Monomer-oligomer equilibrium of sarcoplasmic reticulum Ca-ATPase and the role of subunit interaction in the Ca2+ pump mechanism

Modulation of the Ca2+,Mg(2+)-ATPase of sarcoplasmic reticulum by the hypothalamic hypophyseal inhibitory factor

We have studied the effect of the endogenous inhibitor of the Na+ and Ca2+ pumps, HHIF, on sarcoplasmic reticulum (SR) vesicles. The effect of HHIF on the SR Ca2+,Mg(2+)-ATPase activity shows a biphasic pattern. Low HHIF concentrations activate the Ca2+,Mg(2+)-ATPase by dissipation of Ca2+ gradient across the SR membrane. Higher concentrations irreversibly inhibit this activity following a slow kinetic process both in intact SR membranes and in purified Ca2+,Mg(2+)-ATPase. Differential scanning calorimetry shows that the Ca2+,Mg(2+)-ATPase is denatured after incubation with HHIF concentrations which produced full inhibition of its activity. Micromolar Ca2+ and millimolar Mg2+ ADP protect against the irreversible inhibition of the Ca2+,Mg(2+)-ATPase by HHIF. The concentration of HHIF which produces 50% inhibition depends upon SR membrane concentration and upon the lipid:protein ratio in purified Ca2+,Mg(2+)-ATPase. From this we have obtained a partition coefficient for binding of HHIF to SR membranes of 0.6 (microgram SR protein/ml)-1.

How do Ca2+ ions pass through the sarcoplasmic reticulum membrane

We propose an overview of the mechanism of Ca2+ transport through the sarcoplasmic reticulum membrane via the Ca(2+)-ATPase. We describe cytoplasmic calcium binding, calcium occlusion in the membrane and lumenal calcium dissociation. A channel-like structure is discussed and related to structural data on the membranous domain of the Ca(2+)-ATPase.

The structure and interactions of Ca(2+)-ATPase

Electron crystallographic studies on membrane crystals of Ca(2+)-ATPase reveal different patterns of ATPase-ATPase interactions depending on enzyme conformation. Physiologically relevant changes in Ca2+ concentration and membrane potential affect these interactions. Ca2+ induced difference FTIR spectra of Ca(2+)-ATPase triggered by photolysis of caged Ca2+ are consistent with changes in secondary structure and carboxylate groups upon Ca2+ binding; the changes are reversed during ATP hydrolysis suggesting that a phosphorylated enzyme form of low Ca2+ affinity is the dominant intermediate during Ca2+ transport. A two-channel model of Ca2+ translocation is proposed involving the membrane-spanning helices M2-M5 and M4, M5, M6 and M8 respectively, with separate but interacting Ca2+ binding sites.

Interaction between glycogen phosphorylase and sarcoplasmic reticulum membranes and its functional implications

Skeletal muscle glycogen phosphorylase b binds to sarcoplasmic reticulum (SR) membranes with a dissociation constant of 1.7 +/- 0.6 mg of phosphorylase/ml at 25 degrees C at physiological pH and ionic strength. Raising the temperature to 37 degrees C produced a 2-3-fold decrease in the dissociation constant. The SR membranes could bind up to 1.1 +/- 0.1 mg of glycogen phosphorylase b/mg of SR protein, whereas liposomes prepared with endogenous SR lipids and reconstituted Ca(2+)-ATPase were unable to bind glycogen phosphorylase. Binding of glycogen phosphorylase b to SR membranes is accompanied by inhibition of its activity in the presence of AMP. The Vmax for glycogen phosphorylase b associated with SR membranes is 40 +/- 5% of that for purified glycogen phosphorylase and shows a decreased affinity for its allosteric activators, AMP and IMP. These kinetic effects are also observed with purified glycogen phosphorylase b when starch or alpha-amylose is used as substrate instead of glycogen. Treatment of SR membranes with alpha-amylase produced dissociation of glycogen phosphorylase b from the SR membranes. Thus, linear polysaccharide fragments of glycogen bound to the SR membranes are likely mediating the binding of glycogen phosphorylase b to these membranes.

