Mimicry of the calcium-induced conformational state of troponin C by low temperature under pressure

Charge neutralization as the major factor for the assembly of nucleocapsid-like particles from C-terminal truncated hepatitis C virus core protein

BACKGROUND

Hepatitis C virus (HCV) core protein, in addition to its structural role to form the nucleocapsid assembly, plays a critical role in HCV pathogenesis by interfering in several cellular processes, including microRNA and mRNA homeostasis. The C-terminal truncated HCV core protein (C124) is intrinsically unstructured in solution and is able to interact with unspecific nucleic acids, in the micromolar range, and to assemble into nucleocapsid-like particles (NLPs) in vitro. The specificity and propensity of C124 to the assembly and its implications on HCV pathogenesis are not well understood.

METHODS

Spectroscopic techniques, transmission electron microscopy and calorimetry were used to better understand the propensity of C124 to fold or to multimerize into NLPs when subjected to different conditions or in the presence of unspecific nucleic acids of equivalent size to cellular microRNAs.

RESULTS

The structural analysis indicated that C124 has low propensity to self-folding. On the other hand, for the first time, we show that C124, in the absence of nucleic acids, multimerizes into empty NLPs when subjected to a pH close to its isoelectric point (pH ≈ 12), indicating that assembly is mainly driven by charge neutralization. Isothermal calorimetry data showed that the assembly of NLPs promoted by nucleic acids is enthalpy driven. Additionally, data obtained from fluorescence correlation spectroscopy show that C124, in nanomolar range, was able to interact and to sequester a large number of short unspecific nucleic acids into NLPs.

DISCUSSION

Together, our data showed that the charge neutralization is the major factor for the nucleocapsid-like particles assembly from C-terminal truncated HCV core protein. This finding suggests that HCV core protein may physically interact with unspecific cellular polyanions, which may correspond to microRNAs and mRNAs in a host cell infected by HCV, triggering their confinement into infectious particles.

Probing conformational and functional substates of calmodulin by high pressure FTIR spectroscopy: influence of Ca2+ binding and the hypervariable region of K-Ras4B

Using pressure perturbation, conformational substates of CaM could be uncovered that conceivably facilitate target recognition by exposing the required binding surfaces.

HD exchange and PLIMSTEX determine the affinities and order of binding of Ca2+ with troponin C

Troponin C (TnC), present in all striated muscle, is the Ca(2+)-activated trigger that initiates myocyte contraction. The binding of Ca(2+) to TnC initiates a cascade of conformational changes involving the constituent proteins of the thin filament. The functional properties of TnC and its ability to bind Ca(2+) have significant regulatory influence on the contractile reaction of muscle. Changes in TnC may also correlate with cardiac and various other muscle-related diseases. We report here the implementation of the PLIMSTEX strategy (protein ligand interaction by mass spectrometry, titration, and H/D exchange) to elucidate the binding affinity of TnC with Ca(2+) and, more importantly, to determine the order of Ca(2+) binding of the four EF hands of the protein. The four equilibrium constants, K(1) = (5 ± 5) × 10(7) M(-1), K(2) = (1.8 ± 0.8) × 10(7) M(-1), K(3) = (4.2 ± 0.9) × 10(6) M(-1), and K(4) = (1.6 ± 0.6) × 10(6) M(-1), agree well with determinations by other methods and serve to increase our confidence in the PLIMSTEX approach. We determined the order of binding to the four EF hands to be III, IV, II, and I by extracting from the H/DX results the deuterium patterns for each EF hand for each state of the protein (apo through fully Ca(2+) bound). This approach, demonstrated for the first time, may be general for determining binding orders of metal ions and other ligands to proteins.

