A conserved binding site on the receptor for polymeric Ig is homologous to CDR1 of Ig Vκ domains

We have previously shown that the binding of human secretory component (SC) to polymeric IgA (PIgA) is inhibited by a synthetic peptide fragment of SC (SC[15-37]), and also by a mAb (mAb 6G11) which specifically recognizes the unbound form of SC and SC[15-37]. In this report we show that the binding of bovine, rabbit, and rat SC to human PIgA can also be inhibited by SC[15-37], and that the relative affinity of these three species of SC for human PIgA correlated with their relative affinity for mAb 6G11. The binding of mAb 6G11 to SC isolated from various species was especially affected by amino acid substitutions at position 26 of SC. Structural modeling revealed that the side chain of residue 26 is located on the external face of a loop in SC domain I which is homologous in size and position to the first complementarity determining region (CDR1) of Ig Vκ domains. However, the surface topography of the CDR1-like loop of SC is distinct from other known Vκ CDR1 loops due to the unique distribution of charged and bulky polar side chains which locate to the exterior face of the loop. Together, these findings indicated that the binding site on the receptor for PIg is highly conserved among species, and is composed, in part, of residues of domain I of SC which may form a distinct CDR1 -like loop on the surface of the molecule.