Calcium binding domains of calmodulin. Sequence of fill as determined with terbium luminescence

Spectroscopic characterization of europium binding to a calmodulin-EF4 hand peptide-polymer conjugate

The emergence of biological ligand as an alternative to chemical ligands enables a sustainable lanthanide extraction route. In this study, a peptide originating from the loop of domain 4 calmodulin (EF4) was synthesized and the interaction with europium ions was monitored using time resolved laser fluorescence spectroscopy (TRLFS). Despite being retracted from its full protein structure, the twelve amino acids of calmodulin-EF4 showed binding to europium. Europium-peptide complex formation was evident by an increase in decay time from 110 to 187 μs. The spectra of europium bound to peptide can be easily distinguished from the free europium ion as the 5D0 → 7F2 peak intensifies. When europium bound to the peptide-polymer conjugate, the decay time was further increased to 259 μs. This suggests that lanthanide binding can be enhanced by immobilizing the short peptide into a polymer matrix. The europium-peptide/conjugate bond was reversible, triggered by pH, promoting peptide reusability. Due to the fact that the study was conducted exclusively in water, it suggests minimal use of chemicals is possible while maintaining peptide affinity. This makes the calmodulin-EF4 peptide an ideal candidate as biological ligand. This study lays the groundwork for developing a peptide-based filter material for lanthanide separation.

Mathematical modeling and biochemical analysis support partially ordered calmodulin-myosin light chain kinase binding

Activation of myosin light chain kinase (MLCK) by calcium ions (Ca2+) and calmodulin (CaM) plays an important role in numerous cellular functions including vascular smooth muscle contraction and cellular motility. Despite extensive biochemical analysis, aspects of the mechanism of activation remain controversial, and competing theoretical models have been proposed for the binding of Ca2+ and CaM to MLCK. The models are analytically solvable for an equilibrium steady state and give rise to distinct predictions that hold regardless of the numerical values assigned to parameters. These predictions form the basis of a recently proposed, multi-part experimental strategy for model discrimination. Here we implement this strategy by measuring CaM-MLCK binding using an in vitro FRET system. Interpretation of binding data in light of the mathematical models suggests a partially ordered mechanism for binding CaM to MLCK. Complementary data collected using orthogonal approaches that assess CaM-MLCK binding further support this conclusion.

Distinct mechanism of Tb3+ and Eu3+ binding to NCS1

Lanthanides have been frequently used as biomimetic compounds for NMR and fluorescence studies of Ca2+ binding proteins due to having similar physical properties and coordination geometry to Ca2+ ions. Here we report that a member of the neuronal calcium sensor family, neuronal calcium sensor 1, complexes with two lanthanide ions Tb3+ and Eu3+. The affinity for Tb3+ is nearly 50 times higher than that for Ca2+ (Kd,Tb3+ = 0.002 ± 0.0001 μM and Kd, Ca2+ = 91 nM) whereas Eu3+ binding is notably weaker, Kd,Eu3+ = 26 ± 1 μM. Interestingly, despite having identical charge and similar ionic radii, Tb3+ and Eu3+ ions exhibit a distinct binding stoichiometry for NCS1 with one Eu3+ and two Tb3+ ions bound per NCS1 monomer, as demonstrated in fluorescence titration and mass spectrometry studies. These results suggest that the lanthanides' affinity for the individual EF hands is fine-tuned by a small variation in the ion charge density as well as EF hand binding loop amino acid sequence. As observed previously for other lanthanide:protein complexes, the emission intensity of Ln3+ is enhanced upon complexation with the protein, likely due to the displacement of water molecules by oxygen atoms from the coordinating amino acid residues. The overall shape of the Tb3+NCS1 and Eu3+NCS1 monomer shows high levels of similarity compared to the Ca2+ bound protein based on their collision cross section. However, the distinct occupation of EF hands impacts NCS1 oligomerization and affinity for the D2R peptide that mimics the NCS1 binding site on the D2R receptor. Specifically, the Tb3+NCS1 complex populates the dimer and has comparable affinity for the D2R peptide, whereas Eu3+ bound NCS1 remains in the monomeric form with a negligible affinity for the D2R peptide.

ASAXS measurements on ferritin and apoferritin at the bioSAXS beamline P12 (PETRA III, DESY)

Small-angle X-ray scattering is widely utilized to study biological macromol-ecules in solution. For samples containing specific (e.g. metal) atoms, additional information can be obtained using anomalous scattering. Here, measuring samples at different energies close to the absorption edges of relevant elements provides specific structural details. However, anomalous small-angle X-ray scattering (ASAXS) applications to dilute macromolecular solutions are challenging owing to the overall low anomalous scattering effect. Here, pilot ASAXS experiments from dilute solutions of ferritin and cobalt-loaded apoferritin are reported. These samples were investigated near the resonance X-ray K edges of Fe and Co, respectively, at the EMBL P12 bioSAXS beamline at PETRA III, DESY. Thanks to the high brilliance of the P12 beamline, ASAXS experiments are feasible on dilute protein solutions, allowing one to extract the Fe- or Co-specific anomalous dispersion terms from the ASAXS data. The data were subsequently used to determine the spatial distribution of either iron or cobalt atoms incorporated into the ferritin/apoferritin protein cages.

