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

Over-expression of Medicago Acyl-CoA-binding 2 genes enhance salt and drought tolerance in Arabidopsis

Acyl-CoA-binding proteins (ACBPs) are mainly involved in acyl-CoA ester binding and trafficking in eukaryotic cells, and they function in lipid metabolism, membrane biosynthesis, cellular signaling, stress response, disease resistance, and other biological activities in plants. However, the roles of ACBP family members in Medicago remain unclear. In this study, a total of eight ACBP genes were identified in the genome of Medicago truncatula and Medicago sativa, and they were clustered into four sub-families (Class I-IV). Many cis-acting elements related to abiotic response were identified in the promoter region of these ACBP genes, in particular light-responsive elements. These ACBP genes exhibited distinct expression pattern in various tissues, and the expression level of MtACBP1/MsACBP1 and MtACBP2/MsACBP2 gene pairs were significantly increased under NaCl treatment. Subcellular localization analysis showed that MtACBP1/MsACBP1 and MtACBP2/MsACBP2 were localized in the endoplasmic reticulum of tobacco epidermal cells. Arabidopsis seedlings over-expressing MtACBP2/MsACBP2 displayed increased root length than the wild type under short light, Cu2+, ABA, PEG, and NaCl treatments. Over-expression of MtACBP2/MsACBP2 also significantly enhanced Arabidopsis tolerance under NaCl and PEG treatments in mature plants. Collectively, our study identified salt and drought responsive ACBP genes in Medicago and verified their functions in increasing resistance against salt and drought stresses.

Acyl-CoA-binding protein (ACBP) genes involvement in response to abiotic stress and exogenous hormone application in barley (Hordeum vulgare L.)

BACKGROUND

Acyl-CoA-Binding proteins (ACBPs) function as coenzyme A transporters and play important roles in regulating plant growth and development in response to abiotic stress and phytohormones, as well as in membrane repair. To date, the ACBP family has not been a comprehensively characterized in barley (Hordeum vulgare L.).

RESULTS

Eight ACBP genes were identified in the barley genome and named as HvACBP1-8. The analysis of the proteins structure and promoter elements of HvACBP suggested its potential functions in plant growth, development, and stress response. These HvACBPs are expressed in specific tissues and organs following induction by abiotic stressors such as drought, salinity, UV-B exposure, temperature extremes, and exposure to exogenous phytohormones. The HvACBP7 and HvACBP8 amino acid sequences were conserved during the domestication of Tibetan Qingke barley.

CONCLUSIONS

Acyl-CoA-binding proteins may play important roles in barley growth and environmental adaptation. This study provides foundation for further analyses of the biological functions of HvACBPs in the barley stress response.

Acyl-CoA binding protein is required for lipid droplet degradation in the diatom Phaeodactylum tricornutum

Diatoms (Bacillariophyceae) accumulate neutral storage lipids in lipid droplets during stress conditions, which can be rapidly degraded and recycled when optimal conditions resume. Since nutrient and light availability fluctuate in marine environments, storage lipid turnover is essential for diatom dominance of marine ecosystems. Diatoms have garnered attention for their potential to provide a sustainable source of omega-3 fatty acids. Several independent proteomic studies of lipid droplets isolated from the model oleaginous pennate diatom Phaeodactylum tricornutum have identified a previously uncharacterized protein with an acyl-CoA binding (ACB) domain, Phatrdraft_48778, here referred to as Phaeodactylum tricornutum acyl-CoA binding protein (PtACBP). We report the phenotypic effects of CRISPR-Cas9 targeted genome editing of PtACBP. ptacbp mutants were defective in lipid droplet and triacylglycerol degradation, as well as lipid and eicosapentaenoic acid synthesis, during recovery from nitrogen starvation. Transcription of genes responsible for peroxisomal β-oxidation, triacylglycerol lipolysis, and eicosapentaenoic acid synthesis was inhibited. A lipid-binding assay using a synthetic ACB domain from PtACBP indicated preferential binding specificity toward certain polar lipids. PtACBP fused to eGFP displayed an endomembrane-like pattern, which surrounded the periphery of lipid droplets. PtACBP is likely responsible for intracellular acyl transport, affecting cell division, development, photosynthesis, and stress response. A deeper understanding of the molecular mechanisms governing storage lipid turnover will be crucial for developing diatoms and other microalgae as biotechnological cell factories.

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.

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.

Molecular characterization, expression and functional analysis of acyl-CoA-binding protein gene family in maize (Zea mays)

BACKGROUND

Acyl-CoA-binding proteins (ACBPs) possess a conserved acyl-CoA-binding (ACB) domain that facilitates binding to acyl-CoA esters and trafficking in eukaryotic cells. Although the various functions of ACBP have been characterized in several plant species, their structure, molecular evolution, expression profile, and function have not been fully elucidated in Zea mays L.

RESULTS

Genome-wide analysis identified nine ZmACBP genes in Z. mays, which could be divided into four distinct classes (class I, class II, class III, and class IV) via construction of a phylogenetic tree that included 48 ACBP genes from six different plant species. Transient expression of a ZmACBP-GFP fusion protein in tobacco (Nicotiana tabacum) epidermal cells revealed that ZmACBPs localized to multiple different locations. Analyses of expression profiles revealed that ZmACBPs exhibited temporal and spatial expression changes during abiotic and biotic stresses. Eight of the nine ZmACBP genes were also found to have significant association with agronomic traits in a panel of 500 maize inbred lines. The heterologous constitutive expression of ZmACBP1 and ZmACBP3 in Arabidopsis enhanced the resistance of these plants to salinity and drought stress, possibly through alterations in the level of lipid metabolic and stress-responsive genes.

CONCLUSION

The ACBP gene family was highly conserved across different plant species. ZmACBP genes had clear tissue and organ expression specificity and were responsive to both biotic and abiotic stresses, suggesting their roles in plant growth and stress resistance.

Characterization and function of a sunflower (Helianthus annuus L.) Class II acyl-CoA-binding protein

Acyl-CoA-binding proteins (ACBP) bind to long-chain acyl-CoA esters and phospholipids, enhancing the activity of different acyltransferases in animals and plants. Nevertheless, the role of these proteins in the synthesis of triacylglycerols (TAGs) remains unclear. Here, we cloned a cDNA encoding HaACBP1, a Class II ACBP from sunflower (Helianthus annuus), one of the world's most important oilseed crop plants. Transcriptome analysis of this gene revealed strong expression in developing seeds from 16 to 30 days after flowering. The recombinant protein (rHaACBP1) was expressed in Escherichia coli and purified to be studied by in vitro isothermal titration calorimetry and for phospholipid binding. Its high affinity for saturated palmitoyl-CoA (16:0-CoA; K_D 0.11 μM) and stearoyl-CoA (18:0-CoA; K_D 0.13 μM) esters suggests that rHaACBP1 could act in acyl-CoA transfer pathways that involve saturated acyl derivatives. Furthermore, rHaACBP1 also binds to both oleoyl-CoA (18:1-CoA; K_D 6.4 μM) and linoleoyl-CoA (18:2-CoA; K_D 21.4 μM) esters, the main acyl-CoA substrates used to synthesise the TAGs that accumulate in sunflower seeds. Interestingly, rHaACBP1 also appears to bind to different species of phosphatidylcholines (dioleoyl-PC and dilinoleoyl-PC), glycerolipids that are also involved in TAG synthesis, and while it interacts with dioleoyl-PA, this is less prominent than its binding to the PC derivative. Expression of rHaACBP in yeast alters its fatty acid composition, as well as the composition and size of the host acyl-CoA pool. These results suggest that HaACBP1 may potentially fulfil a role in the transport and trafficking of acyl-CoAs during sunflower seed development.

