Structure, evolution, and regulation of a fast skeletal muscle troponin I gene

The complete structure of a quail fast skeletal muscle troponin I gene was determined by nucleotide sequence comparison of troponin I genomic and cDNA sequences. This 4.5-kilobase troponin I gene has eight exons. The actin-binding domain of troponin I is encoded by a single exon, whereas the troponin C-binding domain is split into at least two exons. The exon organization of the fast troponin I gene suggests that gene conversion directs the nonrandom conservation of the carboxyl-terminal halves of troponin I isoforms and that the amino-terminal extension of the cardiac isoform originated by splice-junction sliding. Comparison of the structure of the troponin I gene with the structures of other contractile protein genes reveals homologous sequences in their 5' flanking regions and similar large introns that separate protein-coding exons from 5' nontranslated exons. These common structural features may function to coordinate the activation of contractile-protein genes during myogenesis.

Energetics of the binding of calcium and troponin I to troponin C from rabbit skeletal muscle

We determined the free energy of interaction between rabbit skeletal troponin I (TNI) and troponin C (TNC) at 10 degrees and 20 degrees C with fluorescently labeled proteins. The sulfhydryl probe 5-iodoacetamidoeosin (IAE) was attached to cysteine (Cys)-98 of TNC and to Cys-133 of TNI, and each of the labeled proteins was titrated with the other unlabeled protein. The association constant for formation of the complex between labeled TNC (TNC*) and TNI was 6.67 X 10(5) M-1 in 0.3 M KCl, and pH 7.5 at 20 degrees C. In the presence of bound Mg2+, the binding constant increased to 4.58 X 10(7) M-1 and in the presence of excess of Ca2+, the association constant was 5.58 X 10(9) M-1. Very similar association constants were obtained when labeled TNI was titrated with unlabeled TNC. The energetics of Ca2+ binding to TNC* and the complex TNI X TNC* were also determined at 20 degrees C. The two sets of results were used to separately determine the coupling free energy for binding TNI and Mg2+, or Ca2+ to TNC. The results yielded a total coupling free energy of -5.4 kcal. This free energy appeared evenly partitioned into the two species: TNI X TNC(Mg)2 or TNI X TNC(Ca)2, and TNI X TNC(Ca)4. The first two species were each stabilized by -2.6 kcal, with respect to the Ca2+ free TNI X TNC complex, and TNI X TNC(Ca)4 was stabilized by -2.8 kcal, respect to TNI X TNC(Ca)2 or TNI X TNC(Mg)2. The coupling free energy was shown to produce cooperatively complexes formed between TNI and TNC in which the high affinity sites were initially saturated as a function of free Ca2+ to yield TNI X TNC(Ca)4. This saturation occurred in the free Ca2+ concentration range 10(-7) to 10(-5) M. The cooperative strengthening of the linkage between TNI and TNC induced by Ca2+ binding to the Ca2+-specific sites of TNC may have a direct relationship to activation of actomyosin ATPase. The nature of the forces involved in the Ca2+-induced strengthening of the complex is discussed.

Cation, trifluoperazine and troponin I binding effects

Proton magnetic resonance spectroscopy has been used to study the cation (Mg2+, Ca2+)-dependent conformational states of the C-terminal domain of rabbit skeletal troponin C under a variety of solution conditions. Nuclear Overhauser data and paramagnetic probe observations provide definition of the configuration of this region of troponin C. Comparative study of homologous proteins identify common features of the tertiary structure relevant to the cation binding reaction. Complex formation with troponin I and the drug trifluoperazine is observed to adjust the solution conformation of the C-terminal domain of troponin C. The interactive conformational response to cation coordination and the binding of the drug and troponin I are discussed.

Rabbit skeletal troponin I

In this study, we have demonstrated the necessity for a combination of size-exclusion, ion-exchange and reversed-phase high-performance liquid-chromatography to resolve completely a protein digest. Our approach minimizes the number of steps, and the column order provides the maximum information about the properties of the fragments. The order is: (1) size-exclusion (Bio-Rad TSK-250 column), (2) strong cation-exchange (Synchropak S300 column), and finally (3) reversed-phase chromatography (Ultrapore C3). It was desirable for the first step of the procedure to be size-exclusion chromatography to produce the least number of fractions. The volatile eluent used in size-exclusion eliminated the need for subsequent sample desalting. Volatile buffers were not necessary for the ion-exchange chromatography, since the fractions were both desalted and purified in the final reversed-phase step. All column effluents were compatible with absorbance measurements at 210 nm to provide maximum sensitivity for peptide detection. The results obtained in this study strongly suggest that the combined use of three methods of separation, which utilize different selectivities (size, charge, hydrophobicity), can provide excellent resolving power for peptide separations. We believe this fast, efficient procedure should be generally applicable to other protein digests.

The functional nature of calcium binding units in calmodulin, troponin C and parvalbumin

Examination of the interaction of major tranquilizers with calmodulin results in the generalization that the functional nature of calcium binding helix-loop-helix regions found in several calcium binding proteins including calmodulin, troponin C and parvalbumin is dependent upon the topography of the hydrophobic and hydrophilic regions on the amphiphilic N-terminal alpha-helix of the helix-loop-helix conformation formed by the binding of the calcium cation to these proteins. The relation of the topography of this amphiphilic alpha-helix to drug binding is delineated at the molecular level and the results obtained are used to describe the interaction of beta-endorphin, dynorphin, alpha-MSH and other peptides with calmodulin. The utility of this hypothesis is further demonstrated by the description of a possible interaction between troponin C, troponin I and troponin T of the troponin complex in skeletal muscle.

