Calcium binding by troponin-C. A proton magnetic resonance study

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.

Structural details of a calcium-induced molecular switch: X-ray crystallographic analysis of the calcium-saturated N-terminal domain of troponin C at 1.75 A resolution

We have solved and refined the crystal and molecular structures of the calcium-saturated N-terminal domain of troponin C (TnC) to 1.75 A resolution. This has allowed for the first detailed analysis of the calcium binding sites of this molecular switch in the calcium-loaded state. The results provide support for the proposed binding order and qualitatively, for the affinity of calcium in the two regulatory calcium binding sites. Based on a comparison with the high-resolution apo-form of TnC we propose a possible mechanism for the calcium-mediated exposure of a large hydrophobic surface that is central to the initiation of muscle contraction within the cell.

Solution secondary structure of calcium-saturated troponin C monomer determined by multidimensional heteronuclear NMR spectroscopy

The solution secondary structure of calcium-saturated skeletal troponin C (TnC) in the presence of 15% (v/v) trifluoroethanol (TFE), which has been shown to exist predominantly as a monomer (Slupsky CM, Kay CM, Reinach FC, Smillie LB, Sykes BD, 1995, Biochemistry 34, forthcoming), has been investigated using multidimensional heteronuclear nuclear magnetic resonance spectroscopy. The 1H, 15N, and 13C NMR chemical shift values for TnC in the presence of TFE are very similar to values obtained for calcium-saturated NTnC (residues 1-90 of skeletal TnC), calmodulin, and synthetic peptide homodimers. Moreover, the secondary structure elements of TnC are virtually identical to those obtained for calcium-saturated NTnC, calmodulin, and the synthetic peptide homodimers, suggesting that 15% (v/v) TFE minimally perturbs the secondary and tertiary structure of this stably folded protein. Comparison of the solution structure of calcium-saturated TnC with the X-ray crystal structure of half-saturated TnC reveals differences in the phi/psi angles of residue Glu 41 and in the linker between the two domains. Glu 41 has irregular phi/psi angles in the crystal structure, producing a kink in the B helix, whereas in calcium-saturated TnC, Glu 41 has helical phi/psi angles, resulting in a straight B helix. The linker between the N and C domains of calcium-saturated TnC is flexible in the solution structure.

Quantification of the calcium-induced secondary structural changes in the regulatory domain of troponin-C

The backbone resonance assignments have been completed for the apo (1H and 15N) and calcium-loaded (1H, 15N, and 13C) regulatory N-domain of chicken skeletal troponin-C (1-90), using multidimensional homonuclear and heteronuclear NMR spectroscopy. The chemical-shift information, along with detailed NOE analysis and 3JHNH alpha coupling constants, permitted the determination and quantification of the Ca(2+)-induced secondary structural change in the N-domain of TnC. For both structures, 5 helices and 2 short beta-strands were found, as was observed in the apo N-domain of the crystal structure of whole TnC (Herzberg O, James MNG, 1988, J Mol Biol 203:761-779). The NMR solution structure of the apo form is indistinguishable from the crystal structure, whereas some structural differences are evident when comparing the 2Ca2+ state solution structure with the apo one. The major conformational change observed is the straightening of helix-B upon Ca2+ binding. The possible importance and role of this conformational change is explored. Previous CD studies on the regulatory domain of TnC showed a significant Ca(2+)-induced increase in negative ellipticity, suggesting a significant increase in helical content upon Ca2+ binding. The present study shows that there is virtually no change in alpha-helical content associated with the transition from apo to the 2Ca2+ state of the N-domain of TnC. Therefore, the Ca(2+)-induced increase in ellipticity observed by CD does not relate to a change in helical content, but more likely to changes in spatial orientation of helices.

