Coil Protein Carriers on Cellular Uptake

We showed previously that a rigid and fibrous-structured cationic coiled-coil artificial protein had cell-penetrating activity that was significantly greater when compared with a less-structured cell-penetrating peptide. Nanomaterials with anisotropic structures often show aspect-ratio-dependent unique physicochemical properties, as well as cell-penetrating activities. In this report, we have designed and demonstrated the cell-penetrating activity of a shorter cationic coiled-coil protein. An aspect ratio at 4.5:1 was found to be critical for ensuring that the cationic coiled-coil protein showed strong cell-penetrating activity. At an aspect ratio of 3.5:1, the cationic coiled-coil protein showed cell-penetrating activity that was similar to a less-structured short cationic cell-penetrating peptide. Interestingly, at an aspect ratio of 4:1, the cationic coiled-coil protein exhibited intermediate cell-penetrating activity. These findings should aid in the principle design of intracellular drug delivery carriers including coiled-coil artificial proteins, their derivatives, and α-helical cell-penetrating peptides as well as provide a framework for developing synthetic nanomaterials, such as metal nanorods and synthetic polymers.

Biophysical Derangements in Genetic Cardiomyopathies

This article focuses on three "bins" that comprise sets of biophysical derangements elicited by cardiomyopathy-associated mutations in the myofilament. Current therapies focus on symptom palliation and do not address the disease at its core. We and others have proposed that a more nuanced classification could lead to direct interventions based on early dysregulation changing the trajectory of disease progression in the preclinical cohort. Continued research is necessary to address the complexity of cardiomyopathic progression and develop efficacious therapeutics.

Recruitment Kinetics of Tropomyosin Tpm3.1 to Actin Filament Bundles in the Cytoskeleton Is Independent of Actin Filament Kinetics

The actin cytoskeleton is a dynamic network of filaments that is involved in virtually every cellular process. Most actin filaments in metazoa exist as a co-polymer of actin and tropomyosin (Tpm) and the function of an actin filament is primarily defined by the specific Tpm isoform associated with it. However, there is little information on the interdependence of these co-polymers during filament assembly and disassembly. We addressed this by investigating the recovery kinetics of fluorescently tagged isoform Tpm3.1 into actin filament bundles using FRAP analysis in cell culture and in vivo in rats using intracellular intravital microscopy, in the presence or absence of the actin-targeting drug jasplakinolide. The mobile fraction of Tpm3.1 is between 50% and 70% depending on whether the tag is at the C- or N-terminus and whether the analysis is in vivo or in cultured cells. We find that the continuous dynamic exchange of Tpm3.1 is not significantly impacted by jasplakinolide, unlike tagged actin. We conclude that tagged Tpm3.1 may be able to undergo exchange in actin filament bundles largely independent of the assembly and turnover of actin.

Localization of the binding interface between leiomodin-2 and α-tropomyosin

The development of some familial dilated cardiomyopathies (DCM) correlates with the presence of mutations in proteins that regulate the organization and function of thin filaments in cardiac muscle cells. Harmful effects of some mutations might be caused by disruption of yet uncharacterized protein-protein interactions. We used nuclear magnetic resonance spectroscopy to localize the region of striated muscle α-tropomyosin (Tpm1.1) that interacts with leiomodin-2 (Lmod2), a member of tropomodulin (Tmod) family of actin-binding proteins. We found that 21 N-terminal residues of Tpm1.1 are involved in interactions with residues 7-41 of Lmod2. The K15N mutation in Tpm1.1, known to be associated with familial DCM, is located within the newly identified Lmod2 binding site of Tpm1.1. We studied the effect of this mutation on binding Lmod2 and Tmod1. The mutation reduced binding affinity for both Lmod2 and Tmod1, which are responsible for correct lengths of thin filaments. The effect of the K15N mutation on Tpm1.1 binding to Lmod2 and Tmod1 provides a molecular rationale for the development of familial DCM.

The structural dynamics of α-tropomyosin on F-actin shape the overlap complex between adjacent tropomyosin molecules

Coiled-coil tropomyosin, localized on actin filaments in virtually all eukaryotic cells, serves as a gatekeeper regulating access of the motor protein myosin and other actin-binding proteins onto the thin filament surface. Tropomyosin's modular pseudo-repeating pattern of approximately 39 amino acid residues is designed to allow binding of the coiled-coil to successive actin subunits along thin filaments. Even though different tropomyosin isoforms contain varying numbers of repeat modules, the pseudo-repeat length, in all cases, matches that of a single actin subunit. Thus, the seven pseudo-repeats of 42nm long muscle tropomyosin bind to seven successive actin subunits along thin filaments, while simultaneously bending into a super-helical conformation that is preshaped to the actin filament helix. In order to form a continuous cable on thin filaments that is free of gaps, adjacent tropomyosin molecules polymerize head-to-tail by means of a short (∼9 residue) overlap. Several laboratories have engineered peptides to mimic the N- and C-terminal tropomyosin association and to characterize the overlap structure. All overlapping domains examined show a compact N-terminal coiled-coil inserting into a partially opened C-terminal partner, where the opposing coiled-coils at the overlap junction face each other at up to ∼90° twist angles. Here, Molecular Dynamics (MD) simulations were carried out to determine constraints on the formation of the tropomyosin overlap complex and to assess the amount of twisting exhibited by full-length tropomyosin when bound to actin. With the exception of the last 20-40 C- and N-terminal residues, we find that the average tropomyosin structure closely resembles a "canonical" model proposed in the classic work of McLachlan and Stewart, displaying perfectly symmetrical supercoil geometry matching the F-actin helix with an integral number of coiled-coil turns, a coiled-coil helical pitch of 137Å, a superhelical pitch of 770Å, and no localized pseudo-rotation. Over the middle 70% of tropomyosin, the average twisting of the coiled-coil deviates only by 10° from the canonical model and the torsional freedom is very small (std. dev. of 7°). This small degree of twisting cannot yield the orthogonal N- and C-termini configuration observed experimentally. In marked contrast, considerable coiled-coil unfolding, splaying and twisting at N- and C-terminal ends is observed, providing the conformational plasticity needed for head-to-tail nexus formation.

