Alternative binding of two sequential glycolytic enzymes to microtubules. Molecular studies in the phosphofructokinase/aldolase/microtubule system

Simultaneous binding of two sequential glycolytic enzymes, phosphofructokinase and aldolase, to a microtubular network was investigated. The binding of the phosphofructokinase to microtubules and its bundling activity has been previously characterized (Lehotzky, A., Telegdi, M., Liliom, K., and Ovádi, J. (1993) J. Biol. Chem. 268, 10888-10894). Aldolase binding to microtubules at near physiological ionic strength is weak (Kd = 20 microM) as compared with that of the kinase (Kd = 1 microM). The interactions of both enzymes with microtubules are modulated by their common intermediate, fructose-1,6-bisphosphate. Pelleting and electron microscopic measurements have revealed that the aldolase binding interferes with that of phosphofructokinase, although they have distinct binding domains on microtubules. The underlying molecular mechanism responsible for this finding is that in the solution phase aldolase and phosphofructokinase form a bienzyme complex that does not bind to the microtubule. The bienzyme complex formation does not influence the catalytic activity of aldolase, however, it inhibits the dissociation-induced inactivation of the kinase by stabilizing a catalytically active molecular form. The present data suggest the first experimental evidence that two sequential glycolytic enzymes do not associate simultaneously to microtubules, but their complexation in solution provides kinetic advantage for glycolysis.

The interaction of a new anti-tumour drug, KAR-2 with calmodulin

1. KAR-2 (3"-(beta-chloroethyl)-2",4"-dioxo-3,5" -spiro-oxazolidino-4-deacetoxy-vinblastine) is a semisynthetic bis-indol derivative, with high anti-microtubular and anti-tumour activities but with low toxicity. KAR-2, in contrast to other biologically active bis-indols (e.g. vinblastine) did not show anti-calmodulin activity in vitro (enzyme kinetic, fluorescence anisotropy and immunological tests). 2. Direct binding studies (fluorescence resonance energy transfer, circular dichroism) provided evidence for the binding of KAR-2 to calmodulin. The binding affinity of KAR-2 to calmodulin (dissociation constant was about 5 microM) in the presence of Ca2+ was comparable to that of vinblastine. 3. KAR-2 was able to interact with apo-calmodulin as well; in the absence of Ca2+ the binding was of cooperative nature. 4. The effect of drugs on Ca2+ homeostasis in human neutrophil cells was investigated by means of a specific fluorescent probe. Trifluoperazine extensively inhibited the elevation of intracellular Ca2+ level, vinblastine did not appreciably affect it, KAR-2 stimulated the Ca2+ influx and after a transient enhancement the Ca2+ concentration reached a new steady-state level. 5. Comparison of the data obtained with KAR-2 and bis-indols used in chemotherapy suggests that the lack of anti-calmodulin potency resides on the spiro-oxazolidino portion of KAR-2. This character of KAR-2 manifested itself in various systems and might result in its low in vivo toxicity, established in an anti-tumour test.

Interaction of a new bis-indol derivative, KAR-2 with tubulin and its antimitotic activity

1. KAR-2 (3"-(beta-chloroethyl)-2",4"-dioxo-3,5"-spiro-oxazolidino- 4-deacetoxy-vinblastine), is a bis-indol derivative; catharantine is coupled with the vindoline moiety which contains a substituted oxazolidino group. Our binding studies showed that KAR-2 exhibited high affinity for bovine purified brain tubulin (Kd-3 microM) and it inhibited microtubule assembly at a concentration of 10 nM. 2. Anti-microtubular activity of KAR-2 was highly dependent on the ultrastructure of microtubules: while the single tubules were sensitive, the tubules cross-linked by phosphofructokinase (ATP: D-fructose-6-phosphate-1-phosphotransferase, EC 2.7.1.11) exhibited significant resistance against KAR-2. 3. The cytoplasmic microtubules of Chinese hamster ovary mammalian and Sf9 insect cells were damaged by 1 microgram ml-1 KAR-2, as observed by indirect immunofluorescence and transmission electron microscopy. Scanning electron microscopy revealed intensive surface blebbing on both types of cells in the presence of KAR-2. 4. KAR-2 was effective in the mouse leukaemia P338 test in vivo without significant toxicity. Studies on a primary cerebro-cortical culture of rat brain and differentiated PC12 cells indicated that the toxicity of KAR-2 was significantly lower than that of vinblastine. The additional property of KAR-2 that distinguishes it from bis-indol derivatives is the lack of anti-calmodulin activity.

