Hydrophobic regions function in calmodulin-enzyme(s) interactions

Purification and cation binding properties of the recombinant human S100 calcium-binding protein A3, an EF-hand motif protein with high affinity for zinc

The calcium-binding protein S100A3 is an unusual member of the S100 family, characterized by its very high content of Cys. In order to study the biochemical, cation-binding, and conformational properties, we produced and purified the recombinant human protein in Escherichia coli. The recombinant protein forms noncovalent homodimers, tetramers, and polymers in vitro with a subunit molecular weight of 11,712. The Zn(2+)-binding parameters of S100A3 were studied by equilibrium gel filtration and yielded a stoichiometry of four Zn2+ per monomer with a [Zn2+]0.5 of 11 microM and a Hill coefficient of 1.4 at physiological ionic strength. The affinity for Ca2+ is too low to be determined by direct methods (KCa > 10 mM). Ca(2+)- and Zn(2+)-binding can be followed by optical methods: the Trp-45 fluorescence is high in the metal-free form and addition of Zn2+ and Ca2+, but not of Mg2+, leads to a 4-fold quenching. Ca2+ and Zn2+ promote also quite similar conformational changes in the Tyr and Trp environment as monitored by difference spectrophotometry. Fluorescence titrations with Zn2+ confirmed that there is one set of high affinity binding sites with a [Zn2+]0.5 of 8 microM and a Hill coefficient of 1.3. Binding of Zn2+ to a second set of low affinity sites induces protein precipitation. Fluorescence titrations with Ca2+ confirmed the very low affinity of S100A3 for this ion with a [Ca2+]0.5 of 30 mM and slight negative cooperativity. Mg2+ has no effect on this binding curve. Of the 10 Cys residues in S100A3, 5 only are free thiols, and accessible to 5,5'-dithiobis(2-nitro-benzoic acid); they display a high reactivity in the metal-free and Ca2+ form, but a 20-fold lowered reactivity in the Zn2+ form of S100A3. Ca(2+)-binding promotes the formation of a solvent-accessible hydrophobic surface as monitored by the 60-fold fluorescence increase of 2-p-toluidinylnaphthalene-6-sulfonate, whereas Zn2+ has no noticeable influence. Our data indicate that Ca2+ and Zn2+ do not bind to the same sites and that under physiological conditions S100A3 is a Zn(2+)-binding rather than a Ca(2+)-binding protein; nevertheless, very specific conformational changes are introduced by either Ca2+ or Zn2+. Since no Zn(2+)-binding motif of known structure was identified in the primary sequence of S100A3, the results are suggestive for a novel Zn(2+)-binding motif.

Role of calmodulin in the activation of carbachol-activated cationic current in guinea-pig gastric antral myocytes

In mammalian gastrointestinal myocytes, it is known that muscarinic stimulation activates nonselective cation channels through a G-protein and a Ca2+-dependent pathway. We recorded inward cationic currents following application of carbachol (ICCh) to guinea-pig gastric myocytes, which were held at -20 mV using the whole-cell patch-clamp method. ICCh was suppressed by nicardipine or removal of Ca2+ from the bath solution. The peak value of inward current induced by repetitive applications of carbachol (CCh) decreased progressively (run-down phenomenon). This run-down was significantly alleviated by the addition of calmodulin to the pipette solution (0.15 mg/ml) or by using the perforated-patch whole-cell voltage-clamp technique. Moreover, W-7[N-6(aminohexyl)-5-chloro-1-naphthalenesulphonamide], a calmodulin antagonist, was a reversible inhibitor of ICCh. However, @-7 had only a weak inhibitory effect on the same cationic current which was induced by guanosine 5'-O-(3-thiotriphosphate) (GTP¿gammaS] 0.2 mM) in the pipette solution. This GTP[gammaS]-induced cationic current was still markedly suppressed by the Ca2+-free bath solution. W-7 itself had a weak inhibitory effect on voltage-operated Ca2+ channels as well as the effects on ICCh. These data suggest that multiple Ca2+-dependent pathways are involved in the activation of CCh-gated cation channels in guinea-pig antral myocytes and a Ca2+/calmodulin-dependent pathway would be one of them.

The anticalmodulin effect of aflatoxin B1 on purified erythrocyte Ca(2+)-ATPase

The genotoxic carcinogen aflatoxin B1 (AFB1) inhibited the calmodulin-stimulated membrane-bound (Ca2+Mg2+)-ATPase. Using the purified enzyme, 12 nmoles per ml of AFB1 caused maximum inhibition of 28% and 50%, of the acidic phospholipid-stimulated and calmodulin-activated Ca(2+)-ATPase activity respectively. Treatment of red cell ghosts with increasing concentrations of Triton X-100, a non-ionic detergent caused a progressive loss of both the basal and calmodulin-stimulated Ca(2+)-ATPase activity. The activity of the phospholipid-free, detergent-solubilized enzyme was almost fully restored by phosphatidyl serine (PS) and its sensitivity to calmodulin was restored in the presence of phosphatidyl choline (PC). Analysis of the results obtained using varying concentrations of ATP shows that AFB1 did not affect the Km and Vmax of the unstimulated enzyme whereas these parameters were reduced by about 75% and 50%, respectively, in the presence of calmodulin. Using the product of limited proteolysis by trypsin i.e. the 90 kDa fragment which still retains its calmodulin binding-domain and the 76 kDa fragment which has lost this domain, kinetic studies on the enzyme activity revealed that AFB1 inhibited the calmodulin-activated 90 kDa fragment by about 50% while the 76 kDa was not affected at all by the toxin and calmodulin. The toxin had no significant affect on the basal activity of the 90 kDa limited proteolysis fragment of the enzyme. These observations suggest that AFB1 inhibits the activated Ca(2+)-ATPase by binding to an important site in the calmodulin-binding domain of the enzyme. It seems likely that the toxin binds to tryptophan in the calmodulin-binding domain, thus causing a reduction in the rate at which this domain can interact with Ca(2+)-calmodulin or acidic phospholipids. The implication of these observations is that Ca(2+)-extrusion and other calmodulin-activated enzymes and processes may be slowed down during prolonged exposure to AFB1 because of its anticalmodulin effect.

