Tropomyosin: a new asymmetric protein component of the muscle fibril

Role of tropomyosin in smooth muscle contraction: effect of tropomyosin binding to actin on actin activation of myosin ATPase

The binding of gizzard tropomyosin to gizzard F-actin is highly dependent on free Mg2+ concentration. At 2 mM free Mg2+, a concentration at which actin-activated ATPase activity was shown to be Ca2+ sensitive, a molar ratio of 1:3 (tropomyosin:actin monomer) is required to saturate the F-actin with tropomyosin to the stoichiometric ratio of 1 mol of tropomyosin to 7 mol of actin monomer. Increasing the Mg2+ could decrease the amount of tropomyosin required for saturating the F-actin filament to the stoichiometric level. Analysis of the binding of smooth muscle tropomyosin to smooth muscle actin by the use of Scatchard plots indicates that the binding exhibits strong positive cooperativity at all Mg2+ concentrations. Calcium has no effect on the binding of tropomyosin to actin, irrespective of the free Mg2+ concentration. However, maximal activation of the smooth muscle actomyosin ATPase in low free Mg2+ requires the presence of Ca2+ and stoichiometric binding of tropomyosin to actin. The lack of effect of Ca2+ on the binding of tropomyosin to actin shows that the activation of actomyosin ATPase by Ca2+ in the presence of tropomyosin is not due to a calcium-mediated binding of tropomyosin to actin.

Regulation in vitro of brush border myosin by light chain phosphorylation

Myosin was purified from chicken brush border cells to greater than 95% homogeneity and in a predominantly non-phosphorylated state. The effects of light chain phosphorylation by a Ca2+-calmodulin-dependent myosin light chain kinase on the conformational, enzymatic and filament assembly properties of this myosin were investigated. The actin-activated MgATPase activity of the non-phosphorylated myosin was low, and upon light chain phosphorylation an eight- to ninefold increase in this activity was observed, which was further potentiated by tropomyosin. Light chain phosphorylation was shown to control the assembly and disassembly of brush border myosin filaments. For example, turbidity measurements and electron microscopy demonstrated that MgATP disassembled non-phosphorylated myosin filaments; the disassembled myosin could reassemble when the light chains were phosphorylated, and could be disassembled again by dephosphorylating the light chains with phosphatase. In the electron microscope, the disassembled non-phosphorylated myosin molecules appeared in a folded conformation, and they were extended when phosphorylated. Proteolytic digestion was used to probe further the conformation of these folded and extended molecules, and their subunit organizations were characterized by a gel overlay technique. Quantitative analysis further demonstrated that light chain phosphorylation alters dramatically the monomer/polymer equilibrium of brush border myosin, shifting it towards filament formation. Comparison of analogous data for myosin from gizzard and thymus shows that each myosin has distinct solubility properties.

Binding of phosphorylase a and b to skeletal muscle thin filament proteins

Phosphorylase plays an important role in energy generation during muscle contraction. We have demonstrated that purified rabbit skeletal muscle phosphorylase a and phosphorylase b bind to rabbit muscle F-actin, F-actin-tropomyosin, F-actin-tropomyosin-troponin, and myofibrils. Neither phosphorylase a nor phosphorylase b binds to myosin. Phosphorylase a and b bind to F-actin with S0.5 values of 1.5 X 10(-6) and 2.1 X 10(-6) M, respectively. At saturation, 0.035 mol of phosphorylase a and b is bound for every seven G-actin monomers in the F-actin polymer. Using the F-actin-tropomyosin-troponin complex as opposed to F-actin as a binding target, there are five- and threefold increases in the maximal binding capacity for phosphorylase a and phosphorylase b, respectively, without a significant change in the S0.5 value for either form of the enzyme. A similar stoichiometry and affinity of phosphorylase binding are observed when myofibrils are used as the binding target. Ca2+ ions and AMP increase the maximal binding capacity for phosphorylase a to myofibrils while ATP decreases the Bmax. Our study suggests that in skeletal muscle, phosphorylase a and phosphorylase b may interact with the thin filament, and that this binding to thin filament proteins may be controlled by changes in sarcoplasmic concentration of Ca2+ and ligands of phosphorylase during muscle contraction.

Muscle connections between imaginal discs in Drosophila

A comparison of the protein content of different imaginal discs of Drosophila revealed that among more than 600 protein species that can be detected, only three show differences in concentration among different types of discs. Two of them form a doublet that can be resolved only by using extended electrophoresis conditions. This doublet was also reported to have a nonhomogeneous distribution within some discs. Here we show that these two proteins are tropomyosin components, and that they are associated not with the discs themselves but with a new type of muscle that connects some of the discs together.

Tropomyosin's end-to-end polymerization is irreversibly lost on exposure to urea or cyanate

Tropomyosin's low-salt viscosity which is due to end-to-end polymerization, is irreversibly lost upon incubation in 8M urea at room temperature. This effect is due to the chemical modification of lysine residues by cyanate in the urea. In the absence of urea, cyanate alone has the same effect. A loss in tropomyosin binding to actin accompanies the loss in viscosity, consistent with the view that tropomyosin's end-to-end interaction is necessary for strong binding to actin. During column chromatography in 8M urea, used to separate the alpha and beta chains of tropomyosin, the loss of viscosity can be minimized by using freshly-prepared urea and by reducing the time during which the protein and urea are in contact.

