Infrared and 31P-NMR studies of the effect of Li+ and Ca2+ on phosphatidylserines

Infrared and 31P-NMR spectra of solid samples of 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS) and 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) have been recorded. Comparison of the spectra of the Na+ salts of these phospholipids with those of complexes formed with Li+ and Ca2+ ions allows the characterization of conformational changes induced by complexation with Li+ and Ca2+. Ca2+ forms tight, crystalline complexes with these phosphatidylserines (PS), irrespective of the degree of unsaturation in the hydrocarbon chains. In these PS-Ca2+ complexes the torsion angles of the two P-O ester bonds exhibit the antiplanar-antiplanar conformation which is significantly different from the standard gauche-gauche conformation commonly found in phosphodiesters. In contrast, complexation with Li+ does not induce this conformational change in the phosphodiester group. It is shown that the degree of unsaturation in the hydrocarbon chains, and related to it, the cross-sectional area of the phospholipid or the surface charge density, determine the affinity of the phosphatidylserine for the metal ion. In general, the affinity of phosphatidylserines for both Li+ and Ca2+ decreases with increasing unsaturation in the hydrocarbon chains or decreasing surface charge density; it is in the order DMPS greater than POPS greater than DOPS.     

Infrared and 31P-NMR studies of the interaction of Mg2+ with phosphatidylserines: effect of hydrocarbon chain unsaturation

Infrared and 31P-NMR spectra of aqueous dispersions of 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS), 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) and ox brain phosphatidylserine in the presence of excess Mg2+ have been recorded. A consistent picture emerges from the application of infrared and 31P-NMR spectroscopy to Mg2+-PS interactions. Mg2+ forms crystalline complexes with saturated phosphatidylserines, such as DMPS, and probably with POPS. In these crystalline PS-Mg2+ complexes the phosphate group loses its water of hydration but the serine carboxylate remains hydrated. Furthermore, there is formation of an additional hydrogen bond to one of the ester carbonyl groups of DMPS, and interchain interactions appear to be enhanced as reflected by a tighter packing of the fatty acyl chains. One main conclusion of this work is that Mg2+ binding to PS bilayers shows a gradation, the binding is in the order DMPS greater than POPS greater than ox brain PS greater than DOPS. The molecular area increases in the order DMPS less than ox brain PS less than POPS less than DOPS and is apparently an important parameter determining the affinity of PS for Mg2+. The general trend is that with increasing molecular area, and hence spacing of the ligands, the binding of Mg2+ decreases. While PS with two saturated fatty acyl chains forms tightly packed, crystalline Mg2+ complexes with an immobilized headgroup, the unsaturated PS molecules such as ox brain PS and DOPS interact only weakly with Mg2+. Their interaction seems to be restricted to electrostatic shielding, since no major changes in molecular conformation, chain packing and headgroup hydration are found. The interaction of POPS with Mg2+ is intermediate between that of saturated PS and that of DOPS. POPS exhibits a higher affinity for Mg2+ than ox brain PS, although their molecular areas (and the surface charge density) are approximately the same. This apparent anomaly is proposed to be due to a discreteness of charge effect. It is proposed that a lipid surface with regularly spaced polar groups has a higher affinity for binding Mg2+.     

Infrared studies of fully hydrated unsaturated phosphatidylserine bilayers. Effect of Li+ and Ca2+

Infrared spectroscopy has been used to characterize the thermal-phase behavior of fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS) and 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) as well as their interaction with Li+ and Ca2+. The order-disorder transition of POPS-NH4+ is at 17 degrees C; in the presence of Li+ a POPS-Li+ complex is formed, and the transition temperature of this complex is 40 degrees C. DOPS-NH4+ has an order-disorder transition at -11 degrees C, and unlike POPS the addition of Li+ has no effect on the thermal behavior of DOPS-NH4+. This indicates that the binding of Li+ to DOPS is negligible or very weak. Li+ binds to the phosphate and carboxylate groups of POPS, and as a result these groups lose their water of hydration. Li+ binding induces a conformational change, probably in the glycerol backbone of POPS; however, the conformation of the two P-O ester bonds remains gauche-gauche as in POPS-NH4+. Both POPS and DOPS form crystalline complexes with Ca2+. As a result of Ca2+ binding to the phosphate, this group loses its water of hydration and there is a conformational change in the P-O ester bonds from gauche-gauche to antiplanar-antiplanar. In contrast to the POPS-Li+ complex, the carboxylate group remains hydrated in the Ca2+ complexes. Furthermore, in these PS-Ca2+ complexes a new hydrogen bond is formed between one of the ester C=O groups and probably water. Such a situation is not found in the NH4+ and Li+ salts of phosphatidylserine.     

