Biological cross-reactivity of rat testis phosphodiesterase activator protein and rabbit skeletal muscle troponin-C

Mapping subdomains in the C-terminal region of troponin I involved in its binding to troponin C and to thin filament

Troponin I (TnI) is the inhibitory component of troponin, the ternary complex that regulates skeletal and cardiac muscle contraction. Previous work showed that the C-terminal region of TnI, when linked to the "inhibitory region" (residues 98-116), possesses the major regulatory functions of the molecule (Farah, C. S., Miyamoto, C. A., Ramos, C. H. I., Silva, A. C. R., Quaggio, R. B., Fujimori, K., Smillie, L. B., and Reinach, F. C. (1994) J. Biol. Chem. 269, 5230-5240). To investigate these functions in more detail, serial deletion mutants of the C-terminal region of TnI were constructed. These experiments showed that longer C-terminal deletions result in lower inhibition of the actomyosin ATPase activity and weaken the interaction with the N-terminal domain of troponin C (TnC), consistent with the antiparallel model for the interaction between these two proteins. The conclusion is that the whole C-terminal region of TnI is necessary for its full regulatory activity. The region between residues 137 and 144, which was shown to have homology with residues 108-115 in the inhibitory region (Farah, C. S., and Reinach, F. C. (1995) FASEB J. 9, 755-767), is involved in the binding to TnC. The region between residues 98 and 129 is involved in modulating the affinity of TnC for calcium. The C-terminal residues 166-182 are involved in the binding of TnI to thin filament. A model for the function of TnI is discussed.

The functional role of the domains of troponin-C investigated with thrombin fragments of troponin-C reconstituted into skinned muscle fibers

Proteolysis of rabbit fast skeletal troponin-C (RSTnC) with thrombin produces four separate fragments containing the following Ca2+-binding site(s): TH1 (residues 1-120) sites I-III; TH2 (121-159) site IV; TH3 (1-100) sites I and II; and TH4 (101-120) site III. We studied the ability of these fragments to restore the steady state isometric force in TnC-depleted skinned skeletal muscle fibers. Interestingly, we found that all investigated fragments of RSTnC possessed some of the properties of native RSTnC, but none of them could fully regulate contraction in the fibers like intact RSTnC. TH1 was the most effective in the force restoration (65%) whereas the smaller fragments developed about 50% (TH3 and TH4) or 20% (TH2) of the initial force of unextracted fibers. Additionally, much higher concentrations of TH2, TH3, and TH4 compared to RSTnC OR TH1 were necessary for force development suggesting a decreased affinity of these fragments to their binding site(s) in the fibers. Like intact RSTnC, TH1 was able to interact with the fibers in a Ca(2+)-independent (Mg(2+)-dependent) manner, indicating that at a minimum, Ca(2+)-binding site III is required for this type of binding. The initial binding of the other fragments to the TnC-depleted fibers occurred only in the presence of Ca2+. TH2 and TH4 appeared to bind to two different binding sites in the fibers. The binding to one of the sites caused partial force restoration. This binding of TH2 and TH4 was abolished when Ca2+ was removed. TH2 and TH4 binding to the second site required Ca2+ initially but was maintained in the presence of Mg2+. This interaction of TH2 and TH4 partially blocked the rebinding of RSTnC to the fibers. The latter results suggest that site III and IV in these small fragments, when removed from the constraints of the parent protein, may assume conformations that allow them to function, to a certain extent, like both the regulatory sites (I and II) and the Ca(2+)-Mg2+ sites(III and IV) of TnC.

Heat capacity and entropy changes of the major isotype of the toad (Bufo) parvalbumin induced by calcium binding