A muscle-type tropomyosin in human fibroblasts: evidence for expression by an alternative RNA splicing mechanism

We have isolated a cDNA clone from a human fibroblast cDNA library that contains the entire protein-coding region of a 1.1-kilobase mRNA. This mRNA encodes a 284-amino acid tropomyosin, the primary structure of which most closely resembles smooth muscle tropomyosin. Thus, the expression of both 284-amino acid muscle-type and 247-amino acid non-muscle-type tropomyosins appears to be a normal feature of human non-muscle cells. We also present evidence to suggest that this cytoskeletal tropomyosin and a human skeletal muscle beta-tropomyosin are derived from a common structural gene by an alternative RNA splicing mechanism.

Isolation, characterization, and distribution of an unusual pancreatic human secretory protein

An unusual protein was isolated from acid extracts of normal human pancreas and pancreatic secretion in the form of uniform 7-10-nm long single threads without visible axial periodicity or other structure, as seen in the electron microscope. It accounts for as much as 300 micrograms/ml in some pancreatic secretions as measured by specific radioimmunoassay. The protein undergoes a freely reversible, pH dependent, globule-fibril transformation, being stable in the fibril form between pH 5.4 and 9.2. The monomer at acid pH has an apparent molecular weight of approximately 14,000 and consists of a single polypeptide chain, the amino acid composition of which is rich in aromatic amino acids and lacks carbohydrate, fatty acid, and phosphate. The amino acid sequence of 45 residues from the amino terminus shows no homology with any other reported protein sequences other than that of the A chain of the bovine pancreas thread protein (reported elsewhere). A sensitive radioimmunoassay employing monoclonal antibodies against human pancreatic thread protein failed to detect the antigen in a wide range of human tissues other than pancreas, nor was the antigen measurable in normal human sera. Immunohistochemistry utilizing these antibodies revealed the antigen as a component of the cytoplasm of some but not all the pancreatic acinar cells. A physiologic function has not yet been determined for this protein.

Novel thick filament protein of chicken pectoralis muscle: the 86 kd protein. I. Purification and characterization

A new thick-filament-associated protein, the 86 kd protein, of chicken pectoralis major muscle was isolated from a crude C-protein preparation by a method similar to that used to purify H-protein from rabbit skeletal muscle. However, the protein with an apparent Mr of 86,000 and 370,000 as estimated by gel electrophoresis and gel permeation, respectively, is not related to C-protein and differs from rabbit H-protein by its elution behaviour from hydroxyapatite columns, by its molecular weight, ultraviolet light spectrum, amino acid composition and localization, and by its amount present in myofibrils. The amino acid composition reveals a high content of proline and gel permeation indicates an either highly asymmetric or polymeric structure of the molecule. Antibodies raised in rabbits against the 86 kd protein were demonstrated by double immunodiffusion and immunoblotting experiments to be specific for this protein. They show no cross-reactivity with any other myofibrillar protein of chicken pectoralis muscle, e.g. myosin, M-band proteins, titin or C-protein, nor did they exhibit a significant cross-reactivity with H-protein from rabbit. The 86 kd protein, which has been purified also by antibody affinity chromatography from a freshly prepared Guba-Straub extract of washed myofibrils, is a specific myofibrillar component located within each half of the A-band.

Factors influencing interaction of phosphorylated and dephosphorylated myosin with actin

The influence of various factors on the interaction of phosphorylated and dephosphorylated myosin with actin was examined. It was found that the difference between the values of specific activity of the two myosin forms of actin-stimulated Mg2+-ATPase is affected by changes in KCl, MgATP and actin concentration. The effect of increased pH on the differences in the rate of ATP hydrolysis by actomyosin containing phosphorylated myosin as compared with that of the dephosphorylated one, observed in the presence of EGTA, is abolished by addition of Ca2+. Tropomyosin strongly inhibits the actin-stimulated Mg2+-ATPase of phosphorylated myosin (by about 60%). The tropomyosin-troponin complex and native tropomyosin lowered the rate of ATP hydrolysis by actomyosin containing both phosphorylated and dephosphorylated myosin by about of 60% of the value obtained in the absence of those proteins. These results indicate that the change of negative charge on the myosin head due to phosphorylation and dephosphorylation of myosin light chains modulates the actin-myosin interaction at different steps of the ATP hydrolysis cycle. Phosphorylation of myosin seems to be a factor decreasing the rate of ATP hydrolysis by actomyosin under physiological conditions.

