Vinson M, van der Merwe P A, Kelm S, May A, Jones E Y, Crocker P R
Imperial Cancer Research Fund Laboratories, University of Oxford, United Kingdom.
J Biol Chem. 1996 Apr 19;271(16):9267-72. doi: 10.1074/jbc.271.16.9267.
The sialoadhesins are a distinct subgroup of the immunoglobulin superfamily, comprising sialoadhesin, CD22, the myelin-associated glycoprotein, and CD33. They can all mediate sialic acid-dependent binding to cells with distinct specificities. Sialoadhesin is a murine macrophage-restricted cell-surface molecule with 17 extracellular immunoglobulin-like domains that recognizes NeuAc alpha 2-3Gal in N- and O-glycans and interacts preferentially with cells of the granulocytic lineage. Its sialic acid-binding site is located within the NH2-terminal (membrane-distal) V-set domain. Here we have carried out site-directed mutagenesis in an attempt to identify the binding site of sialoadhesin. A subset of nonconservative mutations disrupted sialic acid-dependent binding without affecting binding of three monoclonal antibodies directed to two distinct epitopes of sialoadhesin. A CD8 alpha-based molecular model predicts that these residues form a contiguous binding site on the GFCC'C" beta-sheet of the V-set domain centered around an arginine in the F strand. A conservative mutation of this arginine to lysine also abolished binding. This amino acid is conserved among all members of the sialoadhesin family and is therefore likely to be a key residue in mediating sialic acid-dependent binding of sialoadhesins to cells.
唾液酸黏附素是免疫球蛋白超家族中一个独特的亚群，包括唾液酸黏附素、CD22、髓鞘相关糖蛋白和CD33。它们都能介导与具有不同特异性的细胞的唾液酸依赖性结合。唾液酸黏附素是一种小鼠巨噬细胞限制性细胞表面分子，具有17个细胞外免疫球蛋白样结构域，可识别N-和O-聚糖中的NeuAcα2-3Gal，并优先与粒细胞系细胞相互作用。其唾液酸结合位点位于NH2末端（膜远端）V-set结构域内。在此，我们进行了定点诱变，试图确定唾液酸黏附素的结合位点。一部分非保守突变破坏了唾液酸依赖性结合，而不影响针对唾液酸黏附素两个不同表位的三种单克隆抗体的结合。基于CD8α的分子模型预测，这些残基在V-set结构域的GFCC'C"β-折叠上形成一个连续的结合位点，该位点以F链中的一个精氨酸为中心。将这个精氨酸保守突变为赖氨酸也消除了结合。这个氨基酸在唾液酸黏附素家族的所有成员中都是保守的，因此可能是介导唾液酸黏附素与细胞的唾液酸依赖性结合的关键残基。
