Crystallographic and Mutational Data Show That the Streptococcal Pyrogenic Exotoxin J Can Use a Common Binding Surface for T-cell Receptor Binding and Dimerization

Abstract

The protein toxins known as superantigens (SAgs), which are expressed primarily by the pathogenic bacteria Staphylococcus aureus and Streptococcus pyogenes, are highly potent immunotoxins with the ability to cause serious human disease. These SAgs share a conserved fold but quite varied activities. In addition to their common role of cross-linking T-cell receptors (TCRs) and major histocompatibility complex class II (MHC-II) molecules, some SAgs can cross-link MHC-II, using diverse mechanisms. The crystal structure of the streptococcal superantigen streptococcal pyrogenic exotoxin J (SPE-J) has been solved at 1.75 A resolution (R = 0.209, R(free) = 0.240), both with and without bound Zn(2+). The structure displays the canonical two-domain SAg fold and a zinc-binding site that is shared by a subset of other SAgs. Most importantly, in concentrated solution and in the crystal, SPE-J forms dimers. These dimers, which are present in two different crystal environments, form via the same face that is used for TCR binding in other SAgs. Site-directed mutagenesis shows that this face is also used for TCR binding SPE-J. We infer that SPE-J cross-links TCR and MHC-II as a monomer but that dimers may form on the antigen-presenting cell surface, cross-linking MHC-II and eliciting intracellular signaling.

Mutagenesis, biochemical, and biophysical characterization of Mycoplasma arthritidis-derived mitogen

“…Third, zinc may indirectly participate in the interactions between SAgs and host receptors, by inducing conformational changes in certain SAgs; such changes could facilitate the binding of such SAgs to their receptors. Although direct evidence to support this hypothesis is lacking, comparisons of crystal structures of several SAgs with/without Zn 2+ indeed revealed that there are conformational changes associated with Zn 2+ binding (Baker et al, 2004a;Hakansson et al, 2000;Papageorgiou et al, 2004). In addition, although there is currently no evidence to support the following possibility, zinc could be required for interactions with co-stimulatory receptors for T-cell activation.…”

Bacterial superantigens and superantigen-like toxins

“…Similarly, homodimerization was also shown for SPE-J. However, structural analysis showed that SPE-J uses the same interface that is used for TcR binding and therefore, the biological significance of SPE-J dimerization is unknown [25]. A similar mechanism for dimerization has also been demonstrated for SSA [187], whereas SPE-A1 was shown to exist in a disulfidelinked dimeric form [188].…”

“…These include the staphylococcal toxins SEA [98], SEB [100], SEC2 [161], SEC3 [102], SED [104], SEG [107], SEH [162], SEI [111], SEK [114], and TSST-1 [163], and the streptococcal toxins SPE-A [16], SPE-C [20], SPE-H [164], SPE-I [24], SPE-J [25], SMEZ-2 [164], SSA [165], and SDM (SPE-M6) [166]. All staphylococcal and streptococcal SAgs share a common fold with a conserved two-domain architecture and the presence of a long, solvent-accessible central α-helix [164,167] (Figures 32.2 a and b).…”

“…The bacterial SAgs form a family of nonglycosylated, low-molecular-weight exotoxins (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28) produced by several bacterial species, including Staphylococcus aureus, group A streptococcus (Streptococcus pyogenes), group C and G streptococci, Yersinia pseudotuberculosis, and Mycoplasma arthritidis.…”

Biochemical and Immunological Mechanisms Underlying Differential Interaction of Superantigens with Host Immunogenetic Factors in Streptococcal Sepsis

“…The majority of currently known Strep SAgs have been well characterized with respect to their structure, biological activity, and Vβ specificity (Table 1.3) (9,100,103,123,124,128,(139)(140)(141). The crystal structure of several Strep SAgs (SpeA, SpeC, SpeH, SpeJ, SSA, and SmeZ-2) has been solved proteins and a larger C-terminal β-grasp motif separated from the other domain by long α-helix (103,146,148,150).…”