Trifluoperazine-induced conformational change in Ca(2+)-calmodulin

Trifluoperazine: a rynodine receptor agonist

Trifluoperazine (TFP), a phenothiazine, is a commonly used antipsychotic drug whose therapeutic effects are attributed to its central anti-adrenergic and anti-dopaminergic actions. However, TFP is also a calmodulin (CaM) antagonist and alters the Ca(2+) binding properties of calsequestrin (CSQ). The CaM and CSQ proteins are known modulators of sarcoplasmic reticulum (SR) Ca(2+) release in ventricular myocytes. We explored TFP actions on cardiac SR Ca(2+) release in cells and single type-2 ryanodine receptor (RyR2) channel activity in bilayers. In intact and permeabilized ventricular myocytes, TFP produced an initial activation of RyR2-mediated SR Ca(2+) release and over time depleted SR Ca(2+) content. At the single channel level, TFP or nortryptiline (NRT; a tricyclic antidepressant also known to modify CSQ Ca(2+) binding) increased the open probability (Po) of CSQ-free channels with an EC(50) of 5.2 microM or 8.9 microM (respectively). This Po increase was due to elevated open event frequency at low drug concentrations while longer mean open events sustained Po at higher drug concentrations. Activation of RyR2 by TFP occurred in the presence or absence of CaM. TFP may also inhibit SR Ca uptake as well as increase RyR2 opening. Our results suggest TFP and NRT can alter RyR2 function by interacting with the channel protein directly, independent of its actions on CSQ or CaM. This direct action may contribute to the clinical adverse cardiac side effects associated with these drugs.

Combined proteomic and cytological analysis of Ca2+-calmodulin regulation in Picea meyeri pollen tube growth

Ca2+-calmodulin (Ca2+-CaM) is a critical molecule that mediates cellular functions by interacting with various metabolic and signaling pathways. However, the protein expression patterns and accompanying serial cytological responses in Ca2+-CaM signaling deficiency remain enigmatic. Here, we provide a global analysis of the cytological responses and significant alterations in protein expression profiles after trifluoperazine treatment in Picea meyeri, which abrogates Ca2+-CaM signaling. Ninety-three differentially displayed proteins were identified by comparative proteomics at different development stages and were assigned to different functional categories closely related to tip growth machinery. The inhibition of Ca2+-CaM signaling rapidly induced an increase in extracellular Ca2+ influx, resulting in dramatically increased cytosolic Ca2+ concentrations and ultrastructural abnormalities in organelles as the primary responses. Secondary and tertiary alterations included actin filament depolymerization, disrupted patterns of endocytosis and exocytosis, and cell wall remodeling, ultimately resulting in perturbed pollen tube extension. In parallel with these cytological events, time-course experiments revealed that most differentially expressed proteins showed time-dependent quantitative changes (i.e. some signaling proteins and proteins involved in organelle functions and energy production changed first, followed by alterations in proteins related to cytoskeletal organization, secretory pathways, and polysaccharide synthesis). Taken together, Ca2+-CaM dysfunction induced serial cytological responses and temporal changes in protein expression profiles, indicating the pivotal role of Ca2+-CaM in the regulation of tip growth machinery.

Interaction of antagonists with calmodulin: insights from molecular dynamics simulations

We report results of 12 ns, all-atom molecular dynamics simulation (MDS) and Poisson-Boltzmann free energy calculations (PBFE) on calmodulin (CaM) bound to two molecules of trifluoperazine (TFP) and of N-(3,3, diphenylpropyl)- N'-[1- R-(3,4-bis-butoxyphenyl)-ethyl]-propylenediamine (DPD). X-ray data show very similar structures for the two complexes, yet the antagonists significantly differ with respect to their CaM binding affinities, the neutral DPD is much more potent. The goal of the study was to unravel the reason why TFP is less potent although its positive charge should facilitate binding. The electrostatic energy terms in CHARMM and binding free energy terms of the PBFE approach showed TFP a better antagonist, while inspection of hydrophobic contacts supports DPD binding. Detailed inspection of the amino acid contributions of PBFE calculations unravel that steric reasons oppose the favorable binding of TFP. Structural conditions are given for a successful drug design strategy, which may benefit also from charge-charge interactions.

Sphingosylphosphorylcholine as a novel calmodulin inhibitor

S1P (sphingosine 1-phosphate) and SPC (sphingosylphosphorylcholine) have been recently recognized as important mediators of cell signalling, regulating basic cellular processes such as growth,differentiation, apoptosis, motility and Ca2+ homoeostasis.Interestingly, they can also act as first and second messengers. Although their activation of cell-surface G-protein-coupled receptors has been studied extensively, not much is known about heir intracellular mechanism of action, and their target proteins are yet to be identified. We hypothesized that these sphingolipids might bind to CaM (calmodulin), the ubiquitous intracellular Ca2+sensor. Binding assays utilizing intrinsic tyrosine fluorescence of the protein, dansyl-labelled CaM and surface plasmon resonance revealed that SPC binds to both apo- and Ca2+-saturated CaM selectively, when compared with the related lysophospholipid mediators S1P, LPA (lysophosphatidic acid) and LPC (lysophosphatidylcholine). Experiments carried out with the model CaM-binding domain melittin showed that SPC dissociates the CaM-target peptide complex, suggesting an inhibitory role. The functional effect of the interaction was examined on two target enzymes, phosphodiesterase and calcineurin, and SPC inhibited the Ca2+/CaM-dependent activity of both. Thus we propose that CaM might be an intracellular receptor for SPC, and raise the possibility of a novel endogenous regulation of CaM.

Characterisation of two calmodulin-like proteins from the liver fluke, Fasciola hepatica

Calmodulin is a calcium ion-sensing signalling protein found in eukaryotics. Two calmodulin-like gene sequences were identified in an EST library from adult liver flukes. One codes for a protein (FhCaM1) homologous to mammalian calmodulins (98% identity), whereas the other protein (FhCaM2) has only 41% identity. These genes were cloned into expression vectors and the recombinant proteins were expressed in Escherichia coli. Gel shift assays showed that both proteins bind to calcium, magnesium and zinc ions. Homology models have been built for both proteins. As expected, FhCaM1 has a highly similar structure to other calmodulins. Although FhCaM2 has a similar fold, its surface charge is higher than FhCaM1. One of the potential metal ion-binding sites has lower metal-ion co-ordination capability, while another has an adjacent lysine residue, both of which may decrease the metal-binding affinity. These differences may reflect a specialised role for FhCaM2 in the liver fluke.