The possible structural changes in the major isotype of parvalbumin from the toad (Bufo bufo japonicus) skeletal muscle caused by Ca2+ and Mg2+ binding have been analyzed by microcalorimetric titrations. Parvalbumin was titrated with Ca2+ in both the absence and presence of Mg2+ and with Mg2+ in the absence of Ca2+, at pH 7.0, and at 5 degrees, 15 degrees, and 25 degrees C. The two sites in a molecule were equivalent on Mg2(+)-Ca2+ exchange, but distinguishable on Ca2+ and Mg2+ binding. The reactions of parvalbumin with Ca2+ are exothermic at every temperature in both the absence and presence of Mg2+, but those with Mg2+ are always endothermic except for the binding to site 1 at 25 degrees C. The magnitudes of the hydrophobic and internal vibrational contributions to the heat capacity and entropy changes of parvalbumin on Ca2+ and Mg2+ binding and Mg2(+)-Ca2+ exchange have been estimated by the empirical method of Sturtevant [Sturtevant, J. M. (1977) Proc. Natl Acad. Sci. USA 74, 2236-2240]. Although no major conformational changes were noted between Ca2(+)- and Mg2(+)-bound forms of toad parvalbumin, the conformational difference was larger in Ca2+ (or Mg2+) binding to site 1 than site 2. This may indicate that the metal-free form is much less stable than any form with Ca2+ (or Mg2+) bound at one site at least. On Mg2(+)-Ca2+ exchange, the vibrational as well as hydrophobic entropy is only slightly increased in a parallel manner. In contrast, on Ca2+ (or Mg2+) binding, the hydrophobic entropy increases but the vibrational entropy decreases; the former indicates the sequestering of nonpolar groups from the surface to the interior of a molecule, and the latter suggests that the overall structures are tightened on Ca2+ (or Mg2+) binding but loosened on Mg2(+)-Ca2+ exchange. Despite the clear distinctions in the thermodynamic features, the conformational changes of toad parvalbumin are essentially the same as those of the two isotypes of bullfrog parvalbumins on Ca2+ binding and Mg2(+)-Ca2+ exchange.

Soluble calcium-binding proteins: parvalbumins and calmodulin from eel skeletal muscle

1. Eel skeletal muscle contains three parvalbumin isoforms. The overall parvalbumin concentration in the muscle is 0.5 mmol kg-1 wet weight. 2. Calmodulin (0.1 mumol kg-1 wet weight) was purified by extraction with ethylenediamine tetraacetate-containing buffer, fractionation with trichloroacetic acid and separation by ion exchange chromatography on DEAE-cellulose and by molecular sieving on Ultrogel AcA 54. 3. Troponin-C-free calmodulin was obtained by fast protein liquid chromatography on a Mono Q column.

1,25 Dihydroxyvitamin D3 affects calmodulin distribution among subcellular fractions of skeletal muscle

1,25 Dihydroxyvitamin D3 has been shown to stimulate calcium fluxes across skeletal muscle membranes. The involvement of calmodulin in the effects of the metabolite was investigated. Primary cultures of chick embryo skeletal muscle myoblasts and soleus muscles from vitamin D-deficient or 1,25 (OH)2D3-treated chicks were used. Culture of myoblasts and vitamin D-deficient soleus with 1,25 (OH)2D3 (0.05 ng/ml) for 24 and 1 hour, respectively, significantly increased 45Ca uptake by the preparations. In the presence of the calmodulin antagonists flufenazine or compound 48/80 in the uptake medium, no differences between control and treated cultures were observed. The calmodulin content of myoblasts and soleus homogenates and subcellular fractions derived therefrom was estimated by measuring their capacity to stimulate calmodulin-depleted cAMP phosphodiesterase. No changes in total calmodulin cellular content could be detected in response to 1,25(OH)2D3. However, the sterol produced an increase in calmodulin levels of microsomes, mitochondria, and crude myofibrillar fraction and a proportional decrease in cytosolic calmodulin concentration. The 1,25(OH)2D3-dependent changes in calmodulin distribution among subcellular fractions of soleus muscle were observed either in vivo or in vitro. The effects in vitro were already detectable after 5 minutes of treatment with the sterol and parallel 1,25(OH)2D3-dependent changes in tissue Ca uptake. The results suggest that changes in calmodulin intracellular distribution may underly part of the mechanism by which 1,25(OH)2D3 affects muscle calcium transport.

