Static and time-resolved fluorescence studies of fluorescent phosphatidylcholine bound to the phosphatidylcholine transfer protein of bovine liver

Structure and function of phosphatidylcholine transfer protein (PC-TP)/StarD2

Phosphatidylcholine transfer protein (PC-TP) is a highly specific soluble lipid binding protein that transfers phosphatidylcholine between membranes in vitro. PC-TP is a member of the steroidogenic acute regulatory protein-related transfer (START) domain superfamily. Although its biochemical properties and structure are well characterized, the functions of PC-TP in vivo remain incompletely understood. Studies of mice with homozygous disruption of the Pctp gene have largely refuted the hypothesis that this protein participates in the hepatocellular selection and transport of biliary phospholipids, in the production of lung surfactant, in leukotriene biosynthesis and in cellular phosphatidylcholine metabolism. Nevertheless, Pctp(-/-) mice exhibit interesting defects in lipid homeostasis, the understanding of which should elucidate the biological functions of PC-TP.

The binding of phosphatidylcholine to the phosphatidylcholine transfer protein: affinity and role in folding

Bovine liver phosphatidylcholine transfer protein (PC-TP) has been expressed in Escherichia coli and purified to homogeneity from the cytosol fraction at a yield of 0.45 mg PC-TP per 10 mg total cytosolic protein. In addition, active PC-TP was obtained from inclusion bodies. An essential factor in the activation of PC-TP was phosphatidylcholine (PC) present in the folding buffer. PC-TP from the cytosol contains phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) with a preference for the di-monounsaturated species over the saturated species as determined by fast atom bombardment mass spectrometry (FAB-MS). By incubation with microsomal membranes the endogenous PE and PG were replaced by PC. Relative to the microsomal PC species composition, PC-TP bound preferentially C16:0/C20:4-PC and C16:0/C18:2-PC (twofold enriched) whereas the major microsomal species C18:0/C18:1-PC and C18:0/C18:2-PC were distinctly less bound. PC-TP is structurally homologous to the lipid-binding domain of the steroidogenic acute regulatory protein (Nat. Struct. Biol. 7 (2000) 408). Replacement of Lys(55) present in one of the beta-strands forming the lipid-binding site, with an isoleucine residue yielded an inactive protein. This suggests that Lys(55) be involved in the binding of the PC molecule.

Phospholipid transfer proteins and physiological functions

Issues of how cells generate and maintain unique lipid compositions in distinct intracellular membrane systems remain the subject of much study. A ubiquitous class of soluble proteins capable of transporting phospholipid monomers from membrane to membrane across an aqueous milieu has been thought to define part of the mechanism by which lipids are sorted in cells. Progress in the study of these phospholipid transfer proteins (PLTPs) raises questions regarding their physiological functions in cells and the mechanisms by which these proteins execute them. It is now clear that across the eukaryotic kingdom, members of this protein family exert essential roles in the regulation of phospholipid metabolism and central aspects of phospholipid-mediated signaling. Indeed, it is now known that dysfunction of specific PLTPs defines the basis of inherited diseases in mammals, and this list is expected to grow. Phospholipid transfer proteins, their biochemical properties, and the emerging clues regarding their physiological functions are reviewed.

The effect of Glu75 of staphylococcal nuclease on enzyme activity, protein stability and protein unfolding

Staphylococcal nuclease mutants, E57G and E75G, were generated. A comparison of the kinetic parameters both for mutants and wild-type protein shows that the Michaelis constants (Km) were almost identical for the wild-type protein and E57G mutant. An approximately 30-fold decrease in Km compared with the wild-type protein was observed for the E75G mutant. The turnover numbers for the enzyme (kcat) were higher with both the wild-type protein and the E57G mutant (3.88 +/- 0.21 x 103 s-1 and 3.71 +/- 0.28 x 103 s-1) than with the E75G mutant (3.04 +/- 0.02 x 102 s-1). The results of thermal denaturation with differential scanning microcalorimetry indicate that the excess calorimetric enthalpy of denaturations, DeltaHcal, was almost identical for the wild-type protein and E57G mutant (84.1 +/- 6.2 kcal.mol-1 and 79.3 +/- 7.1 kcal.mol-1, respectively). An approximately twofold decrease in DeltaHcal compared with the wild-type protein was observed for the E75G mutant (42.7 +/- 5.5 kcal.mol-1). These outcomes imply that Glu at position 75 plays a significant role in maintaining enzyme activity and protein stability. Further study of the unfolding of the wild-type protein and E75G mutant was conducted by using time-resolved fluorescence with a picosecond laser pulse. Two fluorescent lifetimes were found in the subnanosecond time range. The faster lifetime (tau2) did not generally vary with either pH or the concentration of guanidinium hydrochloride (GdmHCl) in the wild-type protein and the E75G mutant. The slow lifetime (tau1), however, did vary with these parameters and was faster as the protein is unfolded by either pH or GdmHCl denaturation. The midpoints of the transition for tau1 are pH 3.5 and 5.8 for the wild-type protein and E75G mutant, respectively, and the GdmHCl concentrations are 1.1 m and 0.6 m for the wild-type protein and E75G mutant, respectively. Parallel steady-state fluorescence measurements have also been carried out and the results are in general agreement with the time-resolved fluorescence experiments, indicating that Glu at position 75 plays an important role in protein unfolding.

