Tropomyosin crystal structure and muscle regulation

Synthetic model proteins: the relative contribution of leucine residues at the nonequivalent positions of the 3-4 hydrophobic repeat to the stability of the two-stranded alpha-helical coiled-coil

Our de novo designed coiled-coil model protein consists of two identical 35-residue polypeptide chains arranged in a parallel and in-register alignment via interchain hydrophobic interactions and a disulfide bridge at the position 2 between two helices. To quantitate the relative contribution of leucine residues at the nonequivalent position of the 3-4 hydrophobic repeat to the stability of the two-stranded alpha-helical coiled-coil, a single alanine was systematically substituted for a leucine in each chain at position "a" (9, 16, 23, or 30) or "d" (5, 12, 19, 26, or 33). The formation and stability of the coiled-coils were determined by circular dichroism studies in the absence and presence of guanidine hydrochloride. All the proteins with an alanine substituted at position a have a similar stability ([Gdn.HCl]1/2 ranges from 2.6 to 2.9 M), while all the proteins with an alanine substituted at position d have similar stability ([Gdn.HCl]1/2 ranges from 3.6 to 4.2 M), except for the proteins with an alanine substituted in the C-terminal heptad. The greater decrease in stability observed for a Leu----Ala mutation at position a (the average delta delta Gu value is 3.3 kcal/mol) compared to those where the substitution was effected at position d (the average delta delta Gu value is 2.0 kcal/mol) indicates that an Ala mutation at position a has a greater effect on the side-chain packing and hydrophobic interactions in the coiled-coil than an Ala mutation at position d.(ABSTRACT TRUNCATED AT 250 WORDS)

Diffuse x-ray scattering from tropomyosin crystals

Diffuse scattering analyses are emerging as a technique to extract additional dynamic information from x-ray diffraction data. In fact, when examined carefully, most protein crystals show significant diffuse scattering in addition to the usual Bragg diffraction. This diffuse scattering contains information about the disorder in the crystal that cannot be obtained from the Bragg diffraction data. Diffraction from tropomyosin crystals shows characteristic diffuse scattering streaks that are directly related to motion of the molecules. The structure of tropomyosin to 15 A resolution shows that the limited molecular contacts between molecules allow large conformational fluctuations of up to 8 A amplitude. Models for the three-dimensional motion of tropomyosin have been tested by comparing their predicted diffuse scattering patterns with the experimental data. From the parameters of the successful simulations, we were able to determine the amplitudes, directions, and distances over which the atomic displacements are correlated.

Analysis of the three-alpha-helix motif in the spectrin superfamily of proteins

Members of the spectrin superfamily of proteins contain different numbers of homologous repeats arranged in tandem. Each of these consists of a three-alpha-helix motif, comprising two similarly and one oppositely directed alpha-helical segment joined by nonhelical linkers of characteristic length. The right-handed alpha-helices each display a heptad repeat in their amino acid sequences indicative of left-handed coiled-coil-like packing. We have calculated the potential number of inter-helix ionic interactions that specify the spatial arrangement of the helices in the motif in terms of both the handedness of helix connectivity (left or right) and the relative axial stagger between the three alpha-helices. All of the models examined were constrained to have optimal coiled-coil packing. For alpha-spectrin and alpha-actinin the results provide strong support for a left-handed connectivity of the three helices and axial repeat lengths of 5.05 and 6.24 nm, respectively. Furthermore, the axial staggers between homologous segments in the preferred models are identical. The insights provided into the topography of this widespread tertiary fold may prove of value to those concerned with the problem of de novo protein design.

Linear dichroism of acrylodan-labeled tropomyosin and myosin subfragment 1 bound to actin in myofibrils

Muscle contraction can be activated by the binding of myosin heads to the thin filament, which appears to result in thin filament structural changes. In vitro studies of reconstituted muscle thin filaments have shown changes in tropomyosin-actin geometry associated with the binding of myosin subfragment 1 to actin. Further information about these structural changes was obtained with fluorescence-detected linear dichroism of tropomyosin, which was labeled at Cys 190 with acrylodan and incorporated into oriented ghost myofibrils. The fluorescence from three sarcomeres of the fibril was collected with the high numerical aperture objective of a microscope and the dichroic ratio, R (0/90 degrees), for excitation parallel/perpendicular to the fibril, was obtained, which gave the average probe dipole polar angle, Theta. For both acrylodan-labeled tropomyosin bound to actin in fibrils and in Mg2+ paracrystals, Theta congruent to 52 degrees +/- 1.0 degrees, allowing for a small degree of orientational disorder. Binding of myosin subfragment 1 to actin in fibrils did not change Theta; i.e., the orientation of the rigidly bound probe on tropomyosin did not change relative to the actin axis. These data indicate that myosin subfragment 1 binding to actin does not appreciably perturb the structure of tropomyosin near the probe and suggest that the geometry changes are such as to maintain the parallel orientation of the tropomyosin and actin axes, a finding consistent with models of muscle regulation. Data are also presented for effects of MgADP on the orientation of labeled myosin subfragment 1 bound to actin in myofibrils.

