Inhibition of peptide chain initiation in lysates from ATP-depleted cells. II. Studies on the mechanism of the lesion and its relation to similar alterations caused by oxidized glutathione and hemin deprivation

Changes in ribosomal properties during adenylate deprivation in the cells of Kluyveromyces lactis

In an adenine-requiring mutant strain of the yeast, Kluyveromyces lactis, the intracellular content of ATP is one-third to one-fifth that in a prototrophic wild strain under growing conditions. The quantitative differences becomes rather small in resting stationary-phase cells. Temporary changes in the two-dimensional protein patterns of mutant ribosomes occur when the ATP content is lowest during the transition phase of growth. The transfer of exponentially growing cells to a synthetic complete medium void of adenine induces the same changes in mutant ribosomes within several hours. Identification of ribosomal proteins by two-dimensional gel electrophoresis indicated all changeable proteins (at least five proteins) to belong to 40S ribosomal subunits. The mutant ribosomes prepared from the transition-phase cells have much lower activity (below 60%) for poly(U)-directed polyphenylalanine synthesis than those in exponentially growing or resting stationary-phase cells. Thus, changes in ribosomal components associated with the differences in ribosome activity in a cell-free system were noted in the adenylate-deprived cells of K. lactis.

Protein synthesis in cell-free reticulocyte lysates on multi-hour incubation

Cell-free protein synthesis in rabbit reticulocyte lysate translation mixtures has been studied during multi-hour incubations. In an impaired lysate obtained from cells stored at 0 degrees C before lysis, and showing a low initial rate of synthesis, translation could be stimulated by a factor of 4 by including RNase inhibitor and additional ATP and GTP. In translation mixtures prepared from normal lysates, protein synthesis could be improved by approximately 50% by the addition of excess GTP. The observed increases in protein synthesis were obtained by improved maintenance of the initial rate of synthesis.

Studies on the activation of the heme-stabilized translational inhibitor of reticulocyte lysates by oxidized glutathione and NADPH depletion

The translational inhibition produced by addition of oxidized glutathione (GSSG) to hemin-containing reticulocyte lysates and the accompanying phosphorylation of the alpha subunit of the polypeptide chain initiation factor eIF-2 can be prevented or reversed by NADPH generators, including glucose 6-phosphate, deoxyglucose 6-phosphate, fructose 6-phosphate, NADPH itself, and also by dithiols, e.g., dithiothreitol, but not by reduced glutathione (GSH) or other monothiols, e.g., 2-mercaptoethanol. The same is true of the inhibition caused by addition of glutamate dehydrogenase, alpha-ketoglutarate, and NH4+, which may be entirely due to NADPH depletion via the reaction.

High pO2-activated inhibitor of protein synthesis in rabbit reticulocytes: its relationship to glutathione disulfide-induced inhibitor and to a approximately 23,000-Mr sulfhydryl protein

The treatment of reticulocyte post-ribosomal supernatant containing ribosome wash with high pO2 or glutathione disulfide resulted in the activation of an inhibitor of protein synthesis of approximately 23,000-Mr as implicated by its elution from Sephadex G-100. This inhibitor could also be directly activated by exposure of the approximately 23,000-Mr fractions of the control eluate to high pO2 or glutathione disulfide. The high pO2-dependent activation of the inhibitor was blocked by the presence of glucose-6-phosphate or cAMP (2 mM). The inhibitor was stable (and activable) during a 5 minute incubation at 80 degrees C. The analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the G-100 (approximately 23,000-Mr) fractions treated with [14C]N-ethylmaleimide revealed the abolishment of the label in a approximately 23,000-Mr protein band in parallel to high pO2-dependent inhibitor activation.

A translational inhibitor activated in rabbit reticulocyte lysates under high pO2

An inhibitor of protein synthesis was activated under high oxygen partial pressure (pO2) in hemin-supplemented and glutathione disulfide-free lysates from rabbit reticulocytes. This inhibitor shared some common features with other translational inhibitors from rabbit reticulocytes; that is, hemin-controlled repressor, glutathione disulfide-activated inhibitor and high pressure-activated inhibitor. It caused biphasic kinetics of inhibition which could be potentiated by ATP. Its activation was prevented by cAMP or glucose 6-phosphate. The high pO2-inhibitor could be partially purified from post-ribosomal supernatant containing ribosomal salt wash by precipitation between 0-50% (NH4)2SO4-saturation, Sephadex G-100, and DEAE-cellulose chromatography.

