Inhibition of peptide chain initiation by a nonhemin-regulated translational repressor from Friend leukemia cells

Characterization of an inhibitor of protein synthesis initiation from mouse erythroleukemia cells

This study describes the partial purification of a translational inhibitor from mouse erythroleukemia (MEL) cells. It is present in MEL cells induced to erythroid differentiation and in uninduced cells in approximately equal amounts. The inhibitor blocks initiation but not elongation of in vitro protein synthesis in the rabbit reticulocyte lysate and in extracts prepared from induced or uninduced MEL cells. Nuclease-resistance, heat-sensitivity and the chromatographic behaviour of the inhibitor indicate that it is a protein with a relative molecular mass of approx. (45-70).10(3). The inhibitor has no eIF-2 alpha phosphorylating activity and does not affect the formation of the ternary complex [eIF-2.GTP.Met-tRNAf] nor the binding of Met-tRNAf to the 40 S ribosomal subunit. The inhibitor interferes with the binding of mRNA to the 43 S preinitiation complex, independent of the presence of the m7GTP cap of the mRNA.

A (re)initiation-dependent cell-free protein-synthesis system from mouse erythroleukemia cells

Cultured mouse erythroleukemia cells (MEL cells) can be induced in vivo to erythroid differentiation which is marked by the onset of globin mRNA and haemoglobin synthesis. When these cells are briefly exposed to hypertonic growth medium prior to lysis, the resulting post-mitochondrial supernatants show a high in vitro protein-synthesis activity. Amino acid incorporation is linear up to 60 min; more than 80% of this is due to (re)initiation, as shown by the inhibition with edeine. Extracts from induced cells reach only a third of overall incorporation as compared to extracts from uninduced cells. This reduction of the protein-synthesizing capacity is also observed in vivo. Polyacrylamide gel electrophoresis shows that extracts from uninduced cells faithfully translate their endogenous mRNA, whereas in extracts from induced cells, non-globin protein synthesis is reduced and globin is preferentially synthesized. Haemin (40 microM) as well as purified eukaryotic initiation factor 2 (eIF-2) from rabbit reticulocytes enhance amino acid incorporation in both kinds of extracts, which suggests that both uninduced and induced MEL cells contain a haemin-controlled eIF-2 alpha kinase. This system should be useful for studying the mechanisms controlling protein synthesis in a nucleated differentiating cell.

Hemin-independent control of globin synthesis in Friend erythroleukemia cells induced to differentiate

A hemin-independent translational inhibitor that prevents synthesis of rabbit globin when uninduced Friend leukemia (FL) cell and rabbit reticulocyte lysates are mixed [Cimadevilla, J. M. & Hardesty, B. (1975) Biochem. Biophys. Res. Commun. 63, 931-937] cannot be detected in FL cells induced to differentiate. Mixing of lysates of FL cells induced with hexamethylene bisacetamide or aminonucleoside of puromycin and rabbit reticulocytes does not cause inhibition of rabbit globin synthesis. Induction also results in the cells acquiring sensitivity to the inhibitor from uninduced FL cells. A reduction in total protein synthesis is observed when uninduced and induced FL cell lysates are mixed. Inhibition does not result from competition by an excess of uninduced FL cell mRNA species for the translational machinery because uninduced FL cell lysates retain their inhibitory activity after treatment with micrococcal nuclease. Rabbit globin mRNA recovered from rabbit reticulocyte lysates that have been incubated with lysates of uninduced FL cells can still be translated effectively, indicating that inhibition does not result from modification of other species of mRNA by uninduced FL cell lysates. A switch to hemin-dependent translational control does not follow induction of differentiation. The rate of amino acid incorporation in induced FL cell lysates--like that in uninduced FL cell lysates--is unaffected by omission of exogenous hemin from the system. Its presence is not required to prevent activation of heme-regulated inhibitor. From these data, it is clear that the control of protein synthesis in FL cells--whether or not they are induced--is different from that regulated by hemin in normal erythroid cells.

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.