The sites of phosphorylation of rabbit cardiac troponin I by adenosine 3':5'-cyclic monophosphate-dependent protein kinase. Effect of interaction with troponin C

AMP-activated protein kinase phosphorylates cardiac troponin I at Ser-150 to increase myofilament calcium sensitivity and blunt PKA-dependent function

AMP-activated protein kinase (AMPK) is an energy-sensing enzyme central to the regulation of metabolic homeostasis. In the heart AMPK is activated during cardiac stress-induced ATP depletion and functions to stimulate metabolic pathways that restore the AMP/ATP balance. Recently it was demonstrated that AMPK phosphorylates cardiac troponin I (cTnI) at Ser-150 in vitro. We sought to determine if the metabolic regulatory kinase AMPK phosphorylates cTnI at Ser-150 in vivo to alter cardiac contractile function directly at the level of the myofilament. Rabbit cardiac myofibrils separated by two-dimensional isoelectric focusing subjected to a Western blot with a cTnI phosphorylation-specific antibody demonstrates that cTnI is endogenously phosphorylated at Ser-150 in the heart. Treatment of myofibrils with the AMPK holoenzyme increased cTnI Ser-150 phosphorylation within the constraints of the muscle lattice. Compared with controls, cardiac fiber bundles exchanged with troponin containing cTnI pseudo-phosphorylated at Ser-150 demonstrate increased sensitivity of calcium-dependent force development, blunting of both PKA-dependent calcium desensitization, and PKA-dependent increases in length dependent activation. Thus, in addition to the defined role of AMPK as a cardiac metabolic energy gauge, these data demonstrate AMPK Ser-150 phosphorylation of cTnI directly links the regulation of cardiac metabolic demand to myofilament contractile energetics. Furthermore, the blunting effect of cTnI Ser-150 phosphorylation cross-talk can uncouple the effects of myofilament PKA-dependent phosphorylation from β-adrenergic signaling as a novel thin filament contractile regulatory signaling mechanism.

Covalent and noncovalent modification of thin filament action: the essential role of troponin in cardiac muscle regulation

Troponin is essential for the regulation of cardiac contraction. Troponin is a sarcomeric molecular switch, directly regulating the contractile event in concert with intracellular calcium signals. Troponin isoform switching, missense mutations, proteolytic cleavage, and posttranslational modifications are known to directly affect sarcomeric regulation. This review focuses on physiologically relevant covalent and noncovalent modifications in troponin as part of a thematic series on cardiac thin filament function in health and disease.

Solution structure of calcium-saturated cardiac troponin C bound to cardiac troponin I

Cardiac troponin C (TnC) is composed of two globular domains connected by a flexible linker. In solution, linker flexibility results in an ill defined orientation of the two globular domains relative to one another. We have previously shown a decrease in linker flexibility in response to cardiac troponin I (cTnI) binding. To investigate the relative orientation of calcium-saturated TnC domains when bound to cTnI, (1)H-(15)N residual dipolar couplings were measured in two different alignment media. Similarity in alignment tensor orientation for the two TnC domains supports restriction of domain motion in the presence of cTnI. The relative spatial orientation of TnC domains bound to TnI was calculated from measured residual dipolar couplings and long-range distance restraints utilizing a rigid body molecular dynamics protocol. The relative domain orientation is such that hydrophobic pockets face each other, forming a latch to constrain separate helical segments of TnI. We have utilized this structure to successfully explain the observed functional consequences of linker region deletion mutants. Together, these studies suggest that, although linker plasticity is important, the ability of TnC to function in muscle contraction can be correlated with a preferred domain orientation and interdomain distance.

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.

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 I phosphorylation in the normal and failing adult human heart

BACKGROUND

In the failing human heart myofibrillar calcium sensitivity of tension development is greater and maximal myofibrillar ATPase activity is less than in the normal heart. Phosphorylation of the cardiac troponin I (cTnI)-specific NH2-terminus decreases myofilament sensitivity to calcium, while phosphorylation of other cTnI sites decreases maximal myofibrillar ATPase activity.

