Subunit interface selectivity of the α-neurotoxins for the nicotinic acetylcholine receptor

Peptide toxins selective for particular subunit interfaces of the nicotinic acetylcholine receptor have proven invaluable in assigning candidate residues located in the two binding sites and for determining probable orientations of the bound peptide. We report here on a short α- neurotoxin from Naja mossambica mossambica (NmmI) that, similar to other α- neurotoxins, binds with high affinity to αγ and αδ subunit interfaces (K(D)~100 pM) but binds with markedly reduced affinity to the αε interface (K(D)~100 nM). By constructing chimeras composed of portions of the γ and ε subunits and coexpressing them with wild type α, β, and δ subunits in HEK 293 cells, we identify a region of the subunit sequence responsible for the difference in affinity. Within this region, γPro-175 and γGlu-176 confer high affinity, whereas Thr and Ala, found at homologous positions in ε, confer low affinity. To identify an interaction between γGlu-176 and residues in NmmI, we have examined cationic residues in the central loop of the toxin and measured binding of mutant toxin-receptor combinations. The data show strong pairwise interactions or coupling between γGlu-176 and Lys- 27 of NmmI and progressively weaker interactions with Arg-33 and Arg-36 in loop II of this three-loop toxin. Thus, loop II of NmmI, and in particular the face of this loop closest to loop III, appears to come into close apposition with Glu-176 of the γ subunit surface of the binding site interface.