An unusual bovine pancreatic protein exhibiting pH-dependent globule-fibril transformation and unique amino acid sequence

An unusual hitherto unreported protein, extracted in acid from fresh bovine pancreas, has been purified and characterized biochemically. It precipitates in the neutral pH range in the form of uniform double-helical threads, each strand of which is smooth and of uniform diameter, about 7-8 nm. The threads dissolve to a nonviscous solution below pH 3.6 and above pH 9.4, and they reconstitute reversibly in the pH range in between. The monomer in acid has an apparent molecular weight of 17,800 and consists of two disulfide-linked nonidentical polypeptide chains of different lengths. It is rich in aromatic amino acids, particularly tryptophan. There is no significant content of carbohydrate, fatty acid, or bound phosphate. The amino acid sequences of the first NH2-terminal 48 residues of the A chain and 35 residues of the B chain appear to be unique, differing from all other reported animal proteins, including those of the pancreas. Thus far, a function has not been found.

Doxorubicin and covalently crosslinked doxorubicin derivatives binding to purified cardiac thin-filament proteins in vitro

The binding of cardiac actin and tropomyosin to a cardiotoxic antineoplastic agent, doxorubicin, and its covalently crosslinked derivatives was investigated. The primary amino group of the daunosamine moiety of doxorubicin was blocked with fluorescein isothiocyanate. This doxorubicin derivative did not bind to Sepharose which was conjugated with cardiac actin. A doxorubicin dimer was made by covalently crosslinking one doxorubicin molecule to another identical doxorubicin molecule through the free amino group of each daunosamine moiety. This derivative demonstrated mobility different from parent doxorubicin on thin-layer chromatography, different elution pattern by column chromatography, and did not show binding affinity for actin. Exploring other purified thin-filament proteins, it was found that doxorubicin did bind to tropomyosin when gel filtration was performed on the protein drug mixture. The ability of tropomyosin to form paracrystal in vitro was not disturbed by a variety of concentrations of doxorubicin. These data support the concept that the doxorubicin solitary free amino group is the site which is responsible for this ligand to bind to actin and may relate to its cardiotoxic effects.

Suppression of tropomyosin synthesis, a common biochemical feature of oncogenesis by structurally diverse retroviral oncogenes

To identify proteins whose production may be altered as a common event in the expression of structurally diverse oncogenes, we compared two-dimensional electropherograms of newly synthesized proteins from NIH/3T3 cell lines transformed by a variety of retroviral oncogenes, from cellular revertant lines, and from a line (433.3) which expresses the v-ras oncogene in response to corticosteroids. Most alterations in the synthesis of specific proteins detected by this approach appeared to be the result of selection during prolonged cultivation and were probably unrelated to the transformation process. However, we detected seven proteins whose synthesis was strongly suppressed in cell lines transformed by each of the six retroviral oncogenes we studied and whose production was fully or partially restored in two cellular revertant lines. Suppression of two of these proteins was also correlated with the initial appearance of morphological alteration during corticosteroid-induced oncogene expression in 433.3 cells. These proteins (p37/4.78 and p41/4.75) were identified as tropomyosins, a group of at least five cytoskeletal proteins. Transformation by the papovaviruses simian virus 40 and polyomavirus caused no suppression of synthesis of these tropomyosins. This indicates that suppression of tropomyosin synthesis is not a nonspecific response by cells to being forced to grow with the transformed phenotype but is specifically associated with oncogenesis by diverse retroviral oncogenes. The results are consistent with the hypothesis that the different biochemical processes initiated by expression of structurally diverse retroviral oncogenes may converge on a limited number of common targets, one of which is the mechanism which regulates the synthesis of tropomyosins.

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.

The effect of leiotonin fraction on stably phosphorylated smooth muscle myosin

This study was designed to determine the effect of leiotonin on the actin-activation once the myosin is stably phosphorylated. Gizzard myosin was stably phosphorylated by ATP-gamma-S using the gizzard light chain kinase. Addition of leiotonin preparation to phosphorylated myosin reconstituted with actin and tropomyosin did not alter the ATPase activity. Furthermore, leiotonin did not confer the calcium sensitivity of the ATPase activity. These experiments show that the actin-activated ATPase activity of stably phosphorylated gizzard myosin is not altered by leiotonin.

Properties of carboxypeptidase A-treated chicken gizzard tropomyosin

Chicken gizzard tropomyosin was digested with carboxypeptidase A at the weight ratios of enzyme to substrate 1:200 and 1:50. Removal of about 16 C-terminal amino acid residues per tropomyosin molecule, at lower enzyme concentration, caused reversion of the effect on skeletal actomyosin ATPase activity from activating to inhibiting without an influence on polymerizability and actin-binding ability. Removal of about 26 C-terminal amino acid residues per molecule, at higher enzyme concentration, resulted in loss of polymerizability and actin binding ability. Digestion of gizzard tropomyosin with carboxypeptidase A has no dramatic effect on its binding to troponin T. The results show that not only the existence of head-to-tail overlapping regions but also their length is important for the functional properties of chicken gizzard tropomyosin.

