Enhanced closed-state inactivation in a mutant Shaker K+ channel

Area of Science:

• Molecular and Cellular Neuroscience
• Ion Channel Physiology
• Biophysics

Background:

• Shaker channels are potassium channels crucial for neuronal excitability.
• Mutations affecting voltage-gated ion channel activation often impact inactivation similarly.
• The I2 mutation (L382I) in Shaker channels presents an atypical voltage-dependence profile.

Purpose of the Study:

• To investigate the distinct gating properties of the I2 mutant Shaker channel compared to wildtype (WT).
• To elucidate the kinetic mechanisms underlying the altered activation and inactivation in the I2 mutant.
• To propose a kinetic model explaining the unique behavior of the I2 Shaker channel.

Main Methods:

• Utilized macropatch recordings in Xenopus oocytes to study WT and I2 Shaker 29-4 channel function.
• Applied kinetic modeling, including a novel "multiple-state inactivation" model, to analyze channel behavior.
• Compared experimental data with predictions from established and new kinetic models.

Main Results:

• The I2 mutation caused a significant 45 mV shift in activation but only a 9 mV shift in inactivation midpoint.
• WT channel inactivation was accurately modeled assuming it occurs only from the open state.
• I2 channel inactivation required the inclusion of closed states, a characteristic typically seen in voltage-gated sodium channels.

Conclusions:

• The I2 mutation disrupts the coupled relationship between activation and inactivation in Shaker channels.
• Inactivation in I2 mutant channels can occur from closed states, suggesting a novel gating mechanism.
• Residue L382 is implicated as a critical component interacting with inactivation particles in Shaker channels.