Controlled proteolysis of the purified Ca2+-ATPase of the erythrocyte membrane. A correlation between the structure and the function of the enzyme

The calcium pump of plasma membranes

The calcium pump of plasma membranes is an ATPase of the E1E2 type; that is, it forms a phosphoenzyme during the reaction cycle and is inhibited by vanadate. It differs from the Ca2+-transporting ATPase of sarcoplasmic reticulum in molecular mass, immunological properties and Ca2+/ATP stoichiometry. Its affinity for calcium, which is low in the absence of calmodulin (Km, 10-20 microM), is increased by the latter (to a Km of about 0.5 microM). The effect of calmodulin is mimicked by acidic phospholipids (including the phosphorylated products of phosphatidylinositol), long-chain polyunsaturated fatty acids, and controlled treatment with a number of proteases. The ATPase has been purified to homogeneity from a number of plasma membranes using calmodulin affinity chromatography. The purified enzyme (a single polypeptide of molecular mass 138 kDa) pumps calcium into reconstituted liposomes in exchange for protons. Controlled trypsin proteolysis has shown that about one-third of the enzyme mass can be removed without impairing calcium transport. It has also indicated that the ability to bind calmodulin and to respond to it resides in a 9 kDa sequence of the enzyme molecule. The sequence contains a 4 kDa domain that binds calmodulin, and a 5 kDa domain which is essential for the stimulation.

Purification and membrane reconstitution of catalytically active Menkes copper-transporting P-type ATPase (MNK; ATP7A)

The MNK (Menkes disease protein; ATP7A) is a major copper- transporting P-type ATPase involved in the delivery of copper to cuproenzymes in the secretory pathway and the efflux of excess copper from extrahepatic tissues. Mutations in the MNK (ATP7A) gene result in Menkes disease, a fatal neurodegenerative copper deficiency disorder. Currently, detailed biochemical and biophysical analyses of MNK to better understand its mechanisms of copper transport are not possible due to the lack of purified MNK in an active form. To address this issue, we expressed human MNK with an N-terminal Glu-Glu tag in Sf9 [Spodoptera frugiperda (fall armyworm) 9] insect cells and purified it by antibody affinity chromatography followed by size-exclusion chromatography in the presence of the non-ionic detergent DDM (n-dodecyl beta-D-maltopyranoside). Formation of the classical vanadate-sensitive phosphoenzyme by purified MNK was activated by Cu(I) [EC50=0.7 microM; h (Hill coefficient) was 4.6]. Furthermore, we report the first measurement of Cu(I)-dependent ATPase activity of MNK (K0.5=0.6 microM; h=5.0). The purified MNK demonstrated active ATP-dependent vectorial 64Cu transport when reconstituted into soya-bean asolectin liposomes. Together, these data demonstrated that Cu(I) interacts with MNK in a co-operative manner and with high affinity in the sub-micromolar range. The present study provides the first biochemical characterization of a purified full-length mammalian copper-transporting P-type ATPase associated with a human disease.

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.

Expression, purification, and characterization of isoform 1 of the plasma membrane Ca2+ pump: focus on calpain sensitivity

The plasma membrane Ca2+ ATPase isoform 1(PMCA1) is ubiquitously distributed in tissues and cells, but only scarce information is available on its properties. The isoform was overexpressed in Sf9 cells, purified on calmodulin columns, and characterized functionally. The level of expression was very low, but sufficient amounts of the protein could be isolated for biochemical characterization. The affinity of PMCA1 for calmodulin was similar to that of PMCA4, the other ubiquitous PMCA isoform. The affinity of PMCA1 for ATP, evaluated by the formation of the phosphorylated intermediate, was higher than that of the PMCA4 pump. The recombinant PMCA1 pump was a much better substrate for the cAMP-dependent protein kinase than the PMCA2 and PMCA4 isoforms. Pulse and chase experiments on Sf9 cells overexpressing the PMCA pumps showed that PMCA1 was much less stable than the PMCA4 and PMCA2 isoforms, i.e. PMCA1 had a much higher sensitivity to degradation by calpain. The effect of calpain was not the result of a general higher susceptibility of the PMCA1 to proteolytic degradation, because the pattern of degradation by trypsin was the same in the three isoforms.