Regulation of protein synthesis and degradation during in vitro cardiac work

Cardiac work increased protein synthesis in hearts supplied glucose (mixture 1), glucose-insulin-glucagon-lactate-beta-hydroxybutyrate (mixture 2) or palmitate-beta-hydroxybutyrate-glucose (mixture 3). In hearts provided mixture 1, acceleration of synthesis involved increased rates of peptide chain initiation. In these hearts intracellular concentrations of 5 amino acids decreased and 13 others were unchanged, indicating that faster protein synthesis did not depend on increased amino acid availability. In hearts supplied mixtures 2, 3, or 4 (lactate-glucose-insulin), intracellular concentrations of branched-chain amino acids were decreased by work, whereas intracellular levels of some acidic and neutral amino acids increased. Protein degradation was decreased by work in hearts supplied mixtures 1 and 2, but not mixtures 3 and 4. In hearts provided mixture 1, nitrogen balance was negative, but less so in working preparations. Nitrogen balance was zero or positive in working hearts provided mixtures 2 and 4. These studies indicated that in hearts supplied some, but not all, of the substrate mixtures, cardiac work maintained efficiently of protein synthesis and inhibited protein degradation. An improved method for perfusion of working hearts with albumin-containing buffer is described.

The role of cyclic 3',5'-AMP in the regulation of aminoacyl-tRNA synthetase activities in mouse uterus and liver following 17beta-oestradiol treatment. Activation of a phosphoaminoacyl-tRNA synthetase phosphatase by phosphorylation with cyclic 3',5'-AMP dependent protein kinase

The changes in the activities of 17 aminoacyl-tRNA synthetases induced by phosphorylation [1] were reversed by the action of cyclic AMP in preparations from both uterus and liver. Cyclic AMP also inhibited the phosphorylation of aminoacyl-tRNA synthetase protein by endogenous non-cyclic AMP-dependent protein kinase and [gamma-32P]ATP. The effect was not due to a stimulation of phosphoaminoacyl-tRNA synthetase phosphatase or to an influence of cyclic AMP on aminoacyl-tRNA synthetases. The activity of phosphoaminoacyl-tRNA synthetase phosphatase was increased by treatment with endogenous cyclic AMP-dependent protein kinase, ATP and cyclic AMP. Affinity chromatography of the 32P-labeled phosphorylated phosphosynthetase phosphatase protein followed by gel electrophoresis showed that the activated phosphatase was phosphorylated. In the uterus, the changes in 17 aminoacyl-tRNA synthetase activities observed 5 min after dibutyryl cyclic AMP administration to ovariectomized mice were similar to those observed after 17beta-oestradiol treatment, whereas in the liver the changes in these activities were the opposite to those found after treatment with 17beta-oestradiol. A mechanism for the regulation of the 17 aminoacyl-tRNA synthetase activities is proposed, which suggests that the synthetase activities inhibited (group I) or stimulated (group II) by phosphorylation with a non-cyclic AMP-dependent aminoacyl-tRNA synthetase kinase are reactivated (group I) or inhibited (group II), respectively, by the action of a cyclic AMP-dependent phosphatase kinase through the increased activity of phosphorylated phosphoaminoacyl-tRNA synthetase phosphatase.

