Effects of phospholipids on binding of calcium to (Ca2(+)-Mg2(+)-ATPase

Sensing membrane thickness: Lessons learned from cold stress

The lipid bilayer component of biological membranes is important for the distribution, organization, and function of bilayer spanning proteins. These physical barriers are subjected to bilayer perturbations. As a consequence, nature has evolved proteins that are able to sense changes in the bilayer properties and transform these lipid-mediated stimuli into intracellular signals. A structural feature that most signal-transducing membrane-embedded proteins have in common is one or more α-helices that traverse the lipid bilayer. Because of the interaction with the surrounding lipids, the organization of these transmembrane helices will be sensitive to membrane properties, like hydrophobic thickness. The helices may adapt to the lipids in different ways, which in turn can influence the structure and function of the intact membrane proteins. We review recent insights into the molecular basis of thermosensing via changes in membrane thickness and consider examples in which the hydrophobic matching can be demonstrated using reconstituted membrane systems. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

A diversity of SERCA Ca2+ pump inhibitors

The SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) is probably the most extensively studied membrane protein transporter. There is a vast array of diverse inhibitors for the Ca2+ pump, and many have proved significant in helping to elucidate both the mechanism of transport and gaining conformational structures. Some SERCA inhibitors such as thapsigargin have been used extensively as pharmacological tools to probe the roles of Ca2+ stores in Ca2+ signalling processes. Furthermore, some inhibitors have been implicated in the cause of diseases associated with endocrine disruption by environmental pollutants, whereas others are being developed as potential anticancer agents. The present review therefore aims to highlight some of the wide range of chemically diverse inhibitors that are known, their mechanisms of action and their binding location on the Ca2+ ATPase. Additionally, some ideas for the future development of more useful isoform-specific inhibitors and anticancer drugs are presented.

Lipid–protein interactions

Intrinsic membrane proteins are solvated by a shell of lipid molecules interacting with the membrane-penetrating surface of the protein; these lipid molecules are referred to as annular lipids. Lipid molecules are also found bound between transmembrane α-helices; these are referred to as non-annular lipids. Annular lipid binding constants depend on fatty acyl chain length, but the dependence is less than expected from models based on distortion of the lipid bilayer alone. This suggests that hydrophobic matching between a membrane protein and the surrounding lipid bilayer involves some distortion of the transmembrane α-helical bundle found in most membrane proteins, explaining the importance of bilayer thickness for membrane protein function. Annular lipid binding constants also depend on the structure of the polar headgroup region of the lipid, and hotspots for binding anionic lipids have been detected on some membrane proteins; binding of anionic lipid molecules to these hotspots can be functionally important. Binding of anionic lipids to non-annular sites on membrane proteins such as the potassium channel KcsA can also be important for function. It is argued that the packing preferences of the membrane-spanning α-helices in a membrane protein result in a structure that matches nicely with that of the surrounding lipid bilayer, so that lipid and protein can meet without either having to change very much.

Local Anesthetics Inhibit Ca-ATPase in Masticatory Muscles

Local anesthetics have myotoxic effects and inhibit Ca-ATPase activity and Ca transport in skeletal muscles. Such effects have not been fully elucidated in masticatory muscles. We tested the hypothesis that local anesthetics increase myoplasmic calcium in masticatory muscles by inhibiting Ca-ATPase at a concentration similar to that of dental cartridges. The effects of lidocaine and bupivacaine on Ca-ATPase from rabbit masseter and medial pterygoid muscles were tested with radioisotopic and colorimetric methods. Bupivacaine had an action similar to that of lidocaine on Ca-ATPase activity, but less effect on calcium transport. The pre-exposure of the membranes to the anesthetics enhanced the Ca-ATPase activity in the absence of calcium ionophore, supporting their permeabilizing effect. The results demonstrate that amide-type anesthetics do not inhibit calcium binding, but do reduce calcium transport and enzyme phosphorylation by ATP, and suggest that the myoplasmic calcium increase induced by lidocaine and bupivacaine might promote masticatory muscle contraction and eventual rigidity.

Energetics of hydrophobic matching in lipid-protein interactions

Lipid chain length modulates the activity of transmembrane proteins by mismatch between the hydrophobic span of the protein and that of the lipid membrane. Relative binding affinities of lipids with different chain lengths are used to estimate the excess free energy of lipid-protein interaction that arises from hydrophobic mismatch. For a wide range of integral proteins and peptides, the energy cost is much less than the elastic penalty of fully stretching or compressing the lipid chains to achieve complete hydrophobic matching. The chain length dependences of the free energies of lipid association are described by a model that combines elastic chain extension with a free energy term that depends linearly on the extent of residual mismatch. The excess free energy densities involved lie in the region of 0.5-2.0 k(B)T x nm(-2). Values of this size could arise from exposure of hydrophobic groups to polar portions of the lipid or protein, but not directly to water, or alternatively from changes in tilt of the transmembrane helices that are energetically comparable to those activating mechanosensitive channels. The influence of hydrophobic mismatch on dimerization of transmembrane helices and their transfer between lipid vesicles, and on shifts in chain-melting transitions of lipid bilayers by incorporated proteins, is analyzed by using the same thermodynamic model. Segmental order parameters confirm that elastic lipid chain distortions are insufficient to compensate fully for the mismatch, but the dependence on chain length with tryptophan-anchored peptides requires that the free energy density of hydrophobic mismatch should increase with increasing extent of mismatch.

