Catalyzes the transfer of a malonyl moiety from malonyl-CoA to the free thiol group of the phosphopantetheine arm of the ACP protein. This suggests the existence of the biosynthesis of fatty acids in mitochondria (By similarity). holo-[ACP] + malonyl-CoA = CoA + malonyl-[ACP] Lipid metabolism; fatty acid biosynthesis. Eggs have a thin shell. Belongs to the type II malonyltransferase family.