Delipidation of the phosphatidylcholine exchange protein from beef liver by detergents

Phospholipid transfer proteins: from lipid monolayers to cells

Eukaryotic cells contain phospholipid transfer proteins that act as carriers of phospholipids between membranes. In mammalian tissues three transfer proteins with different specificities have been identified: the phosphatidylcholine transfer protein (PC-TP), the phosphatidylinositol transfer protein (PI-TP) and the non-specific lipid transfer protein (nsL-TP) that transfers all common diacyl-phospholipids and cholesterol. Properties of these transfer proteins have been discussed with a special emphasis on the lipid binding site of bovine liver PC-TP. Application of photoactivatable and fluorescent analogues of PC have indicated that PC-TP contains specific and independent hydrophobic binding sites for the sn-1- and sn-2-fatty acyl chains. Because these sites have different properties, PC-TP can discriminate between positional isomers of PC and displays a distinct preference for those molecular species that carry a polyunsaturated fatty acid chain at the sn-2-position. Recent studies on bovine brain PI-TP have strongly suggested that this protein may be well-suited to maintain the levels of PI in natural membranes. Besides this proposed role, evidence has become available from studies on Swiss mouse 3T3 fibroblasts that, apart from its occurrence in cytosol, PI-TP is present in nuclei.

Lipid transfer proteins

Translocations of various lipid species between membranes have been extensively studied. The transport of water-insoluble lipids is thought to require the participation of lipid transfer proteins (LTP). Several LTP, differing in their physiochemical properties and substrate specificities, have been purified to homogeneity from blood plasma, eucaryotic and procaryotic cells. Depending on their site of activity, they can be classified as extracellular and intracellular LTP. Extracellular LTP are found in the blood plasma and intracellular LTP, which were originally characterized as phospholipid exchange proteins, are ubiquitous in nature. Despite the enormous knowledge about their physicochemical properties and their function in vitro their physiological role has not been clearly demonstrated. However, their ubiquitous occurrence indicates an important role in cellular events. This review gives an overview of this interesting category of proteins, which are able to catalyze inter-membrane transfer and exchange of lipids.

Properties and modes of action of specific and non-specific phospholipid transfer proteins

We have described the mode of action of the phosphatidylcholine transfer protein (PC-TP), the phosphatidylinositol transfer protein (PI-TP) and the non-specific lipid transfer protein (nsL-TP) isolated from bovine and rat tissues. PC-TP and PI-TP specifically bind one phospholipid molecule to be carried between membranes. PC-TP, and most likely PI-TP as well, have independent binding sites for the sn-1- and sn-2-fatty acyl chains. These sites have different properties, which may explain the ability of PC-TP and PI-TP to discriminate between positional phospholipid isomers. nsL-TP, which is identical to sterol carrier protein 2, transfers all common phospholipids, cholesterol and oxysterol derivatives between membranes. This protein is very efficient in mediating a net mass transfer of lipids to lipid-deficient membranes. Models for its mode of action, which is clearly different from that of PC-TP and PI-TP, are presented.

Identification of the lipid-binding site of phosphatidylcholine-transfer protein with phosphatidylcholine analogs containing photoactivable carbene precursors

The lipid binding site of the phosphatidylcholine transfer protein from bovine liver has been investigated by use of phosphatidylcholine analogs which carry a diazirinophenoxy group linked to the omega-carbon of either the sn-2-[1-14C]hexanoyl (PC I) or sn-2-[1-14C]undecanoyl chain (PC II). Photolysis of the PC I(PC II)-transfer protein complex resulted in a covalent coupling of 30-40% of the label to the protein as shown by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Upon mild alkaline treatment of the photolysed complex the protein containing covalently coupled 14C-label was separated from the noncoupled 14C-label by gel permeation chromatography. The 14C-labeled protein was degraded with protease from Staphylococcus aureus, trypsin and cyanogen bromide and specific 14C-labeled peptides were sequenced by automated Edman degradation. Major sites of coupling shown by release of radioactivity were identified as Tyr54 and the peptide segment Val171-Phe-Met-Tyr-Tyr-Phe-Asp177. Both PC I and PC II coupled extensively to Tyr54 (90% and 50% of total labeling, respectively). The remainder of the radioactivity was released from the peptide Val171-Asp177 with a distinct difference in in the pattern of release depending on whether PC I or PC II were used. Thus, coupling occurred preferentially to Tyr175 and Asp177 with PC I while Val171 and Met173 were labeled preferentially with PC II. This shift in coupling is compatible with an increase of 0.6 nm for the sn-2-fatty-acyl chains of PC I and II, assuming that the peptide Val171-Asp177 has adopted the strongly predicted beta-strand configuration. These data have been interpreted in terms of the localization of phosphatidylcholine in the phosphatidylcholine transfer protein.