Cholesterol affects divalent cation-induced fusion and isothermal phase transitions of phospholipid membranes

The influence of cholesterol on divalent cation-induced fusion and isothermal phase transitions of large unilamellar vesicles composed of phosphatidylserine (PS) was investigated. Vesicle fusion was monitored by the terbium/dipicolinic acid assay for the intermixing of internal aqueous contents, in the temperature range 10-40 degrees C. The fusogenic activity of the cations decreases in the sequence Ca2+ greater than Ba2+ greater than Sr2+ much greater than Mg2+ for cholesterol concentrations in the range 20-40 mol%, and at all temperatures. Increasing the cholesterol concentration decreases the initial rate of fusion in the presence of Ca2+ and Ba2+ at 25 degrees C, reaching about 50% of the rate for pure PS at a mole fraction of 0.4. From 10 to 25 degrees C, Mg2+ is ineffective in causing fusion at all cholesterol concentrations. However, at 30 degrees C, Mg2+-induced fusion is observed with vesicles containing cholesterol. At 40 degrees C, Mg2+ induces slow fusion of pure PS vesicles, which is enhanced by the presence of cholesterol. Increasing the temperature also causes a monotonic increase in the rate of fusion induced by Ca2+, Ba2+ and Sr2+. The enhancement of the effect of cholesterol at high temperatures suggests that changes in hydrogen bonding and interbilayer hydration forces may be involved in the modulation of fusion by cholesterol. The phase behavior of PS/cholesterol membranes in the presence of Na+ and divalent cations was studied by differential scanning calorimetry. The temperature of the gel-liquid crystalline transition (Tm) in Na+ is lowered as the cholesterol content is increased, and the endotherm is broadened. Addition of divalent cations shifts the Tm upward, with a sequence of effectiveness Ba2+ greater than Sr2+ greater than Mg2+. The Tm of these complexes decreases as the cholesterol content is increased. Although the transition is not detectable for cholesterol concentrations of 40 and 50 mol% in the presence of Na+, Sr2+ or Mg2+, the addition of Ba2+ reveals endotherms with Tm progressively lower than that observed at 30 mol%. Although the presence of cholesterol appears to induce an isothermal gel-liquid crystalline transition by decreasing the Tm, this change in membrane fluidity does not enhance the rate of fusion, but rather decreases it. The effect of cholesterol on the fusion of PS/phosphatidylethanolamine (PE) vesicles was investigated by utilizing a resonance energy transfer assay for lipid mixing. The initial rate of fusion of PS/PE and PS/PE/cholesterol vesicles is saturated at high Mg2+ concentrations. With Ca2+, saturation is not observed for cholesterol-containing vesicles.(ABSTRACT TRUNCATED AT 400 WORDS)     

The importance of an asymmetric distribution of acidic lipids for synaptotagmin 1 function as a Ca2+ sensor

Syt1 (synaptotagmin 1) is a major Ca2+ sensor for synaptic vesicle fusion. Although Syt1 is known to bind to SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) complexes and to the membrane, the mechanism by which Syt1 regulates vesicle fusion is controversial. In the present study we used in vitro lipid-mixing assays to investigate the Ca2+-dependent Syt1 function in proteoliposome fusion. To study the role of acidic lipids, the concentration of negatively charged DOPS (1,2-dioleoyl-sn-glycero-3-phospho-L-serine) in the vesicle was varied. Syt1 stimulated lipid mixing by 3-10-fold without Ca2+. However, with Ca2+ there was an additional 2-5-fold enhancement. This Ca2+-dependent stimulation was observed only when there was excess PS (phosphatidylserine) on the t-SNARE (target SNARE) side. If there was equal or more PS on the v-SNARE (vesicule SNARE) side the Ca2+-dependent stimulation was not observed. We found that Ca2+ at a concentration between 10 and 50?μM was sufficient to give rise to the maximal enhancement. The single-vesicle-fusion assay indicates that the Ca2+-dependent enhancement was mainly on docking, whereas its effect on lipid mixing was small. Thus for Syt1 to function as a Ca2+ sensor, a charge asymmetry appears to be important and this may play a role in steering Syt1 to productively trans bind to the plasma membrane.     

