The interaction of equine platelet tropomyosin with skeletal muscle actin

Tropomodulins and tropomyosins: working as a team

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

Functional diversity of actin cytoskeleton in neurons and its regulation by tropomyosin

Neurons comprise functionally, molecularly, and spatially distinct subcellular compartments which include the soma, dendrites, axon, branches, dendritic spines, and growth cones. In this chapter, we detail the remarkable ability of the neuronal cytoskeleton to exquisitely regulate all these cytoplasmic distinct partitions, with particular emphasis on the microfilament system and its plethora of associated proteins. Importance will be given to the family of actin-associated proteins, tropomyosin, in defining distinct actin filament populations. The ability of tropomyosin isoforms to regulate the access of actin-binding proteins to the filaments is believed to define the structural diversity and dynamics of actin filaments and ultimately be responsible for the functional outcome of these filaments.

Mg2+ ions bind at the C-terminal region of skeletal muscle alpha-tropomyosin

Tropomyosin (Tm) is a dimeric coiled-coil protein that polymerizes through head-to-tail interactions. These polymers bind along actin filaments and play an important role in the regulation of muscle contraction. Analysis of its primary structure shows that Tm is rich in acidic residues, which are clustered along the molecule and may form sites for divalent cation binding. In a previous study, we showed that the Mg(2+)-induced increase in stability of the C-terminal half of Tm is sensitive to mutations near the C-terminus. In the present report, we study the interaction between Mg(2+) and full-length Tm and smaller fragments corresponding to the last 65 and 26 Tm residues. Although the smaller Tm peptide (Tm(259-284(W269))) is flexible and to large extent unstructured, the larger Tm(220-284(W269)) fragment forms a coiled coil in solution whose stability increases significantly in the presence of Mg(2+). NMR analysis shows that Mg(2+) induces chemical shift perturbations in both Tm(220-284(W269)) and Tm(259-284(W269)) in the vicinity of His276, in which are located several negatively charged residues.

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

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

The third and fourth tropomyosin isoforms of Caenorhabditis elegans are expressed in the pharynx and intestines and are essential for development and morphology

The tropomyosin gene tmy-1/lev-11 of Caenorhabditis elegans spans 14.5 kb and encodes three isoforms by alternative splicing. To identify, characterize and compare the genome and tissue expression of a fourth isoform, the technique of rapid amplification of cDNA ends and microinjection with lacZ and gfp fusion plasmids were employed. We elucidated CeTMIV, a fourth isoform of tmy-1, which encoded a 256 residue polypeptide. CeTMIV isoform had a similar promoter region to CeTMIII isoform, but was alternatively spliced to generate a cDNA that differed in two exons. The tmy-1::lacZ and tmy-1::gfp fusion genes, with 3.2 kb promoter sequence and 1.1 kb of CeTMIV isoform specific exons, were expressed in the pharyngeal and intestinal cells. Further unidirectional deletion of the sequence located the primary promoter region 853 bp upstream from the initial codon. We show within the upstream region, the presence of B and C subelement-like sequences of myo-2, which may be used to stimulate pharyngeal expression. Despite the presence of a ges-1 like sequence, we were unable to locate the two GATA sites required for intestinal expression. Reassessing tissue expression for CeTMIII isoform with newly constructed fusion plasmids, we showed further expression in germ-line tissue and intestinal cells in addition to pharyngeal expression. Finally, to demonstrate that tropomyosin is essential for development, we inactivated the body wall and pharynx-specific isoforms by RNA-mediated interference. In addition to 50-75 % embryonic lethality in both cases, the worms that survived body wall interference had abnormal body morphology and uncoordinated movements, and those that survived pharynx interference had deformed pharynges and gut regions. These results show the function of tropomyosin in normal muscle filament assembly and embryonic development, and illustrate the different expression patterns characteristic of tropomyosin isoforms in C. elegans.