Comparison of ligand binding and conformational stability of human calmodulin with its homolog from the malaria parasite Plasmodium falciparum

Calmodulin (CaM), the key calcium sensor of eukaryotic cells regulating a great number of target proteins, belongs to the most conserved proteins. We compared function and properties of CaMs from two evolutionarily distant species, the human (Homo sapiens) representing vertebrates, and the malaria parasite Plasmodium falciparum (Pf). The biophysical characterization revealed higher stability of Pf CaM attributed to the more stable C-terminal domain in both Ca2+ free and saturated states. In vitro binding and functional assays demonstrated that human and Pf CaM exhibit similar biochemical features involving small molecule inhibitor binding and target enzyme activation as illustrated by comparable affinities differing only within a factor of three. It has been reported that CaM antagonists proved to be antimalarials, so Pf CaM could be a potential target to combat malaria parasites. Indeed, we observed that phenotypically active compounds from the Malaria Box could show inhibitory action on Pf CaM, among them the most potent exhibited comparable inhibition to known antagonists of vertebrate CaM. However, based on the minor binding differences in Pf CaM to human CaM, we conclude that CaM is an unsuited target for human intervention against malaria, due to the likely interference with the host protein.

Zinc Increases ABCA1 by Attenuating Its Clearance Through the Modulation of Calmodulin Activity

Aim: We previously revealed that Ca⁺⁺-activated calmodulin binds to ABCA1 by the region near the PEST sequence and retards its calpain-mediated degradation to increase HDL biogenesis. Calmodulin activity is reportedly modulated also by other nutritional divalent cations; thus, we attempted to determine whether Zn⁺⁺ is involved in the regulation of ABCA1 stability through the modulation of calmodulin activity.

Methods: The effects of Zn⁺⁺ on ABCA1 expression was investigated in J774 mouse macrophage cell-line cells and HepG2 human hepatoma cell-line cells.

Results: Zn⁺⁺ increased ABCA1 expression, not by increasing the mRNA but by attenuating its decay rate, more prominently in the presence of cAMP. Accordingly, it enhanced cell cholesterol release with extracellular apolipo-protein A-I. Calmodulin binding to ABCA1 was increased by Zn⁺⁺ and Ca⁺⁺. Zn⁺⁺ suppressed calpain-mediated hydrolysis of the peptide of ABCA1 cytosolic loop, including the PEST sequence and the calmodulin-binding site, in a calmodulin-dependent fashion, in the presence of the minimum amount of Ca⁺⁺ to activate calpain, but not calmodulin. Calpain activity was not directly inhibited by Zn⁺⁺ at the concentration for enhancing calmodulin binding to ABCA1.

Conclusion: Nutritional divalent cation Zn⁺⁺ is involved in the regulation of ABCA1 activity and biogenesis of HDL through the modulation of calmodulin activity. The results were consistent with previous clinical findings that Zn⁺⁺ increased plasma HDL in the conditions of sympathetic activation, such as type 2 diabetes and chronic hemodialysis.

Non-Additive Effects of Binding Site Mutations in Calmodulin

Despite decades of research on ion-sensing proteins, gaps persist in the understanding of ion binding affinity and selectivity even in well-studied proteins such as calmodulin. Site-directed mutagenesis is a powerful and popular tool for addressing outstanding questions about biological ion binding and is employed to selectively deactivate binding sites and insert chromophores at advantageous positions within ion binding structures. However, even apparently nonperturbative mutations can distort the binding dynamics they are employed to measure. We use Fourier transform infrared (FTIR) and ultrafast two-dimensional infrared (2D IR) spectroscopy of the carboxylate asymmetric stretching mode in calmodulin as a mutation- and label-independent probe of the conformational perturbations induced in calmodulin's binding sites by two classes of mutation, tryptophan insertion and carboxylate side-chain deletion, commonly used to study ion binding in proteins. Our results show that these mutations not only affect ion binding but also induce changes in calmodulin's conformational landscape along coordinates not probed by vibrational spectroscopy, remaining invisible without additional perturbation of binding site structure. Comparison of FTIR line shapes with 2D IR diagonal slices provides a clear example of how nonlinear spectroscopy produces well-resolved line shapes, refining otherwise featureless spectral envelopes into more informative vibrational spectra of proteins.

Lanthanides compete with calcium for binding to cadherins and inhibit cadherin-mediated cell adhesion

Substitutions with lanthanides reveal a high sensitivity of cadherin structure, dynamics and function to metal ion chemistry.

Coordination to lanthanide ions distorts binding site conformation in calmodulin

The Ca²⁺-sensing protein calmodulin (CaM) is a popular model of biological ion binding since it is both experimentally tractable and essential to survival in all eukaryotic cells. CaM modulates hundreds of target proteins and is sensitive to complex patterns of Ca²⁺ exposure, indicating that it functions as a sophisticated dynamic transducer rather than a simple on/off switch. Many details of this transduction function are not well understood. Fourier transform infrared (FTIR) spectroscopy, ultrafast 2D infrared (2D IR) spectroscopy, and electronic structure calculations were used to probe interactions between bound metal ions (Ca²⁺ and several trivalent lanthanide ions) and the carboxylate groups in CaM's EF-hand ion-coordinating sites. Since Tb³⁺ is commonly used as a luminescent Ca²⁺ analog in studies of protein-ion binding, it is important to characterize distinctions between the coordination of Ca²⁺ and the lanthanides in CaM. Although functional assays indicate that Tb³⁺ fully activates many Ca²⁺-dependent proteins, our FTIR spectra indicate that Tb³⁺, La³⁺, and Lu³⁺ disrupt the bidentate coordination geometry characteristic of the CaM binding sites' strongly conserved position 12 glutamate residue. The 2D IR spectra indicate that, relative to the Ca²⁺-bound form, lanthanide-bound CaM exhibits greater conformational flexibility and larger structural fluctuations within its binding sites. Time-dependent 2D IR lineshapes indicate that binding sites in Ca²⁺-CaM occupy well-defined configurations, whereas binding sites in lanthanide-bound-CaM are more disordered. Overall, the results show that binding to lanthanide ions significantly alters the conformation and dynamics of CaM's binding sites.

