Kinetic analysis of the calmodulin-binding region of the plasma membrane calcium pump isoform 4b

The sequence L(1086)RRGQILWFRGLNRIQTQIKVVKAFHSS(1113) (peptide C28) is responsible for calmodulin binding to PMCA4b. In this work, peptides following the above sequence were progressively shortened either at the N-terminus (C28NDelta3, C28NDelta5, or C28NDelta6) or at the C-terminus (C20, C22, C23, and C25). Competitive inhibition of PMCA activity was used to measure apparent dissociation constants of the complexes between calmodulin and C28 or progressively shortened peptides. Additionally, equilibrium titrations were used to measure the apparent dissociation constants of the various peptides with TA-calmodulin by changes in TA-calmodulin fluorescence and Trp fluorescence of the peptides. At the N-terminus, deletion of five residues did not change calmodulin affinity, but deletion of six residues resulted in a 5-fold decrease in affinity. There were no major differences in the time course of TA-CaM binding, but C28NDelta6 exhibited a different time course of Trp fluorescence change. At the C-terminus, deletion of five residues (C23) or more resulted in a net increase in fluorescence of TA-CaM upon binding, while longer peptides (C25 and C28) produced both a transient increase and a net decrease in the fluorescence of TA-CaM. Global regression analysis revealed that binding of TA-CaM to the C23 peptide could be fit by a two-step model, while longer peptides required three-step models for adequate fitting. TA-calmodulin dissociated rapidly from C23, C22, and C20, resulting in a marked increase in apparent K(d). Thus, the sequence I(1091)LWFRGLNRIQTQIKVVKAF(1110) (C25NDelta5) is required to reproduce the calmodulin-binding properties of C28. When F(1110) was replaced by A, the TA-calmodulin association and dissociation kinetics resembled C23 kinetics, but changing V(1107) to A produced a smaller effect, suggesting that F(1110), rather than V(1107), is the main anchor for the N-terminal lobe of calmodulin in PMCA4b.

Calmodulin activity and cAMP signalling modulate growth and apical secretion in pollen tubes

Our present understanding implicates both calmodulin (CaM) and 3',5'-cyclicAMP (cAMP) in the regulation of pollen tube growth. However, downstream molecules of these signalling pathways and the cellular processes they modulate remain largely unknown. In order to elucidate the role of CaM, we mapped its activity in growing pollen tubes. 2-chloro-(epsilon-amino-Lys(75))-[6-4-(N,N'-diethylaminophenyl)-1,3,5-triazin-4-yl]-calmodulin (TA-CaM) and fluorescein-calmodulin (FL-CaM), fluorescent analogues of CaM, were loaded into pollen tubes and CaM activity was mapped by fluorescence ratio imaging. It was found that CaM activity exhibits a tip-focused gradient, similar to the distribution of cytosolic-free calcium ([Ca(2+)](c)). In long pollen tubes, apical CaM activity was also found to oscillate with a period similar to [Ca(2+)](c) (40-80 sec). This oscillatory behaviour was not observed in small pollen tubes or in tubes that had stopped growing. Changes in CaM activity within the dome of the pollen tube apex resulting from the photolysis of caged photolysis of RS-20 (a peptide antagonist of CaM) induced re-orientation of the growth axis, suggesting that CaM is also involved in the guidance mechanism. CaM activity was strongly modulated by intracellular changes in cAMP (induced by activators and antagonists of adenylyl cyclase). These results indicate that the action of this protein is dependent not solely on [Ca(2+)](c) but also on a cross-talk with other signalling pathways. A putative target of this cross-talk is the secretory machinery as observed in pollen tubes loaded with the FM (N-(3-triethylammoniumpropyl)-4-(4-dibutylamino)styryl)pyridinium dibromide 1-43 dye and exposed to different antagonists and activators of these molecules. Our data thus suggest that pollen tube growth and orientation depend on an intricate cross-talk between multiple signalling pathways in which CaM is a key element.

Calmodulin Interacts with the Third Intracellular Loop of the Serotonin 5-Hydroxytryptamine1A Receptor at Two Distinct Sites

The serotonin 5-HT(1A) receptor couples to heterotrimeric G proteins and intracellular second messengers, yet no studies have investigated the possible role of additional receptor-interacting proteins in 5-HT(1A) receptor signaling. We have found that the ubiquitous Ca(2+)-sensor calmodulin (CaM) co-immunoprecipitates with the 5-HT(1A) receptor in Chinese hamster ovary fibroblasts. The human 5-HT(1A) receptor contains two putative CaM binding motifs, located in the N- and C-terminal juxtamembrane regions of the third intracellular loop of the receptor. Peptides encompassing both the N-terminal (i3N) and C-terminal (i3C) CaM-binding domains were tested for CaM binding. Using in vitro binding assays in combination with gel shift analysis, we demonstrated Ca(2+)-dependent formation of complexes between CaM and both peptides. We determined kinetic data using a combination of BIAcore surface plasmon resonance (SPR) and dansyl-CaM fluorescence. SPR analysis gave an apparent K(D) of approximately 110 nm for the i3N peptide and approximately 700 nm for the i3C peptide. Both peptides also caused characteristic shifts in the fluorescence emission spectrum of dansyl-CaM, with apparent affinities of 87 +/- 23 nm and 1.70 +/- 0.16 microm. We used bioluminescence resonance energy transfer to show that CaM interacts with the 5-HT(1A) receptor in living cells, representing the first in vivo evidence of a G protein-coupled receptor interacting with CaM. Finally, we showed that CaM binding and phosphorylation of the 5-HT(1A) receptor i3 loop peptides by protein kinase C are antagonistic in vitro, suggesting a possible role for CaM in the regulation of 5-HT(1A) receptor phosphorylation and desensitization. These data suggest that the 5-HT(1A) receptor contains high and moderate affinity CaM binding regions that may play important roles in receptor signaling and function.

A model for the activation of plasma membrane calcium pump isoform 4b by calmodulin

Overexpression of the plasma membrane calcium pump (PMCA) isoform 4b by means of the baculovirus system enabled us, for the first time, to study the kinetics of calmodulin binding to this pump. This was done by stopped-flow fluorescence measurements using 2-chloro-(amino-Lys(75))-[6-[4-(N,N-diethylamino)phenyl]-1,3,5-triazin-4-yl]calmodulin (TA-calmodulin). Upon mixing with PMCA, the fluorescence of TA-calmodulin changed along a biphasic curve: a rapid and small increase in fluorescence was followed by a slow and large decrease that lasted about 100 s. The experiment was done at several PMCA concentrations. Global fitting nonlinear regression analysis of these results led to a model in which PMCA is present in two forms: a closed conformation and an open conformation. Calmodulin reacts with both conformations but reacts faster and with higher affinity for the open conformation. Measurements of the ATPase activity of PMCA under similar conditions revealed that the open form has higher ATPase activity than the closed one. Contrasting with the reaction with the whole pump, TA-calmodulin reacted rapidly (in about 2 s) with a calmodulin-binding peptide made after the sequence of the calmodulin-binding domain of PMCA (C28). Results of TA-calmodulin binding to C28 are explained by a simpler model, in which only an open conformation exists.

Calcium: just a chemical switch?

The calcium-signature hypothesis has evolved as a concept to explain specificity in signaling pathways that utilise calcium as a second messenger. In plant biology, this hypothesis was purely conceptual and based only upon correlative observations until recently. In the past few years, however, empirical data have emerged from experiments that were specifically designed to tackle the question of how specificity is encoded by calcium. In light of the attractive calcium-signature hypothesis, other potential explanations for signalling specificity have been overshadowed and ignored: it has been assumed that the calcium-signature dogma will explain all plant calcium signaling. However, there is a good deal of evidence supporting a counter-hypothesis in which calcium does not itself encode specificity but is merely an essential 'switch' in signaling. At the very least, both hypotheses are likely to be true in different situations, and it may well be that the calcium-signature hypothesis describes the exception rather than the rule.