Novel aspects of calmodulin target recognition and activation

Several crystal and NMR structures of calmodulin (CaM) in complex with fragments derived from CaM-regulated proteins have been reported recently and reveal novel ways for CaM to interact with its targets. This review will discuss and compare features of the interaction between CaM and its target domains derived from the plasma membrane Ca2+-pump, the Ca2+-activated K+-channel, the Ca2+/CaM-dependent kinase kinase and the anthrax exotoxin. Unexpected aspects of CaM/target interaction observed in these complexes include: (a) binding of the Ca2+-pump domain to only the C-terminal part of CaM (b) dimer formation with fragments of the K+-channel (c) insertion of CaM between two domains of the anthrax exotoxin (d) binding of Ca2+ ions to only one EF-hand pair and (e) binding of CaM in an extended conformation to some of its targets. The mode of interaction between CaM and these targets differs from binding conformations previously observed between CaM and peptides derived from myosin light chain kinase (MLCK) and CaM-dependent kinase IIalpha (CaMKIIalpha). In the latter complexes, CaM engulfs the CaM-binding domain peptide with its two Ca2+-binding lobes and forms a compact, ellipsoid-like complex. In the early 1990s, a model for the activation of CaM-regulated proteins was developed based on this observation and postulated activation through the displacement of an autoinhibitory or regulatory domain from the target protein upon binding of CaM. The novel structures of CaM-target complexes discussed here demonstrate that this mechanism of activation may be less general than previously believed and seems to be not valid for the anthrax exotoxin, the CaM-regulated K+-channel and possibly also not for the Ca2+-pump.

3-O-methylfluorescein phosphate as a fluorescent substrate for plasma membrane Ca2+-ATPase

3-O-methylfluorescein phosphate hydrolysis, catalyzed by purified erythrocyte Ca2+-ATPase in the absence of Ca2+, was slow in the basal state, activated by phosphatidylserine and controlled proteolysis, but not by calmodulin. p-Nitrophenyl phosphate competitively inhibits hydrolysis in the absence of Ca2+, while ATP inhibits it with a complex kinetics showing a high and a low affinity site for ATP. Labeling with fluorescein isothiocyanate impairs the high affinity binding of ATP, but does not appreciably modify the binding of any of the pseudosubstrates. In the presence of calmodulin, an increase in the Ca2+ concentration produces a bell-shaped curve with a maximum at 50 microM Ca2+. At optimal Ca2+ concentration, hydrolysis of 3-O-methylfluorescein phosphate proceeds in the presence of fluorescein isothiocyanate, is competitively inhibited by p-nitrophenyl phosphate and, in contrast to the result observed in the absence of Ca2+, it is activated by calmodulin. In marked contrast with other pseudosubstrates, hydrolysis of 3-O-methylfluorescein phosphate supports Ca2+ transport. This highly specific activity can be used as a continuous fluorescent marker or as a tool to evaluate partial steps from the reaction cycle of plasma membrane Ca2+-ATPases.

Molecular aspects of higher plant P-type Ca(2+)-ATPases

Recent genomic data in the model plant Arabidopsis thaliana reveal the existence of at least 11 Ca(2+)-ATPase genes, and an analysis of expressed sequence tags suggests that the number of calcium pumps in this organism might be even higher. A phylogenetic analysis shows that 11 Ca(2+)-ATPases clearly form distinct groups, type IIA (or ECA for ER-type Ca(2+)-ATPase) and type IIB (ACA for autoinhibited Ca(2+)-ATPase). While plant IIB calcium pumps characterized so far are localized to internal membranes, their animal homologues are exclusively found in the plasma membrane. However, Arabidopsis type IIB calcium pump isoforms ACA8, ACA9 and ACA10 form a separate outgroup and, based on the high molecular masses of the encoded proteins, are good candidates for plasma membrane bound Ca(2+)-ATPases. All known plant type IIB calcium ATPases seem to employ an N-terminal calmodulin-binding autoinhibitor. Therefore it appears that the activity of type IIB Ca(2+)-ATPases in plants and animals is controlled by N-terminal and C-terminal autoinhibitory domains, respectively. Possible functions of plant calcium pumps are described and - beside second messenger functions directly linked to calcium homeostasis - new data on a putative involvement in secretory and salt stress functions are discussed.

The N-terminal region of the plasma membrane Ca(2+) pump does not separate from the main catalytic fragments after proteolysis

The purified plasma membrane Ca(2+) pump (PMCA) was digested with trypsin, and the proteolytic products were identified by immunoblotting with monoclonal antibodies JA9 or 5F10 directed against the extreme N-terminal segment and the central portion of the molecule, respectively. After a short treatment with low concentrations of the protease, JA9 reacted predominantly with a peptide of 35 kDa whereas 5F10 detected a peptide of 90 kDa. The trypsin cut leading to the production of these fragments had no effect on the maximal activity of the enzyme. At higher concentrations of trypsin, JA9 detected a main fragment of 33 kDa and smaller fragments of 19 and 15 kDa. The persistence of fragments reacting with JA9 indicates that the N-terminal region containing its epitope (residues 51-75) was not easily accessible to the protease in the native PMCA. However, the reactivity with JA9 was rapidly lost during proteolysis of the denatured protein. The passage of the mixture of PMCA fragments through a calmodulin-Sepharose column resulted in the retention of the N-terminal 35 kDa fragment together with that of 90 kDa, despite the fact that only the latter binds calmodulin. The ethylenediaminetetraacetic acid (EDTA) eluate, which contained about equal amounts of both fragments, had a Ca(2+) ATPase activity similar to that of the intact enzyme. The tight association between the two peptides was evidenced by the fact that concentrations of polyoxyethylene 10 lauryl ether (C(12)E(10)), sodium dodecyl sulfate (SDS) high enough for inactivating the enzyme and dissociate the pump from calmodulin were unable of breaking the interaction between the 35 and 90 kDa fragments. Altogether, these results show that after digestion with trypsin, the N-terminal portion of the PMCA, including the extreme N-terminal segment, remains part of a fully functional catalytic complex.

