The crystal structure of staphylococcal enterotoxin type D reveals Zn2+-mediated homodimerization.

Abstract

Bacterial superantigens, including the staphylococcal enterotoxins, are the most potent activators of T cells known and have been suggested as a causative factor in Gram‐positive shock in humans. Staphylococcal enterotoxin D (SED) is dependent upon Zn2+ for high affinity interactions with MHC class II molecules and thus SED was co‐crystallized with Zn2+. The crystal structure of SED has been determined in two different space groups, at 2.3 and 3.0 A resolution respectively. The three‐dimensional structure of SED is similar to structures of other bacterial superantigens, although this study has revealed that SED has the unique capability of forming dimers in the presence of Zn2+. The high affinity Zn2+ site used in dimer formation is located on the surface of the beta‐sheet in the C‐terminal domain. Two bound metal ions are coordinated by residues from both molecules in the dimer interface and thus contribute directly to formation of the dimer. A second Zn2+ site is located on the surface close to the domain interface of the molecule. The unique feature of SED in forming a Zn2+‐dependent homodimer seems to facilitate novel and biologically relevant multimeric interactions with MHC class II molecules, as shown by the induction of cytokine mRNA in human monocytes when exposed to SED and SED mutants.

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

“…Such interactions result in a so-called "high-affinity" SAg-binding site, via a zinc bridge, on the MHC class II molecules (Hudson et al, 1995;Kozono et al, 1995;Li et al, 2001;Petersson et al, 2001). Second, some SAgs can form zinc-dependent homodimers, which could in turn cause dimerization of the MHC class II molecules on the APC surface (Al-Daccak et al, 1998;Baker et al, 2004a,b;Li et al, 1997;Papageorgiou et al, 1995Papageorgiou et al, , 2004Roussel et al, 1997;Sundstrom et al, 1996). 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.…”

The Staphylococcal Enterotoxins

“…A pair of a helices lie between the two domains to form a long groove on one side of the molecule and a third a helix contributes to a shallower groove near the top of the toxin structure. These same structural motifs are conserved in SEA, SEC and SED (Papageorgiou et al, 1995;Sundstrom et al, 1996aSundstrom et al, , 1996b. The crystal structure of the nonemetic superantigenTSST-1 (Prasad etal., 1993) has a similar two-domain structure, but without the disulphide loop, the N-terminal residue^overlie of The Staphylococcal Enterotoxins 5 A the second domain and one of the a-helices in the groove between the two domains.…”

Zinc Binding and Dimerization of Streptococcus pyogenes Pyrogenic Exotoxin C Are Not Essential for T-cell Stimulation

“…The high affinity zinc coordination complexes of SEA and Spe-C consist of three amino acid side chains contributed by the superantigen and the fourth by the MHCII receptor. As observed in structural data for the Spe-C tetrahedral zinc coordination complex (3)(4)(5), His and/or Asp contribute the zinc ligands from the superantigen and His 81 from the ␤-chain of MHCII molecule. The recently determined crystal structure of SEH with the MHCII HLA-DR1 indicates a potentially lower affinity zinc coordination site, present at the interface with the receptor (6), where one of the ligands is a water molecule.…”

“…In addition, the resulting nonspecific stimulation of the host's T-cells can induce pathological levels of cytokines. There is a substantial amount of structural data available for the superantigens, both free and in complex with their TCR and MHCII receptors (1)(2)(3)(4)(5)(6). The superantigen fold is highly conserved despite low overall sequence similarity among protein family members.…”