Amino acid sequence of a non-specific wheat phospholipid transfer protein and its conformation as revealed by infrared and Raman spectroscopy. Role of disulfide bridges and phospholipids in the stabilization of the alpha-helix structure

Loading characteristics of streptavidin on polypropylene capillary channeled polymer fibers and capture performance towards biotinylated proteins

The development of higher-throughput, potentially lower-cost means to isolate proteins, for a variety of end uses, is of continuing emphasis. Polypropylene (PP) capillary-channeled polymer (C-CP) fiber columns are modified with the biotin-binding protein streptavidin (SAV) to capture biotinylated proteins. The loading characteristics of SAV on fiber supports were determined using breakthrough curves and frontal analysis. Based on adsorption data, a 3-min on-column loading at a flow rate of 0.5 mL min-1 (295.2 cm h-1) with a SAV feed concentration of 0.5 mg mL-1 produces a SAV loading capacity of 1.4 mg g-1 fiber. SAV has an incredibly high affinity for the small-molecule biotin (10-14 M), such that this binding relationship can be exploited by labeling a target protein with biotin via an Avi-tag. To evaluate the capture of the biotinylated proteins on the modified PP surface, the biotinylated versions of bovine serum albumin (b-BSA) and green fluorescent protein (b-GFP) were utilized as probe species. The loading buffer composition and flow rate were optimized towards protein capture. The non-ionic detergent Tween-20 was added to the deposition solutions to minimize non-specific binding. Values of 0.25-0.50% (v/v) Tween-20 in PBS exhibited better capture efficiency, while minimizing the non-specific binding for b-BSA and b-GFP, respectively. The C-CP fiber platform has the potential to provide a fast and low-cost method to capture targeted proteins for applications including protein purification or pull-down assays.

Structural characterization and in vitro lipid binding studies of non-specific lipid transfer protein 1 (nsLTP1) from fennel (Foeniculum vulgare) seeds

Non-specific lipid transfer proteins (nsLTPs) are cationic proteins involved in intracellular lipid shuttling in growth and reproduction, as well as in defense against pathogenic microbes. Even though the primary and spatial structures of some nsLTPs from different plants indicate their similar features, they exhibit distinct lipid-binding specificities signifying their various biological roles that dictate further structural study. The present study determined the complete amino acid sequence, in silico 3D structure modeling, and the antiproliferative activity of nsLTP1 from fennel (Foeniculum vulgare) seeds. Fennel is a member of the family Umbelliferae (Apiaceae) native to southern Europe and the Mediterranean region. It is used as a spice medicine and fresh vegetable. Fennel nsLTP1 was purified using the combination of gel filtration and reverse-phase high-performance liquid chromatography (RP-HPLC). Its homogeneity was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry. The purified nsLTP1 was treated with 4-vinyl pyridine, and the modified protein was then digested with trypsin. The complete amino acid sequence of nsLTP1 established by intact protein sequence up to 28 residues, overlapping tryptic peptides, and cyanogen bromide (CNBr) peptides. Hence, it is confirmed that fennel nsLTP1 is a 9433 Da single polypeptide chain consisting of 91 amino acids with eight conserved cysteines. Moreover, the 3D structure is predicted to have four α-helices interlinked by three loops and a long C-terminal tail. The lipid-binding property of fennel nsLTP1 is examined in vitro using fluorescent 2-p-toluidinonaphthalene-6-sulfonate (TNS) and validated using a molecular docking study with AutoDock Vina. Both of the binding studies confirmed the order of binding efficiency among the four studied fatty acids linoleic acid > linolenic acid > Stearic acid > Palmitic acid. A preliminary screening of fennel nsLTP1 suppressed the growth of MCF-7 human breast cancer cells in a dose-dependent manner with an IC50 value of 6.98 µM after 48 h treatment.

Specificity of synapse formation in Aplysia: paracrine and autocrine signaling regulates bidirectional molecular interactions between sensory and non-target motor neurons

The formation of appropriate neural connections during development is critical for the proper wiring and functioning of the brain. Although considerable research suggests that the specificity of synapse formation is supported by complex intercellular signaling between potential presynaptic and postsynaptic partners, the extracellular factors and the intracellular signal transduction pathways engaged in this process remain largely unknown. Using the sensory-motor neural circuit that contributes to learning in defensive withdrawal reflexes in Aplysia californica, we investigated the molecular processes governing the interactions between sensory neurons and both target and non-target motor neurons during synapse formation in culture. We found that evolutionarily-conserved intercellular and intracellular signaling mechanisms critical for learning-related plasticity are also engaged during synaptogenesis in this in vitro model system. Our results reveal a surprising bidirectional regulation of molecular signaling between sensory neurons and non-target motor neurons. This regulation is mediated by signaling via both paracrine and autocrine diffusible factors that induce differential effects on transcription and on protein expression/activation in sensory neurons and in target and non-target motor neurons. Collectively, our data reveal novel molecular mechanisms that could underlie the repression of inappropriate synapse formation, and suggest mechanistic similarities between developmental and learning-related plasticity.

Orthology Analysis and In Vivo Complementation Studies to Elucidate the Role of DIR1 during Systemic Acquired Resistance in Arabidopsis thaliana and Cucumis sativus

AtDIR1 (Defective in Induced Resistance1) is an acidic lipid transfer protein essential for systemic acquired resistance (SAR) in Arabidopsis thaliana. Upon SAR induction, DIR1 moves from locally infected to distant uninfected leaves to activate defense priming; however, a molecular function for DIR1 has not been elucidated. Bioinformatic analysis and in silico homology modeling identified putative AtDIR1 orthologs in crop species, revealing conserved protein motifs within and outside of DIR1's central hydrophobic cavity. In vitro assays to compare the capacity of recombinant AtDIR1 and targeted AtDIR1-variant proteins to bind the lipophilic probe TNS (6,P-toluidinylnaphthalene-2-sulfonate) provided evidence that conserved leucine 43 and aspartic acid 39 contribute to the size of the DIR1 hydrophobic cavity and possibly hydrophobic ligand binding. An Arabidopsis-cucumber SAR model was developed to investigate the conservation of DIR1 function in cucumber (Cucumis sativus), and we demonstrated that phloem exudates from SAR-induced cucumber rescued the SAR defect in the Arabidopsis dir1-1 mutant. Additionally, an AtDIR1 antibody detected a protein of the same size as AtDIR1 in SAR-induced cucumber phloem exudates, providing evidence that DIR1 function during SAR is conserved in Arabidopsis and cucumber. In vitro TNS displacement assays demonstrated that recombinant AtDIR1 did not bind the SAR signals azelaic acid (AzA), glycerol-3-phosphate or pipecolic acid. However, recombinant CsDIR1 and CsDIR2 interacted weakly with AzA and pipecolic acid. Bioinformatic and functional analyses using the Arabidopsis-cucumber SAR model provide evidence that DIR1 orthologs exist in tobacco, tomato, cucumber, and soybean, and that DIR1-mediated SAR signaling is conserved in Arabidopsis and cucumber.

