A novel extracellular calcium sensing mechanism in voltage-gated potassium ion channels

Potassium (K+) channels influence neurotransmitter release, burst firing rate activity, pacing, and critical dampening of neuronal circuits. Internal and external factors that further modify K+ channel function permit fine-tuning of neuronal circuits. Human ether-a-go-go-related gene (HERG) K+ channels are unusually sensitive to external calcium concentration ([Ca2+](o)). Small changes in [Ca2+](o) shift the voltage dependence of channel activation to more positive membrane potentials, an effect that cannot be explained by nonspecific surface charge screening or channel pore block. The HERG-calcium concentration-response relationship spans the physiological range for [Ca2+](o). The modulatory actions of calcium are attributable to differences in the Ca2+ affinity between rested and activated channels. Adjacent extracellular, negatively charged amino acids (E518 and E519) near the S4 voltage sensor influence both channel gating and Ca2+ dependence. Neutralization of these charges had distinct effects on channel gating and calcium sensitivity. A change in the degree of energetic coupling between these amino acids on transition from closed to activated channel states reveals movement in this region during channel gating and defines a molecular mechanism for protein state-dependent ligand interactions. The results suggest a novel extracellular [Ca2+](o) sensing mechanism coupled to allosteric changes in channel gating and a mechanism for fine-tuning cell repolarization.