The phosphorylation sites of troponin T from white skeletal muscle and the effects of interaction with troponin C on their phosphorylation by phosphorylase kinase

Role of the interaction between troponin T and AMP deaminase by zinc bridge in modulating muscle contraction and ammonia production

The N-terminal region of troponin T (TnT) does not bind any protein of the contractile machinery and the role of its hypervariability remains uncertain. In this review we report the evidence of the interaction between TnT and AMP deaminase (AMPD), a regulated zinc enzyme localized on the myofibril. In periods of intense muscular activity, a decrease in the ATP/ADP ratio, together with a decrease in the tissue pH, is the stimulus for the activation of the enzyme that deaminating AMP to IMP and NH3 displaces the myokinase reaction towards the formation of ATP. In skeletal muscle subjected to strong tetanic contractions, a calpain-like proteolytic activity produces the removal in vivo of a 97-residue N-terminal fragment from the enzyme that becomes desensitized towards the inhibition by ATP, leading to an unrestrained production of NH3. When a 95-residue N-terminal fragment is removed from AMPD by trypsin, simulating in vitro the calpain action, rabbit fast TnT or its phosphorylated 50-residue N-terminal peptide binds AMPD restoring the inhibition by ATP. Taking in consideration that the N-terminus of TnT expressed in human as well as rabbit white muscle contains a zinc-binding motif, we suggest that TnT might mimic the regulatory action of the inhibitory N-terminal domain of AMPD due to the presence of a zinc ion connecting the N-terminal and C-terminal regions of the enzyme, indicating that the two proteins might physiologically associate to modulate muscle contraction and ammonia production in fast-twitching muscle under strenuous conditions.

Phosphoproteomics of three exercise modalities identifies canonical signaling and C18ORF25 as an AMPK substrate regulating skeletal muscle function

Exercise induces signaling networks to improve muscle function and confer health benefits. To identify divergent and common signaling networks during and after different exercise modalities, we performed a phosphoproteomic analysis of human skeletal muscle from a cross-over intervention of endurance, sprint, and resistance exercise. This identified 5,486 phosphosites regulated during or after at least one type of exercise modality and only 420 core phosphosites common to all exercise. One of these core phosphosites was S67 on the uncharacterized protein C18ORF25, which we validated as an AMPK substrate. Mice lacking C18ORF25 have reduced skeletal muscle fiber size, exercise capacity, and muscle contractile function, and this was associated with reduced phosphorylation of contractile and Ca2+ handling proteins. Expression of C18ORF25 S66/67D phospho-mimetic reversed the decreased muscle force production. This work defines the divergent and canonical exercise phosphoproteome across different modalities and identifies C18ORF25 as a regulator of exercise signaling and muscle function.

Comprehensive Characterization of Swine Cardiac Troponin T Proteoforms by Top-Down Mass Spectrometry

Cardiac troponin T (cTnT) regulates the Ca²⁺-mediated interaction between myosin thick filaments and actin thin filaments during cardiac contraction and relaxation. cTnT is released into the blood following injury, and increased serum levels of the protein are used clinically as a biomarker for myocardial infarction. Moreover, mutations in cTnT are causative in a number of familial cardiomyopathies. With the increasing use of large animal (swine) model to recapitulate human diseases, it is essential to characterize species-dependent protein sequence variants, alternative RNA splicing, and post-translational modifications (PTMs), but challenges remain due to the incomplete database and lack of validation of the predicted splicing isoforms. Herein, we integrated top-down mass spectrometry (MS) with online liquid chromatography (LC) and immunoaffinity purification to comprehensively characterize miniature swine cTnT proteoforms, including those arising from alternative RNA splicing and PTMs. A total of seven alternative splicing isoforms of cTnT were identified by LC/MS from swine left ventricular tissue, with each isoform containing un-phosphorylated and mono-phosphorylated proteoforms. The phosphorylation site was localized to Ser1 for the mono-phosphorylated proteoforms of cTnT1, 3, 4, and 6 by online MS/MS combining collisionally activated dissociation (CAD) and electron transfer dissociation (ETD). Offline MS/MS on Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer with CAD and electron capture dissociation (ECD) was then utilized to achieve deep sequencing of mono-phosphorylated cTnT1 (35.2 kDa) with a high sequence coverage of 87%. Taken together, this study demonstrated the unique advantage of top-down MS in the comprehensive characterization of protein alternative splicing isoforms together with PTMs. Graphical Abstract ᅟ.