Understanding the roles of Lys33 and Arg45 in the binding-site stability of LjLTP10, an LTP related to drought stress in Lotus japonicus

In Lotus japonicus, as in most plants, long-chain fatty acids are important components of cuticular wax, one of the principal functions of which is to act as a barrier to water loss in response to drought stress. It is thought that lipid transfer proteins (LTPs) are involved in the process of cuticle formation. We previously described LjLTP10 as an LTP involved in cuticle formation during acclimation response to drought stress in L. japonicus. The structural model of LjLTP10 had two residues (K33 and R45) in the hydrophobic cavity, although the role of these residues was unclear. In the present work, we investigated the molecular mechanism involved in the transport of lipid precursors in L. japonicus and clarified the importance of the residues K33 and R45. First, in silico site-directed mutagenesis studies were carried out on the LjLTP10 structure. Structural analysis showed that LjLTP10 mutants possess similar structures but their hydrophobic cavities are somewhat different. Unfavorable energies for the interactions of the mutant proteins with different ligands were found by molecular docking and molecular dynamics simulations. We also examined the contributions of energetic parameters to the free energy of the protein-ligand complex using the MM-GBSA method. Results showed that the different complexes present similar, favorable van der Waals interactions, whereas electrostatic interactions were not favored in the mutant structures. Our study indicates that the residues K33 and R45 play a crucial role in maintaining the binding pocket structure required for lipid transport.

Efficient expression of a soluble lipid transfer protein (LTP) of Platanus orientalis using short peptide tags and structural comparison with the natural form

Successful recombinant allergen-based immunotherapy has drawn a great deal of attention to use recombinant allergens for new therapeutic and/or diagnostic strategies. The Escherichia coli expression system is frequently used to produce recombinant allergens; however, protein expression in E. coli often results in inclusion bodies. Here, we focused on the expression of two recombinant soluble forms of Pla or 3 using solubility-enhancing peptide tags, human immune deficiency virus type 1 transactivator of transcription core domain and poly-arginine-lysine: rTAT-Pla or 3 and rPoly-Arg-Lys-Pla or 3. Structural characteristics and IgE reactivity of purified recombinant proteins were compared with natural Pla or 3 (nPla or 3) isolated from Platanus orientalis using circular dichroism spectra, fluorescence spectroscopy, and immunoblotting. Likewise, intrinsic viscosity and Stokes radius of the natural and recombinant Pla or 3 allergens were determined to analyze structural compactness in aqueous media. The results indicate high-level solubility and efficient expression of the fusion proteins (rTAT-Pla or 3 and rPoly-Arg-Lys-Pla or 3) compared with the wild-type recombinant. Furthermore, the similar structural characteristics and IgE-binding activities of the fusion proteins to nPla or 3 provide a promising tool for allergy diagnosis and treatment.

Wheat (Triticum aestivum L. and T. turgidum L. ssp. durum) Kernel Hardness: I. Current View on the Role of Puroindolines and Polar Lipids

Wheat hardness has major consequences for the entire wheat supply chain from breeders and millers over manufacturers to, finally, consumers of wheat-based products. Indeed, differences in hardness among Triticum aestivum L. or between T. aestivum L. and T. turgidum L. ssp. durum wheat cultivars determine not only their milling properties, but also the properties of flour or semolina endosperm particles, their preferential use in cereal-based applications, and the quality of the latter. Although the mechanism causing differences in wheat hardness has been subject of research more than once, it is still not completely understood. It is widely accepted that differences in wheat hardness originate from differences in the interaction between the starch granules and the endosperm protein matrix in the kernel. This interaction seems impacted by the presence of either puroindoline a and/or b, polar lipids on the starch granule surface, or by a combination of both. We focus here on wheat hardness and its relation to the presence of puroindolines and polar lipids. More in particular, the structure, properties, and genetics of puroindolines and their interactions with polar lipids are critically discussed as is their possible role in wheat hardness. We also address future research needs as well as the presence of puroindoline-type proteins in other cereals.

[Isolation of the lipid-transporting protein Ns-LTP1 from seeds of the garden fennel flower (Nigella sativa)]

A novel lipid-transporting protein (Ns-LTP1) has been isolated from seeds of the garden fennel flower Nigella sativa. The molecular mass, N-terminal amino acid sequence, and amino acid composition of the protein have been determined. Ns-LTP1 has a molecular mass of 9602 Da and contains eight cysteine residues which form four disulfide bridges. The protein is capable of suppressing the development of some phytopathogenic fungi and oomycetes.

Isolation, purification and characterization of a nonspecific lipid transfer protein from Cuminum cyminum

Cuminum cyminum, an aromatic plant from the family Umbelliferae, is used as a flavoring and seasoning agent in foods. This communication reports the characterization of a nonspecific lipid transfer protein nsLTP1 from its seeds. Plant nsLTPs are small basic proteins involved in transport of lipids between membranes. These proteins are known to participate in plant defense; however, the exact mechanism of their antimicrobial action against fungi or bacteria is still unclear. The cumin nsLTP1 has been purified using a combination of chromatographic procedures and further characterized using mass spectrometry, circular dichroism spectroscopy and Edman degradation. Amino acid sequence has been used to predict homology model of cumin nsLTP1 in complex with myristic acid, and lyso-myristoyl phosphatidyl choline (LMPC). Cumin nsLTP1 is a monomeric protein with a molecular weight of 9.7 kDa as estimated by SDS-PAGE and ESIMS. The protein shows an isoelectric point of 7.8 on 6% PAGE. The primary structure consists of 92 amino acids with eight conserved cysteine residues. The global fold of cumin nsLTP1 includes four alpha-helices stabilized by four disulfide bonds and a C-terminal tail. The role of internal hydrophobic cavity of the protein in lipid transfer is discussed.

Comparison of different extraction solutions for the analysis of allergens in hen's egg

An important requirement for the correct procedure of allergen analysis in hen's egg is to obtain complete and unaltered protein extracts. Besides the aim of a quantitative extraction of the allergens from the matrix, it is equally important not to alter their allergenic potential during the extraction process. This paper describes and compares six extraction solutions for the analysis of whole-egg proteins and allergens. These requirements were examined via protein determination according to Bradford [Bradford, M. M. (1976). Rapid and sensitive method for quantitation of microgram quantities of protein utilizing principle of protein-dye binding. Analytical Biochemistry, 72, 248-254] and Kjeldahl [Meyer, A. H. (2006). Lebensmittelrecht, Verlag C.H. Beck München, Stand: 1. February 2006, § 64, Lebensmittel- und Futtermittelgesetzbuch, Amtliche Sammlung von Untersuchungsmethoden, Nr. L 06.00-7] as well as the EAST-inhibition method. It could be demonstrated that the extraction with a urea solution (8M) led to significant interferences during the protein determination, and substantially reduced the allergenic potential of egg proteins. With all other extraction solutions adequate protein contents could be extracted. The highest protein content was achieved by the extraction with phosphate buffered saline followed by a Tween 20 solution, physiological saline, water, and acetate buffer. The results show that none of these extracts - except for the urea solution (8M) - was altered in its' allergenic potential.