Calcium binding to troponin C and troponin: effects of Mg2+, ionic strength and pH

Calcium binding to troponin C and troponin was examined by a metallochromic indicator method under various conditions to obtain a further understanding of the regulatory roles of these proteins in muscle contraction. Troponin C has four Ca binding sites, of which 2 sites have a high affinity of 4.5 X 10(6) M-1 for Ca2+ and the other 2 sites have a low affinity of 6.4 X 10(4) M-1 in a reaction medium consisting of 100 mM KCl, 20 mM MOPS-KOH pH 6.80 and 0.13 mM tetramethylmurexide at 20 degrees C. Magnesium also binds competitively to both the high and low affinity sites: the apparent binding constants are 1,000 M-1 and 520 M-1, respectively. Contrary to the claim by Potter and Gergely (J. Biol. Chem. 250, 4628-4633, 1975), the low affinity sites are not specific only for Ca2+. The high and low affinity sites of troponin C showed different dependence on the ionic strength: the high affinity sites were similar to GEDTA, while the low affinity sites were similar to calmodulin, which has a steeper ionic strength dependence than GEDTA. Ca binding to troponin C was not affected by change of pH between 6.5 and 7.2. Troponin I enhanced the apparent affinity of troponin C for Ca2+ to a value similar to that for troponin. Trifluoperazine also increased Ca binding to troponin C. Troponin has four Ca binding sites as does troponin C, but the affinities are so high that the precise analysis was difficult by this method. The apparent binding constants for Ca2+ and Mg2+ were determined to be 3.5 X 10(6) M-1 and 440 M-1, respectively, for low affinity sites under the same conditions as for troponin C, being independent of change in pH between 6.5 and 7.2. The competitive binding of Mg2+ to the low affinity sites of troponin is consistent with the results of Kohama (J. Biochem. 88, 591-599, 1980). The estimate for the high affinity sites is compatible with the reported results.

The interaction of rabbit skeletal muscle troponin-T fragments with troponin-I

The interactions of troponin-I (Tn-I) with a variety of fragments spanning the length of the troponin-T (Tn-T) polypeptide chain have been reinvestigated at physiological ionic strength by affinity chromatographic, gel filtration, and circular dichroism methodologies. Strong binding was observed with fragment T2 (residues 159-259) mimicking that observed with whole Tn-T and Tn-I. Partial binding was seen with the shorter cyanogen bromide (CB) fragments of Tn-T in the order CB4 (residues 176-230) greater than CB6 (residues 239-259) or CB5 (residues 152-175). No interaction with Tn-I was observed with fragments (CB2, CB3, T1) encompassing residues 1-158 of Tn-T. Based on the present results and the work of others, the binding region for Tn-I includes residues 159-259 and perhaps extends into the highly helical CB2 region (residues 71-151) of Tn-T. No evidence has been obtained by ourselves or others for the interaction of the CB3 region (1-70) with Tn-I. A significant increase (11.6%) in alpha-helical content was observed when an equimolar amount of fragment T2 (residues 159-259) was mixed with Tn-I, a result similar to that seen with whole Tn-T and Tn-I.

Troponin-C-mediated calcium-sensitive changes in the conformation of troponin I detected by pyrene excimer fluorescence

Troponin I (TnI) from rabbit white skeletal muscle was labeled at cysteines 48 and 64 with the fluorescent reagent N-(1-pyrene)maleimide. The fluorescence spectra of pyrene-labeled TnI (pyr-TnI) exhibit peaks characteristic of pyrene in its monomeric form and an additional peak resulting from formation of excited dimers (excimers), indicating that the labeled cysteines are close together. Formation of a pyr-TnI-TnC complex in the absence of Ca2+ has little effect on the spectrum, but when Ca2+ is bound to the low-affinity sites of TnC there is a substantial decrease in excimer and a corresponding increase in monomer fluorescence. The involvement of the low-affinity sites in the Ca2+-induced effect is consistent with the fact that Mg2+ has no effect on pyrene fluorescence. On rapid mixing of the pyr-TnI-TnC complex with Ca2+ in a stopped-flow apparatus, most of the excimer decrease is complete within the instrumental dead time, indicating a rate constant k greater than 350 s-1, which is comparable to that of the conformational change in TnC resulting from Ca2+ binding to the low-affinity sites. Rapid mixing of the Mg2-TnC-pyr-TnI complex with Ca2+ yields similar results, suggesting that the type of metal ion present at the high-affinity sites has little, if any, effect on the probe. It has been suggested previously that Cys 48 and 64 are located in a TnT-binding region of TnI (Chong P.C.S. and Hodges, R.S. (1982) J. Biol. Chem. 255, 3757). Our results suggest that a Ca2+-induced structural change in the TnI-binding region of TnC could be transmitted to TnT by affecting the TnT-binding region of TnI as part of the chain of events in the regulation of muscle contraction.

Ca2+-calmodulin-dependent regulation of F-actin-myelin basic protein interaction

Myelin basic proteins (MBP) interacts with F-actin resulting in the precipitation of a complex of both proteins. Electron microscope observations of this complex reveal the presence of ordered bundles of F-actin filaments similar to those obtained from F-actin and troponin I. In addition to the bundles, there also appear short fragments of F-actin filaments. In the presence of Ca2+ calmodulin causes a release of MBP from its complex with F-actin, accompanied by dissociation of F-actin bundles into separate filaments. Parallel to the binding of MBP to F-actin the ATPase activity of actomyosin is progressively reduced. This inhibition is reversed by calmodulin but only in the presence of Ca2+. Studies of the binding of S-1 to F-actin and to the F-actin-MBP complex indicate that the interaction sites for MBP and S-1 on the actin molecule are different.

Identification of the crosslink formed between troponin C and troponin I in the absence of Ca2+

The single SH-group of rabbit skeletal muscle troponin C (Cys-98) was reacted with the bifunctional reagent, 1,3-difluoro-4,6-dinitrobenzene. This labelled troponin C was used to reconstitute the troponin complex by the addition of equimolar amounts of troponin T and troponin I. The second function of the bifunctional reagent was triggered in the complex by an increase of pH. A crosslink was formed between troponin C and troponin I both in the presence and absence of Ca2+, but the probability of crosslinking was decreased by Ca2+. Covalently linked troponin C-troponin I was isolated from the complex crosslinked without Ca2+, and cleaved by CNBr. The analysis of crosslinked peptides has revealed that in the presence of Mg2+ and absence of Ca2+ the crosslink in the troponin complex is formed between Cys-98 of troponin C and Cys-133 of troponin I.

