Anion permeation in Ca(2+)-activated Cl(-) channels.

Area of Science:

• Molecular Biology
• Ion Channel Physiology
• Biophysics

Background:

• Calcium(2+)-activated chloride channels (Cl(Ca)Cs) are crucial for cellular processes, responding to elevated intracellular calcium.
• Understanding their ion permeation mechanisms is key to elucidating their physiological roles.

Purpose of the Study:

• To investigate the biophysical properties of anion permeation through Cl(Ca)Cs in Xenopus oocytes.
• To characterize the pore diameter, ion selectivity, and voltage-dependent block of Cl(Ca)Cs.
• To explore the influence of pH and permeant anions on channel function and calcium affinity.

Main Methods:

• Utilized excised inside-out and outside-out membrane patches from Xenopus oocytes.
• Performed electrophysiological recordings to measure ion permeation and channel block.
• Analyzed current-voltage relationships and employed bi-ionic conditions to determine relative ion permeabilities and affinities.

Main Results:

• Cl(Ca)Cs demonstrated moderate chloride selectivity over sodium (P(Na)/P(Cl) = 0.1) and a low apparent affinity for chloride (Kd = 73 mM).
• The pore diameter was estimated to be greater than 0.6 nm, with a specific sequence of anion relative permeabilities and conductances observed.
• Permeant anions caused voltage-dependent block, and channel affinity for calcium increased with anion occupancy. pH significantly affected channel current, indicating potential proton titration or hydroxyl ion block.

Conclusions:

• Anion selectivity is influenced by hydration energy but not solely determined by it.
• Cl(Ca)Cs may function as multi-ion pores with weak ion interactions.
• The distinct permeation properties of Cl(Ca)Cs differentiate them from CFTR and ClC-1 channels, providing novel insights into their pore structure and function.