J chain is covalently bound to both monomer subunits in human secretory IgA

Previous work has established that the secretory component (SC) in human secretory IgA is covalently linked to only one of the two IgA monomer subunits, but it has not been clear whether the J chain is covalently linked to one or to both of these subunits. In view of the asymmetry in the disulfide bonding between SC and the IgA subunits, an arrangement which follows disulfide interchange, several models for the disulfide linkage of J chain and the bonds between IgA subunits were envisaged and investigated. When sIgA was gel filtered through Sephadex G-200 in acetic acid, a single major symmetrical peak eluted at the front. This material contained SC, alpha and L chains, and all of the J chain. The greater resolution afforded by polyacrylamide gel electrophoresis in detergent confirmed that human sIgA contains no major noncovalently linked components in the 150,000-200,000 molecular weight range. In another series of experiments the Fc monomer, which is not covalently attached to SC, isolated after treatment of sIgA with IgA protease and cyanogen bromide, was investigated to learn whether alpha chain COOH-terminal octapeptides could be released by reduction. The results were negative. The available data thus favor a model in which J chain is disulfide-bonded to both IgA monomer subunits in sIgA.