Tropomodulins and tropomyosins: working as a team

Actin filaments are major components of the cytoskeleton in eukaryotic cells and are involved in vital cellular functions such as cell motility and muscle contraction. Tmod and TM are crucial constituents of the actin filament network, making their presence indispensable in living cells. Tropomyosin (TM) is an alpha-helical, coiled coil protein that covers the grooves of actin filaments and stabilizes them. Actin filament length is optimized by tropomodulin (Tmod), which caps the slow growing (pointed end) of thin filaments to inhibit polymerization or depolymerization. Tmod consists of two structurally distinct regions: the N-terminal and the C-terminal domains. The N-terminal domain contains two TM-binding sites and one TM-dependent actin-binding site, whereas the C-terminal domain contains a TM-independent actin-binding site. Tmod binds to two TM molecules and at least one actin molecule during capping. The interaction of Tmod with TM is a key regulatory factor for actin filament organization. The binding efficacy of Tmod to TM is isoform-dependent. The affinities of Tmod/TM binding influence the proper localization and capping efficiency of Tmod at the pointed end of actin filaments in cells. Here we describe how a small difference in the sequence of the TM-binding sites of Tmod may result in dramatic change in localization of Tmod in muscle cells or morphology of non-muscle cells. We also suggest most promising directions to study and elucidate the role of Tmod-TM interaction in formation and maintenance of sarcomeric and cytoskeletal structure.

The role of tropomyosin domains in cooperative activation of the actin-myosin interaction

To establish α-tropomyosin (Tm)'s structure-function relationships in cooperative regulation of muscle contraction, thin filaments were reconstituted with a variety of Tm mutants (Δ2Tm, Δ3Tm, Δ6Tm, P2sTm, P3sTm, P2P3sTm, P1P5Tm, and wtTm), and force and sliding velocity of the thin filament were studied using an in vitro motility assay. In the case of deletion mutants, Δ indicates which of the quasi-equivalent repeats in Tm was deleted. In the case of period (P) mutants, an Ala cluster was introduced into the indicated period to strengthen the Tm-actin interaction. In P1P5Tm, the N-terminal half of period 5 was substituted with that of period 1 to test the quasi-equivalence of these two Tm periods. The reconstitution included bovine cardiac troponin. Deletion studies revealed that period 3 is important for the positive cooperative effect of Tm on actin filament regulation and that period 2 also contributes to this effect at low ionic strength, but to a lesser degree. Furthermore, Tm with one extra Ala cluster at period 2 (P2s) or period 3 (P3s) did not increase force or velocity, whereas Tm with two extra Ala clusters (P2P3s) increased both force and velocity, demonstrating interaction between these periods. Most mutants did not move in the absence of Ca(2+). Notable exceptions were Δ6Tm and P1P5Tm, which moved near at the full velocity, but with reduced force, which indicate impaired relaxation. These results are consistent with the mechanism that the Tm-actin interaction cooperatively affects actin to result in generation of greater force and velocity.

Electron microscopy and persistence length analysis of semi-rigid smooth muscle tropomyosin strands

The structural mechanics of tropomyosin are essential determinants of its affinity and positioning on F-actin. Thus, tissue-specific differences among tropomyosin isoforms may influence both access of actin-binding proteins along the actin filaments and the cooperativity of actin-myosin interactions. Here, 40 nm long smooth and striated muscle tropomyosin molecules were rotary-shadowed and compared by means of electron microscopy. Electron microscopy shows that striated muscle tropomyosin primarily consists of single molecules or paired molecules linked end-to-end. In contrast, smooth muscle tropomyosin is more a mixture of varying-length chains of end-to-end polymers. Both isoforms are characterized by gradually bending molecular contours that lack obvious signs of kinking. The flexural stiffness of the tropomyosins was quantified and evaluated. The persistence lengths along the shaft of rotary-shadowed smooth and striated muscle tropomyosin molecules are equivalent to each other (approximately 100 nm) and to values obtained from molecular-dynamics simulations of the tropomyosins; however, the persistence length surrounding the end-to-end linkage is almost twofold higher for smooth compared to cardiac muscle tropomyosin. The tendency of smooth muscle tropomyosin to form semi-rigid polymers with continuous and undampened rigidity may compensate for the lack of troponin-based structural support in smooth muscles and ensure positional fidelity on smooth muscle thin filaments.

