Elevation of external [K(+)] potentiates outward K(+) current through several voltage-gated
K(+) channels. This increase in current magnitude is paradoxical in that it occurs
despite a significant decrease in driving force. We have investigated the mechanisms
involved in K(+)-dependent current potentiation in the Kv2.1 K(+) channel. With holding
potentials of -120 to -150 mV, which completely removed channels from the voltage-sensitive
inactivated state, elevation of external [K(+)] up to 10 mM produced a concentration-dependent
increase in outward current magnitude. In the absence of inactivation, currents were
maximally potentiated by 38%. At more positive holding potentials, which produced
steady-state inactivation, K(+)-dependent potentiation was enhanced. The additional
K(+)-dependent potentiation (above 38%) at more positive holding potentials was precisely
equal to a K(+)-dependent reduction in steady-state inactivation. Mutation of two
lysine residues in the outer vestibule of Kv2.1 (K356 and K382), to smaller, uncharged
residues (glycine and valine, respectively), completely abolished K(+)-dependent potentiation
that was not associated with inactivation. These mutations did not influence steady-state
inactivation or the K(+)-dependent potentiation due to reduction in steady-state inactivation.
These results demonstrate that K(+)-dependent potentiation can be completely accounted
for by two independent mechanisms: one that involved the outer vestibule lysines and
one that involved K(+)-dependent removal of channels from the inactivated state. Previous
studies demonstrated that the outer vestibule of Kv2.1 can be in at least two conformations,
depending on the occupancy of the selectivity filter by K(+) (Immke, D., M. Wood,
L. Kiss, and S. J. Korn. 1999. J. Gen. Physiol. 113:819-836; Immke, D., and S. J.
Korn. 2000. J. Gen. Physiol. 115:509-518). This change in conformation was functionally
defined by a change in TEA sensitivity. Similar to the K(+)-dependent change in TEA
sensitivity, the lysine-dependent potentiation depended primarily (>90%) on Lys-356
and was enhanced by lowering initial K(+) occupancy of the pore. Furthermore, the
K(+)-dependent changes in current magnitude and TEA sensitivity were highly correlated.
These results suggest that the previously described K(+)-dependent change in outer
vestibule conformation underlies the lysine-sensitive, K(+)-dependent potentiation
mechanism.