Lipid transfer proteins

Multi-Omics Analysis of a Chromosome Segment Substitution Line Reveals a New Regulation Network for Soybean Seed Storage Profile

Soybean, a major source of oil and protein, has seen an annual increase in consumption when used in soybean-derived products and the broadening of its cultivation range. The demand for soybean necessitates a better understanding of the regulatory networks driving storage protein accumulation and oil biosynthesis to broaden its positive impact on human health. In this study, we selected a chromosome segment substitution line (CSSL) with high protein and low oil contents to investigate the underlying effect of donor introgression on seed storage through multi-omics analysis. In total, 1479 differentially expressed genes (DEGs), 82 differentially expressed proteins (DEPs), and 34 differentially expressed metabolites (DEMs) were identified in the CSSL compared to the recurrent parent. Based on Gene Ontology (GO) term analysis and the Kyoto Encyclopedia of Genes and Genomes enrichment (KEGG), integrated analysis indicated that 31 DEGs, 24 DEPs, and 13 DEMs were related to seed storage functionality. Integrated analysis further showed a significant decrease in the contents of the seed storage lipids LysoPG 16:0 and LysoPC 18:4 as well as an increase in the contents of organic acids such as L-malic acid. Taken together, these results offer new insights into the molecular mechanisms of seed storage and provide guidance for the molecular breeding of new favorable soybean varieties.

Non-Vesicular Lipid Transport Machinery in Leishmania donovani: Functional Implications in Host-Parasite Interaction

Eukaryotic cells have distinct membrane-enclosed organelles, each with a unique biochemical signature and specialized function. The unique identity of each organelle is greatly governed by the asymmetric distribution and regulated intracellular movement of two important biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid-transfer proteins (LTPs) plays essential roles in intra-cellular lipid trafficking and cellular lipid homeostasis, while vesicular transport regulates protein trafficking. A comparative analysis of non-vesicular lipid transport machinery in protists could enhance our understanding of parasitism and basis of eukaryotic evolution. Leishmania donovani, the trypanosomatid parasite, greatly depends on receptor-ligand mediated signalling pathways for cellular differentiation, nutrient uptake, secretion of virulence factors, and pathogenesis. Lipids, despite being important signalling molecules, have intracellular transport mechanisms that are largely unexplored in L. donovani. We have identified a repertoire of sixteen (16) potential lipid transfer protein (LTP) homologs based on a domain-based search on TriTrypDB coupled with bioinformatics analyses, which signifies the presence of well-organized lipid transport machinery in this parasite. We emphasized here their evolutionary uniqueness and conservation and discussed their potential implications for parasite biology with regards to future therapeutic targets against visceral leishmaniasis.

Seed Storage Protein, Functional Diversity and Association with Allergy

Plants are essential for humans as they serve as a source of food, fuel, medicine, oils, and more. The major elements that are utilized for our needs exist in storage organs, such as seeds. These seeds are rich in proteins, show a broad spectrum of physiological roles, and are classified based on their sequence, structure, and conserved motifs. With the improvements to our knowledge of the basic sequence and our structural understanding, we have acquired better insights into seed proteins and their role. However, we still lack a systematic analysis towards understanding the functional diversity associated within each family and their associations with allergy. This review puts together the information about seed proteins, their classification, and diverse functional roles along with their associations with allergy.

Non-vesicular Lipid Transport Machinery in Entamoeba histolytica

Eukaryotic cells are organized into separate membrane-bound compartments that have specialized biochemical signature and function. Maintenance and regulation of distinct identity of each compartment is governed by the uneven distribution and intra-cellular movement of two essential biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid transfer proteins plays a pivotal role in intra-cellular lipid trafficking and homeostasis whereas vesicular transport plays a central role in protein trafficking. Comparative study of lipid transport machinery in protist helps to better understand the pathogenesis and parasitism, and provides insight into eukaryotic evolution. Amebiasis, which is caused by Entamoeba histolytica, is one of the major enteric infections in humans, resulting in 40–100 thousand deaths annually. This protist has undergone remarkable alterations in the content and function of its sub-cellular compartments as well represented by its unique diversification of mitochondrion-related organelle, mitosome. We conducted domain-based search on AmoebaDB coupled with bioinformatics analyses and identified 22 potential lipid transfer protein homologs in E. histolytica, which are grouped into several sub-classes. Such in silico analyses have demonstrated the existence of well-organized lipid transport machinery in this parasite. We summarized and discussed the conservation and unique features of the whole repertoire of lipid transport proteins in E. histolytica.

Lipids in the cell: organisation regulates function

Lipids are fundamental building blocks of all cells and play important roles in the pathogenesis of different diseases, including inflammation, autoimmune disease, cancer, and neurodegeneration. The lipid composition of different organelles can vary substantially from cell to cell, but increasing evidence demonstrates that lipids become organised specifically in each compartment, and this organisation is essential for regulating cell function. For example, lipid microdomains in the plasma membrane, known as lipid rafts, are platforms for concentrating protein receptors and can influence intra-cellular signalling. Lipid organisation is tightly regulated and can be observed across different model organisms, including bacteria, yeast, Drosophila, and Caenorhabditis elegans, suggesting that lipid organisation is evolutionarily conserved. In this review, we summarise the importance and function of specific lipid domains in main cellular organelles and discuss recent advances that investigate how these specific and highly regulated structures contribute to diverse biological processes.

Defense-related proteins involved in sugarcane responses to biotic stress

Sugarcane is one of the most important agricultural crops in the world. However, pathogen infection and herbivore attack cause constant losses in yield. Plants respond to pathogen infection by inducing the expression of several protein types, such as glucanases, chitinases, thaumatins, peptidase inhibitors, defensins, catalases and glycoproteins. Proteins induced by pathogenesis are directly or indirectly involved in plant defense, leading to pathogen death or inducing other plant defense responses. Several of these proteins are induced in sugarcane by different pathogens or insects and have antifungal or insecticidal activity. In this review, defense-related proteins in sugarcane are described, with their putative mechanisms of action, pathogen targets and biotechnological perspectives.

Size-dependent, stochastic nature of lipid exchange between nano-vesicles and model membranes

The interaction of nanoscale lipid vesicles with cell membranes is of fundamental importance for the design and development of vesicular drug delivery systems. Here, we introduce a novel approach to study vesicle-membrane interactions whereby we are able to probe the influence of nanoscale membrane properties on the dynamic adsorption, exchange, and detachment of vesicles. Using total internal reflection fluorescence (TIRF) microscopy, we monitor these processes in real-time upon the electrostatically tuned attachment of individual, sub-100 nm vesicles to a supported lipid bilayer. The observed exponential vesicle detachment rate depends strongly on the vesicle size, but not on the vesicle charge, which suggests that lipid exchange occurs during a single stochastic event, which is consistent with membrane stalk formation. The fluorescence microscopy assay developed in this work may enable measuring of the probability of stalk formation in a controlled manner, which is of fundamental importance in membrane biology, offering a new tool to understand nanoscale phenomena in the context of biological sciences.

