Analysis of functional regions of YPM, a superantigen derived from gram‐negative bacteria

The bacterial superantigens, staphylococcal enterotoxins and streptococcal pyrogenic exotoxins, are grouped in a family by the conservation of amino acid sequence and polypeptide folding patterns. In the case of Yersinia pseudotuberculosis ‐derived mitogen (YPM), however, there is no noticeable homology with this family, although many of the in vitro functional features conform to the criteria for a superantigen. To study the mode of action of YPM at the molecular level, we first generated a number of YPM point mutants with reduced T‐cell proliferative activity using random mutagenesis and localized the amino acid positions involved in either major histocompatibility complex class II or T‐cell receptor Vβ‐interaction. Plotting the elucidated positions on the hydrophilicity profile suggested that they reside mostly on the outer portion of the molecule. We also report that the two cysteines positioned almost at opposing ends of the YPM molecule are connected by an S–S bond the destruction of which causes fatal damage. Finally, we obtained evidence that YPM partially competes with staphylococcal enterotoxin E for human leukocyte antigen‐DR binding. This raises the question of whether these different types of superantigens have acquired the same function by genetic convergence or originated from a common ancestral gene.