Top-down mass spectrometry of cardiac myofilament proteins in health and disease

Myofilaments are composed of thin and thick filaments that coordinate with each other to regulate muscle contraction and relaxation. PTMs together with genetic variations and alternative splicing of the myofilament proteins play essential roles in regulating cardiac contractility in health and disease. Therefore, a comprehensive characterization of the myofilament proteins in physiological and pathological conditions is essential for better understanding the molecular basis of cardiac function and dysfunction. Due to the vast complexity and dynamic nature of proteins, it is challenging to obtain a holistic view of myofilament protein modifications. In recent years, top-down MS has emerged as a powerful approach to study isoform composition and PTMs of proteins owing to its advantage of complete sequence coverage and its ability to identify PTMs and sequence variants without a priori knowledge. In this review, we will discuss the application of top-down MS to the study of cardiac myofilaments and highlight the insights it provides into the understanding of molecular mechanisms in contractile dysfunction of heart failure. Particularly, recent results of cardiac troponin and tropomyosin modifications will be elaborated. The limitations and perspectives on the use of top-down MS for myofilament protein characterization will also be briefly discussed.

Phosphorylation, but not alternative splicing or proteolytic degradation, is conserved in human and mouse cardiac troponin T

Cardiac troponin T (cTnT), the tropomyosin binding subunit of the troponin complex, plays a pivotal regulatory role in the Ca(2+)-mediated interaction between actin thin filament and myosin thick filament. The post-translational modifications (PTMs) and alternative splicing of cTnT may represent important regulatory mechanisms of cardiac contractility. However, a complete characterization of PTMs and alternatively spliced isoforms in cTnT present in vivo is lacking. Top-down protein mass spectrometry (MS) analyzes whole proteins, thus providing a global view of all types of modifications, including PTMs and sequence variants, simultaneously in one spectrum without a priori knowledge. In this study, we applied an integrated immunoaffinity chromatography and top-down MS approach to comprehensively characterize PTMs and alternatively spliced isoforms of cTnT purified from healthy human and wild-type mouse heart tissue. High-resolution Fourier transform MS revealed that human cTnT (hcTnT) and mouse cTnT (mcTnT) have similar phosphorylation patterns, whereas higher molecular heterogeneity was observed for mcTnT than hcTnT. Further MS/MS fragmentation of monophosphorylated hcTnT and mcTnT by electron capture dissociation and collisionally activated dissociation unambiguously identified Ser1 as the conserved in vivo phosphorylation site. In contrast, we identified a single spliced isoform for hcTnT but three alternatively spliced isoforms for mcTnT. Moreover, we observed distinct proteolytic degradation products for hcTnT and mcTnT. This study also demonstrates the advantage of top-down MS/MS with complementary fragmentation techniques for the identification of modification sites in the highly acidic N-terminal region of cTnT.

Evidence for the location of the allosteric activation switch in the multisubunit phosphorylase kinase complex from mass spectrometric identification of chemically crosslinked peptides

