Isolation and sequence of a cDNA clone for rabbit fast skeletal muscle troponin C. Homology with calmodulin and parvalbumin

Functional importance of Ca2+-deficient N-terminal lobe of molluscan troponin C in troponin regulation

Ca(2+)-binding sites I and II in the N-terminal lobe of molluscan troponin C (TnC) have lost the ability to bind Ca(2+) due to substitutions of the amino acid residues responsible for Ca(2+) liganding. To evaluate the functional importance of the Ca(2+)-deficient N-terminal lobe in the Ca(2+)-regulatory function of molluscan troponin, we constructed chimeric TnCs comprising the N-terminal lobes from rabbit fast muscle and squid mantle muscle TnCs and the C-terminal lobe from akazara scallop TnC, TnC(RA), and TnC(SA), respectively. We characterized their biochemical properties as compared with those of akazara scallop wild-type TnC (TnC(AA)). According to equilibrium dialysis using (45)Ca(2+), TnC(RA), and TnC(SA) bound stoichiometrically 3 mol Ca(2+)/mol and 1 mol Ca(2+)/mol, respectively, as expected from their primary structures. All the chimeric TnCs exhibited difference-UV-absorption spectra at around 280-290 nm upon Ca(2+) binding and formed stable complexes with akazara scallop troponin I, even in the presence of 6M urea, if Ca(2+) was present. However, when the troponin complexes were constructed from chimeric TnCs and akazara scallop troponin T and troponin I, they showed different Ca(2+)-regulation abilities from each other depending on the TnC species. Thus, the troponin containing TnC(SA) conferred as high a Ca(2+) sensitivity to Mg-ATPase activity of rabbit actomyosin-akazara scallop tropomyosin as did the troponin containing TnC(AA), whereas the troponin containing TnC(RA) conferred virtually no Ca(2+) sensitivity. Our findings indicate that the N-terminal lobe of molluscan TnC plays important roles in molluscan troponin regulation, despite its inability to bind Ca(2+).

Calcium-bindings of wild type and mutant troponin Cs of Caenorhabditis elegans

Apparent Ca(2+)-binding constant (K(app)) of Caenorhabditis elegans troponin C (CeTnC) was determined by a fluorescence titration method. The K(app) of the N-domain Ca(2+)-binding site of CeTnC was 7.9+/-1.6 x 10(5) M(-1) and that of the C-domain site was 1.2+/-0.6 x 10(6) M(-1), respectively. Mg(2+)-dependence of the K(app) showed that both Ca(2+)-binding sites did not bind competitively Mg(2+). The Ca(2+) dissociation rate constant (k(off)) of CeTnC was determined by the fluorescence stopped-flow method. The k(off) of the N-domain Ca(2+)-binding site of CeTnC was 703+/-208 s(-1) and that of the C-domain site was 286+/-33 s(-1), respectively. From these values we could calculate the Ca(2+)-binding rate constant (k(on)) as to be 5.6+/-2.8 x 10(8) M(-1) s(-1) for the N-domain site and 3.4+/-2.1 x 10(8) M(-1) s(-1) for the C-domain site, respectively. These results mean that all Ca(2+)-binding sites of CeTnC are low affinity, fast dissociating and Ca(2+)-specific sites. Evolutional function of TnC between vertebrate and invertebrate and biological functions of wild type and mutant CeTnCs are discussed.

Calcium-induced flexibility changes in the troponin C-troponin I complex

The contraction of vertebrate striated muscle is modulated by Ca(2+) binding to the regulatory protein troponin C (TnC). Ca(2+) binding causes conformational changes in TnC which alter its interaction with the inhibitory protein troponin I (TnI), initiating the regulatory process. We have used the frequency domain method of fluorescence resonance energy transfer (FRET) to measure distances and distance distributions between specific sites in the TnC-TnI complex in the presence and absence of Ca(2+) or Mg(2+). Using sequences based on rabbit skeletal muscle proteins, we prepared functional, binary complexes of wild-type TnC and a TnI mutant which contains no Cys residues and a single Trp residue at position 106 within the TnI inhibitory region. We used TnI Trp-106 as the FRET donor, and we introduced energy acceptor groups into TnC by labeling at Met-25 with dansyl aziridine or at Cys-98 with N-(iodoacetyl)-N'-(1-sulfo-5-naphthyl)ethylenediamine. Our distance distribution measurements indicate that the TnC-TnI complex is relatively rigid in the absence of Ca(2+), but becomes much more flexible when Ca(2+) binds to regulatory sites in TnC. This increased flexibility may be propagated to the whole thin filament, helping to release the inhibition of actomyosin ATPase activity and allowing the muscle to contract. This is the first report of distance distributions between TnC and TnI in their binary complex.

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.

Interaction of a troponin I inhibitory peptide with both domains of troponin C

Skeletal muscle contraction is regulated by Ca2+ binding to troponin (Tn), a complex of three proteins attached to the actin-tropomyosin filaments. We have been investigating key interactions of the Ca(2+)-binding protein TnC and the inhibitory protein TnI. Previously, we used 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) to produce zero-length cross-links in the complex of rabbit skeletal muscle TnC and TnI, and found that the N-terminal, regulatory domain of TnC formed cross-links to the inhibitory region of TnI (Leszyk, J., Grabarek, Z., Gergely, J. and Collins, J.H. (1990) Biochemistry 29, 299-304). In the present study we have used EDC to form cross-links between TnC and a synthetic peptide, based on residues 104-115 of TnI, which mimics intact TnI in its ability to inhibit actomyosin ATPase activity. Prior to cross-linking, we acetylated the epsilon-amino groups of the nine lysine residues of TnC in order to prevent intramolecular cross-linking. Cross-linked TnC-peptide products were cleaved with CNBr and several proteinases. The resulting cross-linked peptides were purified by HPLC and characterized by amino-acid sequence analysis. Our results indicate that the TnI peptide interacted most strongly with two sites in TnC: Glu-60 and/or Glu-61 in the N-terminal domain, and acidic residue(s) in segment 84-94 of the linker region which connects the N- and C-terminal domains of TnC.

