pH-Dependent Conformational Changes of KcsA Tetramer and Monomer Probed by Raman Spectroscopy.

Area of Science:

• Biophysics
• Structural Biology
• Molecular Biology

Background:

• KcsA is a model tetrameric potassium channel crucial for selective ion transport.
• Its pH-dependent gating mechanism and monomer conformational changes upon gating are debated.
• Previous studies on KcsA monomer response during gating present conflicting results.

Purpose of the Study:

• To investigate the conformational states of KcsA potassium channel tetramers and monomers.
• To clarify the monomer's conformational response during channel gating under physiological conditions.
• To determine if KcsA monomer conformation is pH-dependent.

Main Methods:

• Confocal Raman microscopy was employed to analyze KcsA tetramers in liposomes and individual monomer subunits.
• Measurements were performed under physiological conditions for both active and closed channel states.
• Channel function was validated using a liposome flux assay, and Raman spectra were acquired using 532 nm excitation.

Main Results:

• The classic fingerprint region (<1800 cm⁻¹) of the KcsA tetramer's Raman spectrum remained unchanged.
• Significant alterations were observed in the CH-stretching region (2800–3200 cm⁻¹) of the Raman spectrum, correlating with conformational changes.
• Raman spectra of KcsA monomer subunits showed no pH dependency and closely matched the tetramer's active conformation spectrum.

Conclusions:

• The KcsA monomer's relaxed state appears to correspond to its open conformation, challenging previous postulates.
• The observed monomer conformation resembles the active state of the KcsA tetramer.
• Confocal Raman microscopy provides insights into KcsA channel gating mechanisms at the molecular level.