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 [14C]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.