Ca2+, Mg2+, and troponin I inhibitory peptide binding to a Phe-154 to Trp mutant of chicken skeletal muscle troponin C

Mutations in Troponin that cause HCM, DCM AND RCM: what can we learn about thin filament function?

Troponin (Tn) is a critical regulator of muscle contraction in cardiac muscle. Mutations in Tn subunits are associated with hypertrophic, dilated and restrictive cardiomyopathies. Improved diagnosis of cardiomyopathies as well as intensive investigation of new mouse cardiomyopathy models has significantly enhanced this field of research. Recent investigations have showed that the physiological effects of Tn mutations associated with hypertrophic, dilated and restrictive cardiomyopathies are different. Impaired relaxation is a universal finding of most transgenic models of HCM, predicted directly from the significant changes in Ca(2+) sensitivity of force production. Mutations associated with HCM and RCM show increased Ca(2+) sensitivity of force production while mutations associated with DCM demonstrate decreased Ca(2+) sensitivity of force production. This review spotlights recent advances in our understanding on the role of Tn mutations on ATPase activity, maximal force development and heart function as well as the correlation between the locations of these Tn mutations within the thin filament and myofilament function.

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.

Effects of Phe-to-Trp mutation and fluorotryptophan incorporation on the solution structure of cardiac troponin C, and analysis of its suitability as a potential probe for in situ NMR studies

19F NMR spectroscopy is potentially a powerful tool for probing protein properties in situ. However, results obtained using this technique are relevant only if the 19F probe offers minimal perturbation to the surrounding environment. In this paper, we examine the effect of 5-fluorotryptophan (5fW) incorporation on the three-dimensional structure of cardiac troponin-C (cTnC), with the intention of developing a 19F-labeled TnC for use in in situ 19FNMR. We find that, in general, 5fW does not perturb the structure of the protein significantly. Replacement of residue Phe 153 with 5fW produces no noticeable change in protein conformation. However, replacement of residue Phe 104 with 5fW produces a folding behavior that is dependent on the Escherichia coli strain used to express the mutant. The orientations of the indole rings in these mutants are such that the Trp residue adopts a chi2 of approximately 90 degrees in the F104W mutant and approximately -100 degrees in the F153W mutant. Using results from 19F-1H heteronuclear NOE experiment, we show the replacement of L-Trp with 5fW at these positions does not change the orientation of the indole ring and the spread of the 5fW side-chain dihedral angles increases moderately for the F104(5fW) mutant and not at all for the F153(5fW) mutant. Based on these structures, we conclude that the substitution of Phe by 5fW at these two positions has minimal effects on the structure of cTnC and that the 5fW indole rings in both mutants have well defined orientation, making the two mutants viable candidates for use in in situ 19F NMR spectroscopy.

Ca(2+) and Mg(2+) binding to weak sites of TnC C-domain induces exposure of a large hydrophobic surface that leads to loss of TnC from the thin filament

