Phosphorylation of troponin and the effects of interactions between the components of the complex

Regulation of Cardiac PKA Signaling by cAMP and Oxidants

Pathologies, such as cancer, inflammatory and cardiac diseases are commonly associated with long-term increased production and release of reactive oxygen species referred to as oxidative stress. Thereby, protein oxidation conveys protein dysfunction and contributes to disease progression. Importantly, trials to scavenge oxidants by systemic antioxidant therapy failed. This observation supports the notion that oxidants are indispensable physiological signaling molecules that induce oxidative post-translational modifications in target proteins. In cardiac myocytes, the main driver of cardiac contractility is the activation of the β-adrenoceptor-signaling cascade leading to increased cellular cAMP production and activation of its main effector, the cAMP-dependent protein kinase (PKA). PKA-mediated phosphorylation of substrate proteins that are involved in excitation-contraction coupling are responsible for the observed positive inotropic and lusitropic effects. PKA-actions are counteracted by cellular protein phosphatases (PP) that dephosphorylate substrate proteins and thus allow the termination of PKA-signaling. Both, kinase and phosphatase are redox-sensitive and susceptible to oxidation on critical cysteine residues. Thereby, oxidation of the regulatory PKA and PP subunits is considered to regulate subcellular kinase and phosphatase localization, while intradisulfide formation of the catalytic subunits negatively impacts on catalytic activity with direct consequences on substrate (de)phosphorylation and cardiac contractile function. This review article attempts to incorporate the current perception of the functionally relevant regulation of cardiac contractility by classical cAMP-dependent signaling with the contribution of oxidant modification.

Further Investigation into the Biochemical Effects of Phosphorylation of Tropomyosin Tpm1.1(α). Serine-283 Is in Communication with the Midregion

The phosphorylated and unphosphorylated forms of tropomyosin Tpm1.1(α) are prepared from adult rabbit heart and compared biochemically. Electrophoresis confirms the high level of enrichment of the chromatography fractions and is consistent with a single site of phosphorylation. Covalently bound phosphate groups at position 283 of Tpm1.1(α) increase the rate of digestion at Leu-169, suggestive of a conformational rearrangement that extends to the midregion. Such a rearrangement, which is supported by ellipticity measurements between 25 and 42 °C, is consistent with a phosphorylation-mediated tightening of the interaction between various myofilament components. In a nonradioactive, co-sedimentation assay [30 mM KCl, 1 mM Mg(II), and 4 °C], phosphorylated Tpm1.1(α) displays a higher affinity for F-actin compared to that of the unphosphorylated control (K_d, 0.16 μM vs 0.26 μM). Phosphorylation decreases the concentration of thin filaments (pCa 4 plus ATP) required to attain a half-maximal rate of release of product from a pre-power stroke complex [myosin-S1-2-deoxy-3-O-(N-methylanthraniloyl)ADP-P_i], as investigated by double-mixing stopped-flow fluorescence, suggestive of a change in the proportion of active (turned on) and inactive (turned off) conformers, but similar maximum rates of product release are observed with either type of reconstituted thin filament. Phosphorylated thin filaments (pCa 4 and 8) display a higher affinity for myosin-S1(ADP) versus the control scenario without affecting isotherm steepness. Specific activities of ATP and Tpm1.1(α) are determined during an in vitro incubation of rat cardiac tissue [12 day-old, 50% phosphorylated Tpm1.1(α)] with [³²P]orthophosphate. The incorporation of an isotope into tropomyosin lags behind that of ATP by a factor of approximately 10, indicating that transfer is a comparatively slow process.

Troponin I modulation of cardiac performance: Plasticity in the survival switch

Signaling complexes targeting the myofilament are essential in modulating cardiac performance. A central target of this signaling is cardiac troponin I (cTnI) phosphorylation. This review focuses on cTnI phosphorylation as a model for myofilament signaling, discussing key gaps and future directions towards understanding complex myofilament modulation of cardiac performance. Human heart cTnI is phosphorylated at 14 sites, giving rise to a complex modulatory network of varied functional responses. For example, while classical Ser23/24 phosphorylation mediates accelerated relaxation, protein kinase C phosphorylation of cTnI serves as a brake on contractile function. Additionally, the functional response of cTnI multi-site phosphorylation cannot necessarily be predicted from the response of individual sites alone. These complexities underscore the need for systematically evaluating single and multi-site phosphorylation on myofilament cellular and in vivo contractile function. Ultimately, a complete understanding of these multi-site responses requires work to establish site occupancy and dominance, kinase/phosphatase signaling balance, and the function of adaptive secondary phosphorylation. As cTnI phosphorylation is essential for modulating cardiac performance, future insight into the complex role of cTnI phosphorylation is important to establish sarcomere signaling in the healthy heart as well as identification of novel myofilament targets in the treatment of disease.

Phosphorylation of myofibrillar proteins in post-mortem ovine muscle with different tenderness

BACKGROUND

Tenderness is one of the most important quality attributes especially for beef and lamb. As protein phosphorylation and dephosphorylation regulate glycolysis, muscle contraction and turnover of proteins within living cells, it may contribute to the conversion of muscle to meat. The changes of myofibrillar protein phosphorylation in post-mortem ovine muscle with different levels of tenderness were investigated in this study.

RESULTS

The protein phosphorylation level (P/T ratio) of the tender group increased from 0.5 to 12 h post mortem and then decreased. The P/T ratio of tough group increased during 24 h post mortem, increasing faster from 0.5 to 4 h post mortem than from 4 to 24 h post mortem.The global phosphorylation level of tough meat was significantly higher than tender meat at 4, 12 and 24 h post mortem (P < 0.05). Protein identification revealed that most of the phosphoproteins were proteins with sarcomeric function; the others were involved in glycometabolism, stress response, etc. The phosphorylation levels of myofibrillar proteins, e.g. myosin light chain 2 and actin, were significantly different among groups of different tenderness and at different post-mortem time points (P < 0.05).