Analysis of Spatio-Temporal Transcriptome Profiles of Soybean (Glycine max) Tissues during Early Seed Development

Soybean (Glycine max) is an important crop providing oil and protein for both human and animal consumption. Knowing which biological processes take place in specific tissues in a temporal manner will enable directed breeding or synthetic approaches to improve seed quantity and quality. We analyzed a genome-wide transcriptome dataset from embryo, endosperm, endothelium, epidermis, hilum, outer and inner integument and suspensor at the global, heart and cotyledon stages of soybean seed development. The tissue specificity of gene expression was greater than stage specificity, and only three genes were differentially expressed in all seed tissues. Tissues had both unique and shared enriched functional categories of tissue-specifically expressed genes associated with them. Strong spatio-temporal correlation in gene expression was identified using weighted gene co-expression network analysis, with the most co-expression occurring in one seed tissue. Transcription factors with distinct spatiotemporal gene expression programs in each seed tissue were identified as candidate regulators of expression within those tissues. Gene ontology (GO) enrichment of orthogroup clusters revealed the conserved functions and unique roles of orthogroups with similar and contrasting expression patterns in transcript abundance between soybean and Arabidopsis during embryo proper and endosperm development. Key regulators in each seed tissue and hub genes connecting those networks were characterized by constructing gene regulatory networks. Our findings provide an important resource for describing the structure and function of individual soybean seed compartments during early seed development.

Subcellular Localization of Rice Acyl-CoA-Binding Proteins ACBP4 and ACBP5 Supports Their Non-redundant Roles in Lipid Metabolism

Acyl-CoA-binding proteins (ACBPs), conserved at the acyl-CoA-binding domain, can bind acyl-CoA esters as well as transport them intracellularly. Six ACBPs co-exist in each model plant, dicot Arabidopsis thaliana (thale cress) and monocot Oryza sativa (rice). Although Arabidopsis ACBPs have been studied extensively, less is known about the rice ACBPs. OsACBP4 is highly induced by salt treatment, but down-regulated following pathogen infection, while OsACBP5 is up-regulated by both wounding and pathogen treatment. Their differential expression patterns under various stress treatments suggest that they may possess non-redundant functions. When expressed from the CaMV35S promoter, OsACBP4 and OsACBP5 were subcellularly localized to different endoplasmic reticulum (ER) domains in transgenic Arabidopsis. As these plants were not stress-treated, it remains to be determined if OsACBP subcellular localization would change following treatment. Given that the subcellular localization of proteins may not be reliable if not expressed in the native plant, this study addresses OsACBP4:GFP and OsACBP5:DsRED expression from their native promoters to verify their subcellular localization in transgenic rice. The results indicated that OsACBP4:GFP was targeted to the plasma membrane besides the ER, while OsACBP5:DsRED was localized at the apoplast, in contrast to their only localization at the ER in transgenic Arabidopsis. Differences in tagged-protein localization in transgenic Arabidopsis and rice imply that protein subcellular localization studies are best investigated in the native plant. Likely, initial targeting to the ER in a non-native plant could not be followed up properly to the final destination(s) unless it occurred in the native plant. Also, monocot (rice) protein targeting may not be optimally processed in a transgenic dicot (Arabidopsis), perhaps arising from the different processing systems for routing between them. Furthermore, changes in the subcellular localization of OsACBP4:GFP and OsACBP5:DsRED were not detectable following salt and pathogen treatment, respectively. These results suggest that OsACBP4 is likely involved in the intracellular shuttling of acyl-CoA esters and/or other lipids between the plasma membrane and the ER, while OsACBP5 appears to participate in the extracellular transport of acyl-CoA esters and/or other lipids, suggesting that they are non-redundant proteins in lipid trafficking.

The overexpression of rice ACYL-CoA-BINDING PROTEIN2 increases grain size and bran oil content in transgenic rice

As Oryza sativa (rice) seeds represent food for over three billion people worldwide, the identification of genes that enhance grain size and composition is much desired. Past reports have indicated that Arabidopsis thaliana acyl-CoA-binding proteins (ACBPs) are important in seed development but did not affect seed size. Herein, rice OsACBP2 was demonstrated not only to play a role in seed development and germination, but also to influence grain size. OsACBP2 mRNA accumulated in embryos and endosperm of germinating seeds in qRT-PCR analysis, while β-glucuronidase (GUS) assays on OsACBP2pro::GUS rice transformants showed GUS expression in embryos, as well as the scutellum and aleurone layer of germinating seeds. Deletion analysis of the OsACBP2 5'-flanking region revealed five copies of the seed cis-element, Skn-I-like motif (-1486/-1482, -956/-952, -939/-935, -826/-822, and -766/-762), and the removal of any adversely affected expression in seeds, thereby providing a molecular basis for OsACBP2 expression in seeds. When OsACBP2 function was investigated using osacbp2 mutants and transgenic rice overexpressing OsACBP2 (OsACBP2-OE), osacbp2 was retarded in germination, while OsACBP2-OEs performed better than the wild-type and vector-transformed controls, in germination, seedling growth, grain size and grain weight. Transmission electron microscopy of OsACBP2-OE mature seeds revealed an accumulation of oil bodies in the scutellum cells, while confocal laser scanning microscopy indicated oil accumulation in OsACBP2-OE aleurone tissues. Correspondingly, OsACBP2-OE seeds showed gain in triacylglycerols and long-chain fatty acids over the vector-transformed control. As dietary rice bran contains beneficial bioactive components, OsACBP2 appears to be a promising candidate for enriching seed nutritional value.

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.

Arabidopsis ACYL-COA-BINDING PROTEIN1 interacts with STEROL C4-METHYL OXIDASE1-2 to modulate gene expression of homeodomain-leucine zipper IV transcription factors

Fatty acids (FAs) and sterols constitute building blocks of eukaryotic membranes and lipid signals. Co-regulation of FA and sterol synthesis is mediated by sterol regulatory element-binding proteins in animals but remains elusive in plants. We reported recently that Arabidopsis ACYL-COA-BINDING PROTEIN1 (ACBP1) modulates sterol synthesis via protein-protein interaction with STEROL C4-METHYL OXIDASE1-1 (SMO1-1). Herein, ACBP1 was demonstrated to co-express and interact with SMO1-2 by yeast two-hybrid, co-localization, pull-down, co-immunoprecipitation and β-glucuronidase assays. SMO1-2 silenced in acbp1 was used in phenotyping, GC-MS and expression profiling. ACBP1 co-expressed with SMO1-2 in embryo sacs, pollen and trichomes, corroborating with cooperative tissue-specific functions unseen with SMO1-1. SMO1-2 silencing in acbp1 impaired seed development, male and female gamete transmission, and pollen function. Genes encoding homeodomain-leucine zipper IV transcription factors (HDG5, HDG10, HDG11 and GLABRA2), which potentially bind phospholipids/sterols, were transcribed aberrantly. GLABRA2 targets (MYB23, MUM4 and PLDα1) were misregulated, causing glabra2-resembling trichome, seed coat mucilage and oil-accumulating phenotypes. Together with altered sterol and FA compositions upon ACBP1 mutation and/or SMO1-2 silencing, ACBP1-SMO1 interaction appears to mediate homeostatic co-regulation of FAs and sterols, which serve as lipid modulators for gene expression of homeodomain-leucine zipper IV transcription factors.

