Tropomyosin crystal structure and muscle regulation

Three-dimensional image reconstruction of reconstituted smooth muscle thin filaments: effects of caldesmon

Caldesmon inhibits actomyosin ATPase and filament sliding in vitro, and therefore may play a role in modulating smooth and non-muscle motile activities. A bacterially expressed caldesmon fragment, 606C, which consists of the C-terminal 150 amino acids of the intact molecule, possesses the same inhibitory properties as full-length caldesmon and was used in our structural studies to examine caldesmon function. Three-dimensional image reconstruction was carried out from electron micrographs of negatively stained, reconstituted thin filaments consisting of actin and smooth muscle tropomyosin both with and without added 606C. Helically arranged actin monomers and tropomyosin strands were observed in both cases. In the absence of 606C, tropomyosin adopted a position on the inner edge of the outer domain of actin monomers, with an apparent connection to sub-domain 1 of actin. In 606C-containing filaments that inhibited acto-HMM ATPase activity, tropomyosin was found in a different position, in association with the inner domain of actin, away from the majority of strong myosin binding sites. The effect of caldesmon on tropomyosin position therefore differs from that of troponin on skeletal muscle filaments, implying that caldesmon and troponin act by different structural mechanisms.

The active state of the thin filament is destabilized by an internal deletion in tropomyosin

The function of three of tropomyosin's sequential quasiequivalent regions was studied by deletion from skeletal muscle alpha-tropomyosin of internal residues 49-167. This deletion mutant tropomyosin spans four instead of the normal seven actins, and most of the tropomyosin region believed to interact with troponin is retained and uninterrupted in the mutant. The mutant tropomyosin was compared with a full-length control molecule that was modified to functionally resemble muscle tropomyosin (Monteiro, P. B., Lataro, R. C., Ferro, J. A., and Reinach, F. C. (1994) J. Biol. Chem. 269, 10461-10466). The tropomyosin deletion suppressed the actin-myosin subfragment 1 MgATPase rate and the in vitro sliding of thin filaments over a heavy meromyosin-coated surface. This inhibition was not reversed by troponin plus Ca2+. Comparable tropomyosin affinities for actin, regardless of the deletion, suggest that the deleted region has little interaction with actin in the absence of other proteins. Similarly, the deletion did not weaken binding of the troponin-tropomyosin complex to actin. Furthermore, Ca2+ had a 2-fold effect on troponin-tropomyosin's affinity for actin, regardless of the deletion. Notably, the deletion greatly weakened tropomyosin binding to myosin subfragment 1-decorated actin, with the full-length tropomyosin having a 100-fold greater affinity. The inhibitory properties resulting from the deletion are attributed to defective stabilization of the myosin-induced active state of the thin filament.

Cooperative effect of calcium binding to adjacent troponin molecules on the thin filament-myosin subfragment 1 MgATPase rate

The myosin subfragment 1 (S1) MgATPase rate was measured using thin filaments with known extents of Ca2+ binding controlled by varying the ratio of native cardiac troponin versus an inhibitory troponin with a mutation in the sole regulatory Ca2+ binding site of troponin C. Fractional MgATPase activation was less than the fraction of troponins that bound Ca2+, implying a cooperative effect of bound Ca2+ on cross-bridge cycling. Addition of phalloidin did not alter cooperative effects between bound Ca2+ molecules in the presence or absence of myosin S1. When the myosin S1 concentration was raised sufficiently to introduce cooperative myosin-myosin effects, lower Ca2+ concentrations were needed to activate the MgATPase rate. MgATPase activation remained less than Ca2+ binding, implying a true, not just an apparent, increase in Ca2+ affinity. MgATPase activation by Ca2+ was more cooperative than could be explained by cooperativeness of overall Ca2+ binding, the discrepancy between Ca2+ binding and MgATPase activation, or interactions between myosins. The results suggest the thin filament-myosin S1 MgATPase cycle requires calcium binding to adjacent troponin molecules and that this binding is cooperatively promoted by a single cycling cross-bridge. This mechanism is a potential explanation for Ca2+-mediated regulation of cross-bridge kinetics in muscle fibers.

Steric-model for activation of muscle thin filaments

The structural basis of thin filament-linked regulation of muscle contraction is not yet understood. Here we have used electron microscopy and three-dimensional image reconstruction to observe the effects of Ca2+ and myosin head binding on thin filament structure, especially on the position of tropomyosin. Thin filaments isolated in EGTA were treated with Ca2+ or myosin heads (S-1) and negatively stained. Tropomyosin strands were directly visualized in electron micrographs, and distinct EGTA, Ca2+ and S-1-dependent positions were apparent in reconstructions. By fitting reconstructions to the atomic model of F-actin, clusters of amino acids on actin lying beneath tropomyosin were defined under each set of conditions. In the presence of Ca2+, tropomyosin moved 25 degrees away from its low Ca2+ position, exposing most, but not all, of the previously blocked myosin-binding sites. Saturation of filaments with myosin heads produced a further 10 degrees shift in tropomyosin position, thereby exposing the entire myosin-binding site. Our results thus suggest that full switching-on of thin filaments by reversal of steric-blocking requires both Ca2+ and the binding of myosin heads, acting in sequence. By using filaments which were partially decorated with heads, tropomyosin movement was shown to be cooperative, and the size of the actin-tropomyosin cooperative unit was estimated directly. Our results provide direct structural support for previous models of thin filament activation based on kinetics of actin-myosin interaction.