CONCLUSION

Protein phosphorylation may influence meat rigor mortis through contractile machinery and glycolysis, which in turn affect meat tenderness.

Functional phosphorylation sites in cardiac myofilament proteins are evolutionarily conserved in skeletal myofilament proteins

Protein phosphorylation plays an important role in regulating cardiac contractile function, but phosphorylation is not thought to play a regulatory role in skeletal muscle. To examine how myofilament phosphorylation arose in the human heart, we analyzed the amino acid sequences of 25 cardiac phosphorylation sites in animals ranging from fruit flies to humans. These analyses indicated that of the 25 human phosphorylation sites examined, 11 have been conserved across vertebrates and four have been sporadically present in vertebrates. Furthermore, all 11 of the cardiac sites found across vertebrates were present in skeletal muscle isoforms, along with three sites that were sporadically present. Based on the conservation of amino acid sequences between cardiac and skeletal contractile proteins, we tested for phosphorylation in mammalian skeletal muscle using several biochemical techniques and found evidence that multiple myofilament proteins were phosphorylated. Several of these phosphorylation sites were validated using mass spectrometry, including one site that is present in slow- and fast-twitch troponin I (TnI), but was lost in cardiac TnI. Thus, several myofilament phosphorylation sites present in the human heart likely arose in invertebrate muscle, have been evolutionarily conserved in skeletal muscle, and potentially have functional effects in both skeletal and cardiac muscle.

The regulatory beta subunit of phosphorylase kinase interacts with glyceraldehyde-3-phosphate dehydrogenase

Skeletal muscle phosphorylase kinase (PhK) is an (alphabetagammadelta) 4 hetero-oligomeric enzyme complex that phosphorylates and activates glycogen phosphorylase b (GP b) in a Ca (2+)-dependent reaction that couples muscle contraction with glycogen breakdown. GP b is PhK's only known in vivo substrate; however, given the great size and multiple subunits of the PhK complex, we screened muscle extracts for other potential targets. Extracts of P/J (control) and I/lnJ (PhK deficient) mice were incubated with [gamma- (32)P]ATP with or without Ca (2+) and compared to identify potential substrates. Candidate targets were resolved by two-dimensional polyacrylamide gel electrophoresis, and phosphorylated glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was identified by matrix-assisted laser desorption ionization mass spectroscopy. In vitro studies showed GAPDH to be a Ca (2+)-dependent substrate of PhK, although the rate of phosphorylation is very slow. GAPDH does, however, bind tightly to PhK, inhibiting at low concentrations (IC 50 approximately 0.45 microM) PhK's conversion of GP b. When a short synthetic peptide substrate was substituted for GP b, the inhibition was negligible, suggesting that GAPDH may inhibit predominantly by binding to the PhK complex at a locus distinct from its active site on the gamma subunit. To test this notion, the PhK-GAPDH complex was incubated with a chemical cross-linker, and a dimer between the regulatory beta subunit of PhK and GAPDH was formed. This interaction was confirmed by the fact that a subcomplex of PhK missing the beta subunit, specifically an alphagammadelta subcomplex, was unable to phosphorylate GAPDH, even though it is catalytically active toward GP b. Moreover, GAPDH had no effect on the conversion of GP b by the alphagammadelta subcomplex. The interactions described herein between the beta subunit of PhK and GAPDH provide a possible mechanism for the direct linkage of glycogenolysis and glycolysis in skeletal muscle.

Monoclonal antibody-based sandwich enzyme-linked immunosorbent assay for sensitive detection of prohibited ruminant proteins in feedstuffs

Regulations aimed to control the epidemic of bovine spongiform encephalopathy have banned the use of certain animal products, i.e., ruminant meat and bone meals, in ruminant animal feeds. A sensitive enzyme-linked immunosorbent assay has been developed to detect prohibited bovine and ovine muscles in feedstuffs. The assay utilizes a pair of monoclonal antibodies (MAbs) against skeletal troponin I (TnI). MAb 5G9, specific to bovine and ovine TnI, was used as the capture antibody and the biotin-conjugated MAb 2G3, reacting to all heterologous TnI, was used as the detection antibody. Quantitative procedures were applied to samples containing 5, 0.5, and 0.05% (wt/wt) of heat-treated (132 degrees C/2 bar, 2 h) bovine and ovine meat meals in three different feeds, coexisting with porcine, chicken, or turkey meat meal. The presence of these nonprohibited species did not affect the detection of bovine and ovine meat meals in the feed samples (P > 0.05). Quantitative determinations of extractable bovine and ovine TnI, with a detection limit of 5.0 and 4.0 ng/ml, respectively, were achieved when the matching feed matrixes were used in the calibration curves. This new assay provides a rapid and reliable way to detect animal protein products containing a trace amount of bovine or ovine muscle tissue in feedstuffs.

Equilibrium unfolding and conformational plasticity of troponin I and T

The structures and stabilities of recombinant chicken muscle troponin I (TnI) and T (TnT) were investigated by a combination of bis-ANS binding and equilibrium unfolding studies. Unlike most folded proteins, isolated TnI and TnT bind the hydrophobic fluorescent probe bis-ANS, indicating the existence of solvent-exposed hydrophobic domains in their structures. Bis-ANS binding to binary or ternary mixtures of TnI, TnT and troponin C (TnC) in solution is significantly lower than binding to the isolated subunits, which can be explained by burial of previously exposed hydrophobic domains upon association of the subunits to form the native troponin complex. Equilibrium unfolding studies of TnT and TnI by guanidine hydrochloride and urea monitored by changes in far-UV CD and bis-ANS fluorescence revealed noncooperative folding transitions for both proteins and the existence of partially folded intermediate states. Taken together, these results indicate that isolated TnI and TnT are partially unstructured proteins, and suggest that conformational plasticity of the isolated subunits may play an important role in macromolecular recognition for the assembly of the troponin complex.

