Molecular cloning of a cDNA from Brassica napus L. for a homologue of acyl-CoA-binding protein

Molecular Characterization of the Acyl-CoA-Binding Protein Genes Reveals Their Significant Roles in Oil Accumulation and Abiotic Stress Response in Cotton

Members of the acyl-CoA-binding protein (ACBP) gene family play vital roles in diverse processes related to lipid metabolism, growth and development, and environmental response. Plant ACBP genes have been well-studied in a variety of species including Arabidopsis, soybean, rice and maize. However, the identification and functions of ACBP genes in cotton remain to be elucidated. In this study, a total of 11 GaACBP, 12 GrACBP, 20 GbACBP, and 19 GhACBP genes were identified in the genomes of Gossypium arboreum, Gossypium raimondii, Gossypium babardense, and Gossypium hirsutum, respectively, and grouped into four clades. Forty-nine duplicated gene pairs were identified in Gossypium ACBP genes, and almost all of which have undergone purifying selection during the long evolutionary process. In addition, expression analyses showed that most of the GhACBP genes were highly expressed in the developing embryos. Furthermore, GhACBP1 and GhACBP2 were induced by salt and drought stress based on a real-time quantitative PCR (RT-qPCR) assay, indicating that these genes may play an important role in salt- and drought-stress tolerance. This study will provide a basic resource for further functional analysis of the ACBP gene family in cotton.

Genome-wide identification of acyl-CoA binding proteins and possible functional prediction in legumes

Acyl-CoA-binding proteins (ACBPs), members of a vital housekeeping protein family, are present in various animal and plant species. They are divided into four classes: small ACBPs (class I), ankyrin-repeat ACBPs (class II), large ACBPs (class III), and kelch-ACBPs (class IV). Plant ACBPs play a pivotal role in intracellular transport, protection, and pool formation of acyl-CoA esters, promoting plant development and stress response. Even though legume crops are important for vegetable oils, proteins, vegetables and green manure, legume ACBPs are not well investigated. To comprehensively explore the functions of ACBPs in nine legumes (Lotus japonicus, Medicago truncatula, Glycine max, Vigna angularis, Vigna radiata, Phaseolus vulgaris, Arachis hypogaea, Arachis duranensis, and Arachis ipaensis), we conducted genome-wide identification of the ACBP gene family. Our evolutionary analyses included phylogenetics, gene structure, the conserved motif, chromosomal distribution and homology, subcellular localization, cis-elements, and interacting proteins. The results revealed that ACBP Orthologs of nine legumes had a high identity in gene structure and conserved motif. However, subcellular localization, cis-acting elements, and interaction protein analyses revealed potentially different functions from previously reported. The predicted results were also partially verified in Arachis hypogaea. We believe that our findings will help researchers understand the roles of ACBPs in legumes and encourage them to conduct additional research.

Mapping of a QTL associated with sucrose content in peanut kernels using BSA-seq

As an important factor affecting the edible quality of peanut kernels, sucrose content is a complex quantitative trait regulated by multiple factors. In this study, an F₂ segregating population and a recombinant inbred line (RIL) population, derived from a cross between the high sucrose content variety Jihuatian 1 and the low sucrose content line PI478819, were used as materials to map a quantitative trait locus (QTL) associated with sucrose content in peanut kernels. Four QTLs were initially located on chromosomes A03 and A06 based on BSA-seq technology, and multiple kompetitive allele-specific PCR markers were developed based on single-nucleotide polymorphisms (SNPs) in the intervals. The markers were genotyped in the RIL population and finely mapped to a stable QTL, qSUCA06, located on chromosome A06 within a 0.29-Mb physical genomic interval (112367085-112662675 bp), which accounted for 31.95%-41.05% of the phenotypic variance explained. SNP and insertion/deletion annotations were performed on genes in the candidate interval, and having screened out those genes with mutations in exons, candidate genes were verified by qRT-PCR. The results revealed that Arahy.Y2LWD9 may be the main gene regulating sucrose content. The QTL identified in this study will not only contribute to marker-assisted breeding for improvement of peanut sucrose content but also paves the way for identifying gene function.

Roles of acyl-CoA-binding proteins in plant reproduction

Acyl-CoA-binding proteins (ACBPs) constitute a well-conserved family of proteins in eukaryotes that are important in stress responses and development. Past studies have shown that ACBPs are involved in maintaining, transporting and protecting acyl-CoA esters during lipid biosynthesis in plants, mammals, and yeast. ACBPs show differential expression and various binding affinities for acyl-CoA esters. Hence, ACBPs can play a crucial part in maintaining lipid homeostasis. This review summarizes the functions of ACBPs during the stages of reproduction in plants and other organisms. A comprehensive understanding on the roles of ACBPs during plant reproduction may lead to opportunities in crop improvement in agriculture.

Plant Acyl-CoA-Binding Proteins-Their Lipid and Protein Interactors in Abiotic and Biotic Stresses

Plants are constantly exposed to environmental stresses during their growth and development. Owing to their immobility, plants possess stress-sensing abilities and adaptive responses to cope with the abiotic and biotic stresses caused by extreme temperatures, drought, flooding, salinity, heavy metals and pathogens. Acyl-CoA-binding proteins (ACBPs), a family of conserved proteins among prokaryotes and eukaryotes, bind to a variety of acyl-CoA esters with different affinities and play a role in the transport and maintenance of subcellular acyl-CoA pools. In plants, studies have revealed ACBP functions in development and stress responses through their interactions with lipids and protein partners. This review summarises the roles of plant ACBPs and their lipid and protein interactors in abiotic and biotic stress responses.