Dimethylsilane polyamines: cytostatic compounds with potentials as anticancer drugs. II. Uptake and potential cytotoxic mechanisms

Dimethylsilane tetramines are structural analogues of spermine with a (CH3)2 Si-group incorporated into the central carbon chain. They have potential as anticancer drugs. Their cytotoxic effect was considered to rely mainly on their polyamine antagonist property. In order to obtain new ideas about cellular mechanisms, which are potential targets of the dimethylsilane polyamines, the effects of these compounds on some basic cell functions, such as protein and DNA synthesis, and calmodulin antagonism were studied. In addition, their mode of accumulation in cells was investigated. It became evident that the intracellular accumulation of dimethylsilane polyamines is almost exclusively achieved via the polyamine transport system. However, the exchange of a part of the intracellular natural polyamines against dimethylsilane polyamines has only a small effect on polyamine uptake. Binding to the endoplasmic reticulum and inhibition of protein synthesis are presumably important for the cytotoxic action of bis(11-amino-4,8-diazaundecyl)dimethylsilane, a hexamine, but seem of no importance for the tetramines. Calmodulin antagonism, however, is likely to contribute to their cytotoxic effect.

Integration of calcium signals by calmodulin in rat sensory neurons

We have used the fluorescently labelled calmodulin TA-CaM to follow calmodulin activation during depolarization of adult rat sensory neurons. Calcium concentration was measured simultaneously using the low affinity indicator Oregon Green BAPTA 5N. TA-CaM fluorescence increased during a 200-ms depolarization but then continued to increase during the subsequent 500 ms, even though total cell calcium was falling at this time. In the next few seconds TA-CaM fluorescence fell, but to a new elevated level that was then maintained for several tens of seconds. During a train of depolarizations that evoked a series of largely independent calcium changes TA-CaM fluorescence was in contrast raised for the duration of the train and for many tens of seconds afterwards. The presence of a peptide corresponding to the calmodulin binding domain of myosin light chain kinase significantly increased the depolarization-induced TA-CaM fluorescence increase and slowed the subsequent fall of fluorescence. We interpret the slow recovery component of the TA-CaM signal as reflecting the slow dissociation of calcium--calmodulin--calmodulin binding protein complexes. Our results show that after brief electrical activity calmodulin's interaction with calmodulin binding proteins persists for approximately one minute.

Spectroscopic characterization of the interaction between calmodulin-dependent protein kinase I and calmodulin

Calmodulin-dependent protein kinase I (CaM kinase I) is a member of the expanding class of protein kinases that are regulated by calmodulin (CaM). Its putative CaM-binding region is believed to occur within a 22-residue sequence (amino acids 299-320). This sequence was chemically synthesized and utilized for CaM interaction studies. Gel band shift assays and densitometry experiments with intact CaM kinase I and the CaM-binding domain peptide (CaMKIp) reveal that they bind in an analogous manner, giving rise to 1:1 complexes. Fluorescence analysis using dansyl-CaM showed that conformational changes in CaM on binding CaM kinase I or CaMKIp were nearly identical, suggesting that the peptide mimicked the CaM-binding ability of the intact protein. In the presence of CaM, the peptide displays an enhancement of its unique Trp fluorescence as well as a marked blue shift of the emission maximum, reflecting a transfer to a more rigid, less polar environment. Quenching studies, using acrylamide, confirmed that the Trp in the peptide on binding CaM is no longer freely exposed to solvent as is the case for the free peptide. Studies with a series of Met mutants of CaM showed that the Trp-containing N-terminal end of CaMKIp was bound to the C-terminal lobe of CaM. Near-UV CD spectra also indicate that the Trp of the peptide and Phe residues of the protein are involved in the binding. These results show that the CaM-binding domain of CaM kinase I binds to CaM in a manner analogous to that of myosin light chain kinase.

Use of fluorescently labelled calmodulins as tools to measure subcellular calmodulin activation in living dorsal root ganglion cells

We have used fluorescently labelled calmodulins to probe the activity of calmodulin in living dorsal root ganglion cells. Calmodulin labelled with the fluorophore 5-([4,6 dichlorotriazin-2yl]amino)-fluorescein (FL-CaM) does not change its fluorescence when it binds calcium, while calmodulin labelled at lysine 75 with 2-chloro-(6-(4-N,N-diethylamino-phenyl)-1,4,5-triazin-4-yl (TA-CaM), an environment-sensitive probe, increases its fluorescence when it binds calcium. We micro-injected FL-CaM or TA-CaM into rat dorsal root ganglion cells and found that both probes localise to the cell nucleus. In contrast, endogenous cellular calmodulin, in dorsal root ganglion cells as in hippocampal neurones, is predominantly cytosolic unless the neurones are depolarised, then it moves to the nucleus. FL-CaM and TA-CaM, introduced into dorsal root ganglion cells via a patch pipette, also immediately move to the nucleus, indicating that the nuclear localisation is a property of the labelled calmodulins. Although the subcellular distribution of FL-CaM and TA-CaM does not necessarily match that of endogenous calmodulin, we show that FL-CaM can be used as a control for TA-CaM when studying calmodulin activation in different cellular compartments.

Thermodynamics of target peptide recognition by calmodulin and a calmodulin analogue: implications for the role of the central linker

The thermodynamics of interaction of two model peptides melittin and mastoparan with bovine brain calmodulin (CAM) and a smaller CAM analogue, a calcium binding protein from Entamoeba histolytica (CaBP) in 10 mM MOPS buffer (pH 7.0) was examined using isothermal titration calorimetry (ITC). These data show that CAM binds to both the peptides and the enthalpy of binding is endothermic for melittin and exothermic for mastoparan at 25 degrees C. CaBP binds to the longer peptide melittin, but does not bind to mastoparan, the binding enthalpy being endothermic in nature. Concurrently, we also observe a larger increase in alpha-helicity upon the binding of melittin to CAM when compared to CaBP. The role of hydrophobic interactions in the binding process has also been examined using 8-anilino-1-naphthalene-sulphonic acid (ANS) binding monitored by ITC. These results have been employed to rationalize the energetic consequences of the binding reaction.

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

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

A method for the purification of phospho(Tyr)calmodulin free of nonphosphorylated calmodulin

Phosphocalmodulin has been shown to have a differential biological activity compared to nonphosphorylated calmodulin when assayed on a variety of calmodulin-dependent systems. However, the phosphocalmodulin preparations used so far in those experiments were not necessarily free of nonphosphorylated calmodulin. Therefore, the results obtained may not unquestionably show the real effect of pure phosphocalmodulin on the systems under study. To solve this problem, we describe here a method for the purification of phospho(Tyr)calmodulin free of nonphosphorylated calmodulin. The procedure consists of the following steps: (i) phosphorylation of calmodulin by a fraction enriched in epidermal growth factor receptor tyrosine kinase from rat liver isolated by calmodulin affinity chromatography, (ii) isolation of a calmodulin/phosphocalmodulin mixture by Ca(2+)-dependent chromatography in phenyl-Sepharose, (iii) purification of phospho(Tyr)calmodulin using an anti-phosphotyrosine antibody immobilized in agarose upon elution with phenyl phosphate, and (iv) removal of phenyl phosphate from the phospho(Tyr)calmodulin preparation by filtration chromatography in a Bio-Gel P-2 column. The obtained phospho(Tyr)calmodulin preparation was highly pure and essentially free of nonphosphorylated calmodulin because of the use of anti-phosphotyrosine affinity chromatography. We demonstrate that this ultrapure phospho(Tyr)calmodulin preparation is totally incapable of activating the calmodulin-dependent cyclic nucleotide phosphodiesterase. In contrast, when a nonpurified phospho(Tyr)calmodulin preparation was used a partial activation of this enzyme was observed.

Imaging the spatial dynamics of calmodulin activation during mitosis

BACKGROUND

Calcium is an important and ubiquitous signalling ion. In most cell types, changes in intracellular calcium concentrations are sensed by calmodulin, a signal transduction protein that regulates cell function through its interactions with kinases and phosphatases. Calcium signals show complex spatiotemporal patterning, but little, if anything, is known about the patterns of calmodulin activation inside cells.

RESULTS

We have measured calmodulin activation continuously during mitosis in living cells with a new probe, a fluorescent adduct of calmodulin termed TA-calmodulin. We found that calmodulin was activated locally and episodically in the nucleus and mitotic spindle. The pattern of calmodulin activation was different from the pattern of calcium signals and could not be predicted from the pattern of calcium increase. Calmodulin activation was essential for mitotic progression: both entry into mitosis and exit from mitosis were blocked by a novel peptide that bound to calmodulin with high affinity and so prevented the interaction of calmodulin with its target proteins.

