Channel catfish soluble FcμR binds conserved linear epitopes present on Cμ3 and Cμ4

A linear epitope on catfish IgM has been identified as the docking site for the catfish soluble FcμR (IpFcRI). Western blot analyses and latex bead binding assays identified the consensus octapeptide motif FxCxVxHE located at the second cysteine that forms the intrachain disulfide bond of the catfish Cμ3 and Cμ4 immunolglobulin (Ig) domains as the IpFcRI binding sites. Furthermore, molecular modeling of catfish Cμ3 and Cμ4 confirmed that the octapeptide in both of these domains is accessible for IpFcRI interactions. In addition, since this octapeptide motif is also found in other vertebrate Ig domains, IpFcRI binding to Ig heavy (H) and light (L) chains from rainbow trout, chicken, mouse, rabbit, and goat were examined by Western blot analyses and latex bead binding assays. IpFcRI readily bound reduced rainbow trout (Igμ), chicken (Igν), mouse (Igμ, Igγ1, Igγ2a, Igγ2b, and Igα), rabbit (Igμ and Igγ) and goat (Igγ) IgH chains, and mouse Igκ and Igλ, and chicken Igλ IgL chains. IpFcRI also bound mouse IgM, IgA and IgG subclasses when examined under native conditions.