The C-domain of troponin C, the Ca(2+)-binding subunit of the troponin complex, has two high-affinity sites for Ca(2+) that also bind Mg(2+) (Ca(2+)/Mg(2+) sites), whereas the N-domain has two low-affinity sites for Ca(2+). Two more sites that bind Mg(2+) with very low affinity (K(a)<10(3)M(-1)) have been detected by several laboratories but have not been localized or studied in any detail. Here we investigated the effects of Ca(2+) and Mg(2+) binding to isolated C-domain, focusing primarily on low-affinity sites. Since TnC has no Trp residues, we utilized a mutant with Phe 154 replaced by Trp (F154W/C-domain). As expected from previous reports, the changes in Trp fluorescence revealed different conformations induced by the addition of Ca(2+) or Mg(2+) (Ca(2+)/Mg(2+) sites). Exposure of hydrophobic surfaces of F154W/C-domain was monitored using the fluorescence intensity of bis-anilino naphthalene sulfonic acid. Unlike the changes reported by Trp, the increments in bis-ANS fluorescence were much greater (4.2-fold) when Ca(2+)+Mg(2+) were both present or when Ca(2+) was present at high concentration. Bis-ANS fluorescence increased as a function of [Ca(2+)] in two well-defined steps: one at low [Ca(2+)], consistent with the Ca(2+)/Mg(2+) sites (K(a) approximately 1.5 x 10(6)M(-1)), and one of much lower affinity (K(a) approximately 52.3M(-1)). Controls were performed to rule out artifacts due to aggregation, high ionic strength and formation of the bis-ANS-TnC complex itself. With a low concentration of Ca(2+) (0.6mM) to occupy the Ca(2+)/Mg(2+) sites, a large increase in bis-ANS binding also occurred as Mg(2+) occupied a class of low-affinity sites (K(a) approximately 59 M(-1)). In skinned fibers, a high concentration of Mg(2+) (10-44 mM) caused TnC to dissociate from the thin filament. These data provide new evidence for a class of weak binding sites for divalent cations. They are located in the C-domain, lead to exposure of a large hydrophobic surface, and destabilize the binding of TnC to the regulatory complex even when sites III and IV are occupied.

The calcium-induced switch in the troponin complex probed by fluorescent mutants of troponin I

The Ca2+-induced transition in the troponin complex (Tn) regulates vertebrate striated muscle contraction. Tn was reconstituted with recombinant forms of troponin I (TnI) containing a single intrinsic 5-hydroxytryptophan (5HW). Fluorescence analysis of these mutants of TnI demonstrate that the regions in TnI that respond to Ca2+ binding to the regulatory N-domain of TnC are the inhibitory region (residues 96-116) and a neighboring region that includes position 121. Our data confirms the role of TnI as a modulator of the Ca2+ affinity of TnC; we show that point mutations and incorporation of 5HW in TnI can affect both the affinity and the cooperativity of Ca2+ binding to TnC. We also discuss the possibility that the regulatory sites in the N-terminal domain of TnC might be the high affinity Ca2+-binding sites in the troponin complex.

Parallel measurement of Ca2+ binding and fluorescence emission upon Ca2+ titration of recombinant skeletal muscle troponin C. Measurement of sequential calcium binding to the regulatory sites

Calcium binding to chicken recombinant skeletal muscle TnC (TnC) and its mutants containing tryptophan (F29W), 5-hydroxytryptophan (F29HW), or 7-azatryptophan (F29ZW) at position 29 was measured by flow dialysis and by fluorescence. Comparative analysis of the results allowed us to determine the influence of each amino acid on the calcium binding properties of the N-terminal regulatory domain of the protein. Compared with TnC, the Ca(2+) affinity of N-terminal sites was: 1) increased 6-fold in F29W, 2) increased 3-fold in F29ZW, and 3) decreased slightly in F29HW. The Ca(2+) titration of F29ZW monitored by fluorescence displayed a bimodal curve related to sequential Ca(2+) binding to the two N-terminal Ca(2+) binding sites. Single and double mutants of TnC, F29W, F29HW, and F29ZW were constructed by replacing aspartate by alanine at position 30 (site I) or 66 (site II) or both. Ca(2+) binding data showed that the Asp --> Ala mutation at position 30 impairs calcium binding to site I only, whereas the Asp --> Ala mutation at position 66 impairs calcium binding to both sites I and II. Furthermore, the Asp --> Ala mutation at position 30 eliminates the differences in Ca(2+) affinity observed for replacement of Phe at position 29 by Trp, 5-hydroxytryptophan, or 7-azatryptophan. We conclude that position 29 influences the affinity of site I and that Ca(2+) binding to site I is dependent on the previous binding of metal to site II.