Fast pressure jumps can perturb calcium and magnesium binding to troponin C F29W

We have used rapid pressure jump and stopped-flow fluorometry to investigate calcium and magnesium binding to F29W chicken skeletal troponin C. Increased pressure perturbed calcium binding to the N-terminal sites in the presence and absence of magnesium and provided an estimate for the volume change upon calcium binding (-12 mL/mol). We observed a biphasic response to a pressure change which was characterized by fast and slow reciprocal relaxation times of the order 1000/s and 100/s. Between pCa 8-5.4 and at troponin C concentrations of 8-28 muM, the slow relaxation times were invariant, indicating that a protein isomerization was rate-limiting. The fast event was only detected over a very narrow pCa range (5.6-5.4). We have devised a model based on a Monod-Wyman-Changeux cooperative mechanism with volume changes of -9 and +6 mL/mol for the calcium binding to the regulatory sites and closed to open protein isomerization steps, respectively. In the absence of magnesium, we discovered that calcium binding to the C-terminal sites could be detected, despite their position distal to the calcium-sensitive tryptophan, with a volume change of +25 mL/mol. We used this novel observation to measure competitive magnesium binding to the C-terminal sites and deduced an affinity in the range 200-300 muM (and a volume change of +35 mL/mol). This affinity is an order of magnitude tighter than equilibrium fluorescence data suggest based on a model of direct competitive binding. Magnesium thus indirectly modulates binding to the N-terminal sites, which may act as a fine-tuning mechanism in vivo.

Free-energy linkage between folding and calcium binding in EF-hand proteins

Troponin is the singular Ca(2+)-sensitive protein in the contraction of vertebrate striated muscles. Troponin C (TnC), the Ca(2+)-binding subunit of the troponin complex, has two distinct domains, C and N, which have different properties despite their extensive structural homology. In this work, we analyzed the thermodynamic stability of the isolated N-domain of TnC using a fluorescent mutant with Phe 29 replaced by Trp (F29W/N-domain, residues 1-90). The complete unfolding of the N-domain of TnC in the absence or presence of Ca(2+) was achieved by combining high hydrostatic pressure and urea, a maneuver that allowed us to calculate the thermodynamic parameters (DeltaV and DeltaG(atm)). In this study, we propose that part of the affinity for Ca(2+) is contributed by the free-energy change of folding of the N- and C-domains that takes place when Ca(2+) binds. The importance of the free-energy change for the structural and regulatory functions of the TnC isolated domains was evaluated. Our results shed light on how the coupling between folding and ion binding contributes to the fine adjustment of the affinity for Ca(2+) in EF-hand proteins, which is crucial to function.

Volume and free energy of folding for troponin C C-domain: linkage to ion binding and N-domain interaction

Troponin C (TnC) is an 18-kDa acidic protein of the EF-hand family that serves as the trigger for muscle contraction. In this study, we investigated the thermodynamic stability of the C-domain of TnC in all its occupancy states (apo, Mg (2+)-, and Ca (2+)-bound states) using a fluorescent mutant with Phe 105 replaced by Trp (F105W/C-domain, residues 88-162) and (1)H NMR spectroscopy. High hydrostatic pressure was employed as a perturbing agent, in combination with urea or without it. On the basis of changes in Trp emission, the C-domain apo state was denatured by pressure (in the range of 1-1000 bar) in the absence of urea. The fluorescence data were corroborated by following the changes in the (1)H NMR signal of Histidine 128. Addition of Ca (2+) or Mg (2+) increased the C-domain stability so that complete denaturation was attained only by the combined use of high hydrostatic pressure and either 7-8 M or 1.5-2 M urea, respectively. The (1)H NMR spectra in the presence of Ca (2+) was typical of a highly structured protein and allowed us to follow the changes in the local environment of several amino-acid residues as a function of pressure at 4 M Urea. Different residues presented different volume changes, but those that are in the hydrophobic core portrayed values very similar to that obtained for tryptophan 105 as measured by fluorescence, indicating that it is indeed a good probe for the overall tertiary structure. From these experiments, we calculated the thermodynamic parameters (Delta G degrees atm and Delta V) that govern the folding of the C-domain in all its possible physiological states and constructed a thermodynamic cycle. Furthermore, a comparison of the volume and free-energy changes of folding of isolated C-domain with those of intact TnC (F105W) revealed that the N-domain has little effect on the structure of the C-domain, even in the presence of Ca (2+). The volume and free-energy diagrams reveal a landscape of different conformations from the less structured, denatured apo form to the highly structured, Ca (2+)-bound form. The large change in folding free energy of the C-domain that takes place when Ca (2+) binds may explain the much higher Ca (2+) affinity of sites III and IV, 2 orders of magnitude higher than the affinity of sites I and II.