Quantification of casein phosphorylation with conformational interpretation using Raman spectroscopy

Raman spectroscopy is emerging as a powerful method for obtaining both quantitative and qualitative information from biological samples. One very interesting area of research, for which the technique has rarely been used, is the detection, quantification and structural analysis of post-translational modifications (PTMs) on proteins. Since Raman spectra can be used to address both of these questions simultaneously, we have developed near infrared Raman spectroscopy with appropriate chemometric approaches (partial least squares regression) to quantify low concentration (4 microM) mixtures of phosphorylated and dephosphorylated bovine alpha(s)-casein. In addition, we have used these data in conjunction with Raman optical activity (ROA) spectra and NMR to assess the structural changes that occur upon phosphorylation.

Characterization of wheat puroindoline proteins

Puroindoline proteins were purified from selected UK-grown hexaploid wheats. Their identities were confirmed on the basis of capillary electrophoresis mobilities, relative molecular mass and N-terminal amino acid sequencing. Only one form of puroindoline-a protein was found in those varieties, regardless of endosperm texture. Three allelic forms of puroindoline-b protein were identified. Nucleotide sequencing of cDNA produced by RT-PCR of isolated mRNA indicated that these were the 'wild-type', found in soft wheats, puroindoline-b containing a Gly-->Ser amino acid substitution (position 46) and puroindoline-b containing a Trp-->Arg substitution (position 44). The latter two were found in hard wheats. Microheterogeneity, due to short extensions and/or truncations at the N-terminus and C-terminus, was detected for both puroindoline-a and puroindoline-b. The type of microheterogeneity observed was more consistent for puroindoline-a than for puroindoline-b, and may arise through slightly different post-translational processing pathways. A puroindoline-b allele corresponding to a Leu-->Pro substitution (position 60) was identified from the cDNA sequence of the hard variety Chablis, but no mature puroindoline-b protein was found in this or two other European varieties known to possess this puroindoline-b allele. Wheats possessing the puroindoline-b proteins with point mutations appeared to contain lower amounts of puroindoline protein. Such wheats have a hard endosperm texture, as do wheats from which puroindoline-a or puroindoline-b are absent. Our results suggest that point mutations in puroindoline-b genes may confer hard endosperm texture through accumulation of allelic forms of puroindoline-b proteins with altered functional properties and/or through lower amounts of puroindoline proteins.

Stability of barley and malt lipid transfer protein 1 (LTP1) toward heating and reducing agents: relationships with the brewing process

Barley lipid transfer protein (LTP1) is a heat-stable and protease-resistant albumin that concentrates in beer, where it participates in the formation and stability of beer foam. Whereas the barley LTP1 does not display any foaming properties, the corresponding beer protein is surface-active. Such an improvement is related to glycation by Maillard reactions on malting, acylation on mashing, and structural unfolding on brewing. The structural stability of purified barley and glycated malt LTP1 toward heating has been analyzed. Whatever the modification, lipid adduction or glycation, barley LTP1s are highly stable proteins that resisted temperatures up to 100 degrees C. Unfolding of LTP1 occurred only when heating was conducted in the presence of a reducing agent. In the presence of sodium sulfite, the lipid-adducted barley and malt LTP1 displayed higher heat stability than the nonadducted protein. Glycation had no or weak effect on heat-induced unfolding. Finally, it was shown that unfolding occurred on wort boiling before fermentation and that the reducing conditions are provided by malt extract.

Tetraploid and hexaploid wheats express identical isoforms of nsLTP1

Nonspecific lipid-transfer proteins (nsLTPs) have been recognized as allergens in several plant species among which are cereals important in human nutrition. In this report, we purified a 9600 +/- 1 Da protein from both soft wheat and farro bran. Mass spectrometric analyses revealed that these proteins are identical, belong to the nsLTP1 class, and have high sequence homology with nsLTP1 isolated from other cereal species. Their identification was further supported by the ability of the soft wheat nsLTP1 to transfer pyrene-labeled lipids between donor and acceptor membranes. The results are discussed in view of the increasing diffusion on the markets of bran-rich products.

Diversity of wheat anti-microbial peptides

From seeds of Triticum kiharae Dorof. et Migusch., 24 novel anti-microbial peptides were isolated and characterized by a combination of three-step HPLC (affinity, size-exclusion and reversed-phase) with matrix-assisted laser-desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry and Edman degradation. Based on sequence similarity and cysteine motifs, partially sequenced peptides were assigned to 7 families: defensins, thionins, lipid-transfer proteins, hevein-like peptides, knottin-like peptides, glycine-rich peptides, and MBP-1 homologs. A novel subfamily of defensins consisting of 6 peptides and a new family of glycine-rich (8 peptides with different repeat motifs) were identified. Three 6-cysteine knottin-like peptides represented by N- and C-terminally truncated variants revealed no sequence homology to any known plant anti-microbial peptides. A new 8-cysteine hevein-like peptide and three 4-cysteine peptides homologous to MBP-1 from maize were isolated. This is the first communication on the occurrence of nearly all families of plant anti-microbial peptides in a single species.

NsLTP1 and NsLTP2 isoforms in soft wheat (Triticum aestivum Cv. Centauro) and farro (Triticum dicoccon Schrank) bran

Isoforms of nonspecific lipid-transfer protein 1 (nsLTP1) and nonspecific lipid-transfer protein 2 (nsLTP2) were investigated in bran tissues isolated from caryopses of two cereal crops quite relevant for the Italian market, the cultivar Centauro of soft wheat (Triticum aestivum) and Italian emmer or farro (Triticum dicoccon Schrank). By sequential separation of the bran extracts on cation-exchange and gel filtration chromatographies, fractions containing only proteins belonging to the nsLTP1 and nsLTP2 classes were obtained. The proteins were roughly identified by SDS-PAGE and by immunoreactions in Western blotting experiments. By MALDI-MS and RP-HPLC/ESI-MS analyses we were able to show the presence of several LTP1 and LTP2 isoforms in the investigated species. Bioinformatic searches based on the determined Mr indicated that (i) two nsLTP1s already identified in T. aestivum have Mr and number of Cys residues identical to that of a 9.6 kDa protein present both in soft wheat cv. Centauro and in farro; (ii) two isoforms of nsLTP2 detected in T. aestivum have the same Mr and number of Cys residues of two 7 kDa proteins found in Centauro; and (iii) a nsLTP1 detected in Ambrosia artemisiifolia has Mr and number of Cys residues coincident to that of a 9.9 kDa protein found both in soft wheat cv. Centauro and in farro.

