Biochemical and mutational analysis of the histidine residues of staphylococcal enterotoxin A

The goal of this study was to examine the role of histidine residues in the biological activities of staphylococcal enterotoxin A (SEA). Carboxymethylated SEA was unable to stimulate murine T-cell proliferation but was resistant to monkey stomach lavage fluid degradation, suggesting that native conformation was intact. Site-directed mutagenesis of the histidine residues of SEA was subsequently performed. SEA-H44A (SEA with histidine 44 replaced with alanine), SEA-H44D, SEA-H50A, SEA-H50D, SEA-H114A, SEA-H114D, SEA-H187A, and SEA-H187D retained superantigen and emetic activities, whereas SEA-H225A and SEA-H225D were defective in the ability to stimulate T-cell proliferation. These mutants were unable to compete with SEA for binding to Raji cells, suggesting that the defect in SEA-H225A and SEA-H225D is due to impaired major histocompatibility complex class II binding. SEA-H225D provoked an emetic response in monkeys only if fed at high doses, while SEA-H225A did not provoke an emetic response at low or high doses. In comparison, SEA-H61A and SEA-H61D were defective in emetic activity but not in the ability to stimulate murine T-cell proliferation. Overall, these studies show that the carboxy-terminal histidine at residue position 225 of SEA is important for both the superantigen and emetic activities of this enterotoxin. Histidine 61 appears to be important for emetic activity but not for superantigen activity, consistent with the hypothesis that the two activities are separable in staphylococcal enterotoxins.