Binary interactions of troponin subunits

The association constants for the formation of the binary complexes of rabbit fast skeletal muscle troponin subunits have been determined for three solution conditions: (a) 1 mM CaCl2, (b) 3 mM MgCl2 and 1 mM EGTA, and (c) 2 mM EDTA. The subunits were labeled with extrinsic fluorescence probes, either 5-(iodoacetamido)eosin (IAE) or dansylaziridine (DANZ), and the binding was detected by enhancement or quenching of the probe fluorescence. The association constant for the TnI X TnT (where TnI and TnT are the inhibitory subunit and the tropomyosin-binding subunit, respectively, of troponin) complex was measured with two different probes, IAE-TnI and IAE-TnT. The measured values were not affected by the presence of Ca2+ or Mg2+, and the mean values for the three buffer conditions are, respectively, 8.0 X 10(6) and 9.0 X 10(6) M-1 for the two probes. The association constant for TnC-TnI (where TnC is the Ca2+-binding subunit of troponin) interaction was measured with three probes, IAE-TnC, DANZ-TnC, and IAE-TnI. Values of 1.7 X 10(9), 1.2 X 10(8), and 1.0 X 10(6) M-1 were obtained, respectively, in the presence of calcium ion, in the presence of magnesium ion (no calcium), and in the absence of divalent metal ions. A mean value of 4.0 X 10(7) M-1 was obtained for the association constant of TnC X TnT using DANZ-TnC and IAE-TnC as probes in the presence of calcium or magnesium ions. A value of 4.5 X 10(6) M-1 was obtained in the absence of divalent metal ions. The results show that the presence of magnesium ion in the Ca2+-Mg2+ sites strengthens the TnC-TnI and the TnC-TnT interactions and suggest that the troponin structure would be stabilized. This likely results from the effect of magnesium ion on the Ca2+-Mg2+ domains of TnC. The presence of calcium ion in the Ca2+-specific sites provides an additional binding free energy for the TnC-TnI interaction which presumably reflects the changes in the subunit interactions required for the calcium regulatory switch.

Changes in the distribution of fast and slow forms of troponin I in some neuromuscular disorders

As judged from the atrophy of the muscle cells, the distribution of slow and fast troponin I in type I and type II cells in peripheral neuropathies in the adults (24-67 years of age) was unaffected for long periods after denervation. Reinnervation by fast or slow nerve in chronic neuropathies usually resulted in typical fibre type grouping with complete segregation of slow and fast troponin I in type I and type II cells, respectively. Intermediate fibres (slow and fast troponin I in the same cell) were present during the course of transformation. Unlike peripheral neuropathies, the number of cells staining for slow troponin I in the very atrophic fascicles in spinal muscular atrophy was considerably reduced. The synthesis of fast troponin I was stimulated in original type I cells containing slow troponin I. These observations support the experimental data (Dhoot and Perry 1982b) that the suppression of fast troponin I in type I or presumptive type I cells requires innervation by the slow nerve. Its absence results in an increased expression of fast troponin I in cells that originally contained only slow troponin I. Once suppressed, fast troponin I is absent in type I cells even after long periods of experimental denervation or cases of human peripheral neuropathies but not so in the cases of spinal muscular atrophy.

Transformation of fibre types in muscular dystrophies

In the normal human skeletal muscle, slow and fast forms of troponin I are segregated in type I and type II cells, respectively. Muscle biopsies from different dystrophies showed a large number of intermediate cells that stained with antibodies to both fast and slow troponin I. Intermediate cells of variable size were scattered at random and did not show a motor unit distribution as generally seen in some neuromuscular disorders. The preponderance of a particular cell type depended on the type of dystrophy. Except for dystrophia myotonica, all cases showed presence of small cells that looked like regenerating cells.

Regulatory proteins of the myocardium. Atrial and ventricular tropomyosin and troponin-I in the developing and adult bovine and human heart

The myofibrillar regulatory proteins, troponin-I and tropomyosin were isolated from human and bovine atria and ventricles and studied in the adult and during foetal development. A number of analytical electrophoretic procedures were employed to detect possible atrial, ventricular or foetal specific forms of these proteins. Biological activity of the regulatory protein complex during development was monitored by measuring the calcium sensitivity of the myofibrillar Mg2+ activated ATPase. No evidence was obtained for unique or foetal specific forms of either troponin I or the alpha and beta subunits of tropomyosin. Although the atria and ventricles possess markedly different contractile properties, no difference was observed between the two chambers at any developmental stage in the relative amounts of alpha and beta tropomyosin present. However, the relative amount of beta tropomyosin increased by 50% in both atria and ventricles during the transition from foetus to adult. A strong inverse correlation existed (r = -0.92) between beta tropomyosin content and heart rate in different species and at different developmental stages in both cardiac chambers. The relative invariance of tropomyosin and troponin I forms in the myocardium was reflected in the high and constant level of calcium sensitivity of myofibrillar Mg2+ ATPase retained in the atria and ventricles throughout development. The implications of these results in relation to control of cardiac contraction are discussed.

Effect of isoproterenol on protein phosphorylation in myocardial ischaemia

Perfused rat hearts prelabelled with 32P were made ischaemic by reducing the medium flow from 12 ml/min to 0.5 ml/min. There was a rapid decrease in the contractile performance, but no significant changes in the phosphorylation state of troponin I, myosin P-light chain, an 11 K protein or in the proportion of phosphorylase in the a form occurring up to 5 min of ischaemia. Control hearts stimulated with a bolus of isoproterenol showed a large increase in the contractile force and in the phosphorylation of troponin I, 11 K protein, and phosphorylase, respectively. These responses were progressively reduced by increasing periods of ischaemia. The reduction and loss of increased phosphorylation of these proteins on exposure to isoproterenol was parallelled with an inhibition of cyclic AMP accumulation in the ischaemic heart. Phosphorylation of the myosin P-light chain remained unchanged under all the conditions studied.

