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

Functional reconstitution of the HIV receptors CCR5 and CD4 in liposomes

Reconstitution of membrane proteins allows their study in a membrane environment that can be manipulated at will. Because membrane proteins have diverse biophysical properties, reconstitution methods have so far been developed for individual proteins on an ad hoc basis. We developed a postinsertion reconstitution method for CCR5, a G protein coupled receptor, with seven transmembrane alpha helices and small ecto- and endodomains. A His6-tagged version of CCR5 was expressed in mammalian cells, purified using the detergent N-dodecyl-beta-d-maltoside (DDM) and reconstituted into preformed liposomal membranes saturated with DDM, removing the detergent with hydrophobic polystyrene beads. We then attempted to incorporate CD4, a protein with a single transmembrane helix and a large hydrophilic ectodomain into liposomal membranes, together with CCR5. Surprisingly, reconstitution of this protein was also achieved by the method. Both proteins were found to be present together in individual liposomes. The reconstituted CCR5 was recognized by several monoclonal antibodies, recognized its natural ligand, and CD4 bound a soluble form of gp120, a subunit of the HIV fusion protein that uses CD4 as a receptor. Moreover, cells expressing the entire fusion protein of HIV bound to the liposomes, indicating that the proteins were intact and that most of them were oriented right side out. Thus, functional coreconstitution of two widely different proteins can be achieved by this method, suggesting that it might be useful for other proteins.

Membrane proteins: functional and structural studies using reconstituted proteoliposomes and 2-D crystals

Reconstitution of membrane proteins into lipid bilayers is a powerful tool to analyze functional as well as structural areas of membrane protein research. First, the proper incorporation of a purified membrane protein into closed lipid vesicles, to produce proteoliposomes, allows the investigation of transport and/or catalytic properties of any membrane protein without interference by other membrane components. Second, the incorporation of a large amount of membrane proteins into lipid bilayers to grow crystals confined to two dimensions has recently opened a new way to solve their structure at high resolution using electron crystallography. However, reconstitution of membrane proteins into functional proteoliposomes or 2-D crystallization has been an empirical domain, which has been viewed for a long time more like "black magic" than science. Nevertheless, in the last ten years, important progress has been made in acquiring knowledge of lipid-protein-detergent interactions and has permitted to build upon a set of basic principles that has limited the empirical approach of reconstitution experiments. Reconstitution strategies have been improved and new strategies have been developed, facilitating the success rate of proteoliposome formation and 2-D crystallization. This review deals with the various strategies available to obtain proteoliposomes and 2-D crystals from detergent-solubilized proteins. It gives an overview of the methods that have been applied, which may be of help for reconstituting more proteins into lipid bilayers in a form suitable for functional studies at the molecular level and for high-resolution structural analysis.

The amyloid precursor protein interacts with neutral lipids

The amyloid protein precursor (APP) was incorporated into liposomes or phospholipid monolayers. APP insertion into liposomes required neutral lipids, such as L-alpha-phosphatidylcholine, in the target membrane. It was prevented in vesicles containing L-alpha-phosphatidylserine. The insertion was enhanced in acidic solutions, suggesting that it is modulated by specific charge/charge interactions. Surface-active properties and behaviour of APP were characterized during insertion of the protein in monomolecular films of L-alpha-phosphatidylcholine, L-alpha-phosphatidylethanolamine or L-alpha-phosphatidylserine. The presence of the lipid film enhanced the rate of adsorption of the protein at the interface, and the increase in surface pressure was consistent with APP penetrating the lipid film. The adsorption of APP on the lipid monolayers displayed a significant head group dependency, suggesting that the changes in surface pressure produced by the protein were probably affected by electrostatic interactions with the lipid layers. Our results indicate that the penetration of the protein into the lipid monolayer is also influenced by the hydrophobic interactions between APP and the lipid. CD spectra showed that a large proportion of the alpha-helical secondary structure of APP remained preserved over the pH or ionic strength ranges used. Our findings suggest that APP/membrane interactions are mediated by the lipid composition and depend on both electrostatic and hydrophobic effects, and that the variations observed are not due to major secondary structural changes in APP. These observations may be related to the partitioning of APP into membrane microdomains.

Sarcolipin uncouples hydrolysis of ATP from accumulation of Ca2+ by the Ca2+-ATPase of skeletal-muscle sarcoplasmic reticulum

Sarcolipin (SLN) is a small peptide found in the sarcoplasmic reticulum of skeletal muscle. It is predicted to contain a single hydrophobic transmembrane alpha-helix. Fluorescence emission spectra for the single Trp residue of SLN suggest that SLN incorporates fully into bilayers of dioleoylphosphatidylcholine, but only partially into bilayers of phosphatidylcholines with long (C(22) or C(24)) fatty acyl chains. The fluorescence of SLN is quenched in bilayers of dibromostearoylphosphatidylcholine, also consistent with incorporation into the lipid bilayer. SLN was reconstituted with the Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum. Even at a 50:1 molar ratio of SLN/ATPase, SLN had no significant effect on the rate of ATP hydrolysis by the ATPase or on the Ca(2+)-dependence of ATP hydrolysis. However, at a molar ratio of SLN/ATPase of 2:1 or higher the presence of SLN resulted in a marked decrease in the level of accumulation of Ca(2+) by reconstituted vesicles. The effect of SLN was structurally specific and did not result from a breakdown in the vesicular structure or from the formation of non-specific ion channels. Vesicles were impermeable to Ca(2+) in the absence of ATP in the external medium. The effects of SLN on accumulation of Ca(2+) can be simulated assuming that SLN increases the rate of slippage on the ATPase and the rate of passive leak of Ca(2+) mediated by the ATPase. It is suggested that the presence of SLN could be important in non-shivering thermogenesis, a process in which heat is generated by hydrolysis of ATP by skeletal-muscle sarcoplasmic reticulum.

Nucleotide-binding sites in the functional unit of sarcoplasmic reticulum Ca2+-ATPase as studied by photoaffinity spin-labeled 2-N3-SL-ATP

2-N3-SL-ATP [2-azido-2',3'-O-(1-oxyl-2,2,5,5-tetramethyl-3-carbonyl-pyrroline) adenosine triphosphate], a photoaffinity spin-labeled derivative of ATP with a nitroxide moiety attached to the ribose ring and an azido group attached to C2 of the adenine ring, was used to study the nucleotide-binding site stoichiometry of sarcoplasmic reticulum (SR) Ca2+-ATPase. The label was shown to bind at the catalytic site of the enzyme, even though the rate of hydrolysis was poor. A maximal binding ratio of 1 mol/mol of ATPase was found. The ESR spectra showed signals from spin-spin interactions between two radicals corresponding to a distance of about 15 A between labels bound to adjacent sites on the enzyme. This indicates that the minimal functional unit of the Ca2+-ATPase is a dimer with the nucleotide-binding sites in close proximity.

