Voltage-independent gating transitions in squid axon potassium channels

We have investigated the actions of internal and external Zn2+ on squid axon K channel ionic and gating currents. As has been noted previously, application of Zn2+ to either membrane surface substantially slowed the activation of these channels with little or no change in deactivation. Internal Zn2+ (near 200-300 nM) slowed channel activation by up to sixfold over the range of membrane voltages from -30 to +50 mV. External Zn2+ (10 mM) produced an approximate twofold slowing of activation from -40 to +40 mV. We found that the changes in ionic current activation kinetics were accompanied by less than a twofold slowing of channel-gating currents in a narrow range of potentials near -30 mV. There was, at most, only a few percent reduction of charge movement associated with Zn2+ application. We conclude that these ions interact with channel components involved in weakly voltage-dependent conformational changes. Although there are some differences in detail, the general similarity of the actions of both internal and external Zn2+ on channel function suggests that the modified channel-gating step involves amino acids accessible to both the internal and external membrane surface.