Interactions among chymotryptic troponin T subfragments, tropomyosin, troponin I and troponin C

The binding of various combinations of chymotryptic troponin T subfragments, troponin I and troponin C to tropomyosin, troponin C and troponin I was examined semiquantitatively by using affinity chromatography. The interaction between troponin T2 and troponin I intensified the interaction between troponin T2 (or troponin T) and tropomyosin. When a mixture of troponin T2 and troponin C was applied to a tropomyosin-Sepharose 4B column, neither troponin T2 nor troponin C was retained in the presence of Ca2+ ion, while only troponin T2 was bound in the absence of Ca2+ ion. Such a Ca2+-dependent effect was not observed with troponin T. Troponin T2 subfragments, except troponin T2 beta III, were retained by troponin C-Sepharose 4B in the presence of troponin I, even in the solution containing 1.0 M NaCl, in the presence and absence of Ca2+ ion. On the basis of these findings, the interactions among troponin components and tropomyosin are discussed.

Phosphorylation of skeletal-muscle troponin I and troponin T by phospholipid-sensitive Ca2+-dependent protein kinase and its inhibition by troponin C and tropomyosin

Skeletal-muscle troponin I and troponin T were found to be rapidly phosphorylated by cardiac phospholipid-sensitive Ca2+-dependent protein kinase, with Km values of 6.66 and 0.13 microM respectively. Stoichiometric phosphorylation of skeletal troponin I (endogenous phosphate content 0.7 mol/mol) indicated that the Ca2+-dependent enzyme and cyclic AMP-dependent protein kinase incorporated 0.9 and 0.8 mol/mol respectively. The same experiments with skeletal troponin T (endogenous phosphate content 1.9 mol/mol) revealed a maximal phosphorylation of 2 mol/mol by the Ca2+-dependent enzyme, whereas the cyclic AMP-dependent enzyme was unable to phosphorylate troponin T. The Ca2+-dependent enzyme phosphorylated both serine and threonine residues in skeletal and cardiac troponin I or troponin T; the cyclic AMP-dependent enzyme, in comparison, phosphorylated only serine in skeletal and cardiac troponin I. Although an equimolar amount of skeletal or cardiac troponin C markedly inhibited (80-90%) phosphorylation of skeletal and cardiac troponin I by the Ca2+-dependent enzyme, these troponin C preparations inhibited only phosphorylation of skeletal troponin I, but not that of cardiac troponin I, by the cyclic AMP-dependent enzyme. Calmodulin and Ca2+-binding protein S-100a could mimic the inhibitory effect of troponin C. A tissue specificity appeared to exist for the skeletal troponin T-skeletal troponin C interaction. Inhibition of troponin T phosphorylation by an equimolar amount of troponin C was lower than that of troponin I phosphorylation; these findings might explain in part why troponin T was the major substrate for the Ca2+-dependent enzyme in the troponin complex. The present studies indicate that skeletal and cardiac troponin I and troponin T were effective substrates for phospholipid-sensitive Ca2+-dependent protein kinase, suggesting a potential involvement of this Ca2+-effector enzyme in the regulation of myofibrillar activity.

Neural influences on the distribution of troponin I isotypes in the cat

An immunohistochemical technique was used to study changes in the distribution of fast and slow forms of troponin I (TN-I) in response to alterations in the nerve supply. Hind limb muscles from normal, spinal-isolated, and cordotomized cats, one leg of which had undergone cross innervation between slow (soleus) and fast (flexor hallucis longus, FHL) muscles, were examined. At 8 months after cross innervation of normal soleus by the FHL nerve, the number of fast TN-I-positive cells had increased from 0.26 to 22.1%. At 8 months after cross innervation of normal FHL muscle with the soleus nerve, the number of fast TN-I-positive cells had decreased from 86.5 to 30.5%. The number of intermediate cells staining for both fast and slow TN-I, increased considerably after cross innervation of both soleus and FHL muscles. Spinal isolation by itself had a dramatic effect on the distribution of fast and slow TN-I, converting almost all the originally slow fibers in the FHL and 60.0% of the soleus fibers to fast TN-I-positive cells in 8 months. Cordotomy, in contrast, produced an increase of only 15.6% in the soleus, and did not change the FHL. There was no quantitative difference in the crossed-and uncrossed muscles of spinal isolated cats. In cordotomized cats, cross innervation of the soleus by the FHL nerve resulted in 32.3% fast TN-I-positive cells, with some fiber type grouping. Thus, distribution of fast and slow forms of TN-I changed after each neural manipulation which altered amounts and patterns of muscle contraction and stretch.

Forskolin inhibits tension development in detergent-treated cardiac muscle fibers

In detergent-treated cardiac muscle fibers, forskolin, a potent activator of adenylate cyclases, inhibits tension development elicited with submaximal [Ca2+] and increases incorporation of 32P into troponin-I. A similar reduced tension development has been observed after treatment with cAMP or the catalytic subunit of the cAMP-dependent protein kinase. It is concluded that these fibers still contain much of the enzyme cascade involved in evoking a contractile response to beta-adrenergic stimulation.

The use of synthetic peptides for defining the specificity of typrosine protein kinases

The tyrosine protein kinases are a large family of enzymes that may be involved in regulating cell growth and differentiation. An important property of these enzymes is their substrate specificity. Defining the specificity of these enzymes will contribute to a greater understanding of their biological functions. Synthetic peptides provide a useful means for studying the specificities of protein kinases. The utility of synthetic peptides for specificity studies is exemplified by the results obtained with the cyclic nucleotide-dependent protein kinases. A large number of synthetic peptides have been tested as substrates for these enzymes. There are three important conclusions from this work. First, in terms of primary sequence, the two cyclic nucleotide-dependent protein kinases both require the presence of a pair of basic residues on the N-terminal side of the phosphorylatable residue. Second, recognition of a specific secondary structure in the substrate is an equally important factor in the substrate specificities of these enzymes. Third, the specificities of the two cyclic nucleotide-dependent protein kinases are remarkably similar in terms of both primary and secondary structure recognition. Sequences at the sites of tyrosine phosphorylation often show the presence of numerous acidic residues on the N-terminal side of the tyrosine. This result led to the suggestion that these acidic residues might be important for the recognition of these sites by the tyrosine protein kinases. Specificity studies using synthetic peptides have provided some experimental verification of this concept. In the case of four tyrosine protein kinases, LSTRA cell tyrosine protein kinase, epidermal growth factor receptor kinase, insulin receptor kinase and pp60gag-yes tyrosine protein kinase, the presence of acidic residues on the N-terminal side of the tyrosine in a synthetic peptide was a favorable determinant. In the case of a fifth member of the tyrosine protein kinase family, namely pp60src kinase, the data are less clear that the presence of acidic residues is involved in substrate recognition. This enzyme readily phosphorylated several peptides that did not have acidic residues on the N-terminal side of the tyrosine. Although there is some indication that the presence of acidic residues on the N-terminal side of the tyrosine is a factor in substrate recognition by several of the tyrosine protein kinases, the changes in kinetic parameters with the various peptide substrates were rather small. In addition, some sites phosphorylated in proteins do not have any acidic residues on the N-terminal side of the phosphorylated tyrosine residue.(ABSTRACT TRUNCATED AT 400 WORDS)