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.

Cytochrome b(5) plays a key role in human microsomal chromium(VI) reduction

The reduction of chromium(VI) to Cr(III) results in the formation of reactive intermediates that contribute to the cytotoxicity, genotoxicity, and carcinogenicity of Cr(VI)-containing compounds. Previous studies suggest that human microsomal Cr(VI) reduction likely proceeds through cytochrome b(5). In order to better understand Cr(VI) toxicity in humans, the role of cytochrome b(5) in combination with P450 reductase was examined in the reductive transformation of Cr(VI). Proteoliposomes containing human recombinant cytochrome b(5) and P450 reductase were constructed. The ability of P450 reductase to mediate efficient electron transfer from NADPH to cytochrome b(5) was confirmed by spectral analysis. The NADPH-dependent Cr(VI) reduction rate mediated by proteoliposomes was then compared to that of human microsomes. When these rates were normalized to equivalent cytochrome b(5) concentrations, the NADPH-dependent Cr(VI) reduction rates mediated by human microsomes were essentially identical to those for proteoliposomes containing cytochrome b(5) plus P450 reductase. Proteoliposomes containing only P450 reductase or cytochrome b(5) exhibited poor Cr(VI) reducing capabilities. Since it had been previously shown that trace amounts of iron (Fe) could dramatically stimulate microsomal Cr(VI) reduction, the ability of Fe to stimulate Cr(VI) reduction by proteoliposomes was examined. Both ferric chloride (FeCl(3)) and ferric adenosine-5'-diphosphate (FeADP) were shown to stimulate Cr(VI) reduction; this stimulation could be abolished by the addition of deferoxamine, a specific Fe(III) chelator. The NADPH-dependent reduction rates of various ferric complexes by proteoliposomes were sufficient to account for the increased Cr(VI) reduction rates seen with the addition of FeCl(3) or FeADP. Cr(V) was detected by electron paramagnetic resonance (EPR) spectroscopy as a transient intermediate formed during NADPH-dependent Cr(VI) reduction mediated by proteoliposomes containing cytochrome b(5) and P450 reductase. Overall, cytochrome b(5) in combination with P450 reductase can account for the majority of the NADPH-dependent Cr(VI) reduction seen with human microsomes.

Vesicle reconstitution from lipid-detergent mixed micelles

The process of formation of lipid vesicles using the technique of detergent removal from mixed-micelles is examined. Recent studies on the solubilization and reconstitution of liposomes participated to our knowledge of the structure and properties of mixed lipid-detergent systems. The mechanisms involved in both the lipid self assembly and the micelle-vesicle transition are first reviewed. The simplistic three step minimum scheme is described and criticized in relation with isothermal as well as a function of the [det]/[lip] ratio, phase diagram explorations. The techniques of detergent elimination are reviewed and criticized for advantages and disadvantages. New methods inducing micelle-vesicle transition using enzymatic reaction and T-jump are also described and compared to more classical ones. Future developments of these techniques and improvements resulting of their combinations are also considered. Proper reconstitution of membrane constituents such as proteins and drugs into liposomes are examined in the light of our actual understanding of the micelle-vesicle transition.

Use of detergents in two-dimensional crystallization of membrane proteins

Structure determination at high resolution is actually a difficult challenge for membrane proteins and the number of membrane proteins that have been crystallized is still small and far behind that of soluble proteins. Because of their amphiphilic character, membrane proteins need to be isolated, purified and crystallized in detergent solutions. This makes it difficult to grow the well-ordered three-dimensional crystals that are required for high resolution structure analysis by X-ray crystallography. In this difficult context, growing crystals confined to two dimensions (2D crystals) and their structural analysis by electron crystallography has opened a new way to solve the structure of membrane proteins. However, 2D crystallization is one of the major bottlenecks in the structural studies of membrane proteins. Advances in our understanding of the interaction between proteins, lipids and detergents as well as development and improvement of new strategies will facilitate the success rate of 2D crystallization. This review deals with the various available strategies for obtaining 2D crystals from detergent-solubilized intrinsic membrane proteins. It gives an overview of the methods that have been applied and gives details and suggestions of the physical processes leading to the formation of the ordered arrays which may be of help for getting more proteins crystallized in a form suitable for high resolution structural analysis by electron crystallography.

Short-chain phospholipids as detergents

The physico-chemical properties of short-chain phosphatidylcholine are reviewed to the extent that its biological activity as a mild detergent can be rationalized. Long-chain diacylphosphatidylcholines are typical membrane phospholipids that form preferentially smectic lamellar phases (bilayers) when dispersed in water. In contrast, the preferred phase of the short-chain analogues dispersed in excess water is the micellar phase. The preferred conformation and the dynamics of short-chain phosphatidylcholines in the monomeric and micellar state present in H(2)O are discussed. The motionally averaged conformation of short-chain phosphatidylcholines is then compared to the single-crystal structures of membrane lipids. The main conclusion emerging is that in terms of preferred conformation and motional averaging short-chain phosphatidylcholines closely resemble their long-chain analogues. The dispersing power of short-chain phospholipids is emphasized in the second part of the review. Evidence is presented to show that this class of compounds is superior to most other detergents used in the solubilization of membrane proteins and the reconstitution of the solubilized proteins to artificial membrane systems (proteoliposomes). The prominent feature of the solubilization/reconstitution of integral membrane proteins by short-chain PC is the retention of the native protein structure and hence the protein function. Due to their special detergent-like properties, short-chain PC lend themselves very well not only to membrane solubilization but also to the purification of integral membrane proteins. The retention of the native protein structure in the solubilized state, i.e. in mixed micelles consisting of the integral membrane protein, intrinsic membrane lipids and short-chain PC, is rationalized. It is hypothesized that short-chain PC interacts primarily with the lipid bilayer of a membrane and very little if at all with the membrane proteins. In this way, the membrane protein remains associated with its preferred intrinsic membrane lipids and retains its native structure and its function.

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.

