Chlamydomonas contains calcium stores that are mobilized when phospholipase C is activated

Mastoparan induces Ca(2+)-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 (InsP(3); T. Munnik et al., 1998, Planta 207: 133-145). Even in the absence of extracellular Ca(2+), 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 InsP(3) mediates Ca(2+) release from intracellular stores. To test this hypothesis, cells were pre-loaded with (45)Ca(2+) and their plasma membranes permeabilized by digitonin. Subsequent treatment of the cells with mastoparan (3.5 microM) induced release of intracellular (45)Ca(2+). Mastoparan also activated phospholipase C in permeabilized cells, as demonstrated by the breakdown of (32)P-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 Ca(2+) store in C. moewusii. Analysis of digitonin-permeabilized cells showed that EDBs lost calcium at digitonin concentrations that released radioactivity from (45)Ca(2+)-labelled cells, suggesting that (45)Ca(2+) monitored the content of EDBs. X-ray microanaysis of living cells treated with mastoparan also revealed that calcium was released from EDBs.