Genomic structure of the sandworm, Perinereis vancaurica tetradentata, troponin C

Troponin C (TnC) superfamily genes essentially possess five introns, the positions of all but the fourth being highly conserved. The fourth intron is frequently absent from protostomian invertebrate genes, such as calmodulin or TnC. We previously proposed that the common ancestor of TnC superfamily genes never possessed an intron corresponding to today's fourth introns, and that members of the superfamily independently gained a fourth intron in the evolutionary pathway of each lineage. In the present study, we isolated the TnC cDNA from the sandworm, Perinereis vancaurica tetradentata and determined its genomic structure. Sandworm TnC appears to exist as a single copy gene consisting of six exons and five introns. The positions of the first, second, third and fifth introns are identical to other TnCs, but that of the fourth intron is unique. This is in good agreement with the above-mentioned scheme, i.e. the gain of the fourth intron of sandworm TnC might have occurred within the annelid lineage after annelida/mollusca divergence.

Amino acid sequence of squid troponin C

The complete amino acid sequence of squid Todarodes pacificus troponin C (TnC), which was shown to bind only 1 mol Ca(2+)/mol, was determined by both the Edman and cDNA methods. The squid TnC is composed of 147 amino acids including an unblocked Pro at the N-terminus and the calculated molecular weight is 17003.9. Among the four potential Ca(2+)-binding sites, namely sites I-IV from the N-terminus, only site IV completely satisfied the consensus amino acid sequence for the active Ca(2+)-binding loop. This indicates that squid TnC possesses a single Ca(2+)-binding site at the site IV as scallop TnCs [Nishita et al., J. Biol. Chem. 269 (1994) 3464-3468; Ojima et al., Arch. Biochem. Biophys. 311 (1994) 272-276). The sequence homology of squid TnC to TnCs of scallop, arthropods, and rabbit was 61%, 31-38%, and 31%, respectively. In the sequence of the central D/E-helix region of squid and scallop TnCs, a deletion of three amino acids was required to maximize the homology with the other TnCs.

Proximity relationships between residue 117 of rabbit skeletal troponin-I and residues in troponin-C and actin

We used resonance energy transfer and site-directed photo-cross-linking to probe the Ca(2+)-dependent proximity relationships between residue 117 next to the C-terminus of the inhibitory region in rabbit skeletal troponin-I (TnI) and residues in troponin-C (TnC) and in actin. A mutant TnI that contains a single cysteine at position 117 (I117) was constructed, and the distance between TnI residue 117 and TnC residue 98 was measured with the following results: for both the binary TnC-TnI complex and the ternary troponin complex, this distance was 30 and 41 A in the presence and absence of Ca(2+), respectively. The distance between TnI residue 117 and Cys374 of actin was 48 and 41 A in the presence and absence of Ca(2+), respectively. Six additional distances from this TnI residue to cysteines in TnC mutants were measured and used to localize this residue with respect to the crystal structure of TnC. The results show that in the presence of Ca(2+) it is localized near the B and C helices of TnC's N-terminal domain. In the absence of Ca(2+) this residue moves away from this location by approximately 8 A. Photo-cross-linking experiments show that I117 labeled with 4-maleimidobenzophenone photo-cross-linked to TnC but not to actin in both the presence and absence of Ca(2+). Taken together these results provide independent experimental support for the proposal (Y. Luo, J. L. Wu, B. Li, K. Langsetmo, J. Gergely, and T. Tao, 2000, J. Mol. Biol. 296:899-910) that upon Ca(2+) removal the region comprising TnI residues 114-125 triggers the movements of residues 89-113 and 130-150 toward actin, but does not itself interact with actin.

Characterization of the biologically important interaction between troponin C and the N-terminal region of troponin I

The N-terminal regulatory region of Troponin I, residues 1-40 (TnI 1-40, regulatory peptide) has been shown to have a biologically important function in the interactions of troponin I and troponin C. Truncated analogs corresponding to shorter versions of the N-terminal region (1-30, 1-28, 1-26) were synthesized by solid-phase methodology. Our results indicate that residues 1-30 of TnI comprises the minimum sequence to retain full biological activity as measured in the acto-S1-TM ATPase assay. Binding of the TnI N-terminal regulatory peptides (TnI 1-30 and the N-terminal regulatory peptide (residues 1-40) labeled with the photoprobe benzoylbenzoyl group, BBRp) were studied by gel electrophoresis and photochemical cross-linking experiments under various conditions. Fluorescence titrations of TnI 1-30 were carried out with TnC mutants that carry a single tryptophan fluorescence probe in either the N- or C-domain (F105W, F105W/C domain (88-162), F29W and F29W/N domain (1-90)) (Fig. 1). Low Kd values (Kd < 10(-7) M) were obtained for the interaction of F105W and F105W/C domain (88-162) with TnI 1-30. However, there was no observable change in fluorescence when the fluorescence probe was located at the N-domain of the TnC mutant (F29W and F29W/N domain (1-90)). These results show that the regulatory peptide binds strongly to the C-terminal domain of TnC.

Structural and functional studies on Troponin I and Troponin C interactions

Troponin I (TnI) peptides (TnI inhibitory peptide residues 104-115, Ip; TnI regulatory peptide resides 1-30, TnI1-30), recombinant Troponin C (TnC) and Troponin I mutants were used to study the structural and functional relationship between TnI and TnC. Our results reveal that an intact central D/E helix in TnC is required to maintain the ability of TnC to release the TnI inhibition of the acto-S1-TM ATPase activity. Ca(2+)-titration of the TnC-TnI1-30 complex was monitored by circular dichroism. The results show that binding of TnI1-30 to TnC caused a three-folded increase in Ca(2+) affinity in the high affinity sites (III and IV) of TnC. Gel electrophoresis and high performance liquid chromatography (HPLC) studies demonstrate that the sequences of the N- and C-terminal regions of TnI interact in an anti-parallel fashion with the corresponding N- and C-domain of TnC. Our results also indicate that the N- and C-terminal domains of TnI which flank the TnI inhibitory region (residues 104 to 115) play a vital role in modulating the Ca(2+)- sensitive release of the TnI inhibitory region by TnC within the muscle filament. A modified schematic diagram of the TnC/TnI interaction is proposed.

Modulation of contractile activation in skeletal muscle by a calcium-insensitive troponin C mutant

Calcium controls the level of muscle activation via interactions with the troponin complex. Replacement of the native, skeletal calcium-binding subunit of troponin, troponin C, with mixtures of functional cardiac and mutant cardiac troponin C insensitive to calcium and permanently inactive provides a novel method to alter the number of myosin cross-bridges capable of binding to the actin filament. Extraction of skeletal troponin C and replacement with functional and mutant cardiac troponin C were used to evaluate the relationship between the extent of thin filament activation (fractional calcium binding), isometric force, and the rate of force generation in muscle fibers independent of the calcium concentration. The experiments showed a direct, linear relationship between force and the number of cross-bridges attaching to the thin filament. Further, above 35% maximal isometric activation, following partial replacement with mixtures of cardiac and mutant troponin C, the rate of force generation was independent of the number of actin sites available for cross-bridge interaction at saturating calcium concentrations. This contrasts with the marked decrease in the rate of force generation when force was reduced by decreasing the calcium concentration. The results are consistent with hypotheses proposing that calcium controls the transition between weakly and strongly bound cross-bridge states.

Calcium- and magnesium-dependent interactions between the C-terminus of troponin I and the N-terminal, regulatory domain of troponin C

The muscle thin filament protein troponin (Tn) regulates contraction of vertebrate striated muscle by conferring Ca2+ sensitivity to the interaction of actin and myosin. Troponin C (TnC), the Ca2+ binding subunit of Tn contains two homologous domains and four divalent cation binding sites. Two structural sites in the C-terminal domain of TnC bind either Ca2+ or Mg2+, and two regulatory sites in the N-terminal domain are specific for Ca2+. Interactions between TnC and the inhibitory Tn subunit troponin I (TnI) are of central importance to the Ca2+ regulation of muscle contraction and have been intensively studied. Much remains to be learned, however, due mainly to the lack of a three-dimensional structure for TnI. In particular, the role of amino acid residues near the C-terminus of TnI is not well understood. In this report, we prepared a mutant TnC which contains a single Trp-26 residue in the N-terminal, regulatory domain. We used fluorescence lifetime and quenching measurements to monitor Ca2+- and Mg2+-dependent changes in the environment of Trp-26 in isolated TnC, as well as in binary complexes of TnC with a Trp-free mutant of TnI or a truncated form of this mutant, TnI(1-159), which lacked the C-terminal 22 amino acid residues of TnI. We found that full-length TnI and TnI(1-159) affected Trp-26 similarly when all four binding sites of TnC were occupied by Ca2+. When the regulatory Ca2+-binding sites in the N-terminal domain of TnC were vacant and the structural sites in the C-terminal domain of were occupied by Mg2+, we found significant differences between full-length TnI and TnI(1-159) in their effect on Trp-26. Our results provide the first indica- tion that the C-terminus of TnI may play an important role in the regulation of vertebrate striated muscle through Ca2+-dependent interactions with the regula- tory domain of TnC.

