Cyclic-nucleotide-gated channels: pore topology in desensitizing E19A mutants

Area of Science:

• Molecular biology
• Ion channel biophysics
• Cellular physiology

Background:

• Cyclic-nucleotide-gated (CNG) channels are crucial for sensory transduction.
• The pore-loop (P-loop) region is critical for ion conduction and channel gating.
• Extracellular divalent cations interact with specific residues, influencing channel function.

Purpose of the Study:

• To investigate the topology of the P-loop in mammalian rod CNG channels.
• To determine if P-loop structure changes during desensitization induced by an E19A substitution.
• To understand the role of residue E19 in channel gating and ion permeation.

Main Methods:

• Serial cysteine mutagenesis was employed to introduce cysteine residues into the P-loop.
• Thiol-specific reagents (Cd2+, MTSET, MTSEA) were used to probe accessibility from the cytoplasmic and extracellular sides.
• Channel function and desensitization kinetics were analyzed in wild-type and mutant channels.

Main Results:

• The E19A substitution in CNG channels led to slower desensitization in several mutants (T15C, T16C, I17C, T20C).
• The accessibility pattern of the P-loop from the cytoplasmic side remained largely unchanged with the E19A substitution.
• Residues C-terminal to E19 were accessible from the extracellular side even after the E19A substitution.

Conclusions:

• The E19A substitution in CNG channels influences desensitization kinetics but does not induce major P-loop structural rearrangements.
• The P-loop maintains accessibility from the extracellular side despite the E19A substitution.
• Residues T15-T20 appear to be involved in structural changes during desensitization, but the overall P-loop architecture remains stable.