Wheat non-specific lipid transfer protein genes display a complex pattern of expression in developing seeds

Nine cDNA clones encoding non-specific lipid transfer proteins (nsLTPs) were isolated from Triticum aestivum and Triticum durum cDNA libraries and characterized. One cDNA is predicted to encode a type 2 nsLTP (7 kDa) while others encode type 1 nsLTPs (9 kDa). All encoded proteins contain an N-terminal signal sequence and possess the characteristic features of nsLTPs. The genomic structures of the wheat nsLtp genes show that type 2 TaLtp7.1a, TaLtp7.2a and type 1 TaLtp9.2b genes lack introns while the other type 1 genes consist of one intron. Construction of a phylogenic tree of Poaceae nsLTPs shows that wheat nsLTPs can be divided into eleven distinct groups and are closely related to barley sequences. Using reverse transcriptase-polymerase chain reaction (RT-PCR) analysis, the expression patterns of nine nsLtp genes were studied during wheat seed development and germination. We identified three different profiles of nsLtp gene transcript accumulation. Whereas TdLtp7.1a, TdLtp9.4a and TdLtp9.7a transcripts were detected during all maturation stages, TdLtp7.2a, TdLtp9.2a, TdLtp9.3a, TdLtp9.5a and TdLtp9.6a transcripts were only present in the first and TdLtp9.1a in the last stages of seed development. Moreover, these nine wheat nsLtp genes are not seed-specific and are also expressed in the coleoptile of young seedlings. The present study revealed the complexity of the wheat nsLtp gene family and showed that the expression of nsLtp genes is developmentally regulated in the seeds, suggesting a specific function for each of the corresponding proteins.

A bacterial expression system revisited for the recombinant production of cystine-rich plant lipid transfer proteins

Non-specific lipid transfer proteins (nsLTPs) are abundant and ubiquitous cystine-rich proteins that are capable, in vitro, of binding lipids and hydrophobic molecules. In view to probe the lipid binding properties of the wheat nsLTP1, mutant variants may represent a powerful tool. To this end, a synthetic gene, encoding a mature wheat nsLTP1 polypeptide, was designed to ensure high level expression in Escherichia coli. The bacterial expression host strain, a translational fusion strategy, and convenient cleavage and purification procedures were optimized to produce in standard fermentation conditions, a significant amount (15 mg/L final yield), of a soluble and correctly folded recombinant nsLTP1. This highly amenable expression system is helpful in order to investigate structure-activity relationships of plant nsLTP.

Correlated clustering and virtual display of gene expression patterns in the wheat life cycle by large-scale statistical analyses of expressed sequence tags

Compared to rice, wheat exhibits characteristic growth habits and contains complex genome constituents. To assess global changes in gene expression patterns in the wheat life cycle, we conducted large-scale analysis of expressed sequence tags (ESTs) in common wheat. Ten wheat tissues were used to construct cDNA libraries: crown and root from 14-day-old seedlings; spikelet from early and late flowering stages; spike at the booting stage, heading date and flowering date; pistil at the heading date; and seeds at 10 and 30 days post-anthesis. Several thousand colonies were randomly selected from each of these 10 cDNA libraries and sequenced from both 5' and 3' ends. Consequently, a total of 116 232 sequences were accumulated and classified into 25 971 contigs based on sequence homology. By computing abundantly expressed ESTs, correlated expression patterns of genes across the tissues were identified. Furthermore, relationships of gene expression profiles among the 10 wheat tissues were inferred from global gene expression patterns. Genes with similar functions were grouped with one another by clustering gene expression profiles. This technique might enable estimation of the functions of anonymous genes. Multidimensional analysis of EST data that is analogous to the microarray experiments may offer new approaches to functional genomics of plants.

Sequence and functional similarities between pro-apoptotic Bid and plant lipid transfer proteins

Pro-apoptotic proteins of the Bcl-2 family are known to act on mitochondria and facilitate the release of cytochrome c, but the biochemical mechanism of this action is unknown. Association with mitochondrial membranes is likely to be important in determining the capacity of releasing cytochrome c. The present work provides new evidence suggesting that some pro-apoptotic proteins like Bid have an intrinsic capacity of binding and exchanging membrane lipids. Detailed analysis indicates a significant sequence similarity between a subset of Bcl-2 family proteins including Bid and Nix and plant lipid transfer proteins. The similar structural signatures could be related to common interactions with membrane lipids. Indeed, isolated Bid shows a lipid transfer activity that is even higher than that of plant lipid transfer proteins. To investigate the possible relevance of these structure-function correlations to the apoptotic action of Bid, cell free assays were established with isolated mitochondria, recombinant Bid and a variety of exogenous lipids. Micromolar concentrations of lysolipids such as lysophosphatidylcholine were found to change the association of Bid with mitochondria and also stimulate the release of cytochrome c promoted by Bid. The changes in mitochondrial association and cytochrome c release were enhanced by the presence of liposomes of lipid composition similar to that of mitochondrial membranes. Thus, a mixture of liposomes, mitochondria and key lysolipids could reproduce the conditions enabling Bid to transfer lipids between donor and acceptor membranes, and also change its reversible association with mitochondria. Bid was also found to enhance the incorporation of a fluorescent lysolipid, but not of a related fatty acid, into mitochondria. On the basis of the results presented here, it is hypothesised that Bid action may depend upon its capacity of exchanging lipids and lysolipids with mitochondrial membranes. The hypothesis is discussed in relation to current models for the integrated action of pro-apoptotic proteins of the Bcl-2 family.

Gene structure, intracellular localization, and functional roles of sterol carrier protein-2

Since its discovery three decades ago, sterol carrier protein-2 (SCP-2) has remained a fascinating protein whose physiological function in lipid metabolism remains an enigma. Its multiple proposed functions arise from its complex gene structure, post-translational processing, intracellular localization, and ligand specificity. The SCP-2 gene has two initiation sites coding for proteins that share a common 13 kDa SCP-2 C-terminus: (1) One site codes for 58 kDa SCP-x which is partially post-translationally cleaved to 13 kDa SCP-2 and a 45 kDa protein. (2) A second site codes for 15 kDa pro-SCP-2 which is completely post-translationally cleaved to 13 kDa SCP-2. Very little is yet known regarding how the relative proportions of the two transcripts are regulated. Although all three proteins contain a C-terminal SKL peroxisomal targeting sequence, it is unclear why all three proteins are not exclusively localized in peroxisomes. However, the recent demonstration that the SCP-2 N-terminal presequence in pro-SCP-2 dramatically modulated the intracellular targeting coded by the C-terminal peroxisomal targeting sequence may account for the observation that as much as half of total SCP-2 is localized outside the peroxisome. The tertiary and secondary structure of the 13 kDa SCP-2, but not that of 15 kDa pro-SCP-2 and 58 kDa SCP-x, are now resolved. Increasing evidence suggests that the 58 kDa SCP-x and 45 kDa proteins are peroxisomal 3-ketoacyl-CoA-thiolases involved in the oxidation of branched chain fatty acids. Since 15 kDa pro-SCP-2 is post-translationally completely cleaved to 13 kDa SCP-2, relatively little attention has been focused on this protein. Finally, although the 13 kDa SCP-2 is the most studied of these proteins, because it exhibits diversity of its ligand partners (fatty acids, fatty acyl CoAs, cholesterol, phospholipids), new potential physiological function(s) are still being proposed and questions regarding potential compensation by other proteins with overlapping specificity are only beginning to be resolved.