The NUF1 gene encodes an essential coiled-coil related protein that is a potential component of the yeast nucleoskeleton

In an attempt to identify structural components of the yeast nucleus, subcellular fractions of yeast nuclei were prepared and used as immunogens to generate complex polyclonal antibodies. One such serum was used to screen a yeast genomic lambda gt11 expression library. A clone encoding a gene called NUF1 (for nuclear filament-related) was identified and extensively characterized. Antibodies to NUF1 fusion proteins were generated, and affinity-purified antibodies were used for immunoblot analysis and indirect immunofluorescence localization. The NUF1 protein is 110 kD in molecular mass and localizes to the yeast nucleus in small granular patches. Intranuclear staining is present in cells at all stages of the cell cycle. The NUF1 protein of yeast is tightly associated with the nucleus; it was not removed by extraction of nuclei with nonionic detergent or salt, or treatment with RNAse and DNAse. Sequence analysis of the NUF1 gene predicts a protein 945 amino acids in length that contains three domains: a large 627 residue central domain predicted to form a coiled-coil structure flanked by nonhelical amino-terminal and carboxy-terminal regions. Disruption of the NUF1 gene indicates that it is necessary for yeast cell growth. These results indicate that NUF1 encodes an essential coiled-coil protein within the yeast nucleus; we speculate that NUF1 is a component of the yeast nucleoskeleton. In addition, immunofluorescence results indicate that mammalian cells contain a NUF1-related nuclear protein. These data in conjunction with those in the accompanying manuscript (Yang et al., 1992) lead to the hypothesis that an internal coiled-coil filamentous system may be a general structural component of the eukaryotic nucleus.

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.

Actin as the generator of tension during muscle contraction

We propose that the key structural feature in the conversion of chemical free energy into mechanical work by actomyosin is a myosin-induced change in the length of the actin filament. As reported earlier, there is evidence that helical actin filaments can untwist into ribbons having an increased intersubunit repeat. Regular patterns of actomyosin interactions arise when ribbons are aligned with myosin thick filaments, because the repeat distance of the myosin lattice (429 A) is an integral multiple of the subunit repeat in the ribbon (35.7 A). This commensurability property of the actomyosin lattice leads to a simple mechanism for controlling the sequence of events in chemical-mechanical transduction. A role for tropomyosin in transmitting the forces developed by actomyosin is proposed. In this paper, we describe how these transduction principles provide the basis for a theory of muscle contraction.

Simple harmonic motion of tropomyosin: proposed mechanism for length-dependent regulation of muscle active tension

A simple harmonic theory is proposed to describe the regulatory mechanism of tropomyosin in the activation of muscle contraction. The theory proposes that activation-associated displacement of tropomyosin is inherent to tropomyosin, a consequence of the molecule's large-scale vibrational motion (5-10 A root mean square displacement). In association with thin filament the vibrational motion may become less complex, approaching the ideal case of simple harmonic motion. The degree of activation increases as the amplitude of the simple harmonic motion increases, causing tropomyosin to shorten lengthwise, shiftings its position from the periphery of thin filament (OFF) to the actin groove (ON). The amplitude may be regulated in a rectilinear manner by the thick filament electrostatic force, the thin filament hydrophobic force, and the Ca(2+)-dependent force of the troponin complex. The radial and tangenital components of the resultant force may vary as the muscle is stretched, regulating maximum active tension and Ca2+ sensitivity, respectively. This may represent the molecular basis for Starling's law of the heart. The mechanism may be important for describing the regulatory mechanism of tropomyosin in smooth muscle and nonmuscle cells and may facilitate a clinically relevant understanding of the effects of pH, Mg2+ concentration, ionic strength, and ethanol on the regulation of active tension.