Temperature dependence of Na+/Ca2+ exchange activity in beef-heart sarcolemmal vesicles and proteoliposomes

Temperature dependence of Na+/Ca2+ exchange activity was studied in beef cardiac sarcolemmal vesicles in the absence and presence of the inhibitor amiloride and in proteoliposomes reconstituted with different lipid mixtures. Arrhenius plots for Na+/Ca2+ exchange activity in both control and amiloride-treated vesicles revealed an apparent energy of activation of 9665 +/- 585 (SE, n = 4) cal/mol, corresponding to a temperature coefficient (Q10) value of 1.70 +/- 0.05 (SE, n = 4) over the range 25-37 degrees C. When Na+/Ca2+ exchange was reconstituted into phosphatidylcholine (PC):phosphatidylserine (PS) (52:48, mol/mol), PC:PS:cholesterol (25:39:36, mol/mol), and PC:PS:distearoylphosphatidylcholine (DSPC) (31:48:21, mol/mol) proteoliposomes, the highest activity was found in PC:PS:cholesterol proteoliposomes. Arrhenius plots of Na+/Ca2+ exchange activity exhibited breakpoints at 23 degrees C (PC:PS), 33 degrees C (PC:PS:cholesterol), and 23 degrees C (PC:PS:DSPC). The increase in the thermotropic transition temperature with cholesterol could result from the condensing effect of this sterol, whereas the breaks observed with PC:PS and PC:PS:DSPC could be caused by a non-lipid-mediated membrane protein conformational change. These results indicate that the lipid microenvironment around the Na+/Ca2+ exchanger and the nature of the specific lipid-protein interactions influence the activity of this antiporter. Further evidence supporting the hypothesis that cholesterol behaves as a specific positive effector for the exchanger is also given.     

Calcium diphosphatidate membrane traversal is inhibited by common phospholipids and cholesterol but not by plasmalogen

Phosphatidate-mediated Ca2+ membrane traversal is inhibited by phospholipids (PL) such a phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidylserine (PS), sphingomyelin and lysoPC, but not by PC-plasmalogen. Kinetics of Ca2+ traversal through a 'passive' bilayer consisting of OH-blocked cholesterol show competition between PC and phosphatidic acid (PA); it appears likely that a Ca(PA.PC) complex is formed which is not a transmembrane ionophore but will reduce the amount of phosphatidic acid available for the formation of the ionophore, Ca(PA)2. PS and PI may inhibit Ca2+-traversal in the same manner by forming Ca(PA.PL) complexes. We suggest that PC-plasmalogen, with one of the Ca2+-chelating ester CO groups missing, cannot engage in calcium cages, i.e., Ca(PA.PL) complexes, and thus does not interfere with Ca(PA)2 formation. Double-reciprocal plotting of Ca2+ traversal rates in cholesterol-containing liposomes vs. calcium concentration suggests that cholesterol inhibits Ca2+ traversal by competing with Ca2+ for PA. The inhibition does not seem to be caused by a restructuring or dehydration of the membrane 'hydrogen belts' affected by cholesterol; most probably, it is due to hydrogen bonding of the cholesterol-OH group to a CO group of PA; this reduces the amount of PA available for the calcium ferry. The inhibition by sphingomyelin and lysoPC may also be explained by their OH group interacting with PA via hydrogen bonding. The pH dependence of Ca2+ traversal suggests that H[Ca(PA)2]- can serve as Ca2+ cross-membrane ferry but that at physiological pH, [Ca(PA)2]2- is the predominant ionophore. In conclusion, the results indicate that Ca2+ traversal is strongly dependent on the structure of the hydrogen belts, i.e., the membrane strata occupied by hydrogen bond acceptors (CO of phospholipids) and donors (OH of cholesterol, sphingosine), and that lipid hydrogen belt structures may regulate storage and passage of Ca2+.     