Nonvesicular lipid transfer from the endoplasmic reticulum

The transport of lipids from their synthesis site at the endoplasmic reticulum (ER) to different target membranes could be mediated by both vesicular and nonvesicular transport mechanisms. Nonvesicular lipid transport appears to be the major transport route of certain lipid species, and could be mediated by either spontaneous lipid transport or by lipid-transfer proteins (LTPs). Although nonvesicular lipid transport has been extensively studied for more than four decades, its underlying mechanism, advantage and regulation, have not been fully explored. In particular, the function of LTPs and their involvement in intracellular lipid movement remain largely controversial. In this article, we describe the pathways by which lipids are synthesized at the ER and delivered to different cellular membranes, and discuss the role of LTPs in lipid transport both in vitro and in intact cells.

Computational study of ligand binding in lipid transfer proteins: Structures, interfaces, and free energies of protein-lipid complexes

Plant nonspecific lipid transfer proteins (nsLTPs) bind a wide variety of lipids, which allows them to perform disparate functions. Recent reports on their multifunctionality in plant growth processes have posed new questions on the versatile binding abilities of these proteins. The lack of binding specificity has been customarily explained in qualitative terms on the basis of a supposed structural flexibility and nonspecificity of hydrophobic protein-ligand interactions. We present here a computational study of protein-ligand complexes formed between five nsLTPs and seven lipids bound in two different ways in every receptor protein. After optimizing geometries in molecular dynamics calculations, we computed Poisson-Boltzmann electrostatic potentials, solvation energies, properties of the protein-ligand interfaces, and estimates of binding free energies of the resulting complexes. Our results provide the first quantitative information on the ligand abilities of nsLTPs, shed new light into protein-lipid interactions, and reveal new features which supplement commonly held assumptions on their lack of binding specificity.

Tobacco NtLTP1, a glandular-specific lipid transfer protein, is required for lipid secretion from glandular trichomes

Glandular trichomes are the phytochemical factories of plants, and they secrete a wide range of commercially important natural products such as lipids, terpenes and flavonoids. Herein, we report that the Nicotiana tabacum LTP1 (NtLTP1) gene, which is specifically expressed in long glandular trichomes, plays a role in lipid secretion from trichome heads. NtLTP1 mRNA is abundantly transcribed in trichomes, but NtLTP3, NtLTP4 and NtLTP5 are not. In situ hybridization revealed that NtLTP1 mRNAs accumulate specifically in long trichomes and not in short trichomes or epidermal cells. X-gluc staining of leaves from a transgenic plant expressing the NtLTP1 promoter fused to a GUS gene revealed that NtLTP1 protein accumulated preferentially on the tops of long glandular trichomes. GFP fluorescence from transgenic tobacco plants expressing an NtLTP1-GFP fusion protein was localized at the periphery of cells and in the excreted liquid droplets from the glandular trichome heads. In vitro assays using a fluorescent 2-p-toluidinonaphthalene-6-sulfonate probe indicated that recombinant NtLTP1 had lipid-binding activity. The overexpression of NtLTP1 in transgenic tobacco plants resulted in the increased secretion of trichome exudates, including epicuticular wax. In transgenic NtLTP1-RNAi lines, liquid secretion from trichomes was strongly reduced, but epicuticular wax secretion was not altered. Moreover, transgenic tobacco plants overexpressing NtLTP1 showed increased protection against aphids. Taken together, these data suggest that NtLTP1 is abundantly expressed in long glandular trichomes, and may play a role in lipid secretion from long glandular trichomes.

Non-vesicular lipid transport by lipid-transfer proteins and beyond

The movement of lipids within and between intracellular membranes is mediated by different lipid transport mechanisms and is crucial for maintaining the identities of different cellular organelles. Non-vesicular lipid transport has a crucial role in intracellular lipid trafficking and distribution, but its underlying mechanisms remain unclear. Lipid-transfer proteins (LTPs), which regulate diverse lipid-mediated cellular processes and accelerate vectorial transport of lipid monomers between membranes in vitro, could potentially mediate non-vesicular intracellular lipid trafficking. Understanding the mechanisms by which lipids are transported and distributed between cellular membranes, and elucidating the role of LTPs in intracellular lipid transport and homeostasis, are currently subjects of intensive study.

Mitochondrial kinases and their molecular interaction with cardiolipin

Mitochondrial isoforms of creatine kinase (MtCK) and nucleoside diphosphate kinase (NDPK-D) are not phylogenetically related but share functionally important properties. They both use mitochondrially generated ATP with the ultimate goal of maintaining proper nucleotide pools, are located in the intermembrane/cristae space, have symmetrical oligomeric structures, and show high affinity binding to anionic phospholipids, in particular cardiolipin. The structural basis and functional consequences of the cardiolipin interaction have been studied and are discussed in detail in this review. They mainly result in a functional interaction of MtCK and NDPK-D with inner membrane adenylate translocator, probably by forming proteolipid complexes. These interactions allow for privileged exchange of metabolites (channeling) that ultimately regulate mitochondrial respiration. Further functions of the MtCK/membrane interaction include formation of cardiolipin membrane patches, stabilization of mitochondria and a role in apoptotic signaling, as well as in case of both kinases, a role in facilitating lipid transfer between two membranes. Finally, disturbed cardiolipin interactions of MtCK, NDPK-D and other proteins like cytochrome c and truncated Bid are discussed more generally in the context of apoptosis and necrosis.

Mutagenesis study of rice nonspecific lipid transfer protein 2 reveals residues that contribute to structure and ligand binding

Plant nonspecific lipid transfer protein 2 (nsLTP2) is a small (7 kDa) protein that binds lipid-like ligands. An inner hydrophobic cavity surrounded by alpha-helices is the defining structural feature of nsLTP2. Although nsLTP2 structures have been reported earlier, the detailed mechanisms of ligand binding and lipid transfer remain unclear. In this study, we used site-directed mutagenesis to determine the role of various hydrophobic residues (L8, I15, F36, F39, Y45, Y48, and V49) in the structure, stability, ligand binding, and lipid transfer activity of rice nsLTP2. Three single mutations (L8A, F36A, and V49A) drastically alter the native tertiary structure and perturb ligand binding and lipid transfer activity. Therefore, these three residues are structurally important. The Y45A mutant, however, retains a native-like structure but has decreased lipid binding affinity and lipid transfer activity, implying that this aromatic residue is critical for these biological functions. The mutants, I15A and Y48A, exhibit quite different ligand binding affinities. Y48 is involved in planar sterol binding but not linear lysophospholipid association. As for I15A, it had the highest dehydroergosterol binding affinity in spite of the lower lipid binding and transfer abilities. Our results suggest that the long alkyl side chain of I15 would restrict the flexibility of loop I (G13-A19) for sterol entry. Finally, F39A can markedly increase the exposed hydrophobic surface to maintain its transfer efficiency despite reduced ligand binding affinity. These findings suggest that the residues forming the hydrophobic cavity play various important roles in the structure and function of rice nsLTP2.