Improved fruit α-tocopherol, carotenoid, squalene and phytosterol contents through manipulation of Brassica juncea 3-HYDROXY-3-METHYLGLUTARYL-COA SYNTHASE1 in transgenic tomato

3-Hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS) in the mevalonate (MVA) pathway generates isoprenoids including phytosterols. Dietary phytosterols are important because they can lower blood cholesterol levels. Previously, the overexpression of Brassica juncea wild-type (wt) and mutant (S359A) BjHMGS1 in Arabidopsis up-regulated several genes in sterol biosynthesis and increased sterol content. Recombinant S359A had earlier displayed a 10-fold higher in vitro enzyme activity. Furthermore, tobacco HMGS overexpressors (OEs) exhibited improved sterol content, plant growth and seed yield. Increased growth and seed yield in tobacco OE-S359A over OE-wtBjHMGS1 coincided with elevations in NtSQS expression and sterol content. Herein, the overexpression of wt and mutant (S359A) BjHMGS1 in a crop plant, tomato (Solanum lycopersicum), caused an accumulation of MVA-derived squalene and phytosterols, as well as methylerythritol phosphate (MEP)-derived α-tocopherol (vitamin E) and carotenoids, which are important to human health as antioxidants. In tomato HMGS-OE seedlings, genes associated with the biosyntheses of C10, C15 and C20 universal precursors of isoprenoids, phytosterols, brassinosteroids, dolichols, methylerythritol phosphate, carotenoid and vitamin E were up-regulated. In OE-S359A tomato fruits, increased squalene and phytosterol contents over OE-wtBjHMGS1 were attributed to heightened SlHMGR2, SlFPS1, SlSQS and SlCYP710A11 expression. In both tomato OE-wtBjHMGS1 and OE-S359A fruits, the up-regulation of SlGPS and SlGGPPS1 in the MEP pathway that led to α-tocopherol and carotenoid accumulation indicated cross-talk between the MVA and MEP pathways. Taken together, the manipulation of BjHMGS1 represents a promising strategy to simultaneously elevate health-promoting squalene, phytosterols, α-tocopherol and carotenoids in tomato, an edible fruit.

Arabidopsis thaliana Acyl-CoA-binding protein ACBP6 interacts with plasmodesmata-located protein PDLP8

In Arabidopsis thaliana, six acyl-CoA-binding proteins (ACBPs), designated as AtACBP1 to AtACBP6, have been identified to function in various events related to plant stress and development. The 10-kDa AtACBP6 is the smallest in this protein family, and recombinant AtACBP6 interacts with lipids in vitro by binding to acyl-CoA esters and phosphatidylcholine. Using anti-AtACBP6 antibodies in immunoelectron microscopy, we have localized AtACBP6 in the Arabidopsis phloem. The detection of immunogold grains in the plasmodesmata suggested that AtACBP6 could move from the companion cells to the sieve elements via the plasmodesmata. As AtACBP6 has been identified in a membrane-based interactome analysis to be a potential protein partner of Plasmodesmata-Localized Protein, PDLP8, AtACBP6-PDLP8 interaction was investigated herein utilizing isothermal titration calorimetry, as well as pull-down and bimolecular fluorescence complementation assays (BiFC). Notably, BiFC data revealed that AtACBP6-PDLP8 interaction occurred at the plasma membrane, which was unexpected as AtACBP6 has been previously identified in the cytosol. AtACBP6 expression was generally higher than PDLP8 in β-glucuronidase (GUS) assays on transgenic Arabidopsis transformed with AtACBP6 or PDLP8 promoter-driven GUS, consistent with qRT-PCR and microarray results. Furthermore, western blot analysis using anti-AtACBP6 antibodies showed a reduction in AtACBP6 expression in the pdlp8 T-DNA insertional mutant, suggesting that PDLP8 may possibly influence AtACBP6 accumulation in the sieve elements, probably in the plasmodesmata, where PDLP8 is confined and to where AtACBP6 has been immunodetected.

The first plant acyl-CoA-binding protein structures: the close homologues OsACBP1 and OsACBP2 from rice

Acyl-CoA-binding proteins (ACBPs) are a family of proteins that facilitate the binding of long-chain acyl-CoA esters at a conserved acyl-CoA-binding domain. ACBPs act to form intracellular acyl-CoA pools, transport acyl-CoA esters and regulate lipid metabolism. In the model plant Arabidopsis thaliana, a family of six ACBPs has been demonstrated to function in stress and development. Six ACBPs (OsACBPs) have also been identified in Oryza sativa (rice), but they are not as well characterized as those in Arabidopsis thaliana. To understand the need in rice for the two 10 kDa ACBPs, namely OsACBP1 and OsACBP2, which share 79% sequence identity, their crystal structures were elucidated and their affinities toward acyl-CoA esters were compared using isothermal titration calorimetry. OsACBP2 was found to display a higher binding affinity for unsaturated acyl-CoA esters than OsACBP1. A difference between the two proteins is observed at helix 3 and is predicted to lead to different ligand-binding modes in terms of the shape of the binding pocket and the residues that are involved. OsACBP1 thus resembles bovine ACBP, while OsACBP2 is similar to human liver ACBP, in both structure and binding affinity. This is the first time that ACBP structures have been reported from plants, and suggests that OsACBP1 and OsACBP2 are not redundant in function despite their high sequence identity and general structural similarity.

Kelch-motif containing acyl-CoA binding proteins AtACBP4 and AtACBP5 are differentially expressed and function in floral lipid metabolism

