Catalyzes the transfer of a malonyl moiety from malonyl-CoA to the free thiol group of the phosphopantetheine arm of the mitochondrial ACP protein (NDUFAB1) (PubMed:12882974, PubMed:19549604). This suggests the existence of the biosynthesis of fatty acids in mitochondria (PubMed:12882974). Also acts as a mitochondrial small ribosomal subunit (mt-SSU) assembly factor (PubMed:36482135).
