CF 0 , the proton channel of chloroplast ATP synthase

The discharge of the flash‐induced transmembrane voltage through the exposed proton channel, CF 0 , of the chloroplast ATP synthase, CF 0 CF 1 was investigated. EDTA treatment of thylakoid membranes exposed approximately 50% of total CF 0 by removal of the CF 1 counterparts. This greatly accelerated the decay of the transmembrane voltage, as was apparent from electrochromic‐absorption changes of intrinsic pigments and by pH‐indicating‐absorption changes of added dyes. Two decay processes were discernible, one rapid with a typical half‐decay time of 2 ms, and a slower one with a half‐decay time variable between 20 – 100 ms. Both were sensitive to CF 0 inhibitors, but only the rapid decay process was also inhibited by added CF 1 . CF 1 was effective in surprisingly small amounts, which were significantly lower than those previously removed by EDTA treatment. This finding corroborated our previous conclusion that the rapid decay of the transmembrane voltage was attributable to only a few high‐conductance channels among many CF 0 molecules, typically in the order of one channel CF 1 ‐depleted EDTA vesicle. Inhibition of photophosphorylation in control thylakoids was measured as function of the concentration of CF 0 inhibitors. It was compared with the inhibition of proton conduction through exposed CF 0 in EDTA vesicles. Photophosphorylation and proton conduction by the high‐conductance form of CF 0 were inhibited by the same low inhibitor concentrations. This suggested that the high‐conducting form of CF 0 with a time‐averaged single‐channel conductance of 1 pS [Lill, H., Althoff, G. & Junge, W. (1987) J. Membrane Biol. 98 , 69–78] represented the proton channel in the integral enzyme, which acted as a low‐impedance access from the thylakoid lumen to the coupling site in CF 0 CF 1 . The slow decay process was attributed to a majority of low‐conductance CF 0 channels, i. e. about 50 molecules/vesicle. The conductance of these channels was more than 100‐fold lower and they did not compete with the very few highly conducting channels for rebinding of added CF 1 . The low proton conduction of the majority of exposed CF 0 molecules, possibly due to a structural rearrangement, may be protecting the thylakoid membrane against rapid energy dissipation caused by accidental loss of CF 1 . It may also explain the low single‐channel conductance of bacterial F 0 reported in the literature.