We herein demonstrated two of the Arabidopsis acyl-CoA-binding proteins (ACBPs), AtACBP4 and AtACBP5, both function in floral lipid metabolism and they may possibly play complementary roles in Arabidopsis microspore-to-pollen development. Histological analysis on transgenic Arabidopsis expressing β-glucuronidase driven from the AtACBP4 and AtACBP5 promoters, as well as, qRTPCR analysis revealed that AtACBP4 was expressed at stages 11-14 in the mature pollen, while AtACBP5 was expressed at stages 7-10 in the microspores and tapetal cells. Immunoelectron microscopy using AtACBP4- or AtACBP5-specific antibodies further showed that AtACBP4 and AtACBP5 were localized in the cytoplasm. Chemical analysis of bud wax and cutin using gas chromatographyflame ionization detector and GC-mass spectrometry analyses revealed the accumulation of cuticular waxes and cutin monomers in acbp4, acbp5 and acbp4acbp5 buds in comparison to the wild type (Col-0). Fatty acid profiling demonstrated a decline in stearic acid and an increase in linolenic acid in acbp4 and acbp4acbp5 buds, respectively, over Col-0. Analysis of inflorescences from acbp4 and acbp5 revealed that there was an increase of AtACBP5 expression in acbp4, and an increase of AtACBP4 expression in acbp5. Deletion analysis of the AtACBP4 and AtACBP5 5'-flanking regions indicated the minimal promoter activity for AtACBP4 (-145/+103) and AtACBP5 (-181/+81). Electrophoretic mobility shift assays identified a pollen-specific cis-acting element POLLEN1 (AGAAA) mapped at AtACBP4 (-157/-153) which interacted with nuclear proteins from flower and this was substantiated by DNase I footprinting. In Arabidopsis thaliana, six acyl-CoA-binding proteins (ACBPs), designated as AtACBP1 to AtACBP6, have been identified to function in plant stress and development. AtACBP4 and AtACBP5 represent the two largest proteins in the AtACBP family. Despite having kelch-motifs and sharing a common cytosolic subcellular localization, AtACBP4 and AtACBP5 differ in spatial and temporal expression. Histological analysis on transgenic Arabidopsis expressing β-glucuronidase driven from the respective AtACBP4 and AtACBP5 promoters, as well as, qRT-PCR analysis revealed that AtACBP4 was expressed at stages 11-14 in mature pollen, while AtACBP5 was expressed at stages 7-10 in the microspores and tapetal cells. Immunoelectron microscopy using AtACBP4- or AtACBP5-specific antibodies further showed that AtACBP4 and AtACBP5 were localized in the cytoplasm. Chemical analysis of bud wax and cutin using gas chromatography-flame ionization detector and GC-mass spectrometry analyses revealed the accumulation of cuticular waxes and cutin monomers in acbp4, acbp5 and acbp4acbp5 buds, in comparison to the wild type. Analysis of inflorescences from acbp4 and acbp5 revealed that there was an increase of AtACBP5 expression in acbp4, and an increase of AtACBP4 expression in acbp5. Deletion analysis of the AtACBP4 and AtACBP5 5'-flanking regions indicated the minimal promoter region for AtACBP4 (-145/+103) and AtACBP5 (-181/+81). Electrophoretic mobility shift assays identified a pollen-specific cis-acting element POLLEN1 (AGAAA) within AtACBP4 (-157/-153) which interacted with nuclear proteins from flower and this was substantiated by DNase I footprinting. These results suggest that AtACBP4 and AtACBP5 both function in floral lipidic metabolism and they may play complementary roles in Arabidopsis microspore-to-pollen development.

Arabidopsis acyl-CoA-binding protein ACBP6 localizes in the phloem and affects jasmonate composition

Arabidopsis thaliana ACYL-COA-BINDING PROTEIN6 (AtACBP6) encodes a cytosolic 10-kDa AtACBP. It confers freezing tolerance in transgenic Arabidopsis, possibly by its interaction with lipids as indicated by the binding of acyl-CoA esters and phosphatidylcholine to recombinant AtACBP6. Herein, transgenic Arabidopsis transformed with an AtACBP6 promoter-driven β-glucuronidase (GUS) construct exhibited strong GUS activity in the vascular tissues. Immunoelectron microscopy using anti-AtACBP6 antibodies showed AtACBP6 localization in the phloem especially in the companion cells and sieve elements. Also, the presence of gold grains in the plasmodesmata indicated its potential role in systemic trafficking. The AtACBP6 protein, but not its mRNA, was found in phloem exudate of wild-type Arabidopsis. Fatty acid profiling using gas chromatography-mass spectrometry revealed an increase in the jasmonic acid (JA) precursor, 12-oxo-cis,cis-10,15-phytodienoic acid (cis-OPDA), and a reduction in JA and/or its derivatives in acbp6 phloem exudates in comparison to the wild type. Quantitative real-time PCR showed down-regulation of COMATOSE (CTS) in acbp6 rosettes suggesting that AtACBP6 affects CTS function. AtACBP6 appeared to affect the content of JA and/or its derivatives in the sieve tubes, which is consistent with its role in pathogen-defense and in its wound-inducibility of AtACBP6pro::GUS. Taken together, our results suggest the involvement of AtACBP6 in JA-biosynthesis in Arabidopsis phloem tissues.

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.

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.

Computational Prediction of acyl-coA Binding Proteins Structure in Brassica napus

Acyl-coA binding proteins could transport acyl-coA esters from plastid to endoplasmic reticulum, prior to fatty acid biosynthesis, leading to the formation of triacylglycerol. The structure and the subcellular localization of acyl-coA binding proteins (ACBP) in Brassica napus were computationally predicted in this study. Earlier, the structure analysis of ACBPs was limited to the small ACBPs, the current study focused on all four classes of ACBPs. Physicochemical parameters including the size and the length, the intron-exon structure, the isoelectric point, the hydrophobicity, and the amino acid composition were studied. Furthermore, identification of conserved residues and conserved domains were carried out. Secondary structure and tertiary structure of ACBPs were also studied. Finally, subcellular localization of ACBPs was predicted. The findings indicated that the physicochemical parameters and subcellular localizations of ACBPs in Brassica napus were identical to Arabidopsis thaliana. Conserved domain analysis indicated that ACBPs contain two or three kelch domains that belong to different families. Identical residues in acyl-coA binding domains corresponded to eight amino acid residues in all ACBPs of B. napus. However, conserved residues of common ACBPs in all species of animal, plant, bacteria and fungi were only inclusive in small ACBPs. Alpha-helixes were displayed and conserved in all the acyl-coA binding domains, representing almost the half of the protein structure. The findings confirm high similarities in ACBPs between A. thaliana and B. napus, they might share the same functions but loss or gain might be possible.

The Arabidopsis cytosolic Acyl-CoA-binding proteins play combinatory roles in pollen development

In Arabidopsis, six acyl-CoA-binding proteins (ACBPs) have been identified and they have been demonstrated to function in plant stress responses and development. Three of these AtACBPs (AtACBP4-AtACBP6) are cytosolic proteins and all are expressed in floral organs as well as in other tissues. The roles of cytosolic AtACBPs in floral development were addressed in this study. To this end, a T-DNA insertional knockout mutant of acbp5 was characterized before use in crosses with the already available acbp4 and acbp6 T-DNA knockout mutants to examine their independent and combinatory functions in floral development. The single-gene knockout mutations did not cause any significant phenotypic changes, while phenotypic deficiencies affecting siliques and pollen were observed in the double mutants (acbp4acbp6 and acbp5acbp6) and the acbp4acbp5acbp6 triple mutant. Vacuole accumulation in the acbp4acbp6, acbp5acbp6 and acbp4acbp5acbp6 pollen was the most severe abnormality occurring in the double and triple mutants. Furthermore, scanning electron microscopy and transmission electron microscopy revealed exine and oil body defects in the acbp4acbp5acbp6 mutant, which also displayed reduced ability in in vitro pollen germination. Transgenic Arabidopsis expressing β-glucuronidase (GUS) driven from the various AtACBP promoters indicated that AtACBP6pro::GUS expression overlapped with AtACBP4pro::GUS expression in pollen grains and with AtACBP5pro::GUS expression in the microspores and tapetal cells. Taken together, these results suggest that the three cytosolic AtACBPs play combinatory roles in acyl-lipid metabolism during pollen development.

Oxygen sensing and signaling

Oxygen is an indispensable substrate for many biochemical reactions in plants, including energy metabolism (respiration). Despite its importance, plants lack an active transport mechanism to distribute oxygen to all cells. Therefore, steep oxygen gradients occur within most plant tissues, which can be exacerbated by environmental perturbations that further reduce oxygen availability. Plants possess various responses to cope with spatial and temporal variations in oxygen availability, many of which involve metabolic adaptations to deal with energy crises induced by low oxygen. Responses are induced gradually when oxygen concentrations decrease and are rapidly reversed upon reoxygenation. A direct effect of the oxygen level can be observed in the stability, and thus activity, of various transcription factors that control the expression of hypoxia-induced genes. Additional signaling pathways are activated by the impact of oxygen deficiency on mitochondrial and chloroplast functioning. Here, we describe the molecular components of the oxygen-sensing pathway.