Erythrocyte tropomodulin isoforms with and without the N-terminal actin-binding domain

Erythrocyte tropomodulin (E-Tmod or Tmod1) of 41 kDa is a tropomyosin (TM)-binding protein that caps the slow-growing end of the actin filaments. Its N-terminal half is flexible, whereas the C-terminal half has a single domain structure. E-Tmod/TM5 complex may function as a "molecular ruler" generating actin protofilaments of ∼37 nm. Here we report the discovery of a short isoform of 29 kDa that lacks the N-terminal actin-binding domain (N-ABD) but retains the C-terminal actin-binding domain (C-ABD). E-Tmod29 can be generated by alternative splicing from an upstream promoter or by multiple transcriptional start sites from a downstream promoter. Promoter switching leads to a surge of E-Tmod41 in reticulocytes, which degrades quickly in the cytosol. We expressed recombinant isoforms in Escherichia coli and tested their binding toward TM5, G-actin, and F-actin. Solid-phase binding assays show that, without the N-terminal 102 residues, E-Tmod29 binds to TM5 or G-actin more strongly than E-Tmod41 does, but barely binds to F-actin after TM5 binding. Differential bindings explain the distinct localizations of E-Tmod29 in the cytosol and E-Tmod41 on the membrane. Sequential bindings and immunofluorescent staining further suggest that 1) TM5 binding to E-Tmod41 may open up the flexible N-terminal half, exposing N-ABD and unblocking C-ABD; 2) N-ABD binds to F-actin and C-ABD binds to G-actin; and 3) F-actin binding to N-ABD may prevent G-actin from binding to C-ABD. E-Tmod29 may thus modulate the availability of TM5 and G-actin for E-Tmod41 to construct the protofilament-based membrane skeletal network for circulating erythrocytes.

Structure of the tropomyosin overlap complex from chicken smooth muscle: insight into the diversity of N-terminal recognition

Tropomyosin is a stereotypical alpha-helical coiled coil that polymerizes to form a filamentous macromolecular assembly that lies on the surface of F-actin. The interaction between the C-terminal and N-terminal segments on adjacent molecules is known as the overlap region. We report here two X-ray structures of the chicken smooth muscle tropomyosin overlap complex. A novel approach was used to stabilize the C-terminal and N-terminal fragments. Globular domains from both the human DNA ligase binding protein XRCC4 and bacteriophage varphi29 scaffolding protein Gp7 were fused to 37 and 28 C-terminal amino acid residues of tropomyosin, respectively, whereas the 29 N-terminal amino acids of tropomyosin were fused to the C-terminal helix bundle of microtubule binding protein EB1. The structures of both the XRCC4 and Gp7 fusion proteins complexed with the N-terminal EB1 fusion contain a very similar helix bundle in the overlap region that encompasses approximately 15 residues. The C-terminal coiled coil opens to allow formation of the helix bundle, which is stabilized by hydrophobic interactions. These structures are similar to that observed in the NMR structure of the rat skeletal overlap complex [Greenfield, N. J., et al. (2006) J. Mol. Biol. 364, 80-96]. The interactions between the N- and C-terminal coiled coils of smooth muscle tropomyosin show significant curvature, which differs somewhat between the two structures and implies flexibility in the overlap complex, at least in solution. This is likely an important attribute that allows tropomyosin to assemble around the actin filaments. These structures provide a molecular explanation for the role of N-acetylation in the assembly of native tropomyosin.

Alternatively spliced N-terminal exons in tropomyosin isoforms do not act as autonomous targeting signals

Tropomyosin (Tm) polymerises head-to-tail to form a continuous polymer located in the major groove of the actin filament. Multiple Tm isoforms are generated by alternative splicing of four genes, and individual isoforms show specific localisation patterns in many cell types, and can have differing effects on the actin cytoskeleton. Fluorescently-tagged Tm isoforms and mutants were expressed in C2C12 cells to investigate the mechanisms of alternative localisation of high molecular weight (HMW) and low molecular weight (LMW) Tms. Fluorescently-tagged Tm constructs show similar localisation to endogenous Tms as observed by antibodies, with the HMW Tm3 relatively diminished at the periphery of cells compared to LMW isoforms Tm5b or Tm5NM1. Tm3 and Tm5b only differ in their N-terminal exons, but these N-terminal exons do not independently direct localisation within the cell, as chimeric mutants Tm3-Tm5NM1 and Tm5b-Tm5NM1 show an increased peripheral localisation similar to Tm5NM1. The lower abundance of Tm3 at the periphery of the cell is not a result of different protein dynamics, as Tm3 and Tm5b show similar recovery after photobleaching. The relative exclusion of Tm3 from the periphery of cells does, however, require interaction with the actin filament, as mutants with truncations at either the N-terminus or the C-terminus are unable to localise to actin stress fibres, and are present in the most peripheral regions of the cell. We conclude that it is the entire Tm molecule which is the unit of sorting, and that the alternatively spliced N-terminal exons do not act as autonomous targeting signals.

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.

Gestalt-binding of tropomyosin to actin filaments

We argue that the overall behavior of tropomyosin on F-actin cannot be easily discerned by examining thin filaments reduced to their smallest interacting units. In isolation, the individual interactions of actin and tropomyosin, by themselves, are too weak to account for the specificity of the system. Instead the association of tropomyosin on actin can only be fully explained after considering the concerted action of the entire acto-tropomyosin system. We propose that the low K ( a ) describing tropomyosin:actin interaction, when taken together with the form-fitting complementarity of tropomyosin strands on F-actin and the tendency for tropomyosin to polymerize end-to-end, make possible unique thin filament functions both locally and at higher levels of filament organization.