The assembly of triacylglycerol-rich lipoproteins: an essential role for the microsomal triacylglycerol transfer protein

Raised plasma triacylglycerol is an independent risk factor for cardiovascular disease, and an understanding of factors which regulate the synthesis and degradation of lipoproteins which carry triacylglycerol in the blood may lead to novel approaches to the treatment of hypertriacylglycerolaemia. An active microsomal triacylglycerol transfer protein (MTP) is essential for the assembly of particles which transport triacylglycerol through the circulation. After absorption in the intestine, dietary fat and fat-soluble vitamins are incorporated into chylomicrons in the intestinal epithelial cells, and these lipoproteins reach the bloodstream via the lymphatic system. Patients with the rare genetic disorder, abetalipoproteinaemia, in which MTP activity is absent, present clinically with fat-soluble vitamin and essential fatty acid deficiency, indicating a key role for MTP in the movement of fat into the body. The triacylglycerol-rich lipoprotein found in fasting blood, VLDL, is assembled in the liver by an MTP-dependent process similar to chylomicron assembly, and transports triacylglycerol to extra-hepatic tissues such as adipose tissue and heart. In the absence of MTP activity, VLDL are not synthesized and only extremely low levels of triacylglycerol are present in the blood. Dietary components, including fat, cholesterol and ethanol, can modify the expression of the MTP gene and, hence, MTP activity. The present review summarizes current knowledge of the role of MTP in the assembly and secretion of triacylglycerol-rich lipoproteins, and the regulation of its activity in both animal and cell systems.

Novel lipid transfer property of two mitochondrial proteins that bridge the inner and outer membranes

This study provides evidence of a novel function for mitochondrial creatine kinase (MtCK) and nucleoside diphosphate kinase (NDPK-D). Both are basic peripheral membrane proteins with symmetrical homo-oligomeric structure, which in the case of MtCK was already shown to allow crossbridging of lipid bilayers. Here, different lipid dilution assays clearly demonstrate that both kinases also facilitate lipid transfer from one bilayer to another. Lipid transfer occurs between liposomes mimicking the lipid composition of mitochondrial contact sites, containing 30 mol % cardiolipin, but transfer does not occur when cardiolipin is replaced by phosphatidylglycerol. Ubiquitous MtCK, but not NDPK-D, shows some specificity in the nature of the lipids transferred and it is not active with phosphatidylcholine alone. MtCK can undergo reversible oligomerization between dimeric and octameric forms, but only the octamer can bridge membranes and promote lipid transfer. Cytochrome c, another basic mitochondrial protein known to bind to anionic membranes but not crosslinking them, is also incapable of promoting lipid transfer. The lipid transfer process does not involve vesicle fusion or loss of the internal contents of the liposomes.

Lipid metabolism, atherogenesis and CD1-restricted antigen presentation

CD1 molecules are a family of major histocompatibility complex (MHC)-related glycoproteins that present lipid and glycolipid antigens to T cells. Interestingly, it has been demonstrated that CD1d-restricted T cells have a pathogenic role in atherosclerosis. Recent studies suggest an association between the cellular machinery that loads CD1 molecules with glycolipids and several key proteins in lipid metabolism. These proteins include the sphingolipid activator proteins (SAPs), microsomal triglyceride transfer protein (MTP) and apolipoprotein E (apoE). MTP and SAPs seem to be crucial for loading CD1d with lipids in the endoplasmic reticulum and endosomal compartments, respectively, whereas apoE facilitates efficient uptake and delivery of exogenous lipid antigens to CD1d in endosomal compartments. These studies reveal new and unexpected relationships between lipid metabolism and antigen presentation by CD1 molecules. Targeting this pathway of immune activation might have therapeutic potential for the treatment of chronic inflammatory diseases.

Changes in the tobacco leaf apoplast proteome in response to salt stress

The apoplast of plant cells is a dynamic compartment involved in many processes, including maintenance of tissue shape, development, nutrition, signalling, detoxification and defence. In this work we used Nicotiana tabacum plants as a model to investigate changes in the soluble apoplast composition induced in response to salt stress. Apoplastic fluid was extracted from leaves of control plants and plants exposed to salt stress, using a vacuum infiltration procedure. Two-dimension electrophoretic analyses revealed about 150 polypeptide spots in the pH range of 3.0 to 10.0, in independent protein extracts, with a high level of reproducibility between the two sample sets. Quantitative evaluation and statistical analyses of the resolved spots in treated and untreated samples revealed 20 polypeptides whose abundance changed in response to salt stress. Mass spectroscopic peptide separation and sequencing was used to identify polypeptides affected by salt stress. While the levels of some proteins were reduced by salt-treatment, an enhanced accumulation of protein species known to be induced by biotic and abiotic stresses was observed. In particular, two chitinases and a germin-like protein increased significantly and two lipid transfer proteins were expressed entirely de novo. Some apoplastic polypeptides, involved in cell wall modifications during plant development, remained largely unchanged. The significance of these components is discussed in the context of stress responses in plants.

Lipid binding in rice nonspecific lipid transfer protein-1 complexes from Oryza sativa

Nonspecific lipid transfer proteins (nsLTPs) facilitate the transfer of phospholipids, glycolipids, fatty acids and steroids between membranes, with wide-ranging binding affinities. Three crystal structures of rice nsLTP1 from Oryza sativa, complexed with myristic (MYR), palmitic (PAL) or stearic acid (STE) were determined. The overall structures of the rice nsLTP1 complexes belong to the four-helix bundle folding with a long C-terminal loop. The nsLTP1-MYR and the nsLTP1-STE complexes bind a single fatty acid while the nsLTP1-PAL complex binds two molecules of fatty acids. The C-terminal loop region is elastic in order to accommodate a diverse range of lipid molecules. The lipid molecules interact with the nsLTP1-binding cavity mainly with hydrophobic interactions. Significant conformational changes were observed in the binding cavity and the C-terminal loop of the rice nsLTP1 upon lipid binding.

