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

Long-range effects of familial hypertrophic cardiomyopathy mutations E180G and D175N on the properties of tropomyosin

Cardiac α-tropomyosin (Tm) single-site mutations D175N and E180G cause familial hypertrophic cardiomyopathy (FHC). Previous studies have shown that these mutations increase both Ca(2+) sensitivity and residual contractile activity at low Ca(2+) concentrations, which causes incomplete relaxation during diastole resulting in hypertrophy and sarcomeric disarray. However, the molecular basis for the cause and the difference in the severity of the manifested phenotypes of disease are not known. In this work we have (1) used ATPase studies using reconstituted thin filaments in solution to show that these FHC mutants result in an increase in Ca(2+) sensitivity and an increased residual level of ATPase, (2) shown that both FHC mutants increase the rate of cleavage at R133, ~45 residues N-terminal to the mutations, when free and bound to actin, (3) shown that for Tm-E180G, the increase in the rate of cleavage is greater than that for D175N, and (4) shown that for E180G, cleavage also occurs at a new site 53 residues C-terminal to E180G, in parallel with cleavage at R133. The long-range decreases in dynamic stability due to these two single-site mutations suggest increases in flexibility that may weaken the ability of Tm to inhibit activity at low Ca(2+) concentrations for D175N and to a greater degree for E180G, which may contribute to differences in the severity of FHC.

Ionic interventions that alter the association of troponin C C-domain with the thin filaments of vertebrate striated muscle

The regulatory complex of vertebrate skeletal muscle integrates information about cross-bridge binding, divalent cations and other intracellular ionic conditions to control activation of muscle contraction. Relatively little is known about the role of the troponin C (TnC) C-domain in the absence of Ca2+. Here, we use a standardized condition for measuring isometric tension in rabbit psoas skinned fibers to track TnC attachment and detachment in the absence of Ca2+ under different conditions of ionic strength, pH and MgATP. In the presence of MgATP and Mg2+, TnC detaches more readily and has a 1.5- to 2-fold lower affinity for the intact thin filament at pH 8 and 250 mM K+ than at pH 6 or in 30 mM K+; changes in affinity are fully reversible. The response to ionic strength is lost when Mg2+ and MgATP are absent, whereas the response to pH persists, suggesting that weaker electrostatic TnC-TnI-TnT interactions can be overridden by strongly bound cross-bridges. In solution, titration of a fluorescent C-domain mutant (F154W TnC) with Mg2+ reveals no significant changes in Mg2+ affinity with pH or ionic strength, suggesting that these parameters influence TnC binding by acting directly on electrostatic forces between TnC and TnI rather than by changing Mg2+ binding to C-domain sites III and IV.

Diversity of conformational states and changes within the EF-hand protein superfamily

The EF-hand motif, which assumes a helix-loop-helix structure normally responsible for Ca2+ binding, is found in a large number of functionally diverse Ca2+ binding proteins collectively known as the EF-hand protein superfamily. In many superfamily members, Ca2+ binding induces a conformational change in the EF-hand motif, leading to the activation or inactivation of target proteins. In calmodulin and troponin C, this is described as a change from the closed conformational state in the absence of Ca2+ to the open conformational state in its presence. It is now clear from structures of other EF-hand proteins that this "closed-to-open" conformational transition is not the sole model for EF-hand protein structural response to Ca2+. More complex modes of conformational change are observed in EF-hand proteins that interact with a covalently attached acyl group (e.g., recoverin) and in those that dimerize (e.g., S100B, calpain). In fact, EF-hand proteins display a multitude of unique conformational states, together constituting a conformational continuum. Using a quantitative 3D approach termed vector geometry mapping (VGM), we discuss this tertiary structural diversity of EF-hand proteins and its correlation with target recognition.

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.

