C-Terminal Truncation of Cardiac Troponin I Causes Divergent Effects on ATPase and Force

Myocardial stunning is a form of reversible myocardial ischemia/reperfusion injury associated with systolic and diastolic contractile dysfunction. In the isolated rat heart model, myocardial stunning is characterized by specific C-terminal proteolysis of the myofilament protein, troponin I (cTnI) that yields cTnI 1-193 . To determine the effect of this particular C-terminal truncation of cTnI, without the confounding factor of other stunning-induced protein modifications, a series of solution biochemical assays has been undertaken using the human homologue of mouse/rat cTnI 1-193 , cTnI 1-192 . Affinity chromatography and actin sedimentation experiments detected little, or no, difference between the binding of cTnI (cTnI 1-209 ) and cTnI 1-192 to actin-tropomyosin, troponin T, or troponin C. Both cTnI and cTnI 1-192 inhibit the actin-tropomyosin–activated ATPase activity of myosin subfragment 1 (S1), and this inhibition is released by troponin C in the presence of Ca 2+ . However, cTnI 1-192 , when reconstituted as part of the troponin complex (cTn 1-192 ), caused a 54±11% increase in the maximum Ca 2+ -activated actin-tropomyosin-S1 ATPase activity, compared with troponin reconstituted with cTnI (cTn). Furthermore, cTn 1-192 increased Ca 2+ sensitivity of both the actin-tropomyosin-activated S1 ATPase activity and the Ca 2+ -dependent sliding velocity of reconstituted thin filaments, in an in vitro motility assay, compared with cTn. In an in vitro force assay, the actin-tropomyosin filaments bearing cTn 1-192 developed only 76±4% ( P <0.001) of the force obtained with filaments composed of reconstituted cTn. We suggest that cTnI proteolysis may contribute to the pathophysiology of myocardial stunning by altering the Ca 2+ -sensing and chemomechanical properties of the myofilaments.

Cardiomyopathic tropomyosin mutations that increase thin filament Ca2+ sensitivity and tropomyosin N-domain flexibility

The relationship between tropomyosin thermal stability and thin filament activation was explored using two N-domain mutants of alpha-striated muscle tropomyosin, A63V and K70T, each previously implicated in familial hypertrophic cardiomyopathy. Both mutations had prominent effects on tropomyosin thermal stability as monitored by circular dichroism. Wild type tropomyosin unfolded in two transitions, separated by 10 degrees C. The A63V and K70T mutations decreased the melting temperature of the more stable of these transitions by 4 and 10 degrees C, respectively, indicating destabilization of the N-domain in both cases. Global analysis of all three proteins indicated that the tropomyosin N-domain and C-domain fold with a cooperative free energy of 1.0-1.5 kcal/mol. The two mutations increased the apparent affinity of the regulatory Ca2+ binding sites of thin filament in two settings: Ca2+-dependent sliding speed of unloaded thin filaments in vitro (at both pH 7.4 and 6.3), and Ca2+ activation of the thin filament-myosin S1 ATPase rate. Neither mutation had more than small effects on the maximal ATPase rate in the presence of saturating Ca2+ or on the maximal sliding speed. Despite the increased tropomyosin flexibility implied by destabilization of the N-domain, neither the cooperativity of thin filament activation by Ca2+ nor the cooperative binding of myosin S1-ADP to the thin filament was altered by the mutations. The combined results suggest that a more dynamic tropomyosin N-domain influences interactions with actin and/or troponin that modulate Ca2+ sensitivity, but has an unexpectedly small effect on cooperative changes in tropomyosin position on actin.

