Carrier of the growing fatty acid chain in fatty acid biosynthesis (By similarity). May be involved in the synthesis of very-long-chain fatty acids. Lipid metabolism; fatty acid 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). Belongs to the acyl carrier protein (ACP) family. Was previously considered as a subunit of the NADH dehydrogenase of the mitochondrial respiratory chain complex I. Due to lack of 38 of the other 40 subunits that are present in that complex in mammals, this attribution is unlikely (PubMed:1518044).