Model for the interaction of amphiphilic helices with troponin C and calmodulin

Crystals of troponin C are stabilized by an intermolecular interaction that involves the packing of helix A from the N-terminal domain of one molecule onto the exposed hydrophobic cleft of the C-terminal domain of a symmetry related molecule. Analysis of this molecular recognition interaction in troponin C suggests a possible mode for the binding of amphiphilic helical molecules to troponin C and to calmodulin. From the template provided by this troponin C packing, it has been possible to build a model of the contact region of mastoporan as it might be bound to the two Ca2+ binding proteins. A possible binding mode of melittin to calmodulin is also proposed. Although some of the characteristics of binding are similar for the two amphiphilic peptides, the increased length of melittin requires a significant bend in the calmodulin central helix similar to that suggested recently for the myosin light chain kinase calmodulin binding peptide (Persechini and Kretsinger: Journal of Cardiovascular Pharmacology 12:501-512, 1988). Not only are the hydrophobic interactions important in this model, but there are several favorable electrostatic interactions that are predicted as a result of the molecular modeling. The regions of troponin-C and calmodulin to which amphiphilic helices bind are similar to the regions to which the neuroleptic drugs such as trifluoperazine have been predicted to bind (Strynadka and James: Proteins 3:1-17, 1988).

Structural and biological studies on synthetic peptide analogues of a low-affinity calcium-binding site of skeletal troponin C

We have synthesized four oligopeptides that are structural analogues of a low-affinity Ca2+-specific binding site (site II) of rabbit skeletal troponin C. One analogue (peptide 3) was a dodecapeptide with a sequence corresponding to the 12-residue Ca2+-binding loop (residues 63-74 in troponin C), two (peptides 4 and 5) were 23-residue in length, corresponding to residues 52-74 of the protein, and the fourth (peptide 6) was a 25-residue peptide corresponding to residues 50-74. All four peptides had one amino acid substitution within the 12-residue binding loop in which phenylalanine at position 10 was replaced by tyrosine to provide a marker for spectroscopic studies. In addition, peptides 3 and 4 each had a second substitution within the binding loop where glycine at position 6 was replaced by alanine. The second substitution was motivated by the conservation of glycine at the position in the Ca2+-binding loops of all four Ca2+-binding sites in troponin C. The peptides were characterized by their intrinsic fluorescence, ability to enhance the emission of bound Tb3+, affinity for Ca2+ and Tb3+, and circular dichroism. The affinity for Ca2+ was in the range 10-10(2) M-1, and the affinity for Tb3+ was in the range 10(4)-10(5) M-1. The binding constants of the longer peptides were several-fold larger than that of the dodecapeptide. With peptides 4 and 5, substitution of glycine by alanine at position 6 within the 12-residue loop decreased the affinity for Ca2+ by a factor of four, but had little effect on the affinity for Tb3+. However, the mean residue ellipticity of peptide 4 was substantially higher than that of peptide 5. Since peptide 4 differs from peptide 5 only in the substitution of glycine at position 6 in the loop segment, the conservation of glycine at that position may serve a role in providing a suitable secondary structure of the binding sites for interaction with troponin I. Peptides 4 and 6, when present in a large excess, mimic troponin C in regulating fully reconstituted actomyosin ATPase by showing partial calcium sensitivity and activation of the ATPase. Since these peptides are the smallest peptides containing the Ca2+-binding loop of site II, their biological activity suggests that a Ca2+-dependent binding site of troponin C for troponin I could be as short as the segment comprising residues 52-62.

Energetics of the binding of calcium and troponin I to troponin C from rabbit skeletal muscle

