Evidence that the intrinsic membrane protein LHCII in thylakoids is necessary for maintaining localized delta mu H+ energy coupling.

This work tested the hypothesis that thylakoid localized proton-binding domains, suggested to be involved in localized delta mu H(+)-driven ATP formation, are maintained with the involvement of several membrane proteins, including the LHCII (Laszlo, J.A., Baker, G.M., and Dilley, R.A. (1984) Biochim. Biophys. Acta 764, 160-169), which comprises about 50% of the total thylakoid protein. The concept we have in mind is that several membrane proteins cooperate to shield a localized proton diffusion pathway from direct contact with the lumen, thus providing a physical barrier to H+ equilibration between the sequestered domains and the lumen. A barely mutant, chlorina f2, that lacks Chl b and does not accumulate some of the LHCII proteins, was tested for its capacity to carry out localized-proton gradient-dependent ATP formation. Two previously developed assays permit clear discrimination between localized and delocalized delta mu H+ gradient-driven ATP formation. Those assays include the effect of a permeable buffer, pyridine, on the number of single-turnover flashes needed to reach the energetic threshold for ATP formation and the more recently developed assay for lumen pH using 8-hydroxy-1,3,6-pyrene trisulfonic acid as a lumenally loaded pH-sensitive fluorescent probe. By those two criteria, the wild-type barley thylakoids revealed either a localized or a delocalized energy coupling mode under low- or high-salt storage conditions, respectively. Addition of Ca++ to the high-salt storage medium caused those thylakoids to maintain a localized energy-coupling response, as previously observed for pea thylakoids.(ABSTRACT TRUNCATED AT 250 WORDS)