A binding-block ion selective mechanism revealed by a Na/K selective channel

Area of Science:

• Biophysics
• Structural Biology
• Membrane Protein Research

Background:

• Mechanosensitive (MS) channels are crucial for cellular homeostasis, regulating ion and solute transport across membranes.
• However, the precise mechanisms governing MS channel gating and ion selectivity remain largely unelucidated.
• Understanding these channels is vital for comprehending cellular responses to mechanical stimuli.

Purpose of the Study:

• To identify and characterize the YnaI channel, a novel sodium/potassium (Na+/K+) cation-selective MS channel.
• To determine the structural basis of YnaI channel's ion selectivity and gating mechanisms.
• To elucidate the role of accompanying anions in modulating channel function.

Main Methods:

• Cryo-electron microscopy (cryo-EM) single-particle analysis to solve the YnaI channel structure at 3.8 Å resolution.
• Structure-based mutagenesis to investigate the role of specific amino acid residues.
• Quantum mechanical simulations and electrophysiological recordings for functional characterization.
• Anion-binding assays to assess interactions within the channel pore.

Main Results:

• The YnaI channel, a heptameric protein, shares structural similarities with MscS channels but exhibits lower conductance.
• A selectivity filter formed by seven hydrophobic methionine residues (YnaI Met158) dictates Na+/K+ selectivity.
• Both ion selectivity and gating are significantly influenced by the presence and type of accompanying anions in the solution.
• A novel 'binding-block' mechanism, involving distinct anion binding energies to YnaI Met158, facilitates Na+/K+ permeation.

Conclusions:

• The YnaI channel represents a unique Na+/K+ selective MS channel with a distinct structural fold.
• The hydrophobic methionine residues in the pore act as a critical ion selectivity filter.
• Anions play a crucial role in modulating MS channel gating and selectivity through a binding-block mechanism.
• This study offers new insights into the fundamental mechanisms of ion permeation and selectivity in mechanosensitive channels.