NH₂-terminal Inactivation Peptide Binding to C-type–inactivated Kv Channels

In many voltage-gated K⁺ channels, N-type inactivation significantly accelerates the onset of C-type inactivation, but effects on recovery from inactivation are small or absent. We have exploited the Na⁺ permeability of C-type–inactivated K⁺ channels to characterize a strong interaction between the inactivation peptide of Kv1.4 and the C-type–inactivated state of Kv1.4 and Kv1.5. The presence of the Kv1.4 inactivation peptide results in a slower decay of the Na⁺ tail currents normally observed through C-type–inactivated channels, an effective blockade of the peak Na⁺ tail current, and also a delay of the peak tail current. These effects are mimicked by addition of quaternary ammonium ions to the pipette-filling solution. These observations support a common mechanism of action of the inactivation peptide and intracellular quaternary ammonium ions, and also demonstrate that the Kv channel inner vestibule is cytosolically exposed before and after the onset of C-type inactivation. We have also examined the process of N-type inactivation under conditions where C-type inactivation is removed, to compare the interaction of the inactivation peptide with open and C-type–inactivated channels. In C-type–deficient forms of Kv1.4 or Kv1.5 channels, the Kv1.4 inactivation ball behaves like an open channel blocker, and the resultant slowing of deactivation tail currents is considerably weaker than observed in C-type–inactivated channels. We present a kinetic model that duplicates the effects of the inactivation peptide on the slow Na⁺ tail of C-type–inactivated channels. Stable binding between the inactivation peptide and the C-type–inactivated state results in slower current decay, and a reduction of the Na⁺ tail current magnitude, due to slower transition of channels through the Na⁺-permeable states traversed during recovery from inactivation.