Effects of Ca2+ and Mg2+ on the actomyosin adenosine-5'-triphosphatase of stably phosphorylated gizzard myosin

There are conflicting reports on the effect of Ca2+ on actin activation of myosin adenosine-triphosphatase (ATPase) once the light chain is fully phosphorylated by a calcium calmodulin dependent kinase. Using thiophosphorylated gizzard myosin, Sherry et al. [Sherry, J. M. F., Gorecka, A., Aksoy, M. O., Dabrowska, R., & Hartshorne, D. J. (1978) Biochemistry 17, 4417-4418] observed that the actin activation of ATPase was not inhibited by the removal of Ca2+. Hence, it was suggested that the regulation of actomyosin ATPase activity of gizzard myosin by calcium occurs only via phosphorylation. In the present study, phosphorylated and thiophosphorylated myosins were prepared free of kinase and phosphatase activity; hence, the ATPase activity could be measured at various concentrations of Ca2+ and Mg2+ without affecting the level of phosphorylation. The ATPase activity of myosin was activated either by skeletal muscle or by gizzard actin at various concentrations of Mg2+ and either at pCa 5 or at pCa 8. The activation was sensitive to Ca2+ at low Mg2+ concentrations with both actins. Tropomyosin potentiated the actin-activated ATPase activity at all Mg2+ and Ca2+ concentrations. The calcium sensitivity of phosphorylated and thiophosphorylated myosin reconstituted with actin and tropomyosin was most pronounced at a free Mg2+ concentration of about 3 mM. The binding of 125I-tropomyosin to actin showed that the calcium sensitivity of ATPase observed at low Mg2+ concentration is not due to a calcium-mediated binding of tropomyosin to F-actin. The actin activation of both myosins was insensitive to Ca2+ when the Mg2+ concentration was increased above 5 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of tropomyosin synthesis, a common biochemical feature of oncogenesis by structurally diverse retroviral oncogenes

To identify proteins whose production may be altered as a common event in the expression of structurally diverse oncogenes, we compared two-dimensional electropherograms of newly synthesized proteins from NIH/3T3 cell lines transformed by a variety of retroviral oncogenes, from cellular revertant lines, and from a line (433.3) which expresses the v-ras oncogene in response to corticosteroids. Most alterations in the synthesis of specific proteins detected by this approach appeared to be the result of selection during prolonged cultivation and were probably unrelated to the transformation process. However, we detected seven proteins whose synthesis was strongly suppressed in cell lines transformed by each of the six retroviral oncogenes we studied and whose production was fully or partially restored in two cellular revertant lines. Suppression of two of these proteins was also correlated with the initial appearance of morphological alteration during corticosteroid-induced oncogene expression in 433.3 cells. These proteins (p37/4.78 and p41/4.75) were identified as tropomyosins, a group of at least five cytoskeletal proteins. Transformation by the papovaviruses simian virus 40 and polyomavirus caused no suppression of synthesis of these tropomyosins. This indicates that suppression of tropomyosin synthesis is not a nonspecific response by cells to being forced to grow with the transformed phenotype but is specifically associated with oncogenesis by diverse retroviral oncogenes. The results are consistent with the hypothesis that the different biochemical processes initiated by expression of structurally diverse retroviral oncogenes may converge on a limited number of common targets, one of which is the mechanism which regulates the synthesis of tropomyosins.

Monoclonal antibodies against chicken tropomyosin isoforms: production, characterization, and application

Eight mouse monoclonal antibodies, CH1, CH106, CH291, CL2, CG1, CG3, CG beta 2 and CG beta 6, against chicken tropomyosin isoforms have been prepared and characterized. The antigens recognized by these isoform-specific monoclonal antibodies were identified by both solid-phase radioimmunoassay and protein immunoblotting. To some extent, most antibodies showed isoform-specific, but one (CG3) recognized all isoforms of tropomyosin from chicken materials. The effects of monoclonal antibodies on the binding of cardiac tropomyosin to F-actin were investigated. Antibodies CH1, CH106, and CH291 had the ability to interfere with the binding of tropomyosin to F-actin, whereas others appeared to have no effect. Monoclonal antibody CL2 was able to distinguish the skeletal muscle tropomyosin-enriched microfilaments from the fibroblastic tropomyosin-enriched microfilaments of differentiating muscle cells. This antibody will be most useful for studying the compartmentalization of microfilaments and microfilament-associated proteins, particularly actin and tropomyosin isoforms during muscle differentiation. Immunofluorescence microscopy with CG1 antibody which recognized CEF tropomyosin isoforms 1 and 3 revealed the continuous staining of stress fibers in some populations of CEF cells. On the other hand, both periodic fluorescent staining and continuous staining of stress fibers were observed with CG3 antibody in all CEF cells.

