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. Accessory and non-catalytic subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I), which functions in the transfer of electrons from NADH to the respiratory chain (By similarity). Lipid metabolism; fatty acid biosynthesis. Complex I is composed of about 30 different subunits. 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). Present with 60500 molecules/cell in log phase SD medium. Belongs to the acyl carrier protein (ACP) family.