Protein interactions at the tight junction. Actin has multiple binding partners, and ZO-1 forms independent complexes with ZO-2 and ZO-3

Defining how the molecular constituents of the tight junction interact is a prerequisite to understanding tight junction physiology. We utilized in vitro binding assays with purified recombinant proteins and immunoprecipitation analyses to define interactions between ZO-1, ZO-2, ZO-3, occludin, and the actin cytoskeleton. Actin cosedimentation studies showed that ZO-2, ZO-3, and occludin all interact directly with F-actin in vitro, indicating that actin is engaged in multiple interactions at the tight junction. Low speed sedimentation analyses demonstrated that neither ZO-2, ZO-3, nor occludin act as F-actin cross-linking proteins, and further evidence indicates that these proteins do not bind to actin filament ends. The binding interactions of ZO-2, ZO-3, and occludin were corroborated in vivo by immunofluorescence colocalization experiments which showed that all three proteins colocalized with actin aggregates at cell borders in cytochalasin D-treated Madin-Darby canine kidney cells. Exploration of other tight junction protein interactions demonstrated that ZO-2 binds directly to both ZO-1 and occludin. Contrary to previous beliefs, our immunoprecipitation results indicate that ZO-1, ZO-2, and ZO-3 exist in situ primarily as independent ZO-1.ZO-2 and ZO-1.ZO-3 complexes rather than a trimeric ZO-1.ZO-2.ZO-3 grouping. These studies elucidate direct binding interactions among tight junction-associated proteins, giving insight into their organization as a multimolecular structure.

Reorganization of stress fiber-like structures in spreading platelets during surface activation

Alpha-Actinin and myosin were associated into reorganized actin cable networks and partly formed stress fiber-like structures in platelets during surface activation. Double-label immunofluorescence staining using antibodies against alpha-actinin and platelet myosin heavy chain (MHC) showed that alpha-actinin and myosin were colocalized in the cell center at the early stage of activation and dynamically redistributed with shape change. In the later stage, two proteins were colocalized around the granulomeres. alpha-Actinin was also seen beneath the surface membrane while myosin was not. Occasionally, both proteins were segregated, revealed granular staining in the cell body of flattened platelets and often aligned irregular alternate arrangement in the actin cables. Immunoelectron microscopy (immunogold) employing antibodies against MHC and myosin light chain (MLC) demonstrated that myosin, associated with actin cytoskeleton was precisely filamentous (328 nm in average length, 15 nm in width) and bipolar with a central bare zone, since MLCs were located at both ends. Myosin formed a cluster composed of several filaments with repeating alignment, suggesting each cluster corresponded to the granular staining pattern of immunofluorescence. These observations indicated that the organization of alpha-actinin and myosin in actin cables in activated platelets resembled that in stress fibers in various cultured cells.

Coagulation factor Va is an actin filament binding and cross-linking protein

Bovine coagulation cofactor factor Va is shown to bind to filament of skeletal muscle actin with a dissociation constant of 40-50 nM in the presence of 50 mM NaCl. At saturation, approximately one molecule of factor Va was bound for every two actin molecules. The binding of factor Va to F-actin was inhibited by increasing ionic strength, being approximately 20-fold weaker at 150 mM NaCl. Addition of factor Va dramatically increased both the low-speed sedimentation and the low-shear viscosity of actin filament solutions, indicating that factor Va cross-linkis actin filaments. Factor Va bound to actin filaments saturated with myosin. The isolated 74-kilodalton light chain of factor Va displayed actin binding and cross-linking properties indistinguishable from those of intact factor Va. The procofactor factor V bound weakly to F-actin, indicating that proteolytic activation is required to uncover the actin binding sites within the light chain domain. Actin filaments had only a slight inhibitory effect on the prothombinase activity of the factor Va-factor Xa-phospholipid complex. Since high concentrations of actin filaments can be exposed to the circulation when cells are damaged, the interaction of factor Va with actin may be of physiological relevance.

In vitro functional characterization of bacterially expressed human fibroblast tropomyosin isoforms and their chimeric mutants