Structures and metal-ion-binding properties of the Ca2+-binding helix–loop–helix EF-hand motifs

The ‘EF-hand’ Ca2+-binding motif plays an essential role in eukaryotic cellular signalling, and the proteins containing this motif constitute a large and functionally diverse family. The EF-hand is defined by its helix–loop–helix secondary structure as well as the ligands presented by the loop to bind the Ca2+ ion. The identity of these ligands is semi-conserved in the most common (the ‘canonical’) EF-hand; however, several non-canonical EF-hands exist that bind Ca2+ by a different co-ordination mechanism. EF-hands tend to occur in pairs, which form a discrete domain so that most family members have two, four or six EF-hands. This pairing also enables communication, and many EF-hands display positive co-operativity, thereby minimizing the Ca2+ signal required to reach protein saturation. The conformational effects of Ca2+ binding are varied, function-dependent and, in some cases, minimal, but can lead to the creation of a protein target interaction site or structure formation from a molten-globule apo state. EF-hand proteins exhibit various sensitivities to Ca2+, reflecting the intrinsic binding ability of the EF-hand as well as the degree of co-operativity in Ca2+ binding to paired EF-hands. Two additional factors can influence the ability of an EF-hand to bind Ca2+: selectivity over Mg2+ (a cation with very similar chemical properties to Ca2+ and with a cytoplasmic concentration several orders of magnitude higher) and interaction with a protein target. A structural approach is used in this review to examine the diversity of family members, and a biophysical perspective provides insight into the ability of the EF-hand motif to bind Ca2+ with a wide range of affinities.

VP4 protein from human rhinovirus 14 is released by pressure and locked in the capsid by the antiviral compound WIN

Rhinoviruses are the major causative agents of the common cold in humans. Here, we studied the stability of human rhinovirus type 14 (HRV14) under conditions of high hydrostatic pressure, low temperature, and urea in the absence and presence of an antiviral drug. Capsid dissociation and changes in the protein conformation were monitored by fluorescence spectroscopy, light scattering, circular dichroism, gel filtration chromatography, mass spectrometry and infectivity assays. The data show that high pressure induces the dissociation of HRV14 and that this process is inhibited by WIN 52084. MALDI-TOF mass spectrometry experiments demonstrate that VP4, the most internal viral protein, is released from the capsid by pressure treatment. This release of VP4 is concomitant with loss of infectivity. Our studies also show that at least one antiviral effect of the WIN drugs involves the locking of VP4 inside the capsid by blocking the dynamics associated with cell attachment.

Role of the undergraduate student research assistant in the new millennium

In this study, we analyze the contribution of the undergraduate student who participates in the process of generating scientific data and developing a research project using Brazilian research as an example. Historically, undergraduate students have performed the critical role of research assistants in developing countries. This aspect has been underappreciated as a means of generating scientific data in Brazilian research facilities. Brazilian educational institutions are facing major age-related generational changes among the science faculty within the next 5-10 yr. A lack of adequate support for graduate students leads to a concern that undergraduates will not be interested in choosing research assistant programs and, subsequently, academic research careers. To remedy this situation it is important to focus on ways to encourage new research careers and enhance university-industry collaborations.

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.

Stabilization of partially folded states in protein folding/misfolding transitions by hydrostatic pressure

In the last few years, hydrostatic pressure has been extensively used in the study of both protein folding and misfolding/aggregation. Compared to other chemical or physical denaturing agents, a unique feature of pressure is its ability to induce subtle changes in protein conformation, which allow the stabilization of partially folded intermediate states that are usually not significantly populated under more drastic conditions (e.g., in the presence of chemical denaturants or at high temperatures). Much of the recent research in the field of protein folding has focused on the characterization of folding intermediates since these species appear to be involved in a variety of disease-causing protein misfolding and aggregation events. The exact mechanisms of these biological phenomena, however, are still poorly understood. Here, we review recent examples of the use of hydrostatic pressure as a tool to obtain insight into the forces and energetics governing the productive folding or the misfolding and aggregation of proteins.

