Phosphatidylcholine transfer protein promotes apolipoprotein A-I-mediated lipid efflux in Chinese hamster ovary cells.

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic protein of unknown function that catalyzes intermembrane transfer of phosphatidylcholines in vitro. Using stably transfected CHO cells, we explored the influence of PC-TP on apolipoprotein A-I- and high density lipoprotein 3 (HDL(3))-mediated lipid efflux. In proportion to its cellular level of expression, PC-TP accelerated apolipoprotein A-I-mediated phospholipid and cholesterol efflux as pre-beta-HDL particles. PC-TP increased rates of efflux of both lipids by >2-fold but did not affect mRNA levels or the activity of ATP-binding cassette A1, a plasma membrane protein that regulates apolipoprotein A-I-mediated lipid efflux. Overexpression of PC-TP was associated with only slight increases in HDL(3)-mediated phospholipid efflux and no changes in cholesterol efflux. In scavenger receptor BI-overexpressing cells, PC-TP expression minimally influenced apolipoprotein A-I- or HDL(3)-mediated lipid efflux. PC-TP did not affect cellular phospholipid compositions, phosphatidylcholine contents, or phosphatidylcholine synthetic rates. These findings suggest that a physiological function of PC-TP is to replenish the plasma membrane with phosphatidylcholines that are removed during pre-beta-HDL particle formation due to the activity of ATP-binding cassette A1.     

Structure of human phosphatidylcholine transfer protein in complex with its ligand

Phosphatidylcholines (PtdChos) comprise the most common phospholipid class in eukaryotic cells. In mammalian cells, these insoluble molecules are transferred between membranes by a highly specific phosphatidylcholine transfer protein (PC-TP) belonging to the steroidogenic acute regulatory protein related transfer (START) domain superfamily of hydrophobic ligand-binding proteins. The crystal structures of human PC-TP in complex with dilinoleoyl-PtdCho or palmitoyl-linoleoyl-PtdCho reveal that a single well-ordered PtdCho molecule occupies a centrally located tunnel. The positively charged choline headgroup of the lipid engages in cation-pi interactions within a cage formed by the faces of three aromatic residues. These binding determinants and those for the phosphoryl group may be exposed to the lipid headgroup at the membrane-water interface by a conformational change involving the amphipathic C-terminal helix and an Omega-loop. The structures presented here provide a basis for rationalizing the specificity of PC-TP for PtdCho and may identify common features used by START proteins to bind their hydrophobic ligands.     

Regulation of ghrelin structure and membrane binding by phosphorylation

The peptide hormone ghrelin requires Ser-3 acylation for receptor binding, orexigenic and anti-inflammatory effects. Functions of desacylghrelin are less well understood. In vitro kinase assays reveal that the evolutionarily conserved Ser-18 in the basic C-terminus is an excellent substrate for protein kinase C. Circular dichroism reveals that desacylghrelin is 12% helical in aqueous solution and 50% helical in trifluoroethanol. Ser-18-phosphorylation, Ser-18-Ala substitution, or Ser-3-acylation reduces the helical character in trifluoroethanol to 24%. Both ghrelin and desacylghrelin bind to phosphatidylcholine:phosphatidylserine sucrose-loaded vesicles in a phosphatidylserine-dependent manner. Phosphoghrelin and phosphodesacylghrelin show greatly diminished phosphatidylserine-dependent binding. These results are consistent with binding of ghrelin and desacylghrelin to acidic lipids via the basic face of an amphipathic helix with Ser-18 phosphorylation disrupting both helical character and membrane binding.     

The structure of the lactose permease derived from Raman spectroscopy and prediction methods.

The secondary structure of the lactose permease of Escherichia coli reconstituted in lipid membranes was determined by Raman spectroscopy. The alpha-helix content is approximately 70%, the beta-strand content below 10% and beta-turns contribute 15%. About 1/3 of the residues in alpha-helices and most other residues are exposed to water. Employing a method for structural prediction which accounts for amphipathic helices, 10 membrane-spanning helices are predicted which are either hydrophobic or amphipathic. They are expected to form an outer ring of helices in the membrane. The interior of the ring would be made of residues which are predominantly hydrophilic and, evoking the analogy to sugar-binding proteins, suited to provide the sugar binding site.     