Structural basis for sequential displacement of Ca(2+) by Yb(3+) in a protozoan EF-hand calcium binding protein

We have studied the displacement of Ca(2+)by the trivalent lanthanide ions (Yb(3+)) in a protozoan (Entamoeba histolytica) Ca(2+)-binding protein (EhCaBP), by NMR and thermodynamics. We have demonstrated, for the first time, how one can use in a combined fashion the utility of NMR and thermodynamics to have an insight to the relative binding specificities/affinity between Ca(2+) and Yb(3+). As revealed by the titration experiments, Yb(3+) displaces Ca(2+) from the four metal binding sites present in EhCaBP in a sequential manner. The study provides a structural origin for such a sequential Ca(2+) displacement by Yb(3+) in EhCaBP.

Kinetic analysis of the interactions between troponin C and the C-terminal troponin I regulatory region and validation of a new peptide delivery/capture system used for surface plasmon resonance

Using surface plasmon resonance (SPR)-based biosensor analysis and fluorescence spectroscopy, the apparent kinetic constants, k(on) and k(off), and equilibrium dissociation constant, K(d), have been determined for the binding interaction between rabbit skeletal troponin C (TnC) and rabbit skeletal troponin I (TnI) regulatory region peptides: TnI(96-115), TnI(96-131) and TnI(96-139). To carry out SPR analysis, a new peptide delivery/capture system was utilized in which the TnI peptides were conjugated to the E-coil strand of a de novo designed heterodimeric coiled-coil domain. The TnI peptide conjugates were then captured via dimerization to the opposite strand (K-coil), which was immobilized on the biosensor surface. TnC was then injected over the biosensor surface for quantitative binding analysis. For fluorescence spectroscopy analysis, the environmentally sensitive fluoroprobe 5-((((2-iodoacetyl)amino)ethyl)amino) naphthalene-1-sulfonic acid (1,5-IAEDANS) was covalently linked to Cys98 of TnC and free TnI peptides were added. SPR analysis yielded equilibrium dissociation constants for TnC (plus Ca(2+)) binding to the C-terminal TnI regulatory peptides TnI(96-131) and TnI(96-139) of 89nM and 58nM, respectively. The apparent association and dissociation rate constants for each interaction were k(on)=2.3x10(5)M(-1)s(-1), 2.0x10(5)M(-1)s(-1) and k(off)=2.0x10(-2)s(-1), 1.2x10(-2)s(-1) for TnI(96-131) and TnI(96-139) peptides, respectively. These results were consistent with those obtained by fluorescence spectroscopy analysis: K(d) being equal to 130nM and 56nM for TnC-TnI(96-131) and TnC-TnI(96-139), respectively. Interestingly, although the inhibitory region peptide (TnI(96-115)) was observed to bind with an affinity similar to that of TnI(96-131) by fluorescence analysis (K(d)=380nM), its binding was not detected by SPR. Subsequent investigations examining salt effects suggested that the binding mechanism for the inhibitory region peptide is best characterized by an electrostatically driven fast on-rate ( approximately 1x10(8) to 1x10(9)M(-1)s(-1)) and a fast off-rate ( approximately 1x10(2)s(-1)). Taken together, the determination of these kinetic rate constants permits a clearer view of the interactions between the TnC and TnI proteins of the troponin complex.

High-resolution nuclear magnetic resonance studies of proteins

The combination of advanced high-resolution nuclear magnetic resonance (NMR) techniques with high-pressure capability represents a powerful experimental tool in studies of protein folding. This review is organized as follows: after a general introduction of high-pressure, high-resolution NMR spectroscopy of proteins, the experimental part deals with instrumentation. The main section of the review is devoted to NMR studies of reversible pressure unfolding of proteins with special emphasis on pressure-assisted cold denaturation and the detection of folding intermediates. Recent studies investigating local perturbations in proteins and the experiments following the effects of point mutations on pressure stability of proteins are also discussed. Ribonuclease A, lysozyme, ubiquitin, apomyoglobin, alpha-lactalbumin and troponin C were the model proteins investigated.

