Association of initiation factor eIF-2 with a rapidly sedimenting fraction from Ehrlich ascites-tumour cells

Effect of alanine supply on hepatic protein synthesis in animals maintained on a protein free diet

In contrast to what it is observed during starvation, animals maintained on a protein-free isocaloric diet showed an increase in the rate of hepatic peptide chain elongation as determined by measuring the ribosomal transit time in vivo. The loss of body nitrogen per se is insufficient to generate the signal(s) which arrests hepatic peptide chain elongation. This observation suggests that it is an increase in gluconeogenic demand, and not the negative nitrogen balance, which is implicated in determining reciprocal changes in the rate of protein synthesis. The rate of protein synthesis, as expressed per mg of DNA, does not change in protein deprived animals, while the RNA to DNA ratio decreased. These data also agree with a higher ribosomal efficiency at the elongation step. The animals maintained on a protein-free diet have a decreased hepatic content of protein and an increased concentration of valine, indicating an increased proteolysis. The enhanced rate of polypeptide elongation observed in animals kept on a protein-free diet was accompanied by decreases in the state of aggregation of polyribosomes and in the ability of liver extracts to form eIF-2 catalyzed ternary complexes. These observations suggest that the activity of the hepatic initiation factor in vivo may not be rate limiting. The administration of alanine in vivo to animals maintained on a protein-free diet showed a preferential effect in reaggregating polyribosomes. This action was neither accompanied by detectable effects on the rate of eIF-2 catalyzed ternary complexes formation nor by significant changes in the rate of elongation.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel role for aminoacyl-tRNA synthetases in the regulation of polypeptide chain initiation

Exposure of the temperature-sensitive leucyl-tRNA synthetase mutant of Chinese hamster ovary cells, tsH1, to the non-permissive temperature of 39.5 degrees C results in a rapid inhibition of polypeptide chain initiation. This inhibition is caused by a reduced ability of the eukaryotic initiation factor eIF-2 to participate in the formation of eIF-2.GTP.Met-tRNAf ternary complexes and thus in the formation of 43S ribosomal pre-initiation complexes. Associated with this decreased eIF-2 activity is an increased phosphorylation of the eIF-2 alpha subunit. It has previously been shown in other systems that phosphorylation of eIF-2 alpha slows the rate of recycling of eIF-2.GDP to eIF-2.GTP catalysed by the guanine nucleotide exchange factor eIF-2B. We show here that phosphorylation of eIF-2 alpha by the reticulocyte haem-controlled repressor also inhibits eIF-2B activity in cell-free extracts derived from tsH1 cells. Thus the observed increased phosphorylation of eIF-2 alpha at the non-permissive temperature in this system is consistent with impaired recycling of eIF-2 in vivo. Using a single-step temperature revertant of tsH1 cells, TR-3 (which has normal leucyl-tRNA synthetase activity at 39.5 degrees C), we demonstrate here that all inhibition of eIF-2 function reverts together with the synthetase mutation. This establishes the close link between synthetase function and eIF-2 activity. In contrast, recharging tRNALeu in vivo in tsH1 cells at 39.5 degrees C by treatment with a low concentration of cycloheximide failed to reverse the inhibition of eIF-2 function. This indicates that tRNA charging per se is not involved in the regulatory mechanism. Our data indicate a novel role for aminoacyl-tRNA synthetases in the regulation of eIF-2 function mediated through phosphorylation of the alpha subunit of this factor. However, in spite of the fact that cell-free extracts from Chinese hamster ovary cells contain protein kinase and phosphatase activities active against either exogenous or endogenous eIF-2 alpha, we have been unable to show any activation of kinase or inactivation of phosphatase following incubation of the cells at 39.5 degrees C.

Effect of vasopressin on the regulation of protein synthesis initiation in liver cells

Vasopressin was found to be an effective inhibitor of protein labelling in isolated liver cells. Its effect shows the following distinct characteristics: (1) in contrast with alpha-adrenergic agonists, its effect is observable under a wide range of cellular Ca2+-loading conditions; (2) it is not influenced by the nutritional state of the animal. The lack of vasopressin effect on valine production, and its ability to decrease protein labelling from near-saturation concentrations of [3H]valine, indicate that the observed variations in protein labelling reflect actual changes in the rate of protein synthesis. The action of vasopressin is primarily exerted on the initiation step of protein synthesis and this effect is accompanied by a decreased activity of eukaryotic initiation factor 2. Activators of protein kinase C showed similar but not additive effects on protein synthesis, as did vasopressin. It seems plausible to conclude that protein kinase C activation may play an important regulatory role in hepatic protein synthesis as a transducer of hormonal and perhaps other type of signals.

Translation in micrococcal nuclease-treated cell-free extracts from Ehrlich ascites tumor cells. Stimulation by initiation factor eIF-2B

Translation of exogenous mRNAs in micrococcal nuclease-treated extracts from Ehrlich ascites tumor cells is greatly stimulated by the addition of crude initiation factors or initiation factors eIF-2B and eIF-2 containing eIF-2B. The requirement for exogenous eIF-2B in micrococcal nuclease-treated extracts does not result from either loss or enhanced phosphorylation of eIF-2 during incubation.