Editing of CD1d-bound lipid antigens by endosomal lipid transfer proteins

It is now established that CD1 molecules present lipid antigens to T cells, although it is not clear how the exchange of lipids between membrane compartments and the CD1 binding groove is assisted. We report that mice deficient in prosaposin, the precursor to a family of endosomal lipid transfer proteins (LTP), exhibit specific defects in CD1d-mediated antigen presentation and lack Valpha14 NKT cells. In vitro, saposins extracted monomeric lipids from membranes and from CD1, thereby promoting the loading as well as the editing of lipids on CD1. Transient complexes between CD1, lipid, and LTP suggested a "tug-of-war" model in which lipid exchange between CD1 and LTP is on the basis of their respective affinities for lipids. LTPs constitute a previously unknown link between lipid metabolism and immunity and are likely to exert a profound influence on the repertoire of self, tumor, and microbial lipid antigens.

Cross-reactivities between date palm (Phoenix dactylifera L.) polypeptides and foods implicated in the oral allergy syndrome

BACKGROUND

Date fruit and pollen antigens share a number of cross-reactive epitopes. Date pollen has been shown to cross-react with antigens from Artemisia, cultivated rye (Secale cereale), Timothy grass (Phleum pratense), Sydney golden wattle (Acacia longifolia) and Bermuda grass (Cynodon dactylon) pollen. The present study was carried out to examine any cross-reactivities between date palm polypeptides and antigens of some common foods and vegetables that have been implicated in the oral allergy syndrome (OAS). Because most of such cross-reactivities in other allergens are attributable to the presence of carbohydrate chains and profilin, their role was also investigated.

METHODS

Fresh extracts of 20 common fruits and vegetables were prepared. Putative date profilins were isolated by affinity chromatography using a poly L-proline column. Date fruit extracts were digested by various endoglycosidases and the immunoglobulin (Ig)E binding of the postdigest products was assessed in immunoblots. Rabbit antisera to whole date fruit extracts, Timothy grass profilin and putative date profilins, as well as human sera from date sensitive individuals were used in immunoblotting, ELISA and in inhibition experiments.

RESULTS

IgG, ELISA and immunoblot results with the different rabbit antisera and date-sensitive atopic sera showed several antigenic cross-reactivities and similar cross-reactivities were seen with birch, date and timothy grass profilins. IgE, ELISA and immunoblot experiments with pooled date sensitive human sera showed a range of cross-reactivities with some food extracts. A number of the IgE cross-reactivities could be inhibited after preabsorption of pooled sera with date extracts. Sixty-six percent of individual date hypersensitive human sera bound IgE in putative date fruit profilin and their pooled sera bound IgE in birch pollen profilin. IgE-binding of the endoglycosidase digested date fruit extracts to atopic serum pool was restricted to only a very low molecular weight band of 6.5-8 kDa.

CONCLUSION

These results indicate that date palm polypeptides share cross-reactive IgG and IgE epitopes with a number of foods implicated in the oral allergy syndrome, bind to birch and Timothy grass profilins and bind IgE through glycosyl residues. The clinical relevance of these cross-reactivities needs to be further elucidated.

Structural basis of non-specific lipid binding in maize lipid-transfer protein complexes revealed by high-resolution X-ray crystallography

Non-specific lipid-transfer proteins (nsLTPs) are involved in the movement of phospholipids, glycolipids, fatty acids, and steroids between membranes. Several structures of plant nsLTPs have been determined both by X-ray crystallography and nuclear magnetic resonance. However, the detailed structural basis of the non-specific binding of hydrophobic ligands by nsLTPs is still poorly understood. In order to gain a better understanding of the structural basis of the non-specific binding of hydrophobic ligands by nsLTPs and to investigate the plasticity of the fatty acid binding cavity in nsLTPs, seven high-resolution (between 1.3 A and 1.9 A) crystal structures have been determined. These depict the nsLTP from maize seedlings in complex with an array of fatty acids.A detailed comparison of the structures of maize nsLTP in complex with various ligands reveals a new binding mode in an nsLTP-oleate complex which has not been seen before. Furthermore, in the caprate complex, the ligand binds to the protein cavity in two orientations with equal occupancy. The volume of the hydrophobic cavity in the nsLTP from maize shows some variation depending on the size of the bound ligands. The structural plasticity of the ligand binding cavity and the predominant involvement of non-specific van der Waals interactions with the hydrophobic tail of the ligands provide a structural explanation for the non-specificity of maize nsLTP. The hydrophobic cavity accommodates various ligands from C10 to C18. The C18:1 ricinoleate with its hydroxyl group hydrogen bonding to Ala68 possibly mimics cutin monomer binding which is of biological importance. Some of the myristate binding sites in human serum albumin resemble the maize nsLTP, implying the importance of a helical bundle in accommodating the non-specific binding of fatty acids.

Cloning and expression in phospholipid containing cultures of the gene encoding the specific phosphatidylglycerol/phosphatidylinositol transfer protein from Aspergillus oryzae: evidence that the pg/pi-tp is tandemly arranged with the putative 3-ketoacyl-CoA thiolase gene

The phosphatidylglycerol/phosphatidylinositol transfer protein (PG/PI-TP) is a new and original phospholipid transfer protein (PLTP) isolated from the Deuteromycete, Aspergillus oryzae. We have isolated a genomic clone of the A. oryzae pg/pi-tp using a probe derived from the corresponding cDNA and sequenced the complete gene. The DNA sequence analysis revealed that pg/pi-tp gene is composed of three exons encoding a 18,823 Da protein of 175 amino acids as previously described and of two introns as deduced by cDNA and genomic sequence alignment. The isolated pg/pi-tp gene do not show similarity with other PLTP genes or the deduced PG/PI-TP protein with proteins already known. Comparison of the encoded PG/PI-TP with other deduced proteins from recent genomic or cDNA sequence from databases revealed that the PG/PI-TP was close to two encoded proteins deduced from the cDNA database of Aspergillus nidulans (54% identity and 68% similarity) and the second from Neurospora crassa (53% identity and 76% similarity). Therefore, we suggested that both proteins might belong to the PLTP family. Southern blot analysis of A. oryzae genomic DNA show that the PG/PI-TP was encoded by a single gene. Expression of pg/pi-tp was performed in phospholipid containing cultures with increasing carbon source concentrations in order to study the regulation of the PLTPs in the filamentous fungus cell. This was done to know if a high density culture could yield a high amount of biomass with high phospholipid transfer activity. Results showed that phospholipids as compared to glucose in standard cultures stimulated mycelial growth and global phospholipid transfer activity, but not the pg/pi-tp transcript accumulation. However, high concentration of both carbon sources yielded an inhibition of the expression of the pg/pi-tp gene and of the global phospholipid transfer activity. In conclusion, both carbon sources are not suitable to increase the PLTP production in high density cultures for biotechnological applications. Finally, using the gene walking sequencing method it is demonstrated that the pg/pi-tp is tandemly arranged on opposite DNA strands in a tail-to-tail orientation with a putative gene encoding the 3-ketoacyl-CoA thiolase (EC 2.3.1.16). Unlike the pg/pi-tp gene, this thiolase gene show a putative 'beta-oxidation box' and encodes a putative 44,150 Da protein of 321 amino acids composed of a putative N-terminal PTS2 (Peroxisomal Targeting Signal) consensus sequence for the peroxisome targeting. Comparison of the amino acid sequence of the A. oryzae thiolase to that of the Yarrowia lipolytica showed a 50% identity and a 69% similarity.

