A tropomyosin-like protein from human platelets

An electrophoretic variant of a human fibroblast protein with characteristics of smooth muscle tropomyosin

A charge variant of a protein (Tm: 3; molecular weight approximately 35,000) that co-migrates with human smooth muscle tropomyosin has been found in whole cell extracts from the fibroblasts of a father and his son. The variant protein co-purifies with Tm: 3, shifts with it to a higher apparent molecular weight on sodium dodecyl sulfate/polyacrylamide gel electrophoresis in the presence of 4 M-urea, is a component of the microfilaments, and has a peptide cleavage pattern identical to that of Tm: 3 and smooth muscle tropomyosin. The results indicate that Tm: 3 and the variant (Tm: 3.1) are smooth muscle tropomyosin, suggesting that normal human fibroblasts synthesize at least two tropomyosins (Tm:3 and the non-muscle tropomyosin Tm:4, molecular weight approximately 30,000) and that they are the products of separate genes.

Identification of a cytoplasmic tropomyosin gene linked to two muscle tropomyosin genes in Drosophila

A Drosophila cytoplasmic tropomyosin gene has been identified and is located on the same genomic DNA clone as two Drosophila muscle tropomyosin genes previously identified. A positive hybrid-selection translation assay using the subcloned gene and RNA from non-muscle cell sources yielded a protein with a size (Mr, 31,000) and isoelectric point (5.0) similar to vertebrate cytoplasmic tropomyosin. A modified protocol for the purification of vertebrate cytoplasmic tropomyosin was used to partially purify cytoplasmic tropomyosin from the Drosophila Kc cell line. The Kc cell protein was identified as a cytoplasmic form of tropomyosin on the basis of its size, isoelectric point, and crossreactivity with a polyclonal vertebrate antitropomyosin antibody in a two-step binding assay. The Kc cell cytoplasmic tropomyosin comigrates in two dimensions with the hybrid-selected in vitro translation product of the region 3 gene, and both proteins show a mobility shift in NaDodSO4/urea/polyacrylamide gels that is characteristic of vertebrate tropomyosins. The cytoplasmic tropomyosin gene hybridizes to both Drosophila muscle tropomyosin genes under decreased stringency conditions. This cross-hybridization spans several internal restriction endonuclease sites in each muscle tropomyosin gene and indicates an overall partial homology among the three Drosophila tropomyosin genes. These results show that Drosophila tropomyosins are encoded by a closely linked family of differentially regulated genes.

Morphological study of early phases of platelet adhesion to foreign surfaces: effect of calcium

The role of calcium in the interfacial reactions occurring during the early phases of blood contact with glass surfaces has been studied using a rheologic technique that permits enumeration of adherent platelets as well as ultrastructural visualization of the blood-glass interface by both transmission and scanning electron microscopy . In the presence of calcium, platelets adhere to the glass surface at a rapid rate. Within 4 minutes, most of the adherent platelets have lost their normal discoid shape and show pseudopod formation and cytoplasmic spreading. These platelets conform with the glass surface and show central apposition of platelet organelles similar to that seen in activated platelets. Fibrin strands and platelet aggregates are also seen. Chelation of calcium using sodium citrate or EDTA results in decreased platelet adhesion or retention to glass. Platelets that are adherent to glass in the presence of citrate and EDTA retain their discoid shape, although an occasional platelet may show pseudopod formation and centralization of organelles. Calcium appears to be an important factor governing the adhesion of platelets to glass and appears to exert this effect on the platelets themselves in their plasma milieu and not on the preceding adsorption of plasma proteins.

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.

Amino acid sequence of equine platelet tropomyosin. Correlation with interaction properties

Equine platelet beta tropomyosin (247 residues), like rabbit skeletal muscle alpha tropomyosin (284 residues) has a repeating pattern of amino acid residues characteristic of a coiled-coil structure. When compared with the muscle protein, it is extended by 5 residues at the NH2-terminus and possesses two 21 residue deletions (positions 23-43 and 60-80 of the muscle sequence). The two proteins are highly conserved from residues 81-260, but are significantly different at their COOH-termini (residues 261-284). These differences in platelet tropomyosin can be correlated with its diminished head-to-tail polymerization, a weaker interaction with F-actin and a reduced affinity for muscle troponin and the T1 fragment of troponin-T.