We determined the free energy of interaction between rabbit skeletal troponin I (TNI) and troponin C (TNC) at 10 degrees and 20 degrees C with fluorescently labeled proteins. The sulfhydryl probe 5-iodoacetamidoeosin (IAE) was attached to cysteine (Cys)-98 of TNC and to Cys-133 of TNI, and each of the labeled proteins was titrated with the other unlabeled protein. The association constant for formation of the complex between labeled TNC (TNC*) and TNI was 6.67 X 10(5) M-1 in 0.3 M KCl, and pH 7.5 at 20 degrees C. In the presence of bound Mg2+, the binding constant increased to 4.58 X 10(7) M-1 and in the presence of excess of Ca2+, the association constant was 5.58 X 10(9) M-1. Very similar association constants were obtained when labeled TNI was titrated with unlabeled TNC. The energetics of Ca2+ binding to TNC* and the complex TNI X TNC* were also determined at 20 degrees C. The two sets of results were used to separately determine the coupling free energy for binding TNI and Mg2+, or Ca2+ to TNC. The results yielded a total coupling free energy of -5.4 kcal. This free energy appeared evenly partitioned into the two species: TNI X TNC(Mg)2 or TNI X TNC(Ca)2, and TNI X TNC(Ca)4. The first two species were each stabilized by -2.6 kcal, with respect to the Ca2+ free TNI X TNC complex, and TNI X TNC(Ca)4 was stabilized by -2.8 kcal, respect to TNI X TNC(Ca)2 or TNI X TNC(Mg)2. The coupling free energy was shown to produce cooperatively complexes formed between TNI and TNC in which the high affinity sites were initially saturated as a function of free Ca2+ to yield TNI X TNC(Ca)4. This saturation occurred in the free Ca2+ concentration range 10(-7) to 10(-5) M. The cooperative strengthening of the linkage between TNI and TNC induced by Ca2+ binding to the Ca2+-specific sites of TNC may have a direct relationship to activation of actomyosin ATPase. The nature of the forces involved in the Ca2+-induced strengthening of the complex is discussed.

Solution conformation of the C-terminal domain of skeletal troponin C. Cation, trifluoperazine and troponin I binding effects

Proton magnetic resonance spectroscopy has been used to study the cation (Mg2+, Ca2+)-dependent conformational states of the C-terminal domain of rabbit skeletal troponin C under a variety of solution conditions. Nuclear Overhauser data and paramagnetic probe observations provide definition of the configuration of this region of troponin C. Comparative study of homologous proteins identify common features of the tertiary structure relevant to the cation binding reaction. Complex formation with troponin I and the drug trifluoperazine is observed to adjust the solution conformation of the C-terminal domain of troponin C. The interactive conformational response to cation coordination and the binding of the drug and troponin I are discussed.

Molecular structure of troponin C from chicken skeletal muscle at 3-angstrom resolution

The x-ray structure of chicken skeletal muscle troponin C (TnC), the Ca2+-binding subunit of the troponin complex, shows that the protein is about 70 angstroms long with an unusual dumbbell shape. The carboxyl and amino domains are separated by a single long alpha helix of about nine turns. Only the two high-affinity Ca2+-Mg2+ sites of the COOH-domain are occupied by metal ions resulting in conformational differences between the COOH- and NH2-domains. These differences are probably important in the triggering of muscle contraction by TnC. Also the structure of TnC is relevant in understanding the function of other calcium-regulated proteins, in particular that of calmodulin because of its strong similarity in amino acid sequence.

Structure of the calcium regulatory muscle protein troponin-C at 2.8 A resolution

Crystals of turkey skeletal muscle troponin-C reveal a molecule of two domains with an unusual structure. Two Ca2+ ions are bound to the C-terminal domain. The two cation-binding sites of the regulatory (N-terminal) domain are Ca2+ free; this domain adopts a markedly different conformation from the C-terminal domain. The two domains are connected by a long nine-turn alpha-helix; three of these turns are exposed fully to solvent.

Thermodynamic study of domain organization in troponin C and calmodulin

Intramolecular melting of troponin C, calmodulin and their proteolytic fragments has been studied microcalorimetrically at various concentrations of monovalent and divalent ions. It is shown by thermodynamic analysis of the experimentally determined excess heat capacity function that the four calcium-binding domains in these two related proteins are not integrated into a single co-operative system, as would be the case if they formed a common hydrophobic core in the molecule, but still interact with each other in a very specific way. There is a positive interaction between domains I and II, which is so strong that they actually form a single co-operative block. The interaction between domains III and IV is positive also, although much less pronounced, while the interaction between the pairs of domains (I and II) and (III and IV) is negative, as if they repel each other. The structure of the co-operative block of domains I and II at room temperature does not depend noticeably on the ionic conditions, which influence its stability to a small extent only. The same applies to domain IV of calmodulin, but in troponin C this domain is unstable in the absence of divalent ions, in solutions of low ionic strength. In both proteins, the least stable is domain III, which forms a compact ordered structure at room temperature only in the presence of Ca2+. In troponin C, calcium ions can be replaced by magnesium ions, although the compact structure of domain III formed by these two ions does not seem to be quite identical. Thus, at conditions close to physiological, with regard to temperature and ionic strength, the removal of free Ca2+ from the solution induces in both proteins a reversible transition of domain III to the non-compact disordered state. This dramatic Ca2+-induced change in the domain III conformation in troponin C and calmodulin might play a key role in the functioning of these proteins as a Ca2+-controlled switch in the molecular mechanisms of living systems.