At least eight tropomyosin isoforms (hTM1, hTM2, hTM3, hTM4, hTM5, hTM5a, hTM5b, and hTMsm alpha) are expressed from four distinct genes in human fibroblasts. In order to elucidate isoform properties, we have subcloned hTM3 and hTM5 full-length cDNAs, as well as their chimeric cDNAs into the bacterial expression pET8C system. Bacterially expressed tropomyosin isoforms (called PEThTM3, PEThTM5, PEThTM5/3, and PEThTM3/5) were purified and characterized. Under optimal binding conditions, the binding of PEThTM5 isoform to F-actin was stronger than the PEThTM3 isoform. However, analysis of actin-binding by the McGhee and von Hippel equation revealed that PEThTM3 exhibits higher cooperativity in binding than PEThTM5 does. Furthermore, the chimera PEThTM5/3 which possessed the N-terminal fragment of hTM5 fused to the C-terminal fragment of hTM3 had even stronger actin binding ability. The reverse chimera PEThTM3/5 which possessed the N-terminal fragment of hTM3 fused to the C-terminal fragment of hTM5 demonstrated greatly reduced affinity to actin filaments. In addition, both chimeras had different KCl requirements for optimal binding to F-actin than their parental tropomyosins. A bacterially made C-terminal fragment of human fibroblast caldesmon (PETCaD39) and native chicken gizzard caldesmon were both able to enhance the actin-binding of these bacterially expressed tropomyosins. However, PETCaD39's enhancement of binding to F-actin was greater for PEThTM5 than PEThTM3. Under 30 mM KCl and 4 mM MgCl2, the low M(r) isoform PEThTM4 appeared to be able to amplify the actin-activated HMM ATPase activity by 4.7 fold, while the high M(r) isoform PEThTM3 stimulated the activity only 1.5 fold. The higher enhancement of ATPase activity by PEThTM5 than by PEThTM3 suggested that the low M(r) isoform hTM5 may be more involved in modulating nonmuscle cell motility than hTM3. These results further suggested that different isoforms of tropomyosin might have finite differences in their specific functions (e.g., cytoskeletal vs. motile) inside the cell.

Histological distribution and developmental changes of tropomyosin isoforms in three chicken digestive organs

Histological localization of tropomyosin isoforms in three digestive organs from embryonic and adult chickens was performed by using rabbit antisera against chicken skeletal muscle tropomyosin and against low-Mr-type tropomyosin from chicken small intestine mucosa. The former antiserum (named TM-SH) reacted with alpha, beta, and high-Mr-type isoforms, and the latter (named TM-HL) reacted with alpha, beta, high-Mr-type and low-Mr-type isoforms, alpha and beta Isoforms were detected in muscle cells of the muscular layer and the muscularis mucosa. Low-Mr-type isoforms, however, were detected along the cell membrane and cytoplasm of almost all nonmuscle cells, especially in terminal webs of epithelial cells. Developmental changes of tropomyosin isoforms in digestive organs were studied by two-dimensional gel electrophoresis and image analysis. The relative amounts of alpha and beta isoforms increased in the course of development, but those of low-Mr-type and high-Mr-type isoforms decreased.

Tropomodulin binding to tropomyosins. Isoform-specific differences in affinity and stoichiometry

Tropomodulin is a human erythrocyte membrane cytoskeletal protein that binds to one end of tropomyosin molecules and inhibits tropomyosin binding to actin filaments [Fowler, V. M. (1990) J. Cell Biol. 111, 471-482]. We have characterized the interaction of erythroid and non-erythroid tropomyosins with tropomodulin by non-denaturing gel electrophoresis and by solid-phase binding assays using 125I-tropomyosin. Non-denaturing gel analysis demonstrates that all tropomodulin molecules are able to bind tropomyosin and that tropomodulin forms complexes with tropomyosin isoforms from erythrocyte, brain, platelet and skeletal muscle tissue. Scatchard analysis of binding data using tropomyosin isoforms from these tissues indicate that tropomodulin binds preferentially to erythrocyte tropomyosin. Specificity is manifested by decreases in the apparent affinity or the saturation binding capacity of tropomodulin for non-erythrocyte tropomyosins. Erythrocyte tropomyosin saturates tropomodulin at approximate stoichiometric ratios of 1:2 and 1:4 tropomyosin/tropomodulin (apparent Kd = 14 nM-1 and 5 nM-1, respectively). Brain tropomyosin saturates tropomodulin at a 1:2 ratio of tropomyosin/tropomodulin, but with a threefold lower affinity than erythrocyte tropomyosin. Platelet tropomyosin saturates tropomodulin at a tropomyosin/tropomodulin ratio of 1:4, but with a sevenfold lower affinity than erythrocyte tropomyosin at the 1:4 ratio. These results correlate with oxidative cross-linking data which indicate that tropomodulin can self-associate to form dimers and tetramers in solution. Since tropomodulin interacts with one of the ends of tropomyosin, varying interactions of tropomyosin isoforms with tropomodulin probably reflect the heterogeneity in N-terminal or C-terminal sequences characteristic of the different tropomyosin isoforms. Isoform-specific interactions of tropomodulin with tropomyosins may represent a novel mechanism for selective regulation of tropomyosin/actin interactions.