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.

Characterization of microtubule-phosphofructokinase complex: specific effects of MgATP and vinblastine

Phosphofructokinase interacts with both microtubules and microtubules containing microtubule-associated proteins to produce bundling and periodical cross-bridging of tubules. Immunoelectron microscopy using anti-phosphofructokinase antibodies provided direct evidence that the kinase molecules are responsible for the cross-bridging of microtubules. Limited proteolysis by subtilisin, a procedure that cleaves the N-terminal segment of the free enzyme as well as the C-terminal "tails" of tubulin subunits exposed on microtubules, showed that while phosphofructokinase becomes resistant, tubulin retains sensitivity against proteolysis within the heterologous complex. These data suggest that the N-terminal segment of the enzyme, but not the C-terminal "tail" of tubulin subunits, is involved in the interaction between the microtubule and the kinase. The phosphorylation of phosphofructokinase or microtubules containing microtubule-associated proteins by the cAMP-dependent protein kinase did not interfere with the heterologous complex formation. MgATP prevents phosphofructokinase binding to the microtubules, and it can displace the enzyme from the single microtubules. However, the bundled microtubules are apparently resistant to the MgATP dissociation effect. Modelling of the assembly process suggests that the tubulin-kinase complex is able to polymerize as the free tubulin. Vinblastine, an anti-mitotic agent, inhibits tubulin assembly; however, its inhibitory effect is partially suppressed in the presence of phosphofructokinase. Fluorescence anisotropy measurements indicated that kinase and vinblastine compete for tubulin binding with no evidence for ternary complex formation. This competitive mechanism and the ability of the tubulin-enzyme complex to polymerize into microtubules may result in the resistance of the tubulin-enzyme complex against the inhibition of assembly induced by vinblastine. Microtubules formed in the presence of vinblastine plus phosphofructokinase can be visualized by electron microscopy. A molecular model is suggested that summarizes the effects of MgATP and vinblastine on the multiple equilibria in the tubulin/microtubules/phosphofructokinase system.

Metabolic consequences of enzyme interactions

The fact that enzyme complexes, stable, quasi-stable, and dynamic, exist in cells can no longer be ignored. Experimental evidence done with a variety of techniques has demonstrated these interactions in vitro and in vivo. There is scarcely a single known metabolic pathway in which no interactions of any of its enzymes exist (see reference 27 for a list of these). Such interactions are not only ubiquitous throughout metabolism, but they exist in all cell types, procaryote and eucaryote. In many of these systems the advantages of and regulatory power of enzyme-enzyme or enzyme-structural protein interactions has been amply demonstrated. The more difficult task is to assess accurately quantitative aspects of a system that varies between the solid, gel, and aqueous state. It is clear that the metabolic paradigm of soluble randomly dispersed metabolic elements in cells must be replaced, and new theoretical and experimental approaches introduced into this vital area of biological research.

Interaction between D-glyceraldehyde-3-phosphate dehydrogenase and calmodulin

The effect of calmodulin on the associative properties of D-glyceraldehyde-3-phosphate dehydrogenase was investigated by means of a covalently attached fluorescent probe. We found that calmodulin shifts the equilibrium between the different forms of glyceraldehyde-3-phosphate dehydrogenase and binds to the subunits with an apparent dissociation constant of 1.8 microM. Within this heterologous complex calmodulin has no effect on the catalytic activity of the enzyme. The formation of the heterocomplex can be modulated by the specific anti-calmodulin drug, trifluoperazine, as well as by aldolase. The possible role of these associations is that they influence the interaction of both glyceraldehyde-3-phosphate dehydrogenase and calmodulin with other soluble proteins or structural elements.

