An improved preparation of ribosomes and polysomes from cardiac muscle

Isolation and characterization of poly(adenylic acid)-containing messenger ribonucleic acid from rat liver polysomes

Undegraded rat liver polysomes were obtained after homogenizing the tissue in a medium containing NH4Cl, heparine, and yeast tRNA. Purification of poly(A)-containing RNA from polysomal RNA was accomplished by affinity chromatography on oligo(dT)-cellulose columns. Poly(A)-containing RNA molecules were monitored by the formation of ribonuclease-resistant hybrids with [3H]poly(U). To improve the separation of messenger RNA and ribosomal RNA by oligo(dT)-cellulose it was found essential to dissociate the aggregates formed between both molecular species by heat treatment in the presence of dimethylsulfoxide (Me2SO) prior to chromatography. Sucrose gradient analysis under denaturing conditions showed that the preparations obtained were virtually free of ribosomal RNA. Poly(A)-containing RNA constituted approx. 2.2% of the total polysomal RNA and the number average size was 1500--1800 nucleotides, as judged by sedimentation analysis on sucrose density gradients containing Me2SO. Approximately 8.2% of the purified preparation obtained was able to anneal with [3H]poly(U); the number average nucleotide length of the poly(A) segment of the RNA population was calculated to be 133 adenylate residues. Based on these values, our preparations appear to be greater than 90% pure. The RNA fractions obtained after oligo(dT)-cellulose chromatography were used to direct the synthesis of liver polypeptides in a heterologous cell-free system derived from wheat-germ. The system was optimized with respect to monovalent and divalent cations, and presence of polyamines (spermine). More than 65% of the translational activity present in the unfractionated polysomal RNA was recovered in the final poly(A)-containing RNA fraction. However, about 25% of the activity was found to be associated with the unbound fraction which was essentially free of poly(A)-containing RNA. Immunoprecipitation analysis with a specific antiserum to rat serum albumin demonstrated that about 6--8% of the labeled synthetic products translated from the poly(A)-containing RNA sample corresponded to serum albumin. Analysis of the translation products by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a heterogeneous distribution of molecular sizes ranging from 15 000 to greater than 70 000 daltons. Spermine not only increased the overall yield and extent of protein synthesis, but also resulted in higher yields of large protein products. Under optimal translation conditions a discrete peak representing about 7% of the total radioactivity was observed to migrate with rat serum albumin.

Protein synthesis by perfused hearts from normal and insulin-deficient rats. Effect of insulin in the presence of glucose and after depletion of glucose, glucose 6-phosphate and glycogen

In the absence of glucose, insulin stimulated the incorporation of (14)C-labelled amino acids into protein by perfused rat hearts that had been previously substantially depleted of endogenous glucose, glucose 6-phosphate and glycogen by substrate-free perfusion. This stimulation was also demonstrated in hearts perfused with buffer containing 2-deoxy-d-glucose, an inhibitor of glucose utilization. It is concluded that insulin exerts an effect on protein synthesis independent of its action on glucose metabolism. Streptozotocin-induced diabetes was found to have no effect either on (14)C-labelled amino acid incorporation by the perfused heart or on the polyribosome profile and amino acid-incorporating activity of polyribosomes prepared from the non-perfused hearts of these insulin-deficient rats, which show marked abnormalities in glucose metabolism. Protein synthesis was not diminished in the perfused hearts from rats treated with anti-insulin antiserum. The significance of these findings is discussed in relation to the reported effects of insulin deficiency on protein synthesis in skeletal muscle.

Myocardial aminoacyl-transfer-ribonucleic acid synthetase and aminoacyl-transferring enzyme activity

The properties of cytoplasmic aminoacyl-tRNA synthetase and aminoacyl-transferring enzymes in the myocardium were examined and methods for the assay of the activity of these enzyme systems were developed. Aminoacyl-tRNA synthetase activity was measured from the rate of incorporation of (14)C-labelled amino acid into aminoacyl-tRNA. Transferase activity was measured from the rate of incorporation of amino[(14)C]acyl-tRNA into protein in the presence of a standard preparation of hepatic ribosomes. Aminoacyl-tRNA synthetase activity is labile once the heart has been homogenized, whereas transferase activity is stable. The source of energy for synthetase activity is ATP; that for transferase is GTP. Transferase activity was inhibited by puromycin and stimulated by dithiothreitol, whereas synthetase activity was unaffected.

