Identification of the lipid binding site of the phosphatidylcholine exchange protein with a photosensitive nitrene and carbene precursor of phosphatidylcholine

Viscosity-dependent energy barriers and equilibrium conformational fluctuations in oxygen recombination with hemerythrin

The recombination kinetics of photo-dissociated oxyhemerythrin (Sipunculus nudus) have been investigated between 298 K and 90 K. Fast geminate recombinations compete with oxygen escape into the solvent, from which a subsequent slower bimolecular rebinding takes place. In phosphate buffer (pH 7.7) at 278 K, the fast and slow processes are exponential and have comparable amplitudes. Whereas the oxygen escape rate rapidly decreases upon increasing the viscosity, the inward rate from the solvent is found to be independent of viscosity, up to about 50 cP (50 mPa.s). The data suggest that a Brownian-motion-driven displacement of one or several side-chain residues is implied in oxygen escape from within the protein and also that hemerythrin undergoes a conformational change in the deoxy state. At higher viscosities and lower temperature only the geminate phase is observed and the kinetics progressively depart from an exponential. Below about 130 K, the kinetics resemble those reported in the literature for heme proteins. They are consistent with a temperature-independent non-equilibrium frozen distribution of conformational substates. However, between 190 K and 130 K, the profile of the kinetics is invariant on a log/log plot and the results simply differ by a translation along the log t axis. It is shown that this property is expected only for a temperature-dependent distribution of substates in a Boltzmann equilibrium. From room temperature, where rebinding is exponential, down to the 'freezing' temperature, the geminate recombinations display a variety of kinetic laws. It can be shown, however, that for a broad class of substate distributions, the initial slope of the kinetic plot follows an Arrhenius relationship. The activation energy is equal to that of the exponential rate constant measured at high temperature. This result establishes the conditions under which protein data obtained from low-temperature kinetics can be extrapolated to physiological temperature.

Purification and characterisation of a non-specific lipid transfer protein from goat liver

A non-specific lipid transfer protein has been purified from the pH 5.1 supernatant of goat liver by DEAE-cellulose, CM-cellulose and Sephadex G-75 column chromatography. The protein shows a single band on polyacrylamide gel electrophoresis and transfers 450 nmol of phosphatidylcholine per min per mg of protein under the present assay condition. This protein has a subunit molecular weight of 12,000 and an isoelectric point of 8.65. Amino acid analysis reveals the absence of methionine. Histidine has been identified as the only N-terminal amino acid. Besides phosphatidylcholine, the protein transfers phosphatidylinositol, phosphatidylethanolamine, phosphatidylserine and cholesterol. Chemical modification studies showed the involvement of free amino and thiol groups in the maintenance of the transfer activity of the goat liver protein.

Common domain structure of Ca2+ and lipid-binding proteins

The phospholipase A2 inhibitor, lipocortin, shares common sequences with three abundant Ca2+-regulated membrane binding proteins of unknown function which are present in many cell and tissue types. A two-domain model for the structure of lipocortin is described and it is suggested that the new Ca2+-regulated proteins each contain at least one lipocortin domain. The structural and biochemical properties of each protein indicate that they all directly interact with phospholipids. Potential sites of interaction with the lipocortin domain are identified on the basis of homology with phospholipid transfer proteins and phospholipase A2.

Phospholipid transfer proteins: mechanism of action

Phospholipid transfer proteins are generally localized in the cytosolic fraction of cells and are capable of catalyzing the flux of phospholipid molecules among membranes. Artificial membranes also participate in protein-catalyzed phospholipid movements. In this review the major phospholipid transfer proteins are discussed with respect to their phospholipid substrate specificity and the contributions of membrane physical properties to this process. The phenomenon of net transfer of phospholipids is described. The use of various kinetic approaches to the study of these catalysts is reviewed. A detailed consideration of the distinct phospholipid binding and membrane interaction domains of one phospholipid transfer protein is presented. Finally, some recent applications of phospholipid transfer proteins to the examination of membrane structure and function and further directions for the continued research activity with this class of proteins are summarized.

