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 that is required to support the assembly of the oxygen-evolving complex of photosystem II, we have investigated the mechanism of ⁴⁵Ca²⁺ transport into 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 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.