Calmodulin-binding protein BP-10, a probable new member of plant nonspecific lipid transfer protein superfamily

CaMBP-10 is a novel plant endogenous calmodulin-binding protein with important physiological functions. The partial cDNA sequence of this protein was cloned using RT-PCR. The deduced peptide (designated PCBP10) is composed of 74 amino acid residues containing a basic amphiphilic alpha-helix typical for calmodulin-binding proteins. PCBP10 shows very high amino acid sequence homology with plant nonspecific lipid-transfer proteins (nsLTPs). Sequence analysis also reveals that PCBP10 has similar amino acid composition to plant nsLTPs, and seven of the eight conserved cysteine residues are found in PCBP10. Furthermore, the secondary structure features of PCBP10 are very similar to those of plant nsLTPs. In addition, there are striking resemblances between CaMBP-10 and plant nsLTPs in their biochemical and physical properties. Our results suggest that CaMBP-10 is a novel member of the plant and nsLTP gene family, and the Ca(2+)/CaM regulative system may also play roles in lipid metabolism, defense reactions, and the adaptation of plants to natural environment.

Isolation of food allergens

The identification of food allergens is a priority in the management of food allergy, because of the need to obtain standardized extracts and pure allergens for diagnosis and therapy. It is thus important to develop methods for purification of allergenic molecules in order to study their biological and immunological characteristics. Protocols for protein extraction from foods and for allergen purification are reviewed in this paper. We report published methods for extraction of allergens from either animal and vegetable foods and detailed purification methodologies including ion-exchange, gel filtration and reversed-phase chromatography of well known allergens.

Identification of a new form of lipid transfer protein (LTP1) in wheat seeds

Recently, this laboratory has isolated from barley and beer extract an isoform of lipid transfer protein (LTP1), which was not fully sequenced (Jégou, S.; Douliez, J. P.; Mollé, D.; Boivin, P.; Marion, D. J. Agric. Food Chem. 2000, 48, 5023--5029). It was named LTP1b and exhibited a molecular weight 294 Da higher than that of the known LTP1. This paper reports the finding of an LTP1 isoform in wheat that also exhibits an excess of 294 Da compared to the native protein. Amino acid sequencing, reduction and alkylation, and mass spectrometry showed that this new LTP1b possesses the same N-terminal sequence as the native LTP1, suggesting that the difference resides in the binding of an adduct which has a molecular weight of 294 Da. The aim of the present paper is to highlight various biophysical techniques that afford the identification of such an isoform-like LTP1 and to correlate this finding with other isoforms of LTP1 that were isolated from other plants but not fully sequenced.

Cabbage cryoprotectin is a member of the nonspecific plant lipid transfer protein gene family

We have recently purified a protein (cryoprotectin) from the leaves of cold-acclimated cabbage (Brassica oleracea) to electrophoretic homogeneity, which protects thylakoids isolated from the leaves of nonacclimated spinach (Spinacia oleracea) from freeze-thaw damage. Sequencing of cryoprotectin showed the presence of at least three isoforms of WAX9 proteins, which belong to the class of nonspecific lipid transfer proteins. Antibodies raised against two synthetic peptides derived from the WAX9 proteins recognized a band of approximately 10 kD in western blots of crude cryoprotectin preparations. This protein and the cryoprotective activity could be precipitated from solution by the antiserum. We show further that cryoprotectin is structurally and functionally different from WAX9 isolated from the surface wax of cabbage leaves. WAX9 has lipid transfer activity for phosphatidylcholine, but no cryoprotective activity. Cryoprotectin, on the other hand, has cryoprotective, but no lipid transfer activity. The cryoprotective activity of cryoprotectin was strictly dependent on Ca(2+) and Mn(2+) and could be inhibited by chelating agents, whereas the lipid transfer activity of WAX9 was higher in the presence of ethylenediaminetetraacetate than in the presence of Ca(2+) and Mn(2+).

Purification and structural characterization of LTP1 polypeptides from beer

We report on the purification of lipid transfer proteins (LTP) from barley seeds and beer with the aim of investigating the chemical modifications that occur during the brewing process. In seeds, the well-known LTP of 9 kDa (LTP1) has been found together with a second form named LTPb that displays comparable amino acid composition but was not fully sequenced. These two forms have been recovered in beer with marked chemical modifications including disulfide bond reduction and rearrangement and especially glycation by Maillard reaction. The glycation is heterogeneous with variable amounts of hexose units bound to LTPs. Circular dichroism shows that glycated LTP1 having all their disulfide bridges reduced are totally unfolded. These results provide a first basis for understanding how barley LTPs become foam-promoting agents during the malting and brewing process.

Comparison of lipid binding and transfer properties of two lipid transfer proteins from plants

Plant lipid transfer proteins (LTPs) are soluble proteins which are characterized by their in vitro ability to transfer phospholipids between two membranes. We have compared the functional properties of two LTPs purified from maize and wheat seeds knowing that, despite a high degree of sequence identity, the two proteins exhibit structural differences. It was found that wheat LTP had a lower transfer activity than the maize LTP, consistent with a lower kinetics of fatty acid binding. The lower affinity for the fatty acids of the wheat LTP could be explained by a narrowing occurring in the middle part of the binding site, as revealed by comparing the fluorescence spectra of various anthroyloxy-labeled fatty acids associated with the two LTPs. The affinity for some natural fatty acids was studied by competition with fluorescent fatty acids toward binding to the protein. Again, wheat LTP had a lower affinity for those molecules. All together, these observations reveal the complexity of the LTP family in plants, probably reflecting the multiple roles played by these proteins.

The crystal structure of a wheat nonspecific lipid transfer protein (ns-LTP1) complexed with two molecules of phospholipid at 2.1 A resolution

Nonspecific lipid transfer proteins (ns-LTP1) form a multigenic protein family in plants. In vitro they are able to bind all sort of lipids but their function, in vivo, remains speculative. A ns-LTP1 isolated from wheat seed was crystallized in the presence of lyso-myristoyl-phosphatidylcholine (LMPC). The structure was solved by molecular replacement and refined to 2.1 A resolution to an R-factor of 16.3% and a free R-factor of 21.3%. It reveals for the first time that the protein binds two LMPC molecules that are inserted head to tail in a hydrophobic cavity. A detailed study of the structure leads to the conclusion that there are two lipid-binding sites, one of which shows a higher affinity for the LMPC than the other. Comparison with other structures of lipid-bound ns-LTP1 suggests that the presence of two binding sites is a general feature of plant ns-LTP1.