Calmodulin stabilizes an amphiphilic alpha-helix within RC3/neurogranin and GAP-43/neuromodulin only when Ca2+ is absent

Two neuronal protein kinase C substrates, RC3/neurogranin and GAP-43/neuromodulin, preferentially bind to calmodulin (CaM) when Ca2+ is absent. We examine RC3.CaM and GAP-43.CaM interactions by circular dichroism spectroscopy using purified, recombinant RC3 and GAP-43, sequence variants of RC3 displaying qualitative and quantitative differences in CaM binding affinities, and overlapping peptides that cumulatively span the entire amino acid sequence of RC3. We conclude that CaM stabilizes a basic, amphiphilic alpha-helix within RC3 and GAP-43 under physiological salt concentrations only when Ca2+ is absent. This provides structural confirmation for two binding modes and suggests that CaM regulates the biological activities of RC3 and GAP-43 through an allosteric, Ca(2+)-sensitive mechanism that can be uncoupled by protein kinase C-mediated phosphorylation. More generally, our observations imply an alternative allosteric regulatory role for the Ca(2+)-free form of CaM.

Calmodulin: effects of cell stimuli and drugs on cellular activation

The activity, localization and cellular content of CaM can be regulated by drugs, hormones and neurotransmitters. Regulation of physiological responses of CaM can depend upon local Ca(2+)-entry domains in the cells and phosphorylation of CaM target proteins, which would either decrease responsiveness of CaM target enzymes or increase CaM availability for binding to other target proteins. Despite the abundance of CaM in many cells, persistent cellular activation by a variety of substances can lead to an increase in CaM, reflected both in the nucleus and other cellular compartments. Increases in CaM-binding proteins can accompany stimuli-induced increases in CaM. A role for CaM in vesicular or protein transport, cell morphology, secretion and other cytoskeletal processes is emerging through its binding to cytoskeletal proteins and myosins in addition to the more often investigated activation of target enzymes. More complete knowledge of the physiological regulation of CaM can lead to a greater understanding of its role in physiological processes and ways to alter its actions through pharmacology.

Evidence that tamoxifen binds to calmodulin in a conformation different to that when binding to estrogen receptors, through structure-activity study on ring-fused analogues

A ring-fused analogue of tamoxifen, which had previously been shown to have practically identical estrogen receptor (ER) affinity and antitumour potency against estrogen responsive cells as tamoxifen, failed to inhibit calmodulin-dependent cyclic AMP phosphodiesterase. The substitution of an extra methyl group into the ring-fused analogue, at a position which the ethyl group of tamoxifen can occupy in one of its conformations, restored the calmodulin inhibition. Also, the replacement of the tamoxifen ethyl group by methyl diminishes calmodulin inhibition. Direct interaction of these tamoxifen analogues with calmodulin was demonstrated through the use of the fluorescent probe, 2-p-toluidinyl-naphthalene-6-sulfonic acid (TNS). These findings lead to the conclusion that tamoxifen binds to calmodulin in a conformation not accessible to the fused analogue and therefore likely to be different to that which binds to the ER. Also, the results on the ring-fused analogues indicate that the calmodulin binding cannot be essential for antitumour activity.

Calcium-supported calpain degradation rates for cardiac myofibrils in diabetes. Sulfhydryl and hydrophobic interactions

OBJECTIVE

The purpose was to investigate the calcium required for calpain-mediated degradation of selected cardiac myofibril proteins modified by diabetes, sulfhydryl (SH) and hydrophobic reagents.

METHODS

After 20 weeks of streptozotocin-induced (55 mg.kg-1) diabetes, calcium sensitive calpain (1.5 U.ml-1) degradation rates of purified cardiac myofibrillar proteins (1 mg.ml-1) were measured, in vitro, and compared to degradation rates for N-ethylmaleimide (NEM) and 2-p-toluidinylnapthalene-6-sulfonate (TNS) treated samples.

RESULTS

Diabetes (blood glucose of 550 +/- 32 mg.dl-1) reduced the yield of purified myofibrillar protein with minimal change in fibril protein composition. Total SH group reactivities (nmol.mg-1.30min) were 220 +/- 21, 163 +/- 17 and 156 +/- 24 for control, diabetic and NEM-treated (0.5 mM) myofibrils (p < or = 0.05). Calpain degradation rates were faster for all diabetic and SH modified myofibrillar proteins (p < or = 0.05), with a 45 and 35% reduction in the pCa50 for a 37 kDa protein of diabetic and NEM-treated fibril complexes. For control myofibrils, both 100 and 200 uM TNS, reduced calpain degradation rates to a similar extent for all substrate proteins. In contrast, diabetic and NEM-treated samples showed a further reduction in calpain degradation rates with increasing TNS from 100 to 200 uM.