Troponin I: inhibitor or facilitator

TN-I occurs as a homologous group of proteins which form part of the regulatory system of vertebrate and invertebrate striated muscle. These proteins are present in vertebrate muscle as isoforms, Mr 21000-24000, that are specific for the muscle type and under individual genetic control. TN-I occupies a central position in the chain of events starting with the binding of calcium to troponin C and ending with activation of the Ca2+ stimulated MgATPase of the actomyosin filament in muscle. The ability of TN-I to inhibit the MgATPase of actomyosin in a manner that is accentuated by tropomyosin is fundamental to its role but the molecular mechanism involved is not yet completely understood. For the actomyosinATPase to be regulated the interaction of TN-I with actin, TN-C and TN-T must undergo changes as the calcium concentration in the muscle cell rises, which result in the loss of its inhibitory activity. A variety of techniques have enabled the sites of interaction to be defined in terms of regions of the polypeptide chain that must be intact to preserve the biological properties of TN-I. There is also evidence for conformational changes that occur when the complex with TN-C binds calcium. Nevertheless a detailed high resolution structure of the troponin complex and its relation to actin/tropomyosin is not yet available. TN-I induces changes in those proteins with which it interacts, that are essential for their function. In the special case of cardiac TN-I its effect on the calcium binding properties of TN-C is modulated by phosphorylation. It has yet to be determined whether TN-I acts directly as an inhibitor or indirectly by interacting with associated proteins to facilitate their role in the regulatory system.

Calcium binding to the regulatory domain of skeletal muscle troponin C induces a highly constrained open conformation

We have used fluorescence resonance energy transfer to investigate the conformation of the apo and calcium-loaded states of the regulatory N-terminal domain of full-length troponin C mutants from skeletal muscle. The mutants studied each contained a single tryptophan residue (position 22 or 90) and a single cysteine residue (position 52 or 101). The intrinsic fluorophore in each mutant served as an energy donor and the cysteine was conjugated to the acceptor probe 5-(iodoacetamidoethyl)amino-naphthalene-1-sulfonic acid. The distributions of two intersite distances (between residues 22 and 52, and residues 90 and 52) were broad in the apo state, indicative of considerable structural dynamics. These distributions were shifted to longer distances and considerably sharpened in the calcium-loaded state. The shifts to longer distances by 8 to 11 A indicate a calcium-induced opening of the N-terminal domain conformation. The transition of the troponin C structure from a closed conformation to an open conformation is accompanied by a substantial reduction of structural fluctuations that dominate in the apo structure as evidenced from the large decrease of the widths of the distributions. This highly constrained open conformation is required as part of the structural basis to facilitate productive interaction between troponin C and troponin I to trigger contraction in skeletal muscle.

Structures of four Ca2+-bound troponin C at 2.0 A resolution: further insights into the Ca2+-switch in the calmodulin superfamily

BACKGROUND

In contrast to Ca2+4-bound calmodulin (CaM), which has evolved to bind to many target sequences and thus regulate the function of a variety of enzymes, troponin C (TnC) is a bistable switch which controls contraction in striated muscles. The specific target of TnC is troponin I (TnI), the inhibitory subunit of the troponin complex on the thin filaments of muscle. To date, only the crystal structure of Ca2+2-bound TnC (i.e. in the 'off' state) had been determined, which together with the structure of Ca2+4-bound CaM formed the basis for the so-called 'HMJ' model of the conformational changes in TnC upon Ca2+ binding. NMR spectroscopic studies of Ca2+4-bound TnC (i.e. in the 'on' state) have recently been carried out, but the detailed conformational changes that take place upon switching from the off to the on state have not yet been described.

RESULTS

We have determined the crystal structures of two forms of expressed rabbit Ca2+4-bound TnC to 2.0 A resolution. The structures show that the conformation of the N-terminal lobe (N lobe) is similar to that predicted by the HMJ model. Our results also reveal, in detail, the residues involved in binding of Ca2+ in the regulatory N lobe of the molecule. We show that the central helix, which links the N and C lobes of TnC, is better stabilized in the Ca2+2-bound than in the Ca2+4-bound state of the molecule. Comparison of the crystal structures of the off and on states of TnC reveals the specific linkages in the molecule that change in the transition from off to on state upon Ca2+-binding. Small sequence differences are also shown to account for large functional differences between CaM and TnC.