Ca2+-induced fusion of large unilamellar phosphatidylserine/cholesterol vesicles

The effect of cholesterol on the Ca2+-induced aggregation and fusion of large unilamellar phosphatidylserine (PS) vesicles has been investigated. Mixing of aqueous vesicle contents was followed continuously with the Tb/dipicolinate assay, while the dissociation of pre-encapsulated Tb/dipicolinate complex was taken as a measure of the release of vesicle contents. Vesicles consisting of pure PS or PS/cholesterol mixtures at molar ratios of 4:1, 2:1 and 1:1 were employed at three different lipid concentrations, each at four different Ca2+ concentrations. The results could be well simulated in terms of a mass-action kinetic model, providing separately the rate constants of vesicle aggregation, c11, and of the fusion reaction itself, f11. In the analyses the possibility of deaggregation of aggregated vesicles was considered explicitly. Values of both c11 and f11 increase steeply with the Ca2+ concentration increasing from 2 to 5 mM. With increasing cholesterol content of the vesicles the value of c11 decreases, while the rate of the actual fusion reaction, f11, increases. Remarkably, the effect of cholesterol on both aggregation and fusion is quite moderate. The presence of cholesterol in the vesicle bilayer does not affect the leakage of vesicle contents during fusion.     

Activation of protein kinase C by Triton X-100 mixed micelles containing diacylglycerol and phosphatidylserine

A mixed micellar assay for protein kinase C was developed to investigate the specificity and stoichiometry of activation by phospholipids and diacylglycerols. Triton X-100 mixed micelles containing 8 mol % phosphatidylserine (PS) and 2.5 mol % sn-1,2-dioleoylglycerol (diC18:1) activated rat brain protein kinase C in the presence of Ca2+ to the same degree as sonicated PS/diC18:1 did in the standard assay. However, protein kinase C activity was more responsive to diC18:1 in the mixed micellar assay than the standard assay. At 8 mol % PS and 100 microM Ca2+, diC18:1 stimulated maximally at 1 mol %. At 2.5 mol % diC18:1 and 100 microM Ca2+, PS did not activate until 3 mol % and then did so cooperatively with maximal stimulation occurring at 6-8 mol %. Direct evidence for a Ca2+-, PS-, and diC18:1-dependent interaction of protein kinase C with mixed micelles was obtained by molecular sieve chromatography on Sephacryl S-200. These data permit inferences pertaining to the number of diC18:1 and PS molecules/micelle which are required for activation. For diC18:1, a single molecule may be sufficient but no more than 2 molecules are required. For PS, greater than 4 but less than 10 molecules are required. These data establish that a phospholipid bilayer is not required for protein kinase C activation and that activation of monomeric protein kinase C occurs.     

Studies on the mechanism of membrane fusion: evidence for an intermembrane Ca2+-phospholipid complex, synergism with Mg2+, and inhibition by spectrin.

