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 nov el membrane-associated ACBP that accumulates in developing seeds, designate d ACBP1, has provided evidence for the existence of membrane-associated for ms of ACBPs (Chye, 1998, Plant Mel. Biol. 38, 827-838). We now report on th e 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, re sembling the promoters of rat, Drosophila and human genes encoding cytosoli c ACBP and suggesting that it is a housekeeping gene. We show by Western bl ot 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 mos t peripheral cells of the cotyledons and hypocotyl. Immunogold labelling lo calized ACBP1 to vesicles, to the plasma membrane especially at epidermal c ells 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 A CBP1 is involved in intermembrane lipid transport from the ER via vesicles to the plasma membrane where it could maintain a membrane-associated acyl p ool; its immunolocalization to the cell wall of outer integument cells at t he seed coat suggests a role in cuticle and cutin formation.