Comparative Transcriptomics Analysis of Brassica napus L. during Seed Maturation Reveals Dynamic Changes in Gene Expression between Embryos and Seed Coats and Distinct Expression Profiles of Acyl-CoA-Binding Proteins for Lipid Accumulation

Production of vegetable oils is a vital agricultural resource and oilseed rape (Brassica napus) is the third most important oil crop globally. Although the regulation of lipid biosynthesis in oilseeds is still not fully defined, the acyl-CoA-binding proteins (ACBPs) have been reported to be involved in such metabolism, including oil accumulation, in several plant species. In this study, progressive changes in gene expression in embryos and seed coats at different stages of seed development were comprehensively investigated by transcriptomic analyses in B. napus, revealing dynamic changes in the expression of genes involved in lipid biosynthesis. We show that genes encoding BnACBP proteins show distinct changes in expression at different developmental stages of seed development and show markedly different expression between embryos and seed coats. Both isoforms of the ankyrin-repeat BnACBP2 increased during the oil accumulation period of embryo development. By contrast, the expression of the three most abundant isoforms of the small molecular mass BnACBP6 in embryos showed progressive reduction, despite having the highest overall expression level. In seed coats, BnACBP3, BnACBP4 and BnACBP5 expression remained constant during development, whereas the two major isoforms of BnACBP6 increased, contrasting with the data from embryos. We conclude that genes related to fatty acid and triacylglycerol biosynthesis showing dynamic expression changes may regulate the lipid distribution in embryos and seed coats of B. napus and that BnACBP2 and BnACBP6 are potentially important for oil accumulation.

Functional and Structural Diversity of Acyl-coA Binding Proteins in Oil Crops

Diversities in structure and function of ACBP were discussed in this review. ACBP are important proteins that could transport newly synthesized fatty acid, activated into -coA, from plastid to endoplasmic reticulum, where oil in the form of triacylglycerol occurs. ACBP were detected in various animal and plants species, which indicated their importance in biological function. In fact, involvement of ACBP in important process such as lipid metabolism, regulation of enzyme and gene expression, and in response to plant stresses has been proven in several studies. In this review, findings on ACBP of 11 well-known oil crops were reviewed to comprehend diversity, comparative analyses on ACBP structure were made, and link between structure and function, tissue expression and subcellular location of ACBP were also observed. Incomplete reports in some species were mentioned, which might be encouraging to start or to perform deeper studies. Similar characteristics were found in paralogs ACBP, and orthologs ACBP had different functions, despite the high identity in amino acid sequence. At the end, it is confirmed that ortholog proteins could not necessarily display the same function, even from closely related species.

Overexpression of Acyl-CoA-Binding Protein 1 (ChACBP1) From Saline-Alkali-Tolerant Chlorella sp. Enhances Stress Tolerance in Arabidopsis

A large proportion of the world's arable land is saline-alkali land, and this is becoming an urgent environmental problem for agriculture. One approach to address this problem is to develop new varieties of stress-resistant plants through genetic engineering. The algae (Chlorella sp.) JB6, which was previously isolated from saline-alkali land, was found to exhibit strong NaHCO₃ tolerance. Here, we explored saline-alkali-tolerance genes in this alga that might be useful for producing abiotic stress-resistant transgenic plants. We identified a gene encoding acyl-CoA-binding protein 1 (ACBP1) from JB6 by screening a full-length cDNA library in yeast under NaHCO₃ stress. Northern blot analyses showed that the ChACBP1 mRNA levels were significantly up-regulated under abiotic stresses such as salinity, oxidation, heavy metals, and low temperature stresses. The recombinant ChACBP1 protein was found to bind phosphatidylcholine in vitro. Green fluorescent protein-labeled ChACBP1 was localized to the cytosol. Overexpression of ChACBP1 in yeast and Arabidopsis increased their resistance to high salinity, oxidation, heavy metals, and low temperature stresses. These results suggested that ChACBP1 may mediate plant abiotic stress adaptation through phospholipid metabolism. Thus, ChACBP1 may be useful to genetically improve the tolerance of plants to saline-alkali soil.

Deciphering the roles of acyl-CoA-binding proteins in plant cells

Lipid trafficking is vital for metabolite exchange and signal communications between organelles and endomembranes. Acyl-CoA-binding proteins (ACBPs) are involved in the intracellular transport, protection, and pool formation of acyl-CoA esters, which are important intermediates and regulators in lipid metabolism and cellular signaling. In this review, we highlight recent advances in our understanding of plant ACBP families from a cellular and developmental perspective. Plant ACBPs have been extensively studied in Arabidopsis thaliana (a dicot) and to a lesser extent in Oryza sativa (a monocot). Thus far, they have been detected in the plasma membrane, vesicles, endoplasmic reticulum, Golgi apparatus, apoplast, cytosol, nuclear periphery, and peroxisomes. In combination with biochemical and molecular genetic tools, the widespread subcellular distribution of respective ACBP members has been explicitly linked to their functions in lipid metabolism during development and in response to stresses. At the cellular level, strong expression of specific ACBP homologs in specialized cells, such as embryos, stem epidermis, guard cells, male gametophytes, and phloem sap, is of relevance to their corresponding distinct roles in organ development and stress responses. Other interesting patterns in their subcellular localization and spatial expression that prompt new directions in future investigations are discussed.

Characterization of a small acyl-CoA-binding protein (ACBP) from Helianthus annuus L. and its binding affinities

Acyl-CoA-binding proteins (ACBPs) bind to acyl-CoA esters and promote their interaction with other proteins, lipids and cell structures. Small class I ACBPs have been identified in different plants, such as Arabidopsis thaliana (AtACBP6), Brassica napus (BnACBP) and Oryza sativa (OsACBP1, OsACBP2, OsACBP3), and they are capable of binding to different acyl-CoA esters and phospholipids. Here we characterize HaACBP6, a class I ACBP expressed in sunflower (Helianthus annuus) tissues, studying the specificity of its corresponding recombinant HaACBP6 protein towards various acyl-CoA esters and phospholipids in vitro, particularly using isothermal titration calorimetry and protein phospholipid binding assays. This protein binds with high affinity to de novo synthetized derivatives palmitoly-CoA, stearoyl-CoA and oleoyl-CoA (Kd 0.29, 0.14 and 0.15 μM respectively). On the contrary, it showed lower affinity towards linoleoyl-CoA (Kd 5.6 μM). Moreover, rHaACBP6 binds to different phosphatidylcholine species (dipalmitoyl-PC, dioleoyl-PC and dilinoleoyl-PC), yet it displays no affinity towards other phospholipids like lyso-PC, phosphatidic acid and lysophosphatidic acid derivatives. In the light of these results, the possible involvement of this protein in sunflower oil synthesis is considered.