Characterization of the Photophysical, Thermodynamic, and Structural Properties of the Terbium(III)-DREAM Complex

DREAM (also known as K(+) channel interacting protein 3 and calsenilin) is a calcium binding protein and an active modulator of KV4 channels in neuronal cells as well as a novel Ca(2+)-regulated transcriptional modulator. DREAM has also been associated with the regulation of Alzheimer's disease through the prevention of presenilin-2 fragmentation. Many interactions of DREAM with its binding partners (Kv4, calmodulin, DNA, and drugs) have been shown to be dependent on calcium. Therefore, understanding the structural changes induced by binding of metals to DREAM is essential for elucidating the mechanism of signal transduction and biological activity of this protein. Here, we show that the fluorescence emission and excitation spectra of the calcium luminescent analogue, Tb(3+), are enhanced upon binding to the EF-hands of DREAM due to a mechanism of energy transfer between Trp and Tb(3+). We also observe that unlike Tb(3+)-bound calmodulin, the luminescence lifetime of terbium bound to DREAM decays as a complex multiexponential (τaverage ∼ 1.8 ms) that is sensitive to perturbation of the protein structure and drug (NS5806) binding. Using isothermal calorimetry, we have determined that Tb(3+) binds to at least three sites with high affinity (Kd = 1.8 μM in the presence of Ca(2+)) and displaces bound Ca(2+) through an entropically driven mechanism (ΔH ∼ 12 kcal mol(-1), and TΔS ∼ 22 kcal mol(-1)). Furthermore, the hydrophobic probe 1,8-ANS shows that Tb(3+), like Ca(2+), triggers the exposure of a hydrophobic surface on DREAM, which modulates ligand binding. Analogous to Ca(2+) binding, Tb(3+) binding also induces the dimerization of DREAM. Secondary structural analyses using far-UV circular dichroism and trapped ion mobility spectrometry-mass spectrometry reveal that replacement of Ca(2+) with Tb(3+) preserves the folding state with minimal changes to the overall structure of DREAM. These findings pave the way for further investigation of the metal binding properties of DREAM using lanthanides as well as the study of DREAM-protein complexes by lanthanide resonance energy transfer or nuclear magnetic resonance.

Characterization of phospho-(tyrosine)-mimetic calmodulin mutants

Calmodulin (CaM) phosphorylated at different serine/threonine and tyrosine residues is known to exert differential regulatory effects on a variety of CaM-binding enzymes as compared to non-phosphorylated CaM. In this report we describe the preparation and characterization of a series of phospho-(Y)-mimetic CaM mutants in which either one or the two tyrosine residues present in CaM (Y99 and Y138) were substituted to aspartic acid or glutamic acid. It was expected that the negative charge of the respective carboxyl group of these amino acids mimics the negative charge of phosphate and reproduce the effects that distinct phospho-(Y)-CaM species may have on target proteins. We describe some physicochemical properties of these CaM mutants as compared to wild type CaM, after their expression in Escherichia coli and purification to homogeneity, including: i) changes in their electrophoretic mobility in the absence and presence of Ca2+; ii) ultraviolet (UV) light absorption spectra, far- and near-UV circular dichroism data; iii) thermal stability in the absence and presence of Ca2+; and iv) Tb3+-emitted fluorescence upon tyrosine excitation. We also describe some biochemical properties of these CaM mutants, such as their differential phosphorylation by the tyrosine kinase c-Src, and their action as compared to wild type CaM, on the activity of two CaM-dependent enzymes: cyclic nucleotide phosphodiesterase 1 (PDE1) and endothelial nitric oxide synthase (eNOS) assayed in vitro.

The Neuronal Calcium Sensor protein Acrocalcin: a potential target of calmodulin regulation during development in the coral Acropora millepora

To understand the calcium-mediated signalling pathways underlying settlement and metamorphosis in the Scleractinian coral Acropora millepora, a predicted protein set derived from larval cDNAs was scanned for the presence of EF-hand domains (Pfam Id: PF00036). This approach led to the identification of a canonical calmodulin (AmCaM) protein and an uncharacterised member of the Neuronal Calcium Sensor (NCS) family of proteins known here as Acrocalcin (AmAC). While AmCaM transcripts were present throughout development, AmAC transcripts were not detected prior to gastrulation, after which relatively constant mRNA levels were detected until metamorphosis and settlement. The AmAC protein contains an internal CaM-binding site and was shown to interact in vitro with AmCaM. These results are consistent with the idea that AmAC is a target of AmCaM in vivo, suggesting that this interaction may regulate calcium-dependent processes during the development of Acropora millepora.

