Differences in the association of ribosomes with heavy rough and light rough endoplasmic reticulum membranes of MPC-11 cells

The treatment of total endoplasmic reticulum membranes of mouse plasmacytoma cells with EDTA resulted in an abolition of the heavy rough (HR) subfraction, while there was a large increase in smooth (S) membranes. When HR and light rough (LR) endoplasmic reticulum membranes were treated individually with EDTA and re-centrifuged on discontinuous sucrose gradients it was observed that HR were converted into S membranes, i.e. membranes virtually devoid of ribosomes. LR membranes were not affected to the same extent but there was a shift to a somewhat lower density. A quantitation of ribosomes released by EDTA showed that 95% of 60 S and 72% of 40 S subunits were removed from HR membranes while for LR membranes the corresponding values were 8.5 and 22.6% respectively. Ratios of radioactivity to absorbance at 260 nm calculated for 40 S and 60 S subunits isolated from HR and LR membranes show that 60 S subunits from LR membranes, in contrast to those from HR membranes, equilibrate only slowly with the free pool of ribosomal subunits. The results indicate that the ribosomes associated with HR membranes are 'loosely bound' and those with LR membranes 'tightly bound'. When poly(A)-containing mRNA isolated from HR and LR membranes was translated in vitro and the products analysed for light-chain immunoglobulin content, it was found that the HR fraction was enriched in light-chain mRNA.