CONCLUSIONS

The two lobes of TnC are designed to respond to Ca2+-binding quite differently, although the structures with bound Ca2+ are very similar. A small number of differences in the sequences of these two lobes accounts for the fact that the C lobe is stabilized only in the open (Ca2+-bound) state, whereas the N lobe can switch between two stable states. This difference accounts for the Ca2+-dependent and Ca2+-independent interactions of the N and C lobe. The C lobe of TnC is always linked to TnI, whereas the N lobe can maintain its regulatory role - binding strongly to TnI at critical levels of Ca2+ - and in contrast, forming a stable closed conformation in the absence of Ca2+.

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.

Characteristics of troponin C binding to the myofibrillar thin filament: extraction of troponin C is not random along the length of the thin filament

Troponin C (TnC) is the Ca(2+)-sensing subunit of troponin responsible for initiating the cascade of events resulting in contraction of striated muscle. This protein can be readily extracted from myofibrils with low-ionic-strength EDTA-containing buffers. The properties of TnC extraction have not been characterized at the structural level, nor have the interactions of TnC with the native myofibrillar thin filament been studied. To address these issues, fluorescein-labeled TnC, in conjunction with high-resolution digital fluorescence microscopy, was used to characterize TnC binding to myofibrils and to determine the randomness of TnC extraction. Fluorescein-5-maleimide TnC (F5M TnC) retained biological activity, as evidenced by reconstitution of Ca(2+)-dependent ATPase activity in extracted myofibrils and binding to TnI in a Ca(2+)-sensitive manner. The binding of F5M TnC to highly extracted myofibrils at low Ca2+ was restricted to the overlap region under rigor conditions, and the location of binding was not influenced by F5M TnC concentration. The addition of myosin subfragment 1 to occupy all actin sites resulted in F5M TnC being bound in both the overlap and nonoverlap regions. However, very little F5M TnC was bound to myofibrils under relaxing conditions. These results suggest that strong binding of myosin heads enhances TnC binding. At high Ca2+, the pattern of F5M TnC binding was concentration dependent: binding was restricted to the overlap region at low F5M TnC concentration, whereas the binding propagated into the nonoverlap region at higher levels. Analysis of fluorescence intensity showed the greatest binding of F5M TnC at high Ca2+ with S1, and these conditions were used to characterize partially TnC-extracted myofibrils. Comparison of partially extracted myofibrils showed that low levels of extraction were associated with greater F5M TnC being bound in the nonoverlap region than in the overlap region relative to higher levels of extraction. These results show that TnC extraction is not random along the length of the thin filament, but occurs more readily in the nonoverlap region. This observation, in conjunction with the influence of rigor heads on the pattern of F5M TnC binding, suggests that strong myosin binding to actin stabilizes TnC binding at low Ca2+.

Relationship between stability and function for isolated domains of troponin C

Results of spectroscopic thermal and chemical denaturation studies and calcium binding studies are presented for a series of five recombinant chicken troponin C fragments. They were designed to assess the effects of domain isolation, N-helix, and D/E linker helix on stability and calcium affinity. Four of the fragments include the N-terminal regulatory domain and one the C-terminal domain. For the regulatory domain, deletion of the N-helix or the D/E linker decreases the stability of the apo form as measured by delta GN-->U,25. Separation of the domains also decreases the stability. Differences in values of delta GN-->U,25 derived from urea and guanidine hydrochloride studies allowed an estimation of the electrostatic component of the free energy of unfolding. Our measurements provide the first quantitative estimate of the stability for the apo-C-domain (delta GN-->U,25 = -1.8 kcal/mol) which was obtained using the interaction free energy formalism of Schellman. There is an inverse correlation between calcium affinity, binding cooperativity, and stability for all of these homologously structured fragments. The calcium affinity and cooperativity are highest for the unstructured C-domain and lowest for the N-domain which has the highest stability. In view of the direct effects on the folding stability of the apo-N-domain, the N-helix and the bilobed domain organization of TnC are necessarily involved in the fine-tuning of the affinity and cooperativity of calcium binding. Though not directly involved in calcium coordination, these structural features are important for signal transmission by troponin C in the troponin complex.