Effect of actin C-terminal modification on tropomyosin isoforms binding and thin filament regulation

Tropomyosins, a family of actin-binding regulatory proteins, are present in muscle and non-muscle cells. Multiple tropomyosin (TM) isoforms differ in actin affinity and regulatory properties, but little is known about the molecular bases of these differences. The C-terminus of actin stabilizes contacts between actin subunits in the filament and interacts with myosin and regulatory proteins. The goal of this work was to reveal how structural changes in actin and differences between TM isoforms affect binding between these proteins and affect thin filament regulation. Actin proteolytically truncated by three C-terminal amino acids exhibited 1.2–1.5 fold reduced affinity for non-muscle and smooth muscle tropomyosin isoforms. The truncation increased the cooperativity of myosin S1-induced tropomyosin binding for short tropomyosins (TM5a and TM1b9a), but it was neutral for long isoforms (smTM and TM2). Actin modification affected regulation of actomyosin ATPase activity in the presence of all tropomyosins by shifting the filament into a more active state. We conclude that the integrity of the actin C-terminus is important for actin–tropomyosin interactions, however the increased affinity of tropomyosin binding in the S1-induced state of the filament appears not to be involved in the tropomyosin isoform-dependent mechanism of the actomyosin ATPase activation.

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

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.

Some binding properties of Omp T digested muscle tropomyosin

Cleavage of vertebrate muscle tropomyosin by bacterial Omp T produces an amino-terminally truncated product (residues 7-284). The proteolysed protein, which is resolved from the parent by electrophoresis in the presence of sodium dodecylsulphate, can be generated from a variety of striated and smooth muscle tropomyosins, including ones from mammal, bird and fish. Edman-based sequencing and mass analysis confirm that the main site of chain hydrolysis is the peptide bond between Lys 6 and Lys 7. Loss of the hexapeptide, together with the blocking group, from tropomyosin weakens its affinity for troponin. Compared to wild type, the shortened forms of rabbit skeletal tropomyosin and Atlantic salmon fast skeletal tropomyosin, as well as the unacetylated (full-length) version of the latter, all display reduced affinity for both troponin and the amino-terminal fragment of troponin-T (residues 1-158), as judged by affinity chromatography. This is consistent with the view that the amino terminal region is required for full interaction with troponin-T. Truncated tropomyosin fails to bind to F-actin at micromolar concentration, as expected. Interestingly, binding is restored by troponin in the presence of either added Ca(2+) or EGTA. Digestion of muscle tropomyosin by Omp T, which can be carried out on quantitative amounts of protein, is concluded to yield a product that has useful biochemical applications.

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.

Changes in end-to-end interactions of tropomyosin affect mouse cardiac muscle dynamics

The ends of striated muscle tropomyosin (TM) are integral for thin filament cooperativity, determining the cooperative unit size and regulating the affinity of TM for actin. We hypothesized that altering the α-TM carboxy terminal overlap end to the β-TM counterpart would affect the amino-terminal association, which would alter the end-to-end interactions of TM molecules in the thin filament regulatory strand and affect the mechanisms of cardiac muscle contraction. To test this hypothesis, we generated transgenic (TG) mouse lines that express a mutant form of α-TM in which the first 275 residues are from α-TM and the last nine amino acids are from β-TM (α-TM9aaΔβ). Molecular analyses show that endogenous α-TM mRNA and protein are nearly completely replaced with α-TM9aaΔβ. Working heart preparations data show that the rates of contraction and relaxation are reduced in α-TM9aaΔβ hearts. Left ventricular pressure and time to peak pressure are also reduced (−12% and −13%, respectively). The ratio of maximum to minimum first derivatives of change in left ventricular systolic pressure with respect to time (ratio of +dP/d t to −dP/d t, respectively) is increased, but τ is not changed significantly. Force-intracellular calcium concentration ([Ca2+]i) measurements from intact papillary fibers demonstrate that α-TM9aaΔβ TG fibers produce less force per given [Ca2+]icompared with nontransgenic fibers. Taken together, the data demonstrate that the rate of contraction is primarily affected in TM TG hearts. Protein docking studies show that in the mutant molecule, the overall carbon backbone is perturbed about 1.5 Å, indicating that end-to-end interactions are altered. These results demonstrate that the localized flexibility present in the coiled-coil structures of TM isoforms is different, and that plays an important role in interacting with neighboring thin filament regulatory proteins and with differentially modulating the myofilament activation processes.

Amino terminal, but not the carboxy terminal, sequences of tropomyosin-1 are essential for the induction of stress fiber assembly in neoplastic cells

The presence of aberrant cytoskeleton, arising from the downregulation of key cytoskeletal proteins such as tropomyosins (TMs), is a prominent feature of many malignant cells and is suggested to promote neoplastic growth. While our previous work demonstrated that tropomyosin-1 (TM1) promotes stress fiber assembly and suppresses malignant growth, the molecular basis of the anti-oncogenic effects of TM1 has not been determined. By employing chimeric TMs, here we demonstrate that the amino terminal portion of TM1, but not the carboxy terminal portion which contains the alternatively spliced exon-coded sequences, is essential for stress fiber assembly and suppression of malignant growth. These studies also indicate that the amino and carboxy termini of TM1 coordinately function to regulate microfilament organization during cytokinesis.