Phosphorylase kinase (PhK), an (alphabetagammadelta)(4) complex, regulates glycogenolysis. Its activity, catalyzed by the gamma subunit, is tightly controlled by phosphorylation and activators acting through allosteric sites on its regulatory alpha, beta and delta subunits. Activation by phosphorylation is predominantly mediated by the regulatory beta subunit, which undergoes a conformational change that is structurally linked with the gamma subunit and that is characterized by the ability of a short chemical crosslinker to form beta-beta dimers. To determine potential regions of interaction of the beta and gamma subunits, we have used chemical crosslinking and two-hybrid screening. The beta and gamma subunits were crosslinked to each other in phosphorylated PhK, and crosslinked peptides from digests were identified by Fourier transform mass spectrometry, beginning with a search engine developed "in house" that generates a hypothetical list of crosslinked peptides. A conjugate between beta and gamma that was verified by MS/MS corresponded to crosslinking between K303 in the C-terminal regulatory domain of gamma (gammaCRD) and R18 in the N-terminal regulatory region of beta (beta1-31), which contains the phosphorylatable serines 11 and 26. A synthetic peptide corresponding to residues 1-22 of beta inhibited the crosslinking between beta and gamma, and was itself crosslinked to K303 of gamma. In two-hybrid screening, the beta1-31 region controlled beta subunit self-interactions, in that they were favored by truncation of this region or by mutation of the phosphorylatable serines 11 and 26, thus providing structural evidence for a phosphorylation-dependent subunit communication network in the PhK complex involving at least these two regulatory regions of the beta and gamma subunits. The sum of our results considered together with previous findings implicates the gammaCRD as being an allosteric activation switch in PhK that interacts with all three of the enzyme's regulatory subunits and is proximal to the active site cleft.

Characterisation of troponin-T from salmonid fish

Five major troponin-T isoforms were isolated from the myotomal muscles of Atlantic salmon: three from fast muscle (Tn-T1F, Tn-T2F and Tn-T3F) and two from slow muscle (Tn-T1S and Tn-T2S). In addition to their presence in troponin preparations, these proteins were also recognised to be Tn-T on the basis of immunoreaction with anti-troponin-T antibodies and partial amino acid sequence. The electrophoretic mobility in the presence of SDS of the various Tn-Ts increases in the order: 1S < 1F < 2S < 2F < or = 3F. Compositional analysis shows that the higher M(r) forms (1F and 1S) contain considerably more proline, glutamic acid and alanine than the lower-M(r) forms (2F, 3F and 2S). Every isoform lacks cysteine and phosphoserine is present only in isoforms 2F and 3F. All of the Tn-Ts, with the exception of isoform 1F, are N-terminally blocked. CNBr fragments from same cell type Tn-Ts yield identical sequences over at least fifteen Edman cycles. Two full-length cDNA sequences, presumed to represent 1S and 3F, or isoforms that are highly similar, are reported. As documented for higher vertebrate Tn-Ts, the predicted primary structures display a non-uniform distribution of charged amino acids and greater divergence at each end than in the central section. The most striking difference between the two salmonid proteins is the presence of a N-terminal (proline-, glutamic acid- and alanine-rich) extension of about fifty amino acids in Tn-T1s (278 amino acids) that is missing from the fast muscle Tn-T (223 amino acids). The sequences also differ in that 1S lacks the known phosphorylation site while the fast-type isoform contains serine next to the initiating methionine. Of the two, the slow isoform has accumulated the greater number of substitutions.

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.

Investigation of the effects of phosphorylation of rabbit striated muscle alpha alpha-tropomyosin and rabbit skeletal muscle troponin-T

FPLC has been employed to prepare the phosphorylated and unphosphorylated forms of rabbit striated muscle alpha alpha-tropomyosin (TM), and the major isoform of rabbit fast-skeletal-muscle troponin-T (Tn-T2f) and corresponding chymotryptic fragment T1 (residues 1-158), in order to investigate the effects which these in vivo modifications have on thin filament function. In all instances, no significance could be attributed to the presence of a phosphate moiety on acetyl serine 1 of Tn-T (or fragment T1). As expected, fragment T1 increased the relative viscosities of solutions of unphosphorylated alpha alpha-TM, but this induction was noticeably lower for phosphorylated alpha alpha-TM. In affinity chromatography experiments, fragment T1 bound equally well to either form of alpha alpha-TM, but the interaction between fragment T2 (residues 159-259) and phosphorylated alpha alpha-TM was strengthened relative to the control. In the presence of alpha alpha-TM (unphosphorylated), fragment T1 was found to down regulate the actin-activated myosin-S1 MgATPase activity, indicating that this portion of Tn-T possesses modulatory properties. Under the same conditions, less inhibition was observed with phosphorylated alpha alpha-TM. When the two different forms of alpha alpha-TM were reconstituted into a complete regulatory system, the activation of myosin-S1 was double for those thin filaments containing the phosphorylated molecule. Dephosphorylation of the phospho alpha alpha-TM reduced the rates to control values. In ATPase Ca2+ titrations, these systems exhibited no difference in the co-operativity of activation and little or no difference in the pCa2+ 1/2 value. Developmentally linked changes in the steady-state phosphorylation of alpha alpha-TM could be a mechanism to increase the activating propensity of thin filaments, by modifying the functional properties of the T1 section of Tn-T.