Distribution of plasma-membrane Ca2+ pump in mandibular condyles from growing and adult rabbits

Chondrocytes may control the mineralization of the extracellular matrix of condylar cartilage by several mechanisms including the release of microvesicles involved in the initial nucleation, the creation or modification of the local matrix to help propagate or restrict mineralization, and the regulation of the ionic environment at the calcifying foci within the matrix. The plasma membrane Ca2+-Mg2+ ATPase (Ca2+ pump) is known to play a part in the vectorial efflux of calcium in a variety of cells including chondrocytes. The purpose here was to study the distribution of Ca2+-pump protein in mandibular condyles from growing and adult rabbits, and compare the expression of that protein in progressively differentiating chondrocytes whose final stage is associated with a mineralized extracellular matrix. Ca2+-pump antigen was identified immunohistochemically in six growing and six adult rabbit mandibular condyles with a Ca2+ pump-specific monoclonal antibody. The presence of Ca2+-pump antigen was established in hypertrophic chondrocytes, and in osteoblasts and osteoclasts of subchondral bone. Slot-blot analysis of nitrocellulose-immobilized chondrocyte homogenates showed that the amount of Ca2+ pump in growing cartilage was more than twice that in adult cartilage (p < 0.05). The demonstration of Ca2+-pump antigen in the hypertrophic chondrocytes of growing rabbit condyles is consistent with a role for the plasma-membrane Ca2+ pump in the calcification of mandibular condylar cartilage.

The membrane topology of the amino-terminal domain of the red cell calcium pump

A systematic study of the membrane-associated regions in the plasma membrane Ca2+ pump of erythrocytes has been performed by hydrophobic photolabeling. Purified Ca2+ pump was labeled with 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)-diazirine ([125I]TID), a generic photoactivatable hydrophobic probe. These results were compared with the enzyme labeled with a strictly membrane-bound probe, [3H]bis-phosphatidylethanolamine (trifluoromethyl) phenyldiazirine. A significant light-dependent labeling of an M(r) 135,000-140,000 peptide, corresponding to the full Ca2+ pump, was observed with both probes. After proteolysis of the pump labeled with each probe and isolation of fragments by SDS-PAGE, a common pattern of labeled peptides was observed. Similarly, labeling of the Ca2+ pump with [125I]TID, either in isolated red blood cell membranes or after the enzyme was purified, yields a similar pattern of labeled peptides. Taken together, these results validate the use of either probe to study the lipid interface of the membrane-embedded region of this protein, and sustain the notion that the conformation of the pump is maintained throughout the procedures of solubilization, affinity purification, and reconstitution into proteoliposomes. In this work, we put special emphasis on a detailed analysis of the N-terminal domain of the Ca2+ pump. A labeled peptide of M(r) 40,000 belonging to this region was purified and further digested with V8 protease. The specific incorporation of [125I]TID to proteolytic fragments pertaining to the amino-terminal region indicates the existence of two transmembrane stretches in this domain. A theoretical analysis based on the amino acid sequence 1-322 predicts two segments with high probability of membrane insertion, in agreement with the experimental data. Each segment shows a periodicity pattern of hydrophobicity and variability compatible with alpha-helical structure. These results strongly suggest the existence of a transmembrane helical hairpin motif near the N-terminus of the Ca2+ pump.

Intermolecular tuning of calmodulin by target peptides and proteins: differential effects on Ca2+ binding and implications for kinase activation