Purification of a troponin I-like factor from pig platelet

A troponin I-like factor has been purified from pig platelet by G150 Sephadex filtration of a low ionic strength extract, acidification at pH 4.2, ion exchange on DE-52 cellulose, and affinity chromatography on calmodulin-Sepharose. This protein (Mr 17000), together with pig brain calmodulin and platelet tropomyosin, is able to participate to the reconstitution in vitro of a thin filament-like complex which modulates with 55% calcium sensitivity the platelet actin-activated Mg2+-dependent ATPase activity of rabbit skeletal muscle myosin.

Novel form of non-muscle tropomyosin in human fibroblasts

The cytoskeletal extracts of cultured human fibroblasts were found to contain at least four distinct polypeptides, each of which demonstrated the resistance to denaturation and the acidic isoelectric point characteristic of tropomyosin. One of these, hscp 36 (heat-stable cytoskeletal protein having an apparent molecular weight of 36,000), cross-reacted efficiently with an antiserum to chicken skeletal muscle tropomyosin. Furthermore, the messenger RNA coding for hscp 36 was selected by a chicken complementary DNA clone containing a tropomyosin sequence. The abundance of mRNA coding for hscp 36 was found to be similar in both normal and simian virus 40 (SV40) transformed human fibroblasts. The apparent molecular weight of hscp 36 is different from non-muscle tropomyosins previously isolated from human sources, which show the apparent molecular weight of 30,000 normally associated with non-muscle tropomyosin. This, together with the complexity of the heat-stable cytoskeletal proteins present in human fibroblasts, suggests the existence of multiple genes coding for human non-muscle tropomyosins.

The effect of cytochalasin and glutaraldehyde on F-actin filaments containing muscle and non-muscle tropomyosin

F-actin filaments are disrupted by the action of cytochalasin and glutaraldehyde. Muscle tropomyosin which is able to polymerize can protect F-actin against fragmentation caused by these two agents. This protective effect does not occur with nonpolymerizable, brain or carboxy-peptidase A-treated skeletal muscle tropomyosins. The protection of F-actin against the action of cytochalasin and glutaraldehyde takes place under conditions where the F-actin filaments are saturated with tropomyosin, that is, at a molar ratio of tropomyosin to actin of 1:7. It is suggested that nonpolymerizable tropomyosin lacks the protective ability because its binding to F-actin is considerably weaker than the polymerizable tropomyosin and does not saturate all of the binding sites on F-actin.

Three-dimensional organization of the platelet cytoskeleton during adhesion in vitro: observations on human and nonhuman primate cells

Adhesion of platelets in vitro resulted in rapid polymerization of the amorphous cytoplasmic ground substance into an organized cytoskeletal superstructure. This cytoskeleton, characterized through the use of whole-mount and stereo (3-D), high-voltage microscopy in conjunction with morphometrics and cytochemistry, comprised four major size classes of filaments organized in distinctive zones. The central matrix, or granulomere, at the center of the cell mass, was an ill-defined meshwork of 80-100-A filaments which enshrouded granules, dense bodies, and elements of the dense tubular system as identified through peroxidase cytochemistry. Demarcating this central matrix was a trabecular zone containing 30-50, 80-100, and 150-170 A filaments in an open and rigid-appearing lattice. Circumscribing the trabecular zone and extending to the margins of the hyalomere was the third region, the peripheral web, in which 70-A filaments were arranged in a tight honeycomb lattice. This organizational pattern was retained in cytoskeletons prepared by Triton x-100 extraction of the adherent cells, and was observed in basally located cells of aggregates which formed subsequent to adhesion. Our observations are consistent with biochemical studies of cytoskeletons prepared from suspended platelets and suggest a contractile protein composition for the superstructure during adhesion.

Purification, properties, and immunohistochemical localisation of human brain 14-3-3 protein

A protein has been purified from human brain that appears to be the human equivalent of bovine 14-3-3 protein. On polyacrylamide gel electrophoresis the protein migrates as a faster major component, termed 14-3-3-2 protein, and a slower minor component, termed 14-3-3-1 protein, which consists of approximately 12% of the total protein. Both 14-3-3-1 and 14-3-3-2 have a native molecular weight of approximately 67,000. 14-3-3-2 appears to have the subunit composition alpha beta; 14-3-3-1 has the composition beta'beta'. Peptide mapping with Staphylococcus aureus V8 proteinase shows that alpha and beta subunits are unrelated but the beta and beta' subunits show some common peptides. Immunoperoxidase labelling shows that 14-3-3 is localised in neurones in the human cerebral cortex. 14-3-3 shows no enolase, creatine kinase, triose phosphate isomerase, ATPase, cyclic nucleotide-dependent protein kinase, or purine nucleoside phosphorylase activity. 14-3-3 does not bind calcium and does not appear to be related to calmodulin, calcineurin, tubulin, neurofilament proteins, clathrin-associated proteins, or tropomyosin. The functional significance of this neuronal protein remains obscure.

