Fig. 1: Overview of the SIgA structure.
From: Secret(ory) revealed: the long-awaited structures of secretory IgA
The IgA subunits are shown in green and blue, J chain is shown as a red cartoon with a tan surface, and SC is shown as a transparent gray surface on the back side of SIgA. Note the unusual, non-globular fold of the J chain, which resembles the Greek letter λ. J chain forms a β-sandwich domain along with the tailpieces from both IgA subunits at the core of the complex; intermolecular disulfide bonds between J chain and IgA tailpieces are illustrated with colored spheres. Extensions from the central core region of J chain interact with the Fc regions of both IgA monomers in the complex. SC forms a structural brace that engages one IgA monomer and J chain via its N-terminal D1 domain; domains D2–D5 form an extended rod that places the D5 domain in position to form a disulfide bond with the Fc region of the opposite IgA monomer. SpsA is not shown in this figure, but it would bind to the back side of SC in the orientation shown.