Structure and complete nucleotide sequence of the gene encoding rat fibroblast tropomyosin 4

We have isolated and determined the complete nucleotide sequence of the gene that encodes the 248 amino acid residue fibroblast tropomyosin, TM-4. The TM-4 sequence is encoded by eight exons, which span approximately 16,000 bases. The position of the intron-exon splice junctions relative to the final transcript are identical to those present in other vertebrate tropomyosin genes and the Drosophila melanogaster TMII gene. We have found no evidence that the rat TM-4 gene is alternatively spliced, unlike all the other tropomyosin genes from multicellular organisms that have been described. Typical vertebrate tropomyosin genes contain some, or all, of alternatively spliced exons 1a and 1b, 2a and 2b, 6a and 6b, and 9a, 9b, 9c and 9d in addition to common exons 3, 4, 5, 7 and 8. The rat fibroblast TM-4 mRNA is encoded by sequences most similar to exons 1b, 3, 4, 5, 6b, 7, 8 and 9d. Two exon-like sequences that are highly similar to alternatively spliced exons 2b and 9a of the rat beta-tropomyosin gene and the human TMnm gene have been located in the appropriate region of the gene encoding rat fibroblast TM-4. However, several mutations in these sequences render them non-functional as tropomyosin coding exons. We have termed these exon-like sequences, vestigial exons. The evolutionary relationship of the rat TM-4 gene relative to other vertebrate tropomyosin genes is discussed.

Tropomyosin-caldesmon/actomyosin systems in platelets and arterial smooth muscle: results from exchange experiments

Tropomyosin and caldesomon reciprocally control the actomyosin system in smooth muscle and some non-muscle cells. To compare this mechanism between arterial smooth muscle and platelets, we carried out extensive exchange experiments. Actin, myosin, tropomyosin from arterial smooth muscle cells and platelets were recombined and the effects of two species of caldesmon ('caldesmon77' and 'caldesmon140') on the ATPase activities of both systems were examined and analyzed by the method of analysis of variance. (a) The actomyosin system itself is different between artery and platelets, the difference being determined by myosin (P less than 0.05) and not by actin. (b) Platelet tropomyosin differentiates platelet actin from arterial actin (P less than 0.01), while arterial tropomyosin does not. Neither does tropomyosin differentiate myosin. (c) The effect of caldesmon77 differentiates the origins of myosin (P less than 0.01), actin (P less than 0.05) and tropomyosin (P less than 0.05). The effect of caldesmon140 differentiates the origin of myosin (P less than 0.05) and the actin-myosin 'interaction' (combination) (P less than 0.01), but not the origin of tropomyosin (P greater than 0.1). (1) It is concluded that actomyosin/tropomyosin-caldesmon system is distinguishable between platelets and artery. (2) It is suggested that caldesmon is an actomyosin inhibitor which may interact with myosin, in addition to actin and tropomyosin.

The spectrin-actin junction of erythrocyte membrane skeletons

High-resolution electron microscopy of erythrocyte membrane skeletons has provided striking images of a regular lattice-like organization with five or six spectrin molecules attached to short actin filaments to form a sheet of five- and six-sided polygons. Visualization of the membrane skeletons has focused attention on the (spectrin)5,6-actin oligomers, which form the vertices of the polygons, as basic structural units of the lattice. Membrane skeletons and isolated junctional complexes contain four proteins that are stable components of this structure in the following ratios: 1 mol of spectrin dimer, 2-3 mol of actin, 1 mol of protein 4.1 and 0.1-0.5 mol of protein 4.9 (numbers refer to mobility on SDS gels). Additional proteins have been identified that are candidates to interact with the junction, based on in vitro assays, although they have not yet been localized to this structure and include: tropomyosin, tropomyosin-binding protein and adducin. The spectrin-actin complex with its associated proteins has a key structural role in mediating cross-linking of spectrin into the network of the membrane skeleton, and is a potential site for regulation of membrane properties. The purpose of this article is to review properties of known and potential constituent proteins of the spectrin-actin junction, regulation of their interactions, the role of junction proteins in erythrocyte membrane dysfunction, and to consider aspects of assembly of the junctions.