Recombinant single chain cardiac troponin I-C polypeptides: superior calibration and control materials for cardiac troponin I immunoassays

There has been a need to create stable and reproducible calibration and control materials for cardiac troponin I assays. Free troponin I, native or recombinant, has been known to be unstable, while troponin CI complex can be easily dissociated in low concentrations or in the presence of chelating agents. In order to overcome these difficulties, two single chain troponin I-C polypeptides have been engineered and expressed separately in Escherichia coli. One consists of a full-length of human cardiac troponin I and C, termed as ScTnI-C and the other consists of a stable fragment (aa28-110) of human cardiac troponin I and a full-length troponin C, termed as ScTnI-C-2. Both ScTnI-C and ScTnI-C-2 were purified to homogeneity by affinity chromatography using anti-cTnI monoclonal antibodies. ScTnI-C and ScTnI-C-2 have apparent molecular weights of 45 kD and 30 kD by SDS-PAGE, respectively. Stability studies by Stratus showed that ScTn I-C and ScTnI-C-2 were stable for 4 months at 2-8 degrees C and at least one year at -20 degrees C. When incubated in human serum at 37 degrees C, ScTnI-C-2 was more resistant to proteolysis than ScTnI-C. ScTnI-C can be recognized by all commercial TnI immunoassays with excellent activity. ScTnI-C-2 can be recognized by all immunoassays that target the stable region of cardiac troponin I. Judging by their performances, ScTnI-C and ScTnI-C-2 are both superior materials to be used as calibrators and controls in clinical laboratories.

Ca2+ - and cross-bridge-dependent changes in N- and C-terminal structure of troponin C in rat cardiac muscle

Linear dichroism of 5'-tetramethylrhodamine (5'ATR)-labeled cardiac troponin C (cTnC) was measured to monitor cTnC structure during Ca2+-activation of force in rat skinned myocardium. Mono-cysteine mutants allowed labeling at Cys-84 (cTnC(C84), near the D/E helix linker); Cys-35 (cTnC(C35), at nonfunctional site I); or near the C-terminus with a cysteine inserted at site 98 (cTnC-C35S,C84S,S98C, cTnC(C98)). With 5'ATR-labeled cTnC(C84) and cTnC(C98) dichroism increased with increasing [Ca2+], while rigor cross-bridges caused dichroism to increase more with 5'ATR-labeled cTnC(C84) than cTnC(C98). The pCa50 values and n(H) from Hill analysis of the Ca2+-dependence of force and dichroism were 6.4 (+/-0.02) and 1.08 (+/-0.04) for force and 6.3 (+/-0.04) and 1.02 (+/-0.09) (n = 5) for dichroism in cTnC(C84) reconstituted trabeculae. Corresponding data from cTnC(C98) reconstituted trabeculae were 5.53 (+/-0.03) and 3.1 (+/-0.17) for force, and 5.39 (+/-0.03) and 1.87 (+/-0.17) (n = 5) for dichroism. The contribution of active cycling cross-bridges to changes in cTnC structure was determined by inhibition of force to 6% of pCa 4.0 controls with 1.0 mM sodium vanadate (Vi). With 5'ATR-labeled cTnC(C84) Vi caused both the pCa50)of dichroism and the maximum value at pCa 4.0 to decrease, while with 5'ATR-labeled cTnC(C98) the pCa50 of dichroism decreased with no change of dichroism at pCa 4.0. The dichroism of 5'ATR-labeled cTnC(C35) was insensitive to either Ca2+ or strong cross-bridges. These data suggest that both Ca2+ and cycling cross-bridges perturb the N-terminal structure of cTnC at Cys-84, while C-terminal structure is altered by site II Ca2+-binding, but not cross-bridges.

Calcium Affinity of Regulatory Sites in Skeletal Troponin-C Is Attenuated by N-Cap Mutations of Helix C

Site-directed mutagenesis was used to make amino acid substitutions at position 54 of skeletal troponin C, testing a relationship between the stability of helix C and calcium ion affinity at regulatory sites in the protein. Normally, threonine at position 54 is the first helical residue, or N-cap, of the C helix; where helices C and D, and the loop between, comprise binding site II. Mutations were made in the context of a previously described phenylalanine 29--> tryptophan (F29W) variant (Trigo-Gonzalez et al., Biochemistry 31, 7009-7015 (1992)), which allows binding events to be monitored through changes in the intrinsic fluorescence of the protein. N-Cap substitutions at position 54 were shown to attenuate the calcium affinity of regulatory sites in the N-terminal domain. Calcium affinities diminished according to the series T54 T54S > T54A > T54V > T54G with dissociation constants of 1.36 x 10(-6), 1.36 x 10(-6), 2.09 x 10(-6), 2.28 x 10(-6), and 4.24 x 10(-6) M, respectively. The steady state binding of calcium to proteins in the mutant series was seen to be monophasic and cooperative. Calcium off-rates were measured by stopped flow fluorescence and in every instance two transitions were observed. The rate constant of the first transition, corresponding to approximately 99% of the change in fluorescence, was between 900+/-20 and 1470+/-100 s(-1), whereas the rate constant of the second transitions was between 94+/-9 and 130+/-23 s(-1). The significance of two transitions remains unclear, though both rate constants occur on a time scale consistent with the regulation of contraction.

The role of the Ca(2+) regulatory sites of skeletal troponin C in modulating muscle fibre reactivity to the Ca(2+) sensitizer bepridil

1. The Ca(2+)-sensor protein troponin C (TnC) exerts a key role in the regulation of muscle contraction, and constitutes a target for Ca(2+) sensitizer compounds, such as bepridil, known to increase its apparent Ca(2+) affinity. Moreover, bepridil has been reported to exert a differential effect in slow and fast skeletal muscle fibres, which express the slow/cardiac and fast TnC isoform, respectively. 2. The role of the TnC isoform in establishing the differential effect of bepridil was assessed in slow soleus and fast tibialis rat skinned fibres, by extraction of endogenous TnC and consecutive reconstitution with either slow or fast recombinant TnC. A mutant (VG2), lacking the regulatory site II, was also used to distinguish the role of each regulatory site. 3. Fast tibialis fibres reconstituted with cardiac TnC exhibited a typical slow bepridil reactivity, while slow soleus fibres reincorporated with fast TnC displayed a typically fast reactivity to bepridil. These results indicated that the differential effect of bepridil in slow and fast fibres is related to the TnC isoform predominantly expressed in a fibre. 4. Experiments with the VG2 mutant demonstrated that BPD can achieve an increase in the Ca(2+) affinity in the absence of a functional site II. Thus, site I was necessary for the BPD effect to be independent of the Ca(2+) concentration. Moreover, the amplitude of the reinforcement in the Ca(2+) affinity, induced by the binding of bepridil to the TnC molecule, is dependent on the number of functional regulatory sites, the larger affinity reinforcement being detected when only one regulatory site (either site I or II) is functional.