Tissue-specific distribution of breast-muscle-type and leg-muscle-type troponin T isoforms in birds

In order to show the tissue-specific distribution of troponin T (TnT) isoforms in avian skeletal muscles, their expression was examined by electrophoresis of the breast and leg muscles of seven avian species and immunoblotting with the antiserum against fast skeletal muscle TnT. It has been reported in the chicken that breast-muscle-type (B-type) and leg-muscle-type (L-type) TnT isoforms are expressed specifically in the adult breast and leg muscles, respectively. Their differential expression patterns were confirmed in all birds examined in this study. The expression of a segment encoded by the exon x series of TnT was also examined by immunoblotting with the antiserum against a synthetic peptide derived from the exon x3 sequence, because the segment has been shown to be included exclusively in the B-type, but not in the L-type TnT. The expression of the segment was found only in the breast muscle, but not in the leg muscle of all birds examined. TnT cDNA sequences from the duck breast and leg muscles were determined and showed that only B-type TnT had an exon x-related sequence, suggesting that the expression of B-type TnT containing the exon x-derived segment is conserved consistently in the birds.

Troponin I: inhibitor or facilitator

TN-I occurs as a homologous group of proteins which form part of the regulatory system of vertebrate and invertebrate striated muscle. These proteins are present in vertebrate muscle as isoforms, Mr 21000-24000, that are specific for the muscle type and under individual genetic control. TN-I occupies a central position in the chain of events starting with the binding of calcium to troponin C and ending with activation of the Ca2+ stimulated MgATPase of the actomyosin filament in muscle. The ability of TN-I to inhibit the MgATPase of actomyosin in a manner that is accentuated by tropomyosin is fundamental to its role but the molecular mechanism involved is not yet completely understood. For the actomyosinATPase to be regulated the interaction of TN-I with actin, TN-C and TN-T must undergo changes as the calcium concentration in the muscle cell rises, which result in the loss of its inhibitory activity. A variety of techniques have enabled the sites of interaction to be defined in terms of regions of the polypeptide chain that must be intact to preserve the biological properties of TN-I. There is also evidence for conformational changes that occur when the complex with TN-C binds calcium. Nevertheless a detailed high resolution structure of the troponin complex and its relation to actin/tropomyosin is not yet available. TN-I induces changes in those proteins with which it interacts, that are essential for their function. In the special case of cardiac TN-I its effect on the calcium binding properties of TN-C is modulated by phosphorylation. It has yet to be determined whether TN-I acts directly as an inhibitor or indirectly by interacting with associated proteins to facilitate their role in the regulatory system.

Stepwise subunit interaction changes by mono- and bisphosphorylation of cardiac troponin I

Four phosphorylation degrees of cardiac troponin I (cTnI) have been characterized, namely, a dephospho, a bisphospho, and two monophospho states. Here we describe for the first time a role of the monophosphorylated forms. We have investigated the interaction between the cardiac troponin subunits dependent on the phosphorylation state of cTnI by surface plasmon resonance (SPR) spectroscopy. The monophosphorylated forms were generated by mutating each of the two serine residues, located in human cTnI at positions 22 and 23, to alanine. Association and dissociation rate constants of binary (cTnI-cTnT and cTnI-cTnC) and ternary (cTnI/cTnC complex-cTnT) complexes were determined. Mono- and consecutive bisphosphorylation of cTnI gradually reduces the affinity to cTnC and cTnT by lowering the association rate constants; the dissociation rate constants remain unchanged. Phosphorylation also affects formation of the ternary complexes; however, in this instance, association rate constants are constant, and dissociation rate constants are enhanced. A model of cardiac troponin is presented describing an induction of distinct conformational changes by mono- and bisphosphorylation of cTnI.

Troponin T: genetics, properties and function

Troponin T (TnT) is present in striated muscle of vertebrates and invertebrates as a group of homologous proteins with molecular weights usually in the 31-36 kDa range. It occupies a unique role in the regulatory protein system in that it interacts with TnC and TnI of the troponin complex and the proteins of the myofibrillar thin filament, tropomyosin and actin. In the myofibril the molecule is about 18 nm long and for much its length interacts with tropomyosin. The ability of TnT to form a complex with tropomyosin is responsible for locating the troponin complex with a periodicity of 38.5 nm along the thin filament of the myofibril. In addition to it structural role, TnT has the important function of transforming the TnI-TnC complex into a system, the inhibitory activity of which, on the tropomyosin-actomyosin MgATPase of the myofibril, becomes sensitive to calcium ions. Different genes control the expression of TnT in fast skeletal, slow skeletal and cardiac muscles. In all muscles, and particularly in fast skeletal, alternative splicing of mRNA produces a series of isoforms in a developmentally regulated manner. In consequence TnT exists in many more isoforms than any of the other thin filament proteins, the TnT superfamily. Despite the general homology of TnT isoforms, this alternative splicing leads to variable regions close to the N- and C-termini. As the isoforms have slightly different effects on the calcium sensitivity of the actomyosin MgATPase, modulation of the contractile response to calcium can occur during development and in different muscle types. TnT has recently aroused clinical interest in its potential for detecting myocardial damage and the association of mutations in the cardiac isoform with hypertrophic cardiomyopathy.