Arylamide derivatives as peptidomimetic inhibitors of calmodulin

[structure: see text] Many peptides bind to calmodulin (CaM) in a helical conformation. Here we describe a group of synthetic inhibitors of CaM based on an arylamide scaffold that is intended to mimic smMLCK, a CaM-binding helical peptide. Compound 1 showed a K(i) value of 7.10 +/- 1.48 nM in a fluorescence polarization assay that monitors the strong association of CaM and its peptide ligand mastoparan X. ((1)H,(15)N)-HSQC NMR spectroscopy experiments suggested that 1 binds to CaM in an analogous fashion to that of smMLCK.

Membrane traffic in outer hair cells of the adult mammalian cochlea

Outer hair cells (OHCs), the sensory-motor cells responsible for the extraordinary frequency selectivity and dynamic range of the cochlea, rapidly endocytose membrane and protein at their apical surface. Endocytosis and transcytosis in isolated OHCs from the mature guinea-pig cochlea were investigated using the amphipathic membrane probe FM1-43. We observed membrane transport from the apical surface to both the basolateral wall and the subnuclear pole. By double-labelling with DiOC6, a stain for endoplasmic reticulum, and aspiration of the plasma membrane, we showed that the basolateral target was the subsurface cisternae. The fluorescent signal was about three times weaker at the basal than at the apical pole. The speed of vesicle transport to the subnuclear pole was approximately 0.4 microm/s. Changing extracellular Ca2+ concentration from 25 microM to 2 mM accelerated rapid endocytosis. Extracellular application of BAPTA-AM (25 microM), an intracellular Ca2+ chelator, and TFP (20 microM), a specific inhibitor of calmodulin, reduced endocytic activity, as did depolarization of the whole cell. The presence of extracellular Cd2+ (200 microM), a Ca2+-channel blocker, had no effect on the voltage dependence of endocytosis at the apical pole, and inhibited the voltage dependence at the subnuclear pole. These results suggest that rapid endocytosis is a Ca2+/calmodulin-dependent process, with extracellular Ca2+ entering through voltage-gated Ca2+ channels at the basal pole. The two distinct destinations of endocytosed membrane are consistent with the functional polarization of the OHC, with the basolateral wall being dedicated to electromechanical transduction and the subnuclear pole being dedicated to electrochemical transduction processes.

Prediction of three dimensional structure of calmodulin

Calmodulin (CaM) is an important human protein, which has multiple structures. Numerous researchers studied the CaM structures in the past, and about 50 different structures in complex with fragments derived from CaM-regulated proteins have been discovered. Discovery and analysis of existing and new CaM structures is difficult due to the inherent complexity, i.e. flexibility of 6 loops and a central linker that constitute part of the CaM structure. The extensive interest in CaM structure analysis and discovery calls for a comprehensive study, which based on the accumulated expertise would design a method for prediction and analysis of future and existing CaM structures. It is also important to find the mechanisms by which the protein adjusts its structure with respect to various factors. To this end, this paper analyzes the known CaM structures and finds four factors that influence CaM structure, which include existence of Ca2+ binding, different binding segments, measuring surroundings, and sequence mutation. The degree of influence of specific factors on different structural regions is also investigated. Based on the analysis of the relation between the four factors and the corresponding CaM structure a novel method for prediction of the CaM structure in complex with novel segments, given that the surroundings of the complex, is developed. The developed prediction method is tested on a set aside, newest CaM structure. The prediction results provide useful and accurate information about the structure verifying high quality of the proposed prediction method and performed structural analysis.

Structure of the Neuronal Protein Calexcitin Suggests a Mode of Interaction in Signalling Pathways of Learning and Memory

The three-dimensional structure of the neuronal calcium-sensor protein calexcitin from Loligo pealei has been determined by X-ray analysis at a resolution of 1.8A. Calexcitin is up-regulated following Pavlovian conditioning and has been shown to regulate potassium channels and the ryanodine receptor. Thus, calexcitin is implicated in neuronal excitation and plasticity. The overall structure is predominantly helical and compact with a pronounced hydrophobic core between the N and C-terminal domains of the molecule. The structure consists of four EF-hand motifs although only the first three EF hands are involved in binding calcium ions; the C-terminal EF-hand lacks the amino acids required for calcium binding. The overall structure is quite similar to that of the sarcoplasmic calcium-binding protein from Amphioxus although the sequence identity is very low at 31%. The structure shows that the two amino acids of calexcitin phosphorylated by protein kinase C are close to the domain interface in three dimensions and thus phosphorylation is likely to regulate the opening of the domains that is probably required for binding to target proteins. There is evidence that calexcitin is a GTPase and the residues, which have been implicated by mutagenesis in its GTPase activity, are in a short but highly conserved region of 3(10) helix close to the C terminus. This helix resides in a large loop that is partly sandwiched between the N and C-terminal domains suggesting that GTP binding may also require or may cause domain opening. The structure possesses a pronounced electropositive crevice in the vicinity of the 3(10) helix, that might provide an initial docking site for the triphosphate group of GTP. These findings elucidate a number of the reported functions of calexcitin with implications for neuronal signalling.

The human Ca2+-activated K+ channel, IK, can be blocked by the tricyclic antihistamine promethazine

Phenothiazines can be used as psychopharmaceutical agents and are known to cause many side effects during treatment since they interfere with many different cellular systems. Recently, phenothiazines were reported to block Ca(2+)-activated potassium channels of the SK type. Therefore we investigated their effect on the functionally related class of Ca(2+)-activated potassium channels of the IK type. The representative phenothiazine derivative promethazine (PTZ) blocked IK channels almost independently from the extracellular pH(o) with an IC(50) of 49 +/- 0.2 microM (pH(o) 7.4, n = 5) and 32 +/- 0.2 microM (pH(o) 6.2, n = 5) in whole cell experiments. The extracellularly applied membrane impermeable PTZ analogue methyl-promethazine (M-PTZ) had a strongly reduced blocking potency compared to PTZ. In contrast, intracellularly applied PTZ and M-PTZ had the same blocking potency on IK channels in excised inside out patch clamp experiments (K(d) = 9.3 +/- 0.5 microM for PTZ, n = 7 and 6.7 +/- 0.4 microM for M-PTZ, n = 5). The voltage dependency of the PTZ and M-PTZ block was investigated in excised inside out patch clamp experiments at a concentration of 100 microM. For both compounds the block was more pronounced at positive membrane potentials. The steepness of the voltage dependency was found to be 70 +/- 10 mV (for PTZ) and 61 +/- 6 mV (for M-PTZ) indicating that both compounds sensed approximately 40% of the entire membrane spanning electrical field from the inside. We conclude that PTZ and M-PTZ bind to a side in IK channels, which is located within the electrical field and is accessible from the intracellular side.