CONCLUSION

Our results support the hypothesis that in diabetes the calcium requirements for calpain degradation rates are reduced and dependent upon sulfhydryl group status and Ca(2+)-induced hydrophobic interactions, implicating a 37 kDa myofbillar-complexed protein.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

Polyamine transport regulation by calcium and calmodulin: role of Ca(2+)-ATPase

The study was conducted on human leukemia (K 562) cells to characterize the mechanisms implicated in the regulation of the polyamine spermidine (Spd) transport process. The antagonists of calmodulin, trifluoperazine (TFP), W-7 (N-[6-aminohexyl]-5-chloro-1-naphthelenesulfonamide), or mellitin inhibited significantly polyamine Spd uptake in these cells. The translocation of calmodulin towards plasma membrane and a concomitant decrease in its contents in cytosol were directly correlated with the time course increases similar to that of Spd uptake, indicating that calmodulin is recruited towards plasma membrane during the Spd transport process. Diminution of free intracellular calcium, (Ca2+)i, by preincubating the cells in BAPTA (bis[2-amino-5-methylphenoxyl]-ethane-N,N,N',N',-tetraacetate) buffer inhibited Spd transport significantly. Addition of lanthanum (LAN), a molecule known to inhibit Ca2+ efflux via Ca(2+)-ATPase, curtailed Spd uptake by these cells. LAN inhibited Vmax, but not the Km, of Spd uptake, indicating that the former does not directly interact with the polyamine transporter; rather it regulates the transport process, probably via its action on Ca(2+)-ATPase. Calmodulin-stimulated uptake of 45Ca2+ by inside-out vesicles of K 562 cells, a measure of Ca(2+)-ATPase activity. Furthermore, addition of LAN inhibited both basal and calmodulin-stimulated activity of Ca(2+)-ATPase. Thapsigargin (THAP), a molecule known to elevate (Ca2+)i due to its action on the endoplasmic reticulum, increased Spd transport whereas addition of LAN inhibited THAP-stimulated Spd transport activity. THAP increased free (Ca2+)i in these cells, and a pre-addition of LAN to these cells curtailed the THAP-stimulated increases of (Ca2+)i concentrations. Addition of Spd brought about elevations in (Ca2+)i contents. Caffeine also increased (Ca2+)i in these cells; however, it failed to stimulate significantly the Spd uptake process, indicating that (Ca2+)i which is involved in the regulation of polyamine transport pathways does not belong to the calcium-induced calcium-release (CICR) pool. Replacement of Ca2+ from the incubation medium (i.e., 0% Ca2+) resulted in higher uptake activity as compared to that in 100% Ca2+ medium, demonstrating that in 100% Ca2+ medium the calcium efflux process is quickly compensated by calcium refilling/influx from the extracellular medium, while in 0% Ca2+ medium there is perpetual efflux of (Ca2+)i which contributes to higher Spd uptake process. The results of this study suggest that an increase in free (Ca2+)i and its release from the cells via Ca(2+)-ATPase, and concomitant activation of calmodulin, which controls Ca(2+)-pump activity, are involved in the regulation of the Spd uptake process in human leukemia cells.

A peptide analog of the calmodulin-binding domain of myosin light chain kinase adopts an alpha-helical structure in aqueous trifluoroethanol

A 22-residue synthetic peptide encompassing the calmodulin (CaM)-binding domain of skeletal muscle myosin light chain kinase was studied by two-dimensional NMR and CD spectroscopy. In water the peptide does not form any regular structure; however, addition of the helix-inducing solvent trifluoroethanol (TFE) causes it to form an alpha-helical structure. The proton NMR spectra of this peptide in 25% and 40% TFE were assigned by double quantum-filtered J-correlated spectroscopy, total correlation spectroscopy, and nuclear Overhauser effect correlated spectroscopy spectra. In addition, the alpha-carbon chemical shifts were obtained from (1H,13C)-heteronuclear multiple quantum coherence spectra. The presence of numerous dNN(i, i + 1), d alpha N(i, i + 3), and d alpha beta(i, i + 3) NOE crosspeaks indicates that an alpha-helix can be formed from residues 3 to 20; this is further supported by the CD data. Upfield alpha-proton and downfield alpha-carbon shifts in this region of the peptide provide further support for the formation of an alpha-helix. The helix induced by TFE appears to be similar to that formed upon binding of the peptide to CaM.

Calmodulin mediates melatonin cytoskeletal effects

In this article, we review the data concerning melatonin interactions with calmodulin. The kinetics of melatonin-calmodulin binding suggest that the hormone modulates cell activity through intracellular binding to the protein at physiological concentration ranges. Melatonin interaction with calmodulin may allow the hormone to modulate rhythmically many cellular functions. Melatonin's effect on tubulin polymerization, and cytoskeletal changes in MDCK and N1E-115 cells cultured with melatonin, suggest that at low concentrations (10(-9) M) cytoskeletal effects are mediated by its antagonism to Ca2+-calmodulin. At higher concentrations (10(-5)M) non-specific binding of melatonin to tubulin occurs thus overcoming the specific melatonin antagonism to Ca2+-calmodulin. Since the structures of melatonin and calmodulin are phylogenetically well preserved, calmodulin-melatonin interaction probably represents a major mechanism for regulation and synchronization of cell physiology.