Cardiac protein phosphorylation: functional and pathophysiological correlates

Protein phosphorylation acts a pivotal mechanism in regulating the contractile state of the heart by modulating particular levels of autonomic control on cardiac force/length relationships. Early studies of changes in cardiac protein phosphorylation focused on key components of the excitation-coupling process, namely phospholamban of the sarcoplasmic reticulum and myofibrillar troponin I. In more recent years the emphasis has shifted towards the identification of other phosphoproteins, and more importantly, the delineation of the mechanistic and signaling pathways regulating the various known phosphoproteins. In addition to cAMP- and Ca(2+)-calmodulin-dependent kinase processes, these have included regulation by protein kinase C and the ever-emerging family of growth factor-related kinases such as the tyrosine-, mitogen- and stress-activated protein kinases. Similarly, the role of protein dephosphorylation by protein phosphatases has been recognized as integral in modulating normal cardiac cellular function. Recent studies involving a variety of cardiovascular pathologies have demonstrated that changes in the phosphorylation states of key cardiac regulatory proteins may underlie cardiac dysfunction in disease states. The emphasis of this comprehensive review will be on discussing the role of cardiac phosphoproteins in regulating myocardial function and pathophysiology based not only on in vitro data, but more importantly, from ex vivo experiments with corroborative physiological and biochemical evidence.

Troponin-T is a calcium-binding protein in insect muscle: in vivo phosphorylation, muscle-specific isoforms and developmental profile in Drosophila melanogaster

Two sets of muscle polypeptides showing calcium-binding capacity and intense labelling in vivo with 32P were purified and characterized from Drosophila melanogaster adult extracts. The polypeptides exhibit crossed immunoreactivity and share similar biochemical properties such as those involved in purification. They have been identified as isoforms of troponin-T (TnT) by sequence analysis of a cDNA clone isolated from an embryonic library. The two sets of TnT polypeptides correspond to the fibrillar and non-fibrillar muscle isoforms, respectively. The non-fibrillar muscle isoforms separate into two bands which are differentially expressed during development. Analysis of TnT isoforms in bee thoraces indicates that the expression of the fibrillar muscle isoform correlates with the acquisition of functional flight capability. In vivo labelling experiments reveal that the two TnT sets are readily phosphorylated. The Drosophila TnTs show calcium-binding properties by three different types of assays. Our results suggest that this property could be specific to insect TnTs and may be related to the long, extremely acidic polyglutamic carboxy-terminus present in these polypeptides, which does not occur in non-arthropod TnTs.

Microanalysis and distribution of cardiac troponin I phospho species in heart areas

Sequential phosphorylation and dephosphorylation of cTnI by the cAMP dependent protein kinase and by protein phosphatase 2A, respectively, produce the non-, mono- and bisphosphorylated species (Jaquet et al., 1995, Eur. J. Biochem. 231, 486-490). The aim of this study was to determine these forms even in small tissue samples, e.g. in biopsy probes of approximately 30 mg which would allow to define the phosphorylation state of cTnI in heart areas. In order to do so a micro isolation procedure for cTnI had to be established. cTnI is extracted from small bovine, rabbit and human heart tissue samples (30-100 mg) under special conditions avoiding dephosphorylation and is isolated by affinity chromatography on cTnC Sepharose. All three species, the bis-, mono- and dephospho cTnI, are precipitated quantitatively by acetone, then they are separated by non-equilibrium isoelectric focusing and quantified by scanning densitometry. The method presented here allows to quantify the three cTnI species reproducibly. No other phosphorylated species are detected. Truncated cTnI forms of each phospho species are found in human biopsy samples due to removal of a approximately 36 amino acid peptide from the C-terminus. In bovine, human and rabbit heart the pattern of the three cTnI phospho species is characteristic for left and right atrium, left and right ventricle and septum.

Skeletal muscle regeneration induced by chorio-allantoic grafting

To examine whether the expression pattern of fast-muscle type troponin-T (TnT) isoforms was fixed in cell lineage, breast muscle pieces (pectoralis major) from chick embryos and young and adult chickens were grafted on to chorio-allantoic membrane of 9-day-old chick embryos and cultured until the host embryos hatched out. Muscle fibre formation of the grafts was investigated by histological and immunohistochemical methods with anti-fast-muscle type and anti-slow-muscle type TnT sera, and the expression of fast-muscle type TnT in the grafts from chick embryos and young chickens was studied by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional SDS-PAGE, and immunoblotting. In the chorio-allantoic grafting, the breast muscle initially degenerated forming pyknotic nuclei and hyaline cytoplasm. The surviving cells, which were supposed to be satellite cells, regenerated new muscle fibres of the same type as those of the grafted muscle in respect of TnT isoform expression. Therefore, we considered that the ability to express specific isoforms of TnT was fixed in the satellite cells, and that chorio-allantoic grafting was a useful technique for studying muscle differentiation.

Reconstitution of skinned cardiac fibres with human recombinant cardiac troponin-I mutants and troponin-C

Troponin C (TnC) could be extracted from skinned porcine cardiac muscle fibres and their Ca2+ sensitivity restored by reconstitution with recombinant human cardiac TnC. After extraction of troponin I (TnI) and TnC using the vanadate treatment method of Strauss et al. [Strauss, J. D., Zeugner, C., Van Eyk, J.E., Bletz, C., Troschka, M. and Rüegg, J.C. (1992) FEBS Lett. 310, 229-234], skinned porcine cardiac muscle fibres were reconstituted with wild-type recombinant human cardiac TnC and either wild-type cardiac TnI or several mutant isoforms of human TnI. Reconstitution with wild-type proteins restored the Ca2+ sensitivity of the tissue and phosphorylation of the TnI with the catalytic subunit of protein kinase A reduced the Ca2+ sensitivity (i.e.-log[Ca2+] for 50% of maximal force) as has been shown by others. However, reconstitution with the TnI mutant Ser-23Asp/Ser-24Asp mimicking the phosphorylated form of cardiac TnI, led to a reduced Ca2+ sensitivity compared with reconstitution with wild-type TnI, whereas the mutant Ser-23Ala/Ser-24Ala behaved as the dephosphorylated form of TnI. These data confirm the importance of negative charge in this region of the TnI molecule in altering the Ca2+ responsiveness in this system.

