Structural evidence for the evolution of pyrogenic toxin superantigens

Pathogenic bacteria have evolved a wide variety of toxins to invade and att ack host organisms. In particular, strains of the bacteria Staphylococcus a ureus and Streptococcus pyogenes produce a family of pyrogenic toxin supera ntigens (PTSAgs) that can cause illness, e.g., toxic shock syndrome, or syn ergize with a number of other immune system disorders. The PTSAgs are all s imilar in size and have a conserved two-domain tertiary fold despite minima l amino acid sequence identity. The tertiary structure of PTSAg domain 1 is similar to the immunoglobulin binding motif of streptococcal proteins G an d L. PTSAg domain 2 resembles members of the oligosaccharide/oligonucleotid e binding fold family that includes the B subunits of the AB(5) heat-labile enterotoxins, cholera toxin, pertussis toxin, and verotoxin. The strong st ructural homology between the pyrogenic toxins and other bacterial proteins suggests that the PTSAgs evolved through the recombination of two smaller beta -strand motifs.