The widely utilized brominated flame retardant tetrabromobisphenol A (TBBPA) is a potent inhibitor of the SERCA Ca2+ pump

TBBPA (tetrabromobisphenol A) is currently the most widely used type of BFR (brominated flame retardant) employed to reduce the combustibility of a large variety of electronic and other manufactured products. Recent studies have indicated that BFRs, including TBBPA, are bio-accumulating within animal and humans. BFRs including TBBPA have also been shown to be cytotoxic and potentially endocrine-disrupting to a variety of cells in culture. Furthermore, TBBPA has specifically been shown to cause disruption of Ca2+ homoeostasis within cells, which may be the underlying cause of its cytotoxicity. In this study, we have demonstrated that TBBPA is a potent non-isoform-specific inhibitor of the SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) (apparent K(i) 0.46-2.3 microM), thus we propose that TBBPA inhibition of SERCA contributes in some degree to Ca2+ signalling disruption. TBBPA binds directly to the SERCA without the need to partition into the phospholipid bilayer. From activity results and Ca2+-induced conformational results, it appears that the major effect of TBBPA is to decrease the SERCA affinity for Ca2+ (increasing the K(d) from approx. 1 microM to 30 microM in the presence of 10 microM TBBPA). Low concentrations of TBBPA can quench the tryptophan fluorescence of the SERCA and this quenching can be reversed by BHQ [2,5-di-(t-butyl)-1,4-hydroquinone] and 4-n-nonylphenol, but not thapsigargin, indicating that TBBPA and BHQ may be binding to similar regions in the SERCA.

Bilayer thickness and membrane protein function: an energetic perspective

The lipid bilayer component of biological membranes is important for the distribution, organization, and function of bilayer-spanning proteins. This regulation is due to both specific lipid-protein interactions and general bilayer-protein interactions, which modulate the energetics and kinetics of protein conformational transitions, as well as the protein distribution between different membrane compartments. The bilayer regulation of membrane protein function arises from the hydrophobic coupling between the protein's hydrophobic domains and the bilayer hydrophobic core, which causes protein conformational changes that involve the protein/bilayer boundary to perturb the adjacent bilayer. Such bilayer perturbations, or deformations, incur an energetic cost, which for a given conformational change varies as a function of the bilayer material properties (bilayer thickness, intrinsic lipid curvature, and the elastic compression and bending moduli). Protein function therefore is regulated by changes in bilayer material properties, which determine the free-energy changes caused by the protein-induced bilayer deformation. The lipid bilayer thus becomes an allosteric regulator of membrane function.

Inhibition of the SERCA Ca2+ pumps by curcumin. Curcumin putatively stabilizes the interaction between the nucleotide-binding and phosphorylation domains in the absence of ATP

Curcumin is a compound derived from the spice, tumeric. It is a potent inhibitor of the SERCA Ca2+ pumps (all isoforms), inhibiting Ca2+-dependent ATPase activity with IC50 values of between 7 and 15 microm. It also inhibits ATP-dependent Ca2+-uptake in a variety of microsomal membranes, although for cerebellar and platelet microsomes, a stimulation in Ca2+ uptake is observed at low curcumin concentrations (<10 microm). For the skeletal muscle isoform of the Ca2+ pump (SERCA1), the inhibition of curcumin is noncompetitive with respect to Ca2+, and competitive with respect to ATP at high curcumin concentrations ( approximately 10-25 microm). This was confirmed by ATP binding studies that showed inhibition in the presence of curcumin: ATP-dependent phosphorylation was also reduced. Experiments with fluorescein 5'-isothiocyanate (FITC)-labelled ATPase also suggest that curcumin stabilizes the E1 conformational state. The fact that FITC labels the nucleotide binding site of the ATPase (precluding ATP from binding), and the fact that curcumin affects FITC fluorescence indicate that curcumin must be binding to another site within the ATPase that induces a conformational change to prevent ATP from binding. This observation is interpreted, with the aid of recent structural information, as curcumin stabilizing the interaction between the nucleotide-binding and phosphorylation domains, precluding ATP binding.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

The effects of phenothiazines and other calmodulin antagonists on the sarcoplasmic and endoplasmic reticulum Ca(2+) pumps