Static and kinetic studies on rabbit skeletal muscle troponin

Fluorescence titration curves of 2-[4'-iodoacetamido)anilino)naphthalene-6-sulfonic acid-labeled troponin (IAANS-labeled Tn) and troponin-1-anilinonaphthalene-8-sulfonic acid (Tn-ANS) complex indicated that the fluorescent moiety, IAANS or ANS, detects conformational change of troponin I (TnI) or Tn due to the Ca2+ binding or removal reaction with the low affinity Ca2+-binding sites of troponin C (TnC) component. A fluorescence stopped-flow study showed that the kinetic behavior of IAANS-labeled Tn reflects a change in state of the TnI component induced by the Ca2+ binding or removal reaction with the low affinity Ca2+-binding sites of TnC component. The state change of TnI induced by the Ca2+ binding was complete within the instrumental dead time. On the other hand, that induced by the Ca2+ removal had a rate constant of around 13 s-1. ANS, which is noncovalently bound to Tn, reflects the kinetic properties of both the TnI component and the low affinity Ca2+-binding region of TnC component. The fluorescence intensity change of ANS induced by Ca2+ binding to the low affinity Ca2+-binding sites of TnC was complete within the instrumental dead time, while that induced by the Ca2+ removal from the same sites was biphasic. The rate constants of the biphasic process were found to be 62 +/- 7 s-1 and 16 +/- 4 s-1. The former value corresponds to the rate constant of the Ca2+ removal reaction from the low affinity Ca2+-binding sites of TnC component, and the latter value to the rate constant observed in the case of IAANS-labeled Tn. Based on these experimental results and on the discussion in our previous paper (Iio, T. & Kondo, H. (1981) J. Biochem. 90, 163-175), we have refined the two-way information-transfer mechanism which we previously proposed in order to explain the biological function of Tn.

High affinity binding of the mastoparans by calmodulin

Calmodulin exhibits high affinity, calcium-dependent binding of the mastoparans--a group of cytoactive tetradecapeptides. The dissociation constants for the peptide-calmodulin complexes determined in 0.20 N KCl, 1.0 mM CaCl2, pH 7.3 are approximately 0.3 nM for mastoparan, approximately 0.9 nM for mastoparan X, and approximately 3.5 nM for Polistes mastoparan. The dissociation constant for the mastoparan-calmodulin complex is the smallest known for any calmodulin binding protein or peptide, suggesting that some type of peptide-calmodulin interaction could be physiologically significant.

Phosphorylation of the myofibrillar proteins and the regulation of contractile activity in muscle

Evidence now exists for the phosphorylation of all the major proteins of the myofibril with the exception of troponin C. Although uncertainty exists in most cases about the role of phosphorylation of the myofibrillar proteins, there is substantial evidence that phosphorylation of serine 20 of rabbit cardiac troponin I leads to a lowering of the sensitivity of the actomyosin ATPase to Ca2+. This process is of special importance in the physiological response of the heart to adrenalin. A well defined enzymic system involving a specific kinase and a phosphatase is present in most muscles for the phosphorylation and dephosphorylation of the P light chain (regulatory, L2 or DTNB light chain) of myosin. Myosin light-chain kinase is very active in fast skeletal muscles, and although it is unlikely that phosphorylation followed by dephosphorylation of the P light chain occurs fast enough to be synchronous with the contractile cycle, phosphorylation may have a modulatory role in this tissue. Both post-tetanic potentiation and the reduced actomyosin ATPase turnover rate observed in fast-twitch muscle as a consequence of sustained forceful contraction have been suggested by different investigators to be consequences of P light chain phosphorylation. Nevertheless, unequivocal evidence associating either of these effects with phosphorylation is not yet available. Kinase activity is also high in vertebrate smooth muscle and it has been suggested that phosphorylation of the P light chain is the process that activates the actomyosin ATPase in this tissue. Evidence from a number of studies indicates, however, that regulation of smooth muscle actomyosin ATPase may not be a simple phosphorylation-dephosphorylation process.

Cardiac function and phosphorylation of contractile proteins

The primary regulation of cardiac contractility is probably through changes in the level of cytoplasmic free Ca 2+ . In the stimulation of contraction by catecholamines, secondary controls may be present at the level of the contractile proteins. Troponin-I, a subunit of the troponin complex of the thin filament, and C-protein, a thick filament component, are both phosphorylated in perfused hearts in response to catecholamines over time courses similar to that for the increase in contraction. Both proteins are also phosphorylated rapidly in vitro by cyclic-AMP-dependent protein kinase. Phosphorylation of troponin-I causes a decrease in the sensitivity of both cardiac myofibrillar ATPase and tension development of skinned fibre preparations to Ca 2+ , and also an increase in the rate of dissociation of Ca 2+ from isolated troponin. These results support the hypothesis that the role of phosphorylation of cardiac troponin-I is to contribute to the increased rate of relaxation of the heart that is observed with catecholamines. C-protein is phosphorylated to a maximum of 4-5 mol phosphate per mole protein both in vivo and in vitro . At present, however, the functions of both C-protein itself and its phosphorylation are unknown. Dephosphorylation of these contractile proteins after catecholamine stimulation is slow in perfused heart, although the rate can be increased by cholinergic agents. Phosphorylase, in contrast, is rapidly dephosphorylated under these circumstances. Phosphoprotein phosphatases relatively specific for phosphorylase have been identified in rat heart, whereas troponin-I appears to be dephosphorylated by general phosphatases. These observations account for the different rates of dephosphorylation of phosphorylase and the contractile proteins, but do not explain the slow dephosphorylation of the latter. In control perfused hearts myosin P-light chain was 50 % phosphorylated, and this was not changed by perfusion with positive inotropic agents or by short-term ischaemia. It was also unchanged during long-term hormonal modifications. Perfusions with 32 P 1 in rat heart give a half-time for the turnover of phosphate bound to the P-light chain of 2-4 min, showing that the myosin light chain kinase and phosphatase are active in the heart. It is hypothesized that under control conditions the kinase is already fully active, and that an increase in cytoplasmic Ca 2+ cannot therefore cause further activation of the enzyme.