Structure-function relationships of cation translocation by Ca(2+)- and Na+, K(+)-ATPases studied by site-directed mutagenesis

Site-directed mutagenesis studies of the sarcoplasmic reticulum Ca(2+)-ATPase have pinpointed five amino acid residues that are essential to Ca2+ occlusion, and these residues have been assigned to different parts of a Ca2+ binding pocket with channel-like structure. Three of the homologous Na+, K(+)-ATPase residues have been shown to be important for binding of cytoplasmic Na+ at transport sites. In addition, three of the above mentioned Ca(2+)-ATPase residues appear to participate in the countertransport of H+, and two of the Na+, K(+)-ATPase residues to participate in the countertransport of K+. Residues involved in energy transducing conformational changes have also been identified by mutagenesis. In the Ca(2+)-ATPase, ATP hydrolysis is uncoupled from Ca2+ transport following mutation of a tyrosine residue located at the top of transmembrane segment M5. This tyrosine, present also in the Na+, K(+)-ATPase, may play a critical role in closing the gate to a transmembrane channel.

Functional consequences of alterations to amino acids at the M5S5 boundary of the Ca(2+)-ATPase of sarcoplasmic reticulum. Mutation Tyr763-->Gly uncouples ATP hydrolysis from Ca2+ transport

The roles of the hydrophobic side chains of residues Phe760, Ile761, Tyr763, Leu764, and Ile765 located at the M5S5 boundary of the Ca(2+)-ATPase of sarcoplasmic reticulum were analyzed by site-directed mutagenesis. Substitution of Tyr763 with glycine resulted in a new phenotypic variant of the Ca(2+)-ATPase that catalyzed a high rate of Ca(2+)-activated ATP hydrolysis without net accumulation of Ca2+ in the microsomal vesicles. The ATPase activity of the Tyr763-->Gly mutant displayed characteristics similar to the ATPase activity of the wild-type enzyme measured in the presence of calcium ionophore, and the mutant was able to form the ADP-insensitive phosphoenzyme intermediate. Mutants Phe760-->Gly, Ile761-->Gly, Leu764-->Gly, and Ile765-->Gly were able to accumulate Ca2+. In mutants Leu764-->Gly and Ile765-->Gly, the turnover rate was low due to inhibition of dephosphorylation of the ADP-insensitive phosphoenzyme intermediate. On the other hand, mutant Leu764-->Lys dephosphorylated rapidly. Mutants Phe760-->Gly and Leu764-->Lys displayed apparent Ca2+ affinities that were reduced two and three orders of magnitude, respectively, relative to that of the wild-type.

Structural changes of sarcoplasmic reticulum Ca(2+)-ATPase upon Ca2+ binding studied by simultaneous measurement of infrared absorbance changes and changes of intrinsic protein fluorescence

Ca2+ binding to sarcoplasmic reticulum Ca(2+)-ATPase was investigated by Fourier transform infrared (FTIR) spectroscopy using the photolytic release of Ca2+ from the photolabile Ca2+ chelator 1-(2-nitro-4,5-dimethoxy)-N,N,N',N',- tetrakis[(oxycarbonyl)]methyl-1,2-ethandiamine (DM-nitrophen). IR absorbance changes in 1H2O and 2H2O were detected in the spectral region from 1800 cm-1 to 1200 cm-1, reflecting photolysis of DM-nitrophen and Ca2+ binding to the Ca(2+)-ATPase. As an independent probe for protein conformational changes, intrinsic fluorescence changes upon Ca2+ release were monitored simultaneously to the FTIR measurements. Both the IR absorbance changes and the fluorescence intensity changes correlated well with the Ca2+ binding activity of the ATPase in this specific step. Ca2+ binding caused IR difference bands mainly in the region of amide I absorption of the polypeptide backbone, reflecting conformational changes of the protein. The small amplitude of the signals indicates that only a few residues perform local structural changes such as changes of bond angles or hydrogen bonding. Other absorbance changes appearing above 1700 cm-1 can be assigned to Ca2+ binding to Glu or Asp side chain carboxyl groups and concomitant deprotonation of these residues. This assignment is strengthened by downshifts of these bands by 4 cm-1 to 6 cm-1 upon 1H2O/2H2O exchange. This is in line with results of mutagenesis studies where such residues containing carboxyl groups were associated with the high affinity Ca2+ binding site (Clarke, D.M., Loo, T.W. and MacLennan, D.H. (1990) J. Biol. Chem. 265, 6262-6267).

