Ion selectivity of epithelial Na channels

Area of Science:

• Biophysics
• Molecular Biology
• Physiology

Background:

• Epithelial sodium channels (ENaC) are crucial membrane proteins responsible for sodium ion transport.
• These channels exhibit remarkable selectivity, preferentially conducting Na+ ions over other cations like K+.
• Understanding ENaC selectivity is vital for comprehending epithelial transport and related physiological processes.

Purpose of the Study:

• To elucidate the molecular determinants governing the ion selectivity of epithelial sodium channels.
• To investigate the roles of specific structural features in channel conductance and ion permeation.
• To propose a model for ENaC selectivity based on biophysical and biochemical evidence.

Main Methods:

• Analysis of ion conductance properties for various ions (Na+, K+, Li+, protons, organic cations).
• Hypothesizing structural components responsible for selectivity, including charged residues and pore dimensions.
• Utilizing a simplified cartoon model to illustrate proposed selectivity mechanisms.

Main Results:

• Epithelial Na channels conduct Na+ 100-1000 times more efficiently than K+.
• Protons and Li+ are the only other ions readily permeating the channel.
• A high-field-strength anionic site (likely carboxyl residues) and a narrow pore restriction are identified as key selectivity determinants.

Conclusions:

• The channel's selectivity is attributed to a negatively charged outer mouth acting as a preliminary filter and a narrow pore limiting ion passage.
• These structural features explain the preferential conductance of Na+ and Li+ and the exclusion of larger ions and molecules.
• Further research is needed to address remaining questions regarding amiloride block, inner mouth structure, and regulation interplay.