Chlamydomonas contains calcium stores that are mobilized when phospholipase C is activated
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Mastoparan induces Ca2+-dependent deflagellation of the unicellular green alga Chlamydomonas moewusii Gerloff, as well as the activation of phospholipase C and the production of inositol 1,4,5-trisphosphate (InsP3; T. Munnik et al., 1998, Planta 207: 133–145). Even in the absence of extracellular Ca2+, mastoparan still induces deflagellation (L.M. Quarmby and H.C. Hartzell, 1994, J Cell Biol 124: 807–815; J.A.J. van Himbergen et al., 1999, J Exp Bot, in press) suggesting that InsP3 mediates Ca2+ release from intracellular stores. To test this hypothesis, cells were pre-loaded with 45Ca2+ and their plasma membranes permeabilized by digitonin. Subsequent treatment of the cells with mastoparan (3.5 μM) induced release of intracellular 45Ca2+. Mastoparan also activated phospholipase C in permeabilized cells, as demonstrated by the breakdown of 32P-phosphatidylinositol 4,5-bisphosphate and the production of diacylglycerol. The mastoparan analogues mas7 and mas17 were also effective and their efficacy was correlated with their biological activity. X-ray microanalysis showed that electron-dense bodies (EDBs) are a major Ca2+ store in C. moewusii. Analysis of digitonin-permeabilized cells showed that EDBs lost calcium at digitonin concentrations that released radioactivity from 45Ca2+-labelled cells, suggesting that 45Ca2+ monitored the content of EDBs. X-ray microanaysis of living cells treated with mastoparan also revealed that calcium was released from EDBs.
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