Diminished Ca2+ sensitivity of skinned cardiac muscle contractility coincident with troponin T-band shifts in the diabetic rat

We have measured the apparent Ca2+ sensitivities of force development in skinned cardiac trabeculae at different sarcome lengths together with shifts in troponin (Tn) T subunits on specimens from the same hearts and drawn insights into the pathogenesis of myocardial dysfunction in the diabetic rat. The Ca(2+)-force relations were measured at a long (2.4-microns) and a short (1.9-microns) sarcomere length. In disease, compared with the control condition, the apparent Ca2+ sensitivity was greatly diminished at a sarcomere length of 1.9 microns but not affected at all at the long length (2.4 microns). We also examined the alterations in contractile regulatory proteins TnT and TnI by both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blots. The TnI band was largely unperturbed, but major changes were discerned in TnT. The normal rat heart indicated two major bands (TnT1 and TnT2) and a faint third band (TnT3); in the diabetic rat heart, there was a significant shift in intensity from TnT1 to TnT3. Since myosin isozyme shifts also accompany diabetes in the rat, we used a prototypical hypothyroid rat as well to evaluate the myosin influence in the length-induced effects on Ca2+ sensitivity. Myosin shifts during hypothyroidism were unaccompanied by significant changes in TnT, and there were also no length-dependent modifications in Ca2+ sensitivity. The findings raise the possibility that diabetic Ca(2+)-sensitivity changes in the myocardium are coupled with TnT alterations. A plausible explanation is offered whereby these TnT alterations modify the length dependence of Ca2+ sensitivity.

Persistent expression of tissue-specific troponin T isoforms in transplanted chicken skeletal muscle

This study attempted to investigate the expression of skeletal muscle troponin T isoforms in chicken reared for six months after muscle transplantations of breast muscle into leg muscle, leg muscle into breast muscle, and slow muscle into breast and leg muscles of the same animal. The regenerated muscle after transplantation was studied by histological observation, two-dimensional SDS-polyacrylamide gel electrophoresis, and immunoblotting with anti-troponin T antibodies. Persistent expression of troponin T isoforms specific to donor tissue was observed in the regenerated muscle, and compared with their expression in the normal developing muscles. During the regeneration, the cells grew up and expressed troponin T isoforms in a manner similar to that in normal developing muscles, and on around the 178th day after the transplantation, the regenerated muscle expressed the adult type troponin T isoforms. Based on the troponin T isoforms expressed in the transplants, we consider that one type of skeletal muscle has some inherent potential to grow in and coexist with other types for a long term.

Time-resolved tryptophan emission study of cardiac troponin I

We have carried out a time-resolved fluorescence study of the single tryptophanyl residue (Trp-192) of bovine cardiac Tnl (CTnl). With excitation at 300 nm, the intensity decay was resolved into three components by a nonlinear least-squares analysis with lifetimes of 0.60, 2.22, and 4.75 ns. The corresponding fractional amplitudes were 0.27, 0.50, and 0.23, respectively. These decay parameters were not sensitive to complexation of CTnl with cardiac troponin C (CTnC), and magnesium and calcium had no significant effect on the decay parameters. After incubation with 3':5'-cyclic AMP-dependent protein kinase, the intensity decay of CTnl required a fourth exponential term for satisfactory fitting with lifetimes of 0.11, 0.81, 1.95, and 6.63 ns and fractional amplitudes of 0.06, 0.37, 0.27, and 0.29, respectively. When bound to CTnC, the intensity decay of phosphorylated CTnl (p-CTnl) also required four exponential terms for satisfactory fitting, but the longest lifetime increased by a factor of 1.7. The decay parameters obtained from the complex formed between p-CTnl and CTnC were not sensitive to either magnesium or calcium. The anisotropy decay was resolved into two components with rotational correlation times of 0.90 and 23.48 ns. Phosphorylation resulted in a decrease of the long correlation time to 14.61 ns. The anisotropy values recovered at zero time suggest that the side chain of the Trp-192 had considerable subnanosecond motional freedom not resolved in these experiments. Within the CTnl.CTnC complex, the unresolved fast motions appeared sensitive to calcium binding to the calcium-specific site of CTnC. The observed emission heterogeneity is discussed in terms of possible excited-state interactions in conjunction with the predicted secondary structure of CTnl. The loss of molecular asymmetry of cardiac troponin I induced by phosphorylation as demonstrated in this work may be related to the known physiological effect of beta-agonists on cardiac contractility.

Expression pattern of skeletal muscle troponin T isoforms is fixed in cell lineage

The expression of fast-muscle-type troponin T isoforms in chicken skeletal muscles was studied by two-dimensional SDS-polyacrylamide gel electrophoresis and immunoblotting. According to the pattern of troponin T isoform expression, chicken fast muscle was classified into two groups: One group expressed breast-fast-muscle-type troponin T in addition to leg-fast-muscle-type troponin T, the other expressed only leg-fast-muscle-type troponin T. To the former group belong breast and wing fast muscles and some of the back fast muscles, and to the latter group belong the fast muscles in leg, abdomen, and neck. Transplantation of breast muscle into leg was performed in order to change the physical environment and to investigate the mechanism of isoform expression. Histological observation of the transplant revealed severe degeneration of muscle cells, followed by differentiation of myoblasts in which breast-muscle-type troponin T was eventually expressed. The results showed that the pattern of troponin T isoform expression is primarily fixed in the cell lineage, although nerves modulate it.