Triosephosphate isomerase deficiency: predictions and facts

Deficiencies in around 20 enzymes, associated with widely different degrees of severity and complexity, have been identified for human erythrocytes. The fact that glycolysis is crucial for erythrocyte function is reflected by the large number of inherited glycolytic enzymopathies. Triosephosphate isomerase (TPI) deficiency, a rare autosomal disease, is usually associated with nonspherocytic hemolytic anemia, progressive neurologic dysfunction, and death in childhood. The two affected Hungarian brothers studied by us have less than 3% TPI activity and enormously (30-50-fold) increased dihydroxyacetone phosphate (DHAP) concentration in their erythrocytes. The well-established concept of the metabolic control theory was used to test the contribution of TPI and some related enzymes to the control of a relevant segment of the glycolytic pathway in normal and deficient cells. Deviation indices, DEJ = (delta J/delta E) E(r)/J(r), which give a good estimation of flux control coefficients using a single large change in enzyme activity, were determined from the fluxes in the absence and presence of exogeneous enzymes. We found that PFK and aldolase are the enzymes that predominantly control the flux, however, the quantitative values depend extensively on the pH: DEJ values are 0.85 and 0.14 at pH 8.0 and 0.33 and 0.67 at pH 7.2 for aldolase and PFK, respectively. Neither the flux rates nor the capacities of the enzymes seem to be significantly different in normal and TPI deficient cells. There is a discrepancy between DHAP levels and TPI activities in the deficient cells. In contrast to the experimental data the theoretical calculations predict elevation in DHAP level at lower than 0.1% of the normal value of TPI activity. Several possibilities suggested fail to explain this discrepancy. Specific associations of glycolytic enzymes to band-3 membrane proteins with their concomitant inactivation have been demonstrated. We propose that the microcompartmentation of TPI that could further decrease the reduced isomerase activity of the deficient cells, is responsible for the high DHAP level.

Specific characteristics of phosphofructokinase-microtubule interaction

Muscle phosphofructokinase interacts with microtubule-associated protein-free microtubules resulting in a reduction of the overall activity of the enzyme [Lehotzky et al. (1993) J. Biol. Chem. 268, 10888-10894] and periodical cross-linking of the tubules [Lehotzky et al. (1994) Biochem. Biophys. Res. Commun. 204, 585-591]. Microtubule polymers of 'tail-free' tubulin obtained by removal of the carboxy-termini with limited subtilisin digestion retain the binding domains for phosphofructokinase that cross-bridges microtubule 'bodies'. Microtubule-associated proteins bound on tubulin 'tails' do not perturb the kinase binding. These data suggest that the tubulin carboxy-terminal domain is not involved in microtubule-phosphofructokinase interactions and phosphofructokinase and microtubule-associated proteins have distinct binding domains on microtubules. Of different isoforms of phosphofructokinase, occurring mainly in brain and tumor cells, the muscle isoform exhibits selective adsorption behaviour on microtubules. Phosphofructokinase M and C isoforms with different associative and allosteric properties may represent an auxiliary pathway to modulate energy production via glycolysis.

Anti-calmodulin potency of indol alkaloids in in vitro systems

We have demonstrated that bis-indol Vinca alkaloids of anti-mitotic activities (vinblastine, vincristine, and navelbine) bind to calmodulin in a Ca(2+)-dependent manner. We designed direct binding tests (fluorescence energy transfer and circular dichroism measurements) to quantify the interactions of bis-indol derivatives with calmodulin. The dissociation constants of calmodulin-navelbine and calmodulin-vinblastine complexes with 1:1 stoichiometry are 0.5 microM and 3 microM, respectively. These values indicate that the binding affinities of these Vinca alkaloids to calmodulin and tubulin are comparable. Immunological, enzyme kinetic and fluorescence anisotropy measurements showed that bis-indol alkaloids inhibit the interactions of calmodulin with target proteins. The results of indirect enzyme-linked immunosorbent assay showed that bis-indol alkaloids effectively antagonize with anti-calmodulin antibody for calmodulin binding (IC50 = 90 microM, 400 microM, and 430 microM for navelbine, vincristine and vinblastine, respectively). According to the fluorescence anisotropy and enzyme kinetic measurements, vinblastine, vincristine and vinblastine, similarly to trifluoperazine, the classic calmodulin antagonist, compete with target enzyme [phosphofructokinase (ATP: D-fructose 6-phosphate 1-phosphotransferase, EC 2.7.1.11)] for an inhibitory effect either on immunocomplex formation or on calmodulin-enzyme interaction. Navelbine appeared in our tests as the most potent drug in inhibiting the association of calmodulin to target proteins in comparison to other bis-indol derivatives. Since navelbine and vinblastine possess identical vindoline moiety, although they differ in the catharantine part, the difference in anti-calmodulin potencies is suggested to reside predominantly on this portion of the molecules. These findings might establish the pharmacological importance of these activities in the specificity and toxicity of the drugs.

