Fluorescence investigations of calmodulin hydrophobic sites

Binding of calcium and magnesium to human cardiac troponin C

Cardiac muscle thin filaments are composed of actin, tropomyosin, and troponin that change conformation in response to Ca²⁺ binding, triggering muscle contraction. Human cardiac troponin C (cTnC) is the Ca²⁺-sensing component of the thin filament. It contains structural sites (III/IV) that bind both Ca²⁺ and Mg²⁺ and a regulatory site (II) that has been thought to bind only Ca²⁺. Binding of Ca²⁺ at this site initiates a series of conformational changes that culminate in force production. However, the mechanisms that underpin the regulation of binding at site II remain unclear. Here, we have quantified the interaction between site II and Ca²⁺/Mg²⁺ through isothermal titration calorimetry and thermodynamic integration simulations. Direct and competitive binding titrations with WT N-terminal cTnC and full-length cTnC indicate that physiologically relevant concentrations of both Ca²⁺/Mg²⁺ interacted with the same locus. Moreover, the D67A/D73A N-terminal cTnC construct in which two coordinating residues within site II were removed was found to have significantly reduced affinity for both cations. In addition, 1 mM Mg²⁺ caused a 1.4-fold lower affinity for Ca²⁺. These experiments strongly suggest that cytosolic-free Mg²⁺ occupies a significant population of the available site II. Interaction of Mg²⁺ with site II of cTnC likely has important functional consequences for the heart both at baseline as well as in diseased states that decrease or increase the availability of Mg²⁺, such as secondary hyperparathyroidism or ischemia, respectively.

X-ray structures of magnesium and manganese complexes with the N-terminal domain of calmodulin: insights into the mechanism and specificity of metal ion binding to an EF-hand

Calmodulin (CaM), a member of the EF-hand superfamily, regulates many aspects of cell function by responding specifically to micromolar concentrations of Ca(2+) in the presence of an ~1000-fold higher concentration of cellular Mg(2+). To explain the structural basis of metal ion binding specificity, we have determined the X-ray structures of the N-terminal domain of calmodulin (N-CaM) in complexes with Mg(2+), Mn(2+), and Zn(2+). In contrast to Ca(2+), which induces domain opening in CaM, octahedrally coordinated Mg(2+) and Mn(2+) stabilize the closed-domain, apo-like conformation, while tetrahedrally coordinated Zn(2+) ions bind at the protein surface and do not compete with Ca(2+). The relative positions of bound Mg(2+) and Mn(2+) within the EF-hand loops are similar to those of Ca(2+); however, the Glu side chain at position 12 of the loop, whose bidentate interaction with Ca(2+) is critical for domain opening, does not bind directly to either Mn(2+) or Mg(2+), and the vacant ligand position is occupied by a water molecule. We conclude that this critical interaction is prevented by specific stereochemical constraints imposed on the ligands by the EF-hand β-scaffold. The structures suggest that Mg(2+) contributes to the switching off of calmodulin activity and possibly other EF-hand proteins at the resting levels of Ca(2+). The Mg(2+)-bound N-CaM structure also provides a unique view of a transiently bound hydrated metal ion and suggests a role for the hydration water in the metal-induced conformational change.

Disease progression in MRL/lpr lupus-prone mice is reduced by NCS 613, a specific cyclic nucleotide phosphodiesterase type 4 (PDE4) inhibitor

Systemic lupus erythematosus is a polymorphic and multigenic inflammatory autoimmune disease. Cyclic AMP (cAMP) modulates inflammation and the inhibition of cyclic nucleotide phosphodiesterase type 4 (PDE4), which specifically hydrolyzes cAMP, inhibits TNFα secretion. This study was aimed at investigating the evolution of PDE activity and expression levels during the course of the disease in MRL/lpr lupus-prone mice, and to evaluate in these mice the biological and clinical effects of treatments with pentoxifylline, denbufylline and NCS 613 PDE inhibitors. This study reveals that compared to CBA/J control mice, kidney PDE4 activity of MRL/lpr mice increases with the disease progression. Furthermore, it showed that the most potent and selective PDE4 inhibitor NCS 613 is also the most effective molecule in decreasing proteinuria and increasing survival rate of MRL/lpr mice. NCS 613 is a potent inhibitor, which is more selective for the PDE4C subtype (IC₅₀ = 1.4 nM) than the other subtypes (PDE4A, IC₅₀ = 44 nM; PDE4B, IC₅₀ = 48 nM; and PDE4D, IC₅₀ = 14 nM). Interestingly, its affinity for the High Affinity Rolipram Binding Site is relatively low (K_i = 148 nM) in comparison to rolipram (K_i = 3 nM). Finally, as also observed using MRL/lpr peripheral blood lymphocytes (PBLs), NCS 613 inhibits basal and LPS-induced TNFα secretion from PBLs of lupus patients, suggesting a therapeutic potential of NCS 613 in systemic lupus. This study reveals that PDE4 represent a potential therapeutic target in lupus disease.

