Fate of reversing factor during restoration of protein synthesis by hemin or GTP in heme-deficient reticulocyte lysates

Cisplatin inhibits protein synthesis in rabbit reticulocyte lysate by causing an arrest in elongation

The mechanism through which cisplatin (cis-diamminedichloroplatinum) inhibits protein synthesis in rabbit reticulocyte lysate was characterized. Cisplatin and transplatin caused a progressive slowing in the rate of protein synthesis culminating in the complete arrest of translation. Inhibition was dependent upon the aquation of the compounds. Addition of eukaryotic initiation factor eIF-2, eIF-2B, cAMP, MgGTP, or dithiothreitol neither prevented nor reversed the inhibition induced by cisplatin, indicating that the mechanism of cisplatin-induced translational inhibition is distinct from the inhibition induced by other toxic heavy metal ions (Hurst, R., Schatz, J. R., and Matts, R. L. (1987) J. Biol. Chem. 262, 15939-15945; Matts, R. L., Schatz, J. R., Hurst, R., and Kagen, R. (1991) J. Biol. Chem. 266, 12695-12702). Analysis of the polyribosome profile of cisplatin-inhibited reticulocyte lysate indicated that cisplatin arrests the elongation stage of protein synthesis. Agarose gel electrophoresis and Northern blot analysis indicated that mRNA and rRNA become crosslinked to form very high-molecular-weight adducts upon extraction of the RNA from polyribosomes of cisplatin-treated lysates. Diethyldithiocarbamate, which reduces the cytotoxicity of cisplatin in vivo, protects protein synthesis in reticulocyte lysate from inhibition by cisplatin. The data suggest that extensive derivatization of reticulocyte lysate RNA by cis- and transplatin results in the arrest of translating ribosomes. Since arrest of translational elongation is a well-defined mechanism of action of several families of toxins, we suggest that it may contribute to the cytotoxic action of cisplatin observed in certain populations of cells.

Purification and characterisation of an initiation-factor-2 kinase from uninduced mouse erythroleukaemia cells

Mouse erythroleukaemia (MEL) cells, which have not been induced into erythroid development, contain a protein kinase (MKu) which phosphorylates the alpha subunit of protein-synthesis-initiation factor 2 (eIF-2 alpha). In this paper, we show that this kinase phosphorylates both eIF-2 alpha and a synthetic peptide based on the phosphorylation site in eIF-2 alpha at Ser51, the target residue for other eIF-2 alpha kinases. Consistent with this, prior treatment of eIF-2 with MKu impaired the exchange of bound GDP for GTP which is catalysed by the exchange factor eIF-2B. Using a modified cell-free translation system, we have shown that MKu inhibits translation, consistent with the above observations concerning the site of phosphorylation and the effect of phosphorylation on eIF-2B-mediated guanine-nucleotide exchange. MKu has been purified and its properties have been compared with those of the haem-controlled repressor eIF-2 alpha kinase (HCR) from rabbit reticulocytes. Its behaviour on gel filtration is similar to that of HCR, while its behaviour on anion exchange resembles that of certain phosphorylated species of HCR. Highly purified preparations of MKu contain a protein with an apparent molecular mass of 98 kDa which comigrates with HCR on SDS/PAGE. This protein undergoes phosphorylation when incubated in the presence of Mg(2+)-ATP, and both this apparent autophosphorylation and the activity of the kinase against eIF-2 alpha are inhibited by the same, low, (10 microM) concentrations of haemin. Phosphorylation of the 98-kDa components present in the MEL-cell kinase preparation and in purified rabbit reticulocyte HCR occurs on serine and threonine residues. Analysis of these phosphoproteins by peptide mapping reveals significant differences in their structures, indicating that they may be closely related, but are certainly not identical.