CONCLUSIONS

These data suggest that calmodulin regulates mitotic transitions and demonstrate the utility of fluorescent adducts for studying protein activation in living cells with good temporal and spatial resolution.

Inhibition of calmodulin-activated smooth-muscle myosin light-chain kinase by calmodulin-binding peptides and fluorescent (phosphodiesterase-activating) calmodulin derivatives

Aspects of the biochemistry of calmodulin have been addressed that bear on its cell biological role as a mediator of Ca2+ regulation. Calmodulin-binding peptides derived from the amino acid sequence of smooth-muscle myosin light-chain kinase (MLCK) were characterized as inhibitors of calmodulin activation of MLCK-catalyzed phosphorylation of the smooth-muscle regulatory light chain (MLC). MLCK activity was determined by measuring the rate of formation of one of the reaction products, ADP, in a coupled enzymatic assay by continuous fluorimetric monitoring of NADH removal in 100 microM CaCl2 at ionic strength 0.15 M, pH 7.0 and 21 degreesC. The Km value of calmodulin was 3.5 nM, a value 16-35-fold greater than the Kd value of calmodulin for MLCK [Török, K., and Trentham D. R. (1994) Biochemistry 33, 12807-12820]. The different Km and Kd values are most likely associated with the rate-limiting step in MLC phosphorylation being associated with product release from MLCK. The values of the inhibition constants, Ki, were the following: Ac-R-R-K-W-Q-K-T-G-H-A-V-R-A-I-G-R-L-CONH2 (Trp peptide), 8.6 (+/-1. 4 sd) pM; Y4-analogue of Trp peptide (Tyr peptide), 7.3 (+/-0.1) nM; and A-R-R-K-W-Q-K-T-G-H-A-V-R-A-I-G-R-L-S-S (RS20-like peptide), 0. 11-0.39 nM. The Ki values were consistent with kinetically determined Kd values of the peptides to calmodulin. Kinetic determination of Kd values required the use of a fluorescently labeled calmodulin, 2-chloro-(epsilon-amino-Lys75)-[6-(4-N, N-diethylamino-phenyl)-1,3,5-triazin-4-yl]-calmodulin (TA-calmodulin).1 Since, as here, Lys75 is a convenient labeling site on calmodulin for the introduction of fluorescent probes, the biological activity of the Lys-modified calmodulins was evaluated. TA-calmodulin and calmodulin selectively modified by 1-N, N-dimethylaminonaphthalene-5-sulfonyl chloride (dansyl-C1) at Lys75 (dansyl-calmodulin) were characterized as activators of cyclic AMP phosphodiesterase (PDE) and inhibitors of MLCK. The Km value for dansyl-calmodulin was equal to that of calmodulin, and that of TA-calmodulin was 3.5-fold greater. TA-calmodulin and Lys75-labeled dansyl-calmodulin thus distinguish between PDE and MLCK being agonists to the former and antagonists to the latter.

Circular dichroism and 1H NMR studies on the structures of peptides derived from the calmodulin-binding domains of inducible and endothelial nitric-oxide synthase in solution and in complex with calmodulin. Nascent alpha-helical structures are stabilized by calmodulin both in the presence and absence of Ca2+

There exist two types of nitric-oxide synthase (NOS); constitutive isozymes that are activated by binding calmodulin in response to elevated Ca2+ and an inducible isozyme that binds calmodulin regardless of Ca2+. To study the structural basis of the difference in Ca2+ sensitivity, we have designed synthetic peptides of minimal lengths derived from the calmodulin-binding domain of endothelial NOS (eNOS) and that of macrophage NOS (iNOS). A peptide, KRREIPLKVLVKAVLFACMLMRK, derived from human iNOS sequence, retained the ability to bind to calmodulin both in the presence and absence of Ca2+, while a peptide derived from human eNOS sequence, RKKTFKEVANAVKISASLMG, bound to calmodulin only in the presence of Ca2+. Circular dichroism and two-dimensional 1H nuclear magnetic resonance studies suggested that both peptides assume nascent alpha-helical structures in aqueous solution. When mixed with calmodulin, both peptides showed circular dichroism spectra characteristic for alpha-helix. In contrast to other target proteins, the addition of iNOS peptide to calmodulin did not affect the Ca2+ binding of calmodulin appreciably. The peptide derived from the calmodulin-binding domain of iNOS, therefore, binds in alpha-helical structures both to Ca2+-calmodulin and apo-calmodulin, which is unique among various target proteins of calmodulin.

Connexin 32 of gap junctions contains two cytoplasmic calmodulin-binding domains

A fluorescent calmodulin derivative, 2-chloro-[4-(epsilon-amino-Lys75)]-[6-(4- diethylaminophenyl)-1,3,5-triazin-4-yl]-calmodulin (TA-calmodulin) [Török and Trentham (1994) Biochemistry 33, 12807-12820], and equilibrium fluorescence methods were used to identify calmodulin-binding domains of connexin subunits of gap junctions. Synthetic peptides corresponding to six extramembrane regions of connexin 32, a major component of rat liver gap junctions, and peptides derived from connexin 43 and 26, were tested. Two cytoplasmically oriented peptides that correspond to an N-terminal 21-amino-acid sequence and a 15-amino-acid sequence at the C-terminal tail of connexin 32 bound TA-calmodulin in a Ca2+-dependent manner. The dissociation constants (Kd) of TA-calmodulin binding to GAP 10 (MNWTGLYTLLSGVNRHSTAIG, residues 1-21) and GAP 8M (ACARRAQRRSNPPSR, residues 216-230) were 27 nM and 1.2 microM respectively at 150 mM ionic strength, 2 mM MgCl2, 100 microM CaCl2, pH 7.0 and 21 degrees C. Both halves of each peptide were required for calmodulin binding. Substitution of Trp3 present in all connexins by Tyr increased Kd for TA-calmodulin by 40-fold. Liver gap junctions (whose connexons contain mainly connexin 32) and recombinant connexons constructed of connexin 26 expressed by baculovirus-infected insect cells exhibited weaker binding of TA-calmodulin with variable Ca2+-dependence. These studies identify two calmodulin-binding amino-acid sequences in connexin 32, and provide independent evidence that calmodulin may function as an intracellular ligand, regulating Ca2+-dependent intercellular communication across gap junctions.

Isolated calcium-binding loops of EF-hand proteins can dimerize to form a native-like structure

Helix-loop-helix fragments of EF-hand proteins are known to dimerize in solution, re-producing the characteristic structure of native protein domains [Shaw, G.S., Hodges, R.S., & Sykes, B. D. (1990) Science 249, 280-283]. In this paper we present evidence that isolated calcium-binding loops can also dimerize, when saturated with lanthanide ions, interacting with each other in a similar way as do loops in intact proteins. A synthetic analogue of calcium binding loop III of calmodulin, AcDKDGDGYISAAE-NH2, has been studied by 1H NMR spectroscopy. For the La(3+)-saturated peptide, concentration dependent broadenings and shifts of certain signals have been observed indicating dimerization process of intermediate rate on the NMR time scale. Analysis of signal shape and position of the Tyr7 ring protons as a function of concentration makes it possible to determine the association and dissociation rate constants of the process for various temperatures within the range of 10-80 degrees C. The dimerization constant changes according to van't Hoff relationship with delta S = 233 J/mol.K and delta H = 62 kJ/mol. A distance of 11.4 +/- 0.4 A between the ions coordinated by dimer molecules has been determined by measurements of Tb(3+)-->Ho3+ luminescence energy transfer. This value suggests that the dimer structure is similar to that of two-loop structural elements in native EF-hand proteins. From a thermodynamic cycle it can be shown that La3+ ion binding to the peptide dimers must be highly cooperative. Therefore, cooperativity of ion binding to domains of EF-hand proteins is, at least partly, due to local interactions between binding loops.