Expression of Arabidopsis acyl-CoA-binding proteins AtACBP1 and AtACBP4 confers Pb(II) accumulation in Brassica juncea roots

In Arabidopsis thaliana, the expression of two genes encoding acyl-CoA-binding proteins (ACBPs) AtACBP1 and AtACBP4, were observed to be induced by lead [Pb(II)] in shoots and roots in qRT-PCR analyses. Quantitative GUS (β-glucuronidase) activity assays confirmed induction of AtACBP1pro::GUS by Pb(II). Electrophoretic mobility shift assays (EMSAs) revealed that Pas elements in the 5'-flanking region of AtACBP1 were responsive to Pb(II) treatment. AtACBP1 and AtACBP4 were further compared in Pb(II) uptake using Brassica juncea, a potential candidate for phytoremediation given its rapid growth, large roots, high biomass and good capacity to accumulate heavy metals. Results from atomic absorption analyses on transgenic B. juncea expressing AtACBP1 or AtACBP4 indicated Pb(II) accumulation in roots. Subsequent Pb(II)-tracing assays demonstrated Pb(II) accumulation in the cytosol of root tips and vascular tissues of transgenic B. juncea AtACBP1-overexpressors (OXs) and AtACBP4-OXs and transgenic Arabidopsis AtACBP1-OXs. Transgenic Arabidopsis AtACBP1-OXs sequestered Pb(II) in the trichomes and displayed tolerance to hydrogen peroxide (H2 O2 ) treatment. In addition, AtACBP1 and AtACBP4 were H2 O2 -induced in the roots of wild-type Arabidopsis, while lipid hydroperoxide (LOOH) measurements of B. juncea AtACBP1-OX and AtACBP4-OX roots suggested that AtACBP1 and AtACBP4 can protect lipids against Pb(II)-induced lipid peroxidation.

Arabidopsis cytosolic acyl-CoA-binding proteins ACBP4, ACBP5 and ACBP6 have overlapping but distinct roles in seed development

Eukaryotic cytosolic ACBPs (acyl-CoA-binding proteins) bind acyl-CoA esters and maintain a cytosolic acyl-CoA pool, but the thermodynamics of their protein–lipid interactions and physiological relevance in plants are not well understood. Arabidopsis has three cytosolic ACBPs which have been identified as AtACBP4, AtACBP5 and AtACBP6, and microarray data indicated that all of them are expressed in seeds; AtACBP4 is expressed in early embryogenesis, whereas AtACBP5 is expressed later. ITC (isothermal titration calorimetry) in combination with transgenic Arabidopsis lines were used to investigate the roles of these three ACBPs from Arabidopsis thaliana. The dissociation constants, stoichiometry and enthalpy change of AtACBP interactions with various acyl-CoA esters were determined using ITC. Strong binding of recombinant (r) AtACBP6 with long-chain acyl-CoA (C16- to C18-CoA) esters was observed with dissociation constants in the nanomolar range. However, the affinity of rAtACBP4 and rAtACBP5 to these acyl-CoA esters was much weaker (dissociation constants in the micromolar range), suggesting that they interact with acyl-CoA esters differently from rAtACBP6. When transgenic Arabidopsis expressing AtACBP6pro::GUS was generated, strong GUS (β-glucuronidase) expression in cotyledonary-staged embryos and seedlings prompted us to measure the acyl-CoA contents of the acbp6 mutant. This mutant accumulated higher levels of C18:1-CoA and C18:1- and C18:2-CoAs in cotyledonary-staged embryos and seedlings, respectively, in comparison with the wild type. The acbp4acbp5acbp6 mutant showed the lightest seed weight and highest sensitivity to abscisic acid during germination, suggesting their physiological functions in seeds.

The binding affinities of the three Arabidopsis cytosolic ACBPs (AtACBP4, AtACBP5 and AtACBP6) with acyl-CoA esters were investigated by ITC. When the biological significance of these AtACBPs was analysed using mutants, results indicated their overlapping functions in seed acyl-lipid metabolism.

Arabidopsis membrane-associated acyl-CoA-binding protein ACBP1 is involved in stem cuticle formation

The membrane-anchored Arabidopsis thaliana ACYL-COA-BINDING PROTEIN1 (AtACBP1) plays important roles in embryogenesis and abiotic stress responses, and interacts with long-chain (LC) acyl-CoA esters. Here, AtACBP1 function in stem cuticle formation was investigated. Transgenic Arabidopsis transformed with an AtACBP1pro::GUS construct revealed β-glucuronidase (GUS) expression on the stem (but not leaf) surface, suggesting a specific role in stem cuticle formation. Isothermal titration calorimetry results revealed that (His)6-tagged recombinant AtACBP1 interacts with LC acyl-CoA esters (18:1-, 18:2-, and 18:3-CoAs) and very-long-chain (VLC) acyl-CoA esters (24:0-, 25:0-, and 26:0-CoAs). VLC fatty acids have been previously demonstrated to act as precursors in wax biosynthesis. Gas chromatography (GC)-flame ionization detector (FID) and GC-mass spectrometry (MS) analyses revealed that an acbp1 mutant showed a reduction in stem and leaf cuticular wax and stem cutin monomer composition in comparison with the wild type (Col-0). Consequently, the acbp1 mutant showed fewer wax crystals on the stem surface in scanning electron microscopy and an irregular stem cuticle layer in transmission electron microscopy in comparison with the wild type. Also, the mutant stems consistently showed a decline in expression of cuticular wax and cutin biosynthetic genes in comparison with the wild type, and the mutant leaves were more susceptible to infection by the necrotrophic pathogen Botrytis cinerea. Taken together, these findings suggest that AtACBP1 participates in Arabidopsis stem cuticle formation by trafficking VLC acyl-CoAs.

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.

Transgenic tobacco overexpressing Brassica juncea HMG-CoA synthase 1 shows increased plant growth, pod size and seed yield

Seeds are very important not only in the life cycle of the plant but they represent food sources for man and animals. We report herein a mutant of 3-hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS), the second enzyme in the mevalonate (MVA) pathway that can improve seed yield when overexpressed in a phylogenetically distant species. In Brassica juncea, the characterisation of four isogenes encoding HMGS has been previously reported. Enzyme kinetics on recombinant wild-type (wt) and mutant BjHMGS1 had revealed that S359A displayed a 10-fold higher enzyme activity. The overexpression of wt and mutant (S359A) BjHMGS1 in Arabidopsis had up-regulated several genes in sterol biosynthesis, increasing sterol content. To quickly assess the effects of BjHMGS1 overexpression in a phylogenetically more distant species beyond the Brassicaceae, wt and mutant (S359A) BjHMGS1 were expressed in tobacco (Nicotiana tabacum L. cv. Xanthi) of the family Solanaceae. New observations on tobacco OEs not previously reported for Arabidopsis OEs included: (i) phenotypic changes in enhanced plant growth, pod size and seed yield (more significant in OE-S359A than OE-wtBjHMGS1) in comparison to vector-transformed tobacco, (ii) higher NtSQS expression and sterol content in OE-S359A than OE-wtBjHMGS1 corresponding to greater increase in growth and seed yield, and (iii) induction of NtIPPI2 and NtGGPPS2 and downregulation of NtIPPI1, NtGGPPS1, NtGGPPS3 and NtGGPPS4. Resembling Arabidopsis HMGS-OEs, tobacco HMGS-OEs displayed an enhanced expression of NtHMGR1, NtSMT1-2, NtSMT2-1, NtSMT2-2 and NtCYP85A1. Overall, increased growth, pod size and seed yield in tobacco HMGS-OEs were attributed to the up-regulation of native NtHMGR1, NtIPPI2, NtSQS, NtSMT1-2, NtSMT2-1, NtSMT2-2 and NtCYP85A1. Hence, S359A has potential in agriculture not only in improving phytosterol content but also seed yield, which may be desirable in food crops. This work further demonstrates HMGS function in plant reproduction that is reminiscent to reduced fertility of hmgs RNAi lines in let-7 mutants of Caenorhabditis elegans.

