Contribution of Electric Field (#DELTA##psi#)to Steady-State Transthylakoid Proton motive Force (pmf) in Vitro and in Vivo.Controlof pmf Parsing into #DELTA##psi# and #DELTA#pH by Ionic Strength

摘要

The observed levels of AGATP in chioroplasts, as well as the activation behavior of the CFICF0-ATP synthase, suggest a minimum transthylakoid proton motive force (prnJ) equivalent to a ApH of ~2.5 units. If, as is commonly believed, all transthylakoid pmf is stored as ApI-I, this would indicate a lumen pH of less than -~-.5. In contrast, we have presented evidence that the pH of the thylakoid lumen does not drop below pH ~5.8 [Kramer, D. M., Sacksteder, C. A., and Cruz, J. A. (1999) Photosynth. Res. 60, 151—163], leading us to propose that Aip can contribute to steady-state pmJ In this work, it is demonstrated, through assays on isolated thylakoids and computer simulations, that thylakoids can store a substantial fraction of pmf as Aip, provided that the activities of ions permeable to the thylakoid membrane in the chioroplast stromal compartment are relatively low and the buffering capacity (/3) for protons of the lumen is relatively high. Measurements of the light-induced electrochromic shift (ECS) confirm the ionic strength behavior of steady-state Aip in isolated, partially uncoupled thylakoids. Measurements of the ECS in intact plants illuminated for 65 s were consistent with low concentrations of permeable ions and ~-~-‘50% storage of pmf as Aip. We propose that the plant cell, possibly at the level of the inner chloroplast envelope, can control the parsing of pmf into A~ and ApH by regulating the ionic strength and balance of the chioroplast. In addition, this work demonstrates that, under certain conditions, the kinetics of the light-induced ECS can be used to estimate the fractions of pmf stored as Aip and ApH both in vitro and in vivo.