Amplification of Bifunctional Ligands for Calmodulin from a Dynamic Combinatorial Library

A well known strategy to prepare high affinity ligands for a biological receptor is to link together low affinity ligands. DCC (dynamic combinatorial chemistry) was used to select bifunctional protein ligands with high affinity relative to the corresponding monofunctional ligands. Thiol to disulfide linkage generated a small dynamic library of bifunctional ligands in the presence of calmodulin, a protein with two independently mobile domains. The binding constant of the bifunctional ligand (disulfide) most amplified by the presence of calmodulin is nearly two orders of magnitude higher than that of the corresponding monofunctional ligand (thiol).

Can conformational change be described by only a few normal modes?

We suggest a simple method to assess how many normal modes are needed to map a conformational change. By projecting the conformational change onto a subspace of the normal-mode vectors and using root mean square deviation as a test of accuracy, we find that the first 20 modes only contribute 50% or less of the total conformational change in four test cases (myosin, calmodulin, NtrC, and hemoglobin). In some allosteric systems, like the molecular switch NtrC, the conformational change is localized to a limited number of residues. We find that many more modes are necessary to accurately map this collective displacement. In addition, the normal-mode "spectra" can provide useful information about the details of the conformational change, especially when comparing structures with different bound ligands, in this case, calmodulin. Indeed, this approach presents normal-mode analysis as a useful basis in which to capture the mechanism of conformational change, and shows that the number of normal modes needed to capture the essential collective motions of atoms should be chosen according to the required accuracy.

Calmodulin antagonists: Structure of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) and comparison with trifluoperazine (TFP) — Calmodulin binding

The crystal structure of the title compound contains one N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride molecule in the asymmetric unit. The molecule adopts an extended conformation with a linear hexyl group. Protonation occurs at the side chain terminal nitrogen atom. Hydrophobic packing and a three-dimensional hydrogen-bond network, involving all the hydrogen atoms capable of making hydrogen-bond contacts, stabilizes the crystal structure. Due to head-to-head and tail-to-tail arrangement of these hydrophobic molecules, an unusually long cell constant (b = 61.27 Å) characterizes the crystal structure. A stereochemical comparison with trifluoperazine suggests similar calmodulin binding mechanisms.Key words: crystal structure, stereochemistry, W-7, calmodulin binding, TFP.

NMR and molecular dynamics studies of the interaction of melatonin with calmodulin

Pineal hormone melatonin (N-acetyl-5-methoxytryptamine) is thought to modulate the calcium/calmodulin signaling pathway either by changing intracellular Ca(2+) concentration via activation of its G-protein-coupled membrane receptors, or through a direct interaction with calmodulin (CaM). The present work studies the direct interaction of melatonin with intact calcium-saturated CaM both experimentally, by fluorescence and nuclear magnetic resonance spectroscopies, and theoretically, by molecular dynamics simulations. The analysis of the experimental data shows that the interaction is calcium-dependent. The affinity, as obtained from monitoring (15)N and (1)H chemical shift changes for a melatonin titration, is weak (in the millimolar range) and comparable for the N- and C-terminal domains. Partial replacement of diamagnetic Ca(2+) by paramagnetic Tb(3+) allowed the measurement of interdomain NMR pseudocontact shifts and residual dipolar couplings, indicating that each domain movement in the complex is not correlated with the other one. Molecular dynamics simulations allow us to follow the dynamics of melatonin in the binding pocket of CaM. Overall, this study provides an example of how a combination of experimental and theoretical approaches can shed light on a weakly interacting system of biological and pharmacological significance.

The Structure of the Complex of Calmodulin with KAR-2

3'-(beta-Chloroethyl)-2',4'-dioxo-3,5'-spiro-oxazolidino-4-deacetoxyvinblastine (KAR-2) is a potent anti-microtubular agent that arrests mitosis in cancer cells without significant toxic side effects. In this study we demonstrate that in addition to targeting microtubules, KAR-2 also binds calmodulin, thereby countering the antagonistic effects of trifluoperazine. To determine the basis of both properties of KAR-2, the three-dimensional structure of its complex with Ca(2+)-calmodulin has been characterized both in solution using NMR and when crystallized using x-ray diffraction. Heterocorrelation ((1)H-(15)N heteronuclear single quantum coherence) spectra of (15)N-labeled calmodulin indicate a global conformation change (closure) of the protein upon its binding to KAR-2. The crystal structure at 2.12-A resolution reveals a more complete picture; KAR-2 binds to a novel structure created by amino acid residues of both the N- and C-terminal domains of calmodulin. Although first detected by x-ray diffraction of the crystallized ternary complex, this conformational change is consistent with its solution structure as characterized by NMR spectroscopy. It is noteworthy that a similar tertiary complex forms when calmodulin binds KAR-2 as when it binds trifluoperazine, even though the two ligands contact (for the most part) different amino acid residues. These observations explain the specificity of KAR-2 as an anti-microtubular agent; the drug interacts with a novel drug binding domain on calmodulin. Consequently, KAR-2 does not prevent calmodulin from binding most of its physiological targets.

Comparison of negative and positive ion electrospray ionization mass spectra of calmodulin and its complex with trifluoperazine

The protein calmodulin (apoCaM) undergoes a conformational change when it binds calcium. This structure of the protein (Ca4CaM) is a dumbbell-shaped molecule that undergoes a further profound conformational change on binding of the antipsychotic drug trifluoperazine (TFP). Experimental conditions were developed to prepare samples of apoCaM, Ca4CaM and Ca4CaM/TFP that were substantially free of sodium. The effects of the conformational changes of calmodulin on the charge-state distributions observed in positive ion and negative ion electrospray ionization (ESI) mass spectra were examined. Conversion of apoCaM into Ca4CaM was concomitant with a change in the negative ion ESI mass spectrum whereby the 16- ion was the most abundant ion observed for the apo form and the 8- ion was the most abundant for the complex. In contrast, in the positive ion ESI mass spectra of apoCaM and Ca4CaM, the most abundant species in each case was the 8+ ion. When a complex of Ca4CaMwith TFP was prepared, the most abundant species was the 5+ ion. This is consistent with a conformational change of Ca4CaM that rendered some basic sites inaccessible to ionization in the ESI process. Using the same Ca4CaM/TFP mixture, no complex with TFP was observed in negative ion ESI mass spectra. These observations are discussed in the context of the structural changes that are known to occur in calmodulin, and suggestions are made to explain the apparently conflicting data. The results reported here reflect on the validity of using differences in charge-state distributions observed in ESI mass spectra to assess conformational changes in proteins.