The protein-synthesizing activity of ribosomes isolated from the mammary gland of lactating and pregnant guinea pigs

1. Polyribosome preparations were made from the deoxycholate-treated post-nuclear fractions obtained by the disruption of mammary glands from lactating and pregnant guinea pigs. 2. A high proportion of large polyribosomes was obtained from the glands of lactating animals whereas mainly small polyribosomes were obtained from the glands of pregnant animals. The isolated preparations incorporated [(14)C]phenylalanine into protein. The polyribosomes from the glands of pregnant animals were less active than those from the glands of lactating animals but the activity of the former was stimulated more by poly(U) than was the latter. 3. The ribosomes from mammary gland could be dissociated into subunits after incubation, under conditions necessary for protein synthesis, in the presence of puromycin. The subunits could be recombined to give a preparation that actively polymerized [(14)C]phenylalanine in the presence of poly(U). The subunits from guinea-pig mammary gland could be combined with subunits from liver of either guinea pig or rat. Hybrid ribosomes were also formed from subunits derived from glands of pregnant and lactating animals. The hybrids were as active as were the ribosomes formed by reassociation of subunits from the same tissue, suggesting that in this respect the ribosomes from pregnant animals were not defective. 4. Polyribosomes from mammary glands of lactating animals when incubated with cell sap from the same source were tested for their ability to synthesize alpha-lactalbumin. The polyribosomes were incubated in the presence of [(3)H]leucine and alpha-lactalbumin was isolated from the supernatant. The protein was finally treated with cyanogen bromide and the C-terminal and N-terminal fragments were separated and their radioactivity was determined. Both fragments were radioactive consistent with the synthesis of alpha-lactalbumin. 5. The results are discussed in relation to protein synthesis in the mammary gland after parturition.

The rate-limiting step of protein synthesis in vivo and in vitro and the distribution of growing peptides between the puromycin-labile and puromycin-non-labile sites on polyribosomes

1. At 3 min after an intravenous injection of radioactive amino acids into the rat, the bulk of radioactivity associated with liver polyribosomes can be interpreted as growing peptides. 2. In an attempt to identify the rate-limiting step of protein synthesis in vivo and in vitro, use was made of the action of puromycin at 0 degrees C, in releasing growing peptides only from the donor site, to study the distribution of growing peptides between the donor and acceptor sites. 3. Evidence is presented that all growing peptides in a population of liver polyribosomes labelled in vivo are similarly distributed between the donor and acceptor sites, and that the proportion released by puromycin is not an artifact of methodology. 4. The proportion released by puromycin is about 50% for both liver and muscle polyribosomes labelled in vivo, suggesting that neither the availability nor binding of aminoacyl-tRNA nor peptide bond synthesis nor translocation can limit the rate of protein synthesis in vivo. Attempts to alter this by starvation, hypophysectomy, growth hormone, alloxan, insulin and partial hepatectomy were unsuccessful. 5. Growing peptides on liver polyribosomes labelled in a cell-free system in vitro or by incubating hemidiaphragms in vitro were largely in the donor site, suggesting that either the availability or binding of aminoacyl-tRNA, or peptide bond synthesis, must be rate limiting in vitro and that the rate-limiting step differs from that in vivo. 6. Neither in vivo nor in the hemidiaphragm system in vitro was a correlation found between the proportion of growing peptides in the donor site and changes in the rate of incorporation of radioactivity into protein. This could indicate that the intracellular concentration of amino acids or aminoacyl-tRNA limits the rate of protein synthesis and that the increased incorporation results from a rise to a higher but still suboptimum concentration.

Protein synthesis by membrane-bound and free ribosomes of secretory and non-secretory tissues

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.