PHOSPHOPANTETHENYLATED PRECURSOR ACYL CARRIER PROTEIN IS IMPORTED INTO SPINACH (SPINACIA-OLERACEA) CHLOROPLASTS

Acyl carrier protein (ACP) is an essential cofactor of fatty acid synt hase. In plants, ACP is synthesized in the cytosol as a larger precurs or protein and then is imported into the plastid where it is processed to a smaller mature form. The active form of ACP uses a covalently li nked 4'-phosphopantetheine prosthetic group derived from coenzyme A to covalently bind the acyl intermediates during fatty acid synthesis. T he prosthetic group is added to ACP by holoACP synthase. This enzyme a ctivity is associated with both the plastidial subcellular fraction an d the soluble, or cytoplasmic, fraction. To gain further insight into potential in vivo pathways for the synthesis and maturation of ACP, in this study we examined whether precursor holoACP can be imported by i solated spinach (Spinacia oleracea) chloroplasts. Precursor holoACP co ntaining a [S-35]phosphopantetheine prosthetic group was prepared, and the radiolabel was used to demonstrate import of the phosphopantethen ylated protein into isolated chloroplasts. In addition, timed chloropl ast import assays indicated that in vitro import of the phosphopanteth enylated protein is at least as efficient as import of the precursor a poprotein. Evidence was also obtained for a low level turnover of the prosthetic group among endogenous plastidial ACPs when coenzyme A was supplied exogenously.