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

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

Influence of temperature and denaturing agents on the structural stability of calmodulin. A 1H-nuclear magnetic resonance study

The structural stability of calmodulin was studied by 1H-nuclear magnetic resonance spectroscopy under different denaturing conditions. The presence of Ca2+ stabilizes the structural properties of the native protein. In the absence of calcium the structural integrity of calmodulin can easily be affected by elevated temperatures or by high concentrations of denaturing agents. The unfolding process under various denaturing conditions is reversible underlining the high degree of structural flexibility of this protein.

Primary sequence analysis and folding behavior of EF hands in relation to the mechanism of action of troponin C and calmodulin

The primary sequence of EF hands encodes for elements of secondary structure which includes the presence of hydrophobic and charged domains in the helical regions of these sites. The hydrophobic and charged surfaces located in the N-terminal region of EF hands offer a potential site of interaction with complimentary surfaces on target proteins. Although the binding of calcium to the EF hands of calmodulin and troponin C may lead to a local exposure of these domains, it is the tertiary structure of these proteins that probably dictates the extent to which these domains are exposed and the selectively of these proteins for target proteins.

Calcium and cadmium binding to troponin C. Evidence for cooperativity

Proton NMR is used to compare the structural changes induced in bovine cardiac troponin C on binding of cadmium and calcium ions. The same spectral changes are observed for both ion species. The rate of the conformational changes associated with cadmium binding to the two high-affinity sites is slow, that associated with cadmium ions binding to the low-affinity site is high. 113Cd-NMR spectra of cardiac troponin C feature two signals interpreted as due to cadmium ions bound to the strong sites. Strong arguments are given in favour of cooperativity in binding of the first two cadmium or calcium ions to cardiac and skeletal muscle troponin C.

Nuclear magnetic resonance studies on calmodulin: spectral assignments in the calcium-free state

The 400-MHz proton magnetic resonance spectra of calcium-free scallop testis calmodulin (CaM) and pig brain CaM were observed. Detailed spectral assignments were made by resolution enhancement, spin decoupling, and nuclear Overhauser enhancement (NOE) experiments as well as pH titration. Comparison between spectra of scallop testis CaM and pig brain CaM were also utilized for the assignment. Previous assignments for tyrosine-99, histidine-107, epsilon-trimethyllysine-115, and tyrosine-138 [Seamon, K. B. (1980) Biochemistry 19, 207; Krebs, J., & Carafoli, E. (1982) Eur. J. Biochem. 124, 619] were confirmed. Phenylalanine-99 and threonine-143 of scallop testis CaM were identified. Sixteen methyl resonances from one isoleucine, two valines, nine methionines, and the amino-terminal acetyl group were identified. First-stage assignments were made of resonances arising from seven phenylalanines. The uniquely high field shifted phenylalanine resonance previously reported by Seamon was found to consist of two doublets from the two pairs of delta protons of two phenylalanines. The NOE experiments showed that the two phenylalanines are located closely to each other. The large high-field shifts of these phenylalanines were accounted for the ring-current effects due to their proximity. An isoleucine and a valine of which methyl resonances appear at high fields were found to be situated closely to each other. It was found that two delta protons and two epsilon protons of almost all aromatic residues are magnetically equivalent, suggesting that the local structure of aromatic residues is so flexible as to permit the rapid flipping motion of the ring.