Oscillations and multiple steady states in active membrane transport models

The dynamic behavior of some non-linear extensions of the six-state alternating access model for active membrane transport is investigated. We use stoichio-metric network analysis to study the stability of steady states. The bifurcation analysis has been done through standard numerical methods. For the usual six-state model we have proved that there is only one steady state, which is globally asymptotically stable. When we added an autocatalytic step we found self-oscillations. For the competition between a monomer cycle and a dimer cycle, with steps of dimer formation, we have also found self-oscillations. We have also studied models involving the formation of a complex with other molecules. The addition of two steps for formation of a complex of the monomer with another molecule does not alter either the number or the stability of steady states of the basic six-state model. The model which combines the formation of a complex with an autocatalytic step shows both self-oscillations and multiple steady states. The results lead us to conclude that oscillations could be produced by active membrane transport systems if the transport cycle contains a sufficiently large number of steps (six in the present case) and is coupled to at least one autocatalytic reaction,. Oscillations are also predicted when the monomer cycle is coupled to a dimer cycle. In fact, the autocatalytic reaction can be seen as a simplification of the model involving competition between monomer and dimer cycles, which seems to be a more realistic description of biological systems. A self-regulation mechanism of the pumps, related to the multiple stationary states, is expected only for a combined effect of autocatalysis and formation of complexes with other molecules. Within the six-state model this model also leads to oscillation.

Changes of protein structure, nucleotide microenvironment, and Ca(2+)-binding states in the catalytic cycle of sarcoplasmic reticulum Ca(2+)-ATPase: investigation of nucleotide binding, phosphorylation and phosphoenzyme conversion by FTIR difference spectroscopy

Changes of infrared absorbance of sarcoplasmic reticulum Ca(2+)-ATPase (EC 3.6.1.38) associated with partial reactions of its catalytic cycle were investigated in the region from 1800 to 950 cm-1 in H2O and 2H2O. Starting from Ca2E1, 3 reaction steps were induced in the infrared cuvette via photolytic release of ATP and ADP: (a) nucleotide binding, (b) formation of the ADP-sensitive phosphoenzyme (Ca2E1P) and (c) formation of the ADP-insensitive phosphoenzyme (E2P). All reaction steps caused distinct changes of the infrared spectrum which were characteristic for each reaction step but comparable for all steps in the number and magnitude of the changes. Most pronounced were absorbance changes in the amide I spectral region sensitive to protein secondary structure. However, they were small--less than 1% of the total protein absorbance--indicating that the reaction steps are associated with small and local conformational changes of the polypeptide backbone instead of a large conformational rearrangement. Especially, there is no outstanding conformational change associated with the phosphoenzyme conversion Ca2E1P-->E2P. ADP-binding induces conformational changes in the ATPase polypeptide backbone with alpha-helical structures and presumably beta-sheet or beta-turn structures involved. Phosphorylation is accompanied by the appearance of a keto group vibration that can tentatively be assigned to the phosphorylated residue Asp351. Phosphoenzyme conversion and Ca(2+)-release produce difference signals which can be explained by the release of Ca2+ from carboxylate groups and a change of hydrogen bonding or protonation state of carboxyl groups.

Thermal unfolding of monomeric Ca(II), Mg(II)-ATPase from sarcoplasmic reticulum of rabbit skeletal muscle

The thermal unfolding of monomeric and delipidated Ca(2+)-ATPase, solubilized in C12E8, can be appropriately described as a non-two-state irreversible denaturation, with only one endothermic peak. In the Ca2+ concentration range (0-0.5 mM) which stimulates the ATPase activity of solubilized monomeric ATPase, Ca2+ shifts the critical temperature midpoint of the denaturation process (Tm) from 42 to 50 degrees C without segregation of the endothermic peak into two separate components. Because 20 mM Mg2+ only shifts the Tm from 42 to 44 degrees C, we conclude that the effect of Ca2+ upon the Tm is likely to be due to binding to the high affinity Ca2+ sites in the ATPase. The effect of Ca2+ upon the enthalpy of denaturation is biphasic, suggesting the presence of low affinity Ca2+ sites (K0.5 in the millimolar range) in monomeric and solubilized ATPase.

Proton ejection as a major feature of the Ca(2+)-pump

H+ ejection and Ca2+ uptake promoted by the sarcoplasmic reticulum (SR) Ca(2+)-pump are similarly stimulated by millimolar Mg2+. This cannot be assigned to enhanced Ca2+ uptake and H+ displacement from internal metal binding sites since: (1) loading SR vesicles with high Mg2+ concentrations does not impair H+ ejection; (2) loading SR vesicles with Mn2+ does not depress H+ ejection occurring during Mn2+ uptake; (3) H+ ejection occurs even when Ca2+ accumulation inside the vesicles is prevented with Ca2+ ionophores. It is concluded that the Ca(2+)-pump promotes an active Ca2+/H+ countertransport stimulated by Mg2+. Finally, a mechanism for Ca2+ translocation is proposed in basic physico-chemical terms.