Positive contribution of hydration on DNA binding by E2c protein from papillomavirus

Protein-nucleic acid interactions are responsible for the regulation of key biological events such as genomic transcription and recombination and viral replication. However, the recognition mechanisms involved in these processes are not completely understood. Here, we investigate the dominant forces involved in protein-protein and protein-DNA interactions for the 80-amino-acid C-terminal domain of the E2 protein (E2c) from human papillomavirus (HPV-16). The E2c protein is a homodimer that specifically binds to double-stranded DNA containing the consensus sequence ACCG-N(4)-CGGT, where N is any nucleotide. DNA binding affinity is reduced by lowering water chemical potential, accompanied by an increase in cooperativity. Wyman linkage relations between affinity and water chemical potential indicate that 11 additional water molecules are bound in the formation of the complex between E2c and DNA. Salt dissociation isotherms showed that 10 counterions are released upon association, even at low water activity, indicating that this latter variable does not change the electrostatic component of the interaction. Further analysis demonstrates a strong dependence of cooperativity of binding on the protein concentration. Altogether, these results reveal a novel binding pathway in which the consolidated complex may achieve its final form via a monomer-DNA intermediate, which favors the binding of a second monomer. This molecular mechanism reveals the contributions of multiple conformers in a tight virus genome modulation that seems to be important in the cell infection scenario.

Stability and folding studies of the N-domain of troponin C. Evidence for the formation of an intermediate

We report here on the stability and folding of the 91 residue alpha-helical F29W N-terminal domain of chicken skeletal muscle troponin C (TnC(1-91)F29W), the thin filament calcium-binding component. Unfolding was monitored by differential scanning calorimetry, circular dichroism, and intrinsic fluorescence spectroscopy using urea, pH, and temperature as denaturants, in the absence and in the presence of calcium. The unfolding of TnC(1-91)F29W was reversible and did not follow a two-state transition, suggesting that an intermediate may be present during this reaction. Our results support the hypothesis that intermediates are likely to occur during the folding of small proteins and domains. The physiological significance of the presence of an intermediate in the folding pathway of troponin C is discussed.

Conversion of wild-type p53 core domain into a conformation that mimics a hot-spot mutant

The wild-type p53 protein can be driven into a conformation corresponding to that adopted by structural mutant forms by heterodimerization with a mutant subunit. To seek partially folded states of the wild-type p53 core domain (p53C) we used high hydrostatic pressure (HP) and subzero temperatures. Aggregation of the protein was observed in parallel with its pressure denaturation at 25 and 37 degrees C. However, when HP experiments were performed at 4 degrees C, the extent of denaturation and aggregation was significantly less pronounced. On the other hand, subzero temperatures under pressure led to cold denaturation and yielded a non-aggregated, alternative conformation of p53C. Nuclear magnetic resonance (1H15N-NMR) data showed that the alternative p53C conformation resembled that of the hot-spot oncogenic mutant R248Q. This alternative state was as susceptible to denaturation and aggregation as the mutant R248Q when subjected to HP at 25 degrees C. Together these data demonstrate that wild-type p53C adopts an alternative conformation with a mutant-like stability, consistent with the dominant-negative effect caused by many mutants. This alternative conformation is likely related to inactive forms that appear in vivo, usually driven by interaction with mutant proteins. Therefore, it can be a valuable target in the search for ways to interfere with protein misfolding and hence to prevent tumor development.