The effects of a number of phenothiazines and other calmodulin antagonists on the Ca(2+)-ATPase activity of sarcoplasmic reticulum (SR) and endoplasmic reticulum (ER) were investigated. The drugs used in this study were trifluoperazine, calmidazolium, fluphenazine, chlorpromazine, W-7, and calmodulin-binding peptide. Our results showed that calmidazolium and calmodulin-binding peptide were the most potent inhibitors of skeletal muscle SR Ca(2+)-ATPase activity (isoform SERCA 1) (IC(50) values of 0.5 and 7 microM, respectively), while W-7 was the least potent inhibitor (IC(50), 125 microM). All of the antagonists had little effect on the cerebellar ER Ca(2+)-ATPase activity (isoform SERCA 2b), except for trifluoperazine, which had a biphasic effect, causing stimulation at low concentrations and inhibition at higher concentrations. Our results suggest that the effects of these calmodulin antagonists are independent of calmodulin and that they inhibit the Ca(2+)-ATPase in an isoform-specific manner. It was found that these antagonists inhibit the skeletal muscle isoform of the Ca(2+) pump by altering the Ca(2+) affinity and the associated Ca(2+)-binding steps, as well as possibly stabilising the E1 conformational state of the enzyme.

The mechanism of inhibition of the Ca2+-ATPase by mastoparan. Mastoparan abolishes cooperative ca2+ binding

The amphiphilic peptide mastoparan, isolated from wasp venom, is a potent inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. At pH 7. 2, ATPase activity is inhibited with an inhibitory constant (Ki) of 1 +/- 0.13 microM. Mastoparan shifts the E2-E1 equilibrium toward E1 and may affect the regulatory ATP binding site. The peptide also decreases the affinity of the ATPase for Ca2+ and abolishes the cooperativity of Ca2+ binding. In the presence of mastoparan, the two Ca2+ ions bind independently of one another. Our results appear to support the model that describes the relationship between the two Ca2+ binding sites as "side-by-side," because this model allows the possibility of independent Ca2+ entry to the two sites. Mastoparan shifts the steady-state equilibrium between E1'Ca2 and E1'Ca2.P toward E1'Ca2.P, by possibly affecting the conformational change that follows ATP binding. The peptide also causes a reduction in the levels of phosphoenzyme formed from [32P]Pi. Some analogues of mastoparan were also tested and were found to cause inhibition of the Ca2+-ATPase in the range of 2-4 microM. The inhibitory action of mastoparan and its analogues appears dependent on their ability to form alpha-helices in membranes.

Evidence for phospholipid microdomain formation in liquid crystalline liposomes reconstituted with Escherichia coli lactose permease

The well-characterized integral membrane protein lactose (lac) permease from Escherichia coli was reconstituted together with trace amounts (molar fraction X = 0.005 of the total phospholipid) of different pyrene-labeled phospholipid analogs into 1-palmitoyl-2-oleoyl-sn-glycero-3-sn-glycero-3-phospho-rac'-glycerol (POPG) liposomes. Effects of lac permease on bilayer lipid dynamics were investigated by measuring the excimer-to-monomer fluorescence intensity ratio IE/IM. Compared to control vesicles, the presence of lac permease (at a protein:phospholipid stoichiometry P/L of 1:4.000) increased the rate of excimer formation by 1-palmitoyl-2[6-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (PPDPC) by approximately fivefold. Decreasing P/L from approximately 1:4.000 to 1:7.600 decreased the IE/IM for PPDPC from 0.16 to 0.05, respectively. An increase in bilayer fluidity due to permease is unlikely, thus implying that the augmented IE/IM should arise from partial lateral segregation of PPDPC in the vesicles. This notion is supported by the further 38% increase in IE/IM observed for the pyrene-labeled Cys-148 lac permease reconstituted into POPG vesicles at P/L 1:4000. The importance of the length of the lipid-protein boundary is implicated by the reduction in IE/IM resulting from the aggregation of the lac permease in vesicles by a monoclonal antibody. Interestingly, excimer formation by 1-palmitoyl-2[6-(pyren-1-yl)hexanoyl-sn-glycero-3-phosphocholine (PPHPC) was enhanced only fourfold in the presence of lac permease. Results obtained with the corresponding pyrenyl phosphatidylglycerols and -methanols were qualitatively similar to those above, thus indicating that lipid headgroup-protein interactions are not involved. Inclusion of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamino-N-(5-fluoresce inthio- carbamoyl) (DPPF, X = 0.005) into reconstituted lactose permease vesicles containing PPDPC caused a nearly 90% decrease in excimer fluorescence, whereas in control vesicles lacking the reconstituted protein only 40% quenching was evident. The addition of 1,2-dipalmitoyl-sn-glycero-3-phospho-rac'-glycerol (DPPG) decreased IE/IM for PPDPC, revealing the driving force for the lateral segregation of this probe to become attenuated. More specifically for protein-free bilayers at XDPPG = 0.10 the rate of lateral diffusion of PPDPC in POPG is diminished, as evidenced by the 24% decrement in IE/IM, under these conditions the increase in IE/IM due to lac permease was strongly reduced, by approximately 84%. The present data are interpreted in terms of the hydrophobic mismatch theory, which predicts that integral membrane proteins will draw lipids of similar hydrophobic thickness into their vicinity. In brief, the approximate lengths of most of the predicted 12 hydrophobic, membrane-spanning alpha-helical segments of lactose permease range between 28.5 and 37.5 A and thus exceed the hydrophobic thickness of POPG of approximately 25.8 A. Therefore, to reduce the free energy of the assembly, longer lipids such as PPDPC and DPPF are accumulated in the immediate vicinity of lactose permease in fluid, liquid crystalline POPG bilayers.