Ca(2+)-activated calmodulin (CaM) regulates many target enzymes by docking to an amphiphilic target helix of variable sequence. This study compares the equilibrium Ca2+ binding and Ca2+ dissociation kinetics of CaM complexed to target peptides derived from five different CaM-regulated proteins: phosphorylase kinase. CaM-dependent protein kinase II, skeletal and smooth myosin light chain kinases, and the plasma membrane Ca(2+)-ATPase. The results reveal that different target peptides can tune the Ca2+ binding affinities and kinetics of the two CaM domains over a wide range of Ca2+ concentrations and time scales. The five peptides increase the Ca2+ affinity of the N-terminal regulatory domain from 14- to 350-fold and slow its Ca2+ dissociation kinetics from 60- to 140-fold. Smaller effects are observed for the C-terminal domain, where peptides increase the apparent Ca2+ affinity 8- to 100-fold and slow dissociation kinetics 13- to 132-fold. In full-length skeletal myosin light chain kinase the inter-molecular tuning provided by the isolated target peptide is further modulated by other tuning interactions, resulting in a CaM-protein complex that has a 10-fold lower Ca2+ affinity than the analogous CaM-peptide complex. Unlike the CaM-peptide complexes, Ca2+ dissociation from the protein complex follows monoexponential kinetics in which all four Ca2+ ions dissociate at a rate comparable to the slow rate observed in the peptide complex. The two Ca2+ ions bound to the CaM N-terminal domain are substantially occluded in the CaM-protein complex. Overall, the results indicate that the cellular activation of myosin light chain kinase is likely to be triggered by the binding of free Ca2(2+)-CaM or Ca4(2+)-CaM after a Ca2+ signal has begun and that inactivation of the complex is initiated by a single rate-limiting event, which is proposed to be either the direct dissociation of Ca2+ ions from the bound C-terminal domain or the dissociation of Ca2+ loaded C-terminal domain from skMLCK. The observed target-induced variations in Ca2+ affinities and dissociation rates could serve to tune CaM activation and inactivation for different cellular pathways, and also must counterbalance the variable energetic costs of driving the activating conformational change in different target enzymes.

Deletion of amino acid residues 18-75 inactivates the plasma membrane Ca2+ pump

A mutant of the plasma membrane Ca2+ pump hPMCA4b(d18-75)(ct120) containing a deletion of the N-terminal amino acid residues 18-75 and lacking the C-terminal 120 amino acid residues was expressed in COS-1 cells. The deletion in the N-terminal region did not significantly affect the level of expression of the Ca2+ pump. Tryptic digestion of the hPMCA4b(d18-75)(ct120) mutant resulted in the appearance of the same fragments obtained by proteolysis of the hPMCA4b(ct120) enzyme, suggesting that deletion of residues 18-75 neither impeded the insertion in the membrane nor extensively affected the folding of the mutant protein. The functional competence of the hPMCA4b(d18-75)(ct120) enzyme was examined by measuring the Ca2+ transport and the Ca2+ ATPase activity of COS-1 cell microsomes expressing the mutant protein. Both tests proved the mutant to be inactive. Under conditions in which hPMCA4b(ct120) becomes phosphorylated, hPMCA4b(d18-75)(ct120) was incapable of reacting with ATP and Ca2+ to form the phosphoenzyme. Taken together these results suggest that the segment of amino acids 18-75 is essential for the activity of the plasma membrane Ca2+ pump.

Calmodulin kinase IV: expression and function during rat brain development

The expression of calmodulin kinase IV (CaMKIV) can be induced by the thyroid hormone T3 in a time- and concentration-dependent manner at a very early stage of brain differentiation using a fetal rat telencephalon primary cell culture system which can grow and differentiate under chemically defined conditions (Krebs et al. (1996) J. Biol. Chem. 271, 11055-11058). After the induction of CaMKIV by T3 we examined the influence of prolonged absence of T3 from the culture medium on the expression of CaMKIV. We could demonstrate that after the T3-dependent induction of CaMKIV, omission of the hormone, even for 8 days, from the medium did not downregulate the expression of CaMKIV indicating that different regulatory mechanisms became important for the expression of the enzyme. We further showed that CaMKIV could be involved in the Ca(2+) -dependent expression of the immediate early gene c-fos, probably via phosphorylation of the transcription factor CREB. Convergence of signal transduction pathways on this transcription factor by using different protein kinases may explain the importance of CREB for the regulation of different cellular processes.

Dynamic structure of the calmodulin-binding domain of the plasma membrane Ca-ATPase in native erythrocyte ghost membranes