Insights into modulation of calcium signaling by magnesium in calmodulin, troponin C and related EF-hand proteins

The Ca(2+)-binding helix-loop-helix structural motif called "EF-hand" is a common building block of a large family of proteins that function as intracellular Ca(2+)-receptors. These proteins respond specifically to micromolar concentrations of Ca(2+) in the presence of ~1000-fold excess of the chemically similar divalent cation Mg(2+). The intracellular free Mg(2+) concentration is tightly controlled in a narrow range of 0.5-1.0mM, which at the resting Ca(2+) levels is sufficient to fully or partially saturate the Ca(2+)-binding sites of many EF-hand proteins. Thus, to convey Ca(2+) signals, EF-hand proteins must respond differently to Ca(2+) than to Mg(2+). In this review the structural aspects of Mg(2+) binding to EF-hand proteins are considered and interpreted in light of the recently proposed two-step Ca(2+)-binding mechanism (Grabarek, Z., J. Mol. Biol., 2005, 346, 1351). It is proposed that, due to stereochemical constraints imposed by the two-EF-hand domain structure, the smaller Mg(2+) ion cannot engage the ligands of an EF-hand in the same way as Ca(2+) and defaults to stabilizing the apo-like conformation of the EF-hand. It is proposed that Mg(2+) plays an active role in the Ca(2+)-dependent regulation of cellular processes by stabilizing the "off state" of some EF-hand proteins, thereby facilitating switching off their respective target enzymes at the resting Ca(2+) levels. Therefore, some pathological conditions attributed to Mg(2+) deficiency might be related to excessive activation of underlying Ca(2+)-regulated cellular processes. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

The flavonoid dioclein is a selective inhibitor of cyclic nucleotide phosphodiesterase type 1 (PDE1) and a cGMP-dependent protein kinase (PKG) vasorelaxant in human vascular tissue

The inhibitory effect of the flavonoid dioclein was assessed on purified vascular cyclic nucleotide phosphodiesterase isoforms (EC 3.1.4.17, PDE1-5) in comparison with 8-methoxymethyl-isobutylmethylxanthine (8-MM-IBMX) and vinpocetine which are currently used as PDE1 inhibitors. The mechanism underlying the vasorelaxant effect of dioclein was investigated in human saphenous vein. Dioclein inhibited PDE1 more selectively than vinpocetine and 8-MM-IBMX, with IC(50) values of 2.47+/-0.26 and 1.44+/-0.35 microM, respectively in basal- and calmodulin-activated states. Dioclein behaved as a competitive inhibitor for cGMP hydrolysis by PDE1 in basal- and calmodulin-activated states (K(i)=0.62+/-0.14 and 0.55+/-0.07 microM, respectively), indicating this inhibitory effect to be independent of calmodulin interactions. In addition, dioclein induced a concentration-dependent relaxation of human saphenous vein which was independent on the presence of functional endothelium (EC(50) values of 7.3+/-3.1 and 11+/-2.7 microM, respectively with and without endothelium). 8-MM-IBMX relaxed human saphenous vein with an EC(50)=31+/-16 microM, whereas vinpocetine did not cause any vasorelaxation at concentrations up to 100 microM. Rp-8-pCPT-cGMPS, which inhibits cGMP-dependent protein kinase (PKG), blocked the vasodilator effect of dioclein, whereas H-89, which is a cAMP-dependent protein kinase (PKA) inhibitor, had a minor inhibitory effect. Our data show that dioclein is a potent calmodulin-independent selective inhibitor of PDE1 and that inhibition of PDE1 is involved in the PKG-mediated vasorelaxant effect of dioclein in human saphenous vein. Furthermore, dioclein may represent a new archetype to develop more specific PDE1 inhibitors.

Inhibition of firefly luciferase by alkane analogues

We reported that anesthetics increased the partial molal volume of firefly luciferase (FFL), while long-chain fatty acids (LCFA) decreased it. The present study measured the actions of dodecanol (neutral), dodecanoic acid (negatively charged), and dodecylamine (positively charged) hydrophobic molecules on FFL. The interaction modes are measured by (1) ATP-induced bioluminescence of FFL and (2) fluorescence of 2-(p-toluidino)naphthalene-6-sulfonate (TNS). TNS fluoresces brightly in hydrophobic media. It competes with the substrate luciferin on the FFL binding. From the Scatchard plot of TNS titration, the maximum binding number of TNS was 0.83, and its binding constant was 8.27 x 10(5) M(-1). Job's plot also showed that the binding number is 0.89. From the TNS titration of FFL, the binding constant was estimated to be 8.8 x 10(5) M(-1). Dodecanoic acid quenched the TNS fluorescence entirely. Dodecanol quenched about 25% of the fluorescence, whereas dodecylamine increased it. By comparing the fluorescence of TNS and bioluminescence of FFL, the binding modes and the inhibition mechanisms of these dodecane analogues are classified in three different modes: competitive (dodecanoic acid), noncompetitive (dodecylamine), and mixed (dodecanol).

