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

J Cell Sci. 2006 Sep 1;119(Pt 17):3705-17. doi: 10.1242/jcs.03075. Epub 2006 Aug 15.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Calcium / metabolism*
  • Calcium Signaling / physiology*
  • Gene Expression Regulation
  • Homeostasis
  • Inositol 1,4,5-Trisphosphate / metabolism
  • Inositol 1,4,5-Trisphosphate Receptors / chemistry
  • Inositol 1,4,5-Trisphosphate Receptors / classification
  • Inositol 1,4,5-Trisphosphate Receptors / genetics
  • Inositol 1,4,5-Trisphosphate Receptors / metabolism*
  • Lithium Chloride / metabolism
  • Models, Molecular
  • Molecular Sequence Data
  • Paramecium tetraurelia / genetics
  • Paramecium tetraurelia / metabolism*
  • Paramecium tetraurelia / ultrastructure
  • Phylogeny
  • Protein Structure, Tertiary
  • Protozoan Proteins / chemistry
  • Protozoan Proteins / classification
  • Protozoan Proteins / genetics
  • Protozoan Proteins / metabolism*
  • Rats
  • Recombinant Fusion Proteins / genetics
  • Recombinant Fusion Proteins / metabolism
  • Sequence Alignment
  • Water-Electrolyte Balance / physiology*


  • Inositol 1,4,5-Trisphosphate Receptors
  • Protozoan Proteins
  • Recombinant Fusion Proteins
  • Inositol 1,4,5-Trisphosphate
  • Lithium Chloride
  • Calcium