Characterization of detergent-solubilized sarcoplasmic reticulum Ca2+-ATPase by high-performance liquid chromatography

Characterization of sarcoplasmic reticulum Ca(2+) ATPase pumps in muscle of patients with myotonic dystrophy and with hypothyroid myopathy

Sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase (SERCA) pumps play the major role in lowering cytoplasmic calcium concentration in skeletal muscle by catalyzing the ATP-dependent transport of Ca(2+) from the cytosol to the lumen of the sarcoplasmic reticulum (SR). Although SERCA abnormalities have been hypothesized to contribute to the dysregulation of intracellular Ca(2+) homeostasis and signaling in muscle of patients with myotonic dystrophy (DM) and hypothyroid myopathy, the characterization of SERCA pumps remains elusive and their impairment is still unclear. We assessed the activity of SR Ca(2+)-ATPase, expression levels and fiber distribution of SERCA1 and SERCA2, and oligomerization of SERCA1 protein in muscle of patients with DM type 1 and 2, and with hypothyroid myopathy. Our data provide evidence that SR Ca(2+) ATPase activity, protein levels and muscle fiber distribution of total SERCA1 and SERCA2, and SERCA1 oligomerization pattern are similar in patients with both DM1 and DM2, hypothyroid myopathy and in control subjects. We prove that SERCA1b, the neonatal isoform of SERCA1, is expressed at protein level in muscle of patients with DM2 and, in lower amount, of patients with DM1. Our present study demonstrates that SERCA function is not altered in muscle of patients with DM and with hypothyroid myopathy.

Chelerythrine inhibits the sarco/endoplasmic reticulum Ca(2+)-ATPase and results in cell Ca(2+) imbalance

The isoquinoline alkaloid chelerythrine is described as an inhibitor of SERCA. The ATPase inhibition presented two non-competitive components, Ki1=1, 2 μM and Ki2=26 μM. Conversely, chelerythrine presented a dual effect on the p-nitrophenylphosphatase (pNPPase) of SERCA. Ca(2+)-dependent pNPPase was activated up to ∼5 μM chelerythrine with inhibition thereafter. Ca(2+)-independent pNPPase was solely inhibited. The phosphorylation of SERCA with ATP reached half-inhibition with 10 μM chelerythrine and did not parallel the decrease of ATPase activity. In contrast, chelerythrine up to 50 μM increased the phosphorylation by Pi. Cross-linking of SERCA with glutaraldehyde was counteracted by high concentrations of chelerythrine. The controlled tryptic digestion of SERCA shows that the low-affinity binding of chelerythrine evoked an E2-like pattern. Our data indicate a non-competitive inhibition of ATP hydrolysis that favors buildup of the E2-conformers of the enzyme. Chelerythrine as low as 0.5-1.5 μM resulted in an increase of intracellular Ca(2+) on cultured PBMC cells. The inhibition of SERCA and the loss of cell Ca(2+) homeostasis could in part be responsible for some described cytotoxic effects of the alkaloid. Thus, the choice of chelerythrine as a PKC-inhibitor should consider its potential cytotoxicity due to the alkaloid's effects on SERCA.

Quality assessment of recombinant proteins by infrared spectroscopy. Characterisation of a protein aggregation related band of the Ca2+-ATPase

FTIR spectroscopy detects aggregates of recombinantly produced protein and can therefore be used for quality control.

SERCA mutant E309Q binds two Ca(2+) ions but adopts a catalytically incompetent conformation

The sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) couples ATP hydrolysis to transport of Ca(2+). This directed energy transfer requires cross-talk between the two Ca(2+) sites and the phosphorylation site over 50 Å distance. We have addressed the mechano-structural basis for this intramolecular signal by analysing the structure and the functional properties of SERCA mutant E309Q. Glu(309) contributes to Ca(2+) coordination at site II, and a consensus has been that E309Q only binds Ca(2+) at site I. The crystal structure of E309Q in the presence of Ca(2+) and an ATP analogue, however, reveals two occupied Ca(2+) sites of a non-catalytic Ca2E1 state. Ca(2+) is bound with micromolar affinity by both Ca(2+) sites in E309Q, but without cooperativity. The Ca(2+)-bound mutant does phosphorylate from ATP, but at a very low maximal rate. Phosphorylation depends on the correct positioning of the A-domain, requiring a shift of transmembrane segment M1 into an 'up and kinked position'. This transition is impaired in the E309Q mutant, most likely due to a lack of charge neutralization and altered hydrogen binding capacities at Ca(2+) site II.

