Identification of a Ca(2+)/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.