Monoclonal antibodies to the Ca2+ + Mg2+-dependent ATPase of sarcoplasmic reticulum identify polymorphic forms of the enzyme and indicate the presence in the enzyme of a classical high-affinity Ca2+ binding site

In order to determine whether polymorphic forms of the Ca2+ + Mg2+-dependent ATPase exist, we have examined the cross-reactivity of five monoclonal antibodies prepared against the rabbit skeletal muscle sarcoplasmic reticulum enzyme with proteins from microsomal fractions isolated from a variety of muscle and nonmuscle tissues. All of the monoclonal antibodies cross-reacted in immunoblots against rat skeletal muscle Ca2+ + Mg2+-dependent ATPase but they cross-reacted differentially with the enzyme from chicken skeletal muscle. No cross-reactivity was observed with the Ca2+ + Mg2+-dependent ATPase of lobster skeletal muscle. The pattern of antibody cross-reactivity with a 100,000 dalton protein from sarcoplasmic reticulum and microsomes isolated from various muscle and nonmuscle tissues of rabbit demonstrated the presence of common epitopes in multiple polymorphic forms of the Ca2+ + Mg2+-dependent ATPase. One of the monoclonal antibodies prepared against the purified Ca2+ + Mg2+-dependent ATPase of rabbit skeletal muscle sarcoplasmic reticulum was found to cross-react with calsequestrin and with a series of other Ca2+-binding proteins and their proteolytic fragments. Its cross-reactivity was enhanced in the presence of EGTA and diminished in the presence of Ca2+. Its lack of cross-reactivity with proteins that do not bind Ca2+ suggests that it has specificity for antigenic determinants that make up the Ca2+-binding sites in several Ca2+-binding proteins including the Ca2+ + Mg2+-dependent ATPase.

Iodination of myofibrils and myosin

The relative reactivity of the tyrosine side chains in the proteins of skeletal muscle myofibrils was determined using iodination techniques. The destruction of ATPase activity of myofibrils and myosin by lactoperoxidase and chloramine-T iodination could be prevented by the attachment of cysteamine to the sulphydryl groups prior to the iodination reaction and subsequent regeneration with thioglycolate or dithiothreitol. Iodination using 1,3,4,6-tetrachloro-3 alpha, 6 alpha-diphenylglycoluril did not require cysteamine treatment for retention of full enzymatic activity. The specific activity of the different proteins varied markedly with desmin, troponin-T, and tropomyosin having the highest labelling with all three iodination procedures. In contrast the myosin light chains had low specific activity when labelled in myofibrils or intact myosin. The isolated light chains, however, were much more highly iodinated. It appears that iodination may be a useful technique for examining protein-protein interactions in the myofibril.

Experimental myocarditis induced in Swiss mice by homologous heart immunization resembles chronic experimental Chagas' heart disease

The Swiss mouse is considered a satisfactory model for experimental chronic chagasic myocarditis and there is some evidence of an immunopathologic mechanism in the development of this disease. To further support this conjecture, 45-day-old albino Swiss mice (40 animals) were immunized with homologous heart in complete Freund's adjuvant. As controls, 20 animals were likewise inoculated with allogeneic testis, as "non-related" antigen. Three mice from the former group died suddenly at 19-21 days postinoculation while the survivors were sacrificed at 60 days for serum samples, and histologic analysis of the heart and skeletal muscle. Electrocardiographic records were taken at Days 0, 30, and 60 postinoculation. Of myocardium-inoculated animals and testis-inoculated mice 33/37 (89%) and 1/20 (5%), respectively, exhibited myocarditis (P less than 0.001). Histologic lesions were highly reminiscent of those observed in chronic experimental Chagas' disease of Swiss mice. Antimuscle antibodies were seen, by indirect immunofluorescence employing cryostat sections, in 30/33 (91%) of the former group and in 3/20 (15%) of the latter (P less than 0.001), some of which recognized a surface antigen of primary cultured fetal rat myocardiocytes. Mice inoculated with myocardium also exhibited electrocardiographic abnormalities consisting in QRS interval widening. Results show that following an autoimmune experimental design the main features of chronic chagasic myocarditis may be reproduced in the Swiss mouse. This agrees with the likely role of an immunopathologic mechanism in heart damage due to Trypanosoma cruzi infection.

Purification and properties of Ca2+-regulated thin filaments and F-actin from sheep aorta smooth muscle

We have investigated the conditions for isolation of Ca2+-regulated thin filaments from sheep aorta. Inhibition of proteolysis by 2 micrograms ml-1 leupeptin and chymostatin and of oxidation with 5 mM dithiothreitol were essential. Washed homogenates were extracted in 10 mM ATP of low ionic strength at pH 6.1 to minimize coextraction of myosin with thin filaments. Thin filaments were separated from myosin by high speed sedimentation; 20% glycol was added to prevent loss of regulatory factors and tropomyosin. The resulting thin filaments (yield 2.5 mg protein g-1 artery wet weight) were made up of actin, tropomyosin and a 120 000 Mr protein (molar ratio 1:1/5:1/29) and were up to 4 micron long. They activated skeletal muscle myosin at least 50 times in presence of Ca2+. Up to 80% inhibition was observed in the absence of Ca2+. We also prepared pure arterial F-actin, which activated skeletal myosin more than the thin filaments, but was similar to skeletal F-actin. We conclude that Ca2+ regulation is negative, involves cooperative interactions between actin, myosin and tropomyosin and suggest that it is mediated by the 120 000 Mr protein.