The importance of carboxyl groups on the lumenal side of the membrane for the function of the Ca(2+)-ATPase of sarcoplasmic reticulum

The conventional model for transport of Ca(2+) by the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR) involves a pair of binding sites for Ca(2+) that change upon phosphorylation of the ATPase from being high affinity and exposed to the cytoplasm to being low affinity and exposed to the lumen. However, a number of recent experiments suggest that in fact transport involves two separate pairs of binding sites for Ca(2+), one pair exposed to the cytoplasmic side and the other pair exposed to the lumenal side. Here we show that the carbodiimide 1-ethyl-3-[3-(dimethylamino)-propyl] carbodiimide (EDC) is membrane-impermeable, and we use EDC to distinguish between cytoplasmic and lumenal sites of reaction. Modification of the Ca(2+)-ATPase in sealed SR vesicles with EDC leads to loss of ATPase activity without modification of the pair of high affinity Ca(2+)-binding sites. Modification of the purified ATPase in unsealed membrane fragments was faster than modification in SR vesicles, suggesting the presence of more quickly reacting lumenal sites. This was confirmed in experiments measuring EDC modification of the ATPase reconstituted randomly into sealed lipid vesicles. Modification of sites on the lumenal face of the ATPase led to loss of the Ca(2+)-induced increase in phosphorylation by P(i). It is concluded that carboxyl groups on the lumenal side of the ATPase are involved in Ca(2+) binding to the lumenal side of the ATPase and that modification of these sites leads to loss of ATPase activity. The presence of MgATP or MgADP leads to faster inhibition of the ATPase by EDC in unsealed membrane fragments than in sealed vesicles, suggesting that binding of MgATP or MgADP to the ATPase leads to a conformational change on the lumenal side of the membrane.

Molecular interactions of peptides with phospholipid vesicle membranes as studied by fluorescence correlation spectroscopy

Interactions of the peptides melittin and magainin with phospholipid vesicle membranes have been studied using fluorescence correlation spectroscopy. Molecular interactions of melittin and magainin with phospholipid membranes are performed in rhodamine-entrapped vesicles (REV) and in rhodamine-labelled phospholipid vesicles (RLV), which did not entrap free rhodamine inside. The results demonstrate that melittin makes channels into vesicle membranes since exposure of melittin to vesicles causes rhodamine release only from REV but not from RLV. It is obvious that rhodamine can not be released from RLV because the inside of RLV is free of dye molecules. In contrast, magainin breaks vesicles since addition of magainin to vesicles results in rhodamine release from both REV and RLV. As the inside of RLV is free of rhodamine, the appearance of rhodamine in solution confirms that these vesicles are broken into rhodamine-labelled phospholipid fragments after addition of magainin. This study is of pharmaceutical significance since it will provide insights that fluorescence correlation spectroscopy can be used as a rapid protocol to test incorporation and release of drugs by vesicles.

Co-reconstitution of phospholamban mutants with the Ca-ATPase reveals dependence of inhibitory function on phospholamban structure

Phospholamban (PLB), a 52-amino acid integral membrane protein, regulates the Ca-ATPase (calcium pump) in cardiac sarcoplasmic reticulum through PLB phosphorylation mediated by beta-adrenergic stimulation. Based on site-directed mutagenesis and coexpression with Ca-ATPase (SERCA2a) in Sf21 insect cells or in HEK 293 cells, and on spin label detection of PLB oligomeric state in lipid bilayers, it has been proposed that the monomeric form of PLB is the inhibitory species, and depolymerization of PLB is essential for its regulatory function. Here we have studied the relationship between PLB oligomeric state and function by in vitro co-reconstitution of PLB and its mutants with purified Ca-ATPase. We compared wild type-PLB (wt-PLB), which is primarily a pentamer on SDS-polyacrylamide gel electrophoresis (PAGE) at 25 degrees C, with two of its mutants, C41L-PLB and L37A-PLB, that are primarily tetramer and monomer, respectively. We found that the monomeric mutant L37A-PLB is a more potent inhibitor than wt-PLB, supporting the previous proposal that PLB monomer is the inhibitory species. On the other hand, C41L-PLB, which has a monomeric fraction comparable to that of wt-PLB on SDS-PAGE at 25 degrees C, has no inhibitory activity when assayed at 25 degrees C. However, at 37 degrees C, a 3-fold increase in the monomeric fraction of C41L-PLB on SDS-PAGE resulted in inhibitory activity comparable to that of wt-PLB. Upon increasing the temperature from 25 to 37 degrees C, no change in fraction monomer or inhibitory activity for wt-PLB and L37A-PLB was observed. Based on these results, the extent of inhibition of Ca-ATPase by PLB or its mutants appears to depend not only on the propensity of PLB to dissociate into monomers but also on the relative potency of the particular PLB monomer when interacting with the Ca-ATPase.

Surfactants in membrane solubilisation

An understanding of the action of many drugs requires a knowledge of how the drug reaches the site of action in a cell. A detailed knowledge of the structure and function of cell membranes is often required to understand the transport of drugs across the plasma membrane. To obtain this information proteins must be isolated. The isolation and characterisation of cell membrane proteins usually requires the solubilisation of the membrane and a method of separation of the various membrane proteins and glycoproteins. The starting point for such an investigation is the choice of a suitable surfactant (detergent) to solubilise the membrane. This review considers the range of surfactants that are available for membrane solubilisation, how surfactants interact with membranes, the part they play in the separation of integral membrane proteins and in the reconstitution of membrane proteins for functional studies. The solubilisation of specific membrane proteins and glycoproteins including the human erythrocyte anion transporter, mitochondrial porin, sarcoplasmic reticulum Ca(2+)-ATPase, the ATPase-active multidrug transporter P-glycoprotein, bacteriorhodopsin and rhodopsin are also discussed.

How lipids interact with an intrinsic membrane protein: the case of the calcium pump

Ca2+-ATPase can be purified from the skeletal muscle of sarcoplasmic reticulum and reconstituted into phospholipid bilayers of defined composition. This allows a detailed study of the interactions between phospholipid molecules and the ATPase, and of the effects of phospholipid structure on the activity of the ATPase. It has been shown that the thickness of the lipid bilayer, its physical phase and the lipid headgroup structure can all be important. The interaction between phospholipids and the ATPase is not structurally specific in that the strength of the phospholipid-ATPase interaction does not depend on headgroup structure or on fatty acyl chain length, but the strength of binding is different for liquid crystalline and gel phase lipid. There are also 'specific' sites for some lipids on the ATPase. There is no unique mechanism explaining the effects of phospholipid on the function of the ATPase; the changes observed with any particular phospholipid follow from a distinct set of changes in the conformational state of the ATPase. The changes in activity are likely to follow from tilting of trans-membrane alpha-helices in the ATPase. In simple model systems it has been shown that the extent to which lipids can distort to match the protein is limited.