Adenosine inhibition of calmodulin-sensitive adenylate cyclase from bovine cerebral cortex

Calmodulin (CaM)-sensitive adenylate cyclase has recently been purified extensively from bovine brain. In this study, the sensitivity of the CaM-sensitive adenylate cyclase to adenosine and adenosine analogs was examined. The highly purified enzyme preparation retained sensitivity to inhibition by adenosine and adenosine analogs with ribose ring modifications, but not to those with purine ring modifications. Adenosine inhibition of this enzyme was not dependent on GTP and was noncompetitive with respect to ATP. Enzyme that had been dissociated from functional guanine nucleotide binding protein interactions by gel filtration in the presence of the zwitterionic detergent 3-[3-(cholamidopropyl)-dimethylammonio]-propanesulfonate and Mn2+ retained sensitivity to adenosine inhibition. The Ki for adenosine inhibition of the CaM-sensitive adenylate cyclase was approximately 2.6 X 10(-4) M. 5'-Guanylylimidodiphosphate and CaM did not affect the Ki of 3'-deoxyadenosine for the enzyme, but the presence of Ca2+ in the millimolar range raised the Ki by a factor of 5. These results show that the CaM-sensitive form of adenylate cyclase from bovine brain is subject to adenosine inhibition, and strongly suggest that this inhibition is due to interaction of ligands with a purine-specific ("P") site located on the catalytic subunit of the enzyme.

Increased sensitivity of adenylate cyclase activity in the striatum of the rat to calmodulin and GppNHp after chronic treatment with haloperidol

Chronic treatment of rats with haloperidol causes behavioral supersensitivity to dopaminergic agonists and an increase in the sensitivity of adenylate cyclase activity in the striatum to stimulation by dopamine. In this study the authors examined whether chronic treatment with haloperidol could elicit a change in sensitivity of adenylate cyclase in the striatum of the rat for guanyl nucleotides and the endogenous Ca2+-binding protein, calmodulin. These agents increase the activation of adenylate cyclase activity by dopamine but act beyond the level of the dopamine receptor. Male, Sprague-Dawley rats were injected subcutaneously with either 0.6 mg/kg haloperidol or vehicle for 14 days. Four days after the last injection, the animals were sacrificed and the activity of adenylate cyclase was measured in a EGTA-washed particulate preparation of the striatum. There was an increase in the activation of adenylate cyclase activity by calmodulin and GppNHp but not by guanosine triphosphate (GTP) in particulate fractions of the striatum from rats treated with haloperidol as compared to controls. The sensitivity of adenylate cyclase to calmodulin was increased 5-fold in particulate fractions from rats treated with haloperidol as opposed to vehicle-treated rats. The lack of change in activation by GTP was not due to an altered activity of GTPase in rats treated with haloperidol. In animals treated for 14 days but not withdrawn from haloperidol there was no statistically significant increase in the sensitivity of adenylate cyclase to calmodulin. There was no change in activation of the enzyme by GppNHp or GTP as compared to control. The activation of adenylate cyclase by calmodulin was not affected when haloperidol was added in vitro to the assay or after the acute injection of rats with haloperidol.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific involvement of calmodulin and non-specific effect of tropomyosin in the sensitivity to ouabain of Na+,K+-ATPase in murine plasmocytoma cells

The Kd for ouabain for inhibition of Na+,K+-ATPase isolated from murine plasmocytoma MOPC 173 cells is 120 microM, but when isolated in the presence of EDTA, it is 100-fold lower (1.2 microM). Simultaneous addition of muscle tropomyosin and calcium to sensitive membranes restored the original insensitivity (tropomyosin bound to the membranes in an irreversible and saturable manner). For comparison 86Rb influx into intact cells, mediated by the Na+,K+-pump, is half-maximally inhibited at 50 microM ouabain. Calcium converts the enzyme to an insensitive form. This appeared to involve calmodulin because after extraction of calmodulin with EDTA and EGTA from sensitive membranes, they could not be made insensitive by the addition of tropomyosin and Ca2+. Addition of exogenous calmodulin to these calmodulin-depleted membranes was required, in addition to tropomyosin and Ca2+, to decrease the ouabain sensitivity. The involvement of calmodulin was further assessed by measuring the range of Ca2+ concentrations required to convert to the insensitive form. At saturating concentrations of tropomyosin, increasing free [Ca2+] up to 3 microM led to an heterogeneous population of Na+,K+-ATPase forms. The calcium dependency was a saturable process. The shift to the insensitive form was half maximal at 0.65 + 0.11 microM free Ca2+ and was abolished by the addition of troponin I or trifluoroperazine (0.1 mM). These results suggest that, in murine plasmocytoma cells, the intrinsic sensitivity of Na+,K+-ATPase to ouabain might be regulated by a calmodulin-dependent process within a submembrane contractile-like environment.