The interaction of Ca2+ and Mg2+ with phosphatidylserine (PS) vesicles in 0.1 M NaCl aqueous solution was studied by equilibrium dialysis binding, X-ray diffraction, batch microcalorimetry, kinetics of cation-induced vesicle aggregation, release of vesicle contents, and fusion. Addition of either cation causes aggregation of PS vesicles and produces complexes with similar stoichiometry (1:2 cation/PS) at saturating concentrations, although the details of the interactions and the resulting complexes are quite different. Addition of Ca2+ to PS vesicles at T greater than or equal to 25 degrees C induces the formation of an "anhydrous" complex of closely apposed membranes with highly ordered crystalline acyl chains and a very high transition temperature (Tc greater than 100 degrees C). The formation of this complex is accompanied by a release of heat (5.5 kcal/mol), rapid release of vesicle contents, and fusion of the vesicles into larger membranous structures. By contrast, addition of Mg2+ produces a complex with PS which is much more hydrated, has no crystallization of the acyl chains at T greater than or equal to 20 degrees C, and has comparatively little fusion. Studies with both Ca2+ and Mg2+ added simultaneously indicate that there is a synergistic effect between the two cations, which results in an enhancement of the ability of Ca2+ to form its specific complex with PS at lower concentrations. The presence of the erythrocyte protein "spectrin" inhibits this synergism and interferes with the formation of the specific PS/Ca complex. It also inhibits the fusion of PS vesicles. It is proposed that the unique PS/Ca complex, which involves close apposition of vesicle membranes, is an intermembrane "trans" complex. We further propose that such a complex is a key step for the resultant phase transition and fusion of PS vesicles. By contrast, the PS/Mg complex is proposed to be a "cis" complex with respect to each membrane. The results are discussed in terms of the mechanism of membrane fusion.     

Activation and inhibition of protein kinase C isozymes alpha and beta by Gd3+.

Gd3+ was evaluated as a probe for Ca2+ sites on protein kinase C (PKC) by studying its ability to replace Ca2+ in activation of PKC isozymes II (beta) and III (alpha) in the lipid systems phosphatidylserine/1,2-dioleoyl-sn-glycerol (PS/DO) and diheptanoylphosphatidylcholine (PC7)/DO. PKC beta was stimulated by Ca2+ or Gd3+ in PS/DO whereas activity in PC7/DO was independent of these metals. Thus, it is suggested that Gd3+ replaces Ca2+ at a site involving metal-lipid interactions. High concentrations of Ca2+ or Gd3+ inhibited activity in both lipid systems. Analysis of the Gd3+ inhibition in the PC7/DO system suggests that it is due to formation of GdATP, which competes at the MgATP site. Activity of PKC alpha was dependent on low concentrations of Ca2+ in both lipid systems. The ability of Gd3+ to substitute for Ca2+ could not be evaluated in the PS system due to the inability to completely remove contaminating Ca2+ without chelating buffers. Successful reduction of contaminating Ca2+ was achieved in the PC7 system but Gd3+ failed to substitute for Ca2+ in activating PKC alpha and only caused inhibition. This is consistent with binding of Gd3+ to a Ca2+ site at or near the active site of the enzyme rather than to a site on the lipid. These results indicate that interactions between PKC and Gd3+ are complex, involving occupation of more than one class of sites. Conditions for separately evaluating the individual sites can be manipulated by selection of isozyme and lipid system.     

Phosphatidylserine binding alters the conformation and specifically enhances the cofactor activity of bovine factor Va

Factor V(a) is a cofactor for the serine protease factor X(a) that activates prothrombin to thrombin in the presence of Ca(2+) and a platelet membrane surface. A platelet membrane lipid, phosphatidylserine (PS), regulates the proteolytic activity of factor X(a) as well as the structure of prothrombin. Here we ask whether PS also regulates the structure and cofactor activity of factor V(a), which is a heterodimer composed of one heavy chain (A1-A2 domains) and one light chain (A3-C1-C2 domains). We use fluorescence, circular dichroism, equilibrium dialysis, and activity measurements to demonstrate the following: (1) Factor V(a) has four sites for dicaproyl-sn-glycero-3-phospho-L-serine (C(6)PS, a soluble form of PS); the heavy and light chains each bind two C(6)PS molecules. (2) In the absence of Ca(2+), only two sites remain, one in the heavy chain and another in the light chain. (3) Binding to these sites causes conformational changes evidenced by changes in intrinsic fluorescence and in CD spectra and changes in cofactor activity. (4) At least some of the four lipid binding sites are nonspecific with respect to soluble lipid species, but the site(s) that regulate(s) cofactor activity is (are) specific for C(6)PS, phosphatidic acid, or phosphatidyl(homo)serine and produce a response comparable to that seen with a PS-containing membrane. (5) Like Ca(2+), C(6)PS also mediates the interaction between factor V(a) heavy (V(a)-HC) and light (V(a)-LC) chains. We conclude that PS regulates both the cofactor and the enzyme of the prothrombin-activating complex.     

