The Unique Reassociation of Human IgA2 Immunoglobulins from Dimer Subunits

Abstract

Abstract The Am2(+) genetic variant of human IgA2 immunoglobulins lacks disulfide bridges between the heavy and light chains. Under dissociating conditions these proteins separate spontaneously into covalent heavy (H2) and light (L2) chain dimer subunits. In this study covalent heavy (H2) and light (L2) chain dimers were prepared from Am2(+) IgA2 myeloma proteins. These subunits recombined readily to give molecules which closely resembled the native proteins in hydrodynamic, antigenic and optical properties. In contrast, in the case of IgG, IgA1 and Am2(-) IgA2 proteins with intact H-L interchain disulfide bridges, covalent heavy and light chain dimers prepared from these proteins failed to recombine, although monomer heavy and light chains recombined readily. Among human immunoglobulins this property of reversible dissociation and reassociation appears to characterize the Am2(+) genetic variant of IgA2. The light chains exist within these molecules as disulfide-bonded dimers. Selective cleavage of this inter-light (L-L) chain disulfide bridge was possible by gentle reduction with cysteine, with the inter-heavy (H-H) chain bridges remaining intact.