Mechanism of inactivation in brain potassium channels: Molecular gates and shutters
Inactivation and recovery from inactivation of Kv4 voltage-gated potassium channels are thought to regulate the interpulse interval in neurons that exhibit repetitive firing. Earlier structural and pharmacological studies suggested that Kv4 inactivation is determined by the hydrophilic N-terminal region and the hydrophobic core region. However, the process by which these channel structures influence inactivation remained unknown. In this study, I attempted to understand the mechanisms of inactivation of Kv4.1, a Kv4 channel restricted to the brain. The work mainly involved generating mutations within the channel polypeptide, recording functional data by electrophysiological methods, and examining associated changes in channel gating behavior. To begin, I applied diagnostic tests for inactivation mechanisms found in the Shaker subfamily Kv4.1 channels and found the results to be inconsistent with their presence in Kv4.1. Deletions at the N- and C-terminal regions both eliminated fast inactivation, and the effects of the terminal deletions were not additive, suggesting that these regions act in concert in mediating fast inactivation. Specific substitutions within the terminal regions inhibited channel expression possibly by interfering with normal physical interaction between the termini. Consistent with the idea of terminal interaction, a peptide corresponding to the distal N-terminal domain suppressed channel expression unless a particular C-terminal domain was removed. To examine the slower processes of inactivation, specific mutations (V404I/V406I and V406I) were introduced at sites possibly involved in the slower processes. These mutations, as well as another mutation (C322S) also putatively located in the inner vestibule, dramatically slowed inactivation. The slowed inactivation in these mutants was associated with slowed channel closing and/or slowed entry into closed-state inactivation. These results suggested that Kv4.1 slow inactivation is a consequence of channel closing and entry into closed-state inactivation. In conclusion, the process of inactivation in Kv4 channels progresses with a short residency in fast inactivated state, followed by rapid transitions between open and closed states and eventual absorbance into the closed-inactivated state. A working structural model includes a cytoplasmic inactivation gate maintained by the termini and a dynamic inner vestibule prepared to undergo transitions between open, closed, and inactivated conformations.
Jerng, Henry Hungtao, "Mechanism of inactivation in brain potassium channels: Molecular gates and shutters" (1998). ETD Collection for Thomas Jefferson University. AAI9829080.