Acyl-CoA-Binding Proteins (ACBPs) in Plant Development

Acyl-CoA-binding proteins (ACBPs) play a pivotal role in fatty acid metabolism because they can transport medium- and long-chain acyl-CoA esters. In eukaryotic cells, ACBPs are involved in intracellular trafficking of acyl-CoA esters and formation of a cytosolic acyl-CoA pool. In addition to these ubiquitous functions, more specific non-redundant roles of plant ACBP subclasses are implicated by the existence of multigene families with variable molecular masses, ligand specificities, functional domains (e.g. protein-protein interaction domains), subcellular locations and gene expression patterns. In this chapter, recent progress in the characterization of ACBPs from the model dicot plant, Arabidopsis thaliana, and the model monocot, Oryza sativa, and their emerging roles in plant growth and development are discussed. The functional significance of respective members of the plant ACBP families in various developmental and physiological processes such as seed development and germination, stem cuticle formation, pollen development, leaf senescence, peroxisomal fatty acid β-oxidation and phloem-mediated lipid transport is highlighted.

The binding versatility of plant acyl-CoA-binding proteins and their significance in lipid metabolism

Acyl-CoA esters are the activated form of fatty acids and play important roles in lipid metabolism and the regulation of cell functions. They are bound and transported by nonenzymic proteins such as the acyl-CoA-binding proteins (ACBPs). Although plant ACBPs were so named by virtue of amino acid homology to existing yeast and mammalian counterparts, recent studies revealed that ligand specificities of plant ACBPs are not restricted to acyl-CoA esters. Arabidopsis and rice ACBPs also interact with phospholipids, and their affinities to different acyl-CoA species and phospholipid classes vary amongst isoforms. Their ligands also include heavy metals. Interactors of plant ACBPs are further diversified due to the evolution of protein-protein interacting domains. This review summarizes our current understanding of plant ACBPs with a focus on their binding versatility. Their broad ligand range is of paramount significance in serving a multitude of functions during development and stress responses as discussed herein. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Plant Cytosolic Acyl-CoA-Binding Proteins

A gene family encoding six members of acyl-CoA-binding proteins (ACBP) exists in Arabidopsis and they are designated as AtACBP1-AtACBP6. They have been observed to play pivotal roles in plant lipid metabolism, consistent to the abilities of recombinant AtACBP in binding different medium- and long-chain acyl-CoA esters in vitro. While AtACBP1 and AtACBP2 are membrane-associated proteins with ankyrin repeats and AtACBP3 contains a signaling peptide for targeting to the apoplast, AtACBP4, AtACBP5 and AtACBP6 represent the cytosolic forms in the AtACBP family. They were verified to be subcellularly localized in the cytosol using diverse experimental methods, including cell fractionation followed by western blot analysis, immunoelectron microscopy and confocal laser-scanning microscopy using autofluorescence-tagged fusions. AtACBP4 (73.2 kDa) and AtACBP5 (70.1 kDa) are the largest, while AtACBP6 (10.4 kDa) is the smallest. Their binding affinities to oleoyl-CoA esters suggested that they can potentially transfer oleoyl-CoA esters from the plastids to the endoplasmic reticulum, facilitating the subsequent biosynthesis of non-plastidial membrane lipids in Arabidopsis. Recent studies on ACBP, extended from a dicot (Arabidopsis) to a monocot, revealed that six ACBP are also encoded in rice (Oryza sativa). Interestingly, three small rice ACBP (OsACBP1, OsACBP2 and OsACBP3) are present in the cytosol in comparison to one (AtACBP6) in Arabidopsis. In this review, the combinatory and distinct roles of the cytosolic AtACBP are discussed, including their functions in pollen and seed development, light-dependent regulation and substrate affinities to acyl-CoA esters.

Genome-wide identification and Phylogenic analysis of kelch motif containing ACBP in Brassica napus

Background

Acyl-coA binding proteins (ACBPs) bind long chain acyl-CoA esters with very high affinity. Their possible involvement in fatty acid transportation from the plastid to the endoplasmic reticulum, prior to the formation of triacylglycerol has been suggested. Four classes of ACBPs were identified in Arabidopsis thaliana: the small ACBPs, the large ACBPs, the ankyrin repeats containing ACBPs and the kelch motif containing ACBPs. They differed in structure and in size, and showed multiple important functions. In the present study, Brassica napus ACBPs were identified and characterized.

Results

Eight copies of kelch motif ACBPs were cloned, it showed that B. napus ACBPs shared high amino acid sequence identity with A. thaliana, Brassica rapa and Brassica oleracea. Furthermore, phylogeny based on domain structure and comparison map showed the relationship and the evolution of ACBPs within Brassicaceae family: ACBPs evolved into four separate classes with different structure. Chromosome locations comparison showed conserved syntenic blocks.

Conclusions

ACBPs were highly conserved in Brassicaceae. They evolved from a common ancestor, but domain duplication and rearrangement might separate them into four distinct classes, with different structure and functions. Otherwise, B. napus inherited kelch motif ACBPs from ancestor conserving chromosomal location, emphasizing preserved synteny block region. This study provided a first insight for exploring ACBPs in B. napus, which supplies a valuable tool for crop improvement in agriculture.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1735-6) contains supplementary material, which is available to authorized users.

Subcellular localization of rice acyl-CoA-binding proteins (ACBPs) indicates that OsACBP6::GFP is targeted to the peroxisomes

Acyl-CoA-binding proteins (ACBPs) show conservation at the acyl-CoA-binding (ACB) domain which facilitates binding to acyl-CoA esters. In Arabidopsis thaliana, six ACBPs participate in development and stress responses. Rice (Oryza sativa) also contains six genes encoding ACBPs. We investigated differences in subcellular localization between monocot rice and eudicot A. thaliana ACBPs. The subcellular localization of the six OsACBPs was achieved via transient expression of green fluorescence protein (GFP) fusions in tobacco (Nicotiana tabacum) epidermal cells, and stable transformation of A. thaliana. As plant ACBPs had not been reported in the peroxisomes, OsACBP6::GFP localization was confirmed by transient expression in rice sheath cells. The function of OsACBP6 was investigated by overexpressing 35S::OsACBP6 in the peroxisomal abc transporter1 (pxa1) mutant defective in peroxisomal fatty acid β-oxidation. As predicted, OsACBP1::GFP and OsACBP2::GFP were localized to the cytosol, and OsACBP4::GFP and OsACBP5::GFP to the endoplasmic reticulum (ER). However, OsACBP3::GFP displayed subcellular multi-localization while OsACBP6::GFP was localized to the peroxisomes. 35S::OsACBP6-OE/pxa1 lines showed recovery in indole-3-butyric acid (IBA) peroxisomal β-oxidation, wound-induced VEGETATIVE STORAGE PROTEIN1 (VSP1) expression and jasmonic acid (JA) accumulation. These findings indicate a role for OsACBP6 in peroxisomal β-oxidation, and suggest that rice ACBPs are involved in lipid degradation in addition to lipid biosynthesis.