METHODS AND RESULTS

We examined cTnI phosphorylation in left ventricular myocardium collected from failing hearts at the time of transplant (n=20) and normal hearts from trauma victims (n=24). The relative amounts of actin, tropomyosin, and TnI did not differ between failing and normal myocardium. Using Western blot analysis with a monoclonal antibody (MAb) that recognizes the striated muscle TnI isoforms, we confirmed that the adult human heart expresses only cTnI. A cTnI-specific MAb recognized two bands of cTnI, designated cTnI1 and cTnI2, while a MAb whose epitope is located in the cTnI-specific NH2-terminus recognized only cTnI1. Alkaline phosphatase decreased the relative amount of cTnl1, while protein kinase A and protein kinase C increased cTnI1. The percentage of cTnI made up of cTnI1, the phosphorylated form of TnI, is greater in the normal than the failing human heart (P<.00).

CONCLUSIONS

This phosphorylation difference could underlie the reported greater myofibrillar calcium sensitivity of failing myocardium. The functional consequence of this difference may be an adaptive or maladaptive response to the lower and longer calcium concentration transient of the failing heart, eg, enhancing force development or producing ventricular diastolic dysfunction.

Regulation of phospholamban and troponin-I phosphorylation in the intact rat cardiomyocytes by adrenergic and cholinergic stimuli: roles of cyclic nucleotides, calcium, protein kinases and phosphatases and depolarization

Protein phosphorylation was investigated in [32P]-labeled cardiomyocytes isolated from adult rat heart ventricles. The beta-adrenergic stimulation (by isoproterenol, ISO) increased the phosphorylation of inhibitory subunit of troponin (TN-I), C-protein and phospholamban (PLN). Such stimulation was largely mediated by increased adenylyl cyclase (AC) activity, increased myoplasmic cyclic AMP and increased cyclic AMP dependent protein kinase (A-kinase)-catalyzed phosphorylation of these proteins in view of the following observations: (a) dibutyryl-and bromo-derivatives of cyclic AMP mimicked the stimulatory effect of ISO on protein phosphorylation while (b) Rp-cyclic AMP was found to attenuate ISO-dependent stimulation. Unexpectedly, 8-bromo cyclic GMP was found to markedly increase TN-I and PLN phosphorylation. Both beta 1- and beta 2-adrenoceptors were present and ISO binding to either receptor was found to stimulate myocyte AC. However, the stimulation of the beta 2-AR only marginally increased while the stimulation of beta 1-AR markedly increased PLN phosphorylation. Other stimuli that increase tissue cyclic AMP levels also increased PLN and TN-I phosphorylation and these included isobutylmethylxanthine (non-specific phosphodiesterase inhibitor), milrinone (inhibits cardiotonic inhibitable phosphodiesterase, sometimes called type III or IV) and forskolin (which directly stimulates adenylyl cyclase). Cholinergic agonists acting on cardiomyocyte M2-muscarinic receptors that are coupled to AC via pertussis toxin(PT)-sensitive G proteins inhibited AC and attenuated ISO-dependent increases in PLN and TN-I phosphorylation. The in vivo PT treatment, which ADP-ribosylated Gi-like protein(s) in the myocytes, markedly attenuated muscarinic inhibitory effect on PLN and TN-I phosphorylation on one hand and, increased the beta-adrenergic stimulation, on the other. Controlled exposure of isolated myocytes to N-ethyl maleimide, also led to the findings similar to those seen following the PT treatment. Exposure of myocytes to phorbol, 12-myristate, 13-acetate (PMA) increased the protein phosphorylation, augmenting the stimulation by ISO, and such augmentation was antagonized by propranolol suggesting modulation of the beta-adrenoceptor coupled AC pathway by PMA. Okadaic acid (OA) exposure of myocytes also increased protein phosphorylation with the results supporting the roles for type 1 and 2A protein phosphatases in the dephosphorylation of PLN and TN-I. Interestingly OA treatment attenuated the muscarinic inhibitory effect which was restored by subsequent brief exposure of myocytes to PMA. While the stimulation of alpha adrenoceptors exerted little effect on the phosphorylation of PLN and TN-I, inactivation of alpha adrenoceptors by chloroethylclonidine (CEC), augmented beta-adrenergically stimulated phosphorylation. KCl-dependent depolarization of myocytes was observed to potentiate ISO-dependent increase in phosphorylation (incubation period 15 sec to 1 min) as well as to accelerate the time-dependent decline in this phosphorylation seen upon longer incubation. Verapamil decreased ISO-stimulated protein phosphorylation in the depolarized myocytes. Depolarization was found to have little effect on the muscarinic inhibitory action on phosphorylation. Prior treatment of myocytes with PMA, was found to augment ISO-stimulated protein phosphorylation in the depolarized myocytes. Such augmented increases were completely blocked by propranolol. Forskolin also stimulated PLN and TN-I phosphorylation. Prior exposure of myocytes to forskolin followed by incubation in the depolarized and polarized media showed that PLN was dephosphorylated more rapidly in the depolarized myocytes. The results support the view that both cyclic AMP and calcium signals cooperatively increase the rates of phosphorylation of TN-I and PLN in the depolarized cardiomyocytes during beta-adrenergic stimulation. (ABSTRACT TRUNCATED)