Properties of tropomyosin from the dual-regulated obliquely striated body wall muscle of the earthworm (Lumbricus terrestris L.)

The obliquely striated body wall muscle of the earthworm Lumbricus terrestris L. possesses a dual actin-linked and myosin-linked regulatory system. Tropomyosin from this muscle has now been purified and its functional properties compared to tropomyosin from vertebrate skeletal muscle. Earthworm tropomyosin has a molecular weight of about 70 000 and is composed of two polypeptide chains of molecular weight of 34 000 and 37 000. Structural and functional similarities to skeletal muscle tropomyosin were demonstrated with respect to the formation and periodicity of paracrystals and nets and the potentiation of skeletal muscle acto-SF1 ATPase activity at low ATP concentration. Likewise, earthworm tropomyosin inhibited skeletal muscle acto-HMM ATPase activity at normal ATP concentrations but to a much greater extent than skeletal muscle tropomyosin; this inhibition was removed by skeletal muscle troponin, in the presence of Ca2+. In a system containing earthworm myosin and skeletal muscle actin, earthworm tropomyosin had no detectable influence on the actin-activated ATPase activity. It is concluded that earthworm tropomyosin plays an active role in the actin-linked troponin-dependent regulatory system and has no measurable effect on the regulation via myosin.

Muscle protein analysis by two-dimensional gel electrophoresis

Two-dimensional electrophoresis was first applied to the analysis of muscle proteins in 1976 when the occurrence of multiple forms of actin was discovered. Since that time the technique has become widely accepted as a new approach to studies of myogenesis, muscle differentiation, and muscle pathology. In addition, two-dimensional electrophoresis now is being used to investigate contractile proteins present in nonmuscle cells. This review will discuss, in general, the technique of two-dimensional electrophoresis in polyacrylamide gels which combines isoelectric focusing and sodium dodecyl sulfate electrophoresis. The application of the technique specifically to muscle protein analysis will be discussed through a review of existing literature on two-dimensional electrophoresis of cultured muscle cells and tissue homogenates. Attention will be given to contractile protein heterogeneities such as alpha, beta, and gamma actin and the embryonic forms of myosin light chains, all discovered through the use of two-dimensional electrophoresis. New information concerning gene expression during muscle differentiation revealed by differences in two-dimensional electrophoresis protein patterns and the use of two-dimensional electrophoresis for studying human muscle pathology through analysis of tissue biopsies will also be discussed.

Hybrids of Physarum myosin light chains and desensitized scallop myofibrils

The two light chains of Physarum myosin have been purified in a 1:1 ratio with a yield of 0.5-1 mg/100 g of plasmodium and a purity of 40-70%; the major contaminant is a 42,000-dalton protein. The 17,700 Mr Physarum myosin light chain (PhLC1) binds to scallop myofibrils, providing the regulatory light chains (ScRLC) have been removed. The 16,500 Mr light (PhLC2) does not bind to scallop myofibrils. The calcium control of scallop myosin ATPase is lost by the removal of one of the two ScRLC's and restored equally well by the binding of either PhLC1 or rabbit skeletal myosin light chains. When both ScRLC's are removed, replacement by two plasmodial light chains does not restore calcium control as platelet or scallop light chains do. Purified plasmodial actomyosin does not bind calcium in 10(-6) M free calcium, 1 mM MgCl2. No tropomyosin was isolated from Physarum by standard methods. Because the Physarum myosin light chains can substitute only partially for light chains from myosin linked systems, because calcium does not bind to the actomyosin, and because tropomyosin is apparently absent, the regulation of plasmodial actomyosin by micromolar Ca++ may involve other mechanisms, possibly phosphorylation.