Organization of the hTMnm gene. Implications for the evolution of muscle and non-muscle tropomyosins

We have isolated clones of human genomic DNA which contain the structural elements of the hTMnm gene. In non-muscle tissue this gene produces a 2.5 kb (1 kb = 10(3) bases or base-pairs) mRNA encoding TM30nm, a 248 amino acid cytoskeletal tropomyosin. In muscle, alternative splicing of this gene results in the expression of a 1.3 kb mRNA encoding a 285 amino acid skeletal muscle alpha-tropomyosin. The hTMnm gene spans at least 42 kb of DNA and consists of 13 exons, only five of which are common to both the 2.5 kb and 1.3 kb transcripts. The boundaries of the exons giving rise to the muscle-specific isoform are identical to the base to those of other genes encoding muscle tropomyosins. A comparison of the structures of exons encoding the amino-terminal sequences of the muscle and non-muscle isoforms suggests that the hTMnm gene has evolved by a specific pattern of exon duplication with alternative splicing.

Fluorescence of equine platelet tropomyosin labeled with acrylodan

Equine platelet tropomyosin was labeled with the sulfhydryl-specific fluorescent reagent 6-acryloyl-2-dimethylaminonaphthalene (acrylodan). The extent of labeling at 4 degrees C could be regulated between 0.5 and 1.3 acrylodans per tropomyosin chain by varying the reaction time from 1 to 4.5 h. Acrylodan-labeled platelet tropomyosin, AD-P-TM, was highly fluorescent, having an emission maximum near 518 nm on excitation at 365 nm. Steady-state measurements of polarization of the fluorescence of AD-P-TM in both low and high ionic strength solutions gave Perrin plots that exhibited sharp changes in slope near 50 degrees C, indicative of a sharp increase in mobility of the label at that temperature. This correlates with the melting temperature of the platelet tropomyosin coiled coil observed by circular dichroism [G. P. Côté, W. G. Lewis, M. D. Pato, and L. B. Smillie, (1978) FEBS Lett. 91, 237-241]. Perrin plots of carboxypeptidase A-treated platelet tropomyosin that was labeled with acrylodan after digestion resembled more closely those of acrylodan-labeled cardiac tropomyosin rather than those of AD-P-TM, suggesting that the observed emission arose from label at Cys-153 on each truncated platelet tropomyosin chain. In solutions containing 150 mM KCl and 5 mM MgCl2, addition of actin at up to a sixfold molar excess over AD-P-TM caused both the fluorescence emission intensities and fluorescence polarization values of samples to increase. In the presence of actin, the wavelength of maximal emission was shifted to shorter values by about 5 to 7 nm. These changes indicate that actin does bind to AD-P-TM and that the binding affects the environment of the label, both by making it more hydrophobic and by reducing the freedom of the label to tumble in solution.

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

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

Caldesmon specifically inhibits the effect of tropomyosin on actomyosin system in platelet

Caldesmon, a calmodulin and actin binding protein, has been shown to exist in platelet. In this report, it is shown that caldesmon specifically inhibits the effect of tropomyosin to enhance the actomyosin ATPase activity in platelet. Platelet tropomyosin enhances the MgATPase activity of platelet actomyosin. This effect is abolished by platelet caldesmon. In the absence of tropomyosin, however, caldesmon has no effect on the ATPase activity. The inhibition is not due to displacement of the binding of tropomyosin to F-actin by caldesmon. The result indicates that caldesmon is the specific inhibitor of tropomyosin in resting platelet.