Sequence mutations in teleost cardiac troponin C that are permissive of high Ca2+ affinity of site II

Cardiac myofibrils isolated from trout heart have been demonstrated to have a higher sensitivity for Ca(2+) than mammalian cardiac myofibrils. Using cardiac troponin C (cTnC) cloned from trout and mammalian hearts, we have previously demonstrated that this comparatively high Ca(2+) sensitivity is due, in part, to trout cTnC (ScTnC) having twice the Ca(2+) affinity of mammalian cTnC (McTnC) over a broad range of temperatures. The amino acid sequence of ScTnC is 92% identical to McTnC. To determine the residues responsible for the high Ca(2+) affinity, the function of a number of ScTnC and McTnC mutants was characterized by monitoring an intrinsic fluorescent reporter that monitors Ca(2+) binding to site II (F27W). The removal of the COOH terminus (amino acids 90-161) from ScTnC and McTnC maintained the difference in Ca(2+) affinity between the truncated cTnC isoforms (ScNTnC and McNTnC). The replacement of Gln(29) and Asp(30) in ScNTnC with the corresponding residues from McNTnC, Leu and Gly, respectively, reduced Ca(2+) affinity to that of McNTnC. These results demonstrate that Gln(29) and Asp(30) in ScTnC are required for the high Ca(2+) affinity of site II.

Converting troponin C into calmodulin: effects of mutations in the central helix and of changes in temperature

Calmodulin (CaM) and troponin C (TnC) are the most similar members of EF-hand family and show few differences in the primary structure. Here, we use mutants of troponin that mimic calmodulin and changes in temperature to investigate the factors that determine their specificity as regulatory proteins. Using a double mutant of troponin that resembles calmodulin in lacking both the N-terminal helix and KGK(91-93) we observe a small difference from troponin in binding to the erythrocyte Ca(2+)-ATPase, and an improvement in enzyme activation. A triple mutant, where in addition, the residues 88-90 are replaced with the corresponding sequence from calmodulin is equivalent to calmodulin in maximal activation, and it restores protein ability to increase Ca(2+) affinity for the enzyme. However, this mutant also binds less tightly (1/100) than calmodulin. Remarkably, a decrease in temperature has a more marked effect in protein binding than either mutation, reducing the difference in affinities to 18-fold, but without any improvement in their ability to increase Ca(2+) affinity for the enzyme. Spectroscopic analysis of hydrophobic domain exposure in EF-hand proteins was carried out using 8-anilino-1-naphthalenesulfonic acid (ANS). The probe shows a much higher fluorescence when bound to the complex Ca(4)-calmodulin than to Ca(4)-troponin. Decreasing the temperature exposes additional hydrophobic regions of troponin. Changing the Mg(2+) concentration does not affect their bindings to the enzyme. It is suggested that the requirements for troponin to mimic calmodulin in binding to the target enzyme, and those for activating it, are met by different regions of the protein.

Investigation of the effect of high hydrostatic pressure on proteins and lipidic membranes by dynamic fluorescence spectroscopy

Dynamic fluorescence spectroscopy brings new insight into the functional and structural changes of biological molecules under moderate and high hydrostatic pressure. The principles of time-resolved fluorescence methods are briefly described and the resulting type of information is summarized. A first set of selected applications of the use of dynamic fluorescence on pressure effects on proteins in terms of denaturation, ternary and quaternary structure, aggregation and also interaction with DNA are presented. A second set of applications is devoted to the effect of pressure and of cholesterol on lateral heterogeneity of lipidic membranes.

Hydrostatic pressure induces the fusion-active state of enveloped viruses

Enveloped animal viruses must undergo membrane fusion to deliver their genome into the host cell. We demonstrate that high pressure inactivates two membrane-enveloped viruses, influenza and Sindbis, by trapping the particles in a fusion-intermediate state. The pressure-induced conformational changes in Sindbis and influenza viruses were followed using intrinsic and extrinsic fluorescence spectroscopy, circular dichroism, and fusion, plaque, and hemagglutination assays. Influenza virus subjected to pressure exposes hydrophobic domains as determined by tryptophan fluorescence and by the binding of bis-8-anilino-1-naphthalenesulfonate, a well established marker of the fusogenic state in influenza virus. Pressure also produced an increase in the fusion activity at neutral pH as monitored by fluorescence resonance energy transfer using lipid vesicles labeled with fluorescence probes. Sindbis virus also underwent conformational changes induced by pressure similar to those in influenza virus, and the increase in fusion activity was followed by pyrene excimer fluorescence of the metabolically labeled virus particles. Overall we show that pressure elicits subtle changes in the whole structure of the enveloped viruses triggering a conformational change that is similar to the change triggered by low pH. Our data strengthen the hypothesis that the native conformation of fusion proteins is metastable, and a cycle of pressure leads to a final state, the fusion-active state, of smaller volume.

