Mutations affecting MHC class II binding of the superantigen streptococcal erythrogenic toxin A

Streptococcal pyrogenic exotoxin A (SPEA) is an important pathogenicity factor of group A streptococci. It is a member of the family of 'superantigens' produced by Staphylococcus aureus and Streptococcus pyogenes, and its T lymphocyte stimulating activity is involved in the pathogenesis of certain diseases caused by pyogenic streptococci. In this study we have generated nine mutant SPEA molecules by substituting amino acids in the regions of homology between different streptococcal and staphylococcal superantigens. An additional mutant was created by deletion of the 10 N-terminal amino acids. The mutants were expressed as fusion proteins. Several mutations led to a loss of function due to a loss of class II-binding activity. Such loss mutations did not cluster to a certain region of the SPEA molecule. Rather, even a substitution of neighboring amino acids had opposite effects. None of the loss mutations affected the binding of neutralizing mAb and all loss mutants could be precipitated in Ouchterlony tests by a polyclonal anti-SPEA serum. We conclude that the functional activities of SPEA, and probably of other superantigens as well, cannot be attributed to a defined region of the molecule.