Structure of the Lethocerus troponin-tropomyosin complex as determined by electron microscopy

Native troponin-tropomyosin complex was isolated from Lethocerus indicus indirect flight muscle and tested for function. It was shown by rotary shadowing and by forming paracrystals on monolayers that the regulatory complex consists of a troponin head region approximately 130 A in diameter and a 400-A-long troponin T-tropomyosin tail. The complex forms paracrystals at the air-water interface on a positively charged monolayer. The globular head packs in rows 380 A apart which are bridged by the tail domain. Filamentous paracrystals were obtained by adding Mg2+ ions to the troponin-tropomyosin sample. These showed globular domains arranged in a regular pattern along "ribbon"-like filaments. The spacing of the repeat was determined to be 380 A.

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.

Heptad breaks in alpha-helical coiled coils: stutters and stammers

The discontinuities found in heptad repeats of alpha-helical coiled-coil proteins have been characterized. A survey of 40 alpha-fibrous proteins reveals that only two classes of heptad breaks are prevalent: the stutter, corresponding to a deletion of three residues, and the newly identified "stammer," corresponding to a deletion of four residues. This restriction on the variety of insertions/deletions encountered gives support to a unifying structural model, where different degrees of supercoiling accommodate the observed breaks. Stutters in the hemagglutinin coiled-coil region have previously been shown to produce an underwinding of the supercoil, and we show here how, in other cases, stammers would lead to overwinding. An analysis of main-chain structure also indicates that the mannose-binding protein, as well as hemagglutinin, contains an underwound coiled-coil region. In contrast to knobs-into-holes packing, these models give rise to non-close-packed cores at the sites of the heptad phase shifts. We suggest that such non-close-packed cores may function to terminate certain coiled-coil regions, and may also account for the flexibility observed in such long alpha-fibrous molecules as myosin. The local underwinding or overwinding caused by these specific breaks in the heptad repeat has a global effect on the structure and can modify both the assembly of the protein and its interaction properties.

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.

Integral repeats and a continuous coiled coil are required for binding of striated muscle tropomyosin to the regulated actin filament

Tropomyosin is a coiled-coil protein that binds along the length of filamentous actin and contains sequence repeats that correspond to actin monomers in the filament. Analysis of striated muscle alpha-tropomyosin mutants in which internal sequence has been deleted or replaced with non-tropomyosin sequence showed that the following parameters are important for high affinity, cooperative binding of tropomyosin-troponin to actin. 1) Tropomyosin must be a coiled coil along its entire length. 2) An integral number of repeats corresponding to the actin monomers along its length is more important than the total number. 3) In comparison, the actin affinity is relatively insensitive to changes in the sequence of the internal regions of tropomyosin. The results suggest that the internal sequence repeats function as weakly interacting spacers to allow proper alignment of the ends on the regulated actin filament.

Regulation of actin filament length in erythrocytes and striated muscle

Actin filaments polymerize in vitro to lengths which display an exponential distribution, yet in many highly differentiated cells they can be precisely maintained at uniform lengths in elaborate supramolecular structures. Recent results obtained using two classic model systems, the erythrocyte membrane cytoskeleton and the striated muscle sarcomere, reveal surprising similarities and instructive differences in the molecules and mechanisms responsible for determining and maintaining actin filament lengths in these two systems. Tropomodulin caps the slow-growing, pointed filament ends in muscle and in erythrocytes. CapZ caps the fast-growing, barbed filament ends in striated muscle, whereas a newly discovered barbed end capping protein, adducin, may cap the barbed filament ends in erythrocytes. The mechanisms responsible for specifying the characteristic filament lengths in these systems are more elusive and may include strict control of the relative amounts of actin filament capping proteins and side-binding proteins, molecular templates (e.g. tropomyosin and nebulin) and/or verniers (e.g. tropomyosin).

Roles of electrostatic interaction in proteins

More than 60 years after the analyses by Linderstrom-Lang and Kirkwood of their hypothetical 'protein' structures, we have now a plethora of experimental evidence and computational estimates of the electrostatic forces in proteins, with very many protein 3D structures at atomic resolution. In the mean time, there were in the beginning, many arguments and suggestions about the roles of electrostatics, mainly from empirical findings and tendencies. A few experimental results indicated that the electrostatic contribution is of the order of several kcal/mol, which was theoretically difficult to reproduce correctly, because a large opposing reaction field should be subtracted from a large, direct Coulombic field. Although the importance of the reaction field was recognized even 70 years ago, appropriate applications to protein molecules were made only in this decade, with the development of numerical computation. Now, an electrostatic molecular surface is one of the most popular pictures in journals of structural biology, indicating that the electrostatic force is one of the important components contributing to molecular recognition, which is a major focus of current biology and biochemistry. The development of NMR techniques has made it possible to observe the individual ionizations of ionizable groups in a protein, in addition to the determination of the 3D structure. Since it does not require any additional probe, each charge state can report the very local and heterogeneous electrostatic potentials working in the protein, without disturbing the original field. From the pKa values, the contributions of electrostatic interactions, ion pairs, charge-dipole interactions, and hydrogen bonds to protein stability have been correctly evaluated. Protein engineering also provides much more information than that obtainable from the native proteins, as the residues concerned can now be easily substituted with other amino acid residues having electrostatically different characteristics. Those experimental results have revealed smaller contributions than previously expected, probably because we underestimated the reaction field effects. Especially, a single ion pair stabilizes a protein only slightly, although a cooperative salt-bridge network can contribute significantly to protein stability. Marginal stabilities of proteins arise from small difference between many factors with driving and opposing forces. In spite of the small contribution of each single electrostatic interaction to the protein stability, the sum of their actions works to maintain the specific 3D structure of the protein. The 'negative' roles of electrostatics, which might destabilize protein conformation, should be pointed out. Unpaired buried charges are energetically too expensive to exit in the hydrophobic core. Isolated hydrogen bond donors and acceptors also exert negative effects, but they are not as expensive as the unpaired buried charges, with costs of a few kcal/mol. Therefore, statistical analyses of protein 3D structures reveal only rare instances of isolated hydrogen bond donors and acceptors. This must be the main reason why alpha-helices and beta-sheets are only observed in protein cores as the backbone structures. Such secondary structures do not leave any backbone hydrogen donors or acceptors unpaired, because of their intrinsically regular packing. Otherwise, it might be very difficult to construct a backbone structure, in which all the backbone amide and carbonyl groups had their own hydrogen bond partners in the protein core. There are two theoretical approaches to protein electrostatics, the macroscopic or continuum model, and the microscopic or molecular model. As described in this article, the macroscopic model has inherent problems because the protein-solvent system is very hetergeneous from the physical point of view...

