| C-type inactivation of Kv channels is thought to involve conformational changes in the outer pore of the channel, culminating in partial constriction of the selectivity filter. Through biophysical characterization of an N-terminally truncated form of Kv1.5, we have demonstrated that the cytoplasmic N-terminus exerts an important regulatory role in this process. Deletion of the N-terminus significantly alters the inactivation properties of Kv1.5, resulting most remarkably in a U-shaped inactivation-voltage relationship, excessive cumulative inactivation (in which more inactivation is observed during multiple repetitive depolarization than during a continuous pulse of the same duration), and markedly enhanced voltage-dependence of recovery from inactivation. These changes were attributed to a shift in the state-dependence of inactivation in N-terminally truncated forms of Kv1.5, suggesting that the N-terminus limits the propensity for closed-state inactivation in full-length channels. Using a deletion scan of the Kv1.5 N-terminus, and analysis of several chimeric channels, we have delimited this effect to the T1 domain, a segment of the N-terminus that is very highly conserved among Kv channels.; Interactions between the Kv1.4 N-terminal inactivation domain and the pore of Kv channels have also been investigated. In experiments using Na + as the primary permeant ion, Kv1.4 and Kv1.5 channels exhibit a pronounced Na+ tail current during recovery from C-type inactivation, as channels transiently occupy one or more states that are highly permeable to Na+. In the presence of the Kv1.4 N-terminal inactivation domain, or intracellular quaternary ammonium ions, the properties of this C-type inactivated tail current are dramatically altered: the magnitude of inward tail current is reduced, and the kinetics of tail current rise and decay are slowed considerably. These observations parallel previously documented effects of the N-terminal inactivation ball on deactivation of open channels. These effects can be explained by stable binding of the N-terminal inactivation domain to C-type inactivated channels, therefore we conclude that the inner pore remains a patent receptor for the inactivation ball after the onset of C-type inactivation. These results constrain the view of conformational changes that take place during C-type inactivation at the inner pore of Kv channels. |
