Synthetic peptide analogs of skeletal troponin C: fluorescence studies of analogs of the low-affinity calcium-binding site II

Lanmodulin peptides – unravelling the binding of the EF-Hand loop sequences stripped from the structural corset

Lanmodulin (LanM), a naturally lanthanide (Ln)-binding protein with a remarkable selectivity for Lns over Ca(ii) and affinities in the picomolar range, is an attractive target to address challenges in Ln separation. Why LanM has such a high selectivity is currently not entirely understood; both specific amino acid sequences of the EF-Hand loops and cooperativity effects have been suggested. Here, we removed the effect of cooperativity and synthesised all four 12-amino acid EF-Hand loop peptides, and investigated their affinity for two Lns (Eu(iii) and Tb(iii)), the actinide Cm(iii) and Ca(ii). Using isothermal titration calorimetry and time-resolved laser fluorescence spectroscopy (TRLFS) combined with parallel factor analysis, we show that the four short peptides behave very similarly, having affinities in the micromolar range for Eu(iii) and Tb(iii). Ca(ii) was shown not to bind to the peptides, which was verified with circular dichroism spectroscopy. This technique also revealed an increase in structural organisation upon Eu(iii) addition, which was supported by molecular dynamics simulations. Lastly, we put Eu(iii) and Cm(iii) in direct competition using TRLFS. Remarkably, a slightly higher affinity for Cm(iii) was found. Our results demonstrate that the picomolar affinities in LanM are largely an effect of pre-structuring and therefore a reduction of flexibility in combination with cooperative effects, and that all EF-Hand loops possess similar affinities when detached from the protein backbone, albeit still retaining the high selectivity for lanthanides and actinides over calcium.

Taking a closer look at Lanmodulin’s remarkable selectivity for lanthanides (Ln) over Ca(ii) and high Ln/actinide affinities on the amino acid level by investigating the four binding-loops as peptides with Ca(ii), Eu(iii), Tb(iii) and Cm(iii).

Lanthanide-binding tags as versatile protein coexpression probes

Comprehensive proteomic analyses require new methodologies to accelerate the correlation of gene sequence with protein function. Key tools for such efforts include biophysical probes that integrate into the covalent architecture of proteins. Lanthanide-binding tags (LBTs) are expressible, multitasking fusion partners that are optimized to bind lanthanide ions and have several desirable attributes, which include long-lived luminescence, excellent X-ray scattering power for phase determination, and magnetic properties to facilitate NMR spectroscopic structure elucidation. Herein, we present peptide sequences with a 40-fold higher affinity for Tb(3+) ions and significantly brighter luminescence intensity compared with existing peptides. Incorporation of an LBT onto ubiquitin as a prototype fusion protein allows the use of powerful protein-visualization techniques, which include rapid luminescence detection of LBT-tagged proteins in SDS-PAGE gels, as well as determination of protein concentrations in complex mixtures. The LBT strategy is a new alternative for expressing fluorescent fusion proteins by routine molecular biological techniques.

An interdomain distance in cardiac troponin C determined by fluorescence spectroscopy

The distance between Ca2+-binding site III in the C-terminal domain and Cys35 in the N-terminal domain in cardiac muscle troponin C (cTnC) was determined with a single-tryptophan mutant using bound Tb3+ as the energy donor and iodoacetamidotetramethylrhodamine linked to the cysteine residue as energy acceptor. The luminescence of bound Tb3+ was generated through sensitization by the tryptophan located in the 12-residue binding loop of site III upon irradiation at 295 nm, and this sensitized luminescence was the donor signal transferred to the acceptor. In the absence of bound cation at site II, the mean interdomain distance was found to be 48-49 A regardless of whether the cTnC was unbound or bound to cardiac troponin I, or reconstituted into cardiac troponin. These results suggest that cTnC retains its overall length in the presence of bound target proteins. The distribution of the distances was wide (half-width >9 A) and suggests considerable interdomain flexibility in isolated cTnC, but the distributions became narrower for cTnC in the complexes with the other subunits. In the presence of bound cation at the regulatory site II, the interdomain distance was shortened by 6 A for cTnC, but without an effect on the half-width. The decrease in the mean distance was much smaller or negligible when cTnC was complexed with cTnI or cTnI and cTnT under the same conditions. Although free cTnC has considerable interdomain flexibility, this dynamics is slightly reduced in troponin. These results indicate that the transition from the relaxed state to an activated state in cardiac muscle is not accompanied by a gross alteration of the cTnC conformation in cardiac troponin.