Production of a Brassica napus Low-Molecular Mass Acyl-Coenzyme A-Binding Protein in Arabidopsis Alters the Acyl-Coenzyme A Pool and Acyl Composition of Oil in Seeds

Low-molecular mass (10 kD) cytosolic acyl-coenzyme A-binding protein (ACBP) has a substantial influence over fatty acid (FA) composition in oilseeds, possibly via an effect on the partitioning of acyl groups between elongation and desaturation pathways. Previously, we demonstrated that the expression of a Brassica napus ACBP (BnACBP) complementary DNA in the developing seeds of Arabidopsis (Arabidopsis thaliana) resulted in increased levels of polyunsaturated FAs at the expense of eicosenoic acid (20:1^cisΔ11) and saturated FAs in seed oil. In this study, we investigated whether alterations in the FA composition of seed oil at maturity were correlated with changes in the acyl-coenzyme A (CoA) pool in developing seeds of transgenic Arabidopsis expressing BnACBP. Our results indicated that both the acyl-CoA pool and seed oil of transgenic Arabidopsis lines expressing cytosolic BnACBP exhibited relative increases in linoleic acid (18:2^cisΔ9,12; 17.9%-44.4% and 7%-13.2%, respectively) and decreases in 20:1^cisΔ11 (38.7%-60.7% and 13.8%-16.3%, respectively). However, alterations in the FA composition of the acyl-CoA pool did not always correlate with those seen in the seed oil. In addition, we found that targeting of BnACBP to the endoplasmic reticulum resulted in FA compositional changes that were similar to those seen in lines expressing cytosolic BnACBP, with the most prominent exception being a relative reduction in α-linolenic acid (18:3^cisΔ9,12,15) in both the acyl-CoA pool and seed oil of the former (48.4%-48.9% and 5.3%-10.4%, respectively). Overall, these data support the role of ACBP in acyl trafficking in developing seeds and validate its use as a biotechnological tool for modifying the FA composition of seed oil.

An acyl-CoA-binding protein from grape that is induced through ER stress confers morphological changes and disease resistance in Arabidopsis

We here report characterization of a grape (Vitis vinifera) acyl-CoA-binding protein (VvACBP). Expression of VvACBP was detected in grape leaves exposed to tunicamycin-induced endoplasmic reticulum (ER) stress as well as cold and heat shock treatments. In tendrils and peduncles, however, high-temperature treatment induced BiP (luminal binding protein) expression, a marker of ER stress in berry skin, but not VvACBP expression. We hypothesize that VvACBP may be sorted to the periphery of plant cells. Transgenic Arabidopsis plants, expressing VvACBP, exhibited slowed-down floral transition. The gene expression of proteins related to the photoperiodic pathway, CONSTANS, FLOWERING LOCUS T (FT), and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), was down-regulated in transgenic seedlings. These results underscore the possibility that VvACBP may affect the regulation of floral transition in Arabidopsis by suppressing the photoperiodic pathway. The transgenic Arabidopsis plants also exhibited morphological changes such as thicker inflorescences and rosette leaves. In addition, the rosette leaves of the transgenic plants had higher anthocyanin, total phenol, and chlorophyll contents than those of the control plants. Finally, the transgenic plants showed disease resistance to Pseudomonas syringae and Colletotrichum higginsianum, suggesting that VvACBP may also enhance disease resistance in grapevine.

The rice acyl-CoA-binding protein gene family: phylogeny, expression and functional analysis

• Acyl-CoA-binding proteins (ACBPs) show conservation in an acyl-CoA-binding domain (ACB domain) which binds acyl-CoA esters. Previous studies on plant ACBPs focused on eudicots, Arabidopsis and Brassica. Here, we report on the phylogeny and characterization of the ACBP family from the monocot Oryza sativa (rice). • Phylogenetic analyses were conducted using 16 plant genomes. Expression profiles of rice ACBPs under normal growth, as well as biotic and abiotic stress conditions, were examined by quantitative real-time reverse-transcription polymerase chain reactions. In vitro acyl-CoA-binding assays were conducted using recombinant (His)₆-tagged ACBPs. • The ACBP family diversified as land plants evolved. Classes I and IV show lineage-specific gene expansion. Classes II and III are closely related phylogenetically. As in the eudicot Arabidopsis, six genes (designated OsACBP1 to OsACBP6) encode rice ACBPs, but their distribution into various classes differed from Arabidopsis. Rice ACBP mRNAs showed ubiquitous expression and OsACBP4, OsACBP5 and OsACBP6 were stress-responsive. All recombinant rice ACBPs bind [¹⁴C]linolenoyl-CoA besides having specific substrates. • Phylogeny, gene expression and biochemical analyses suggest that paralogues within and across classes are not redundant proteins. In addition to performing conserved basal functions, multidomain rice ACBPs appear to be associated with stress responses.

Involvement of low molecular mass soluble acyl-CoA-binding protein in seed oil biosynthesis

Acyl-CoA-binding protein (ACBP), a low molecular mass (m) (∼ 10 kDa) soluble protein ubiquitous in eukaryotes, plays an important housekeeping role in lipid metabolism by maintaining the intracellular acyl-CoA pool. ACBP is involved in lipid biosynthesis and transport, gene expression, and membrane biogenesis. In plants, low m ACBP and high m ACBPs participate in response mechanisms to biotic and abiotic factors, acyl-CoA transport in phloem, and biosynthesis of structural and storage lipids. In light of current research on the modification of seed oil, insight into mechanisms of substrate trafficking within lipid biosynthetic pathways is crucial for developing rational strategies for the production of specialty oils with the desired alterations in fatty acid composition. In this review, we summarize our knowledge of plant ACBPs with emphasis on the role of low m ACBP in seed oil biosynthesis, based on in vitro studies and analyses of transgenic plants. Future prospects and possible applications of low m ACBP in seed oil modification are discussed.