Relationship between alternatively spliced exons and functional domains in tropomyosin

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

A model for human cardiac troponin C and for modulation of its Ca2+ affinity by drugs

Calcium sensitizers are drugs which increase force development in striated muscle by sensitizing myofilaments to Ca2+. This can happen by increasing Ca2+ affinity of the regulatory domain of Ca2+ binding protein troponin C. High resolution crystal structures of two calcium binding proteins, calmodulin (Babu et al.: J. Mol. Biol. 203:191-204, 1988) and skeletal troponin C (Satyshur et al.: J. Biol. Chem. 263:1628-1647, 1988; Herzber et al.: J. Mol. Biol. 203:761-779, 1988), have recently been published. This makes it possible to model in detail the calcium-sensitizing action of drugs on troponin C. In this study a model of human cardiac troponin C in three-calcium state has been constructed. When calcium is bound to calcium site II of cardiac troponin C an open conformation of the protein results, which has a hydrophobic pocket surrounded by a few polar side chains. Complexation of three drugs, trifluoperazine, bepridil, and pimobendan, to the hydrophobic pocket is studied using energy minimization techniques. Two different binding modes are found, which differ in the location of a strong electrostatic interaction. In analogy with the crystal structure of skeletal troponin C it is hypothezed that in cardiac troponin C an interaction occurs between Gln-50 and Asp-88, which has a long-range effect on calcium binding. The binding modes of drugs, where a strong interaction with Asp-88 exists, can effectively prevent the interaction between Asp-88 and Gln-50 in the protein, and are proposed to be responsible for the calcium-sensitizing properties of the studied drugs.

Structural studies of tropomyosin by cryoelectron microscopy and x-ray diffraction

A comparison has been made between cryoelectron microscope images and the x-ray structure of one projection of the Bailey tropomyosin crystal. The computed transforms of the electron micrographs extend to a resolution of approximately 18 A compared with the reflections from x-ray crystallography which extend to 15 A. After correction of the images for lattice distortions and the contrast transfer function, the structure factors were constrained to the plane group (pmg) symmetry of this projection. Amplitude and phase data for five images were compared with the corresponding view from the three-dimensional x-ray diffraction data (Phillips, G.N., Jr., J.P. Fillers, and C. Cohen. 1986. J. Mol. Biol. 192: 111-131). The average R factor between the electron microscopy and x-ray amplitudes was 15%, with an amplitude-weighted mean phase difference of 4.8 degrees. The density maps derived from cryoelectron microscopy contain structural features similar to those from x-ray diffraction: these include the width and run of the filaments and their woven appearance at the crossover regions. Preliminary images obtained from frozen-hydrated tropomyosin/troponin cocrystals suggest that this approach may provide structural details not readily obtainable from x-ray diffraction studies.

Cross-helix separation of tropomyosin molecules in acto-tropomyosin as determined by neutron scattering

The cross-helix separation of Tm molecules in acto-tropomyosin has been determined using neutron scattering. Deuterated Dictyostelium discoideum actin was density matched in a 93% D2O buffer so that effectively only the protonated tropomyosin was "visible" to neutrons. Analysis of the solution scattering pattern in the region of the first oscillation yielded a value for the cross-helix separation of 7.9 +/- 0.3 nm. The implications of this value for the mechanism of the regulation of muscle contraction are discussed in light of recent results by others.

Molecular analysis of protein assembly in muscle development

The challenge presented by myofibril assembly in striated muscle is to understand the molecular mechanisms by which its protein components are arranged at each level of organization. Recent advances in the genetics and cell biology of muscle development have shown that in vivo assembly of the myofilaments requires a complex array of structural and associated proteins and that organization of whole sarcomeres occurs initially at the cell membrane. These studies have been complemented by in vitro analyses of the renaturation, polymerization, and three-dimensional structure of the purified proteins.

Drosophila melanogaster troponin-T mutations engender three distinct syndromes of myofibrillar abnormalities

In vertebrates troponin complexes interact co-operatively with tropomyosin dimers to modulate skeletal muscle contraction. In order further to investigate troponin assembly and function in vivo, we are developing molecular genetic approaches. Here we report characterization of the gene that encodes Drosophila tropinin-T and analyses of muscle defects engendered by several mutant alleles. We found that the Drosophila troponin-T locus specifies at least three proteins having sequences similar to vertebrate troponin-T. All are significantly larger than any avian or mammalian isoforms, however, due to a highly acidic carboxy-terminal extension. Comparisons of the chromosomal arrangements of vertebrate and Drosophila troponin-T genes revealed that the location of one intron-exon boundary is conserved. This observation and the similarity of vertebrate and Drosophila troponin-T primary sequences suggest that the respective proteins are homologous, and that troponin-T pre-dates the divergence of vertebrate and invertebrate organisms. In situ hybridization of the Drosophila troponin-T gene to polytene chromosomes demonstrated that it resides within subdivision 12A of the X chromosome, precisely where upheld and indented thorax flight muscle mutations have been mapped previously. We determined the nucleotide sequences of troponin-T genes in five extant mutants. All have deleterious alterations, directly establishing that upheld and indented thorax muscle abnormalities are due to defective troponin-T. Two of the alleles, upheld2 and upheld3, apparently disrupt RNA splicing and eliminate most or all troponin-T from flight and jump muscles, while the remaining three alleles change the identities of single amino acids of troponin-T. Electron microscopy of mutant muscles revealed that the two null alleles eliminate thin filaments, except where they are bound by electron-dense material presumed to be Z-disc proteins. Two of the point mutations, upheld101 and indented thorax3, do not perturb assembly of myofibrils, but cause their degeneration within days after muscles begin to be utilized. The final mutation, upheldwhu, reduces the diameter of the myofibril lattice by approximately one-half. We propose hypotheses to explain how each troponin-T mutation engenders the observed myofibrillar defects.