Troponin I gene expression during human cardiac development and in end-stage heart failure

Recent reports have demonstrated the presence of two isoforms of troponin I in the human fetal heart, namely, cardiac troponin I and slow skeletal muscle troponin I. Structural and physiological considerations indicate that these isoforms would confer differing contractile properties on the myocardium, particularly on the phosphorylation-mediated regulation of contractility by adrenergic agonists. We have investigated the developmental expression of these isoforms in the human heart from 9 weeks of gestation to 9 months of postnatal life, using Western blots revealed with troponin I antibodies to detect troponin protein isoforms and Northern blots to detect the corresponding mRNAs. The results show the following: 1) Slow skeletal muscle troponin I is the predominant isoform throughout fetal life. 2) After birth, the slow skeletal isoform is lost, with cardiac troponin I being the only isoform detectable by 9 months of postnatal development. 3) The protein isoforms and their corresponding mRNAs follow the same pattern of accumulation, suggesting that the transition in troponin expression is regulated at the level of gene transcription. The developmental transition in troponin I isoform content has implications for contractility of the fetal and postnatal myocardium. We further analyzed right and left ventricular muscle samples from 17 hearts in end-stage heart failure resulting from pulmonary hypertension, ischemic heart disease, or dilated cardiomyopathy. Cardiac troponin I mRNA remained abundant in each case, and slow skeletal muscle troponin I mRNA was not detectable in any of sample. We conclude that alterations in troponin I isoform content do not therefore contribute to the altered contractile characteristics of the adult failing ventricle.

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.

Force-pCa relation and troponin T isoforms of rabbit myocardium

We have previously reported the existence of at least four troponin T isoforms in rabbit ventricular muscle and described the changes in their distribution with development. In this report we test whether the proportions of the troponin T isoforms are related to the sensitivity of the myofilaments to calcium. We measured the force-pCa relations in 12 detergent-skinned ventricular strands of cardiac muscle from newborn (2-5-day-old) rabbits. We determined from each strand the amount of each troponin T isoform relative to the total amount of troponin T by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and densitometric scans of Western blots probed with a cardiac-specific troponin T monoclonal antibody, MAb 13-11. To assess the presence of different relative amounts of cardiac and slow skeletal troponin I among the strands, we determined the amount of cardiac troponin I relative to tropomyosin. We determined the Hill coefficient and the pCa for half-maximal force, pCa50, for each strand. pCa50 was related directly to the relative amount of troponin T2 (pslope = 0.037). Our results do not indicate a relation between the Hill coefficient and troponin T2. We also did not find a relation between pCa50 and the cardiac troponin I/tropomyosin ratio, which suggests that the correlation between pCa50 and troponin T2 was not a result of changes in the relative amounts of cardiac and slow skeletal muscle troponin I. Our findings indicate that a relation exists between the force-pCa characteristics of rabbit myocardium and the troponin T isoforms that it expresses, suggesting a role for troponin T in modulating the sensitivity of cardiac myofilaments to calcium.