Characterization of the smooth muscle calponin and calmodulin complex

Calponin interacts with several Ca2+ binding proteins in a Ca(2+)-dependent manner. In order to determine the possible biological relevance of these interactions in smooth muscle function, it is necessary to characterize the strength and stoichiometry of the complexes formed. The interaction between calponin and calmodulin can be monitored through an acrylodan label on a cysteine of calponin. The fluorescently labeled calponin possesses the same biological function and physical behavior in binding to calmodulin as the native calponin. This probe is very environment-sensitive and responds to the calponin-calmodulin interaction by the emission peak blue-shifting 20 nm and by the fluorescent quantum yield increasing 3.5 times at 460 nm. The stoichiometric nature of this complex has been determined using analytical ultracentrifugation and is two calmodulins to one calponin, and the interaction is Ca(2+)-sensitive with a Kd1 of < or = 0.22 microM and a Kd2 of 2.5-3.4 microM. Calmodulin is not the only protein which interacts with calponin in this manner, but rather this interaction seems to be a general feature attributable to all hydrophobic patch exposing proteins, suggesting that it may be nonspecific, occurring because of a generalized mode of interaction. Two other proteins, S-100b from bovine brain and SMCaBP-11 from smooth muscle, had stronger affinities for calponin, and in particular interaction of SMCaBP-11 with calponin may be biologically relevant. In determining the nature of calponin's interaction with these Ca2+ binding proteins, it was apparent there was no effect of Ca2+ upon calponin itself and physical studies could find no evidence that calponin interacts with calcium.

Purification and characterization of a novel 12,000-Da calcium binding protein from smooth muscle

A new low molecular weight calcium binding protein, designated 12-kDa CaBP, has been isolated from chicken gizzard using a phenyl-Sepharose affinity column followed by ion-exchange and gel filtration chromatographies. The isolated protein was homogeneous and has a molecular weight of 12,000 based on sodium dodecyl sulfate-gel electrophoresis. The amino acid composition of this protein is similar to but distinct from other known low molecular weight Ca2+ binding proteins. Ca2+ binding assays using Arsenazo III (Sigma) indicated that the protein binds 1 mol of Ca2+/mol of protein. The 12-kDa CaBP underwent a conformational change upon binding Ca2+, as revealed by uv difference spectroscopy and circular dichroism studies in the aromatic and far-ultraviolet range. Addition of Ca2+ to the 12-kDa CaBP labeled with 2-p-toluidinylnaphthalene-6-sulfonate (TNS) resulted in a sevenfold increase in fluorescence intensity, accompanied by a blue shift of the emission maximum from 463 to 445 nm. Hence, the probe in the presence of Ca2+ moves to a more nonpolar microenvironment. Like calmodulin and other related Ca2+ binding proteins, this protein also exposes a hydrophobic site upon binding calcium. Fluorescence titration with Ca2+ using TNS-labeled protein revealed the presence of a single high affinity calcium binding site (kd approximately 1 x 10(-6) M).

Purification and characterization of S-modulin, a calcium-dependent regulator on cGMP phosphodiesterase in frog rod photoreceptors

S-modulin is a 26 kDa protein that regulates light sensitivity of cGMP phosphodiesterase in a Ca(2+)-dependent manner in frog rod outer segments (ROSs). In the present study, we purified S-modulin by taking advantage of a hydrophobic interaction between Phenyl Sepharose and S-modulin at high Ca2+ concentrations. The yield was greater than 90%. 45Ca(2+)-binding experiment showed that S-modulin is a Ca(2+)-binding protein. At high Ca2+ concentrations, S-modulin binds to ROS membranes. The binding target of the Ca2+/S-modulin complex is possibly a ROS membrane lipid(s), but it was difficult to identify. The binding was observed mainly at greater than 1 microM Ca2+. The amino acid sequence deduced from proteolytic fragments of S-modulin was approximately 80% and 60% identical to those of recovering and visinin, respectively.

The solution structures of calmodulin and its complexes with synthetic peptides based on target enzyme binding domains

Small-angle X-ray and neutron scattering experiments have given important information on the solution structures of calmodulin and its complexes with synthetic peptides used to model target enzyme interactions. In combination with crystallographic data, site directed mutagenesis and various spectroscopic studies, these experiments have contributed to our understanding of the solution structure of calmodulin in different functional states. We have gained important insights into the conformational flexibility in calmodulin that appears to be crucial to its regulatory functions. Specifically, flexibility in the interconnecting helix region of calmodulin has been shown to play a critical role in facilitating calmodulin's binding to a wide variety of target enzymes whose activities are thus regulated. This review will focus mainly on the contributions small-angle scattering has made to our understanding of the solution structure of calmodulin in the context of other studies, with particular regard to circular dichroism and Fourier transform infrared studies that complement the small-angle scattering data.

Identification of two domains which mediate the binding of activating phospholipids to the plasma-membrane Ca2+ pump

The stimulation of the purified human erythrocyte calcium pump by acidic phospholipids was investigated using synthetic peptides corresponding to a putative phospholipid-responsive domain [Zvaritch, E., James, P., Vorherr, T., Falchetto, R., Modyanov, N. & Carafoli, E. (1990) Biochemistry 29, 8070-8076] and to the calmodulin-binding domain of the pump. The peptides interfered with the activation of the enzyme by phosphatidylserine and phosphatidic acid in competition assays. The peptide corresponding to the calmodulin-binding domain was found to be the most efficient antagonist. Direct binding measurements using fluorescent derivatives of the peptides confirmed the interaction between the acidic phospholipids and the peptides, and fluorescence titrations of dansylated calmodulin with the purified ATPase showed a direct effect of acidic phospholipids on calmodulin binding. A proteolyzed preparation of the Ca(2+)-ATPase lacking the calmodulin-binding domain confirmed that the phospholipid-induced stimulation is mediated by two sites, one located in the C-terminal portion of the previously identified 44-amino-acid phospholipid-responsive domain, the other in the calmodulin-binding domain.

