Carrier of the growing fatty acid chain in fatty acid biosynthesis (By similarity). May be involved in the synthesis of short and medium 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 at least 49 different subunits. 4'-phosphopantetheine is transferred from CoA to a specific serine of the apo-ACP-like protein. Belongs to the acyl carrier protein (ACP) family.