Effects of alcohol-induced lipid interdigitation on proton permeability in L-alpha-dipalmitoylphosphatidylcholine vesicles

6-Carboxyfluorescein was employed to examine the effect of alcohol-induced lipid interdigitation on proton permeability in L-alpha-dipalmitoylphosphatidylcholine (DPPC) large unilamellar vesicles. Proton permeability was measured by monitoring the decrease of 6-carboxyfluorescein fluorescence after a pH gradient from 3.5 (outside the vesicle) to 8.0 (inside the vesicle) was established. At 20 degrees C and below 1.2 M ethanol, the fluorescence decrease is best described by a single exponential function. Above 1.2 M ethanol, the intensity decrease is better described by a two-exponential decay law. Using the fitted rate constants and the vesicle radii determined from light-scattering measurements, the proton permeability coefficient, P, in DPPC vesicles was calculated as a function of ethanol concentration. At 20 degrees C, P increases monotonically with increasing ethanol content up to 1.0 M, followed by an abrupt increase at 1.2 M. The vesicle size also exhibits a sudden increase at around 1.2 M ethanol, which has been shown to result from vesicle aggregation rather than vesicle fusion. The abrupt increases in P and in vesicle size occur at the concentration region close to the critical ethanol concentration for the formation of the fully interdigitated gel state of DPPC. At 14 degrees C, the abrupt change in P shifts to 1.9-2.0 M ethanol, completely in accordance with the ethanol-temperature phase diagram of interdigitated DPPC. Effects of methanol and benzyl alcohol on lipid interdigitation have also been examined. At 20 degrees C, DPPC large unilamellar vesicles exhibit a dramatic change in P at 3 M methanol and at 40 mM benzyl alcohol. These concentrations come close to the critical methanol and benzyl alcohol concentrations for the formation of fully interdigitated DPPC structures determined previously by others. It can be concluded that proton permeability increases dramatically as DPPC is transformed from the noninterdigitated gel to the fully interdigitated gel state by high concentrations of alcohol. This marked increase in proton permeability can be attributed to the combined effect of the changes in membrane thickness and surface charge density, due to the ethanol-induced lipid interdigitation. The possible effects of the increased proton permeability caused by ingested ethanol on gastric mucosal membranes are discussed.

Functional consequences of substitution of the seven-residue segment LysIleArgAspGlnMetAla240 located in the stalk helix S3 of the Ca(2+)-ATPase of sarcoplasmic reticulum

Site-directed mutagenesis was used to substitute the seven-residue segment LysIleArgAspGlnMetAla240 located at the NH2- terminal end of the "stalk" helix S3, near the beta-strand domain, in the sarcoplasmic reticulum Ca(2+)-ATPase of rabbit fast twitch muscle, with the corresponding Na+,K(+)-ATPase segment ArgIleAlaThrLeuAlaSer. This led to a new phenotypic variant of Ca(2+)-ATPase. The overall turnover rates for Ca2+ transport and ATP hydrolysis measured at 27 and 37 degrees C, respectively, were reduced to 30-40% of the wild-type rates. Analysis of the phosphoenzyme intermediates at 0 degrees C showed that the ADP-insensitive phosphoenzyme intermediate accumulated under conditions where the ADP-sensitive phosphoenzyme intermediate predominated in the wild-type Ca(2+)-ATPase. The rate of dephosphorylation of the ADP-insensitive phosphoenzyme intermediate formed through the forward reaction with ATP, or in the "backdoor" reaction with Pi, was reduced severalfold in the mutant relative to the dephosphorylation rate measured in the wild type, but there was no significant difference between the mutant and the wild type with respect to the apparent affinity for Pi measured under equilibrium conditions. The mutant was much less susceptible to inhibition by vanadate than the wild type, under equilibrium conditions as well as during turnover with ATP and Ca2+. These observations suggest that the transition state in the hydrolysis of the aspartyl phosphate bond in the ADP-insensitive phosphoenzyme intermediate was destabilized in the mutant.