Kinetic studies of calcium binding to the regulatory site of troponin C from cardiac muscle

We have studied the kinetics of the structural transitions induced by calcium binding to the single, regulatory site of cardiac troponin C by measuring the rates of calcium-mediated fluorescence changes with a monocysteine mutant of the protein (C35S) specifically labeled at Cys-84 with the fluorescent probe 2(-)[4'-(iodoacetamido)anilino]naphthalene-6-sulfonic acid. At 4 degrees C, the binding kinetics determined in the presence of Mg2+ was resolved into two phases with positive amplitude, which were completed in less than 100 ms. The rate of the fast phase increased linearly with [Ca2+] reaching a maximum of approximately 590 s-1, and that of the slow phase was approximately 100 s-1 and did not depend on Ca2+ concentration. Dissociation of bound Ca2+ from the regulatory site occurred with a rate of 102 s-1, whereas the dissociation from the two high affinity sites was about two orders of magnitude slower. These results are consistent with the following scheme for the binding of Ca2+ to the regulatory site: [formula: see text] where the asterisks denote states with enhanced fluorescence. The apparent second-order rate constant for calcium binding is Kok1 = 1.4 x 10(8) M 1 s-1. The two first-order transitions occur with observed rates of k1 + kappa-1 approximately 590 s-1 and kappa 2 + kappa-2 approximately 100 s-1, and the binding of Ca2+ to the regulatory site is not a simple diffusion-controlled reaction. These transitions provide the first information on the rates of Ca(2+)-induced conformational changes involving helix movements in the regulatory domain.

Concerted action of the high affinity calcium binding sites in skeletal muscle troponin C

Mutants of each of the four divalent cation binding sites of chicken skeletal muscle troponin C (TnC) were constructed using site-directed mutagenesis to convert Asp to Ala at the first coordinating position in each site. With a view to evaluating the importance of site-site interactions both within and between the N- and C-terminal domains, in this study the mutants are examined for their ability to associate with other components of the troponin-tropomyosin regulatory complex and to regulate thin filaments. The functional effects of each mutation in reconstitution assays are largely confined to the domain in which it occurs, where the unmutated site is unable to compensate for the defect. Thus the mutants of sites I and II bind to the regulatory complex but are impaired in ability to regulate tension and actomyosin ATPase activity, whereas the mutants of sites III and IV regulate activity but are unable to remain bound to thin filaments unless Ca2+ is present. When all four sites are intact, free Mg2+ causes a 50-60-fold increase in TnC's affinity for the other components of the regulatory complex, allowing it to attach firmly to thin filaments. Calcium can replace Mg2+ at a concentration ratio of 1:5000, and at this ratio the Ca2.TnC complex is more tightly bound to the filaments than the Mg2.TnC form. In the C-terminal mutants, higher concentrations of Ca2+ (above tension threshold) are required to effect this transformation than in the recombinant wild-type protein, suggesting that the mutants reveal an attachment mediated by Ca2+ in the N-domain sites.

Interaction of troponin I and troponin C: use of the two-dimensional transferred nuclear Overhauser effect to determine the structure of a Gly-110 inhibitory troponin I peptide analog when bound to cardiac troponin C

The structure of a peptide analog of the inhibitory region of cardiac troponin-I (N-acetyl-G110-TnI(104-115) amide) when bound to cardiac troponin-C has been determined by 2-dimensional 1H-NMR techniques. The bound structure determined for this peptide is similar to that determined previously for the skeletal peptide (which has a proline at position 110) bound to skeletal troponin-C (Campbell and Sykes (1991) J. Mol. Biol. 222, 405-421). This structure shows a helical like peptide backbone 'bent' around P109-G110 to bring the hydrophobic residues F106, L111 and V114 closer together. The other 'side' of this structure is surrounded by the basic residues extending outwards towards the protein or solution. While the bound structures of the cardiac and skeletal peptides are shown to be quite similar, the cardiac peptide appears more flexible near the central glycine residue.