Troponin I isoform expression in human heart

Troponin I is the inhibitory component of troponin, the thin filament regulatory complex in striated muscle. Separate genes encode cardiac-specific fast and slow skeletal-specific isoforms of this protein. We have previously described gene switching from the slow skeletal to the cardiac troponin I mRNA expression in developing rat heart. The purpose of this work was to characterize the expression of the different troponin isoforms in the human heart. Human cardiac and slow skeletal troponin I cDNA probes were obtained by screening an adult cardiac cDNA library and by Taq polymerase amplification of RNA from an infant's heart, respectively. We found that the cardiac troponin I isoform is tissue-specific in its expression in normal adult tissues. RNA blot analysis of cardiac ventricular RNA from infants with congenital heart disease and from an adult with cardiomyopathy revealed expression of human cardiac troponin I in all analyzed specimens. In addition, we found expression of slow skeletal troponin I mRNA and protein in infant hearts but no detectable mRNA expression in the adult heart. We conclude that troponin I isoforms are developmentally regulated in the human heart by a mechanism similar to that in the rat heart.

Inhibition of phosphorylation of troponin I in rat heart by adenosine and 5'-chloro-5'-deoxyadenosine

We have investigated the effects of adenosine on protein phosphorylation in extracts of rat heart. Incubation of a myofibrillar fraction with [gamma-32P]ATP resulted in the phosphorylation of several proteins by endogenous protein kinases. The adenosine analog 5'-chloro-5'-deoxyadenosine inhibited the phosphorylation of a 29 kD protein in this preparation. The protein was identified as cardiac troponin I (cTnI) by two-dimensional gel electrophoresis, using purified cTnI as standard. Addition of the catalytic subunit of cAMP-dependent protein kinase to the myofibrillar fraction increased phosphorylation of cTnI; this increase was inhibited by 5'-chloro-5'-deoxyadenosine and adenosine. Phosphorylation of purified cTnI by the catalytic subunit was also inhibited by 5'-chloro-5'-deoxyadenosine. Under these conditions used, 50% inhibition of phosphorylation by either endogenous or exogenous kinase was observed at approximately 50 microM 5'-chloro-5'-deoxyadenosine or adenosine. The inhibition described here occurred independently of catecholamines. The effects of ADP, AMP, and adenine on cTnI phosphorylation are also described.

A common motif of two adjacent phosphoserines in bovine, rabbit and human cardiac troponin I

From rabbit and human cardiac troponin I N-terminal mono and bisphosphorylated peptides were isolated which were obtained from Lys-C proteinase digests. Two adjacent phosphoserine residues could be localized in each phosphopeptide following further tryptic digestion. The previously published sequence of rabbit cardiac troponin I had to be corrected. Two adjacent phosphoserine residues are a common motif in the very similar sequences of bovine, rabbit and human cardiac troponin I. The N-terminal sequences are: AcADRSGGSTAG DTVPAPPPVR RRS(P)S(P)ANYRAY ATEPHAK (bovine), AcADESTDA-AG EARPAPAPVR RRS(P)S(P)ANYRAY ATEPHAK (rabbit), (Ac,A,D/N,G,S,S,D/N,A,A,R) EPRPAPAPIR RRS(P)S(P)-NYRAY ATEPHAK (human).