Characterization of tubulin-alkaloid interactions by enzyme-linked immunosorbent assay

An indirect enzyme-linked immunosorbent assay has been developed to characterize the interactions of drugs (bisindol alkaloids and colchicine) with immobilized tubulin. The binding of polyclonal antibodies raised in rabbits to tubulin heterodimers and monoclonal antibodies against the C-terminal regions of alpha- and beta-tubulin subunits is tested at various concentrations of the drugs. The results of the displacement experiments showed that (i) the monomeric alkaloids compete with neither polyclonal nor monoclonal antibodies for tubulin binding; (ii) the dimer alkaloids displace the polyclonal but not the monoclonal antibodies from tubulin; and (iii) the inhibitory potencies of the bisindol alkaloids of different chemical structures are different. A new semisynthetic derivative of bisindol alkaloids, KAR-2, was found to be a powerful ligand in inhibiting both tubulin polymerization and immunocomplex formation. Colchicine did not inhibit binding of the antibodies to the immobilized tubulin. Competitive-displacement experiments were also designed to test the anti-tubulin activity of drugs in solution. The results suggest that while bisindol alkaloids interact with tubulin bound on surface or in solution, colchicine binds exhaustively to tubulin in solution and enhances the affinity of polyclonal antibodies probably via long-distance interactions between the binding domains in tubulin.

Binding of malate dehydrogenase and NADH channelling to complex I

As previously reported, mitochondrial malate dehydrogenase (MDH) binds to purified complex I of the electron transport system. With conditions used in previous reports, MDH binds even more extensively, but probably predominantly non-specifically, to the matrix side of the inner mitochondrial membrane of submitochondrial particles (SMP). Herein we report experimental conditions for highly specific binding of malate dehydrogenase to complex I within SMP. These conditions permit us to demonstrate NADH channelling from malate dehydrogenase to complex I using the competing reaction test. This test, though not ideal for all situations, has several advantages over the enzyme buffering test previously used. These advantages should facilitate further studies elucidating NADH channelling to complex I from MDH and other dehydrogenases. Independent evidence of NADH channelling to the electron transport chain and the potential advantages of substrate channelling in general are also discussed. Substrate channelling from MDH in particular may be especially beneficial because of the unfavourable equilibrium and kinetics of this enzyme reaction.

Ligand-modulated cross-bridging of microtubules by phosphofructokinase

The interaction of phosphofructokinase and microtubules results in mutual effects: decreases overall activity of the kinase and alters the ultrastructural organization of microtubules. Electron microscopic studies provide direct evidence for the periodical cross-bridges of microtubules by the kinase. 3-4 closely aligned tubules are connected by rows of highly periodic lateral arms about 13 nm long and 12 nm wide. The bundling activity of the enzyme seems to be specific since aldolase, which also interacts with microtubules, does not cross-link tubules, but it impedes the binding of the kinase to tubules. ATP, ADP and fructose bisphosphates inhibit the cross-bridges of microtubules by phosphofruktokinase to a different extent and concentration dependent manner. The kinase complexed with specific metabolites inducing distinct conformers does not interact with tubules. Microtubules cross-linked by the kinase became partly resistant to the depolymerizing action of vinblastine.