Mechanical properties of tropomyosin and implications for muscle regulation

Tropomyosin is a protein that controls the interactions of actin and myosin as a part of the regulation of muscle contraction. The 420 A long alpha-helical coiled-coil molecules form long filaments, both in muscle and in crystals. The x-ray diffraction data from tropomyosin crystals have indicated large scale motions of the filaments that can be related to the inherent mechanical properties of the molecule, and by extension, to the role of tropomyosin in the cooperative activation of the thin filaments of muscle. Diffuse scattering analysis has provided information about the amplitudes of the motions that has been used to calculate the intrinsic flexibility of the molecule. It can then be shown that each tropomyosin molecule by itself can only mediate interactions of the nearest-neighboring tropomyosin molecules along the filament. The repeating nature of the thin filament, however, allows the entire filament to activate cooperatively.

Diffuse scattering in protein crystallography

The understanding of flexibility and deformability in proteins is one of the current major challenges of structural molecular biology. The knowledge of the average atomic positions of three-dimensional folding of proteins, which is obtained either by X-ray diffraction or n.m.r. spectroscopy, is generally not sufficient to explain their functional mechanisms. Very often it is necessary to consider the existence of other concerted atomic motions as, for example, in the well-known case of the CO molecule fixation at the active site of myoglobin which requires the concerted displacement of a large number of atoms in order to open a channel down to this site. This opening, which depends on the physico-chemical conditions, plays the role of a regulator in the biochemical reactions (Janin & Wodak, 1983; Tainer et al. 1984; Westhof et al. 1984; Ormos et al. 1988).

Forced expression of chimeric human fibroblast tropomyosin mutants affects cytokinesis

Human fibroblasts generate at least eight tropomyosin (TM) isoforms (hTM1, hTM2, hTM3, hTM4, hTM5, hTM5a, hTM5b, and hTMsm alpha) from four distinct genes, and we have previously demonstrated that bacterially produced chimera hTM5/3 exhibits an unusually high affinity for actin filaments and a loss of the salt dependence typical for TM-actin binding (Novy, R.E., J. R. Sellers, L.-F. Liu, and J.J.-C. Lin, 1993. Cell Motil. & Cytoskeleton. 26: 248-261). To examine the functional consequences of expressing this mutant TM isoform in vivo, we have transfected CHO cells with the full-length cDNA for hTM5/3 and compared them to cells transfected with hTM3 and hTM5. Immunofluorescence microscopy reveals that stably transfected CHO cells incorporate force-expressed hTM3 and hTM5 into stress fibers with no significant effect on general cell morphology, microfilament organization or cytokinesis. In stable lines expressing hTM5/3, however, cell division is slow and sometimes incomplete. The doubling time and the incidence of multinucleate cells in the stable hTM5/3 lines roughly parallel expression levels. A closely related chimeric isoform hTM5/2, which differs only in the internal, alternatively spliced exon also produces defects in cytokinesis, suggesting that normal TM function may involve coordination between the amino and carboxy terminal regions. This coordination may be prevented in the chimeric mutants. As bacterially produced hTM5/3 and hTM5/2 can displace hTM3 and hTM5 from actin filaments in vitro, it is likely that CHO-expressed hTM5/3 and hTM5/2 can displace endogenous TMs to act dominantly in vivo. These results support a role for nonmuscle TM isoforms in the fine tuning of microfilament organization during cytokinesis. Additionally, we find that overexpression of TM does not stabilize endogenous microfilaments, rather, the hTM-expressing cells are actually more sensitive to cytochalasin B. This suggests that regulation of microfilament integrity in vivo requires stabilizing factors other than, or in addition to, TM.

Visualization of dynamic simulations of muscle thin filaments

Following a novel computational formalism, the thin filament of muscle can be modeled by a computational machine containing a large number of finite automata that have one-to-one correspondence with the constituent protein molecules. Computer graphics can be used to visualize the correspondence between the states of finite automata and the configurations of protein molecules according to the structural data. The dynamic simulation of the muscle filament that corresponds to the concurrent state transitions of finite automata can be represented as a sequence of video images. The kinetic and structural knowledge of individual protein molecules is, therefore, integrated into a coherent and functional system. This type of computation and visualization can also be useful for the investigation of molecular structure, function, and interaction in various complex biological systems.

