RELEASE of intracellular Ca2+ by inositol 1,4,5-trisphosphate (InsP3) occurs through specific receptor proteins' which are ligand-activated Ca2+ channels2. Changes in intracellular Ca2+ regulate many cellular functions3. This Ca2+ release is a discontinuous quantal process in which successive increments of InsP3 transiently release precise amounts of Ca2+ (refs 4-6). Possible explanations of quantal Ca2+ release have included rapid degradation of InsP3, reciprocity of Ca2+ release and sequestration, desensitization of InsP3 receptors7, or actions of InsP3 on discrete compartments of Ca2+ with variable sensitivity to InsP3 (ref. 4). We successfully reconstituted InsP3-induced Ca2+ flux in vesicles containing only purified InsP3 receptor protein2. The reconstituted vesicles retain the regulatory features of the InsP3 receptor, including phosphorylation sites8 and modulation of Ca2+ release by adenine nucleotides9. Using these reconstituted vesicles, we show here that quantal flux of Ca2+ elicited by InsP3 is a fundamental property of its receptor.
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