Interaction of troponin I and troponin C. Use of the two-dimensional nuclear magnetic resonance transferred nuclear Overhauser effect to determine the structure of the inhibitory troponin I peptide when bound to skeletal troponin C

Anomalous structural dynamics of minimally frustrated residues in cardiac troponin C triggers hypertrophic cardiomyopathy

Cardiac TnC (cTnC) is highly conserved among mammals, and genetic variants can result in disease by perturbing Ca2+-regulation of myocardial contraction. Here, we report the molecular basis of a human mutation in cTnC's αD-helix (TNNC1-p.C84Y) that impacts conformational dynamics of the D/E central-linker and sampling of discrete states in the N-domain, favoring the "primed" state associated with Ca2+ binding. We demonstrate cTnC's αD-helix normally functions as a central hub that controls minimally frustrated interactions, maintaining evolutionarily conserved rigidity of the N-domain. αD-helix perturbation remotely alters conformational dynamics of the N-domain, compromising its structural rigidity. Transgenic mice carrying this cTnC mutation exhibit altered dynamics of sarcomere function and hypertrophic cardiomyopathy. Together, our data suggest that disruption of evolutionary conserved molecular frustration networks by a myofilament protein mutation may ultimately compromise contractile performance and trigger hypertrophic cardiomyopathy.

Single histidine button in cardiac troponin I sustains heart performance in response to severe hypercapnic respiratory acidosis in vivo

Intracellular acidosis is a profound negative regulator of myocardial performance. We hypothesized that titrating myofilament calcium sensitivity by a single histidine substituted cardiac troponin I (A164H) would protect the whole animal physiological response to acidosis in vivo. To experimentally induce severe hypercapnic acidosis, mice were exposed to a 40% CO(2) challenge. By echocardiography, it was found that systolic function and ventricular geometry were maintained in cTnI A164H transgenic (Tg) mice. By contrast, non-Tg (Ntg) littermates experienced rapid and marked cardiac decompensation during this same challenge. For detailed hemodymanic assessment, Millar pressure-conductance catheterization was performed while animals were treated with a beta-blocker, esmolol, during a severe hypercapnic acidosis challenge. Survival and load-independent measures of contractility were significantly greater in Tg vs. Ntg mice. This assay showed that Ntg mice had 100% mortality within 5 min of acidosis. By contrast, systolic and diastolic function were protected in Tg mice during acidosis, and they had 100% survival. This study shows that, independent of any beta-adrenergic compensation, myofilament-based molecular manipulation of inotropy by histidine-modified troponin I maintains cardiac inotropic and lusitropic performance and markedly improves survival during severe acidosis in vivo.

Interaction of cardiac troponin with cardiotonic drugs: a structural perspective

Over the 40 years since its discovery, many studies have focused on understanding the role of troponin as a myofilament based molecular switch in regulating the Ca(2+)-dependent activation of striated muscle contraction. Recently, studies have explored the role of cardiac troponin as a target for cardiotonic agents. These drugs are clinically useful for treating heart failure, a condition in which the heart is no longer able to pump enough blood to other organs. These agents act via a mechanism that modulates the Ca(2+)-sensitivity of troponin; such a mode of action is therapeutically desirable because intracellular Ca(2+) concentration is not perturbed, preserving the regulation of other Ca(2+)-based signaling pathways. This review describes molecular details of the interaction of cardiac troponin with a variety of cardiotonic drugs. We present recent structural work that has identified the docking sites of several cardiotonic drugs in the troponin C-troponin I interface and discuss their relevance in the design of troponin based drugs for the treatment of heart disease.

Determination of the Bound Conformation of a Competitive Nanomolar Inhibitor ofMycobacterium tuberculosis Type II Dehydroquinase by NMR Spectroscopy

The synergy between tuberculosis and the AIDS epidemic, along with the surge of multidrug-resistant isolates of M. tuberculosis, has reaffirmed tuberculosis as a primary public health threat. It is therefore necessary to discover new, safe, and more efficient antibiotics against this disease. On the other hand, mapping the dynamic interactions of inhibitors of a target protein can provide information for the development of more potent inhibitors and consequently, more potent potential drugs. In this context, the conformational binding of our previously reported nanomolar inhibitor of M. tuberculosis type II dehydroquinase, the 3-nitrophenyl derivative 1, was studied using saturation transfer difference (STD) and transferred NOESY experiments. These studies have shown that in the bound state, one conformation of those present in solution of the competitive nanomolar inhibitor 3-nitrophenyl derivative 1 is selected. In the bound conformation, the aromatic ring is slightly shifted from coplanarity, with the double bond and the nitro group of 1 oriented towards the double bond side.

The role of electrostatics in the interaction of the inhibitory region of troponin I with troponin C

We have addressed the electrostatic interactions occurring between the inhibitory region of cardiac troponin I with the C-lobe of troponin C using scanning glycine mutagenesis of the inhibitory region. We report variations in the electric potentials due to mutation of charged residues within this complex based upon the solved NMR structure (1OZS). These results demonstrate the importance of electrostatics within this complex, and it is proposed that electrostatic interactions are integral to the formation and function of larger ternary troponin complexes. To address this hypothesis, we report (15)N NMR relaxation measurements, which suggest that, within a ternary complex involving the C-lobe and the N-terminal region of troponin I (residues 34-71), the inhibitory region maintains the electrostatic interactions with the E-helix of the C-lobe as observed within the binary complex. These results imply that, in solution, the cardiac troponin complex behaves in a manner consistent with that of the crystal structure of the skeletal isoform (1YTZ). A cardiac troponin complex possessing domain orientations similar to that of the skeletal isoform provides structural insights into altered troponin I activities as observed for the familial hypertrophic cardiomyopathy mutation R144G and phosphorylation of Thr142.

Inherited cardiomyopathies as a troponin disease

Troponin, one of the sarcomeric proteins, plays a central role in the Ca(2+) regulation of contraction in vertebrate skeletal and cardiac muscles. It consists of three subunits with distinct structure and function, troponin T, troponin I, and troponin C, and their accurate and complex intermolecular interaction in response to the rapid rise and fall of Ca(2+) in cardiomyocytes plays a key role in maintaining the normal cardiac pump function. More than 200 mutations in the cardiac sarcomeric proteins, including myosin heavy and light chains, actin, troponin, tropomyosin, myosin-binding protein-C, and titin/connectin, have been found to cause various types of cardiomyopathy in human since 1990, and more than 60 mutations in human cardiac troponin subunits have been identified in dilated, hypertrophic, and restrictive forms of cardiomyopathy. In this review, we have focused on the mutations in the genes for human cardiac troponin subunits and discussed their functional consequences that might be involved in the primary mechanisms for the pathogenesis of these different types of cardiomyopathy.

