Streptococcus pyogenes (group A streptococcus) is a virulent organism that causes scarlet fever, rheumatic fever and streptococcal toxic shock syndrome (STSS). While modern antibiotics have effectively combated the two former illnesses, STSS seems to be occurring with increasing frequency. STSS is often caused by a bacterial infection of a minor skin lesion or bruise and is characterized by rash, fever, hypotension and multiorgan failure; 30% to 70% of infected individuals die in spite of aggressive modern medicine (Stevens, 1999).
Streptococcal pyrogenic exotoxin A1 (SpeA1) is a superantigen commonly isolated from streptococcal strains infecting individuals with STSS. Superantigens are a family of proteins able to simultaneously bind major histocompatibility complex (MHC) class II molecules and the T-cell receptor (TcR). Superantigens bind outside the MHC-TcR binding groove occupied by conventional antigens. (Papageorgiou and Acharya 1997). Such binding specifically stimulates Vß expressing T-cells, mitigating a massive release of cytokines by lymphocytes and monocytes. (Papageorgiou et al, 1999). While a normal antigen stimulates 0.001% of T-cells, a superantigen may stimulate nearly 20% (Papageorgiou and Acharya, 1997). This catastrophic buildup of cytokines leads to acute shock, a symptom typical of STSS.
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SpeA1 crystallizes in an asymmetric unit of four individual complete molecules.< >
Each monomer of SpeA1 is comprised of a single polypeptide
chain of 221 amino acids. Similar to other superantigens, the N terminal domain
contains a ß-barrel tertiary structure called the 'oligosaccharide
/ oligonucleotide fold '<
which is comprised of five ß sheets. ß strands 2 and 3 are antiparallel,
while the ß1 strand runs parallel to 5 and anti-parallel to 4. A long
runs down the center of the protein connecting the N and C terminal domains.
ß sheets 6, 7, 12, 9, and 10 comprise a ß
characteristic of C terminal domains in the superantigen family (Papageorgiou
et al, 1999).
Each molecule of SpeA1 may potentially bind a zinc ion using a binding site located at the interface between the N and C terminal domains< >. Glu 33, Asp 77, His 106, and His 110 move slightly to coordinate the binding of the single zinc ligand (Baker et al, 2001).
SpeA1 also contains an intramolecular disulfide bridge between Cys-87 and Cys-98 < >. This bond connects the ß4 and ß5 sheets at the top of the N-terminal ß-barrel and forms a 'flexible loop' < >. Located on this loop is Cys-90 < > which is involved in dimerization (Papageorigou et al, 1999).
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The dimeric structure forms through the creation of a disulfide bond between two identical monomers. Cysteines located on the flexible loop (residues 87-98) at position 90 create an intermolecular disulfide bond, providing the foundation for dimeric quaternary structure < > (Papageorgiou et al, 1999). Formation of this disulfide linkage requires the movement of the flexible loop between 1.62 and 1.96 Å from its position in the native monomeric structure. While the individual amino acid sequence of this loop may vary from one molecule to another, the Cys-90 is conserved, indicating its importance in the establishment of the dimer (Baker et al, 2004).
Toxins in the streptococcal family each exhibit a high affinity for binding a particular subset of MHC class II molecules. The SpeA1 toxin possesses a high affinity for the HLA-DQ type MHCs. Binding of these molecules occurs through one of two binding sites (Papageorgiou, 1999).
The generic MHC binding site <> provides a non-ligand mediated method of MHC binding. Mutational studies have identified the following amino acids as crucial to generic MHC class II binding in SpeA1: Leu 42, Asp 45, Leu 46, Ile 47, Tyr 48, and Tyr 83 < > (Papageorgiou et al, 1999).
Dimerization appears to block the generic MHC binding site, sequestering the essential residues within the interface between the two monomers < >. If this steric hinderance of MHC binding does occur, then zinc-mediated MHC binding would be the only available method of TcR-MHC complex formation. (Baker et al, 2001)
Ligand mediated binding of MHC class II molecules occurs at the zinc binding site < >, located in the region near the N and C terminal domains.
In SpeA1, mutagenesis studies have concluded that the zinc site is crucial to MHC binding, as mutations within any of the key residues significantly decreases ability to bind MHC II molecules, even to the point of nonexistent affinity (Hartwig and Fleischer, 1993). Additionally, in other protein members of the superantigen family, metal ion binding sites have been shown to not only influence MHC binding, but also dimer formation and thermostability (Baker et al, 2001).