Reconstitution of calmodulin-sensitive adenylate cyclase from bovine brain with phosphatidylcholine liposomes

A partially purified calmodulin (CaM)-sensitive adenylate cyclase from bovine cerebral cortex was reconstituted with a series of phosphatidylcholine liposomes having variable fatty acid composition. The enzyme was successfully associated with dimyristoyl, dipalmitoyl, distearoyl, and dioleoylphosphatidylcholine liposomes. The specific activity of the enzyme in the various liposomes varied over a 4.6-fold range indicating some degree of specificity for fatty acid composition. The adenylate cyclase-liposome preparation retained sensitivity to both CaM and 5'-guanylylimidodiphosphate (GppNHp). Arrhenius plots of enzyme activity in the four different liposome preparations all exhibited a pronounced discontinuity at 30 degrees C +/- 2, even though the bulk-phase thermal transition points for the liposomes varied from -20 to 54 degrees C. Fluorescence anisotropy studies of reconstituted liposome systems illustrated that incorporation of protein did not alter the normal-phase transition point of these lipids. Since Arrhenius plots of the enzyme in Lubrol PX, prior to reconstitution with lipids, were strictly linear, it is concluded that the breaks at 30 degrees C may be the effect of a local enzyme-phospholipid environment. It appears that this adenylate cyclase is not particularly sensitive to phase transitions of the bulk lipid phase. The phospholipid reconstituted enzyme system appears suitable for examination of the influence of lipids on the CaM-sensitive adenylate cyclase.

Calmodulin-dependent elevation of calcium transport associated with calmodulin-dependent phosphorylation in cardiac sarcoplasmic reticulum

The rate of calcium transport by sarcoplasmic reticulum vesicles from dog heart assayed at 25 degrees C, pH 7.0, in the presence of oxalate and a low free Ca2+ concentration (approx. 0.5 microM) was increased from 0.091 to 0.162 mumol . mg-1 . min-1 with 100 nM calmodulin, when the calcium-, calmodulin-dependent phosphorylation was carried out prior to the determination of calcium uptake in the presence of a higher concentration of free Ca2+ (preincubation with magnesium, ATP and 100 microM CaCl2; approx. 75 microM free Ca2+). Half-maximal activation of calcium uptake occurs under these conditions at 10-20 nM calmodulin. The rate of calcium-activated ATP hydrolysis by the Ca2+-, Mg2+-dependent transport ATPase of sarcoplasmic reticulum was increased by 100 nM calmodulin in parallel with the increase in calcium transport; calcium-independent ATP splitting was unaffected. The calcium-, calmodulin-dependent phosphorylation of sarcoplasmic reticulum, preincubated with approx. 75 microM Ca2+ and assayed at approx. 10 microM Ca2+ approaches maximally 3 nmol/mg protein, with a half-maximal activation at about 8 nM calmodulin; it is abolished by 0.5 mM trifluperazine. More than 90% of the incorporated [32P]phosphate is confined to a 9-11 kDa protein, which is also phosphorylated by the catalytic subunit of the cAMP-dependent protein kinase and most probably represents a subunit of phospholamban. The stimulatory effect of 100 nM calmodulin on the rate of calcium uptake assayed at 0.5 microM Ca2+ was smaller following preincubation of sarcoplasmic reticulum vesicles with calmodulin in the presence of approx. 75 microM Ca2+, but in the absence of ATP, and was associated with a significant degree of calmodulin-dependent phosphorylation. However, the stimulatory effect on calcium uptake and that on calmodulin-dependent phosphorylation were both absent after preincubation with calmodulin, without calcium and ATP, suggestive of a causal relationship between these processes.

