THE INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR - KINETIC-PROPERTIES AND REGULATION

Inositol 1,4,5-trisphosphate (InsP(3)) is a second messenger responsib le for the mobilization of intracellular Ca2+ after receptor-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate. InsP(3) binds to a specific receptor located on the membrane of an intracellular compar tment and opens a Ca2+ channel causing the cytosolic Ca2+ concentratio n to increase. Measurement of radiolabelled InsP(3) binding and InsP(3 )-induced Ca2+ release in parallel experiments indicated that the live r InsP(3) receptor exists in two main states: an active state (A) and an inactive one (I). The ''I'' form of the receptor is found in the pr esence of high Ca2+ concentrations (above 1 mu M). The binding propert ies of the ''A'' and the ''I'' states of the receptor have been charac terized by analysing a membrane fraction enriched in InsP(3) receptors . The inactive ''I'' state displays a high affinity (K-d = 2 nM) and s low rates of association and dissociation. The active state ''A'' of t he receptor displays complex kinetic properties. The rate of associati on and the rate of dissociation of labelled InsP, are rapid phenomena probably involving several components. The apparent K-d for the InsP(3 ) binding is about 40 nM in a low Ca2+ medium. The affinity of the ''A '' state of the receptor is increased by Ca2+ (at concentrations lower than 0.5 mu M) and by thiol reagents. The increase of the affinity of the receptor is due to a decrease of the dissociation rate constants. This lowers the threshold such that Ca2+ is released at lower concent rations of InsP(3). These data indicate that the binding of InsP(3) to its receptor is a complex phenomenon involving the transition among s everal states. These changes in the receptor properties may be involve d in the mechanisms underlying the positive and negative feedback for the InsP(3)-induced Ca2+ release found in several cell types.