Enhanced Ligand Affinity for Receptors in which Components of the Binding Site Are Independently Mobile

Using calmodulin antagonism as a model, it is demonstrated that, under circumstances in which binding sites are motionally independent, it is possible to create bifunctional ligands that bind with significant affinity enhancement over their monofunctional counterparts. Suitable head groups were identified by using a semiquantitative screen of monofunctional tryptophan analogs. Two bifunctional ligands, which contained two copies of the highest-affinity head group tethered by rigid linkers, were synthesized. The bifunctional ligands bound to calmodulin with a stoichiometry of 1:1 and with an affinity enhancement over their monofunctional counterparts; the latter bound with a stoichiometry of 2:1 ligand:protein. A lower limit to the effective concentrations of the domains of calmodulin relative to each other (0.2-2 mM) was determined. A comparable effective concentration was achieved for bifunctional ligands based on higher-affinity naphthalene sulphonamide derivatives.

Evaluation of the relative stability of liganded versus ligand-free protein conformations using Simplicial Neighborhood Analysis of Protein Packing (SNAPP) method

Many proteins change their conformation upon ligand binding. For instance, bacterial periplasmic binding proteins (bPBPs), which transport nutrients into the cytoplasm, generally consist of two globular domains connected by strands, forming a hinge. During ligand binding, hinge motion changes the conformation from the open to the closed form. Both forms can be crystallized without a ligand, suggesting that the energy difference between them is small. We applied Simplicial Neighborhood Analysis of Protein Packing (SNAPP) as a method to evaluate the relative stability of open and closed forms in bPBPs. Using united residue representation of amino acids, SNAPP performs Delaunay tessellation of the protein, producing an aggregate of space-filling, irregular tetrahedra with nearest neighbor residues at the vertices. The SNAPP statistical scoring function is derived from log-likelihood scores for all possible quadruplet compositions of amino acids found in a representative subset of the Protein Data Bank, and the sum of the scores for a given protein provides the total SNAPP score. Results of scoring for bPBPs suggest that in most cases, the unliganded form is more stable than the liganded form, and this conclusion is corroborated by similar observations of other proteins undergoing conformation changes upon binding their ligands. The results of these studies suggest that the SNAPP method can be used to predict the relative stability of accessible protein conformations. Furthermore, the SNAPP method allows delineation of the role of individual residues in protein stabilization, thereby providing new testable hypotheses for rational site-directed mutagenesis in the context of protein engineering.

Interaction of two phenothiazine derivatives with phospholipid monolayers

This paper addresses the cooperative interaction of two phenothiazine drugs, viz. trifluoperazine (TFP) and chlorpromazine (CPZ), with phospholipid monolayers as the model membrane system. Surface pressure and surface potential isotherms were obtained for mixed Langmuir monolayers of either dipalmitoyl-phosphatidyl-choline (DPPC) or dipalmitoyl-phosphatidyl-glycerol (DPPG) co-spread with TFP or CPZ. The changes in monolayer behavior caused by incorporation of a few molar ratio of drug molecules were practically within the experimental dispersion for the zwitterionic DPPC, and therefore a more refined analysis will be required to probe the interactions in an unequivocal way. For the charged DPPG, on the other hand, the surface pressure and the dipole moment were significantly affected even for TFP or CPZ concentrations as low as 0.002 molar ratio. Overall, the effects from CPZ and TFP are similar, but small differences exist which are probably due to the different protonation properties of the two drugs. For both drugs, changes are more prominent at the liftoff of the surface pressure, i.e. at the gas-condensed phase transition, with the surface pressure and surface potential isotherms becoming more expanded with the drug incorporation. With DPPG/CPZ monolayers, in particular, an additional phase transition appears at higher CPZ concentrations, which resembles the effects from increasing the subphase temperature for a pure DPPG monolayer. The dipole moment for DPPG/CPZ and DPPG/TFP monolayers decreases with the drug concentration, which means that the effects from the charged drugs are not associated with changes in the double-layer potential. Otherwise, the effective dipole moment should increase with the drug concentration. The changes caused in surface pressure and dipole moment by small concentrations of TFP or CPZ can only be explained by some cooperative effect through which the contribution from DPPG molecules changes considerably, i.e. even DPPG molecules that are not neighbor to a CPZ or TFP molecule are also affected. Such changes may occur either through a significant reorientation of the DPPG molecules or to a change in their hydration state. We discuss the cooperativity semi-quantitatively by estimating the number of lipid molecules affected by the drug interaction. CPZ and TFP also affect the morphology of DPPG monolayers, which was confirmed with Brewster angle microscopy. The biological implications from the cooperative, non-specific interaction of CPZ and TFP with membranes are also commented upon.

Investigation into the Applicability of the Centrifugal Microfluidics Platform for the Development of Protein−Ligand Binding Assays Incorporating Enhanced Green Fluorescent Protein as a Fluorescent Reporter

The incorporation of a protein-ligand binding assay into a centrifugal microfluidics platform is described. The platform itself is a disc-shaped polymer substrate, upon which a series of microfluidic channels and reservoirs have been machined. Centrifugal microfluidics platforms require no internal moving parts, and fluid propulsion is achieved solely through rotation of the disc. Fluid flow is controlled by passive valves, the opening of which is dependent on the angular frequency of the rotating platform, the channel dimensions, and the physical properties of the fluid. To evaluate the effectiveness of incorporating a protein-based assay onto the centrifugal microfluidics analytical platform, a class-selective, homogeneous assay for the detection of phenothiazine antidepressants was employed. This class of drugs is known to bind to calmodulin, a calcium binding protein. Specifically, a fusion protein between calmodulin and enhanced green fluorescent protein was utilized. Calmodulin undergoes a conformational change upon binding to phenothiazines that alters the fluorescence properties of the attached fluorescent protein, which can be correlated to the concentration of the drug present. Another important aspect of this work was to study the efficacy of the platform to perform reconstitution assays. To do this, the biological reagent was dried on the platform and rehydrated to carry out the assay. The ability to prealiquot reagents on the platform should enhance its versatility and portability. The integration of protein-based assays in this platform should be useful in the design of analytical systems for high-throughput screening of pharmaceuticals and clinical diagnostics.

