Flow-force relationships in lettuce thylakoids. 2. Effect of the uncoupler FCCP on local proton resistances at the ATPase level.

The relationship between the steady-state proton gradient (delta pH) and the rate of phosphorylation was investigated in thylakoids under various conditions. Under partial uncoupling by carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), the rate of ATP synthesis was reduced by less than expected from the decrease of delta pH. This was observed in the case of the pyocyanine-mediated cyclic electron flow around photosystem 1, but not with the H2O-->photosystem 2-->cytochrome b6f-->photosystem 1-->methyl viologen system. In state 4, a unique relation was found between delta pH and the "phosphate potential", delta Gp, regardless of whether the energy level was controlled by light input or FCCP. The anomalous effect of FCCP on the rate of ATP synthesis disappeared when the ATPase was partially blocked by the reversible inhibitor venturicidin, but not in the presence of tentoxin, an irreversible inhibitor. These results are consistent with the existence of a small kinetic barrier for protons, limiting their access to the ATPase. This resistance would be collapsed by FCCP.