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

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 (InsP(3); 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 Bet. in press) suggesting that InsP(3) mediates Ca2+ release from intracellular stores. To test this hypothesis, cells were pre-loaded with Ca-45(2+) and their plasma membranes permeabilized by digitonin. Subsequent treatment of the cells with mastoparan (3.5 mu M) induced release of intracellular Ca-45(2+). Mastoparan also activated phospholipase C in permeabilized cells, as demonstrated by the breakdown of P-32-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 Ca-45(2+)-labelled cells, suggesting that Ca-45(2+) monitored the content of EDBs. X-ray microanaysis of living cells treated with mastoparan also revealed that calcium was released from EDBs.