A mouse model of familial hypertrophic cardiomyopathy caused by a alpha-tropomyosin mutation

Familial hypertrophic cardiomyopathy, a disease caused by mutations in cardiac contractile proteins, is characterized by left and/or right ventricular hypertrophy, myocyte disarray, fibrosis, and cardiac arrhythmias that may lead to premature sudden death. Five distinct point mutations within alpha-tropomyosin are associated with the development of familial hypertrophic cardiomyopathy. Two of these mutations are found within a troponin T binding site, located at amino acids 175 and 180. In this study, we analyze a transgenic mouse model for one of the mutations that occur at codon 180: a substitution of a glutamic acid for a glycine. These mice develop severe cardiac hypertrophy, substantial interstitial fibrosis, and have an increased heart weight/ body weight ratio. Results show that calcium-handling proteins associated with the sarcoplasmic reticulum exhibit decreased expression. These alterations in gene expression, coupled with the structurally-altered tropomyosin, may contribute to the demonstrated decreased physiological performance exhibited by these transgenic mice. A DNA hybridization microarray analysis of the transgenic vs. control ventricular RNAs shows that 50 transcripts are differentially expressed by more than 100% during the onset of the hypertrophic process, many of which are associated with the extracellular matrix. This study demonstrates that mutations within tropomyosin can be severely disruptive of sarcomeric function, triggering a hypertrophic response coupled with a cascade of alterations in gene expression.

Cloning and characterization of Taenia saginata paramyosin cDNA

A lambdaZAP-express cDNA library of Taenia saginata metacestodes was constructed. Antibody screening yielded a clone with an insert of 3,408 bp, an open reading frame of 2,589 bp, a deduced sequence of 863 amino acid and a molecular mass of 98.89 kDa. Alignments of the predicted amino acid sequence showed identity with paramyosins from several species: 98.8% with Taenia solium, 96.3% with Echinococcus.granulosus and about 70% with Schistosoma spp. The insert was expressed and purified. A collagen binding assay was performed which showed that T. saginata GST-paramyosin retained this property in a dose-dependent manner. Problems were encountered due to high backgrounds in serological assays in the homologous T. saginata system. However, the recombinant paramyosin was recognized by antibodies present in 31.6% of sera from T. solium seropositive cysticercosis patients and 100% of the sera from acute cysticercosis patients. The immunodominant epitope was the carboxyl-terminal fragment of the molecule.

Regulatory proteins alter nucleotide binding to acto-myosin of sliding filaments in motility assays

The sliding speed of unregulated thin filaments in motility assays is only about half that of the unloaded shortening velocity of muscle fibers. The addition of regulatory proteins, troponin and tropomyosin, is known to increase the sliding speed of thin filaments in the in vitro motility assay. To learn if this effect is related to the rate of MgADP dissociation from the acto-S1 cross-bridge head, the effects of regulatory proteins on nucleotide binding and release in motility assays were measured in the presence and absence of regulatory proteins. The apparent affinity of acto-heavy meromyosin (acto-HMM) for MgATP was reduced by the presence of regulatory proteins. Similarly, the regulatory proteins increase the concentration of MgADP required to inhibit sliding. These results suggest that regulatory proteins either accelerate the rate of MgADP release from acto-HMM-MgADP or slow its binding to acto-HMM. The reduction of temperature also altered the relationship between thin filament sliding speed and the regulatory proteins. At lower temperatures, the regulatory proteins lost their ability to increase thin filament sliding speed above that of unregulated thin filaments. It is hypothesized that structural changes in the actin portion of the acto-myosin interface are induced by regulatory protein binding to actin.

Structure of the core domain of human cardiac troponin in the Ca(2+)-saturated form

Troponin is essential in Ca(2+) regulation of skeletal and cardiac muscle contraction. It consists of three subunits (TnT, TnC and TnI) and, together with tropomyosin, is located on the actin filament. Here we present crystal structures of the core domains (relative molecular mass of 46,000 and 52,000) of human cardiac troponin in the Ca(2+)-saturated form. Analysis of the four-molecule structures reveals that the core domain is further divided into structurally distinct subdomains that are connected by flexible linkers, making the entire molecule highly flexible. The alpha-helical coiled-coil formed between TnT and TnI is integrated in a rigid and asymmetric structure (about 80 angstrom long), the IT arm, which bridges putative tropomyosin-anchoring regions. The structures of the troponin ternary complex imply that Ca(2+) binding to the regulatory site of TnC removes the carboxy-terminal portion of TnI from actin, thereby altering the mobility and/or flexibility of troponin and tropomyosin on the actin filament.

