Håkansson M, Antonsson P, Björk P, Svensson L A
Department of Molecular Biophysics, Center for Chemistry and Chemical Engineering, P.O. Box 124, 221 00 Lund, Sweden.
J Biol Inorg Chem. 2001 Oct;6(8):757-62. doi: 10.1007/s007750100251.
The structure of a mutant form of staphylococcal enterotoxin A (SEA) has been determined to 2.1 A resolution. The studied SEA substitution H187-->A187 (SEAH187A) leads to an almost 10-fold reduction of the binding to major histocompatibility complex (MHC) class II. H187 is important for this interaction since it coordinates Zn2+. The zinc ion is thought to hold MHC class II and SEA together in a complex. Interestingly, only one of two molecules in the asymmetric unit binds Zn2+. H225, D227, a water molecule, and H44 from a symmetry-related molecule ligate Zn2+. The symmetry-related histidine is necessary for this substituted Zn2+ site to bind to Zn2+ at low zinc concentration (no Zn2+ added). Since a water molecule replaces the missing H187, H44 binds Zn2+ at the position where betaH81 from MHC class II probably will bind. Dynamic light scattering analysis reveals that in solution as well as in the crystal lattice the SEA(H187A) mutant forms aggregates. The substitution per se does not cause aggregation since wild-type SEA also forms aggregates. Addition of EDTA reduces the size of the aggregates, indicating a cross-linking function of Zn2+. In agreement with the biological function, the aggregation is weak (i.e. not revealed by gel filtration) and non-specific.
已确定葡萄球菌肠毒素A（SEA）突变体形式的结构，分辨率达到2.1埃。所研究的SEA替代物H187→A187（SEAH187A）导致与主要组织相容性复合体（MHC）II类的结合减少近10倍。H187对这种相互作用很重要，因为它能与Zn2+配位。锌离子被认为能使MHC II类和SEA在复合物中结合在一起。有趣的是，不对称单元中的两个分子中只有一个结合Zn2+。来自对称相关分子的H225、D227、一个水分子和H44与Zn2+配位。对称相关的组氨酸对于这个替代的Zn2+位点在低锌浓度（未添加Zn2+）下结合Zn2+是必需的。由于一个水分子取代了缺失的H187，H44在MHC II类的βH81可能结合的位置结合Zn2+。动态光散射分析表明，在溶液和晶格中，SEA（H187A）突变体都会形成聚集体。这种替代本身不会导致聚集，因为野生型SEA也会形成聚集体。添加EDTA会减小聚集体的大小，表明Zn2+具有交联功能。与生物学功能一致，这种聚集很弱（即通过凝胶过滤未显示）且是非特异性的。