Structural based insights into the role of troponin in cardiac muscle pathophysiology

Troponin is a molecular switch, directly regulating the Ca2+-dependent activation of myofilament in striated muscle contraction. Cardiac troponin is subject to covalent and noncovalent modifications; phosphorylation modulates myofilament physiology, mutations are linked to familial hypertrophic cardiomyopathy, intracellular acidification causes myocardial infarction, and cardiotonic drugs modify myofilament response to Ca2+. The structure of troponin provides insights into the mechanism of this molecular switch and an understanding of the effects of protein modification under pathophysiological conditions. Although the structure of troponin C has been solved in various Ca2+-bound states for some time, structural information on troponin I and troponin T has only emerged recently. This review summarizes recent advances on the structure of complexes of troponin subunits with the aim of assessing how these proteins interact with each other to execute its role as a molecular switch and how covalent and noncovalent modifications affect the structure of troponin and the switch mechanism. We focus on pinpointing the specific amino acid residues involved in phosphorylation and mutation and the pH sensitive regions in the structure of troponin. We also present recent structural work that have identified the docking sites of several cardiotonic drugs on cardiac troponin C and discuss their relevance in the direction of troponin based drug design in the therapy of heart disease.

Solution Structure of the Chicken Skeletal Muscle Troponin Complex Via Small-angle Neutron and X-ray Scattering

Troponin is a Ca2+-sensitive switch that regulates the contraction of vertebrate striated muscle by participating in a series of conformational events within the actin-based thin filament. Troponin is a heterotrimeric complex consisting of a Ca2+-binding subunit (TnC), an inhibitory subunit (TnI), and a tropomyosin-binding subunit (TnT). Ternary troponin complexes have been produced by assembling recombinant chicken skeletal muscle TnC, TnI and the C-terminal portion of TnT known as TnT2. A full set of small-angle neutron scattering data has been collected from TnC-TnI-TnT2 ternary complexes, in which all possible combinations of the subunits have been deuterated, in both the +Ca2+ and -Ca2+ states. Small-angle X-ray scattering data were also collected from the same troponin TnC-TnI-TnT2 complex. Guinier analysis shows that the complex is monomeric in solution and that there is a large change in the radius of gyration of TnI when it goes from the +Ca2+ to the -Ca2+ state. Starting with a model based on the human cardiac troponin crystal structure, a rigid-body Monte Carlo optimization procedure was used to yield models of chicken skeletal muscle troponin, in solution, in the presence and in the absence of regulatory calcium. The optimization was carried out simultaneously against all of the scattering data sets. The optimized models show significant differences when compared to the cardiac troponin crystal structure in the +Ca2+ state and provide a structural model for the switch between +Ca2+ and -Ca2+ states. A key feature is that TnC adopts a dumbbell conformation in both the +Ca2+ and -Ca2+ states. More importantly, the data for the -Ca2+ state suggest a long extension of the troponin IT arm, consisting mainly of TnI. Thus, the troponin complex undergoes a large structural change triggered by Ca2+ binding.

The anthrax protective antigen (PA63) bound conformation of a peptide inhibitor of the binding of lethal factor to PA63: as determined by trNOESY NMR and molecular modeling

Anthrax protective antigen (PA) is one of the three proteins produced by the gram positive bacteria Bacillus anthracis collectively known as the "anthrax toxin" (Ascenzi, P.; Visca, P.; Ippolito, G.; Spallarossa, A.; Bolognesi, M.; et al. Anthrax toxin: a tripartite lethal combination. FEBS Lett. 2002, 531, 384-388). The role played by PA in anthrax intoxication is to transport the two enzymes lethal factor (LF) and edema factor (EF) into the cell. Collier and co-workers (Mourez, M.; Kane, R. S.; Mogridge, J.; Metallo, S.; Deschatelets, P.; et al. Designing a polyvalent inhibitor of anthrax toxin. Nat. Biotechnol. 2001, 958). reported the isolation of two peptides via phage display that bind to the PA63 heptamer and inhibit its interaction with LF and EF, and thereby prevent the transport of LF and EF into the cell. One of these peptides, His-Thr-Ser-Thr-Try-Trp-Trp-Leu-Asp-Gly-Ala-Pro (P1), was selected for structural investigation on the basis of its ability to prevent the binding of LF to the PA63 heptamer bundle. Two-dimensional trNOESY experiments coupled with NOE restrained simulated annealing calculations were used to determine the PA63-bound conformation of P1. On binding to PA63, P1 adopts a helical conformation involving residues 3-9 while the C- and N-terminal residues exhibit dynamic fraying.

Structure and dynamics of the C-domain of human cardiac troponin C in complex with the inhibitory region of human cardiac troponin I

Cardiac troponin C is the Ca2+-dependent switch for heart muscle contraction. Troponin C is associated with various other proteins including troponin I and troponin T. The interaction between the subunits within the troponin complex is of critical importance in understanding contractility. Following a Ca2+ signal to begin contraction, the inhibitory region of troponin I comprising residues Thr128-Arg147 relocates from its binding surface on actin to troponin C, triggering movement of troponin-tropomyosin within the thin filament and thereby freeing actin-binding site(s) for interactions with the myosin ATPase of the thick filament to generate the power stroke. The structure of calcium-saturated cardiac troponin C (C-domain) in complex with the inhibitory region of troponin I was determined using multinuclear and multidimensional nuclear magnetic resonance spectroscopy. The structure of this complex reveals that the inhibitory region adopts a helical conformation spanning residues Leu134-Lys139, with a novel orientation between the E- and H-helices of troponin C, which is largely stabilized by electrostatic interactions. By using isotope labeling, we have studied the dynamics of the protein and peptide in the binary complex. The structure of this inhibited complex provides a framework for understanding into interactions within the troponin complex upon heart contraction.