Nuclear magnetic resonance studies on calmodulin: calcium-induced conformational change

The 400-MHz 1H nuclear magnetic resonance (NMR) studies were carried out on the Ca2+-induced conformational change of calmodulins (CaM's) isolated from scallop testis and pig brain. The resonances were found to be classified approximately into three groups. The resonances of group I, which are perturbed by the binding of Ca2+ to the high-affinity sites, include those of tyrosine-138, epsilon-trimethyllysine-115, histidine-107, tyrosine-99, etc. The previous assignments for tyrosine- (Tyr) 138 [Seamon, K. B. (1980) Biochemistry 19, 207] were corrected. The resonances of group II, which are affected by the binding of Ca2+ to the low-affinity sites, include those of a phenylalanine (Phe), a high field shifted methyl, and a low field shifted alpha-methine. Group III (related to the binding of Ca2+ to both the high-and low-affinity sites) includes the resonances of a Phe, a high field shifted methyl, and threonine-143. It is concluded that sites III and IV are the high-affinity sites. The off-rate of Ca2+ from the high-affinity sites is slower than 50 s-1 while the off-rate from the low-affinity sites is faster than 600 s-1. In the Ca2+-free state, there exists a hydrophobic region containing three phenylalanine (probably Phe-89, Phe-92, and Phe-141), a valine, and an isoleucine in the vicinity of sites III and IV. Tyr-138 is distant from these amino acids. Upon binding of Ca2+ to the high-affinity sites, one of the Phe residues and the valine approach Tyr-138. Similar structural changes were observed between CaM and troponin C when Ca2+ ions are bound to the high-affinity sites. CaM changes in a somewhat different way from troponin C when Ca2+ ions are bound to the low-affinity sites.

Lanthanide-induced peptide folding: variations in lanthanide affinity and induced peptide conformation

The present work has demonstrated the utility of the diamagnetic lanthanides lutetium and lanthanum as metal binding probes for a synthetic 13-residue fragment representing calcium binding site 3 of rabbit skeletal troponin C (residues 103-115). The peptide conformation induced by these metals was monitored by the proton magnetic resonance at 270 MHz. The peptide affinity for these rare earths is 50-400 times higher than that for calcium (KLu3+, 1.3 X 10(4) M-1; KLa3+, 1.1 X 10(5) M-1; KCa2+, 3 X 10(2) M-1) which is related to the change in cation charge from 2+ to 3+. The peptide conformation induced by the presence of La3+ generates a different 1H NMR spectrum than the one observed for the lutetium-saturated peptide. Thus, it appears that these metals do not fold the peptide into exactly the same conformation. The resonance shifts observed during the Lu3+ titration are much smaller than those seen in the case of La3+ addition. The fact that lutetium binds less tightly than lanthanum to the peptide may be linked directly or indirectly to the difference in ionic radius between these metals (Lu3+, 0.86 A; La3+, 1.03 A). This may in turn indicate that the peptide primary sequence encodes for some aspects of metal ion specificity. The 1H NMR results also demonstrate that glycine-108 adopts a restricted geometry in the absence of metal such that its two alpha-carbon protons are in different environments which are further affected by the addition of either metal. These observations support the concept that geometric constraints arising from the particular peptide folding pattern near this residue correlate with the highly conserved nature of this site of the EF hand. This position remains occupied by glycine in most EF hand domains with the exception of known distorted calcium binding sites present in intestine calcium binding proteins and S-100.

Spectral studies on the calcium binding properties of bovine brain S-100b protein

The effect of Ca2+ binding on the circular dichroism (CD) and 270-MHz proton nuclear magnetic resonance (NMR) spectra of brain-specific S-100b calcium binding protein has been examined at two pH values, 8.5 and 7.5. At pH 8.5, S-100b protein binds two Ca2+ per monomer with Kd values of 6 x 10(-5) and 2 x 10(-4) M, whereas at pH 7.5, the protein binds only one Ca2+ per monomer with a Kd of 2 x 10(-4) M. The presence of K+ inhibits the binding of Ca2+ to the higher affinity site at pH 8.5, and the affinity for calcium is lowered to Kd = 8.5 x 10(-4) M. Mg2+ has no effect on protein conformation. In the absence of Ca2+, S-100b undergoes a conformational change when the protein is titrated from pH 8.6 to 6.0. Addition of Ca2+ perturbed the environment of tyrosine and phenylalanine residues as measured by ultraviolet difference spectroscopy and 1H NMR. CD melt experiments and far-ultraviolet CD studies at alkaline pH and NMR experiments suggest that the protein is more stable in the presence of Ca2+. The single tyrosine residue in the protein ionizes only after the protein is denatured by exposure to high pH.