Isolation of human cardiac troponin T and localization of epitopes recognized by monoclonal antibodies to cardiac troponin T

Human cardiac troponin T has been isolated and its properties compared with those of rabbit skeletal and bovine cardiac troponin T. Seven monoclonal antibodies to troponin T have been obtained. Two antibodies cross-reacted with both cardiac and skeletal troponin T and recognized epitopes located between residues 98-177 of bovine cardiac troponin T. Five other antibodies were specific for cardiac troponin T and recognized antigenic determinants located between residues 180-258 of bovine cardiac troponin T. Localization of antigenic determinants in the central part of troponin T seems to be due to the high hydrophilicity and flexibility of this part of the molecule. The monoclonal antibodies thus obtained may be used for diagnosing various types of human heart diseases.

The partial amino acid sequence of the NAD(+)-dependent glutamate dehydrogenase of Clostridium symbiosum: implications for the evolution and structural basis of coenzyme specificity

The amino acid sequence is reported for CNBr and tryptic peptide fragments of the NAD(+)-dependent glutamate dehydrogenase of Clostridium symbiosum. Together with the N-terminal sequence, these make up about 75% of the total sequence. The sequence shows extensive similarity with that of the NADP(+)-dependent glutamate dehydrogenase of Escherichia coli (52% identical residues out of the 332 compared) allowing confident placing of the peptide fragments within the overall sequence. This demonstrated sequence similarity with the E. coli enzyme, despite different coenzyme specificity, is much greater than the similarity (31% identities) between the GDH's of C. symbiosum and Peptostreptococcus asaccharolyticus, both NAD(+)-linked. The evolutionary implications are discussed. In the 'fingerprint' region of the nucleotide binding fold the sequence Gly X Gly X X Ala is found, rather than Gly X Gly X X Gly. The sequence found here has previously been associated with NADP+ specificity and its finding in a strictly NAD(+)-dependent enzyme requires closer examination of the function of this structural motif.

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.

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.

Cardiac troponin I, isolated from bovine heart, contains two adjacent phosphoserines. A first example of phosphoserine determination by derivatization to S-ethylcysteine

Bovine cardiac troponin containing approximately 3 mol P/mol protein could be separated into its subunits without loss of phosphate. Troponin I and troponin T each contain about 1.5 mol P/mol protein. In troponin I two phosphorylated serine residues could be localized in the N-terminal region by conversion of phosphoserine to S-ethylcysteine. They are located in adjacent positions in the following sequence: -Arg-Arg-Ser(P)-Ser(P)-Ala-Asn-Tyr-Tyr-Arg-Ala-Tyr-Ala-Thr-Glu-Pro- His-Ala-Lys. This sequence shows that the first phosphoserine residue in bovine cardiac troponin I occupies a homologous position to phosphoserine-20 of rabbit cardiac troponin I.

Calcium-dependent phosphorylation of bovine cardiac C-protein by phosphorylase kinase

Phosphorylase kinase catalyzed the calcium-dependent phosphorylation of bovine cardiac C-protein. Phosphorylation of C-protein by phosphorylase kinase reached nearly 2 mol [32P]/mol C-protein. Tryptic phosphopeptide mapping and phosphoamino acid analysis indicated that phosphorylase kinase maybe phosphorylating some of the same seryl residues that undergo phosphorylation by cAMP-dependent protein kinase and that C-protein from bovine and chicken heart are structurally different. Bovine cardiac C-protein was not a substrate for a number of calcium and cyclic nucleotide-independent protein kinases, suggesting that phosphorylation of cardiac C-protein is restricted to protein kinases which are modulated by calcium and cAMP.