Effects of phosphatidylethanolamines on the activity of the Ca(2+)-ATPase of sarcoplasmic reticulum

ATPase activities for the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum reconstituted into dioleoylphosphatidylethanolamine [di(C18:1)PE] are, at temperatures higher than 20 degrees C, lower than in dioleoylphosphatidylcholine [di(C18:1)PC], whereas in egg yolk phosphatidylethanolamine the activities are the same as in di(C18:1)PC up to 25 degrees C, suggesting that low ATPase activities occur when the phosphatidylethanol-amine species is in the hexagonal H11 phase. ATPase activities measured in mixtures of di(C18:1)PC and di(C18:1)PE do not change with changing di(C18:1)PE content up to 80%. It is concluded that curvature frustration in bilayers containing di(C18:1)PE has no effect on ATPase activity. The rates of phosphorylation and of Ca2+ transport are identical for the native ATPase and for the ATPase in di(C18:1)PE. Dephosphorylation of the phosphorylated ATPase in di(C18:1)PE at 25 degrees C is, however, slower than for the native ATPase, explaining the lower steady-state rate of ATP hydrolysis; in egg yolk phosphatidylethanolamine at 25 degrees C the rate of dephosphorylation is equal to that for the unreconstituted ATPase. Phosphorylation of the ATPase by P1 in the absence of Ca2+ is unaffected by reconstitution in di(C18:1)RE. The stoichiometry of Ca2+ binding to the ATPase is also unaltered. Studies of the effect of di(C18:1)PE on the fluorescence intensity of the ATPase labelled with 7-chloro-4-nitro-2,1,3-benzoxadiazole are consistent with an increase in the E1/E2 equilibrium constant, where E1 is the conformation of the ATPase with two high-affinity binding sites for Ca2+ exposed to the cytoplasm, and E2 is a conformation unable to bind cytoplasmic Ca2+. A slight increase in affinity for Ca2+ can be attributed to the observed increase in the E1/E2 equilibrium constant.

Separate effects of long-chain phosphatidylcholines on dephosphorylation of the Ca(2+)-ATPase and on Ca2+ binding

The steady-state activity of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR) is low when reconstituted into bilayers of the long-chain phosphatidylcholines dierucyl phosphatidylcholine [di(C22:1)PC] or dinervonyl phosphatidylcholine [di(C24:1)PC]. In di(C24:1)PC the ATPase binds a single Ca2+ ion, whereas in di(C22:1)PC it binds two, as in the native SR [Starling, East and Lee (1993) Biochemistry 32, 1593-1600]. In di(C22:1)PC, rates of phosphorylation of the ATPase by ATP and the rate of ATP-induced Ca2+ dissociation are slightly lower than in the native ATPase. However, a much more marked decrease is observed in di(C22:1)PC in the rate of dephosphorylation of the phosphorylated ATPase, which explains the low steady-state ATPase activity. The level of phosphorylation of the ATPase by Pi was little affected by reconstitution in di(C22:1)PC, suggesting that the rate of phosphorylation by Pi is also decreased. The very similar effects of di(C22:1)PC and di(C24:1)PC (Starling, East and Lee (1995) Biochem. J. 310, 875-879) on phosphorylation and dephosphorylation suggest that changes in these steps and the change in Ca2+ binding stoichiometry observed in di(C24:1)PC represent independent changes on the ATPase.

Labelling the Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum with the cross-linker o-phthalaldehyde

The Ca(2+)-ATPase in the sarcoplasmic reticulum of skeletal muscle reacts with o-phthalaldehyde (OPA) to form a fluorescent isoindole product. The stoichiometry of labelling of the ATPase is 9 nmol of isoindole/mg of ATPase, corresponding to a 1:1 molar ratio of isoindole: ATPase. There is no evidence for any intermolecular cross-linking. Isoindole formation is faster in the presence of methylamine, but the stoichiometry of labelling is unchanged, whereas in the presence of 2-mercaptoethanol the level of labelling is much higher. It is concluded that OPA reacts with a single Cys residue (defining the specificity of the reaction) in a fast step, subsequent reaction with a Lys residue to form the isoindole being rate-controlling. Labelling the ATPase with OPA in the absence of methylamine leads to total loss of ATPase activity, whereas in the presence of methylamine, the decrease in ATPase activity on reaction is small. We conclude that the loss of ATPase activity probably follows from formation of the intramolecular cross-link rather than from the initial modification of the Cys residue. Reaction with OPA is not affected by the presence of ATP, ADP or Ca2+, so that the reactive Cys is not part of a ligand-binding site. The fluorescence emission spectrum of the labelled ATPase indicates a hydrophobic environment for the isoindole ring.