Phalloidin and tropomyosin do not prevent actin filament shortening by the 90 kD protein-actin complex from brain

Previously we reported the purification from bovine brain of the 90 kD protein-actin complex that shortens actin filaments. In the present work we study the effect of this complex on actin polymerized in the presence of phalloidin (PL) or tropomyosin (TM) which are known to stabilize actin filaments. The effect of the complex has been compared with that of cytochalasin D (CD), a fungal metabolite that also shortens actin filaments. Low shear viscosimetry and electron microscopy showed that PL or TM could not prevent the shortening of actin filaments in the presence of 90 kD protein-actin complex whereas they effectively protected actin filaments from shortening by CD. We conclude that the 90 kD protein-actin complex is a more potent filament-shortening factor than CD.

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.

Separation of muscle proteins

This review covers various methods used in the separation and isolation of individual muscle contractile proteins. It is shown which methods have been most useful for the separation of contractile proteins and their fragments and in extending our knowledge of muscle biochemistry and physiology.

Is the SII portion of the cross-bridge in glycerinated rabbit psoas fibers compliant in the rigor state?

To see whether the SII portion of the cross-bridge in rigor fibers is longitudinally compliant, we chemically cross-linked with dimethyl suberimidate the entire rod portion (including the SII portion) of myosin onto the surface of thick filaments in glycerinated rabbit psoas fibers, and studied the effect of the SII fixation on the stiffness of the rigor fibers. The cross-linking of fiber segments with full filament overlap increased the rigor stiffness by approximately 25%. Almost the same absolute amount of the stiffness increase was also observed in rigor fibers with half- or no filament overlap after the cross-linking, and a similar but somewhat larger increment of stiffness was observed in fiber segments cross-linked in relaxing solution. These results indicate that the stiffness increase is not produced by the fixation of the SII portion onto the thick filament surface, but is caused instead by the cross-linking of some parallel elastic elements in muscle, and therefore indicate that the SII portion of the cross-bridge is hardly longitudinally compliant in rigor fibers.

An enzyme-probe method to detect structural changes in the myosin rod

The temperature-dependence of local melting within the alpha-helical, coiled-coil structure of rabbit myosin rod has been investigated by following changes in the rate constants of proteolytic digestion. The kinetics of fragmentation of the rod by three different enzymes (alpha-chymotrypsin, trypsin and papain) over the temperature range 5 to 40 degrees C (pH 7, I = 0.5) has been monitored by electrophoresis of the digestion products on sodium dodecyl sulfate/polyacrylamide gels. All rate constants were corrected for the intrinsic temperature-dependence of the enzyme by comparison with model substrates. Results from the three enzyme-probes are similar in showing that local melting within the rod occurs in two distinct stages. At temperatures between 5 and 25 degrees C, melting is confined to a restricted segment of the rod structure near the light meromyosin/heavy meromyosin junction. At temperatures between 25 and 40 degrees C, a wider segment of the rod lysing between the junction and the short subfragment-2 segment (the hinge domain) appears to be melting, judging from the broad spectrum of cleavage sites observed in this region. Results are compared with those from other physicochemical methods that measure the hinging or opening of the coiled-coil structure of the rod.

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.

Studies on the spectrin-like protein from the intestinal brush border, TW 260/240, and characterization of its interaction with the cytoskeleton and actin

The terminal web of the intestinal brush border contains a spectrin-like protein, TW 260/240 (Glenney, J. R., Jr., P. Glenney, M. Osborne, and K. Weber, 1982, Cell, 28:843-854.) that interconnects the "rootlet" ends of microvillar filament bundles in the terminal web (Hirokawa, N., R. E. Cheng, and M. Willard, 1983, Cell, 32:953-965; Glenney J. R., P. Glenney, and K. Weber, 1983, J. Cell Biol., 96:1491-1496). We have investigated further the structural properties of TW 260/240 and the interaction of this protein with actin. Salt extraction of TW 260/240 from isolated brush borders results in a loss of terminal web cross-linkers primarily from the apical zone directly beneath the plasma membrane. Morphological studies on purified TW 260/240 using the rotary shadowing technique confirm earlier results that this protein is spectrin-like and is in the tetrameric state in buffers of low ionic strength. However, examination of TW 260/240 tetramers by negative staining revealed a molecule much straighter and more uniform in diameter than rotary-shadowed molecules. At salt concentrations at (150 mM KCl) and above (300 mM KCl) the physiological range, we observed a partial dissociation of tetramers into dimers that occurred at both 0 degree and 37 degrees C. We also observed (in the presence of 75 mM KCl) a concentration-dependent self-association of TW 260/240 into sedimentable aggregates. We have studied the interaction of TW 260/240 with actin using techniques of co-sedimentation, viscometry, and both light and electron microscopy. We observed that TW 260/240 can bind and cross-link actin filaments and that this interaction is salt- and pH-dependent. Under optimum conditions (25-75 mM KCl, at pH 7.0) TW 260/240 cross-linked F-actin into long, large-diameter bundles. The filaments within these bundles were tightly packed but loosely ordered. At higher pH (7.5) such bundles were not observed, although binding and cross-linking were detectable by co-sedimentation and viscometry. At higher salt (greater than 150 mM KCl), the binding of TW 260/240 to actin was inhibited. The presence of skeletal muscle tropomyosin had no significant effect on the salt-dependent binding of TW 260/240 to F-actin.