We have used frequency-domain fluorescence resonance energy transfer (FRET) and anisotropy measurements to identify the structural properties of wheat germ calmodulin (CaM) bound to either the plasma membrane Ca-ATPase (PM-Ca-ATPase) in native erythrocyte ghost membranes or a peptide (C25W) that has an identical sequence to the CaM-binding domain on the PM-Ca-ATPase. Cross-linking experiments using benzophenone labeled CaM in conjunction with immunoblots using antibodies specific for either CaM or the PM-Ca-ATPase indicate that one molecule of CaM selectively binds one PM-Ca-ATPase polypeptide chain in native erythrocyte ghost membranes. There are no other proteins in the erythrocyte membrane that bind CaM with high affinity, permitting the measurement of the structural properties of CaM bound to the PM-Ca-ATPase in native erythrocyte ghost membranes. FRET measurements between the fluorophore pyrene maleimide (PMal) located at Cys27 in calcium binding loop I and nitrotyrosine139 in calcium binding loop IV on wheat germ CaM indicate that the average spatial separation and conformational heterogeneity associated with the two opposing globular domains of CaM are virtually identical upon CaM binding to either the PM-Ca-ATPase or C25W. Measurements of the solvent accessibility and segmental rotational dynamics of PMal-CaM bound to either the PM-Ca-ATPase or C25W further indicate that the local environment around the pyrene label located at Cys27 is very similar. However, the overall rotational dynamics of CaM bound to the PM-Ca-ATPase is much slower (phi 2 = 83 +/- 14 ns) than observed when CaM binds C25W (phi 2 = 10.3 +/- 0.5 ns). This implies that CaM is tightly associated with the CaM-binding domain of the PM-Ca-ATPase and that the observed rotational motion of pyrenylmaleimide labeled CaM is characteristic of the global motion of the CaM-binding domain on the PM-Ca-ATPase. The similar conformational heterogeneity and local environment of CaM bound to either the PM-Ca-ATPase or C25W indicates that CaM binds to a contiguous sequence of amino acids on the Ca-ATPase that are analogous to C25W and that there are no significant interactions with other structural elements within the PM-Ca-ATPase. The rate of rotational motion associated with CaM bound to the PM-Ca-ATPase is consistent with hydrodynamic calculation in which the calmodulin-binding domain located at the carboxyl-terminus of the PM-Ca-ATPase has a stable and defined tertiary structure that is independent of the other cytoplasmic domains of the PM-Ca-ATPase.

Induction of calmodulin kinase IV by the thyroid hormone during the development of rat brain

This communication reports the specific induction of calmodulin kinase IV by the thyroid hormone 3,3',5-triiodo-L-thyronine (T3) in a time- and concentration-dependent manner at a very early stage of brain differentiation using a fetal rat telencephalon primary cell culture system, which can grow and differentiate under chemically defined conditions. The induction of the enzyme that can be observed both on the mRNA and on the protein level is T3-specific, i.e. it cannot be induced by retinoic acid or reverse T3, and can be inhibited on both the transcriptional and the translational level by adding to the culture medium actinomycin D or cycloheximide, respectively. The earliest detection of calmodulin kinase IV in the fetal brain tissue of the rat is at days E16/E17, both on the mRNA as well as on the protein level. This is the first report in which a second messenger-dependent kinase involved in the control of cell regulatory processes is itself controlled by a primary messenger, the thyroid hormone.

Activation of a recombinant petunia glutamate decarboxylase by calcium/calmodulin or by a monoclonal antibody which recognizes the calmodulin binding domain

To date, only plants have been shown to possess a form of glutamate decarboxylase (GAD) that binds calmodulin. In the present study, a recombinant calmodulin-binding 58-kDa petunia GAD produced in Escherichia coli was purified to homogeneity using calmodulin-affinity chromatography, and its responsiveness to calcium and calmodulin was examined in vitro. At pH 7.0-7.5, the purified recombinant enzyme was essentially inactive in the absence of calcium and calmodulin, but it could be stimulated to high levels of activity (Vmax = 30 micromol of CO2 min-1 mg of protein-1) by the addition of exogenous calmodulin (K0.5 = 15 nM) in the presence of calcium (K0.5 = 0.8 microM). Neither calcium nor calmodulin alone had any effect on GAD activity. Recombinant GAD displayed hyperbolic kinetics at pH 7.3 (Km = 8.2 mM). A monoclonal antibody directed against the carboxyl-terminal region, which contains the calmodulin-binding domain of GAD, was able to fully activate GAD in a dose-dependent manner in the absence of calcium and calmodulin, whereas an antibody recognizing an epitope outside of this region was unable to activate GAD. This study provides the first evidence that the activity of the purified 58-kDa GAD polypeptide is essentially calcium/calmodulin-dependent at physiological pH. Furthermore, activation of GAD by two different proteins that interact with the calmodulin-binding domain, a monoclonal antibody or calcium/calmodulin, suggests that this domain plays a major role in the regulation of plant GAD activity.

Thapsigargin protects human erythrocyte Ca(2+)-ATPase from proteolysis

The effect of thapsigargin on the activation by partial proteolysis of the plasma membrane Ca(2+)-ATPase was studied in intact human erythrocyte membranes and in the purified enzyme. The enzyme was maximally activated in the absence of thapsigargin within 1 min of exposure to trypsin. However, in the presence of thapsigargin maximal activation was achieved only after 5 min trypsin digestion. Thapsigargin did not alter the pattern of proteolysis as revealed by SDS-PAGE of the tryptic fragments, although it slowed down the rate of appearance of the fragments. Thapsigargin also enhanced the activation of the enzyme by calmodulin. These findings suggest that, although thapsigargin at low concentrations has no effect on the catalytic activity of the Ca(2+)-ATPase in vitro in the absence of calmodulin, it could interfere with its regulation in vivo.

Calmodulin-binding domains: just two faced or multi-faceted?