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.

Lipid transfer protein: a pan-allergen in plant-derived foods that is highly resistant to pepsin digestion

BACKGROUND

Lipid transfer proteins (LTPs) are small molecules of approximately 10 kD that demonstrate high stability. They have recently been identified as allergens in the Rosaceae subfamilies of the Prunoideae (peach, apricot, plum) and of the Pomoideae (apple). They belong to a family of structurally highly conserved proteins that are also present in non-Rosaceae vegetable foods.

OBJECTIVE

The aim of this study was to investigate the cross-reactivity to non-Rosaceae LTPs, and to study the role of protein stability in allergenicity.

METHODS

Thirty-eight patients with a positive SPT to Rosaceae fruit extracts enriched for LTP were characterized by interview and SPT. To investigate IgE cross-reactivity between Rosaceae and non-Rosaceae LTPs, RAST and RAST inhibition as well as ELISA and ELISA inhibition were performed, using whole food extracts and purified LTPs. Both purified natural LTPs (peach, carrot and broccoli) and Pichia pastoris recombinant LTPs (carrot and wheat) were included. Pepsin digestion was used to address the role of stability in the allergenicity of LTPs.

RESULTS

IgE antibodies to Rosaceae LTPs reacted to a broad range of vegetable foods, including Gramineae (cereals), Leguminosae (peanut), Juglandaceae (walnut), Anacardiaceae (pistachio), Brassicaceae (broccoli), Umbelliferae (carrot, celery), Solanaceae (tomato), Cucurbitaceae (melon), and Actinidiaceae (kiwi). Binding and inhibition studies with purified natural and recombinant LTPs confirmed their role in this cross-reactivity. Many of these cross-reactivities were accompanied by clinical food allergy, frequently including systemic reactions. Antibody binding to LTP was shown to be resistant to pepsin treatment of whole extract or purified LTP.

CONCLUSION

LTP is a pan-allergen with a degree of cross-reactivity comparable to profilin. Due to its extreme resistance to pepsin digestion, LTP is a potentially severe food allergen.

Complete amino acid sequence determination of the major allergen of peach (Prunus persica) Pru p 1

The major protein allergen of peach (Prunus persica), Pru p 1, has recently been identified as a lipid transfer protein (LTP). The complete primary structure of Pru p 1, obtained by direct amino acid sequence and liquid chromatography-mass spectrometry (LC-MS) analyses with the purified protein, is described here. The protein consists of 91 amino acids with a calculated molecular mass of 9178 Da. The amino acid sequence contains eight strictly conserved cysteines, as do all known LTPs, but secondary structure predictions failed to classify the peach 9 kDa protein as an 'all-alpha type', due to the high frequency of amino acids (nine prolines) disrupting alpha helices. Although the sequence similarity with maize LTP is only 63%, out of the 25 amino acids forming the inner surface of the tunnel-like hydrophobic cavity in maize ns-LTP 16 are identical and 7 similar in the peach homolog, supporting the hypothesis of a similar function.

Characterization and expression of the cDNA encoding a new kind of phospholipid transfer protein, the phosphatidylglycerol/phosphatidylinositol transfer protein from Aspergillus oryzae: evidence of a putative membrane targeted phospholipid transfer protein in fungi

The full-length cDNA of a phospholipid transfer protein (PLTP) was isolated from Aspergillus oryzae by a RACE-PCR procedure using degenerated primer pool selected from the N-terminal sequence of the purified phosphatidylinositol/phosphatidylglycerol transfer protein (PG/PI-TP). The cDNA encodes a 173 amino acid protein of 18823 Da. The deduced amino acid sequence from position 38 to 67 is 100% identical to the N-terminal sequence (first 30 amino acids) of the purified PG/PI-TP. This amino acid sequence is preceded by a leader peptide of 37 amino acids which is predicted to be composed of a signal peptide of 21 amino acids followed by an extra-sequence of 16 amino acids, or a membrane anchor protein signal (amino acid 5-29). This strongly suggests that the PG/PI-TP is a targeted protein. The deduced mature protein is 138 amino acids long with a predicted molecular mass of 14933 Da. Comparison of the deduced PG/PI-TP sequence with other polypeptide sequences available in databases revealed a homology with a protein deduced from an open reading frame coding for an unknown protein in Saccharomyces cerevisiae (36% identity and 57% similarity). Apart from this homology, the PG/PI-TP is unique and specific to the filamentous fungi on the basis of comparison of PLTP protein sequences. Northern blot analysis of RNA isolated from A. oryzae cultures grown on glucose or glucose supplemented with phospholipids suggests that the PG/PI-TP is transcribed by only one RNA species and allows us to show that expression of the protein is regulated at the messenger RNA level.

Solution structure of a lipid transfer protein extracted from rice seeds. Comparison with homologous proteins

Nuclear magnetic resonance (NMR) spectroscopy was used to determine the three dimensional structure of rice nonspecific lipid transfer protein (ns-LTP), a 91 amino acid residue protein belonging to the broad family of plant ns-LTP. Sequence specific assignment was obtained for all but three HN backbone 1H resonances and for more than 95% of the 1H side-chain resonances using a combination of 1H 2D NOESY; TOCSY and COSY experiments at 293 K. The structure was calculated on the basis of four disulfide bridge restraints, 1259 distance constraints derived from 1H-1H Overhauser effects, 72 phi angle restraints and 32 hydrogen-bond restraints. The final solution structure involves four helices (H1: Cys3-Arg18, H2: Ala25-Ala37, H3: Thr41-Ala54 and H4: Ala66-Cys73) followed by a long C-terminal tail (T) with no observable regular structure. N-capping residues (Thr2, Ser24, Thr40), whose side-chain oxygen atoms are involved in hydrogen bonds with i + 3 amide proton additionally stabilize the N termini of the first three helices. The fourth helix involving Pro residues display a mixture of alpha and 3(10) conformation. The rms deviation of 14 final structures with respect to the average structure is 1.14 +/- 0.16 A for all heavy atoms (C, N, O and S) and 0.72 +/- 0.01 A for the backbone atoms. The global fold of rice ns-LTP is close to the previously published structures of wheat, barley and maize ns-LTPs exhibiting nearly identical pattern of the numerous sequence specific interactions. As reported previously for different four-helix topology proteins, hydrophobic, hydrogen bonding and electrostatic mechanisms of fold stabilization were found for the rice ns-LTP. The sequential alignment of 36 ns-LTP primary structures strongly suggests that there is a uniform pattern of specific long-range interactions (in terms of sequence), which stabilize the fold of all plant ns-LTPs.