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.

Fish fast skeletal muscle tropomyosins show species-specific thermal stability

Tropomyosin (TM) was isolated from the fast skeletal muscle of six fish species, whose amino acid sequences of this protein have already been revealed. The thermal stability of these TMs was measured by differential scanning calorimetry (DSC) and circular dichroism (CD), while the molecular weights were measured by mass spectrometry. The results showed clear differences in thermostability among these fish TMs, though the identity of amino acid sequences was more than 93.3%. Therefore, only a few amino acid substitutions could affect the overall stability of the TM molecule. Especially, several residues located on the molecular surface were considered to be responsible for such stability difference. In contrast, the molecular weights of these TMs as measured by mass spectrometry were higher than those calculated from amino acid composition, suggesting the presence of post-translational modification(s) which could also affect their thermal stability.

Expression of tropomyosin from Blattella germanica as a recombinant non-fusion protein in Pichia pastoris and comparison of its IgE reactivity with its native counterpart

Tropomyosins derived from invertebrates are well-known pan allergens. However, the allergenicities of recombinant tropomyosins are variable. Here, we undertook to compare the IgE-binding reactivities of native and recombinant German cockroach tropomyosins. Native tropomyosin was purified by ammonium sulfate fractionation, hydroxyapatite column chromatography, and electroelution, and recombinant tropomyosin was expressed in Pichia pastoris. The allergenicities of the native and recombinant tropomyosins were compared by ELISA inhibition analysis. Native German cockroach tropomyosin showed 18% IgE-binding reactivity to German cockroach sensitized sera. Recombinant tropomyosin was produced without fusion protein and its N-terminus was blocked like that of the native counterpart. The IgE-binding reactivity of the recombinant was found to be comparable to that of native tropomyosin over the concentration range 1-1000 ng/ml by ELISA inhibition testing. Recombinant German cockroach tropomyosin expressed in Pichia pastoris showed better allergenicity than that expressed in Escherichia coli. Other factors in addition to the structural differences of native and recombinant proteins may also influence the IgE reactivities of tropomyosins.

N Terminus Is Essential for Tropomyosin Functions

Down-regulation of several key actin-binding proteins, such as alpha-actinin, vinculin, gelsolin, and tropomyosins (TMs), is considered to contribute to the disorganized cytoskeleton present in many neoplastic cells. TMs stabilize actin filaments against the gel severing actions of proteins such as cofilin. Among multiple TMs expressed in non-muscle cells, tropomyosin-1 (TM1) isoform induces stress fibers and functions as a suppressor of malignant transformation. However, the molecular mechanisms of TM1-mediated cytoskeletal effects and tumor suppression remain poorly understood. We have hypothesized that the ability of TM1 to stabilize microfilaments is crucial for tumor suppression. In this study, by employing a variant TM1, which contains an N-terminal hemagglutinin epitope tag, we demonstrate that the N terminus is a key determinant of tropomyosin-1 function. Unlike the wild type TM1, the modified protein fails to restore stress fibers and inhibit anchorage-independent growth in transformed cells. Furthermore, the N-terminal modification of TM1 disorganizes the cytoskeleton and delays cytokinesis in normal cells, abolishes binding to F-actin, and disrupts the dimeric associations in vivo. The functionally defective TM1 allows the association of cofilin to stress fibers and disorganizes the microfilaments, whereas wild type TM1 appears to restrict the binding of cofilin to stress fibers. TM1-induced cytoskeletal reorganization appears to be mediated through preventing cofilin interaction with microfilaments. Our studies provide in vivo functional evidence that the N terminus is a critical determinant of TM1 functions, which in turn determines the organization of stress fibers.

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.

Allergenicity of recombinant Bla g 7, German cockroach tropomyosin

BACKGROUND

Cockroach infestation may sensitize and elicit allergic responses to genetically predisposed individuals. Invertebrate tropomyosins are a frequent cause of allergy and highly cross-reactive in nature. In this study, we aimed to produce recombinant German cockroach tropomyosin and investigate its allergenicity.

METHODS

German cockroach tropomyosin (Bla g 7) was cloned by reverse transcriptase polymerase chain reaction (RT-PCR). The cloned cDNA was over-expressed in Escherichia coli and purified by affinity chromatography using Ni-nitrilotriacetic (NTA) acid resin. The allergenicity of the recombinant tropomyosin was examined by enzyme-linked immunosorbent assay (ELISA).

RESULTS

The cloned Bla g 7 shared up to 91% amino acid sequence identity with other cockroach tropomyosins. ELISA showed a recombinant Bla g 7 sensitization rate of 16.2% to German cockroach allergic sera. Recombinant tropomyosin was able to inhibit 32.4% of the specific IgE binding to cockroach extract.

CONCLUSIONS

Tropomyosin represents a minor allergen in cockroach extracts. It is hoped that recombinant tropomyosin will be useful for further studies and clinical applications.