Delayed embryonic development of mouse masseter muscle correlates with delayed MyoD family expression

While the masseter muscle is known to have several unique developmental characteristics as compared with other skeletal muscles, little is known about its myogenesis. Thus, we examined the expression of myogenic marker and of myoD family gene mRNA from embryonic day (E) 11 to birth. The obtained results were compared with our earlier results of the mouse tongue muscle, which is also involved in oral functions. The mRNA quantities were determined by means of the reverse-transcription and competitive-polymerase chain-reaction techniques. The expression of myogenic marker mRNA indicated that differentiation and maturation in the masseter began at E13 as in the tongue, and were not yet completed at birth, although they were completed in the tongue. The expression of myoD, myogenin, and myf5 mRNA peaked later in the masseter (E17) than in the tongue (E13). The expression of MRF4 mRNA began later in the masseter (E15) than in the tongue (E13). These results suggest that the delayed expression of the myoD family genes in the masseter correlates with delayed differentiation and maturation, probably due to the later functional requirements of the masseter than of the tongue.

Ca(2+) binding to cardiac troponin C: effects of temperature and pH on mammalian and salmonid isoforms

A reduction in temperature lowers the Ca(2+) sensitivity of skinned cardiac myofilaments but this effect is attenuated when native cardiac troponin C (cTnC) is replaced with skeletal TnC. This suggests that conformational differences between the two isoforms mediate the influence of temperature on contractility. To investigate this phenomenon, the functional characteristics of bovine cTnC (BcTnC) and that from rainbow trout, Oncorhynchus mykiss, a cold water salmonid (ScTnC), have been compared. Rainbow trout maintain cardiac function at temperatures cardioplegic to mammals. To determine whether ScTnC is more sensitive to Ca(2+) than BcTnC, F27W mutants were used to measure changes in fluorescence with in vitro Ca(2+) titrations of site II, the activation site. When measured under identical conditions, ScTnC was more sensitive to Ca(2+) than BcTnC. At 21 degrees C, pH 7.0, as indicated by K(1/2) (-log[Ca] at half-maximal fluorescence, where [Ca] is calcium concentration), ScTnC was 2.29-fold more sensitive to Ca(2+) than BcTnC. When pH was kept constant (7.0) and temperature was lowered from 37.0 to 21.0 degrees C and then to 7.0 degrees C, the K(1/2) of BcTnC decreased by 0.13 and 0.32, respectively, whereas the K(1/2) of ScTnC decreased by 0.76 and 0.42, respectively. Increasing pH from 7.0 to 7.3 at 21.0 degrees C increased the K(1/2) of both BcTnC and ScTnC by 0.14, whereas the K(1/2) of both isoforms was increased by 1.35 when pH was raised from 7.0 to 7.6 at 7.0 degrees C.

Energetics of the induced structural change in a Ca2+ regulatory protein: Ca2+ and troponin I peptide binding to the E41A mutant of the N-domain of skeletal troponin C

Structural studies have shown that the regulatory domains of skeletal and cardiac troponin C (sNTnC and cNTnC) undergo different conformational changes upon Ca(2+) binding; sNTnC "opens" with a large exposure of the hydrophobic surface, while cNTnC retains a "closed" conformation similar to that in the apo state. This is mainly due to the fact that there is a defunct Ca(2+)-binding site I in cNTnC. Despite the striking difference, the two proteins bind their respective troponin I (TnI) regions (sTnI(115-131) and cTnI(147-163), respectively) in a similar open fashion. Thus, there must exist a delicate energetic balance between Ca(2+) and TnI binding and the accompanying conformational changes in TnC for each system. To understand the coupling between Ca(2+) and TnI binding and the concomitant structural changes, we have previously engineered an E41A mutant of sNTnC and demonstrated that this mutation drastically reduced the Ca(2+)-binding affinity of site I in sNTnC, and as a result, E41A-sNTnC remains closed in the Ca(2+)-bound state. In the present work, we investigated the interaction of E41A-sNTnC with the sTnI(115-131) peptide and found that the peptide binds to the Ca(2+)-saturated E41A-sNTnC with a 1:1 stoichiometry and a dissociation constant of 300 +/- 100 microM. The peptide-induced chemical shift changes resemble those of Ca(2+) binding to sNTnC, suggesting that sTnI(115-131) induces the "opening" of E41A-sNTnC. In addition, the binding of sTnI(115-131) appears to be accompanied by a conformational change in site I of E41A-sNTnC so that the damaged regulatory site can bind Ca(2+) more tightly. Without Ca(2+), sTnI(115-131) only interacts with E41A-sNTnC nonspecifically. When Ca(2+) is titrated into E41A-sNTnC in the presence of sTnI(115-131), the Ca(2+)-binding affinity of site I was enhanced by approximately 5-fold as compared to when sTnI(115-131) was not present. These observations suggest that the binding of Ca(2+) and TnI is intimately coupled to each other. Together with our previous studies on Ca(2+) and TnI peptide binding to sNTnC and cNTnC, these results allow us to dissect the mechanism and energetics of coupling of ligand binding and structural opening intricately involved in the regulation of skeletal and cardiac muscle contraction.

Systematic mapping of regions of human cardiac troponin I involved in binding to cardiac troponin C: N- and C-terminal low affinity contributing regions

The Spot method of multiple peptide synthesis was used to map in a systematic manner regions of the human cardiac troponin I sequence (hcTnI) involved in interactions with its physiological partner, troponin C (cTnC). Ninety-six 20-mer peptides describing the entire hcTnI sequence were chemically assembled; their reactivity with [125I]cTnC, in the presence of 3 mM Ca2+, enabled the assignment of six sites of interaction (residues 19-32, 45-54, 129-138, 145-164, 161-178 and 191-210). For several sites, a good correlation with literature data was obtained, thus validating this methodological approach. Synthetic peptides, each containing in their sequence an interaction site, were prepared. As assessed by BIACORE, all of them exhibited an affinity for cTnC in the range of 10(-6)-10(-7) M, except for hcTnI [39-58] which showed a nanomolar affinity. This peptide was also able to block the interaction between hcTnI and cTnC. We therefore postulate that despite the existence of multiple cTnC interaction sites on the hcTnI molecule, only that region of hcTnI allows a stabilization of the complex. Residues 19-32 from the N-terminal cardio-specific extension of hcTnI were also found to be involved in interaction with cTnC; residues 19-32 may correspond to the minimal sequence of the extension which could switch between the N- and C-terminal TnC domains, depending on its phosphorylation state. Finally, two Ca(2+)-dependent cTnC binding domains within the C-terminal part of hcTnI (residues 164-178 and 191-210) were also mapped. The latter site may be linked with the cardiac dysfunction observed in stunned myocardium.

Molecular cloning and characterization of an IgE-reactive protein from Anisakis simplex: Ani s 1

Ingestion of the parasitic nematode Anisakis simplex in undercooked fish can cause severe allergic reactions in some individuals. Using pooled human sera from sensitized patients we have probed an expression library for A. simplex antigens. One positive clone was found to encode a full length 21 kDa protein with strong homology to nematode troponins. The recombinant protein was expressed as a GST-fusion protein and found by immunoblot analysis to react with sera from 20% of allergic patients. The presence of functional EF-hand Ca(2+) binding motifs was demonstrated by gel-shift analysis.

The genomic structure of the scallop, Patinopecten yessoensis, troponin C gene: a hypothesis for the evolution of troponin C

Two cDNAs encoding troponin C (TnC) isoforms are isolated from the scallop, Patinopecten yessoensis, striated adductor muscle. The sequential differences between these isoforms, named TnC(long) and TnC(short), are restricted in several residues of the C-terminal region. TnC(long) is commonly expressed in both the striated and the smooth adductor muscle; however, TnC(short) is only in the striated adductor muscle. The TnC gene is a single copy gene in the scallop, thus they are expressed through the alternative splicing from the same gene. The scallop TnC gene is constructed from five exons and four introns, and positions of introns are identical with chordate TnC genes, although the scallop TnC possesses no corresponding intron to the fourth intron of chordates. The loss of this intron is also observed in Drosophila TnC; these may be remnants of their ancestor, namely the early metazoan TnC gene might be a five exons-four introns structure. In addition, the absence of the corresponding intron is also observed among protostomian calmodulins (CaMs), a molecule closely related to TnC. This suggests that the common ancestor gene of the TnC superfamily might also be a five exons-four introns structure. Assuming this to be true, the discordance of the fourth intron positions observed among members of the family is well explained by the evolutionary independent gain of the intron on each member's lineage.