The Ca(2+)-binding protein calmodulin binds to and activates several cellular enzymes in response to a rise in Ca2+ concentration. It binds certain basic amphiphilic helices within these enzymes, which also act as autoinhibitory domains. The modulation of the binding equilibrium of these helices between intramolecular (inhibition) and intermolecular (activation) sites forms a focal point for crosstalk between various signalling pathways.

Ethanol stimulates the plasma membrane calcium pump from human erythrocytes

The plasma membrane Ca(2+)-ATPase from human erythrocytes can be stimulated by different treatments such as addition of calmodulin or acidic phospholipids and controlled proteolysis. In this report we show that short chain alkyl alcohols also stimulated this enzyme. At 5% (v/v) ethanol, the maximal velocity of the enzyme was about 2.4-fold higher than in the control, and thus, was also higher than the maximal velocity obtained in the presence of calmodulin (about 2-fold). When ethanol and calmodulin were present simultaneously, the stimulatory effect was additive (3.4-fold stimulation). On the other hand, the stimulatory effect of ethanol was preserved after treatment of the enzyme with trypsin to stimulate the Ca(2+)-ATPase and render it independent of calmodulin, thus suggesting that the interaction of ethanol and calmodulin with the Ca(2+)-ATPase occurred through a different mechanism. Other short chain alkyl alcohols (methanol, n-propanol and n-butanol) stimulated the Ca(2+)-ATPase activity to the same extent than ethanol but with different efficacy. Thus, the larger the carbon number, the lower the concentration needed to get the same maximal stimulation. Ethanol also increased the affinity of the enzyme for ATP to a larger extent and additively, when compared to calmodulin. All the effects of ethanol mentioned above were identically observed on the membrane-bound enzyme (i.e., erythrocyte ghosts) ruling out any effect of the alcohols attributable to the solubilized purified enzyme. Furthermore, Ca2+ transport by inside-out vesicles was also stimulated by ethanol, showing both the same concentration-dependence as the Ca(2+)-ATPase activity and the additive effect observed when calmodulin was also present. The stimulatory effect of ethanol was significant at pharmacological concentrations, thus suggesting potential implications of toxicological relevance.

Regulation of the erythrocyte Ca(2+)-ATPase at high pH

The activation of the Ca(2+)-ATPase from erythrocyte membranes at high pH has been investigated. Following alkalinization and in the absence of regulators, the enzyme exhibits a very high affinity for Ca2+ and a decreased maximal velocity. Either addition of calmodulin, addition of acidic phospholipids, or controlled trypsinization decreases the concentration of effector required to elicit half-maximal activation of the enzyme for calcium to similar values. The increase in affinity for Ca2+, however, is smaller than that observed at neutral pH. The maximal velocity at high pH becomes insensitive to both calmodulin and controlled proteolysis, although calmodulin binds to the protein with similar affinities at pH 7.0 and 8.0, as indicated by similarity in binding to a calmodulin-Sepharose resin and in dependence on calmodulin concentrations when the pH is increased. In contrast to the attenuated effects of calmodulin and proteolysis, at pH 8.0 the enzyme is susceptible to stimulation by phospholipids, indicating that the pathway for transduction of the signal from phospholipids is distinct from that pathway engaged by calmodulin and/or trypsinization. At pH 8.0, phosphatidylinositol induces the modulatory effect of ATP at the regulatory site but calmodulin does not. We suggest that the intraenzymic connection between the calmodulin-binding, autoinhibitory peptide and the nucleotide domain of the enzyme is impaired upon alkalinization, which would account for the differing abilities of the activators to modulate the ATP effects.

The plasma membrane calcium pump: functional domains, regulation of the activity, and tissue specificity of isoform expression

The plasma membrane Ca2+ pump is responsible for the fine regulation of the intracellular Ca2+ level and is thus involved in the control of several cellular processes. The activity of the pump is regulated by a multiplicity of mechanisms, among which are calmodulin, acidic phospholipids, kinase-mediated phosphorylation, or an oligomerization process. The C-terminal part of the molecule interacts with the region of the pump close to the active site, leading to the decrease of the activity in the resting state. Four genes coding for different isoforms of the plasma membrane Ca2+ ATPase are known in humans. Isoform 1 and 4 represent housekeeping isoforms, whereas isoforms 2 and 3 are only present in specialized tissues. The variability of the protein is further increased by alternative RNA splicing at two sites (A, C). Alternative splicing occurs within (splice site C) or near (splice site A) regions coding for regulatory domains of the protein. In all isoforms a corresponding splice form exists at both splice sites. These common splice forms are present in all tissues, whereas isoform unique splice forms are normally only present in specialized tissues. In neuronal tissues all isoforms and almost the complete set of splice forms are found. The transcripts of the different isoforms are distributed in a region-specific manner in neuronal tissues.

