| Type: | Family | Name: | Superantigen, staphylococcal/streptococcal toxin, bacterial |
| Description: | The prokaryote Staphylococcus aureusis a well-characterised and specialised human pathogen, expressing a variety of virulence factors to allow successful infection of the host []. Symptoms usually manifest in cases of food poisoning, pyrogenic fever and toxic shock syndrome, and can prove lethal in immunocompromised patients []. Of all the exotoxins secreted from the bacterial cell, superantigens and haemolysins are amongst the most studied []. Of these, the former are well characterised, and several S. aureus superantigenic enterotoxins exist that trigger excessive and aberrant T-cell activation in the host. Homologues of these S. aureus proteins have been found in Streptococcus pyogenes, and cause similar effects []. In conventional Major Histocompatibility Complex (MHC)-II-restricted antigen processing, a peptide epitope is presented to a specific T-cell receptor (TCR) by the antigen presenting cell, and up to 0.0001% of the host T-cell repertoire is activated []. By contrast, a bacterial superantigen (SAg) can bypass this process, binding non-specifically to constant regions on both the MHC-II and TCR []. This results in up to 25% of the total host T-cell population being activated, with a consequent massive release of inflammatory cytokines. A recent study into the origins and functions of the S. aureus and S. pyogenes superantigens has identified distinct protein domains that are responsible for the slightly different actions of each protein subgroup []. Based on these criteria, the staphylococcal and streptococcal entero/exo toxins discovered so far can be placed into several groups/subfamilies []. Apart from the S. aureus toxic shock syndrome toxins, which form their own unique group, all superantigens described so far share a common motif that may bind to an as yet unknown ligand. Each different subgroup also contains a combination of domains and motifs that sets it apart []. However, all of these toxins share a similar 2-domain fold, a beta-barrel towards the N terminus and a beta-grasp domain towards the C terminus []. | Short Name: | Superantigen_bac |