Co-reconstitution and co-crystallization of phospholamban and Ca(2+)-ATPase

Significant advances have recently been made in understanding the regulation of Ca(2+)-ATPase by phospholamban and in modeling their structures. However, these insights would be furthered by determining the 3-D structure of both proteins within the membrane, thus revealing the structural basis for their interaction. To this end, we have developed methods for reconstituting purified Ca(2+)-ATPase with recombinant phospholamban. After reconstitution at high lipid-to-protein ratios, we have verified their functional association by measuring calcium transport and ATPase activity. Furthermore, we have grown co-crystals after reconstitution at low lipid-to-protein ratios. The structure of Ca(2+)-ATPase has recently been solved by cryoelectron microscopy at 8-A resolution, thus revealing transmembrane alpha-helices. Using a variety of constraints, we have associated these helices with the predicted transmembrane sequences to produce a detailed model for the packing of transmembrane helices. Structure determination of the co-crystals is currently underway, which we hope will eventually reveal the interaction of phospholamban with Ca(2+)-ATPase at a similar level of detail.

Two-dimensional crystallization of Ca-ATPase by detergent removal

By using Bio-Beads as a detergent-removing agent, it has been possible to produce detergent-depleted two-dimensional crystals of purified Ca-ATPase. The crystallinity and morphology of these different crystals were analyzed by electron microscopy under different experimental conditions. A lipid-to-protein ratio below 0.4 w/w was required for crystal formation. The rate of detergent removal critically affected crystal morphology, and large multilamellar crystalline sheets or wide unilamellar tubes were generated upon slow or fast detergent removal, respectively. Electron crystallographic analysis indicated unit cell parameters of a = 159 A, b = 54 A, and gamma = 90 degrees for both types of crystals, and projection maps at 15-A resolution were consistent with Ca-ATPase molecules alternately facing the two sides of the membrane. Crystal formation was also affected by the protein conformation. Indeed, tubular and multilamellar crystals both required the presence of Ca2+; the presence of ADP gave rise to another type of packing within the unit cell (a = 86 A, b = 77 A, and gamma = 90 degrees), while maintaining a bipolar orientation of the molecules within the bilayer. All of the results are discussed in terms of nucleation and crystal growth, and a model of crystallogenesis is proposed that may be generally true for asymmetrical proteins with a large hydrophilic cytoplasmic domain.

Ca2+ transport by reconstituted synaptosomal ATPase is associated with H+ countertransport and net charge displacement

The synaptosomal plasma membrane Ca2+-ATPase (PMCA) purified from pig brain was reconstituted with liposomes prepared by reverse phase evaporation at a lipid to protein ratio of 150/1 (w/w). ATP-dependent Ca2+ uptake and H+ ejection by the reconstituted proteoliposomes were demonstrated by following light absorption and fluorescence changes undergone by arsenazo III and 8-hydroxy-1,3, 6-pyrene trisulfonate, respectively. Ca2+ uptake was increased up to 2-3-fold by the H+ ionophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone, consistent with relief of an inhibitory transmembrane pH gradient (i.e. lumenal alkalinization) generated by H+ countertransport. The stoichiometric ratio of Ca2+/H+ countertransport was 1.0/0.6, and the ATP/Ca2+ coupling stoichiometry was 1/1 at 25 degrees C. The electrogenic character of the Ca2+/H+ countertransport was demonstrated by measuring light absorption changes undergone by oxonol VI. It was shown that a 20 mV steady state potential (positive on the lumenal side) was formed as a consequence of net charge transfer associated with the 1/1 Ca2+/H+ countertransport. Calmodulin stimulated ATPase activity, Ca2+ uptake, and H+ ejection, demonstrating that these parameters are linked by the same mechanism of PMCA regulation.

A new "gel-like" phase in dodecyl maltoside-lipid mixtures: implications in solubilization and reconstitution studies

The interaction of dodecyl maltoside with lipids was investigated through the studies of solubilization and reconstitution processes. The solubilization of large unilamellar liposomes was analyzed through changes in turbidity and cryo-transmission electron microscopy. Solubilization was well described by the three-stage model previously reported for other detergents, and the critical detergent/phospholipid ratios at which lamellar-to-micellar transition occurred (Rsat = 1 mol/mol) and finished (Rsol = 1.6 mol/mol) were determined. The vesicle-micelle transition was further observed in the vitrified hydrated state by cryo-transmission electron microscopy. A striking feature of the solubilization process by dodecyl maltoside was the discovery of a new phase consisting of a very viscous "gel-like" sample. It is shown that this equilibrium cohesive phase is composed of long filamentous thread-like micelles, over microns in length. Similar structures were observed upon solubilization of sonicated liposomes, multilamellar liposomes, or biological Ca2+ ATPase membranes. This "gel-like" phase was also visualized during the process of liposome reconstitution after detergent removal from lipid-dodecyl maltoside micelles. The rate of detergent removal, controlled through the use of SM2 Bio-Beads, was demonstrated to drastically influence the morphology of reconstituted liposomes with a propensity for multilamellar liposome formation upon slow transition through the "gel-like" phase. Finally, on the basis of these observations, the mechanisms of dodecyl maltoside-mediated reconstitution of bacteriorhodopsin were analyzed, and optimal conditions for reconstitution were defined.

Generation of proteoliposomes from subcellular fractions

Intracellular membranes are highly dynamic, yet they retain their identity and functional characteristics. Integral membrane proteins, which must confer this specific membrane identity, remain poorly characterized at the biochemical level, largely because detergent-mediated solubilization is required for purification and analysis, and several properties of integral membrane proteins can only be investigated when the molecule is properly embedded in a lipid bilayer. We present a method for the efficient reconstitution into proteoliposomes of integral membrane proteins from subcellular fractions. Integral membrane proteins were identified on high-resolution two-dimensional gels after selective extraction of soluble and peripheral membrane proteins; they accounted for 8% of the number of resolved polypeptides. A reconstitution procedure based on membrane solubilization with dodecyl-octaoxyethylene (C12E8) and subsequent detergent removal with BioBeads SM-2 resulted in the efficient reconstitution of several membrane proteins into proteoliposomes of uniform density. The generated proteoliposomes strongly resemble the starting membrane fraction in protein composition. This reconstitution allows the functional characterization of integral membrane proteins after enrichment and/or specific (immuno)depletion.