The sarcoplasmic Ca2+-ATPase: design of a perfect chemi-osmotic pump

The sarcoplasmic (SERCA 1a) Ca2+-ATPase is a membrane protein abundantly present in skeletal muscles where it functions as an indispensable component of the excitation-contraction coupling, being at the expense of ATP hydrolysis involved in Ca2+/H+ exchange with a high thermodynamic efficiency across the sarcoplasmic reticulum membrane. The transporter serves as a prototype of a whole family of cation transporters, the P-type ATPases, which in addition to Ca2+ transporting proteins count Na+, K+-ATPase and H+, K+-, proton- and heavy metal transporting ATPases as prominent members. The ability in recent years to produce and analyze at atomic (2·3-3 Å) resolution 3D-crystals of Ca2+-transport intermediates of SERCA 1a has meant a breakthrough in our understanding of the structural aspects of the transport mechanism. We describe here the detailed construction of the ATPase in terms of one membraneous and three cytosolic domains held together by a central core that mediates coupling between Ca2+-transport and ATP hydrolysis. During turnover, the pump is present in two different conformational states, E1 and E2, with a preference for the binding of Ca2+ and H+, respectively. We discuss how phosphorylated and non-phosphorylated forms of these conformational states with cytosolic, occluded or luminally exposed cation-binding sites are able to convert the chemical energy derived from ATP hydrolysis into an electrochemical gradient of Ca2+ across the sarcoplasmic reticulum membrane. In conjunction with these basic reactions which serve as a structural framework for the transport function of other P-type ATPases as well, we also review the role of the lipid phase and the regulatory and thermodynamic aspects of the transport mechanism.

Crystallization and preliminary structural analysis of the Listeria monocytogenes Ca(2+)-ATPase LMCA1

Ca(2+)-ATPases are ATP-driven membrane pumps that are responsible for the transport of Ca(2+) ions across the membrane. The Listeria monocytogenes Ca(2+)-ATPase LMCA1 has been crystallized in the Ca(2+)-free state stabilized by AlF(4)(-), representing an occluded E2-P(i)-like state. The crystals belonged to space group P2(1)2(1)2 and a complete data set extending to 4.3 Å resolution was collected. A molecular-replacement solution was obtained, revealing type I packing of the molecules in the crystal. Unbiased electron-density features were observed for AlF(4)(-) and for shifts of the helices, which were indicative of a reliable structure determination.

Protein–protein contacts in solubilized membrane proteins, as detected by cross-linking

The amount of detergent required for the solubilization of membrane proteins needs to be optimised as an excess may cause loss of activity and insufficiency may result in poor solubilization or heterogeneous samples. With sarcoplasmic reticulum Ca2+ -ATPase as an example we show by cross-linking that it can be misleading to choose the proper amount of detergent based on clarification of membrane suspensions, because clarification -as detected by turbidity measurements, for instance- precedes full protein solubilization as monomers. We demonstrate that to assess the extent of sample homogeneity at a given detergent/protein ratio, cross-linking followed by HPLC gel filtration in detergent usefully complements cross-linking followed by SDS-PAGE.

Solubilization, purification and reconstitution of Ca2+-ATPase from bovine pulmonary artery smooth muscle microsomes by different detergents: Preservation of native structure and function of the enzyme by DHPC

The properties of Ca(2+)-ATPase purified and reconstituted from bovine pulmonary artery smooth muscle microsomes {enriched with endoplasmic reticulum (ER)} were studied using the detergents 1,2-diheptanoyl-sn-phosphatidylcholine (DHPC), poly(oxy-ethylene)8-lauryl ether (C(12)E(8)) and Triton X-100 as the solubilizing agents. Solubilization with DHPC consistently gave higher yields of purified Ca(2+)-ATPase with a greater specific activity than solubilization with C(12)E(8) or Triton X-100. DHPC was determined to be superior to C(12)E(8); while that the C(12)E(8) was determined to be better than Triton X-100 in active enzyme yields and specific activity. DHPC solubilized and purified Ca(2+)-ATPase retained the E1Ca-E1*Ca conformational transition as that observed for native microsomes; whereas the C(12)E(8) and Triton X-100 solubilized preparations did not fully retain this transition. The coupling of Ca(2+) transported to ATP hydrolyzed in the DHPC purified enzyme reconstituted in liposomes was similar to that of the native micosomes, whereas that the coupling was much lower for the C(12)E(8) and Triton X-100 purified enzyme reconstituted in liposomes. The specific activity of Ca(2+)-ATPase reconstituted into dioleoyl-phosphatidylcholine (DOPC) vesicles with DHPC was 2.5-fold and 3-fold greater than that achieved with C(12)E(8) and Triton X-100, respectively. Addition of the protonophore, FCCP caused a marked increase in Ca(2+) uptake in the reconstituted proteoliposomes compared with the untreated liposomes. Circular dichroism analysis of the three detergents solubilized and purified enzyme preparations showed that the increased negative ellipticity at 223 nm is well correlated with decreased specific activity. It, therefore, appears that the DHPC purified Ca(2+)-ATPase retained more organized and native secondary conformation compared to C(12)E(8) and Triton X-100 solubilized and purified preparations. The size distribution of the reconstituted liposomes measured by quasi-elastic light scattering indicated that DHPC preparation has nearly similar size to that of the native microsomal vesicles whereas C(12)E(8) and Triton X-100 preparations have to some extent smaller size. These studies suggest that the Ca(2+)-ATPase solubilized, purified and reconstituted with DHPC is superior to that obtained with C(12)E(8) and Triton X-100 in many ways, which is suitable for detailed studies on the mechanism of ion transport and the role of protein-lipid interactions in the function of the membrane-bound enzyme.