Immunohistochemical localization of troponin-C in cultured neurons

Our previous immunofluorescence studies on neurons have demonstrated the presence of myosin in regions of neurons which contained actin. To determine if a system similar to the troponin complex of striated muscle is present in neurons, antibody shown to be specific for the calcium-binding component of troponin (troponin-C) was applied to cultures of embryonic chick and rat dorsal root ganglia. Neurites treated with anti-troponin-C exhibited a bright fluorescence. Accompanying non-neuronal cells were less reactive than the neuronal elements. Immunodiffusion and immunofluorescence showed that the anti-troponin-C did not react with calmodulin, whereas homogenates of the ganglia elicited a positive immunochemical reaction with the anti-troponin-C in Ouchterlony tests. Our results suggest that some intra-axonal movements may be generated by the interaction of actin and myosin and controlled in part by a calcium-troponin-C-dependent mechanism.

Localization of a trifluoperazine binding site on troponin C

Trifluoperazine (TFP) was shown to interact with the cyanogen bromide fragment 9 (CB9) (residues 84-135) of rabbit skeletal troponin C and with a synthetic peptide representing the N-terminal region of CB9. The phenothiazine did not affect the calcium binding property of CB9 as observed by proton magnetic resonance and circular dichroism spectroscopies. The calculated calcium binding constants for CB9 in the presence and absence of trifluoperazine were identical (KCa2+ = 1.3 X 10(5) M-1). Localization of the trifluoperazine binding site was achieved by analyzing the 1H NMR spectrum of CB9 and of a synthetic fragment corresponding to residues 90-104 of CB9. Drug-induced shifting and broadening of the ring protons of phenylalanine residues and the methyl resonances of alanine, leucine, and isoleucine residues suggest that the segment 95-102 is in close proximity to the phenothiazine aromatic region. The neighboring negative side chains in the peptide sequence also suggest that the single positive charge present on the piperazine nitrogens of trifluoperazine may interact with them and sterically block a region of interaction of calmodulin (CaM) and troponin C (TnC) with modulated proteins such as phosphodiesterase. Primary sequence analysis of CaM and troponin C reveals that a homologous hydrophobic region to site 3 is also found in the N-terminal region of site 1 of both calcium binding proteins. Binding of TFP to CB9 occurs both in the presence and absence of calcium since the hydrophobic region in these small fragments is completely accessible to TFP whether calcium is present or not. The dissociation constant of the drug to apoCB9 (8 microM) was obtained by ellipticity measurements at 222 nm and was comparable to the 5 microM value obtained by Levin and Weiss [Levin, R. M., & Weiss, B. (1978) Biochim. Biophys. Acta 540, 197-204] for calcium-saturated rabbit skeletal troponin C.

Association of calmodulin inhibition, erythrocyte membrane stabilization and pharmacological effects of drugs

The present study was designed to determine whether there is an association of drug-induced inhibition of calmodulin functions, drug-induced membrane stabilization (protection against osmotic lysis), and pharmacological effects of drugs. First, data on drugs which have been studied for both calmodulin inhibition and membrane antihemolysis were collected from the literature and an association of the two properties was established. Second, ten additional drugs were selected for study of all three properties. Four drugs, with known antihemolytic effects, were studied for calmodulin inhibition. One drug, which was a known calmodulin inhibitor, was studied for antihemolysis. Our results show that membrane-stabilizing drugs are usually calmodulin inhibitors, and vice versa; that drugs in certain therapeutic classes inhibit calmodulin-activated functions and protect against osmotic lysis; and finally, that there is a significant correlation (P less than 0.01) in terms of potency between these two actions of drugs. Data from the literature which bear on these mechanisms of drug actions suggest that the interactions between drugs and calmodulin, and drugs and the membrane, appear to be hydrophobic in nature. At this point, we do not know whether there is some causal relationship between calmodulin inhibition and the antihemolytic effect of drugs, or whether the two are simply a result of hydrophobic properties of drugs. Similarly, the roles of calmodulin inhibition and/or membrane antihemolysis in producing therapeutic efficacy are unknown.