Sites phosphorylated in bovine cardiac troponin T and I. Characterization by 31P-NMR spectroscopy and phosphorylation by protein kinases

Bovine cardiac troponin isolated in a highly phosphorylated form shows four 31P-NMR signals [Beier, N., Jaquet, K., Schnackerz, K. & Heilmeyer, L.M.G. Jr (1988) Eur. J. Biochem. 176, 327-334]. Troponin I, which contains phosphate covalently linked to serine-23 and/or -24 [Swiderek, K., Jaquet, K., Meyer, H. E. & Heilmeyer, L. M. G. Jr (1988) Eur. J. Biochem. 176, 335-342], shows three resonances. Mg2(+)-saturation of holotroponin shifts these troponin I resonances to higher fields. Direct binding of Mg2+ to the phosphate groups can be excluded. Both these serine residues of troponin I, 23 and 24, are substrates for cAMP- and cGMP-dependent protein kinases as well as for protein kinase C. Isolated bovine cardiac troponin T contains 1.5 mol phosphoserine/mol protein, indicating that minimally two serine residues are phosphorylated. One phosphoserine residue is located at the N-terminus. An additional phosphoserine is located in the C-terminal cyanogen bromide fragment, CN4, which contains covalently bound phosphate. Protein kinase C phosphorylates serine-194, thus demonstrating exposure of this residue on the surface of holotoponin.

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.

Identification of the multifunctional calmodulin-dependent protein kinase in the cytosol, sarcoplasmic reticulum, and sarcolemma of rabbit skeletal muscle

A multifunctional calmodulin-dependent protein kinase (calmodulin kinase) was purified from the cytosol of rabbit skeletal muscle as a subunit of 58 kDa. A 58-kDa protein in sarcoplasmic reticulum (SR) and sarcolemma (SL) of rabbit skeletal muscle was endogenously phosphorylated in a calmodulin-dependent manner. The 58-kDa protein in SR and SL was considered to be identical to the subunit of cytosol calmodulin kinase on the basis of immunoreactivity, calmodulin binding, and autophosphorylation studies and on the patterns of protease-treated phosphopeptides. Calmodulin kinase showed broad substrate specificity and phosphorylated troponins I and T.

Quantitative determination of phosphoserine by high-performance liquid chromatography as the phenylthiocarbamyl-S-ethylcysteine. Application to picomolar amounts of peptides and proteins

A method is described that permits the phosphoserine content of proteins and peptides to be determined in picomolar amounts. A micro-batch reaction first converts phosphoserine into S-ethylcysteine. Hydrolysis with 6 M hydrochloric acid then yields the free amino acid, which is coupled with phenyl isothiocyanate to give the corresponding phenylthiocarbamylamino acid. This derivative is determined quantitatively in the range 10-20 pmol by reversed-phase high-performance liquid chromatography. The method works well with either small peptides or proteins in the low picomole range.

Purification and characterization of a multifunctional calmodulin-dependent protein kinase from canine myocardial cytosol

A calmodulin-dependent protein kinase from canine myocardial cytosol was purified 1150-fold to apparent homogeneity with a 1.5% yield. The purified enzyme had a Mr of 550,000 with a sedimentation coefficient of 16.6 S, and showed a single protein band with a Mr of 55,000 (55K protein), determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified enzyme had a specific activity of 1.6 mumol/mg protein/min, and Ka values of 67 nM and 1.1 microM for calmodulin and Ca2+, respectively, using chicken gizzard myosin light chain as substrate. Calmodulin bound to the 55K protein. The purified enzyme had a broad substrate specificity. Endogenous proteins including glycogen synthase, phospholamban, and troponin I from the canine heart were phosphorylated by the enzyme. These results suggest that the purified enzyme works as a multifunctional protein kinase in the Ca2+, calmodulin-dependent cellular functions of the canine myocardium, and that the enzyme resembles enzymes detected in the brain, liver, and skeletal muscle.

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.

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

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

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.

cGMP-dependent protein kinase decreases calcium sensitivity of skinned cardiac fibers

Chemically skinned (Lubrol WX) cardiac muscle fibers produce half-maximum isometric tension at pCa 6.18 (pH 6.7) in presence of MgATP (10 mM). After addition of cGMP (5 microM) and cGMP-dependent protein kinase (0.1 microM), the pCa required for half-maximum activation is 5.96, while maximum tension is not affected. Similar shifts in the tension/pCa-relationship have been observed after incubation of skinned cardiac muscle fibers with cAMP of catalytic subunit of the cAMP-dependent protein kinase. The shift in the Ca2+-sensitivity is associated with an increased incorporation of radioactivity into a Mr 28000 band (presumably troponin-I) and a Mr 145000 band.