Three-dimensional structure of a cloning vector. X-ray diffraction studies of filamentous bacteriophage M13 at 7 A resolution.

Filamentous bacteriophage M13 is a single-stranded DNA phage about 65 A in diameter and 9300 A long. X-ray diffraction studies of magnetically oriented fibers of native, mercury and iodine-labeled phage particles have been used to determine the arrangement of the major coat protein, the gene 8 product, in the virion. The coat protein is made up of a single gently curving alpha-helix extending from approximately Pro6 to near the carboxyl terminus. The axis of the alpha-helix is tilted about 20 degrees from the viral axis and wraps around the axis in a right-handed helical sense. The surface of the virus is made up largely of polar residues in the amino-terminal half of the protein including the segment of alpha-helix extending from Pro6 to Tyr24. The interior surface of the protein coat faces the DNA and consists of an amphipathic helical segment extending from Thr36 to Ser50. The alpha-helices form a tightly packed 15 to 20 A thick cylindrical coat around the DNA. This structural model provides insight into the potential sites for incorporating foreign protein domains that may act as functional binding sites on the surface of M13.     

Induction of transient ion channel-like pores in a cancer cell by antibiotic peptide

The anticancer activity of anti-bacterial cecropins makes them potentially useful as peptide anti-cancer drugs. We used the cell-attached patch to study the effect of cecropin B (CB; having one hydrophobic and one amphipathic alpha-helix) and its derivative, cecropin B3 (CB3; having two hydrophobic alpha-helices) on the membrane of Ags cancer cells. Application of 10-60 microM CB onto the membrane of the cancer cell produces short outward currents. Comparative study with CB3, which induces no outward currents, shows that the amphipathic group of CB is necessary for the pore formation. The results provide a rationale to study the cell-killing activity of antimicrobial peptides at the single cancer cell level.     

Expression, purification, and activities of full-length and truncated versions of the integral membrane protein Vpu from HIV-1

Vpu is an 81-residue accessory protein of HIV-1. Because it is a membrane protein, it presents substantial technical challenges for the characterization of its structure and function, which are of considerable interest because the protein enhances the release of new virus particles from cells infected with HIV-1 and induces the intracellular degradation of the CD4 receptor protein. The Vpu-mediated enhancement of the virus release rate from HIV-1-infected cells is correlated with the expression of an ion channel activity associated with the transmembrane hydrophobic helical domain. Vpu-induced CD4 degradation and, to a lesser extent, enhancement of particle release are both dependent on the phosphorylation of two highly conserved serine residues in the cytoplasmic domain of Vpu. To define the minimal folding units of Vpu and to identify their activities, we prepared three truncated forms of Vpu and compared their structural and functional properties to those of full-length Vpu (residues 2-81). Vpu(2-37) encompasses the N-terminal transmembrane alpha-helix; Vpu(2-51) spans the N-terminal transmembrane helix and the first cytoplasmic alpha-helix; Vpu(28-81) includes the entire cytoplasmic domain containing the two C-terminal amphipathic alpha-helices without the transmembrane helix. Uniformly isotopically labeled samples of the polypeptides derived from Vpu were prepared by expression of fusion proteins in E. coli and were studied in the model membrane environments of lipid micelles by solution NMR spectroscopy and oriented lipid bilayers by solid-state NMR spectroscopy. The assignment of backbone resonances enabled the secondary structure of the constructs corresponding to the transmembrane and the cytoplasmic domains of Vpu to be defined in micelle samples by solution NMR spectroscopy. Solid-state NMR spectra of the polypeptides in oriented lipid bilayers demonstrated that the topology of the domains is retained in the truncated polypeptides. The biological activities of the constructs of Vpu were evaluated. The ion channel activity is confined to the transmembrane alpha-helix. The C-terminal alpha-helices modulate or promote the oligomerization of Vpu in the membrane and stabilize the conductive state of the channel, in addition to their involvement in CD4 degradation.     

