Electron Microscope Examination of Free IgA Molecules and of their Complexes with Antigen

Abstract

THE molecules of different classes of immunoglobulin are built from units of similar structure, containing two light and two heavy polypeptide chains linked through disulphide bridges¹. The IgG molecule, which contains one such unit of molecular weight 150,000, has been shown by electron microscopy to be a flexible Y-shaped molecule^2,3 with two arms corresponding to the Fab regions carrying the two antigen-binding sites⁴ and a third (Fc) arm. The IgM molecule consists of five Y-shaped units, each with a molecular weight of 180,000, linked cyclically by disulphide bridges through the Fc arms^5–8. IgA commonly occurs in the form of a bridged dimer. Such an IgA dimer preparation has been examined in the electron microscope, but the molecules shown and described had no distinct Fc feature⁹. We have examined two preparations of dimeric IgA, one isolated from the serum of mice carrying plasmacytoma MOPC 315, and the other from the serum of a patient (W.E.) with multiple myelomatosis. 10 to 15 ml. of each serum was pumped through a column (2.5 × 100 cm) of ‘Sephadex G-200’ (Pharmacia). Fractions were selected from the trailing side of the breakthrough peak, pooled on the basis of their antigenic purity, and concentrated by ultrafiltration (Amicon). Each preparation sedimented in the analytical ultracentrifuge as a single component with a sedimentation coefficient (S_20,w) of about 9.5S and after treatment with 5 mM dithiothreitol dissociated to the monomer of 6.3S.