The calmodulin-binding site of the plasma membrane Ca2+ pump interacts with the transduction domain of the enzyme

Calpain proteolysis of the plasma membrane Ca2+ pump removes a C-terminal 14-kDa portion which includes the calmodulin-binding domain. This produces a fully activated 124-kDa fragment, which can be inhibited by synthetic versions of the calmodulin-binding domain. The inhibition is strongest when Trp-8 in the latter domain is replaced by a Tyr residue (Falchetto, R., Vorherr, T., Brunner, J., & Carafoli, E., 1991, J. Biol. Chem. 266, 2930-2936). In the present study, the N-terminus of the 28-residue synthetic calmodulin-binding domain was acetylated with 3H-acetic anhydride, and Phe in position 25 was replaced by a phenylalanine derivatized with a diazirine-based, photoactivatable carbene precursor. This peptide (C28WC*) inhibited the fully active 124-kDa fragment of the pump and became cross-linked to it upon photolysis. After proteolysis of the fragment with Asp-N or Staphylococcus aureus V8 (Glu-C) protease, labeled peptides were isolated by reversed-phase high-performance liquid chromatography and subjected to Edman sequence analysis. The peptides originated from a region of the pump located within the unit protruding into the cytoplasm between transmembrane domain two and three. This unit has been proposed to be the site of the energy transduction domain, which would couple the ATP hydrolysis to Ca2+ translocation.

Altered plasma membrane H(+)-ATPase from the Dio-9-resistant pma1-2 mutant of Schizosaccharomyces pombe

The pma1-2 mutation affecting the plasma membrane H(+)-ATPase of Schizosaccharomyces pombe has been selected for resistance to the antibiotic Dio-9. In membrane fractions purified from glucose-starved cells, the mutant ATPase activity is reduced by 96%, is insensitive to inhibition by vanadate and has a pH profile displaced in the acidic pH range when compared to the wild type. The maximum velocity of the H(+)-ATPase activity of plasma membranes from glucose-activated pma1-2 cells is activated 20-fold. This is in striking contrast with the wild-type ATPase activity, the maximal velocity of which is not affected by glucose. However, similar to the wild-type enzyme, glucose activation of the pma1-2 mutant H(+)-ATPase reduces the Km for MgATP 9-2 mM and shifts the optimal pH from 4.8 to 6.0-6.5. The pma1-2 mutation modifies Lys250 to a threonine, which is highly conserved in fungal and plant H(+)-ATPases. These results, compared to those reported for mutations of neighbour residues in yeast or mammalian P-type ATPases, suggest that Lys250 could play a significant role, not only in phosphate binding and/or in the E1P-E2P conformational isomerisation, but also in glucose activation of the H(+)-ATPase.

Distinct mechanisms of inhibition of purified cardiac sarcolemma Ca(2+)-ATPase by two calmodulin antagonists

The effects of calmidazolium and compound 48/80 were studied in four different states of activation of the purified Ca(2+)-ATPase from cardiac sarcolemma: "basal" or unactivated, activated by calmodulin, activated by phosphatidylserine, and activated by controlled trypsinization. When assayed in the presence of phosphatidylcholine as the sole phospholipid (basal state), the purified enzyme was resistant to inhibition by calmidazolium (0.1 to 3 microM). In the same range, calmidazolium inhibited the enzyme activated by controlled proteolysis as well as the calmodulin-activated enzyme regardless of the calmodulin concentration. The phosphatidylserine-activated enzyme was inhibited at higher calmidazolium concentrations due to non-specific trapping of the inhibitor by the excess of phospholipid. Addition of calmidazolium did not modify the K0.5 for calcium activation of ATP hydrolysis by the enzyme. The inhibition by calmidazolium was counteracted by Pi. Compound 48/80 also had no effect on the enzyme when only phosphatidylcholine was present and, like calmidazolium, it inhibited the calmodulin-activated enzyme and the phosphatidylserine-activated enzyme. The apparent Ki for inhibition by compound 48/80 was dependent on the calmodulin concentration. However, the enzyme activated by controlled trypsinization was insensitive to compound 48/80. Binding of 48/80 to the enzyme in the presence of phosphatidylserine or calmodulin reversed the increased affinity for Ca2+ caused by these activators.

