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). {ECO:0000269|PubMed:12882974, ECO:0000269|PubMed:19549604, ECO:0000269|PubMed:36482135}. This is the function of Ensembl gene identifier ENSG00000100294 (MCAT, malonyl-CoA-acyl carrier protein transacylase).