Localization of unique functional determinants in the calmodulin lobes to individual EF hands

We have investigated the functional interchangeability of EF hands I and III or II and IV, which occupy structurally analogous positions in the native I-II and III-IV EF hand pairs of calmodulin. Our approach was to functionally characterize four engineered proteins, made by replacing in turn each EF hand in one pair by a duplicate of its structural analog in the other. In this way functional determinants we define as unique were localized to the component EF hands in each pair. Replacement of EF hand I by III reduces calmodulin-dependent activation of cerebellar nitric oxide synthase activity by 50%. Replacement of EF hand IV by II reduces by 60% activation of skeletal muscle myosin light chain kinase activity. There appear to be no major unique determinants for activation of these enzyme activities in the other EF hands. Replacement of EF hand III by I or IV by II reduces by 50-80% activation of smooth muscle myosin light chain kinase activity, and replacement of EF hand I by III or II by IV reduces by 90% activation of this enzyme activity. Thus, calmodulin-dependent activation of each of the enzyme activities examined, even the closely related kinases, is dependent upon a distinct pattern of unique determinants in the four EF hands of calmodulin. All the engineered proteins examined bind four Ca2+ ions with high affinity. Comparison of the Ca2+-binding properties of native and engineered CaMs indicates that the Ca2+-binding affinity of an engineered I-IV EF hand pair and a native I-II pair are similar, but an engineered III-II EF hand pair is intermediate in affinity to the native III-IV and I-II pairs, minimally suggesting that EF hands I and III contain unique determinants for the formation and function of EF hand pairs. The residues directly coordinating Ca2+ ion appear to play little or no role in establishing the different Ca2+-binding properties of the EF hand pairs in calmodulin.

Mechanism of 2-chloro-(epsilon-amino-Lys75)-[6-[4-(N,N- diethylamino)phenyl]-1,3,5-triazin-4-yl]calmodulin interactions with smooth muscle myosin light chain kinase and derived peptides

The mechanism of the interactions of 2-chloro-(epsilon-amino-Lys75)-[6-[4-(N,N-diethylamino)phenyl]- 1,3,5-triazin-4-yl]calmodulin (TA-calmodulin) with smooth muscle myosin light-chain kinase (MLCK) and two 17-residue peptides, Ac-R-R-K-W-Q-K-T-G-H-A-V-R-A-I-G-R-L-CONH2 (Trp peptide) and Tyr peptide, in which W is replaced by Y, were studied by measurements of equilibrium and transient fluorescence changes in the nanomolar range. Most reactions were carried out in 100 microM CaCl2 at ionic strength 0.15 M, pH 7.0, and 21 degrees C. In each case association of MLCK or peptide to TA-calmodulin could be described by a two-step process, a bimolecular step and an isomerization. In the case of the interaction between TA-calmodulin and Tyr peptide it was shown that the isomerization involved the binary complex of TA-calmodulin and Tyr peptide as opposed to an isomerization of either TA-calmodulin or Tyr peptide in isolation. These distinctions depended in part on development for transient kinetic experiments of a general theory to quantify relative phase amplitudes in two-step mechanisms. The kinetics for all three association reactions were then interpreted in terms of a bimolecular association (rate constants k+1 and k-1) followed by an isomerization of the binary complex (rate constants k+2 and k-2). For the interaction of TA-calmodulin and Tyr peptide, values of the rate constants are k+1, 8.8 x 10(8) M-1 s-1; k-1, 5.7 s-1; k+2, 0.38 s-1; and k-2, 0.65 s-1. The fluorescence intensities (lambda ex 365 nm, lambda ex 365 nm, lambda em > 400 nm) of TA-calmodulin, the initial binary complex of TA-calmodulin and Tyr peptide, and the isomerized binary complex are in the ratio 1:2.8:1.3. Analogous mechanisms were found for TA-calmodulin binding to Trp peptide and to MLCK, but values for the rate constants and relative fluorescence intensities of the binary complexes were generally not so completely defined. Values for the Trp peptide and MLCK, respectively, are k+1, 8.8 x 10(8) M-1 s-1 and 1.1 x 10(8) M-1 s-1; (k+2 + k-2), 0.97 s-1 and 1.3 s-1; and k-1k-2/(k+2 + k-2), 0.0079 s-1 and 0.025-0.056 s-1. Equilibrium dissociation constants (Kd) for interactions of TA-calmodulin and targets determined from these data are Tyr peptide, 4.1 nM; Trp peptide, 0.011 nM; and MLCK, 0.23-0.51 nM.(ABSTRACT TRUNCATED AT 400 WORDS)

Biphasic effect of estrogen on neuronal constitutive nitric oxide synthase via Ca(2+)-calmodulin dependent mechanism

Estrogen is known to retard the development of atherosclerosis and to work in the brain, but the mechanism of hormonal action is completely unknown. We investigated the effect of estrogen on the activity of neuronal constitutive nitric oxide synthase (NNOS). A low concentration of estrogen (10(-10)(-7) M) enhanced the activity of homogenates of the cytosol fraction of rabbit cerebellums and also that of partially purified NNOS, and high dose (10(-6)(-5) M) attenuated them. The study using estrogen receptor antagonists, tamoxifen, clomiphene, and ICI182780 suggested that estrogen receptor did not relate significantly to those effects of 17 beta-estradiol. 17 alpha-estradiol or progesterone did not change significantly it in low doses, although moderately inhibited it in high doses. Estrogen enhanced the fluorescence of dansyl-calmodulin in low doses and attenuated it in high doses, suggesting that estrogen affects Ca(2+)-calmodulin directly. This study demonstrated that estrogen has a biphasic effect on the activity of NNOS through a Ca(2+)-calmodulin.

Analysis of calcium-dependent interaction between amino-terminal conserved region of calpastatin functional domain and calmodulin-like domain of mu-calpain large subunit

Calpain requires Ca2+ both for proteolysis of its substrates and for interaction with its endogenous inhibitor, calpastatin. Although calmodulin-like domains (CaMLDs) of large and small subunits of calpain have been suggested to be the sites for Ca(2+)-dependent interaction with calpastatin, specificity and molecular basis of the interaction have remained unclear. We investigated the interaction between the CaMLD of human mu-calpain large subunit expressed in Escherichia coli and a 19-residue synthetic oligopeptide corresponding to the region A (the amino-terminal conserved acidic region) of one of the four repetitive functional domains of calpastatin. The recombinant CaMLD bound to the oligopeptide immobilized on Sepharose beads in a Ca(2+)-dependent manner. The CaMLD failed in binding to a mutant oligopeptide with one amino acid substitution. Enhancement of fluorescence intensity of a hydrophobic probe, 2-(p-toluidino)naphthalene-6-sulfonate, was observed upon incubating with the CaMLD and further increased by Ca2+. The Ca(2+)-dependent enhancement of fluorescence intensity was strongly suppressed by the wild type oligopeptide, but not by the mutant one. Kinetic experiments were performed with BIAcore where binding of the CaMLD to the oligopeptide immobilized on a biosensor chip was detected as real time signals of surface plasmon resonance. The determined dissociation constant (KD) was 3.1 x 10(-9) M. These results suggest that the region A of calpastatin binds to the CaMLD in a specific manner similar to interactions between calmodulin-binding peptides and calmodulin where hydrophobic properties are known to be important.

Two-dimensional NMR studies of selenomethionyl calmodulin

Calmodulin (CaM) is a ubiquitous calcium regulatory protein that can interact with almost 30 different target proteins. The majority of the CaM-binding domains of the target proteins are believed to interact with two hydrophobic surfaces on Ca(2+)-CaM; these two regions are very rich in Met residues. To obtain more information about the role of these residues, we have biosynthetically incorporated selenomethionine (SeMet) in place of the nine Met residues of CaM. Amino acid analysis shows that the SeMet-CaM contains 15% Met and 85% SeMet. SeMet-CaM retains many of the properties of the wild-type protein; it activates the enzyme cyclic nucleotide phosphodiesterase, it binds to phenyl-Sepharose and myosin light chain kinase (MLCK) in a calcium-dependent manner, and it experiences a calcium-dependent band shift during SDS-gel electrophoresis. Moreover, by comparing the natural abundance (1H,13C)-heteronuclear multiple quantum coherence (HMQC) spectra of the calcium, apo and target peptide-bound forms of wild-type CaM and SeMet-CaM, we have found that the two proteins have very similar, if not identical, structures. Thus, the substitution of SeMet for Met does not cause a change in the conformation and function of CaM, in agreement with the results obtained for other proteins. The apo, calcium and target peptide-bound forms of SeMet-CaM were subsequently studied by natural abundance (1H,77Se)-heteronuclear multiple bond correlation (HMBC) and (1H,13C)-HMQC NMR. Nine well-resolved 77Se resonances could be observed. Substitution of SeMet for Met gave rise to the same 1H and 13C chemical shift changes for each individual Met residue, this facilitated making the assignments from known 1H,13C assignments of the Met residues. Some of these assignments were confirmed by studying Met-->Leu mutants of CaM. With the exception of Met76, which always remains solvent exposed, all resonances experienced large 77Se chemical shift changes upon the addition of Ca2+ and the MLCK peptide. The large shift changes indicate that the electron distribution in the SeMet side-chain can be adjusted for the different states of CaM, suggesting that the polarizability of sulfur or selenium may be important for the proper functioning of CaM. This study also shows that the natural abundance (1H,77Se)-HMBC experiment provides a sensitive approach for the study of SeMet proteins.