Structural characterization of a recombinant flagellar calcium-binding protein from Trypanosoma cruzi

Calflagin are flagellar calcium-binding proteins belonging to the EF-hand super family described in several protozoa, including Trypanosoma cruzi. Evidences have shown that Ca(2+) may play an important regulatory role in trypanosomatid flagellar mobility. In these parasites, the response of the cell to variations of Ca(2+) levels is determined by a variety of calcium-modulated proteins. Starting from T. cruzi cDNA lambdagt11 library trypomastigote, a clone encoding a 29-kDa flagellar protein designated recombinant calflagin (rC29) was selected. rC29 is a calcium-acyl switch protein modified by the addition of myristate and palmitate at its amino terminal segment. In this work, unmyristoylated rC29 was expressed in Escherichia coli as an intein fusion protein and purified by affinity chromatography. Circular dichroism (CD) and fluorescence measurements showed conformational changes of rC29 due to Ca(2+) binding. The Ca(2+) binding constants were obtained by tryptophan intrinsic fluorescence spectroscopy. Fluorescence titration exhibited two classes of Ca(2+)-binding sites in the unmyristoylated rC29, which bind calcium with apparent association constant of K(a) of 3.3+/-0.5 (10(6)) and 1.9+/-0.2 (10(4)) M(-1). Experiment using 8-anilinonaphthalene-1-sulfonic acid (ANS) as hydrophobic probe showed that the Ca(2+)-loaded form of rC29 contains exposed hydrophobic surfaces, thus suggesting that rC29 is probably functioning as a calcium sensor.

Effect of sildenafil on cyclic nucleotide phosphodiesterase activity, vascular tone and calcium signaling in rat pulmonary artery

(1) Sildenafil (viagra) is a potent PDE5 inhibitor and thus a relaxant drug in corpus carvernosum smooth muscle. In the present work, we evidenced the presence of PDE5 isozyme and investigated the effect of sildenafil on the specific cyclic nucleotide phosphodiesterase (PDE) activity, smooth muscle tone and calcium signaling in the rat main pulmonary artery (MPA). (2) The PDE activity was measured in cytosolic and microsomal fractions. Total cAMP and cGMP-PDE activities were mainly present in the cytosolic fraction. Sildenafil (0.1 micro M) reduced by 72% cGMP-PDE activity, whereas zaprinast (10 micro M), a relatively selective PDE5 inhibitor, reduced this activity by 63%. Sildenafil (0.1 micro M) also inhibited significantly (22%) the cAMP-PDE activity. (3) Western blot analysis revealed the expression of PDE5 mainly in the cytosolic fraction of MPA. Sildenafil concentration-dependently inhibited (IC(50)=3.4 nM) the activity of MPA PDE5 partially purified by HPLC. (4) Sildenafil (0.1 nM-50 micro M) concentration-dependently relaxed MPA rings precontracted with phenylephrine (0.5 micro M). The potency of sildenafil (IC(50)=11 nM) was similar to that of a nitric oxide donor, sodium nitroprusside, but higher than that of zaprinast (IC(50)=600 nM). The vasorelaxant effect of sildenafil was not altered by endothelium removal or in the presence of KT 5823 (1 micro M) and H89 (1 micro M), potent inhibitors of PKG and PKA, respectively. (5) In isolated MPA myocytes, which had been loaded with the calcium fluorophore indo-1, sildenafil (10-100 nM) antagonized ATP- and endothelin-1-induced calcium oscillations but had no effect on the transient caffeine-induced [Ca(2+)](i) response. (6) This study demonstrates the presence of a functional and highly sildenafil-sensitive PDE5 isozyme in rat MPA. Inhibition of this isozyme mainly accounts for the potent pulmonary vasodilator action of sildenafil, which involves alteration in the inositol triphosphate-mediated calcium signaling pathway.