Calcium-dependent association of calmodulin with the rubella virus nonstructural protease domain

The rubella virus (RUBV) nonstructural (NS) protease domain, a Ca(2+)- and Zn(2+)-binding papain-like cysteine protease domain within the nonstructural replicase polyprotein precursor, is responsible for the self-cleavage of the precursor into two mature products, P150 and P90, that compose the replication complex that mediates viral RNA replication; the NS protease resides at the C terminus of P150. Here we report the Ca(2+)-dependent, stoichiometric association of calmodulin (CaM) with the RUBV NS protease. Co-immunoprecipitation and pulldown assays coupled with site-directed mutagenesis demonstrated that both the P150 protein and a 110-residue minidomain within NS protease interacted directly with Ca(2+)/CaM. The specific interaction was mapped to a putative CaM-binding domain. A 32-mer peptide (residues 1152-1183, denoted as RUBpep) containing the putative CaM-binding domain was used to investigate the association of RUBV NS protease with CaM or its N- and C-terminal subdomains. We found that RUBpep bound to Ca(2+)/CaM with a dissociation constant of 100-300 nm. The C-terminal subdomain of CaM preferentially bound to RUBpep with an affinity 12.5-fold stronger than the N-terminal subdomain. Fluorescence, circular dichroism and NMR spectroscopic studies revealed a "wrapping around" mode of interaction between RUBpep and Ca(2+)/CaM with substantially more helical structure in RUBpep and a global structural change in CaM upon complex formation. Using a site-directed mutagenesis approach, we further demonstrated that association of CaM with the CaM-binding domain in the RUBV NS protease was necessary for NS protease activity and infectivity.

Activation of the SK potassium channel-calmodulin complex by nanomolar concentrations of terbium

Small conductance Ca(2+)-activated K(+) (SK) channels sense intracellular Ca(2+) concentrations via the associated Ca(2+)-binding protein calmodulin. Structural and functional studies have revealed essential properties of the interaction between calmodulin and SK channels. However, it is not fully understood how the binding of Ca(2+) to calmodulin leads to channel opening. Drawing on previous biochemical studies of free calmodulin using lanthanide ions as Ca(2+) substitutes, we have used the lanthanide ion, Tb(3+), as an alternative ligand to study the activation properties of SK channels. We found that SK channels can be fully activated by nanomolar concentrations of Tb(3+), indicating an apparent affinity >100-fold higher than Ca(2+). Competition experiments show that Tb(3+) binds to the same sites as Ca(2+) to activate the channels. Additionally, SK channels activated by Tb(3+) demonstrate a remarkably slow deactivation process. Comparison of our results with previous biochemical studies suggests that in the intact SK channel complex, the N-lobe of calmodulin provides ligand-binding sites for channel gating, and that its ligand-binding properties are comparable to those of the N-lobe in isolated calmodulin.

The spectral studies on the effect of Glu 101 to the metal binding characteristic of Euplotes octocarinatus centrin

Glu is highly conserved as the first amino acid of E-helix of the EF-hand protein. In this paper, Glu 101, the first amino acid of E-helix of the third EF-hand motif in Euplotes octocarinatus centrin (EoCen) was mutated to be Lys by the method of site direct mutation. Tb3+ and TNS were used as fluorescence probes in the study of the effect of this mutation to the metal binding characteristic of EoCen by fluorescence spectra. Results indicate that compared with EoCen, the mutation protein (E101K) displays a different Tb3+ binding characteristic and an increased hydrophobic exposure surface. Polyacrylamide gels electrophoresis indicated that the electrophoretic mobilities of EoCen and E101K are distinctly different. It can be deduced that the conformation of EoCen has been altered by this mutation. The general conditional binding constant of Tb3+ to the three loops of EF-hand sites I-III in E101K was calculated to be (5.64+/-0.57)x10(5)M(-1) according to the modified equation of the single binding process.

Characterisation of tyrosine-phosphorylation-defective calmodulin mutants

Using site-directed mutagenesis, we have produced three calmodulin (CaM) mutants in which one or the two tyrosine residues of native CaM were substituted by phenylalanine. The three variants, denoted CaM(Y99F), CaM(Y138F), and CaM(Y99F/Y138F), were highly expressed in transformed Escherichia coli BL21(DE3)pLysS and purified in high yield. The three CaM mutants were able to activate the cyclic nucleotide phosphodiesterase and the plasma membrane Ca(2+)-ATPase, and present the characteristic Ca(2+)-induced electrophoretic mobility shift of native CaM. CaM(Y138F) and CaM(Y99F/Y138F), however, showed a slightly higher electrophoretic mobility than CaM(Y99F) or wild type CaM. The molar extinction coefficient of native CaM at 276 nm decreases 50% in CaM(Y99F) and CaM(Y138F), while the 276nm peak disappears in CaM(Y99F/Y138F). Terbium fluorescence studies with the different CaM mutants indicate that Y99 (but not Y138) closely interacts with Ca(2+) in the III Ca(2+)-binding domain. The epidermal growth factor receptor (EGFR) and the non-receptor tyrosine kinase c-Src phosphorylate CaM(Y99F) and CaM(Y138F) at a lesser extent than wild type CaM, while they fail to phosphorylate CaM(Y99F/Y138F) as expected. All resulting phospho-(Y)CaM species present the characteristic Ca(2+)-induced electrophoretic mobility shift observed in non-phosphorylated CaM. Quantitative analysis of the different phospho-(Y)CaM species suggests that the relative phosphorylation of Y99 and Y138 in wild type CaM by both the EGFR and c-Src is different than the respective phosphorylation of either Y99 in CaM(Y138F) or Y138 in CaM(Y99F).