The Arabidopsis acbp1acbp2 double mutant lacking acyl-CoA-binding proteins ACBP1 and ACBP2 is embryo lethal

*In Arabidopsis thaliana, the amino acid sequences of membrane-associated acyl-CoA-binding proteins ACBP1 and ACBP2 are highly conserved. We have shown previously that, in developing seeds, ACBP1 accumulates in the cotyledonary cells of embryos and ACBP1 is proposed to be involved in lipid transfer. We show here by immunolocalization, using ACBP2-specific antibodies, that ACBP2 is also expressed in the embryos at various stages of seed development in Arabidopsis. *Phenotypic analyses of acbp1 and acbp2 single mutants revealed that knockout of either ACBP1 or ACBP2 alone did not affect their life cycle as both single mutants exhibited normal growth and development similar to the wild-type. However, the acbp1acbp2 double mutant was embryo lethal and was also defective in callus induction. *On lipid and acyl-CoA analyses, the siliques, but not the leaves, of the acbp1 mutant accumulated galactolipid monogalactosyldiacylglycerol and 18:0-CoA, but the levels of most polyunsaturated species of phospholipid, such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine, declined. *As recombinant ACBP1 and ACBP2 bind unsaturated phosphatidylcholine and acyl-CoA esters in vitro, we propose that ACBP1 and ACBP2 are essential in lipid transfer during early embryogenesis in Arabidopsis.

A 10-kDa acyl-CoA-binding protein (ACBP) from Brassica napus enhances acyl exchange between acyl-CoA and phosphatidylcholine

The gene encoding a 10-kDa acyl-CoA-binding protein (ACBP) from Brassica napus was over-expressed in developing seeds of Arabidopsis thaliana. Biochemical analysis of T(2) and T(3) A. thaliana seeds revealed a significant increase in polyunsaturated fatty acids (FAs) (18:2(cisDelta9,12) and 18:3(cisDelta9,12,15)) at the expense of very long monounsaturated FA (20:1(cisDelta11)) and saturated FAs. In vitro assays demonstrated that recombinant B. napus ACBP (rBnACBP) strongly increases the formation of phosphatidylcholine (PC) in the absence of added lysophosphatidylcholine in microsomes from DeltaYOR175c yeast expressing A. thaliana lysophosphatidylcholine acyltransferase (AthLPCAT) cDNA or in microsomes from microspore-derived cell suspension cultures of B. napus L. cv. Jet Neuf. rBnACBP or bovine serum albumin (BSA) were also shown to be crucial for AthLPCAT to catalyse the transfer of acyl group from PC into acyl-CoA in vitro. These data suggest that the cytosolic 10-kDa ACBP has an effect on the equilibrium between metabolically active acyl pools (acyl-CoA and phospholipid pools) involved in FA modifications and triacylglycerol bioassembly in plants. Over-expression of ACBP during seed development may represent a useful biotechnological approach for altering the FA composition of seed oil.

An Arabidopsis family of six acyl-CoA-binding proteins has three cytosolic members

In Arabidopsis thaliana, a gene family of six members encodes acyl-CoA-binding proteins (ACBPs). These Arabidopsis ACBPs (designated ACBP1 to ACBP6) range in size from 10.4kDa to 73.1kDa and display varying affinities for acyl-CoA esters, suggesting that they have different roles in plant lipid metabolism. In contrast, only the 10-kDa ACBPs have been well-characterized from other eukaryote species. Our previous studies have revealed that ACBP1 and ACBP2 are membrane-associated proteins, while ACBP3 is extracellularly-targeted. More recently, we have reported that the remaining three members in this protein family (namely ACBP4, ACBP5 and ACBP6) are subcellularly localized to the cytosol in Arabidopsis. The subcellular localizations of ACBP4, ACBP5 and ACBP6 in the cytosol were demonstrated using a number of different approaches incorporating biochemical fractionation, confocal microscopy of transgenic Arabidopsis expressing autofluorescence-tagged fusions and immunoelectron microscopy using ACBP-specific antibodies. Our results indicate that all three ACBPs in the cytosol are potential candidates for acyl-CoA binding and trafficking in plant cells. In this review, the functional redundancy and differences among the three cytosolic ACBPs are discussed by comparison of their light-regulated expression and substrate affinities to acyl-CoA esters, and from biochemical analyses on their knockout mutants and/or overexpression in transgenic Arabidopsis. The transcriptionally light-induced ACBP4 and ACBP5, which encode the two largest forms of Arabidopsis ACBPs, bind oleoyl-CoA esters and likely transfer oleoyl-CoAs from the plastids (the site of de novo fatty acid biosynthesis) to the endoplasmic reticulum for the biosynthesis of non-plastidial membrane lipids in Arabidopsis.

Proteomic analysis of Chelidonium majus milky sap using two-dimensional gel electrophoresis and tandem mass spectrometry

Milky sap, a milky-like orange fluid, isolated from the Greater Celandine (Chelidonium majus L.), family Papaveraceae, serves as a rich source of various biologically active substances such as alkaloids, several flavonoids, phenolic acids and proteins. The objective of this study was to separate Ch. majus milky sap extract proteins using two-dimensional gel electrophoresis (2-DE) to demonstrate for the first time the protein composition in the sap and to identify them using liquid chromatography-tandem mass spectrometry analysis (LC-ESI-MS/MS). It was possible to identify 21 proteins, which comprise disease/defence-related, signalling, Krebs cycle, nucleic acid binding and other proteins. The majority of the identified proteins can be linked to direct and indirect stress and defence reactions, e.g. against different pathogens. The specific protein composition of the milky sap suggests an important role of these proteins for the whole plant physiology and development.

A Novel Arabidopsis thaliana Protein is a Functional Peripheral-Type Benzodiazepine Receptor

A key element in the regulation of mammalian steroid biosynthesis is the 18 kDa peripheral-type benzodiazepine receptor (PBR), which mediates mitochondrial cholesterol import. PBR also possess an affinity to the tetrapyrrole metabolite protoporphyrin. The bacterial homolog to the mammalian PBR, the Rhodobacter TspO (CrtK) protein, was shown to be involved in the bacterial tetrapyrrole metabolism. Looking for a similar mitochondrial import mechanism in plants, protein sequences from Arabidopsis and several other plants were found with significant similarities to the mammalian PBR and to the Rhodobacter TspO protein. A PBR-homologous Arabidopsis sequence was cloned and expressed in E. coli. The recombinant gene product showed specific high affinity benzodiazepine ligand binding. Moreover, the protein applied to E. coli protoplasts caused an equal benzodiazepine-stimulated uptake of cholesterol and protoporphyrin IX. These results suggest that the PBR like protein is involved in steroid import and is directing protoporphyrinogen IX to the mitochondrial site of protoheme formation.