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.

A model of troponin-I in complex with troponin-C using hybrid experimental data: the inhibitory region is a beta-hairpin

We present a model for the skeletal muscle troponin-C (TnC)/troponin-I (TnI) interaction, a critical molecular switch that is responsible for calcium-dependent regulation of the contractile mechanism. Despite concerted efforts by multiple groups for more than a decade, attempts to crystallize troponin-C in complex with troponin-I, or in the ternary troponin-complex, have not yet delivered a high-resolution structure. Many groups have pursued different experimental strategies, such as X-ray crystallography, NMR, small-angle scattering, chemical cross-linking, and fluorescent resonance energy transfer (FRET) to gain insights into the nature of the TnC/TnI interaction. We have integrated the results of these experiments to develop a model of the TnC/TnI interaction, using an atomic model of TnC as a scaffold. The TnI sequence was fit to each of two alternate neutron scattering envelopes: one that winds about TnC in a left-handed sense (Model L), and another that winds about TnC in a right-handed sense (Model R). Information from crystallography and NMR experiments was used to define segments of the models. Tests show that both models are consistent with available cross-linking and FRET data. The inhibitory region TnI(95-114) is modeled as a flexible beta-hairpin, and in both models it is localized to the same region on the central helix of TnC. The sequence of the inhibitory region is similar to that of a beta-hairpin region of the actin-binding protein profilin. This similarity supports our model and suggests the possibility of using an available profilin/actin crystal structure to model the TnI/actin interaction. We propose that the beta-hairpin is an important structural motif that communicates the Ca2+-activated troponin regulatory signal to actin.

The role of the NH(2)- and COOH-terminal domains of the inhibitory region of troponin I in the regulation of skeletal muscle contraction

The role of the inhibitory region of troponin (Tn) I in the regulation of skeletal muscle contraction was studied with three deletion mutants of its inhibitory region: 1) complete (TnI-(Delta96-116)), 2) the COOH-terminal domain (TnI-(Delta105-115)), and 3) the NH(2)-terminal domain (TnI-(Delta95-106)). Measurements of Ca(2+)-regulated force and relaxation were performed in skinned skeletal muscle fibers whose endogenous TnI (along with TnT and TnC) was displaced with high concentrations of added troponin T. Reconstitution of the Tn-displaced fibers with a TnI.TnC complex restored the Ca(2+) sensitivity of force; however, the levels of relaxation and force development varied. Relaxation of the fibers (pCa 8) was drastically impaired with two of the inhibitory region deletion mutants, TnI-(Delta96-116).TnC and TnI-(Delta105-115).TnC. The TnI-(Delta95-106).TnC mutant retained approximately 55% relaxation when reconstituted in the Tn-displaced fibers. Activation in skinned skeletal muscle fibers was enhanced with all TnI mutants compared with wild-type TnI. Interestingly, all three mutants of TnI increased the Ca(2+) sensitivity of contraction. None of the TnI deletion mutants, when reconstituted into Tn, could inhibit actin-tropomyosin-activated myosin ATPase in the absence of Ca(2+), and two of them (TnI-(Delta96-116) and TnI-(Delta105-115)) gave significant activation in the absence of Ca(2+). These results suggest that the COOH terminus of the inhibitory region of TnI (residues 105-115) is much more critical for the biological activity of TnI than the NH(2)-terminal region, consisting of residues 95-106. Presumably, the COOH-terminal domain of the inhibitory region of TnI is a part of the Ca(2+)-sensitive molecular switch during muscle contraction.