Nanosecond study of fluorescently labeled troponin C

The time-resolved extrinsic fluorescence of rabbit skeletal troponin C was studied with the protein labeled at Cys-98 with N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine. Both the intensity and anisotropy decays followed a biexponential decay law, regardless of the ionic condition, pH, viscosity or temperature. The lifetimes and their fractional amplitudes were insensitive to Mg2+, and the lifetimes were also insensitive to Ca2+. In response to Ca2+ binding to all four sites, the fractional amplitude (alpha 1) associated with the short lifetime (tau 1) decreased by a factor of two, thus increasing the ratio of the two amplitudes alpha 2/alpha 1 from 1.6 to 4.3. These amplitude changes suggest the existence of two conformational states of TnC-IAEDANS, with the conformation associated with the long-decay component (tau 2) being promoted by saturation of the two Ca(2+)-specific sites. At pH 5.2 the ratio alpha 2/alpha 1 for the apo-protein was 3.5 indicating different relative populations of the two decay components when compared with pH 7.2. In the presence of Ca2+ at the lower pH, alpha 2/alpha 1 decreased to 2.1, suggesting a shift of the conformations in favor of the short-decay component. Thus Ca2+ elicited different conformational changes in TnC at the two pH values. The recovered anisotropies suggest that there were fast molecular motions that were not resolved in the present experiments, and some of these motions were sensitive to Ca2+ binding to the specific sites. These results support the notion of communication between the N-domain and the C-terminal end of the central helix of troponin C.

A comparative study of the interactions of synthetic peptides of the skeletal and cardiac troponin I inhibitory region with skeletal and cardiac troponin C

The cardiac and skeletal TnI inhibitory regions have identical sequences except at position 110 which contains Pro in the skeletal sequence and Thr in the cardiac sequence. The effect of the synthetic TnI inhibitory peptides [skeletal TnI peptide (104-115), cardiac TnI peptide (137-148), and a single Gly-substituted analogue at position 110] on the secondary structure of skeletal and cardiac TnC was investigated. The biphasic increases in ellipticity and tyrosine fluorescence were analyzed to determine the Ca2+ binding constants for the high- and low-affinity Ca2+ binding sites of TnC. Importantly, the skeletal and cardiac TnI peptides altered Ca2+ binding at the low-affinity sites of TnC, but the magnitude and direction of the pCa shifts depended on whether the peptides were bound to skeletal or cardiac TnC. For example, binding of skeletal TnI peptide to skeletal TnC (monitored by CD) caused a pCa shift of +0.30 unit such that a lower Ca2+ concentration was required to fill sites I and II, while binding of this peptide to cardiac TnC caused a pCa shift of -0.35 unit such that a higher Ca2+ concentration was required to fill site II. This is the first report of the alteration at the low-affinity regulatory sites (located in the N-terminal domain) by the skeletal TnI inhibitory peptide, even though the primary peptide binding site is located in the C-terminal domain of TnC, a finding which strongly indicates that there is communication between the two halves of the TnC molecule.(ABSTRACT TRUNCATED AT 250 WORDS)

Distance distributions and anisotropy decays of troponin C and its complex with troponin I

We used frequency domain measurements of fluorescence resonance energy transfer to recover the distribution of distances between Met 25 and Cys 98 in rabbit skeletal troponin C. These residues were labeled with dansylaziridine as energy donor and 5-(iodoacetamido)eosin as acceptor and are located on the N- and C-terminal lobes of the two-domain protein, respectively. We developed a procedure to correct for the fraction of the sample that was incompletely labeled with the acceptor independent of chemical data. At pH 7.5 and in the presence of Mg2+, the mean distance was near 15 A with a half-width of the distribution of 15 A; when Mg2+ was replaced by Ca2+, the mean distance increased to 22 A with a decrease in the half-width by 4 A. Similar but less pronounced differences in the mean distance and half-width between samples containing Mg2+ and Ca2+ were also observed with troponin C complexed to troponin I. The results suggest that the conformation of troponin C is altered by Ca2+ binding to the Ca(2+)-specific sites and displacing bound Mg2+ at the Ca2+/Mg2+ sites. This alteration may play an important role in Ca2+ signaling in muscle. At pH 7.5, the anisotropy decays of the donor-labeled troponin C showed two components, with the long rotational correlation time (12 ns) reflecting the overall motion of the protein. When the pH was lowered from 7.5 to 5.2, the mean distribution distance of apotroponin C increased from 22 to 32 A and the half-width decreased by a factor of 2 from 13 to 7 A. The long correlation time of apotroponin C increased to 19 ns at the acidic pH. These results are discussed in terms of a model in which skeletal troponin C is a dimer at low pH and enable comparison of the solution conformation of the protein at neutral pH with a crystal structure obtained at pH 5.2. While the conformation of the monomeric unit of troponin C dimer at pH 5.2 is extended and consistent with the crystal structure, the conformation at neutral pH is likely more compact than the crystal structure predicts.

