Scanlon C.H., Martinec J., Machackova I., Rolph C.E., Lumsden P.J.: Identification and preliminary characterisation of Ca2+ - dependent high affinity binding site for inositol-1,4,5-trisphosphate in Chenopodium rubrum. Plant Physiology 1996, 110, 867-874.

Using a radioligand-binding assay we have identified a Ca²⁺- dependent high-affinity D-myo-inositol-1,4,5-trisphosphate (InsP₃) binding site in a membrane vesicle preparation from Chenopodium rubrum. Millimolar concentrations of Ca²⁺ were required to observe specific binding of [³H]InsP₃. A stable equilibrium between bound and free ligand was established within 5 min and bound [³H]InsP₃ could be completely displaced by InsP₃ in a time- and concentration-dependent manner. Displacement assays indicated a single class of binding sites with an estimated dissociation constant of 142 [plus or minus] 17 nM. Other inositol phosphates bound to the receptor with much lower affinity. The glycosaminoglycan heparin was an effective competitor for the binding site (inhibitor concentration for 50% displacement = 534 nM). ATP at higher, although physiologically relevant, concentrations (inhibitor concentration for 50% displacement = 241 μM) also displaced [³H]InsP₃ from the receptor. Recent studies in animals have highlighted the importance of Ca²⁺ regulation of InsP₃-induced Ca²⁺ release. The potential for the operation of similar regulatory mechanisms in plants is discussed.