Effect of temperature and the F27W mutation on the Ca2+ activated structural transition of trout cardiac troponin C

The Ca(2+) sensitivity of cardiac contractile element is reduced at lower temperatures, in contrast to that in fast skeletal muscle. Cardiac troponin C (cTnC) replacement in mammalian skinned fibers showed that TnC plays a critical role in this phenomenon (Harrison and Bers, (1990), Am. J. Physiol. 258, C282-8). Understanding the differences in affinity and structure between cTnCs from cold-adapted ectothermic species and mammals may bring new insights into how the different isoforms provide different resistances to cold. We followed the Ca(2+) titration to the regulatory domain of rainbow trout cTnC by NMR (wild type at 7 and 30 degrees C and F27W mutant at 30 degrees C) and fluorescence (F27W mutant, at 7 and 30 degrees C) spectroscopies. Using NMR spectroscopy, we detected Ca(2+) binding to site I of trout cTnC at high concentrations. This places trout cTnC between mammalian cTnC, in which site I is completely inactive, and skeletal TnC, in which site I binds Ca(2+) during muscle activation, and which is not as much affected by lower temperatures. This binding was seen both at 7 and at 30 degrees C. Despite the low Ca(2+) affinity, trout TnC site I may increase the likelihood of an opening of the regulatory domain, thus increasing the affinity for TnI. This way, it may be responsible for trout cTnC's capacity to function at lower temperatures.

Studies of the receptor-bound conformation of alphaIIbbeta3 antagonists by 15N-edited NMR spectroscopy

We report the results of NMR studies and computer simulations of potent antagonists reflective of the alpha(IIb)beta(3) receptor-bound conformations. The peptides c[Mpa-(15)N-Arg(1)-(15)N-Gly(2)-(15)N-Asp(3)-(15)N-Phe(4)-(15)N-Arg(5)-Cys]-NH(2) (Phe-Arg analog) (Mpa: 3-mercaptopropionic acid) and c[Mpa-(15)N-Arg(1)-(15)N-Gly(2)-(15)N-Asp(3)-(15)N-Asp(4)-(15)N-Val(5)-Cys]-NH(2) (Asp-Val analog) were subjected to (15)N-edited NMR experiments to study the conformations of these peptides in the absence and in the presence of alpha(IIb)beta(3) receptor. The NMR studies of the Phe-Arg analog, a selective alpha(IIb)beta(3) antagonist, resulted in distinctly different experimental data in the presence and absence of the receptor. The computer simulations for this peptide resulted in one large family of structures consistent with the experimental data. This conformation suggests a type I beta-turn spanning residues Arg(1) and Gly(2) when bound to the receptor and we were able to establish a model for the three dimensional arrangement of the pharmacophores. The studies on the Asp-Val analog, an alpha(v)beta(3) antagonist that binds to the alpha(IIb)beta(3) with moderate affinity, resulted in conformations that are not as well defined as those for the Phe-Arg analog but are consistent with the model established for this analog. These results are important for the design of novel alpha(IIb)beta(3) antagonists.

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.

Effects of T142 phosphorylation and mutation R145G on the interaction of the inhibitory region of human cardiac troponin I with the C-domain of human cardiac troponin C

Cardiac troponin I (cTnI) is the inhibitory component of the troponin complex, and its interaction with cardiac troponin C (cTnC) plays a critical role in transmitting the Ca(2+) signal to the other myofilament proteins in heart muscle contraction. The switch between contraction and relaxation involves a movement of the inhibitory region of cTnI (cIp) from cTnC to actin-tropomyosin. This region of cTnI is prone to missense mutations in heart disease, and a specific mutation, R145G, has been associated with familial hypertrophic cardiomyopathy. It also contains the unique cardiac PKC phosphorylation site at residue T142. To determine the structural consequences of the mutation R145G and the T142 phosphorylation on the interaction of cIp with cTnC, we have utilized 2D [(1)H, (15)N]-HSQC NMR spectroscopy to monitor the binding of native cIp, cIp-R (R145G), and cIp-P (phosphorylated T142), respectively, to the Ca(2+)-saturated C-domain of cTnC (cCTnC.2Ca(2+)). We also report a strategy for cloning, expression, and purification of cTnI peptide, and both synthetic and recombinant peptides are used in this study. NMR chemical shift mapping indicates that the binding epitope of cIp on cCTnC.2Ca(2+) is not greatly affected, but the affinity is reduced by approximately 14-fold by the T142 phosphorylation and approximately 4-fold by the mutation R145G, respectively. This suggests that these modifications of cIp have an adverse effect on the binding of cIp to cCTnC.2Ca(2+). These perturbations may correlate with the impairment or loss of cTnI function in heart muscle contraction.

Ca(2+) induces an extended conformation of the inhibitory region of troponin I in cardiac muscle troponin

The inhibitory region of troponin I (TnI) plays a central regulatory role in the contraction and relaxation cycle of skeletal and cardiac muscle through its Ca(2+)-dependent interaction with actin. Detailed structural information on the interface between TnC and this region of TnI has been long in dispute. We have used fluorescence resonance energy transfer (FRET) to investigate the global conformation of the inhibitory region of a full-length TnI mutant from cardiac muscle (cTnI) in the unbound state and in reconstituted complexes with the other cardiac troponin subunits. The mutant contained a single tryptophan residue at the position 129 which was used as an energy transfer donor, and a single cysteine residue at the position 152 labeled with IAEDANS as energy acceptor. The sequence between Trp129 and Cys152 in cTnI brackets the inhibitory region (residues 130-149), and the distance between the two sites was found to be 19.4 A in free cTnI. This distance was insensitive to reconstitution of cTnI with cardiac troponin T (cTnT), cTnC, or cTnC and cTnT in the absence of bound regulatory Ca(2+) in cTnC. An increase of 9 A in the Trp129-Cys152 separation was observed upon saturation of the Ca(2+) regulatory site of cTnC in the complexes. This large increase suggests an extended conformation of the inhibitory region in the interface between cTnC and cTnI in holo cardiac troponin. This extended conformation is different from a recent model of the Ca(2+)-saturated skeletal TnI-TnC complex in which the inhibitory region is modeled as a beta-turn. The observed Ca(2+)-induced conformational change may be a switch mechanism by which movement of the regulatory region of cTnI to the exposed hydrophobic patch of the open regulatory N-domain of cTnC pulls the inhibitory region away from actin upon Ca(2+) activation in cardiac muscle.