The low-affinity lipid binding site of the non-specific lipid transfer protein. Implications for its mode of action

The non-specific lipid transfer protein (nsL-TP) from bovine liver was studied by using the following fluorescent lipid analogs: phosphatidylcholine species with a sn-2-pyrenylacyl-chain of different length [Pyr(x)PC], sn-2-pyrenyldecanoyl-labelled phosphatidylinositol [Pyr(10)PI], -phosphatidylinositol 4-phosphate [Pyr(10)PIP], -phosphatidylinositol 4,5-bisphosphate [Pyr(10)PIP2] and dehydroergosterol. These analogs provided information on the effect of hydrophobicity and charge on lipid binding and transfer by nsL-TP. Binding of the Pyr(x)PC species decreased with increasing sn-2 acyl-chain length. Under equilibrium conditions, the fraction of nsL-TP that carried a PC molecule did not exceed 8%, which is consistent with a low affinity binding site. Also nsL-TP-mediated transfer of the Pyr(x)PC species decreased with increasing sn-2 acyl-chain length and was highly correlated with spontaneous transfer. Binding of the phosphoinositides increased in the order Pyr(10)PI less than Pyr(10)PIP less than Pyr(10)PIP2, indicating that an increase in lipid negative charge stimulates binding. The transfer of the phosphoinositides, however, decreased in the same order, which suggests that a high negative charge impairs the dissociation of the phospholipid from nsL-TP. Cholesterol, at concentrations up to 50 mol% in the donor membrane, hardly affected binding and transfer of Pyr(6)PC, strongly suggesting that nsL-TP has no high binding affinity for cholesterol. In agreement with this, binding of dehydroergosterol to nsL-TP was not detectable. Despite this apparently negligible affinity, nsL-TP-mediated transfer of dehydroergosterol was in the same order as that of Pyr(6)PC. The results are interpreted to indicate that transfer of lipids by nsL-TP involves the formation of a putative low-affinity lipid-protein complex. This formation is enhanced when lipid hydrophobicity decreases or lipid negative charge increases. Based on the binding and transfer data, the mode of action of nsL-TP is discussed in terms of change in free energy.

Interaction of fatty acids with recombinant rat intestinal and liver fatty acid-binding proteins

Intestinal enterocytes contain two homologous fatty acid-binding proteins, intestinal fatty acid-binding protein (I-FABP)2 and liver fatty acid-binding protein (L-FABP). Since the functional basis for this multiplicity is not known, the fatty acid-binding specificity of recombinant forms of both rat I-FABP and rat L-FABP was examined. A systematic comparative analysis of the 18 carbon chain length fatty acid binding parameters, using both radiolabeled (stearic, oleic, and linoleic) and fluorescent (trans-parinaric and cis-parinaric) fatty acids, was undertaken. Results obtained with a classical Lipidex-1000 binding assay, which requires separation of bound from free fatty acid, were confirmed with a fluorescent fatty acid-binding assay not requiring separation of bound and unbound ligand. Depending on the nature of the fatty acid ligand, I-FABP bound fatty acid had dissociation constants between 0.2 and 3.1 microM and a consistent 1:1 molar ratio. The dissociation constants for L-FABP bound fatty acids ranged between 0.9 and 2.6 microM and the protein bound up to 2 mol fatty acid per mole of protein. Both fatty acid-binding proteins exhibited relatively higher affinity for unsaturated fatty acids as compared to saturated fatty acids of the same chain length. cis-Parinaric acid or trans-parinaric acid (each containing four double bonds) bound to L-FABP and I-FABP were displaced in a competitive manner by non-fluorescent fatty acid. Hill plots of the binding of cis- and trans- parinaric acid to L-FABP showed that the binding affinities of the two sites were very similar and did not exhibit cooperativity. The lack of fluorescence self-quenching upon binding 2 mol of either trans- or cis-parinaric acid/mol L-FABP is consistent with the presence of two binding sites with dissimilar orientation in the L-FABP. Thus, the difference in binding capacity between I-FABP and L-FABP predicts a structurally different binding site or sites.