Rotational and translational motion of troponin C

Time resolved fluorescence anisotropy and sedimentation velocity has been used to study the rotational and translational hydrodynamic behavior of two mutants of chicken skeletal troponin C bearing a single tryptophan residue at position 78 or 154 in the metal-free-, metal-bound-, and troponin I peptide (residues 96-116 of troponin I)-ligated states. The fluorescence anisotropy data of both mutants were adequately described by two rotational correlation times, and these are compared with the theoretically expected values based on the rotational diffusion of an idealized dumbbell. These data imply that the motion of the N- and C-terminal domains of troponin C are independent. They also suggest that in the metal-free, calcium-saturated and calcium-saturated troponin I peptide-bound states, troponin C is elongated, having an axial ratio of 4-5. Calcium or magnesium binding to the high affinity sites alone reduces the axial ratio to approximately 3. However, with calcium bound to sites III and IV and in the presence of a 1:1 molar ratio of the troponin I peptide, troponin C is approximately spherical. The metal ion and troponin I peptide-induced length changes in troponin C may play a role in the mechanism by which the regulatory function of troponin C is effected.

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.

Ca2+-dependent interaction of the inhibitory region of troponin I with acidic residues in the N-terminal domain of troponin C

Ca2+ regulation of vertebrate striated muscle contraction is initiated by conformational changes in the N-terminal, regulatory domain of the Ca2+-binding protein troponin C (TnC), altering the interaction of TnC with the other subunits of troponin complex, TnI and TnT. We have investigated the role of acidic amino acid residues in the N-terminal, regulatory domain of TnC in binding to the inhibitory region (residues 96-116) of TnI. We constructed three double mutants of TnC (E53A/E54A, E60A/E61A and E85A/D86A), in which pairs of acidic amino acid residues were replaced by neutral alanines, and measured their affinities for synthetic inhibitory peptides. These peptides had the same amino acid sequence as TnI segments 95-116, 95-119 or 95-124, except that the natural Phe-100 of TnI was replaced by a tryptophan residue. Significant Ca2+-dependent increases in the affinities of the two longer peptides, but not the shortest one, to TnC could be detected by changes in Trp fluorescence. In the presence of Ca2+, all the mutant TnCs showed about the same affinity as wild-type TnC for the inhibitory peptides. In the presence of Mg2+ and EGTA, the N-terminal, regulatory Ca2+-binding sites of TnC are unoccupied. Under these conditions, the affinity of TnC(E85A/D86A) for inhibitory peptides was about half that of wild-type TnC, while the other two mutants had about the same affinity. These results imply a Ca2+-dependent change in the interaction of TnC Glu-85 and/or Asp-86 with residues (117-124) on the C-terminal side of the inhibitory region of TnI. Since Glu-85 and/or Asp-86 of TnC have also been demonstrated to be involved in Ca2+-dependent regulation through interaction with TnT, this region of TnC must be critical for troponin function.

Mapping of a second actin-tropomyosin and a second troponin C binding site within the C terminus of troponin I, and their importance in the Ca2+-dependent regulation of muscle contraction