Are there different water requirements in different steps of a catalytic cycle? Hydration effects at the E1 and E2 conformers of sarcoplasmic reticulum Ca(2+)-ATPase studied in organic solvents with low amounts of water

The Ca(2+)-ATPase from sarcoplasmic reticulum was transferred in an active form to a low-water system composed of toluene, phospholipids, and Triton X-100 (TPT). The Ca(2+)-ATPase activity in the TPT system with 4.0% water (by vol. was about 50% of the activity observed in all-aqueous mixtures. Phosphate formation was linear with time up to 20% of ATP hydrolysis and, as expected from an enzyme-catalysed reaction, activity was linear with protein concentration. No ATPase activity was detected in the presence of 3 mM EGTA, indicating that the enzyme retained its Ca2+ dependence in the TPT system. A hyperbolic response to ATP concentration was observed with a Km of 0.15 mM. There was no detectable ATPase activity at water concentrations below 1.5% (by vol.). With 2.0% water, activity became detectable and increased as the water content was progressively raised to 7.0% (by vol.). Higher amounts of water produced unstable emulsions. Enzyme phosphorylation by ATP and dephosphorylation took place in the TPT system. The velocities of both enzyme phosphorylation and dephosphorylation increased with increments in the water content. The enzyme could also be phosphorylated in the TPT system by inorganic phosphate. However, in comparison to ATP, phosphorylation by phosphate took place with significantly lower amounts of water. It is suggested that at low amounts of water, the enzyme is in a relatively rigid conformation and, as the water content is increased, the ATPase acquires more flexibility and, hence, the capacity to carry out catalysis at higher rates. Nevertheless, the release of conformational constraints of the catalytic site of the E2 conformer takes place at water concentrations much lower than those needed for the expression of catalytic activity by the E1 conformer.

Autocatalytic cooperativity and self-regulation of ATPase pumps in membrane active transport

We investigate the effect of autocatalysis on the conformational changes of membrane pumps during active transport driven by ATP. The translocation process is described by means of an alternating access model. The usual kinetic scheme is extended by introducing autocatalytic steps and allowing for dynamic formation of enzyme complexes. The usual features of cooperative models are recovered, i.e., sigmoid shapes of flux versus concentration curves. We show also that two autocatalytic steps lead to a mechanism of inhibition by the substrate as experimentally observed for some ATPase pumps. In addition, when the formation of enzyme complexes is allowed, the model exhibits a multiple stationary states regime, which can be related to a self-regulation mechanism of the active transport in biological systems.

Deduced amino acid sequence and E1-E2 equilibrium of the sarcoplasmic reticulum Ca(2+)-ATPase of frog skeletal muscle. Comparison with the Ca(2+)-ATPase of rabbit fast twitch muscle

The cDNA encoding a Ca(2+)-transport ATPase of frog (Rana esculenta) skeletal muscle was isolated and characterized. The deduced amino acid sequence, consisting of 994 residues, showed 89% identity to the fast twitch muscle sarcoplasmic reticulum Ca(2+)-ATPases of chicken and rabbit. Northern blot analysis using a fragment of this cDNA as probe detected a 5.0 kb message in frog skeletal muscle but did not detect any mRNA encoding sarcoplasmic reticulum Ca(2+)-ATPase in frog cardiac muscle. The enzymatic properties of the amphibian skeletal muscle Ca(2+)-ATPase were compared with those of the rabbit fast twitch muscle Ca(2+)-ATPase by functional expression of the cDNAs in COS-1 cells. The amphibian Ca(2+)-ATPase displayed a reduced apparent affinity for Ca2+ and an increased apparent affinity for the inhibitors, vanadate and thapsigargin, relative to the mammalian enzyme. This may be explained by a mechanism in which relatively more of the E2 conformation accumulated in the frog Ca(2+)-ATPase than in the mammalian enzyme.

Mutational analysis of the role of Glu309 in the sarcoplasmic reticulum Ca(2+)-ATPase of frog skeletal muscle

Site-specific mutagenesis was used to analyse the role of the residue, Glu309, in the function of the Ca(2+)-ATPase of frog skeletal muscle sarcoplasmic reticulum by substitution with Ala or Lys. At pH 6.0, 100 microM Ca2+ was unable to prevent phosphorylation from Pi, consistent with previous observations on the Ca(2+)-ATPase of rabbit fast twitch muscle [Clarke, D.M., Loo, T.W, Inesi, G. and MacLennan, D.H. (1989) Nature 339, 476-478]. At neutral pH, however, micromolar concentrations of Ca2+ were sufficient to inhibit phosphorylation of the Glu309----Lys mutant from inorganic phosphate, suggesting that at least one high-affinity Ca2+ site was relatively intact in this mutant. The Glu309----Lys mutant was unable to form a phosphoenzyme from ATP at all Ca2+ concentrations studied (up to 12.5 mM), whereas phosphorylation of the Glu309----Ala mutant occurred at 12.5 mM Ca2+, but not at Ca2+ concentrations in the submillimolar range. Kinetic studies demonstrated a reduced rate of dephosphorylation of the E2P intermediate in the Glu309----Lys mutant. A less pronounced stabilization of E2P was observed with the Glu309----Ala mutant, suggesting a possible role of the charge at the position of Glu309 in phosphoenzyme hydrolysis.

