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PDBsum entry 1sxt
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Contents |
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* Residue conservation analysis
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References listed in PDB file
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Key reference
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Title
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The co-Crystal structure of staphylococcal enterotoxin type a with zn2+ at 2.7 a resolution. Implications for major histocompatibility complex class ii binding.
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Authors
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M.Sundström,
D.Hallén,
A.Svensson,
E.Schad,
M.Dohlsten,
L.Abrahmsén.
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Ref.
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J Biol Chem, 1996,
271,
32212-32216.
[DOI no: ]
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PubMed id
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Abstract
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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.
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Figure 1.
Fig. 1. A representative part of the 2F[o]F[c] electron
density map of SEA-Zn2+ in the core of the superantigen
molecule. Observe the interactions between tyrosine residues
that possibly contribute to the resistance^ of SEA to
environmental factors such as high temperature or protease^
degradation. The 2F[o]F[c] electron density map is contoured at
1.2^  above the
mean.
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Figure 3.
Fig. 3. The zinc binding site of SEA co-crystallized with
Zn2+. A, tetrahedral zinc co-ordination in molecule one (yellow)
in the asymmetric unit. Note that the use of His61 from the
neighboring molecule (cyan) as zinc ligand leads to the loop
59-63, absent in the SEA-Cd^2+ structure, here becoming ordered.
B, tetrahedral zinc co-ordination in the second molecule of the
asymmetric unit. The three high affinity SEA ligands are used
and in addition a water molecule^ (H[2]O) is used as the fourth
Zn2+ ligand. The figures were drawn using Molscript (30) and
Raster3D^ (31).
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(1996,
271,
32212-32216)
copyright 1996.
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