On the Attachment of Ribosomes to Heavy Rough and Light Rough Endoplasmic Reticulum in Eukaryotic Cells
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A 48 hr period of starvation resulted in an almost complete conversion of rough endoplasmic reticulum (ER) membranes to the smooth (S) type. The order of re-appearance of the light rough (LR) and heavy rough (HR) ER fractions after feeding a starved culture was dependent on the nutritional conditions. Inhibition of protein synthesis by cycloheximide resulted in a conversion of HR to S and/or LR membranes. The HR fraction re-appeared within 1/2 hr after removal of the block. RNase treatment of isolated HR membranes resulted in a release of ribosomes sufficient to lower their density causing them to migrate to the position of LR membranes upon re-centrifugation. The migration of LR membranes was unaffected by this treatment. The migratory behaviour of HR membranes was also affected by raising the salt concentration of the gradient buffer from 25mM to 100mM KCl: the membranes appearing in the LR region of the gradient. The results add further support to the suggestion that the HR and LR fractions represent distinct compartments of the rough ER. In addition differences in the mechanisms of association of ribosomes with HR and IR membranes are indicated.
KeywordsEndoplasmic Reticulum Membrane RNase Treatment Discontinuous Sucrose Gradient Polysome Profile Identical Aliquot
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