Biological function and site II Ca2+-induced opening of the regulatory domain of skeletal troponin C are impaired by invariant site I or II Glu mutations

To investigate the roles of site I and II invariant Glu residues 41 and 77 in the functional properties and calcium-induced structural opening of skeletal muscle troponin C (TnC) regulatory domain, we have replaced them by Ala in intact F29W TnC and in wild-type and F29W N domains (TnC residues 1-90). Reconstitution of intact E41A/F29W and E77A/F29W mutants into TnC-depleted muscle skinned fibers showed that Ca(2+)-induced tension is greatly reduced compared with the F29W control. Circular dichroism measurements of wild-type N domain as a function of pCa (= -log[Ca(2+)]) demonstrated that approximately 90% of the total change in molar ellipticity at 222 nm ([theta](222 nm)) could be assigned to site II Ca(2+) binding. With E41A, E77A, and cardiac TnC N domains this [theta](222 nm) change attributable to site II was reduced to < or =40% of that seen with wild type, consistent with their structures remaining closed in +Ca(2+). Furthermore, the Ca(2+)-induced changes in fluorescence, near UV CD, and UV difference spectra observed with intact F29W are largely abolished with E41A/F29W and E77A/F29W TnCs. Taken together, the data indicate that the major structural change in N domain, including the closed to open transition, is triggered by site II Ca(2+) binding, an interpretation relevant to the energetics of the skeletal muscle TnC and cardiac TnC systems.

Ca(2+) binding to cardiac troponin C: effects of temperature and pH on mammalian and salmonid isoforms

A reduction in temperature lowers the Ca(2+) sensitivity of skinned cardiac myofilaments but this effect is attenuated when native cardiac troponin C (cTnC) is replaced with skeletal TnC. This suggests that conformational differences between the two isoforms mediate the influence of temperature on contractility. To investigate this phenomenon, the functional characteristics of bovine cTnC (BcTnC) and that from rainbow trout, Oncorhynchus mykiss, a cold water salmonid (ScTnC), have been compared. Rainbow trout maintain cardiac function at temperatures cardioplegic to mammals. To determine whether ScTnC is more sensitive to Ca(2+) than BcTnC, F27W mutants were used to measure changes in fluorescence with in vitro Ca(2+) titrations of site II, the activation site. When measured under identical conditions, ScTnC was more sensitive to Ca(2+) than BcTnC. At 21 degrees C, pH 7.0, as indicated by K(1/2) (-log[Ca] at half-maximal fluorescence, where [Ca] is calcium concentration), ScTnC was 2.29-fold more sensitive to Ca(2+) than BcTnC. When pH was kept constant (7.0) and temperature was lowered from 37.0 to 21.0 degrees C and then to 7.0 degrees C, the K(1/2) of BcTnC decreased by 0.13 and 0.32, respectively, whereas the K(1/2) of ScTnC decreased by 0.76 and 0.42, respectively. Increasing pH from 7.0 to 7.3 at 21.0 degrees C increased the K(1/2) of both BcTnC and ScTnC by 0.14, whereas the K(1/2) of both isoforms was increased by 1.35 when pH was raised from 7.0 to 7.6 at 7.0 degrees C.