Detergent-mediated reconstitution of a glycosyl-phosphatidylinositol-protein into liposomes

A three-step detergent-mediated reconstitution has been applied to the incorporation of a glycosyl-phosphatidylinositol-protein into liposomes. The protein studied was alkaline phosphatase from bovine intestine. Liposomes prepared by dialysis were treated with various amounts of two detergents, either n-octyl beta-D-glucoside or Triton X-100. At different steps of the solubilization process, protein was added and the detergent was removed by hydrophobic resins. The most efficient reconstitutions were obtained with an octyl glucoside concentration corresponding to the onset of liposome solubilization and with a Triton X-100 concentration leading to partial solubilization of the liposomes. The involvement of the glycosyl-phosphatidylinositol anchor in alkaline phosphatase reconstitution was demonstrated by the inability of phosphoinositol-specific phospholipase-C-hydrolysed alkaline phosphatase to incorporate into liposomes. Between 70-85% of the protein associated with liposomes were anchored in the outer leaflet of the bilayer, oriented towards the outside of the liposome. The remainder was trapped within the lumen of the liposomes.

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.

Expression and purification of the Saccharomyces cerevisiae alpha-factor receptor (Ste2p), a 7-transmembrane-segment G protein-coupled receptor

A plasmid vector was developed that permitted high-level expression of a functional form of the Saccharomyces cerevisiae alpha-factor receptor (the STE2 gene product) tagged at its C-terminal end with an epitope (FLAG) and a His6 tract. When expressed in yeast from this plasmid, Ste2p was produced at a level at least 3-fold higher than that reported previously for any other 7-transmembrane-segment receptor expressed in the same cells. For purification, isolated cell membranes containing the overexpressed receptor were solubilized with detergent under specific conditions and subjected to immobilized metal affinity chromatography. Yields as high as 1 mg of nearly homogeneous (95%) receptor were routinely obtained even from relatively small scale preparations (60 g of frozen cell paste). The purified receptor was reconstituted into artificial phospholipid vesicles. Radioligand binding studies demonstrated that the purified receptor, in the reconstituted vesicles, bound its tridecapeptide ligand (alpha-factor) with a KD (155 nM) consistent with the affinity expected for this receptor in the absence of its associated G protein. Efficient restoration of ligand binding activity upon reconstitution required the addition of solubilized membranes prepared from a yeast strain lacking the receptor. Sufficient amounts of active material can be obtained by this procedure to allow physical studies of this receptor and other 7-transmembrane-segment receptors expressed in this system.

How to make tubular crystals by reconstitution of detergent-solubilized Ca2(+)-ATPase

In an attempt to better define the parameters governing reconstitution and two-dimensional crystallization of membrane proteins, we have studied Ca2(+)-ATPase from rabbit sarcoplasmic reticulum. This ion pump forms vanadate-induced crystals in its native membrane and has previously been reconstituted at high lipid-to-protein ratios for functional studies. We have characterized the reconstitution of purified Ca2(+)-ATPase at low lipid-to-protein ratios and discovered procedures that produce long, tubular crystals suitable for helical reconstruction. C12E8 (n-dodecyl-octaethylene-glycol monoether) was used to fully solubilize various mixtures of lipid and purified Ca2(+)-ATPase, and BioBeads were then used to remove the C12E8. Slow removal resulted in two populations of vesicles, and the proteoliposome population was separated from the liposome population on a sucrose density gradient. These proteoliposomes had a lipid-to-protein ratio of 1:2, and virtually 100% of molecules faced the outside of vesicles, as determined by fluorescein isothiocyanate labeling. Cycles of freeze-thaw caused considerable aggregation of these proteoliposomes, and, if phosphatidyl ethanolamine and phosphatidic acid were included, or if the bilayers were doped with small amounts of C12E8, vanadate-induced tubular crystals grew from the aggregates. Thus our procedure comprised two steps-reconstitution followed by crystallization-allowing us to consider mechanisms of bilayer formation separately from those of crystallization and tube formation.

Comparison of the effects of phospholamban and jasmone on the calcium pump of cardiac sarcoplasmic reticulum. Evidence for modulation by phospholamban of both Ca2+ affinity and Vmax (Ca) of calcium transport

Regulation of the calcium pump of the cardiac sarcoplasmic reticulum by phosphorylation/dephosphorylation of phospholamban is central to the inotropic and lusitropic effects of beta-adrenergic agonists on the heart. In order to study the mechanism of this regulation, we first obtained purified ruthenium red-insensitive microsomes enriched in sarcoplasmic reticulum membranes. The kinetics of microsomal Ca2+ uptake after phospholamban phosphorylation or trypsin treatment, which cleaves the inhibitory cytoplasmic domain of phospholamban, were then compared with those in the presence of jasmone, whose effects on the kinetics of fast skeletal muscle Ca2+-ATPase are largely known. All three treatments increased Vmax (Ca) at 25 degrees C and millimolar ATP; phosphorylation and trypsin decreased the Km (Ca), while jasmone increased it. Trypsin and jasmone increased the rate of E2P decomposition 1.8- and 3. 0-fold, respectively. The effects of phospholamban phosphorylation and jasmone on the Ca2+-ATPase activity paralleled their effects on Ca2+ uptake. Our data demonstrate that phospholamban regulates E2P decomposition in addition to the known increase in the rate of a conformational change in the Ca2+-ATPase upon binding the first of two Ca2+. These steps in the catalytic cycle of the Ca2+-ATPase may contribute to or account for phospholamban's effects on both Vmax (Ca) and Km (Ca), whose relative magnitude may vary under different experimental and, presumably, physiological conditions.