Interaction of the wheat endosperm lipid-binding protein puroindoline-a with phospholipids

Puroindoline-a is the main component of a new family of proteins that has been suggested to exert an antimicrobial activity in plant seeds through an interaction with lipid membranes. Here the interaction of puroindoline-a with model phospholipid membranes and micelles has been studied using intrinsic tryptophan fluorescence, fluorescence polarization of diphenyl hexatriene, and proteolysis experiments. The protein appears to interact with both zwitterionic and negative phospholipids. The interaction with phosphatidylcholine is characterized by low-affinity surface binding with very limited penetration into the hydrophobic membrane interior. On the other hand, the interaction with phosphatidylglycerol displays a high affinity and involves a partial penetration of the protein into the bilayer interior that disrupts acyl chain packing. The specificity appears to be due to the presence of a stretch of positively charged residues in the protein sequence. In all, the lipid-binding properties of puroindoline-a resemble those of cardiotoxins, another family of proteins for which a disruptive effect on the membrane structure has been involved to explain their biological function.

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.

Phospholipid transfer proteins revisited

Phosphatidylinositol transfer protein (PI-TP) and the non-specific lipid transfer protein (nsL-TP) (identical with sterol carrier protein 2) belong to the large and diverse family of intracellular lipid-binding proteins. Although these two proteins may express a comparable phospholipid transfer activity in vitro, recent studies in yeast and mammalian cells have indicated that they serve completely different functions. PI-TP (identical with yeast SEC14p) plays an important role in vesicle flow both in the budding reaction from the trans-Golgi network and in the fusion reaction with the plasma membrane. In yeast, vesicle budding is linked to PI-TP regulating Golgi phosphatidylcholine (PC) biosynthesis with the apparent purpose of maintaining an optimal PI/PC ratio of the Golgi complex. In mammalian cells, vesicle flow appears to be dependent on PI-TP stimulating phosphatidylinositol 4,5-bisphosphate (PIP2) synthesis. This latter process may also be linked to the ability of PI-TP to reconstitute the receptor-controlled PIP2-specific phospholipase C activity. The nsL-TP is a peroxisomal protein which, by its ability to bind fatty acyl-CoAs, is most likely involved in the beta-oxidation of fatty acids in this organelle. This protein constitutes the N-terminus of the 58 kDa protein which is one of the peroxisomal 3-oxo-acyl-CoA thiolases. Further studies on these and other known phospholipid transfer proteins are bound to reveal new insights in their important role as mediators between lipid metabolism and cell functions.

Peroxisomes in adrenal steroidogenesis

Peroxisomes, cytoplasmic organelles limited by a single membrane and with a matrix of moderate electron density, are present in a great number of cells, namely in adrenal cortex and other steroid-secreting organs. Presently peroxisomes are considered to be involved in important metabolic processes. They intervene in: (1) the production and degradation of H2O2; (2) biosynthesis of ether-phospholipids, cholesterol, dolichol, and bile acids; (3) oxidation of very long chain fatty acids, purines, polyamines, and prostaglandins; (4) catabolism of pipecolic, phythanic and glyoxylic acids; and (5) gluconeogenesis. Recent studies demonstrated that the experimental alterations in the normal steroidogenesis, produce significant morphological and biochemical changes in peroxisomes. Besides this, the presence of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (the key enzyme in the de novo cholesterol synthesis from acetate) and of sterol carrier protein-2 (SCP2), which is involved in the cholesterol metabolism and steroid metabolic pathways, are located in peroxisomes of steroid-secreting cells. In addition, patients with peroxisome diseases present deficiency in steroidogenesis, as well as reduced levels of SCP2. These data pointed out the important role of peroxisomes in steroid biosynthesis.

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.

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).

Cloning and expression of human cDNA encoding phosphatidylinositol transfer protein beta

cDNA encoding the beta isoform of human phosphatidylinositol transfer protein was cloned from a human brain cDNA library. The deduced sequence of the protein comprised 271 amino acids with a calculated molecular mass of 31,539 Da, and showed 98.1% identity to that of the beta isoform of rat phosphatidylinositol transfer protein. The cDNA hybridized to a 3.4-kb mRNA, which was widely expressed in various human tissues including brain.

Mutant rat phosphatidylinositol/phosphatidylcholine transfer proteins specifically defective in phosphatidylinositol transfer: implications for the regulation of phospholipid transfer activity

The mammalian phosphatidylinositol/phosphatidylcholine transfer proteins (PI-TPs) catalyze exchange of phosphatidylinositol (PI) or phosphatidylcholine (PC) between membrane bilayers in vitro. We find that Ser-25, Thr-59, Pro-78, and Glu-248 make up a set of rat (r) PI-TP residues, substitution of which effected a dramatic reduction in the relative specific activity for PI transfer activity without significant effect on PC transfer activity. Thr-59 was of particular interest as it is a conserved residue in a highly conserved consensus protein kinase C phosphorylation motif in metazoan PI-TPs. Replacement of Thr-59 with Ser, Gln, Val, Ile, Asn, Asp, or Glu effectively abolished PI transfer capability but was essentially silent with respect to PC transfer activity. These findings identify rPI-TP residues that likely cooperate to form a PI head-group binding/recognition site or that lie adjacent to such a site. Finally, the selective sensitivity of the PI transfer activity of rPI-TP to alteration of Thr-59 suggests a mechanism for in vivo regulation of rPI-TP activity.