Interaction of phosphofructokinase with tubulin and microtubules. Quantitative evaluation of the mutual effects

The linked equilibria involved in the binding of phosphofructokinase (EC 2.7.1.11, ATP:D-fructose-6-phosphate 1-phosphotransferase) to tubulin and microtubules were studied at high ionic strength in vitro. The concentration-dependent dissociation of phosphofructokinase was analyzed in the absence and presence of tubulin or microtubules, and the binding of kinase to the tubulin dimer and microtubules was compared. Enzyme activity of phosphofructokinase was inhibited by both tubulin and microtubules: the relative inhibition increased with decreasing enzyme concentration. The complex formation between phosphofructokinase and tubulin was demonstrated by means of fluorescent anisotropy. Concentration-dependent copelleting of the kinase with taxol-stabilized microtubules revealed binding of the enzyme to microtubules as well as phosphofructokinase-enhanced pelleting of microtubules. The binding data agree with the enzyme kinetic findings that the inactive dissociated forms of phosphofructokinase (monomer-dimer) are involved in the heterologous complex formation. Microtubule reorganization (bundle formation) by phosphofructokinase was established by turbidity measurements and sedimentation experiments. The binding data are consistent with a simple molecular model for the interactions in phosphofructokinase-tubulin/microtubules systems.

Effects of calmodulin antagonists on antibody binding to calmodulin. Distinct conformers of calmodulin induced by the binding of drugs

An indirect enzyme-linked immunosorbent assay has been used to study the interactions between calmodulin and two calmodulin antagonists, trifluoperazine and a neuropeptide isolated from the hypothalamus. The binding of a monospecific anti-calmodulin antibody, raised in rabbit against dinitrophenylated calmodulin, to calmodulin was tested at various concentrations of these drugs under equilibrium conditions. Trifluoperazine at low concentrations stimulated, but at relatively high concentrations inhibited, immunocomplex formation. The neuropeptide displaced the antibody from calmodulin at nanomolar concentrations. Enzyme-linked immunosorbent assays were also carried out with the large tryptic fragments of calmodulin. The results suggest that (i) the C-terminal fragment binds the antibody with an affinity which is comparable with that of intact calmodulin; (ii) the neuropeptide can form complexes with both N- and C-terminal fragments, but with two orders of magnitude less activity in case of the C-terminal fragment; and (iii) trifluorperazine does not stimulate antibody binding to the C-terminal fragment. Therefore the tertiary structure of calmodulin must be intact to ensure long-distance interactions between the binding sites of trifluoperazine, the neuropeptide and the antibody. These interactions may produce distinct conformers of calmodulin which may exhibit altered potency, not only for antibody binding but also for stimulation/inhibition of target enzymes.

Quantitative evaluation of indirect ELISA. Effect of calmodulin antagonists on antibody binding to calmodulin

A simple linearization procedure has been developed to determine the apparent dissociation constant of the interaction between antigen and antibody from the data of indirect, non-competitive enzyme-linked immunosorbent assays (ELISA). Applying this dissociation constant the binding constant of ligands to antigen can be determined and the quantitative evaluation of the competitive ELISA experiments makes it possible to analyse the affinity of antibody to antigen on the surface and in solution. The binding of the monospecific anti-calmodulin antibody to calmodulin and to solid-phase bound calmodulin has been tested by non-competitive and competitive assays. We have developed an experimental system where binding of the antibody to the solid-phase bound calmodulin has been studied under equilibrium conditions. Competitive ELISA experiments showed that the affinity of antibody to calmodulin on the surface and in solution was almost the same. The binding constant of a hypothalamic neuropeptide to calmodulin was determined using the quantitative ELISA approach. The neuropeptide was found to be of very high inhibitory potency (Kd = 2 nM) and competed with the antibody for calmodulin binding. This simple and sensitive procedure is suitable for screening molecules with anti-calmodulin activity and comparing their efficacy.