Rice acyl-CoA-binding proteins OsACBP4 and OsACBP5 are differentially localized in the endoplasmic reticulum of transgenic Arabidopsis

Acyl-CoA-binding proteins (ACBPs) are known to bind and transport acyl-CoA esters and phospholipids intracellularly. In our recent paper in the New Phytologist, we reported that the six acyl-CoA-binding proteins (OsACBPs) in rice (Oryza sativa) are distributed across various subcellular compartments in transgenic Arabidopsis (Arabidopsis thaliana) such as the cytosol (OsACBP1, OsACBP2 and OsACBP3), the endoplasmic reticulum (ER) including the tubules (OsACBP4 and OsACBP5) and the cisternae (OsACBP4), and the peroxisomes (OsACBP6). Localization of OsACBP4::GFP to the peripheral ER cisternae and the central cisternal ER-like structures in transgenic Arabidopsis distinguished it from OsACBP5::GFP. We further report that besides the ER, OsACBP4::GFP and OsACBP5::GFP were also targeted to the membrane of ER bodies and ER-derived spherical structures, respectively, in transgenic Arabidopsis. These findings support our previous conclusion that OsACBP4 and OsACBP5 are not redundant proteins in the ER.

Interactions between Arabidopsis acyl-CoA-binding proteins and their protein partners

Protein-protein interactions are at the core of cellular interactomics and are essential for various biological functions. Since proteins commonly function as macromolecular complexes, it is important to identify their interacting partners to better understand their function and the significance in these interactions. The acyl-CoA-binding proteins (ACBPs) of eukaryotes show conservation in the presence of a lipid-binding acyl-CoA-binding domain. In Arabidopsis thaliana, four of six members from the AtACBP family possess ankyrin repeats (AtACBP1 and AtACBP2) or kelch motifs (AtACBP4 and AtACBP5), which can potentially mediate protein-protein interactions. Through yeast two-hybrid screens, a dozen putative protein partners interacting with AtACBPs have been isolated from an Arabidopsis cDNA library. Investigations in the past decade on the interaction between AtACBPs and their protein partners have revealed novel roles for AtACBPs, including functions in mediating oxidative stress responses, heavy metal tolerance and oxygen sensing. Recent progress and current questions on AtACBPs and their interactors are discussed in this review.

Molecular cloning and characterization of acyl-CoA binding protein (ACBP) gene from shrimp Penaeus monodon exposed to salinity stress

Acyl-CoA binding protein (ACBP), a protein present ubiquitously in wide range of organisms play significant role in transport of acyl groups for macromolecular biosynthesis involved in various functional and regulatory processes. In crustaceans, ACBP has functional role in growth, reproduction and temperature tolerance. In the present study, two suppression subtractive hybridization (SSH) cDNA libraries were performed using gut tissues of shrimp Penaeus monodon exposed to low (3 ppt) and high (55 ppt) salinity stress conditions. SSH library resulted in identification of differentially expressed genes that belonged to various functional classes such as the nucleic acid regulation and replication, defence proteins, allergen protein, signal transduction pathways, apoptosis, energy and metabolism, cell cycle regulation and hypothetical proteins. ACBP was identified as one of the differentially expressed gene in both the SSH libraries of shrimp P. monodon subjected to low and high salinity stress. The full-length cDNA of P. monodon ACBP gene was isolated and the sequence revealed 273 bp open reading frame encoding 90 amino acids with molecular mass of 10 kDa and pI 6.8. The ORF showed presence of four phosphorylation sites, with absence of signal peptide sequence and glycosylation sites. The deduced amino acid sequence of ACBP exhibited high sequence identity (92%) with ACBP class of protein identified from Fenneropenaeus chinensis. Real time PCR analysis of shrimps subjected to 3 ppt salinity conditions after 2 weeks revealed an increase in expression of ACBP transcripts, in the gut (28.08-folds), gills (11.71-folds) and in the muscle tissues (1.70-folds). Whereas, shrimps exposed to 55 ppt salinity conditions after 2 weeks exhibited increased ACBP transcript levels in the gut (11.95-folds), gills (1.052-folds) and muscle tissues (7.35-folds). The significant increase in expression levels of ACBP in various tissues of shrimps suggests a functional role of this gene in salinity stress tolerance and adaptation.

Arabidopsis acyl-CoA-binding protein ACBP1 participates in the regulation of seed germination and seedling development

A family of six genes encoding acyl-CoA-binding proteins (ACBPs), ACBP1-ACBP6, has been characterized in Arabidopsis thaliana. In this study, we demonstrate that ACBP1 promotes abscisic acid (ABA) signaling during germination and seedling development. ACBP1 was induced by ABA, and transgenic Arabidopsis ACBP1-over-expressors showed increased sensitivity to ABA during germination and seedling development, whereas the acbp1 mutant showed decreased ABA sensitivity during these processes. Subsequent RNA assays showed that ACBP1 over-production in 12-day-old seedlings up-regulated the expression of PHOSPHOLIPASE Dα1 (PLDα1) and three ABA/stress-responsive genes: ABA-RESPONSIVE ELEMENT BINDING PROTEIN1 (AREB1), RESPONSE TO DESICCATION29A (RD29A) and bHLH-TRANSCRIPTION FACTOR MYC2 (MYC2). The expression of AREB1 and PLDα1 was suppressed in the acbp1 mutant in comparison with the wild type following ABA treatment. PLDα1 has been reported to promote ABA signal transduction by producing phosphatidic acid, an important lipid messenger in ABA signaling. Using lipid profiling, seeds and 12-day-old seedlings of ACBP1-over-expressing lines were shown to accumulate more phosphatidic acid after ABA treatment, in contrast to lower phosphatidic acid in the acbp1 mutant. Bimolecular fluorescence complementation assays indicated that ACBP1 interacts with PLDα1 at the plasma membrane. Their interaction was further confirmed by yeast two-hybrid analysis. As recombinant ACBP1 binds phosphatidic acid and phosphatidylcholine, ACBP1 probably promotes PLDα1 action. Taken together, these results suggest that ACBP1 participates in ABA-mediated seed germination and seedling development.

Molecular properties of the class III subfamily of acyl-coenyzme A binding proteins from tung tree (Vernicia fordii)

Acyl-CoA binding proteins (ACBPs) have been identified in most branches of life, and play various roles in lipid metabolism, among other functions. Plants contain multiple classes of ACBP genes. The most diverse group is the class III proteins. Tung tree (Vernicia fordii) contains two such genes, designated VfACBP3A and VfACBP3B. The two proteins are significantly different in length and sequence. Analysis of tung ACBP3 genes revealed significant evolution, suggesting relatively ancient divergence of the two genes from a common ancestor. Phylogenetic comparisons of multiple plant class III proteins suggest that this group is the most evolutionarily dynamic class of ACBP. Both tung ACBP3 genes are expressed at similar levels in most tissues tested, but ACBP3A is stronger in leaves. Three-dimensional modeling predictions confirmed the presence of the conserved four α-helix bundle acyl-CoA binding (ACB); however, other regions of these proteins likely fold much differently. Acyl-CoA binding assays revealed different affinities for different acyl-CoAs, possibly contradicting the redundancy of function suggested by the gene expression studies. Subcellular targeting of transiently-expressed plant ACBP3 proteins contradicted earlier studies, and suggested that at least some class III ACBPs may be predominantly targeted to endoplasmic reticulum membranes, with little or no targeting to the apoplast.

