Structural features associated with the ability of a monoclonal antibody (mAb) 
to discriminate between protein variants are identified and engineered. The 
variants are the curaremimetic toxin alpha from Naja nigricollis and erabutoxin 
a or b from Laticauda semifasciata, which differ from each other by 16 
substitutions and one insertion. The neutralizing mAb M alpha 1 recognizes with 
high affinity a topographical epitope on the surface of toxin alpha, but fails 
to recognize the erabutoxins although they possess most of the residues forming 
the presumed epitope. Examinations of the toxin alpha and erabutoxin 3-D 
structures and molecular dynamics simulations reveal several differences between 
the variants. In particular, the region involving the beta-turn 17-24 is 
organized differently. Analysis of the differences found in this region suggest 
that the insertion (or deletion) at position 18 of the variant amino acid 
sequences is particularly important in determining the differential 
cross-reactivity. To test this proposal, residue 18 was deleted in one 
erabutoxin using site-directed mutagenesis, and the biological properties of the 
resulting mutant were examined. We found that full antigenicity was restored in 
the previously unrecognized variant. The implications of this finding are 
discussed.