Functional reconstitution of photosystem I reaction center from cyanobacterium Synechocystis sp PCC6803 into liposomes using a new reconstitution procedure

Photosystem I reaction center from the cyanobacterium Synechocystis sp PCC6803 was reconstituted into phosphatidylcholine/phosphatidic acid liposomes. Liposomes prepared by reversephase evaporation were treated with various amounts of different detergents and protein incorporation was analyzed at each step of the solubilization process. After detergent removal the activities of the resulting proteoliposomes were measured. The most efficient reconstitution was obtained by insertion of the protein complex into preformed liposomes destabilized by saturating amounts of octylglucoside. In the presence of N-methylphenazonium methosulfate and ascorbic acid, liposomes containing the reaction center catalyzed a light-dependent net H+ uptake as measured by the 9-aminoacridine fluorescence quenching and the pH meter. An important benefit of the new reconstitution procedure is that it produces a homogeneous population of large-size proteoliposomes with a low ionic permeability and with a majority inwardly directed H+ transport activity. In optimal conditions, a light-induced delta pH of about 1.8 units could be sustained at 20 degrees C in the presence of valinomycin. In the absence of valinomycin, a "back-pressure" effect of an electrical transmembrane potential decreased both the rate and the extent of the H+ transport. The reaction center was also co-reconstituted with F0F1 H(+)-ATPases from chloroplasts and from the thermophilic bacterium, PS3. The co-reconstituted system was shown to catalyze a light-dependent phosphorylation which could only be measured in the presence of a high concentration of PSI (low lipid/PSI ratios) while no delta pH could be detected.

Unidirectional reconstitution into detergent-destabilized liposomes of the purified lactose transport system of Streptococcus thermophilus

The lactose transport protein (LacS) of Streptococcus thermophilus was amplified to levels as high as 8 and 30% of total membrane protein in Escherichia coli and S. thermophilus, respectively. In both organisms the protein was functional and the expression levels were highest with the streptococcal lacS promoter. Also a LacS deletion mutant, lacking the carboxyl-terminal regulatory domain, could be amplified to levels >20% of membrane protein. Membranes from S. thermophilus proved to be superior in terms of efficient solubilization and ease and extent of purification of LacS; >95% of LacS was solubilized with relatively low concentrations of Triton X-100, n-octyl-beta-D-glucoside, n-dodecyl-beta-D-maltoside, or C12E8. The LacS protein carrying a poly-histidine tag was purified in large quantities (approximately 5 mg/liter of culture) and with a purity >98% in a two-step process involving nickel chelate affinity and anion exchange chromatography. The membrane reconstitution of LacS was studied systematically by stepwise solubilization of preformed liposomes, prepared from E. coli phospholipid and phosphatidylcholine, and protein incorporation at the different stages of liposome solubilization. The detergents were removed by adsorption onto polystyrene beads and H+-lactose symport and lactose counterflow were measured. Highest transport activities were obtained when Triton X-100 was used throughout the solubilization/purification procedure, whereas activity was lost irreversibly with n-octyl-beta-D-glucoside. For reconstitutions mediated by n-dodecyl-beta-D-maltoside, C12E8, and to a lesser extent Triton X-100, the highest transport activities were obtained when the liposomes were titrated with low amounts of detergent (onset of liposome solubilization). Importantly, under these conditions proteoliposomes were obtained in which LacS was reconstituted in an inside-out orientation, as suggested by the outside labeling of a single cysteine mutant with a membrane impermeable biotin-maleimide. The results are consistent with a mechanism of reconstitution in which the hydrophilic regions of LacS prevent a random insertion of the protein into the membrane. Consistent with the in vivo lactose/galactose exchange catalyzed by the LacS protein, the maximal rate of lactose counterflow was almost 2 orders of magnitude higher than that of H+-lactose symport.

Purification by Ni2+ affinity chromatography, and functional reconstitution of the transporter for N-acetylglucosamine of Escherichia coli

The N-acetyl-D-glucosamine transporter (IIGlcNAc) of the bacterial phosphotransferase system couples vectorial translocation to phosphorylation of the transported GlcNAc. IIGlcNAc of Escherichia coli containing a carboxyl-terminal affinity tag of six histidines was purified by Ni2+ chelate affinity chromatography. 4 mg of purified protein was obtained from 10 g (wet weight) of cells. Purified IIGlcNAc was reconstituted into phospholipid vesicles by detergent dialysis and freeze/thaw sonication. IIGlcNAc was oriented randomly in the vesicles as inferred from protein phosphorylation studies. Import and subsequent phosphorylation of GlcNAc were measured with proteoliposomes preloaded with enzyme I, histidine-containing phosphocarrier protein, and phosphoenolpyruvate. Uptake and phosphorylation occurred in a 1:1 ratio. Active extrusion of GlcNAc entrapped in vesicles was also measured by the addition of enzyme I, histidine-containing phosphocarrier protein, and phosphoenolpyruvate to the outside of the vesicles. The Km for vectorial phosphorylation and non-vectorial phosphorylation were 66. 6 +/- 8.2 microM and 750 +/- 19.6 microM, respectively. Non-vectorial phosphorylation was faster than vectorial phosphorylation with kcat 15.8 +/- 0.9 s-1 and 6.2 +/- 0.7 s-1, respectively. Using exactly the same conditions, the purified transporters for mannose (IIABMan, IICMan, IIDMan) and glucose (IICBGlc, IIAGlc) were also reconstituted for comparison. Although the vectorial transport activities of IICBAGlcNAc and IICBGlc. IIAGlc are inhibited by non-vectorial phosphorylation, no such effect was observed with the IIABMan.IICMan.IIDMan complex. This suggests that the molecular mechanisms underlying solute transport and phosphorylation are different for different transporters of the phosphotransferase system.

Purified, reconstituted cardiac Ca2+-ATPase is regulated by phospholamban but not by direct phosphorylation with Ca2+/calmodulin-dependent protein kinase

Regulation of calcium transport by sarcoplasmic reticulum provides increased cardiac contractility in response to beta-adrenergic stimulation. This is due to phosphorylation of phospholamban by cAMP-dependent protein kinase or by calcium/calmodulin-dependent protein kinase, which activates the calcium pump (Ca2+-ATPase). Recently, direct phosphorylation of Ca2+-ATPase by calcium/calmodulin-dependent protein kinase has been proposed to provide additional regulation. To investigate these effects in detail, we have purified Ca2+-ATPase from cardiac sarcoplasmic reticulum using affinity chromatography and reconstituted it with purified, recombinant phospholamban. The resulting proteoliposomes had high rates of calcium transport, which was tightly coupled to ATP hydrolysis (approximately 1.7 calcium ions transported per ATP molecule hydrolyzed). Co-reconstitution with phospholamban suppressed both calcium uptake and ATPase activities by approximately 50%, and this suppression was fully relieved by a phospholamban monoclonal antibody or by phosphorylation either with cAMP-dependent protein kinase or with calcium/calmodulin-dependent protein kinase. These effects were consistent with a change in the apparent calcium affinity of Ca2+-ATPase and not with a change in Vmax. Neither the purified, reconstituted cardiac Ca2+-ATPase nor the Ca2+-ATPase in longitudinal cardiac sarcoplasmic reticulum vesicles was a substrate for calcium/calmodulin-dependent protein kinase, and accordingly, we found no effect of calcium/calmodulin-dependent protein kinase phosphorylation on Vmax for calcium transport.

