Barley acyl carrier protein: its amino acid sequence and assay using purified malonyl-CoA:ACP transacylase

Malonyl-CoA:ACP transacylase from barley (Hordeum vulgare L.) has been purified to homogeneity and used in an assay for acyl carrier protein (ACP). The transacylase is an acidic, monomeric protein with a molecular weight of 34,500 very similar to the analogous E. coli enzyme. A heat and acid stable acyl carrier protein from barley has been purified to homogeneity and its chemical composition determined. The ACP consists of a continuous stretch of the following 72 amino acids H2N-A-A-M-G-E-A-Q-A-K-K-E-T-V-D-K-V-(C?)-M-I-V-K-K-Q-L-A-V-P-D-G-T-P-V-T-A-E-S-K-F-S-E-L-G-A-D-S-L-D-T-V-E-I-V-M-G-L-E-E-E-F-N-I-T-V-D-E-T-S-A-Q-D-I-A72...A87-COOH. A comparison of the primary structure of this plant ACP and bacterial ACP reveals two identical sequences (underlined) in the midregion of the molecule containing the 4′-phosphopantetheine attachment site, while differences occur outside this region. Nine extra residues (italicized) are present at the N-terminal end of the barley protein thereby accounting for its larger size. Identical products are obtained when barley chloroplast fatty acid synthetase is incubated with either barley or E. coli ACP, but the latter is twice as active as the former in fatty acid synthesis. The possible significance of the N-terminal part of the ACP is discussed in relation to the reported differences in biochemical activities of plant and bacterial ACPs