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.

Affinity of phosphatidylcholine molecular species for the bovine phosphatidylcholine and phosphatidylinositol transfer proteins. Properties of the sn-1 and sn-2 acyl binding sites

Both the phosphatidylcholine transfer protein (PC-TP) and the phosphatidylinositol transfer protein (PI-TP) act as carriers of phosphatidylcholine (PC) molecules between membranes. To study the structure of the acyl binding sites of these proteins, the affinity of 32 distinct natural and related PC molecular species was determined by using a previously developed fluorometric competition assay. Marked differences in affinity between species were observed with both proteins. Affinity vs lipid hydrophobicity (determined by reverse-phase HPLC) plots displayed a well-defined maximum indicating that the acyl chain hydrophobicity is an important determinant of binding of a phospholipid molecule by these transfer proteins. However, besides the overall lipid hydrophobicity, steric properties of the individual acyl chains contribute considerably to the affinity, and PC-TP and PI-TP respond differently to modifications of the acyl chain structure. The affinity of PC-TP increased steadily with increasing unsaturation of the sn-2 acyl moiety, resulting in high affinity for species containing four and six double bonds in the sn-2 chain, whereas the affinity of PI-TP first increased up to two to three double bonds and then declined. These data, as well as the distinct effects of sn-2 chain double bond position and bromination, indicate that the sn-2 acyl chain binding sites of the two proteins are structurally quite different. The sn-1 acyl binding sites are dissimilar as well, since variation of the length of saturated sn-1 chain affected the affinity differently. The data are discussed in terms of the structural organization of the sn-1 and sn-2 acyl binding sites of PC-TP and PI-TP.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Time-resolved fluorescence spectroscopy of lumazine protein from Photobacterium phosphoreum using synchrotron radiation

Time-resolved fluorescence on lumazine protein from Photobacterium phosphoreum was performed with synchrotron radiation as a source of continuously tunable excitation. The experiments yielded structural and dynamic details from which two aspects became apparent. From fluorescence anisotropy decay monitoring of lumazine fluorescence with different excitation wavelengths, the average correlation times were shown to change, which must indicate the presence of anistropic motion of the protein. A similar study with 7-oxolumazine as the fluorescent ligand led to comparable results. The other remarkable observation dealt with the buildup of acceptor fluorescence, also observed with 7-oxolumazine although much less pronounced, which is caused by the finite energy transfer process between the single donor tryptophan and the energy accepting lumazine derivatives. Global analytical approaches in data analysis were used to yield realistic correlation times and reciprocal transfer rate constants. It was found that the tryptophan residue has a large motional freedom as also reported previously for this protein and for the related protein from P. leiognathi (Lee et al. 1985; Kulinski et al. 1987). The average distance between the tryptophan residue and the ligand donor-acceptor couple has been determined to be 2.7 nm for the same donor and two different acceptors.

Dynamic fluorescence study of the interaction of lumazine protein with bacterial luciferases

The equilibrium association of lumazine protein from Photobacterium phosphoreum with luciferases from either P. phosphoreum or an aldehyde-requiring dark mutant of Vibrio harveyi is measured from changes of the rotational correlation time which is derived from the decay of the lumazine ligand's fluorescence anisotropy. The rotational correlation time of lumazine protein is 23 ns (2 degrees C, 0.25 M Pi) and is increased on addition of luciferase due to the formation of a higher molecular weight complex. The V. harveyi luciferase exhibits full competence for the association and a 1:1 stoichiometry with a Kd in the range 40-90 microM. At lower ionic strength (0.05 M Pi), the Kd increases but is reduced again by the addition of dodecanol or dimyristoyllecithin. In contrast, tetradecanal, a substrate for the bioluminescence reaction, exerts no influence on the association. The equilibration rate is found to be too slow and for both luciferases the Kd values are too high for the interaction of the native proteins to account quantitatively for the spectral shifting of the bioluminescence by lumazine protein.