Purification and characterization of a novel specific phosphatidylglycerol-phosphatidylinositol transfer protein with high activity from Aspergillus oryzae

A novel phospholipid transfer protein has been purified to homogeneity 406-fold from the filamentous fungus Aspergillus oryzae. The successive steps of purification comprised ultrafiltration, gel filtration on Sephadex G-75, ion exchange chromatographies on DEAE-Sepharose and Mono Q. The active protein is a monomer with a molecular mass of 19,000, estimated from SDS electrophoresis, amino acid composition as well as gel filtration. The isoelectric point is 4.8. The amino acid composition is characterized by a high amount of Gly, Leu, Ser, Asx and Glx residues and 4 Cys residues. N-terminal sequence was determined and compared with M. mucedo sequence. The purified protein was found to transfer preferentially phosphatidylglycerol and phosphatidylinositol over phosphatidylcholine > phosphatidylethanolamine > phosphatidylserine and no phosphatidic acid. Optimal temperature for in vitro transfer was 25-30 degrees C and optimal pH 4-7. Heating protein at 100 degrees C does not inactivate protein whereas a denaturation with urea is irreversible.

High-resolution crystal structure of the non-specific lipid-transfer protein from maize seedlings

BACKGROUND

The movement of lipids between membranes is aided by lipid-transfer proteins (LTPs). Some LTPs exhibit broad specificity, transferring many classes of lipids, and are termed non-specific LTPs (ns-LTPs). Despite their apparently similar mode of action, no sequence homology exists between mammalian and plant ns-LTPs and no three-dimensional structure has been reported for any plant ns-LTP.

RESULTS

We have determined the crystal structure of ns-LTP from maize seedlings by multiple isomorphous replacement and refined the structure to 1.9 A resolution. The protein comprises a single compact domain with four alpha-helices and a long C-terminal region. The eight conserved cysteines form four disulfide bridges (assigned as Cys4-Cys52, Cys14-Cys29, Cys30-Cys75, and Cys50-Cys89) resolving the ambiguity that remained from the chemical determination of pairings in the homologous protein from castor bean. Two of the bonds, Cys4-Cys52 and Cys50-Cys89, differ from what would have been predicted from sequence alignment with soybean hydrophobic protein. The complex between maize ns-LTP and hexadecanoate (palmitate) has also been crystallized and its structure refined to 1.8 A resolution.

CONCLUSIONS

The fold of maize ns-LTP places it in a new category of all-alpha-type structure, first described for soybean hydrophobic protein. In the absence of a bound ligand, the protein has a tunnel-like hydrophobic cavity, which is large enough to accommodate a long fatty acyl chain. In the structure of the complex with palmitate, most of the acyl chain is buried inside this hydrophobic cavity.

Amino acid sequences of three acyl-binding/lipid-transfer proteins from rape seedlings

The complete amino acid sequence of three acyl-binding/lipid-transfer proteins, AB/LTP I, AB/LTP II and AB/LTP III from germinated rape seeds were determined. AB/LTP I and AB/LTP II consist of 93 residues and the M(r) was determined as 9408 by mass spectrometry and calculated as 9406.8 from the sequence. AB/LTP III consists of 92 residues and the M(r) was determined as 9424 by mass spectrometry and calculated as 9422.8 from the sequence. The primary structures were determined by automated Edman degradations of the intact proteins and peptides obtained from digestion with trypsin and endoproteinase Asp-N and cyanogen bromide cleavage. Use of 252Cf plasma-desorption mass spectrometry facilitated the identification and verification of peptides.

The Saccharomyces cerevisiae phosphatidylinositol-transfer protein effects a ligand-dependent inhibition of choline-phosphate cytidylyltransferase activity

The Saccharomyces cerevisiae protein SEC14p is required for Golgi function and cell viability in vivo. This requirement is obviated by mutations that specifically inactivate the CDP-choline pathway for phosphatidylcholine biosynthesis. The biochemical basis for the in vivo relationship between SEC14p function and the CDP-choline pathway has remained obscure. We now report that SEC14p effects an in vivo depression of CDP-choline pathway activity by inhibiting choline-phosphate cytidylyltransferase (CCTase; EC 2.7.7.15), the rate-determining enzyme of the CDP-choline pathway. Moreover, this SEC14p-mediated inhibition of CCTase was recapitulated in vitro and was saturable. Finally, whereas the SEC14p-dependent inhibition of CCTase in vitro was markedly reduced under assay conditions that were expected to increase levels of phosphatidylinositol-bound SEC14p, assay conditions expected to increase levels of phosphatidylcholine-bound SEC14p resulted in significant potentiation of CCTase inhibition. The collective data suggest that the phosphatidylcholine-bound form of SEC14p effects an essential repression of CDP-choline pathway activity in Golgi membranes by inhibiting CCTase and that the phospholipid-binding/exchange activity of SEC14p represents a mechanism by which the regulatory activity of SEC14p is itself controlled.

Solubilization of a membrane-associated protein from rat nervous system tissues which binds anionic glycolipids and phospholipids

An anionic glycolipid and phospholipid binding protein was characterized in detergent-solubilized rat brain synaptosomes using a synthetic, polyvalent radiolabeled ganglioside-protein conjugate as radioligand. Gangliosides are prominent cell surface glycoconjugates in vertebrate brain, where they may function in membrane protein regulation or in cell-cell recognition. The neoganglioprotein (GT1b)13BSA was radioiodinated and used to probe solubilized synaptosomal proteins for ganglioside binding activity using a receptor-ligand precipitation assay. Binding data revealed a high affinity (KD = 1 nM), saturable (Bmax = 173 pmol/mg protein) binding activity that was proteinase sensitive, calcium independent and maximal at neutral pH. Size exclusion chromatography of the synaptosomal (GT1b)13BSA binding activity indicated a M(r) of approximately 28 kDa. Binding activity with similar characteristics was solubilized from other rat tissues, with activity from sciatic nerve = muscle > synaptosomes > central nervous system myelin = liver. Gangliosides added as mixed detergent-lipid micelles inhibited (GT1b)13BSA binding: GT1b, GD1a and GD1b were the most effective inhibitors (IC50 approximately 200 nM), while GM1 and GM3 were 5-fold less effective. In addition, the sphingolipids sulfatide and sialylneolactotetraosylceramide were effective inhibitors, with IC50 values of 300 nM and 200 nM, respectively. The neutral sphingolipid GA1 did not block (GT1b)13BSA binding. Phosphatidylcholine and phosphatidylethanolamine were non-inhibitory, however phosphatidylglycerol, phosphatidylserine, and phosphatidylinositol inhibited half-maximally at 200-300 nM. Inhibition by both gangliosides and anionic phospholipids was competitive with (GT1b)13BSA. We conclude that a binding protein for anionic glycolipids and phospholipids is distributed on nerve and muscle membranes.