Characterization of cation-binding sequences in the platelet integrin GPIIb-IIIa (alpha IIb beta 3) by terbium luminescence

The binding of cations to purified GPIIb-IIIa (alpha IIb beta 3) and synthetic peptides corresponding to the potential cation-binding sites within this integrin has been assessed by terbium luminescence spectroscopy. Tb3+ supported fibrinogen binding to purified GPIIb-IIIa, at lower concentrations than Ca2+, consistent with its higher affinity for cation-binding motifs. Titration analyses indicated the presence of five Tb(3+)-binding sites of relatively high affinity in the receptor. These sites also could be filled by divalent cations. Six sequences within GPIIb-IIIa have the appropriate spacing of five of the usual six coordination sites for cations in functional Ca(2+)-binding EF-hand motifs. Peptides containing Tyr and/or Trp at selected positions as fluorescence energy donors were synthesized, and their Tb(3+)-binding capacity was assessed. The four potential Ca(2+)-binding sequences in the GPIIb subunit, GPIIb 242-255, 296-309, 364-377, and 425-438, were functional, despite lacking the usual Glu residue at the terminal coordination position. These peptides bound Tb3+ with the same affinity as typical Ca(2+)-binding loop peptides and also bound Ca2+ and other divalent cations without preference. Of the two candidate GPIIIa sequences, 118-131 and 208-221, the former bound Tb3+ and divalent cations with an affinity similar to that of the GPIIb peptides, whereas the latter peptide was not functional. This functional difference, as well as data obtained with substituted peptides, emphasizes the importance of the first coordination position for interaction of synthetic peptide loops with cations. Together, these data identify the five cation-binding sites within intact GPIIb-IIIa.

A possible calcium binding site in animal lectins: a 1H-NMR study of the interaction between lanthanides and a synthetic peptide from a highly conserved domain of Pleurodeles lectin

1H-NMR techniques have been used to study the metal binding properties of a synthetic peptide of 15 amino acids corresponding to a highly conserved domain of Pleurodeles lectin. The addition of lanthanum chloride or praseodymium chloride in a peptide solution induces some conformational changes as displayed by several concerted variations of peptide resonances. The Ln3+ concentration dependence of the chemical shifts was used to calculate the Ln3+ binding constants. The dissociation constants of 95 microM and 280 microM were found for La3+ and Pr3+, respectively.

Stoichiometry of calcium binding to a synthetic heterodimeric troponin-C domain

In this work we describe calcium binding to two synthetic 34-residue peptides, determined by 1H-nmr spectroscopy. The peptides investigated, SCIII and SCIV, encompass the calcium-binding sites III and IV, respectively, of troponin-C. In the absence of calcium it has previously been shown that each of these peptides possesses little regular secondary structure. Further, the 1H-nmr spectra of an equimolar mixture of both of these apo-peptides (apo-SCIII/SCIV) shows that little interaction occurs between peptides. Upon calcium binding the spectral changes that occur to SCIII/SCIV are consistent with global conformational changes in both peptides. We have shown previously that these conformational changes are a product of calcium binding to SCIII and SCIV to form a two-site heterodimer Ca2-SCIII/SCIV. It is proposed that this calcium-induced folding proceeds via calcium binding to SCIII to form Ca-SCIII, peptide association with apo-SCIV to form the heterodimer Ca-SCIII/SCIV, and calcium binding to form Ca2-SCIII/SCIV. The dissociation constants involved in this pathway, K1, Kd, and K2, respectively, have been determined by stoichiometric calcium titration of SCIII/SCIV, monitored by 1H-nmr spectroscopy. Using this procedure it has been determined that K1 = 3 microM, Kd = 10 microM, and K2 = 2 microM.

Luminescence of peptide-bound terbium ions. Determination of binding constants

Luminescence of Tb3+ ions bound to a calmodulin fragment has been studied. It is shown that during their lifetime excited ions dissociate from the peptide. If concentration of free peptide is high enough they can be coordinated again. As a consequence, observed terbium luminescence lifetime and intensity depends not only on binding equilibrium, but also on concentration of free peptide molecules. In such a system terbium binding constant cannot be correctly determined by simple steady-state measurements of luminescence intensities. Instead, terbium luminescence decay curves measured at various peptide concentrations must be analysed. Such an analysis has been made for a fragment of the IIIrd calcium binding domain of rat testis calmodulin. Rate constant of terbium association and the equilibrium binding constant corresponding to the best fit of theoretical functions to experimental points have been determined.