Calmodulin Mediates Ca2+ Sensitivity of Sodium Channels

Ca2+ has been proposed to regulate Na+ channels through the action of calmodulin (CaM) bound to an IQ motif or through direct binding to a paired EF hand motif in the Nav1 C terminus. Mutations within these sites cause cardiac arrhythmias or autism, but details about how Ca2+ confers sensitivity are poorly understood. Studies on the homologous Cav1.2 channel revealed non-canonical CaM interactions, providing a framework for exploring Na+ channels. In contrast to previous reports, we found that Ca2+ does not bind directly to Na+ channel C termini. Rather, Ca2+ sensitivity appears to be mediated by CaM bound to the C termini in a manner that differs significantly from CaM regulation of Cav1.2. In Nav1.2 or Nav1.5, CaM bound to a localized region containing the IQ motif and did not support the large Ca(2+)-dependent conformational change seen in the Cav1.2.CaM complex. Furthermore, CaM binding to Nav1 C termini lowered Ca2+ binding affinity and cooperativity among the CaM-binding sites compared with CaM alone. Nonetheless, we found suggestive evidence for Ca2+/CaM-dependent effects upon Nav1 channels. The R1902C autism mutation conferred a Ca(2+)-dependent conformational change in Nav1.2 C terminus.CaM complex that was absent in the wild-type complex. In Nav1.5, CaM modulates the Cterminal interaction with the III-IV linker, which has been suggested as necessary to stabilize the inactivation gate, to minimize sustained channel activity during depolarization, and to prevent cardiac arrhythmias that lead to sudden death. Together, these data offer new biochemical evidence for Ca2+/CaM modulation of Na+ channel function.

Metal ion selectivity for formation of the calmodulin-metal-target peptide ternary complex studied by surface plasmon resonance spectroscopy

Ion selectivities for Ca(2+) signaling pathways of 33 metal ions were examined based on the Ca(2+)-dependent on/off switching mechanism of calmodulin (CaM): Ca(2+) ion-induced selective binding of CaM-Ca(2+) ion complex to the target peptide was observed as an increase in surface plasmon resonance (SPR) signals. As the target peptide, M13 of 26-amino-acid residues derived from skeletal muscle myosin light-chain kinase was immobilized in the dextran matrix, over which sample solutions containing CaM and each metal ion were injected in a flow system. Large changes in SPR signals were also observed for Sr(2+), Ba(2+), Cd(2+), Pb(2+), Y(3+) and trivalent lanthanide ions, thereby indicating that not only Ca(2+) but also these metal ions induce the formation of CaM-M13-metal ion ternary complex. No SPR signal was, however, induced by Mg(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+) and all monovalent metal ions examined. The latter silent SPR signal indicates that these ions, even if they bind to CaM, are incapable of forming the CaM-M13-metal ion ternary complex. Comparing the obtained SPR results with ionic radii of those metal ions, it was found that all cations examined with ionic radii close to or greater than that of Ca(2+) induced the formation of the CaM-metal-M13 ternary complex, whereas those with smaller ionic radii were not effective, or much less so. Since these results are so consistent with earlier systematic data for the effects of various metal ions on the conformational changes of CaM, it is concluded that the present SPR analysis may be used for a simple screening and evaluating method for physiologically relevant metal ion selectivity for the Ca(2+) signaling via CaM based on CaM/peptide interactions.

Ion-binding and pharmacological properties of Tyr6 and Tyr9 antamanide analogs

In order to investigate the antiproliferative properties of antamanide, we have synthesized and studied two antamanide analogs where the phenylalanine residue in positions 6 or 9 is substituted by tyrosine, their corresponding linear forms and the cyclic and linear des Phe5,Phe6-Tyr9-analogs. Antamanide and its biologically active synthetic analogs are able to form highly stable complexes with metal ions, particularly Na+, K+ and Ca2+. We studied the ion-binding properties of the Tyr-antamanide analogs by CD and Tb3+ -mediated fluorescence in acetonitrile. In this medium the far-and near-UV CD spectra of the neat Tyr6-antamanide analog are very similar to that of the parent cyclic decapeptide. Substantial differences occur on the contrary in the CD spectra of the neat Tyr9-antamanide, particularly in the regions at 220 nm and 270-290 nm. In acetonitrile, as already found for antamanide, the interaction with the above-mentioned metal ions always produces evident changes in the far- and near-UV CD spectra of both analogs. On the contrary, the CD spectra of the linear deca- and octa- and of the cyclic octa-analogs are affected by the presence of metal ions only in the near-UV region. In the same solvent the Tb3+ -mediated fluorescence spectra of all the synthetic peptides are remarkably affected by the addition of ions. On the basis of the spectral total changes, by using either or both the spectroscopic techniques, it has been possible to determine the ion binding constants for all the linear and cyclic Tyr-antamanide analogs and to compare them with that of the parent peptide. The antitoxic and antiproliferative activities of these antamanide analogs have been tentatively correlated to their ion-binding properties. A preliminary account of this work was given in (1).

Binding of terbium and cisplatin to C13* human ovarian cancer cells using time-resolved terbium luminescence

Terbium (Tb3+) has been shown to increase the cellular accumulation and cytotoxicity of cisplatin in cisplatin-resistant human breast and ovarian cancer cells. Time-resolved Tb3+ luminescence was used to describe the binding of cisplatin to cisplatin-resistant C13* cells. A high-affinity Tb3+ binding site was identified in the plasma membrane of the C13* cells (n=105+/-2 fmol/cell and Kd=36. 3+/-5.2 microM). The binding of Tb3+ is suggested to occur through a cation-pi interaction with tryptophan residues in the plasma membrane, resulting in an enhancement of the intensity and lifetime of Tb3+. Stern-Volmer quenching analysis revealed that the Tb3+ binding site is not readily accessible to the aqueous environment. The quenching of the Tb3+-C13* intensity by cisplatin occurred by static quenching processes, involving both a direct electron-exchange interaction as well as an indirect dipole-dipole resonant energy transfer mechanism. Formation of the Tb3+-C13*-cisplatin complex does not interfere with the high-affinity binding of Tb3+; cisplatin and Tb3+ bind within 5 to 10 A of each other. A specific terbium/cisplatin binding protein is suggested to play a role in the cellular accumulation and cytotoxicity of cisplatin. Therefore, the transport of cisplatin across the plasma membrane must also involve a facilitated diffusion process. Our results indicate that the binding of Tb3+ to the plasma membrane may be potentially useful in the reversal of cisplatin resistance.