Investigating the role of Ca2+-binding site IV in barnacle troponin C

Two genetically engineered, recombinant versions of native barnacle troponin C (TnC) (BTnC,) were created from the bacterially expressed, recombinant, wild-type BTnC (BTnCWT) to investigate the role of the Ca(2+)-specific sites in force regulation. The mutant BTnC4- contains a single amino acid mutation in site IV which results in the inactivation of site IV Ca2+ binding; the mutant BTnCTrunc lacks the last II amino acids of the C-terminal, and hence most of site IV. Both mutant proteins, which retain an active site II, bind to native TnC-depleted myofibrillar bundles and restore approximately 40% of the tension-generating capacity, about half that seen with purified native BTnC1 or BTnC2. This observation implies that the Mg(2+)-dependent interaction with troponin I (TnI) is at a location on TnC other than the C-terminal Ca(2+)-binding sites of BTnC2. Replacement with BTnCTrunc increases the sensitivity of the myofibrillar bundle to changes in ionic strength. Decreasing the ionic strength from 0.15 to 0.075 M increased force by 34%, a value much greater that the 8% increase seen in control bundles or bundles substituted with BTnC4-. These findings implicate TnC in determining this fibre characteristic, although this cannot be simply due to the alteration in the numbers of Ca2+ ions bound by the troponin complex since both BTnC4- and BTnCTrunc bind only 1 mol Ca2+/mol protein.

Further evidence for the cardiac troponin C mediated calcium sensitization by levosimendan: structure-response and binding analysis with analogs of levosimendan

Levosimendan, an inodilatory drug discovered using troponin C as a target protein, has a cardiac effect deriving from the calcium sensitization of contractile proteins. The aim of this study was to give further evidence that levosimendan binds to cardiac troponin C and that the binding involves amino acid residues on helixepsilon of the N-terminal domain of this calcium-binding protein. Nine organic molecules, obtained by chemical modification of levosimendan, were tested both for their calcium-dependent binding to troponin C and troponin complex affinity HPLC columns, and for their ability to increase the calcium sensitivity of myofilaments in cardiac skinned fibers. A good correlation between the calcium sensitization and the calcium-dependent binding to troponin complex (r=0.90) and to cardiac troponin C (r=0.91) for the analogs of levosimendan was shown. In addition, the effect of levosimendan on the calcium-induced conformational changes in native and point-mutated cTnC was studied. Cys84-->Ser, Asp87-->Lys and Asp88-->Ala point-mutated cTnC were shown to maintain a high affinity to calcium, but their Ca(2+)titration curves were not influenced by levosimendan as for the native protein. Finally, it was demonstrated that the NMR chemical shifts of the terminal methyl groups of Met47, Met81, and Met85 on calcium-saturated cTnC were changed after addition of levosimendan in water solution at pH 7.4. This effect was not seen when adding an analog of levosimendan, which did not bind to the troponin C affinity HPLC column and did not increase the calcium-induced tension in cardiac skinned fibers.

Functional consequences of the deletion mutation deltaGlu160 in human cardiac troponin T

To explore the functional consequences of a deletion mutation of troponin T (DeltaGlu160) found in familial hypertrophic cardiomyopathy, the mutant human cardiac troponin T, and wild-type troponins T, I, and C were expressed in Escherichia coli and directly incorporated into isolated porcine cardiac myofibrils using our previously reported troponin exchange technique. The mutant troponin T showed a slightly reduced potency in replacing the endogenous troponin complex in myofibrils and did not affect the inhibitory action of troponin I but potentiated the neutralizing action of troponin C, suggesting that the deletion of a single amino acid, Glu-160, in the strong tropomyosin-binding region affects the tropomyosin binding affinity of the entire troponin T molecule and alters the interaction between troponin I and troponin C within ternary troponin complex in the thin filament. This mutation also increased the Ca(2+) sensitivity of the myofibrillar ATPase activity, as in the case of other mutations in troponin T with clinical phenotypes of poor prognosis similar to that of Glu160. These results provide strong evidence that the increased Ca(2+) sensitivity of cardiac myofilament is a typical functional consequence of the troponin T mutation associated with a malignant form of hypertrophic cardiomyopathy.

An interdomain distance in cardiac troponin C determined by fluorescence spectroscopy

The distance between Ca2+-binding site III in the C-terminal domain and Cys35 in the N-terminal domain in cardiac muscle troponin C (cTnC) was determined with a single-tryptophan mutant using bound Tb3+ as the energy donor and iodoacetamidotetramethylrhodamine linked to the cysteine residue as energy acceptor. The luminescence of bound Tb3+ was generated through sensitization by the tryptophan located in the 12-residue binding loop of site III upon irradiation at 295 nm, and this sensitized luminescence was the donor signal transferred to the acceptor. In the absence of bound cation at site II, the mean interdomain distance was found to be 48-49 A regardless of whether the cTnC was unbound or bound to cardiac troponin I, or reconstituted into cardiac troponin. These results suggest that cTnC retains its overall length in the presence of bound target proteins. The distribution of the distances was wide (half-width >9 A) and suggests considerable interdomain flexibility in isolated cTnC, but the distributions became narrower for cTnC in the complexes with the other subunits. In the presence of bound cation at the regulatory site II, the interdomain distance was shortened by 6 A for cTnC, but without an effect on the half-width. The decrease in the mean distance was much smaller or negligible when cTnC was complexed with cTnI or cTnI and cTnT under the same conditions. Although free cTnC has considerable interdomain flexibility, this dynamics is slightly reduced in troponin. These results indicate that the transition from the relaxed state to an activated state in cardiac muscle is not accompanied by a gross alteration of the cTnC conformation in cardiac troponin.

Expression of myogenic regulatory factors during the development of mouse tongue striated muscle

While the role of myogenic regulatory factors (MRFs) in skeletal myogenesis has been well evaluated in limb and trunk muscles, very little is known about their role in tongue myogenesis. Here the expression of MRF mRNA in mouse tongue muscle was examined during development from embryonic day (E)11 to birth and compared them with that in hind-limb muscle. Desmin, muscle creatine kinase and troponin C mRNAs were used as markers for myoblast determination, myotubule formation and myofibre maturation, respectively. The mRNA quantities were determined by competitive reverse transcriptase-polymerase chain reaction. The expression profile of desmin mRNA indicated that myoblast determination occurred before E11 in both the tongue and hind-limb muscles; the profile of muscle creatine kinase and troponin C mRNAs indicated that myotubule formation and myofibre maturation began between E11 and 13 in both tongue and hind-limb muscles, but ended 2 days earlier in the tongue than in the hind limb. Expression of myoD and myogenin mRNAs began at E11, increased, and showed peak values earlier in the tongue muscle (E13) than in the hind-limb muscle (E15). Expression of MRF4 mRNA appeared earlier in the tongue (E13) than in the hind-limb muscle (E15) and increased in both muscles after that. These results suggest that myotubule formation and myofibre maturation in the tongue muscle progress faster than in the hind-limb muscle, a result of earlier expression of myoD, myogenin, and MRF4 in response to earlier functional demands such as suckling immediately after birth.

Investigation of a genetically engineered mutant of barnacle troponin C containing a central helix deletion

To examine the importance of the central alpha-helix of troponin C (TnC) we have bacterially expressed one of the isoforms of barnacle TnC (BTnC2), BTnCWT, but without the aspartate residue at position 80 in the central helix (BTnC80-). This manipulation is expected to produce an approximately 100 degrees axial rotation of the C-domain with respect to the N-domain, and a net charge change of -1. BTnC80- mutant was able to restore force to TnC-depleted skinned barnacle myofibrillar bundles to a greater extent than wild-type protein (approximately = 170%). Competition experiments between BTnC80- and BTnC2-4-, a mutant lacking both of the calcium-specific sites (sites II and IV), shows that deletion of a single amino acid in the central helix results in a protein with increased affinity for the thin filament and one that is bound preferentially compared to BTnC2-4- when at equimolar concentrations.