Characterization of cationic lipid DNA transfection complexes differing in susceptability to serum inhibition

Background

Cationic lipid DNA complexes based on DOTAP (1,2-dioleoyl-3-(trimethyammonium) propane) and mixtures of DOTAP and cholesterol (DC) have been previously optimized for transfection efficiency in the absence of serum and used as a non-viral gene delivery system. To determine whether DOTAP and DC lipid DNA complexes could be obtained with increased transfection effciency in the presence of high serum concentrations, the composition of the complexes was varied systematically and a total of 162 different complexes were analyzed for transfection efficiency in the presence and absence of high serum concentrations.

Results

Increasing the ratio of DOTAP or DC to DNA led to a dose dependent enhancement of transfection efficiency in the presence of high serum concentrations up to a ratio of approximately 128 nmol lipid/μg DNA. Transfection efficiency could be further increased for all ratios of DOTAP and DC to DNA by addition of the DNA condensing agent protamine sulfate (PS). For DOTAP DNA complexes with ratios of ≤ 32 nmol/μg DNA, peak transfection efficiencies were obtained with 4 μg PS/μg DNA. In contrast, increasing the amount of PS of DC complexes above 0.5 μg PS /μg DNA did not lead to significant further increases in transfection efficiency in the presence of high serum concentrations. Four complexes, which had a similar high transfection efficiency in cell culture in the presence of low serum concentrations but which differed largely in the lipid to DNA ratio and the amount of PS were selected for further analysis. Intravenous injection of the selected complexes led to 22-fold differences in transduction efficiency, which correlated with transfection efficiency in the presence of high serum concentrations. The complex with the highest transfection efficiency in vivo consisted of 64 nmol DC/ 16 μg PS/ μg DNA. Physical analysis revealed a predicted size of 440 nm and the highest zeta potential of the complexes analyzed.

Conclusions

Optimization of cationic lipid DNA complexes for transfection efficiency in the presence of high concentrations of serum led to the identification of a DC complex with high transduction efficiency in mice. This complex differs from previously described ones by higher lipid to DNA and PS to DNA ratios. The stability of this complex in the presence of high concentrations of serum and its high transduction efficiency in mice suggests that it is a promising candidate vehicle for in vivo gene delivery.     

Calcium ion binding between lipid bilayers: the four-component system of phosphatidylserine, phosphatidylcholine, calcium chloride, and water

Ca2+ binding between lamellae of phosphatidylserine (PS) and phosphatidylcholine (PC) gives rise to a rigid phase of Ca(PS)2. When aqueous Ca2+, hydrated PS/PC, and Ca(PS)2 coexist at equilibrium, the aqueous Ca2+ concentration is invariant and is characteristic of the PS/PC ratio. This characteristic Ca2+ concentration is 0.040 microM for palmitoyloleoylphosphatidylserine without PC and increases as the inverse square of the PS mole fraction at high PS concentration (Raoult's law) and as the inverse square of the PS mole fraction multiplied by a constant at low PS concentration (Henry's law). For example, for palmitoyloleoylphosphatidylserine/palmitoyloleoylphosphatidylcholi ne = 0.6/0.4 or 0.2/0.8, this characteristic Ca2+ concentration is about 0.1 or about 6 microM, respectively. These observations at constant temperature are summarized in a quaternary phase diagram for the four-component system CaCl2/PS/PC/water.     

1,2-Dioleoylglycerol promotes calcium-induced fusion in phospholipid vesicles.

The effect of 1,2-dioleoyglycerol (1,2-DOG) on the promotion of Ca(2+)-induced fusion of phosphatidylserine/phosphatidylcholine (PS/PC) vesicles was studied. 1,2-DOG is able to induce the mixing of membrane lipids at concentrations of 10 mol% without mixing of vesicular contents. At concentrations of 20 mol% or higher, 1,2-DOG promotes fusion, lipid and content mixing, of LUV composed of an equimolar mixture of PS and PC, which otherwise are unable to fuse in the presence of Ca2+. Fusion was demonstrated by fluorescence assays monitoring mixing of aqueous vesicular contents and mixing of membrane lipids. Studies by Fourier transform infrared spectroscopy provided evidence for a fusion mechanism different to that of Ca(2+)-induced fusion of pure PS vesicles. Final equilibrium structures were characterized by 31P-NMR and freeze-fracture electron microscopy. Ca(2+)-induced fusion of 1,2-DOG containing vesicles is accompanied by the formation of isotropic structures which are shown to correspond to structures with lipidic particle morphology. The possible fusion mechanisms and implications are discussed.     

