N-Terminal deletions of rKv1.4 channels affect the voltage dependence of channel availability

Rat Kv1.4 potassium channels undergo rapid inactivation, which is mediated by the N-terminal structure of the polypeptide. This inactivation can be removed by N-terminal deletion of about 20 residues. However, more substantial deletion (e.g. 37 residues) restores inactivation suggesting a second inactivating domain [Kondoh et al. J Biol Chem 272:19333-19338, 1997]. Here we provide evidence that this inactivation shares all properties with N-type inactivation. Pore mutations, which are supposed to affect C-type inactivation, have no effect. In addition, the redox regulation of inactivation, which is typical for Kv1.4 channels, can be conferred to the inactivation of the deleted constructs by incorporation of an N-terminal cysteine residue. The most remarkable feature of this secondary inactivation is the existence of two components in the steady-state voltage dependence of inactivation. For mutant rKv1. 4Delta2-37 about 90% of the channels only activate when the holding membrane potential is more negative than about -120 mV; the remaining 10% show the typical threshold at -60 mV. Mutagenesis of the truncated channel affected the relative amplitudes of these two components, but not the voltage dependence. The results suggest that the secondary ball structures of rKv1.4 channels interact with the protein structures responsible for activation.