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.
Ladenburger
Eva-Maria
EM
Department of Biology, University of Konstanz, 78457 Konstanz, Germany. Eva.Landenburger@uni-konstanz.de
Korn
Iris
I
Kasielke
Nicole
N
Wassmer
Thomas
T
Plattner
Helmut
H
D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D005786:Gene Expression Regulation; D006706:Homeostasis; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000737:chemistry / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D018021:Lithium Chloride / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016805:Paramecium tetraurelia / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D010802:Phylogeny; D017434:Protein Structure, Tertiary; D015800:Protozoan Proteins / Q000737:chemistry / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D014882:Water-Electrolyte Balance / Q000502:physiology*
D053496:Inositol 1,4,5-Trisphosphate Receptors; D015800:Protozoan Proteins; D011993:Recombinant Fusion Proteins; D015544:Inositol 1,4,5-Trisphosphate; D018021:Lithium Chloride; D002118:Calcium