Culture Age, Intracellular Ca2+ Concentration, and Protein Phosphorylation in Encystment-Induced Colpoda cucullus

In Colpoda cucullus, intracellular Ca2+ mediates the encystment induction and protein phosphorylation that occur just prior to morphogenetic transformation into the resting form. When rapidly growing cells were stimulated to encyst, encystment was not readily induced, and the protein phosphorylation level was lower. On the other hand, in post-growing cells stimulated to encyst, the encystment rate and protein phosphorylation level were elevated. These results suggest that protein phosphorylation is closely linked to encystment induction. Why, then, are the protein phosphorylation level and encystment rate difficult to elevate in the rapidly growing cells? Fura 2 ratiometry showed that the intracellular Ca2+ concentration (F-340/F-380 ratio) was raised in rapidly growing cells as well as in post-growing cells when the cells were stimulated to encyst. It is presumed that the Ca2+-mediated signal transduction pathways for protein phosphorylation and encystment may be triggered in rapidly growing cells, but downstream certain steps may be suppressed by certain intracellular components.