Collapse of Conductance Is Prevented by a Glutamate Residue Conserved in Voltage-Dependent K⁺ Channels

Voltage-dependent K⁺ channel gating is influenced by the permeating ions. Extracellular K⁺ determines the occupation of sites in the channels where the cation interferes with the motion of the gates. When external [K⁺] decreases, some K⁺ channels open too briefly to allow the conduction of measurable current. Given that extracellular K⁺ is normally low, we have studied if negatively charged amino acids in the extracellular loops of Shaker K⁺ channels contribute to increase the local [K⁺]. Surprisingly, neutralization of the charge of most acidic residues has minor effects on gating. However, a glutamate residue (E418) located at the external end of the membrane spanning segment S5 is absolutely required for keeping channels active at the normal external [K⁺]. E418 is conserved in all families of voltage-dependent K⁺ channels. Although the channel mutant E418Q has kinetic properties resembling those produced by removal of K⁺ from the pore, it seems that E418 is not simply concentrating cations near the channel mouth, but has a direct and critical role in gating. Our data suggest that E418 contributes to stabilize the S4 voltage sensor in the depolarized position, thus permitting maintenance of the channel open conformation.