Protein Synthesis in Rabbit Reticulocytes

Inhibition of translation in lysates of mouse L cells infected with vesicular stomatitis virus: presence of a defective ribosome-associated factor

Lysates of L cells infected for 4 h with vesicular stomatitis virus were inhibited in their in vitro translational activity to about the same extent as protein synthesis was inhibited in vivo in infected L cells. Inhibition of translation occurred at the level of the ribosome as determined by reciprocal cross-reconstitution studies with polyribosomes and postribosomal supernatant fractions isolated from virus-infected and mock-infected cells. Inhibition of protein synthesis in reconstituted lysates of virus-infected cells was found to be at the level of initiation of translation as evidenced by reduction in incorporation into acid-precipitable proteins of formylatable [35S]methionine. Ribosomes from virus-infected and mock-infected cells were exposed to 0.5 M KCl and fractionated by centrifugation into salt-washed polyribosomes and supernatant fractions containing ribosome-associated proteins; reciprocal reconstitution of translational activity by a mixing of salt-washed polyribosomes and ribosome-associated proteins revealed that the defect in initiation of translation was in the ribosome-associated proteins released by salt wash from the infected-cell ribosomes. Differential ammonium sulfate precipitation of the supernatant ribosome-associated proteins from virus-infected and mock-infected cells indicated by reciprocal reconstitution studies that the defective ribosomal initiation factor(s) was (were) present primarily in the 0-40% ammonium sulfate fraction that is considered to contain primarily eIF-3 and eIF-4B. These results are similar to those found in earlier studies of defective initiation factors responsible for impaired protein synthesis in cells infected with plus-strand viruses quite different from the rhabdovirus studied in these experiments.

Mammalian epidermal protein synthesis: initiation factors

There is a deficiency of initiation in protein-synthesizing systems prepared from mammalian epidermis. These systems do not respond to inhibitors of initiation although they remain sensitive to elongation inhibitors and exogenous ribonuclease. In spite of this deficiency of initiation, active protein factors which support initiation reactions are present in the potassium chloride extract of mammalian epidermal ribosomes. A factor corresponding to the reticulocyte factor IF-MP has been isolated. An inhibitor of initiation is also present in the epidermal KCl wash.

Binding of Met-tRNA-f to native and derived 40S ribosomal subunits

Our previous work has shown that the native 40S ribosomal subunits (those found free in the cell sap) but not polyribosomal 40S subunits have additional associated proteins that are removed by 0.5 M KCl. In this communication we present evidence that in the Ehrlich cell one of the native subunit associated proteins is the mammalian initiation factor that forms a Met-tRNA-f-factor-GTP complex, and is required for the binding of Met-tRNA-f to the 40S subunit. Initial examination of the KCl wash of the Ehrlich cell total ribosomal pellet revealed a factor which (1) shifted the elution of Met-tRNA-f and of GTP from the included to the excluded volume on Sephadex G-100 chromatography, (2) stimulated the binding of Met-tRNA-f to Millipore filters, and (3) stimulated the binding of Met-tRNA-f to salt-washed 40S subunits. These activities were dependent upon or enhanced by GTP; were inhibited by GDP; were much greater for Met-tRNA-f than for Met-tRNA-m or for lysyl-tRNA; and were concentrated in the KCl ribosomal wash and were not detected in the cell soluble fraction. Met-tRNA-f bound in conjunction with a specific amount of KCl wash protein, to form a distinctive particle of bouyant density 1.40 g cm minus 3 in CsCl, identical in density to one form of the native 40S subunit. Native 40S subunits, but no other subunits, contained a factor which was eluted by 0.5 M KCl and which (1) stimulated the binding of Met-tRNA-f to Millipore filters, and (2) stimulated the binding of Met-tRNA-f to salt-washed 40S subunits. The factor appeared to be localized on the native 40S subunit of density 1.40 g cm minus 3.

The nucleotide sequence of a methionine tRNA which functions in protein elongation in mouse myeloma cells

The major form of methionine tRNA operational in the elongation of protein synthesis in mouse myeloma cells was purufied from these cells after they had been cultured in the presence of [32P]-phosphate. This [32P]tRNA4-Met species was then digested with T1 RNase or pancreatic RNase so as to obtain both complete and partial RNase digestion products. The nucleotide sequences of these fragments were analysed to enable the derivation of the complete primary structure of this tRNA. tRNA4-Met of mouse myeloma cells is 76 nucleotides in length and contains 15 modified nucleotides. It is the only tRNA yet sequenced which has been found to possess the minor nucleoside 2-methylguanosine (m2G) within the amino acid (a) stem, and also to have an anticodon (c) stem of only 4 and not 5 base-pairs. The loop IV sequence of eukaryotic initiator methionine tRNA (tRNAf-Met) species, -A-U-C-G-m1A-A-A-, IS NOT FOUND IN TRNA4-Met and is therefore absent from at least one of the methionine tRNAs functioning in polypeptide elongation in mammalian cells. This is consistent with the suggested importance of this loop structure in the initiator function of tRNAf-Met in eukaryotic organisms. Three distinct regions of the tRNA cloverleaf, the (b) stem, the anticodon loop (loop II), and loop III, are substantially conserved in structure between tRNAf-Met and tRNA4-Met of mouse myeloma cells. These regions of the structures of mammalian methionine tRNAs probably do not determine whether a certain tRNA-Met will function in the initiation or elongation of protein synthesis, although they might be important in tRNA-Met recognition if the different cytoplasmic tRNA-Met species of mammalian cells are aminoacylated by a single activating enzyme.

Inhibition of Protein Synthesis in Cell-Free Systems from Interferon-Treated, Infected Cells: Further Characterization and Effect of Formylmethionyl-tRNA(F)

The translation of encephalomyocarditis virus (EMC) RNA is markedly inhibited in cell-free systems from interferon-treated, vaccinia virus-infected L-cells (10, 11). The polypeptide products synthesized in response to EMC RNA in cell-free systems from these and untreated infected cells have been analyzed by electrophoresis on polyacrylamide gels. Qualitatively, the same EMC-specific polypeptides were synthesized throughout. In experiments using preincubated microsomes from normal Krebs cells to assay cell sap from L-cells which had been exposed to interferon prior to infection, only the amount of the EMC-specific polypeptide products was reduced. This result suggests that there is an inhibition very early in translation in interferon-treated, infected cells. Initiation seems a priori the more attractive site for this inhibition, but an effect shortly after initiation cannot be excluded. With unfractionated cell-free systems from interferon-treated infected L-cells, however, there appeared to be an additional minor inhibitory effect on polypeptide chain elongation, in that the EMC-specific polypeptides synthesized showed not only a reduction in amount but also a bias towards lower molecular weight. The formylated methionyl initiator tRNA (Fmet-tRNA(F)) was used as a further probe into the apparent effect on intiation. With this reagent we have confirmed that there is one major initiation site for the translation of EMC RNA in these cell-free systems. In addition, the results have shown that EMC-specific polypeptide chains initiated with Fmet escape the major interferon-mediated inhibition at or shortly after initiation.