Mutational studies using a cation-conducting GABAA receptor reveal the selectivity determinants of the Cys-loop family of ligand-gated ion channels.

The present study evaluates how four key amino acid residue positions (- 4' to - 1') within the M1-M2 linker of the GABA(A) receptor beta subunit influences ion selectivity of a cation-conducting GABA receptor. Cation selectivity was found to be highly dependent on the side-chains of the amino acid residues present. The critical factor for cation selectivity was the presence of a negatively charged Glu or Asp residue in the -1' position. Receptors containing the neutral amino acids Gln or Asn or a positively charged Arg residue were anion selective. In the presence of a -1' Glu residue, the amino acids in adjacent positions were also found to be important determinants of cation selectivity. Moreover, the length of the M1-M2 linker as well as the presence of a Pro residue within this segment also affected ion selectivity, suggesting that the local environment and three-dimensional position of the -1' Glu are essential determinants of cation permeation. Conversely, no specific amino acid residues were found to be essential for anion selectivity, suggesting that the basic architecture of the selectivity segment of this class of receptor channels is optimally suited for anion conduction.