Protein-synthesising activity of free and membrane-bound ribosomes in vitro and their differential sensitivity to protein synthesis inhibitors

Initial inhibition and recovery of protein synthesis in cycloheximide-treated hepatocytes

Previous studies conducted with intact rats had demonstrated that protein synthesis was reversibly inhibited by cycloheximide. Polysome aggregation occurred during inhibition with a return to normal during recovery. Suggesting that the block of translational activity involved termination and release of polypeptides. This study involving freshly isolated hepatocytes was undertaken to clarify the mechanism of the biphasic response to cycloheximide. Cycloheximide at 1 microM inhibited [3H]leucine incorporation into both cellular and secreted proteins by at least 86%, without having deleterious effects on membrane integrity as indicated by trypan blue uptake and lactate dehydrogenase (LDH) (EC 1.1.1.27) release. After removal of cycloheximide, incorporation of labeled amino acids into cellular protein and protein secreted into the medium returned to control levels. Kinetically, incorporation into secreted protein exhibited a lag of 30-45 min, indicating that a longer recovery period for restoration of proteosynthetic ability is required for membrane-bound polysomes. During the first 100 min of the recovery period, 30% of the cellular protein, which had been prelabeled during cycloheximide inhibition, was secreted into the medium; treated cells, however, secreted prelabeled protein at a lower initial rate. To elucidate the mechanism of action of cycloheximide, the content of the cytoplasmic ribonucleoprotein complexes (RPC), polysome size classes, and the distribution of radioactivity among the various ribosome classes were determined during inhibition and recovery. Larger size class polysomes (7+) were increased by cycloheximide treatment and remained increased during recovery. During inhibition, there was enhanced [3H]leucine labeling with increasing polysome size, implicating termination as the rate-limiting step, whereas during the recovery phase the labeled nascent polypeptides were removed from the ribonucleoprotein complex at a 3- to 4-fold greater rate than control, indicating an accelerated release of completed proteins.

Protein synthesis on membrane-bound and free ribosomes in growing yeast

1. A method for the study of membrane-binding of ribosomes and the rate of synthesis of protein on free and bound ribosomes has been examined critically. 'Brij 58' has been found to be more suitable for detaching ribosomes from membranes than 'Triton X-100' or sodium deoxycholate since it does not alter the structure or biological activity of the ribosomes. Triton and deoxycholate remove amino acid- and ribonucleotide-containing material from ribosomes, thus decreasing their activity during protein synthesis in a cell free system. 2. The rate of protein synthesis on membrane-bound and free ribosomes, the total amount of ribosomes, and the percentage distribution of free and membrane-bound ribosomes were determined in a culture of synchronously growing yeast cells. The results show that in all probability protein synthesis in yeast cells occurs mainly on membrane-bound ribosomes. Free ribosomes are considered here to be a ribosome pool which allows the cell to increase its rate of protein synthesis rapidly. Membrane-bound and free ribosomes are viewed as different functional states, one being a state of protein synthesis and the other a latent state.

Regulation of mammalian protein synthesis in vivo. Stimulated protein synthesis in liver in vivo after cycloheximide treatment

Protein synthesis in rat liver in vivo was measured between 0 and 72 h after administration of a non-lethal dose of cycloheximide. There was a period of inhibition of [3H]leucine incorporation into both intra- and extra-cellular proteins at 2 h after administration of the drug, which was followed by a recovery phase in which amino acid incorporation varied significantly among the various proteins evaluated. At 12 h there was a marked stimulation of incorporation into nascent polypeptides released from polyribosomes and plasma fibrinogen, but incorporation into ribosomal proteins as well as albumin was still inhibited. Between 12 and 48 h, nascent-polypeptide synthesis remained elevated, but ribosomal-protein synthesis recovered slowly from the inhibition to normal rates only, and plasma-albumin synthesis increased slowly to above control values up to 48 h before returning to normal. A differential pattern of incorporation was also observed for incorporation into free and membrane-bound polyribosomes.

Attachment of ribosomes to endoplasmic membranes in mouse pancreas. Degranulation in vivo caused by the inducers of autophagocytosis neutral red, vinblastine, puromycin, and cadmium ions, and prevention by cycloheximide

The present investigation was undertaken to discover whether in vivo inducers of autophagocytosis such as neutral red (0.40 mg/g body weight), cadmium chloride (0.15 mg/animal), vinblastine sulfate (7.5 mg/kg b.w.), and puromycin dihydrochloride (0.20 mg/g b.w.) are able to produce degranulation of rough surfaced endoplasmic reticulum (ER) of pancreatic acinar cells as we suggested earlier. Using a modified method of Blobel and Potter about 30% of the total ribosomes of untreated control pancreas were recovered in the free form, and 70% in the membrane-bound form. Cycloheximide (0.20 mg/g body weight) had no effect on this distribution of ribosomes, while neutral red, cadmium ions, vinblastine, and puromycin led to the presence of more free ribosomes: thus up to 70% of the total cytoplasmic ribosomes were recovered in the free form 30 to 60 min after treatments with the autophagic inducers. Pretreatments with cycloheximide prevented this shift and the distribution of ribosomes remained normal. On the basis of these and previous results we conclude that in pancreas an initial degranulation of the ER caused by these agents is a precondition of membrane transformation in autophagocytosis which is preventable by cycloheximide. Confirming the ultrastructural data of Longnecker et al. on rat pancreas, puromycin was shown to cause different forms of cellular injury including autophagocytosis. All forms of cellular injury were also prevented by pretreatment with cycloheximide in mouse pancreas.

The in vitro reconstitution of a functional rough membrane active in protein synthesis

Rough endoplasmic reticulum was reconstituted from free polyribosomes and rough membrane stripped from its ribosomes by KCl and puromycin. The reconstituted rough membrane resembled the native rough membrane in the following aspects: RNA/protein ratio, buoyant density in a continuous sucrose gradient, amino acid incorporation capacity and sensitivity towards protein synthesis inhibitors. When the reconstitution was done with double labelled polyribosomes ([32-P] polyribosomes, [3-H] leucine labelling of nascent peptide chain before or after the attachment of the polyribosomes to the membrane) both labels banded together with the reconstituted rough membrane band. Hybrid rough membrane could be formed from rat liver stripped rough membrane and wheat germ ribosomes. This hybrid membrane could translate globin mRNA.