Recycling and phosphorylation of eukaryotic initiation factor 2 on 60S subunits of 80S initiation complexes and polysomes

Phosphorylation of the alpha-subunit (38 kDa) of eukaryotic initiation factor 2 (eIF-2 alpha) regulates initiation of protein synthesis in eukaryotic cells. This phosphorylation is enhanced in cycloheximide-treated heme-deficient reticulocyte lysates in which polysomes are maintained. In early heme deficiency prior to polysome disaggregation, eIF-2(alpha P) accumulates primarily on the 60S subunits of polysomes. Further, isolated polysomes contain eIF-2 alpha that is efficiently phosphorylated in vitro by heme-regulated inhibitor (HRI). Immunoblot analysis of eIF-2 distribution in sucrose gradients of actively protein-synthesizing lysates indicates that eIF-2 is distributed at low levels throughout the polysome profiles. These findings suggest that polysome-bound eIF-2 alpha is a target of HRI under physiological conditions. The presence of eIF-2 on the 60S subunits of polysomes is incompatible with the conventional model in which eIF-2 is recycled during the joining of the 48S preinitiation complex and the 60S subunit to form the 80S initiation complex. A modified model is presented with emphasis on the translocation of eIF-2 from the 40S ribosomal subunit of the 48S preinitiation complex (eIF-2.GTP.Met-tRNA(f).40S.mRNA) to the 60S subunit of the 80S initiation complex.

Partial purification of a novel N-ethylmaleimide-activated translational inhibitor from adult rat brain

A translational inhibitor that is activated by N-ethylmaleimide treatment can be found in the postmicrosomal fraction prepared from the brain of adult rats, but it is almost undetectable in the same fraction prepared from suckling animals. The inhibitor is thermolabile and remains in the supernatant fraction after precipitation at pH 5. During the purification procedure, the inhibitor in its unactivated state binds to the anion exchanger (diethylaminoethyl-cellulose) but not to the cation exchanger (phosphocellulose). Treatment with N-ethylmaleimide increases inhibitor affinity for the cation exchanger, and this chromatographic step purifies the inhibitor by 143-fold. Both the thermolabile nature and the behavior of the inhibitory activity during the different steps of the purification procedure suggest that this activity is most probably due to a protein. Although the addition of initiation factor 2 reverses the inhibition of protein synthesis in the presence of ATP and Mg2+, the inhibitor does not phosphorylate any of the initiation factor subunits "in vitro," which indicates that it does not contain any intrinsic protein kinase activity. However, its presence in both a crude and a purified preparation of a kinase of the alpha subunit of a brain eukaryotic initiation factor increases the phosphorylation of the alpha subunit of the initiation factor. The mechanism of action of this inhibitor is discussed.

Tissue distribution and immunoreactivity of heme-regulated eIF-2 alpha kinase determined by monoclonal antibodies

A highly purified preparation of heme-regulated inhibitor (HRI), an eIF-2 alpha kinase, from rabbit reticulocyte lysates has been used for generating monoclonal antibodies (mAB). Two hybridoma clones secreting HRI-specific antibodies (mAB A and mAB F) were obtained. Both antibodies immunoprecipitated biosynthetically labeled as well as phosphorylated HRI in reticulocyte lysates and also recognized denatured HRI in a Western blot. In in vitro protein kinase assays, preincubation of HRI with the antibodies significantly diminished both autokinase and eIF-2 alpha kinase activities. HRI from reticulocyte lysates could be quantitatively removed by immunoprecipitation with mAB F, and such HRI-depleted lysates were able to maintain protein synthesis under conditions of heme deficiency. With these monoclonal antibodies, HRI was detected only in the reticulocytes and bone marrow of anemic rabbits, among several rabbit tissues tested. The antibodies did not detect cross-reacting HRI in rat or human reticulocytes or in mouse erythroleukemic cells or human K562 cells even after induction of differentiation, although eIF-2 alpha kinase activity was detected in them. Polyclonal anti-rabbit HRI antibody detected HRI in rat reticulocytes. However, no cross-reacting HRI was detected by polyclonal antibody in human reticulocytes or other cell types tested. These findings suggest that HRI is not ubiquitous, and may be erythroid-specific, and that it is antigenically different in different species.