Ion binding to calmodulin. A comparison with other intracellular calcium-binding proteins

Over the past few years calcium has emerged as an important bioregulator. Upon external stimulation, the cell generates a transient Ca2+ increase, which is transformed into a cellular event through a molecular cascade. The first step in this cascade is the binding of calcium to proteins present in the cytosol. These proteins capable of binding Ca2+ under physiological conditions all belong to the same evolutionary family that evolved from a common ancestor. However, they strongly differ in the properties of their calcium binding sites. Calmodulin, the ubiquitous calcium binding protein present in all eukaryotic cells, is very close to the ancestor protein, presents four calcium binding sites which bind calcium, magnesium and monovalent ions competitively and is involved in the triggering of cellular processes. Parvalbumin, another member of the family, is more specialized and found mostly in fast-twitch skeletal muscle. It binds calcium and magnesium with high affinity and seems to be involved in muscle relaxation. On the other hand, troponin C which confers Ca2+ sensitivity to acto-myosin interaction exhibits both triggering and relaxing sites. The study of intracellular Ca2+ binding proteins has shown that calcium binding proteins have evolved from a simple common structure to fulfill different functions.

Calcium binding proteins and cellular regulation

An important feature of cellular regulation is the precise control of intracellular calcium levels. This is accomplished both by dynamic organelle release and sequestration of calcium and by specific calcium active transport mechanisms located in the plasma membrane. The actual calcium signal for mediation of a cellular response is carried out by specific intracellular proteins, the most widely studied examples are calmodulin and troponin C. The recent discovery of phospholipid protein kinase and calcimedins suggests receptor mediation via several independent proteins. The physiological importance of a particular protein as a calcium messenger rests on several features: 1) calcium binding is of the order of 1-10 microns, 2) the protein is known to be localized at the site of proposed action, 3) if translocation occurs upon activation, the time required is consistent with the time course of the physiologic response and 4) substrates or effectors at the next level of action when isolated can be demonstrated to have similar activation kinetics as in situ.

Proton magnetic resonance studies on peptide fragments of troponin-C containing single calcium-binding sites

Proton magnetic resonance spectroscopy has been employed to study the solution conformation of three cleavage fragments of troponin-C, each containing a single Ca(II)-binding site and corresponding to different regions in the primary sequence; viz. CB8 (residues 46-77), CB9 (residues 85-134) and TH2 (residues 121-159). Although all three peptides lack a well-defined tertiary fold in the absence of metal ions, several spectral features indicate the presence of local conformational constraints in each apo-peptide. Ca(II) binding led to spectral changes consistent with increased restriction of backbone motility and the adoption of a more compact conformation. Studies using paramagnetic ions as conformational probes support current views concerning the nature of the ligands at the metal binding sites. The nature and kinetics of the structural influence of metal binding suggest that the conformational constraints existing in the CB8 apo-peptide provide an adequate Ca(II)-binding configuration. In contrast, the CB9 and TH2 peptides exhibit spectral changes consistent with an increased local structure in the region of helix E (residues 94-102) in the case of CB9 and helix H (residues 148-159) in the case of TH2. In CB9, conformation changes also appear to be transmitted to a portion of the sequence (residues 87-93) preceding helix E, a putative site of interaction between troponin-C and troponin-I. These data are discussed with reference to the contribution of long-range (interdomain) interactions within troponin-C and the modulation of troponin subunit protein-protein interactions by Ca(II) binding.