Conformation of the regulatory domain of cardiac muscle troponin C in its complex with cardiac troponin I

Calcium activation of fast striated muscle results from an opening of the regulatory N-terminal domain of fast skeletal troponin C (fsTnC), and a substantial exposure of a hydrophobic patch, essential for Ca(2+)-dependent interaction with fast skeletal troponin I (fsTnI). This interaction is obligatory to relieve the inhibition of strong, force-generating actin-myosin interactions. We have determined intersite distances in the N-terminal domain of cardiac TnC (cTnC) by fluorescence resonance energy transfer measurements and found negligible increases in these distances when the single regulatory site is saturated with Ca(2+). However, in the presence of bound cardiac TnI (cTnI), activator Ca(2+) induces significant increases in the distances and a substantial opening of the N-domain. This open conformation within the cTnC.cTnI complex has properties favorable for the Ca(2+)-induced interaction with an additional segment of cTnI. Thus, the binding of cTnI to cTnC is a prerequisite to achieve a Ca(2+)-induced open N-domain similar to that previously observed in fsTnC with no bound fsTnI. This role of cardiac TnI has not been previously recognized. Our results also indicate that structural information derived from a single protein may not be sufficient for inference of a structure/function relationship.

Optical spectroscopic characterization of single tryptophan mutants of chicken skeletal troponin C: evidence for interdomain interaction

The effects of metal ion binding on the optical spectroscopic properties and temperature stability of two single tryptophan mutants of chicken skeletal TnC, F78W and F154W, have been examined. The absence of tyrosine and other tryptophan residues allowed the unambiguous assignment of the spectral signal from the introduced Trp residue. Changes in the molar ellipticity values in the far-UV CD spectra of the mutant proteins on metal ion binding were similar to those of wild-type TnC suggesting that the introduction of the Trp residue had no effect on the total secondary structure content. The fluorescence and near-UV absorbance data reveal that, in the apo state, Trp-78 is buried while Trp-154 is exposed to solvent. Additionally, the highly resolved (1)L(b) band of Trp-78 seen in the near-UV absorbance and CD spectra of the apo state of F78W suggest that this residue is likely in a rigid molecular environment. In the calcium-saturated state, Trp-154 becomes buried while the solvent accessibility of Trp-78 increases. The fluorescence emission and near-UV CD of Trp-78 in the N-terminal domain were sensitive to calcium binding at the C-terminal domain sites. Measurements of the temperature stability reveal that events occurring in the N-terminal domain affect the stability of the C-terminal domain and vice versa. This, coupled with the titration data, strongly suggests that there are interactions between the N- and C-terminal domains of TnC.

Genomic organization, expression, and analysis of the troponin C gene pat-10 of Caenorhabditis elegans

We have cloned and characterized the troponin C gene, pat-10 of the nematode Caenorhabditis elegans. At the amino acid level nematode troponin C is most similar to troponin C of Drosophila (45% identity) and cardiac troponin C of vertebrates. Expression studies demonstrate that this troponin is expressed in body wall muscle throughout the life of the animal. Later, vulval muscles and anal muscles also express this troponin C isoform. The structural gene for this troponin is pat-10 and mutations in this gene lead to animals that arrest as twofold paralyzed embryos late in development. We have sequenced two of the mutations in pat-10 and both had identical two mutations in the gene; one changes D64 to N and the other changes W153 to a termination site. The missense alteration affects a calcium-binding site and eliminates calcium binding, whereas the second mutation eliminates binding to troponin I. These combined biochemical and in vivo studies of mutant animals demonstrate that this troponin is essential for proper muscle function during development.

Binding of cardiac troponin-I147-163 induces a structural opening in human cardiac troponin-C

The interaction of troponin-C (TnC) with troponin-I (TnI) plays a central role in skeletal and cardiac muscle contraction. We have recently shown that the binding of Ca2+ to cardiac TnC (cTnC) does not induce an "opening" of the regulatory domain in order to interact with cTnI [Sia, S. K., et al. (1997) J. Biol. Chem. 272, 18216-18221; Spyracopoulos et al. (1997) Biochemistry 36, 12138-12146], which is in contrast to the regulatory N-domain of skeletal TnC (sTnC). This implies that the mode of interaction between cTnC and cTnI may be different than that between sTnC and sTnI. In sTnI, a region downstream from the inhibitory region (residues 115-131) has been shown to bind the exposed hydrophobic pocket of Ca2+-saturated sNTnC [McKay, R. T., et al. (1997) J. Biol. Chem. 272, 28494-28500]. The present study demonstrates that the corresponding region in cTnI (residues 147-163) binds to the regulatory domain of cTnC only in the Ca2+-saturated state to form a 1:1 complex, with an affinity approximately six times weaker than that between the skeletal counterparts. Thus, while Ca2+ does not cause opening, it is required for muscle regulation. The solution structure of the cNTnC.Ca2+.cTnI147-163 complex has been determined by multinuclear multidimensional NMR spectroscopy. The structure reveals an open conformation for cNTnC, similar to that of Ca2+-saturated sNTnC. The bound peptide adopts a alpha-helical conformation spanning residues 150-157. The C-terminus of the peptide is unstructured. The open conformation for Ca2+-saturated cNTnC in the presence of cTnI (residues 147-163) accommodates hydrophobic interactions between side chains of the peptide and side chains at the interface of A and B helices of cNTnC. Thus the mechanistic differences between the regulation of cardiac and skeletal muscle contraction can be understood in terms of different thermodynamics and kinetics equilibria between essentially the same structure states.

Characterisation of a mutant of barnacle troponin C lacking Ca2+-binding sites at positions II and IV

This study investigates a mutant barnacle troponin C (TnC) protein (BTnC2-4-) in which the Ca2+-binding sites (sites II and IV) have been rendered non-functional. Eliminating Ca2+ binding at both Ca2+-binding sites of barnacle TnC did not prevent the incorporation of BTnC2-4- into TnC-depleted myofibrillar bundles, although, as expected, the mutant was not able to effect muscle regulation. We conclude that the Mg2+ involved in stabilising the interaction between TnC and TnI in the barnacle must bind at a separate location to the Ca2+-binding sites. Competition experiments between BTnC2-4- and wild-type barnacle TnC (BTnCWT) or the native isoform BTnC2 indicate that BTnC2-4- has an approximately fourfold higher affinity for barnacle TnI than BTnCWT but a lower affinity for TnI compared to BTnC2. These results indicate that disabling sites II and IV changes the affinity of BTnC2-4- for TnC-denuded barnacle myofibrils, altering the stability of the bond formed between TnC and the thin filament.

Phylogenetic relationship of muscle tissues deduced from superimposition of gene trees

Muscle tissues can be divided into six classes; smooth, fast skeletal, slow skeletal and cardiac muscle tissues for vertebrates, and striated and smooth muscle tissues for invertebrates. We reconstructed phylogenetic trees of six protein genes that are expressed in muscle tissues and, using a newly developed program, inferred the phylogeny of muscle tissues by superimposition of five of those gene trees. The proteins used are troponin C, myosin essential light chain, myosin regulatory light chain, myosin heavy chain, actin, and muscle regulatory factor (MRF) families. Our results suggest that the emergence of skeletal-cardiac muscle type tissues preceded the vertebrate/arthropod divergence (ca. 700 MYA), while vertebrate smooth muscle seemed to evolve independent of other muscles. In addition, skeletal muscle is not monophyletic, but cardiac and slow skeletal muscles make a cluster. Furthermore, arthropod striated muscle, urochordate smooth muscle, and vertebrate muscles except for smooth muscle share a common ancestor. On the other hand, arthropod nonmuscle and vertebrate smooth muscle and nonmuscle share a common ancestor.

Residues 48 and 82 at the N-terminal hydrophobic pocket of rabbit skeletal muscle troponin-C photo-cross-link to Met121 of troponin-I

It has been proposed [Herzberg et al. (1986) J. Biol. Chem. 261, 2638-2644], and confirmed by structural studies [Gagne et al. (1995) Nat. Struct. Biol. 2, 784-789], that the binding of Ca2+ to the triggering sites in troponin-C (TnC) causes the opening of the N-terminal hydrophobic pocket bound by the B, C, and D helices. This conformational change is believed to provide an additional binding site for troponin-I (TnI) and to lead to further events in the Ca2+ regulation process. To answer the question of which part of TnI interacts with this hydrophobic patch of TnC, we constructed two TnC mutants, each with a single cysteine, one at residue 48 between helices B and C and the other at residue 82 on the D helix. Each mutant was labeled with the photoactivatable cross-linker benzophenone-4-iodoacetamide, followed by reconstitution and UV irradiation. Studies were made in the binary complex composed of TnC and TnI, the ternary complex composed of TnC, TnI, and troponin-T (TnT), and the synthetic thin filament composed of troponin, tropomyosin, and F-actin. TnC-TnI photo-cross-linking was observed for both mutants and for all three types of complexes. Although no Ca2+ dependence in the photo-cross-linking was observed on the binary and ternary complexes, the extent of cross-linking was reduced in the absence vs the presence of Ca2+ in the thin filament. TnI Met121, five residues from the C-terminus of the inhibitory region, was identified as the cross-linking site for both TnC mutants using microsequencing and mass spectrometry following proteolysis. These results, obtained with intact TnC.TnI complexes, indicate that the TnI segment containing Met121 is in close contact with the N-terminal hydrophobic patch of TnC, and that in the thin filament the segment containing this residue moves away slightly from the hydrophobic patch in the absence of Ca2+, possibly triggering the translocation of the actin-binding region(s) of TnI toward actin.