Construction of bacterial plasmids containing sequences complementary to chicken alpha-tropomyosin mRNA

Recombinant plasmids have been constructed with contain sequences complementary to the mRNA coding for skeletal muscle alpha-tropomyosin. These recombinants were detected initially using a selective cDNA probe and subsequently using a messenger RNA selection assay. alpha-TM plasmids hybridize to a singly mRNA species smaller than 18S ribosomal RNA and found only in skeletal muscle. Cross-hybridization with mRNA's coding for other tropomyosins could not be detected under normal conditions. However, under conditions of reduced stringency alpha- TM plasmids cross-hydridize with an RNA species in heart muscle which may code for cardiac tropomyosin.

Platelet calcium-dependent proteins: identification and localization of the calcium-dependent regulator, calmodulin, in platelets

The calcium-dependent regulator protein, calmodulin, is a 17,000 molecular weight polypeptide which binds calcium and has been shown to confer calcium sensitivity on contractile and other proteins. In the present study, we have examined the presence and subcellular distribution of this protein in preparations of human platelets. Calmodulin was quantified using a two-stage phosphodiesterase assay. Whole platelets contained 1.33 +/- 0.06 units calmodulin per 10(6) platelets or 26.5 +/- 3.4 fg calmodulin per platelet. The distribution of calmodulin in the platelet was predominantly soluble with over 80 percent of calmodulin activity in the soluble fraction of the cell. There was no apparent difference in the distribution of calmodulin between soluble and particulate compartments in recalcified platelet homogenates compared to homogenates in EDTA. Indirect immunofluorescent studies with monospecific antisera to dinitrophenylated calmodulin showed intense staining of platelets in a diffuse pattern. The identification of calmodulin in platelets raises the possibility that this protein may participate in calcium-dependent reactions important in platelet aggregation and release.

Microfilaments and tropomyosin of cultured mammalian cells: isolation and characterization

Microfilaments were isolated from cultured mammalian cells, utilizing procedures similar to those for isolation of "native" thin filaments from muscle. Isolated microfilaments from rat embryo, baby hamster kidney (BHK- 21), and Swiss mouse 3T3 cells appeared structurally similar to muscle thin filaments, exhibiting long, 6 nm Diam profiles with a beaded, helical substructure. An arrowhead pattern was observed after reaction of isolated microfilaments with rabbit skeletal muscle myosin subfragment 1. Under appropriate conditions, isolated microfilaments will aggregate into a form that resembles microfilament bundles seen in situ cultured cells. Isolated microfilaments represent a complex of proteins including actin. Some of these components have been tentatively identified, based on coelectrophoresis with purified proteins, as myosin, tropomyosin, and a high molecular weight actin-binding protein. The tropomyosin components of isolated microfilaments were unexpected; polypeptides comigrated on SDS-polyacrylamide gels with both muscle and nonmuscle types of tropomyosin. In order to identify more specifically these subunits, we isolated and partially characterized tropomyosin from three cell types. BHK-21 cell tropomyosin was similar to other nonmuscle tropomyosins, as judged by several criteria. However, tropomyosin isolated from rate embryo and 3T3 cells contained subunits that comigrated with both skeletal muscle and nonmuscle types of myosin, whereas the BHK cell protein consistently contained a minor muscle-like subunit. The array of tropomyosin subunits present in a cell culture was reflected in the polypeptide chain pattern seen on SDS-polyacrylamide gels of microfilaments isolated from that culture. These studies provide a starting point for correlating changes in the ultrastructural organization of microfilaments with alterations in their protein composition.

Effect of the state of oxidation of cysteine 190 of tropomyosin on the assembly of the actin-tropomyosin complex

Tropomyosin, cross-linked at cysteine 190, was found to bind more weakly to actin filaments than uncross-linked tropomyosin. Cross-linking of tropomyosin can cause actin filaments nearly completely covered with tropomyosin to be uncovered almost completely. The critical monomer concentration of actin is not significantly changed by binding of cross-linked or uncross-linked tropomyosin to actin filaments. The binding curves were analyzed quantitatively, thereby taking into account the polar end-to-end contact of tropomyosin molecules bound by actin and the overlap of the seven subunit binding sites along the actin filament. Under the conditions of the experiment (80 mM KCl, 1 mM MgCl2, pH 7.5, 38-42 degrees C), the equilibrium constant for isolated binding of tropomyosin to actin filaments is in the range 1 x 10(3)-3 x 10(3) M-1. The equilibrium constants for binding of tropomyosin to binding sites along the actin filament with one or two neighbouring tropomyosin molecules are in the range of 10(6) or 10(8) to 10(9) M-1, respectively. The equilibrium constants for binding of tropomyosin to binding sites along the actin filament with one or two neighbouring tropomyosin molecules are in the range of 10(6) or 10(8) to 10(9) M-1, respectively. The equilibrium constants for cross-linked and uncross-linked tropomyosin differ by a factor of only about two. Owing to the highly cooperative binding, these differences are sufficient so that actin filaments nearly completely covered with uncross-linked tropomyosin are uncovered almost completely by cross-linking tropomyosin at cysteine 190.