Overexpression of Arabidopsis acyl-CoA-binding protein ACBP2 enhances drought tolerance

Arabidopsis thaliana acyl-CoA-binding protein 2 (ACBP2) is a stress-responsive protein that is also important in embryogenesis. Here, we assign a role for ACBP2 in abscisic acid (ABA) signalling during seed germination, seedling development and the drought response. ACBP2 was induced by ABA and drought, and transgenic Arabidopsis overexpressing ACBP2 (ACBP2-OXs) showed increased sensitivity to ABA treatment during germination and seedling development. ACBP2-OXs also displayed improved drought tolerance and ABA-mediated reactive oxygen species (ROS) production in guard cells, thereby promoting stomatal closure, reducing water loss and enhancing drought tolerance. In contrast, acbp2 mutant plants showed decreased sensitivity to ABA in root development and were more sensitive to drought stress. RNA analyses revealed that ACBP2 overexpression up-regulated the expression of Respiratory Burst Oxidase Homolog D (AtrbohD) and AtrbohF, two NAD(P)H oxidases essential for ABA-mediated ROS production, whereas the expression of Hypersensitive to ABA1 (HAB1), an important negative regulator in ABA signalling, was down-regulated. In addition, transgenic plants expressing ACBP2pro:GUS showed beta-glucuronidase (GUS) staining in guard cells, confirming a role for ACBP2 at the stomata. These observations support a positive role for ACBP2 in promoting ABA signalling in germination, seedling development and the drought response.

The gene encoding Arabidopsis acyl-CoA-binding protein 3 is pathogen inducible and subject to circadian regulation

In Arabidopsis thaliana, acyl-CoA-binding protein 3 ( ACBP3), one of six ACBPs, is unique in terms of the C-terminal location of its acyl-CoA-binding domain. It promotes autophagy-mediated leaf senescence and confers resistance to Pseudomonas syringae pv. tomato DC3000. To understand the regulation of ACBP3, a 1.7 kb 5'-flanking region of ACBP3 and its deletion derivatives were characterized using β-glucuronidase (GUS) fusions. A 374 bp minimal fragment (-151/+223) could drive GUS expression while a 1698 bp fragment (-1475/+223) conferred maximal activity. Further, histochemical analysis on transgenic Arabidopsis harbouring the largest (1698 bp) ACBP3pro::GUS fusion displayed ubiquitous expression in floral organs and vegetative tissues (vascular bundles of leaves and stems), consistent with previous results showing that extracellularly localized ACBP3 functions in plant defence. A 160 bp region (-434/-274) induced expression in extended darkness and caused down-regulation in extended light. Electrophoretic mobility shift assay (EMSA) and DNase I footprinting assay showed that the DNA-binding with one finger box (Dof-box, -341/-338) interacted specifically with leaf nuclear proteins from dark-treated Arabidopsis, while GT-1 (-406/-401) binds both dark- and light-treated Arabidopsis, suggesting that Dof and GT-1 motifs are required to mediate circadian regulation of ACBP3. Moreover, GUS staining and fluorometric measurements revealed that a 109 bp region (-543/-434) was responsive to phytohormones and pathogens. An S-box of AT-rich sequence (-516/-512) was identified to bind nuclear proteins from pathogen-infected Arabidopsis leaves, providing the basis for pathogen-inducible regulation of ACBP3 expression. Thus, three cis-responsive elements (Dof, GT-1, and the S-box) in the 5'-flanking region of ACBP3 are proven functional in the regulation of ACBP3.

Novel anti-Cryptosporidium activity of known drugs identified by high-throughput screening against parasite fatty acyl-CoA binding protein (ACBP)

BACKGROUND

Cryptosporidium parvum causes an opportunistic infection in AIDS patients, and no effective treatments are yet available. This parasite possesses a single fatty acyl-CoA binding protein (CpACBP1) that is localized to the unique parasitophorous vacuole membrane (PVM). The major goal of this study was to identify inhibitors from known drugs against CpACBP1 as potential new anti-Cryptosporidium agents.

METHODS

A fluorescence assay was developed to detect CpACBP1 activity and to identify inhibitors by screening known drugs. Efficacies of top CpACBP1 inhibitors against Cryptosporidium growth in vitro were evaluated using a quantitative RT-PCR assay.

RESULTS

Nitrobenzoxadiazole-labelled palmitoyl-CoA significantly increased the fluorescent emission upon binding to CpACBP1 (excitation/emission 460/538 nm), which was quantified to determine the CpACBP1 activity and binding kinetics. The fluorescence assay was used to screen a collection of 1040 compounds containing mostly known drugs, and identified the 28 most active compounds that could inhibit CpACBP1 activity with sub-micromolar IC(50) values. Among them, four compounds displayed efficacies against parasite growth in vitro with low micromolar IC(50) values. The effective compounds were broxyquinoline (IC(50) 64.9 μM), cloxyquin (IC(50) 25.1 μM), cloxacillin sodium (IC(50) 36.2 μM) and sodium dehydrocholate (IC(50) 53.2 μM).

CONCLUSIONS

The fluorescence ACBP assay can be effectively used to screen known drugs or other compound libraries. Novel anti-Cryptosporidium activity was observed in four top CpACBP1 inhibitors, which may be further investigated for their potential to be repurposed to treat cryptosporidiosis and to serve as leads for drug development.

Overexpression of Arabidopsis ACBP3 enhances NPR1-dependent plant resistance to Pseudomonas syringe pv tomato DC3000

ACBP3 is one of six Arabidopsis (Arabidopsis thaliana) genes, designated ACBP1 to ACBP6, that encode acyl-coenzyme A (CoA)-binding proteins (ACBPs). These ACBPs bind long-chain acyl-CoA esters and phospholipids and are involved in diverse cellular functions, including acyl-CoA homeostasis, development, and stress tolerance. Recombinant ACBP3 binds polyunsaturated acyl-CoA esters and phospholipids in vitro. Here, we show that ACBP3 plays a role in the plant defense response to the bacterial pathogen Pseudomonas syringae pv tomato DC3000. ACBP3 mRNA was up-regulated upon pathogen infection and treatments using pathogen elicitors and defense-related phytohormones. Transgenic Arabidopsis ACBP3 overexpressors (ACBP3-OEs) showed constitutive expression of pathogenesis-related genes (PR1, PR2, and PR5), cell death, and hydrogen peroxide accumulation in leaves. Consequently, ACBP3-OEs displayed enhanced resistance to the bacterial pathogen P. syringae DC3000. In contrast, the acbp3 T-DNA insertional mutant was more susceptible and exhibited lower PR gene transcript levels upon infection. Using the ACBP3 OE-1 line in combination with nonexpressor of PR genes1 (npr1-5) or coronatine-insensitive1 (coi1-2), we concluded that the enhanced PR gene expression and P. syringae DC3000 resistance in the ACBP3-OEs are dependent on the NPR1-mediated, but not the COI1-mediated, signaling pathway. Given that ACBP3-OEs showed greater susceptibility to infection by the necrotrophic fungus Botrytis cinerea while the acbp3 mutant was less susceptible, we suggest that ACBP3 plays a role in the plant defense response against biotrophic pathogens that is distinct from necrotrophic pathogens. ACBP3 function in plant defense was supported further by bioinformatics data showing up-regulation of many biotic and abiotic stress-related genes in ACBP3 OE-1 in comparison with the wild type.