Properties of the binding sites for the sn-1 and sn-2 acyl chains on the phosphatidylinositol transfer protein from bovine brain

We have studied the properties of the fatty acyl binding sites of the phosphatidylinositol transfer protein (PI-TP) from bovine brain, by measuring the binding and transfer of pyrenylacyl-containing phosphatidylinositol (PyrPI) species and pyrenylacyl-containing phosphatidylcholine (PyrPC) species as a function of the acyl chain length. The PyrPI species carried a pyrene-labeled acyl chain of variable length in the sn-2 position and either palmitic acid [C(16)], palmitoleic acid [C(16:1)], or stearic acid [C(18:1)] in the sn-1 position. Binding and transfer of the PI species increased in the order C(18) less than C(16) less than C(16:1), with a distinct preference for those species that carry a pyrenyloctanoyl [Pyr(8)] or a pyrenyldecanoyl [Pyr(10)] chain. The PyrPC species studied consisted of two sets of positional isomers: one set contained a pyrenylacyl chain of variable length and a C(16) chain, and the other set contained an unlabeled chain of variable length and a Pyr(10) chain. The binding and transfer experiments showed that PI-TP discriminates between positional isomers with a preference for the species with a pyrenylacyl chain in the sn-1 position. This discrimination is interpreted to indicate that separate binding sites exist for the sn-1 and sn-2 acyl chains. From the binding and transfer profiles it is apparent that the binding sites differ in their preference for a particular acyl chain length. The binding and transfer vs chain length profiles were quite similar for C(16)Pyr(x)PC and C(16)Pyr(x)PI species, suggesting that the sn-2 acyl chains of PI and PC share a common binding site in PI-TP.

Effect of acceptor membrane phosphatidylcholine on the catalytic activity of bovine liver phosphatidylcholine transfer protein

Protein-mediated transfer of phosphatidylcholine (PC) by bovine liver phosphatidylcholine transfer protein (PC-TP) was examined using a vesicle-vesicle assay system. Donor and acceptor membranes were prepared from Escherichia coli phospholipids and limiting amounts of egg yolk PC. PC transfer between vesicles of E. coli lipid/egg PC was markedly higher than transfer of PC from vesicles of E. coli lipid/egg PC to vesicles of E. coli lipid. Kinetic parameters of the interaction between PC-TP and E. coli lipid vesicles with or without PC was investigated. The apparent dissociation constants of the complex formed between PC-TP and these vesicles were determined kinetically and from double-reciprocal plots of intrinsic PC-TP fluorescence intensity increase versus vesicle concentration. The magnitude of the dissociation constant decreased as the PC content of the vesicles increased from 0 to 5 mol%. In addition, kinetic analysis revealed that the presence of PC in acceptor vesicles increased both the association and dissociation of PC-TP from vesicles. The effect of membrane PC molecules on transfer rates was examined using bis-phosphatidylcholine, a dimeric PC molecule which is not transferred by PC-TP. Rates of PC transfer to acceptor vesicles comprised of E. coli lipid/bis-PC were virtually identical to rates observed with acceptors vesicles prepared from E. coli lipid. The results suggest that transfer of PC by PC-TP is enhanced only when insertion of protein-bound PC occurs concurrently with the extraction of a molecule of membrane PC, i.e., a concerted, one-step catalytic mechanism for phospholipid exchange.

Design, synthesis, and characterization of bis-phosphatidylcholine, a mechanistic probe of phosphatidylcholine transfer protein catalytic activity

The design, synthesis, and characterization of 1-(17,18-dithiatetratriacontandioyl)-bis(2-hexadecanoyl-sn-glycero -3- phosphocholine) is described. Bis-phosphatidylcholine is a dimeric phospholipid comprised of two glycerophosphocholine groups linked together by a disulfide bond at the distal ends of the sn-1 fatty acyl chains. Electron microscopy and [14C]glucose trapping studies indicate that hydrated dispersions of bis-phosphatidylcholine form closed, spherical structures which have diameters in the range of 125-500 nm. Sensitivity to phospholipase hydrolysis suggests that this bipolar lipid is organized in a membrane such that the two polar head groups of the molecular are oriented at the same surface of the membrane. Using conditions in which bovine liver phosphatidylcholine transfer protein transfers both unsaturated and saturated diacyl phosphatidylcholines between fluid phosphatidylcholine vesicles, no transfer of the bipolar phospholipid is observed. The lack of activity toward bis-phosphatidylcholine suggests that this molecule may be a useful tool for elucidating the role of membrane phosphatidylcholine in the catalytic mechanism of the phosphatidylcholine transfer protein.