Molecular structure of F-actin and location of surface binding sites

Comparisons of three-dimensional maps of vertebrate muscle thin filaments obtained by cryo-electron microscopy and image analysis, reveal the molecular structure of F-actin, the location of the C terminus of the monomer and the positions of the binding sites of tropomyosin, the myosin head and the N-terminal portion of the myosin A1 light chain on the filament. These data provide strong constraints for evaluating models built from the atomic structure of the monomer and the subsequent identification of molecular contacts.

Immunoelectron microscopic observations on tropomyosin localization in striated muscle

Tropomyosin localization in striated muscle was studied by means of immunoelectron microscopy. Polyclonal and monoclonal antibodies to tropomyosin were allowed to diffuse into mechanically skinned single fibres dissected from frog semitendinosus muscle. Antibodies produced transverse I-band stripes with the expected periodicity of 38 nm. However, some differences were revealed among the various antibodies. While polyclonal antibodies generally showed 23 stripes, monoclonal antibodies showed an extra 24th stripe immediately adjacent to the Z-line, implying some structural/functional uniqueness of this terminal tropomyosin. Furthermore, the stripes did not always lie parallel to the Z-line. When the Z-line was straight or slightly skewed, the stripes generally were parallel to it. However, when Z-line skew was more severe, the stripes remained perpendicular to the fibre axis, indifferent to the Z-line skew. This may implay that the coupling of tropomyosin to the thin filament is not tight. Finally, the monoclonal antibodies themselves exerted an anomalous effect on the Z-line, apparently extracting or shifting some of its mass.

High-resolution spot-scan electron microscopy of microcrystals of an alpha-helical coiled-coil protein

We describe the electron microscopy of a crystalline assembly of an alpha-helical coiled-coil protein extracted from the ootheca of the praying mantis. Electron diffraction patterns of unstained crystals show crystal lattice sampling of the coiled-coil molecular transform to a resolution beyond 1.5 A. Using a "spot-scan" method of electron imaging, micrographs of unstained crystals have been obtained that visibly diffract laser light from crystal spacings as small as 4.3 A. A projection map was calculated to 4 A using electron diffraction amplitudes and phases from computer-processed images. The projection map clearly shows modulations in density arising from the 5.1 A alpha-helical repeat, the first time this type of modulation has been revealed by electron microscopy. The crystals have p2 plane group symmetry with a = 92.4 A, b = 150.7 A, y = 92.4 degrees. Examination of tilted specimens shows that c is approximately 18 A, indicating that the unit cell is only one molecule thick. A preliminary interpretation shows tightly packed molecules some 400 A long lying with their long axes in the plane of the projection. The molecules have a coiled-coil configuration for most of their length. The possible modes of packing of the molecules in three dimensions are discussed.

Tropomyosin has discrete actin-binding sites with sevenfold and fourteenfold periodicities

Analysis of the periodic distribution of amino acids in tropomyosin has revealed the presence of seven or 14 quasi-equivalent actin-binding sites. We tested the hypothesis of periodic actin-binding sites by making deletions of chicken striated alpha-tropomyosin cDNA using oligonucleotide-directed mutagenesis. The deletions corresponded to one-half (amino acid residues 47 to 67), two-thirds (residues 47 to 74) and one actin-binding site (residues 47 to 88), on the basis of there being seven sites. The mutant cDNAs were expressed as fusion and non-fusion proteins in Escherichia coli and analyzed for actin binding and regulatory function. Fusion tropomyosin binds to actin with an affinity similar to that of muscle tropomyosin. Of the mutant fusion tropomyosins, only that with a full site deleted retained actin affinity and the ability to inhibit the actomyosin S1 ATPase, though it was less effective than wild-type. We conclude that an integral number of half-turns of the tropomyosin coiled-coil, and the consequential sevenfold periodicity, as well as the correct orientation of the ends with respect to each other, are important for actin binding. On the other hand, non-fusion tropomyosin binds well to actin only in the presence of troponin, and the binding is calcium-sensitive. Assay of non-fusion mutant tropomyosins showed that mutants with deletion of one-half and one actin binding site both had high affinity for actin, equal to or slightly less than wild-type. The ability of these two mutants to regulate the actomyosin or acto-S1 ATPase with troponin in the absence of calcium was indistinguishable from that of the wild-type. The normal regulatory function of the mutant with a 1/14 deletion (removal of a quarter turn or half a site) indicates that a 14-fold periodicity is adequate for regulation, consistent with the presence of two sets of seven alpha and seven beta quasi-equivalent actin-binding sites. An alternative explanation is that the alpha-sites are of primary importance and that proper alignment of the alpha-sites in every second tropomyosin, as when half a site is deleted, is sufficient for normal regulatory function. Deletion of a non-integral period (2/3 of a site) severely compromised actin-binding and regulatory function, presumably due to the inability of the mutant to align properly on the actin filament.