Prediction of secondary structural elements in the phosphatidylcholine-transfer protein from bovine liver

Secondary structural elements of the phosphatidylcholine-transfer protein from bovine liver have been predicted from its primary structure with the aid of two computerized methods. The predicted alpha-helix and beta-strand content have been compared with the values derived from circular dichroism spectra. The hydrophobicity profile (Rose plot) of the protein indicated that the supposed lipid-binding site occurs in the most hydrophobic region. The predicted secondary structural elements have been folded in a tentative model of the protein molecule according to its hydrophobicity profile.

Interaction of C-reactive protein with artificial phosphatidylcholine bilayers

C-Reactive protein (CRP), the most characteristic of the 'acute phase proteins' (ref. 1) is thought to participate in the mediation and/or modulation of acute inflammatory processes, but its exact function is unknown. CRP has a Ca2+-dependent binding specificity for phosphorylcholine, the polar head group of two widely distributed lipids, lecithin (phosphatidylcholine, PC) and sphingomyelin (SM). A number of observations suggest that at least some of the biological activities of CRP depend on its interaction with phospholipids of cell membranes. In addition, interaction of CRP with PC- and SM-containing lipid dispersions and with PC-containing liposomes can activate the complement system. We report here that binding of CRP to model membranes of PC requires the incorporation into the bilayer of lysophosphatidylcholine (LPC). Thus, a disturbance of the molecular organisation of the bilayer appears to be necessary for binding of CRP. These findings provide a possible biochemical explanation for binding of CRP to damaged but not intact cell membranes and might be relevant to its biological function.

Determination of the hydrophobic binding site of phosphatidylcholine exchange protein with photosensitive phosphatidylcholine

1-Acyl-2-(7-(4-azido-2-nitrophenoxy)-[1-14C]heptanoly)-sn-glycero-3-phosphocholine was synthesized in order to study the lipid-binding site of the phosphatidylcholine exchange protein from bovine liver. Photosensitive phosphatidylcholine was incorporated into the protein by incubation with vesicles of this phosphatidylcholine derivative. The lipid-protein complex was separated from the vesicles by chromatography on Biogel A-0.5m. Photolysis of the complex by irradiation with light of a high pressure mercury lamp at a wavelength above 340 nm generated the highly reactive nitrene. Sodium dodecyl sulfate gel electrophoresis of the photolysed complex indicated that 30% of the endogenous 14C-labeled phosphatidylcholine was covalently linked to the protein. Peptides were isolated after digestion of the photolysed complex with protease from Staphylococcus aureus and trypsin. It was determined that the 2-acyl chain of the phosphatidylcholine molecule was linked to the peptide segment -Gly-Ser-Lys-Val-Phe-Met-Tyr-Tyr-. This segment was part of a protease peptide of about 65 residues of which the sequence was determined by Edman degradation for the first 38 residues. This peptide contains a cluster of apolar residues -Val-Phe-Met-Tyr-Tyr-Phe with an extremely high hydrophobicity index and with a predicted beta-sheet conformation. It is concluded that this hydrophobic cluster forms part of the binding site.

Affinity chromatography of the phosphatidylcholine exchange protein from bovine liver

Affinity chromatography has been used to purify the phosphatidylcholine exchange protein from bovine liver. The affinity resin consisted of 1-acyl-2-(9-carboxy)nonyl-glycero-3-phosphocholine linked to AH-Sepharose 4 B via the carboxyl group. Application of a crude exchange protein fraction to the affinity column resulted in a complete adsorption of the phosphatidylcholine exchange protein. The exchange protein eluted with a buffer containing 0.15% sodium deoxycholate. The most active fraction was 130-fold purified and accounted for 62% of the activity.

Interaction of the phosphatidylcholine exchange protein with phospholipids. A fluorescence and circular dichroism study

The phosphatidylcholine exchange protein from bovine liver has a fluorescence emission maximum at 327 nm. Fluorescence was enhanced in the presence of vesicles containing phosphatidylcholine and various amounts of either phosphatidic acid or phosphatidylglycerol. From the increase in fluorescence it was derived that the apparent dissociation constant of the exchange protein-vesicle complex decreased with an increased vesicle content of acidic phospholipids. Fluorescence indicated that the exchange protein interacted with lysophosphatidylcholine micelles at pH 3.5, 5.9 and 8.5. The increase in fluorescence was most prominent at the acidic pH. Circular dichroism indicated that the alpha-helix content of the native protein was low between pH 3.6 and 8.0. Interaction with lysophosphatidylcholine micelles had a negligible effect on the secondary structure of the protein, except at pH 3.6 where distinct minima at 208 nm and 220 nm in the circular dichroic spectrum became apparent.