Interaction of membrane proteins and lipids with solubilizing detergents

Detergents are indispensable in the isolation of integral membrane proteins from biological membranes to study their intrinsic structural and functional properties. Solubilization involves a number of intermediary states that can be studied by a variety of physicochemical and kinetic methods; it usually starts by destabilization of the lipid component of the membranes, a process that is accompanied by a transition of detergent binding by the membrane from a noncooperative to a cooperative interaction already below the critical micellar concentration (CMC). This leads to the formation of membrane fragments of proteins and lipids with detergent-shielded edges. In the final stage of solubilization membrane proteins are present as protomers, with the membrane inserted sectors covered by detergent. We consider in detail the nature of this interaction and conclude that in general binding as a monolayer ring, rather than as a micelle, is the most probable mechanism. This mode of interaction is supported by neutron diffraction investigations on the disposition of detergent in 3-D crystals of membrane proteins. Finally, we briefly discuss the use of techniques such as analytical ultracentrifugation, size exclusion chromatography, and mass spectrometry relevant for the structural investigation of detergent solubilized membrane proteins.

Interaction of amphipols with sarcoplasmic reticulum Ca2+-ATPase

Amphipols are short-chain amphipathic polymers designed to keep membrane proteins soluble in aqueous solutions. We have evaluated the effects of the interaction of amphipols with sarcoplasmic reticulum Ca(2+)-ATPase either in a membrane-bound or a soluble form. If the addition of amphipols to detergent-solubilized ATPase was followed by removal of detergent, soluble complexes formed, but these complexes retained poor ATPase activity, were not very stable upon long incubation periods, and at high concentrations they experienced aggregation. Nevertheless, adding excess detergent to diluted detergent-free ATPase-amphipol complexes incubated for short periods immediately restored full activity to these complexes, showing that amphipols had protected solubilized ATPase from the rapid and irreversible inactivation that otherwise follows detergent removal. Amphipols also protected solubilized ATPase from the rapid and irreversible inactivation observed in detergent solutions if the ATPase Ca(2+) binding sites remain vacant. Moreover, in the presence of Ca(2+), amphipol/detergent mixtures stabilized concentrated ATPase against inactivation and aggregation, whether in the presence or absence of lipids, for much longer periods of time (days) than detergent alone. Our observations suggest that mixtures of amphipols and detergents are promising media for handling solubilized Ca(2+)-ATPase under conditions that would otherwise lead to its irreversible denaturation and/or aggregation.

pH and ligand binding modulate the strength of protein-protein interactions in the Ca(2+)-ATPase from sarcoplasmic reticulum membranes

The Ca(2+)-ATPase from sarcoplasmic reticulum (SR) membranes couples the Ca(2+) transport to ATP hydrolysis through phosphorylation in its cytoplasmic catalytic domain. Interactions between protein domains and the role of monomer-monomer interactions remain unclear. Here, we report a differential scanning calorimetric study of the thermal unfolding of this protein. In the pH range 6-8, thermal unfolding of the Ca(2+)-ATPase in glycogen phosphorylase-free SR membranes shows a major endothermic peak with a critical temperature midpoint ranging between 51 and 55 degrees C, depending on pH, Ca(2+), Mg(2+)-ADP and KCl concentrations. The enthalpy change of the overall unfolding process ranged between 250 and 300 kcal/mol of Ca(2+)-ATPase monomer. Thermal denaturation of the Ca(2+)-ATPase in SR membranes is well fitted to an irreversible process that can be rationalized in terms of a non-two state process, N (native)right harpoon over left harpoon I (intermediate)-->D (denatured). Thermodynamic analysis show that this protein has a compact structure, implying a tight structural interconnection between catalytic and Ca(2+) transport domains. The apparent cooperative unit, defined by the van 't Hoff enthalpy to the overall unfolding enthalpy ratio, increased from 1.1 at pH 6 to 1.8 at pH 8, showing that monomer-monomer interactions are stronger at weakly basic pH than at weakly acidic pH. While micromolar Ca(2+) concentrations had only a weak effect on the cooperativity of the unfolding process, this is clearly increased by millimolar Mg(2+)-ADP. In addition, high ionic strength lowered the apparent cooperative unit to approximately 1.0 in the pH range 6-8. Taken together, these results suggest that protein-protein interactions are altered by variables that modulate the catalytic activity of this enzyme.