A New Curcumin Derivative, HBC, Interferes with the Cell Cycle Progression of Colon Cancer Cells via Antagonization of the Ca2+/Calmodulin Function

HBC (4-[3,5-Bis-[2-(4-hydroxy-3-methoxy-phenyl)-ethyl]-4,5-dihydro-pyrazol-1-yl]-benzoic acid) is a recently developed curcumin derivative which exhibits potent inhibitory activities against the proliferation of several tumor cell lines. In the present study, we identified Ca2+/calmodulin (Ca2+/CaM) as a direct target protein of HBC using phage display biopanning. Ca2+/CaM-expressing phages specifically bound to the immobilized HBC, and the binding was Ca2+ dependent. Moreover, flexible docking modeling demonstrated that HBC is compatible with the binding cavity for a known inhibitor, W7, in the C-terminal hydrophobic pocket of Ca2+/CaM. In biological systems, HBC induced prolonged phosphorylation of ERK1/2 and activated p21(WAF1) expression, resulting in the induction of G0/G1 cell cycle arrest in HCT15 colon cancer cells. These results suggest that HBC inhibits the cell cycle progression of colon cancer cells via antagonizing of Ca2+/CaM functions.

A molecular dynamics study of Ca(2+)-calmodulin: evidence of interdomain coupling and structural collapse on the nanosecond timescale

A 20-ns molecular dynamics simulation of Ca(2+)-calmodulin (CaM) in explicit solvent is described. Within 5 ns, the extended crystal structure adopts a compact shape similar in dimension to complexes of CaM and target peptides but with a substantially different orientation between the N- and C-terminal domains. Significant interactions are observed between the terminal domains in this compact state, which are mediated through the same regions of CaM that bind to target peptides derived from protein kinases and most other target proteins. The process of compaction is driven by the loss of helical structure in two separate regions between residues 75-79 and 82-86, the latter being driven by unfavorable electrostatic interactions between acidic residues. In the first 5 ns of the simulation, a substantial number of contacts are observed between the first helix of the N-terminal domain and residues 74-77 of the central linker. These contacts are correlated with the closing of the second EF-hand, indicating a mechanism by which they can lower calcium affinity in the N-terminal domain.

A closed compact structure of native Ca(2+)-calmodulin

Calmodulin has been a subject of intense scrutiny since its discovery because of its unusual properties in regulating the functions of about 100 diverse target enzymes and structural proteins. The original and to date only crystal conformation of native eukaryotic Ca(2+)-calmodulin (Ca(2+)-CaM) is a very extended molecule with two widely separated globular domains linked by an exposed long helix. Here we report the 1.7 A X-ray structure of a new native Ca(2+)-CaM that is in a compact ellipsoidal conformation and shows a sharp bend in the linker helix and a more contracted N-terminal domain. This conformation may offer advantages for recognition of kinase-type calmodulin targets or small organic molecule drugs.

Crystal structure of a myristoylated CAP-23/NAP-22 N-terminal domain complexed with Ca2+/calmodulin

A variety of viral and signal transduction proteins are known to be myristoylated. Although the role of myristoylation in protein-lipid interaction is well established, the involvement of myristoylation in protein-protein interactions is less well understood. CAP-23/NAP-22 is a brain-specific protein kinase C substrate protein that is involved in axon regeneration. Although the protein lacks any canonical calmodulin (CaM)-binding domain, it binds CaM with high affinity. The binding of CAP-23/NAP-22 to CaM is myristoylation dependent and the N-terminal myristoyl group is directly involved in the protein-protein interaction. Here we show the crystal structure of Ca2+-CaM bound to a myristoylated peptide corresponding to the N-terminal domain of CAP-23/NAP-22. The myristoyl moiety of the peptide goes through a hydrophobic tunnel created by the hydrophobic pockets in the N- and C-terminal domains of CaM. In addition to the myristoyl group, several amino-acid residues in the peptide are important for CaM binding. This is a novel mode of binding and is very different from the mechanism of binding in other CaM-target complexes.

Disruption of a calmodulin central helix-like region of 10-formyltetrahydrofolate dehydrogenase impairs its dehydrogenase activity by uncoupling the functional domains

10-Formyltetrahydrofolate dehydrogenase (FDH) is composed of three domains and possesses three catalytic activities but has only two catalytic centers. The amino-terminal domain (residue 1-310) bears 10-formyltetrahydrofolate hydrolase activity, the carboxyl-terminal domain (residue 420-902) bears an aldehyde dehydrogenase activity, and the full-length FDH produces 10-formyltetrahydrofolate dehydrogenase activity. The intermediate linker (residues 311-419) connecting the two catalytic domains does not contribute directly to the enzyme catalytic centers but is crucial for 10-formyltetrahydrofolate dehydrogenase activity. We have identified a region within the intermediate domain (residues 384-405) that shows sequence similarity to the central helix of calmodulin. Deletion of either the entire putative helix or the central part of the helix or replacement of the six residues within the central part with alanines resulted in total loss of the 10-formyltetrahydrofolate dehydrogenase activity, whereas the full hydrolase and aldehyde dehydrogenase activities were retained. Alanine-scanning mutagenesis revealed that neither of the six residues alone is required for FDH activity. Analysis of the predicted secondary structures and circular dichroic and fluorescence spectroscopy studies of the intermediate domain expressed as a separate protein showed that this region is likely to consist of two alpha-helices connected by a flexible loop. Our results suggest that flexibility within the putative helix is important for FDH function and could be a point for regulation of the enzyme.