Effects of troponin on thermal unfolding of actin-bound tropomyosin

Differential scanning calorimetry (DSC) was used to study the effect of troponin (Tn) and its isolated components on the thermal unfolding of skeletal muscle tropomyosin (Tm) bound to F-actin. It is shown that in the absence of actin the thermal unfolding of Tm is expressed in two well-distinguished thermal transitions with maxima at 42.8 and 53.8 degrees C. Interaction with F-actin affects the character of thermal unfolding of Tm leading to appearance of a new Tm transition with maximum at about 48 degrees C, but it has no influence on the thermal denaturation of F-actin stabilized by aluminum fluoride, which occurs within the temperature region above 70 degrees C. Addition of troponin leads to significant increase in the cooperativity and enthalpy of the thermal transition of the actin-bound Tm. The most pronounced effect of Tn was observed in the absence of calcium. To elucidate how troponin complex affects the properties of Tm, we studied the influence of its isolated components, troponin I (TnI) and troponin T (TnT), on the thermal unfolding of actin-bound Tm. Isolated TnT and TnI do not demonstrate cooperative thermal transitions on heating up to 100 degrees C. However, addition of TnI, and especially of TnT, to the F-actin-Tm complex significantly increased the cooperativity of the thermal unfolding of actin-bound tropomyosin.

Functional importance of the carboxyl-terminal region of striated muscle tropomyosin

Striated muscle tropomyosin (TM) interacts with actin and the troponin complex to regulate calcium-mediated muscle contraction. Previous work by our laboratory established that alpha- and beta-TM isoforms elicit physiological differences in sarcomeric performance. Heart myofilaments containing beta-TM exhibit an increased sensitivity to calcium that is associated with a decrease in the rate of relaxation and a prolonged time of relaxation. To address whether the carboxyl-terminal, troponin T binding domain of beta-TM is responsible for these physiological alterations, we exchanged the 27 terminal amino acids of alpha-TM (amino acids 258 -284) for the corresponding region in beta-TM. Hearts of transgenic mice that express this chimeric TM protein exhibit significant decreases in their rates of contraction and relaxation when assessed by ex vivo work-performing cardiac analyses. There are increases in the time to peak pressure and a dramatic increase in end diastolic pressure. In myofilaments, this chimeric protein induces depression of maximum tension and ATPase rate, together with a significant decrease in sensitivity to calcium. Our data are the first to demonstrate that the TM isoform-specific carboxyl terminus is a critical determinant of sarcomere performance and calcium sensitivity in both the whole heart and in isolated myofilaments.

Identification, characterization, and expression of a novel alpha-tropomyosin isoform in cardiac tissues in developing chicken

Tropomyosins are present in various muscle (skeletal, cardiac, and smooth) and non-muscle cells with different isoforms characteristic of specific cell types. We describe here a novel smooth/striated chimeric isoform that was expressed in developing chick heart in addition to the classically described TM-4 type. This novel alpha-Tm tropomyosin isoform, designated as alpha-Tm-2, contains exon 2a (in place of exon 2b). The known striated muscle isoform (alpha-Tm-1) was also expressed in embryonic hearts along with the striated muscle isoform of TM-4. In adult heart, TM-4 was expressed, however, expression of both alpha-Tm-1 and alpha-Tm-2 isoforms was drastically reduced or downregulated. Interestingly, we were unable to detect the expression of alpha-Tm-2 in embryonic and adult skeletal muscle, however, the alpha-Tm-1 isoform is expressed in embryonic and adult skeletal muscle. Examination of other possible isoforms of the alpha-TM gene, i.e., alpha-smooth muscle tropomyosin (alpha-Sm), alpha-Fibroblast-1 (alpha-F1), and alpha-Fibroblast-2 (alpha-F2) revealed expression in embryonic hearts and a significant reduction of each of these isoforms in adult heart. In order to elucidate the role of the newly discovered tropomyosin isoform in chicken, we ectopically expressed the GFP fusion protein of alpha-Tm-1 and alpha-Tm-2 separately into cardiomyocytes isolated from neonatal rats. Each isoform was incorporated into organized myofibrils. Our results suggest that the alpha-TM gene may undergo both positive and negative transcriptional control in chicken hearts during development.