Microdomain arrangement of the SERCA-type Ca2+ pump (Ca2+-ATPase) in subplasmalemmal calcium stores of paramecium cells

We localized SERCA pumps to the inner region of alveolar sac membranes, facing the cell interior, by combining ultrastructural and biochemical methods. Immunogold labeling largely predominated in the inner alveolar sac region which displayed aggregates of intramembrane particles (IMPs). On image analysis, these represented oligomeric arrangements of approximately 8-nm large IMP subunits, suggesting formation of SERCA aggregates (as known from sarcoplasmic reticulum). We found not only monomers of typical molecular size ( approximately 106 kD) but also oligomeric forms on Western blots (using anti-SERCA antibodies, also against endogenous SERCA from alveolar sacs) and on electrophoresis gelautoradiographs of 32P-labeled phosphoenzyme intermediates. Selective enrichment of SERCA-pump molecules in the inner alveolar sac membrane region may eliminate Ca2+ after centripetal spread observed during exocytosis activation, while the plasmalemmal Ca2+ pump may maintain or reestablish [Ca2+] in the narrow subplasmalemmal space between the outer alveolar sac membrane region and the cell membrane. We show for the first time the microzonal arrangement of SERCA molecules in a Ca2+ store of a secretory system, an intensely discussed issue in stimulus-secretion coupling research.

In vivo aging of rat skeletal muscle sarcoplasmic reticulum Ca-ATPase. Chemical analysis and quantitative simulation by exposure to low levels of peroxyl radicals

Sarcoplasmic reticulum (SR) Ca-ATPase of young adult (5 months) and aged (28 months) Fischer 344 male rat skeletal muscle was analyzed for posttranslational modifications as a result of biological aging and their potential functional consequences. The significant differences in the amino acid composition were a 6.8% lower content of sulfhydryl groups and a ca. 4% lower content of Arg residues of the Ca-ATPase from old as compared to young rats. Based on a total of 24 Cys residues the difference in protein thiols corresponds to a loss of 1.5 mol Cys/mol Ca-ATPase as a result of in vivo aging. The loss of Cys residues was not accompanied by a loss of enzyme activity though the 'aged' Ca-ATPase was more sensitive to heat inactivation, aggregation, and tryptic digestion. A comparison of the total sulfhydryl content of all SR proteins present revealed a 13% lower amount for SR vesicles isolated from aged rats. Compared to the alterations of Cys and Arg, there was only a slight and probably physiologically insignificant increase of protein carbonyls with aging, i.e. from 0.32 to 0.46 mol carbonyl groups per mol of Ca-ATPase. When SR vesicles from young rats were exposed to AAPH-derived peroxyl radicals, there was a loss of ca. 1.38 x 10(-4) M total SR sulfhydryl groups per 4 mg SR protein/ml (corresponding to ca. 25%) and a loss of 9.6 x 10(-5) M Ca-ATPase sulfhydryl groups (corresponding to ca. 31%) per 1.6 x 10(-5) M initiating peroxyl radicals, indicating that the stoichiometry of sulfhydryl oxidation was > or = 6 oxidized thiols per initiating AAPH-derived peroxyl radical. Besides Cys, the exposure to AAPH-derived radicals caused a slight loss of Ca-ATPase Arg, Met, and Ser residues. Most importantly, the SR Ca-ATPase exposed to this low concentration of peroxyl radicals displayed physical and functional properties quantitatively comparable to those of SR Ca-ATPase isolated from aged rats, i.e. no immediate loss of activity, increased susceptibility to heat inactivation, aggregation, and tryptic digestion. Moreover, a comparison of kinetically early tryptic fragments by HPLC-electrospray MS and N-terminal sequencing revealed that similar peptide fragments were produced from 'aged' and AAPH-oxidized Ca-ATPase which were not (or kinetically significantly later) generated from the 'young' Ca-ATPase, suggesting some conformational changes of the Ca-ATPase as a result of aging and AAPH-exposure. All except one of these peptides originated from locations remote from the nucleotide-binding and calcium-binding sites. The latter results suggest that aging and AAPH-exposure may target similar Cys residues, mainly at locations remote from the nucleotide-binding and calcium-binding sites, rationalizing the fact that Cys oxidation did not immediately cause inactivation of the Ca-ATPase. Our results provide a quantitative estimate of a net concentration of reactive oxygen species, here peroxyl radicals, which induces physical and chemical alterations of the SR Ca-ATPase quantitatively comparable to those induced by in vivo aging.

Preservation of the native structure and function of Ca2+-ATPase from sarcoplasmic reticulum: solubilization and reconstitution by new short-chain phospholipid detergent 1,2-diheptanoyl-sn-phosphatidylcholine