Novel aspects of calmodulin target recognition and activation

Several crystal and NMR structures of calmodulin (CaM) in complex with fragments derived from CaM-regulated proteins have been reported recently and reveal novel ways for CaM to interact with its targets. This review will discuss and compare features of the interaction between CaM and its target domains derived from the plasma membrane Ca2+-pump, the Ca2+-activated K+-channel, the Ca2+/CaM-dependent kinase kinase and the anthrax exotoxin. Unexpected aspects of CaM/target interaction observed in these complexes include: (a) binding of the Ca2+-pump domain to only the C-terminal part of CaM (b) dimer formation with fragments of the K+-channel (c) insertion of CaM between two domains of the anthrax exotoxin (d) binding of Ca2+ ions to only one EF-hand pair and (e) binding of CaM in an extended conformation to some of its targets. The mode of interaction between CaM and these targets differs from binding conformations previously observed between CaM and peptides derived from myosin light chain kinase (MLCK) and CaM-dependent kinase IIalpha (CaMKIIalpha). In the latter complexes, CaM engulfs the CaM-binding domain peptide with its two Ca2+-binding lobes and forms a compact, ellipsoid-like complex. In the early 1990s, a model for the activation of CaM-regulated proteins was developed based on this observation and postulated activation through the displacement of an autoinhibitory or regulatory domain from the target protein upon binding of CaM. The novel structures of CaM-target complexes discussed here demonstrate that this mechanism of activation may be less general than previously believed and seems to be not valid for the anthrax exotoxin, the CaM-regulated K+-channel and possibly also not for the Ca2+-pump.

Analysis of protein sequence/structure similarity relationships

Current analyses of protein sequence/structure relationships have focused on expected similarity relationships for structurally similar proteins. To survey and explore the basis of these relationships, we present a general sequence/structure map that covers all combinations of similarity/dissimilarity relationships and provide novel energetic analyses of these relationships. To aid our analysis, we divide protein relationships into four categories: expected/unexpected similarity (S and S(?)) and expected/unexpected dissimilarity (D and D(?)) relationships. In the expected similarity region S, we show that trends in the sequence/structure relation can be derived based on the requirement of protein stability and the energetics of sequence and structural changes. Specifically, we derive a formula relating sequence and structural deviations to a parameter characterizing protein stiffness; the formula fits the data reasonably well. We suggest that the absence of data in region S(?) (high structural but low sequence similarity) is due to unfavorable energetics. In contrast to region S, region D(?) (high sequence but low structural similarity) is well-represented by proteins that can accommodate large structural changes. Our analyses indicate that there are several categories of similarity relationships and that protein energetics provide a basis for understanding these relationships.

Quaternary structure built from subunits combining NMR and small-angle x-ray scattering data

A new principle in constructing molecular complexes from the known high-resolution domain structures joining data from NMR and small-angle x-ray scattering (SAXS) measurements is described. Structure of calmodulin in complex with trifluoperazine was built from N- and C-terminal domains oriented based on residual dipolar couplings measured by NMR in a dilute liquid crystal, and the overall shape of the complex was derived from SAXS data. The residual dipolar coupling data serves to reduce angular degrees of freedom, and the small-angle scattering data serves to confine the translational degrees of freedom. The complex built by this method was found to be consistent with the known crystal structure. The study demonstrates how approximate tertiary structures of modular proteins or quaternary structures composed of subunits can be assembled from high-resolution structures of domains or subunits using mutually complementary NMR and SAXS data.

Trisamine C(60)-fullerene adducts inhibit neuronal nitric oxide synthase by acting as highly potent calmodulin antagonists

C(60)-Fullerene trisamine adducts inhibit neuronal nitric oxide synthase and calcineurin phosphatase activities in a manner completely reversible by calmodulin. As measured by difference spectroscopy, D(3)-trisamine and C(3)-semiamine fullerene adducts displace trifluoperazine bound to calmodulin coincident with their binding. These binding events are complete at a molar ratio of 4 mol added fullerene per mole calmodulin. Trisamine fullerene adducts alter the native electrophoretic mobility of calmodulin, producing a heterogeneity of bands with associated fullerene. D(3)- and C(3)-trisamine fullerene adducts interact with dansylated calmodulin, producing a 50% loss of maximal fluorescence at concentrations of 30 nM. At higher concentrations than those required to inhibit neuronal nitric oxide synthase, trisamine fullerene adducts inhibit nitric oxide formation by the cytokine-inducible nitric oxide synthase isoform. These inhibitions are fully reversible by calmodulin and skeletal muscle troponin C but not by skeletal muscle parvalbumin. Of the trisamine fullerene adducts tested only the C(3)- and D(3)-semiamine adducts inhibit Ca(2+)-dependent nitric oxide production in GH(3) pituitary cells. These observations support the proposal that trisamine C(60)-fullerene adducts are potent calmodulin antagonists, some of which display activity in intact cellular systems.

Probing the role of calmodulin in Al toxicity in maize

The role of calmodulin on Al toxicity was studied in two maize (Zea mays L.) inbred lines, Cat 100-6 (Al-tolerant) and S 1587-17 (Al-sensitive). Increasing levels of Al induced the release of malate at similar rate by roots of both genotypes, while the exudation of citrate, a stronger Al-binding compound, was 3.5 times higher in Cat 100-6 seedlings exposed to 16.2x10(-6) Al(3+) activity. The calmodulin inhibitor trifluoperazine significantly reduced the root growth in both genotypes, mimicking the main effect of Al. However, when Cat 100-6 and S 1587-17 seedlings were challenged with Al in conjunction with trifluoperazine, no further reduction in root growth or any other effect of Al toxicity was observed. The rate of Al-induced citrate exudation by both genotypes was not affected by treatment with trifluoperazine or calmidazolium, another calmodulin inhibitor. The Al(3+) interaction with cytoplasmic CaM was estimated using models for the binding of Al(3+) and Mg(2+) with CaM and physiological concentrations of citrate, CaM, InsP(3), ATP, ADP, Al(3+) and Mg(2+). In this simulation, Al(3+) associated with citrate and InsP(3), but not with CaM. We conclude that calmodulin is not relevant to the physiological processes leading to the Al tolerance in maize, nor is it a primary target for Al toxicity.

Molecular dynamics simulation of the calmodulin-trifluoperazine complex in aqueous solution

Trifluoperazine (TFP) has been widely studied in relation to its mode of binding and its inactivation of calmodulin (CaM). Most studies in solution have indicated that CaM has two high-affinity binding sites for TFP. The crystal structure of the 1:4 CaM-TFP complex (CaM-4TFP) shows that three TFP molecules bind to the C-domain of CaM, and that one TFP molecule binds to the N-domain. In contrast, the crystal structure of the 1:1 CaM-TFP complex (CaM-1TFP) shows that one TFP molecule binds to the C-domain. It has been thought that the binding of one TFP molecule to the C-domain is followed by binding to the N-domain. The crystal structure of the 1:2 CaM-TFP complex (CaM-2TFP), moreover, has recently been determined, showing that two TFP molecules bind to the C-domain. In order to determine the structure of the CaM-TFP complex and to clarify the interaction between CaM and TFP in solution, we performed a molecular dynamics simulation of the CaM-TFP complex in aqueous solution starting from the CaM-4TFP crystal structure. The obtained solution structure is very similar to the CaM-2TFP crystal structure. The computer simulation showed that the binding ability of the secondary binding site of the C-domain is higher than that of the primary binding site of the N-domain.