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SpeA1 binds T-cells expressing the Vß subset of T-cell receptor proteins, specifically Vß 2.1, 12.2, 14.1, and 15.1 (Kline and Collins, 1997). The variability within the structures of these different receptors indicates that the residue used for specific receptor recognition varies across a range of amino acids (Papageorgiou et al, 1999). The TcR binding site is located in two α helices as well as the flexible disulfide loop < >. This binding site is adjacent to the MHC II binding site < >, providing further structural support for the conclusion that superantigens bind both the MHC II and TcR molecules outside of the standard binding groove motif. A depiction of the complex can be seen here: MHC-TcR SpeA1 Complex
Similar to the MHC II binding site, dimerization appears to sterically inhibit the access of TCR molecules to the requisite binding site < >. Given that only one TcR binding site exists within the protein, such dimeric structure would appear to prevent MHC-TcR complex formation. As mentioned above, experimental data contradicts this conclusion, indicating equal viability of both the monomeric and dimeric forms. An explanation for this apparent discrepancy lies in the postulated flexibility of the disulfide loop, a quality not reflected in a rigid protein crystal structure (Baker et al, 2004). Such flexibility of the loop and the dimer-mediating residues it includes allows for novel orientations of both the TcR and MHC II binding sites. The prevalence of such variable orientations in general toxin structure increase the possibility of binding of multiple MHC and TcR molecules, imparting greater potency on the dimeric form.
Recent years have witnessed an increase in the frequency of infections due to Streptococcus pyrogenes and deaths from STSS. The SpeA1 toxin is released by Streptococcus, stimulating a response on the immunocellular level. Research into the structure of SpeA1 and its ability to simulanteously bind MHC II and TcR molecules has revealed a dynamic protein potent in both monomeric and dimeric forms.
While evidence from the crystal structure indicates that dimeric binding blocks formation of MHC-TcR complex, recent experimental data contradicts this conclusion. Both forms have been shown to exhibit equally potent activity in vitro. One prediction even postulates a greater potency of the dimeric form in vivo (Baker et al, 2004). This discrepancy is explained through the flexibility of the linker region. Linker elasticity allows the protein to take a number of different orientations, opening multiple binding sites to both MHC and TcR molecules. In order understand and combat this protein, further research is needed to elucidate the true nature of TcR-MHC complex formation in monomeric and dimeric forms.
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 Baker, Matthew D., Inessa Gendlina, Carleen M. Collins, and K. Ravi Acharya. 2004. Crystal structure of a dimeric form of streptococcal pyrogenic exotoxin A (SpeA1). Protein Science 13:2285-2290.
 Baker, Matthew, Delia M. Gutman, Anastassios C. Papageorgiou, Carleen M. Collins, and K. Ravi Acharya. 2001. Structural features of a zinc binding site in the superantigen streptococcal pyrogenic exotoxin A (SpeA1): Implications for MHC class II recognition. Protein Science 10:1268-1273.
 Hartwig, U.F., and B. Fleischer. 1993. Mutations affecting MHC class II binding of the superantigen streptococcal erythrogenic toxin A. International Immunology 5: 869-875.
 Kline, J. Bradford, and Carleen M. Collins. 1997. Analysis of the interaction between the bacterial superantigen streptococcal pyrogenic exotoxin A (SpeA) and the human T-cell receptor. Molecular Microbiology 24:191-202.
 Papageorgiou, Anastassios C., Carleen M. Collins, Delia M. Gutman, J. Bradford Kline, Susan M. O'Brien, Howard S. Tranter, and K. Ravi Acharya. 1999. Structural basis for the recognition of superantigen streptococcal pyrogenic exotoxin A (SpeA1) by MHC class II molecules and T-cell receptors. The EMBO Journal 18:9-21.
Papageorgiou, Anastassios C., and K. Ravi Acharya. 1997. Superantigens as immunomodulators: recent structural insights. Structure 5: 991-996.
 Stevens, Dennis L. 1995. Streptococcal Toxic-Shock Syndrome: Spectrum of Disease, Pathogenesis, and New Concepts in Treatment. Emerging Infectious Diseases 1:69-78.