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Calcium release from Synechocystis cells induced by depolarization of the plasma membrane: MscL as an outward Ca²⁺ channel

Institute of Plant Physiology, Russian Academy of Science, Botanicheskaya Street 35, 127276 Moscow, Russian Federation

Correspondence
Dmitry A. Los
losnet{at}ippras.ru

Cells of the cyanobacterium Synechocystis sp. PCC 6803 are equipped with a mechanosensitive ion channel MscL that is located in their plasma membrane. However, the exact function of the channel in this freshwater cyanobacterium is unknown. This study shows that cells of Synechocystis are capable of releasing Ca²⁺ in response to depolarization of the plasma membrane by the K⁺ ionophore valinomycin in the presence of K⁺ or by tetraphenylphosphonium (TPP⁺). A fluorescent dye, diS-C₃-(5), sensitive to membrane potential and the metallochromic Ca²⁺ indicator arsenazo III were used to follow the plasma membrane depolarization and the Ca²⁺ release, respectively. The Ca²⁺ release from wild-type cells was temperature-dependent and it was strongly inhibited by the Ca²⁺ channel blocker verapamil and by the mechanosensitive channel blocker amiloride. In MscL-deficient cells, Ca²⁺ release was about 50 % of that from the wild-type cells. The mutant cells had lost temperature sensitivity of Ca²⁺ release completely. However, verapamil and amiloride inhibited Ca²⁺ release from these cells in same manner as in the wild-type cells. This suggests the existence of additional Ca²⁺ transporters in Synechocystis, probably of a mechanosensitive nature. Evidence for the putative presence of intracellular Ca²⁺ stores in the cells was obtained by following the increase in fluorescence intensity of the Ca²⁺ indicator chlortetracycline. These results suggest that the MscL of Synechocystis might operate as a verapamil/amiloride-sensitive outward Ca²⁺ channel that is involved in the plasma-membrane depolarization-induced Ca²⁺ release from the cells under temperature stress conditions.

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