Calcium-induced troponin flexibility revealed by distance distribution measurements between engineered sites

The contraction of vertebrate striated muscle is regulated by Ca2+ binding to troponin C (TnC). This causes conformational changes which alter the interaction of TnC with the inhibitory protein TnI and the tropomyosin-binding protein TnT. We have used the frequency domain method of fluorescence resonance energy transfer to measure TnT-TnC and TnT-TnI distances and distance distributions, in the presence of Ca2+, Mg2+, or EGTA, in TnC.TnI.TnT complexes. We reconstituted functional, ternary troponin complexes using the following recombinant subunits whose sequences were based on those of rabbit skeletal muscle: wild-type TnC; TnT25, a mutant C-terminal 25-kDa fragment of TnT containing a single Trp212 which was used as the sole donor for fluorescence energy transfer measurements; Trp-less TnI mutants which contained either no Cys or a single Cys at position 9, 96, or 117. Energy acceptor groups were introduced into TnC or TnI by labeling with dansyl aziridine or N-(iodoacetyl)-N'-(1-sulfo-5-naphthyl)ethylenediamine. Our results indicate that the troponin complex is relatively rigid in relaxed muscle, but becomes much more flexible when Ca2+ binds to regulatory sites in TnC. This increased flexibility may be propagated to the whole thin filament, releasing the inhibition of actomyosin ATPase activity and allowing the muscle to contract. This is the first report of distance distribution measurements between troponin subunits.

Determination of the Ca2+ and Mg2+ affinity constants of troponin C from eel skeletal muscle and positioning of the single tryptophan in the primary structure

The complete amino acid sequence of troponin C (ETnC) from the white muscle of the European eel has been determined by Edman degradation procedures. Its single tryptophan residue is situated in helix H at amino acid position 152 of the aligned sequence; the tryptophan is the first residue on the C-terminal side of Ca2+ binding loop IV. The increase of tryptophan fluorescence emission intensity occurring upon titration of ETnC with Ca2+ has been used to determine the affinity constants of ETnC for Ca2+. The calculated affinity of ETnC for Ca2+ results in a K(Ca) of 1.3 10(7) M-1, typical of the Ca(2+)-Mg2+ sites of the second domain of fast skeletal muscle TnCs. Moreover, a direct competition between Ca2+ and Mg2+ was also observed. The calculated affinity of ETnC for Mg2+ is K(Mg) = 1.2 10(3) M-1. In order to probe the affinity constants of the Ca2+ binding sites of the regulatory domain, ETnC was labelled with dansylaziridine (Danz). The Danz fluorescent signal was used to estimate the affinity constants of ETnC-Danz for Ca2+ and also for Mg2+ (assuming a competitive behaviour between these two metal ions). The calculated affinity constants are K(Ca) = 9.4 10(5) M-1 and K(Mg) = 2.0 10(2) M-1, respectively. These values are typical of the Ca(2+)-specific sites of the regulatory domain of fast skeletal muscle TnCs.

Evolution of EF-hand calcium-modulated proteins. III. Exon sequences confirm most dendrograms based on protein sequences: calmodulin dendrograms show significant lack of parallelism

In the first report in this series we presented dendrograms based on 152 individual proteins of the EF-hand family. In the second we used sequences from 228 proteins, containing 835 domains, and showed that eight of the 29 subfamilies are congruent and that the EF-hand domains of the remaining 21 subfamilies have diverse evolutionary histories. In this study we have computed dendrograms within and among the EF-hand subfamilies using the encoding DNA sequences. In most instances the dendrograms based on protein and on DNA sequences are very similar. Significant differences between protein and DNA trees for calmodulin remain unexplained. In our fourth report we evaluate the sequences and the distribution of introns within the EF-hand family and conclude that exon shuffling did not play a significant role in its evolution.

Evolution of EF-hand calcium-modulated proteins. I. Relationships based on amino acid sequences

The relationships among 153 EF-hand (calcium-modulated) proteins of known amino acid sequence were determined using the method of maximum parsimony. These proteins can be ordered into 12 distinct subfamilies--calmodulin, troponin C, essential light chain of myosin, regulatory light chain, sarcoplasmic calcium binding protein, calpain, aequorin, Stronglyocentrotus purpuratus ectodermal protein, calbindin 28 kd, parvalbumin, alpha-actinin, and S100/intestinal calcium-binding protein. Eight individual proteins--calcineurin B from Bos, troponin C from Astacus, calcium vector protein from Branchiostoma, caltractin from Chlamydomonas, cell-division-cycle 31 gene product from Saccharomyces, 10-kd calcium-binding protein from Tetrahymena, LPS1 eight-domain protein from Lytechinus, and calcium-binding protein from Streptomyces--are tentatively identified as unique; that is, each may be the sole representative of another subfamily. We present dendrograms showing the relationships among the subfamilies and uniques as well as dendrograms showing relationships within each subfamily. The EF-hand proteins have been characterized from a broad range of organismal sources, and they have an enormous range of function. This is reflected in the complexity of the dendrograms. At this time we urge caution in assigning a simple scheme of gene duplications to account for the evolution of the 600 EF-hand domains of known sequence.