To investigate the functional importance of the C-terminal residues 116 to 148 of troponin I (TnI) in the Ca2+-dependent regulation of vertebrate skeletal muscle contraction, we have prepared several synthetic TnI peptide analogs corresponding to various regions within residues 96 to 148 of rabbit skeletal TnI, and analyzed each of these peptides in reconstituted thin filament assays. Our results show that the TnI peptide 96 to 148 (TnI96-148) constitutes the minimal sequence of TnI capable of mediating an inhibitory activity similar to that of intact TnI protein. Truncation of residues 140 to 148 from this region (TnI96-139) or substitution of residues K141, K142 and K144 with alanine (TnI96-148A2) completely abolishes the enhanced inhibitory effect of this region when compared with TnI96-115. A synthetic peptide, residues 128 to 148 of TnI, containing residues 140 to 148, now termed the "second actin-tropomyosin (actin-Tm) binding site" is able to bind specifically to the actin-Tm filament and can induce a weak inhibitory activity on its own. Residues 116 to 131 of TnI do not appear to be important for inhibition, but are critical for interacting with troponin C (TnC). Specific investigations into this region have shown that residues 116 to 126, located directly adjacent to the "inhibitory region" (residues 96 to 115), are critical for allowing TnC to neutralize fully and rapidly the acto-S1-Tm inhibition caused by the various TnI peptides. Furthermore, residues 116 to 131 of TnI, now termed the "second TnC binding site", can significantly enhance the binding affinity of the inhibitory region, residues 96 to 115, for TnC in a Ca2+-dependent manner as determined by affinity chromatography analysis. The implication that TnI residues 116 to 131 bind to the N domain of TnC, and thus the inhibitory region (residues 96 to 115) binds to the C domain of TnC, has made us re-investigate the structural/functional role of the NH2-terminal region of TnI. Studies of competition between the N terminus of TnI (Rp1-40, residues 1 to 40) with the C-terminal peptides TnI96-115, TnI96-131 and TnI96-148 showed that only TnI96-115 could be easily displaced from TnC. These results thus suggest that Ca2+ binding to the regulatory sites of TnC (N domain) alters the binding affinity between the NH2 terminus and the C terminus of TnI for TnC, i.e. a Ca2+-dependent switch between these two sites of TnI for the C domain of TnC. These results have been incorporated into a general model describing the Ca2+-dependent regulation of muscle contraction.

Ca(2+)-induced conformational shift of the COOH-domain of eel skeletal muscle troponin C in the presence of physiological concentrations of Mg2+

The spectroscopic properties of Trp152 of eel skeletal muscle troponin C have been studied under conditions in which the COOH-domain is depleted of metal ions, titrated with Mg2+ and subsequently with Ca2+. This spectroscopic study clearly shows that the Mg2+ or Ca(2+)-bound states substitution lead to distinct conformations of the COOH-domain. The analysis of eel troponin C absorption and Trp152 fluorescence emission spectra indicates a more polar environment in the Mg(2+)-bound state of the protein. Steady state tryptophan polarization and lifetime distribution data indicate that the motion of the indole moiety is more restricted in the Mg2+ state of the protein than in the Ca(2+)-bound state. However, fluorescence quenching data using I- and Cs+ show that Trp152 is more accessible to the solvent in the Mg(2+)-bound state of eel troponin C. This spectroscopic analysis of the distinct Ca2+ and Mg(2+)-bound states of eel troponin C is consistent with the description of the three-dimensional structure of the corresponding states of pike (pI = 4.10) parvalbumin which is structurally homologous to the COOH-domain of troponin C. Since it appears that during muscular contraction, magnesium ions, which occupy the binding sites of the COOH-domain of troponin C in the resting cell are replaced by calcium ions, the structural shift occurring upon Mg2+/Ca2+ substitution, must have a physiological significance. The role of this domain is probably not limited to a structural role.

Distinct regions of troponin I regulate Ca2+-dependent activation and Ca2+ sensitivity of the acto-S1-TM ATPase activity of the thin filament