Solution structure of phosphorylase kinase studied using small-angle X-ray and neutron scattering

Small-angle X-ray and neutron scattering have been used to characterize the solution structure of rabbit skeletal phosphorylase kinase. The radius of gyration of the unactivated holoenzyme determined from neutron scattering is 94 A, and its maximum dimension is approximately 275-295 A. A planar model has been constructed that is in general agreement with the dimensions of the transmission electron microscope images of negatively stained phosphorylase kinase and that gives values for the radius of gyration, maximum linear dimension, and a pair distribution function for the structure that are consistent with the scattering data.

In vitro effects of organophosphorus compounds on calmodulin activity

In vitro effects of organophosphorus compounds (OP), such as malathion (M), methyl parathion (MP) and ethyl parathion (EP), on calmodulin (CaM) activity and its active conformation were studied to understand the mechanism(s) of neurotoxicity, since CaM is known to regulate Ca2+ transport and the enzymes involved in signal transduction and nucleotide metabolism. The biological activity of CaM was assessed as a measure of phosphodiesterase (PDE) stimulation. The effect of OP compounds on the active conformation of CaM was determined by studying the binding of fluorescence probes, namely N-phenyl-1-naphthylamine (NPN), and changes in dansyl-calmodulin fluorescence. Dansylated calmodulin was also used to study the effect of OP compounds on complex formation between CaM and PDE. All three OP compounds inhibited the CaM activity and its active conformation in a concentration-dependent manner. Malathion was less effective in comparison to EP and MP, with IC50 values of 37 microM, 34.5 microM and 32 microM, respectively, for CaM activity. EP and MP significantly altered NPN and dansyl-calmodulin fluorescence (50 microM concentrations of OP compounds), whereas M did not show any significant effect on NPN fluorescence. All these compounds significantly affected complex formation between the dansylated CaM and PDE. These results suggest that OP compounds may be interacting with CaM, altering its active conformation, and thus may be inhibiting its biological activity.

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.

Fluorescence analysis of calmodulin mutants containing tryptophan: conformational changes induced by calmodulin-binding peptides from myosin light chain kinase and protein kinase II

Peptide-induced conformational changes in five isofunctional mutants of calmodulin (CaM), each bearing a single tryptophan residue either at the seventh position of each of the four calcium-binding loops (i.e., amino acids 26, 62, 99, and 135) or in the central helix (amino acid 81) were studied by using fluorescence spectroscopy. The peptides RS20F and RS20CK correspond to CaM-binding amino acid sequence segments of either nonmuscle myosin light chain kinase (nmMLCK) or calmodulin-dependent protein kinase II (CaMPK-II), respectively. Both steady-state and time-resolved fluorescence data were collected from the various peptide-CaM complexes. Steady-state fluorescence intensity measurements indicated that, in the presence of an excess of calcium, both peptides bind to the calmodulin mutants with a 1:1 stoichiometry. The tryptophans located in loops I and IV exhibited red-shifted emission maxima (356 nm), high quantum yields (0.3), and long average lifetimes (6 ns). They responded in a similar manner to peptide binding, by only slight changes in their fluorescence features. In contrast, the fluorescence intensity of the tryptophans in loops II and III decreased markedly, and their fluorescence spectrum was blue-shifted upon peptide binding. Analysis of the tryptophan fluorescence decay of the last mentioned calmodulins supports a model in which the equilibrium between two (Trp-99) or three (Trp-62) states of these tryptophan residues, each characterized by a different lifetime, was altered toward the blue-shifted short lifetime component upon peptide binding. Taken together, these data provide new evidence that both lobes of calmodulin are involved in peptide binding. Both peptides induced similar changes in the fluorescence properties of the tryptophan residues located in the calcium-binding loops, with the exception of calmodulin with Trp-135. For this last mentioned calmodulin, slight differences were observed. Tryptophan in the central helix responded differently to RS20F and RS20CK binding. RS20F binding induced a red-shift in the emission maximum of Trp-81 while RS20CK induced a blue-shift. The quenching rate of Trp-81 by iodide was slightly reduced upon RS20CK binding, while RS20F induced a 2-fold increase. These results provide evidence that the environment of Trp-81 is different in each case and are, therefore, consistent with the hypothesis that the central helix can play a differential role in the recognition of, or response to, CaM-binding structures.

Small-angle X-ray scattering study of calmodulin bound to two peptides corresponding to parts of the calmodulin-binding domain of the plasma membrane Ca2+ pump

The interaction between calmodulin (CaM) and two synthetic peptides, C20W and C24W, corresponding to parts of the calmodulin-binding domain of the Ca2+ pump of human erythrocytes, has been studied by using small-angle X-ray scattering (SAXS). The total length of the CaM-binding domain of the enzyme is estimated to be 28 amino acids. C20W contains the 20 N-terminal amino acids of this domain, C24W the 24 C-terminal amino acids. The experiments have shown that the binding of either peptide results in a complex with a radius of gyration (Rg) smaller than that of CaM. The complex between CaM and C20W revealed an interatomic length distribution function, P(r), similar to that of calmodulin alone, indicating that the complex retains an extended, dumbbell-shaped structure. By contrast, the binding of C24W resulted in the formation of a globular structure similar to those observed with many other CaM-binding peptides.

