Coupling between voltage sensor activation, Ca2+ binding and channel opening in large conductance (BK) potassium...

Area of Science:

• Molecular Biology
• Biophysics
• Ion Channel Physiology

Background:

• Large conductance Ca(2+)-activated K(+) (BK) channels are crucial for regulating membrane potential and cellular excitability.
• BK channel gating is influenced by both intracellular calcium (Ca2+) and membrane voltage.
• Previous models suggest allosteric coupling between voltage sensor activation and channel gating.

Purpose of the Study:

• To elucidate the distinct roles of intracellular Ca2+ and membrane voltage in regulating BK channel (mSlo1) gating.
• To investigate the steady-state and kinetic properties of mSlo1 ionic and gating currents under varying Ca2+ concentrations and voltages.
• To determine the interplay between Ca2+ binding and voltage sensor activation in BK channel opening.

Main Methods:

• Examined steady-state and kinetic properties of mSlo1 ionic and gating currents.
• Utilized patch-clamp electrophysiology in the presence and absence of Ca2+.
• Analyzed charge-voltage (Q-V) and conductance-voltage (G-V) relationships across a wide voltage range.

Main Results:

• In the absence of Ca2+, the Q-V relationship is shallower and shifted negatively compared to the G-V relationship.
• Ca2+ shifts both Q-V and G-V relationships to more negative voltages, leading to their near-superimposition at 70 μM Ca2+.
• Ca2+ minimally affects voltage sensor activation but significantly increases the closed-to-open equilibrium, particularly at negative voltages.

Conclusions:

• Ca2+ binding and voltage sensor activation act largely independently to enhance BK channel opening.
• A dual-allosteric mechanism, with additive effects on the closed-to-open transition energy, explains steady-state BK channel behavior.
• Complex energy landscapes likely govern the Ca2+ and voltage-dependent rates of the closed-to-open conformational change.