Tropomyosin is essential in yeast, yet the TPM1 and TPM2 products perform distinct functions

Sequence analysis of chromosome IX of Saccharomyces cerevisiae revealed an open reading frame of 166 residues, designated TPM2, having 64.5% sequence identity to TPM1, that encodes the major form of tropomyosin in yeast. Purification and characterization of Tpm2p revealed a protein with the characteristics of a bona fide tropomyosin; it is present in vivo at about one sixth the abundance of Tpm1p. Biochemical and sequence analysis indicates that Tpm2p spans four actin monomers along a filament, whereas Tpmlp spans five. Despite its shorter length, Tpm2p can compete with Tpm1p for binding to F-actin. Over-expression of Tpm2p in vivo alters the axial budding of haploids to a bipolar pattern, and this can be partially suppressed by co-over-expression of Tpm1p. This suggests distinct functions for the two tropomyosins, and indicates that the ratio between them is important for correct morphogenesis. Loss of Tpm2p has no detectable phenotype in otherwise wild type cells, but is lethal in combination with tpm1 delta. Over-expression of Tpm2p does not suppress the growth or cell surface targeting defects associated with tpm1 delta, so the two tropomyosins must perform an essential function, yet are not functionally interchangeable. S. cerevisiae therefore provides a simple system for the study of two tropomyosins having distinct yet overlapping functions.

Dissecting titin into its structural motifs: identification of an alpha-helix motif near the titin N-terminus

Titin, also known as connectin, is a giant modular protein specifically found in vertebrate striated muscle. Since the huge size of titin does not allow a direct structure determination, we have started a long-term project to characterize the protein by cutting it into smaller domains or structural units. The major part of the titin sequence is assembled by modules approximately 100 amino acids long that belong to two major protein superfamilies. Most of these modules are linked together by stretches of variable length with unique sequence. No direct structural characterization has been achieved so far for any of these linkers. We present here a study of a stretch located in the titin N-terminus and part of a linker between two modules. Our attention was drawn toward this region because it shows 100% probability to form a coiled coil when analyzed by a prediction program. A synthetic 38 amino acid peptide spanning such a sequence was studied in aqueous solution by circular dichroism, nuclear magnetic resonance, and analytical ultracentrifugation at various pH, salt, and peptide concentrations. Under all conditions, it shows a strong tendency to form alpha-helical structures. In the presence of salt, this conformation is associated with the formation of helical bundles below pH 5. Above pH 5, any aggregate breaks, and the titin peptide is a monomeric helix in equilibrium with its random coil conformation. We discuss the factors which stabilize the helical conformation and the possible role of this stretch in vivo.

Direct measurement of stiffness of single actin filaments with and without tropomyosin by in vitro nanomanipulation

In order to explain the molecular mechanism of muscle contraction, it is crucial to know the distribution of the sarcomere compliance of active muscle. Here, we directly measure the stiffness of single actin filaments with and without tropomyosin, using a recently developed technique for nanomanipulation of single actin filaments with microneedles. The results show that the stiffness for 1-micron-long actin filaments with and without tropomyosin is 65.3 +/- 6.3 and 43.7 +/- 4.6 pN/nm, respectively. When the distribution of crossbridge forces along the actin filament is taken into account, the elongation of a 1-micron-long thin filament during development of isometric contraction is calculated to be approximately 0.23%. The time constant of force in response to a sudden length change is < 0.2 ms, indicating that the viscoelasticity is negligible in the millisecond time range. These results suggest that approximately 50% of the sarcomere compliance of active muscle is due to extensibility of the thin filaments.

Tropomyosin isoforms in rat neurons: the different developmental profiles and distributions of TM-4 and TMBr-3 are consistent with different functions

Antipeptide antisera specific for TM-4 and TMBr-3, the two tropomyosin isoforms in neurons, were used to investigate the concentrations and distributions of these F-actin-binding proteins in neurons in vitro and in vivo. TM-4 and TMBr-3 tropomyosins had different developmental profiles. TM-4 was found mainly in immature stages, while the concentration of TMBr-3 increased with maturation. The two isoforms also had different subcellular distributions. TM-4 was concentrated in the growth cones of cultured neurons and, in vivo, in areas where neurites were growing. Later, when development was complete, TM-4 was restricted to postsynaptic sites in the cerebellar cortex, whereas TMBr-3 was found in the presynaptic terminals. These data suggest that the tropomyosin isoforms have different functions, through their interaction with the actin cytoskeleton. TM-4 may be involved in the motile events of neurite growth and synaptic plasticity, while TMBr-3 could play a role in stabilizing neuronal networks and synaptic functioning.

Dynamics of the muscle thin filament regulatory switch: the size of the cooperative unit

Actin thin filaments containing bound tropomyosin (Tm) or tropomyosin troponin (Tm.Tn) exist in two states ("off" and "on") with different affinities for myosin heads (S1), which results in the cooperative binding of S1. The rate of S1 binding to, and dissociating from, actin, Tm.actin, and Tm.Tn.actin, monitored by light scattering (LS), was compared with the rate of change in state, monitored by the excimer fluorescence (Fl) of a pyrene label attached to Tm. The ATP-induced S1 dissociation showed similar exponential decreases in LS for actin.S1, Tm.actin.S1, and Tm.Tn.actin.S1 +/- Ca2+. The Fl change, however, showed a delay that was greater for Tm.Tn.actin than Tm.actin, independent of Ca2+. The S1 binding kinetics gave observed rate constants for the S1-induced change in state that were 5-6 times the observed rate constants of S1 binding to Tm.actin, which were increased to 10-12 for Tm.Tn.actin, independent of Ca2+. The rate of the Fl signals showed that the on/off states were in rapid equilibrium. These data indicate that the apparent cooperative unit for Tm.actin is 5-6 actin subunits rather than the minimum structural unit size of 7, and is increased to 10-12 subunits for Tm.Tn.actin, independent of the presence of Ca2+. Thus, Tm appears semi-flexible, and Tn increases communication between neighboring structural units. A general model for the dynamic transitions involved in muscle regulation is presented.