Studies of myosin and its proteolytic fragments by laser Raman spectroscopy

Two bands in the Raman spectrum of myosin, at 1,304 cm-1 and 1,270 cm-1, are attributable to alpha-helical structure. The first of these, also present in the spectrum of light meromyosin (LMM) but not in that of subfragment-1 (S-1), is assigned to the coiled-coil tail region of myosin; the second, seen in spectra of S-1 or heavy meromyosin (HMM), is largely absent from the spectrum of light meromyosin and is likely to correspond to the alpha-helical segments of the head region. When myosin or LMM aggregates, spectral bands attributable to backbone and sidechain groups sharpen suggesting a reduction in motional freedom. This sharpening is particularly apparent in the 902 cm-1 C--C stretching mode. Mg2+ broadens and shifts the peak at 1,244 cm-1 to 1,237 cm-1 and diminishes the intensity from 1,230 to 1,240 cm-1, changes which appear to be associated the S-1 region. MgPPi produces changes in the 1,300 cm-1 region attributable to alpha-helical regions in coiled-coil structures suggesting that MgPPi affects not only S-1, but also some part of the myosin rod.

Regulation of contraction and thick filament assembly-disassembly in glycerinated vertebrate smooth muscle cells

Isolated smooth muscle cells and cell fragments prepared by glycerination and subsequent homogenization will contract to one-third their normal length, provided Ca++ and ATP are present. Ca++-independent contraction was obtained by preincubation in Ca++ and ATP gamma S, or by addition of trypsin-treated myosin light chain kinase (MLCK) that no longer requires Ca++ for activation. In the absence of Ca++, myosin was rapidly lost from the cells upon addition of ATP. Glycerol-urea-PAGE gels showed that none of this myosin is phosphorylated. The extent of myosin loss was ATP- and pH-dependent and occurred under conditions similar to those previously reported for the in vitro disassembly of gizzard myosin filaments. Ca++-dependent contraction was restored to extracted cells by addition of gizzard myosin under rigor conditions (i.e., no ATP), followed by addition of MLCK, calmodulin, Ca++, and ATP. Function could also be restored by adding all these proteins in relaxing conditions (i.e., in EGTA and ATP) and then initiating contraction by Ca++ addition. Incubation with skeletal myosin will restore contraction, but this was not Ca++-dependent unless the cells were first incubated in troponin and tropomyosin. These results strengthen the idea that contraction in glycerinated cells and presumably also in intact cells is primarily thick filament regulated via MLCK, that the myosin filaments are unstable in relaxing conditions, and that the spatial information required for cell length change is present in the thin filament-intermediate filament organization.

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.

Quantitative determination of myosin and actin in rabbit skeletal muscle

The myosin and actin content of muscle tissue and purified myofibrils from rabbit psoas muscle has been determined. Myofibrils were purified using Percoll gradients, which allowed rapid separation from nuclei and connective tissue proteins. Myosin and actin were quantitated by amino acid analysis of the appropriate bands from sodium dodecyl sulfate/polyacrylamide gels. Muscle tissue contained 94 and 619 nmol/g wet weight of myosin and actin, respectively, while myofibrils had 0.82 and 5.37 mumol/g protein. Thus myosin contributed 43% and actin 22% of the myofibril protein mass. The value of 2.5 myosins per 14.3 nm repeat as calculated from these results suggests that thick filament models with mixtures of two and three crossbridges per repeat should be considered.

Ca2+ and Mg2+-dependent complex formation of tropomyosin with phosphotroponin (P1TI2C) or dephosphotroponin (TI2C)

The reduced viscosity of troponin and dephosphotroponin is independent of the protein concentration in both states, either metal-free or with troponin C saturated with Ca2+ or Mg2+; that of tropomyosin increases linearly as function of the protein concentration, indicating aggregation. Addition of troponin to tropomyosin increases the reduced viscosity over the expected value being maximal at a 1:1 molar ratio of both proteins. The reduced viscosity of a 1:1 molar mixture of phosphotroponin-Mg4 or dephosphotroponin-Mg3 increases in two phases as function of the total protein concentration, indicating the formation of two kinds of troponin-tropomyosin complexes. In the first phase, troponin and tropomyosin form a non-aggregating 1:1 complex, which is characterized by a value of 0.45 dl/g for the intrinsic viscosity and a sedimentation coefficient of 3.6 S. Employing these two values a molecular weight of 150 000 can be calculated, which is in the range of the sum of molecular weights for troponin and tropomyosin (156 000). In the second phase the troponin-tropomyosin complex aggregates further, a process described by:n (troponin-tropomyosin) leads to (troponin-tropomyosin)n. This further aggregation occurs upon saturation of the Ca2+-specific sites in troponin C. A model is discussed which explains the shortening of 1.5 nm per tropomyosin molecule upon the shift of tropomyosin from the periphery into the groove of the actin filament by tropomyosin aggregation.