The helical structure of surfactant peptide KL4 when bound to POPC: POPG lipid vesicles

KL 4 is a 21-residue peptide employed as a functional mimic of lung surfactant protein B, which successfully lowers surface tension in the alveoli. A mechanistic understanding of how KL 4 affects lipid properties has proven elusive as the secondary structure of KL 4 in lipid preparations has not been determined at high resolution. The sequence of KL 4 is based on the C-terminus of SP-B, a naturally occurring helical protein that binds to lipid interfaces. The spacing of the lysine residues in KL 4 precludes the formation of a canonical amphipathic alpha-helix; qualitative measurements using Raman, CD, and FTIR spectroscopies have given conflicting results as to the secondary structure of the peptide as well as its orientation in the lipid environment. Here, we present a structural model of KL 4 bound to lipid bilayers based on solid state NMR data. Double-quantum correlation experiments employing (13)C-enriched peptides were used to quantitatively determine the backbone torsion angles in KL 4 at several positions. These measurements, coupled with CD experiments, verify the helical nature of KL 4 when bound to lipids, with (phi, psi) angles that differ substantially from common values for alpha-helices of (-60, -45). The average torsion angles found for KL 4 bound to POPC:POPG lipid vesicles are (-105, -30); this deviation from ideal alpha-helical structure allows KL 4 to form an amphipathic helix at the lipid interface.     

The role of the N terminus in Tet repressor for tet operator binding determined by a mutational analysis.

The N-terminal residues preceding the alpha-helix-turn-alpha-helix motif on the Tn10 Tet repressor protein were probed by oligonucleotide-directed deletion mutagenesis for their role in protein activity. All deletion mutants showed decreased repression in vivo, emphasizing the importance of the N terminus for tet operator binding. Only two of the mutants, TetR delta 2-23 and TetR delta 3-8 displayed a reduced intracellular protein level. The remaining deletion mutants showed either reduced binding to tet operator and inducibility by tetracycline or transdominance. We conclude that these deletions do not affect stability and overall protein structure. DNA binding activities of residue-wise increasing deletions, TetR delta 9 through TetR delta 9-13, reveal a pattern consistent with an alpha-helical structure of the affected residues. This conclusion is supported by the helical wheel projection and the hydrophobic moment profile calculated for the protein segment ranging from residues S7-V20. We propose that these residues form an amphipathic alpha-helix which packs closely against the alpha-helix-turn-alpha-helix motif and is essential for Tet repressor activity. The residues preceding this putative alpha-helix contribute to DNA binding, but no direct interactions with base pairs of tet operator were revealed in a loss of contact analysis. Individual mutation of the 4 charged residues to alanine at the N terminus shows that no single residue can account for the reduction in repression observed for the deletion mutants.     

Conformations of yeast alpha-mating factor and analog peptides as bound to phospholipid bilayer. Correlation of membrane-bound conformation with physiological activity

The transferred nuclear Overhauser effects of yeast alpha-mating factor [(1-13)peptide] in the presence of various spin-labeled phosphatidylcholines in small unilamellar vesicles of perdeuterated phosphatidylcholine have been analyzed. From the analysis of the quenching effect by spin-labels, the depth of amino acid side chains of the mating factor in phospholipid bilayer has been elucidated. The Leu4 and Leu6 residues are buried deeply in the apolar region of the phospholipid bilayer while the hydrophilic residues such as Gln5 and Lys7 are in the shallow region of the bilayer. The interaction of the side chains of Trp1 and Trp3 residues of alpha-mating factor with the hydrophobic interior of the bilayer contributes to the binding of this peptide with the phosphatidylcholine bilayer. The conformation of des-Trp1-alpha-mating-factor [(2-13)peptide] in the membrane-bound state has been found to be similar to that of (1-13)peptide from the analysis of transferred nuclear Overhauser effects in the presence of mixed vesicles of perdeuterated phosphatidylcholine and perdeuterated phosphatidylserine. The incorporation of this acidic phospholipid in the vesicle remarkably enhances the binding of (1-13)peptide and analog peptides. However, such modifications that weaken the interaction with phospholipid bilayer (deletion of Trp1 and substitution of Trp3 by Gly or Ala) appreciably lower the physiological activity. Transferred nuclear Overhauser effect analyses have also been made of [DHis2]peptide, [DLeu6]peptide and [DLys7]peptide in the presence of the vesicles of perdeuterated phosphatidylcholine. The main-chain conformations of these three analogs in the membrane-bound state have been found to be similar to that of (1-13)peptide, although the side-chain conformations of the D-amino acid residues are naturally different from those of the L-amino acid ones. Thus, the physiological activities of the (1-13)peptide and a variety of analog peptides are found to correlate with the affinities to the phosphatidylcholine/phosphatidylserine membrane and with the molecular conformations in the membrane-bound state.     