Specificity of the phosphatidylcholine exchange protein from bovine liver

The phosphatidylcholine exchange protein from bovine liver stimulates the specific transfer of phosphatidylcholine (PC) from rat liver microsomes to mitochondria or phospholipid vesicles (Wirtz, K.W.A., Kamp, H.H., and van Deenen, L.L.M. (1972), Biochim. Biophys. Acta 274, 606). In the present study, it has been established which components of the PC molecule are essential to the specific interaction with the protein. Radiochemically labeled analogues of PC have been synthesized with modifications in the polar and apolar moiety, and their transfer was measured between donor and acceptor vesicles. Relative to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphorylcholine (egg yolk PC), transfer is inhibited or abolished when (a) the distance between phosphorus and nitrogen is decreased or increased and (b) a methyl group on the quaternary nitrogen is removed or substituted by an ethyl or propyl group. Transfer is much less affected when (a) the ester bonds are replaced by ether or carbon-carbon bonds, (b) the PC molecule contains two saturated fatty acids, and (c) the D stereoisomer is used. It is concluded that the protein has a binding site which interacts specifically with the phosphorylcholine head group and which cannot accommodate substantial configurational changes. Interaction with the apolar moiety of PC is less specific. However, lyso-PC is not transferred, suggesting that two hydrocarbon chains are required to stabilize the exchange protein-phospholipid complex. Interaction of [14C]PC-labeled exchange protein with vesicles of different phospholipid compositon has been analyzed by measuring the release of [14C]PC into these vesicles. Vesicles of egg PC or dimethylphosphatidylethanolamine function as acceptors, in contrast to vesicles of sphingomyelin or phosphatidylethanolamine.

Immunological comparison of phosphatidylinostiol and phosphatidylcholine exchange proteins in bovine brain, liver and heart

The two phosphatidylinositol exchange proteins isolated from bovine cerebral cortex, I (isoelectric point pH 5.2) and II (isoelectric point pH 5.5), had essentially identical amino acid compositions. Rabbit antisera preparations specific to each of these brain proteins were equally effective in inhibiting the phosphatidylinositol transfer activity of both protein I and II. Judged by double diffusion on agar gels, immunoprecipitation was not observed between either of the brain phosphatidylinositol exchange proteins and anti-liver phosphatidylcholine exchange protein antibody or between liver phosphatidylcholine exchange protein and anti-brain phosphatidylinositol exchange protein antibody. Phosphatidylinositol and phosphatidylcholine transfer activity was measured in microsome-liposome assay systems. For membrane-free tissue preparations phosphatidylinositol activity increased in the order: brain greater than heart greater than liver, while phosphatidylinositol exchange proteins transferred phosphatidylinositol and phosphatidylcholine in the ratio 1.4: liver phosphatidylcholine exchange protein transferred exclusively phosphatidylcholine. Phosphatidylinositol transfer activity in brain, heart and liver was more than 80% inhibited by anti-brain phosphatidylinositol exchange protein antibody. The proportion of phosphatidylcholine transfer activity sensitive to anti-liver phosphatidylcholine exchange protein antibody was 15% for brain, 75% for liver and 20% for heart, while the proportion sensitive to anti-brain phosphatidylinositol exchange protein antibody was 65% for brain, 10% for liver and 60% for heart. Together these two classes of phospholipid exchange proteins accounted for approx. 80% of the phosphoatidylcholine transfer activity in selected bovine tissues. A protein which was chemically, immunologically, and catalytically similar to liver phosphatidylcholine exchange protein was identified in brain and contributed about 20% of the phosphatidylcholine transfer activity in that tissue.

The protein-mediated transfer of phosphatidylcholine between membranes. The effect of membrane lipid composition and ionic composition of the medium

The phosphatidylcholine exchange protein from bovine liver catalyzes the transfer of phosphatidylcholine between rat liver mitochondria and sonicated liposomes. The effect of changes in the liposomal lipid composition and ionic composition of the medium on the transfer have been determined. In addition, it has been determined how these changes affected the electrophoretic mobility i.e. the surface charge of the membrane particles involved. Transfer was inhibited by the incorporation of negatively charged phosphatidic acid, phosphatidylserine, phosphatidylglycerol and phosphatidylinositol into the phosphatidylcholine-containing vesicles; zwitterionic phosphatidyl-ethanolamine had much less of an inhibitory effect while positively charged stearylamine stimulated. The cation Mg2+ and, to a lesser extent, K+ overcame the inhibitory effect exerted by phosphatidic acid, in that concentration range where these ions neutralized the negative surface charge most effectively. Under conditions where Mg2+ and K+ affected the membrane surface charge relatively little inhibition was observed. In measuring the protein-mediated transfer between a monolayer and vesicles consisting of only phosphatidylcholine, cations inhibited the transfer in the order La3+ greater than Mg2+ larger than or equal to Ca2+ greater than K+ = Na+. Inhibition was not related to the ionic strength, and very likely reflects an interference of these cations with an electrostatic interaction between the exchange protein and the polar head group of phosphatidylcholine.

Action of phospholipases on the phosphatidylcholine exchange protein from beef liver

The phospholipases A2, C and D have been used to investigate the localization of phosphatidylcholine in the phosphatidylcholine exchange protein from beef liver. The rate of enzymatic hydrolysis of the protein-bound phosphatidylcholine was found to be very low. Addition of deoxycholate, isobutanol or dioxane to the native protein, under conditions where delipidation did not occur, greatly enhanced the hydrolytic action of the phospholipases. From these results it is concluded that phosphatidylcholine may be buried in the protein molecule.