Identification of a Ca2+/H+ antiport in the plant chloroplast thylakoid membrane

To assess the availability of Ca2+ in the lumen of the thylakoid membrane t hat is required to support the assembly of the oxygen-evolving complex of p hotosystem II, we have investigated the mechanism of Ca-45(2+) transport in to the lumen of pea (Pisum sativum) thylakoid membranes using silicone-oil centrifugation. Transthylakoid Ca2+ transport is dependent on light or, in the dark, on exogenously added ATP. Both light and ATP hydrolysis are coupl ed to Ca2+ transport through the formation of a transthylakoid pH gradient. The H+-transporting ionophores nigericin/K+ and carbonyl cyanide 3-chlorop henylhydrazone inhibit the transport of Ca2+. Thylakoid membranes are capab le of accumulating up to 30 nmol Ca2+ mg(-1) chlorophyll from external conc entrations of 15 mu M over the course of a 15-min reaction. These results a re consistent with the presence of an active Ca2+/H+ antiport in the thylak oid membrane. Ca2+ transport across the thylakoid membrane has significant implications for chloroplast and plant Ca2+ homeostasis. We propose a model of chloroplast Ca2+ regulation whereby the activity of the Ca2+/H+ antipor ter facilitates the light-dependent uptake of Ca2+ by chloroplasts and redu ces stromal Ca2+ levels.