Developmental changes of protein-bound radioactivity distribution in rat brain slices incubated with labelled leucine

The distribution of protein-bound labelled leucine in cortical slices prepared from young rats was investigated by autoradiography and complemented with measurements of the rate of protein synthesis in slices. During the second and third postnatal weeks the rate of protein synthesis in cortical slices from rat brain declined rapidly. The distribution of protein-bound radioactivity was fairly homogenous within the neuronal cell population of cortical slices from 7-day-old rats and was not influenced by the level of labelled leucine in the incubation medium (0.01 and 1 mM). On the contrary, the gradient-like distribution of incorporated radioactivity appeared in slices from 14-day-old rats, at both trace and 1 mM levels of the precursor. The laminar analysis of neuronal cell labelling confirmed the preponderance of incorporation into the neurones localized near the slice surface and the considerable inhibition of neuronal protein synthesis in the slice centre. Therefore, these limitations on the use of cortical slices for the study of brain protein synthesis in the rat should be respected from the beginning of the second postnatal week.

Studies of the protein synthetic activity of lysates from HeLa cells incubated in hypertonic medium

When HeLa S-3 cells are incubated with medium made hypertonic by adding NaCl, protein synthesis is inhibited. When the cells are returned to normal conditions protein synthesis is restored. To study the molecular mechanism of this regulation of protein synthesis, lysates were prepared from HeLa S-3 cells grown in minimum essential medium (normal, N); from cells which were incubated with additional (100 mM) NaCl (hypertonic, H); and from cells which were treated similarly in hypertonic medium and then restored to isotonic conditions (hypertonic-isotonic, H-I). Lysates of H cells exhibited reduced endogenous protein synthesis. Studies with mixed lysates from H and N cells implicated that the H lysate did not contain a soluble, non-labile macromolecule (greater than 10 000 daltons) with an inhibitory activity upon the protein synthesis. Analysis by Edman reaction of H lysates showed a reduced incorporation of [35S]methionine into N-terminal suggesting that the initiation step of protein synthesis was affected. However, sucrose gradietn analysis indicated that lysates of H cells were still able to form 80-S initiation complexes with [35S] methionyl-tRNAIMet. The block in initiation was not complete. The lesion could not be reversed by adding post-ribosomal supernatant or a ribosomal salt wash from N cells to ribosomes from the H cells. The data show that the ribosomal fraction is primarily involved in the inhibition.

Inhibition of peptide chain initiation in lysates from ATP-depleted cells.I. Stages in the evolution of the lesion and its reversal by thiol compounds, cyclic AMP or purine derivatives and phosphorylated sugars

Anaerobic incubation of rabbit reticulocytes at 37 degrees C in Krebs-Ringer solution supplemented with hemin but devoid of glucose resulted at the end of 1-2h in a drastic decline of their ATP content and an attendant arrest of protein synthesis. Subsequent provision of glucose and reoxygenation of the cells was followed by a rapid replenishment of the ATP pool, while resumption of protein synthesis was markedly delayed. This lag period could be considerably reduced by addition of 5-10 mM adenine or 2,6-diaminopurine to the incubation medium. Lysates prepared from ATP-depleted cells exhibited disaggregation of the polysomes and an inhibition of the nedogenously coded protein synthesis, when tested in a cell-free system supplied with an adequate ATP generator. Both alterations increased in severity with the progressive decay of the intracellular ATP pool. The early phase of partial inhibition following a 40-70% decrease of the cellular ATP level was fully reversible by fortifying the cell-free preparation with dithiothreitol or a suitable NADPH-generating system. Aternative, the inhibition could be also overcome by millimolar amounts of adenine, 2,6-diaminopurine and a variety of other purine derivatives or cyclic AMP. The effect of these compounds was unrelated to the endogenous cyclic AMP pool. Joint addition of both dithiothreitol and cyclic AMP or adenine was necessary for relieving the initiation block in lysates derived from cells depleted of 80-90% of their ATP content. On further aggravating the conditions of energy starvation, an additional requirement for phosphorylated sugars, e.g. glucose 6-phosphate or fructose 1,6-diphosphate, became apparent. ATP depletion brought about by exposing the cells to Antimycin A or 2,4-dinitrophenol resulted in a lesion which was indistinguishable from that induced by anaerobic incubation. On the other hand, energy deprivation in cell-free lysates from untreated reticulocytes, preincubated in the absence of an ATP-generating system failed to duplicate the deleterious effect of intracellular ATP depletion. Some aspects bearing on the biochemical mechanism of the lesion and its reversal are discussed in the light of the available data.