Inactivation gating and 4-AP sensitivity in human brain Kv1.4 potassium channel

Voltage-gated K+ channels vary in sensitivity to block by 4-aminopyridine ( 4-AP) over a 1000-fold range. Most K+ channel phenotypes with leucine at th e fourth position (L4) in the leucine heptad repeat region. spanning the S4 -S5 linker, exhibit low 4-AP sensitivity, while channels with phenylalanine exhibit high sensitivity. Mutational analysis on delayed rectifier type K channels demonstrate increased 4-AP sensitivity upon mutation of the L4 he ptad leucine to phenylalanine. This mutation can also influence inactivatio n gating, which is known to compete with 4-AP in rapidly inactivating A-typ e K+ channels. Here, in a rapidly inactivating human brain Kv1.4 channel, w e demonstrate a 400-fold increase in 4-AP sensitivity following substitutio n of L4 with phenylalanine. Accompanying this mutation is a slowing of inac tivation, an acceleration of deactivation, and depolarizing shifts in the v oltage dependence of activation and steady-state inactivation. To test the relative role of fast inactivation in modulating 4-AP block, N-terminal del etions of thr: fast inactivation gate were carried out in both channels. Th ese deletions produced no change in 4-AP sensitivity in the mutant channel and approximately a six-fold increase in the wild type channel. These resul ts support the view that changes at L4 which increase 4-AP sensitivity are largely due to 4-AP binding and may, in part, arise from alterations in cha nnel conformation. Primarily, this study demonstrates that the fast inactiv ation gate is not a critical determinant of 4-AP sensitivity in Kv1.4 chann els. (C) 1999 Elsevier Science B.V. All rights reserved.