Isolation of phosphorylated calmodulin from rat liver and identification of the in vivo phosphorylation sites

A procedure is described for the isolation of calmodulin (CaM) from rat liver which produces a fraction containing non-phosphorylated, mono-, di-, and triphosphocalmodulin as determined by mass spectrometric analysis. The distribution of CaM between the various phospho-species varies from preparation to preparation even though the isolation procedure is rigidly defined, suggesting that CaM phosphorylation may be a very labile phenomenon dependent on the state of the liver as it is removed from the animal. Approximately 15% of CaM in the cell is phosphorylated. The in vivo phosphorylation sites were determined by mass spectrometric analysis of a combined CNBr and trypsin digestion of the phosphocalmodulin (phospho-CaM)-containing fractions. Phosphorylated peptides were sequenced using two mass scanning devices linked together for collisionally activated fragmentation studies to determine peptide sequences, and the phosphorylation sites were determined as Thr-79, Ser-81, and Ser-101. These correspond to three of the four in vitro target sites of calmodulin phosphorylation by casein kinase II, which indicates that this may be the enzyme responsible for the phosphorylation in vivo. A preliminary study on the modulatory activity of phosphorylated calmodulin using a sample extensively phosphorylated in vitro with casein kinase II confirmed that phospho-CaM has an altered biological activity, i.e. reduced activation of the erythrocyte plasma membrane Ca2+ pump.

The interaction of calmodulin with regulatory peptides of phosphorylase kinase

The regulatory peptides Phk13 (301-327) and Phk5 (342-367) have been synthesized and their interaction with calmodulin studied. In the case of Phk13 modified forms were also synthesized in which a tryptophan group was placed at position 4 or 21, as well as a form with tryptophan at position 4 and nitrotyrosine at position 21. From tryptic digestion, circular dichroism, and radiationless energy transfer measurements, it appears that Phk13 forms an elongated complex with calmodulin in which the peptide is in a non-helical conformation, probably bent into a hairpin-shaped structure, the connecting strand of calmodulin is extended and exposed to the action of proteolytic enzymes, and the peptide makes contact with both the N- and C-terminal half-molecules of calmodulin. In contrast, the Phk5 peptide has an alpha-helical conformation in the complex, which is relatively compact in shape.

A structure/activity study of calcium affinity and selectivity using a synthetic peptide model of the helix-loop-helix calcium-binding motif

The acid pair hypothesis predicts the calcium affinity of the helix-loop-helix calcium-binding motif based on the number and location of acidic amino acid residues in chelating positions of the calcium-binding loop region. This study investigates the effects of the number and position of acidic residues in the loop region on calcium affinity and selectivity using 33-residue synthetic models of single helix-loop-helix calcium-binding motifs. Increasing the number of acidic residues in the octahedrally arranged chelating positions of the loop region from 3 to 4 by replacing an asparagine in the +y position with an aspartic acid increases the calcium affinity of the models between 2- and 38-fold. Differences in affinities are more pronounced in the models containing an x axis acid pair. The calcium affinities of peptide models containing 3 or 4 acidic residues in chelating positions of the loop region and an x axis acid pair are reduced when the residue in the +z position is changed from asparagine to serine. A similar reduction in calcium affinity occurs in the z axis acid paired peptides when the -x chelating residue is changed from serine to asparagine. Models with 3 acidic residues in chelating positions containing a z axis acid pair have greater calcium affinity than comparable peptide models with an x axis acid pair. The presence of x or z axis acid pairs in comparable peptides containing 4 acidic residues in chelating positions does not greatly alter calcium affinity. Calcium selectivity resides in x axis acid paired peptides, whereas z axis acid paired peptides exhibit both magnesium- and calcium-induced structural changes. This ion selectivity may be explained by postulating that the z axis residue side chains produce the initial, rate-limiting interactions with the cation, causing hydration shell destabilization and initiating the subsequent ligand interactions.

The linker of des-Glu84-calmodulin is bent

The crystal structure of a mutant calmodulin (CaM) lacking Glu-84 has been refined to R = 0.23 using data measured to 2.9-A resolution. In native CaM the central helix is fully extended, and the molecule is dumbbell shaped. In contrast, the deletion of Glu-84 causes a bend of 95 degrees in the linker region of the central helix at Ile-85. However, EF-hand domains 1 and 2 (lobe 1,2) do not touch lobe 3,4. The length, by alpha-carbon separation, of des-Glu84-CaM is 56 A; that of native CaM is 64 A. The shape of des-Glu84-CaM is similar to that of native CaM, as it is bound to the target peptide of myosin light-chain kinase. This result supports the proposal that the linker region of the central helix of CaM functions as a flexible tether.

Activation of enzymes by calmodulins containing intramolecular cross-links

We have reacted calmodulins containing cysteines substituted at positions 3 and 146 or 5 and 146 with bismaleimidohexane (BMH) to generate intramolecularly cross-linked proteins termed BMHCM or BMHCM1, respectively. Reactions were also performed with N-ethylmaleimide (NEM) in place of BMH to generate corresponding S-ethylsuccinimidylated proteins termed NEMCM or NEMCM1. The abilities of these proteins to activate plant NAD kinase, erythrocyte Ca(2+)-ATPase and bovine brain calcineurin activities were assessed. The BMH- or NEM-reacted proteins activate calcineurin activity as does control calmodulin. Kact values for Ca(2+)-ATPase activation by BMHCM and BMHCM1 are increased 10-fold relative to the control value, with no corresponding change in Vmax values. Activation of this enzyme by NEMCM or NEMCM1 is not different from the control. In NAD kinase activation experiments BMHCM and BMHCM1 are associated with a 10 to 20-fold increase in Kact values and a 60-75% reduction in Vmax values relative to the control. NEMCM1 is not associated with any apparent changes in NAD kinase activation, however, NEMCM is associated with a 10-fold increase in the Kact value. NEM-reacted calmodulin containing a cysteine only at position 3 is not associated with an increased Kact value, implying that this change is due to interactions between S-(ethylsuccinimido)cysteines at positions 3 and 146. In conclusion, cross-linking and associated distortions in the structure of calmodulin appear to have little or no effect on activation of calcineurin enzyme activity. However, bending in the central helix and/or steric restrictions associated with cross-linking increase significantly the Kact value for Ca(2+)-ATPase and NAD kinase activation, and dramatically reduce maximal activation of NAD kinase activity.

The lipid-binding peptide from the plasma membrane Ca2+ pump binds calmodulin, and the primary calmodulin-binding domain interacts with lipid

Peptide G25 (KKAVKVPKKEKSVLQGKLTRLAVQI) representing the putative lipid-binding region (G region) of the erythrocyte Ca2+ pump was synthesized. This peptide interacted with acidic lipids, as shown by the increase in size of phosphatidylserine liposomes in its presence. This lipid interaction is consistent with the previous evidence suggesting that the portion of the pump from which this peptide was taken is responsible for the activation of the pump by acidic lipid. G25 also bound to calmodulin, as was shown by its cause of a shift in the fluorescence of 5-dimethylamino naphthalene-1-sulfonyl- (dansyl)-calmodulin, and by its competition with Ca2+ pump for calmodulin. Its Kd for dansyl-calmodulin was much higher (0.8 microM) than that of the peptides representing the primary calmodulin-binding region (C region) of the Ca2+ pump. Although the presence of the G region provided the possibility of a second calmodulin-binding site, activation of the pump by calmodulin always could be fitted by simple saturation kinetics. The calmodulin-binding peptide from the C region of the pump, C28R2, also interacted with lipid with even greater effectiveness than G25. When the C region of the pump was saturated with calmodulin, acidic lipid activation of the pump followed simple saturation kinetics. However, when calmodulin was omitted, a higher concentration of lipid was needed for saturation and the kinetics became complex. The data are consistent with the idea that calmodulin activates the pump only by interaction at the C region, but that acidic lipid activates by interaction at both of the C and G regions.