Production in Escherichia coli and site-directed mutagenesis of a 9-kDa nonspecific lipid transfer protein from wheat

The sequence encoding a wheat (Triticum durum) nonspecific lipid transfer protein of 9 kDa (nsLTP1) was inserted into an Escherichia coli expression vector, pET3b. The recombinant protein that was expressed accumulated in insoluble cytoplasmic inclusion bodies and was purified and refolded from them. In comparison with the corresponding protein isolated from wheat kernel, the refolded recombinant protein exhibits a methionine extension at its N-terminus but has the same structure and activity as demonstrated by CD, lipid binding and lipid transfer assays. Using the same expression system, four mutants with H5Q, Y16A, Q45R and Y79A replacements were produced and characterized. No significant changes in structure or activity were found for three of the mutants. By contrast, lipid binding experiments with the Y79A mutant did not show any increase of tyrosine fluorescence as observed with the wild-type nsLTP1. Comparison of the two tyrosine mutants suggested that Tyr79 is the residue involved in this phenomenon and thus is located close to the lipid binding site as expected from three-dimensional structure data.

Solution structure of barley lipid transfer protein complexed with palmitate. Two different binding modes of palmitate in the homologous maize and barley nonspecific lipid transfer proteins

The structure of a nonspecific lipid transfer protein from barley (ns-LTPbarley) in complex with palmitate has been determined by NMR spectroscopy. The structure has been compared to the structure of ns-LTPbarley in the absence of palmitate, to the structure of ns-LTPbarley in complex with palmitoyl coenzyme A, to the structure of ns-LTPmaize in its free form, and to the maize protein complexed with palmitate. Binding of palmitate only affects the structure of ns-LTPbarley moderately in contrast to the binding of palmitoyl coenzyme A, which leads to a considerable expansion of the protein. The modes of binding palmitate to the maize and barley protein are different. Although in neither case there are major conformational changes in the protein, the orientation of the palmitate in the two proteins is exactly opposite.

Cloning and expression of ragweed allergen Amb a 6

We have cloned the protein coding region of an isoform of short ragweed allergen Amb a 6 (Ra6) and expressed the secreted product in Pichia pastoris at mg/l levels. 5' RACE was performed using sequence obtained from a partial Amb a 6 clone. This yielded a product whose deduced protein sequence has a characteristic signal sequence motif at the N-terminus followed by sequence consistent with that previously published for highly purified Amb a 6 [Roebber et al. J Immunol 1983;131:706-11]. The region encoding the secreted product was amplified by PCR and cloned into pPICZ alpha a, an expression vector for the yeast Pichia pastoris. Yeast transformed with this vector secrete a protein which migrates near Amb a 6 in SDS-PAGE. This secreted protein reacts with polyclonal anti-Amb a 6 antisera as well as an anti-Amb a 6 monoclonal antibody, and has the N-terminal sequence of Amb a 6. By time-of-flight mass spectrometry, recombinant Amb a 6 has a molecular weight of 9884 +/- 0.2%. In addition to the deduced amino acid sequence of an Amb a 6 clone, the amino acid sequence of Amb a 6 protein is reported for comparison. The amino acid sequence was obtained by aligning overlapping tryptic and chymotryptic peptides from enzymatic digests of extensively reduced and alkylated Amb a 6. Sequences from at least three closely related Amb a 6 isoforms are present among these peptides. The amino acid sequence closely matches the deduced amino acid sequence of the Amb a 6 clone.

Comparison of solution and crystal structures of maize nonspecific lipid transfer protein: a model for a potential in vivo lipid carrier protein

The three-dimensional solution structure of maize nonspecific lipid transfer protein (nsLTP) obtained by nuclear magnetic resonance (NMR) is compared to the X-ray structure. Although both structures are very similar, some local structural differences are observed in the first and the fourth helices and in several side-chain conformations. These discrepancies arise partly from intermolecular contacts in the crystal lattice. The main characteristic of nsLTP structures is the presence of an internal hydrophobic cavity whose volume was found to vary from 237 to 513 A3 without major variations in the 15 solution structures. Comparison of crystal and NMR structures shows the existence of another small hollow at the periphery of the protein containing a water molecule in the X-ray structure, which could play an important structural role. A model of the complexed form of maize nsLTP by alpha-lysopalmitoylphosphatidylcholine was built by docking the lipid inside the protein cavity of the NMR structure. The main structural feature is a hydrogen bond found also in the X-ray structure of the complex maize nsLTP/palmitate between the hydroxyl of Tyr81 and the carbonyl of the lipid. Comparison of 12 primary sequences of nsLTPs emphasizes that all residues delineating the cavities calculated on solution and X-ray structures are conserved, which suggests that this large cavity is a common feature of all compared plant nsLTPs. Furthermore several conserved basic residues seem to be involved in the stabilization of the protein architecture.

Solution structure of Ace-AMP1, a potent antimicrobial protein extracted from onion seeds. Structural analogies with plant nonspecific lipid transfer proteins

The three-dimensional solution structure of Ace-AMP1, an antifungal protein extracted from onion seeds, was determined using 1H NMR spectroscopy and molecular modeling. This cationic protein contains 93 amino acid residues and four disulfide bridges. Its structure was determined from 1260 NOE-derived distance restraints and 173 dihedral restraints derived from NOEs and 3JCaHNH coupling constants. The global fold involves four helical segments connected by three loops and a C-terminal tail without regular secondary structures, except for a 3(10)-helix turn and a beta-turn. The most striking feature is the absence of any continuous cavity running through the whole molecule as found in recently determined structures of nonspecific transfer proteins extracted from wheat and maize seeds, although their global folds are very similar. Consistent with the absence of a cavity in the core of Ace-AMP1, it was found that this protein, in contrast to ns-LTPs, does not bind fluorescently labeled phospholipids in solution. On the other hand, Ace-AMP1 is able to interact with phospholipid membranes as shown by the release of carboxyfluorescein from the lumen of artificial liposomes and by the induction of alterations in fluorescence polarization of fluorescently labeled phospholipids embedded in artificial liposomes.

Hydrophobins and repellents: proteins with fundamental roles in fungal morphogenesis

Fungal hydrophobins are secreted proteins which react to interfaces between fungal cell walls and the air or between fungal cell walls and solid surfaces. They have been shown to be important in many morphogenetic processes, including sporulation, fruit body development, and infection structure formation. Hydrophobins form hydrophobic surface layers by self-assembly of secreted protein monomers in response to the environment. This process results in amphipathic polymers of interwoven rodlets on surfaces of fungal aerial structures and hyphal aggregations. Hydrophobin self-assembly is also involved in attachment of hyphae to hydrophobic surfaces and this may act as a conformational cue for certain developmental processes. Although hydrophobins appear to be ubiquitous among fungal taxa, a second class of fungal protein with very different biochemical characteristics could fulfill a similar role. These proteins, called repellents, have been identified in only one fungal species so far, but clearly help to make aerial hyphae hydrophobic. The functional similarities between hydrophobins and repellents highlight the importance of aerial development to the fungal lifestyle.