Structure of a human Clara cell phospholipid-binding protein-ligand complex at 1.9 A resolution

The Clara cell phospholipid-binding protein, previously referred to as CC10, is a homodimeric protein of M(r) 15,800. It is secreted into the bronchioalveolar lining layer in mammalian lung. A combination of X-ray crystallography and chemical analysis was used to determine that phosphatidylcholine and phosphatidylinositol are bound to the protein as isolated from human lung lavage. We now report the crystal structure of the protein-phospholipid complex at 1.9 A resolution. The phospholipid is bound inside the protein's large hydrophobic cavity. A model is proposed for the manner in which a channel may open to provide access to the cavity, allowing the binding or potential release of phospholipid.

Two-dimensional 1H-NMR studies of maize lipid-transfer protein. Sequence-specific assignment and secondary structure

Correlation spectroscopy (COSY), total correlation spectroscopy (TOCSY) and NOE spectroscopy (NOESY) experiments have been used to assign sequentially the 1H 500-MHz NMR spectra of a non-specific (ns) lipid-transfer protein extracted from maize seeds. The spin-system identification and sequential assignment were combined with secondary-structure determination to identify most of the proton resonances of this 93-residue protein. From the sequential connectivities it was established that the secondary structure mainly involved four helical fragments: H1, H2, H3 and H4. This secondary structure was compared with that of wheat ns-lipid-transfer protein recently determined. The four helices are located in nearly the same regions, but helix H4 is appreciably longer in the maize protein than in the wheat protein. Comparison of the transfer activities reveals that the maize protein is more efficient than the wheat ns-lipid-transfer protein and that this difference is probably due to the affinity of the lipid for the binding site and not to the interfacial activation, i.e. adsorption of the ns-lipid-transfer protein to the membrane. From these results, it is suggested that helix H4 is a part of the lipid-binding site or contributes to the folding of this site. The present data define the basis for a further modelling of the three-dimensional structure of the maize ns-lipid-transfer protein which will be compared with that of the wheat ns-lipid-transfer protein in order to establish structure/activity relationships for this class of carriers by using natural ns-lipid-transfer protein mutants.

Properties of human plasma lipid transfer protein in aqueous solution and at interfaces

Human plasma lipid transfer protein (cholesteryl ester transfer protein) has been characterized for its solution and surface properties. The protein is monomeric in aqueous solution up to 0.62 g/L (11.7 microM) as demonstrated by sedimentation equilibrium. It binds to the surface of a lipid microemulsion having an average diameter of 26 nm made from triolein and egg yolk phosphatidylcholine, with an estimated dissociation constant 1.2 x 10(-8) M, and the maximum saturation binding level is 8 protein molecules per particle regardless of the presence of apolipoprotein A-I. Circular dichroism measurement indicated that the protein in solution is predominantly in the beta-sheet/beta-turn conformation with some alpha-helix, and this profile does not undergo drastic change by its binding to the lipid surface. The analysis of the behavior of the protein in its monomolecular layer at the air-buffer interface indicated that it is also monomeric at the interface. LTP molecules occupied the same area per amino acid as other apolipoproteins in the monolayer but had a higher collapse pressure of its monolayer (18 dyn/cm), and the protein stayed at the interface even after the overcompressing monolayer far beyond the collapsing pressure up to 40 dyn/cm.

Sequence of a human cDNA encoding phosphatidylinositol transfer protein and occurrence of a related sequence in widely divergent eukaryotes

Phosphatidylinositol (PtdIns) transfer protein (PtdInsTP) is a phospholipid transfer protein that has been detected in all mammalian tissues examined. It catalyzes the transfer in vitro of PtdIns and phosphatidylcholine between membranes in a number of natural and artificial membrane systems and may be involved in secretion in vivo. In previous studies, we isolated and sequenced a cDNA encoding a rat PtdInsTP. A rat cDNA probe was used to isolate clones from a lambda gt11 human testis cDNA library which encoded full-length human PtdInsTP. The cDNA sequence defines a 270-amino-acid, 31.8-kDa protein whose sequence shares 98.9% identity to that of rat, making it one of the most conserved proteins known between the two species. DNA blot hybridization studies suggest that there may be more than one gene encoding this protein in humans. A comparison of rat and human PtdInsTP cDNAs revealed strong sequence similarity (88 and 84%) in portions of the corresponding 5'- and 3'-untranslated regions (UTR) of the rat and human mRNAs.

Band 3, the anion exchanger of the erythrocyte membrane, is also a flippase

The transbilayer reorientation (flip-flop) of the long-chain amphiphilic anion DENSA (5-(N-decyl)aminonaphthalene-2-sulfonic acid) in the erythrocyte membrane was studied by fluorescence spectroscopy. DENSA intercalates into the membrane at a high membrane/water partition coefficient (3.2.10(5)) and rapidly reorients from the outer to the inner layer in a first order process (k = 0.11 min-1, 37 degrees C, pH 7.4) leading to a steady-state distribution inner:outer layer of about 30:70. The activation energy of the fully reversible and symmetric flip process is about 110 kJ/mol. DIDS and various other established covalent and non-covalent inhibitors of anion transport via the erythrocyte anion exchanger, band 3 (AE 1), suppress the flip to a minimum of about 30-35% of the control. The flip is also inhibited by Cl- with a half maximal inhibitory concentration equal to that required for the inhibition of the exchange flux of ordinary anions via band 3. These findings indicate the involvement of a band 3 mediated (DIDS-sensitive) component of the flip and a DIDS-insensitive one, possibly involving, at least to some extent, simple transbilayer 'diffusion'. This latter component is stimulated by diamide, an SH oxidant known to increase the permeability of the membrane lipid domain of the erythrocyte. Alcohols (butanol, hexanol) accelerate both flip components. Papain treatment, known to inhibit 'ordinary' anion exchange, accelerates both flip and flop. The results suggest that band 3 protein, besides being a conventional transporter of anions, can act as a flippase translocating anionic, membrane-intercalated amphiphiles approaching the transporter from the lipid domain. The flippase mode of operation of band 3 must, however, differ in its mechanism from the conventional exchange mode.

A phosphatidylinositol transfer protein controls the phosphatidylcholine content of yeast Golgi membranes

SEC14p is required for protein transport from the yeast Golgi complex. We describe a quantitative analysis of yeast bulk membrane and Golgi membrane phospholipid composition under conditions where Golgi secretory function has been uncoupled from its usual SEC14p requirement. The data demonstrate that SEC14p specifically functions to maintain a reduced phosphatidylcholine content in Golgi membranes and indicate that overproduction of SEC14p markedly reduces the apparent rate of phosphatidylcholine biosynthesis via the CDP-choline pathway in vivo. We suggest that SEC14p serves as a sensor of Golgi membrane phospholipid composition through which the activity of the CDP-choline pathway in Golgi membranes is regulated such that a phosphatidylcholine content that is compatible with the essential secretory function of these membranes is maintained.