Mapping the interacting regions between troponins T and C. Binding of TnT and TnI peptides to TnC and NMR mapping of the TnT-binding site on TnC

Muscular contraction is triggered by an increase in calcium concentration, which is transmitted to the contractile proteins by the troponin complex. The interactions among the components of the troponin complex (troponins T, C, and I) are essential to understanding the regulation of muscle contraction. While the structure of TnC is well known, and a model for the binary TnC.TnI complex has been recently published (Tung, C.-S., Wall, M. E., Gallagher, S. C., and Trewhella, J. (2000) Protein Sci. 9, 1312-1326), very little is known about TnT. Using non-denaturing gels and NMR spectroscopy, we have analyzed the interactions between TnC and five peptides from TnT as well as how three TnI peptides affect these interactions. Rabbit fast skeletal muscle peptide TnT-(160-193) binds to TnC with a dissociation constant of 30 +/- 6 microm. This binding still occurs in the presence of TnI-(1-40) but is prevented by the presence of TnI-(56-115) or TnI-(96-139), both containing the primary inhibitory region of TnI. TnT-(228-260) also binds TnC. The binding site for TnT-(160-193) is located on the C-terminal domain of TnC and was mapped to the surface of TnC using NMR chemical shift mapping techniques. In the context of the model for the TnC.TnI complex, we discuss the interactions between TnT and the other troponin subunits.

Structure, dynamics, and thermodynamics of the structural domain of troponin C in complex with the regulatory peptide 1-40 of troponin I

The structure of the calcium-saturated C-domain of skeletal troponin C (CTnC) in complex with a regulatory peptide comprising residues 1-40 (Rp40) of troponin I (TnI) was determined using nuclear magnetic resonance (NMR) spectroscopy. The solution structure determined by NMR is similar to the structure of the C-domain from intact TnC in complex with TnI(1)(-)(47) determined by X-ray crystallography [Vassylyev, D. G., Takeda, S., Wakatsuki, S., Maeda, K., and Maeda, Y. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 4847-4852]. Changes in the dynamic properties of CTnC.2Ca2+ induced by Rp40 binding were investigated using backbone amide (15)N NMR relaxation measurements. Analysis of NMR relaxation data allows for extraction of motional order parameters on a per residue basis, from which the contribution of changes in picosecond to nanosecond time scale motions to the conformational entropy associated with complex formation can be estimated. The results indicate that binding of Rp40 decreases backbone flexibility in CTnC, particularly at the end of the C-terminal helix. The backbone conformational entropy change (-TDeltaS) associated with binding of Rp40 to CTnC.2Ca2+ determined from (15)N relaxation data is 9.6 +/- 0.7 kcal mol(-1) at 30 degrees C. However, estimation of thermodynamic quantities using a structural approach [Lavigne, P., Bagu, J. R., Boyko, R., Willard, L., Holmes, C. F., and Sykes, B. D. (2000) Protein Sci. 9, 252-264] reveals that the change in solvation entropy upon complex formation is dominant and overcomes the thermodynamic "cost" associated with "stiffening" of the protein backbone upon Rp40 binding. Additionally, backbone amide (15)N relaxation data measured at different concentrations of CTnC.2Ca2+.Rp40 reveal that the complex dimerizes in solution. Fitting of the apparent global rotational correlation time as a function of concentration to a monomer-dimer equilibrium yields a dimerization constant of approximately 8.3 mM.

Structure of the C-domain of human cardiac troponin C in complex with the Ca2+ sensitizing drug EMD 57033

Ca(2+) binding to cardiac troponin C (cTnC) triggers contraction in heart muscle. In heart failure, myofilaments response to Ca(2+) are often altered and compounds that sensitize the myofilaments to Ca(2+) possess therapeutic value in this syndrome. One of the most potent and selective Ca(2+) sensitizers is the thiadiazinone derivative EMD 57033, which increases myocardial contractile function both in vivo and in vitro and interacts with cTnC in vitro. We have determined the NMR structure of the 1:1 complex between Ca(2+)-saturated C-domain of human cTnC (cCTnC) and EMD 57033. Favorable hydrophobic interactions between the drug and the protein position EMD 57033 in the hydrophobic cleft of the protein. The drug molecule is orientated such that the chiral group of EMD 57033 fits deep in the hydrophobic pocket and makes several key contacts with the protein. This stereospecific interaction explains why the (-)-enantiomer of EMD 57033 is inactive. Titrations of the cCTnC.EMD 57033 complex with two regions of cardiac troponin I (cTnI(34-71) and cTnI(128-147)) reveal that the drug does not share a common binding epitope with cTnI(128-147) but is completely displaced by cTnI(34-71). These results have important implications for elucidating the mechanism of the Ca(2+) sensitizing effect of EMD 57033 in cardiac muscle contraction.

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.

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

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

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Role of the structural domain of troponin C in muscle regulation: NMR studies of Ca2+ binding and subsequent interactions with regions 1-40 and 96-115 of troponin I

The interaction between the calcium binding and inhibitory components of troponin is central to the regulation of muscle contraction. In this work, two-dimensional heteronuclear single-quantum coherence nuclear magnetic resonance (2D-¿1H,15N¿-HSQC NMR) spectroscopy was used to determine the stoichiometry, affinity, and mechanisms for binding of Ca2+ and two synthetic TnI peptides [TnI1-40 (or Rp40) and TnI96-115] to the isolated C-domain of skeletal troponin C (CTnC). The Ca2+ titration revealed that 2 equiv of Ca2+ binds to sites III and IV of CTnC with strong positive cooperativity and high affinity [dissociation constant (KD) </= 0.1 microM]. In this process, CTnC folds from a largely unstructured state to a compact domain capable of interacting with TnI. Titration of CTnC x 2Ca2+ with Rp40 occurs with a 1:1 stoichiometry and a KD of 2 +/- 1 microM. Titration of CTnC x 2Ca2+ with a peptide corresponding to the inhibitory region of TnI (TnI96-115) also reveals a 1:1 ratio, but weaker affinity (KD = 47 +/- 7 microM). Both Rp40- and TnI96-115-induced backbone amide chemical shift changes of CTnC x 2Ca2+ are similarly distributed along the sequence, indicating that these two regions of TnI may compete for the same binding site on CTnC x 2Ca2+. The changes induced by Rp40 are much larger, however, and define the interaction sites on TnC and regions where the flexibility of hinge and terminal residues is altered. To investigate the possibility of direct competition, TnI(96-115) was titrated into the CTnC x 2Ca(2+) x Rp40 complex, whereas Rp40 was titrated into the CTnC x 2Ca2+. TnI96-115 complex. The results show that Rp40 can displace TnI96-115 completely, while TnI96-115 has no effect on CTnC x 2Ca2+ x Rp40. Recent proposals for the mechanism of muscle regulation [Tripet, B. P., Van Eyk, J. E., and Hodges, R. S. (1997) J. Mol. Biol. 271, 728-750] suggest that the N-terminal and inhibitory regions of TnI competitively bind the structural domain of TnC. The findings presented here indicate that additional factors, such as interactions between the N-domain of TnC with the C-domain of TnI or the C-domain of TnT, are required, if the inhibitory region is going to successfully compete for the structural domain of TnC.