Using micropatterned lipid bilayer arrays to measure the effect of membrane composition on merocyanine 540 binding

The lipophilic dye merocyanine 540 (MC540) was used to model small molecule-membrane interactions using micropatterned lipid bilayer arrays (MLBAs) prepared using a 3D Continuous Flow Microspotter (CFM). Fluorescence microscopy was used to monitor MC540 binding to fifteen different bilayer compositions simultaneously. MC540 fluorescence was two times greater for bilayers composed of liquid-crystalline (l.c.) phase lipids (1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC),1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)) compared to bilayers in the gel phase (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)). The effect cholesterol (CHO) had on MC540 binding to the membrane was found to be dependent on the lipid component; cholesterol decreased MC540 binding in DMPC, DPPC and DSPC bilayers while having little to no effect on the remaining l.c. phase lipids. MC540 fluorescence was also lowered when 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (sodium salt) (DOPS) was incorporated into DOPC bilayers. The increase in the surface charge density appears to decrease the occurrence of highly fluorescent monomers and increase the formation of weakly fluorescent dimers via electrostatic repulsion. This paper demonstrates that MLBAs are a useful tool for preparing high density reproducible bilayer arrays to study small molecule-membrane interactions in a high-throughput manner.     

Structure-activity relationship of synthetic branched-chain distearoylglycerol (distearin) as protein kinase C activators

Several representative branched-chain analogues of distearin (DS) were synthesized and tested for their abilities to activate protein kinase C (PKC) and to compete for the binding of [3H]phorbol 12,13-dibutyrate (PDBu) to the enzyme. Substitutions of stearoyl moieties at sn-1 and sn-2 with 8-methylstearate decreased activities on these parameters, relative to those of the parental diacylglycerol DS, a weak PKC activator. Substitutions with 8-butyl, 4-butyl, or 8-phenyl derivatives, on the other hand, increased activities of the resulting analogues to levels comparable to those seen for diolein (DO), a diacylglycerol prototype shown to be a potent PKC activator. Kinetic analysis indicated that 8-methyldistearin (8-MeDS) acted by decreasing, whereas 8-butyldistearin (8-BuDS) and 8-phenyldistearin (8-PhDS) acted by increasing, the affinities of PKC for phosphatidylserine (PS, a phospholipid cofactor) and Ca2+ compared to the values seen in the absence or presence of DS. The stimulatory effect of 8-BuDS and 8-PhDS on PKC, as DO, was additive to that of 1,2-(8-butyl)distearoylphosphatidylcholine [1,2(8-Bu)DSPC] and, moreover, they abolished the marked inhibition of the enzyme activity caused by high concentrations of 1,2(8-Bu)DSPC. The present findings demonstrated a structure-activity relationship of the branched-chain DS analogues in the regulation of PKC, perhaps related to their abilities to specifically modify interactions of PKC with PS and/or Ca2+ critically involved in enzyme activation/inactivation.     

Thermal stability and DPPC/Ca2+ interactions of pulmonary surfactant SP-A from bulk-phase and monolayer IR spectroscopy.