Interaction with troponin T from white skeletal muscle restores in white skeletal muscle AMP deaminase those allosteric properties removed by limited proteolysis

Limited proteolysis of rabbit skeletal muscle AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) with trypsin results in conversion of the enzyme to a form which is no longer inhibited by ATP and exhibits hyperbolic kinetics even at low K+ concentration and in the absence of ADP. The interaction with troponin T from white skeletal muscle or with the phosphorylated 42-residue N-terminal peptide of troponin T restores in the trypsin-treated AMP deaminase the sensitivity to adenine nucleotides and increases the KA for K+ activation of the enzyme from 1 mM to 12 mM, this effect being diametrically opposite to that exerted by limited proteolysis on the native enzyme. Treatment of the N-terminal peptide of troponin T with alkaline phosphatase abolishes the modulating properties of the peptide, suggesting that phosphorylation-dephosphorylation processes may be involved in the regulation of the enzyme.

Comparison of the structure of two cardiac troponin T isoforms

Two isoforms of troponin T have been isolated from bovine cardiac muscle. One isoform has an Mr of 31000 and a pI at about 7.1, the corresponding values for the second isoform being 33000 and 6.5. Both isoforms have identical C- and N-terminal sequences, and, according to the data from tryptic-peptide mapping, a similar structure of the central and C-terminal domains. The large N-terminal peptides of troponin T isoforms differ in the content of glutamine/glutamic acid and alanine. It is concluded that the isoform with Mr 33000 has an additional peptide enriched with glutamic acid and alanine that is inserted between the N-terminal pentapeptide and the cysteine located 40-60 residues from the N-terminus.

Sequential phosphorylation of skeletal muscle troponin

Phosphorylation of the isolated rabbit skeletal muscle holotroponin complex at troponin-T by phosphorylase kinase is unusual in that it shows maxima and minima. These oscillations are due to protein phosphatase activity present in the preparations. Following tryptic digestion two phosphorylated peptides, I and II, can be isolated. Their amino-acid compositions are identical and correspond to that of the tryptic peptide which contains the two known phosphorylatable sites 149/150 and 156/7 of troponin-T. Peptide I is phosphorylated on both sites and peptide II only on one site. During phosphorylation the doubly phosphorylated peptide I appears first; after a short lag phase peptide II is formed containing only one phosphate. These phenomena probably cause the observed oscillations in the degree of the holotroponin phosphorylation.

The expression of multiple forms of troponin T in chicken-fast-skeletal muscle may result from differential splicing of a single gene

Troponin T isolated from chicken fast skeletal muscle has been shown to be present in three different molecular forms, one in breast and two in leg muscle. The three forms differ in both size and charge. Troponin T from breast muscle has a molecular mass of 33.5 kDa and a pI of about 7. Of the two leg muscle forms the larger has a molecular mass of 30.5 kDa and a pI of about 8.5 and the smaller a molecular mass of 29.8 kDa and a pI of about 10. Considerably more heterogeneity has been found in the leg than in the breast muscle proteins although this is not reflected in their N-terminal sequences. The reason for this is not clear. Troponin T from breast or leg muscle can be phosphorylated with troponin T kinase at the single serine residue at the N-terminus. No difference in the rate or extent of phosphorylation could be found between proteins from breast or leg muscle. The three proteins have been shown to differ only in the amino acid sequence of their N-terminal tryptic peptides. These peptides are of different length, that from breast troponin T being 58 residues and those from leg troponin T being 36 and 42 residues, these differences account for the difference in molecular mass of the parent proteins. Despite this difference the sequence of the first 12 and last 14 residues is identical in all three N-terminal peptides. The remainder of the sequence of the smallest peptide is also repeated in the other two but they each contain an extra piece of unique sequence. On the basis of these sequences it is proposed that chicken troponin T is coded for by a single gene containing, at the 5' end, a number of small exons and that three different mRNA molecules may be produced by alternative pathways of RNA splicing. The possible significance of these N-terminal sequence variations is 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.