Acid-induced dimerization of skeletal troponin C

We have investigated pH-dependent changes of the properties of troponin C from rabbit skeletal muscle. At pH 7.5 this protein is a monomer and at pH 5.2 it is a dimer. In contrast, bovine cardiac troponin C remains essentially monomeric at pH 5.2. Bovine brain calmodulin is not a dimer, but significantly aggregated at the same acidic pH. The dimerization of skeletal troponin C was demonstrated by low-speed (16,000 rpm) sedimentation equilibrium measurements carried out at 20 degrees C and by polyacrylamide gel electrophoresis under nondenaturing conditions. Dimer formation was significantly inhibited in the ultracentrifuge at rotor speeds of 30,000 and 40,000 rpm at 20 degrees C, and was completely prevented at a rotor speed of 40,000 rpm and 4 degrees C. This temperature and pressure dependence of dimerization strongly suggests that hydrophobic bonding is a major factor in promoting skeletal troponin C association at pH 5.2. The intramolecular distance between Met-25 and Cys-98 of rabbit skeletal troponin C deduced from fluorescence resonance energy transfer measurements increased by a factor of two upon lowering the pH from 7.5 to 5.2, indicating a pH-dependent transition in which the protein changed from a relatively compact conformation to an elongated conformation. The proton-induced increase in the energy transfer distance is related to the acid-induced dimerization of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Theoretical estimation of the calcium-binding constants for proteins from the troponin C superfamily based on a secondary structure prediction method. I. Estimation procedure

Proteins belonging to the TNC superfamily are known to be built of two, three, four, or six domains of closely similar amino acid sequences. Each domain binds no more than one calcium ion and shows a characteristic helix-loop-helix structure when in the calcium-bound state. Conformational properties of all the domains known so far have been analysed by us using a secondary structure prediction method (Garnier, J., Osguthorpe, D.J. & Robson, B. (1978). J. molec. Biol. 120, 97). Significant differences in distribution of residues predicted as being in the helical, beta-turn, and coil conformations have been found between the strongly, weakly, and non-binding domains. We could determine the ideal prediction pattern characteristic for the domains with the highest affinity for calcium. On the basis of our analysis and observations made by other authors we worked out a few simple rules which made it possible to compare conformational properties of a given domain with the ideal reference pattern and estimate, in this way, the Ca2+-binding constant of the domain. In native proteins the domains are known to be organized in pairs. The Ca2+-binding constant for a two-domain region could be evaluated from the sum of the estimation points attributed to each of its components. Using our method it is possible to predict the binding constants of typical domains and two-domain regins with a precision of one order of magnitude. Data on amino acid sequences and calcium-binding constants of all known proteins, believed to be the members of the TNC superfamily, have been reviewed. References to virtually all papers published on this subject before the end of 1987 are given.

Resolution of end-to-end distance distributions of flexible molecules using quenching-induced variations of the Forster distance for fluorescence energy transfer