Preparation, characterization and biological properties of biotinylated derivatives of calmodulin

Biotinylated derivatives of calmodulin (CaM) were prepared and their biological properties characterized by using enzyme assays, affinity and hydrophobic-interaction chromatography. Several N-hydroxysuccinimidobiotin derivatives [sulphosuccinimidobiotin (sulpho-NHS) and sulphosuccinimido-6-(biotinamido)hexanoate (BNHS-LC)] differing in spacer arm length were used to modify CaM. The shorter-spacer-arm CaM derivative (sulpho-CaM) activated CaM-dependent cyclic nucleotide phosphodiesterase and CaM-dependent protein kinase II; preincubation with avidin blocked its ability to activate these enzymes. The extended-spacer-arm derivative (BNHS-LC-CaM) activated CaM-dependent enzymes both in the presence and in the absence of avidin, suggesting that the longer spacer arm diminished steric effects from avidin preincubation. Other biotinylated CaM derivatives were prepared with biotinylated tyrosine and/or histidine residues (diazobenzoylbiocytin; DBB-CaM) or nucleophilic sites (photobiotin acetate; photo-CaM). These derivatives activated CaM-dependent enzymes in the presence and in the absence of avidin. Oriented affinity columns were constructed with covalently immobilized avidin complexed to each biotinylated CaM derivative. The chromatographic profiles obtained revealed that each column interacted with a specific subset of CaM-binding proteins. Elution profiles of biotinyl CaM derivatives on phenyl-Sepharose hydrophobic-interaction chromatography suggested that several derivatives displayed diminished binding to the matrix in the presence of Ca2+. Development and characterization of a series of biotinylated CaM molecules can be used to identify domains of CaM that interact with specific CaM-dependent enzymes.

A calcium-sensitive fluorescent analog of calmodulin based on a novel calmodulin-binding fluorophore

Structure-activity studies of tetramethinemerocyanine fluorophores enabled the synthesis of novel dyes which showed spectral changes during reversible, calcium-dependent association with calmodulin. These spectral changes were greatly enhanced in dyes with a quaternary nitrogen and specifically placed hydrophobic chains. One such dye was covalently attached to calmodulin, producing a calmodulin analog with calcium-sensitive fluorescence. The analog, MeroCaM, showed a calcium-induced 3.4-fold increase in excitation ratio (608/532 nm excitation, 623 nm emission), which was fully reversed by lowering free calcium levels. MeroCaM's excitation ratio showed a half-maximal change at 300-400 nM calcium, below calcium concentrations reported to produce half-maximal saturation of calcium-calmodulin binding. However, the calcium dependence of MeroCaM's phosphodiesterase activation paralleled that of calmodulin. MeroCaM's fluorescence changes therefore appear to reflect primarily calcium binding to high affinity sites. MeroCaM's maximal phosphodiesterase activation was 30-40% that of calmodulin. In myosin light chain kinase activation, MeroCaM and calmodulin displayed indistinguishable maximal activation levels and concentration dependence of activation. Changes in MeroCaM's calcium affinity induced by magnesium, phosphodiesterase, and melittin were similar to those reported for calmodulin. Experiments with melittin revealed that target protein interaction could alter the fluorescence changes produced by calcium binding. MeroCaM showed promising brightness and photostability when imaged in individual living fibroblasts. The long excitation and emission wavelengths of MeroCaM, and the strong dependence of its excitation ratio on calcium concentrations, suit it well for use as a probe of calmodulin-dependent calcium signaling in living cells, as well as for experiments in vitro.

Differential inhibition of calcium-dependent and calmodulin-dependent enzymes by drug-calmodulin adducts

Most of the currently available calmodulin (CaM) antagonists inhibit the actions of CaM by binding directly to it. These CaM-binding drugs tend to be relatively nonselective, because they inhibit the interaction of CaM with most, if not all, of its target enzymes. In order to develop more selective CaM antagonists, we synthesized covalent adducts of CaM and several drugs, including chlorpromazine (CPZ), fluphenazine-N-mustard (FNM), and phenoxybenzamine (PBZ), and examined the effects of these adducts on various CaM and Ca2(+)-dependent enzymes. One of the adducts (CPZ-CaM) selectively inhibited the CaM-induced activation of phosphodiesterase and myosin light chain kinase, without affecting the basal activity of either enzyme. The inhibition of these enzymes by CPZ-CaM was competitive with respect to CaM. CPZ-CaM did not inhibit CaM-sensitive Ca2(+)-ATPase or CaM-dependent protein kinase or the CaM-insensitive enzyme protein kinase C. The FNM-CaM and PBZ-CaM adducts did not inhibit the effects of CaM on any of the enzymes, but they selectively activated two of the enzymes; FNM-CaM slightly activated the CaM-dependent protein kinase, and PBZ-CaM slightly activated phosphodiesterase. These results show that certain covalently linked drug-CaM adducts can differentially inhibit or activate various CaM-sensitive enzymes, and they provide further evidence that it may be possible to develop new classes of CaM antagonists that are directed against the CaM recognition sites on CaM-sensitive enzymes.

Interaction of calmodulin with the calmodulin binding domain of the plasma membrane Ca2+ pump

Peptides corresponding to the calmodulin binding domain of the plasma membrane Ca2+ pump (James et al., 1988) were synthesized, and their interaction with calmodulin was studied with circular dichroism, infrared spectroscopy, nuclear magnetic resonance, and fluorescence techniques. They corresponded to the complete calmodulin binding domain (28 residues), to its first 15 or 20 amino acids, and to its C-terminal 14 amino acids. The first three peptides interacted with calmodulin. The K value was similar to that of the intact enzyme in the 28 and 20 amino acid peptides, but increased substantially in the shorter 15 amino acid peptide. The 14 amino acid peptide corresponding to the C-terminal portion of the domain failed to bind calmodulin. 2D NMR experiments on the 20 amino acid peptides have indicated that the interaction occurred with the C-terminal half of calmodulin. A tryptophan that is conserved in most calmodulin binding domains of proteins was replaced by other amino acids, giving rise to modified peptides which had lower affinity for calmodulin. An 18 amino acid peptide corresponding to an acidic sequence immediately N-terminal to the calmodulin binding domain which is likely to be a Ca2+ binding site in the pump was also synthesized. Circular dichroism experiments have shown that it interacted with the calmodulin binding domain, supporting the suggestion (Benaim et al., 1984) that the latter, or a portion of it, may act as a natural inhibitor of the pump.

Conformational changes of (Ca2+-Mg2+)-ATPase of erythrocyte plasma membrane caused by calmodulin and phosphatidylserine as revealed by circular dichroism and fluorescence studies

Two spectroscopic techniques, circular dichroism and steady-state fluorescence, were employed in order to study conformational changes of the purified, detergent-solubilized (Ca2+-Mg2+)-ATPase of porcine erythrocyte ghost membranes. Circular dichroism (CD) spectra in the peptide region were obtained from the purified (Ca2+-Mg2+)-ATPase of porcine erythrocyte ghost membranes with the aim to investigate the secondary structure of the enzyme in the presence of calmodulin (CaM) or phosphatidylserine (PS), as well as in the E1 and E2 states. The E1 conformation was stabilized by 10 microM free Ca2+, while the E2 conformation was stabilized by 0.1 mM ethylene glycol bis(2-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). It was found that the E1 and E2 states of the enzyme strikingly differed in their secondary structure (66% and 46% of calculated alpha-helix content, respectively). In the presence of Ca2+, PS decreased the helical content of the ATPase to 61%, while CaM to 55%. Quenching of intrinsic fluorescence of (Ca2+-Mg2+)-ATPase by acrylamide, performed in the presence of Ca2+, gave evidence for a single class of tryptophan residues with Stern-Volmer constant (KSV) of 10 M-1. Accessibility of tryptophan residues varied depending on the conformational status of the enzyme. Addition of PS and CaM decreased the KSV value to 7.6 M-1 and 8.5 M-1, respectively. In the absence of Ca2+, KSV was 7.0 M-1. KI and CsCl were less effective as quenchers. The fluorescence energy transfer between (Ca2+-Mg2+)-ATPase tryptophan residues and dansyl derivative of covalently labeled CaM occurred in the presence of EGTA, but was further promoted by Ca2+. It is concluded that the interaction of CaM and PS with (Ca2+-Mg2+)-ATPase results in different conformational states of the enzyme. CD and fluorescence spectroscopy allowed to distinguish these states from the E1 and E2 conformational forms of the ATPase.

