Stabilization of Dissociable IgA2 Proteins by Secretory Component

The Am2(+) genetic variant of human IgA2 immunoglobulins is potentially unstable because it lacks disulfide bridges between the heavy (H) and light (L) chains. It dissociates spontaneously under denaturing conditions into heavy and light chain dimer subunits (H2, L2), without the need for prior reduction. Isolated secretory component (SC) binds to human Am2(+), IgA2 myeloma proteins in vitro, and stabilizes them so that they will no longer dissociate into subunits in the presence of denaturants. Both human and bovine SC will bind to and stabilize the polymer forms of these myeloma proteins, but only human SC will bind to and stabilize monomeric proteins. The reaction involves the whole IgA2 molecule rather than just its subunits. Moreover, SC will stabilize a pepsin F(ab′)2 fragment. Since stabilization is prevented by iodoacetic acid, covalent disulfide bonding appears to be involved. The ability of SC to stabilize the potentially dissociable Am2(+) genetic variant of IgA2 may enhance its capacity to function in external secretions.