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

To assess the availability of Ca 2+ in the lumen of the thylakoid membrane that is required to support the assembly of the oxygen-evolving complex of photosystem II, we have investigated the mechanism of 45 Ca 2+ transport into the lumen of pea ( Pisum sativum ) thylakoid membranes using silicone-oil centrifugation. Trans-thylakoid Ca 2+ transport is dependent on light or, in the dark, on exogenously added ATP. Both light and ATP hydrolysis are coupled to Ca 2+ transport through the formation of a transthylakoid pH gradient. The H + -transporting ionophores nigericin/K + and carbonyl cyanide 3-chlorophenylhydrazone inhibit the transport of Ca 2+ . Thylakoid membranes are capable of accumulating up to 30 nmol Ca 2+ mg −1 chlorophyll from external concentrations of 15 μm over the course of a 15-min reaction. These results are consistent with the presence of an active Ca 2+ /H + antiport in the thylakoid membrane. Ca 2+ transport across the thylakoid membrane has significant implications for chloroplast and plant Ca 2+ homeostasis. We propose a model of chloroplast Ca 2+ regulation whereby the activity of the Ca 2+ /H + antiporter facilitates the light-dependent uptake of Ca 2+ by chloroplasts and reduces stromal Ca 2+ levels.