Nat3p and Mdm20p are required for function of yeast NatB Nalpha-terminal acetyltransferase and of actin and tropomyosin

NatB Nalpha-terminal acetyltransferase of Saccharomyces cerevisiae acts cotranslationally on proteins with Met-Glu- or Met-Asp- termini and subclasses of proteins with Met-Asn- and Met-Met- termini. NatB is composed of the interacting Nat3p and Mdm20p subunits, both of which are required for acetyltransferase activity. The phenotypes of nat3-Delta and mdm20-Delta mutants are identical or nearly the same and include the following: diminished growth at elevated temperatures and on hyperosmotic and nonfermentable media; diminished mating; defective actin cables formation; abnormal mitochondrial and vacuolar inheritance; inhibition of growth by DNA-damaging agents such as methyl methanesulfonate, bleomycin, camptothecin, and hydroxyurea; and inhibition of growth by the antimitotic drugs benomyl and thiabendazole. The similarity of these phenotypes to the phenotypes of certain act1 and tpm1 mutants suggests that such multiple defects are caused by the lack of acetylation of actin and tropomyosins. However, the lack of acetylation of other unidentified proteins conceivably could cause the same phenotypes. Furthermore, unacetylated actin and certain N-terminally altered actins have comparable defective properties in vitro, particularly actin-activated ATPase activity and sliding velocity.

Mdm20 protein functions with Nat3 protein to acetylate Tpm1 protein and regulate tropomyosin-actin interactions in budding yeast

The evolutionarily conserved Mdm20 protein (Mdm20p) plays an important role in tropomyosin-F-actin interactions that generate actin filaments and cables in budding yeast. However, Mdm20p is not a structural component of actin filaments or cables, and its exact function in cable stability has remained a mystery. Here, we show that cells lacking Mdm20p fail to N-terminally acetylate Tpm1p, an abundant form of tropomyosin that binds and stabilizes actin filaments and cables. The F-actin-binding activity of unacetylated Tpm1p is reduced severely relative to the acetylated form. These results are complemented by the recent report that Mdm20p copurifies with one of three acetyltransferases in yeast, the NatB complex. We present genetic evidence that Mdm20p functions cooperatively with Nat3p, the catalytic subunit of the NatB acetyltransferase. These combined results strongly suggest that Mdm20p-dependent, N-terminal acetylation of Tpm1p by the NatB complex is required for Tpm1p association with, and stabilization of, actin filaments and cables.

Tropomyosin ends determine the stability and functionality of overlap and troponin T complexes

Tropomyosin binds end to end along the actin filament. Tropomyosin ends, and the complex they form, are required for actin binding, cooperative regulation of actin filaments by myosin, and binding to the regulatory protein, troponin T. The aim of the work was to understand the isoform and structural specificity of the end-to-end association of tropomyosin. The ability of N-terminal and C-terminal model peptides with sequences of alternate alpha-tropomyosin isoforms, and a troponin T fragment that binds to the tropomyosin overlap, to form complexes was analyzed using circular dichroism spectroscopy. Analysis of N-terminal extensions (N-acetylation, Gly, AlaSer) showed that to form an overlap complex between the N-terminus and the C-terminus requires that the N-terminus be able to form a coiled coil. Formation of a ternary complex with the troponin T fragment, however, effectively takes place only when the overlap complex sequences are those found in striated muscle tropomyosins. Striated muscle tropomyosins with N-terminal modifications formed ternary complexes with troponin T that varied in affinity in the order: N-acetylated > Gly > AlaSer > unacetylated. The circular dichroism results were corroborated by native gel electrophoresis, and the ability of the troponin T fragment to promote binding of full-length tropomyosins to filamentous actin.

Structure and interactions of the carboxyl terminus of striated muscle alpha-tropomyosin: it is important to be flexible

Tropomyosin (TM) binds to and regulates the actin filament. We used circular dichroism and heteronuclear NMR to investigate the secondary structure and interactions of the C terminus of striated muscle alpha-TM, a major functional determinant, using a model peptide, TM9a(251-284). The (1)H(alpha) and (13)C(alpha) chemical shift displacements show that residues 252 to 277 are alpha-helical but residues 278 to 284 are nonhelical and mobile. The (1)H(N) and (13)C' displacements suggest that residues 257 to 269 form a coiled coil. Formation of an "overlap" binary complex with a 33-residue N-terminal chimeric peptide containing residues 1 to 14 of alpha-TM perturbs the (1)H(N) and (15)N resonances of residues 274 to 284. Addition of a fragment of troponin T, TnT(70-170), to the binary complex perturbs most of the (1)H(N)-(15)N cross-peaks. In addition, there are many new cross-peaks, showing that the binding is asymmetric. Q263, in a proposed troponin T binding site, shows two sets of side-chain (15)N-(1)H cross-peaks, indicating conformational flexibility. The conformational equilibrium of the side chain changes upon formation of the binary and ternary complexes. Replacing Q263 with leucine greatly increases the stability of TM9a(251-284) and reduces its ability to form the binary and ternary complexes, showing that conformational flexibility is crucial for the binding functions of the C terminus.