Bepridil opens the regulatory N-terminal lobe of cardiac troponin C

Cardiac troponin C (cTnC) is the calcium-dependent switch for contraction in heart muscle and a potential target for drugs in the therapy of congestive heart failure. This calmodulin-like protein consists of two lobes connected by a central linker; each lobe contains two EF-hand domains. The regulatory N-terminal lobe of cTnC, unlike that of skeletal troponin C (sTnC), contains only one functional EF-hand and does not open fully upon the binding of Ca(2+). We have determined the crystal structure of cTnC, with three bound Ca(2+) ions, complexed with the calcium-sensitizer bepridil, to 2.15-A resolution. In contrast to apo- and 3Ca(2+)-cTnC, the drug-bound complex displays a fully open N-terminal lobe similar to the N-terminal lobes of 4Ca(2+)-sTnC and cTnC bound to a C-terminal fragment of cardiac troponin I (residues 147-163). The closing of the lobe is sterically hindered by one of the three bound bepridils. Our results provide a structural basis for the Ca(2+)-sensitizing effect of bepridil and reveal the details of a distinctive two-stage mechanism for Ca(2+) regulation by troponin C in cardiac muscle.

A new potent calmodulin antagonist with arylalkylamine structure: crystallographic, spectroscopic and functional studies

An arylalkylamine-type calmodulin antagonist, N-(3, 3-diphenylpropyl)-N'-[1-R-(3, 4-bis-butoxyphenyl)ethyl]-propylene-diamine (AAA) is presented and its complexes with calmodulin are characterized in solution and in the crystal. Near-UV circular dichroism spectra show that AAA binds to calmodulin with 2:1 stoichiometry in a Ca(2+)-dependent manner. The crystal structure with 2:1 stoichiometry is determined to 2.64 A resolution. The binding of AAA causes domain closure of calmodulin similar to that obtained with trifluoperazine. Solution and crystal data indicate that each of the two AAA molecules anchors in the hydrophobic pockets of calmodulin, overlapping with two trifluoperazine sites, i.e. at a hydrophobic pocket and an interdomain site. The two AAA molecules also interact with each other by hydrophobic forces. A competition enzymatic assay has revealed that AAA inhibits calmodulin-activated phosphodiesterase activity at two orders of magnitude lower concentration than trifluoperazine. The apparent dissociation constant of AAA to calmodulin is 18 nM, which is commensurable with that of target peptides. On the basis of the crystal structure, we propose that the high-affinity binding is mainly due to a favorable entropy term, as the AAA molecule makes multiple contacts in its complex with calmodulin.

Locking regulatory myosin in the off-state with trifluoperazine

Scallop striated adductor muscle myosin is a regulatory myosin, its activity being controlled directly through calcium binding. Here, we show that millimolar concentrations of trifluoperazine were effective at removal of all regulatory light chains from scallop myosin or myofibrils. More important, 200 microM trifluoperazine, a concentration 10-fold less than that required for light-chain removal, resulted in the reversible elimination of actin-activated and intrinsic ATPase activities. Unlike desensitization induced by metal ion chelation, which leads to an elevation of activity in the absence of calcium concurrent with regulatory light-chain removal, trifluoperazine caused a decline in actin-activated MgATPase activity both in the presence and absence of calcium. Procedures were equally effective with respect to scallop myosin, myofibrils, subfragment-1, or desensitized myofibrils. Increased alpha-helicity could be induced in the isolated essential light chain through addition of 100-200 microM trifluoperazine. We propose that micromolar concentrations of trifluoperazine disrupt regulation by binding to a single high-affinity site located in the C-terminal domain of the essential light chain, which locks scallop myosin in a conformation resembling the off-state. At millimolar trifluoperazine concentrations, additional binding sites on both light chains would be filled, leading to regulatory light-chain displacement.

IQGAP1 and calmodulin modulate E-cadherin function

Ca(2+)-dependent cell-cell adhesion is mediated by the cadherin family of transmembrane proteins. Adhesion is achieved by homophilic interaction of the extracellular domains of cadherins on adjacent cells, with the cytoplasmic regions serving to couple the complex to the cytoskeleton. IQGAP1, a novel RasGAP-related protein that interacts with the cytoskeleton, binds to actin, members of the Rho family, and E-cadherin. Calmodulin binds to IQGAP1 and regulates its association with Cdc42 and actin. Here we demonstrate competition between calmodulin and E-cadherin for binding to IQGAP1 both in vitro and in a normal cellular milieu. Immunocytochemical analysis in MCF-7 (E-cadherin positive) and MDA-MB-231 (E-cadherin negative) epithelial cells revealed that E-cadherin is required for accumulation of IQGAP1 at cell-cell junctions. The cell-permeable calmodulin antagonist CGS9343B significantly increased IQGAP1 at areas of MCF-7 cell-cell contact, with a concomitant decrease in the amount of E-cadherin at cell-cell junctions. Analysis of E-cadherin function revealed that CGS9343B significantly decreased homophilic E-cadherin adhesion. On the basis of these data, we propose that disruption of the binding of calmodulin to IQGAP1 enhances the association of IQGAP1 with components of the cadherin-catenin complex at cell-cell junctions, resulting in impaired E-cadherin function.

Internal calcium modulates apparent affinity of metabotropic GABA receptors

The metabotropic GABA receptor (GABA(B)R) regulates calcium influx in neurons. Whole cell voltage-clamp techniques were employed to determine the effects of internal calcium on the activity of GABA(B)Rs. GABA(B)R receptor apparent affinity was maximal when bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) maintained internal calcium below 70 nM. Apparent affinity was reduced as internal calcium increased. EGTA did not produce similar effects, suggesting that localized increases in calcium influenced GABA(B)R apparent affinity. Confocal imaging disclosed relatively high internal calcium just below the plasma membrane of isolated neurons. BAPTA, but not EGTA, reduced this ring of high calcium. Heparin, dantrolene, and ryanodine increased GABA(B)R apparent affinity, effects similar to that of BAPTA. Calmodulin inhibitors also increased receptor apparent affinity. These results suggest that internally released calcium activates calmodulin, which reduces GABA(B)R apparent affinity. This identifies a reciprocal system in which the metabotropic GABA receptor can reduce calcium influx, but internal calcium can suppress this receptor pathway. Metabotropic glutamate receptors linked to inositol 1,4,5 trisphosphate (InsP(3)) raised internal calcium and suppressed the action of GABA(B)Rs. Thus negative feedback systems control the balance between excitatory and inhibitory metabotropic receptor pathways in retinal neurons.

