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Acyl carrier protein (ACP) dependent fatty acid synthesis (FAS) in plant leaf tissues is exclusively localized in the chloroplast (1). Two forms of ACP have been purified from spinach and barley leaf, but, only one ACP isoform predominates in spinach seed tissue (2, 3). Oleic acid is the major product of fatty acid synthesis by isolated chloroplasts (4). The regulation of plant lipid metabolism is believed to involve the export of oleic acid from its site of synthesis (plastid) to sites of complex lipid metabolism (eg. ER). This pathway requires the initial release of oleic acid from ACP by oleoyl-ACP thioesterase (5). In addition, the oleoyl moiety may also enter glycerolipid synthesis within the plastid through the action of acyl-ACP acyl-transferase (6). The preferred substrate for both the thioesterase and glycerol-3-phosphate acyl transferase reactions is 18: 1-ACP (6, 7).

Acyl carrier protein (ACP) from spinach leaves has been purified to homogeneity by high-performance liquid chromatography with an anion-exchange column. The amino acid sequence of one major ACP in spinach leaves, ACP-I, has been determined by automated Edman degradation. It consists of the following 82 amino acids: H 2 NAK KETIDKVSDIVKEKLA LGADVVVTADSEFSKL GA DSLDTVEIVMNLEE EFGINVDEDKAQDIST IQQAA DVIEXLLEKKACOOH. Sequencing of the intact polypeptide provided data for the first 57 residues. Cleavage of the succinylated ACP with CNBr at Met-46, followed by sequencing of the fragment mixture, provided data for the final 36 residues. The C-terminal alanine was confirmed by carboxypeptidase Y digestion. The spinach ACP has 40, 70, and 25% homology with Escherichia coli , barley, and rabbit ACPs, respectively. The results not only provide the first complete sequence of a plant ACP, but also provide insight into the structural and evolutionary relationships among plant, animal, and bacterial ACPs.