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Protein synthesis by membrane-bound and free ribosomes of the developing rat cerebral cortex.

ABSTRACT: 1. Rates of RNA and protein synthesis were measured in rat cerebral-cortex slices, and compared with amino acid incorporation into protein by membrane-bound and free ribosomes from the same tissue, in the first 3 weeks of life. 2. A rapid age-dependent decline in the incorporation of labelled precursors into both RNA and protein was observed, which was more marked for amino acid incorporation into protein. 3. Although membrane-bound ribosomes comprise only a small fraction of total ribosomes, they were more active in incorporating amino acids into protein than were free ribosomes, especially immediately after birth. The decline in activity with age was more marked in the membrane-bound fraction than in free ribosomes. This loss of activity was largely independent of alterations in soluble factors or endogenous mRNA content and appeared to involve some alteration of the function of the ribosome itself, with relatively small alterations in the ratio of membrane-bound to free ribosomes. 4. Thyroidectomy, performed soon after birth, had no effect on the incorporation of radioactive precursors into RNA or protein by either slices or the cell-free preparations during the first 3-4 weeks of life.

SUBMITTER: Andrews TM

PROVIDER: S-EPMC1177275 | biostudies-other | 1971 Oct

REPOSITORIES: biostudies-other

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Project description:1. Methods for the separation of membrane-bound and free ribosomes from rat brain (cortex) and skeletal muscle were described and the preparations characterized by chemical analysis and electron microscopy. The attachment of ribosomes to membranes is not an artifact of the separation procedure. 2. The rate of incorporation of l-[(14)C]leucine into protein in vitro by the membrane-bound and free ribosomes from these two predominantly non-protein-secreting tissues is compared with that by similar preparations from rat liver. With all three tissues the initial rate was higher for the membrane-bound preparations. 3. By using the technique of discharging nascent polypeptide chains by incubation with puromycin followed by treatment with sodium deoxycholate (Redman & Sabatini, 1966), a major difference was observed for the vectorial discharge of nascent protein synthesized both in vivo and in vitro on membrane-bound ribosomes from liver, on the one hand, and brain and muscle, on the other. Whereas a large part of nascent protein synthesized on membrane-bound liver ribosomes was discharged into the membranous vesicles (presumably destined for export from the cell), almost all nascent protein from membrane-bound ribosomes from brain and muscle was released directly into the supernatant. Incorporation of [(3)H]puromycin into peptidyl-[(3)H]puromycin confirmed these findings. There was thus no difference between membrane-bound and free ribosomes from brain on the one hand, and from free polyribosomes from liver on the other, as far as the vectorial release of newly synthesized protein was concerned. 4. Incubation with puromycin also showed that the nascent chains, pre-formed in vivo and in vitro, are not involved in the attachment of ribosomes to membranes of the endoplasmic reticulum. 5. The differences in vectorial discharge from membrane-bound ribosomes from liver as compared with brain and muscle are not due to the different types of messenger RNA in the different tissues. Polyphenylalanine synthesized on incubation with polyuridylic acid was handled in the same way as polypeptides synthesized with endogenous messenger. 6. It is concluded that there is a major difference in the attachment of ribosomes to the membranes of the endoplasmic reticulum of secretory and non-secretory tissues, which results in a tissue-specific difference in the vectorial discharge of nascent proteins.

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Protein synthesis by membrane-bound and free ribosomes of the developing rat cerebral cortex.

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