Amino acid microsequencing of internal tryptic peptides of heme-regulated eukaryotic initiation factor 2 alpha subunit kinase: homology to protein kinases

We have purified the heme-regulated eukaryotic initiation factor 2 alpha subunit (eIF-2 alpha) kinase (HRI) from rabbit reticulocytes for amino acid microsequencing. This kinase is a single 92-kDa polypeptide and migrates in perfect alignment with 32P-labeled HRI on SDS/PAGE. Its functions of binding ATP and of autophosphorylation and eIF-2 alpha phosphorylation are inhibited by hemin. The amino acid sequences of three tryptic peptides of HRI have been obtained. A search of the data base of the National Biomedical Research Foundation reveals that these amino acid sequences are unique and that two of these three sequences show homology to protein kinases. HRI peptide P-52 contains Asp-Phe-Gly, which is the most highly conserved short stretch of amino acids in catalytic domain VII of protein kinases. HRI peptide P-74 contains the conserved amino acid residues Asp-(Met)-Tyr-Ser-(Val)-Gly-Val found in catalytic domain IX of protein kinases [Hanks, S. K., Quinn, A. M. & Hunter, T. (1988) Science 241, 42-52]. These findings are consistent with the autokinase and eIF-2 alpha kinase activities of HRI. Synthetic HRI peptide P-74 is a very potent inhibitor of eIF-2 alpha phosphorylation by HRI. Since little is known about the function of conserved domain IX, P-74 peptide may be useful in elucidating the role of this domain of protein kinases.

Correlation between the distribution of the reversing factor and eukaryotic initiation factor 2 in heme-deficient or double-stranded RNA-inhibited reticulocyte lysates

The recycling of eukaryotic initiation factor eIF-2 requires the exchange of GDP for GTP, in a reaction catalyzed by the reversing factor (RF). Recent studies have suggested that a 60 S ribosomal subunit-bound eIF-2.GDP complex is an intermediate in protein chain initiation. We have monitored the distribution of RF in heme-deficient and dsRNA-inhibited lysates by immunoblot analysis of sucrose gradient fractions and have compared the distribution with that of eIF-2(alpha-32P). RF and eIF-2(alpha P) were both found to be tightly associated with 60 S and 80 S ribosomes, as their distribution did not change in gradients containing up to 0.1 M K+. The association of eIF-2(alpha-32P) and RF with 60 S and 80 S ribosomes was enhanced in the presence of F-, indicating the presence of an endogenous ribosome-associated phosphatase activity which is capable of dephosphorylating eIF-2(alpha P) in the absence of F-. These observations are consistent with the hypothesis that under physiologic conditions, RF interacts with the 60 S-bound eIF-2.GDP complex to promote the dissociation of GDP from eIF-2 and the release of eIF-2 from the 60 S subunit as a complex with RF.

The association of NADPH with the guanine nucleotide exchange factor from rabbit reticulocytes: a role of pyridine dinucleotides in eukaryotic polypeptide chain initiation

The guanine nucleotide exchange factor (GEF) was purified to apparent homogeneity from postribosomal supernatants of rabbit reticulocytes by chromatography on DEAE-cellulose and phosphocellulose, fractionation by glycerol gradients, and chromatography on Mono S and Mono Q (Pharmacia). At the Mono S step GEF is isolated as a complex with the eukaryotic polypeptide chain initiation factor 2 (eIF-2) and is separated from this factor by column chromatography on Mono Q. An emission spectrum characteristic of a reduced pyridine dinucleotide was observed when GEF was subjected to fluorescence analysis. By both coupled enzymatic analysis and chromatography on reverse-phase or Mono Q columns, the bound dinucleotide associated with GEF was determined to be NADPH. The GEF-catalyzed exchange of eIF-2-bound GDP for GTP was markedly inhibited by NAD+ and NADP+. This inhibition was not observed in the presence of equimolar concentrations of NADPH. Similarly, the stimulation of ternary complex (eIF-2 X GTP X Met-tRNAf) formation by GEF in the presence of 1 mM Mg2+ was abolished in the presence of oxidized pyridine dinucleotide. These results demonstrate that pyridine dinucleotides may be directly involved in the regulation of polypeptide chain initiation by acting as allosteric regulators of GEF activity.