The properties of Ca2+-ATPase purified and reconstituted from rabbit skeletal sarcoplasmic reticulum (SR) has been studied in comparison with the preparations obtained by the commonly used detergent poly(oxyethylene)8-lauryl ether (C12E8) and the bile salt detergents cholate and deoxycholate. 1,2-Diheptanoyl-sn-phosphatidylcholine (DHPC) has been shown to be excellent for solubilizing a wide variety of membrane proteins [Kessi, Poiree, Wehrli, Bachofen, Semenza and Hauser (1994) Biochemistry 33, 10825-10836]. The DHPC method consistently gave higher yields of purified Ca2+-ATPase with a greater specific activity than the methods with C12E8, cholate, or deoxycholate. DHPC and C12E8 were superior to cholate and deoxycholate in active enzyme yields and specific activity. DHPC-solubilized Ca2+-ATPase purified on a density gradient retained the E1Ca-E1(*)Ca conformational transition, whereas the enzyme from the C12E8 purification did not retain this transition. The coupling of Ca2+ transported to ATP hydrolysed in the DHPC-purified enzyme was maximal and matched the values obtained with native SR, whereas the coupling was much lower for the C12E8-purified enzyme. The specific activity of Ca2+-ATPase reconstituted into dioleoylphosphatidylcholine vesicles with DHPC was up to 2-fold greater than that achieved with C12E8, and is comparable to that measured in the native SR. Finally, the dissociation of Ca2+-ATPase into monomers by DHPC preserved the ATPase activity, whereas similar dissociation by C12E8 gave only one-sixth the activity of that obtained with DHPC. These studies show that the Ca2+-ATPase solubilized, purified and reconstituted with DHPC is superior to that obtained with C12E8 in significant ways, making it a preparation suitable for detailed studies on the mechanism of ion transport and the role of protein-lipid interactions in the function of membrane proteins.

Negative or positive cooperation in calcium binding to detergent-solubilized ATPase of the sarcoplasmic reticulum. Its modulation by a high concentration of ATP

Two different conformations of chemically equivalent Ca(2+)-ATPase molecules in the sarcoplasmic reticulum have been shown to non- and positive cooperatively bind two calcium ions, respectively (Nakamura, J. (1994) J. Biol. Chem. 269, 30822-30827). At pH 7.40, these ATPase molecules split into E1 (high affinity state for calcium), and E2 (low affinity state for calcium), respectively, before calcium binding. At this pH, calcium binding to the monomeric ATPase, solubilized with dodecyloctaethylenglycol monoether, was studied by examining 45Ca2+ binding to the ATPase and calcium dependence of its phosphorylation, fluorescence intensity, ATP-hydrolysis at a low (5 microM) concentration of ATP, and acetyl phosphate hydrolysis. The results suggest that the solubilized ATPase molecules predominantly preexist in E2 and negative cooperatively (the Hill value (nH) = 0.5-0.6) bind 2 mol of calcium/mol of the ATPase with an apparent calcium affinity (K0.5) of 3-5 microM. The nonequivalences of calcium bindings at the membranous ATPase molecules seem to result from the intermolecular interaction of the molecules. A high concentration (5 mM) of ATP modulated the binding manner so that it became positively cooperative (nH approximately 2) and increased the K0.5 to 0.1 microM.

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.

Reconstitution of the sarcoplasmic reticulum Ca(2+)-ATPase: mechanisms of membrane protein insertion into liposomes during reconstitution procedures involving the use of detergents

The Ca(2+)-ATPase from skeletal muscle sarcoplasmic reticulum was reconstituted into sealed phospholipid vesicles using the method recently developed for bacteriorhodopsin (Rigaud, J.L., Paternostre, M.T. and Bluzat, A. (1988) Biochemistry 27, 2677-2688). Liposomes prepared by reverse-phase evaporation were treated with various amounts of Triton X-100, octyl glucoside, sodium cholate or dodecyl octa(oxyethylene) glycol ether (C12E8) and protein incorporation was studied at each step of the liposome solubilization process by each of these detergents. After detergent removal by SM-2 Bio-Beads the resulting vesicles were analyzed with respect to protein incorporation by freeze-fracture electron microscopy, sucrose density gradients and Ca2+ pumping measurements. The nature of the detergent used for reconstitution proved to be important for determining the mechanism of protein insertion. With octyl glucoside, direct incorporation of Ca(2+)-ATPase into preformed liposomes destabilized by saturating levels of this detergent was observed and gave proteoliposomes homogeneous in regard to protein distribution. With the other detergents, optimal Ca(2+)-ATPase pumping activities were obtained when starting from Ca(2+)-ATPase/detergent/phospholipid micellar solutions. However, the homogeneity of the resulting recombinants was shown to be dependent upon the detergent used and in the presence of Triton X-100 or C12E8 different populations were clearly evidenced. It was further demonstrated that the rate of detergent removal drastically influenced the composition of resulting proteoliposomes: upon slow detergent removal from samples solubilized with Triton X-100 or C12E8, Ca(2+)-ATPase was found seggregated and/or aggregated in very few liposomes while upon rapid detergent removal compositionally homogeneous proteoliposomes were obtained with high Ca2+ pumping activities. The reconstitution process was further analyzed by centrifugation experiments and the results demonstrated that the different mechanisms of reconstitution were driven predominantly by the tendency for self-aggregation of the Ca(2+)-ATPase. A model for Ca(2+)-ATPase reconstitution was proposed which accounted for all our results. In summary, the advantage of the systematic studies reported in this paper was to allow a rapid and easy determination of the experimental conditions for optimal detergent-mediated reconstitution of Ca(2+)-ATPase. Proteoliposomes prepared by the present simple method exhibited the highest Ca2+ pumping activities reported to date in Ca(2+)-ATPase reconstitution experiments performed in the absence of Ca2+ precipitating agents.

