By applying immobilised metal affinity chromatography to a free secretory component (FSC) preparation from bovine milk, we identified a novel FSC variant with lower affinity to dimeric immunoglobulin A. Using cDNA analysis, we found a novel His to Arg mutation at the 81st residue where the amino acid is well conserved among species. The IgA-binding portion of polymeric immunoglobulin receptor (pIgR) consists of five Ig-like domains (D1–D5); the first complementarity-determining region (CDR1) of D1 is especially critical to the noncovalent binding of pIgR to IgA. The mutation at position 81 occurred within D1, but at the beginning of the E/F loop on the opposite side of CDRl; this region is located at the interface with D2. Given that residues in D2 may form hydrogen bonds with D1, the mutation to Arg may have caused a positive charge that disturbed hydrogen bonding, altered domain relative orientation, and thereby lowered affinity.

通过将固定化金属亲和层析应用于来自牛乳的游离分泌成分 (FSC) 制剂，我们鉴定了一种对二聚免疫球蛋白 A 亲和力较低的新型 FSC 变体。使用 cDNA 分析，我们在第 81 个残基处发现了一个新的 His 到 Arg 突变，其中该氨基酸在物种间是高度保守的。聚合免疫球蛋白受体 (pIgR) 的 IgA 结合部分由五个 Ig 样结构域 (D1–D5) 组成；D1 的第一个互补决定区 (CDR1) 对于 pIgR 与 IgA 的非共价结合尤其重要。第 81 位的突变发生在 D1 内，但位于 CDR1 另一侧的 E/F 环的开始处；该区域位于与 D2 的接口处。鉴于 D2 中的残基可能与 D1 形成氢键，