Identification and pattern of transitions of some developmental and adult isoforms of fast troponin T in some human and rat skeletal muscles

Using a monoclonal antibody (F24) in an immunoblotting procedure, the composition of fast troponin T in several adult and developing skeletal muscles of rat and human was studied. With the exception of diaphragm, four isoforms of fast troponin T (HF1-HF4) were detected in all the adult human skeletal muscles investigated. Another isoform of fast troponin T undetectable in the adult human skeletal muscles, designated the fetal isoform (HFF1), was found to be present in all fetal skeletal muscles at 20 weeks of gestation except the diaphragm. Unlike isoform HF4 that was undetectable in all the fetal skeletal muscles, isoforms HF1-HF3 were present in all the human fetal skeletal muscles including the diaphragm. At least five isoforms of fast troponin T (AF1-AF5) could be detected in adult rat skeletal muscles. An additional isoform designated (D) appeared to be present in the rat diaphragm. In some muscles one of the isoforms, AF1, could be further resolved into two to three variants. The proportions and the level of expression of AF1-AF5 isoforms varied not only in different muscles but in some cases also in different parts of the same muscle. In addition to the adult isoforms, four other developmental isoforms termed fetal (FF1 and FF2) and neonatal (NF1 and NF2), were detected during the early development in the rat skeletal muscles. Their presence was first detected during the late fetal to early neonatal period and these isoforms were generally undetectable in a majority of the muscles after 1-2 months of age although their low level of expression persisted in a small number of muscles.

Identification of and pattern of transitions of cardiac, adult slow and slow skeletal muscle-like embryonic isoforms of troponin T in developing rat and human skeletal muscles

Using a monoclonal antibody (CDC4) that recognizes both the cardiac and slow skeletal isoforms of troponin T in an immunoblotting procedure, the composition of troponin T isoforms in adult and developing skeletal muscles of the rat and human were studied. Two major isoforms of slow troponin T (HS1 and HS2) were detected in all the adult human skeletal muscles investigated. Significant amounts of another isoform (HS3) in addition to HS1 and HS2 were also detectable in a subgroup of these muscles. All the human fetal skeletal muscles at 20 weeks of gestation expressed HS1 and HS2 isoforms but not HS3. The fetal skeletal muscles, also expressed cardiac troponin T in addition. Unlike the human skeletal muscles, only a single isoform of slow troponin T was detected by antibody CDC4 in both the adult and neonatal rat skeletal muscles. The investigation of fetal rat skeletal muscles using the same antibody, however, detected the presence of not only the embryonic cardiac and adult slow skeletal isoforms but also five additional not previously described isoforms (Es1-Es5) of troponin T. These embryonic isoforms, Es1-Es5, were undetectable in the postnatal skeletal muscles although their small amounts could be detected in the neonatal rat hearts. The analysis of individual skeletal muscles from different parts of the body at different stages of fetal development showed marked variations in both the composition of troponin T isoforms and the time sequence of their transitions in each muscle.

Distance distributions and anisotropy decays of troponin C and its complex with troponin I

We used frequency domain measurements of fluorescence resonance energy transfer to recover the distribution of distances between Met 25 and Cys 98 in rabbit skeletal troponin C. These residues were labeled with dansylaziridine as energy donor and 5-(iodoacetamido)eosin as acceptor and are located on the N- and C-terminal lobes of the two-domain protein, respectively. We developed a procedure to correct for the fraction of the sample that was incompletely labeled with the acceptor independent of chemical data. At pH 7.5 and in the presence of Mg2+, the mean distance was near 15 A with a half-width of the distribution of 15 A; when Mg2+ was replaced by Ca2+, the mean distance increased to 22 A with a decrease in the half-width by 4 A. Similar but less pronounced differences in the mean distance and half-width between samples containing Mg2+ and Ca2+ were also observed with troponin C complexed to troponin I. The results suggest that the conformation of troponin C is altered by Ca2+ binding to the Ca(2+)-specific sites and displacing bound Mg2+ at the Ca2+/Mg2+ sites. This alteration may play an important role in Ca2+ signaling in muscle. At pH 7.5, the anisotropy decays of the donor-labeled troponin C showed two components, with the long rotational correlation time (12 ns) reflecting the overall motion of the protein. When the pH was lowered from 7.5 to 5.2, the mean distribution distance of apotroponin C increased from 22 to 32 A and the half-width decreased by a factor of 2 from 13 to 7 A. The long correlation time of apotroponin C increased to 19 ns at the acidic pH. These results are discussed in terms of a model in which skeletal troponin C is a dimer at low pH and enable comparison of the solution conformation of the protein at neutral pH with a crystal structure obtained at pH 5.2. While the conformation of the monomeric unit of troponin C dimer at pH 5.2 is extended and consistent with the crystal structure, the conformation at neutral pH is likely more compact than the crystal structure predicts.

Isolation and partial characterization of a cystine-rich basic heparin-binding protein from bovine platelets

We report the isolation and partial characterization of a so far unidentified basic platelet protein. Delipidated bovine platelets were extracted at pH 2.1. The extract was subjected to differential precipitation at pH 5.4-5.5 and by ammonium sulfate, then it was further purified by ion exchange chromatography on DEAE and CM cellulose columns in an urea containing medium. The major protein peak eluted from the CM cellulose column by NaCl gradient contained a protein in electrophoretically homogeneous form. It consists of a single polypeptide chain with an Mr of 28,000 as estimated by SDS PAGE. It was shown to be extremely rich in lysine and cystine and possessed a highly basic character (pI 9.8-10.1). On this basis the term cystine-rich basic protein (CRBP) was proposed for the new protein. Unlike some other low Mr basic proteins it did not bind calmodulin and troponin C, however, it showed significant heparin neutralizing activity.

Structure-function relationships in cardiac troponin T

Regions of rabbit and bovine cardiac troponin T that are involved in binding tropomyosin, troponin C and troponin I have been identified. Two sites of contact for tropomyosin have been located, situated between residues 92-178 and 180-284 of troponin T. A cardiac-specific binding site for troponin I has been identified between residues 1-68 of cardiac troponin T, within a region of the protein that has previously been shown to be encoded by a series of exons that are expressed in a tissue-specific and developmentally regulated manner. The binding site for troponin C is located between residues 180-284 of cardiac troponin T. When isolated from fresh bovine hearts, cardiac troponin T contained 0.21 +/- 0.11 mol phosphate per mol; incubation with phosphorylase kinase increased the phosphate content to approx. 1 mol phosphate per mol. One site of phosphorylation was identified as serine-1; a second site of phosphorylation was located within peptide CB3 (residues 93-178) and has been tentatively identified as serine-176. Addition of troponin C to cardiac troponin T does not inhibit the phosphorylation of this latter protein that is catalysed by phosphorylase b kinase.