A cellular automaton model for the regulatory behavior of muscle thin filaments

Regulation of skeletal muscle contraction is achieved through the interaction of six different proteins: actin, myosin, tropomyosin, and troponins C, I, and T. Many experiments have been performed on the interactions of these proteins, but comparatively less effort has been spent on attempts to integrate the results into a coherent description of the system as a whole. In this paper, we present a new way of approaching the integration problem by using a cellular automaton. We assign rate constants for state changes within each constituent molecule of the muscle thin filament as functions of the states of its neighboring molecules. The automaton shows how the interactions among constituent molecules give rise to the overall regulatory behavior of thin filaments as observed in vitro and is extendable to in vivo measurements. The model is used to predict myosin binding and ATPase activity, and the result is compared with various experimental data. Two important aspects of regulation are revealed by the requirement that the model fit the experimental data: (1) strong interactions must exist between two successively bound myosin heads, and (2) the cooperative binding of calcium to the thin filament can be attributed in a simple way to the interaction between neighboring troponin-tropomyosin units.

Coiled-coil packing in spermine-induced tropomyosin crystals. A comparative study of three forms

Electron microscope images of highly ordered spermine-induced microcrystals of tropomyosin have been analyzed to determine the packing of the molecular filaments. Negatively stained microcrystals terminate in a distinctive "double fringe", which reveals the location of the molecular ends. This information, together with the symmetry of the structure in projection, shows that the microcrystals can be accounted for by a packing scheme of four layers of molecules in the unit cell. Knowing the position of the symmetry elements relating the layers then allows the three-dimensional space group of the microcrystals to be established as C222(1). Using cryo-electron microscopy and simulation studies, the run of the filaments and their packing in the C222(1) form have been shown to be related to those in the spermine-induced C2 crystal of tropomyosin whose structure has been solved to 9 A by X-ray crystallography. This result allows us to infer the location of the molecular ends in the C2 crystal as well, and this inference has been confirmed by analysis of thin sections of the C2 crystal. The C222(1) microcrystal has also been shown to be closely related to the classical divalent cation tropomyosin paracrystal. Based on knowledge of the molecular packing in the divalent cation paracrystal, the polarity of the molecules has been deduced in the other two crystal forms. The tropomyosin filament packing in all these forms may be accounted for by coiled-coil close packing and specific cationic bridging of negatively charged zones on the molecule. Taken together the results reveal a hierarchy of interactions in these close-packed crystalline forms, whose principles may apply to the packing in other fibrous proteins. This study also shows the usefulness of co-ordinating results from cryo-electron microscopy with negative staining in the structure analysis of such ordered arrays, and how these findings can complement the results of low resolution X-ray crystallographic studies.

The effect of N-terminal acetylation on the structure of an N-terminal tropomyosin peptide and alpha alpha-tropomyosin

We have used a synthetic peptide consisting of the first 30 residues of striated muscle alpha-tropomyosin, with GlyCys added to the C-terminus, to investigate the effect of N-terminal acetylation on the conformation and stability of the N-terminal domain of the coiled-coil protein. In aqueous buffers at low ionic strength, the reduced, unacetylated 32mer had a very low alpha-helical content (approximately 20%) that was only slightly increased by disulfide crosslinking or N-terminal acetylation. Addition of salt (> 1 M) greatly increased the helical content of the peptide. The CD spectrum, the cooperativity of folding of the peptide, and sedimentation equilibrium ultracentrifugation studies showed that it formed a 2-chained coiled coil at high ionic strength. Disulfide crosslinking and N-terminal acetylation both greatly stabilized the coiled-coil alpha-helical conformation in high salt. Addition of ethanol or trifluoroethanol to solutions of the peptide also increased its alpha-helical content. However, the CD spectra and unfolding behavior of the peptide showed no evidence of coiled-coil formation. In the presence of the organic solvents, N-terminal acetylation had very little effect on the conformation or stability of the peptide. Our results indicate that N-terminal acetylation stabilizes coiled-coil formation in the peptide. The effect cannot be explained by interactions with the "helix-dipole" because the stabilization is observed at very high salt concentrations and is independent of pH. In contrast to the results with the peptide, N-terminal acetylation has only small effects on the overall stability of tropomyosin.

Differential regulation of skeletal muscle myosin-II and brush border myosin-I enzymology and mechanochemistry by bacterially produced tropomyosin isoforms