Bovine cardiac troponin subunits: binary complexes and reconstitution of whole troponin

The reconstitution of bovine cardiac troponin from its subunits has been investigated using hydrodynamic techniques. Gel filtration (Sephacryl S-300) and sedimentation velocity experiments indicate that troponin-C and troponin-I form a stable binary complex (1:1 mole ratio) with an apparent Stokes' radius of 36 A (frictional ratio = 1.6). Troponin-C and troponin-T do not interact significantly while troponin-I and troponin-T undergo partial complex formation. The effect of subunit ratio on the reconstitution of whole troponin has been examined by SDS-polyacrylamide gel electrophoresis and gel filtration and the results suggest that native troponin contains the subunits in an equimolar ratio.

Phosphorylation of rabbit cardiac-muscle troponin I by phosphorylase kinase. The effect of adrenaline

1. Incubation of rabbit cardiac-muscle troponin I with phosphorylase b kinase leads to the incorporation of .07-1.2 mol of Pi/mol. 2. The major site of phosphorylation is a serine residue at position 72. 3. Lesser amounts of phosphate are incorporated into threonine-138, threonine-162 and serine 20. 4. Serine-20 is the only site that contains a significant amount of phosphate before incubation with phosphorylase b kinase. 5. Unlike the situation with serine-20, the extent of phosphorylation of serine-72 and threonine-138 in the perfused rabbit heart does not change when the heart is exposed to adrenaline (4 microM).

Comparison of the Mg2+ and Ca2+ binding properties of troponin complexes P1-TI2C and TI2C

The phosphoserine present in troponin T of freshly isolated skeletal muscle troponin P1-TI2C was dephosphorylated by alkaline phosphatase and the resulting troponin TI2C characterized by phosphorous content and gel electrophoresis in presence of sodium dodecylsulfate. Both complexes bind Ca2+ in an identical manner with a K0.5 of 5.3 X 10(-9) M for the Ca2+/Mg2+ binding sites and of 1.1 X 10(-6) M for the Ca2+-specific sites. 3.5 mM Mg2+ lowers the K0.5 value at the Ca2+/Mg2+ binding sites of 1.3 X 10(-7) M in the phospho-troponin P1-TI2C and leaves nearly unchanged the value of the dephosphorylated troponin TI2C at 1.2 X 10(-8) M. At 10 mM Mg2+ only one dissociation constant of about 1.0 X 10(-6) M is determined with both complexes. In analogy dephosphorylation of troponin P1-TI2C reduces the affinity for Mg2+ at the Ca2+/Mg2+ binding sites from 6.7 X 10(-5) M to 2.0 X 10(-3) M. Again the Mg2+-specific sites are uninfluenced. The possibility is discussed that removal of the phosphate group from troponin T allows the interaction of the N-terminal domain of troponin T with other amino acid side chains of troponin.

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.

Protein-protein interaction sites in the calcium modulated skeletal muscle troponin complex

The sequence domains that contribute to the surfaces of contact between Troponin-C and the other regulatory protein subunits of skeletal muscle troponin are proposed on the basis of data obtained by proton magnetic resonance and other physicochemical studies on the interaction with Troponin-I of both Troponin-C and its peptide fragments. Marked sequence homology in Troponin-C from various species is found for the residues involved in subunit linkage. The role of the recognition sites is discussed.

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.

Comparison of amino acid sequence of troponin I from different striated muscles

The sequence of troponin I from fast and slow skeletal and cardiac muscle shows strong homology in the region which binds to actin and is responsible for inhibition of the actomyosin AT Pase. More differences are found in the N-terminal region which binds to troponin C.