Effect of ganglioside GM3 on the activity and conformation of reconstituted Ca2+-ATPase

Trace amounts of gangliosides were found in rabbit skeletal muscle sarcoplasmic reticulum and their main part was shown, by high performance thin layer chromatography. to be GM3. Addition of GM3 to the soybean phospholipids used for reconstitution of proteoliposomes markedly increased ATP hydrolysis as well as Ca2+ uptake activity of sarcoplasmic reticulum Ca2+-ATPase incorporated into the proteoliposomes. Conformation changes of Ca2+-ATPase induced by GM3 were also observed by intrinsic fluorescence and circular dichroism measurements.

Contribution of plasma membrane and endoplasmic reticulum Ca(2+)-ATPases to the synaptosomal [Ca2+]i increase during oxidative stress

In the present study we analyzed the effect of ascorbate (0.8 mM)/Fe2+ (2.5 microM)-induced membrane lipid peroxidation on the levels of intracellular free calcium,[Ca2+]i and on the possible mechanisms involved in the perturbation of intracellular calcium homeostasis during oxidative stress. For this purpose, the influence of the ascorbate/iron oxidant system on the plasma membrane and endoplasmic reticulum Ca(2+)-dependent ATPases of brain cortical synaptosomes was studied. In addition, the influence of the peroxidative process on the uptake of calcium (45Ca2+) and on the Na+/Ca2+ exchange activity at the plasma membrane was evaluated. After ascorbate/Fe(2+)-induced membrane lipid peroxidation of the order of 18.05 +/- 4.20 nmol TBARS/mg protein, an increase in [Ca2+]i occurred, under basal or depolarizing conditions (30 mM KCl), which was dependent on the extracellular calcium concentration. Thus, for 1 and 3 mM extracellular calcium concentration, an increase of the resting [Ca2+]i values of 19.8% and 33.7% was observed, while after the K(+)-depolarization the enhancement of the [Ca2+]i was 18.4% and 29.5%, respectively. The Na+/Ca2+ exchange activity and the time-dependent influx of 45Ca2+ observed in basal conditions and after the 30 mM K(+)-depolarization, were not affected under the peroxidative conditions. The Ca(2+)-dependent ATPase activity of the synaptosomal plasma membrane was significantly depressed following peroxidation of membrane lipids, decreasing the V(max) by 48.1%, without significant changes in the affinity of the enzyme for calcium (K(m) for Ca2+ was 0.54 +/- 0.04 microM in control conditions and 0.56 +/- 0.034 microM in peroxidized conditions). The Ca(2+)-ATPase activity of the endoplasmic reticulum was also affected during ascorbate/iron-induced oxidative stress; thus, an inhibition of 45.2% was observed 5 min after adding ATP. These data suggest that the increase in synaptosomal [Ca2+]i due to oxidative stress may result from the inhibition of the plasma membrane and the endoplasmic reticulum membrane Ca(2+)-ATPase activities, probably as a result of the alteration of the lipid environment required for the maximal activity of these membrane enzymes. The consequent increase in [Ca2+]i may be responsible for the injury of the nervous tissue observed during several pathological conditions in which free radical generation seems to be involved.

Lipid structure and Ca(2+)-ATPase function

Effects of lipid structure on the function of the Ca(2+)-ATPase of skeletal muscle of sarcoplasmic reticulum are reviewed. Binding of phospholipids to the ATPase shows little specificity. Phosphatidylcholines with short (C14) or long (C24) fatty acyl chains have marked effects on the activity of the ATPase, including a change in the stoichiometry of Ca binding. Low ATPase activity in gel phase lipid follows from low rate of phosphorylation. Phosphatidylinositol 4-phosphate increases ATPase activity by increasing the rate of dephosphorylation of the phosphorylated ATPase. Stimulation is not seen with other anionic phospholipids; phosphatidic acid decreases ATPase activity in a Mg(2-)-dependent manner.

Effects of phospholipid fatty acyl chain length on phosphorylation and dephosphorylation of the Ca(2+)-ATPase

The kinetics of the Ca(2+)-ATPase purified from sarcoplasmic reticulum have been studied after reconstitution into bilayers of dimyristoleoylphosphatidylcholine [di(C14:1)PC], dioleoylphosphatidylcholine[di(C18:1)PC] and dinervonylphosphatidylcholine [di(C24:1)PC]. In di(C24:1)PC the rate of phosphorylation of the ATPase by ATP was comparable with that in di(C18:1)PC (about 70 s-1), but in di(C14:1)PC the rate was much lower (21 s-1). Fluorescence responses of the ATPase suggest changes in the phosphoryl-transfer step rather than in the preceding conformational change E1Ca2ATP<-->E1'Ca2ATP. The rate of dephosphorylation of the phosphorylated ATPase was found to decrease in the order di(C24:1)PC < di(C14:1)PC < di(C18:1)PC. For the ATPase in di(C24:1)PC the rate of dephosphorylation (3.3 s-1) was slow enough to be the rate-limiting step for ATP hydrolysis; in di(C14:1)PC, it is suggested that both phosphorylation and dephosphorylation contribute to rate limitation. Phosphorylation of the ATPase in di(C24:1)PC by Pi was normal, but no phosphoenzyme could be detected in di(C14:1)PC. The rate of the Ca(2+)-transport step was normal in di(C24:1)PC, suggesting that the single Ca2+ ion bound to the ATPase in di(C24:1)PC could be transported.