Tropomyosin requires an intact N-terminal coiled coil to interact with tropomodulin

Tropomodulins (Tmods) are tropomyosin (TM) binding proteins that bind to the pointed end of actin filaments and modulate thin filament dynamics. They bind to the N termini of both "long" TMs (with the N terminus encoded by exon 1a of the alpha-TM gene) and "short" nonmuscle TMs (with the N terminus encoded by exon 1b). In this present study, circular dichroism was used to study the interaction of two designed chimeric proteins, AcTM1aZip and AcTM1bZip, containing the N terminus of a long or a short TM, respectively, with protein fragments containing residues 1 to 130 of erythrocyte or skeletal muscle Tmod. The binding of either TMZip causes similar conformational changes in both Tmod fragments promoting increases in both alpha-helix and beta-structure, although they differ in binding affinity. The circular dichroism changes in the Tmod upon binding and modeling of the Tmod sequences suggest that the interface between TM and Tmod includes a three- or four-stranded coiled coil. An intact coiled coil at the N terminus of the TMs is essential for Tmod binding, as modifications that disrupt the N-terminal helix, such as removal of the N-terminal acetyl group from AcTM1aZip or striated muscle alpha-TM, or introduction of a mutation that causes nemaline myopathy, Met-8-Arg, into AcTM1aZip destroyed Tmod binding.

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.

Tropomodulin-binding site mapped to residues 7-14 at the N-terminal heptad repeats of tropomyosin isoform 5

Tropomodulin is a globular protein that caps the pointed end of actin filaments by complexing with the N-terminus of a tropomyosin (TM) molecule. TM consists of coiled coils except for the N-terminus, which may be globular. Here we report that human TM isoform 5 (hTM5) lacking the N-terminal 18 residues lost its binding activity toward tropomodulin. We further characterized the tropomodulin-binding site by creating a series of deletion and missense mutations within this region, followed by a solid-phase binding assay. I7, V10, and I14, hydrophobic residues located at the a and d positions of N-terminal heptad repeats involving intertwine, are essential for tropomodulin binding. R12, a positively charged residue at the f position, is also involved in recognition. In contrast, A2R and G3Y mutations, each creating a bulky N-terminus, did not alter the binding. In addition, rat TM5b, which differs from hTM5 in residues 4-6, exhibits a similar binding affinity. The tropomodulin-binding site, therefore, is mapped to residues 7-14 at the beginning of the long heptad repeats. Column chromatography revealed that hTM5 mutants remained capable of dimerization. Results also suggest tropomodulin has a groove-type, rather than a cavity-type, binding site for hTM5. We also mapped the epitope of monoclonal antibody LC1 to residues 4-10 of hTM5 and showed the competition between mAb LC1 and tropomodulin in hTM5 binding. Since the N-terminal residues need to overlap with the C-terminus of TM in their head-to-tail association, this investigation elucidates the mechanisms by which the tropomodulin-hTM5 complex is formed and functions in regulating the actin filaments.

A nemaline myopathy mutation in alpha-tropomyosin causes defective regulation of striated muscle force production

Nemaline myopathy (NM) is a rare autosomal dominant skeletal muscle myopathy characterized by severe muscle weakness and the subsequent appearance of nemaline rods within the muscle fibers. Recently, a missense mutation inTPM3, which encodes the slow skeletal alpha-tropomyosin (alphaTm), was linked to NM in a large kindred with an autosomal-dominant, childhood-onset form of the disease. We used adenoviral gene transfer to fully differentiated rat adult myocytes in vitro to determine the effects of NM mutant human alphaTm expression on striated muscle sarcomeric structure and contractile function. The mutant alphaTm was expressed and incorporated correctly into sarcomeres of adult muscle cells. The primary defect caused by expression of the mutant alphaTm was a decrease in the sensitivity of contraction to activating Ca(2+), which could help explain the hypotonia seen in NM. Interestingly, NM mutant alphaTm expression did not directly result in nemaline rod formation, which suggests that rod formation is secondary to contractile dysfunction and that load-dependent processes are likely involved in nemaline rod formation in vivo.

Effects of tropomyosin internal deletions on thin filament function

Striated muscle tropomyosin spans seven actin monomers and contains seven quasi-repeating regions with loose sequence similarity. Each region contains a hypothesized actin binding motif. To examine the functions of these regions, full-length tropomyosin was compared with tropomyosin internal deletion mutants spanning either five or four actins. Actin-troponin-tropomyosin filaments lacking tropomyosin regions 2-3 exhibited calcium-sensitive regulation in in vitro motility and myosin S1 ATP hydrolysis experiments, similar to filaments with full-length tropomyosin. In contrast, filaments lacking tropomyosin regions 3-4 were inhibitory to these myosin functions. Deletion of regions 2-4, 3-5, or 4-6 had little effect on tropomyosin binding to actin in the presence of troponin or troponin-Ca(2+), or in the absence of troponin. However, all of these mutants inhibited myosin cycling. Deletion of the quasi-repeating regions diminished the prominent effect of myosin S1 on tropomyosin-actin binding. Interruption of this cooperative, myosin-tropomyosin interaction was least severe for the mutant lacking regions 2-3 and therefore correlated with inhibition of myosin cycling. Regions 3, 4, and 5 each contributed about 1.5 kcal/mol to this process, whereas regions 2 and 6 contributed much less. We suggest that a myosin-induced conformational change in actin facilitates the azimuthal repositioning of tropomyosin which is an essential part of regulation.