Characterization of the single tyrosine containing troponin C from lungfish white muscle. Comparison with several fast skeletal muscle troponin C's from fish species

Troponin C molecules from fast skeletal muscle of the following fish species (trout, whiting, lungfish, tilapia, and cod) have been purified to homogeneity. Upon binding of Ca2+ or Mg2+, lungfish troponin C is the only troponin C from fish white muscle to show the typical increase of tyrosine fluorescence emission quantum yield reported for rabbit fast skeletal muscle troponin C. The increase of tyrosine fluorescence signal occurring upon Ca2+ and Mg2+ titration of lungfish troponin C has been used to determine the corresponding affinity constants. With K(Ca) = 7.0 10(7) M-1 and K(Mg) = 3.6 10(3) M-1, the sites probed by the tyrosine residue of lungfish troponin C are typical of the COOH-terminal domain of fast skeletal troponin C's. The amino acid sequencing of the tyrosine containing tryptic peptides has allowed us to position the single tyrosine residue at position 7 in the Ca2+ binding loop of the third site, in identical position to Tyr109 of troponin C from rabbit fast skeletal muscle. Metal ion binding studies followed by intrinsic fluorescence or Tb3+ luminescence indicate that the conformation of the structural domain of lungfish troponin C with one metal ion bound is close to the physiological conformation of this domain.

Circularly polarized luminescence from terbium(III) as a probe of metal ion binding in calcium-binding proteins

A number of different experimental techniques have been used to probe the details of structural changes on the binding of Ca(II) to the large number of known calcium-binding proteins. The use of luminescent lanthanide(III) ions, especially terbium(III) and europium(III), as substitutional replacement for calcium(II), has led to a number of useful experiments from which important details concerning the metal ion coordination sites have been obtained. This work is concerned with the measurement of the circularly polarized luminescence (CPL) from the 5D4----7F5 transition of Tb(III) bound to the calcium binding sites of bovine trypsin, bovine brain calmodulin, and frog muscle parvalbumin. It is demonstrated that it is possible to make these polarization measurements from very dilute solutions (less than 20 microM) and monitor structural changes as equivalents of Tb(III) are added. It is shown that the two proteins that belong to the class of "EF-hand" structures (calmodulin and parvalbumin) possess quite similar CPL line shapes, whereas Tb(III) bound to trypsin has a much different band structure. CPL results following competitive and consecutive binding of Ca(II) and Tb(III) bound to calmodulin are also reported and yield information concerning known differences between the sequence of binding of these two species.

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.

The binding of terbium ions to gelsolin reveals two classes of metal ion binding sites

Spectroscopically active terbium ions have been used to probe the Ca2+ ion-binding sites on human plasma gelsolin. The luminescence of Tb3+ ions bound to gelsolin is markedly enhanced when excited indirectly at 295 nm due to Förster type dipole-dipole energy transfer from neighboring tryptophan residues. Titration of this luminescence with increasing concentrations of Tb3+ ions was saturable although the shape of this titration curve was complex indicating the involvement of multiple classes of sites. Luminescence lifetime measurements (obtained by indirect excitation at 295 nm) demonstrate the presence of two classes of sites characterized by a major lifetime of 1.0-1.1 ms and a minor lifetime of 0.7-0.8 ms. However, while the amplitude of the minor lifetime showed a hyperbolic dependence on the Tb3+ ion concentration, the amplitude of the major lifetime showed a strongly sigmoidal dependence. Different classes of Tb3+ ion binding sites can also be distinguished by the different Ca2+ ion concentrations needed to displace Tb3+ ions from these sites on gelsolin. It is proposed that the occupancy of one class of Tb3+ ion binding sites on gelsolin causes a conformational change in gelsolin which then allows a second class of cryptic Tb3+ ion binding sites to be expressed. The implications of these results in terms of the binding of Ca2+ ions to gelsolin and the regulation of the activities of gelsolin by calcium are discussed.

Conformational and binding studies on peptides related to domains I and III of calmodulin

The conformational and ion-binding properties of two peptide fragments of 25 amino acid residues corresponding to the helix-loop sequences of domains I and III of calmodulin (CaM) were investigated by CD and Tb(3+)-mediated fluorescence spectroscopy. Both peptides exhibit very similar ion binding properties either in water or trifluoroethanol (TFE), and do not allow the differentiation of the two domains in the native protein in terms of their binding capacity. An aggregation phenomenon was observed in TFE with increase of the alpha-helical content. We suggest that the aggregation involves an interaction between the hydrophilic surfaces of amphiphilic alpha-helices in a way similar to inverse micelle formation.

