Functional interaction between InsP(3) receptors and store-operated Htrp3 channels

Calcium ions are released from intracellular stores in response to agonist- stimulated production of inositol 1,4,5-trisphosphate (InsP(3)), a second m essenger generated at the cell membrane. Depletion of Ca2+ from internal st ores triggers a capacitative influx of extracellular Ca2+ across the plasma membrane(1,2). The influx of Ca2+ can be recorded as store-operated channe ls (SOC) in the plasma membrane or as a current known as the Ca2+-release-a ctivated current (I-crac)(3-5). A critical question in cell signalling is h ow SOC and I-crac sense and respond to Ca2+-store depletion: in one model, a messenger molecule is generated that activates Ca2+ entry in response to store depletion(1,6); in an alternative model(7), InsP(3) receptors in the stores are coupled to SOC and I-crac. The mammalian Htrp3 protein(8) forms a well defined store-operated channel(8,9) and so provides a suitable syste m for studying the effect of Ca2+-store depletion on SOC and I-crac. We sho w here that Htrp3 channels stably expressed in HEK293 cells are in a tight functional interaction with the InsP(3) receptors. Htrp3 channels present i n the same plasma membrane patch can be activated by Ca2+ mobilization in i ntact cells and by InsP(3) in excised patches. This activation of Htrp3 by InsP(3) is lost on extensive washing of excised patches but is restored by addition of native or recombinant InsP(3)-bound InsP(3) receptors. Our resu lts provide evidence for the coupling hypothesis(7), in which InsP(3) recep tors activated by InsP(3) interact with SOC and regulate I-crac.