Distribution of fluorescently labeled alpha-actinin in living and fixed fibroblasts

The distribution of flourescently labeled alpha-actinin after microinjection into fibroblasts has been determined in both living and fixed cells. We have found that the distribution of the injected tetramethylrhodamine isthiocyanate-labeled protein (TMRITC-alpha-actinin) in living cells, which is in ruffling membranes, actin microfilament bundles, and polygonal microfilament networks (Feramisco, 1979, Proc. Natl. Acad. Sci. U. S. A. 76:3967-3971), was virtually unaffected by the fixation (3.5 percent formaldehyde) and extraction (absolute acetone) used for the preparation of the cells for immunoflourescence. Also, these patterns were found to coincide with the alpha-actinin revealed by immunoflourescence. Also, these patterns were found to coincide with the alpha-actinin revealed by immunoflourescence. These findings offer, for the first time, evidence indicating the validity of the immunoflourescence technique in the localization of alpha-actinin in cultured cells. With the combination of the injection procedure and the immunoflourescence localization of endogenous structural proteins, it was determined that nearly all of the actin stress fibers were decorated in a periodic manner with the injected alpha-actinin. Endogenous tropomyosin in the injected cells was found to be distributed with a periodic pattern along the stress fibers that was antiperiodic to the pattern observed for the microinjected alpha-actinin. The tropomyosin antibody stained the polygonal microfilament networks and was excluded from the foci, whereas the microinjected alpha-actinin was incorporated into the foci of the networks. Thus, the microinjected fluorescent derivative of alpha-actinin appears to be incorporated into the functional pools of alpha-actinin within the living cell and to be utilized by the cell with fidelity.

Tropomyosin-like seven residue periodicity in three immunologically distinct streptococal M proteins and its implications for the antiphagocytic property of the molecule

Partial sequences of three immunologically distinct group A streptococcal M proteins (M5, M6, and M24) revealed significant homology with each other, certain amino acid residues being conserved within the three molecules. In addition, a common feature of the sequenced regions of these M proteins was their high alpha-helical potential and the presence of a repeating seven residue periodicity that is characteristic of the double helical coiled-coil molecule, tropomyosin. The existence of a tropomyosin-like seven residue periodicity strongly suggests that regions of these three M proteins may participate in intra- and/or intermolecular coiled-coil interactions. Because of the constraints imposed by such a repeating periodicity, certain conserved residues within the M proteins would occupy spatially equivalent positions in the tertiary structure of these molecules. This common characteristic could play an important role in the common antiphagocytic property of the immunologically diverse M molecules. In addition to similarities in the secondary structure of M proteins and tropomyosin, significant sequence homology has also been observed between certain regions of these molecules with up to 50% identical residues. As a result of the striking structural similarity with tropomyosin, M proteins may play a regulatory role in the contractile mechanisms involved in phagocytosis.

Isolation of human platelet and red blood cell plasma membrane proteins by preparative detergent electrophoresis

High resolution polyacrylamide gel electrophoretic techniques have been applied to the preparative isolation and analysis of plasma membrane proteins and glycoproteins from human platelets and red blood cells. The techniques presented allow relatively simple, direct, rapid and quantitative purification of a broad molecular weight range of membrane proteins, by means of continuous elution preparative gel electrophoresis of protein solubilized with sodium dodecyl sulfate. Spectrophotometric and fluorophotometric (fluorescamine) profiling, and high resolution gel electrophoretic analysis (SDS-acrylamide gradient slab gels, and gel electrofocusing) of eluted protein species indicate that purified membrane proteins of a broad molecular weight range may be obtained in a one step procedure, and in quantities and concentrations sufficient for further analytical or experimental procedures.