Disulfide Bonding of Secretary Component to a Single Monomer Subunit in Human Secretory IgA

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Abstract
The arrangement of disulfide bonds joining secretory component (SC) to the α chains in secretory IgA was studied by determining the molecular size of the principal fragments resulting from CNBr digestion of secretory dimeric Fc fragments from IgA ((Fc)2α fragments). In vitro complexes formed by incubating 125I-free SC and myeloma 131I-(Fc)2α fragments were isolated by gel filtration and subsequently digested with cyanogen bromide. The CNBr digests of SC·(Fc)2α fragments were analyzed by gel filtration in 5 M guanidine. Two principal fragments were obtained, one containing a monomeric Fc fragment from IgA (Fcα) associated with SC (m.w. ≃ 110,000) and a second containing the second Fcα monomer (m.w. ≃ 50,000) from the dimeric SC·(Fc)2α. Similar results were obtained when secretory (Fc)2α fragments isolated from native secretory IgA dimer were subjected to CNBr digestion. The data indicate that SC is disulfide bonded to a single monomer subunit in secretory IgA dimer.

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