The interaction of peripheral proteins and membranes studied with alpha-lactalbumin and phospholipid bilayers of various compositions

To characterize the interaction of peripheral proteins and membranes at the molecular level, we studied the reversible association of bovine alpha-lactalbumin (BLA) with lipid bilayers composed of different molecular forms of phosphatidylserine or equimolar mixtures of these phosphatidylserine forms and egg yolk phosphatidylcholine. At pH 4.5, almost all BLA (>90%) associates to negatively charged small unilamellar vesicles. The conformational changes that binding to these bilayers induced on the protein were characterized by circular dichroism and fluorescence spectroscopy. Because binding of BLA to negatively charged vesicles is reverted by adjusting the pH back to >6.0, we also investigated the conformation of the membrane-bound protein by NMR-monitored H-D exchange of the backbone amide protons. The conformation adopted by BLA bound to these bilayers resembles a molten globule-like state but the negative ellipticity at 222 nm and the apparent alpha-helix content of the bound protein senses the changes in the physical properties of the membrane. Binding to bilayers in the gel state appears to correlate with an increased amount of alpha-helical structure and with a lower extent of integration into the membrane, corresponding to the adsorbed protein, while the opposite is found for BLA bound to vesicles in the liquid-crystalline phase, corresponding to the embedded conformation. A common feature for the membrane-bound conformations of BLA is that the amphipathic helix C (residues 86 to 99) is an important determinant for the adsorption and further integration of the protein into the membrane.     

A helical hairpin region of soluble annexin B12 refolds and forms a continuous transmembrane helix at mildly acidic pH

Annexins are soluble proteins that are best known for their ability to undergo reversible Ca(2+)-dependent binding to the surface of phospholipid bilayers. Recent studies, however, have shown that annexins also reversibly bind to membranes in a Ca(2+)-independent manner at mildly acidic pH. We investigated the structural changes that occur upon pH-dependent membrane binding by performing a nitroxide scan on the helical hairpin encompassing helices A and B in the fourth repeat of annexin B12. Residues 251-273 of annexin B12 were replaced, one at a time, with cysteine and then labeled with a nitroxide spin label. Electron paramagnetic resonance (EPR) mobility and accessibility analyses of soluble annexin B12 derivatives were in excellent agreement with the known crystal structure of annexin B12. However, EPR studies of annexin B12 derivatives bound to membranes at pH 4.0 indicated major structural changes in the scanned region. The helix-loop-helix structure present in the soluble protein was converted into a continuous transmembrane alpha-helix that was exposed to the hydrophobic core of the bilayer on one side and exposed to an aqueous pore on the other side. Asp-264 was on the hydrophobic membrane-exposed face of the amphipathic transmembrane helix, thereby suggesting that protonation of its carboxylate group stabilized the transmembrane form. Inspection of the amino acid sequence of annexin B12 revealed several other helical hairpin regions that might refold and form continuous amphipathic transmembrane helices in response to protonation of Asp or Glu switch residues on or near the hydrophobic face of the helix.     

