Site-specific interaction of ATPase-pumped protons with photosystem II in chloroplast thylakoid membranes

The chloroplast thylakoid ATPase proton pump-driven H+ accumulation in the dark was compared to the light-dependent proton pump driven by either photosystem II or I, in regard to the effects of the resultant acidity on chemical modification reactions. The assays used to detect the acidity effects were: (a)the incorporation of [3H]-acetic anhydride into membrane protein -NH2 groups, and (b) the effect of a certain level of that chemical modification on inhibition of photosystem II water oxidation activity. Based on labeling data with [3H]-acetic anhydride, 20-30 nmol.(mg chl)-1 of -NH3+ groups appear to be metastable in the dark in untreated membranes. The term metastable is used because proton leak-inducing treatments in the dark lead to about 20-30 nmol . (mg chl)-1 increase in acetic anhydride labeling probably due to reaction with the -NH2 form of amine groups. Addition of low levels of uncoupler or a brief thermal treatment caused a loss of protons from the membrane equivalent to the increase in acetic anhydride derivatization. The increase in acetic anhydride derivatization caused inhibition of water oxidation activity. Using thermally sensitized membranes, photosystem II but not photosystem I electron transport (each giving a steady-state proton accumulation of about 50 nmol H+ . (mg chl)-1 restored the lower level of acetic anhydride reactivity as in previous results (Baker et al., 1981). In dark-maintained, thermally treated membranes, ATPase activity, i.e., the proton pump associated with it, also restored the lower level of acetic anhydride labeling, and again acetic anhydride no longer inhibited water oxidation. Because photosystem I activity did not elicit this type of response to acetic anhydride, there appears to be a pathway for ATPase pumped protons which allows them to reach a restricted domain, perhaps intramembrane, common with the photosystem II water oxidation mechanism and unavailable to protons pumped by photosystem I. The membrane structure(s) which determines this site specificity is not yet understood.