Intra- and interchain disulfide bridges of the human J chain in secretory immunoglobulin A.

All intra J chain disulfide bridges of human sIgA, the disulfide bonds between the J chain and the two IgA monomers, and one inter IgA monomer disulfide bridge were determined. sIgA was isolated from colostrum of healthy women and digested with IgA1-specific protease followed by cyanogen bromide cleavage. This procedure generated fragments of 140 kDa, 60 kDa, and 28 kDa. The 28-kDa polypeptide comprised the complete J chain covalently bound to two alpha 1 chain octapeptides derived from the C-termini of two alpha 1 chains. The 28-kDa fragment was digested with trypsin. The resulting peptides were purified by RP-HPLC, and subsequently characterized by amino-acid analysis, mass spectrometry, and gas phase sequencing. These data unequivocally show that the J chain cysteines C1-C6, C4-C5, and C7-C8 form intra chain disulfide bridges. The second (C2) and the third (C3) J chain cysteines are disulfide linked to two alpha chain cysteines (C17) joining the two IgA monomers of sIgA tail to tail. The remaining two alpha chains of the two monomers are directly bound to each other via their ultimate cysteines (C17-C17). A new model for the J chain in sIgA is presented.