The preferred stoichiometry of c subunits in the rotary motor sector of Escherichia coli ATP synthase is 10

The stoichiometry of c subunits in the H+-transporting F-o rotary motor of ATP synthase is uncertain, the most recent suggestions varying from 10 to 1 4, The stoichiometry will determine the number of H+ transported per ATP sy nthesized and will directly relate to the P/O ratio of oxidative phosphoryl ation, The experiments described here show that the number of c subunits in functional complexes of FoF1 ATP synthase from Escherichia coli can be man ipulated, but that the preferred number is 10, Mixtures of genetically fuse d cysteine-substituted trimers (c(3)) and tetramers (c(4)) of subunit c wer e coexpressed and the c subunits crosslinked in the plasma membrane. Promin ent products corresponding to oligomers of c(7) and c(10) were observed in the membrane and purified FoF1 complex, indicating that the c(10) oligomer formed naturally. Oligomers larger than c(10) were also observed in the mem brane fraction of cells expressing c(3) or c(4) individually, or in cells c oexpressing c(3) and c(4) together, but these larger oligomers did not copu rify with the functional FoF1 complex and were concluded to be aberrant pro ducts of assembly in the membrane.