Binding of phospholipids to the phosphatidylinositol transfer protein from bovine brain as studied by steady-state and time-resolved fluorescence spectroscopy

The phosphatidylinositol transfer protein isolated from brain, liver, heart and platelets was found to be present in two subforms which could be distinguished on the basis of the isoelectric points. In this study we have demonstrated that the two subforms isolated from bovine brain are due to the presence of either phosphatidylinositol or phosphatidylcholine in the lipid binding site of the protein. The transfer protein accommodates one phosphatidylinositol molecule in the binding site. The binding site for the sn-2 fatty acyl chain was investigated by incorporating in the transfer protein either phosphatidylinositol or phosphatidylcholine carrying a parinaroyl-chain attached at the sn-2 position. Time-resolved fluorescence spectroscopy revealed that the sn-2 fatty acyl chains for both phospholipids in the lipid-protein complex were completely immobilized (i.e., rotational correlation times of 17.4 ns for phosphatidylcholine and 16.3 ns for phosphatidylinositol). The similarity in correlation times suggests that the sn-2 fatty acyl chains of both phospholipids are accommodated in the same hydrophobic binding site of the protein.

The phosphatidylcholine transfer protein from bovine liver discriminates between phosphatidylcholine isomers. A monolayer study

The activity of the phosphatidylcholine transfer protein from bovine liver toward phosphatidylcholine isomers carrying a long and a short fatty acyl chain on either the sn-1- or sn-2-position was determined by way of the monolayer-vesicle assay. In this assay equimolar mixtures of the isomers were spread at the air/water interface and their transfer measured to the vesicles in the subphase initiated by addition of the transfer protein. The following isomers were tested: 1-decanoyl-2-[3H]oleoyl-sn-glycero-3-phosphocholine (C10:0/[3H]C18:1-PC) and 1-oleoyl-2-decanoyl-sn-glycero-3-phospho[14C]choline (C18:1/C10:0-[14C]PC); 1-lauroyl-2-[3H]oleoyl-sn-glycero-3-phosphocholine (C12:0/[3H]C18:1-PC) and 1-oleoyl-2-[14C]lauroyl-sn-glycero-3-phosphocholine (C18:1/[14C]C12:0-PC); 1-myristoyl-2-[3H]oleoyl-sn-glycero-3-phosphocholine (C14:0/[3H]C18:1-PC) and 1-oleoyl,2-myristoyl-sn-glycero-3-phospho[14C]choline (C18:1/C14:0-[14C]PC). It was found that the protein transferred C10:0/[3H]C18:1-PC twice as fast as C18:1/C10:0-[14C]PC. Similar differences in rate were observed for C12:0/[3H]C18:1-Pc and C18:1/[14C]C12:0-PC but not for the isomers carrying myristic acid. We propose that the transfer protein can discriminate between PC isomers due to the presence of distinct binding sites for the sn-1- and sn-2-acyl chain (Berkhout et al. (1984) Biochemistry, 23, 1505-1513).

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.

The lipid binding site of the phosphatidylcholine transfer protein from bovine liver

The phosphatidylcholine transfer protein (PC-TP) from bovine liver has a binding site for phosphatidylcholine (PC). Structural and molecular characteristics of this site were investigated by binding PC-analogues carrying photolabile, fluorescent and short-chain fatty acids. Analysis of the photolabeled PC/PC-TP adduct showed that the hydrophobic peptide segment Val171-Phe-Met-Tyr-Tyr-Phe-Asp177 is part of the lipid binding site for the 2-acyl chain. This site was further studied by binding PC carrying cis-parinaric acid at the sn-2-position. Time resolved fluorescence anisotropy measurements indicated that the 2-acyl chain was immobilized following the rotation of PC-TP. Similar experiments with PC carrying cis-parinaric acid at the sn-1-position demonstrated that the 1-acyl chain was immobilized as well but at a site distinctly different from that of the 2-acyl chain. Binding sites for the 1- and 2-acyl chain were then explored by use of PC-isomers carrying decanoic, lauric and myristic acid at the sn-1- (or sn-2-)-position and oleic acid at the sn-2- (or sn-1-)-position. Incubation with vesicles prepared of these PC-species indicated that binding to PC-TP diminished with decreasing acyl chain length but more so for species with short-chain fatty acids on the sn-2-position than on the sn-1-position. Transfer experiments confirmed that PC-TP discriminates between PC-isomers of apparently equal hydrophobicity favouring the transfer of these species carrying oleic acid at the sn-2-position.