PDB 2G9H deposited: 2006-03-06 modified: 2011-07-13
Title Crystal Structure of Staphylococcal Enterotoxin I (SEI) in Complex with a Human MHC class II Molecule
Authors Fernandez, M.M., Guan, R., Malchiodi, E.L., Mariuzza, R.A., Swaminathan, C.P.
Structure factors resolution 2.0 rfactor 0.21327 rfree 0.2524
DPI 0.39 theoretical min: 0.20

Superantigens are bacterial or viral proteins that elicit massive T cell activation through simultaneous binding to major histocompatibility complex (MHC) class II and T cell receptors. This activation results in uncontrolled release of inflammatory cytokines, causing toxic shock. A remarkable property of superantigens, which distinguishes them from T cell receptors, is their ability to interact with multiple MHC class II alleles independently of MHC-bound peptide. Previous crystallographic studies have shown that staphylococcal and streptococcal superantigens belonging to the zinc family bind to a high affinity site on the class II beta-chain. However, the basis for promiscuous MHC recognition by zinc-dependent superantigens is not obvious, because the beta-chain is polymorphic and the MHC-bound peptide forms part of the binding interface. To understand how zinc-dependent superantigens recognize MHC, we determined the crystal structure, at 2.0 A resolution, of staphylococcal enterotoxin I bound to the human class II molecule HLA-DR1 bearing a peptide from influenza hemagglutinin. Interactions between the superantigen and DR1 beta-chain are mediated by a zinc ion, and 22% of the buried surface of peptide.MHC is contributed by the peptide. Comparison of the staphylococcal enterotoxin I.peptide.DR1 structure with ones determined previously revealed that zinc-dependent superantigens achieve promiscuous binding to MHC by targeting conservatively substituted residues of the polymorphic beta-chain. Additionally, these superantigens circumvent peptide specificity by engaging MHC-bound peptides at their conformationally conserved N-terminal regions while minimizing sequence-specific interactions with peptide residues to enhance cross-reactivity.

J.Biol.Chem. 2006 Sep; 281(35):25356-25364 doi:10.1074/jbc.M603969200

Cross References
Database source Identifier Description
PubMed 16829512 JBCHA3
Biomolecule Structure Assembly Serial Assembly Type Conformational State Chains Ligands Atoms
2G9H/1 2G9H 1 tetramer 0 4 8 5041