ABSTRACT
BACKGROUND: Staphylococcus aureus enterotoxin C2 (SEC2) belongs to a family of proteins, termed 'superantigens', that form complexes with class II MHC molecules enabling them to activate a substantial number of T cells. Although superantigens seem to act by a common mechanism, they vary in many of their specific interactions and biological properties. Comparison of the structure of SEC2 with those of two other superantigens--staphylococcal enterotoxin B (SEB) and toxic shock syndrome toxin-1 (TSST-1)--may provide insight into their mode of action. RESULTS: The crystal structure of SEC2 has been determined at 2.0 A resolution. The overall topology of the molecule resembles that of SEB and TSST-1, and the regions corresponding to the MHC class II and T-cell receptor binding sites on SEB are quite similar in SEC2. A unique feature of SEC2 is the presence of a zinc ion located in a solvent-exposed region at the interface between the two domains of the molecule. The zinc ion is coordinated to Asp83, His118, His122 and Asp9* (from the neighbouring molecule in the crystal lattice). Atomic absorption spectrometry demonstrates that zinc is also bound to SEC2 in solution. CONCLUSIONS: SEC2 appears to be capable of binding to MHC class II molecules in much the same manner as SEB. However, structure-function studies have suggested an alternative binding mode that involves a different site on the toxin. The zinc ion of SEC2 lies within this region and thus may be important for complex formation, for example by acting as a bridge between the two molecules. Other possible roles for the metal cation, including a catalytic one, are also considered.
Subject(s)
Bacterial Toxins , Enterotoxins/chemistry , Protein Structure, Secondary , Staphylococcus aureus , Superantigens/chemistry , Zinc/metabolism , Amino Acid Sequence , Binding Sites , Crystallography, X-Ray/methods , Enterotoxins/metabolism , Models, Molecular , Molecular Sequence Data , Sequence Homology, Amino Acid , Staphylococcus aureus/immunology , Superantigens/metabolismABSTRACT
Superantigens stimulate T cells bearing particular T-cell receptor V beta sequences, so they are extremely potent polyclonal T-cell mitogens. T-cell activation is preceded by binding of superantigens to class II major histocompatibility complex (MHC) molecules. To further the structural characterization of these interactions, the crystal structure of a toxin associated with toxic-shock syndrome, TSST-1, which is a microbial superantigen, has been determined at 2.5 A resolution. The N- and C-terminal domains of the structure both contain regions involved in MHC class II association; the C-terminal domain is also implicated in binding the T-cell receptor. Despite low sequence conservation, the TSST-1 topology is similar to the structure reported for the superantigen staphylococcal enterotoxin B4. But TSST-1 lacks several of the structural features highlighted as central to superantigen activity in the staphylococcal enterotoxin B and we therefore reappraise the structural basis of superantigen action.
Subject(s)
Bacterial Toxins , Enterotoxins/chemistry , Superantigens/chemistry , Amino Acid Sequence , Computer Graphics , Crystallography, X-Ray , Enterotoxins/immunology , Histocompatibility Antigens Class II/immunology , Molecular Sequence Data , Protein Conformation , Protein Structure, Secondary , Receptors, Antigen, T-Cell, alpha-beta/immunology , Staphylococcus aureus , Structure-Activity Relationship , Superantigens/immunologyABSTRACT
High yields of staphylococcal enterotoxin C2, from Staphylococcus aureus, have been purified using dye ligand chromatography. Crystals suitable for X-ray diffraction work were obtained by vapour diffusion using ammonium sulphate and polyethylene glycol as precipitants. They belong to the tetragonal space group P4(1)22 with unit cell dimensions a = b = 43.2 A and c = 290.9 A with one molecule per asymmetric unit. The crystals diffract to at least 2.8 A resolution and are suitable for three-dimensional X-ray structural analysis.
Subject(s)
Antigens, Bacterial/chemistry , Enterotoxins/chemistry , Antigens, Bacterial/isolation & purification , Enterotoxins/isolation & purification , X-Ray DiffractionABSTRACT
High yields of toxic shock syndrome toxin-1, from Staphylococcus aureus, have been purified (> 99%) using a novel, simple, two-step procedure involving dye ligand chromatography. Crystals suitable for X-ray diffraction work were obtained by vapour diffusion using ammonium sulphate and polyethylene glycol as precipitants. They belong to the orthorhombic space group C222(1), with unit cell dimensions a = 108.6 A, b = 177.6 A and c = 97.5 A, with three molecules per asymmetric unit. The crystals diffract to at least 2.5 A resolution and are suitable for three-dimensional X-ray structural analysis.