On the stiffness of the natural actin filament decorated with alexa fluor tropomyosin

Natural, phalloidin-free, actin filaments were decorated with tropomyosin made fluorescent by reaction with alexa fluor (R) 488 C(5) maleimide. The elastic modulus by stretching of these filaments was then determined and found to span between 38.2 MPa and 61.48 MPa. We tried also to determine the yield strength of the same filaments in the laser light trap operated at 920 mW, the maximum power of the apparatus. Only two out of the 10 filaments tested were broken under these conditions, yield strength being 50.5 and 55 pN, respectively.

Cooperative regulation of myosin-actin interactions by a continuous flexible chain II: actin-tropomyosin-troponin and regulation by calcium

The model of myosin regulation by a continuous tropomyosin chain is generalized to a chain of tropomyosin-troponin units. Myosin binding to regulated actin is cooperative and initially inhibited by the chain as before. In the absence of calcium, myosin is further inhibited by the binding of troponin-I to actin, which through the whole of troponin pins the tropomyosin chain in a blocking position; myosin and TnI compete for actin and induce oppositely-directed chain kinks. The model predicts equilibrium binding curves for myosin-S1 and TnI as a function of their first-order affinities K(S1) and L(TI). Myosin is detached by the actin binding of TnI, but TnI is more efficiently detached by myosin when the kink size (typically nine to ten actin sites) spans the seven-site spacing between adjacent TnI molecules. An allosteric mechanism is used for coupling the detachment of TnI to calcium binding by TnC. With thermally activated TnI kinks (kink energy B approximately k(B)T), TnI also binds cooperatively to actin, producing cooperative detachment of myosin and biphasic myosin-calcium Hill plots, with Hill coefficients of 2 at high calcium and 4-6 at low calcium as observed in striated muscle. The theory also predicts the cooperative effects observed in the calcium loading of TnC.

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.

Cooperative regulation of myosin-actin interactions by a continuous flexible chain I: actin-tropomyosin systems

We present a model for cooperative myosin binding to the regulated actin filament, where tropomyosins are treated as a weakly-confined continuous flexible chain covering myosin binding sites. Thermal fluctuations in chain orientation are initially required for myosin binding, leaving kinked regions under which subsequent myosins may bind without further distortion of the chain. Statistical mechanics predicts the fraction of sites with bound myosin-S1 as a function of their affinities. Published S1 binding curves to regulated filaments with different tropomyosin isoforms are fitted by varying the binding constant, chain persistence length nu (in actin monomers), and chain kink energy A from a single bound S1. With skeletal tropomyosin, we find an S1 actin-binding constant of 2.2 x 10(7) M(-1), A = 1.6 k(B)T and nu = 2.7. Similar persistence lengths are found with yeast tropomyosin. Larger values are found for tropomyosin-troponin in the presence of calcium (nu = 3.7) and tropomyosins from smooth muscle and fibroblasts (nu = 4.5). The relationship of these results to structural information and the rigid-unit model of McKillop and Geeves is discussed.