We describe a new method to recover the distribution of donor-to-acceptor (D-A) distances in flexible molecules using steady-state measurements of the efficiency of fluorescence energy transfer. The method depends upon changes in the Forster distance (Ro) induced by collisional quenching of the donor emission. The Ro-dependent transfer efficiencies are analyzed using nonlinear least squares to recover the mean D-A distance and the width of the distribution. The method was developed and tested using three synthetic D-A pairs, in which the chromophores were separated by alkyl chains of varying lengths. As an example application we also recovered the distribution of distances from the single tryptophan residue in troponin I (trp 158) to acceptor-labeled cysteine 133. The half-width of the distribution increases from 12 A in the native state to 53 A when unfolded by guanidine hydrochloride. For both TnI and the three model compounds the distance distributions recovered from the steady-state transfer efficiencies were in excellent agreement with the distributions recovered using the more sophisticated frequency-domain method (Lakowicz, J.R., M.L. Johnson, W. Wiczk, A. Bhat, and R.F. Steiner. 1987. Chem. Phys. Lett. 138:587-593). The method was found to be reliable and should be generally useful for studies of conformational distributions of macromolecules.

Interactions of troponin subunits: free energy of binary and ternary complexes

We have determined the free energy of formation of the binary complexes formed between skeletal troponin C and troponin T (TnC.TnT) and between troponin T and troponin I (TnT.TnI). This was accomplished by using TnC fluorescently modified at Cys-98 with N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine for the first complex and TnI labeled at Cys-133 with the same probe for the other complex. The free energy of the ternary complex formed between troponin C and the binary complex TnT.TnI [TnC.(TnT.TnI)] was also measured by monitoring the emission of 5-(iodoacetamido)eosin attached to Cys-133 of the troponin I in TnT.TnI. The free energies were -9.0 kcal.mol-1 for TnC.TnT, -9.2 kcal.mol-1 for TnT.TnI, and -8.7 kcal.mol-1 for TnC.(TnT.TnI). In the presence of Mg2+ the free energies of TnC.TnT and TnC.(TnT.TnI) were -10.3 and -10.9 kcal.mol-1, respectively; in the presence of Ca2+ the corresponding free energies were -10.6 and -13.5 kcal.mol-1. Mg2+ and Ca2+ had negligible effect on the free energy of TnT.TnI. From these results the free energies of the formation of troponin from the three subunits were found to be -16.8 kcal.mol-1, -18.9 kcal.mol-1, and -21.6 kcal.mol-1 in the presence of EGTA, Mg2+, and Ca2+, respectively. Most of the free energy decrease caused by Ca2+ binding to the Ca2+-specific sites is derived from stabilization of the TnI-TnC linkage.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Proximity relationship in the binary complex formed between troponin I and troponin C

We have determined six molecular distances among four sites in the binary complex formed between troponin C (TnC) and troponin I (TnI) by fluorescence resonance energy transfer between donor and acceptor probes that were either an intrinsic fluorophore (Trp158 of TnI) or extrinsic probes attached to the sites. The three extrinsic probes were dansylaziridine (DNZ), N'-(iodoacetyl)-N'-(8-sulfo-1-naphthyl)ethylenediamine (IAEDANS) and 5-(iodoacetamido)eosin (IAE). The four fluorophores provided four donor-acceptor pairs: DNZ----IAE, Trp----IAEDANS, IAEDANS----IAE, and Trp----DNZ. They allowed determinations of separations between specific sites from measurements of energy transfer from (1) Met25 (DNZ) to Cys98 (IAE) in TnC, (2) Trp158 to Cys133 (IAEDANS) in TnI, (3) Cys98 (IAEDANS) of TnC to Cys133(IAE) of TnI, (4) Trp158 of TnI to Cys98(IAEDANS) of TnC, and (6) Met25(DNZ) of TnC to Cys133(IAE) of TnI. Distance (1) in TnC was little affected when the isolated protein was complexed with TnI, whereas distance (2) in TnI increased by 6A (29%) when TnI was incorporated into the binary complex. In the presence of EGTA, the six donor-acceptor separations (R) in the complex were in the range 28 to 57 A based on kappa 2 = 2/3. Mg2+ had only small effects on R, but Ca2+ induced substantial increases or decreases of R in five of the six distances. These changes were not accompanied by significant changes in the axial depolarization of the fluorophores. The results indicate global structural perturbations of regions of the two proteins in the complex by Ca2+ binding to the TnC, and suggest that large-scale movements of domains of troponin subunits may be the initial molecular events that occur in the transmission of the Ca2+ signal in the regulation of contraction by calcium.