Enhancement by Mg2+ of domain specificity in Ca2+-dependent interactions of calmodulin with target sequences

Mg2+ binds to calmodulin without inducing the changes in secondary structure that are characteristic of Ca2+ binding, or the exposure of hydrophobic surfaces that are involved in typical Ca2+-dependent target interactions. The binding of Mg2+ does, however, produce significant spectroscopic changes in residues located in the Ca2+-binding loops, and the Mg-calmodulin complex is significantly different from apo-calmodulin in loop conformation. Direct measurement of Mg2+ binding constants, and the effects of Mg2+ on Ca2+ binding to calmodulin, are consistent with specific binding of Mg2+, in competition with Ca2+. Mg2+ increases the thermodynamic stability of calmodulin, and we conclude that under resting, nonstimulated conditions, cellular Mg2+ has a direct role in conferring stability on both domains of apo-calmodulin. Apo-calmodulin binds typical target sequences from skeletal muscle myosin light chain kinase and neuromodulin with Kd approximately 70-90 nM (at low ionic strength). These affinities are virtually unchanged by 5 mM Mg2+, in marked contrast to the strong enhancement of peptide affinity induced by Ca2+. Under conditions of stimulation and increased [Ca2+], Mg2+ has a role in directing the mode of initial target binding preferentially to the C-domain of calmodulin, due to the opposite relative affinities for binding of Ca2+ and Mg2+ to the two domains. Mg2+ thus amplifies the intrinsic differences of the domains, in a target specific manner. It also contributes to setting the Ca2+ threshold for enzyme activation and increases the importance of a partially Ca2+-saturated calmodulin-target complex that can act as a regulatory kinetic and equilibrium intermediate in Ca2+-dependent target interactions.

Inhibition of type 4 phosphodiesterase by rolipram and Ginkgo biloba extract (EGb 761) decreases agonist-induced rises in internal calcium in human endothelial cells

The effects of Gingko biloba extract EGb 761 on 5 isolated, vascular, cyclic nucleotide phosphodiesterase (PDE) isoforms were evaluated. EGb 761 preferentially inhibited PDE4 (IC(50)=25.1 mg/L), the isoform that is mainly present in endothelial cells, in a competitive manner (K:(i)=12.5 mg/L). Because changes in cyclic nucleotide levels may affect intracellular calcium ([Ca(2+)](i)) levels in endothelial cells, we examined the effects of EGb 761 on both resting [Ca(2+)](i) levels and agonist-induced rises in [Ca(2+)](i) in single human umbilical vein endothelial cells (HUVECs) in culture. The effects of EGb 761 were compared with those of rolipram, a selective PDE4 inhibitor that increases cellular cAMP levels, and the cAMP analogue dibutyryl cAMP (db-cAMP). EGb 761 (20 and 100 mg/L), rolipram (50 micromol/L), and db-cAMP (100 micromol/L) significantly inhibited histamine-, ATP-, and thrombin-induced [Ca(2+)](i) increases in HUVECs without modifying resting [Ca(2+)](i) levels. Similar results were obtained by using a Ca(2+)-free bath solution. EGb 761 (100 mg/L), but not rolipram (50 micromol/L) or db-cAMP (100 micromol/L), also inhibited Ca(2+) influx into cells having thapsigargin-depleted internal Ca(2+) stores and bathed in a Ca(2+)-free external solution. Our results are consistent with an inhibition of PDE activity that causes a reduction of agonist-induced increases in [Ca(2+)](i) in HUVECs, mainly by inhibition of Ca(2+) mobilization from internal stores. It thus may be that the cardiovascular effects of EGb 761 involve inhibition of PDE4 activity and subsequent modification of Ca(2+) signaling in endothelial cells.

Characterization of cyclic nucleotide phosphodiesterase isoforms associated to isolated cardiac nuclei

The identity and location of nuclear cyclic nucleotide phosphodiesterases (PDE) has yet to be ascertained. Intact cardiac nuclei and subnuclear fractions from ovine hearts were isolated to determine cAMP-specific PDE activity which was 3-fold greater than that of cGMP PDE, the latter being insensitive to Ca-calmodulin and zaprinast. Specific hydrolytic activities of the cardiac nuclear envelopes (NE) were similar to those measured in the corresponding intact nuclei, thus suggesting that most PDE activity is associated with the nuclear membrane. Moreover, the main hydrolytic activities in cardiac nuclei were attributed to PDE4 (56%) and PDE3 (44%). The pharmacological sensitivity of each isoform in terms of IC(50), K(m) and K(i) values was typical of previously characterized cardiac PDE 3 and 4 isoforms. PDE2 (cGMP-stimulated PDE) represented a minor component (8-9%) of total hydrolytic activity. Solubilization of nuclear envelopes and HPLC separation also yielded rolipram-sensitive PDE activities. Upon 1% Triton X-100 extractions, the presence of PDE3 and PDE4 was revealed in a low speed, nucleopore complex-enriched, P1 pellet. In addition, Western blot analysis demonstrated the presence of PDE4B and PDE4D subtypes in the nuclei as well as enrichment in NE. However, in the same preparations, the presence of PDE4A could not be ascertained. Altogether, these results suggest an intrinsic and predominant association of these nuclear PDEs with the NE and much likely with nucleopore complexes.