The effect of conformation on the CD of interacting helices: a theoretical study of tropomyosin

A recent report [M. E. Holtzer, et al. (1988) Biophysics Journal, 53, 96a] of the anomalous CD spectrum of the tropomyosin (TM) fragment 11TM127 motivated us to model the system as two 21-residue alpha-helices distorted to a coiled-coil conformation. We used strong-coupling exciton theory to model the optical properties of the system. Two backbone amide excited states (n pi* and pi pi*) were considered, as well as four excited states (Lb, La, Bb, Ba) for the phenolic side chain. We calculated the effect of superhelix formation on the backbone CD spectrum. The decrease in molar ellipticity of the alpha-helix parallel-polarized transition at 208 nm was found to be a simple function of superhelix tilt angle. We then modeled a coiled coil (radius = 5.5 A, pitch = -140 A) with one aromatic ring per superhelix. Steric interactions between aromatic side chains in a coiled coil were calculated as a function of side-chain conformation and heptet position. Steric interactions between phenolic rings will be significant for heptet positions a and d, but not for positions b, c, e, f, or g. We calculated the phenolic Lb transition rotational strength as a function of position within the heptet repeats, and of all possible side-chain dihedral angles, chi 1 and chi 2. When tyrosines were placed at heptet positions b, c, e, f, or g, the rotational-strength surface was nearly identical to that of a single tyrosine in an undistorted helix. In contrast, the rotational-strength surface for tyrosines in heptet positions a or d showed substantial intertyrosine coupling components. The rotational-strength surfaces for the three types of heptet positions (position a, position d, and the others) allowed an interpretation of the aromatic CD spectra of TM and its fragments. It was predicted that the three types of heptet positions will be spectroscopically distinguishable.

Location of the head-tail junction of myosin

The tails of double-headed myosin molecules consist of an alpha-helical/coiled-coil structure composed of two identical polypeptides with a heptad repeat of hydrophobic amino acids that starts immediately after a conserved proline near position 847. Both muscle and nonmuscle myosins have this heptad repeat and it has been assumed that proline 847 is physically located at the head-tail junction. We present two lines of evidence that this assumption is incorrect. First, we localized the binding sites of several monoclonal antibodies on Acanthamoeba myosin-II both physically, by electron microscopy, and chemically, with a series of truncated myosin-II peptides produced in bacteria. These data indicate that the head-tail junction is located near residue 900. Second, we compared the lengths of two truncated recombinant myosin-II tails with native myosin-II. The distances from the NH2 termini to the tips of these short tails confirms the rise per residue (0.148 nm/residue) and establishes that the 86-nm tail of myosin-II must start near residue 900. We propose that the first 53 residues of heptad repeat of Acanthamoeba myosin-II and other myosins are located in the heads and the proteolytic separation of S-1 from rod occurs within the heads.

The Drosophila melanogaster tropomyosin II gene produces multiple proteins by use of alternative tissue-specific promoters and alternative splicing

The structure of the Drosophila melanogaster tropomyosin II (TmII) gene has been determined by DNA sequencing of cDNA clones and the genomic DNA coding for the gene. Two overlapping transcriptional units produce at least four different tropomyosin isoforms. A combination of developmentally regulated promoters and alternative splicing produces both muscle and cytoskeletal tropomyosin isoforms. One promoter is a muscle-specific promoter and produces three different tropomyosin isoforms by alternative splicing of the last three 3' exons. The second promoter has the characteristics of a housekeeping promoter and produces a cytoskeletal tropomyosin isoform. Several internal exons along with a final 3' exon are alternatively spliced in the cytoskeletal transcript. The intron-exon boundaries of the TmII gene are identical to the intron-exon boundaries of all vertebrate tropomyosin genes reported, but are very different from the intron-exon boundaries of the D. melanogaster tropomyosin I gene. The TmII gene is the only reported tropomyosin gene that has two promoters and a quadruple alternative splice choice for the final exon. Models for the mechanism of D. melanogaster tropomyosin gene evolution are discussed.