K(+)-site-directed pyridine derivative, AU-1421, activates hydrolysis of the K(+)-sensitive phosphoenzyme of sarcoplasmic reticulum Ca(2+)-ATPase and inactivates that of K(+)-transporting ATPases

(Z)-5-Methyl-2-[2-(1-naphthyl)ethenyl]-4-piperidinopyridine, AU-1421, interacted at 0 degree C with the K(+)-sensitive phosphoenzymes of three transport ATPases, Ca(2+)-, H+/K(+)- and Na+/K(+)-ATPase. In the case of Ca(2+)-ATPase, AU-1421 at about 80 microM stimulated 6-fold the rate of splitting of the phosphoenzyme, on which K+ simply functions as an accelerator from one side of the membrane. Probably AU-1421 also simply interacts with the K(+)-binding site of the phosphoenzyme that is easily accessible from the aqueous phase. In the cases of H(+)/K(+)- and Na(+)/K(+)-ATPases, AU-1421 stabilized the phosphoenzymes which accept K+ as the translocating ion. The rate constants of dephosphorylation for H(+)/K(+)-ATPase and Na(+)/K(+)-ATPase were decreased to half by AU-1421 at about 5 and 10 microM, respectively. Presumably after binding of AU-1421 to a K(+)-recognition site of the phosphoenzyme, local motion of the peptide region near the binding site that serves to move the bound ion into the ion-transport pathway (occlusion center) might be inhibited. Thus AU-1421 may be able to distinguish two modes of K+ action on the K(+)-sensitive phosphoenzymes.

Infrared spectroscopic signals arising from ligand binding and conformational changes in the catalytic cycle of sarcoplasmic reticulum calcium ATPase

Fourier transform infrared spectroscopy was used to investigate ligand binding and conformational changes in the Ca2(+)-ATPase of sarcoplasmic reticulum during the catalytic cycle. The ATPase reaction was started in the infrared sample by release of ATP from the inactive, photolabile ATP derivative P3-1-(2-nitro)phenylethyladenosine 5'-triphosphate (caged ATP). Absorption spectroscopy in the visible spectral region using the Ca2(+)-sensitive dye Antipyrylazo III ensured that the infrared samples were able to transport Ca2+ in spite of their low water content, which is required for mid-infrared measurements (1800-950 cm-1). Small, but characteristic and highly reproducible infrared absorbance changes were observed upon ATP release. These infrared absorbance changes exhibit different kinetic properties. Comparison with model compound infrared spectra indicates that they are related to photolysis of caged ATP, hydrolysis of ATP in consequence of ATPase activity and to molecular changes in the active ATPase. The absorbance changes due to alterations in the ATPase were observed mainly in the region of Amide I and Amide II protein absorbance and presumably reflect the molecular processes upon phosphoenzyme formation. Since the absorbance changes were small compared to the overall ATPase absorbance, no major rearrangement of ATPase conformation as the result of catalysis could be detected.

Purification and characterization of a calsequestrin-like calcium-binding protein from carp (Cyprinus carpio) sarcoplasmic reticulum

1. A calsequestrin-like calcium-binding protein was purified from carp sarcoplasmic reticulum by column chromatographies using DEAE-cellulose and Butyl-Toyopearl 650S. 2. The mol. wt was estimated to be 50 kDa, which was larger than that of rabbit calsequestrin (42 kDa). 3. Carp calsequestrin-like protein bound Ca2+ with a higher affinity (apparent Kd = 400 microM) and lower capacity (25 mol/mol) compared with rabbit calsequestrin (1 mM and 40-50 mol/mol, respectively). 4. Anti-carp calsequestrin-like protein rabbit antiserum reacted with rabbit calsequestrin in immunoblotting analysis. 5. Carp calsequestrin-like protein was rich in acidic amino acids, as was rabbit calsequestrin.