Extracellular signal-regulated kinase mediates phosphorylation of tropomyosin-1 to promote cytoskeleton remodeling in response to oxidative stress: impact on membrane blebbing

Oxidative stress induces in endothelial cells a quick and transient coactivation of both stress-activated protein kinase-2/p38 and extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases. We found that inhibiting the ERK pathway resulted, within 5 min of oxidative stress, in a misassembly of focal adhesions characterized by mislocalization of key proteins such as paxillin. The focal adhesion misassembly that followed ERK inhibition with the mitogen-activated protein kinase kinase (MEK) inhibitor PD098059 (2'-amino-3'-methoxyflavone) or with a kinase negative mutant of ERK in the presence of H(2)O(2) resulted in a quick and intense membrane blebbing that was associated with important damage to the endothelium. We isolated by two-dimensional gel electrophoresis a PD098059-sensitive phosphoprotein of 38 kDa that we identified, by mass spectrometry, as tropomyosin-1. In fact, H(2)O(2) induced a time-dependent phosphorylation of tropomyosin that was sensitive to inhibition by PD098059 and UO126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butanediane). Tropomyosin phosphorylation was also induced by expression of a constitutively activated form of MEK1 (MEK(CA)), which confirms that its phosphorylation resulted from the activation of ERK. In unstimulated cells, tropomyosin-1 was found diffuse in the cells, whereas it quickly colocalized with actin and stress fibers upon stimulation of ERK by H(2)O(2) or by expression of MEK(CA). We propose that phosphorylation of tropomyosin-1 downstream of ERK by contributing to formation of actin filaments increases cellular contractility and promotes the formation of focal adhesions. Incidentally, ML-7 (1-[5iodonaphthalene-1-sulfonyl]homopiperazine, HCl), an inhibitor of cell contractility, inhibited phosphorylation of tropomyosin and blocked the formation of stress fibers and focal adhesions, which also led to membrane blebbing in the presence of oxidative stress. Our finding that tropomyosin-1 is phosphorylated downstream of ERK, an event that modulates its interaction with actin, may lead to further understanding of the role of this protein in regulating cellular functions associated with cytoskeletal remodeling.

The role of tropomyosin in the regulation of myocardial contraction and relaxation

Studies over the last 30 years have demonstrated the essential nature of the evolutionarily highly conserved tropomyosin (TM) protein. TM-deficient cells neither function properly nor survive, and mutations within this protein impair severely its function within the sarcomere. The ability to manipulate TM isoform expression genetically within functioning cardiomyocytes and the whole heart has proven essential in deciphering the physiological significance of the different TM isoforms. It is now apparent that alpha-TM and actin serve as the requisite backbone of the thin filament, with varying levels of beta- and gamma-TM, together with the troponin complex serving to modulate sarcomere function. Defining the mechanisms whereby specific TM isoform amino acid differences alter thin filament dynamics will enhance greatly the understanding of muscle contraction during both normal and pathological states.

Overexpression of kinase suppressor of Ras upregulates the high-molecular-weight tropomyosin isoforms in ras-transformed NIH 3T3 fibroblasts

The down-regulation of the high-molecular-weight isoforms of tropomyosin (TM) is considered to be an essential event in cellular transformation. In ras-transformed fibroblasts, the suppression of TM is dependent on the activity of the Raf-1 kinase; however, the requirement for other downstream effectors of Ras, such as MEK and ERK, is less clear. In this study, we have utilized the mitogen-activated protein kinase scaffolding protein Kinase Suppressor of Ras (KSR) to further investigate the regulation of TM and to clarify the importance of MEK/ERK signaling in this process. Here, we report that overexpression of wild-type KSR1 in ras-transformed fibroblasts restores TM expression and induces cell flattening and stress fiber formation. Moreover, we find that the transcriptional activity of a TM-alpha promoter is decreased in ras-transformed cells and that the restoration of TM by KSR1 coincides with increased transcription from this promoter. Although ERK activity was suppressed in cells overexpressing KSR1, ERK inhibition alone was insufficient to upregulate TM expression. The KSR1-mediated effects on stress fiber formation and TM transcription required the activity of the ROCK kinase, because these effects could be suppressed by the ROCK inhibitor, Y27632. Overexpression of KSR1 did not directly regulate ROCK activity, but did permit the recoupling of ROCK to the actin polymerization machinery. Finally, all of the KSR1-induced effects were mediated by the C-terminal domain of KSR1 and were dependent on the KSR-MEK interaction.