Binding of simple peptides, hormones, and neurotransmitters by calmodulin

We have prepared a fluorescent conjugate of porcine calmodulin with 5-(dimethylamino)-1-naphthalene-sulfonyl chloride that is highly sensitive to both calcium binding and protein binding. We have used the fluorescence of this conjugate in addition to the intrinsic peptide fluorescence to show that adrenocorticotropic hormone (ACTH), beta-endorphin, glucagon, and substance P undergo calcium-dependent binding by calmodulin, with competition for common binding sites. The dissociation constants determined in the presence of 0.85 mM CaCl2 and 0.2 N KC1, pH 7.3 at 25 degrees C, range from 1.5 muM to 3.4 muM. The alpha-melanocyte-stimulating hormone, bombesin, and somatostatin also bind, with dissociation constants between 60 muM and 90 muM. Angiotensins I and III, bradykinin, neurotensin, physalaemin, substance P octapeptide, insulin, and Leu- and Met-enkephalin show little or no binding. Sequence comparisons show that the peptides that bind calmodulin well contain regions structurally similar to the recognition sequence for the cAMP-dependent protein kinase and to the sequences surrounding phosphorylated serine residues in several calmodulin binding proteins. This result suggests that modification of calmodulin binding sites in calmodulin-dependent proteins is one of the functions of protein kinase. Calcium has a dual role in peptide binding by calmodulin. The occupation of calcium binding sites having a pK approximately 4 results in a 2-fold increase in peptide binding affinity.

The control of pigment migration in isolated erythrophores of Holocentrus ascensionis (Osbeck). II. The role of calcium

The integumental pigment cells (erythrophores) of the squirrel fish, Holocentrus ascensionis, are specialized for rapid radial transport of the pigment granules contained within their cytoplasm. Pigment granules in isolated denervated erythrophores alternate spontaneously between a centrally aggregated state and a radially dispersed state. In the absence of external calcium, pigment aggregation does not occur spontaneously and cannot be induced by the aggregating agents epinephrine or high concentration of external K+. Pigment aggregation is also impaired in the presence of D600 or papaverine, compounds reported to antagonize calcium influx into the cell. Pigment aggregation can be induced by experimental elevation of the concentration of cytoplasmic free Ca2+, with a Ca-EGTA buffer system in conjunction with ionophore A23187. The threshold concentration of Ca2+ required to produce this effect is 5 X 10(-6) M. These results suggest that cytoplasmic free Ca2+ is involved in mediating pigment aggregation and that some, if not all, the Ca2+ is supplied by influx from the extracellular space.

Platelet calcium-dependent proteins: identification and localization of the calcium-dependent regulator, calmodulin, in platelets

The calcium-dependent regulator protein, calmodulin, is a 17,000 molecular weight polypeptide which binds calcium and has been shown to confer calcium sensitivity on contractile and other proteins. In the present study, we have examined the presence and subcellular distribution of this protein in preparations of human platelets. Calmodulin was quantified using a two-stage phosphodiesterase assay. Whole platelets contained 1.33 +/- 0.06 units calmodulin per 10(6) platelets or 26.5 +/- 3.4 fg calmodulin per platelet. The distribution of calmodulin in the platelet was predominantly soluble with over 80 percent of calmodulin activity in the soluble fraction of the cell. There was no apparent difference in the distribution of calmodulin between soluble and particulate compartments in recalcified platelet homogenates compared to homogenates in EDTA. Indirect immunofluorescent studies with monospecific antisera to dinitrophenylated calmodulin showed intense staining of platelets in a diffuse pattern. The identification of calmodulin in platelets raises the possibility that this protein may participate in calcium-dependent reactions important in platelet aggregation and release.

The purification and identification of calmodulin from human placenta

A protein which showed similarity to bovine brain calmodulin in electrophoretic mobilities on polyacrylamide gels in the presence of 40% glycerol (pH 8.6) and 0.1% sodium dodecyl sulfate (pH 7.2) was isolated from human placenta. Its final yield was approx. 4 mg per kg human placenta. The placenta protein was similar to bovine brain calmodulin in stimulating bovine brain calmodulin-deficient cyclic nucleotide phosphodiesterase in the presence of calcium. However, its stimulating activity was eliminated by ethyleneglycol-bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) or trifluoperazine. In addition, there is a close resemblance in amino acid composition between the placental protein and bovine brain calmodulin. These results indicate that calmodulin is present in human placenta.