Proteomics of curcurbit phloem exudate reveals a network of defence proteins

Many different proteins can be separated from the sap of mature sieve tubes of different plant species. To date, only a limited number of those have been identified and functionally characterised. Due to sieve tubes inability of transcription and translation, the proteins are most probably synthesised in the intimately connected companion cells and transported into the sieve elements through plasmodesmata. The specific protein composition of phloem sap suggests an important role of these proteins not only for sieve tube maintenance, but also for whole plant physiology and development. Here we describe a comprehensive analysis of the phloem protein composition employing one- and high-resolution two-dimensional gel electrophoresis and partial sequencing by mass spectrometry. In this study more than 300 partial sequences generated by hybrid mass spectrometry were used to identify a total of 45 different proteins from the phloem exudates of cucumber (Cucumis sativus L. cv. Hoffmanns Giganta) and pumpkin (Cucurbita maxima Duch. cv. Gelber Zentner) plants. In addition to previously described phloem proteins, it was possible to localise proteins with high similarity to an acyl-CoA binding protein, a glyoxalase, a malate dehydrogenase, a rhodanese-like protein, a drought-induced protein, and a beta-glucosidase. The results indicate that the majority of the so far identified proteins are involved in stress and defence reactions.

ACBP4 and ACBP5, novel Arabidopsis acyl-CoA-binding proteins with kelch motifs that bind oleoyl-CoA

In plants, fatty acids synthesized in the chloroplasts are exported as acyl-CoA esters to the endoplasmic reticulum (ER). Cytosolic 10-kDa acyl-CoA-binding proteins (ACBPs), prevalent in eukaryotes, are involved in the storage and intracellular transport of acyl-CoAs. We have previously characterized Arabidopsis thaliana cDNAs encoding membrane-associated ACBPs with ankyrin repeats, designated ACBP1 and ACBP2, which show conservation to cytosolic ACBPs at the acyl-CoA-binding domain. Analysis of the Arabidopsis genome has revealed the presence of three more genes encoding putative proteins with acyl-CoA-binding domains, designated ACBP3, ACBP4 and ACBP5. Homologues of ACBP1 to ACBP5 have not been reported in any other organism. We show by reverse-transcriptase polymerase chain reaction (RT-PCR) analysis that ACBP3 , ACBP4 and ACBP5 are expressed in all plant organs, like ACBP1 and ACBP2 . ACBP4 and ACBP5 that share 81.4 identity and which contain kelch motifs were further investigated. To demonstrate their function in binding acyl-CoA, we have expressed them as (His)6-tagged recombinant proteins in Escherichia coli for in vitro binding assays. Both (His)6-ACBP4 and (His)6-ACBP5 bind [14C]oleoyl-CoA with high affinity, [14C]palmitoyl-CoA with lower affinity and did not bind [14C]arachidonyl-CoA. Eight mutant forms of each protein with single amino acid substitutions within the acyl-CoA-binding domain were produced and analyzed. On binding assays, all mutants were impaired in oleoyl-CoA binding. Hence, these novel ACBPs with kelch motifs have functional acyl-CoA-binding domains that bind oleoyl-CoA. Their predicted cytosol localization suggests that they could maintain an oleoyl-CoA pool in the cytosol or transport oleoyl-CoA from the plastids to the ER in plant lipid metabolism.

Properties of lysophosphatidylcholine acyltransferase from Brassica napus cultures

Acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT; EC 2.3.1.23) catalyzes the acyl-CoA-dependent acylation of lysophosphatidylcholine (LPC) to produce PC and CoA. LPCAT activity may affect the incorporation of fatty acyl moieties at the sn-2 position of PC where PUFA are formed and may indirectly influence seed TAG composition. LPCAT activity in microsomes prepared from microspore-derived cell suspension cultures of oilseed rape (Brassica napus L. cv Jet Neuf) was assayed using [1-14C]acyl-CoA as the fatty acyl donor. LPCAT activity was optimal at neutral pH and 35 degrees C, and was inhibited by 50% at a BSA concentration of 3 mg mL(-1). At acyl-CoA concentrations above 20 microM, LPCAT activity was more specific for oleoyl (18:1)-CoA than stearoyl (18:0)- and palmitoyl (16:0)-CoA. Lauroyl (12:0)-CoA, however, was not an effective acyl donor. LPC species containing 12:0, 16:0, 18:0, or 18:1 as the fatty acyl moiety all served as effective acyl acceptors for LPCAT, although 12:0-LPC was somewhat less effective as a substrate at lower concentrations. The failure of LPCAT to catalyze the incorporation of a 12:0 moiety from acyl-CoA into PC is consistent with the tendency of acyltransferases to discriminate against incorporation of this fatty acyl moiety at the sn-2 position of TAG from the seed oil of transgenic B. napus expressing a medium-chain thioesterase.

Overexpression of acyl-coA binding protein and its effects on the flux of free fatty acids in McA-RH 7777 cells

Overexpression of acyl-CoA binding protein (ACBP) was induced in a rat hepatoma cell line (McA-RH 7777) by stable integration of rat ACBP cDNA. The transfected cells (ACBP-27) had 3.5-fold higher concentrations of ACBP than control cells (14 vs. 4 ng/microg DNA). Both ACBP-27 and control cells were cultured in the presence of various concentrations of radiolabeled palmitic acid; and the effects of ACBP on lipogenesis and beta-oxidation were studied. Incubation of the cells with 100 microM palmitic acid resulted in 42% greater incorporation of the fatty acid in ACBP-27 cells as compared to that in the control cells. This increased incorporation of the fatty acid was observed predominantly in the triglyceride fraction. Higher concentrations of palmitic acid (200 to 400 microM) were associated with a significant decrease in the production of 14CO2 in the ACBP-27 cell line than in the control cells, while lower concentrations had no effect. Our data suggest a role for ACBP in the partitioning of fatty acids between esterification reactions leading to the formation of neutral lipids and beta-oxidation. ACBP may play a regulatory role by influencing this important branch point in intermediary lipid metabolism.

