Carrier of the growing fatty acid chain in fatty acid biosynthesis. Is involved in the transfer of long hydroxylated fatty acids to lipid A. Is acylated predominantly with 27-hydroxyoctacosanoic acid (By similarity). Glycolipid biosynthesis; KDO(2)-lipid A biosynthesis. 4'-phosphopantetheine is transferred from CoA to a specific serine of apo-ACP by AcpS. This modification is essential for activity because fatty acids are bound in thioester linkage to the sulfhydryl of the prosthetic group (By similarity). Extended N-terminus.