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.

Acyl-CoA-binding protein 2 binds lysophospholipase 2 and lysoPC to promote tolerance to cadmium-induced oxidative stress in transgenic Arabidopsis

Lysophospholipids are intermediates of phospholipid metabolism resulting from stress and lysophospholipases detoxify lysophosphatidylcholine (lysoPC). Many lysophospholipases have been characterized in mammals and bacteria, but few have been reported from plants. Arabidopsis thaliana lysophospholipase 2 (lysoPL2) (At1g52760) was identified as a protein interactor of acyl-CoA-binding protein 2 (ACBP2) in yeast two-hybrid analysis and co-immunoprecipitation assays. BLASTP analysis indicated that lysoPL2 showed approximately 35% amino acid identity to the lysoPL1 family. Co-localization of autofluorescence-tagged lysoPL2 and ACBP2 by confocal microscopy in agroinfiltrated tobacco suggests the plasma membrane as a site for their subcellular interaction. LysoPL2 mRNA was induced by zinc (Zn) and hydrogen peroxide (H(2)O(2)), and lysoPL2 knockout mutants showed enhanced sensitivity to Zn and H(2)O(2) in comparison to wild type. LysoPL2-overexpressing Arabidopsis was more tolerant to H(2)O(2) and cadmium (Cd) than wild type, suggesting involvement of lysoPL2 in phospholipid repair following lipid peroxidation arising from metal-induced stress. Lipid hydroperoxide (LOOH) contents in ACBP2-overexpressors and lysoPL2-overexpressors after Cd-treatment were lower than wild type, indicating that ACBP2 and lysoPL2 confer protection during oxidative stress. A role for lysoPL2 in lysoPC detoxification was demonstrated when recombinant lysoPL2 was observed to degrade lysoPC in vitro. Filter-binding assays and Lipidex competition assays showed that (His)(6)-ACBP2 binds lysoPC in vitro. Binding was disrupted in a (His)(6)-ACBP2 derivative lacking the acyl-CoA-binding domain, confirming that this domain confers lysoPC binding. These results suggest that ACBP2 can bind both lysoPC and lysoPL2 to promote the degradation of lysoPC in response to Cd-induced oxidative stress.

Overexpression of Arabidopsis acyl-CoA binding protein ACBP3 promotes starvation-induced and age-dependent leaf senescence

In Arabidopsis thaliana, a family of six genes (ACBP1 to ACBP6) encodes acyl-CoA binding proteins (ACBPs). Investigations on ACBP3 reported here show its upregulation upon dark treatment and in senescing rosettes. Transgenic Arabidopsis overexpressing ACBP3 (ACBP3-OEs) displayed accelerated leaf senescence, whereas an acbp3 T-DNA insertional mutant and ACBP3 RNA interference transgenic Arabidopsis lines were delayed in dark-induced leaf senescence. Acyl-CoA and lipid profiling revealed that the overexpression of ACBP3 led to an increase in acyl-CoA and phosphatidylethanolamine (PE) levels, whereas ACBP3 downregulation reduced PE content. Moreover, significant losses in phosphatidylcholine (PC) and phosphatidylinositol, and gains in phosphatidic acid (PA), lysophospholipids, and oxylipin-containing galactolipids (arabidopsides) were evident in 3-week-old dark-treated and 6-week-old premature senescing ACBP3-OEs. Such accumulation of PA and arabidopsides (A, B, D, E, and G) resulting from lipid peroxidation in ACBP3-OEs likely promoted leaf senescence. The N-terminal signal sequence/transmembrane domain in ACBP3 was shown to be essential in ACBP3-green fluorescent protein targeting and in promoting senescence. Observations that recombinant ACBP3 binds PC, PE, and unsaturated acyl-CoAs in vitro and that ACBP3 overexpression enhances degradation of the autophagy (ATG)-related protein ATG8 and disrupts autophagosome formation suggest a role for ACBP3 as a phospholipid binding protein involved in the regulation of leaf senescence by modulating membrane phospholipid metabolism and ATG8 stability in Arabidopsis. Accelerated senescence in ACBP3-OEs is dependent on salicylic acid but not jasmonic acid signaling.

Expression of ACBP4 and ACBP5 proteins is modulated by light in Arabidopsis

In our recent paper in Plant Physiology and Biochemistry, we reported that the mRNAs encoding Arabidopsis thaliana cytosolic acyl-CoA-binding proteins, ACBP4 and ACBP5, but not ACBP6, are modulated by light/dark cycling. The pattern of circadian-regulated expression in ACBP4 and ACBP5 mRNAs resembles that of FAD7 which encodes omega-3-fatty acid desaturase, an enzyme involved in plastidial fatty acid biosynthesis. Recombinant ACBP4 and ACBP5 proteins were observed to bind oleoyl-CoA ester comparably better than recombinant ACBP6, suggesting that ACBP4 and ACBP5 are promising candidates in the trafficking of oleoyl-CoA from the plastids to the endoplasmic reticulum (ER) for the biosynthesis of non-plastidial membrane lipids. By western blot analyses using the ACBP4 and ACBP5-specific antibodies, we show herein that the levels of ACBP4 and ACBP5 proteins peak at the end of the light period, further demonstrating that they, like their corresponding mRNAs, are tightly controlled by light to satisfy demands of lipids in plant cells.

Light-regulated Arabidopsis ACBP4 and ACBP5 encode cytosolic acyl-CoA-binding proteins that bind phosphatidylcholine and oleoyl-CoA ester

In Arabidopsis thaliana, six genes encode acyl-CoA-binding proteins (ACBPs) that show conservation of an acyl-CoA-binding domain. These ACBPs display varying affinities for acyl-CoA esters, suggesting of different cellular roles. We have recently reported that three members (ACBP4, ACBP5 and ACBP6) are subcellularly localized to the cytosol by biochemical fractionation, confocal microscopy of transgenic Arabidopsis expressing autofluorescence-tagged fusions and immuno-electron microscopy using ACBP-specific antibodies. In this study, we observed by Northern blot analysis that ACBP4 and ACBP5 mRNAs in rosettes were up-regulated by light and dampened-off in darkness, mimicking FAD7 which encodes omega-3-fatty acid desaturase, an enzyme involved in plastidial lipid metabolism. Results from in vitro binding assays indicate that recombinant ACBP4 and ACBP5 proteins bind [(14)C]oleoyl-CoA esters better than recombinant ACBP6, suggesting that light-regulated ACBP4 and ACBP5 encode cytosolic ACBPs that are potential candidates for the intracellular transport of oleoyl-CoA ester exported from the chloroplast to the endoplasmic reticulum for the biosynthesis of non-plastidial membrane lipids. Nonetheless, His-tagged ACBP4 and ACBP5 resemble ACBP6 in their ability to bind phosphatidylcholine suggesting that all three ACBPs are available for the intracellular transfer of phosphatidylcholine.