A 1H NMR determination of the solution conformation of a synthetic peptide analogue of calcium-binding site III of rabbit skeletal troponin C

NMR techniques have been used to determine the structure in solution of acetyl (Asp 105) skeletal troponin C (103-115) amide, one of a series of synthetic peptide analogues of calcium-binding site III of rabbit skeletal troponin C [Marsden et al. (1988) Biochemistry 27, 4198-4206]. The NMR measurements include 1H-1H nuclear Overhauser enhancements and gadolinium-induced 1H relaxation measurements. The former yield short-range internuclear distances (less than 4 A); the latter, once properly corrected for chemical exchange, yield longer range metal to proton distances (5-10 A). These measurements were then used as pseudo potential energy restraints in energy minimization and molecular dynamics calculations to determine the solution structure. Further information was provided by NMR coupling constants, amide proton exchange rates, and the temperature dependences of amide proton chemical shifts. The solution structure of the peptide analogue is very similar to that of the calcium-binding loop in the protein, the root-mean-square deviation between the backbone atoms being approximately 1.1 A.

Conformation and ion binding properties of peptides related to calcium binding domain III of bovine brain calmodulin

The conformational and ion binding properties of the sequences 93-104, 96-104, and 93-98 of domain III of bovine brain calmodulin (CaM) have been studied by CD and Tb3+-mediated fluorescence. In aqueous solution the interaction of all fragments with Ca2+ and Mg2+ ions is very weak and without any effect on the peptide conformation, which remains always random. In trifluoroethanol the interaction is very strong and the different fragments exhibit very distinct binding properties. In particular, the dodecapeptide fragment 93-104, and its N-terminal hexapeptide 98-104, bind calcium and magnesium with a very high binding constant (Kb greater than 10(5) M-1), undergoing a substantial conformational change. The structural rearrangement is particularly evident in the hexapeptide fragment, which tend to form a beta-bend. The C-terminal nonapeptide fragment 96-104 interacts with calcium and magnesium more weakly, and the binding process causes a decrease of ordered structure. These results suggest that, even in the entire dodecapeptide sequence corresponding to the loop of domain III of CaM, the calcium binding site is shifted toward the N-terminal hexapeptide segment. This interpretation is consistent with the results of crystallographic studies of CaM, which show that the calcium ions are located toward the amino terminal portion of the loop.

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

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

Total sequential solid phase synthesis of rabbit skeletal troponin C calcium binding site III

The total sequential solid phase synthesis of AcA98STnC(90-123)amide is described. The yield is comparable to that obtained previously using fragment condensation on the solid support. HPLC purification results in a product which displays greater calcium induced conformational changes than the peptide prepared by the fragment condensation method. However, the former has a dissociation constant for calcium (28 microM) seven times higher than the latter (4 microM).

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

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

Peptides related to the calcium binding domains II and III of calmodulin. Synthesis and calmodulin-like features

Three hexadecapeptides which correspond to the putative Ca2+ binding domains II and III of calmodulin were synthesized employing solid phase methodology. One of the peptides contained an internal cystine bridge which was formed while the corresponding linear peptide was still attached to the polymeric carrier. The interaction of the synthetic peptides with calcium ions was investigated using Tb3+-mediated fluorescence. Binding was of the order Ca12 greater than Ca13 greater than Ca13C (Fig. 1) with binding constants KTb3+ = 0.68 X 10(-5), 0.54 X 10(-5), and 0.21 X 10(-5) M-1 respectively. Biological activity of the compounds was assessed by measuring their stimulatory effect on erythrocyte membrane (Ca2+ + Mg2+)-ATPase activity. For 50% activity as compared with CaM, the concentration of peptides required was for Ca12, Ca13 and Ca13C, 50, 100 and 167 times higher than CaM, respectively. The results suggest that the three synthetic peptides possess certain calmodulin-like features.

Investigation of cation-binding properties of cardiac troponin C peptides by circular-dichroism spectroscopy

Bovine cardiac troponin C was cleaved at residues cysteine-35 and cysteine-84. Three peptides, N-terminal (residues 1-34), central (residues 35-83) and C-terminal (residues 84-161), of cardiac troponin C were obtained in a homogeneous state. Saturation of troponin C or its C-terminal peptide with Ca2+ or Mg2+ is accompanied by an increase in the ellipticity at 222 nm in the c.d. spectrum. The half-maximal changes in the ellipticity of troponin C were observed at 32 nM-Ca2+ or 56 microM-Mg2+. The corresponding values for the C-terminal peptide are 7.1 nM for Ca2+ and 4.5 microM for Mg2+. The ellipticity of the central peptide (residues 35-83) containing the second cation-binding site was decreased on saturation with Ca2+. The half-maximal changes in the ellipticity occur at 80 microM-Ca2+. Study of the c.d. spectra suggests that the alpha-helices flanking the second cation-binding site of cardiac troponin C exist independently of Ca2+. Saturation of the third and fourth sites with these cations is associated with a considerable increase in the alpha-helix content, probably due to the formation of an alpha-helix flanking the third site on the N-terminus.