The extracellular disulfide loop motif of the inhibitory glycine receptor does not form the agonist binding site

Inhibitory (glycine and gamma-aminobutyric acid type A) and excitatory (nicotinic acetylcholine and serotonin 5-hydroxytryptamine type 3) receptors of the ligand-gated ion channel superfamily are related by both structural similarities and primary sequence identity. One invariant feature of all members of this receptor superfamily is the presence of an extracellular disulfide loop motif. This structural motif has been modeled, and Cockcroft et al. [Proteins 8:386-397 (1990)] have suggested that it forms the agonist binding site of the ligand-gated ion channel receptors. Using site-directed mutagenesis of the inhibitory glycine receptor (GlyR), we have specifically tested this hypothesis. The lysine residue at position 143 is proposed to form the binding site for the negatively charged carboxyl group of the agonist glycine. Differing residues at this position in other ligand-gated receptors are proposed to confer agonist specificity. The aspartic acid residue at position 148 is an invariant residue in every known subunit of the ligand-gated ion channel receptor superfamily. This residue has been proposed as the binding site for the positively charged amino group of the various agonists. Mutation of the lysine at position 143 to alanine resulted in essentially unaltered GlyRs, showing only modest decreases in strychnine affinity (Kd, 8.1 +/- 1.4 nM versus 13.4 +/- 1.3 nM), glycine displacement of strychnine binding (Ki, 25 +/- 5 microM versus 49 +/- 9 microM), and glycine activation of chloride currents (EC50, 27 +/- 6 microM versus 114 +/- 14 microM). Thus, we conclude that Lys-143 does not play a major role in either agonist or antagonist binding or agonist activation of the GlyR. Mutation of Asp-148 to either alanine or asparagine disrupted the expression and/or assembly of the receptor, and no binding sites or ion channels were expressed on the cell surface. The conservative mutation of the aspartic acid at position 148 to glutamic acid (D148E) allowed the expression of receptors, although with reduced efficiency. The D148E GlyRs showed a 1 order of magnitude decrease in strychnine affinity (Kd, 8.1 +/- 1.4 nM versus 82 +/- 21 nM), without any change in the glycine displacement of strychnine binding (Ki, 25 +/- 5 microM versus 29 +/- 8 microM) or glycine activation of chloride currents (EC50, 27 +/- 6 microM versus 20 +/- 1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)