Modulation of the interaction between aldolase and glycerol-phosphate dehydrogenase by fructose phosphates

Kinetics of fructose-1,6-disphosphate aldolase (EC 4.1.2.13) catalyzed conversion of fructose phosphates was analyzed by coupling the aldolase reactions to the metabolically sequential enzyme, glycerol-3-phosphate dehydrogenase (EC 1.1.1.8), which interacts with aldolase. At low enzyme concentration poly(ethylene glycol) was added to promote complex formation of aldolase and glycerol-phosphate dehydrogenase resulting in a 3-fold increase in KM of fructose-1,6-bisphosphate and no change in Vmax. Kinetic parameters for fructose-1-phosphate conversion changed inversely upon complex formation: Vmax increased while KM remained unchanged. Gel penetration and ion-exchange chromatographic experiments showed positive modulation of the interaction of aldolase and dehydrogenase by fructose-1,6-bisphosphate. The dissociation constant of the heterologous enzyme complex decreased 10-fold in the presence of this substrate. Fructose-1-phosphate or dihydroxyacetone phosphate had no effect on the dissociation constant of the aldolase-dehydrogenase complex. In addition, titration of fluorescein-labelled glycerol-phosphate dehydrogenase with aldolase indicated that both fructose-1,6-bisphosphate and fructose-2,6-biphosphate enhanced the affinity of aldolase to glycerol-phosphate dehydrogenase. The results of the kinetic and binding experiments suggest that binding of the C-6 phosphate group of fructose-1,6-bisphosphate to aldolase complexed with dehydrogenase is sterically impeded while saturation of the C-6 phosphate group site increases the affinity of aldolase for dehydrogenase. The possible molecular mechanism of the fructose-1,6-bisphosphate modulated interaction is discussed.

Calmodulin is a potent target for new hypothalamic neuropeptides

Recently, five glycopeptides with coronaro-constrictory properties were isolated from bovine hypothalamus [(1988) Neurochemistry (USSR) 7, 519-524]. Calmodulin has been recognized in our laboratory as a target protein for the neuropeptides isolated from hypothalamus. The results of indirect enzyme-linked immunosorbent assay have shown that the new hypothalamic neuropeptides antagonize with the monospecific anti-calmodulin antibody for calmodulin binding although they are not fragments of calmodulin. The inhibitory potency of the peptides is dependent on their concentration and the length of the polypeptide chain. Four out of five peptides are effective in nM concentration range. Ca2+ stimulates the binding of peptides to calmodulin; however, immunocomplex can be formed in the absence of Ca2+ as well. The effects of trifluoperazine and peptides on the calmodulin/antibody interaction are not additive, suggesting the cooperativity between the binding sites on calmodulin. Under physiological conditions the presence of the peptides could produce distinct conformers of calmodulin which may exhibit altered potency for stimulation/inhibition of target enzymes.

Dissimilar mechanisms of action of anticalmodulin drugs: quantitative analysis

A novel molecule from the arylalkylamine family of drugs, KHL-8430, has been identified as a potent and specific inhibitor of calmodulin activity. The effect of this drug on calmodulin-mediated enzymatic actions has been analyzed to exemplify how to model the mechanism of action of a functional calmodulin antagonist. The approach used includes both binding and enzyme kinetic studies. In both types of experiments, the effects of drugs on calmodulin-phosphofructokinase [ATP:D[fructose-6-phosphate-1-phosphotransferase, EC 2.7.1.11] and calmodulin-phosphodiesterase (3':5' cyclic nucleotide phosphodiesterase, EC 3.6.1.3) interactions have been investigated. We have found that KHL-8430, in contrast to trifluoperazine, a classical anticalmodulin drug, competes with neither phosphofructokinase nor phosphodiesterase for calmodulin binding, yet it liberates phosphofructokinase from calmodulin inhibition and phosphodiesterase from calmodulin stimulation. The anticalmodulin activity occurs at lower KHL-8430 than trifluoperazine concentrations. These findings might establish the functional importance of these differences in the specificity of these drugs. The synthesis of the data suggests that (i) whereas trifluoperazine antagonizes both phosphofructokinase and phosphodiesterase binding to calmodulin, KHL-8430 interacts with calmodulin complexed with enzymes; (ii) KHL-8430 binds to the calmodulin-phosphofructokinase complex with an affinity constant of 0.8 microM, whereas the binding constant of trifluoperazine is 2.5 microM (iii) within the ternary complex the dimeric form of the kinase preserves activity that is otherwise inactive; and (iv) the binding of trifluoperazine and KHL-8430 to calmodulin exhibits negative cooperativity. The approach used in this study makes it possible to screen for the calmodulin antagonist effect of other drugs as well.