Molecular changes in the sarcoplasmic reticulum calcium ATPase during catalytic activity. A Fourier transform infrared (FTIR) study using photolysis of caged ATP to trigger the reaction cycle

Fourier transform infrared spectroscopy was used to study ligand binding and conformational changes in the Ca2(+)-ATPase of sarcoplasmic reticulum. Novel in infrared difference spectroscopy, the catalytic cycle in the IR sample was started by photolytic release of ATP from an inactive, photolabile ATP-derivative (caged ATP). Small, but characteristic infrared absorbance changes were observed upon ATP release. On the basis of model spectra, the absorbance changes corresponding to the trigger and substrate reactions, i.e. to photolysis of caged ATP and hydrolysis of ATP, were separated from the absorbance changes due to the active ATPase reflecting formation of the phosphorylated Ca2E1P enzyme form. A major rearrangement of ATPase conformation as the result of catalysis can be excluded.

Distances between functional sites of the Ca2+ + Mg2(+)-ATPase from sarcoplasmic reticulum using Co2+ as a spectroscopic ruler

Cobalt ion inhibits the Ca2+ + Mg2(+)-ATPase activity of sealed sarcoplasmic reticulum vesicles, of solubilized membranes and of the purified enzyme. To use Co2+ appropriately as a spectroscopic ruler to map functional sites of the Ca2+ + Mg2(+)-ATPase, we have carried out studies to obtain the kinetic parameters needed to define the experimental conditions to conduct the fluorimetric studies. 1. The apparent K0.5 values of inhibition of this ATPase are 1.4 mM, 4.8 mM and 9.5 mM total Co2+ at pH 8.0, 7.0 and 6.0, respectively. The inhibition by Co2+ is likely to be due to free Co2+ binding to the enzyme. Millimolar Ca2+ can fully reverse this inhibition, and also reverses the quenching of the fluorescence of fluorescein-labeled sarcoplasmic reticulum membranes due to Co2+ binding to the Ca2+ + Mg2(+)-ATPase. Therefore, we conclude that Co2+ interacts with Ca2+ binding sites. 2. Co2+.ATP can be used as a substrate by this enzyme with Vmax of 2.4 +/- 0.2 mumol ATP hydrolyzed min-1 (mg protein)-1 at 20-22 degrees C and pH 8.0, and with a K0.5 of 0.4-0.5 mM. 3. Co2+ partially quenches, about 10 +/- 2%, the fluorescence of fluorescein-labeled sarcoplasmic reticulum Ca2+ + Mg2(+)-ATPase upon binding to this enzyme at pH 8.0. From the fluorescence data we have estimated an average distance between Co2+ and fluorescein in the ATPase of 1.1-1.8 nm or 1.3-2.1 nm for one or two equidistant Co2+ binding sites, respectively. 4. Co2+.ATP quenches about 20-25% of the fluorescence of fluorescein-labeled Ca2+ + Mg2(+)-ATPase, from which we obtain a distance of 1.1-1.9 nm between Co2+ and fluorescein located at neighbouring catalytic sites.

Stoichiometries of calcium and strontium transport coupled to ATP and acetyl phosphate hydrolysis by skeletal sarcoplasmic reticulum

The stoichiometries of Ca2+ and of Sr2+ transport by the Ca2(+)-ATPase of skeletal muscle sarcoplasmic reticulum have been previously reported to be 2 and 1, respectively, when determined by flux ratio methods (Mermier, P. and Hasselbach, W. (1976) Eur. J. Biochem. 69, 79-86; Holguin, J.A. (1986) Arch. Biochem. Biophys. 251, 9-16). We have measured transport of Ca2+ and Sr2+ by the pulsed pH-stat method, when supported by ATP or the pseudo-substrate acetyl phosphate (AcP). The stoichiometry of ATP-supported Ca2+ transport, Ca2+/ATP, was pH dependent and varied from 2.0 at pH 6.5 to 1.0 at pH 8.0. Sr2+/ATP ratios showed a similar pH dependence and were approx. 7-18% lower. Ca2+/AcP ratios showed little pH dependence and varied from 2.0 to 1.7 in the pH range 6.5 to 8.0. Sr2+/AcP ratios were 17-34% lower, with maximum differences at the pH extremes. Ruthenium red, which blocks calcium efflux from calcium release channels, increased measured stoichiometries by less than 10%. It is concluded that the transport of both Ca2+ and Sr2+, when supported by either ATP or a pseudo-substrate, have similar stoichiometrics and occurs via identical mechanisms. The relatively low Sr2+ transport ratios have been related to uncoupled reverse flux through the Ca2(+)-ATPase cation transport channel. Subintegral M2+/substrate ratios appear to be an intrinsic feature of active transport by the Ca2+ pump of skeletal muscle sarcoplasmic reticulum.