Regulation of muscle phosphorylase kinase by actin and calmodulin

The activation of muscle phosphorylase kinase b by actin has been studied. F-actin which is polymerized by 2 mM MgCl2 is a more effective activator of phosphorylase kinase than F-actin polymerized by 50 mM KCl. There is evidence suggesting that the activation of phosphorylase kinase by actin is not due to trace contamination of actin preparations with calmodulin: (1) Troponin I and trifluoperazine inhibit the activation of phosphorylase kinase by calmodulin but do not inhibit the activation of phosphorylase kinase by F-actin. (2) The activation induced by saturating concentrations of calmodulin and actin is additive both at pH 8.2 and at pH 6.8. (3) The activation of phosphorylase kinase by calmodulin and actin has different pH profiles. An addition of F-actin does not affect the apparent Km value for ATP but increases the sensitivity to phosphorylase b and the value of Vmax.

Cyclic adenosine monophosphate effects on the myocardium: a man who blows hot and cold with one breath

Agents that increase cellular cyclic adenosine monophosphate (cyclic AMP) levels exert several effects on cardiac function. These include increases in heart rate and both the force of contraction and rate of relaxation. The latter effects, which might appear to be contradictory, actually represent essential components of the response needed to allow the heart to increase its stroke volume when heart rate is accelerated. This article reviews the mechanisms by which cyclic AMP exerts both contraction-promoting and relaxation-promoting effects in the integrated response of the heart to sympathetic stimulation.

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.

Troponin subunit stoichiometry and content in rabbit skeletal muscle and myofibrils

The quantity and molar ratio of the three troponin subunits to actin were determined in rabbit psoas muscle, muscle homogenates (800 X g pellet), and purified myofibrils. Proteins were separated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The quantities of the separated proteins were determined directly from the gel slices by amino acid analysis after correction for losses and background. The molar ratio of actin, troponin T, troponin I, and troponin C was found to be 6.99:1:05:1:04:0.92 in purified myofibrils and was not significantly different (p greater than 0.05) from those obtained from 800 X g pellets of muscle homogenates or intact muscle tissue. Isolated troponin purified by several different procedures also had a 1:1:1 subunit ratio although the variability was much greater than that found in myofibrils. The troponin content of rabbit psoas muscle and myofibrils was 91 +/- 16 and 770 +/- 110 pmol/mg, respectively.

On the interrelation between calmodulin and EGTA in the regulation of the affinity to Ca2+ and the maximal activity of the erythrocyte-membrane calcium pump

1. Calmodulin distribution in rat erythrocytes and the interrelation between calmodulin and EGTA in regulation of the rate of 45Ca accumulation by erythrocyte-membrane inside-out vesicles were studied. 2. The total content of calmodulin in rat erythrocytes is about 24 mumol/l of cells. About 60% of calmodulin is localized in cytoplasm and 40% (10 mumol/l of cells) of calmodulin is loosely bound to membranes; after subsequent washing by hypotonic solution (membranes A) the content of membrane-bound calmodulin decreases up to 0.1 mumol/l of cells. 3. The addition of exogenous calmodulin to membranes A results in increase of the maximal activity of the Ca-pump and does not influence its affinity to Ca2+. Troponin I (30 microM) completely abolishes the calmodulin effect on the Ca-pump activity without significant alterations in its basal activity. 4. The addition of EGTA in the membrane-washing solution results in decrease of the membrane-found pool calmodulin up to 0.01 mumol/l of cells (membranes B). This procedure is accompanied by decrease of the affinity of Ca-pump to Ca2+ and does not influence its maximal rate. 5. The effect of EGTA treatment (membranes B) on the affinity of Ca-pump to Ca is abolished after addition of micromolar concentrations of calmodulin or millimolar concentration of EGTA in the incubation medium. The increase of EGTA concentration in the incubation medium results in decrease of the affinity of Ca-pump to calmodulin. 6. It is assumed that two essentially different pools of calmodulin participate in the regulation of the activity of Ca-pump. (a) The pool of calmodulin which is loosely bound to membrane (this size is dependent on calmodulin concentration in cytoplasm) determines the maximal activity of Ca-pump. The effect of this calmodulin pool is blocked by troponin I. (b) The tightly bound pool of calmodulin which is removed by EGTA treatment determines the affinity of Ca-pump to Ca. 7. In this connection the reasons for contradictory data on estimation of calmodulin effect on the kinetic parameters of plasma membrane Ca-pump are discussed.

