Identification of a Ca2+/H+ Antiport in the Plant Chloroplast Thylakoid Membrane1

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Abstract
To assess the availability of Ca2+ 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 of45Ca2+ transport into the lumen of pea (Pisum sativum) thylakoid membranes using silicone-oil centrifugation. Trans-thylakoid Ca2+ transport is dependent on light or, in the dark, on exogenously added ATP. Both light and ATP hydrolysis are coupled to Ca2+ transport through the formation of a transthylakoid pH gradient. The H+-transporting ionophores nigericin/K+ and carbonyl cyanide 3-chlorophenylhydrazone inhibit the transport of Ca2+. Thylakoid membranes are capable of accumulating up to 30 nmol Ca2+ 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 Ca2+/H+ antiport in the thylakoid 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+antiporter facilitates the light-dependent uptake of Ca2+by chloroplasts and reduces stromal Ca2+ levels.