Fourier transform ion cyclotron resonance mass spectrometric detection of small Ca(2+)-induced conformational changes in the regulatory domain of human cardiac troponin C

Troponin C (TnC), a calcium-binding protein of the thin filament of muscle, plays a regulatory role in skeletal and cardiac muscle contraction. NMR reveals a small conformational change in the cardiac regulatory N-terminal domain of TnC (cNTnC) on binding of Ca2+ such that the total exposed hydrophobic surface area increases very slightly from 3090 +/- 86 A2 for apo-cNTnC to 3108 +/- 71 A2 for Ca(2+)-cNTnC. Here, we show that measurement of solvent accessibility for backbone amide protons by means of solution-phase hydrogen/deuterium (H/D) exchange followed by pepsin digestion, high-performance liquid chromatography, and electrospray ionization high-field (9.4 T) Fourier transform Ion cyclotron resonance mass spectrometry is sufficiently sensitive to detect such small ligand binding-induced conformational changes of that protein. The extent of deuterium incorporation increases significantly on binding of Ca2+ for each of four proteolytic segments derived from pepsin digestion of the apo- and Ca(2+)-saturated forms of cNTnC. The present results demonstrate that H/D exchange monitored by mass spectrometry can be sufficiently sensitive to detect and identify even very small conformational changes in proteins, and should therefore be especially informative for proteins too large (or too insoluble or otherwise intractable) for NMR analysis.

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.

Cavity defects in the procapsid of bacteriophage P22 and the mechanism of capsid maturation

Bacteriophage P22 belongs to a family of double-stranded DNA viruses that share common morphogenetic features like DNA packaging into a procapsid precursor and maturation. Maturation involves cooperative expansion of the procapsid shell with concomitant lattice stabilization. The expansion is thought to be mediated by movement of two coat protein domains around a hinge. The metastable conformation of subunit within the procapsid lattice is considered to constitute a late folding intermediate. In order to understand the mechanism of expansion it is necessary to characterize the interactions stabilizing procapsid and mature capsid lattices, respectively. We employ pressure dissociation to compare subunit packing within the procapsid and expanded lattice. Procapsid shells contain larger cavities than the expanded shells, presumably due to polypeptide packing defects. These defects contribute to the metastable nature of the procapsid lattice and are cured during expansion. Improved packing contributes to the increased stability of the expanded shell. Comparison of two temperature-sensitive folding (tsf) mutants of coat protein (T294I and W48Q) with wild-type coat revealed that both mutations markedly destabilized the procapsid shell and yet had little effect on relative stability of the monomeric subunit. Thus, the regions affected by these packing defects constitute subunit interfaces of the procapsid shell. The larger activation volume of pressure dissociation observed for both T294I and W48Q indicates that the decreased stability of these particles is due to increase of cavity defects. These defects in the procapsid lattice are cured upon expansion suggesting that the intersubunit contacts affected by tsf mutations are absent or rearranged in the mature shell. The energetics of the in vitro expansion reaction also suggests that entropic stabilization contributes to the large free energy barrier for expansion.

Hydrostatic pressure and calcium-induced dissociation of calpains

The dissociation of mu- and m-calpains was studied by fluorescence spectroscopy under high hydrostatic pressure (up to 650 MPa). Increasing pressure induced a red shift of the tryptophan fluorescence of the calcium-free enzyme. The concentration dependence of the spectral transition was consistent with a pressure-induced dissociation of the subunits. Rising temperature increased the stability of calpain heterodimers and confirmed the predominance of hydrophobic interactions between monomers. At saturating calcium, the spectral transition was not observed for native or iodoacetamide-inactivated calpains, indicating that they were already dissociated by calcium. The reaction volume was about -150 ml mol-1 for both isoforms, and the dissociation constants at atmospheric pressure are approximately 10-12 M and 10-15 M for mu- and m-calpains, respectively. This result indicates a tighter interaction in the isoform that requires higher calcium concentration for activity.

