An Ins(1,4,5)P₃ receptor in Paramecium is associated with the osmoregulatory system

In the ciliate Paramecium, a variety of well characterized processes are regulated by Ca²⁺, e.g. exocytosis, endocytosis and ciliary beat. Therefore, among protozoa, Paramecium is considered a model organism for Ca²⁺ signaling, although the molecular identity of the channels responsible for the Ca²⁺ signals remains largely unknown. We have cloned - for the first time in a protozoan - the full sequence of the gene encoding a putative inositol (1,4,5)-trisphosphate (Ins(1,4,5)P₃) receptor from Paramecium tetraurelia cells showing molecular characteristics of higher eukaryotic cells. The homologously expressed Ins(1,4,5)P₃-binding domain binds [³H]Ins(1,4,5)P₃, whereas antibodies unexpectedly localize this protein to the osmoregulatory system. The level of Ins(1,4,5)P₃-receptor expression was reduced, as shown on a transcriptional level and by immuno-staining, by decreasing the concentration of extracellular Ca²⁺ (Paramecium cells rapidly adjust their Ca²⁺ level to that in the outside medium). Fluorochromes reveal spontaneous fluctuations in cytosolic Ca²⁺ levels along the osmoregulatory system and these signals change upon activation of caged Ins(1,4,5)P₃. Considering the ongoing expulsion of substantial amounts of Ca²⁺ by the osmoregulatory system, we propose here that Ins(1,4,5)P₃ receptors serve a new function, i.e. a latent, graded reflux of Ca²⁺ to fine-tune [Ca²⁺] homeostasis.