A calcium/calmodulin-dependent cyclic adenosine monophosphate phosphodiesterase from Drosophila heads

A Ca2+-activated cycl AMP phosphodiesterase from Drosophila melanogaster heads was studied. The enzyme accounted for approx. 40% of the total, soluble cyclic AMP phosphodiesterase activity in heads. After gel filtration, Ca2+ stimulation of the enzyme was no longer apparent, but Ca2+ activation could be restored by the addition of boiled Drosophila extract to the column-fractionated phosphodiesterase. The protein responsible for restoring Ca2+ activation was purified and shown to have some characteristics of calmodulin. In addition, porcine calmodulin was able to activate the Drosophila phosphodiesterase. Thus, the phosphodiesterase-calmodulin system in Drosophila appears analogous to similar systems in mammals.

The identification of calmodulin-binding sites on mitochondria in cultured 3T3 cells

We have uniformly labeled calmodulin with tetramethyl rhodamine isothiocyanate (CaM-RITC) and used the derivative as a molecular probe in order to identify available, unoccupied calmodulin-binding sites. In mildly fixed (3% formalin) cultured 3T3 cells, the biologically active CaM-RITC bound predominantly to mitochondria. Binding was markedly reduced in the presence of 1 mM EGTA. Stelazine, a phenothiozine which binds to calmodulin, prevented the interaction of CaM-RITC with mitochondrial sites. A 10 fold excess of unlabeled CaM competitively inhibited binding. Fluorescently labeled troponin C and parvalbumin did not bind to mitochondria on any other cellular organelle. Rhodamine (TMRITC) alone did not bind to 3T3 mitochondria. Similar results were obtained using 125I-calmodulin binding to isolated rat liver mitochondria. When solubilized mitochondrial proteins were subjected to calmodulin-Sepharose affinity chromatography and eluted with 1 mM EGTA, there were two major polypeptides 120,000 and 67,000 daltons and at least three minor species (100,000, 60,000 and 40,000 daltons). The interaction required an active Ca2+-CaM complex and is specific for CaM. Double fluorescent staining with CaM-RITC and fluorescein-labeled antibodies to tubulin and DNAase I revealed a mitochondrial distribution pattern similar to that of microtubule arrays but unrelated to actin cabling. There was no evidence that CaM-RITC directly interacted with either microtubules or microfilaments.

Calcium-induced exposure of a hydrophobic surface on calmodulin

Interactions between calmodulin (CaM) and several hydrophobic fluorescent probes were characterized in order to determine if CaM expresses hydrophobic binding sites in the presence of Ca2+. Several classes of fluorescent probes capable of sensing exposure of hydrophobic binding sites on proteins were found to bind to CaM, and these interactions were greatly enhanced by Ca2+. In the presence of Ca2+, the fluorescence intensity of 9-anthroylcholine (9AC) was increased 24-fold by CaM, with a shift in the fluorescence emission maximum from 514 to 486 nm. The fluorescence intensity of 8-anilino-1-naphthalenesulfonate (Ans) was enhanced 27-fold with an emission maximum shift from 540 to 488 nm in the presence of CaM and Ca2+. Similar results were obtained with the uncharged fluorescent ligand, N-phyenyl-1-naphthylamine. With all three fluorescent dyes, the fluorescence changes caused by CaM in the absence of Ca2+ were minor compared to those observed with CaM and Ca2+. Direct binding studies using equilibrium dialysis demonstrated that CaM can bind four to six molecules of 9AC or two to three molecules of Ans in a calcium-dependent manner. The effects of various amphiphilic compounds on the Ca2+-dependent complex formation between CaM and the Ca2+-sensitive phosphodiesterase or troponin I were investigated. Trifluoperazine (TFP) and 9AC inhibited CaM stimulation of the Ca2+-sensitive phosphodiesterase. The Ca2+-dependent binding of the phosphodiesterase to CaM-Sepharose was also inhibited by TFP, 9AC, and Ans. Furthermore, binding of CaM to troponin I-Sepharose was inhibited by these ligands. Consistent with these data was the observation that troponin I antagonized binding of 9AC to CaM. These data indicate that binding of Ca2+ to CaM results in exposure of a domain with considerable hydrophobic character, and binding of hydrophobic ligands to this domain antagonizes CaM-protein interactions. It is proposed that this hydrophobic domain may serve as the interface for the Ca2+-dependent binding of CaM to the phosphodiesterase or troponin I.