Low temperature aqueous electrospray ionization mass spectrometry of noncovalent complexes

In the present study we describe conditions that permit the characterization of noncovalent protein-substrate complexes in aqueous solution by microspray electrospray ionization-mass spectrometry (ESI-MS), using a heated transfer capillary at low temperature (45 degrees C). Specifically, we examined the binding of calmodulin to two polypeptides; the calmodulin-binding domain of calmodulin-dependent protein kinase II (CamK-II) and melittin. Calmodulin, a well known calcium-binding protein, binds to a number of small amphipathic peptides in a calcium-dependent manner. Our results directly show that both peptides form equimolar complexes with calmodulin only in the presence of calcium. The stoichiometry necessary for the formation of each complex was 1:1:4 for calmodulin:peptide (melittin or CamK-II):Ca2+, respectively. Furthermore, it is demonstrated that the detection of the complex in ESI-MS is source temperature dependent.

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.

Cytosolic and membrane-bound cyclic nucleotide phosphodiesterases from guinea pig cardiac ventricles

Cyclic nucleotide phosphodiesterase (PDE) activities were characterized in the cytosolic and post-nuclear membrane preparations of guinea pig cardiac ventricles. The cytosolic PDE activities were stimulated 5-fold by calmodulin (CaM) on both substrates (1 microM) and 1.2-fold by cGMP (5 microM) on cAMP hydrolysis. Conversely, in the membrane preparation, CaM only stimulated PDE activities 1.2- to 1.4-fold, but cGMP induced a 3-fold increase of the hydrolysis of cAMP. In both the cytosolic and the membrane preparations, the hydrolysis of cAMP was inhibited by 100 microM of either the PDE III inhibitor SK&F 94120 (27% and 31% respectively) or the PDE IV inhibitor rolipram (14% and 23% respectively). Four peaks were resolved from the cytosolic preparation by chromatography. Peak A and peak B hydrolyzed both cAMP and cGMP and were stimulated respectively by CaM and cGMP. Peak C and peak D selectively hydrolyzed cAMP. Peak C had an apparent Km value for cAMP of 3.3 microM and was inhibited by PDE IV inhibitors. Peak D showed an apparent Km value for cAMP of 0.43 microM and was inhibited by cGMP and by cardiotonic inhibitors of PDE III. Similar potencies of these inhibitors were observed in the membrane preparation. These results suggest that in guinea pig cardiac ventricles: (1) PDE I (CaM-activated) is almost exclusively cytosolic; (2) PDE II (cGMP-stimulated), PDE III (cGMP-inhibited and cardiotonic-sensitive) and PDE IV (rolipram-sensitive) are present in cytosolic and membrane preparations; (3) PDE III and PDE IV differ in their apparent Km values for cAMP. The latter observation could explain the differential effects of PDE III and PDE IV inhibitors in the regulation of cardiac contraction.

The interaction of auramine O with calmodulin: location of the binding site on the connecting strand

The cationic dye auramine O forms a fluorescent complex with Ca(2+)-liganded calmodulin. One moderately strong binding site is present, as well as one or more weaker sites. The binding site for auramine O is different from those for toluidinyl-naphthalene sulfonate. The dependence of binding upon electrolyte concentration suggests a substantial electrostatic component of the free energy of binding. The splitting of the bond between residues 77 and 78 by trypsin digestion abolishes auramine O binding; the N- and C-terminal half-molecules have virtually no binding capacity. This suggests that the primary binding site is located near the midpoint of the connecting strand and includes elements of both half-molecules. Thrombin digestion, which splits calmodulin between residues 106 and 107, also substantially reduces auramine O binding; this may be interpreted in terms of the stabilization of the structure of the connecting strand by interaction with residues within binding domain IV. The binding affinity at pH 5.0, where the helical organization of the connecting strand may be intact, is greater than at neutral pH.