Terbium simulates the action of calcium on electrophysiological activity in the isolated fish retina

It has been suggested that terbium (Tb) could be substituted for calcium (Ca) in biological tissue and, because of its useful spectroscopic properties, provide an assessment of calcium distribution in the tissue. In order to assess their physiological effects, we have examined the action of free terbium ions on the membrane potential and light-evoked responses of horizontal cells, and on the massed light evoked response (ERG) of the neural retina. Measurements were performed by intracellular and extracellular recording from the isolated fish retina, superfused with Ringers of various ionic compositions, and certain technical difficulties which occurred in the use of terbium are discussed. We show that Tb ions mimic Ca ions in suppression of the ERG and in hyperpolarization of the horizontal cell membrane potential. The effects of Tb persist after its removal from the superfusate for a longer period than do those of calcium, which indicates that terbium binds more firmly at its site(s) activity. Tb and Ca both suppress the light-evoked S-potentials from the horizontal cells, although some differences were observed in the waveform of the S-potentials during the onset of suppression. We conclude that the physiological effects of Tb on the retina are essentially similar to those of Ca, and that Tb could, therefore, prove a useful marker of Ca distribution in neural tissue.

The conformation of the CD3 complex on T lymphocytes is modified by calcium ions

While calcium ions are known to play a prominent role in signal transduction in the activation of T lymphocytes, its mechanism of action, target and function have not been elucidated. One crucial event in the calcium-dependent process is the activation of the CD3 complex, and this, too, is not understood. While studying the release of Ca2+ from intracellular stores by several monoclonal antibodies (mAb) against CD3, we found that one of them, mAb 141, was ineffective unless free Ca2+ was present in the external medium. By flow cytometric analysis of the binding of this mAb to Jurkat T cells and peripheral blood lymphocytes we showed that 141 does not recognize the CD3 complex when external Ca2+ is chelated by EGTA. The binding was restored by addition of Ca2+ but not Mg2+. Finally at least one subunit of the CD3 complex displayed a modified electrophoretic migration rate when immunoprecipitated by Leu-4 in the absence of external free Ca2+. These results suggest that the conformation of the CD3 complex depends on Ca2+, the epitope recognized by 141 being concealed at low Ca2+ concentration.

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.

Activation of calmodulin by the essential trace element chromium

Chromium at very low concentrations is an essential trace element--at higher concentrations it is associated with contact dermatitis and other toxicity problems. Its ionic radius is just outside that of other metal cations which have been found to activate calmodulin in vitro. We found that chromium was able to activate calmodulin at two different concentration ranges--over the micromolar range (which would probably never be achieved in man) a small degree of activation was found--but a much greater activation (76% of the maximum possible) was also found at nanomolar concentrations of chromium. In welders, who work with stainless steel and who were not reporting any physical symptoms of chromium toxicity, red cell chromium levels were 28.2 +/- 3.3 nM (n = 22) compared to 7.5 +/- 0.7 nM (n = 11) for normal controls. Thus, the concentration of chromium experienced within the cell can be of the order which will activate calmodulin in vitro. The possibility exists, therefore, that inappropriate activation of calmodulin could be relevant to chromium biology possibly contributing to the symptoms of chromium toxicity.

Aluminum-induced conformational changes in calmodulin alter the dynamics of interaction with melittin

Studies were undertaken to examine the impact of aluminum-induced structural changes in bovine brain calmodulin on the protein's interface region with melittin, a model for calmodulin's target enzymes. Both steady-state and time-dependent fluorescence characteristics of the single tryptophanyl residue of melittin were employed to derive information on aluminum-related changes in the fluorophore's microenvironment. In the presence of stoichiometric amounts of aluminum ions, calmodulin's target region with melittin appears to be more polar than that with aluminum absent. As a result, upon association of melittin with aluminum-calmodulin, the enhancement of helical arrays is less pronounced. The fluorophore's average microenvironment also is modified such that its apparent lifetime is shortened when aluminum is present. In the presence of aluminum ions, the solvation structure of calmodulin is possibly changed, which may be unfavorable for a proper fit between calmodulin and target proteins.

Calmodulin-mediated cadmium inhibition of phosphodiesterase activity, in vitro

Ion-stripped bovine brain calmodulin (CaM) binds 4 moles Cd2+ as well as 4 moles Ca2+ per mole protein, with similar affinity; in the presence of 1 mM Mg2+ the molar binding ratio of CaM for Ca2+ decreased to 3, the apparent K0.5 for Ca2+ nearly doubled, but the binding characteristics of CaM for Cd2+ were not changed. Saturating concentrations Ca2+ did not affect the molar binding ratio of CaM for Cd2+, but increased the apparent K0.5 for Cd2+; vice versa, saturating concentrations Cd2+ decreased the molar binding ratio for Ca2+ to 2 without affecting the apparent K0.5 for Ca2+. CaM-independent phosphodiesterase (PDE) activity was inhibited at [Cd2+] greater than 10(-5) M. Cd2+-CaM as well as Ca2+-CaM activated PDE. However, the Cd2+-CaM complex is less effective than the Ca2+-CaM complex in stimulating CaM-dependent enzyme activities. Cd2+ inhibits Ca2+- and CaM-dependent PDE in a competitive way. Introduction of Cd2+ in a medium containing Ca2+ and CaM may, therefore, result in a reduction of CaM-dependent enzyme stimulation. By its interference with Ca2+- and CaM- dependent PDE activity, Cd2+ could upset the catabolic pathway of cellular cyclic nucleotide metabolism.

