The three mitochondrially translated ATP synthase subunits of Saccharomyces cerevisiae were extracted from the enzyme and from whole mitochondria using an organic solvent mixture and then purified by reverse-phase HPLC. The amino acid composition of subunit 6 is close to the one predicted from the oli2 gene. The partial amino terminal sequence of subunit 6 reveals a post-translational cleavage site between the Thr-10 and Ser-11 residues of the precursor. Thus, mature subunit 6 contains 249 amino acid residues and displays a molecular mass of 27943 Da.