Structural analysis of human skeletal beta-tropomyosin produced in E. coli

We have cloned the cDNA coding the beta-tropomyosin of human muscle in an expression vector whose expression depends upon a promotor that can be induced by isopropyl-beta-thiogalactopyranoside. We show that a new protein was synthesized by bacteria containing the engineered plasmid. This protein was heat stable and reacted with antibodies against tropomyosin. We have purified this protein and further identified it by determining its amino acid composition and sequencing the NH2 terminal. Unlike the native muscle tropomyosin, the NH2 terminal is not acetylated and contains a methionine. The circular dichroism spectrum is compatible with 100% alpha-helices. These results show that the protein synthesized in E. coli possesses a native structure.

Protein design, a minimalist approach

The question of how the amino acid sequence of a protein specifies its three-dimensional structure remains to be answered. Proteins are so large and complex that it is difficult to discern the features in their sequences that contribute to their structural stability and function. One approach to this problem is de novo design of model proteins, much simpler than their natural counterparts, yet containing sufficient information in their sequences to specify a given function (for example, folding in aqueous solution, folding in membranes, or formation of ion channels). Designed proteins provide simple model systems for understanding protein structure and function.

Segmented alpha-helical coiled-coil structure of the protein giardin from the Giardia cytoskeleton

The parasitic flagellate Giardia is the source of a filamentous protein, giardin, which binds to microtubules. The primary sequence of one giardin chain has been decoded from the base sequences of cDNAs isolated by antibody screens of a library constructed in the expression vector lambda gt11. The amino acid sequence favours a continuous alpha-helical fold for the protein without any inserts of a non-helical character. Analysis of apolar residue positions revealed 35 repeating heptads consistent with coiled-coil structure. This conformation relates giardin to the alpha-type fibrous proteins (k-m-e-f class) like tropomyosin and myosin (also found in Giardia). The giardin sequence has a regular series of skip residues like those at certain positions in the rod section of nematode myosin where the internal apolar seam of the coiled coil is shifted on the helix surface. The skips divide the giardin coil into quasi-equivalent structural segments about 4 nm in length, which might be domains for combining with tubulin subunits in the microtubule surface lattice.

Structure of tropomyosin-troponin T cocrystals

Crystals formed from a mixture of tropomyosin and troponin T have an open double-stranded lattice structure with a diamond-shaped repeat. In some regions the appearance in electron micrographs of negatively stained specimens changes from this double-diamond lattice to a more condensed banded crystal form. The double-diamond lattice has plane group symmetry cmm with unit cell 76.3 by 21.7 nm. The molecules form continuous chains along the diagonal of the unit cell and the diagonal length (79.4 nm) is that expected for two tropomyosin molecules joined end-to-end. Computer filtering of the micrographs shows that the strands of the lattice are thicker from the acute vertex of the large diamond to a point about half-way along the side of the diamond, where there is a small blob of density. At the acute vertex of the diamond is a large blob of density which is accentuated, however, by being at the lattice node where strands cross each other, and which is much weaker in regions of the micrographs where the crystals have condensed laterally. The results indicate that troponin T is a long thin molecule running in contact with the tropomyosin strands over 40-50% of the tropomyosin molecular length. The small globular region may represent the end-to-end overlap of tropomyosin but is more likely to be a globular region at the C-terminal region of troponin T.

An X-ray diffraction study of alpha-tropomyosin magnesium tactoid

The structure of the needle-shaped aggregate of alpha-tropomyosin formed in the presence of Mg2+ ions (the Mg-tactoid) was studied by X-ray diffraction. Orientated specimens were prepared by magnetic orientation. The meridional reflections corresponding to a Bragg spacing of up to 2.6 nm were recorded and phased by the isomorphous replacement method using p-chloromercuribenzoate bound to the unique cysteine residue of the alpha-chain of tropomyosin. The axial electron density profile thus obtained was compared with the model proposed from electron microscopic investigations. With an adequate phase combination for the observed intensities, the agreement was satisfactory. Comparison with electron micrographs of negatively stained Mg-tactoids suggests that the C-terminus of the molecule has an extended conformation and penetrates into the N-N overlap region. The principal repeat length along the tactoid was 39.0 nm, which was about 5% shorter than the expected periodicity of the tropomyosin molecules with an end-to-end overlap of eight residues, suggesting supercoiling. The equatorial reflections consisted of the diffuse peaks at 1/8 nm-1 and 1/2.3 nm-1. The former indicates, for the first time, the presence of a large structural unit with low crystallinity. The spacing of the latter probably corresponds to the average centre-to-centre distance between neighbouring tropomyosin molecules.