Phenothiazine-binding and attachment sites of CAPP1-calmodulin

In the presence of Ca2+ norchlorpromazine isothiocyanate forms a monocovalent complex with calmodulin: CAPP1-calmodulin (Newton et al, 1983). Trypsin digestion of [3H]CAPP1-calmodulin yields as the major radioactive peptide N epsilon-CAPP-Lys-Met-Lys, corresponding to residues 75-77 of calmodulin. Stoichiometric amounts of all other expected tryptic peptides are also found, indicating that norchlorpromazine isothiocyanate selectively acylates Lys 75. A second molecule of CAPP-NCS can react, albeit slowly, with calmodulin to form CAPP2-calmodulin. Fragments 38-74 and 127-148 are completely missing from the trypsin digests of CAPP2-calmodulin without deliberate exposure to UV irradiation. Possibly the lengthy preparation of CAPP2-calmodulin favors photolysis, caused by room lights, of the putative CAPP-binding domains located in these two peptides. Lys 148, the sole lysyl residue in fragment 127-148, is a probable site of attachment of the second molecule of CAPP. UV irradiation of CAPP1-calmodulin, followed by digestion with trypsin, results in the selective loss of 50% each of peptides containing residues 38-74 and 127-148, suggesting that these peptides contain the hydrophobic amino acids that form the phenothiazine-binding sites. The loss of peptides encompassing residues 38-74 and 127-148, located in the amino and carboxyl halves of calmodulin, respectively, suggests that the hydrophobic rings of CAPP can bind at either one of the two phenothiazine sites. Computer modeling of CAPP1-calmodulin with the X-ray coordinates of calmodulin (Babu et al., 1986) indicates that CAPP attached to Lys 75 cannot interact with the carboxyl-terminal phenothiazine-binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Topographical mapping of calmodulin-target enzyme interaction domains

Calmodulin derivatives, specifically biotinylated in domains I and III, were synthesized to address the structures of calmodulin necessary for binding to its target enzymes in active conformations. By binding avidin to these biotinylated calmodulins, the role of specific sequences of the calmodulin molecule in target enzyme interactions could then be evaluated. The role of domain I in these interactions was assessed by biotinylation of Cys-27 of wheat germ calmodulin with N-ethylmaleimidobiotin. This modification did not affect the ability of this calmodulin to activate 3'-5'-cyclic nucleotide phosphodiesterase (PDE) or human erythrocyte Ca2+-Mg2+ ATPase. The addition of avidin to form a stable calmodulin-avidin complex also did not affect activation. Bovine testes calmodulin was biotinylated on Lys-94 by calcium-dependent reaction with N-hydroxysuccinimido ester-biotin at pH 6.0. This derivative was used to probe the Ca+2 binding region of domain III. The incorporation of biotin at Lys-94 of bovine calmodulin did not affect calmodulin activation of PDE. However, compared to unmodified calmodulin, a 4-fold higher concentration of this derivative was required to fully activate the ATPase. The addition of excess avidin to this derivative abolished all activation for both PDE and the ATPase. Sites of modification were determined by sequence analysis of labeled peptides.

Calmodulin stabilization of kinetochore microtubule structure to the effect of nocodazole

To investigate the function of calmodulin (CaM) in the mitotic apparatus, the effect of microinjected CaM and chemically modified CaMs on nocodazole-induced depolymerization of spindle microtubules was examined. When metaphase PtK1 cells were microinjected with CaM or a CaM-TRITC conjugate, kinetochore microtubules (kMTs) were protected from the effect of nocodazole. The ability of microinjected CaM to subsequently protect kMTs from the depolymerizing effect of nocodazole was dose dependent, and was effective for approximately 45 min, with protection decreasing if nocodazole treatment was delayed for more than 60 min after injection of CaM. The CaM-TRITC conjugate, similar to native CaM, displayed the ability to activate bovine brain CaM-dependent adenylate cyclase in a Ca++-dependent manner and showed a Ca++-dependent mobility shift when subjected to PAGE. A heat-altered CaM-TRITC conjugate also protected kMTs from the effect of nocodazole. However, this modified CaM was not able to activate adenylate cyclase nor did it display a Ca++-dependent mobility shift when electrophoresed. In a permeabilized cell model system, both CaM analogs were observed to bind to the spindle in a Ca++-independent manner. In contrast, a performic acid-oxidized CaM did not have a protective effect on spindle structure when microinjected into metaphase cells before nocodazole treatment. The oxidized CaM did not activate adenylate cyclase and did not exhibit Ca++-dependent mobility on polyacrylamide gels. These results are interpreted as supporting the hypothesis that CaM binds to the mitotic spindle in a Ca++-independent manner and that CaM may serve in the spindle, at least in part, to stabilize kMTs.

Inhibition of mitosis in PtK2 cells by CAPP1-calmodulin

Various indirect evidence has indicated that calcium ions and the calcium-binding regulator protein, calmodulin, may regulate mitosis in higher eukaryotes. We have used the competitive antagonist, CAPP1-calmodulin, to antagonize intracellular calmodulin and test the hypothesis that calmodulin serves as a regulator of mitosis. We find that CAPP1-calmodulin inhibits the transit of cells through metaphase at estimated intracellular concentrations up to that of native calmodulin; beyond that level, the inhibition of mitosis vanishes. The membrane-permeant anticalmodulin agents, W7 and calmidazolium, also inhibit the progress of cells through metaphase. The similarity of the inhibitory curves for CAPP1-calmodulin, W7, and calmidazolium suggests that all these agents inhibit mitosis by antagonizing intracellular calmodulin. In order to test whether this inhibition of metaphase transit is due to an effect of the agents on intracellular free calcium, we used the calcium indicator Fura-2 to measure intracellular calcium levels after CAPP1-calmodulin injection or during calmidazolium treatment. We found that, while intracellular calcium levels are modestly elevated during calmidazolium treatment, they were unaffected by CAPP1-calmodulin, a result suggesting that mitosis inhibition was not due to an effect on intracellular free calcium. The reasons for the anomalous dose-response behavior of these drugs are not known; however, the behavior of cells at drug levels below the point of anomaly supports the hypothesis that calmodulin acts as a regulator of mitosis in these cells.

Interaction of Bordetella pertussis adenylate cyclase with calmodulin. Identification of two separated calmodulin-binding domains

The structural organization of Bordetella pertussis adenylate cyclase was examined by limited proteolysis with trypsin and/or cross-linking with azido-calmodulin a photoactivable derivative of its activator, calmodulin (CaM). Adenylate cyclase (which consists of three structurally related peptides of 50, 45, and 43 kDa as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) formed a 1:1 complex with CaM or azido-CaM. CaM-bound adenylate cyclase was cleaved by trypsin into two separate trypsin-resistant fragments of 25 and 18 kDa which both interacted with CaM as judged by their ability to be cross-linked with azido-CaM. These two fragments remained associated with CaM in a catalytically active conformation resembling that of the undigested complex. When proteolysis was carried out in the absence of CaM, the adenylate cyclase was completely inactivated in less than 3 min. Sodium dodecyl sulfate-polyacrylamide gel revealed a single 24-kDa trypsin-resistant fragment. Since this fragment cannot be cross-linked with azido-CaM we suggest that the CaM-binding site on the 25-kDa moiety of the adenylate cyclase is located on a short segment of 1 kDa.