Effects of the anesthetics heptanol, halothane and isoflurane on gap junction conductance in crayfish septate axons: a calcium- and hydrogen-independent phenomenon potentiated by caffeine and theophylline, and inhibited by 4-aminopyridine

This study has monitored junctional and nonjunctional resistance, [Ca2+]i and [H+]i, and the effects of various drugs in crayfish septate axons exposed to neutral anesthetics. The uncoupling efficiency of heptanol and halothane is significantly potentiated by caffeine and theophylline. The modest uncoupling effects of isoflurane, described here for the first time, are also enhanced by caffeine. Heptanol causes a decrease in [Ca2+]i and [H+]i both in the presence and absence of either caffeine or theophylline. A similar but transient effect on [Ca2+]i is observed with halothane. 4-Aminopyridine strongly inhibits the uncoupling effects of heptanol. The observed decrease in [Ca2+]i with heptanol and halothane and negative results obtained with different [Ca2+]o, (Ca2+)-channel blockers (nisoldipine and Cd2+) and ryanodine speak against a Ca2+ participation. Negative results obtained with 3-isobutyl-1-methylxanthine, forskolin, CPT-cAMP, 8Br-cGMP, adenosine, phorbol ester and H7, superfused in the presence and absence of caffeine and/or heptanol, indicate that neither the heptanol effects nor their potentiation by caffeine are mediated by cyclic nucleotides, adenosine receptors and kinase C. The data suggest a direct effect of anesthetics, possibly involving both polar and hydrophobic interactions with channel proteins. Xanthines and 4-aminopyridine may participate by influencing polar interactions. The potentiating effect of xanthines on cell-to-cell uncoupling by anesthetics may provide some clues on the nature of cardiac arrhythmias in patients treated with theophylline during halothane anesthesia.

Modulation of calmodulin properties by amiodarone and its major metabolite desethylamiodarone

Long-term amiodarone therapy is invariably associated with some side effects. Although its mechanism of action, as an antiarrhythmic drug is well understood, the side effect profile of amiodarone is not yet established. To determine possible mechanisms, the interaction of amiodarone and its major metabolite desethylamiodarone with calmodulin was investigated, since calmodulin is known to regulate Ca2+ transport, cell proliferation and the enzymes involved in signal transduction and nucleotide metabolism. The interaction between the drugs and calmodulin was studied by monitoring intrinsic tyrosine fluorescence of calmodulin and by using a fluorescent probe, N-phenyl-1-naphthylamine (NPN). 14C-Chlorpromazine displacement studies were conducted to differentiate the specific binding sites. The effect on the biological activity of calmodulin was determined with calmodulin dependent phosphodiesterase and Ca2(+)-ATPase. The dansyl calmodulin was used as fluorescent probe to study the effect of these drugs on complex formation between calmodulin and phosphodiesterase. Both amiodarone and desethylamiodarone decreased tyrosine fluorescence of calmodulin with IC50 of 4.9 and 4.4 microM respectively and these interactions were Ca2(+)-dependent. NPN fluorescence was also affected in a concentration dependent manner. These drugs also displaced bound 14C-chlorpromazine from calmodulin and the effect was biphasic. However, desethylamiodarone was more potent than amiodarone. The binding of 3H-amiodarone to calmodulin was modified by a variety of compounds, one class of compounds decreased and the other increased 3H-amiodarone binding to calmodulin. Only, desethylamiodarone inhibited the phosphodiesterase activation by calmodulin with IC50 of 13.2 microM without changing the basal enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of the dihydropyridine calcium antagonist, CD-349, with calmodulin

The characteristics of the binding of the 1,4-dihydropyridine Ca2+ antagonist, 2-nitratopropyl 3-nitratopropyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine 3,5-dicarboxylate (CD-349), to calmodulin (CaM) and the effect of CD-349 on the Ca2+/CaM-dependent enzyme, cyclic GMP (cGMP) phosphodiesterase (PDE), were investigated. CD-349 showed a Ca2(+)-dependent binding to CaM, in equilibrium column binding studies. CD-349 inhibited the [3H]CD-349 binding to CaM, at a concentration producing a 50% inhibition (IC50) of 2.4 microM, whereas the CaM antagonist, trifluoperazine hydrochloride (TFP), stimulated the [3H]CD-349 binding to CaM. Scatchard plot analysis of the binding of CD-349 to CaM revealed that the apparent dissociation constant (Kapp) of CD-349 was 2.1 microM and the maximal number of binding sites (Bmax) of CD-349 was 1.0 nmol/nmol CaM. In the presence of TFP, the Kapp and Bmax values of CD-349 binding to CaM were changed to 1.1 microM and 1.5 nmol/nmol CaM respectively. Although the CaM antagonists, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) and TFP, decreased and increased, respectively, the fluorescence intensity caused by 2-p-toluidinylnaphthalene-6-sulfonic acid (TNS)-CaM binding, CD-349 only slightly decreased the fluorescence of TNS bound CaM. CD-349 inhibited both basal and Ca2+/CaM-activated cGMP PDE activity. However, CaM did not competitively antagonize the CD-349-induced inhibition of the Ca2+/CaM-activated PDE activity. In addition, the kinetic study showed that CD-349 inhibited both basal and Ca2+/CaM-activated cGMP PDE activity, competitively with cGMP, with almost the same inhibition constant (Ki). These results suggest that CD-349 binds to CaM, with Ca2+ dependency, at sites differing from those which bind to the CaM antagonist. The inhibitory activity of CD-349 on Ca2+/CaM-dependent PDE does not seem to be due to a CaM antagonistic action.

