BIOLOGICAL-ACTIVITIES OF STAPHYLOCOCCAL-ENTEROTOXIN TYPE-A MUTANTS WITH N-TERMINAL SUBSTITUTIONS

The purpose of this study was to examine the importance of certain N-t erminal amino acid residues of staphylococcal enterotoxin type A (SEA) for biological activity. The results confirm our previous observation that Asn-25, Phe-47, and Leu-48 are important for SEA's emetic and su perantigen activities. Substitutions at six other sites (Leu-12, Lys-1 4, Ser-16, Asp-45, Gln-46, and Thr-51) did not reveal any additional r esidues required for biological activity. Mutant SEAs with substitutio ns at 25, 47, or 48 all had decreased T-cell stimulatory activity, wit h the mutants at position 47 being the most defective. Results of a co mpetition assay for binding to the major histocompatibility complex (M HC) class II-expressing cell line Raji suggested that the decreased su perantigen activities of the mutants with substitutions at positions 4 7 and 48 are due to poor interactions with MHC class II molecules, whe reas the defects of the mutants at position 25 are a consequence of fa ulty interactions with T-cell receptors. With respect to emetic activi ty in rhesus monkeys, the mutants at position 25 or 48 exhibited decre ased but significant activity. Interestingly, the two mutants at posit ion 47 had different emetic activities; SEA-F47G was nonemetic when ad ministered intragastrically at 500 mu g per animal, hereas SEA-F47S wa s emetic at this dosage. Since the mutants at position 47 were equally defective for superantigen activity, this further supports our previo us suggestion of an incomplete correlation between SEA's emetic and su perantigen activities.