Regulation of skeletal muscle tension redevelopment by troponin C constructs with different Ca2+ affinities

In maximally activated skinned fibers, the rate of tension redevelopment (ktr) following a rapid release and restretch is determined by the maximal rate of cross-bridge cycling. During submaximal Ca2+ activations, however, ktr regulation varies with thin filament dynamics. Thus, decreasing the rate of Ca2+ dissociation from TnC produces a higher ktr value at a given tension level (P), especially in the [Ca2+] range that yields less than 50% of maximal tension (Po). In this study, native rabbit TnC was replaced with chicken recombinant TnC, either wild-type (rTnC) or mutant (NHdel), with decreased Ca2+ affinity and an increased Ca2+ dissociation rate (koff). Despite marked differences in Ca2+ sensitivity (>0.5 DeltapCa50), fibers reconstituted with either of the recombinant proteins exhibited similar ktr versus tension profiles, with ktr low (1-2 s-1) and constant up to approximately 50% Po, then rising sharply to a maximum (16 +/- 0.8 s-1) in fully activated fibers. This behavior is predicted by a four-state model based on coupling between cross-bridge cycling and thin filament regulation, where Ca2+ directly affects only individual thin filament regulatory units. These data and model simulations confirm that the range of ktr values obtained with varying Ca2+ can be regulated by a rate-limiting thin filament process.

Involvement of conserved, acidic residues in the N-terminal domain of troponin C in calcium-dependent regulation

We have mutated eight conserved, charged amino acid residues in the N-terminal, regulatory domain of troponin C (TnC) so we could investigate their role in troponin-linked Ca2+ regulation of muscle contraction. These residues surround a hydrophobic pocket in the N-terminal domain of TnC which, when Ca2+ binds to regulatory sites in this domain, is exposed and interacts with the inhibitory region of troponin I (TnI). We constructed three double mutants (E53A/E54A, E60A/E61A, and E85A/D86A) and two single mutants (R44A and R81A) of rabbit fast skeletal muscle troponin C (TnC) in which the charged residues were replaced with neutral alanines. All five of these mutants retained TnC's ability to bind TnI in a Ca2+-dependent manner, to neutralize TnI's inhibition of actomyosin S1 ATPase activity, and to form a ternary complex with TnI and troponin T (TnT). Ternary complexes formed with TnC(R44A) or TnC(R81A) regulated actomyosin S1 ATPase activity normally, with TnI-based inhibition in the absence of Ca2+ and TnT-based activation in the presence of Ca2+. TnC(E53A/E54A) and TnC(E85A/D86A) interacted weakly with TnT, as judged by native gel electrophoresis. Ternary complexes formed with these mutants inhibited actomyosin S1 ATPase activity in both the presence and absence of Ca2+, and did not undergo Ca2+-dependent structural changes in TnI which can be detected by limited chymotryptic digestion. TnC(E60A/E61A) interacted normally with TnT. Its ternary complex showed Ca2+-dependent structural changes in TnI, inhibited actomyosin S1 ATPase in the absence of Ca2+, but did not activate ATPase in the presence of Ca2+. This is the first demonstration that selective mutation of TnC can abolish the activating effect of troponin while its inhibitory function is retained. Our results suggest the existence of an elaborate network of protein-protein interactions formed by TnI, TnT, and the N-terminal domain of TnC, all of which are important in the Ca2+-dependent regulation of muscle contraction.

Effects of high pressure and temperature on the wild-type and F29W mutant forms of the N-domain of avian troponin C

The N-domain of troponin C (residues 1-90) regulates muscle contraction through conformational changes induced by Ca2+ binding. A mutant form of the isolated domain of avian troponin C (F29W) has been used in previous studies to observe conformational changes that occur upon Ca2+ binding, and pressure and temperature changes. Here we set out to determine whether the point mutation itself has any effects on the protein structure and its stability to pressure and temperature in the absence of Ca2+. Molecular dynamics simulations of the wild-type and mutant protein structures suggested that both structures are identical except in the main chain and the loop I region near the mutation site. Also, the simulations proposed that an additional cavity had been created in the core of the mutant protein. To determine whether such a cavity would affect the behavior of the protein when subjected to high pressures and temperatures, we performed 1H-NMR experiments at 300, 400, and 500 MHz on the wild-type and F29W mutant forms of the chicken N-domain troponin C in the absence of Ca2+. We found that the mutant protein at 5 kbar pressures had a destabilized beta-sheet between the Ca2+-binding loops, an altered environment near Phe-26, and reduced local motions of Phe-26 and Phe-75 in the core of the protein, probably due to a higher compressibility of the mutant. Under the same pressure conditions, the wild-type domain exhibited little change. Furthermore, the hydrophobic core of the mutant protein denatured at temperatures above 47 degrees C, while the wild-type was resistant to denaturation up to 56 degrees C. This suggests that the partially exposed surface mutation (F29W) significantly destabilizes the N-domain of troponin C by altering the packing and dynamics of the hydrophobic core.

Characterization of the cardiac holotroponin complex reconstituted from native cardiac troponin T and recombinant I and C

Cardiac troponin I (cTnI), the inhibitory subunit of cardiac troponin (cTn), is phosphorylated by the cAMP-dependent protein kinase A at two adjacently located serine residues within the heart-specific N-terminal elongation. Four different phosphorylation states can be formed. To investigate each monophosphorylated form cTnI mutants, in which each of the two serine residues is replaced by an alanine, were generated. These mutants, as well as the wild-type cardiac troponin I (cTnI-WT) have been expressed in Escherichia coli, purified and characterized by isoelectric focusing, MS and CD-spectroscopy. Monophosphorylation induces conformational changes within cTnI that are different from those induced by bisphosphorylation. Functionality was assessed by measuring the calcium dependence of myosin S1 binding to thin filaments containing reconstituted native, wild-type and mutant cTn complexes. In all cases a functional holotroponin complex was obtained. Upon bisphosphorylation of cTnI-WT the pCa curve was shifted to the right to the same extent as that observed with bisphosphosphorylated native cTnI. However, the absolute values for the midpoints were higher when recombinant cTn subunits were used for reconstitution. Reconstitution itself changed the calcium affinity of cTnC: pCa50-values were higher than those obtained with the native cardiac holotroponin complex. Apparently only bisphosphorylation of cTnI influences the calcium sensitivity of the thin filament, thus monophosphorylation has a function different from that of bisphosphorylation; this function has not yet been identified.

