AFFINITY LABELING OF CYSTEINE-MUTANTS EVIDENCES CONTACT RESIDUES IN MODELED RECEPTOR BINDING SITES

ABSTRACT

To investigate the topology of binding sites in two ionotropic receptors, we have initiated a strategy combining affinity labeling with cysteine-scanning mutagenesis. For the GABA_A receptor we have used reactive derivatives of non-competitive blockers (NCBs) to explore interacting positions in its channel. The polypeptide positions of the M₂ segment of the α₁ subunit which we mutated into cysteine were selected for their established accessibility, as determined by the substituted-cysteine accessibility method (SCAM). Using the Xenopus oocyte expression system, we show that receptors containing mutations V257C and S272C are inactivated by several reactive NCBs. These position-selective inactivations lead to an analysis of NCB binding in the channel. For the NMDA receptor glycine-binding site, the prototype antagonist L-701,324 was derivatized at different positions with different reactive groups. The receptor positions to mutate into cysteine were selected after a 3-D homology model. The observed receptor inactivations are mutant- and probe-selective, leading to an unambiguous chemical docking of the antagonist pharmacophore and supporting the model. The site-specificity of the inactivating reactions is assessed by protection experiments and by mutant to wild-type (WT) comparisons. The scope and limitations of the method are briefly discussed.