The regions of troponin I (TnI) responsible for Ca2+-dependent activation and Ca2+ sensitivity of the actin-myosin subfragment 1-tropomyosin ATPase (acto-S1-TM) activity have been determined. A colorimetric ATPase assay at pH 7.8 has been applied to reconstituted skeletal muscle thin filaments at actin:S1:TM ratios of 6:1:2. Several TnI fragments (TnI-(104-115), TnI-(1-116), and TnI-(96-148)) and TnI mutants with single amino acid substitutions within the inhibitory region (residues 104-115) were assayed to determine their roles on the regulatory function of TnI. TnI-(104-115) is sufficient for achieving maximum inhibition of the acto-S1-TM ATPase activity and its importance was clearly shown by the reduced potency of TnI mutants with single amino acid substitutions within this region. However, the function of the inhibitory region is modulated by other regions of TnI as observed by the poor inhibitory activity of TnI-(1-116) and the increased potency of the inhibitory region by TnI-(96-148). The regulatory complex composed of TnI-(96-148) plus troponin T-troponin C complex (TnT.C) displays the same Ca2+ sensitivity (pCa50) as intact troponin (Tn) or TnI plus TnT.C while those regulatory complexes composed of TnT.C plus either TnI-(104-115) or TnI-(1-116) had an increase in their pCa50 values. This indicates that the Ca2+ sensitivity or responsiveness of the thin filament is controlled by TnI residues 96-148. The ability of Tn to activate the acto-S1-TM ATPase activity in the presence of calcium to the level of the acto-S1 rate was mimicked by the regulatory complex composed of TnI-(1-116) plus TnT.C and was not seen with complexes composed with either TnI-(104-115) or TnI-(96-148). This indicates that the N terminus of TnI in conjunction with TnT controls the degree of activation of the ATPase activity. Although the TnI inhibitory region (104-115) is the Ca2+-sensitive switch which changes binding sites from actin-TM to TnC in the presence of calcium, its function is modulated by both the C-terminal and N-terminal regions of TnI. Thus, distinct regions of TnI control different aspects of Tn's biological function.

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.

Characterizing the microenvironment surrounding protein sites

Sites are microenvironments within a biomolecular structure, distinguished by their structural or functional role. A site can be defined by a three-dimensional location and a local neighborhood around this location in which the structure or function exists. We have developed a computer system to facilitate structural analysis (both qualitative and quantitative) of biomolecular sites. Our system automatically examines the spatial distributions of biophysical and biochemical properties, and reports those regions within a site where the distribution of these properties differs significantly from control nonsites. The properties range from simple atom-based characteristics such as charge to polypeptide-based characteristics such as type of secondary structure. Our analysis of sites uses non-sites as controls, providing a baseline for the quantitative assessment of the significance of the features that are uncovered. In this paper, we use radial distributions of properties to study three well-known sites (the binding sites for calcium, the milieu of disulfide bridges, and the serine protease active site). We demonstrate that the system automatically finds many of the previously described features of these sites and augments these features with some new details. In some cases, we cannot confirm the statistical significance of previously reported features. Our results demonstrate that analysis of protein structure is sensitive to assumptions about background distributions, and that these distributions should be considered explicitly during structural analyses.

Anti-peptide monoclonal antibody imaging of a common binding domain involved in muscle regulation

Multiple-component regulatory protein systems function through a generalized mechanism where a single regulatory protein or ligand binds to a variety of receptors to modulate specific functions in a physiologically sensitive context. Muscle contraction is regulated by the interaction of actin with troponin I (TnI) or myosin in a Ca(2+)-sensitive manner. Actin utilizes a single binding domain (residues 1-28) to bind to residues 104-115 of TnI (Van Eyk JE, Sönnichsen FD, Sykes BD, Hodges RS, 1991, In: Rüegg JC, ed, Peptides as probes in muscle research, Heidelberg, Germany: Springer-Verlag, pp 15-31) and to myosin subfragment 1 (S1, an enzymatic fragment of myosin containing both the actin and ATP binding sites) (Van Eyk JE, Hodges RS, 1991, Biochemistry 30:11676-11682) in a Ca(2+)-sensitive manner. We have utilized an anti-TnI peptide (104-115) monoclonal antibody, Mab B4, that binds specifically to TnI, to image the common binding domain of actin and thus mimic the activity of actin including activation of the S1 ATPase activity and TnI-mediated regulation of the S1 ATPase. Mab B4 has also been utilized to identify a receptor binding domain on myosin (residues 633-644) that is recognized by actin. Interestingly, Mab B4 binds to the native protein receptors TnI and S1 with relative affinities of 100- and 25,000-fold higher than the binding affinity to the 12-residue peptide immunogen. Thus, anti-peptide monoclonal antibodies prepared against a receptor binding domain can mimic the ligand binding domain and be utilized as a powerful tool for the detailed analysis of complex multiple-component regulatory systems.