Surfactant protein A (SP-A), the most abundant pulmonary surfactant protein, is implicated in multiple biological functions including surfactant homeostasis, biophysical activity, and host defense. SP-A forms ternary complexes with lipids and Ca2+ which are important for protein function. The current study uses infrared (IR) transmission spectroscopy to investigate the bulk-phase interaction between SP-A, 1,2-dipalmitoylphosphatidylcholine (DPPC), and Ca2+ ions along with IR reflection-absorption spectroscopy (IRRAS) to examine protein secondary structure and lipid orientational order in monolayer films in situ at the air/water interface. The amide I contour of SP-A reveals two features at 1653 and 1636 cm(-1) arising from the collagen-like domain and a broad feature at 1645 cm(-1) suggested to arise from the carbohydrate recognition domain (CRD). SP-A secondary structure is unchanged in lipid monolayers. Thermal denaturation of SP-A in the presence of either DPPC or Ca2+ ion reveals a sequence of events involving the initial melting of the collagen-like region, followed by formation of intermolecular extended forms. Interestingly, these spectral changes were inhibited in the ternary system, showing that the combined presence of both DPPC and Ca2+ confers a remarkable thermal stability upon SP-A. The ternary interaction was revealed by the enhanced intensity of the asymmetric carboxylate stretching vibration. The IRRAS measurements indicated that incorporation of SP-A into preformed DPPC monolayers at a surface pressure of 10 mN/m induced a decrease in the average acyl chain tilt angle from 35 degrees to 28 degrees. In contrast, little change in chain tilt was observed at surface pressures of 25 or 40 mN/m. These results are consistent with and extend the fluorescence microscopy studies of Keough and co-workers [Ruano, M. L. F., et al. (1998) Biophys. J. 74, 1101-1109] in which SP-A was suggested to accumulate at the liquid-expanded/liquid-condensed boundary. Overall these experiments reveal the remarkable stability of SP-A in diverse, biologically relevant environments.     

A new infrared spectroscopoic marker for cochleate phases in phosphatidylserine-containing model membranes.

Fourier transform-infrared (IR) spectroscopic and electron microscopic studies are reported for 1,2-dimyristoylphosphatidylserine (DMPS) and for DMPS/1,2-dimyristoylphosphatidylcholine mixtures in the presence and absence of Ca2+ ion. The frequency of the methyl symmetric deformation mode near 1,378 cm-1, previously assumed insensitive to changes in lipid morphology, has been found to respond to cochleate phase formation by undergoing an approximately 8 cm-1 increase. The new IR spectroscopic marker at 1,386 cm-1 has been used to identify and verify structures suggested from the phase diagram of J. R. Silvius and J. Gagné (1984. Biochemistry. 23:3241-3247) for this system. In addition, the ability of Mg2+ ion to induce cochleate formation has been demonstrated. Higher Mg2+ than Ca2+ levels are required for this process. Finally, IR spectroscopy has been used to monitor dehydration of the lipid surface through changes in the asymmetric PO2- stretching mode. Dehydration precedes cochleate phase formation (i.e., occurs at a lower Ca2+/phosphatidylserine level).     

Effective production of monoclonal antibodies against phosphatidylserine: stereo-specific recognition of phosphatidylserine by monoclonal antibody

A system based on the direct immunization of phospholipid Ag into mouse spleen has been used to produce mAb against phosphatidylserine (PS). mAb that bind to PS but not to phosphatidylcholine were selected. Remarkable frequency of the production of mAb against PS was observed with the immunization protocol. The mAb exhibited three distinct reactivity profiles ranging from highly specific to broadly cross-reactive. Among 61 hybridomas, 15 mAb were established for further analysis. The reactivities of three typical mAb, designated PS4A7, PS3A, and PSC8, are described. PS4A7 is highly specific to PS and no cross-reaction with other acidic phospholipids was observed. In the experiments using PS derivatives with a modified polar head group, PS4A7 was shown to bind to 1,2-diacyl-sn-glycero-3-phospho-L-serine (PS) but not to 1,2-diacyl-sn-glycero-3-phospho-D-serine or 1,2-diacyl-sn-glycero-3-phospho-L-homoserine, indicating that the antibody recognizes the stereo-specific configuration of serine residue in PS. PS3A binds to both PS and phosphatidylethanolamine, whereas no cross-reaction with other acidic phospholipids was observed. The analysis using the derivatives of PS and phosphatidylethanolamine shows that the antibody recognizes the amino group of the phospholipid Ag and cannot distinguish the conformational structure of serine residue in PS. PSC8 represents the family of mAb that cross-react considerably with other acidic phospholipids.