Regulation of N- and C-type inactivation of Kv1.4 by pH_o and K⁺: Evidence for transmembrane communication

Kv1.4 encodes a slowly recovering transient outward current (I_to), which inactivates by a fast N-type (intracellular ball and chain) mechanism but has slow recovery due to C-type inactivation. C-type inactivation of the NH₂-terminal deletion mutant (fKv1.4ΔN) was inhibited by 98 mM extracellular K⁺ concentration ([K⁺]_o), whereas N-type was unaffected. In 98 mM [K⁺]_o, removal of intracellular K⁺ concentration ([K⁺]_i) speeded C-type inactivation but had no effect on N-type inactivation, suggesting that C-type inactivation is sensitive to K⁺ binding to intracellular sites. C-type inactivation is thought to involve closure of the extracellular pore mouth. However, a valine to alanine mutation on the intracellular side of S6 (V561A) of fKv1.4ΔN alters recovery and results in anomalous speeding of C-type inactivation with increasing [K⁺]_o. Extracellular pH (pH_o) modulated both N-and C-type inactivation through an S5-H5 linker histidine (H508) with acidosis speeding both N- and C-type inactivation. Mutation of an extracellular lysine to a tyrosine (K532Y) slowed C-type inactivation and inhibited the pH dependence of both N- and C-type inactivation. These results suggest that mutations, [K⁺], and pH modulate inactivation through membrane-spanning mechanisms involving S6.