Conservation of Complete Trimethylation of Lysine-43 in the Rotor Ring of c-Subunits of Metazoan Adenosine Triphosphate (ATP) Synthases

The rotors of ATP synthases turn about 100 times every second. One essential component of the rotor is a ring of hydrophobic c-subunits in the membrane domain of the enzyme. The rotation of these c-rings is driven by a transmembrane proton-motive force, and they turn against a surface provided by another membrane protein, known as subunit a. Together, the rotating c-ring and the static subunit a provide a pathway for protons through the membrane in which the c-ring and subunit a are embedded. Vertebrate and invertebrate c-subunits are well conserved. In the structure of the bovine F-1-ATPase-c-ring subcomplex, the 75 amino acid c-subunit is folded into two transmembrane alpha-helices linked by a short loop. Each bovine rotor-ring consists of eight c-subunits with the N- and C-terminal alpha-helices forming concentric inner and outer rings, with the loop regions exposed to the phospholipid head-group region on the matrix side of the inner membrane. Lysine-43 is in the loop region and its alpha-amino group is completely trimethylated. The role of this modification is unknown. If the trimethylated lysine-43 plays some important role in the functioning, assembly or degradation of the c-ring, it would be expected to persist throughout vertebrates and possibly invertebrates also. Therefore, we have carried out a proteomic analysis of c-subunits across representative species from different classes of vertebrates and from invertebrate phyla. In the twenty-nine metazoan species that have been examined, the complete methylation of lysine-43 is conserved, and it is likely to be conserved throughout the more than two million extant metazoan species. In unicellular eukaryotes and prokaryotes, when the lysine is con-served it is unmethylated, and the stoichiometries of c-subunits vary from 9-15. One possible role for the trimethylated residue is to provide a site for the specific binding of cardiolipin, an essential component of ATP synthases in mitochondria.