Cardiac troponin I inhibitory peptide: location of interaction sites on troponin C

Cardiac troponin I(129-149) binds to the calcium saturated cardiac troponin C/troponin I(1-80) complex at two distinct sites. Binding of the first equivalent of troponin I(129-149) was found to primarily affect amide proton chemical shifts in the regulatory domain, while the second equivalent perturbed amide proton chemical shifts within the D/E linker region. Nitrogen-15 transverse relaxation rates showed that binding the first equivalent of inhibitory peptide to the regulatory domain decreased conformational exchange in defunct calcium binding site I and that addition of the second equivalent of inhibitory peptide decreased flexibility in the D/E linker region. No interactions between the inhibitory peptide and the C-domain of cardiac troponin C were detected by these methods demonstrating that the inhibitory peptide cannot displace cTnI(1-80) from the C-domain.

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.

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

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

Troponin I inhibitory peptide (96-115) has an extended conformation when bound to skeletal muscle troponin C

We have utilized CD and NMR spectroscopy to study the conformation of the troponin I (TnI) inhibitory peptide [TnI(96-115)] free in solution and when bound to troponin C (TnC). Analysis of the CD spectrum of the free peptide in aqueous solution indicates it is only approximately 3% helix. Upon complex formation with TnC, there is no change in total helix content compared to the sum of the free components. The NMR data support a predominantly extended conformation for the free peptide. TnI(96-115) bound to TnC was selectively observed by NMR using deuterated TnC (dTnC). For the 1:1 ratio of TnI(96-115) to dTnC used, 95% of the peptide was bound to dTnC. The chemical shifts of the TnC-bound peptide resonances are similar to those of the free peptide, indicating that the change in peptide conformation as a consequence of binding to TnC is small. For the TnC-bound TnI(96-115) peptide, the ratios of sequential Halpha-HN to intraresidue HN-Halpha NOE cross-peak volumes support a predominantly extended conformation, possibly kinked at Gly104. The results presented here are in agreement with sequence analysis predictions for TnI(96-115) as a free peptide or within the intact TnI sequence. The predominantly extended structure for the 96-115 inhibitory sequence segment of TnI with a kink at Gly104 may facilitate its binding alternately to actin or TnC in response to the Ca2+ signals that control thick and thin filament interactions during the contractile cycle.

Defining the region of troponin-I that binds to troponin-C

The kinetics and energetics of the binding of three troponin-I peptides, corresponding to regions 96-131 (TnI96-131), 96-139 (TnI96-139), and 96-148 (TnI96-148), to skeletal chicken troponin-C were investigated using multinuclear, multidimensional NMR spectroscopy. The kinetic off-rate and dissociation constants for TnI96-131 (400 s-1, 32 microM), TnI96-139 (65 s-1, <1 microM), and TnI96-148 (45 s-1, <1 microM) binding to TnC were determined from simulation and analysis of the behavior of 1H,15N-heteronuclear single quantum correlation NMR spectra taken during titrations of TnC with these peptides. Two-dimensional 15N-edited TOCSY and NOESY spectroscopy were used to identify 11 C-terminal residues from the 15N-labeled TnI96-148 that were unperturbed by TnC binding. TnI96-139 labeled with 13C at four positions (Leu102, Leu111, Met 121, and Met134) was complexed with TnC and revealed single bound species for Leu102 and Leu111 but multiple bound species for Met121 and Met134. These results indicate that residues 97-136 (and 96 or 137) of TnI are involved in binding to the two domains of troponin-C under calcium saturating conditions, and that the interaction with the regulatory domain is complex. Implications of these results in the context of various models of muscle regulation are discussed.

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.

Antibody imprint of a membrane protein surface. Phagocyte flavocytochrome b

Structural features of the integral membrane protein flavocytochrome b (Cyt b) were discovered using an antibody "imprint" of the Cyt b surface. Amino acid sequences were selected from a random nonapeptide phage-display library by their affinity for the monoclonal antibody 44.1 binding site, which recognizes the native conformation of the p22 subunit of Cyt b. Transferred nuclear Overhauser effect spectroscopy and rotating frame Overhauser effect spectroscopy NMR were used to study the antibody-bound conformation of a synthetic peptide derived from phage-displayed sequences. The NMR data supported the phage-display analysis suggesting the existence of a complex epitope and allowed the modeling of the close spatial proximity of the epitope components 29TAGRF33 and 183PQVNPI188 from discontinuous regions of p22. Although these regions are separated by two putative membrane-spanning domains and are 150 residues apart in the sequence, they appear to combine to form a complex epitope on the cytosolic surface of the transmembrane protein. NMR constraints, measured from the antibody-bound conformation of a composite peptide mimetic of the Cyt b epitope, and one constraint inferred from the phage-display results, were used to demonstrate the close proximity of these two regions. This information provides a low resolution view of the tertiary structure of the native discontinuous epitope on the Cyt b surface. Given additional antibodies, such imprint analysis has the potential for producing structural constraints to help support molecular modeling of this and other low abundance or noncrystallizable proteins.

Structure and interaction site of the regulatory domain of troponin-C when complexed with the 96-148 region of troponin-I

The structure of the regulatory domain of chicken skeletal troponin-C (residues 1-90) when complexed with the major inhibitory region (residues 96-148) of chicken skeletal troponin-I was determined using multinuclear, multidimensional NMR spectroscopy. This complex represents the first interaction formed between the regulatory domain of troponin-C and troponin-I after calcium binding in the regulation of muscle contraction. The stoichiometry of the complex was determined to be 1:1, with a dissociation constant in the 1-40 microM range. The structure of troponin-C in the complex was calculated from 1039 NMR distance and 111 dihedral angle restraints. When compared to the structure of this domain in the calcium saturated "open" form but in the absence of troponin-I, the bound structure appears to be slightly more "closed". The troponin-I peptide-binding site was found to be in the hydrophobic pocket of calcium saturated troponin-C, using edited/filtered NMR experiments and chemical shift mapping of changes induced in the regulatory domain upon peptide binding. The troponin-I peptide (residues 96-148) was found to bind to the regulatory domain of troponin-C very similarly, but not identically, to a shorter troponin-I peptide (region 115-131) thought to represent the major interaction site of troponin-I for this domain of troponin-C.

