ATP-dependent adenophostin activation inositol 1,4,5-trisphosphate receptor channel gating -: Kinetic implications for the durations of calcium puffs in cells

The inositol 1.3,5-trisphosphate (InsP(3)) receptor (InsP(3)R) is a ligand-gated intracellular Ca2+ release channel that plays a central role in modulating cytoplasmic free Ca2+ concentration ([Ca2+](i)). The fungal metabolite adenophostin A (AdA) is a potent agonist of the InsP(3)R that is structurally different from InsP(3) and elicits distinct calcium signals in cells. We have investigated the effects of AdA and its analogues on single-channel activities of the InsP,,R in the outer membrane of isolated Xenopus laevis oocyte nuclei. InsP(3)R activated by either AdA or InsP(3) have identical channel conductance properties. Furthermore, AdA, like InsP(3), activates the channel by tuning Ca2+ inhibition of gating. However er. gating of the AdA-liganded InsP(3)R has a critical dependence on cytoplasmic ATP free acid concentration not observed for InsP(3)-liganded channels. Channel gating activated by AdA is indistinguishable from that elicited by InsP(3) in the presence of 0.5 mM ATP, although the functional affinity of the channel is 60-fold higher for AdA. However, in the absence of ATP. gating kinetics of AdA-liganded InsP,R were very different. Channel open time was reduced by 50%. resulting in substantially lower maximum open probability than channels activated by in the presence of ATP, or by InsP(3) in the presence or absence of ATP. Also, the higher Functional affinity of InsP(3)R for AdA than for InsP(3) is nearly abolished in the absence of ATP. Low affinity; AdA analogues furanophostin and ribophostin activated InsP(3)R channels with gating properties similar to those of AdA These results provide novel insights for interpretations of observed effects of Adh oil calcium signaling, including the mechanisms that determine the durations: of elementary Ca2+ release events in cells. Comparisons of single-channel gating kinetics of the InsP(3)R activated by InsP(3). AdA, and its analogues also identify molecular elements in InsP(3)R ligands that contribute to binding and activation of channel gating.