The N-terminal sequence of paratropomyosin binding fragments from beta-connectin

In order to clarify the position where paratropomyosin binds to connectin in the A-I junction region of a sarcomere, chicken beta-connectin was digested by Staphylococcus aureus V8 protease under denaturing conditions and the digested peptides were electrophoretically separated. Five peptides, 150-kDa, 100-kDa, 70-kDa, and 43-kDa fragments, were simultaneously detected by biotinylated paratropomyosin and an anti-connectin monoclonal antibody. The N-terminal sequence of the 43-kDa fragment was found to be YQFRVYAVNK, similar to the sequence of 7556-7565 amino acids in the I51 fibronectin type 3 domain that was located at the A-I junction region of human cardiac titin/connectin. Therefore, we propose that paratropomyosin binds to the 43-kDa fragment from beta-connectin at the A-I junction region in both living muscle and in muscle immediately postmortem, and the N-terminus of the 43-kDa fragment is localized in the I51 domain.

Dual wavelength parametric test of two-state models for circular dichroism spectra of helical polypeptides: anomalous dichroic properties of alanine-rich peptides

A two-state helix-coil model underlies all calculations of fractional helicities FH from CD spectra of helical polypeptides. The presence of an isodichroic point near 203 nm is widely assumed to validate this model, but is shown here to provide inadequate validation for alanine-rich peptides. A parametric correlation with constant slope B between CD ellipticities at a pair of wavelengths is introduced as a more rigorous two-state test. Correlations of temperature-dependent [theta](222) vs [theta](208) values are reported for a variety of peptides. Constant slopes B are observed for literature CD data obtained from fragments of helical proteins and dimeric helical coiled coils, but parametric correlations of CD data for alanine-rich peptides consistently exhibit anomalous concave upward curvature, characterized by local slopes that are linearly temperature dependent. Low-temperature CD studies together with parametric correlations at a series of wavelengths demonstrate that the curvature anomaly is maximal at 222 nm and localized in the 215-230 nm wavelength region. Precedented structural variation of the phi, psi dihedral angles of the alpha-helix is suggested as a possible explanation. For the important case of alanine-rich peptides, experiments are proposed that may yield temperature corrections for [theta](222) and permit reliable calculations of FH from [theta](222) values.

The structure of the carboxyl terminus of striated alpha-tropomyosin in solution reveals an unusual parallel arrangement of interacting alpha-helices

Coiled coils are well-known as oligomerization domains, but they are also important sites of protein-protein interactions. We determined the NMR solution structure and backbone (15)N relaxation rates of a disulfide cross-linked, two-chain, 37-residue polypeptide containing the 34 C-terminal residues of striated muscle alpha-tropomyosin, TM9a(251-284). The peptide binds to the N-terminal region of TM and to the tropomyosin-binding domain of the regulatory protein, troponin T. Comparison of the NMR solution structure of TM9a(251-284) with the X-ray structure of a related peptide [Li, Y., Mui, S., Brown, J. H., Strand, J., Reshetnikova, L., Tobacman, L. S., and Cohen, C. (2002) Proc. Natl. Acad. Sci. U.S.A. 99, 7378-7383] reveals significant differences. In solution, residues 253-269 (like most of the tropomyosin molecule) form a canonical coiled coil. Residues 270-279, however, are parallel, linear helices, novel for tropomyosin. The packing between the parallel helices results from unusual interface residues that are atypical for coiled coils. Y267 has poor packing at the coiled-coil interface and a lower R(2) relaxation rate than neighboring residues, suggesting there is conformational flexibility around this residue. The last five residues are nonhelical and flexible. The exposed surface presented by the parallel helices, and the flexibility around Y267 and the ends, may facilitate binding to troponin T and formation of complexes with the N-terminus of tropomyosin and actin. We propose that unusual packing and flexibility are general features of coiled-coil domains in proteins that are involved in intermolecular interactions.