The effect of regulatory Ca2+ on the in situ structures of troponin C and troponin I: a neutron scattering study

The effects of regulatory amounts of Ca2+ on the in situ structures of troponin C (TnC) and troponin I (TnI) in whole troponin have been investigated by neutron scattering. In separate difference experiments, 97% deuterated TnC and TnI within whole troponin were studied +/-Ca2+ in 41.6% 2H2O buffers in which protonated subunits were rendered "invisible". We found that the radius of gyration (Rg) of TnI decreased by approximately 10% upon addition of regulatory Ca2+ indicating that it was significantly more compact in the presence of Ca2+. The apparent cross-sectional radius of gyration (Rc) of TnI increased by about 9% when regulatory Ca2+ was bound to TnC. Modeling studies showed that the high-Q scattering patterns of TnI could be fit by a TnI which consisted of two subdomains: one, a highly oblate ellipsoid of revolution containing about 65% of the mass and the other, a highly prolate ellipsoid of revolution consisting of about 35% of the mass. No other fits could be found with this class of models. Best fits were achieved when the axes of revolution of these ellipsoids were steeply inclined with respect to each other. Ca2+ addition decreased the center of mass separation by about 1.5 nm. The Rg of TnI, its high-Q scattering pattern, and the resultant structure were different from previous results on neutron scattering by TnI in the (+Ca2+) TnC.TnI complex. The Rg of TnC indicated that it was elongate in situ. The Rg of TnC was not sensitive to the Ca2+ occupancy of its regulatory sites. However, Rc increased upon Ca2+ addition in concert with expectations from NMR and crystallography of isolated TnC. The present observations indicate that TnI acts like a molecular switch which is controlled by smaller Ca2+-induced changes in TnC.

Crystal structure of troponin C in complex with troponin I fragment at 2.3-A resolution

Troponin (Tn), the complex of three subunits (TnC, TnI, and TnT), plays a key role in Ca2+-dependent regulation of muscle contraction. To elucidate the interactions between the Tn subunits and the conformation of TnC in the Tn complex, we have determined the crystal structure of TnC (two Ca2+ bound state) in complex with the N-terminal fragment of TnI (TnI1-47). The structure was solved by the single isomorphous replacement method in combination with multiple wavelength anomalous dispersion data. The refinement converged to a crystallographic R factor of 22.2% (Rfree = 32.6%). The central, connecting alpha-helix observed in the structure of uncomplexed TnC (TnCfree) is unwound at the center (residues Ala-87, Lys-88, Gly-89, Lys-90, and Ser-91) and bent by 90 degrees. As a result, TnC in the complex has a compact globular shape with direct interactions between the N- and C-terminal lobes, in contrast to the elongated dumb-bell shaped molecule of uncomplexed TnC. The 31-residue long TnI1-47 alpha-helix stretches on the surface of TnC and stabilizes its compact conformation by multiple contacts with both TnC lobes. The amphiphilic C-end of the TnI1-47 alpha-helix is bound in the hydrophobic pocket of the TnC C-lobe through 38 van der Waals interactions. The results indicate the major difference between Ca2+ receptors integrated with the other proteins (TnC in Tn) and isolated in the cytosol (calmodulin). The TnC/TnI1-47 structure implies a mechanism of how Tn regulates the muscle contraction and suggests a unique alpha-helical regulatory TnI segment, which binds to the N-lobe of TnC in its Ca2+ bound conformation.

Insights into tyrosine phosphorylation control of protein-protein association from the NMR structure of a band 3 peptide inhibitor bound to glyceraldehyde-3-phosphate dehydrogenase

A protein-protein association regulated by phosphorylation of tyrosine is examined by NMR structural studies and biochemical studies. Binding of glyceraldehyde-3-phosphate dehydrogenase (G3PDH) and aldolase to the N-terminus of human erythrocyte anion transporter, band 3, inhibits enzyme activity. This inhibition is reversed upon phosphorylation of band 3 Y8, as shown by kinetic studies on purified components, as well as in vivo studies. Thus, tyrosine phosphorylation mediates against the intermolecular protein-protein association, in contrast to the positive control involving SH2 and PTB domains where phosphorylation is required for binding. To elucidate the basis of recognition and negative control by tyrosine phosphorylation, the structure of a synthetic peptide, B3P, corresponding to the first 15 residues of band 3 (MEELQDDYEDMMEEN-NH2), bound to G3PDH has been determined using the exchange-transferred nuclear Overhauser effect. The G3PDH-bound B3P structure was found to be very similar to the structure recognized by aldolase. A hydrophobic triad forms from side chains within a loop structure of residues 4 through 9 in both bound species. Another structural feature stabilizing the loop, in the case of the B3P-G3PDH complex, is a hydrogen bond between the side chains of Y8 and D10 associated with a beta-turn of residues 8-11. Based on the structure of this phosphorylation sensitive interaction (PSI) loop, it is suggested that tyrosine phosphorylation disrupts protein-protein association, in part, by intramolecular electrostatic destabilization. The inhibition by B3P is competitive with respect to the coenzyme NAD+ and noncompetitive with the substrate analog arsenate. Specific binding of B3P to G3PDH is demonstrated by reversion of the NMR spectral properties of bound B3P to those of the free peptide upon addition of coenzyme and substrate analog. The stoichiometry of binding for the B3P-G3PDH complex was determined from Sephadex G-50 displacement experiments to be 4:1. Collectively, these results are consistent with B3P binding the active site of G3PDH.

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+.