In this report, we have compared the physical properties and actin-binding characteristics of several bacterially produced nonmuscle and striated muscle tropomyosins, and we have examined the effects of these isoforms on the interactions of actin with two structurally distinct classes of myosin: striated muscle myosin-II and brush border (BB) myosin-I. All of the bacterially produced nonmuscle tropomyosins bind to F-actin with the expected stoichiometry and with affinities comparable to that of a tissue produced alpha-tropomyosin, although the striated muscle tropomyosin CTm7 has a lower affinity for F-actin than a tissue-purified striated muscle alpha tropomyosin. The bacterially produced isoforms also protect F-actin from severing by villin as effectively as tissue-purified striated muscle alpha-tropomyosin. The bacterially produced 284 amino acid striated muscle tropomyosin isoform CTm7, the 284 amino acid nonmuscle tropomyosin isoform CTm4, and two chimeric tropomyosins (CTm47 and CTm74) all inhibit the actin-activated MgATPase activity of muscle myosin S1 by approximately 70-85%, comparable to the inhibition seen with tissue-purified striated muscle alpha tropomyosin. The 248 amino acid tropomyosin XTm4 stimulated the actin-activated MgATPase activity of muscle myosin S1 approximately two- to threefold. The in vitro sliding of actin filaments translocated by muscle myosin-II (2.4 microns/sec at 19 degrees C, 5.0 microns/s at 24 degrees C) increased 25-65% in the presence of XTm4. Tropomyosins CTm4, CTm7, CTm47, and CTm74 had no detectable effect on myosin-II motility. The actin-activated MgATPase activity of BB myosin-I was inhibited 75-90% by all of the tropomyosin isoforms tested, including the 248 amino acid tropomyosin XTm4. BB myosin-I motility (50 nm/s) was completely inhibited by both the 248 and 284 amino acid tropomyosins. These results demonstrate that bacterially produced tropomyosins can differentially regulate myosin enzymology and mechanochemistry, and suggest a role for tropomyosin in the coordinated regulation of myosin isoforms in vivo.

Conformational intermediates in the folding of a coiled-coil model peptide of the N-terminus of tropomyosin and alpha alpha-tropomyosin

Circular dichroism was used to study the folding of alpha alpha-tropomyosin and AcTM43, a 43-residue peptide designed to serve as a model for the N-terminal domain of tropomyosin. The sequence of the peptide is AcMDAIKKKMQMLKLDVENLLDRLEQLEADLKALEDRYKQLEGGC. The peptide appeared to form a coiled coil at low temperatures (< 25 degrees C) in buffers with physiological ionic strength and pH. The folding and unfolding of the peptide, however, were noncooperative. When CD spectra were examined as a function of temperature, the apparent degree of folding differed when the ellipticity was followed at 222, 208, and 280 nm. Deconvolution of the spectra suggested that at least three component curves contributed to the CD in the far UV. One component curve was similar to the CD spectrum of the coiled-coil alpha-helix of native alpha alpha-tropomyosin. The second curve resembled the spectrum of single-stranded short alpha-helical segments found in globular proteins. The third was similar to that of polypeptides in the random coil conformation. These results suggested that as the peptide folded, the alpha-helical content increased before most of the coiled coil was formed. When the CD spectrum of striated muscle alpha alpha-tropomyosin was examined as a function of temperature, the unfolding was also not totally cooperative. As the temperature was raised from 0 to 25 degrees C, there was a decrease in the coiled coil and an increase in the conventional alpha-helix type spectrum without formation of random coil. The major transition, occurring at 40 degrees C, was a cooperative transition characterized by the loss of all of the remaining coiled coil and a concomitant increase in random coil.

The effect of Ca2+ on the conformation of tropomyosin and actin in regulated actin filaments with or without bound myosin subfragment 1

The effects of Ca2+ and myosin subfragment 1 on the conformation of tropomyosin and actin in regulated actin filaments in ghost fibers were investigated by means of the polarized fluorescence technique. Regulated thin filaments were reconstituted in skeletal muscle ghost fibers by incorporation into the fibers of either skeletal muscle troponin-tropomyosin or smooth-muscle caldesmon-calmodulin-tropomyosin complexes. Tropomyosin and actin were specifically labeled with fluorescent probes, 1,5-IAEDANS and phalloidin-rhodamine, respectively. Analysis of the fluorescence parameters indicated that the binding of Ca2+ to regulated actin filaments induces conformational changes in tropomyosin and actin that lead to the strengthening of the interaction between these two proteins and weakening of the binding of actin monomers in the filament. These changes become larger when regulated actin forms rigor links with myosin subfragment 1. No notable alterations in the position of tropomyosin relative to actin in the frontal plane of the fiber were detected either upon binding of Ca2+ or upon the additional binding of myosin subfragment 1 to regulated actin.

Packing and hydrophobicity effects on protein folding and stability: effects of beta-branched amino acids, valine and isoleucine, on the formation and stability of two-stranded alpha-helical coiled coils/leucine zippers

The aim of this study was to examine the differences between hydrophobicity and packing effects in specifying the three-dimensional structure and stability of proteins when mutating hydrophobes in the hydrophobic core. In DNA-binding proteins (leucine zippers), Leu residues are conserved at positions "d," and beta-branched amino acids, Ile and Val, often occur at positions "a" in the hydrophobic core. In order to discern what effect this selective distribution of hydrophobes has on the formation and stability of two-stranded alpha-helical coiled coils/leucine zippers, three Val or three Ile residues were simultaneously substituted for Leu at either positions "a" (9, 16, and 23) or "d" (12, 19, and 26) in both chains of a model coiled coil. The stability of the resulting coiled coils was monitored by CD in the presence of Gdn.HCl. The results of the mutations of Ile to Val at either positions "a" or "d" in the reduced or oxidized coiled coils showed a significant hydrophobic effect with the additional methylene group in Ile stabilizing the coiled coil (delta delta G values range from 0.45 to 0.88 kcal/mol/mutation). The results of mutations of Leu to Ile or Val at positions "a" in the reduced or oxidized coiled coils showed a significant packing effect in stabilizing the coiled coil (delta delta G values range from 0.59 to 1.03 kcal/mol/mutation). Our results also indicate the subtle control hydrophobic packing can have not only on protein stability but on the conformation adopted by the amphipathic alpha-helices. These structural findings correlate with the observation that in DNA-binding proteins, the conserved Leu residues at positions "d" are generally less tolerant of amino acid substitutions than the hydrophobic residues at positions "a."