Evidence for calmodulin inter-domain compaction in solution induced by W-7 binding

Small-angle X-ray scattering and nuclear magnetic resonance were used to investigate the structural change of calcium-bound calmodulin (Ca2+/CaM) in solution upon binding to its antagonist, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7). The radius of gyration was 17.4+/-0.3 A for Ca2+/CaM-W-7 with a molar ratio of 1:5 and 20.3+/-0.7 A for Ca2+/CaM. Comparison of the radius of gyration and the pair distance distribution function of the Ca2+/CaM-W-7 complex with those of other complexes indicates that binding of two W-7 molecules induces a globular shape for Ca2+/CaM, probably caused by an inter-domain compaction. The results suggest a tendency for Ca2+/CaM to form a globular structure in solution, which is inducible by a small compound like W-7.

Lack of calmodulin antagonism of melatonin in T-lymphocyte activation

Despite various reported effects of the pineal hormone melatonin on the immune system, its mechanism of action on immune cells is still unknown. Since melatonin has been suggested as a physiological antagonist to calmodulin in certain cell types, we investigated effects of melatonin on calmodulin-dependent IL-2 production and proliferation of activated T-lymphocytes. It was found, however, that, in contrast to the calmodulin antagonists trifluoperazine and W7, melatonin neither inhibited the IL-2 production of activated lymphoblastoid Jurkat T-cells nor decreased the mitogen response of peripheral blood mononuclear leukocytes. Preincubation of Jurkat cells with melatonin did not influence trifluoperazine effects on IL-2 production indicating that melatonin does not bind to the same sites of calmodulin as trifluoperazine, as has been postulated. In conclusion, these results did not give any evidence for a calmodulin antagonism of melatonin in T-lymphocyte activation. Thus, melatonin as a calmodulin antagonist appears not to be a universal phenomenon.

Metabotropic and ionotropic glutamate receptors regulate calcium channel currents in salamander retinal ganglion cells

1. Glutamate suppressed high-voltage-activated barium currents (IBa, HVA) in tiger salamander retinal ganglion cells. Both ionotropic (iGluR) and metabotropic (mGluR) receptors contributed to this calcium channel inhibition. 2. Trans-ACPD (1-aminocyclopentane-trans-1S,3R-dicarboxylic acid), a broad-spectrum metabotropic glutamate receptor agonist, suppressed a dihydropyridine-sensitive barium current. Kainate, an ionotropic glutamate receptor agonist, reduced an omega-conotoxin GVIA-sensitive current. 3. The relative effectiveness of selective agonists indicated that the predominant metabotropic receptor was the L-2-amino-4-phosphonobutyrate (L-AP4)-sensitive, group III receptor. This receptor reversed the action of forskolin, but this was not responsible for calcium channel suppression. l-AP4 raised internal calcium concentration. Antagonists of phospholipase C, inositol trisphosphate (IP3) receptors and ryanodine receptors inhibited the action of metabotropic agonists, indicating that group III receptor transduction was linked to this pathway. 4. The action of kainate was partially suppressed by BAPTA, by calmodulin antagonists and by blockers of calmodulin-dependent phosphatase. Suppression by kainate of the calcium channel current was more rapid when calcium was the charge carrier, instead of barium. The results indicate that calcium influx through kainate-sensitive glutamate receptors can activate calmodulin, which stimulates phosphatases that may directly suppress voltage-sensitive calcium channels. 5. Thus, ionotropic and metabotropic glutamate receptors inhibit distinct calcium channels. They could act synergistically, since both increase internal calcium. These pathways provide negative feedback that can reduce calcium influx when ganglion cells are depolarized.

Solution structure of calmodulin-W-7 complex: the basis of diversity in molecular recognition

The solution structure of calcium-bound calmodulin (CaM) complexed with an antagonist, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), has been determined by multidimensional NMR spectroscopy. The structure consists of one molecule of W-7 binding to each of the two domains of CaM. In each domain, the W-7 chloronaphthalene ring interacts with four methionine methyl groups and other aliphatic or aromatic side-chains in a deep hydrophobic pocket, the site responsible for CaM binding to CaM-dependent enzymes such as myosin light chain kinases (MLCKs) and CaM kinase II. This competitive binding at the same site between W-7 and CaM-dependent enzymes suggests the mechanism by which W-7 inhibits CaM to activate the enzymes. The orientation of the W-7 naphthalene ring in the N-terminal pocket is rotated approximately 40 degrees with respect to that in the C-terminal pocket. The W-7 ring orientation differs significantly from the Trp800 indole ring of smooth muscle MLCK bound to the C-terminal pocket and the phenothiazine ring of trifluoperazine bound to the N or C-terminal pocket. These comparative structural analyses demonstrate that the two hydrophobic pockets of CaM can accommodate a variety of bulky aromatic rings, which provides a plausible structural basis for the diversity in CaM-mediated molecular recognition.

Crystallization and preliminary diffraction analysis of Ca(2+)-calmodulin-drug and apocalmodulin-drug complexes

Ca(2+)-calmodulin is crystallized with two new and potent drugs: a bisindol derivative (KAR-2, 3"-(beta-chloroethyl)-2",4"-dioxo-3,5"- spiro-oxazolidino-4-deacetoxy-vinblastine) with antitumor activity and an arylalkylamine fendiline analogue (N-(3,3-diphenylpropyl)-N'-[1-(3,4- di-n-butoxy-phenyl)-ethyl]-1,3-diaminopropane) with anticalmodulin activity. The crystals diffract beyond 2.8 A and differ in unit cell parameters from each other as well as from crystals of Ca(2+)-calmodulin or Ca(2+)-calmodulin-ligand complexes, as reported thus far. Attempts to crystallize Ca(2+)-free calmodulin without drugs failed, in consonance with earlier results; however, single Ca(2+)-free calmodulin crystals diffracting-beyond 2.5 A resolution were grown in the presence of KAR-2. Results indicate that binding of the two drugs to apocalmodulin or Ca(2+)-calmodulin may induce unique novel protein conformers, targets of further detailed X-ray studies.