The Na,K-ATPase

The energy dependent exchange of cytoplasmic Na+ for extracellular K+ in mammalian cells is due to a membrane bound enzyme system, the Na,K-ATPase. The exchange sustains a gradient for Na+ into and for K+ out of the cell, and this is used as an energy source for creation of the membrane potential, for its de- and repolarisation, for regulation of cytoplasmic ionic composition and for transepithelial transport. The Na,K-ATPase consists of two membrane spanning polypeptides, an alpha-subunit of 112-kD and a beta-subunit, which is a glycoprotein of 35-kD. The catalytic properties are associated with the alpha-subunit, which has the binding domain for ATP and the cations. In the review, attention will be given to the biochemical characterization of the reaction mechanism underlying the coupling between hydrolysis of the substate ATP and transport of Na+ and K+.

Identification of H+/K(+)-ATPase alpha,beta-heterodimers

Glutaraldehyde treatment of the C12E8 solubilized H+/K(+)-ATPase crosslinks the catalytic subunit with an apparent molecular mass of 94 kDa in SDS polyacrylamide gels into two Coomassie stained particles migrating at approx. 147 and 173 kDa. The subunit composition of these particles was determined from the comparative distribution of FITC fluorescence, wheat germ agglutinin and anti-beta antibody reactivity in control and crosslinked preparations. FITC exclusively labelled the catalytic monomer of the native preparation and its fluorescence was initially distributed into two broad bands centered at approx. 147 and 173 kDa after crosslinking. These fluorescent bands coincided with the Coomassie stained particles. A glycoprotein(s) detected by wheat germ agglutinin reactivity was present in diffuse areas between 65 and 86 kDa and 95 to 134 kDa in the control preparation. This area was also labelled by the anti-beta antibodies. With crosslinking, the distribution of the wheat germ agglutinin reactive protein and anti-beta antibodies coincided with the crosslinked particles labelled by FITC. The presence of both the catalytic monomer and the beta subunit glycoprotein in the crosslinked particles indicated that these proteins were closely associated in the C12E8 solution. This suggests that the minimal structural particle of the H+/K(+)-ATPase is an alpha,beta-heterodimer.

Long-term stabilization and crystallization of (Ca2+ + Mg2+)-ATPase of detergent-solubilized erythrocyte plasma membrane

Conditions which were optimal for the stabilization of Ca2(+)-transporting ATPase in solubilized sarcoplasmic reticulum membranes (Pikułla, S., Mullner, N., Dux, L. and Martonosi, A. (1988) J. Biol. Chem. 263, 5277-5286) were also found conducive for preservation of (Ca2+ + Mg2+)-ATPase activity in detergent-solubilized erythrocyte plasma membrane for up to 60 days. Of particular importance for the stabilization of calmodulin-stimulated Ca2(+)-dependent activity of (Ca2+ + Mg2+)-ATPase of solubilized erythrocyte plasma membrane was the presence of Ca2+ (10-20 mM), glycerol, anti-oxidants, proteinase inhibitors and appropriate detergents. Among eight detergents tested octaethylene glycol dodecyl ether, polyoxyethylene glycol(10) lauryl alcohol and polydocanol were found to be promotive in long-term preservation of the enzyme activity. Under these conditions (Ca2+ + Mg2+)-ATPase of erythrocyte ghosts became highly stable and developed microcrystalline arrays after storage for 35 days. Electron micrographs of the negatively stained and thin sectioned material indicated that crystals of purified, detergent-solubilized, lipid-stabilized erythrocyte (Ca2+ + Mg2+)-ATPase differ from those of Ca2(+)-ATPase of detergent-solubilized sarcoplasmic reticulum microsomes.

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.