Influence of Ca2+ and trifluoperazine on the structure of calmodulin. A 1H-nuclear magnetic resonance study

Ca2+-induced conformational changes of calmodulin under a variety of different experimental conditions have been studied by 1H-nuclear magnetic resonance techniques. The assignment for Tyr-99 has been corrected. Ca2+ titration performed at pH 7.5 and greater than 9.5 apparently induces a different sequence of the protein folding process as can be monitored by the resonances of His-107. These two structural forms cannot be interconverted. The phenylalanine residue(s) responsible for the resonances at 6.47 ppm (Ca2+-free form) and 6.64 ppm (Ca2+-saturated form) respectively, are apparently located close to Ca2+-binding sites III and IV. This can be recognized from nuclear Overhauser enhancement and Gd3+-broadening techniques. Gd3+-broadening experiments classify Ca2+-binding site IV as the site with the highest Gd3+/ca2+-exchange rate. The antipsychotic drug trifluoperazine, which is known to bind to calmodulin in a calcium-dependent way [Levin, R. M. and Weiss, B. (1977) Mol. Pharmacol. 13, 690-697], has been found to induce a conformational change of the Ca2+-saturated form of calmodulin. The methionine and phenylalanine residues were especially affected. Possible binding site(s) for trifluoperazine are discussed.

Digestion of troponin C with trypsin in the presence and absence of Ca2+. Identification of cleavage points

The rate of tryptic digestion of troponin C has been shown to be dependent on Ca2+ (Drabikowski et al., Biochim. Biophys. Acta 490, 216-224). We have characterized the tryptic peptides produced both in the presence and absence of Ca2+ using amino acid composition and end-group analyses. In the presence of Ca2+ trypsin cleaves TnC at Arg-8, Lys-84 and Lys-88, leading to the formation of two large peptides, one containing the two low-affinity sites (TR1C), the other, the two high-affinity Ca2+-binding sites (TR2C). In the absence of Ca2+ (1 mM EDTA), digestion proceeds much more rapidly and takes place first at Arg-100, followed by Arg-104, Arg-120, Lys-153, Arg-8 and others. The data suggest that the points of cleavage are determined by the Ca2+-dependent conformational states of TnC, particularly in the C-terminal half of the protein where the cation is known to induce secondary structure.

Laser photochemically induced dynamic nuclear polarization proton nuclear magnetic resonance studies on three homologous calcium binding proteins: cardiac troponin-C, skeletal troponin-C, and calmodulin

Laser photo-CIDNP 1H NMR experiments were performed with rabbit skeletal troponin-C (sTn-C), bovine cardiac troponin-C (cTn-C), and bovine brain calmodulin to study the exposure of histidine and tyrosine residues. In cTn-C, tyrosine residues, 5, 111, and 150 were exposed in the apoprotein, becoming buried as Ca2+ was bound. A similar phenomenon was observed for tyrosine residues 10 and 109 of sTn-C. In calmodulin, only tyrosine-99 was accessible in the apoprotein. The lack of exposure of tyrosine-138 observed with this technique correlates with the buried nature of this residue implied by other criteria. In 6 M urea each of the apoproteins were observed to be unfolded from the standpoint of the tyrosine environments. A large tyrosyl CIDNP effect was obtained for each protein which decreased as Ca2+ was bound, with a stoichiometry of one metal ion per protein. This was correlated for cTn-C with the appearance of "native" resonances representing tyrosine residues 111 and 150 in Ca2+-saturated cTn-C, also with a stoichiometry of one. Analysis of our NMR findings, in the light of other spectroscopic and model building studies on these systems, suggests that the sole high-affinity Ca2+ binding site of cTn-C and sTn-C remaining in 6 M urea is site IV.