Interaction of the second binding region of troponin I with the regulatory domain of skeletal muscle troponin C as determined by NMR spectroscopy

Two dimensional 1H,15N-heteronuclear single quantum correlation NMR was used to monitor the resonance frequency changes of the backbone amide groups belonging to the 15N-labeled regulatory domain of calcium saturated troponin C (N-TnC) upon addition of synthetic skeletal N-acetyl-troponin I 115-131-amide peptide (TnI115-131). Utilizing the change in amide chemical shifts, the dissociation constant for 1:1 binding of TnI115-131 to N-TnC in low salt and 100 mM KCl samples was determined to be 28 +/- 4 and 24 +/- 4 microM, respectively. The off rate of TnI115-131 was determined to be 300 s-1 from observed N-TnC backbone amide 1H,15N-heteronuclear single quantum correlation cross-peak line widths, which is on the order of the calcium off rates (Li, M. X., Gagné, S. M., Tsuda, S., Kay, C. M., Smillie, L. B., and Sykes, B. D. (1995) Biochemistry 34, 8330-8340), and agrees with kinetic expectations for biological regulation of muscle contraction. The TnI115-131 binding site on N-TnC was determined by mapping of chemical shift changes onto the N-TnC NMR structure and was demonstrated to be in the "hydrophobic pocket" (Gagné, S. M., Tsuda, S., Li, M. X., Smillie, L. B., and Sykes, B. D. (1995) Nat. Struct. Biol. 2, 784-789).

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.

Interaction of human neutrophil flavocytochrome b with cytosolic proteins: transferred-NOESY NMR studies of a gp91phox C-terminal peptide bound to p47phox

During activation of the neutrophil NADPH oxidase, cytosolic p47(phox) is translocated to the membrane where it associates with flavocytochrome b via multiple binding regions, including a site in the C-terminus of gp91(phox). To investigate this binding site further, we studied the three-dimensional structure of a gp91(phox) C-terminal peptide (551SNSESGPRGVHFIFNKEN568) bound to p47(phox) using transferred nuclear Overhauser effect spectroscopy (Tr-NOESY) NMR. Using MARDIGRAS analysis and simulated annealing, five similar sets of structures of the p47(phox)-bound peptide were obtained, all containing an extended open bend from Ser5 to Phe14 (corresponding to gp91(phox) residues 555-564). The ends of the peptide were poorly defined, however, suggesting they were more flexible. Therefore further refinement was performed on the Ser5-Phe14 region of the peptide after omitting the ends of the peptide from consideration. In this case, two similar structures were obtained. Both structures again exhibited extended open-bend conformations. In addition, the amino acid side chains that showed evidence of immobilization on binding to p47(phox) correlated directly with those that were found previously to be essential for biological activity. Thus during NADPH oxidase assembly, the C-terminus of gp91(phox) binds to 47(phox) in an extended conformation between gp91(phox) residues 555 and 564, with immobilization of all of the amino acid side chains in the 558RGVHFIF564 region except for His561.

Interactions of structural C and regulatory N domains of troponin C with repeated sequence motifs in troponin I

The actomyosin ATPase inhibitory protein troponin I (TnI) plays a central regulatory role in skeletal and cardiac muscle contraction and relaxation through its calcium-dependent interactions with troponin C (TnC) and actin. Previously we have demonstrated the utility of F29W and F105W mutants of TnC for measurement of binding affinities of inhibitory peptide TnI(96-116) to its regulatory N and structural C domains, both in isolation and in the intact TnC molecule [Pearlstone, J. R. & Smillie, L. B. (1995) Biochemistry 34, 6932-6940]. This approach is now extended to fragment TnI(96-148). Curve-fitting analyses of fluorescence changes induced in the intact TnC mutants and the isolated N and C domains by increasing [TnI(96-148)] have permitted the assignments of K(D) values (designated K(D,N) and K(D,C)) to the interaction of TnI(96-148) with the N and C domains, respectively, of intact TnC. Taken together with the previous data for TnI(96-116) binding, it can be concluded that, within TnI(96-148), residues 96-116 are primarily responsible for binding to C domain of intact TnC and residues 117-148 to its N domain. Inspection of the available mammalian and avian skeletal muscle TnI amino acid sequences reveals a previously unrecognized conserved motif repeated 3-fold, once in the inhibitory peptide region (approximately residues 101-114; designated alpha) and twice more in the region of residues approximately 121-132 (beta) and approximately 135-146 (gamma). The number and distribution of these motifs have important structural implications for the TnI x C complex. In the beta motif of cardiac TnI, as compared with skeletal, several changes in charged amino acids are suggested as candidates responsible for the greater sensitivity of cardiac Ca2+-regulated actomyosin to acidic pH as in ischemia.

Differential binding of desmopressin and vasopressin to neurophysin-II

Desmopressin is a synthetic analog of the peptide hormone vasopressin in which the N-terminal alpha-amino group has been removed and L-arginine in position 8 has been replaced by D-arginine. Using 1H-NMR spectroscopy, we show that desmopressin binds to neurophysin-II, whereas deamino-vasopressin does not bind. Thus, the change in configuration at Arg8 causes a significant difference in the binding of these hormones to neurophysin-II. We have determined the structure of desmopressin bound to neurophysin-II using two-dimensional 1H nuclear magnetic resonance-transferred nuclear Overhauser effect techniques. A common binding motif for vasopressin and desmopressin is proposed that includes a positive charge group along with the hydrophobic surface formed by the side chains of Tyr2 and a beta-methylene provided by Phe-3. In vasopressin, the positive charge is provided by the N-terminal NH3+, whereas in desmopressin, the side chain of Arg-8 contributes the positive charge. The type II beta-turn found in residues Cys6-Pro7-D-Arg8-Gly9 of the bound structure of desmopressin folds the Arg8 side chain back toward the disulfide-bond loop, which allows the positive charged side chain of Arg8 to participate in binding. Such a type II beta-turn is not found in deamino-vasopressin in the presence of neurophysin-II.