Pig platelet tropomyosin: interactions with the other thin-filament proteins

Pig platelet tropomyosin exhibits many of the functional activities of skeletal tropomyosin. At low ionic strength it forms end-to-end aggregates similar to those formed by skeletal tropomyosins. It forms a 1:1 complex with muscle troponin or with a troponin I-pig brain calmodulin complex, as well as a 1:6 association with platelet filamentous actin. Electron microscopy of paracrystals shows that the troponin binding site is slightly C-terminal of the unique cysteine, corresponding to position 190 of the rabbit skeletal alpha-tropomyosin sequence. The effect of a complex comprising platelet actin and tropomyosin on the ATPase activity of rabbit skeletal muscle myosin subfragment-1 was similar to that displayed by its skeletal muscle counterpart. Platelet tropomyosin decreased the activity by roughly half in a calcium-independent manner. Addition of troponin to the actin-tropomyosin in the absence of calcium results in further inhibition and allows the full activity of the complex to be restored by Ca2+. These results differ from those obtained by Côté & Smillie for horse platelet tropomyosin and this may reflect the different isomeric nature of pig platelet tropomyosin. These results suggest that the functional properties of non-muscle tropomyosins may differ when comparisons are made between proteins isolated from the same type of cell but in different species. Differences in self-association and actin-binding properties may be finely graded between different isoforms.

Adrenal medullary tropomyosins: purification and biochemical characterization

Tropomyosins have been isolated from bovine adrenal medulla. Purified from a heat-stable extract, the adrenal medullary tropomyosins show the same chromatographic patterns as platelet tropomyosin components purified under very similar conditions on ion-exchange (DEAE-Sephacel) and hydroxylapatite columns. When analyzed by polyacrylamide gel electrophoresis, the purified fraction, reduced and denatured, yielded three polypeptides with apparent molecular weights of 38,000, 35,500, and 32,000. The molar ratio of the two major polypeptides (38 kd and 32 kd) was 2:1. The predominant form of 38 kd is different from other nonmuscle tropomyosins previously isolated and with which an apparent molecular weight of 30,000 is normally associated. The three adrenal medullary tropomyosins have similar isoelectric points of about 4.7. When adrenal tropomyosins were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the presence of 8 M urea, each form showed a shift to a higher molecular weight, which is a characteristic of muscle tropomyosin. The 38,000 adrenal medullary tropomyosin exhibits a stronger affinity for F-actin than the other forms. Peptide profiles obtained after limited proteolytic digestion show some similarity between the two predominant tropomyosins of the bovine adrenal medulla and also between these and the alpha and beta forms of bovine skeletal muscle tropomyosin.

Effect of ethanol on tropomyosin in solution and in reconstituted thin filaments

The effects of ethanol at concentrations below 10% on the conformation of tropomyosin, its end-to-end polymerization, its binding to F-actin, and its effects on actomyosin ATPase activity were studied. Ethanol stabilized the tropomyosin conformation by shifting the helix thermal unfolding profile to higher temperatures, and increased the end-to-end polymerization of tropomyosin. Ethanol-induced changes in the excimer fluorescence of pyrene-tropomyosin indicated that its conformation was stabilized by ethanol both free and bound to F-actin. Effects of tropomyosin and tropomyosin-troponin on actomyosin ATPase activity were measured under conditions for which tropomyosin binding to F-actin increases the activity. Under conditions for which the binding of tropomyosin to F-actin is optimum, in the presence of tropomyosin, the actomyosin ATPase activity decreased as the ethanol concentration increased, further indicating that ethanol induces a structural change in the tropomyosin-F-actin complex. Under conditions for which the binding of tropomyosin to F-actin is weak (low salt or high temperature), addition of ethanol increased the ATPase activity due to increased binding of tropomyosin to F-actin. Thus, ethanol appears to modify actomyosin ATPase activity by increasing the binding of tropomyosin to F-actin and affecting the structure of tropomyosin in the tropomyosin-F-actin filament.