Interactions at the NH2-terminal interface of cardiac troponin I modulate myofilament activation

Cardiac troponin I (cTnI) is an essential element in activation of myofilaments by Ca2+ binding to cardiac troponin C (cTnC). Yet, its role in transduction of the Ca2+ binding signal to cardiac troponin T (cTnT) and tropomyosin-actin remain poorly understood. We have recently discovered that regions of cTnI C-terminal to a previously defined inhibitory peptide are essential for full inhibitory activity and Ca(2+)-sensitivity of cardiac myofilaments (Rarick et al., 1997). However, apart from its role in structural binding to cTnC, there is little knowledge concerning the role of the N-terminus of cTnI in the activation and regulation of cardiac myofilaments. To address this question, we generated wild-type mouse cardiac TnI (WT-cTnI; 211 residues) and two N-terminal deletion mutants of mouse cTnI, cTnI54-211 (missing 53 residues), and cTnI80-211 (missing 79 residues). The cTnI54-211 mutant retained the ability to bind to cTnT, but lost the ability to bind to cTnC, whereas the cTnI80-211 mutant lost the ability to bind to cTnT, but bound weakly to cTnC. Both mutants bound to F-actin. In the absence of Ca2+, cTnI54-211 was able to inhibit the unregulated MgATPase activity of myofibrils lacking endogenous cTnI-cTnC to the same extent as WT-cTnI, whereas cTnI80-211 had some impairment of its inhibitory capability. Reconstitution with cTnI54-211/cTnC complex did not restore Ca(2+)-activation of myofibrillar MgATPase activity at all, however, the cTnI80-211/cTnC complex restored Ca(2+)-activation to nearly 50% of that obtained with WT-cTnI/cTnC. These data provide the first evidence of a significant function of a cTnT-binding domain on cTnI. They also indicate that the structural cTnC binding site on cTnI is required for Ca(2+)-dependent activation of cardiac myofilaments, and that cTnT binding to the N-terminus of cTnI is a negative regulator of activation.

Mutations in the N- and D-helices of the N-domain of troponin C affect the C-domain and regulatory function

Troponin C contains a 14-residue alpha-helix at the amino terminus, the N-helix, that calmodulin lacks. Deletion of the first 11-14 residues of troponin C alters function. In the present investigation a mutant lacking residues 1-7 of the N-helix has normal conformation, Ca2+ binding, and regulatory function. Thus, residues 8-14 of the N-helix are generally sufficient for troponin C function. In the x-ray structures of troponin C there is a salt bridge between Arg 11 in the N-helix and Glu 76 in the D-helix. Destroying the salt bridge by individually mutating the residues to Cys has no effect on function. However, mutation of both residues to Cys reduces troponin C's affinity for the troponin complex on the thin filament, reduces the stability of the N-domain in the absence of divalent cations, increases the Ca2+ affinity and reduces the cooperativity of the Ca2+Mg2+ sites in the C-domain, and alters the conformational change that takes place upon Ca2+ binding (but not Mg2+ binding) to the C-domain. Cross-linking with bis-(maleimidomethylether) partially restores function. The Ca2+-specific sites in the N-domain, those closest to the sites of the mutations, are unaffected in the assays employed. These results show that the N-helix is a critical structural element for interaction with and activation of the thin filament. Moreover, mutations in the N-helix affect the C-terminal domain, consistent with recent structural studies showing that the N-helix and C-terminal domain are physically close.

The kinetic cycle of cardiac troponin C: calcium binding and dissociation at site II trigger slow conformational rearrangements

The goal of this study is to characterize the kinetic mechanism of Ca2+ activation and inactivation of cardiac troponin C (cTnC), the Ca2+ signaling protein which triggers heart muscle contraction. Previous studies have shown that IAANS covalently coupled to Cys84 of wild-type cTnC is sensitive to conformational change caused by Ca2+ binding to the regulatory site II; the present study also utilizes the C35S mutant, in which Cys84 is the lone cysteine, to ensure the specificity of IAANS labeling. Site II Ca2+ affinities for cTnC-wt, cTnC-C35S, cTnC-wt-IAANS2, and cTnC-C35S-IAANS were similar (KD = 2-5 microM at 25 degrees C; KD = 2-8 microM at 4 degrees C), indicating that neither the IAANS label nor the C35S mutation strongly perturbs site II Ca2+ affinity. To directly determine the rate of Ca2+ dissociation from site II, the Ca2+-loaded protein was rapidly mixed with a spectroscopically sensitive chelator in a stopped flow spectrometer. The resulting site II Ca2+ off-rates were k(off) = 700-800 s(-1) (4 degrees C) for both cTnC-wt and cTnC-C35S, yielding calculated macroscopic site II Ca2+ on-rates of k(on) = k(off)/KD = 2-4 x 10(8) M(-1) s(-1) (4 degrees C). As observed for Ca2+ affinities, neither the C35S mutation nor IAANS labeling significantly altered the Ca2+ on- and off-rates. Using IAANS fluorescence as a monitor of the protein conformational state, the intramolecular conformational changes (delta) induced by Ca2+ binding and release at site II were found to be significantly slower than the Ca2+ on- and off-rates. The conformational rate constants measured for cTnC-wt-IAANS2 and cTnC-C35S-IAANS were k(delta on) = 120-210 s(-1) and k(delta off) = 90-260 s(-1) (4 degrees C) . Both conformational events were slowed in cTnC-wt-IAANS2 relative to cTnC-C35S-IAANS, presumably due to the bulky IAANS probe coupled to Cys35. Together, the results provide a nearly complete kinetic description of the Ca2+ activation cycle of isolated cTnC, revealing rapid Ca2+ binding and release at site II accompanied by slow conformational steps that are likely to be retained by the full troponin complex during heart muscle contraction and relaxation.

Stabilization of slow troponin C polypeptide compensates for its reduced synthesis in antisense oligodeoxynucleotide-treated cells

The expression of genes for contractile proteins during myogenesis is coordinately regulated. Uncoupling the expression of the slow/cardiac troponin C (sTnC) gene from this process with an antisense phosphorothioate oligodeoxynucleotide (ODN) was used to examine the presence of any post-transcriptional mechanisms for regulating muscle protein synthesis. Approximately 70 and 50% decreases in sTnC polypeptide synthesis and mRNA levels, respectively, were achieved after 4 days antisense treatment. This decrease in sTnC polypeptide synthesis was not reflected in a similar decline in the steady-state level of this polypeptide. Extension of the ODN treatment to 7 days was required to produce a substantial decrease in the steady-state level of sTnC polypeptide. Our investigation suggests that during the 4-day treatment, the affected cells stabilized the sTnC polypeptide level by increasing its half-life. However, the stabilizing effect appears to be overridden during prolonged (7 days) antisense ODN treatment. Measurement of the polypeptide synthesis and mRNA levels of several contractile proteins showed no evidence of cross-regulation among the genes to coordinately regulate their expression levels.

Structural determinants of Ca2+ exchange and affinity in the C terminal of cardiac troponin C

The C terminal of cardiac troponin C (TnC) has two Ca2+-Mg2+ sites which exhibit approximately 20-fold higher Ca2+ affinity than the two C-terminal Ca2+ specific sites in calmodulin (CaM). Substitution of the third EF-hand of TnC for the corresponding EF-hand of CaM produced a mutant (CaM[3TnC]) with a 10-fold higher C-terminal Ca2+ and Mg2+ affinity. Substitution of loop 3 of TnC for loop 3 of CaM produced a mutant (CaM[loop3TnC]) with a 10-fold faster Ca2+ on rate and a 5-fold faster Ca2+ off rate than CaM. A mutant CaM (CaM[loop3X, Z]) which contained the identical coordinating amino acids and X and Z acid pairs of TnC loop 3 had a 3-fold higher C-terminal Ca2+ affinity without the increased Ca2+ exchange rates exhibited by CaM[loop3TnC]. Thus, loop factors other than the acid pairs must be responsible for the rapid Ca2+ exchange rates of CaM[loop3TnC]. Helix 6 and helix 5 in the third EF-hand of TnC support the rapid Ca2+ on rate of TnC's loop 3 and produce an approximately 4-fold reduction in its Ca2+ off rate, explaining the high Ca2+ affinity of the third EF-hand of TnC. Exchanging loop 3 or helix 5 of TnC into CaM increased the Mg2+ affinity by decreasing the Mg2+ off rate. Our results are consistent with the high Ca2+ and Mg2+ affinity of the third EF-hand of TnC resulting from the two (X and Z) acid pairs in loop 3, coupled with the greater hydrophobicity of helix 6 and helix 5 compared to that of the third EF-hand of CaM.

A high-pressure, high-resolution NMR probe for experiments at 500 MHz

A novel high-pressure, high-resolution NMR probe is described which operates at a frequency of 500 MHz. The design features an alternative RF coil (8 mm sample tube) for high frequency, sensitivity, probe power, and resolution (< 3.0 x 10(-9)). The probe is capable of pressures to at least 5 kbar over a temperature range of -30 to 80 degreesC, and has a double-tuned 1H/2H circuit which can tune at 1H frequencies of either 300 or 500 MHz. The sensitivity of the 300-MHz circuit is over twice that of previous 10-mm high-pressure NMR probe designs, while at 500 MHz the sensitivity is nearly five times that of previous 300-MHz pressure probes. Potential biochemical applications are demonstrated by 2D NOESY spectra of a Troponin C mutant.