Phosphatidylinositol 4-phosphate increases the rate of dephosphorylation of the phosphorylated Ca(2+)-ATPase

Incubation of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum with ATP in the absence of Ca2+ leads to phosphorylation of phosphatidylinositol (PtdIns) to phosphatidylinositol 4-phosphate (PtdIns-4P) and to a doubling of ATPase activity. Similarly, reconstitution of the ATPase with mixtures of dioleoylphosphatidylcholine and PtdIns-4P also led to a doubling of activity; ATPase activity increased with increasing PtdIns-4P content, up to 10% beyond which no further increase was observed. Reconstitution with PtdIns had a much smaller effect on activity. Changes in the Ca2+ affinity of the ATPase following incubation with ATP or reconstitution with PtdIns-4P were small. The rates of phosphorylation of the ATPase by ATP and of the Ca2+ transport step were unaffected, but the rate of dephosphorylation of the phosphorylated ATPase increased by a factor of 2 either following incubation with ATP or following reconstitution with PtdIns-4P. Activation of the ATPase led to a decrease in the level of phosphorylation of the ATPase by Pi corresponding to a 10-fold decrease in the equilibrium constant E2PMg/E2PiMg.

Evidence that the effects of phospholipids on the activity of the Ca(2+)-ATPase do not involve aggregation

The Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum, solubilized in monomeric from in C12E8, has been reconstituted by dialysis into sealed vesicles of dioleoyl phosphatidylcholine [di(C18:1)PC], dimyristoleoyl phosphatidylcholine [di(C14:1)PC], dinervonyl phosphatidylcholine [di(C24:1)PC] or dipalmitoyl phosphatidylcholine [di(C16:0)PC] in the gel phase, at a phospholipid/ATPase molar ratio of 10,000: 1. Cross-linking experiments show that ATPase molecules are present in these reconstituted vesicles as isolated monomeric species. ATPase activities for the reconstituted vesicles are about half of those for the ATPase reconstituted with the same lipid in unsealed membrane fragments, attributed to a close to random orientation for the ATPase molecules in the reconstituted vesicles. ATPase activities for the ATPase in reconstituted vesicles of di(C14:1)PC or di(C24:1)PC are less than in vesicles of di(C18:1)PC, and no activity could be detected for the ATPase in di(C16:0)PC in the gel phase. It is concluded that effects of lipids on the activity of the ATPase are independent of any changes in the state of aggregation of the ATPase. Inhibition of ATPase activity by spermine and by the hydrophilic domain of phospholamban are observed both for the unreconstituted ATPase and for the ATPase in reconstituted vesicles, so that inhibition is independent of any aggregation caused by these polycationic species. Stimulation of ATPase activity by jasmone is also observed both for the unreconstituted ATPase and for the ATPase in reconstituted vesicles, so that stimulation of the ATPase also does not follow from any change in the state of aggregation of the ATPase.

Characterization of the single Ca(2+)-binding site on the Ca(2+)-ATPase reconstituted with short- or long-chain phosphatidylcholines

On reconstitution of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum into bilayers of dimyristoleoylphosphatidylcholine [di(C14:1)PC] or dinervonylphosphatidylcholine [di(C24:1)PC] the stoichiometry of Ca2+ binding changes from the usual two Ca2+ ions bound per ATPase molecule to one Ca2+ ion bound per ATPase molecule. For the ATPase in di(C24:1)PC, removal of Ca2+ from the Ca(2+)-bound ATPase results in a decrease in tryptophan fluorescence intensity, as observed for the ATPase in dioleoylphosphatidylcholine [di(C18:1)PC]. For the ATPase in di(C14:1)PC removal of Ca2+ results in no change in tryptophan fluorescence intensity. In the presence of Mg2+, removal of Ca2+ from the ATPase in di(C18:1)PC or di(C24:1)PC results in a decrease in tryptophan fluorescence intensity, but for the ATPase in di(C14:1)PC this results in an increase in intensity. Fluorescence of the ATPase labelled with 4-nitrobenzo-2-oxa-1,3-diazole (NBD) is the same for the ATPase in di(C18:1)PC or di(C24:1)PC, but is markedly greater in di(C14:1)PC, consistent with a 4-fold increase in the E1/E2 equilibrium constant. Addition of Mg2+ to NBD-labelled ATPase in di(C18:1) PC or di(C24:1)PC results in an increase in NBD fluorescence, attributed to stronger binding of Mg2+ to the E1 than to the E2 conformation; addition of Mg2+ had no effect on the fluorescence of the NBD-labelled ATPase in di(C14:1)PC. In the absence of Ca2+, Mg2+ increased the tryptophan fluorescence of the ATPase in di(C14:1)PC, di(C18:3)PC or di(C24:1)PC, with the same binding-constant for Mg2+ in all three lipids. Addition of Mg2+ to the ATPase labelled with 4-(bromomethyl)-6,7-dimethoxycoumarin resulted in a decrease in fluorescence in di(C18:1)PC or di(C24:1)PC but had no effect in di(C14:1)PC. These effects are interpreted in terms of binding of Ca2+ at a single outer Ca2+ binding-site on the ATPase in di(C14:1)PC and di(C24:1)PC, in a conformation in which the inner site is occluded [in di(C14:1)PC] or modified in its affinity for Ca2+ [in di(C24:1)PC]. Thapsigargin binds to the ATPase, reducing its affinity for Ca2+ both in di(C14:1)PC and di(C24:1)PC.

