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

Plant Physiology, ISSN: 0032-0889, Vol: 119, Issue: 4, Page: 1379-1385
1999
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To assess the availability of Ca 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 Ca transport into the lumen of pea (Pisum sativum) thylakoid membranes using silicone-oil centrifugation. Transthylakoid Ca transport is dependent on light or, in the dark, on exogenously added ATP. Both light and ATP hydrolysis are coupled to Ca 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. Thylakoid membranes are capable of accumulating up to 30 nmol Ca mg 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/H antiport in the thylakoid membrane. Ca transport across the thylakoid membrane has significant implications for chloroplast and plant Ca homeostasis. We propose a model of chloroplast Ca regulation whereby the activity of the Ca/H antiporter facilitates the light-dependent uptake of Ca by chloroplasts and reduces stromal Ca levels.

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