Identification of the photocrosslinking sites in troponin-I with 4-maleimidobenzophenone labelled mutant troponin-Cs having single cysteines at positions 158 and 21

Our previous studies have shown that 4-maleimidobenzophenone (BP-Mal) attached to troponin-C (TnC) mutants with single cysteines at positions 12, 57, 89 and 98 forms crosslinks to troponin-I (TnI), and the identified crosslinking regions indicate an antiparallel course of the two interacting polypeptide chains, in agreement with other studies using fragments of TnC and TnI. In this work we extended the mapping of the TnC-TnI interface by analysing photocrosslinking between TnI and BP-Mal labelled TnC mutants with single Cys residues at positions 21 (TnC21) and 158 (TnC158). We determined the sites of these photocrosslinks in TnI by progressive proteolysis of the crosslinked product, followed by N-terminal sequencing and mass spectrophotometric analyses. The results show that whereas TnC158 forms a specific crosslink with Met-21, TnC21 forms multiple crosslinks in the range of residues 96 to 134 of TnI. The results are discussed in light of the antiparallel model of the TnI-TnC complex and a structural model derived from low-angle X-ray and neutron scattering studies.

Diversity of the troponin C genes during chordate evolution

To elucidate the diversity of troponin C (TnC) during chordate evolution, we determined the organization of TnCs from the amphioxus, the lamprey, and the frog. Like the ascidian, the amphioxus possesses a single gene of TnC, and the fundamental gene structure is identical with the ascidian TnC. However, because alternative splicing does not occur in amphioxus, the potential for generation of TnC isoforms through this event arises only in the ascidian lineage. From the frog Xenopus laevis, two distinct cDNAs encoding fTnC isoforms and a single s/cTnC cDNA were determined. The duplication of the fTnC gene may be a character of only Xenopus or closely related species. The lamprey possesses two cDNAs each encoding fTnC and s/cTnC. The lamprey is the earliest diverged species among vertebrates, and thus it is supposed that the presence of both fTnC and s/cTnC is universal among vertebrate species, and that the gene duplication might have occurred at a vertebrate ancestor after the protochordate/vertebrate divergence. The position of the 4th intron is 3.24/0 in protochordate TnC genes, but at 3. 11/2 in vertebrate fTnCs and s/cTnCs. It is suggested that the 4th intron sliding might have occurred prior to the gene duplication.

Identification and mutagenesis of a highly conserved domain in troponin T responsible for troponin I binding: potential role for coiled coil interaction

Troponin T (TnT), a thin filament myofibrillar protein, is essential for the Ca2+ regulation of striated muscle contraction in vertebrates, both in vivo and in vitro. To understand the role of TnT in this process, its interaction with two other troponin components, troponin I (TnI) and troponin C (TnC) was examined by using the yeast two hybrid system, which is a genetic approach to detect protein-protein interactions. Computer assisted analysis of phylogenetically distant TnT amino acid sequences unveiled a highly conserved protein domain that is characterized by a heptad repeat (HR) motif with a potential for alpha-helical coiled coil formation. A similar, potentially coiled coil forming domain is also conserved in all known TnI sequences. These protein motifs appeared to be the regions where TnI-TnT interaction may take place. Deletions and point mutations in TnT, which disrupted its HR motif, severely reduced or abolished TnI binding, but binding to TnC was not affected, indicating that the TnT-TnI and TnT-TnC binary interactions can be uncoupled. Remarkably, the truncated fragments of TnT and TnI in which the HR motifs were retained showed binary interaction in the yeast two hybrid system. It was also observed that the formation of the TnT-TnI heterodimers is favored over the homodimers TnT-TnT and TnI-TnI. These results indicate that the evolutionarily conserved HR motifs may play a role in TnT-TnI dimerization, presumably through the formation of alpha-helical coiled coils.

Effects of protein kinase A phosphorylation on signaling between cardiac troponin I and the N-terminal domain of cardiac troponin C

During beta-adrenergic stimulation of the heart, there is a decrease in myofilament Ca2+ sensitivity mediated by the protein kinase A-(PKA-) induced phosphorylation of troponin I (cTnI). Phosphorylation, which occurs at Ser 23 and Ser 24 in an amino-terminal extension unique to cTnI, decreases the Ca2+ affinity of the amino-terminal regulatory site of cardiac troponin C (cTnC). In view of the antiparallel organization of the cTnI-cTnC complex [Krudy, G. A., Kleerekoper, Q., Guo, X., Howarth, J. W., Solaro, R. J., and Rosevear, P. R. (1994) J. Biol. Chem. 269, 23731-23735], it is not clear how the phosphorylation signal at one end of the complex affects the Ca2+ binding site at the other end. To address this question, we probed the interaction between cTnI and cTnC fragments, cTnC1-89 and cTnC90-162 (recombinant peptides corresponding to the N- and C-domains of cTnC). cTnI-Cys 5 mutant (S5C/C81I/C98S) and cTnC1-89 were fluorescently labeled with IAANS. When cTnI was phosphorylated, the affinity of Ca2+ for the cTnI-cTnC1-89 complex decreased significantly as indicated by a shift in the pCa50 value from 6.65 to 5.25. Upon phosphorylation, the affinity of cTnI for cTnC1-89 decreased by 3.8-fold in the absence of Ca2+ and 1.7-fold in the presence of Ca2+. In contrast to the case with full-length cTnC, neither cTnC1-89 nor cTnC90-162 induced significant structural changes in cTnI-Cys 5 as determined from intersite distance measurements between Cys 5 and Trp 192. Moreover, neither fragment of cTnC could significantly restore Ca2+ regulation of force generation, when exchanged into fiber bundles from which cTnC had been extracted. Our findings indicate that the transduction of PKA-induced phosphorylation signal from cTnI to the regulatory site of cTnC involves a global change in cTnI structure.

Assignment of the human fast skeletal muscle troponin C gene (TNNC2) between D20S721 and GCT10F11 on chromosome 20 by somatic cell hybrid analysis

The chromosomal location of the human fast skeletal muscle troponin C (TNNC2) gene was determined using somatic cell hybrids. PCR-based analysis of a 'monochromosomal' hybrid panel identified the presence of the TNNC2 gene on human chromosome 20 and subsequent analysis of the Genebridge4 radiation hybrid panel located the gene between D20S721 and GCT10F11 with a lod score of > 3.

Mutations in the cardiac troponin I gene associated with hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM), the most common cause of sudden death in the young, is an autosomal dominant disease characterized by ventricular hypertrophy accompanied by myofibrillar disarrays. Linkage studies and candidate-gene approaches have demonstrated that about half of the patients have mutations in one of six disease genes: cardiac beta-myosin heavy chain (c beta MHC), cardiac troponin T (cTnT), alpha-tropomyosin (alpha TM), cardiac myosin binding protein C (cMBPC), ventricular myosin essential light chain (vMLC1) and ventricular myosin regulatory light chain (vMLC2) genes. Other disease genes remain unknown. Because all the known disease genes encode major contractile elements in cardiac muscle, we have systematically characterized the cardiac sarcomere genes, including cardiac troponin I (cTnI), cardiac actin (cACT) and cardiac troponin C (cTnC) in 184 unrelated patients with HCM and found mutations in the cTnI gene in several patients. Family studies showed that an Arg145Gly mutation was linked to HCM and a Lys206Gln mutation had occurred de novo, thus strongly suggesting that cTnI is the seventh HCM gene.

Assignment of the human cardiac/slow skeletal muscle troponin C gene (TNNC1) between D3S3118 and GCT4B10 on the short arm of chromosome 3 by somatic cell hybrid analysis

We have determined the chromosomal location of the human cardiac/slow skeletal muscle troponin C gene (TNNC1) to the short arm of chromosome 3 using somatic cell hybrids. PCR-based analysis of a 'monochromosomal' hybrid panel identified the presence of the TNNC1 gene on human chromosome 3 and subsequent analysis of the Genebridge4 radiation hybrid panel located the gene between D3S3118 (7.3cR) and GCT4B10 (4.2cR) with a lod score of > 3.0.