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 a cid sequence and polypeptide folding patterns. In the case of Yersinia pseu dotuberculosis-derived mitogen (YPM), however, there is no noticeable homol ogy with this family, although many of the in vitro functional features con form 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 w ith reduced T-cell proliferative activity using random mutagenesis and loca lized the amino acid positions involved in either major histocompatibility complex class II or T-cell receptor V beta-interaction. Plotting the elucid ated positions on the hydrophilicity profile suggested that they reside mos tly on the. outer portion of the molecule. We also report that the two cyst eines 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 ob tained evidence that YPM partially competes with staphylococcal enterotoxin E for human leukocyte antigen-DR binding. This raises the question of whet her these different types of superantigens have acquired the same function by genetic convergence or originated from a common ancestral gene.