Glutaraldehyde crosslinking analysis of the C12E8 solubilized H,K-ATPase

A soluble porcine H,K-ATPase preparation was obtained with the nonionic detergent, C12E8. ATP hydrolysis by the soluble H,K-ATPase was stimulated with respect to the native preparation at pH 6.1, while the K(+)-phosphatase activity was comparable to the native enzyme. The soluble enzyme demonstrated characteristic ligand-dependent effects on ATP hydrolysis, including ATP activation of K(+)-stimulated hydrolysis with a K0.5 of 28 +/- 4 microM ATP, and inhibition with an IC50 of 2.1 mM ATP. The activation and inhibition of ATP hydrolysis by K+ was also observed with a K0.5 for activation of 2.8 +/- 0.4 mM KCl at 2.0 mM ATP (pH 6.1) and inhibition with an IC50 of 135 mM KCl at 0.05 mM ATP. 2-Methyl-8-(phenylmethoxy)imidazo[1,2a]pyridine-3-acetonitrile (SCH 28080), a specific inhibitor of the native H,K-ATPase, competitively inhibited the K(+)-stimulated activity with a Ki of 0.035 microM. The soluble enzyme was stable with a t0.5 for ATPase activity of 6 h between 4 and 11 degrees C. The demonstration of these related ligand responses in the catalytic reactions of the soluble preparation indicates that it is an appropriate medium for investigation of the subunit associations of the functional H,K-ATPase. Subunit associations of the active soluble enzyme were assessed following treatment with the crosslinking reagent, glutaraldehyde. The distribution of crosslinked particles was independent of the soluble protein concentration in the crosslinking buffer within the protein range 0.3 to 2.0 mg/ml or the detergent to protein ratio varied from 1 to 15 (w/w). The crosslinked pattern was unaffected by the presence or absence of K during crosslinking or nucleotide concentration. These observations suggest that crosslinking occurs in associated subunits that do not undergo rapid associations dependent upon enzyme turnover. Phosphorylation of the soluble enzyme with 0.1 mM MgATP produced a phosphoprotein at 94 kDa. A phosphoprotein obtained after glutaraldehyde treatment exhibited identical electrophoretic mobility to the crosslinked particle identified by silver stain. Glutaraldehyde treatment of soluble protein fractions resolved on a linear 10-35% glycerol gradient revealed several smaller peptides partially resolved from the crosslinked pump particle, but no active fraction enriched in the monomeric H,K-ATPase. This data indicates that the functional porcine gastric H,K-ATPase is organized as a structural dimer.

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.

Size exclusion chromatography and universal calibration of gel columns

We have tested the proposal (M. Potschka (1987) Anal. Biochem. 162, 47-64) that the elution position of macromolecules by gel chromatography is better correlated with the viscosity-based Stokes radius (R eta) than with the Stokes radius (RS) calculated from the frictional coefficient. By the use of different gel matrices (agarose, Sephadex, TSK silica gel columns) we found that the elution of dextran fractions and reduced proteins, denatured with guanidinium hydrochloride, agreed with their R eta, using water-soluble, globular proteins for gel calibration. However, the elution of large sodium dodecyl sulfate-protein complexes and other elongated macromolecules (fibrinogen, tropomyosin) was systematically retarded while polyethylene glycol and polyethylene glycol detergents eluted earlier than water-soluble, globular protein as a function of R eta. The same was the case for bacteriorhodopsin, solubilized by C12E8 or Triton X-100. It is concluded that for steric reasons size exclusion chromatography is more sensitive than hydrodynamic measurements to the detailed conformation of macromolecules (rods and random coils) and that for this reason gels with inert pores cannot be universally calibrated for all kinds of macromolecules.

Radiation inactivation analysis of sarcoplasmic reticulum Ca-ATPase in membrane-bound form and in detergent-solubilized monomeric states

The sarcoplasmic reticulum Ca-ATPase was subjected to target size analysis by radiation inactivation in various buffer conditions and after solubilization in monomeric form in non-ionic detergent and in SDS. The target size was also determined for Ca-ATPase in bidimensional crystals formed in the presence of decavanadate or lanthanide. The standardization obtained with defined monomers of Ca-ATPase shows that the target size of Ca-ATPase in the functional membrane-bound state may be ascribed to a single peptide chain, possibly with surrounding lipid. Further analysis of the radiation inactivation sizes of various partial reactions of the pump cycle, including phosphorylation and Ca2+ occlusion, indicated much smaller values than the target size pertaining to decomposition of the whole peptide chain. This is consistent with the existence of separate functional domains within a single peptide chain.

Use of a small TSK GSW high-performance liquid chromatographic column for large-zone chromatographic studies of monomer-oligomer equilibria of membrane protein

A TSK GSWP size-exclusion high-performance liquid chromatography "pre-column" (75 mm x 7.5 mm) was used for large-zone chromatographic studies of the effect of ligands on the monomer-oligomer equilibrium in sarcoplasmic reticulum Ca-ATPase solubilized in a non-ionic detergent. The monomer-dimer association constants determined for the Ca2+-occluded E1 P[Ca2] and vanadate-reacted E2V forms were 2-3 fold lower than the association constant of the E1Ca2 form (bound Ca2+ is freely exchangeable). With the TSK GSWP column, large-zone chromatography was rapid and required only small quantities of protein. This column was found to be useful also for studies in the presence of phospholipid, provided that the influence of lipid binding, per se, on the partition coefficient of the protein in the column was taken into consideration. This opens new possibilities for studies of membrane proteins under non-denaturing conditions.

