Catalyzes the post-translational modification of target proteins by phosphopantetheine. Can transfer the 4'-phosphopantetheine moiety from coenzyme A, regardless of whether the CoA is presented in the free thiol form or as an acetyl thioester, to a serine residue of a broad range of acceptors including the acyl carrier domain of FASN. apo-[ACP] + CoA = adenosine 3',5'-bisphosphate + H(+) + holo-[ACP] acetyl-CoA + apo-[ACP] = acetyl-[ACP] + adenosine 3',5'-bisphosphate + H(+) Binds 1 Mg(2+) ion. Monomer. Belongs to the P-Pant transferase superfamily. AcpS family.