Differential inhibition of calmodulin-sensitive phosphodiesterase and Ca++-adenosine triphosphatase by chlorpromazine-linked calmodulin

Upon irradiation with UV light, chlorpromazine binds irreversibly to calmodulin and inactivates it. To determine whether this chlorpromazine-calmodulin (CPZ-CaM) complex can inhibit the actions of native calmodulin, we examined its effects on the activity of calmodulin-sensitive cyclic nucleotide phosphodiesterase from rat brain and on the Ca++-adenosine triphosphatase (ATPase) of human erythrocyte membranes. The CPZ-CaM complex was prepared by irradiating purified bovine brain calmodulin in the presence of chlorpromazine and Ca++. The sample was then dialyzed extensively to remove reversibly bound chlorpromazine and then assayed for its ability to activate calmodulin-sensitive phosphodiesterase and Ca++-ATPase, and for its ability to block the stimulatory effects of native calmodulin on these enzymes. The CPZ-CaM complex had no effect on the basal activity of either enzyme; it neither activated nor inhibited the enzymes when assayed in the absence of calmodulin. However, it affected differentially the activation of the two enzymes by native calmodulin. The CPZ-CaM complex totally inhibited calmodulin-stimulated phosphodiesterase but had no effect on the activation of the ATPase by calmodulin. Other studies showed that CPZ-CaM increased the activation constant (Ka) for the interaction of calmodulin with phosphodiesterase but did not affect the maximal activation (Vmax) of the enzyme by calmodulin. Neither calmodulin nor CPZ-CaM altered the Km for the interaction between phosphodiesterase and cyclic AMP. These results suggest that CPZ-CaM inhibits the calmodulin-induced activation of phosphodiesterase by competing with calmodulin for regulatory sites on the enzyme and not by interacting with calmodulin itself or by blocking the interaction of cyclic AMP with the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

The interaction of calmodulin with human erythrocyte spectrin. Inhibition of protein 4.1-stimulated actin binding

The functional significance of calmodulin binding to human erythrocyte spectrin has been investigated under native conditions. Both native calmodulin and calmodulin derivatized with the photoactivable cross-linker methyl 4-azidobenzimidate (azidocalmodulin) have been used. When azidocalmodulin is photolyzed in the presence of erythrocyte ghosts, ghost extracts, or purified protein, it cross-links predominately to the beta subunit of erythrocyte spectrin. This cross-linking is calcium-dependent, requires photolysis, and is inhibited by 100 microM trifluoperazine or unlabeled calmodulin. Calmodulin labeled spectrin exhibits a specific and non-calcium-dependent inhibition of its ability to bind actin, even in the presence of protein 4.1. Its ability to self-associate or to bind spectrin-depleted membrane vesicles is unperturbed. Native calmodulin also inhibits protein 4.1-stimulated spectrin-actin binding, but unlike that of covalently bound calmodulin, inhibition by the uncross-linked calmodulin requires calcium. The degree of inhibition of spectrin-actin-4.1 binding induced by native calmodulin is significant since 109 microM calmodulin inhibits over 63% of the spectrin-actin binding induced by 4.5 microM protein 4.1. These results demonstrate a specific effect of calmodulin on erythroid spectrin function and suggest that calmodulin may influence the binding of protein 4.1 and actin to spectrin within the cytoskeleton.

Activation of a cyclic nucleotide phosphodiesterase and of a protein kinase by chemically modified calmodulin

1. Several calmodulin derivatives prepared by chemical modification of lysine residues were tested using bovine heart cyclic nucleotide phosphodiesterase and wheat germ calmodulin-dependent protein kinase. 2. The effect of chemical modification on the activation capacity of calmodulin for the two studied enzymes was different. 3. This was particularly noticeable in the case of alkylated derivatives which exhibited a higher affinity than native calmodulin towards phosphodiesterase but a lower affinity towards protein kinase. 4. The efficiency of these derivatives (maximal activation) was higher than that of native calmodulin in relation with the protein kinase.

Binding of trifluoperazine and fluorene-containing compounds to calmodulin and adducts

Calmodulin can be specifically acylated with a fluorene-containing hydrophobic spin-labeling reagent at just Lys 75 or at Lys 75 and Lys 148. The binding of trifluoperazine to calmodulin and the two adducts was determined using a Hummel-Dreyer procedure, and binding of the phenothiazine was found to be characterized by apparent positive cooperativity and an apparent limiting stoichiometry of about seven binding sites per protein molecule. Two non-reactive fluorene-containing compounds were synthesized, and both reagents exhibited far less binding to calmodulin than did trifluoperazine. One of these was also assayed for binding to the monolabeled adduct, and this binding was about half that observed with calmodulin and was non-cooperative. Thus, the qualitative and quantitative binding parameters of hydrophobic groups to calmodulin can be quite different.

Association of calmodulin with peptide analogues of the inhibitory region of the heat-stable protein inhibitor of adenosine cyclic 3',5'-phosphate dependent protein kinase

A 20-residue peptide analogue (IASGRTGRRNAIHDILVSSA) of the 8000-dalton heat-stable cAMP-dependent protein kinase inhibitor undergoes efficient calcium-dependent binding by calmodulin, with Kd approximately 70 nM when calcium is present. It is a potent inhibitor of smooth muscle myosin light chain kinase and of the calmodulin-dependent phosphatase activity of calcineurin. At concentrations above 3 microM, the peptide stimulates the basal activity of calcineurin. The native protein kinase inhibitor has no effect on the catalytic activity of myosin light chain kinase and is moderately inhibitory to both the calmodulin-dependent and -independent phosphatase activity of calcineurin. Competition experiments using excess concentrations of calcineurin and calmodulin suggest that the primary interaction of the native heat-stable inhibitor is with the catalytic subunit of protein kinase. Dansylcalmodulin exhibits only a weak interaction with the inhibitor. Observations on deletion peptides of the 20-residue analogue help to delineate the overlapping peptide binding specificities of the cAMP-dependent protein kinase [Scott, J. D., Glaccum, M. B., Fischer, E. H., & Krebs, E. G. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 1613-1616] and calmodulin. In both cases, the most effectively bound peptides contain the RTGRR sequence.

Pathophysiology of panic: a review of pharmacologic provocative tests and naturalistic monitoring data

Panic is a psychologic symptom with prominent physiologic manifestations. Pathophysiologic theories need to explain both the central emotional state and peripheral symptoms. This paper reviews current findings and hypotheses from studies using two strategies for understanding panic. First, recent results from provocative tests using sodium lactate, caffeine, CO2, yohimbine, and isoproterenol suggest panic patients have a special sensitivity to these substances. Underlying mechanisms may be related to abnormal functioning of the central noradrenergic (locus ceruleus) system. However, cognitive and behavioral aspects of pharmacologic provocative testing need further study. Second, naturalistic studies support findings of peripheral physiologic activation during panic, but do not show evidence for sustained hyperactivity throughout the day. Studies of phobic patients demonstrate the potential for imagined fear to provoke physiologic changes, again underlining the need to pay attention to psychologic aspects of panic.

Trimethyllysine and protein function. Effect of methylation and mutagenesis of lysine 115 of calmodulin on NAD kinase activation

Unmethylated calmodulins have been enzymatically methylated at lysine 115, and a direct effect of this methylation on NAD kinase activation has been shown. Similar to naturally occurring calmodulins with trimethyllysine 115, the enzymatically methylated calmodulins activated an NAD kinase preparation to a maximal level that was at least 3-fold lower than the level of activation obtained with the corresponding unmethylated calmodulins. Methylation did not alter the cyclic nucleotide phosphodiesterase activator properties of these calmodulins. A genetically engineered calmodulin containing an arginine at position 115 instead of a lysine was produced by site-specific mutagenesis of a cloned synthetic calmodulin gene. The arginine derivative retained the higher maximal NAD kinase activator properties of the unmethylated calmodulins but was no longer susceptible to the effects of the methyltransferase. The data indicate that the reduction in the level of NAD kinase activation is the direct result of trimethylation of lysine 115 of calmodulin, provide a precedent for a functional effect of trimethyllysine in a protein, and raise the possibility that some of calmodulin's physiological activities may be affected by lysine methylation.