Assembly of tropomyosin isoforms into the cytoskeleton of avian muscle cells

Tropomyosin (TM) is a component of microfilaments of most eukaryotic cells. In striated muscle, TM helps confer calcium sensitivity to the actin-myosin interaction. TM is a fibrillar, self-associating protein that binds to the extended actin filament system. We hypothesized that these structural features would permit TM to undergo assembly into the cytoskeleton during translation, or cotranslational assembly. Pulse-chase experiments with [35S]methionine and pulse experiments with [3H]puromycin followed by extraction and immunoprecipitation of TM were performed to examine the mechanism of assembly of TM into the cytoskeleton in cultured avian muscle cells. Pulse-chase experiments provide kinetic evidence for cotranslational assembly of TM in skeletal and cardiac muscle. Demonstration of a large majority of completed TM on purified skeletal muscle microfilaments after a short labeling period confirms that these kinetic data are not related to trapping of TM within the actin network of the cytoskeleton. Nascent TM peptides are demonstrated on the cytoskeleton of muscle cells after a short metabolic pulse followed by puromycin treatment to release nascent peptides from ribosomes or after labeling with [3H]puromycin. Nascent chain localization to the cytoskeleton independent of ribosomal attachment further confirms the high degree of cotranslational assembly of this protein. The extent of cotranslational assembly is similar before and after the formation of significant myofibril in myotubes, suggesting that cotranslational assembly of TM is active during contractile apparatus assembly in muscle differentiation. This is the first report where assembly mechanism has been predicted to be cotranslational based upon structural features of a cytoskeletal protein.

Transformation-related expression of a low-molecular-mass tropomyosin isoform TM5/TM30nm in transformed rat fibroblastic cell lines

We cloned a full-length rat TM5/TM30nm cDNA. Using this cDNA as a probe, we demonstrated that expression of TM5/TM30nm mRNA was higher in the tumorigenic rat fibroblastic cell lines SR-3Y1-2 and fos-SR-3Y1-202 than in the normal cell line 3Y1. High expression of TM5/TM30nm protein in SR-3Y1-2 and fos-SR-3Y1-202 cells was also detected by Western blot analysis using anti-TM5/TM30nm antiserum. In addition, the cellular localization of this protein differed between 3Y1 cells and tumorigenic ones. These findings suggest that TM5/TM30nm is involved in malignant transformation of rat fibroblastic cells.

Molecular mechanisms regulating the myofilament response to Ca2+: implications of mutations causal for familial hypertrophic cardiomyopathy

In this chapter we consider a current perception of the molecular mechanisms controlling myofilament activation with emphasis on alterations that may occur in familial hypertrophic cardiomyopathy (FHC). FHC is a sarcomeric disease (100) with an autosomal dominant pattern of heritability (27, 51). There is a substantial body of evidence implicating missense mutations in the beta-MHC gene as causal for the development of this disease. Recently, mutations in genes of two thin filament regulatory proteins, cardiac troponin T(cTnT) and alpha-tropomyosin (alpha-Tm), have also been linked to FHC. The commonality among the functional consequences of these mutations remains an important question. This review discusses how these pathological mutations may impact the activation process by disrupting critical structure function relations in both the thick and thin filaments.

Tropomyosin isoforms in nonmuscle cells

Vertebrate nonmuscle cells, such as human and rat fibroblasts, express multiple isoforms of tropomyosin, which are generated from four different genes and a combination of alternative promoter activities and alternative splicing. The amino acid variability among these isoforms is primarily restricted to three alternatively spliced exon regions; an amino-terminal region, an internal exon, and a carboxyl-terminal exon. Recent evidence reveals that these variable exon regions encode amino acid sequences that may dictate isoform-specific functions. The differential expression of tropomyosin isoforms found in cell transformation and cell differentiation, as well as the differential localization of tropomyosin isoforms in some types of culture cells and developing neurons suggest a differential isoform function in vivo. Tropomyosin in striated muscle works together with the troponin complex to regulate muscle contraction in a Ca(2+)-dependent fashion. Both in vitro and in vivo evidence suggest that multiple isoforms of tropomyosin in nonmuscle cells may be required for regulating actin filament stability, intracellular granule movement, cell shape determination, and cytokinesis. Tropomyosin-binding proteins such as caldesmon, tropomodulin, and other unidentified proteins may be required for some of these functions. Strong evidence for the distinct functions carried out by different tropomyosin isoforms has been generated from genetic analysis of yeast and Drosophila tropomyosin mutants.

Exchange of beta- for alpha-tropomyosin in hearts of transgenic mice induces changes in thin filament response to Ca2+, strong cross-bridge binding, and protein phosphorylation

Despite its potential as a key determinant of the functional state of striated muscle, the impact of tropomyosin (Tm) isoform switching on mammalian myofilament activation and regulation in the intact lattice remains unclear. Using a transgenic approach to specifically exchange beta-Tm for the native alpha-Tm in mouse hearts, we have been able to uncover novel functions of Tm isoform switching in the heart. The myofilaments containing beta-Tm demonstrated an increase in the activation of the thin filament by strongly bound cross-bridges, an increase in Ca2+ sensitivity of steady state force, and a decrease in the rightward shift of the Ca2+-force relation induced by cAMP-dependent phosphorylation. Our results are the first to demonstrate the specific effects of Tm isoform switching on mammalian thin filament activation in the intact lattice and suggest an important role for Tm in modulation of myofilament activity by phosphorylation of troponin.