Effects of calcium and calmodulin antagonists on calpain II subunit conformations

Only the 80-kD catalytic subunit of smooth muscle calpain II shows a change in intrinsic fluorescence on binding calcium, but both the 80-kD and 30-kD subunits show fluorescence changes in bound toluidinyl-naphthalenesulphonate as a result of calcium binding. Both subunits also show changes in intrinsic fluorescence in the presence of calmidazolium and felodipine. These studies indicate that both subunits have binding sites for calcium and the calmodulin antagonists, which are probably located in the calmodulin-like domain of each subunit.

19F-n.m.r. study of trifluoperazine-S100 protein interaction: effects of Ca2+ and Zn2+

19F-n.m.r. spectra were measured to investigate the effects of Ca2+ and Zn2+ on the interaction of trifluoperazine (TFP) with three S100 proteins. It was found that TFP binds to S100a and S100ao proteins irrespective of the presence of Ca2+ and Zn2+, while in the presence of Ca2+ the apparent affinity of TFP to the proteins was greater than that in its absence or in the presence of Zn2+. In contrast, the binding affinity of TRP to S100b protein in the presence and absence of metal ions was lower than to S100a and S100ao proteins. These results suggested that TFP binds to each S100 protein in two ways: one is Ca2(+)- or Zn2(+)-dependent specific manner and another is Ca2(+)- or Zn2(+)-independent non-specific manner.

Differential effects of triorganotins on calmodulin activity

In vitro effects of three triorganotins--tributyltin (TBT), triethyltin (TET), and trimethyltin (TMT)--on calmodulin (CaM) activity were studied. Stimulation of Ca2(+)-ATPase of rat brain synaptic membranes and phosphodiesterase (PDE) of bovine brain were assayed as indicators of CaM activity. The rat synaptic membranes were prepared and CaM was depleted by washing with 1 mM EGTA. All the three organotins inhibited the basal as well as CaM-stimulated Ca2(+)-ATPase in a concentration-dependent manner, suggesting their interaction with calcium pump. However, CaM-stimulated Ca2(+)-ATPase was more sensitive than the basal enzyme. The order of potency of the three organotin compounds was TBT greater than TET greater than TMT. The IC50 values of Ca2(+)-ATPase (basal) were 0.63, 35, and approximately 800 microM, respectively, whereas the values for CaM-stimulated Ca2(+)-ATPase were 0.05, 0.8, and 18 microM for TBT, TET, and TMT, respectively. CaM-deficient PDE did not show any sensitivity to these three organotin compounds, while TBT and TET significantly decreased the CaM-stimulated PDE activity. TMT, which was the least effective inhibitor of Ca2+ pump, did not alter PDE activity. Further, the inhibition of CaM-stimulated Ca2(+)-ATPase activity by these organotins could be reversed by excess addition of CaM. These results suggest that the organotins interact with CaM activity, as evidenced by their potent effect on CaM-dependent Ca2(+)-ATPase and PDE activities.

Isolation and characterization of a novel molecular weight 11,000 Ca2(+)-binding protein from smooth muscle

A new low molecular weight calcium-binding protein, designated as SMCaBP-11, has been isolated from chicken gizzard using a phenyl-Sepharose affinity column followed by ion-exchange and gel filtration chromatographies. The isolated protein was homogeneous by the criteria of gel electrophoresis in the absence and presence of sodium dodecyl sulfate (NaDodSO4). Molecular weight studies by both sedimentation equilibrium in 6 M guanidine hydrochloride and 15% polyacrylamide-SDS gels indicated the subunit molecular weight to be 11,000, and since a molecular weight of 21,000 was obtained in native solvents, the protein exists as a dimer in benign medium. The amino acid composition of this protein is similar but distinct from other known low molecular weight Ca2(+)-binding proteins. Ca2(+)-binding assays using Arsenazo III (Sigma) indicated the protein to bind 2 mol of Ca2+/subunit. In non-SDS gels, the protein moved faster in the presence of EDTA, suggesting that Ca2+ binding affects its mobility in a manner similar to other smooth muscle calcium-binding proteins such as calmodulin and 67-kDa calcimedin. Upon binding calcium, the protein underwent a conformational change as revealed by UV difference spectroscopy and circular dichroism studies in the aromatic and far-ultraviolet range. When the protein was excited at 280 nm, the tyrosine fluorescence emission maximum was centered at 306 nm. Ca2+ addition resulted in a nearly 15% decrease in intrinsic fluorescence intensity. Fluorescence titration with Ca2+ exhibited two classes of calcium-binding sites with Kd values of 0.2 and 80 microM, in agreement with UV difference spectral data.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence spectroscopic analysis of calpain II interactions with calcium and calmodulin antagonists

1. The intrinsic fluorescence of epoxysuccinyl-inhibited calpain II undergoes a Ca2(+)-dependent decrease which contrasts with the increase observed for calmodulin. 2. Calpain II was inhibited by the calmodulin antagonist toluidinylnaphthalenesulfonate (TNS), and a Ca2(+)-dependent increase in TNS fluorescence intensity was observed for epoxysuccinyl-inhibited calpain II. 3. The calmodulin antagonists calmidazolium CDZ and felodipine both caused decreases in the intrinsic fluorescence of epoxysuccinyl-inhibited calpain II. 4. Increasing concentrations of Ca2+ caused an increase in the fluorescence intensity of the inhibited enzyme in the presence of (CDZ), and a decrease in the presence of felodipine. 5. It is concluded from these studies that Ca2+ and calmodulin antagonists induce conformational changes in calpain II, and that changes occur in regions other than the Ca2(+)-binding domains.