Isolation of a gene encoding Arabidopsis membrane-associated acyl-CoA binding protein and immunolocalization of its gene product

Until recently, only cytosolic acyl-CoA binding proteins (ACBPs) have been characterized. The isolation of an Arabidopsis thaliana cDNA encoding a novel membrane-associated ACBP that accumulates in developing seeds, designated ACBP1, has provided evidence for the existence of membrane-associated forms of ACBPs (Chye, 1998, Plant Mol. Biol. 38, 827-838). We now report on the isolation of its corresponding gene from an A. thaliana Columbia genomic library using the ACBP1 cDNA as a hybridization probe. Nucleotide sequence analysis of Arabidopsis ACBP1 showed that its promoter lacks a TATA box, resembling the promoters of rat, Drosophila and human genes encoding cytosolic ACBP and suggesting that it is a housekeeping gene. We show by Western blot analysis that ACBP1 expression in developing seeds coincides with lipid deposition and that homologues of membrane-associated ACBP1 exist in other plants. Using light microscopy, we show that ACBP1 is strongly expressed in the embryo at the cotyledons, hypocotyl, procambium of the axis and in most peripheral cells of the cotyledons and hypocotyl. Immunogold labelling localized ACBP1 to vesicles, to the plasma membrane especially at epidermal cells of heart, torpedo and cotyledonary stage embryos, and to the cell wall of the outer integument cells at the seed coat. Our results suggest that ACBP1 is involved in intermembrane lipid transport from the ER via vesicles to the plasma membrane where it could maintain a membrane-associated acyl pool; its immunolocalization to the cell wall of outer integument cells at the seed coat suggests a role in cuticle and cutin formation.

Structure and function of normal and transformed murine acyl-CoA binding proteins

Acyl-CoA binding protein (ACBP) is a ubiquitous cytosolic protein found in high levels in tumorigenic cells. However, the molecular basis for the elevated levels of ACBP in malignant cells, ligand binding characteristics, and function in microsomal phospholipid synthesis have not been resolved. To address whether tumorigenic ACBP differs from the native protein, ACBP was purified from LM cells, a tumorigenic subline of mouse L-929 fibroblasts, and its primary structure was examined by delayed-extraction MALDI-linear TOF mass spectrometry. Proteolytic digestion and peptide sequence analysis confirmed that ACBP from LM cells was identical to native mouse ACBP (based on cDNA-derived amino acid sequence) with no amino acid substitutions, deletions, or posttranslational modifications. A fluorescent binding assay revealed that mouse ACBP bound cis-parinaroyl-CoA with high affinity, Kd 7.6 +/- 2.3 nM, at a single binding site. Furthermore, mouse ACBP enhanced microsomal phosphatidic acid formation from oleoyl-CoA 2.3-fold. Mouse ACBP also inhibited microsomal phospholipid acyl chain remodeling of choline-containing phospholipids, phosphatidylcholine and sphingomyelin, by 50 and 64%, respectively. These effects were specific compared to those of native rat liver or recombinant rat ACBP. Mouse and rat ACBPs differed by three amino acid substitutions at positions 4, 68, and 78. Although these small differences in amino acid sequence did not alter binding affinity for cis-parinaroyl-CoA, rat liver ACBP stimulated utilization of oleoyl-CoA 3.8-fold by microsomal glycerol-3-phosphate acyltransferase, significantly higher than that observed with mouse ACBP, but did not alter microsomal phospholipid acyl chain remodeling from oleoyl-CoA. In addition, these ACBPs protected oleoyl-CoA against hydrolysis. Finally, both mouse and rat ACBP shifted the incorporation of oleoyl-CoA from microsomal phospholipid acyl chain remodeling to phosphatidic acid biosynthesis. These data for the first time show a role for ACBP in stimulating microsomal phosphatidic acid biosynthesis and acyl chain remodeling in vitro. While ACBP from tumorigenic cells did not differ from normal, ACBPs from different murine species displayed subtle differences in their effects on microsomal phospholipid metabolism in vitro.

Saccharomyces carlsbergensis contains two functional genes encoding the acyl-CoA binding protein, one similar to the ACB1 gene from S. cerevisiae and one identical to the ACB1 gene from S. monacensis

Saccharomyces carlsbergensis is an amphiploid, and it has previously been suggested that the genomes of S. carlsbergensis originate from S. cerevisiae and S. monacensis. We have cloned the ACB1 genes encoding the acyl-CoA binding protein (ACBP) from S. carlsbergensis, S. cerevisiae and S. monacensis. Two genes were found in S. carlsbergensis and named ACB1 type 1 and type 2, respectively. The type 1 gene is identical to the S. cerevisiae ACB1 gene except for three substitutions, one single base pair deletion and one double base pair insertion, all located in the promoter region. The type 2 gene is completely identical to the S. monacensis ACB1 gene. These findings substantiate the notion that S. carlsbergensis is a hybrid between S. cerevisiae and S. monacensis. Both ACB1 type 1 and type 2 are actively transcribed in S. carlsbergensis and transcription is initiated at sites identical to those used for transcriptional initiation of the ACB1 genes in S. cerevisiae and S. monacensis, respectively. Two polyadenylation sites, spaced 225 bp apart, are present in the S. cerevisiae ACB1 gene. The upstream polyadenylation site is used exclusively during exponential growth, whereas both sites are utilized during later stages of growth.

REGULATION OF FATTY ACID SYNTHESIS

All plant cells produce fatty acids from acetyl-CoA by a common pathway localized in plastids. Although the biochemistry of this pathway is now well understood, much less is known about how plants control the very different amounts and types of lipids produced in different tissues. Thus, a central challenge for plant lipid research is to provide a molecular understanding of how plants regulate the major differences in lipid metabolism found, for example, in mesophyll, epidermal, or developing seed cells. Acetyl-CoA carboxylase (ACCase) is one control point that regulates rates of fatty acid synthesis. However, the biochemical modulators that act on ACCase and the factors that in turn control these modulators are poorly understood. In addition, little is known about how the expression of genes involved in fatty acid synthesis is controlled. This review evaluates current knowledge of regulation of plant fatty metabolism and attempts to identify the major unanswered questions.