Cytoskeletal organization affects cellular responses to cytochalasins: comparison of a normal line and its transformant

The relationships between cytoskeletal network organization and cellular response to cytochalasin D (CD) in a normal rat fibroblast cell line (Hmf-n) and its spontaneous transformant (tHmf-e), with markedly different cytoskeletal phenotypes, were compared (using immunofluorescence, electron microscopy, and DNAse I assay for actin content). Hmf-n have prominent, polar stress fiber (SF) arrays terminating in vinculin adhesion plaques whereas tHmf-e, which are apolar, epithelioid cells with dense plasma membrane-associated actin networks, lack SF and adhesion plaques. Hmf-n exposed to CD become markedly retracted and dendritic, SF-derived actin aggregates form large endoplasmic masses, and discrete tabular aggregates at the distal ends of retraction processes. Prolonged exposure leads to recession of process, cellular rounding, and development of large cystic vacuoles. tHmf-e cells exposed to similar doses of CD display a diagnostically different response; retraction is less drastic, cells retain broad processes containing scattered actin aggregates in discrete foci often associated with plasma membrane, large tabular aggregates are never found and processes persist throughout long exposure, vacuolation is uncommon. The CD-induced microfilamentous aggregates in Hmf-n are composed of short, kinky filament fragments forming a felt-like skein, often aggregates contain a more ordered array of roughly parallel fragments, while those of tHmf-e are very short, kinky, randomly orientated filaments imparting a distinctly granular nature to the mass. Total actin content and the amount of actin associated with detergent-resistant cytoskeletons increase following CD exposure in both cell types. Throughout exposure to CD, the actin-associated contractile proteins tropomyosin, myosin, and alpha-actinin co-localize within the actin aggregates in both cell types. Fodrin, the protein linking cortical actin to membrane, co-localizes with actin aggregates in tHmf-e cells and most, but not all, such aggregates in Hmf-n cells, consistent with their stress fiber derivation. Vinculin is lost from the tabular aggregates at the distal ends of retraction processes in Hmf-n cells concomitant with the fragmentation and contraction of SF. The aborized processes in both cells types contain strikingly similar axial cores of bundled vimentin filaments associated with passively compressed microtubules. The characteristic CD-induced distribution of actin filament aggregates and redistribution of vimentin in these cell types also occur when cells are allowed to respread from the rounded state in the presence of CD.

Isolation from bovine brain of tropomyosins that bind to actin filaments with different affinities

Tropomyosin was isolated from bovine brain using mild conditions thereby avoiding heat precipitation. Separation by DEAE ion exchange chromatography yielded a 33 kDa tropomyosin and a mixture of 30 and 32 kDa tropomyosin. Binding of the tropomyosins to actin filaments was measured by a newly developed method. The binding was assayed by the retarding effect of tropomyosin on actin polymerization. The 33 kDa tropomyosin was found to bind to actin filaments with considerably higher affinity than the 30 and 32 kDa tropomyosin.

Enhancement of actomyosin ATPase activity by tropomyosin. Recombination of myosin and tropomyosin between muscles and platelet

In skeletal muscle, the physiological role of tropomyosin has been assumed to be the 'blocking' of the actin-myosin interaction. In smooth muscle and platelet, however, tropomyosin was shown to 'enhance' the interaction. To investigate the reason for this apparent contradiction, we carried out recombination experiments using reconstituted actomyosins and different tropomyosins. Tropomyosins from skeletal muscle, arterial smooth muscle and platelet were recombined with skeletal, arterial and platelet myosins. The effects of tropomyosins on the actin-activated ATPase activities of myosins were then examined. The results are as follows. (i) Although tropomyosins from artery and platelet are distinctively different in molecular weight, they are interchangeable in enhancing the ATPase activities of both arterial and platelet actomyosins. The enhancement, however, is reduced by increasing the concentration of Mg X ATP and decreasing the concentration of myosin. (ii) Arterial and platelet tropomyosins are not capable of inhibiting the ATPase activity of skeletal actomyosin. (iii) Skeletal tropomyosin enhances arterial and platelet actomyosin ATPase activities in the same way as arterial and platelet tropomyosins. The results indicate that the major determinant of the effect of tropomyosin on the actomyosin-ATPase activity is the state of actomyosin. We suggest that any tropomyosin enhances the actin-activated ATPase activity of myosin recombined with skeletal actin, under the condition where actin and myosin form a 'rigor' (tight) complex.