The location of ubiquitin in Lethocerus arthrin

Arthrin is a ubiquitinated actin that is present in flight muscles of some insects. In addition, it has been found in the malaria parasite Plasmodium falciparum. The role of this monoubiquitylation is not clear, and it does not appear to be associated with proteolytic degradation. The stoichiometry of arthrin to actin in Lethocerus indirect flight muscle, 1:6, suggests that there would be one arthrin molecule for each Tm-Tn (tropomyosin-troponin) complex. The appearance of arthrin after tropomyosin and troponin in Drosophila development is consistent with the Tm-Tn complex determining which actin subunit is targeted for conjugation with ubiquitin. We have used a new approach of three-dimensional reconstruction of helical filaments, the iterative helical real space reconstruction method, to extract segments of homogeneous arthrin out of long filaments where the conformation of the ubiquitin is more heterogeneous. Surprisingly, the location of the ubiquitin is on the face of actin subdomain 1, opposite to where tropomyosin binds in the "off" state, suggesting that there could not be a direct interaction between the ubiquitin and the tropomyosin. It is possible that the troponin complex in the "on" state that is bound to one actin strand makes an unfavorable contact with a ubiquitin molecule attached to the opposite actin strand. This might be the basis for a destabilization of the on state at rest length. Lys118 is the most likely residue to which the ubiquitin is conjugated, based upon fitting atomic structures of actin and ubiquitin into the reconstruction.

Functions of tropomyosin's periodic repeats

Tropomyosin binds along actin filaments and regulates actin-myosin interaction in muscle and nonmuscle cells. Seven periodic amino acid repeats are proposed to correspond to actin binding sites, and the middle periods are important for cooperative activation of actin by myosin. The functional contributions of individual periods were studied in mutants in which periods 2-6 were individually deleted from rat striated muscle alphaalpha-tropomyosin or replaced with a leucine zipper sequence. Unacetylated recombinant tropomyosins were assayed for actin binding, regulation of the actomyosin ATPase with troponin, cooperative myosin S1-induced binding to actin, and thermal stability. Tropomyosin function is relatively insensitive to deletion of period 2, but loss increases as the deletion is shifted toward the C-terminus. Retention of function upon deletion of the periodic repeats is in the order of 2 > 3 approximately 4 approximately 6 >> 5. Internal periods are important for specific functions and are not quasiequivalent. Deletion of period 5 (residues 166-207), and especially deletion or replacement of residues 166-188, a constitutively expressed region encoded by exon 5, had severe consequences on actin affinity and cooperative myosin S1-induced binding to actin. Period 6, residues 208-242, part of the troponin binding site, is required for full inhibition of the actomyosin ATPase in the absence of calcium. The effect of the deletion can depend on its context, suggesting that sequence alone is not the only factor important for function. We propose that the local structure and stability, and consequent flexibility, of the coiled coil are major determinants of actin affinity.

Tropomyosin isoforms present in the sea anemone, Anthopleura japonica (Anthozoa, Cnidaria)

Five isoforms of tropomyosin, designated as TMa, TMb, TMc, TMd, and TMe, were detected in the sea anemone, Anthopleura japonica. The apparent molecular weights of these isoforms were estimated to be approximately 30 kD to 37.5 kD, and their pI values were approximately 4.55 (TMa and TMb) and 4.65 (TMc, TMd, and TMe). Although sea anemone tropomyosin isoforms have the ability to bind to rabbit skeletal muscle actin, they preferably bind to actin at higher concentrations of Mg(2+) (10-20 mM) and slightly lower pH (6.2-7.2) than those used in conventional conditions. Antigenic properties of sea anemone tropomyosin seemed to be considerably specific to each isoform. Distribution of tropomyosin isoforms in the sea anemone body was somewhat portion-specific. TMa, TMb, and TMe were detected similarly in the extracts from tentacle, oral disc, column, mouth, and pedal disc. Although TMc and TMd were detected abundantly in the tentacle extract and moderately in the column and mouth extracts, these components were not contained in the pedal disc extract and detected only faintly in the oral disc extract.