Pitch diversity in alpha-helical coiled coils

Two complementary methods for measuring local pitch based on heptad position in alpha-helical coiled coils are described and applied to six crystal structures. The results reveal a diversity of pitch values: two-stranded coiled coils appear to have pitch values near 150 A; the values for three- and four-stranded coiled coils range closer to 200 A. The methods also provide a rapid and sensitive gauge of local coiled-coil conformation. Polar or charged residues in the apolar interface between coiled-coil helices markedly affect local pitch values, suggesting a connection between pitch uniformity and coiled-coil stability. Moreover, the identification of a skip residue (heptad frame shift) in the hemagglutinin glycoprotein of influenza virus (HA) allows interpretation of local pitch changes. These results on relatively short coiled-coil structures have relevance for the much longer fibrous proteins (many of which have skip residues) whose detailed structures are not yet established. We also show that local pitch values from molecular dynamics predictions of the GCN4 leucine zipper are in striking agreement with the high-resolution crystal structure--a result not readily discerned by direct comparison of atomic coordinates. Taken together, these methods reveal specific aspects of coiled-coil structure which may escape detection by global analyses of pitch.

A non-flight muscle isoform of Drosophila tropomyosin rescues an indirect flight muscle tropomyosin mutant

The tropomyosin I(TmI) gene of Drosophila melanogaster encodes two isoforms of tropomyosin. The Ifm-TmI isoform is expressed only in indirect flight and jump muscles; the Scm-TmI isoform is found in other muscles of the larva and adult. The level of Ifm-TmI is severely reduced in the flightless mutant Ifm(3)3, which also is unable to jump. To explore the functional significance of tropomyosin isoform diversity in Drosophila, we have used P element-mediated transformation to express Scm-TmI in the indirect flight and jump muscles of Ifm(3)3 flies. Transformants gained the ability to jump and fly. The mechanical properties of isolated indirect flight muscle myofibres, and the ultrastructure of indirect flight and jump muscles from the transformants were comparable to wildtype. Thus, the Scm-TmI isoform can successfully substitute for Ifm-TmI in the indirect flight and jump muscles of the Ifm(3)3 strain.

Incorporation of microinjected mutant and wildtype recombinant tropomyosins into stress fibers in fibroblasts

The structural requirements for assembly of tropomyosin into stress fibers were investigated by microinjecting wildtype and four mutant striated chicken muscle alpha-tropomyosins expressed in E. coli as fusion and nonfusion proteins into cultured rat embryo fibroblasts, followed by localization of tropomyosin using indirect immunofluorescence. The results show that the determinants for stress fiber incorporation in living cells correlate with the in vitro actin affinity of these tropomyosins. Wildtype recombinant protein incorporated into stress fibers both when the amino terminus was unacetylated and when it was blocked with an 80-residue fusion protein [Hitchcock-DeGregori, S.E., and Heald, R.W. (1987): J. Biol. Chem. 262:9730-9735]. The pattern of incorporation was indistinguishable from that of tropomyosin isolated from chicken pectoral muscle. The striated alpha-tropomyosin incorporated into stress fibers, even though this isoform is not found in nonmuscle cells. Three recombinant mutant tropomyosins in which one-half, two-thirds, or one actin binding site was deleted were tested [Hitchcock-DeGregori, S.E., and Varnell, T.A. (1990): J. Mol. Biol. 214:885-896]. Only the fusion protein with a full actin binding site deleted incorporated into stress fibers. However, the unacetylated, nonfusion proteins with one half and one actin binding site deleted incorporated into stress fibers, consistent with the ability of troponin to promote the actin binding in vitro. A fourth mutant, in which the conserved amino-terminal nine residues were deleted, did not incorporate into stress fibers, consistent with the complete loss of function of this mutant [Cho, Y.J., Liu, J., and Hitchcock-DeGregori, S.E. (1990): J. Biol. Chem. 265:538-545].

A comparison of the atomic model of F-actin with cryo-electron micrographs of actin and decorated actin

We compare the atomic model calculated from the crystal structure and the X-ray fiber diagram of orientated F-actin1) with the 3-D reconstructions produced from cryo-electron microscopy of actin2). Out to 30A resolution the two structures are essentially identical. Furthermore, by combining the atomic model with the reconstruction of S1-decorated actin filaments2) one can establish the nature of the actin binding site for myosin in the rigor complex. Each myosin head binds to two actin molecules on two distinct sites. Some of the actin residues involved in each of these binding sites can be identified. Furthermore, the atomic model of actin may be combined with the reconstruction of the S1 decorated thin filament to establish the tropomyosin binding site in the rigor complex. This result is compared with the model of tropomyosin-actin derived from an analysis of the X-ray fibre diagram of a reconstituted thin filament and are shown to be very similar.

A new crystal form of tropomyosin

Tropomyosin crystals with a new morphology have been obtained from lobster tail muscle tropomyosin from which 11 residues at the carboxyl-terminus have been proteolytically removed to avoid head-to-tail polymerization. In contrast to the conventional Bailey crystal form in which the elongated tropomyosin molecules form a mesh, in the present crystals the molecules are packed side-to-side with the long axes parallel to the c-axis of the crystal. The unit cell is tetragonal with a = b = 109 A, c = 509 A, and the symmetry is either P4(1)2(1)2 or P4(3)2(1)2, with 4(1)(4(3)) helical axes parallel to the c-axis. This suggests that a group of molecules surrounding a local 4(1)(4(3)) axis is regarded as the building unit of the crystal. It is likely that the unit cell contains eight molecules with one molecule per asymmetric unit.