Identification of the ATP transporter of rat liver rough endoplasmic reticulum via photoaffinity labeling and partial purification

In order to identify the ATP transporter in rat liver rough endoplasmic reticulum (RER), a photoreactive azido derivative of ATP, 3'-O-(p-azidobenzoyl)-ATP (AB-ATP), was synthesized by the reaction of ATP with N-hydroxysuccinimido 4-azidobenzoate (NHS-AB). The activity of the ATP transporter was determined by measuring the influx of [8-14C]ATP. The ATP transport had an apparent Km value of 6.5 microM and a Vmax of 1 nmol min-1 (mg of protein)-1. The transport of ATP was specifically inhibited by AB-ATP and 4, 4'-diisothiocyanatostilbene-2', 2'-disulfonic acid (DIDS). Under a dim light, AB-ATP was a competitive inhibitor of the ATP transport with Ki value of 0.19 microM, which indicates that AB-ATP has a high affinity for the ATP transporter, so it can be utilized as a photoaffinity probe for the identification of the ATP transporter in rat liver RER. An SDS--PAGE analysis of RER vesicles photolabeled with [gamma-32P]AB-ATP indicates the presence of a 56-kDa protein. The 56-kDa protein was completely protected from photoaffinity labeling by 10 microM ATP but not by 30 microM GTP. The specific labeling of the 56-kDa protein was sensitive to the anion transport inhibitor DIDS. In order to confirm whether the apparent uptake of ATP was due to the 56-kDa protein, the ATP transporter was partially purified through two successive ion-exchange chromatography steps (DEAE and Mono-S). The fraction showing the high activity of the ATP transporter also contained the 56-kDa protein photolabeled with [gamma-32P]AB-ATP. On the basis of the photoaffinity labeling and reconstitution experiment, we conclude that the 56-kDa protein represents the ATP transporter in rat liver RER.

Changes in phosphatidylcholine liposomes caused by a mixture of Triton X-100 and sodium dodecyl sulfate

The mechanisms governing the interaction of equimolecular mixtures of Triton X-100 (Tx-100) and sodium dodecyl sulfate (SDS) with phosphatidylcholine liposomes were investigated. Permeability alterations were determined as a change in 5(6)-carboxyfluorescein released from the interior of vesicles and bilayer solubilization as a decrease in the static light-scattered by liposome suspensions. At subsolubilizing level, a maximum bilayer/water partitioning of surfactant mixture was reached at 30% CF release, which correlated with the increased presence of SDS in the bilayers. However, transition stages between 70% CF release and 100% light-scattering corresponded to the increased presence of Tx-100 in these structures. These findings may be correlated with the reduced deleterious effects caused by this mixture in different tissues versus pure SDS, given that the presence of Tx-100 may modulate the level of SDS partitioning in the human stratum corneum. At subsolubilizing level, the mixture showed higher affinity with bilayers than those reported for single components, whereas at solubilizing level this affinity was slightly lower and higher than those reported for Tx-100 and SDS respectively. A direct relationship was established in the initial interaction steps between the growth of vesicles, the leakage of entrapped CF and the effective molar ratio of surfactant to phospholipid in bilayers (Re). This dependence was also detected during solubilization, where the decrease in the vesicle size and in the scattered light of the system depended on the Re parameter and hence on the bilayer composition. The fact that the free surfactant concentration at subsolubilizing and solubilizing levels showed respectively lower and similar values than the critical micelle concentration (c.m.c.) of the surfactant mixture indicates that permeability alterations and solubilization were determined respectively by the action of surfactant monomer and by the formation of mixed micelles. This finding supports the generally admitted assumption, for single surfactants, that the concentration of free surfactant must reach the c.m.c. for solubilization to occur and highlights the influence of the negative synergism of this surfactant mixture on the free surfactant concentration needed to saturate or solubilize liposomes.

The micelle to vesicle transition of lipids and detergents in the presence of a membrane protein: towards a rationale for 2D crystallization

The assembly of two-dimensional membrane protein crystals in the presence of lipids was analyzed with quasielastic light scattering and electron microscopy. Mixtures of detergent-solubilized lipids and/or proteins were submitted to slow or rapid dilution while measuring the hydrodynamic radii of the aggregates. Lipids alone exhibited lambda-shaped dilution curves with intermediate rod-shaped particles that converted into small vesicles. Depending on the protein-protein and protein-lipid interactions, detergent-solubilized protein-lipid mixtures showed a sharp transition from micelles to large densely packed proteoliposomes. Electron microscopy revealed that formation of crystals occurred shortly after this phase transition.

ATP synthesis by the F0F1 ATP synthase from thermophilic Bacillus PS3 reconstituted into liposomes with bacteriorhodopsin. 1. Factors defining the optimal reconstitution of ATP synthases with bacteriorhodopsin

Optimal conditions for the reconstitution of bacteriorhodopsin and H+-transporting ATP synthase from thermophilic Bacillus PS3 (TF0F1) were determined. Phosphatidylcholine/phosphatidic acid liposomes prepared by reverse-phase evaporation were treated with various amounts of Triton X-100, octyl glucoside, octaethylene glycol n-dodecylether, sodium cholate or sodium deoxycholate and the incorporation of proteins by these detergents was studied at each step of the solubilization process. After removal of detergent by means of SM-2 Bio-Beads, the light-driven ATP synthase activities of the resulting proteoliposomes were analyzed at 40 degrees C. The nature of the detergent used for reconstitution was important for determining the mechanism of protein insertions. The most efficient reconstitutions were obtained with octyl glucoside or Triton X-100 by insertion of the proteins into detergent-saturated liposomes. The conditions for reconstitutions were further optimized with regard to functional coupling between bacteriorhodopsin and TF0F1. It was demonstrated that one of the main factors limiting the production of efficient reconstituted proteoliposomes was related to activation of the highly stable TFO-F1. Activation was accomplished by total solubilization of phospholipids and proteins in a Triton X-100/octyl glucoside mixture containing 20 mM octyl glucoside, leading to a threefold stimulation of the ATP synthase activity. Final ATP synthase activities depended greatly on the lipid/bacteriorhodopsin and the lipid/TF0F1 ratios as well as on the phospholipid used. In particular, light-driven ATP synthesis depended upon the presence of negatively charged phospholipids. Cholesterol was found to induce a fourfold increase in ATP synthase activity with a concomitant 65% decrease in the Km for ADP, suggesting that sterols can modulate catalytic events mediated by F1. Preparations obtained by this step-by-step reconstitution procedure displayed activities up to 20-fold higher (500-800 nmol ATP x min(-1) x mg TF0F1(-1) in the presence of cholesterol) than the maximal values reported in the literature for light-driven ATP synthesis TF0F1 measured under similar conditions. This study also allowed rationalization of the different parameters involved in reconstitution experiments and the present simple method is shown to be of general use for preparation of efficient proteoliposomes containing bacteriorhodopsin and choloroplast or mitochondrial F0F1-type ATP synthases.