Cancellation of the cooperativity of Ca2+ binding to sarcoplasmic reticulum Ca(2+)-ATPase by the non-ionic detergent dodecylmaltoside

The perturbation of the kinetics of the sarcoplasmic reticulum (SR) membranous Ca(2+)-ATPase cycle by the non-ionic detergent dodecylmaltoside (DM) has been shown to exhibit specific features which were not observed with the related detergents octa(ethylene glycol) monododecylether and Triton X-100 [de Foresta, B., Henao, F. & Champeil, P. (1992) Eur. J. Biochem. 209, 1023-1034]. This previous study has been completed here by a detailed analysis of the perturbation by DM of the interaction of Ca2+ with membranous ATPase, both in its unphosphorylated and phosphorylated form. Equilibrium binding measurements, performed at pH 7.5 and 20 degrees C, showed that only one 45Ca2+ was bound with high affinity to the ATPase in the presence of maximally perturbing concentrations of DM, as compared to two 45Ca2+ in the absence of detergent. This binding was also assessed by a small decrease in the tryptophan fluorescence intensity. Binding of a second Ca2+ occurred only with a much lower affinity. In the presence of DM, the pCa dependence of the phosphorylation by [gamma-32P]ATP of the ATPase shifted towards 50-fold higher Ca2+ concentrations than in its absence. Furthermore, DM completely inhibited the cooperativity of this dependence. This shift strongly suggests that the phosphorylation of DM-perturbed ATPase requires the binding of this second, low-affinity Ca2+. In order to assess this, samples of ATPase were intramolecularly cross-linked with glutaraldehyde. This treatment stabilized the phosphorylated intermediated with occluded Ca2+ [Ross, D. C., Davidson, G.A. & McIntosh, D. B. (1991) J. Biol. Chem. 266, 4613-4621]. Both in the absence and presence of DM, the cross-linked enzyme occluded close to two Ca2+/phosphorylated molecule. Finally, the pCa dependences of the ATPase hydrolytic activity, measured with two different high-energy substrates, ATP or p-nitrophenylphosphate (PNpP), were also found to shift towards higher Ca2+ concentrations in the presence of DM, which was again consistent with a normal coupling ratio, i.e. two bound Ca2+/substrate hydrolyzed. As compared to other detergents, the maltoside head group of DM might favor a stronger interaction with membranous ATPase, resulting in its high perturbing effect on Ca2+ binding. The loss of cooperativity of Ca2+ binding evidenced here makes DM a useful tool in the analysis of the sequence of events occurring during Ca2+ binding.

Localization of the hinge region of the Ca(2+)-ATPase of sarcoplasmic reticulum using resonance energy transfer

The Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum can be labelled at Cys-670 and Cys-674 with 5-[[2-[(iodoacetyl) amino]ethyl]amino]naphthalene-1-sulphonic acid (IAEDANS). Resonance energy transfer has been used to measure the distance between Cys-670/Cys-674 and Glu-439 labelled with 5-(bromomethyl)fluorescein as 40 A. The height of Cys-670/Cys-674 above the phospholipid/water interface has been measured by resonance energy transfer between IAEDANS-labelled ATPase and fluorescein-labelled phosphatidylethanolamine as 54 A. This locates the hinge region of the ATPase close to the mouth of the pore observed in the cytoplasmic region of the ATPase in electron micrographs. No significant changes in these distances can be detected by resonance energy transfer on binding Ca2+ or vanadate. The height of the IAEDANS label above the phospholipid/water interface is the same for bilayers of dimyristoleoylphosphatidylcholine and dioleoylphosphatidylcholine. Conformation changes on the Ca(2+)-ATPase appear to be localised to small regions of the ATPase.