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

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

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.

Phosphorylation of troponin and myosin light chain by cAMP-independent casein kinase-2 from rabbit skeletal muscle

Casein kinase-2 from rabbit skeletal muscle was found to phosphorylate, in addition to glycogen synthase, troponin from skeletal muscle, and myosin light chain from smooth muscle. Troponin T and the 20,000 Mr myosin light chain are phosphorylated by casein kinase-2 at much greater rates than glycogen synthase. The V values for the phosphorylation of troponin and myosin light chain are nearly an order of magnitude greater than that of glycogen synthase; however, the Km values for these two substrates are greater than that for glycogen synthase. The kinase activities with the various protein substrates are stimulated approximately three- and fivefold by 5 mM spermidine and 3 mM spermine, respectively. Heparin is a potent inhibitor of the kinase when casein, glycogen synthase, or myosin light chain is the substrate. However, with troponin as substrate the kinase is relatively insensitive to inhibition by heparin. The amount of heparin required for 50% inhibition with troponin as substrate is at least 10 times greater than with casein as substrate. The phosphorylation of troponin by casein kinase-2 results in the incorporation of phosphate into two major tryptic peptides, which are different from those phosphorylated by casein kinase-1. The site in myosin light chain phosphorylated by casein kinase-2 is different from that phosphorylated by myosin light chain kinase.

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.

Non-correlation of phosphorylation of the P-light chain and the actin activation of the ATPase of chicken gizzard myosin

1. The enzymic properties of myosin isolated from chicken gizzard by three different methods have been compared. 2. Although the specific Ca2+-stimulated ATPases of all preparations were similar and high, there were significant differences in the specific activities of the Mg2+-stimulated actomyosin ATPases. 3. There was no direct correlation between the Mg2+-stimulated actomyosin ATPase activity and the extent of P-light-chain phosphorylation in any of the three myosin preparations. 4. A fraction that activates the Mg2+-stimulated actomyosin ATPase of gizzard muscle has been isolated from a gizzard muscle filament preparation. 5. The activator was specific for the Mg2+-activated actomyosin ATPase of smooth muscle. 6. The activator required the addition of calmodulin for full effect.

Phosphorylase kinase phosphorylation of skeletal-muscle troponin T

Rabbit skeletal-muscle troponin T was phosphorylated by a standard preparation of phosphorylase kinase [Cohen (1973) Eur. J. Biochem. 34, 1--14] and by fractions obtained after chromatography of phosphorylase kinase on phosphocellulose. The original preparation of phosphorylase kinase phosphorylated at least two sites, one of which was serine-1. The second and probably the third sites were presumably located in the peptide flanked by amino-acid residues 147 and 161 of troponin T. Fractions of phosphorylase kinase was adsorbed on phosphocellulose phosphorylated only the second site. Tightly adsorbed fractions possessed high troponin T kinase and phosvitin kinase activities and phosphorylated only serine-1 of troponin T. The results suggest that standard preparations of phosphorylase kinase are contaminated by troponin T kinase, which can phosphorylate serine-1 of troponin T.

Millisecond photo-cross-linking of protein components in vertebrate striated muscle thin filaments

Troponin I (TnI) was reacted with a photosensitive heterobifunctional reagent, methyl 4-azidobenzimidate (ABI), and then troponin was reconstituted with the ABI-modified TnI. Flash irradiation of the reconstituted troponin resulted in the formation of cross-links between TnI and other components of troponin, troponin C (TnC) and troponin T (TnT), suggesting that TnI is in contact with TnC and TnT when troponin is free in solution. No effect of calcium on the cross-linking could be detected. When the reconstituted troponin was complexed with F-actin-tropomyosin, flash irradiation of the reconstituted then filament yielded the cross-linked products of TnC-TnI, TnI-TnT, and TnI-actin in the presence ans absence of calcium, indicating that TnI is in contact with TnC, TnT, and actin in the thin filament complex irrespective of calcium concentration. No cross-linking could be detected between TnI and tropomyosin. Calcium was found to affect the cross-linking of TnC-TnI and TnI-actin; when TnC was saturated with calcium, the extent of the TnC-TnI cross-linking increased, while that of the TnI-actin cross-linking decreased. Calcium did not affect the TnI-TnT cross-linking.