Ca2+-independent interaction of the gamma subunit of phosphorylase kinase with dansyl-calmodulin

A strong Ca2+-independent interaction between the isolated, active gamma subunit of phosphorylase kinase and dansyl-calmodulin (dansyl-CaM) was observed by monitoring changes in fluorescence intensity in the absence of calcium ion. The pure, active gamma subunit of phosphorylase kinase was simply prepared by dialyzing the HPLC-purified, inactive gamma subunit against 8 M urea, containing 0.1 mM DTT, 0.1 M Hepes at pH 6.8 or 0.1 M Tris at pH 8.2, followed by dilution of urea with pH 6.8 or 8.2 buffer. The dissociation constants determined by fluorescence spectroscopy for the gamma subunit to dansyl-CaM are 25.7 +/- 0.6 and 104 +/- 12 nM at pH 6.8 in the presence and absence of CaCl2. At pH 8.2, these values are 4.9 +/- 0.3 and 29 +/- 8 nM in the presence and absence of CaCl2. As the free Ca2+ decreases to as low as 10(-9) M, the fluorescence intensity and the fluorescence polarization of the gamma subunit and dansyl-CaM complex do not decrease in parallel, indicating that the complex does not come apart at low Ca2+ concentration. The presence of Mg2+ affects the interaction between dansyl-CaM and the gamma subunit, as indicated by the increase in the polarization of fluorescence of dansyl-CaM. Mn2+ interferes with the interaction of the gamma subunit and dansyl-CaM. Free ATP has little effect.

Purification and characterization of 81K, heat stable calmodulin-binding protein from bovine brain

Heat stable calmodulin-binding protein has been purified from Triton X-100 soluble particulate fraction of bovine brain. Considerable purification was achieved with calmodulin coupled Sepharose 4B affinity chromatography. SDS-PAGE of the purified protein revealed the apparent homogeneity being 92% at Mr 81,000. Isoelectric focusing of purified 81K protein gave isoelectric point of 4.3. The amino acid composition was notable for high contents of acidic amino acids (15.0 mol% of glutamic acid and 8.1 mol% of aspartic acid) and 17.4 mol% of alanine. On alkaline 1 M urea gel electrophoresis, mobility of the purified 81K protein in the presence of Ca2+ and calmodulin became lower than 81K protein alone toward the anode; however, Ca2+ solely did not affect the mobility of this protein. Similarly, S-100 protein and troponin C showed the interaction with 81K protein and a decrease of mobility in the presence of Ca2+ in alkaline urea PAGE. Binding assay of 125I-labeled calmodulin revealed that 81K protein could bind to an equimolar of 125I-calmodulin as apparent dissociation constant (Kd) of 0.65 x 10(-6) M.

Purification of heat shock protein 90 from calf uterus and rat liver and characterization of the highly hydrophobic region

Heat shock protein 90 was purified from calf uterus and rat liver. Both heat shock protein 90s had similar molecular weights, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, of Mr 87,000 and 88,000, isoelectric points of 5.2, and Stokes radii of 6.7 and 6.5 nm, respectively. Heat shock protein 90 bound to phenyl-Sepharose CL-4B even at low ionic strength, and also bound to butyl-Toyopearl at high ionic strength. Heat shock protein 90 bound to phenyl-Sepharose could be eluted with a buffer containing organic solvents or detergents such as 2-propanol, dioxane, dimethylformamide, methyl cellosolve, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate or Triton X-100, but not with ionic salts such as 1 M KCl. These results suggest that heat shock protein 90 possesses a significant hydrophobic region on the surface of the molecule. Hydrophobicities of heat shock protein 90 and 4S calf uterine estrogen receptor were both decreased by formation of a 8 S estrogen receptor complex. The role of the hydrophobic region of heat shock protein 90 in the interaction with estrogen receptor and other proteins is discussed.

Phosphatidylinositol modulates the response of calmodulin-dependent phosphatase to calmodulin

Phosphatidylinositol (PtdIns) and many other phospholipids activated calmodulin (CaM)-dependent phosphatase in the presence or absence of Ca2+, and the stimulation was more pronounced in the presence of Ca2+. In addition, PtdIns modulated the response of phosphatase to CaM: at low and nonstimulatory concentrations (less than 70 microM), PtdIns augmented the activity of phosphatase by a submaximum concentration of CaM, giving a synergistic effect; and at high concentrations (greater than 100 microM), PtdIns suppressed the synergistic effect. Kinetic experiments indicated that PtdIns (both nonstimulatory and stimulatory concentrations) increased the affinity of phosphatase for CaM. In addition to the CaM regulatory site, phosphatase appears to have two PtdIns regulatory sites: a high-affinity site the occupation of which does not stimulate enzyme activity, and a low-affinity site the occupation of which stimulates enzyme activity in the absence of CaM and inhibits it in the presence of CaM. Modulating the response of phosphatase to CaM is not unique to PtdIns, and was observed with other phospholipids, including some that did not stimulate the enzyme. This raises the possibility that certain phospholipids may regulate phosphatase in two ways: (i) direct activation of the enzyme and (ii) modulation of its response to CaM.