F0F1-ATPase/Synthase Is Geared to the Synthesis Mode by Conformational Rearrangement of ϵ Subunit in Response to Proton Motive Force and ADP/ATP Balance

The ϵ subunit in F0F1-ATPase/synthase undergoes drastic conformational rearrangement, which involves the transition of two C-terminal helices between a hairpin “down”-state and an extended “up”-state, and the enzyme with the up-fixed ϵ cannot catalyze ATP hydrolysis but can catalyze ATP synthesis (Tsunoda, S. P., Rodgers, A. J. W., Aggeler, R., Wilce, M. C. J., Yoshida, M., and Capaldi, R. A. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 6560–6564). Here, using cross-linking between introduced cysteine residues as a probe, we have investigated the causes of the transition. Our findings are as follows. (i) In the up-state, the two helices of ϵ are fully extended to insert the C terminus into a deeper position in the central cavity of F1 than was thought previously. (ii) Without a nucleotide, ϵ is in the up-state. ATP induces the transition to the down-state, and ADP counteracts the action of ATP. (iii) Conversely, the enzyme with the down-state ϵ can bind an ATP analogue, 2′,3′-O-(2,4,6-trinitrophenyl)-ATP, much faster than the enzyme with the up-state ϵ. (iv) Proton motive force stabilizes the up-state. Thus, responding to the increase of proton motive force and ADP, F0F1-ATPase/synthase would transform the ϵ subunit into the up-state conformation and change gear to the mode for ATP synthesis.