Biosynthesis of immunoglobulins on polyribosomes and assembly of the IgG molecule

Abstract

Immunoglobulin G formation was studied using as model system an ascitic form of the murine plasmacytoma 5563. Following pulse labelling of the cells with $^{3}$H-leucine, polyribosomes were fractionated on sucrose gradients. By the use of antisera specific for various parts of the IgG molecule, nascent heavy and light chains were detected on distinct polyribosomes of different size. Polyribosomes carrying heavy chain determinants were present in clusters with maximum sedimentation constants of around 300 S.; a much lower proportion of radioactivity was detectable as light chain determinants on polyribosomes up to 200 S. This observation is consistent with the independent synthesis of each chain as one polypeptide unit. Release of light chains appears to be an intermediate stage in the assembly of the IgG molecule. After pulse labelling of cells soluble IgG determinants were analysed by sucrose gradient centrifugation and precipitation with specific antisera. Only the light chains were released into a small pool which may control the release of heavy chains from polyribosomes. The radioactivity of the light chain pool reached a maximum at 10 min, whereas that of whole myeloma protein increased linearly with time. These results fit the interpretation that light chains form a small, rapidly turning over, pool before being incorporated into whole IgG molecules.