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

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Abstract
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 14. The stoichiometry will determine the number of H + transported per ATP synthesized and will directly relate to the P/O ratio of oxidative phosphorylation. The experiments described here show that the number of c subunits in functional complexes of F o F 1 ATP synthase from Escherichia coli can be manipulated, but that the preferred number is 10. Mixtures of genetically fused cysteine-substituted trimers ( c 3 ) and tetramers ( c 4 ) of subunit c were coexpressed and the c subunits crosslinked in the plasma membrane. Prominent products corresponding to oligomers of c 7 and c 10 were observed in the membrane and purified F o F 1 complex, indicating that the c 10 oligomer formed naturally. Oligomers larger than c 10 were also observed in the membrane fraction of cells expressing c 3 or c 4 individually, or in cells coexpressing c 3 and c 4 together, but these larger oligomers did not copurify with the functional F o F 1 complex and were concluded to be aberrant products of assembly in the membrane.