Effect of phospholipid, detergent and protein-protein interaction on stability and phosphoenzyme isomerization of soluble sarcoplasmic reticulum Ca-ATPase

The purpose of the present study was to elucidate the separate roles of lipid, detergent and protein-protein interaction for stability and catalytic properties of sarcoplasmic reticulum Ca-ATPase solubilized in the non-ionic detergent octa(ethylene glycol) monododecyl ether (C12E8). The use of large-zone high-performance liquid chromatography permitted us to define the self-association state of Ca-ATPase peptide at various detergent, phospholipid and protein concentrations, and also during enzymatic turnover with ATP. Conditions were established for monomerization of Ca-ATPase in the presence of a high concentration of phospholipid relative to detergent. The lipid-saturated monomeric preparation was relatively resistant to inactivation in the absence of Ca2+, whereas delipidated enzyme in monomeric or in oligomeric form was prone to inactivation. Kinetics of phosphoenzyme turnover were examined in the presence and absence of Mg2+. Dephosphorylation rates were sensitive to Mg2+, irrespective of whether the peptide was present in soluble monomeric form or was membrane-bound. C12E8-solubilized monomer without added phospholipid was, however, characterized by a fast initial phase of dephosphorylation in the absence of Mg2+. This was not observed with monomer saturated with phospholipid or with monomer solubilized in myristoylglycerophosphocholine or deoxycholate. The mechanism underlying this difference was shown to be a C12E8-induced acceleration of conversion of ADP-sensitive phosphoenzyme (E1P) to ADP-insensitive phosphoenzyme (E2P). The phosphoenzyme isomerization rate was also found to be enhanced by low-affinity binding of ATP. This was demonstrated both in membrane-bound and in soluble monomeric Ca-ATPase. Our results indicate that a single peptide chain constitutes the target for modulation of phosphoenzyme turnover by Mg2+ and ATP, and that detergent effects, distinct from those arising from disruption of protein-protein contacts, are the major determinants of kinetic differences between C12E8-solubilized and membrane-bound enzyme preparations.

State of aggregation of the (Ca2+ + Mg2+)-ATPase studied using chemical cross-linking

We have studied cross-linking of the (Ca2+ + Mg2+)-ATPase in sarcoplasmic reticulum and in reconstituted systems, using glutaraldehyde, cupric-1,10-phenanthroline and 3,3'-dithiobis (sulphosuccinimidylpropionate). All reagents produce extensive cross-linking, forming aggregates too large to enter polyacrylamide gels. Only traces of cross-linked dimeric ATPase species are formed. Saturation transfer electron spin resonance spectra of spin-labelled sarcoplasmic reticulum cross-linked with glutaraldehyde are also consistent with the formation of extensively cross-linked aggregates in the membrane. The results are interpreted in terms of dynamic clusters of ATPase molecules in the membrane, probably in the form of rows of ATPase molecules.

Column liquid chromatography of integral membrane proteins

Biological membranes have as a major function the compartmentation of biological processes in cells and organelles. They consist of a bilayer of phospholipid molecules in which proteins are embedded. These integral membrane proteins, which cross the bilayer once or several times, generally have a higher than average hydrophobicity and tend to aggregate. Detergents are needed to remove integral membrane proteins from the lipid bilayer and they have to be present during further chromatographic purification. Predominantly, four modes of HPLC have been used alone or in combination for the purification of integral membrane proteins. These are based on differences of proteins in size (size-exclusion chromatography, SEC), electrostatic interaction (ion-exchange chromatography, IEC), bioaffinity (bioaffinity chromatography, BAC) and hydrophobic interaction (reversed-phase chromatography, RPC, and hydrophobic-interaction chromatography, HIC). SEC, IEC, BAC and HIC are used under relatively mild conditions, and buffer systems generally contain a non-ionic detergent. RPC generally has a denaturing effect on the protein and should preferably be used for the purification of integral membrane proteins smaller than 50 kD.

Characterization of CrATP-induced calcium occlusion in membrane-bound and soluble monomeric sarcoplasmic reticulum Ca2+-ATPase

Occlusion of Ca2+ induced by beta, gamma-bidentate CrATP in membrane bound and in soluble monomeric sarcoplasmic reticulum Ca2+-ATPase was studied by previously developed filtration and HPLC techniques (Vilsen and Andersen (1986) Biochim. Biophys. Acta 855, 429-431). Activation of Ca2+ occlusion occurred at micromolar free Ca2+ and depended on the concentration of Ca2+, H+ and Mg2+ in a similar way as activation of Ca2+ transport and equilibrium Ca2+ binding to high-affinity Ca2+ transport sites. The slopes of the Ca2+ titration curves indicated that Ca2+ binding is a cooperative process both in membraneous and in soluble monomeric enzyme. At alkaline pH and absence of Mg2+, occlusion of Ca2+ was inhibited by 1 mM Ca2+ in membrane-bound, but not in soluble monomeric Ca2+-ATPase. Parallel studies of phosphorylation from [gamma-32P]CrATP indicated a stoichiometry of 2 mol Ca2+ occluded per mol Ca2+-dependent EP formed, at saturating as well as at desaturating Ca2+ concentrations. Tryptic digestion of the CrATP induced Ca2+ occluded complex indicated that it belongs to the E1 conformational class (E1P). In the absence of Ca2+ and Mg2+, but presence of CrATP the conformational state was E2. When Mg2+ was added together with CrATP at alkaline pH the conformation was shifted in direction of E1.