The interaction of troponin C with phosphofructokinase. Comparison with calmodulin

Phosphofructokinase (PFK) is a calmodulin (CaM)-binding protein [Mayr & Heilmeyer (1983) FEBS Lett. 195, 51-57]. We found that troponin C (TnC), which is homologous to CaM, also binds PFK and affects PFK's catalytic activity, aggregation states and conformational changes as CaM does in most cases. PFK titration of N-acetylaminoethyl-5-naphthylamido-1-sulphonate ('AEDANS')-TnC showed that its apparent dissociation constant is comparable with that of PFK-CaM. Fluorescent labels were also used to probe contact regions on TnC and CaM. It is likely that the C-terminal end of the connecting strand of the TnC molecule is close to PFK in the binary complex. Hydrophobic regions of TnC and CaM also possibly play roles in the binding and polymerization of PFK. TnC and CaM deactivate PFK through accelerating PFK conformational change as well as through accelerating PFK tetramer dissociation, as implied in the results of activity, light-scattering, fluorescence and c.d. experiments. The intact molecule of CaM appears to be required to deactivate PFK, because neither half of the CaM molecule has an effect on PFK activity.

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.

Retinoic acid inhibits calmodulin binding to human erythrocyte membranes and reduces membrane Ca2(+)-adenosine triphosphatase activity

Ca2(+)-ATPase activity in human red cell membranes is dependent on the presence of calmodulin. All trans-retinoic acid inhibited human red cell membrane Ca2(+)-ATPase activity in vitro in a concentration-dependent manner (10(-8) to 10(-4) M). In contrast, retinol, retinal, 13-cis-retinoic acid and the benzene ring analogue of retinoic acid did not alter enzyme activity. Purified calmodulin (up to 500 ng/ml, 3 X 10(-8) M) added to red cell membranes, in the presence of inhibitory concentrations of retinoic acid, only partially restored Ca2(+)-ATPase activity. 125I-Calmodulin bound to red cell membranes was displaced by unlabeled retinoic acid (50% reduction at 10(-8) M retinoic acid), as effectively as by unlabeled calmodulin. Another calmodulin-stimulable enzyme, bovine brain cyclic nucleotide phosphodiesterase, was unaffected by retinoic acid. 8-Anilino-1-naphthalene sulfonic acid bound to calmodulin, studied spectrofluorometrically, was not displaced by retinoic acid. Thus, retinoic acid inhibits calmodulin binding to red cell membranes, reducing calmodulin-stimulable Ca2(+)-ATPase activity. Retinoic acid does not directly interact with calmodulin, but rather exerts its effect by interfering with calmodulin access to the membrane enzyme. These effects occur at physiological concentrations of the retinoid.

The interaction of cyclosporin and calmodulin

The interaction of cyclosporin A and dansyl cyclosporin A with bovine and wheat germ calmodulin has been monitored by measurements of induced changes in dansyl and bound toluidinyl naphthalene sulfonate fluorescence. The interaction is Ca2(+)-dependent and 1:1. Measurements of the efficiency of radiationless energy transfer from bound dansyl cyclosporin A to an acceptor group located on Cys-27 of wheat germ calmodulin suggest that the primary binding site is not located on the N-terminal lobe (residues 1-65). However, studies with proteolytic fragments of calmodulin indicate that elements of the N-terminal half-molecule (residues 1-77) may be involved in the stabilization of the binding site. The binding of cyclosporin alters the physical properties of calmodulin and, in particular, reduces the localized rotational mobility of a fluorescent probe.

Characterization of cyclic nucleotide phosphodiesterases from cultured bovine aortic endothelial cells

Experiments were carried out in order to isolate and characterize the cyclic nucleotide phosphodiesterase activities in primary and low passages of cultured bovine aortic endothelial cells. The subcellular characterization of the cyclic nucleotide hydrolytic activity showed that both cAMP and cGMP hydrolytic activities were predominant in the cytosolic rather than the particulate fraction of the endothelial cell homogenate. At a low substrate concentration (0.25 microM), the major hydrolytic activity was for cAMP while at a high concentration (20 microM) it was for both cAMP and cGMP. Both cAMP and cGMP hydrolytic activities were insensitive to calmodulin. Cytosolic cyclic nucleotide phosphodiesterase activity was resolved into two distinct phosphodiesterase forms using HPLC. The first eluted form was designated cGS-PDE: it hydrolysed both cAMP and cGMP and its cAMP hydrolytic activity was markedly enhanced by the presence of cGMP. The second form was designated cAMP-PDE: it selectively hydrolysed cAMP. The cytosolic cAMP-PDE was inhibited by micromolar concentrations of cAMP-PDE inhibitors such as trequinsin, rolipram, dipyridamole or papaverine. The cGS-PDE was inhibited by micromolar concentrations of trequinsin, dipyridamole and papaverine and was insensitive to rolipram, except for the hydrolysis of cAMP which was inhibited in the micromolar range. Both the cAMP-PDE and the cGS-PDE were relatively insensitive to the selective cGMP-PDE inhibitor, zaprinast which was about 750-fold less potent on endothelial PDEs than on smooth muscle cGMP-PDE. The identification of selective and specific PDE inhibitors of the different PDE forms may allow a better understanding of the regulation and the role of cyclic nucleotides in endothelial cells.