Sensitivity of actomyosin ATPase to calcium and strontium ions. Effect of hybrid troponins

1. Hybrid or reconstituted troponins were prepared from troponin components of rabbit skeletal muscle and porcine cardiac muscle and their effect on the actomyosin ATPase activity was measured at various concentrations of Ca2+ or Sr2+. The Ca2+ concentration required for half-maximum activation of actomyosin ATPase with troponin containing cardiac troponin I was slightly higher than that with troponin containing skeletal troponin I. The Sr2+ concentration required for half-maximum activation of actomyosin ATPase with troponin containing skeletal troponin C was higher than that with troponin containing cardiac troponin C. 2. Reconstituted cardiac troponin was phosphorylated by cyclic AMP-dependent protein kinase. The Ca2+ sensitivity of actomyosin ATPase with cardiac troponin decreased upon phosphorylation of troponin I; maximum ATPase activity was depressed and the Ca2+ concentration at half-maximum activation increased. On the other hand, phosphorylation of troponin I did not change Sr2+ sensitivity. 3. The inhibitory effect of cardiac troponin I on the actomyosin ATPase activity was neutralized by increasing the amount of brain calmodulin at high Ca2+ and Sr2+ concentrations but not at low concentrations. 4. ATPase activity of actomyosin with a mixture of troponin I and calmodulin was assayed at various concentrations of Ca2+ or Sr2+. The Ca2+ or Sr2+ sensitivity of actomyosin ATPase containing skeletal troponin I was approximately the same as that of actomyosin ATPase containing cardiac troponin I. Phosphorylation of cardiac troponin I did not change the Ca2+ sensitivity of the ATPase. 5. The Ca2+ or Sr2+ concentration required for half-maximum activation of actomyosin ATPase with troponin I-T-calmodulin was higher than that of actomyosin ATPase with the mixture of troponin I and calmodulin. Maximum ATPase activity was lower than that with the mixture of troponin I and calmodulin.

Hydrodynamic properties of bovine cardiac troponin-I and troponin-T

Bovine cardiac troponin-I (TN-I) and troponin-T (TN-T) have been examined in solution using ultracentrifugation, gel filtration, and viscosity. A new method of purifying TN-T, employing hydroxylapatite chromatography in 6 M urea, is reported. Cardiac TN-T (Mr = 36,000) undergoes a reversible, concentration-dependent association in nondenaturing buffers, probably to a tetramer. The Stokes radius (Rs) of aggregated TN-T, determined by sedimentation velocity and gel chromatography on Sephacryl S-300, is 80 A and the reduced viscosity of the subunit ranges from 20 to 25 ml/g at protein concentrations between 0.5 and 2.5 mg/ml. These data suggest that TN-T forms highly asymmetric aggregates in solution. Bovine cardiac TN-I also has a tendency toward self-association, but is essentially monomeric (Mr = 23,000) at protein concentrations below 1 mg/ml. The presence of reducing agent is necessary to avoid intermolecular disulfide bond formation. From gel filtration experiments, the value of Rs is 29 A indicating that TN-I is a moderately asymmetric protein (frictional ratio = 1.5). Similar properties are observed when both sulfhydryl groups of TN-I are modified by carboxamidomethylation.

Effects of troponin-I plus-C on the binding of troponin-T and its fragments to alpha-tropomyosin. Ca2+ sensitivity and cooperativity

The binding of troponin-I to tropomyosin, as well as its effects on the binding of troponin-T and its fragments T1 (residues 1-158) and T2 (residues 159-259) to tropomyosin, have been studied using immobilized alpha-tropomyosin. When applied alone, troponin-I exhibited weak interaction with tropomyosin and was eluted with a NaCl gradient at 0.12 M. Intact troponin-T was eluted at 0.40 M NaCl, while its fragment T1 was eluted from site 1 (near the COOH terminus of tropomyosin) at 0.32 M, independently from T2, which was eluted from site 2 (near Cys-190) at 0.22 M NaCl. However, the simultaneous presence of troponin-I and T2 resulted in formation of a strong troponin-I/T2/tropomyosin ternary complex at site 2 such that troponin-I/T2 complex was now eluted at 0.45 to 0.5 M NaCl. This binding was Ca2+-sensitive in the presence of troponin-C. An additional effect of troponin-I binding at site 2 was the strengthening of the T1/tropomyosin interaction at site 1, such that T1 was now eluted at the higher value of 0.45 to 0.50 M NaCl. Troponin-I also enhanced the binding of intact troponin-T to tropomyosin. These results suggest that cooperativity exists between the two sites, presumably induced by the binding of troponin-I to tropomyosin and mediated by a conformational change in the latter.

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.

Interaction with tropomyosin, troponin I and troponin C

The binding of the chymotryptic troponin T subfragments to tropomyosin, troponin I, and troponin C was semiquantitatively examined by using affinity chromatography, and also by co-sedimentation with F-actin and polyacrylamide gel electrophoresis in 14 mM Tris/90 mM glycine. Circular dichroism spectra of the subfragments were measured to confirm that the subfragments retained their conformational structures. Based on these results, the binding sites of tropomyosin, troponin I, and troponin C on the troponin T sequence were elucidated. Tropomyosin bound mainly to the region of troponin T1 (residues 1-158) with the same binding strength as to the original troponin T. The C-terminal region of troponin T (residues 243-259) was the second binding site to tropomyosin under physiological conditions. The binding site of troponin I was concluded to be the region including residues 223-227. The binding of troponin C was dependent on Ca2+ ion concentration. The C-terminal region of troponin T2 (residues 159-259) was indicated to be the Ca2+-independent troponin C-binding site and the N-terminal side of troponin T2 to be the Ca2+-dependent site.

Studies of the interaction of troponin I with proteins of the I-filament and calmodulin

1. All lysine residues in native troponin I from rabbit fast-twitch skeletal muscle reacted with methyl acetimidate and ethyl acetimidate. 2. The reactivity of lysine-18 of troponin I to acetimidate was much diminished when the troponin I was complexed in the presence of Ca2+ with troponin C alone or in the whole troponin complex. 3. In the presence of EGTA, lysine-18 of troponin I in the troponin I-troponin C complex was more reactive to acetimidate than it was in the presence of Ca2+. 4. No masking of lysine residues could be detected when troponin I interacted with calmodulin or actin. 5. Sedimentation-equilibrium studies indicated that the complex of troponin I with calmodulin was more readily dissociated in the absence of Ca2+ than was its complex with troponin C under otherwise identical conditions. 6. These studies suggest that the nature of the involvement of the N-terminal region of troponin I is a major difference between its modes of interaction with calmodulin and with troponin C.