Crystal structure of a secondary vitamin D3 binding site of milk beta-lactoglobulin

Beta-lactoglobulin (beta-LG), one of the most investigated proteins, is a major bovine milk protein with a predominantly beta structure. The structural function of the only alpha-helix with three turns at the C-terminus is unknown. Vitamin D(3) binds to the central calyx formed by the beta-strands. Whether there are two vitamin D binding-sites in each beta-LG molecule has been a subject of controversy. Here, we report a second vitamin D(3) binding site identified by synchrotron X-ray diffraction (at 2.4 A resolution). In the central calyx binding mode, the aliphatic tail of vitamin D(3) clearly inserts into the binding cavity, where the 3-OH group of vitamin D(3) binds externally. The electron density map suggests that the 3-OH group interacts with the carbonyl of Lys-60 forming a hydrogen bond (2.97 A). The second binding site, however, is near the surface at the C-terminus (residues 136-149) containing part of an alpha-helix and a beta-strand I with 17.91 A in length, while the span of vitamin D(3) is about 12.51 A. A remarkable feature of the second exosite is that it combines an amphipathic alpha-helix providing nonpolar residues (Phe-136, Ala-139, and Leu-140) and a beta-strand providing a nonpolar (Ile-147) and a buried polar residue (Arg-148). They are linked by a hydrophobic loop (Ala-142, Leu-143, Pro-144, and Met-145). Thus, the binding pocket furnishes strong hydrophobic force to stabilize vitamin D(3) binding. This finding provides a new insight into the interaction between vitamin D(3) and beta-LG, in which the exosite may provide another route for the transport of vitamin D(3) in vitamin D(3) fortified dairy products. Atomic coordinates for the crystal structure of beta-LG-vitamin D(3) complex described in this work have been deposited in the PDB (access code 2GJ5).     

Relative spatial positions of tryptophan and cationic residues in helical membrane-active peptides determine their cytotoxicity

The cytotoxic activity of 10 analogs of the idealized amphipathic helical 21-mer peptide (KAAKKAA)3, where three of the Ala residues at different positions have been replaced with Trp residues, has been investigated. The peptide's cytotoxic activity was found to be markedly dependent upon the position of the Trp residues within the hydrophobic sector of an idealized α-helix. The peptides with Trp residues located opposite the cationic sector displayed no antitumor activity, whereas those peptides with two or three Trp residues located adjacent to the cationic sector exhibited high cytotoxic activity when tested against three different cancer cell lines. Dye release experiments revealed that in contrast to the peptides with Trp residues located opposite the cationic sector, the peptides with Trp residues located adjacent to the cationic sector induced a strong permeabilizing activity from liposomes composed of a mixture of zwitterionic phosphatidylcholine and negatively charged phosphatidylserine (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS)) (2:1) but not from liposomes composed of zwitterionic phosphatidylcholine, POPC. Fluorescence blue shift and quenching experiments revealed that Trp residues inserted deeper into the hydrophobic environment of POPC/POPS liposomes for peptides with high cytotoxic activity. Through circular dichroism studies, a correlation between the cytotoxic activity and the α-helical propensity was established. Structural studies of one inactive and two active peptides in the presence of micelles using NMR spectroscopy showed that only the active peptides adopted highly coiled to helical structures when bound to a membrane surface.     

Lipid-protein interactions studied by introduction of a tryptophan residue: the mechanosensitive channel MscL