The interaction of calmodulin with the C-terminal M5 peptide of myosin light chain kinase

The interaction with calmodulin of the 17-residue C-terminal fragment M5 of myosin light chain kinase has been studied by several physical techniques. Circular dichroism measurements suggest that M5 exists within the complex primarily as an alpha-helix. Fluorescence intensity measurements of the single tryptophan of M5 (Trp-4) indicate that it is in a relatively nonpolar environment and is shielded from solvent. Dynamic measurements of fluorescence anisotropy decay indicate that Trp-4 changes from a freely rotating fluorophore to one which is largely immobilized upon complex formation. Static fluorescence measurements show that 2,6-TNS is displaced from its binding site on calmodulin by M5. The binding of M5 also partially inhibits the proteolytic scission by trypsin of the bond between residues 77 and 78.

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.

Amiodarone is a potent calmodulin antagonist

The possible interaction between amiodarone, a potent antiarrhythmic and antianginal agent, and calmodulin (CaM) was investigated by three avenues of approach: (a) Effect of amiodarone on cardiac and vascular Ca2+/calmodulin-activated cyclic nucleotide phosphodiesterase (CaM-PDE); (b) Effect on the CaM-activated (Ca2+ + Mg2+)-ATPase from human erythrocytes; (c) Direct interaction between amiodarone and calmodulin measured by the effect of the drug on the fluorescence of 9-anthroylcholine (9AC) bound to calmodulin. Results show that amiodarone did not interact with basal activities of CaM-PDE and other isolated CaM-insensitive PDE forms as well as with (Ca2+ + Mg2+)-ATPase. Amiodarone inhibited calmodulin-activation of aortic CaM-PDE (Ki = 650 nM, substrate cGMP) and calmodulin-activation of erythrocyte ghosts (Ca2+ + Mg2+)-ATPase (IC50 = 4.5 microM) in an apparently competitive manner. Amiodarone decreased the fluorescence of the hydrophobic probe 9AC bound to calmodulin (IC50 = 5 microM). It is concluded that amiodarone is a potent calmodulin antagonist.

The interaction of delta-hemolysin with calmodulin

Delta-Hemolysin forms a 1:1 complex with Ca2+ -liganded calmodulin. Probably because of the pronounced tendency of delta-hemolysin to self-associate, the apparent binding affinity is much less than that for melittin. Complex formation is reflected by an increase in quantum yield of Trp-15 of delta-hemolysin and by increased shielding from acrylamide quenching. There is, however, no indication of a change in peptide molecular ellipticity. The binding of 2-toluidinyl-naphthalene-6-sulfonate is reduced by complex formation, suggesting the involvement of a hydrophobic region. Complex formation also blocks the proteolysis by trypsin of the bond between residues 77 and 78. The time decays of fluorescence intensity and anisotropy for tryptophan are multiexponential for both free and complexed delta-hemolysin; the average decay time for intensity is substantially increased for the complex. The localized mobility of tryptophan is greatly reduced in the complex. Complex formation appears to involve both the C-terminal lobe and the connecting strand of calmodulin.

Rotational modes of Ca2+-liganded calmodulin, as determined by time-domain fluorescence

The time decay of fluorescence anisotropy was monitored as a function of pH and temperature for complexes of 2,6-toluidinylnaphthalenesulfonate with calmodulin, with its proteolytic fragments, and with the 1:1 complex of calmodulin and melittin. For all the conditions examined the anisotropy decay of native calmodulin involved at least two rotational modes. These corresponded to a short correlation time of 2-3 ns, reflecting a localized motion in the vicinity of the binding site and a longer correlation time which arises from the rotation of a major portion of the molecule. The relative amplitudes of the two rotational modes were dependent upon temperature in the range 11-40 degrees C, the contribution of the more rapid mode increasing with temperature. The maximum immobilization of the probe occurred at pH 5.0 and 12 degrees C. While these results indicate the presence of internal rotations in Ca2+-liganded calmodulin, the magnitude of the longer correlation time is consistent with the crystallographic structure.

