Staphylococcal and Streptococcal Superantigens: Basic Biology of Conserved Protein Toxins

Abstract

Pore-forming toxins (PFTs) represent the largest known group of bacterial protein toxins to date. Membrane insertion and subsequent pore-formation occurs after initial binding to cell-surface receptor and oligomerization. Aerolysin, a toxin produced by the Gram-negative bacterium Aeromonas hydrophila and related species, belongs to the PFT group and shares a common mechanism of action involving -barrel structures resulting from the assembly ofhairpins from individual toxin monomers into a heptamer. Aerolysin is also the name given to structurally and mechanistically related toxins called the aerolysin-like toxin family. A universal characteristic of this toxin family involves the diverse life forms that synthesize these proteins throughout Nature. Examples include: 1) epsilon-toxin and septicum-alpha-toxin produced by anaerobic, Gram-positive Clostridium species; 2) enterolobin by the Brazilian tree Enterolobium contortisiliquum; 3) a mushroom toxin Laetiporus sulphureus lectin (LSL); 4) mosquitocidal toxins (Mtxs) from the Gram-positive bacteria Bacillus sphaericus and parasporine-2 from Bacillus thuringiensis; and 6) hydralysins from the tiny aquatic animal Chlorohydra viridis. The following review provides an overview of the different members within the aerolysin-like toxin family.