Characterization of a novel inositol 1,4,5-trisphosphate receptor in isolated olfactory cilia

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Abstract
Inositol 1,4,5-trisphosphate (InsP3), a product of G-protein-mediated receptor activation of phosphoinositide turnover, plays the role of a second messenger when olfactory neurons are stimulated with certain olfactory stimuli. In this paper we examine the specific binding of [3H]InsP3 to isolated olfactory cilia, microsomes and brain membranes from the channel catfish (Ictalurus punctatus) and, by photoaffinity labelling with an InsP3 analogue (125I-labelled 1-[3-(4-azidosalicyloxy)-aminopropyl]inositol 1,4,5-trisphosphate (125I-ASA-InsP3)], we tentatively identify the major InsP3-binding protein in catfish olfactory cilia. InsP3 binding to ciliary membranes is specific and saturable, with a Kd of 1.10 +/- 0.31 microM and a maximum number of binding sites (Bmax) of 17.6 +/- 5.8 pmol/mg. The rank order for potency of inhibition of [3H]InsP3 binding is Ins(1,4)P2 less than Ins(1,3,4)P3 less than Ins(1,3,4,5)P4 = Ins(1,4,5)P3 less than Ins(2,4,5)P3. Exposure of cilia membranes to u.v. light in the presence of 125I-ASA-InsP3 results in the labelling of a protein with apparent Mr 107,000. Labelling is specifically prevented by Ins(1,4,5)P3, Ins(2,4,5)P3 and Ins(1,3,4,5)P4, but not by Ins(1,4)P2 or Ins(1,3,4)P3. Both specific [3H]InsP3 binding and photoaffinity labelling of the Mr-107,000 protein were displaced by heparin. The Kd and the inhibition of [3H]InsP3 binding and of photoaffinity labelling by inositol phosphates and heparin are consistent with the ability of micromolar concentrations of Ins(1,4,5)P3 [but not Ins(1,3,4)P3] to activate the InsP3-gated currents in patch-clamp experiments with olfactory neurons. These results suggest that InsP3 binding to a Mr-107,000 cilia membrane protein may represent binding to the olfactory InsP3-gated cation channel.