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 modul ating cytoplasmic free Ca2+ concentration ([Ca2+](i)). The fungal metabolit e adenophostin A (AdA) is a potent agonist of the InsP(3)R that is structur ally different from InsP(3) and elicits distinct calcium signals in cells. We have investigated tile effects of AdA and its analogues on single-channe l activities of the InsP,,R in the outer membrane of isolated Xenopus laevi s 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 aci d 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 tile 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 channel s activated by in the presence of ATP, or by InsP(3) in the presence or abs ence of ATP. Also, the higher Functional affinity of InsP(3)R for AdA than for InsP(3) is nearly abolished in tile absence of ATP. Low affinity; AdA a nalogues furanophostin and ribophostin activated InsP(3)R channels with gat ing properties similar to those of AdA These results provide novel insights for interpretations of observed effects of Adh oil calcium signaling, incl uding the mechanisms that determine the durations: of elementary Ca2+ relea se events in cells. Comparisons of single-channel gating kinetics of the In sP(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.