1H NMR and fluorescence studies of the complexation of DMPG by wheat non-specific lipid transfer protein. Global fold of the complex

Plant non-specific lipid transfer proteins (LTPs) are proteins which transfer lipids between membranes in vitro and are believed to be involved in the transport of cutin monomers to the cuticle layer in vivo or in the plant defence against phytopathogens. The complexation of DMPG, a diacyl phospholipid, by wheat ns-LTP, a protein extracted from wheat seeds, was followed by 1H NMR and fluorescence spectroscopy. The global fold of the protein was calculated using the DIANA software package from a list of 968 distance constraints. The internal cavity volume, a feature common to all known ns-LTP structures, was estimated to be 750 A3 using the 'CAVITE' program. This model of the complex was obtained by inserting a lipid molecule in the cavity and was energy minimized. The study showed that the protein fold described for the free form was only weakly affected by the insertion of the bulky lipid. Observation of some intermolecular NOEs between the protein and the lipid glycerol moiety revealed that the cavity entrance was located between residues His35 and Arg44. The resulting solution structure was compared to the crystal structure of the maize ns-LTP/palmitate complex.

Barley lipid-transfer protein complexed with palmitoyl CoA: the structure reveals a hydrophobic binding site that can expand to fit both large and small lipid-like ligands

BACKGROUND

. Plant nonspecific lipid-transfer proteins (nsLTPs) bind a variety of very different lipids in vitro, including phospholipids, glycolipids, fatty acids and acyl coenzyme As. In this study we have determined the structure of a nsLTP complexed with palmitoyl coenzyme A (PCoA) in order to further our understanding of the structural mechanism of the broad specificity of these proteins and its relation to the function of nsLTPs in vivo.

RESULTS

. 1H and 13C nuclear magnetic resonance spectroscopy (NMR) have been used to study the complex between a nsLTP isolated from barley seeds (bLTP) and the ligand PCoA. The resonances of 97% of the 1H atoms were assigned for the complexed bLTP and nearly all of the resonances were assigned in the bound PCoA ligand. The palmitoyl chain of the ligand was uniformly 13C-labelled allowing the two ends of the hydrocarbon chain to be assigned. The comparison of a subset of 20 calculated structures to an average structure showed root mean square deviations of 1.89 +/- 0.19 for all C, N, O, P and S atoms of the entire complex and of 0.57 +/- 0.09 for the peptide backbone atoms of the four alpha helices of the complexed bLTP. The four-helix topology of the uncomplexed bLTP is maintained in the complexed form of the protein. The bLTP only binds the hydrophobic parts of PCoA with the rest of the ligand remaining exposed to the solvent. The palmitoyl chain moiety of the ligand is placed in the interior of the protein and bent in a U-shape. This part of the ligand is completely buried within a hydrophobic pocket of the protein.

CONCLUSIONS

. A comparison of the structures of bLTP in the free and bound forms suggests that bLTP can accommodate long olefinic ligands by expansion of the hydrophobic binding site. This expansion is achieved by a bend of one helix, HA, and by conformational changes in both the C terminus and helix HC. This mode of binding is different from that seen in the structure of maize nsLTP in complex with palmitic acid, where binding of the ligand is not associated with structural changes.

Homology modelling of an antimicrobial protein, Ace-AMP1, from lipid transfer protein structures

BACKGROUND

Plant nonspecific lipid transfer proteins (ns-LTPs) are small basic proteins that facilitate lipid shuttling between membranes in vitro. The function of ns-LTPs in vivo is still unknown. It has been suggested, in relation to their lipid binding ability, that they may be involved in cutin formation. Alternatively, they may act in the plant defence system against pathogenic agents. Ace-AMP1 is an antimicrobial protein extracted from onion seed that shows sequence homology with ns-LTPs but that is unable to transfer lipids. We have recently determined the three-dimensional structure of wheat and maize ns-LTPs. In order to compare the structural features of Ace-AMP1 and ns-LTPs, we have used the comparative modelling software MODELLER to predict the structure of Ace-AMP1.

RESULTS

The global fold of Ace-AMP1 is very similar to those of ns-LTPs, involving four helices and a C-terminal tail without secondary structure elements. The structure of maize and wheat ns-LTP is characterized by the existence of a tunnel-like hydrophobic cavity in which a lipid molecule can be inserted. In the Ace-AMP1 structure, this cavity is blocked by a number of bulky residues. Similarly, the electrostatic potential contours of ns-LTPs show some common features that were not observed in Ace-AMP1.

CONCLUSIONS

Although Ace-AMP1 displays a similar global fold to ns-LTPs, it does not present a hydrophobic cavity, which may explain why Ace-AMP1 cannot shuttle lipids between membranes in vitro. The large differences in the electrostatic properties of Ace-AMP1 and ns-LTPs suggest a different mode of interaction with membranes.

Purification and amino acid sequences of piscicocins V1a and V1b, two class IIa bacteriocins secreted by Carnobacterium piscicola V1 that display significantly different levels of specific inhibitory activity

Two bacteriocins produced by Carnobacterium piscicola V1 were purified and characterized. Piscicocin V1a (molecular mass = 4,416 Da) and piscicocin V1b (molecular mass = 4,526 Da) are nonlantibiotic, small, heat-stable antibacterial peptides. Piscicocin V1b is identical to carnobacteriocin BM1, while piscicocin V1a is a new bacteriocin. Its complete sequence of 44 amino acid residues has been determined. Piscicocin V1a belongs to the class IIa bacteriocins having the consensus YGNGV motif. These peptides inhibit various gram-positive bacteria, including Listeria monocytogenes. Piscicocin V1a is approximately 100 times more active than piscicocin V1b against indicator strains. However, the antagonistic spectrum is the same for both piscicocins. Comparison of these results with the analysis of the amino acid sequence and secondary structure predictions suggests that (i) the conserved N-terminal conserved domain is involved in the receptor recognition and therefore in an "all-or-none" response against target bacterial cells and (ii) the C-terminal variable and hydrophobic domain determines membrane anchoring and therefore the intensity of the antagonist response.

