Gross conformation of human secretory immunoglobulin A and its component parts.

Abstract

Human secretory immunoglobulin A, several immunoglobulin A dimers and monomers and secretory component have been characterized with regard to size, shape and conformation. The molecular weights obtained were 420000 for secretory IgA, 360000 for IgA dimer, 160000 for IgA monomer and 75000 for secretory component. Secretory IgA has a markedly extended shape as evident from its frictional ratio and intrinsic viscosity. Moreover, the hydrodynamic parameters of secretory IgA are similar to those of dimeric serum IgA, which indicates similar molecular shapes of the two proteins and thus that the presence of the secretory component does not have a significant influence on the shape of the IgA part of the secretory IgA molecule. This is in contrast to results of electron‐microscopic investigations, which have suggested different shapes for secretory IgA and IgA dimer. The hydrodynamic data are compatible with an extended model for both proteins, in which the carboxy‐terminal halves of the IgA mononiers are joined end to end via a rigid or flexible link, but are in less agreement with an alternative, more compact model, in which the two IgA monomers are superimposed on each other. The IgA monomer itself has hydrodynamic characteristics suggesting a significantly more extended shape than that of IgG. Optical rotatory dispersion and circular dichroism spectra show large similarities between the conformations of IgG and both secretory and serum IgA, although minor differences do exist. As in IgG, evidence for the presence of β‐structure in the different types of IgA was obtained. The similarities of the ORD and CD spectra of secretory IgA and IgA dimer suggest that the secretory component does not exert a major effect on the internal folding of the IgA backbone.Human secretory component also has a somewhat extended shape. Its conformation, as measured by ORD and CD spectra, differs markedly from that of the immunoglobulins, although the spectra do seem to suggest the presence of some β‐structure also in secretory component. The main ORD minimum and a prominent CD band exhibited by this protein are very rarely seen in proteins and suggest that a unique type of conformation is present in secretory component.