Acyl selectivity in the transfer of molecular species of phosphatidylcholines from human erythrocytes

This report describes the molecular species composition of phosphatidylcholines (PC) transferred from human erythrocytes to acceptor vesicles composed of cholesterol and single PC species in the presence of PC-specific transfer protein from bovine liver. The compositions of the PC isolated from the vesicles were determined by capillary GLC as the diacylglycerol trimethylsilyl ethers. The cellular PC species appearing in the acceptor vesicles were enriched in unsaturated species and showed a low content of dipalmitoyl PC compared to untreated erythrocytes. This trend was independent of the composition of the PC used to construct the acceptor vesicles and it was possible to determine that the relative rates of efflux of the palmitoyl-containing phosphatidylcholines decreased in the order: palmitoyl-linoleoyl greater than palmitoyl-oleoyl greater than dipalmitoyl and in the stearoyl series, stearoyl-linoleoyl greater than stearoyl-oleoyl. No clear trend was distinguished for the influence of chain-length on the efflux, thus preventing an unambiguous assignment of the order of removal of all species from the cell membrane. Results derived for arachidonoyl-containing species were compromised by evidence for oxidation occurring during incubations at 37 degrees C. To confirm that acyl selectivity was also possible during transfer in the absence of the transfer protein, the efflux of 14C-labeled soya PC and [14C]dipalmitoyl PC from prelabeled erythrocytes was measured using plasma as the acceptor. As predicted by the chromatographic analyses, 14C-labeled soya PC effused up to 10-times faster than [14C]dipalmitoyl PC from the red cell membrane. Thus, the more rapid transfer of unsaturated PC cannot be explained entirely as a specificity of the transfer protein and is consistent with the hypothesis that intermolecular interactions involving PC molecules within the erythrocyte membrane, become weaker with increasing unsaturation. The results suggest a potential role of PC-specific transfer protein as a probe of the nature of PC interactions within biological membranes.

Agglutination of mouse erythrocytes by binding of non-choline phospholipids to a 70 000-Dalton protein

The structure of some phospholipids that cause agglutination of mouse erythrocytes has been studied. Haemagglutination is a property of non-choline-containing phospholipids; the phosphate group is essential and unsaturated fatty acids optimal. A protein of Mr 70 000 was isolated from mouse erythrocyte membranes which completely inhibited phospholipid-mediated erythrocyte agglutination. It is proposed that this protein is the phospholipid binding site on mouse erythrocytes and the ligand for the human B-lymphocyte receptor for mouse erythrocytes. Preliminary investigations suggest that a similar inhibitor of phospholipid-mediated agglutination is found in serum. Agglutination of mouse erythrocytes by phospholipid and specific inhibition by the 70 kDa membrane protein constitute a simple system for studying the interaction of phospholipid with protein.

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.

Modification of the phosphatidylcholine-transfer protein from bovine liver with butanedione and phenylglyoxal. Evidence for one essential arginine residue

1. Modification of arginine residues with 2,3-butanedione and phenylglyoxal completely inhibits the transfer activity of the phosphatidylcholine transfer protein from bovine liver. Removal of borate and butanedione leads to a slow reactivation of the protein. 2. Both alpha-dicarbonyl reagents modify three of the ten arginine residues present per protein molecule. The extent of modification is linearly related to the loss of activity. 3. Inactivation with butanedione is greatly diminished when the protein is bound to strongly negatively charged vesicles. Under these conditions a rapid modification of two arginine residues is observed. This suggests that the transfer protein contains one arginine residue essential for activity, probably as a binding site for the negatively charged phosphate group of the phosphatidylcholine molecule. 4. This study provides convincing evidence that arginine residues may play an essential role in phospholipidprotein interactions.