Characterization of a critical binding site for human polymeric Ig on secretory component

Secretory component (SC) is an integral membrane glycoprotein of secretory epithelial cells which is responsible for the specific transport of polymeric Ig (PIg) to external mucosal surfaces. The ectoplasmic segment which binds polymeric Ig is comprised of five Ig-type domains. Chemically and enzymatically modified forms of the ectoplasmic portion of SC (FSC) were produced and tested for their ability to bind to PIgA and PIgM. Deglycosylated FSC bound specifically to PIg, indicating that N-linked carbohydrate moieties on FSC are not required for binding. Denatured, reduced, and alkylated FSC did not bind to PIgA, and bound to PIgM with significantly reduced affinity, suggesting that native conformation of the polypeptide backbone of SC was important to binding. Tryptic fragments of FSC which bound to PIg were isolated and identified to be derived from domain I of SC. Synthetic peptides comprising overlapping portions of domain I bound to PIg to varying degrees. The strongest affinity was demonstrated by a peptide comprised of residues 15 to 37 of SC. A comparison of the amino acid sequences of human, rabbit, and rat SC indicated that this region contained a high degree of residue identity (78%) and may represent a consensus sequence for binding of FSC to PIg. Importantly, the peptide comprised of residues 15 to 37 was also recognized by a monoclonal antibody, 6G11, which inhibited the binding of FSC to PIgA. These results demonstrate that the binding of human SC to PIg is critically dependent on a highly conserved peptide region within the first domain of SC centering at residues 15-37.