Mechanism of action of the platelet aggregation inhibitor purified from Agkistrodon halys (mamushi) snake venom

The platelet aggregation inhibitor purified from Agkistrodon halys snake venom inhibited rabbit platelet aggregations induced by thrombin, sodium arachidonate, collagen or ionophore A-23187. The IC50 was about 11 micrograms/ml in platelet aggregation regardless of which aggregation inducer was used. beta-Mercaptoethanol abolished both the phospholipase A enzymatic and platelet aggregation inhibitory activities of this venom inhibitor. p-Bromophenacyl bromide-treated venom inhibitor lost almost completely its phosphilipase A enzymatic activity, but retained its platelet aggregation inhibitory effect. In the presence of EGTA, the venom inhibitor still showed the same inhibitory activity on thrombin-, sodium arachidonate-, collagen- or ionophore A23187-induced platelet aggregations triggered by successive addition of Ca2+. The activation of platelet phospholipase A and the serotonin release reaction triggered by Ca2+ influx were unaffected by this venom inhibitor. It also inhibited the clot retraction of platelet-rich plasma. It is concluded that the inhibitory effect of the venom inhibitor on platelet aggregation is independent of its phospholipase A enzymatic activity. Its mode of action is different from those of other known platelet inhibitory drugs. This venom inhibitor possibly acts on a common step subsequent to platelet shape change, leading to inhibition of platelet aggregation.

Thin filament proteins and thin filament-linked regulation of vertebrate muscle contraction

Recent developments in the field of myofibrillar proteins will be reviewed. Consideration will be given to the proteins that participate in the contractile process itself as well as to those involved in Ca-dependent regulation of striated (skeletal and cardiac) and smooth muscle. The relation of protein structure to function will be emphasized and the relation of various physiologically and histochemically defined fiber types to the proteins found in them will be discussed.

Structural and functional properties of the non-muscle tropomyosins

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

A processed gene defining a gene family encoding a human non-muscle tropomyosin

We have isolated and characterized a human genomic DNA sequence that defines a family of closely related sequences. At least one member of this family expresses a 2.5 X 10(3) base messenger RNA transcript encoding a 30,000 molecular weight tropomyosin in human fibroblasts. The coding sequence of this mRNA but not the non-coding sequence is also related to that of a 1.1 X 10(3) base mRNA encoding a 36,000 molecular weight non-muscle tropomyosin. This demonstrates the existence of at least two functional genes encoding human non-muscle tropomyosins.

Identification of a troponin-I like protein in platelet preparations as histone H2B

A tropomyosin-binding protein (app. Mr 17000) was detected in equine platelet preparations by a gel overlay technique. Its isolation, amino acid and partial sequence analyses have shown it to be histone H2B. As with a similar protein from pig platelet preparations [der Terrossian et al. (1983) FEBS Lett. 152, 202-206], it inhibits Mg2+-dependent actomyosin S1 ATPase. This inhibition is partially reversed in the presence of calmodulin and Ca2+ but is not potentiated, unlike troponin-I, by tropomyosin. This protein, along with the other histones, is almost certainly derived from a low level of contaminating nucleated cells in most platelet preparations.

Some functional properties of nonpolymerizable and polymerizable tropomyosin

The binding of 125I-labelled nonpolymerizable (brain or carboxypeptidase A-treated skeletal muscle) and polymerizable (intact skeletal muscle) tropomyosin to muscle F-actin was studied by ultracentrifugation under various conditions. The amount of nonpolymerizable tropomyosin bound to F-actin both in 0.1 M KCl and in 7 mM MgCl2 was much lower than that of the polymerizable one. In the presence of MgCl2 the amount of nonpolymerizable tropomyosin bound to F-actin approached saturation level. Under these conditions, however, the amount of skeletal muscle tropomyosin bound exceeded saturation, suggesting formation of both head-to-tail polymers and side-to-side aggregates. The latter seems to be responsible for the inhibition of acto-heavy meromyosin ATPase activity which is caused by skeletal muscle tropomyosin but not by nonpolymerizable tropomyosin. Nonpolymerizable tropomyosin can substitute for the rabbit skeletal muscle tropomyosin in the regulatory system operating in skeletal muscle. Inhibition of ATPase activity of acto-heavy meromyosin by nonpolymerizable tropomyosin in the presence of troponin and the absence of calcium ions is less than that obtained with polymerizable tropomyosin. The inhibition of ATPase activity is directly correlated with the extent of binding of nonpolymerizable tropomyosin to F-actin under the conditions of the ATPase assay.