Comparative effects of calmodulin inhibitors on calmodulin's hydrophobic sites and on the activation of cyclic nucleotide phosphodiesterase by calmodulin

Experiments were designed to investigate the effect of inhibitors on calmodulin's hydrophobic sites and their consequences on the activation of a target enzyme, cyclic nucleotide phosphodiesterase. Two fluorescent probes, 2-(p-toluidinyl)-naphthalene-6-sulfonate (TNS) and 9-anthroylcholine (9AC) were used to study the interactions with calmodulin of inhibitors devoid of direct effect on the probes. Contrary to W-7, nicergoline, nicardipine and quercetin, which decreased the fluorescence of the two probes bound to calmodulin, bepridil only decreased 9AC fluorescence but increased the fluorescence intensity at the wavelength of the emission maximum of TNS. In spite of this difference, bepridil as well as W-7 and nicergoline competitively inhibited calmodulin activation of phosphodiesterase. In addition, nicergoline also inhibited phosphodiesterase activity competitively to cyclic GMP. These results show differences in the interactions of inhibitors with calmodulin; these differences are not detected in functional studies of the effect of inhibitors on phosphodiesterase activation.

Calcium release from intact calmodulin and calmodulin fragment 78-148 measured by stopped-flow fluorescence with 2-p-toluidinylnaphthalene sulfonate. Effect of calmodulin fragments on cardiac sarcoplasmic reticulum

Calcium release from high and low-affinity calcium-binding sites of intact bovine brain calmodulin (CaM) and from the tryptic fragment 78-148, purified by high-pressure liquid chromatography, containing only the high-affinity calcium-binding sites, was determined by fluorescence stopped-flow with 2-p-toluidinylnaphthalene sulfonate (TNS). The tryptic fragments 1-77 and 78-148 each contain a calcium-dependent TNS-binding site, as shown by the calcium-dependent increase in TNS fluorescence. The rate of the monophasic fluorescence decrease in endogenous tyrosine on calcium dissociation from intact calcium-saturated calmodulin (kobs 10.8 s-1 and 3.2 s-1 at 25 degrees C and 10 degrees C respectively) as well as the rate of equivalent slow phase of the biphasic decrease in TNS fluorescence (kobsslow 10.6 s-1 and 3.0 s-1 at 25 degrees C and 10 degrees C respectively) and the rate of the solely monophasic decrease in TNS fluorescence, obtained with fragment 78-148 (kobs 10.7 s-1 and 3.5 s-1 at 25 degrees C and 10 degrees C respectively), were identical, indicating that the rate of the conformational change associated with calcium release from the high-affinity calcium-binding sites on the C-terminal half of calmodulin is not influenced by the N-terminal half of the molecule. The fast phase of the biphasic decrease of TNS fluorescence, observed by the N-terminal half of the molecule. The fast phase of the biphasic decrease of TNS fluorescence, observed with intact calmodulin only (kobsfast 280 s-1 at 10 degrees C) but not with fragment 78-148, is most probably due to the conformational change associated with calcium release from low-affinity sites on the N-terminal half. The calmodulin fragments 1-77 and 78-148 neither activated calcium/calmodulin-dependent protein kinase of cardiac sarcoplasmic reticulum nor inhibited calmodulin-dependent activation at a concentration approximately 1000-fold greater (5 microM) than that of the calmodulin required for half-maximum activation (5.9 nM at 0.8 mM Ca2+ and 5 mM Mg2+) of calmodulin-dependent phosphoester formation.

Fluorescence study of brevin, the Mr 92 000 actin-capping and -fragmenting protein isolated from serum. Effect of Ca2+ on protein conformation

The fluorescence characteristics of brevin and the effects of Ca2+ on the protein conformation were fully investigated. Brevin contains 18 tryptophans and 27 tyrosines. Analysis of the fluorescence spectra and the accessibility to quenching molecules indicate that the emitting tryptophans are located in a hydrophobic environment (lambda max = 324 nm) close to the protein surface. In native brevin, tyrosyl residues do not contribute to the fluorescence emission. Partial quenching of these chromophores has to be attributed to tyrosine----tryptophan resonance energy transfer which is highly efficient. The effect of brevin on actin polymerization has been shown to be Ca2+ sensitive [Harris, D. A., & Schwartz, J. H. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 6798-6802; Thorstensson, R., Utter, G., & Norberg, R. (1982) Eur. J. Biochem. 126, 11-16; Wilkins, J. A., Schwartz, J. H. & Harris, D. A. (1983) Cell Biol. Int. Rep. 7, 1097-1104; Harris, H. E., & Weeds, A. G. (1983) Biochemistry 22, 2728-2741] and brevin binding to hydrophobic matrices to be Ca2+ dependent (Z. Soua, personal communication). Ca2+ binding to brevin decreases the tryptophan fluorescence polarization degree (without affecting the excited-state lifetime), which suggests a higher chromophore mobility. This effect may be partly related to the slight unshielding of the tryptophan residues observed in fluorescence quenching experiments. Moreover, the reactivity of brevin sulfhydryl groups toward 5,5'-dithiobis(2-nitrobenzoic acid) increases in the presence of Ca2+. On the other hand, fluorescence spectra, quantum yields, excited-state lifetimes, and thermostability remain unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)