Coordinate accumulation of troponin subunits in chicken breast muscle

The accumulation of troponin subunits in developing chicken breast muscle was determined by two-dimensional gel electrophoresis and an image analyzing system. Many troponin T isoforms, including those hidden behind creatine kinase, were detected on the two-dimensional pattern by the addition of 6 M urea in the second dimension. These troponin T isoforms were classified into four types by developmental order, isoelectric point, and molecular weight: leg-muscle type (L), neonatal breast-muscle type (BN), young chicken breast-muscle type (BC), and adult breast-muscle type (BA). The L-, BN-, and BC-type troponin Ts were transiently expressed at specific developmental stages. Quantitative analysis of two-dimensional patterns of troponin subunits including troponin I and troponin C showed moderate coordination in accumulation among the three subunits throughout postnatal development, when the total amount of all isoforms of troponin T was taken into account.

N-terminal amino acid sequences of three functionally different troponin T isoforms from rabbit fast skeletal muscle

The different isoforms of fast skeletal muscle troponin T (TnT) are generated by alternative splicing of several 5' exons in the fast TnT gene. In rabbit skeletal muscle this process results in three major fast TnT species, TnT1f, TnT2f and TnT3f, that differ in a region of 30 to 40 amino acid residues near the N terminus. Differential expression of these three isoforms modulates the activation of the thin filament by calcium. To establish a basis for further structure-function studies, we have sequenced the N-terminal region of these proteins. TnT2f is the fast TnT sequenced by Pearlstone et al. The larger species TnT1f contains six additional amino acid residues identical in sequence and position to those encoded by exon 4 in the rat fast skeletal muscle TnT gene. TnT3f also contains that sequence but lacks 17 amino acid residues spanning the region encoded by exons 6 and 7 of the rat gene. These three TnTs appear to be generated by discrete alternative splicing pathways, each differing by a single event. Comparison of these TnT sequences with those from chicken fast skeletal muscle and bovine heart shows that the splicing pattern resulting in the excision of exon 4 is evolutionarily conserved and leads to a more calcium-sensitive thin filament.

The multiplicity of troponin T isoforms. Distribution in normal rabbit muscles and effects of chronic stimulation

Polyclonal antibodies were raised in guinea pigs against troponin-T (TnT) isoforms purified from fast- and slow-twitch rabbit muscles. With the use of these antibodies and immunoblots of one- and two-dimensional electrophoreses, the distribution of fast and slow TnT isoforms was investigated in normal and chronically stimulated hindlimb muscles of the rabbit. According to differences in their apparent molecular masses, six fast TnT isoforms (TnTcf, TnT1f, TnT2f, TnT3f, TnT4f, TnT5f) were distinguished in normal tibialis anterior and extensor digitorum longus muscles. These muscles also contained low amounts of TnT1s and TnT2s which were the predominant TnT isoforms in slow-twitch soleus muscle. Fast and slow TnT isoforms were found to exist in several charge variants, i.e. one for TnTcf, three different charge forms for TnT1f, seven for TnT2f, four for TnT3f, three for TnT4f, one for TnT5f, four for TnT1s, and three for TnT2s. Some charge variants were phosphorylated isoforms because treatment with alkaline phosphatase reduced the number of the 19 fast and 7 slow variants to 12 and 3, respectively. The stimulation-induced fast-to-slow transition caused progressive decreases in fast and increases in slow isoforms. The decrease and the disappearance of the major fast isoforms followed a sequence of TnT2f, TnTcf, TnT4f, TnT1f, and TnT3f. This decrease in fast isoforms fits well with the reduction of fast TnT mRNAs assessed by Northern blot analysis. Prolonged stimulation ultimately created a TnT isoform pattern similar to that found in normal slow-twitch muscle. Stimulation also induced changes in the tropomyosin subunit pattern with a decrease in the fast and an increase in the slow alpha-tropomyosin subunit without altering the alpha/beta-tropomyosin subunit ratio. Similar to slow-twitch soleus muscle, long-term stimulated muscles contained appreciable amounts of the fast alpha-tropomyosin subunit, but only traces of fast TnT isoforms. This combination indicated that the predominant slow TnT isoforms may be capable of interacting with fast tropomyosin in these muscles.

Distance distributions in proteins recovered by using frequency-domain fluorometry. Applications to troponin I and its complex with troponin C

We used resonance energy transfer to examine the distribution of distances between two sites on troponin I (TnI). The donor (D) was the single tryptophan residue at site 158 (Trp 158), and the acceptor (A) was cysteine 133 (Cys 133) which was labeled with N-(iodoacetyl)-N'-(1-sulfo-5-naphthyl)ethylenediamine (IE). A distribution of D-A distances results in a distribution of donor decay times, which were resolved by using frequency-domain fluorometry. In the native state we recovered a relatively narrow distribution of D-A distances. The widths of the distance distributions were found to increase progressively and dramatically with increasing concentrations of guanidine hydrochloride. Binding of calcium-free troponin C (TnC) to troponin I did not alter the distance distribution. Addition of Ca2+ to the TnI.TnC complex resulted in a sharper distance distribution and protected against the guanidine hydrochloride induced increase in the width of the distance distribution. Additionally, the same distance distributions were recovered for native and denatured TnI when the Forster distance for energy transfer was decreased by acrylamide quenching. These results demonstrate that distance distributions can be recovered with good accuracy, to the extent of revealing modest changes due to binding of other components. This technique should have widespread applications in studies of protein folding.