Calmodulin antagonists' binding sites on calmodulin

Troponin I inhibited, concentration-dependently, [3H]-N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and [3H]-trifluoperazine (TFP) binding to purified bovine brain calmodulin (CaM). Selective oxidation of methionine residues of CaM by N-chlorosuccinimide resulted in a rapid decrease in [3H]-W-7, [3H]-TFP and [14C]-chlorpromazine binding concomitant with the loss of CaM activity. Carbethoxylation of histidine residues, nitration of tyrosine residues and chemical modification of arginine residues with 1,2-cyclohexanedione produced no significant changes either in [3H]-W-7 binding to CaM or in the ability of CaM to stimulate phosphodiesterase. Our results suggest that the binding sites of these CaM antagonists on CaM may be located between the second and third Ca2+-binding loops.

Phosphorylation of cardiac troponin inhibitory subunit (troponin I) and tropomyosin-binding subunit (troponin T) by cardiac phospholipid-sensitive Ca2+-dependent protein kinase

Cardiac phospholipid-sensitive Ca2+-dependent protein kinase phosphorylated cardiac troponin inhibitory subunit (troponin I) and tropomyosin-binding subunit (troponin T), present either as the free form or as the troponin-tropomyosin complex. Exhaustive phosphorylation of troponin I and of troponin T revealed that 1.7 and 2 mol of phosphate was incorporated/mol of the subunits respectively. Cyclic AMP-dependent protein kinase, though incorporating 0.8 mol of phosphate/mol of troponin I, was unable to phosphorylate troponin T. Phosphorylation of troponin I (apparent Km = 3.4 microM; Vmax. = 2.6 mumol/min per mg of enzyme) or troponin T (apparent Km = 0.3 microM; Vmax. = 0.5 mumol/min per mg of enzyme) by the Ca2+-dependent enzyme was inhibited by various agents, such as adriamycin, palmitoylcarnitine, trifluoperazine, melittin and N-(6-aminohexyl)-5-chloronaphthalene-1-sulphonamide (compound W-7). Ca2+ antagonists (such as verapamil), forskolin and ouabain were ineffective. These findings indicate that troponin I and troponin T were effective substrates for this species of Ca2+-dependent protein kinase, suggesting its potential regulatory role in the contractile activity of myofibrils modulated by troponin.

Interaction of a fluorescent N-dansylaziridine derivative of troponin I with calmodulin in the absence and presence of calcium

Rabbit skeletal muscle troponin I was covalently labeled with N-dansylaziridine, resulting in a fluorescent labeled protein. This derivative (DANZTnI) and native troponin I (TnI) inhibited calmodulin (CaM) stimulation of bovine heart Ca2+-sensitive cyclic nucleodite phosphodiesterase with identical inhibition constants. Association of DANZTnI with calmodulin was monitored directly by changes in flourescence intensity in the presence of Ca2+ and by changes in fluorescence anisotropy in the absence of Ca2+. Quantitation of the affinity of calmodulin for calmodulin-binding proteins in both the presence and absence of Ca2+ is necessary for prediction of the extent of interaction of both Ca2+ and calmodulin-binding proteins with calmodulin in vivo. The dissociation constants for the DANZTnI-calmodulin-l4Ca2+ and DANZTnI-calmodulin complexes were 20 nM and 70 micrometers, respectively. These dissociation constants define a free energy coupling of-4.84 kcal/mol of troponin I for binding of Ca2+ and troponin I to calmodulin. The Ca2+ dependence for troponin I-calmodulin complex formation predicted from these experimentally determined parameters was closely approximated by the Ca2+ dependence for complex formation between troponin I and fluorescent 5-[[[(iodoacetyl)amino]ethyl]-amino]-1-napthalenesulfonic acid derivatized calmodulin as determined by fluorescence anisotropy. Complex formation occurred over a relatively narrow range of Ca2+ concentration, indicative of positive heterotropic cooperativity for Ca2+ and troponin I binding to calmodulin.

Troponin I-troponin T interactions

The heterobifunctional photoaffinity probe, AGTC (N-(4-azidobenzoyl-[2-3H]glycyl)-S-(2-thiopyridyl) cysteine), was attached to cysteines 48 and 64 of troponin I to determine which component of rabbit skeletal troponin was in close proximity to these cysteines. The reconstituted troponin complex (AGC labeled CM-TnI, TnT, and TnC) was photolyzed and separated using DEAE-Sephadex chromatography in the presence of 8 M urea and absence of reducing agent. Radioactivity measurements indicated that 12% of the cross-linker reacted with solvent and 88% with protein. The percentage of radiolabel found in TnI, TnI-TnT, and TnI-TnC complexes was 35, 55, and 10%, respectively. These results have shown that both TnT and TnC are within 14 A of one or both cysteines 48 and 64 of TnI. Of the total radiolabel found in TnT, 33 and 23% was found in the two cyanogen bromide fragments, CB4 (residues 176-230) and CB2 (residues 71-151). The most likely interpretation of the cross-linking results is that one of the interaction sites between TnI and TnT is an ionic interaction involving the region around cysteines 48 and 64 of TnI (residues 28-82) with the CB5 region of TnT (residues 135-185).

Study of the structure of troponin-I by measuring the relative reactivities of lysines with acetic anhydride

A competitive labeling method that measures the relative reactivity of lysines was used to study the structure of troponin-I. Troponin-I was acetylated free and complexed with troponin-C and troponin-T in the native state with [3H]acetic anhydride. The [3H]troponin-I was combined with [14C]troponin-I that had been acetylated in 6 M guanidine HCl and completely chemically labeled. Peptides containing labeled lysines were isolated following digestion with trypsin and Staphylococcus aureus protease and identified in the published sequence. The 3H/14C ratio of these peptides was used as a measure of the relative reactivity of the lysines. Troponin-I contains 24 lysines; we have identified 23 of these in 16 peptides. When troponin-I is labeled in a native complex, the lysines in the region from residues 40 to 98 are influenced: five become relatively less reactive (40, 65, 70, 78, and 90) and three become relatively more reactive (84, 87), and 98). All of these changes except Lys 70 can be seen when troponin-I binds to troponin-T. Lys 70 is reduced in reactivity when it binds to troponin-C. The lysines that appear to be important in binding of troponin-I to troponin-T are influenced by the binding of Ca2+ to troponin-C in the native troponin complex (in the presence of 2 mM MgCl2), suggesting for the first time that the troponin-IT interaction is affected by Ca2+.