Abstract
A large proton leak not coupled to ATP synthesis (slip) occurs at alkaline pH through the chloroplast ATP synthase (Y. Evron, M. Avron [1990] Biochim Biophys Acta 1019: 115-120). The involvement of the ATP synthase [gamma]-subunit in the regulation of proton conductance was analyzed by measuring the effect of thiolalkylating agents on proton slip. Alkylation by N-ethylmaleimide of [gamma]-cysteine (Cys)-89, which is exposed upon energization of thylakoids, increases the slip only at alkaline pH. The slip is partially suppressed by low concentrations of adenine nucleotides and is completely eliminated by venturicidin, a blocker of the hydrophobic polypeptide complex of the chloroplast ATP synthase (CF0). Conversely, cross-linking of [gamma]-Cys-89 with [gamma]-Cys-322 renders the ATP synthase leaky to protons and insensitive to ATP also at neutral pH. The accessibility of [gamma]-Cys-89 to alkylation by fluorescein maleimide is completely suppressed by N,N-dicyclohexylcarbodiimide and by venturicidin, which block proton conductance through CF0 and increase the pH gradient. These results suggest that the [gamma]-subunit has a dominant role in proton gating through the ATP synthase and responds to changes in pH and ligands taking place on either side of the thylakoid membrane. It is proposed that the conformational changes that induce the proton slip and the exposure of [gamma]-Cys-89 reflect the conversion of the enzyme from a catalytically latent to an active state, and depend on the deprotonation of a stromal site at alkaline pH and on protonation of an intrathylakoid inner site upon energization. Therefore, conditions that induce the conformational activation also provide the driving force for ATP synthesis.