Hydrogen-1 nuclear magnetic resonance investigation on bovine cardiac troponin C. Comparison of tyrosyl assignments and calcium-induced structural changes to those of two homologous proteins, rabbit skeletal troponin C and bovine brain calmodulin

The effect of Ca2+ binding on the 270-MHz proton nuclear magnetic resonance spectrum of bovine cardiac troponin C (cTnC) has been examined. Assignment of resonances in the aromatic spectral region to tyrosine residues 10, 111, and 150 has been made for apo-cTnC and calcium-bound cTnC on the basis of decoupling experiments, pH titrations, temperature-induced changes, and gadolinium broadening experiments. The sequence homology which these tyrosine residues display with residues in two previously studied proteins, rabbit skeletal troponin C (sTnC) [Seamon, K. B., Hartshorne, D. J., & Bothner-By, A. A. (1977) Biochemistry 16, 4039] and bovine brain calmodulin [Seamon, K. B. (1980) Biochemistry 19, 207], was also used in assignments. High-affinity calcium binding (up to 2 mol/cTnC) causes large alterations in the environments of tyrosines-10 and -150, indicating that the N terminus is probably buried in the protein interior. The evidence suggests that the environment of tyrosine-150 in calcium-saturated cTnC must closely resemble that of tyrosine-138 in calmodulin in that it experiences the hydrophobic core of the protein. However, there is no similarity between these environments in the apoproteins. Dramatic alterations in phenylalanine resonances are seen during the binding of the third mole of calcium, corresponding to filling the sole low affinity site. Comparison of the spectral calmodulin reveals many structural similarities which stem from their high degree of primary sequence homology.

Competitive membrane adsorption of Na+, K+, and Ca2+ in smooth muscle cells

A theory for Na+, K+ and Ca2+ competitive adsorption to a charged membrane is used to explain a number of experimental observations in smooth muscle. Adsorption is described by Langmuir isotherms for mono- and divalent cations which in turn are coupled in a self-consistent way to the bulk solution through the diffuse double layer theory and the Boltzman equations. We found that the dissociation constants for binding of Na+, K+ and Ca2+ in guinea pig taenia coli are ca. 0.009, 1.0, and 4 X 10(-8) M, respectively. Furthermore, the effect of a Ca2+ pump that maintains free surface Ca2+ concentration constant is investigated. A decrease in intracellular Na+ content results in an increased Ca2+ uptake; part of this uptake is due to an increase in surface-bound Ca2+ in an intracellular compartment which is in contact with the myofilaments. Variations in the amount of charge available to bind Ca2+ and the surface charge density are studied and their effect interpreted in terms of different pharmacological agents.

Stability of troponin C

The stability of the structure of troponin C (calcium-binding components of troponin from rabbit skeletal muscle) has been studied by the scanning microcalorimetry method. It has been shows that: 1. In the presence of divalent ions the protein structure is represented by two practically independent cooperative blocks, one of which contains Ca2+-specific binding sites, and the other (Ca2+, Mg2+)-binding sites. 2. The stability of the cooperative block containing Ca2+-specific binding sites depends only on the concentration of Ca2+ and in its absence the melting temperature of the block decreases to 58 degrees C at neutral pH and low ionic strength. 3. The stability of the cooperative block containing (Ca2+, Mg2+)-binding sites depends on the concentration of Ca2+ or Mg2+. In their absence the stability of the block is so low that its structure is already disrupted at 25 degrees C. The conformational transition observed by different methods when divalent ions are removed is nothing else than the breaking down of the structure of this cooperative block.

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.

Proton magnetic resonance studies on proteolytic fragments of troponin-C. Structural homology with the native molecule

Comparison of proton magnetic resonance spectra of a tryptic and a thrombin fragment of troponin-C with that of the native protein has identified the domain of the molecule influenced by Ca2+ binding to the lower affinity regions I and II of troponin-C. The binding of Ca2+ to these sites results in a subtle alteration of the tertiary fold of the N-terminal half of troponin-C involving weakened contacts between several hydrophobic groups. The role and kinetics of the movements within the troponin-C molecule associated with binding at the regulatory sites are discussed.

A laser Raman spectroscopic study of Ca2+ binding to troponin C

Laser Raman spectroscopy has been used detect structural changes in troponin C induced by Ca2+ binding. Addition of Ca2+ - Mg2+ sites produces perturbations in the amide III region of the spectrum indicative of increased alpha-helical content, and in regions of the spectrum corresponding to carboxylate, thiol, and phenol side chains. However, Ca2+ binding to the low affinity Ca2+ - specific sites is not detected by laser Raman spectral changes.