Photo-cross-linking of rabbit skeletal troponin I deletion mutants with troponin C and its thiol mutants: the inhibitory region enhances binding of troponin I fragments to troponin C

Contraction of vertebrate striated muscle is regulated by the strong Ca(2+)-dependent interaction between troponin I (TnI) and troponin C (TnC). To critically evaluate this interaction, we generated four recombinant deletion fragments of rabbit fast skeletal TnI: the NH2-terminal fragment (TnI1-94), the NH2 terminus and the inhibitory region (TnI1-120), the inhibitory region and the COOH terminus (TnI96-181), and the COOH-terminal fragment (TnI122-181) containing amino acid residues 1-94, 1-120, 96-181, and 122-181, respectively. Native TnC and seven thiol mutants, containing single cysteine residues in the two globular domains and in the central helix of TnC, e.g., Cys-12, Cys-21, Cys-57, Cys-89, Cys-122, Cys-133, and Cys-158, were labeled with 4-maleimidobenzophenone, and their interaction with the recombinant TnI fragments and the synthetic inhibitory peptide (TnI98-114, residues 98-114) was studied by photo-cross-linking. Extensive cross-linking occurred between various domains of TnC and TnI. The cross-linking patterns (a) showed that both NH2- and COOH-terminal fragments of TnI are accessible to both of the globular domains of TnC, (b) indicated that linkage of the NH2- and COOH-terminal sequences to the inhibitory region of TnI (TnIir) caused marked enhancement of cross-linking with native TnC and all seven thiol mutants, and (c) identified the region in TnC where TnIir binds as that containing residues 98, 133, 158, and 57. Thus, the results suggest that TnI and TnC may adopt flexible and dynamic conformations in which multiple interactions involving various domains of the two polypeptides occur and TnIir acting as a linker facilitates these interactions. The interaction of TnI and its fragments with actin, TnC, and TnT, considered together with the biological activity indicates that residues 96-120 represent a key structural and functional region of TnI. Whereas the NH2-terminal region of TnI stabilizes binding to TnC and TnT, the COOH-terminal region stabilizes TnC and actin binding.

Assignment of the helical structure in neuropeptide Y by HPLC studies of methionine replacement analogues and 1H-NMR spectroscopy

The HPLC retention behavior of three complete single methionine and methionine sulfoxide replacement sets of two 18-mer model peptides and neuropeptide Y (NPY) were investigated. All peptides were prepared by multiple solid-phase peptide synthesis. Plotting the retention time differences between methionine and methionine sulfoxide analogues vs the position of replacement shows that potentially alpha-helical peptides become helical on binding during reversed-phase high performance liquid chromatography. In the case of an amphipathic alpha-helix, the retention time differences change periodically with a 3-4 repeat pattern, which allow the location of amphipathic helical structures. Replacements in nonamphipathic alpha-helical domains cause local preferential binding areas and lead to sequence-dependent retention time profiles. Methionine replacement studies of NPY suggest an unstructured or extended conformation from Tyr1 to Ala12 connected to a well-defined amphipathic alpha-helix from Pro13 to Arg35. The assignment is confirmed by comparison of nuclear Overhauser effects based two-dimensional 1H-nmr spectroscopy and utilization of the C alpha H shift index method in 50% trifluoroethanol/50% water.

Nuclear magnetic resonance (NMR) analysis of ligand receptor interactions: the cholinergic system--a model

Elucidation of the molecular mechanisms that govern ligand-receptor recognition is essential to the rational design of specific pharmacological reagents. Whereas often the receptor and its binding site are the target of investigation, study of the ligand in its free and bound state can also reveal important information regarding this recognition process. Nuclear magnetic resonance (NMR) spectroscopy can be extremely useful for such studies. In this review, we discuss the attributes of NMR in the study of ligand receptor interactions. The cholinergic receptor and its binding to the neurotransmitter, acetylcholine, and cholinergic antagonists serve as a model system, illustrating the power of ligand analysis by NMR. The results discussed prove that the region of residues alpha 180-205 of the nicotinic acetylcholine receptor are an essential component of the cholinergic binding site and that ligand binding involves a positively charged hydrophobic motif.

An NMR and spin label study of the effects of binding calcium and troponin I inhibitory peptide to cardiac troponin C

The paramagnetic relaxation reagent, 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy (HyTEMPO), was used to probe the surface exposure of methionine residues of recombinant cardiac troponin C (cTnC) in the absence and presence of Ca2+ at the regulatory site (site II), as well as in the presence of the troponin I inhibitory peptide (cTnIp). Methyl resonances of the 10 Met residues of cTnC were chosen as spectral probes because they are thought to play a role in both formation of the N-terminal hydrophobic pocket and in the binding of cTnIp. Proton longitudinal relaxation rates (R1's) of the [13C-methyl] groups in [13C-methyl]Met-labeled cTnC(C35S) were determined using a T1 two-dimensional heteronuclear single- and multiple-quantum coherence pulse sequence. Solvent-exposed Met residues exhibit increased relaxation rates from the paramagnetic effect of HyTEMPO. Relaxation rates in 2Ca(2+)-loaded and Ca(2+)-saturated cTnC, both in the presence and absence of HyTEMPO, permitted the topological mapping of the conformational changes induced by the binding of Ca2+ to site II, the site responsible for triggering muscle contraction. Calcium binding at site II resulted in an increased exposure of Met residues 45 and 81 to the soluble spin label HyTEMPO. This result is consistent with an opening of the hydrophobic pocket in the N-terminal domain of cTnC upon binding Ca2+ at site II. The binding of the inhibitory peptide cTnIp, corresponding to Asn 129 through Ile 149 of cTnI, to both 2Ca(2+)-loaded and Ca(2+)-saturated cTnC was shown to protect Met residues 120 and 157 from HyTEMPO as determined by a decrease in their measured R1 values. These results suggest that in both the 2Ca(2+)-loaded and Ca(2+)-saturated forms of cTnC, cTnIp binds primarily to the C-terminal domain of cTnC.

A model structure of the muscle protein complex 4Ca2+.troponin C.troponin I derived from small-angle scattering data: implications for regulation

We report here a model structure for 4Ca2+.troponin C.troponin I derived from small-angle X-ray and neutron scattering data using a Monte Carlo modeling method. In this model, troponin I appears as a spiral structure that wraps around 4Ca2+.troponin C which adopts an extended dumbbell conformation similar to that observed in the crystal structures of troponin C. The troponin I spiral has the approximate dimensions of an alpha-helix and winds through the hydrophobic "cups" in each globular domain of troponin C. The model is consistent with a body of previously published biochemical data on the interactions between troponin C and troponin I, and suggests the molecular mechanism for the Ca(2+)-sensitive switch that regulates the muscle contraction/relaxation cycle involves a signal transmitted via the central spiral region of troponin I.

Protein dynamics

Modern NMR has revitalized the study of protein dynamics. Multidimensional spectra and the heteronuclear spectroscopy allow a substantial gain in resolution. Dynamics can be analyzed at individual sites and data on segmental and sequence-dependent flexibility are accumulating. This review summarizes the wide variety of NMR approaches for observing internal motions, including the folding processes, and the attempts to correlate dynamics to the biological activity of proteins. The implications of mobility on structure determination by NMR is also discussed.