NMR-Derived Dynamic Aspects of N-Type Inactivation of a Kv Channel Suggest a Transient Interaction with the T1 Domain

Some eukaryotic voltage-gated K+ (Kv) channels contain an N-terminal inactivation peptide (IP), which mediates a fast inactivation process that limits channel function during membrane depolarization and thus shapes the action potential. We obtained sequence-specific nuclear magnetic resonance (NMR) assignments for the polypeptide backbone of a tetrameric N-terminal fragment (amino acids 1-181) of the Aplysia Kv1.1 channel. Additional NMR measurements show that the tetramerization domain 1 (T1) has the same globular structure in solution as previously determined by crystallography and that the IP (residues 1-20) and the linker (residues 21-65) are in a flexibly disordered, predominantly extended conformation. A potential contact site between the T1 domain and the flexible tail (residues 1-65) has been identified on the basis of chemical-shift changes of individual T1 domain amino acids, which map to the T1 surface near the interface between adjacent subunits. Paramagnetic perturbation experiments further indicate that, in the ensemble of solution conformers, there is at least a small population of species with the IP localized in close proximity to the proposed interacting residues of the T1 tetramer. Electrophysiological measurements show that all three mutations in this pocket that we tested slow the rate of inactivation and speed up recovery, as predicted from the preinactivation site model. These results suggest that specific, short-lived transient interactions between the T1 domain and the IP or the linker segment may play a role in defining the regulatory kinetics of fast channel inactivation.