The Drosophila melanogaster tropomyosin II gene produces multiple proteins by use of alternative tissue-specific promoters and alternative splicing

The structure of the Drosophila melanogaster tropomyosin II (TmII) gene has been determined by DNA sequencing of cDNA clones and the genomic DNA coding for the gene. Two overlapping transcriptional units produce at least four different tropomyosin isoforms. A combination of developmentally regulated promoters and alternative splicing produces both muscle and cytoskeletal tropomyosin isoforms. One promoter is a muscle-specific promoter and produces three different tropomyosin isoforms by alternative splicing of the last three 3' exons. The second promoter has the characteristics of a housekeeping promoter and produces a cytoskeletal tropomyosin isoform. Several internal exons along with a final 3' exon are alternatively spliced in the cytoskeletal transcript. The intron-exon boundaries of the TmII gene are identical to the intron-exon boundaries of all vertebrate tropomyosin genes reported, but are very different from the intron-exon boundaries of the D. melanogaster tropomyosin I gene. The TmII gene is the only reported tropomyosin gene that has two promoters and a quadruple alternative splice choice for the final exon. Models for the mechanism of D. melanogaster tropomyosin gene evolution are discussed.

Lysine reactivities of tropomyosin complexed with troponin

The relative reactivities of lysine residues of tropomyosin complexed with troponin have been measured in order to locate the binding site of troponin on tropomyosin in a complex between the two native proteins. The lysines were labeled with acetic anhydride using a competitive labeling procedure and the relative reactivities of tropomyosin lysine containing peptides were compared to those from tropomyosin labeled in the absence of troponin (S. E. Hitchcock-DeGregori, S. F. Lewis, and T. M.-T. Chou, (1985) Biochemistry 24, 3305-3314). Analysis of about two-thirds of the lysines indicates that troponin affects the reactivities of lysines along the length of the tropomyosin, indicating long-range effects. The inferred binding site is more extensive than previously reported, about 25 nm, extending from res. 136 to the carboxy-terminus and to res. 30 beyond the end-to-end overlap in the amino-terminal region of the next tropomyosin molecule.

Force measurements by micromanipulation of a single actin filament by glass needles

Single actin filaments (approximately 7 nm in diameter) labelled with fluorescent phalloidin can be clearly seen by video-fluorescence microscopy. This technique has been used to observe motions of single filaments in solution and in several in vitro movement assays. In a further development of the technique, we report here a method to catch and manipulate a single actin filament (F-actin) by glass microneedles under conditions in which external force on the filament can be applied and measured. Using this method, we directly measured the tensile strength of a filament (the force necessary to break the bond between two actin monomers) and the force required for a filament to be moved by myosin or its proteolytic fragment bound to a glass surface in the presence of ATP. The first result shows that the tensile strength of the F-actin-phalloidin complex is comparable with the average force exerted on a single thin filament in muscle fibres during isometric contraction. This force is increased only slightly by tropomyosin. The second measurement shows that the myosin head (subfragment-1) can produce the same ATP-dependent force as intact myosin. The magnitude of this force is comparable with that produced by each head of myosin in muscle during isometric contraction.

Birefringence of tropomyosin crystals

The birefringence of tropomyosin crystals was measured in the temperature range 5 degrees-35 degrees C. The experimental results are compared with a simple model calculation based on the theory developed by Wiener for the optical properties of colloidal systems. The difference between experimental and theoretical values is less than 15%, which denotes a good agreement given the simplicity of the model. A value of 0.011 was obtained for the intrinsic birefringence of the tropomyosin molecule. The temperature dependence of the crystal birefringence could be accounted for in part by a change of the unit cell parameters; this change was experimentally observed by others in x-ray diffraction experiments.

Macromolecular changes accompanying immortalization and tumorigenic conversion in a human fibroblast model system

Mutagenesis of a diploid human fibroblast strain, KD, with the chemical carcinogen 4 nitroquinolin-1-oxide led to the isolation of stably immortalized neoplastic substrains. Four of these transformed strains, HuT-11, -12, -13, and -14, have been characterized in great detail with regard to morphology and changes in gene expression from the parental KD strain. The HuT-11, -12 and -13 substrains are immortalized and non-tumorigenic, in contrast to HuT-14 which is both immortalized and tumorigenic. The HuT-14 substrain expresses a defective beta-actin as a consequence of a point mutation in 1 of the 2 functional beta-actin alleles. All 4 HuT strains have induced expression of the phosphoprotein plastin and 2 EGF-related polypeptides, and down-regulated expression of the transformation-sensitive tropomyosin isoforms. KD and HuT cells expressing high levels of exogenous mutant beta-actin after gene transfection show morphological alterations. HuT-12 transfectants with excessive mutant beta-actin expression exhibit an elevated tumorigenic potential and tropomyosin-isoform switching characteristic of the tumorigenic HuT-14 strain.