What is the pitch of the alpha-helical coiled coil?

The alpha-helical, coiled-coil protein motif is increasingly recognized in a variety of functional classes of proteins. The pitch of a coiled coil, or rate of winding of the alpha-helices around each other, is a key determinant of both intra- and intermolecular interactions. Experimental measurements of the pitch of parallel two-stranded coiled coils of muscle proteins, and examination of the recently determined structure of another two-stranded coiled coil, the GCN4 transcription factor protein, suggest that the pitch has an average value of about 140 A. This value is consistent with the observed number of residues per turn in alpha-helices of globular proteins, the determinant of the interhelical packing within the coiled-coil motif. An understanding of the structural determinants of this value for the pitch and possible variations will be important in defining the interactions of coiled-coil proteins with other macromolecules.

Unfolding domains of recombinant fusion alpha alpha-tropomyosin

The thermal unfolding of the coiled-coil alpha-helix of recombinant alpha alpha-tropomyosin from rat striated muscle containing an additional 80-residue peptide of influenza virus NS1 protein at the N-terminus (fusion-tropomyosin) was studied with circular dichroism and fluorescence techniques. Fusion-tropomyosin unfolded in four cooperative transitions: (1) a pretransition starting at 35 degrees C involving the middle of the molecule; (2) a major transition at 46 degrees C involving no more than 36% of the helix from the C-terminus; (3) a major transition at 56 degrees C involving about 46% of the helix from the N-terminus; and (4) a transition from the nonhelical fusion domain at about 70 degrees C. Rabbit skeletal muscle tropomyosin, which lacks the fusion peptide but has the same tropomyosin sequence, does not exhibit the 56 degrees C or 70 degrees C transition. The very stable fusion unfolding domain of fusion-tropomyosin, which appears in electron micrographs as a globular structural domain at one end of the tropomyosin rod, acts as a cross-link to stabilize the adjacent N-terminal domain. The least stable middle of the molecule, when unfolded, acts as a boundary to allow the independent unfolding of the C-terminal domain at 46 degrees C from the stabilized N-terminal unfolding domain at 56 degrees C. Thus, strong localized interchain interactions in coiled-coil molecules can increase the stability of neighboring domains.

Structure of tropomyosin at 9 angstroms resolution

We have used molecular replacement followed by a highly parameterized refinement to determine the structure of tropomyosin crystals to a resolution to 9 A. The shape, coiled-coil structure and interactions of the molecules in the crystals have been determined. These crystals have C2 symmetry with a = 259.7 A, b = 55.3 A, c = 135.6 A and beta = 97.2 degrees. Because of the unusual distribution of intensity in X-ray diffraction patterns from these crystals, it was possible to solve the rotation problem by inspection of qualitative aspects of the diffraction data and to define unequivocally the general alignment of the molecules along the (332) and (3-32) directions of the unit cell. The translation function was then solved by a direct search procedure, while electron microscopy of a related crystal form indicated the probable location of molecular ends in the asymmetric unit, as well as the anti-parallel arrangement. The structural model we have obtained is much clearer than that obtained previously with crystals of extraordinarily high solvent content and shows the two alpha-helices of the coiled coil over most of the length of the molecules and establishes the coiled-coil pitch at 140(+/- 10) A. Moreover, the precise value of the coiled-coil pitch varies along the molecule, probably in response to local variations in the amino acid sequence, which we have determined by sequencing the appropriate cDNA. The crystals are constructed from layers of tropomyosin filaments. There are two molecules in the crystallographic asymmetric unit and the molecules within a layer are bent into an approximately sinusoidal profile. Molecules in consecutive layers in the crystal lie at an angle relative to one another as found in crystalline arrays of actin and myosin rod. There are three classes of interactions between tropomyosin molecules in the spermine-induced crystals and these give some insights into the molecular interactions between coiled-coil molecules that may have implications for assemblies such as muscle thick filaments and intermediate filaments. In interactions within a layer, the geometry of coiled-coil contacts is retained, whereas in contacts between molecules in adjacent layers the coiled-coil geometry varies and these interactions instead appear to be dominated by the repeating pattern of charged zones along the molecule.

Design, synthesis and structural characterization of model heterodimeric coiled-coil proteins

We report the design and synthesis of model heterodimeric coiled-coil proteins and the packing contribution of interchain hetero-hydrophobic side-chains to coiled-coil stability. The heterodimeric coiled-coils are obtained by oxidizing two 35-residue polypeptide chains, each containing a cysteine residue at position 2 and differing in amino acid sequences in the hydrophobic positions ("a" and "d") responsible for the formation and stabilization of the coiled-coil. In each peptide, a single Ala residue was substituted for Leu at position "a" or "d". The formation and stability of heterodimeric coiled-coils were investigated by circular dichroism studies in the presence and absence of guanidine hydrochloride and compared to the corresponding homodimeric coiled-coils. The coiled-coil proteins with an Ala substitution at position "a" were less stable than those with an Ala substitution at position "d" in both the homodimeric (Ala-Ala interchain interactions) and heterodimeric (Leu-Ala interchain interactions ) coiled-coils. The 70-residue disulfide bridged peptides (homo- and heterodimeric coiled-coils) can be readily separated by reversed-phase chromatography (RPC) even though they have identical amino acid compositions as well as in the hydrophobic "a" and "d" positions. The elution of the 70-residue peptides prior to their corresponding 35-residue monomers suggests that these proteins are retaining a large portion of their coiled-coil structure during RPC at pH2 and their retention behavior correlates with protein stability.