The Co-crystal structure of staphylococcal enterotoxin type A with Zn2+ at 2.7 A resolution. Implications for major histocompatibility complex class II binding.
Superantigens form complexes with major histocompatibility complex (MHC) class II molecules and T-cell receptors resulting in extremely strong immunostimulatory properties. Staphylococcus aureus enterotoxin A (SEA) belongs to a subgroup of the staphylococcal superantigens that utilizes Zn2+ in the high affinity interaction with MHC class II molecules. A high affinity metal binding site was described previously in SEA co-crystallized with Cd2+ in which the metal ion was octahedrally co-ordinated, involving the N-terminal serine. We have now co-crystallized SEA with its native co-factor Zn2+ and determined its crystal structure at 2.7 A resolution. As expected for a Zn2+ ion, the co-ordination was found to be tetrahedral. Three of the ligands are located on the SEA surface on a C-terminal domain beta-sheet, while the fourth varies with the conditions. Further analysis of the zinc binding event was performed using titration microcalorimetry, which showed that SEA binds Zn2+ with an affinity of KD = 0.3 microM in an entropy driven process. The differential Zn2+ co-ordination observed here has implications for the mechanism of the SEA-MHC class II interaction.
Sundström
M
M
Department of Structural Biochemistry, Pharmacia & Upjohn, Stockholm, Sweden.
Hallén
D
D
Svensson
A
A
Schad
E
E
Dohlsten
M
M
Abrahmsén
L
L
D002151:Calorimetry; D018360:Crystallography, X-Ray; D004768:Enterotoxins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D013211:Staphylococcus aureus; D018089:Superantigens / Q000737:chemistry* / Q000378:metabolism; D015032:Zinc / Q000378:metabolism*
D004768:Enterotoxins; D018089:Superantigens; C026066:enterotoxin A, Staphylococcal; D015032:Zinc