Isolation of a novel variant of secretory component with low affinity to dimeric immunoglobulin a by immobilised metal ion affinity chromatography

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.