Contribution of electric field (delta psi) to steady-state transthylakoid proton motive force in vivo.

Abstract

The observed levels of DGATP in chloroplasts, as well as the activation behavior of the CF1CFO-ATP synthase, suggest a minimum transthylakoid proton motive force (pmf) equivalent to a DpH of ca. 2.5 units. If, as is commonly believed, all transthylakoid pmf is stored as DpH, this would predict a lumen pH of less than ca. 5. In contrast, we have presented evidence that the pH of the thylakoid lumen does not drop below about pH 5.8 (Kramer, D. M., Sacksteder, C. A., and Cruz, J. A. (1999) Photosynth. Res. 60, 151-163), leading us to propose that Dy can contribute to steady-state pmf. As demonstrated through assays on isolated thylakoids and computer simulations, thylakoids can store a substantial fraction of pmf as Dy, provided that the activities of ions permeable to the thylakoid membrane in the chloroplast stroma is relatively low and the buffering capacity (b) for protons of the lumen is relatively high. Measurements of light-induced electrochromic shift (ECS) confirm the influence of ionic-strength on the behavior of steady-state Dy in isolated, partially-uncoupled thylakoids. Measurements of ECS in intact plants illuminated for 65 s were consistent with low concentrations of permeable ions and ca. 50% storage of pmf as Dy. We propose that the plant cell, possibly at the level of the inner chloroplast envelope, can control the parsing of pmf into Dy and DpH by regulating the ionic strength and balance of the chloroplast. We further propose that balance of DpH and Dy may serve to modulate down regulation of photosynthesis. Finally, under certain conditions, the kinetics of light-induced ECS can be used to estimate the fractions of pmf stored as Dy and DpH both in vitro and in vivo