Functional reconstitution of recombinant phospholamban with rabbit skeletal Ca(2+)-ATPase

Phospholamban (PLB) is a small, transmembrane protein that resides in the cardiac sarcoplasmic reticulum (SR) and regulates the activity of Ca(2+)-ATPase in response to beta-adrenergic stimulation. We have used the baculovirus expression system in Sf21 cells to express milligram quantities of wild-type PLB. After purification by antibody affinity chromatography, the function of this recombinant PLB was tested by reconstitution with Ca(2+)-ATPase purified from skeletal SR. The results obtained with recombinant PLB were indistinguishable from those obtained with purified, canine cardiac PLB. In particular, PLB reduced the apparent calcium affinity of Ca(2+)-ATPase but had no effect on Vmax. At pCa 6.8, PLB inhibited both calcium uptake and ATPase activity of Ca(2+)-ATPase by 50%. This inhibition was fully reversed by addition of a monoclonal antibody to PLB, which mimics the physiological effects of PLB phosphorylation. Maximal PLB regulatory effects occurred at a molar stoichiometry of approximately 3:1, PLB/Ca(2+)-ATPase. We also investigated peptides corresponding to the two main domains of PLB. The membrane-spanning domain, PLB26-52, appeared to uncouple ATPase hydrolysis from calcium transport, even though the permeability of the reconstituted vesicles was not altered. The cytoplasmic peptide, PLB1-31, had little effect, even at a 300:1 molar excess over Ca(2+)-ATPase.

Variable stoichiometric efficiency of Ca2+ and Sr2+ transport by the sarcoplasmic reticulum ATPase

In comparative experiments with Ca2+ ATPase in native sarcoplasmic reticulum vesicles and reconstituted proteoliposomes, we find that a variable stoichiometry of Ca2+ or Sr2+ transport per ATPase cycle is observed in the absence of passive leak through independent channels. The observed ratio is commonly lower than the optimal value of 2 and depends on the composition of the reaction mixture. In all cases, a progressive rise in the lumenal concentration of Ca2+ and Sr2+ is accompanied by a parallel reduction of coupling ratios. Significant ATPase activity remains even after asymptotic levels of Ca2+ accumulation are reached. This residual activity subsides if the Ca2+ concentration in the outer medium is reduced below activating levels (as it would following Ca2+ transients in muscle fibers). The reduction of stoichiometric coupling is explained with a reaction scheme, including a branched pathway for hydrolytic cleavage of phosphorylated intermediate before release of Ca2+ into the lumen of the vesicles. Flux through this pathway is favored when net lumenal Ca2+ dissociation from the phosphoenzyme is impeded and results in P(i) production accompanied by lumenal and medium Ca2+ exchange. Occurrence of reactions through branched pathways may have general implications for the stoichiometric efficiency of energy-transducing enzymes.

Reconstitution of ATP-dependent aminophospholipid translocation in proteoliposomes

In addition to ion-pumping ATPases, most plasma membranes of animal cells contain a Mg2+ ATPase activity, the function of which is unknown. This enzyme, of apparent molecular mass 110 kDa, was purified from human erythrocyte membranes by a series of column chromatographic procedures after solubilization in Triton X-100. When reincorporated into artificial bilayers formed from phosphatidylcholine, it was able to transport a spin-labeled phosphatidylserine analogue from the inner to the outer membrane leaflet provided Mg2+ ATP was present in the incubation mixture. The ATP-dependent transport of the phosphatidylethanolamine analogue required the presence of an anionic phospholipid (e.g., phosphatidylinositol) in the outer membrane leaflet. In contrast the transmembrane distribution of spin-labeled phosphatidylcholine was unaffected in the same experimental conditions. This transmembrane movement of aminophospholipid analogues was inhibited by treatment of the proteoliposomes with a sulfhydryl reagent. We conclude that the Mg2+ ATPase is sufficient for the biochemical expression of the aminophospholipid translocase activity, which is responsible for the inward transport of phosphatidylserine and phosphatidylethanolamine within the erythrocyte membrane. The presence of this transport activity in many animal cell plasma membranes provides a function for the Mg2+ ATPase borne by these membranes.

Reconstitution of the myometrial oxytocin receptor into proteoliposomes. Dependence of oxytocin binding on cholesterol

The requirements for regaining high-affinity binding of the myometrial oxytocin receptor after detergent solubilization were investigated by reconstitution experiments. Large unilamellar liposomes were prepared by reverse-phase evaporation from different mixtures of phospholipids, cholesterol and cholesteryl hemisuccinate. In the presence of the oxytocin receptor solubilized from myometrial membranes from pregnant guinea pig uterus, liposomes were treated with 3-[(3-cholamidopropyl)-dimethylammonio]-2-hydroxy-1-propanesulfonate (Chapso) throughout the range of detergent concentrations that cause the transformation of lamellar structures to mixed micelles. Detergent removal was achieved using bio-beads SM-2 as adsorbent. The presence of cholesterol was a prerequisite for regaining high-affinity binding of [3H]oxytocin and 125I-oxytocin antagonist to reconstituted proteoliposomes. Binding of [3H]oxytocin but not of the antagonist was dependent on the presence of Mn2+ ions. Reconstitution after lectin chromatography and photoaffinity labeling of reconstituted vesicles resulted in the exclusive labeling of the oxytocin receptor with a molecular mass of 68-80 kDa.