Determination of the secondary structure and conformation of puroindolines by infrared and Raman spectroscopy

The conformation of puroindoline-a and -b, two basic lipid-binding proteins isolated from wheat seedlings, has been studied for the first time by infrared and Raman spectroscopy. The infrared results show that puroindoline-a and -b have similar secondary structure composed of approximately 30% alpha-helices, 30% beta-sheets, and 40% unordered structure at pH 7. The conformation of both puroindolines is significantly pH-dependent. The reduction of the disulfide bridges leads to a decrease of the solubility of puroindolines in water and to an increase of the beta-sheet content by about 15% at the expense of the alpha-helix content. Raman spectroscopy confirms the structure similarity between the two puroindolines with little differences in the side chains' environment. All the disulfide bridges are in a gauche-gauche-gauche conformation, and the unique tyrosine residue present in both puroindolines is hydrogen-bonded to water. Raman spectra have been recorded in both H2O and D2O media, thus providing additional information concerning the accessibility of certain residues to water. We have also observed that puroindoline-a tends to form some aggregates under acidic and high ionic strength conditions. Near-ultraviolet circular dichroism measurements suggest that the tryptophan-rich domain is involved in this aggregate formation. Finally, on the basis of a combined infrared and sequence conformational analysis, we propose a secondary structure assignment for both puroindolines. The results show that puroindolines exhibit a similar folding pattern with plant nonspecific lipid-transfer protein and some amylase-protease inhibitors. These proteins could form a homogeneous structural family of plant proteins involved in the defense against pathogens that are probably derived from a common "helicoidal" protein ancestor.

LIPID-TRANSFER PROTEINS IN PLANTS

Lipid-transfer proteins (LTP) are basic, 9-kDa proteins present in high amounts (as much as 4% of the total soluble proteinss) in higher plants. LTPs can enhance the in vitro transfer of phospholipids between membranes and can bind acyl chains. On the basis of these properties, LTPs were thought to participate in membrane biogenesis and regulation of the intracellular fatty acid pools. However, the isolation of several cDNAs and genes revealed the presence of a signal peptide indicating that LTPs could enter the secretory pathway. They were found to be secreted and located in the cell wall. Thus, novel roles were suggested for plant LTPs: participation in cutin formation, embryogenesis, defense reactions against phytopathogens, symbiosis, and the adaptation of plants to various environmental conditions. The validity of these suggestions needs to be determined, in the hope that they will elucidate the role of this puzzling family of plant proteins.

Solution structure and lipid binding of a nonspecific lipid transfer protein extracted from maize seeds

The three-dimensional solution structure of a nonspecific lipid transfer protein extracted from maize seeds determined by 1H NMR spectroscopy is described. This cationic protein consists of 93 amino acid residues. Its structure was determined from 1,091 NOE-derived distance restraints, including 929 interresidue connectivities and 197 dihedral restraints (phi, psi, chi 1) derived from NOEs and 3J coupling constants. The global fold involving four helical fragments connected by three loops and a C-terminal tail without regular secondary structures is stabilized by four disulfide bridges. The most striking feature of this structure is the existence of an internal hydrophobic cavity running through the whole molecule. The global fold of this protein, very similar to that of a previously described lipid transfer protein extracted from wheat seeds (Gincel E et al., 1994, Eur J Biochem 226:413-422) constitutes a new architecture for alpha-class proteins. 1H NMR and fluorescence studies show that this protein forms well-defined complexes in aqueous solution with lysophosphatidylcholine. Dissociation constants, Kd, of 1.9 +/- 0.6 x 10(-6) M and > 10(-3) M were obtained with lyso-C16 and -C12, respectively. A structure model for a lipid-protein complex is proposed in which the aliphatic chain of the phospholipid is inserted in the internal cavity and the polar head interacts with the charged side chains located at one end of this cavity. Our model for the lipid-protein complex is qualitatively very similar to the recently published crystal structure (Shin DH et al., 1995, Structure 3:189-199).

Structure in solution of a four-helix lipid binding protein

Because of the low solubility of lipids in water, intercellular and intracellular pathways of lipid transfer are necessary, e.g., for membrane formation. The mechanism by which lipids in vivo are transported from their site of biogenesis (endoplasmatic reticulum and the chloroplasts) to their place of action is unknown. Several small plant proteins with the ability to mediate transfer of radiolabeled phospholipids in vitro from liposomal donor membranes to mitochondrial and chloroplast acceptor membranes have been isolated, and a protein with this ability, the nonspecific lipid transfer protein (nsLTP) isolated from barley seeds (bLTP), has been studied here. The structure and the protein lipid interactions of lipid transfer proteins are relevant for the understanding of their function, and here we present the three-dimensional structure in solution of bLTP as determined by NMR spectroscopy. The 1H NMR spectrum of the 91-residue protein was assigned for more than 97% of the protein 1H atoms, and the structure was calculated on the basis of 813 distance restraints from 1H-1H nuclear Overhauser effects, four disulfide bond restraints, from dihedral angle restraints for 66 phi-angles, 61 chi 1 angles, and 2 chi 2 angles, and from 31 sets of hydrogen bond restraints. The solution structure of bLTP consists of four well-defined alpha-helices A-D (A, Cys 3-Gly 19; B, Gly 25-Ala 38; C, Arg 44-Gly 57; D, Leu 63-Cys 73), separated by three short loops that are less well defined and concluded by a well defined C-terminal peptide segment with no observable regular secondary structure. For the 17 structures that are used to represent the solution structure of bLTP, the RMS deviation to an average structure is 0.63 A +/- 0.04 A for backbone atoms and 0.93 A +/- 0.06 A for all heavy atoms. The secondary structure elements and their locations in the sequence resemble those of nsLTP from two other plant species, wheat and maize, whose structures were previously determined (Gincel E et al, 1995, Eur J Biochem 226:413-422; Shin DH et al, 1995, Structure 3:189-199). In bLTP, the residues analogous to those in maize nsLTP that constitute the palmitate binding site are forming a similar hydrophobic cavity and a potential acyl group binding site. Analysis of the solution structure of bLTP and bLTP in complex with a ligand might provide information on the conformational changes in the protein upon ligand binding and subsequently provide information on the mode of ligand uptake and release. In this work, we hope to establish a foundation for further work of determining the solution structure of bLTP in complex with palmitoyl coenzyme A, which is a suitable ligand, and subsequently to outline the mode of ligand binding.

Cholesterol interaction with recombinant human sterol carrier protein-2

The interaction of human recombinant sterol carrier protein-2 (SCP-2) with sterols was examined. Two independent ligand binding methods, Lipidex 1000 binding of [3H]cholesterol and a fluorescent dehydroergosterol binding assay, were used to determine the affinity of SCP-2 for sterols. Binding analysis indicated SCP-2 bound [3H]cholesterol and dehydroergosterol with a Kd of 0.3 and 1.7 microM, respectively, and suggested the presence of a single binding site. Phase fluorometry and circular dichroism were used to characterize the SCP-2 sterol binding site. Alterations in dehydroergosterol lifetime, SCP-2 tryptophan lifetime, and SCP-2 tryptophan quenching by acrylamide upon cholesterol binding demonstrated a shielding of the SCP-2 tryptophan from the aqueous solvent by bound sterol. Differential polarized phase fluorometry revealed decreased SCP-2 tryptophan rotational correlation time upon cholesterol binding. Circular dichroism of SCP-2 indicated that cholesterol elicited a small decrease in SCP-2 alpha helical content. The data suggest that SCP-2 binds sterols with affinity consistent with a lipid transfer protein that may act either as an aqueous carrier or at a membrane surface to enhance sterol desorption.