Developmental change of protein constituents in chicken gizzards

Developmental change of protein constituents of chick gizzard smooth muscle was described by the fluorescent antibody technique and two-dimensional polyacrylamide gel electrophoresis. Myosin heavy chain, tropomyosin, and desmin were immunohistologically detected in 5-day-old gizzard primordia, but myoglobin was detected after 19 days of incubation. Two-dimensional polyacrylamide gel electrophoresis revealed that most structural proteins including beta- and gamma-actin are synthesized almost simultaneously in the primordium, and accumulate in three patterns by which the proteins examined are classified: (1) gradually increasing protein (gamma-actin, tropomyosin, desmin), (2) abruptly increasing protein at a certain stage (myosin, myoglobin), (3) decreasing or constantly kept protein (tubulin, beta-actin). Based on the quantitative analysis of protein constituents, the nature of regulatory system of protein synthesis in smooth muscle and the possible functional difference between beta- and gamma-actin are discussed.

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.

Direct electron microscopic visualization of barbed end capping and filament cutting by intestinal microvillar 95-kdalton protein (villin): a new actin assembly assay using the Limulus acrosomal process

We have re-examined the Ca(++)-dependent interaction of an intestinal microvillar 95- kdalton protein (MV-95K) and actin using the isolated acrosomal process bundles from limulus sperm. Making use of the processes as nuclei for assembling actin filaments, we quantitatively and qualitatively examined MV-95K's effect on filament assembly and on F- actin, both in the presence and in the absence of Ca(++). The acrosomal processes are particularly advantageous for this approach because they nucleate large numbers of filaments, they are extremely stable, and their morphology can be used to determine the polarity of any nucleated filaments. When filament nucleation was initiated in the presence of MV-95K and the absence of Ca(++), there was biased filament assembly from the bundle ends. The calculated elongation rates from both the barbed and pointed filament ends were virtually indistinguishable from control preparations. In the presence of Ca(++), MV-95K completely inhibited filament assembly from the barbed filament end without affecting the initial rate of assembly from the pointed filament end. The inhibition of assembly results from MV-95K binding to and capping the barbed filament end, thereby preventing monomer addition. This indicates that, while MV-95K is a potent nucleator of actin assembly, it is also a potent inhibitor of actin filament elongation. To examine the effects of MV-95K on F-actin in the presence of Ca(++), we developed an assay where MV-95K is added to filaments previously assembled from acrosomal processes without causing filament breakage during mixing. These results clearly demonstrated that rapid filament shortening by MV-95K results through a mechanism of disrupting intrafilament monomer-monomer interactions. Finally, we show that tropomyosin-containing actin filaments are insensitive to cutting, but not to capping, by MV-95K in the presence of Ca(++).

Dimerization of the polypeptide chains of skeletal muscle tropomyosin

The composition of alpha and beta chains in tropomyosin dimers present in fetal and adult skeletal muscle of cow has been analysed by SDS-polyacrylamide gel electrophoresis after cross-linking of the chains by disulphide bridges. The results indicate that in vivo alpha beta heterodimers of tropomyosin are assembled preferentially and only the excess of particular chains forms homodimers, i.e., alpha alpha dimers in adult and beta beta ones in fetal muscle. The original dimers of tropomyosin were dissociated with urea in the presence of dithiothreitol. Subsequent reassembly of the tropomyosin dimers from the mixture of alpha and beta chains approaches the random model.

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.

Regional differences in the expression of myosin light chains and tropomyosin subunits during development of chicken breast muscle

Types of myosin light chains and tropomyosins present in various regions and at different developmental stages of embryonic and posthatched chicken breast muscle (pectoralis major) have been characterized by two-dimensional gel electrophoresis. In the embryonic muscle all areas appear to accumulate both slow and fast forms of myosin light chains in addition to alpha and beta forms of tropomyosin. During development regional differences in myosin and tropomyosin expression become apparent. Slow myosin subunits become gradually restricted to areas of the anterior region of the muscle and finally become localized to a small red strip found on its anterior deep surface. This red region is characterized by the presence of slow and fast myosin light chains, alpha-fast, alpha-slow, and beta-tropomyosin. In all other areas of the muscle examined only fast myosin light chains, beta-tropomyosin and the alpha-fast form of tropomyosin, are found. In addition, beta-tropomyosin also gradually becomes lost in the posterior regions of the developing breast muscle. In the adult, the red strip area represents less than 1% of the total pectoralis major mass and of the myosin extracted from this area approximately 15% was present as an isozyme that comigrated on nondenaturing gels with myosin from a slow muscle (anterior latissimus dorsi). The red region accumulates therefore fast as well as slow muscle myosin. Thus while the adult chicken pectoralis major is over 99% fast white muscle, the embryonic muscle displays a significant and changing capacity to accumulate both fast and slow muscle peptides.