GABA(A) receptor M2-M3 loop secondary structure and changes in accessibility during channel gating

Area of Science:

• Neuroscience
• Molecular Biology
• Biochemistry

Background:

• The gamma-aminobutyric acid type A (GABA(A)) receptor is a crucial ligand-gated ion channel.
• Understanding its M2-M3 loop structure is key to comprehending receptor gating mechanisms.

Purpose of the Study:

• To investigate the structure of the GABA(A) receptor M2-M3 loop.
• To elucidate the role of this loop in channel gating.

Main Methods:

• Substituted cysteine accessibility method was employed.
• Specific residues within the M2-M3 loop (alpha(1)Arg-273 to alpha(1)Ile-289) were mutated to cysteine.
• Reactions with MTSET(+) and MTSES(-) were analyzed in the absence and presence of GABA.

Main Results:

• Residues alpha(1)R273C to alpha(1)Y281C showed differential reactivity with MTSET(+) and MTSES(-).
• A subset of residues (alpha(1)N274C, alpha(1)S275C, alpha(1)K278C, alpha(1)Y281C) exhibited fast reaction rates, suggesting proximity to the channel.
• GABA presence accelerated reaction rates at alpha(1)S275C, alpha(1)V279C, alpha(1)A280C, and alpha(1)A284C, indicating GABA-induced conformational changes.

Conclusions:

• The M2 segment's alpha-helix extends beyond its predicted extracellular end.
• Gating involves conformational changes in the N-terminal half of the M2-M3 loop.
• These findings provide insights into coupling extracellular ligand binding to membrane-spanning domain channel gating.