Isolation and structure-functional characterization of phage display library-derived mimotopes of noxiustoxin, a neurotoxin of the scorpion Centruroides noxius Hoffmann.

Noxiustoxin (NTX) is a short-chain toxin from the venom of the scorpion Centruroides noxius Hoffmann, whose molecular structure and physiological effects have been characterized in detail, whereas the antigenic properties of this and other K(+) channel-blocking toxins are poorly studied. A monoclonal antibody against NTX, BNTX18, able to inhibit the binding of NTX to rat brain synaptosomes, was used in the present study for selecting immunoreactive peptides, mimotopes, from a 12mer and a 7mer phage library. The peptides were characterized immunologically and used for mapping the epitope on NTX. In total, 75 phage clones carrying 43 different peptides were analyzed of which 42 clones carrying 17 different peptides, twelve 12mer and five 7mer peptides, presented a single consensus motif: Leu(Ile, Val)-Tyr(Phe, Trp, Leu)-Gly-Met(Ala). All but three of the peptides containing this motif were reactive with selected mAb BNTX18 in a dot-blot assay of which eight were clearly positive in ELISA and exhibited in competition-inhibition assay the antibody binding specificity of the NTX epitope recognized by BNTX18. The two most reactive mimotopes injected into mice showed the ability to induce antibodies reacting with NTX, thus, to mimic the epitope of NTX antigenically. Sequence comparison and the analysis of the three-dimensional structure of NTX led to the proposal that residues Glu19-Leu20-Tyr21-Gly22 and the hydrophobic part of the side chain of Lys18 form the C-terminal part of the epitope. Due to the frequent presence of residues Pro, Leu, Thr, Arg, and Gln in the N-terminal part of the mimotopes, corresponding homologous residues in the N-terminal proximity of the partial epitope may be part of an additional more hydrophilic epitope element.