Structural basis for the recognition of superantigen streptococcal pyrogenic exotoxin A (SpeA1) by MHC class II molecules and T-cell receptors

Abstract

Streptococcal pyrogenic exotoxin A (SpeA) is a superantigen produced by Streptococcus pyogenes and is associated with severe infections characterized by rash, hypotension, multiorgan failure and a high mortality rate. In this study, an allelic form of this toxin, SpeA1, was crystallized with four molecules in the crystallographic asymmetric unit and its crystal structure was determined at 2.6 Å resolution. The crystallographic R-factor was 19.4% (33 497 reflections) for 7031 protein atoms and 88 water molecules. The overall structure of SpeA1 is considerably similar to that of other prototype microbial superantigens, either of staphylococcal or streptococcal origin, but has greatest similarity to staphylococcal enterotoxin C (SEC). Based on structural and mutagenesis data, we have mapped several important residues on the toxin molecule, which are involved in the recognition of major histocompatibility complex (MHC) class II molecules and T-cell receptors. Also, the toxin appears to possess a potential zinc-binding site which may have implications in binding to particular MHC class II molecules. Finally, we propose models for SpeA1-MHC class II and SpeA1-T-cell receptor association and the relevance of this phenomenon to the superantigenic action of this toxin is considered.

Mutational effects on protein folding stability and antigenicity: the case of streptococcal pyrogenic exotoxin A

“…Of the mutant SPEA proteins assayed, only the C90A mutant, and combinations thereof, resulted in complete elimination of the dimer species. These results strongly indicate that the observed protein dimer was formed by an intermolecular disulfide bond between the C90 residues of two separate SPEA molecules, as was previously proposed by Papageorgiou et al [6].…”

“…The X-ray crystal structure of SPEA has been solved at 2.6-Å resolution [6], revealing N-and C-terminal subdomains, and a surface loop constrained by a disulfide bond between residues C87 and C98. In addition, another cysteine (located at position 90) is also inside this loop (Fig.…”

“…Chromatographic separation of recombinantly produced SPEA protein isolated at pH 7.9 revealed the presence of both monomer and dimer species, leading Papageorgiou et al [6] to speculate that residue C90 in one toxin monomer forms a disulfide bond to the C90 of another SPEA molecule. We confirm here using dynamic light scattering measurements on a C90A mutant protein at pH 7.0 that this residue does indeed form such a disulfide-linked dimer.…”

Molecular Requirements for MHC Class II α-Chain Engagement and Allelic Discrimination by the Bacterial Superantigen Streptococcal Pyrogenic Exotoxin C

“…Of the group II (SEB and SEC 3 ) and III (SEA) SAgs that have been structurally characterized in complex with MHC␣, each have similar "footprints" on MHC␣, although SEA is rotated slightly away from HLA-DR1, presenting a smaller contact surface (8) with a reduced affinity (31,59). Not surprisingly, the molecular surfaces of the ␤-barrel domains in group II and group III SAgs, which also includes SpeA and SSA (60,61), are generally similar.…”

Group A Streptococcus: allelic variation, population genetics, and host-pathogen interactions

“…In this regard, Kline and Collins reported (27) that the new SpeA3 toxin variant was more mitogenic and had tenfold higher affinity for the class II MHC molecule than the old SpeA1 variant. Crystal structure analysis revealed that the two amino acid replacements, Gly80Ser in SpeA2 and Val76Ile in SpeA3, are located close to a zinc-binding site in SpeA1, a region postulated to be involved in class II MHC recognition (28,29). Taken together, the results suggest that clones expressing the mutant toxins might have more severe effects in vivo.…”