Protein folding and unfolding: proline cis-trans isomerization at the c subunits of F 1 F O -ATPase might open a high conductance ion channel.

The c subunits, which constitute the c-ring apparatus of the F 1 F O -ATPase, could be the main components of the mitochondrial permeability transition pore (mPTP). The well-known modulator of the mPTP formation and opening is the cyclophilin D (CyPD), a peptidyl-prolyl cis-trans isomerase. On the loop, which connects the two hairpin α-helix of c subunit, is present the unique proline residue (Pro 40 ) that could be a biological target of CyPD. Indeed, the proline cis-trans isomerization might provide the switch that interconverts the open/closed states of the pore by pulling out the c-ring lipid plug.