Studies on polysome-membrane interactions in mouse myeloma cells.

Abstract

The nature of the interaction of membrane‐bound polysomes with the endoplasmic reticulum has been studied in MOPC 21 mouse plasmocytoma tissue‐culture cells which secrete an immunoglobulin G. Treatment of microsomes at high salt concentrations (500 mM KCl) results in the detachment of 40–45% of the bound ribosomes. This released (“loose”) fraction is predominantly monosomal in contrast to the remaining “tight” polysomes. It is shown that both ribosomal fractions contain immunoglobulin light‐chain mRNA which argues against a functional distinction between them in the cell.Polysomes have been dissociated to apparently undegraded ribosomal subunits at 0°C by incubation with puromycin in the presence of a magnesium ion buffer. Application of this procedure to membrane‐bound polysomes results in partial release of ribosomal subunits from the membrane. The ribosomal material which is not so released sediments predominantly at 60 S and this fraction can be detached by subsequent washing at 500 mM KCl. Thus, by use of the polysome‐dissociating procedure together with high salt concentrations, essentially total release of the ribosomal components can be obtained.The results indicate that the salt‐labile interactions are mediated via the large ribosomal subunits and are the only anchoring forces to the membrane at early stages of protein synthesis. The tight membrane‐bound polysomes result from the additional anchoring effect of the polypeptide chains as they grow through the membrane.