Extracellular Mg2+ modulates slow gating transitions and the opening of Drosophila ether-a-go-go potassium channels

We have characterized the effects of prepulse hyperpolarization and extrace llular Mg2+ on the ionic and gating currents of the Drosophila ether-a-go-g o K+ channel (eag). Hyperpolarizing prepulses significantly slowed channel opening elicited by a subsequent depolarization, revealing rate-limiting tr ansitions for activation of the ionic currents. Extracellular Mg2+ dramatic ally slowed activation of eag ionic currents evoked with or without prepuls e hyperpolarization and regulated the kinetics of channel opening from a ne arby closed state(s). These results suggest that Mg2+ modulates voltage-dep endent gating and pore opening in eag channels. To investigate the mechanis m of this modulation, eag gating currents were recorded using the cut-open oocyte voltage clamp. Prepulse hyperpolarization and extracellular Mg2+ slo wed the time course of ON gating currents. These kinetic changes resembled the results at the ionic current level, but were much smaller in magnitude, suggesting that prepulse hyperpolarization and Mg2+ modulate gating transi tions that occur slowly and/or move relatively little gating charge. To det ermine whether quantitatively different effects on ionic and gating current s could be obtained from a sequential activation pathway, computer simulati ons were performed. Simulations using a sequential model for activation rep roduced the key features of eag ionic and gating currents and their modulat ion by prepulse hyperpolarization and extracellular Mg2+. We have also iden tified mutations in the S3-S4 loop that modify or eliminate the regulation of eag gating by prepulse hyperpolarization and Mg2+, indicating an importa nt role for this region in the voltage-dependent activation of eag.