Calcium effects on calmodulin lysine reactivities

The differential reactivities of individual lysines on porcine testicular calmodulin were determined by trace labeling with high specific activity [3H]acetic anhydride as a function of the molar ratio of Ca2+ to calmodulin. In progressing from the Ca2+-depleted form of the protein to a Ca2+:calmodulin molar ratio of 5:1, six of the seven lysyl residues exhibited a modest 1.5- to 3.0-fold increase in reactivity. Lys 75, in contrast, was enhanced in reactivity greater than 20-fold. When the change in reactivity of each lysine was normalized as a percentage of the maximum change, most of the residues were found to fall into two distinct classes. One class, comprising lysines 94 and 148 from the two carboxy terminal Ca2+-binding domains 3 and 4, respectively, exhibited about 90% of their reactivity change when the Ca2+:calmodulin molar ratio was 2:1, and these residues were perturbed very little upon further addition of Ca2+. The other class, encompassing lysines 13, 21, and 30 from the amino terminal domain 1 and Lys 75 from the extended helix connecting the two globular lobes of calmodulin, underwent most of their overall reactivity change (55-70%) between 2 and 5 equivalents of Ca2+ per mol of calmodulin. Lys 77 was distinct in its pattern of change, undergoing approximately equal changes with each Ca2+ increment. These results are consistent with a model where Ca2+ first binds to the two carboxy terminal sites of calmodulin with no apparent preference, concomitant with minor alterations in the microenvironments of lysines in the unoccupied amino terminal domains. The third and fourth Ca2+ ions then bind to these latter two domains, again with no evidence of preference, with little change in the lysine reactivities at the carboxy terminus of the molecule. The environments of groups in the central helix appear to undergo changes in a manner that reflects their proximity to the amino and carboxy terminal domains. In the course of this work, it was found that Lys 94 in apocalmodulin is specifically perturbed by the addition of EGTA, suggesting that the chelating agent may interact with calmodulin at or near the third Ca2+-binding domain.

Kinetics of cadmium and terbium dissociation from calmodulin and its tryptic fragments

The kinetics of cadmium and terbium dissociation from bovine testis calmodulin and its tryptic fragments have been studied by stopped-flow fluorescence methods, using the calcium indicator Quin 2. Studies of the tryptic fragments TR1C and TR2C, comprising the N-terminal or C-terminal half of calmodulin, have clearly identified cadmium binding sites I and II as the low-affinity (rapidly dissociating) sites and sites III and IV as the high-affinity (slowly dissociating) sites. Thus the site preference of cadmium is the same as that of calcium. For terbium, however, sites I and II are the high-affinity sites and sites III and IV are the low-affinity sites. Thus, the site preference or terbium is not the same as that of calcium and cadmium. In contrast to previous studies with calcium, we observe two kinetic processes for dissociation from sites III and IV for experiments with both cadmium and terbium. Possible models for the binding of metal ions are discussed.

Changes in conformation of spin-labeled calmodulin by phospholipids

Spin-labeled calmodulin was synthesized and the effects of phospholipids on its conformation were examined by ESR spectroscopy. Phosphatidylserine (0.1-1.0 mM) increased the signal intensity of the ESR spectrum of spin-labeled calmodulin and decreased the apparent rotational correlation time in the presence of 0.1 mM CaCl2. This change was reversed by addition of excess calcium, and in the absence of calcium phosphatidylserine did not change the spectrum, suggesting that the change in spin-labeled calmodulin brought about by phosphatidylserine was not induced by a hydrophobic interaction of the two, but by inhibition of the binding of calcium to calmodulin. L-Serine and O-phospho-L-serine had no effect on the ESR signals of spin-labeled calmodulin. The effects of various other phospholipids were also examined. Their inhibitory activities were in the order phosphatidic acid greater than phosphatidylserine greater than phosphatidylglycerol = phosphatidylinositol; phosphatidylethanolamine and phosphatidylcholine had no effect on the spectra. The effects of these phospholipids were dependent on their binding activities toward calcium. Furthermore, phosphatidic acid and phosphatidylserine at 1 mM reduced the activity of calmodulin-dependent phosphodiesterase by 16.4 and 8.7%, respectively. These findings indicate that spin-labeled calmodulin did not interact with the phospholipids by a hydrophobic interaction, but that calcium binding to spin-labeled calmodulin interfered with phosphatidic acid, phosphatidylserine, phosphatidylglycerol and phosphatidylinositol, and some of these phospholipids inactivated calmodulin. Thus the activity of calmodulin may be regulated in part by some phospholipids.

Affinity of heavy metal ions to intracellular Ca2+-binding proteins

Parvalbumin, troponin C and vitamin D dependent Ca2+-binding proteins (CaBP type I and II) share the property of calmodulin to interact with some heavy metal ions. In flow dialysis and in spot tests the affinities of Cd2+ and Pb2+ to these proteins were comparable to those of Ca2+. The relative affinities were for calmodulin: Pb2+ greater than Ca2+ greater than Cd2+, for troponin C: Ca2+ greater than Cd2+ greater than Pb2+, for CaBP I: Ca2+ approximately Pb2+, for CaBP II; Ca2+ greater than Pb2+ greater than Cd2+, and for parvalbumin: Cd2+ approximately Ca2+ greater than Pb2+. Upon gel filtration of the supernatant of a pig mucosal homogenate, binding for both Pb2+ and Ca2+ appeared in the MW range of 10,000, together with CaBP II. We conclude that the investigated proteins bind heavy metal ions, in particular Pb2+ and Cd2+, similar to calmodulin. Their role in transport, storage and toxicity remains to be assessed.