Structural basis of voltage-dependent gating in BK channels.

Area of Science:

• Molecular and Cellular Neuroscience
• Ion Channel Physiology
• Structural Biology

Background:

• The calcium- and voltage-activated potassium channel (BK) is crucial for signal integration in excitable cells.
• BK channel function relies on allosteric communication between its pore, voltage sensors, and calcium-binding sites.
• The molecular mechanisms underlying BK's fast gating and voltage sensitivity remain poorly understood.

Purpose of the Study:

• To elucidate the mechanistic basis of coupling between voltage-sensing domains (VSDs) and calcium sensors in Aplysia BK.
• To investigate the structural rearrangements associated with BK channel activation and calcium binding.
• To understand the structural determinants of BK channel's unique gating properties.

Main Methods:

• Utilized cryo-electron microscopy (Cryo-EM) to determine high-resolution structures.
• Employed site-directed mutagenesis to lock VSDs in specific conformational states (activated and resting).
• Investigated structures with and without calcium to observe its effect on channel conformation.

Main Results:

• Cryo-EM structures revealed unique S4 C-terminal tilts and gating charge side-chain movements in VSDs.
• The resting state mutant (R202Q) provided a structure of the elusive fully closed BK channel.
• Observed reciprocal relationships between voltage and calcium sensors, influencing the pore domain conformation.

Conclusions:

• The study provides structural evidence for the energetic coupling between voltage and calcium sensors in BK channels.
• These findings offer a plausible mechanism for how voltage and calcium collectively regulate the pore domain and BK channel function.
• The structures illuminate the molecular basis for BK channel's role as a signal integrator.