Trp fluorescence spectroscopy is a powerful tool to study the structures of membrane proteins and their interactions with the surrounding lipid bilayer. Many membrane proteins contain more than one Trp residue, making analysis of the fluorescence data more complex. The mechanosensitive channels MscL's of Mycobacterium tuberculosis (TbMscL) and Escherichia coli (EcMscL) contain no Trp residues. We have therefore introduced single Trp residues into the transmembrane regions of TbMscL and EcMscL to give the Trp-containing mutants F80W-TbMscL and F93W-EcMscL, respectively, which we show are highly suitable for measurements of lipid binding constants. In vivo cell viability assays in E. coli show that introduction of the Trp residues does not block function of the channels. The Trp-containing mutants have been reconstituted into lipid bilayers by mixing in cholate followed by dilution to re-form membranes. Cross-linking experiments suggest that the proteins retain their pentameric structures in phosphatidylcholines with chain lengths between C14 and C24, phosphatidylserines, and phosphatidic acid. Quenching of Trp fluorescence by brominated phospholipids suggests that the Trp residue in F80W-TbMscL is more exposed to the lipid bilayer than the Trp residue in F93W-EcMscL. Binding constants for phosphatidylcholines change with changing fatty acyl chain length, the strongest interaction for both TbMscL and EcMscL being observed with a chain of length C16, corresponding to a bilayer of hydrophobic thickness ca. 24 A, compared to a hydrophobic thickness for TbMscL of about 26 A estimated from the crystal structure. Lipid binding constants change by only a factor of 1.5 in the chain length range from C12 to C24, much less than expected from theories of hydrophobic mismatch in which the protein is treated as a rigid body. It is concluded that MscL distorts to match changes in bilayer thickness. The binding constants for dioleoylphosphatidylethanolamine for both TbMscL and EcMscL relative to those for dioleoylphosphatidylcholine are close to 1. Quenching experiments suggest a single class of binding sites for phosphatidylserine, phosphatidylglycerol, and cardiolipin on TbMscL; binding constants are greater than those for phosphatidylcholine and decrease with increasing ionic strength, suggesting that charge interactions are important in binding these anionic phospholipids. Quenching experiments suggest two classes of lipid binding sites on TbMscL for phosphatidic acid, binding of phosphatidic acid being much less dependent on ionic strength than binding of phosphatidylserine.     

Phosphatidylcholine transfer protein regulates size and hepatic uptake of high-density lipoproteins

Phosphatidylcholine transfer protein (PC-TP) is a steroidogenic acute regulatory-related transfer domain protein that is enriched in liver cytosol and binds phosphatidylcholines with high specificity. In tissue culture systems, PC-TP promotes ATP-binding cassette protein A1-mediated efflux of cholesterol and phosphatidylcholine molecules as nascent pre-beta-high-density lipoprotein (HDL) particles. Here, we explored a role for PC-TP in HDL metabolism in vivo utilizing 8-wk-old male Pctp(-/-) and wild-type littermate C57BL/6J mice that were fed for 7 days with either chow or a high-fat/high-cholesterol diet. In chow-fed mice, neither plasma cholesterol concentrations nor the concentrations and compositions of plasma phospholipids were influenced by PC-TP expression. However, in Pctp(-/-) mice, there was an accumulation of small alpha-migrating HDL particles. This occurred without changes in hepatic expression of ATP-binding cassette protein A1 or in proteins that regulate the intravascular metabolism and clearance of HDL particles. In Pctp(-/-) mice fed the high-fat/high-cholesterol diet, HDL particle sizes were normalized, whereas plasma cholesterol and phospholipid concentrations were increased compared with wild-type mice. In the absence of upregulation of hepatic ATP-binding cassette protein A1, reduced HDL uptake from plasma into livers of Pctp(-/-) mice contributed to higher plasma lipid concentrations. These data indicate that PC-TP is not essential for the enrichment of HDL with phosphatidylcholines but that it does modulate particle size and rates of hepatic clearance.     

Statistical and functional analyses of viral and cellular proteins with N-terminal amphipathic alpha-helices with large hydrophobic moments: importance to macromolecular recognition and organelle targeting.

A total of 1,911 proteins with N-terminal methionyl residues were computer screened for potential N-terminal alpha-helices with strong amphipathic character. By the criteria of D. Eisenberg (Annu. Rev. Biochem. 53:595-623, 1984), only 3.5% of nonplastid, nonviral proteins exhibited potential N-terminal alpha-helices, 18 residues in length, with hydrophobic moment values per amino acyl residue ([muH]) in excess of 0.4. By contrast, 10% of viral proteins exhibited corresponding [muH] values in excess of 0.4. Of these viral proteins with known functions, 55% were found to interact functionally with nucleic acids, 30% were membrane-interacting proteins or their precursors, and 15% were structural proteins, primarily concerned with host cell interactions. These observations suggest that N-terminal amphipathic alpha-helices of viral proteins may (i) function in nucleic acid binding, (ii) facilitate membrane insertion, and (iii) promote host cell interactions. Analyses of potential amphipathic N-terminal alpha-helices of cellular proteins are also reported, and their significance to organellar or envelope targeting is discussed.