Role of long-chain fatty acyl-CoA esters in the regulation of metabolism and in cell signalling

The intracellular concentration of free unbound acyl-CoA esters is tightly controlled by feedback inhibition of the acyl-CoA synthetase and is buffered by specific acyl-CoA binding proteins. Excessive increases in the concentration are expected to be prevented by conversion into acylcarnitines or by hydrolysis by acyl-CoA hydrolases. Under normal physiological conditions the free cytosolic concentration of acyl-CoA esters will be in the low nanomolar range, and it is unlikely to exceed 200 nM under the most extreme conditions. The fact that acetyl-CoA carboxylase is active during fatty acid synthesis (Ki for acyl-CoA is 5 nM) indicates strongly that the free cytosolic acyl-CoA concentration is below 5 nM under these conditions. Only a limited number of the reported experiments on the effects of acyl-CoA on cellular functions and enzymes have been carried out at low physiological concentrations in the presence of the appropriate acyl-CoA-buffering binding proteins. Re-evaluation of many of the reported effects is therefore urgently required. However, the observations that the ryanodine-senstitive Ca2+-release channel is regulated by long-chain acyl-CoA esters in the presence of a molar excess of acyl-CoA binding protein and that acetyl-CoA carboxylase, the AMP kinase kinase and the Escherichia coli transcription factor FadR are affected by low nanomolar concentrations of acyl-CoA indicate that long-chain acyl-CoA esters can act as regulatory molecules in vivo. This view is further supported by the observation that fatty acids do not repress expression of acetyl-CoA carboxylase or Delta9-desaturase in yeast deficient in acyl-CoA synthetase.

The transcriptional and translational control of diazepam binding inhibitor expression in rat male germ-line cells

The diazepam binding inhibitor [DBI, also known as acyl-CoA-binding protein, (ACBP), or endozepine] is a 10-kD protein that has been suggested to be involved in the regulation of several biological processes such as acyl-CoA metabolism, steroidogenesis, insulin secretion, and gamma-aminobutyric acid type A (GABA(A))/benzodiazepine receptor modulation. DBI has been cloned from vertebrates, insects, plants, and yeasts. In mammals, DBI is expressed in almost all the tissues studied. Nevertheless, DBI expression is restricted to specific cell types. Here we have studied DBI gene expression in the germ-line cells of rat testis. The DBI gene was intensively transcribed in postmeiotic round spermatids from stages VI to VIII of the seminiferous epithelial cycle. A prominent, spermatid-specific upstream transcription initiation site was identified in addition to the multiple common transcriptional initiation sites found in the somatic tissues. However, no DBI protein was detected in round spermatids, suggesting that the DBI transcripts were translationally arrested. The DBI protein was detected in the late spermatogenic stages starting from elongating spermatids from step 18 (stage VI) onward. The DBI protein was also detected in mature spermatozoa and in ejaculated human sperms. The majority of DBI was located at the middle piece of the spermatozoons tail enriched with mitochondria. On the basis of this observation and the well-established role of DBI in acyl-CoA metabolism, we propose that DBI expression in spermatozoa reflects the usage of fatty acids as a primary energy source by spermatozoa. The biological function of DBI in spermatozoa could thus be related to the motility function of sperm.

Disruption of the gene encoding the acyl-CoA-binding protein (ACB1) perturbs acyl-CoA metabolism in Saccharomyces cerevisiae

The ACB1 gene encoding the acyl-CoA-binding protein (ACBP) was disrupted in Saccharomyces cerevisiae. The disruption did not affect the growth rate on glucose but reduced the growth rate on ethanol slightly. Although the growth rate of the acb1-disrupted cells was unaffected or only slightly affected, the acb1-disrupted strain was unable to compete with wild type cells when grown in mixed culture. The acyl-CoA level in the disrupted cells was increased from 1.5- to 2.5-fold during exponential growth. The increase in the acyl-CoA level was caused solely by an increase in de novo synthesized stearoyl-CoA. Experiments with purified yeast fatty acid synthetase show that it will synthesize long chain acyl-CoAs in the absence of acyl-CoA-binding protein. The addition of ACBP to the incubation medium resulted in a dramatic decrease in the chain length of the synthesized acyl-CoA esters. Despite the fact that the stearoyl-CoA concentration was increased 7-fold and the Delta9-desaturase mRNA level was increased 3-fold, the synthesis of oleic acid was unchanged in the acb1-disrupted strain. The results strongly indicate that ACBP in yeast is involved in the transport of newly synthesized acyl-CoA esters from the fatty acid synthetase to acyl-CoA-consuming processes.

Acyl-CoA binding proteins: multiplicity and function

The physiological role of long-chain fatty acyl-CoA is thought to be primarily in intermediary metabolism of fatty acids. However, recent data show that nM to microM levels of these lipophilic molecules are potent regulators of cell functions in vitro. Although long-chain fatty acyl-CoA are present at several hundred microM concentration in the cell, very little long-chain fatty acyl-CoA actually exists as free or unbound molecules, but rather is bound with high affinity to membrane lipids and/or proteins. Recently, there is growing awareness that cytosol contains nonenzymatic proteins also capable of binding long-chain fatty acyl-CoA with high affinity. Although the identity of the cytosolic long-chain fatty acyl-CoA binding protein(s) has been the subject of some controversy, there is growing evidence that several diverse nonenzymatic cytosolic proteins will bind long-chain fatty acyl-CoA. Not only does acyl-CoA binding protein specifically bind medium and long-chain fatty acyl-CoA (LCFA-CoA), but ubiquitous proteins with multiple ligand specificities such as the fatty acid binding proteins and sterol carrier protein-2 also bind LCFA-CoA with high affinity. The potential of these acyl-CoA binding proteins to influence the level of free LCFA-CoA and thereby the amount of LCFA-CoA bound to regulatory sites in proteins and enzymes is only now being examined in detail. The purpose of this article is to explore the identity, nature, function, and pathobiology of these fascinating newly discovered long-chain fatty acyl-CoA binding proteins. The relative contributions of these three different protein families to LCFA-CoA utilization and/or regulation of cellular activities are the focus of new directions in this field.

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.