Binding of Ca2+ to the (Ca(2+)-Mg2+)-ATPase of sarcoplasmic reticulum: equilibrium studies

Equilibrium fluorescence methods have been used to establish a model for Ca2+ binding to the (Ca(2+)-Mg2+)-ATPase of skeletal muscle sarcoplasmic reticulum and to define the effects of H+ and Mg2+ on Ca2+ binding. The basic scheme proposed is: E2 <--> E1 <--> E1Ca <--> El'Ca <--> E1'Ca2. The E1 conformation of the ATPase initially has one high-affinity binding site for Ca2+ exposed to the cytoplasmic side of the sarcoplasmic reticulum, but in the E2 conformation this site is unable to bind Ca2+; Ca2+ does not bind to luminal sites on E2. The second, outer, Ca(2+)-binding site on the ATPase is formed after binding of Ca2+ to the first, inner, site on E1 and the E1Ca <--> E1'Ca conformation change. The pH- and Mg(2+)-dependence of the E2 <--> E1 equilibrium has been established after changes in the fluorescence of the ATPase labelled with 4-nitrobenzo-2-oxa-1,3-diazole. It is proposed that Mg2+ from the cytoplasmic side of the sarcoplasmic reticulum can bind to the first Ca(2+)-binding site on both E1 and E2. It is proposed that the change in tryptophan fluorescence intensity after binding of Ca2+ follows from the E1Ca <--> E1'Ca change. The pH- and Mg(2+)-dependence of this change defines H(+)- and Mg(2+)-binding constants at the two Ca(2+)-binding sites. It is proposed that the change in tryptophan fluorescence observed on binding Mg2+ follows from binding at the second Ca(2+)-binding site. Effects of pH and Mg2+ on the fluorescence of the ATPase labelled with 4-(bromomethyl)-6,7-dimethoxycoumarin are proposed to follow from binding to a site on the ATPase, the 'gating' site, which affects the affinity of the first Ca(2+)-binding site for Ca2+ and affects the rate of dissociation of Ca2+ from the ATPase.

Kinetic characterization of the perturbation by dodecylmaltoside of sarcoplasmic reticulum Ca(2+)-ATPase

We investigated the functional aspects of the interaction between the sarcoplasmic reticulum (SR) membranous Ca(2+)-ATPase and the non-ionic detergent dodecylmaltoside, using detergent concentrations allowing perturbation of the membrane but not its solubilization. At pH 7.5, the effects of dodecylmaltoside on ATPase activity and delipidation had previously been shown to resemble, in some respects, those of octa(ethylene glycol) monododecylether (C12E8), an appropriate detergent for ATPase studies. Our aim here was to explore the specific effects of dodecylmaltoside on the different steps in the ATPase catalytic cycle, which may owe their specificity to the difference between the polar head groups of dodecylmaltoside and C12E8. This was done at 20 degrees C, both at pH 6 in the absence of KCl and at pH 7.5 in the presence of 100 mM KCl, two conditions under which the characteristics of unperturbed ATPase have already been well defined. Preliminary estimation of dodecylmaltoside partition between water and SR membranes at pH 6 yielded a partition coefficient K close to 4 x 10(5) (ratio of the molar fraction of dodecylmaltoside in the lipid to that in the aqueous phase at a low detergent concentration, assuming that most of this detergent was present in the lipid phase). At near saturation of SR membranes, bound dodecylmaltoside was roughly equimolar with the constituent phospholipids. Non-solubilizing concentrations of dodecylmaltoside inhibited SR ATPase activity by up to 65-70% at pH 7.5, but not at pH 6, unlike the results of similar experiments with C12E8. The rates of the four main steps in the ATPase catalytic cycle were measured by fast kinetic techniques; they were similarly modified at both pH. Dodecylmaltoside slowed down both the rate of calcium-saturated ATPase phosphorylation and the rate of ATPase isomerization after phosphorylation, two steps which were not targets of perturbation by C12E8. The slowing down of the isomerization step by dodecylmaltoside might well explain why it inhibited overall ATPase activity at pH 7.5. In contrast to C12E8, dodecylmaltoside did not affect the dephosphorylation step, which was the main target of inhibition by C12E8 and the main rate-limiting step at pH 6. However, like C12E8, dodecylmaltoside accelerated the calcium binding-induced transition of nonphosphorylated ATPase. Another striking feature of the perturbation induced by dodecylmaltoside was that it significantly altered the binding of 45Ca2+ to the ATPase and the corresponding conformational changes. At pCa 5-5.5, it almost halved calcium binding to the ATPase but ATPase phosphorylation was unimpaired.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of inhibition of the calcium pump of sarcoplasmic reticulum by thapsigargin

The steady-state ATPase activity of sarcoplasmic-reticulum (Ca(2+)-Mg2+)-ATPase is inhibited by thapsigargin at a molar ratio of 1:1, with a dissociation constant for thapsigargin estimated to be in the sub-nanomolar range. In the presence of thapsigargin, only a single Ca2+ ion binds to the ATPase. Similarly, addition of thapsigargin to the ATPase incubated in the presence of Ca2+ results in the release of one of the two originally bound Ca2+ ions. As monitored by the fluorescence of nitrobenzo-2-oxa-1,3-diazole-labelled ATPase, thapsigargin appears to shift the transition between E1 and E2 conformations towards E2. Addition of thapsigargin prevents phosphorylation of the ATPase by P(i) and results in a very low steady-state level of phosphorylation of the ATPase by ATP, as observed previously for nonylphenol.