Sodium-23 nuclear magnetic resonance as an indicator of sodium binding to calmodulin and tryptic fragments, in relation to calcium content

The relaxation rate enhancements of the 23Na nuclei for NaHCO3 solutions of calmodulin and its tryptic peptides TR-1 and TR-2 indicate true binding of Na+ ions to these biomolecules. With both TR-1 and TR-2, Na+ binding occurs in competition with Ca2+ and Mg2+ binding: log KNa approximately equal to 2, log KMg apoproximately equal to 4, log KCa approximately equal to 6 for TR-1; and log KNa approximately equal to 2, log KMg approximately equal to 3, log KCa approximately equal to 5 for TR-2. All the binding constants are systemically greater for binding to TR-1, as compared to TR-2. There is also an increase in KNa for TR-1 of calmodulin as compared to the homologous tryptic fragment of troponin C. The increased binding is identified tentatively with site I of calmodulin. The binding constants KNa, KCa and KMg of calmodulin appear to be finely tuned to the intracellular concentrations of these cations.

Detection of a calmodulin-sensitive cyclic nucleotide phosphodiesterase in rat parotid gland

Calmodulin coupled to Sepharose has provided a rapid and sensitive means of isolating a cyclic nucleotide phosphodiesterase activity which is stimulated by the calmodulin-Ca2+ complex, from rat parotid gland. Initial experiments established that phosphodiesterase activity sensitive to calmodulin and Ca2+ could not be demonstrated in crude extracts of rat parotid gland or after partial purification of rat parotid phosphodiesterase over DEAE-cellulose. However, it was possible to readily demonstrate the presence of a cyclic nucleotide phosphodiesterase activity regulated by calmodulin if the extracts were first purified by batch ion-exchange chromatography over DEAE-cellulose followed by affinity chromatography with calmodulin coupled to Sepharose. The batch ion-exchange chromatography step removed the major portion of free parotid calmodulin which could compete with calmodulin-coupled Sepharose for the proteins regulated by calmodulin. Thus, by employing an initial chromatography step over DEAE-cellulose to separate phosphodiesterase activity from calmodulin, it was possible to increase the recovery of calmodulin-sensitive phosphodiesterase after affinity chromatrography with calmodulin coupled to Sepharose. This approach should be useful for demonstrating the presence of and for purifying other parotid proteins regulated by calmodulin.

Characterization of the plant nicotinamide adenine dinucleotide kinase activator protein and its identification as calmodulin

A protein activator of plant NAD kinase has been extracted from plant sources (peanuts and peas), purified to homogeneity, characterized, and identified as calmodulin. A comparison of the properties of calmodulin isolated from either plant or animal sources shows that they are strikingly similar proteins. The similarities include molecular weight, Stokes radii, amino acid composition, Ca2+-dependent enhancement of tyrosine fluorescence, Ca2+-dependent interaction with troponin I, equal abilities to activate cyclic nucleotide phosphodiesterase, Ca2+-dependent inhibition of calmodulin action by the phenothiazine drugs, and electrophoretic mobility. We discuss the possibility that plant cells may undergo Ca2+-dependent regulatory events that are mediated by calmodulin in a manner similar to those found in animals.

The purification of a unique calcium-binding protein from Morris hepatoma 5123 tc

A heat-stable Ca2+-binding protein was purified to homogeneity from Morris hepatoma 5123 tc. It had an apparent molecular weight of 11 000, and isoelectric point of 3.9, and bound two atoms of calcium per molecule of protein. The spectral and amino acid analysis indicated the tumour protein to be similar to the parvalbumins. This protein has been shown to be absent in liver.

Structure and evolution of calcium-modulated proteins

This review suggests that the intracellular functions of calcium are best understood in terms of calcium's functioning as a second messenger. Further, when functioning as a second messenger, calcium completes its mission not by transferring charge nor by binding to lipid but by binding to specific targets, calcium-modulated proteins. This concept is broadly interpreted to include proteins involved in calcium transport. There is strong evidence that many, if not all, of these calcium-modulated proteins are homologs. Their structures and properties are contrasted to those of extracellular calcium-binding proteins which are not homologous to one another or to the intracellular calcium-modulated proteins. Finally, this line of thought leads to a suggestion of the evolutionary reason for the choice of calcium as the sole inorganic second messenger.