Characterization of a partially folded monomer of the DNA-binding domain of human papillomavirus E2 protein obtained at high pressure

The pressure-induced dissociation of the dimeric DNA binding domain of the E2 protein of human papillomavirus (E2-DBD) is a reversible process with a Kd of 5.6 x 10(-8) M at pH 5.5. The complete exposure of the intersubunit tryptophans to water, together with the concentration dependence of the pressure effect, is indicative of dissociation. Dissociation is accompanied by a decrease in volume of 76 ml/mol, which corresponds to an estimated increase in solvent-exposed area of 2775 A2. There is a decrease in fluorescence polarization of tryptophan overlapping the red shift of fluorescence emission, supporting the idea that dissociation of E2-DBD occurs in parallel with major changes in the tertiary structure. The dimer binds bis(8-anilinonaphthalene-1-sulfonate), and pressure reduces the binding by about 30%, in contrast with the almost complete loss of dye binding in the urea-unfolded state. These results strongly suggest the persistence of substantial residual structure in the high pressure state. Further unfolding of the high pressure state was produced by low concentrations of urea, as evidenced by the complete loss of bis(8-anilinonaphthalene-1-sulfonate) binding with less than 1 M urea. Following pressure dissociation, a partially folded state is also apparent from the distribution of excited state lifetimes of tryptophan. The combined data show that the tryptophans of the protein in the pressure-dissociated state are exposed long enough to undergo solvent relaxation, but the persistence of structure is evident from the observed internal quenching, which is absent in the completely unfolded state. The average rotational relaxation time (derived from polarization and lifetime data) of the pressure-induced monomer is shorter than the urea-denatured state, suggesting that the species obtained under pressure are more compact than that unfolded by urea.

NMR studies of Ca2+ binding to the regulatory domains of cardiac and E41A skeletal muscle troponin C reveal the importance of site I to energetics of the induced structural changes

Ca2+ binding to the N-domain of skeletal muscle troponin C (sNTnC) induces an "opening" of the structure [Gagné, S. M., et al. (1995) Nat. Struct. Biol. 2, 784-789], which is typical of Ca2+-regulatory proteins. However, the recent structures of the E41A mutant of skeletal troponin C (E41A sNTnC) [Gagné, S. M., et al. (1997) Biochemistry 36, 4386-4392] and of cardiac muscle troponin C (cNTnC) [Sia, S. K., et al. (1997) J. Biol. Chem. 272, 18216-18221] reveal that both of these proteins remain essentially in the "closed" conformation in their Ca2+-saturated states. Both of these proteins are modified in Ca2+-binding site I, albeit differently, suggesting a critical role for this region in the coupling of Ca2+ binding to the induced structural change. To understand the mechanism and the energetics involved in the Ca2+-induced structural transition, Ca2+ binding to E41A sNTnC and to cNTnC have been investigated by using one-dimensional 1H and two-dimensional {1H,15N}-HSQC NMR spectroscopy. Monitoring the chemical shift changes during Ca2+ titration of E41A sNTnC permits us to assign the order of stepwise binding as site II followed by site I and reveals that the mutation reduced the Ca2+ binding affinity of the site I by approximately 100-fold [from KD2 = 16 microM [sNTnC; Li, M. X., et al. (1995) Biochemistry 34, 8330-8340] to 1.3 mM (E41A sNTnC)] and of the site II by approximately 10-fold [from KD1 = 1.7 microM (sNTnC) to 15 microM (E41A sNTnC)]. Ca2+ titration of cNTnC confirms that cNTnC binds only one Ca2+ with a determined dissociation constant KD of 2.6 microM. The Ca2+-induced chemical shift changes occur over the entire sequence in cNTnC, suggesting that the defunct site I is perturbed when site II binds Ca2+. These measurements allow us to dissect the mechanism and energetics of the Ca2+-induced structural changes.

Mechanism of direct coupling between binding and induced structural change in regulatory calcium binding proteins

The structural transition in troponin C induced by the binding of two calcium ions involves an "opening" of the structure, an event that triggers skeletal muscle contraction. We have solved the solution structure of a mutant (E41A) of the regulatory domain of skeletal troponin C wherein one bidentate ligand to the calcium in site I is missing. This structure remains "closed" upon calcium binding, indicating that the linkage between calcium binding and the induced conformational change has been broken. This provides a snapshot of skeletal troponin C between the off and on state and thereby valuable insight into the mechanism of regulation within skeletal TnC. Although several factors contribute to the triggering mechanism, the opening of the troponin C structure is ultimately dependent on one amino acid, Glu41. Insights into the structure of cardiac troponin C can also be derived from this skeletal mutant.