The rat alpha-tropomyosin gene generates a minimum of six different mRNAs coding for striated, smooth, and nonmuscle isoforms by alternative splicing

Tropomyosin (TM), a ubiquitous protein, is a component of the contractile apparatus of all cells. In nonmuscle cells, it is found in stress fibers, while in sarcomeric and nonsarcomeric muscle, it is a component of the thin filament. Several different TM isoforms specific for nonmuscle cells and different types of muscle cell have been described. As for other contractile proteins, it was assumed that smooth, striated, and nonmuscle isoforms were each encoded by different sets of genes. Through the use of S1 nuclease mapping, RNA blots, and 5' extension analyses, we showed that the rat alpha-TM gene, whose expression was until now considered to be restricted to muscle cells, generates many different tissue-specific isoforms. The promoter of the gene appears to be very similar to other housekeeping promoters in both its pattern of utilization, being active in most cell types, and its lack of any canonical sequence elements. The rat alpha-TM gene is split into at least 13 exons, 7 of which are alternatively spliced in a tissue-specific manner. This gene arrangement, which also includes two different 3' ends, generates a minimum of six different mRNAs each with the capacity to code for a different protein. These distinct TM isoforms are expressed specifically in nonmuscle and smooth and striated (cardiac and skeletal) muscle cells. The tissue-specific expression and developmental regulation of these isoforms is, therefore, produced by alternative mRNA processing. Moreover, structural and sequence comparisons among TM genes from different phyla suggest that alternative splicing is evolutionarily a very old event that played an important role in gene evolution and might have appeared concomitantly with or even before constitutive splicing.

The rat alpha-tropomyosin gene generates a minimum of six different mRNAs coding for striated, smooth, and nonmuscle isoforms by alternative splicing

Tropomyosin (TM), a ubiquitous protein, is a component of the contractile apparatus of all cells. In nonmuscle cells, it is found in stress fibers, while in sarcomeric and nonsarcomeric muscle, it is a component of the thin filament. Several different TM isoforms specific for nonmuscle cells and different types of muscle cell have been described. As for other contractile proteins, it was assumed that smooth, striated, and nonmuscle isoforms were each encoded by different sets of genes. Through the use of S1 nuclease mapping, RNA blots, and 5' extension analyses, we showed that the rat alpha-TM gene, whose expression was until now considered to be restricted to muscle cells, generates many different tissue-specific isoforms. The promoter of the gene appears to be very similar to other housekeeping promoters in both its pattern of utilization, being active in most cell types, and its lack of any canonical sequence elements. The rat alpha-TM gene is split into at least 13 exons, 7 of which are alternatively spliced in a tissue-specific manner. This gene arrangement, which also includes two different 3' ends, generates a minimum of six different mRNAs each with the capacity to code for a different protein. These distinct TM isoforms are expressed specifically in nonmuscle and smooth and striated (cardiac and skeletal) muscle cells. The tissue-specific expression and developmental regulation of these isoforms is, therefore, produced by alternative mRNA processing. Moreover, structural and sequence comparisons among TM genes from different phyla suggest that alternative splicing is evolutionarily a very old event that played an important role in gene evolution and might have appeared concomitantly with or even before constitutive splicing.

Genetic rescue of muscle defects associated with a mutant Drosophila melanogaster tropomyosin allele

We describe a genetic transformation system which should prove useful for investigating tropomyosin assembly and function. Muscle abnormalities associated with a defective tropomyosin allele were corrected by integrating the wild-type gene into germ line chromosomes. The transformation protocol permits application of directed mutagenesis techniques in investigations of contractile regulatory mechanisms.

Three-dimensional structure of the surface protein of Desulfurococcus mobilis

The spherical cells of the thermophilic, sulfur-dependent archaebacterium Desulfurococcus mobilis are completely covered with a relatively poorly ordered, tetragonally arrayed surface protein. The structure of this surface protein was examined by using three-dimensional electron microscopy. The protein lattice forms an open meshwork composed of cross-shaped morphological units, which are released when glycerol is added. These subunits make contact at the distal ends of their four arms. The p4 symmetry requires that each of these morphological subunits represents a tetramer. The strong interaction of the monomers within the crosses and the relatively weak interaction of the intersecting arms of the crosses within the lattice structure suggest that the tetramers are assembled before their incorporation into the lattice.