Diagram of proposed ρ1 GABA receptor subunit topology and transmembrane domain sequences. (A) Endogenously charged residues are indicated by the appropriate charge sign and these residues were neutralized in this study to examine the role of endogenous charges on ionic selectivity. The TM2 numbering system used in the manuscript begins with the 0′ TM2 arginine with increasing numbers proceeding toward the C terminus. Charge-scanning mutants covered the region from −12′ (intracellular valine, V) to 20′ (extracellular alanine, A) and are indicated with gray-filled circles. This figure is to merely show the extent of the residues examined in this study and the secondary structure or positions of these residues with respect to the membrane should not be taken literally. (B) Aligned TM1–TM2 linker and TM2 sequences from several representative members of the receptor family. Anionic members are above the dashed line, cationic members are below. The GABA ρ1 receptor sequence is human, all others are rat.