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The binding of ribosomal subunits to endoplasmic reticulum membranes
Abstract:
The binding of ribosomes and ribosomal subunits to endoplasmic reticulum preparations of mouse liver was studied. (1) Membranes prepared from rough endoplasmic reticulum by preincubation with 0.5m-KCl and puromycin bound 60-80% of added 60S subunits and 10-15% of added 40S subunits. Membranes prepared with pyrophosphate and citrate showed less clear specificity for 60S subunits particularly when assayed at low ionic strengths. (2) Ribosomal 40S subunits bound efficiently to membranes only in the presence of 60S subunits. The reconstituted membrane-60S subunit-40S subunit complex was active in synthesis of peptide bonds. (3) No differences in binding to membranes were seen between subunits derived from free and from membrane-bound ribosomes. (4) It is concluded that the binding of ribosomes to membranes does not require that they be translating a messenger RNA, and that the mechanism whereby bound and free ribosomes synthesize different groups of proteins does not depend on two groups of ribosomes that differ in their ability to bind to endoplasmic reticulum.
Insights
Ribosomes bind to the endoplasmic reticulum independently of mRNA translation. This suggests that differences in protein synthesis between free and bound ribosomes are not due to varying binding abilities.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- Ribosomes are responsible for protein synthesis.
- Ribosomes can be found free in the cytoplasm or bound to the endoplasmic reticulum (ER).
- The location of ribosomes influences the proteins they synthesize.
Purpose of the Study:
- To investigate the binding mechanisms of ribosomes and ribosomal subunits to the endoplasmic reticulum (ER).
- To determine if ribosome binding to ER requires active translation (mRNA).
- To explore potential differences in binding affinity between free and membrane-bound ribosomes.
Main Methods:
- Preparation of rough endoplasmic reticulum (RER) membranes from mouse liver.
- Incubation of RER membranes with ribosomal subunits (40S and 60S) under various conditions (e.g., KCl, puromycin, pyrophosphate, citrate).
- Assay of subunit binding efficiency to membranes.
- Reconstitution of membrane-ribosomal subunit complexes and assessment of peptide bond synthesis activity.
Main Results:
- ER membranes pretreated with KCl and puromycin showed specific binding of 60S ribosomal subunits.
- Efficient binding of 40S subunits to membranes required the presence of 60S subunits, forming an active complex.
- No significant differences in membrane binding were observed between subunits from free and membrane-bound ribosomes.
Conclusions:
- Ribosome binding to the ER does not necessitate concurrent mRNA translation.
- The distinct protein synthesis roles of free versus membrane-bound ribosomes are not attributable to inherent differences in their ER-binding capabilities.
- The study elucidates key aspects of ribosome-ER interaction and its implications for protein localization.