Identification and distribution of the fast class of troponin T in the adult and developing avian skeletal muscle

In the adult chicken, two groups of fast troponin T, the higher molecular weight (B1 and B2) present only in the pectoral muscle and the lower molecular weight (L1-L5) the only group present in the leg muscle, were identified by the immunoblotting procedure using monoclonal antibodies against fast skeletal troponin T. The presence of significant amounts of three major variants of leg muscle type troponin T (L3-L5), however, could also be detected in the adult chicken pectoral muscle. Although none of the antibodies cross-reacted with slow troponin T itself, the proportions of both leg and pectoral type troponin T variants belonging to the fast class varied in fast muscles that contained slow muscle fibres or fast muscles devoid of slow muscle cells. The troponin T present in the early embryonic skeletal muscles did not react with any of the antibodies raised against adult fast isoforms. The gradual expression of some of the adult isoforms of troponin T was detected at about day 13 in ovo. However, the adult isoforms did not all appear simultaneously and their full complement was not achieved until after hatching. In addition to the increased number of bands in the leg type troponin T region, the presence of two other protein bands (neonatal forms) with slower electrophoretic mobility than the other fast isoforms of troponin T, was detected in post-hatch pectoral muscle tested at 1-12 weeks of age. These neonatal forms (N1 and N2) in the pectoral muscle were undetectable at eight months of age. The presence of breast type troponin T in the leg muscle was not detected with these antibodies at any stage of development.

Isolation and characterization of a highly phosphorylated troponin from bovine heart

A modified procedure for isolation of troponin from bovine heart is described, which results in a stable and highly phosphorylated protein. 31P-NMR spectra show up to four phosphoserine signals indicating that at least four serine residues of cardiac troponin are phosphorylated in the intact organ. The hydrodynamic parameters of phosphotroponin are almost identical to those previously published. Characteristically cardiac troponin shows a strong tendency to associate that is dependent on protein concentration. Mg2+ may specifically induce an aggregation, which can be observed during sedimentation. This phenomenon seems to be analogous to the Mg2+-induced dimerization of cardiac troponin C [Jaquet, K. and Heilmeyer, L. M. G., Jr (1987) Biochem. Biophys. Res. Commun. 145, 1390-1396]. Upon Mg2+ saturation a shift of one of the four 31P-NMR signals is observed. The affinity of troponin to Ca2+ is reduced when the protein concentration is enhanced only in the presence of Mg2+. This effect of Mg2+ suggests a model for the regulation of the Ca2+-binding affinity of cardiac troponin.

Vascular smooth muscle calponin. A novel troponin T-like protein

In a search for additional Ca2+ regulatory components in vascular smooth muscle, a novel troponin T-like protein was purified from bovine aorta smooth muscle. The isolated protein was separated into several isoforms on isoelectric focusing. The major isoelectric variants were focused in the pH region of 8.4 to 9.1. The protein had slightly different molecular masses in the Mr range of 35,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Its molar ratio relative to tropomyosin in the muscle extract was estimated to be 0.9:1.0. The novel protein bound to the immobilized calmodulin and exhibited a number of common physicochemical properties with gizzard (Mr = 34,000) calmodulin-binding and F-actin-binding protein. The aorta and gizzard proteins were immunologically cross-reactive. Both proteins shared a common antigenic determinant with COOH-terminal segments of rabbit skeletal and bovine cardiac troponin T and bound to the immobilized smooth muscle tropomyosin. Both proteins interacted with rabbit skeletal troponin C in the presence and absence of Ca2+, but they did not interact with troponin I. These results suggest that the novel protein, which is designated calponin, may be a specialized component of smooth muscle thin filament involved in the regulation of contractile apparatus.

The interaction of troponin-I with the N-terminal region of actin

The interaction between troponin-I and actin that underlies thin-filament regulation in striated muscle has been studied using proton magnetic resonance spectroscopy. A restricted portion of skeletal muscle troponin-I (residues 96-116) has previously been shown to be capable of inhibiting the MgATPase activity of actomyosin in a manner enhanced by tropomyosin [Syska et al. (1976) Biochem. J. 153, 375-387]. On the basis of homologous spectral effects for signals of specific groups observed in different complexes formed using the native proteins and a variety of defined peptides, it is concluded that the segment of troponin-I which has inhibitory activity interacts with the N-terminal region of actin. The surface of contact of the inhibitory segment of troponin-I with actin involves two regions of the N-terminal of actin. These are located between residues 1-7 and 19-44. The data are discussed in the context of a structural mechanism for the inhibition of myosin ATPase activation.

Phosphorylation of troponin I by protein kinase C: mechanism of inhibition by calmodulin and troponin C

The mechanism by which calmodulin and troponin C influence phosphorylation of troponin I (TnI) by protein kinase C was investigated. The phosphorylation of TnI by protein kinase C requires the presence of acidic phospholipid, calcium and diacylglycerol. Light scattering intensity and fluorescence intensity experiments showed that TnI associated with the phospholipid membranes and caused extensive aggregation. In the presence of Ca2+, TnI-phospholipid interactions were prevented by approximately stoichiometric amounts of either troponin C or calmodulin. Troponin C was shown to completely inhibit phosphorylation of TnI by either protein kinase C or by phosphorylase b kinase. In contrast, calmodulin completely inhibited phosphorylation of TnI by protein kinase C, but had only little effect on TnI phosphorylation by phosphorylase b kinase. Inhibition by calmodulin did not appear to be due to interaction with PKC, since calmodulin mildly increased protein kinase C phosphorylation of histone III-S. The ratio of phosphoserine to phosphothreonine in protein kinase C-phosphorylated TnI remained approximately constant for reactions inhibited by up to 90% by calmodulin. TnI interactions with phospholipid and phosphorylation of TnI by PKC were also prevented by high salt concentrations. However, salt concentrations adequate to inhibit phosphorylation were sufficient to dissociate only TnI, but not protein kinase C from the membrane. These results suggest that the binding of TnI to phospholipid is required for phosphorylation by protein kinase C and that prevention of this binding by any means completely inhibited phosphorylation of TnI by protein kinase C.