Isolation and some properties of troponin T kinase from rabbit skeletal muscle

A method for isolation of troponin T kinase (ATP-protein phosphotransferase, EC 2.7.1.37) from rabbit skeletal muscles in proposed. The method gives a 7000-10 000-fold purification and results in an enzyme with specific activity of 400-800-nmol x min-1 x mg-1 of protein. The molecular weight of tropin T kinase as determined by gel filtration exceeds 500 000. Electrophoresis in polyacrylamide gel in the presence of sodium dodecyl sulphate revealed that isolated preparations of the enzyme consisted of at least three distinct proteins with apparent mol.wt. of 50 000, 46 000 and 31 000. The enzyme phosphorylates isolated troponin T at a rate which exceeds the phosphorylation rates of casein, phosvitin, histones, phosphorylase b and protamine 5-30-fold. Within the whole troponin complex, only troponin T is phosphorylated by the enzyme. The enzyme phosphorylates only the N-terminal serine residue of troponin T, i.e. the site that is normally phosphorylated in the whole troponin complex isolated from rabbit skeletal muscles.

Direct evidence for the calcium-induced change in the quaternary structure of troponin in situ. Millisecond cross-linking of troponin components by a photosensitive heterobifunctional reagent

Flash irradiation of the reconstituted troponin of thin filament complex in which one of their components, troponin C, was modified with a heterobifunctional photosensitive reagent before reconstitution of the troponin complex resulted in the formation of cross-links between troponin C and other components in contact with it. Quantitative analysis of the cross-linked products by gel electrophoresis has revealed interesting features of the quaternary structure of troponin. When the reconstituted troponin was photo-cross-linked with a xenon flash, an appreciable amount of cross-linking was detected between troponin C and troponin I and also between troponin C and troponin T. No effect of calcium on the cross-linking could be detected. This arrangement of components was found to change when troponin was complexed with F-actin-tropomyosin. The arrangement of troponin components in the thin filament complex was sensitive to calcium and magnesium; maximum cross-linking of troponin C and troponin I was observed when the thin filament was cross-linked in the presence of calcium and magnesium, while an appreciable decrease in the extent of the cross-linking was detected when calcium alone or calcium and magnesium were removed from the cross-linking medium. The cross-linking of troponin C and troponin T remained marginal irrespective of the concentration of calcium and magnesium.

The site of phosphorylation of troponin I in the perfused rabbit heart. The effect of adrenaline

1. On treatment of the perfused rabbit heart with adrenaline, the total covalently bound phosphate of troponin I increased from 1.14 mol of phosphate/mol to 1.86 mol of phosphate/mol. 2. Covalently bound phosphate could be identified only in the region of the molecule of cardiac troponin I consisting of residues 1--48. 3. When 32P-labelled orthophosphate was present in the perfusion medium the phosphate at serine-20 became radioactively labelled. This residue was the only significant site of phosphorylation that could be identified. 4. The addition of adrenaline caused a 4--5-fold increase in covalently bound [32P]phosphate. Virtually all of the 32P was located at serine-20. 5. It was concluded from these studies that the extent of phosphorylation of serine-20 of cardiac troponin I increased from 30--40% in the control perfused heart to about 100% in the presence of adrenaline.

Structure and evolution of calcium-modulated proteins

This review suggests that the intracellular functions of calcium are best understood in terms of calcium's functioning as a second messenger. Further, when functioning as a second messenger, calcium completes its mission not by transferring charge nor by binding to lipid but by binding to specific targets, calcium-modulated proteins. This concept is broadly interpreted to include proteins involved in calcium transport. There is strong evidence that many, if not all, of these calcium-modulated proteins are homologs. Their structures and properties are contrasted to those of extracellular calcium-binding proteins which are not homologous to one another or to the intracellular calcium-modulated proteins. Finally, this line of thought leads to a suggestion of the evolutionary reason for the choice of calcium as the sole inorganic second messenger.