THE COVALENT LINKAGE OF SECRETORY COMPONENT TO IGA - STRUCTURE OF SIGA

Immunoglobulin A which is secreted into external fluids is synthesized in plasma cells as an (IgA)2-J-chain complex. This complex docks on t o the polyimmunoglobulin receptor which is located at the basolateral surface of epithelial cells. After docking the (IgA)2-J-receptor compl ex is internalized and processed. The polyimmunoglobulin receptor lose s its C-terminal tail and thus becomes the secretory component. This s ecretory component is then covalently linked to the (IgA)2-J-chain com plex by a disulfide bond, and protects the so formed sIgA from denatur ation and proteolysis in external fluids. In order to establish this d isulfide bond between IgA and the secretory component, sIgA, purified from human colostrum, was subjected to several enzymatic and chemical fragmentation reactions. One of the resulting polypeptides allowed us to characterize the covalent linkage of the secretory component to IgA in sIgA. IgA was found to be covalently linked to the secretory piece by a single disulfide bond between Cys 311 of one alpha-chain and Cys 467 of the secretory component. Cys 501 of the secretory component an d Cys 311 of the other alpha-chain are blocked by cysteines. With this last paper of a series the structure of an entire sIgA molecule has b een elucidated.