The amino acid sequence of toxin IV from the Androctonus australis scorpion: differing effects of natural mutations in scorpion alpha-toxins on their antigenic and toxic properties.

The complete amino acid sequence (64 residues) of the AaH IV toxin from the scorpion Androctonus australis Hector was determined by automated Edman degradation and was compared with the sequences of other Androctonus toxins. AaH IV was also tested by radioimmunoassay for binding to antisera raised against other toxins of the same species. The results indicated that AaH IV shares some of the antigenic properties of AaH I and AaH III toxins, but does not cross-react with anti-AaH II antibodies. The structural basis for the observed antigenic relationships can be found in the high degree of homology displayed by AaH IV with regard to AaH I and III, the changes in amino acid residues equally affecting regions included or excluded from the main predicted antigenic sites of AaH IV. The lower biological potency of AaH IV is presumably the result of some of the sequence differences. In particular, substitution affecting the charge and bulkiness of residue 61 could account for the poor receptor binding and consequential weak toxic properties of this molecule.