Electrostatic domino effect in the Shaker K channel turret

Area of Science:

• Molecular Biology
• Biophysics
• Ion Channel Physiology

Background:

• Voltage-gated potassium (K+) channels are crucial for cellular electrical excitability.
• Extracellular divalent cations, like magnesium (Mg2+), modulate K+ channel gating mechanisms.
• Different K+ channel subtypes exhibit varying sensitivities to divalent cations, impacting their physiological working ranges.

Purpose of the Study:

• To identify critical amino acid residues responsible for Mg2+-induced shifts in the conductance-voltage (G-V) relationship of K+ channels.
• To investigate how specific residue substitutions in Shaker K+ channels alter their response to Mg2+.
• To elucidate the mechanism by which surface charges modulate K+ channel gating and working range.

Main Methods:

• Site-directed mutagenesis was used to introduce Kv2.1 residues into the Shaker K+ channel.
• K+ channels were expressed in Xenopus laevis oocytes.
• Two-electrode voltage-clamp electrophysiology was employed to measure channel activity and G-V relationships.

Main Results:

• Three neutral-to-positive amino acid substitutions in extracellular loops (S5-S6) conferred Mg2+-dependent gating shifts.
• The effects of these residue exchanges were additive, with residues 425 and 419 showing the most significant impact.
• Mutations at positions 425 and 419 shifted the G-V relationship by 17 mV, altering the channel's working range.
• Residue 425 was found to influence K427, initiating an electrostatic cascade affecting the S4 voltage sensor.

Conclusions:

• Specific extracellular residues, particularly at positions 425 and 419, are critical determinants of Mg2+ sensitivity in K+ channels.
• Strategic placement of surface charges can significantly modulate K+ channel gating and physiological operating parameters.
• An electrostatic 'domino effect' initiated by residue 425 explains the mechanism of Mg2+-induced gating modulation.