Secretory component is covalently bound to a single sub-unit in human secretory IgA

Abstract

The binding of secretory component (SC) ∗ ∗ SC, secretory component; sIgA, secretory immunoglobulin A; CNBr, cyanogen bromide. to the α chain in human secretory IgA has been studied in order to decide whether SC is covalently attached to one or both IgA sub-units. Secretory IgA was digested with streptococcal ( Streptococcus sanguis) IgA protease, and the purified SC-(Fc) 2 · J fragment from the IgA 1 sub-class was treated with cyanogen bromide. The resulting products were separated on Sephadex G-150 in 1 M acetic acid, and two main peaks were obtained. The first had a mol. wt of 115,000, and by reduction was shown to contain SC (80,000 mol. wt) and a fragment of the a chain (26,000 mol. wt). The second Sephadex peak contained the Fc portion of the other IgA monomer (51,000 mol. wt), and no SC; its mol. wt was halved on reduction. Furthermore, the NH 2-terminal amino acid sequences of the a chain fragments from both peaks confirmed their identities as Fc fragments of IgA, beginning at residue position 228, the same position where serum IgA is split by IgA protease. The results indicate that SC is disulfide-bonded to only one of the two monomer IgA sub-units of secretory IgA. The sequence studies additionally revealed at position 230, in comparison to previously studied IgA myeloma proteins, an Ala/Ser substitution, whose possible significance is discussed.