A single amino acid in the second transmembrane domain of GABA ρ receptors regulates channel conductance

GABAC receptors, expressed predominately in vertebrate retina, are thought to be formed mainly by GABA ρ subunits, each of which exhibits distinct physiological and pharmacological properties. In this study, the receptors formed by perch GABA ρ subunits were expressed in HEK cells, and their single channel conductances were determined using noise analysis techniques. The receptors formed by the perch ρ1A subunit gate a channel with a conductance of 0.2 pS, whereas the receptors formed by GABA ρ2 subunits exhibit much higher channel conductances, i.e., 3.2 and 3.5 pS for perch ρ2A and ρ2B receptors, respectively. A comparison of the amino acid sequences of the channel-forming TMII regions of the various subunits suggested that a single amino acid at position 2′ was a potential site for the large differential in conductance. We found that switching the serine residue at that site in the GABA ρ2 subunit to the proline residue present in the ρ1 subunit reduced the channel conductance to a level similar to that of the wild type ρ1 receptor. Conversely, mutating proline to serine in the amino acid sequence of the ρ1 receptor significantly increased its unitary conductance. These results indicate that a single amino acid in the TMII region plays an important role in determining the single channel conductance of the GABAC receptors.