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

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

In vitro enzyme activation with calbindin-D28k, the vitamin D-dependent 28 kDa calcium binding protein

Purified porcine erythrocyte membrane Ca(2+)-ATPase and 3':5'-cyclic nucleotide phosphodiesterase were stimulated in a dose-dependent, saturable manner with the vitamin D-dependent calcium binding protein from rat kidney, calbindin-D28k (CaBP-D28k). The concentration of CaBP-D28k required for half-maximal activation (K0.5 act.) of the Ca(2+)-ATPase was 28 nM compared to 2.2 nM for calmodulin (CaM), with maximal activation equivalent upon addition of either excess CaM or CaBP-D28k. 3':5'-Cyclic nucleotide phosphodiesterase (PDE) also showed equivalent maximum saturable activation by calbindin (K0.5 act. = 90 nM) or calmodulin (K0.5 act. = 1.2 nM). CaBP-D28k was shown to effectively compete with CaM-Sepharose for PDE binding. Immunoprecipitation with CaBP-D28k antiserum completely inhibited calbindin-mediated activation of PDE but had no effect on calmodulin's ability to activate PDE. While the physiological significance of these results remains to be established, they do suggest that CaBP-D28k can activate enzymes and may be a regulator of yet to be identified target enzymes in certain tissues.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Cellular and segmental distribution of Ca2(+)-pump epitopes in rat intestine

We used a monoclonal antibody (5F10) specific for the human erythrocyte plasma membrane Ca(++)-pump to demonstrate the presence and distribution of Ca(++)-pump epitopes in rat intestine. In paraffin embedded tissue sections, antibody 5F10 binds to epitopes in the basolateral membranes of absorptive cells in rat duodenum and portions of jejunum but not ileum. Western blot analysis of intestinal mucosal proteins with antibody 5F10 shows binding of antibody to major bands of Mr approximately 135,000 and Mr approximately 72,000, and to lesser bands of Mr approximately 125,000 and Mr approximately 27,000. This pattern was seen in mucosal homogenates of rat duodenal and jejunal cells and to a lesser extent in ileal cells. The Mr approximately 135,000 band corresponds to the molecular weight of Ca(++)-pumps in other tissues. The other bands correspond in size to known proteolytic fragments of the Ca(++)-pump. Slot-blot analysis of nitrocellulose immobilized mucosal homogenates shows binding of 5F10 to be greatest in duodenum and least in ileum. Ca(++)-transport studies by the everted gut sac technique show a correlation between vitamin D induction of active Ca(++)-transport and the segmental distribution of Ca(++)-pump epitopes.

Similarities between the effects of dimethyl sulfoxide and calmodulin on the red blood cell Ca2(+)-ATPase

The Ca2(+)-ATPase of the erythrocyte plasma membrane can be activated by calmodulin, acidic phospholipids, limited proteolysis and self-association. Recently, it has been shown that different organic solvents increase both the Vmax and the Ca2+ affinity of the enzyme (Benaim, G. and De Meis, L. (1989) FEBS Lett. 244, 484-486). In this report the effects of calmodulin and dimethyl sulfoxide (20%, v/v) on the Ca2(+)-ATPase are compared. Dimethyl sulfoxide also elicits the appearance of the low-affinity binding site, which in this enzyme is strictly dependent on calmodulin. Dimethyl sulfoxide increases the Ca2+ affinity of the enzyme in a manner similar to that observed with the use of calmodulin and of acidic phospholipids. This was tested using both native and partially trypsinized ATPase. When activated by calmodulin the enzyme is inhibited by compound 48/80, trifluoperazine and calmidazolium. When activated by dimethyl sulfoxide the enzyme is still inhibited by calmidazolium but is no longer inhibited by either compound 48/80 or trifluoperazine. Activation of the ATPase promoted by either calmodulin or dimethyl sulfoxide is abolished when the Ca2+ concentration is raised from 10 microM to 2 mM. The effect of dimethyl sulfoxide is also abolished by 20 mM Pi. In the presence of 1 to 10 mM Ca2+ the ATPase catalyzes an ATP in equilibrium Pi exchange. The rate of exchange increases several fold when dimethyl sulfoxide is included in the assay medium.

A study to see whether phosphatidylserine, partial proteolysis and EGTA substitute for calmodulin during activation of the Ca2+-ATPase from red cell membranes by ATP

(1) The effects of treatments that mimic calmodulin in increasing the apparent affinity for Ca2+ were tested to see whether, like calmodulin, they also change the activation of the Ca2+-ATPase from human red cell membranes by ATP at the low-affinity site. (2) Short incubations with either trypsin or acidic phospholipids such as phosphatidylserine increased the apparent affinity for ATP at the low-affinity site. (3) Under conditions in which it increased the apparent affinity of the Ca2+-ATPase for Ca2+, EGTA failed to change the activation by ATP. (4) As in calmodulin-bound Ca2+-ATPase, compound 48/80 inhibited the activity of the enzyme in the presence of phosphatidylserine by lowering the apparent affinity for ATP at the low-affinity site, leaving the maximum velocity of the enzyme unaltered. (5) Compound 48/80 also inhibited the Ca2+-ATPase after partial proteolysis, but in this case it lowered the maximum activity, leaving the apparent affinity of the enzyme for ATP at the low-affinity site unaltered. (6) Inhibition of the Ca2+-ATPase by compound 48/80 in the absence of calmodulin suggests that the inhibitor can act directly on the enzyme.