Protein-protein interaction of detergent-solubilized Ca2(+)-ATPase during ATP hydrolysis analyzed by low-angle laser light scattering photometry coupled with high-performance gel chromatography

Protein-protein interaction of detergent-solubilized Ca2(+)-ATPase was examined, employing low-angle laser light scattering photometry coupled with high-performance gel chromatography. When solubilized with octa(ethylene glycol) mono-n-dodecyl ether (C12E8) and chromatographed in the presence of 0.3 mg/ml C12E8, the Ca2(+)-ATPase emerged as a single peak with an intermediate molecular weight between the monomer and the dimer, showing a dissociation-association equilibrium of the two components. In the presence of 50 micrograms/ml phosphatidylcholine and 0.3 mg/ml C12E8 at 0 degrees C, the Ca2(+)-ATPase (0.8 mg) emerged as the two distinct components with molecular weights of 125,000 +/- 2100 (n = 3) and 211 300 +/- 7300 (n = 3), indicating that there was no rapid interconversion between the monomer and the dimer. Under the latter conditions, addition of ATP induced fusion of two components. The apparent molecular weight of the fused peak shifted from the monomer to the dimer as the amount of protein increased. Addition of ADP or adenosine 5'-(beta, gamma-methylene triphosphate), however, did not induce such fusion of the peaks. The ATP-induced fusion of the peaks was not observed either in 5 mM CaCl2, the conditions in which the rate of ATP hydrolysis was extremely slow. Thus, the solubilized Ca2(+)-ATPase underwent a rapid interconversion between the monomer and the dimer during ATP hydrolysis. These results suggest that the protein-protein interaction during ATP hydrolysis is an intrinsic nature of Ca2(+)-ATPase and that such interaction may be important for Ca2+ transport by Ca2(+)-ATPase in the sarcoplasmic reticulum membranes.

Participation of a non-covalent phosphointermediate in ATP hydrolysis by the sarcoplasmic reticulum Ca2(+)-cotransport ATPase

With increasing SDS/protein ratios, covalent phosphorylation by ATP and Pi is abolished before ATP hydrolysis (Pi production) ceases. We have shown that the SDS-dependent profiles of the decline in covalent phosphorylation by either substrate are virtually identical, reflecting a common mechanism of detergent interaction, while ATP can be hydrolysed via a non-covalent phosphointermediate. Our studies support that the transfer of both terminal Pi from ATP, as well as Pi to its final binding site, is a multistep reaction involving electrostatic interaction with one or more amino acid side chains, including a Lys residue.

Effect of calcium on the interactions between Ca2+-ATPase molecules in sarcoplasmic reticulum

The interaction between Ca2+-ATPase molecules in the native sarcoplasmic reticulum membrane and in detergent solutions was analyzed by chemical crosslinking, high performance liquid chromatography (HPLC), and by the polarization of fluorescence of fluorescein 5'-isothiocyanate (FITC) covalently attached to the Ca2+-ATPase. Reaction of sarcoplasmic reticulum vesicles with glutaraldehyde causes the crosslinking of Ca2+-ATPase molecules with the formation of dimers, tetramers and higher oligomers. At moderate concentrations of glutaraldehyde solubilization of sarcoplasmic reticulum by C12 E8 or Brij 36T (approximately equal to 4 mg/mg protein) decreased the formation of higher oligomers without significant interference with the appearance of crosslinked ATPase dimers. These observations are consistent with the existence of Ca2+-ATPase dimers in detergent-solubilized sarcoplasmic reticulum. Ca2+ (2-20 mM) and glycerol (10-20%) increased the degree of crosslinking at pH 6.0 both in vesicular and in solubilized sarcoplasmic reticulum, presumably by promoting interactions between ATPase molecules; at pH 7.5 the effect of Ca2+ was less pronounced. In agreement with these observations, high performance liquid chromatography of sarcoplasmic reticulum proteins solubilized by Brij 36T or C12 E10 revealed the presence of components with the expected elution characteristics of Ca2+-ATPase oligomers. The polarization of fluorescence of FITC covalently attached to the Ca2+-ATPase is low in the native sarcoplasmic reticulum due to energy transfer, consistent with the existence of ATPase oligomers (Highsmith, S. and Cohen, J.A. (1987) Biochemistry 26, 154-161); upon solubilization of the sarcoplasmic reticulum by detergents, the polarization of fluorescence increased due to dissociation of ATPase oligomers. Based on its effects on the fluorescence of FITC-ATPase, Ca2+ promoted the interaction between ATPase molecules, both in the native membrane and in detergent solutions.