Polypeptide chain initiation in eukaryotes. IV. Purification and properties of supernatant initiation factor from Artemia salina embryos

Removal of beta subunit of the eukaryotic polypeptide chain initiation factor 2 by limited proteolysis

It is generally considered that the eukaryotic polypeptide chain initiation factor 2 (eIF-2) from rabbit reticulocytes consists of three nonidentical subunits termed alpha, beta, and gamma, in order of increasing molecular weight. However, a recent report [Stringer, E. A., Chaudhuri, A., Valenzuela, D. & Maitra, U. (1980) Proc. Natl. Acad. Sci. USA 77, 3356-3359] suggested that this factor is made up of only two subunits. In this paper we show that limited proteolysis of rabbit reticulocyte eIF-2 leads to loss of the beta subunit. This modified eIF-2 has the same activity as the native factor in promoting ternary (eIF-2.GTP.Met-tRNAi) and 40S (eIF-2.GTP.Met-tRNAi.40S ribosome) initiation complex formation. Like native eIF-2, the protease-treated factor can restore translation in heme-deficient lysates. On the other hand, the treated factor is less stable than the native protein.

Identification of initiation factors and ribosome-associated phosphoproteins by two-dimensional polyacrylamide gel electrophoresis

A two-dimensional polyacrylamide gel electrophoresis procedure has been used to identify initiation factors rapidly in the high-salt-wash fraction from reticulocyte ribosomes. Initiation factors are identified by relative mobility and by co-electrophoresis with purified factors. A creatine phosphate/ATP/GTP/Pi exchange system is described which has been used to maintain [gamma-32P]ATP and [gamma-32P]GTP at constant specific activity in the cell-free protein-synthesizing system. Phosphorylated proteins associated with the protein-synthesizing complex have been identified using a combination of the two procedures. The salt-wash fraction contains eight major phosphorylated proteins and a number of minor ones. Two phosphorylated proteins are observed to comigrate with two of the three subunits of eukaryotic initiation factor 2 (eIF-2), the initiation factor involved in binding Met-tRNAf onto the 40-S subunit and promoting dissociation of 80-S ribosomes. eIF-4B, one of the proteins involved in binding mRNA to 40-S subunits is also phosphorylated. The remainder of phosphorylated proteins in the high-salt-wash fraction are not previously characterized initiation factors and have not been identified further. Two of the six phosphoproteins associated with the salt-washed ribosomes comigrate with ribosomal proteins; one is the major phosphorylated protein in 40-S ribosomal subunits, the other is an acidic protein.

Elongation factor 1 from Artemia salina: properties and disaggregation of the enzyme

Elongation factor 1 has been purified from undeveloped embryos of Artemia salina. The purified enzyme appears to be an aggregate (molecular weight approximately to 200 000) which on sodium dodecylsulfate gels shows the presence of two major protein bands whose estimated molecular weights are 52 000 and 47 000. Lipid material appears to be associated with the purified protein. In aminoacyl-tRNA binding to ribosomes, there is only a limited turnover of the enzyme, but the protein acts catalytically in amino acid polymerization. The enzyme is disaggregated by a partially purified phospholipase C preparation, elastase and under certain conditions, by guanosine nucleotides. The significance of these results is discussed with respect to the overall role of elongation faction 1 in aminoacyl-tRNA binding to ribosomes.

On the mode of action of eucaryotic initiation factor 1

A factor isolated from the human tonsillar ribosomal wash specifically stimulated the poly (U)-dependent binding of Phe-tRNA to 40S subunits at low Mg2+ concentration and without any requirement for GTP. The stimulated binding of Phe-tRNA to 40S particles was inhibited in proportion to added deacylated tRNA. The factor was inactivated by N-ethylmaleimide, but in the presence of 40S subunits a considerable protection was observed. 40S subunits, incubated with the factor and isolated by centrifugation, carried significant factor activity. The results imply that the human tonsillar factor, which shows a great functional analogy to the eucaryotic initiation factor 1 from other sources, exerts its effect by an initial interaction with the 40S subunit.

A protein binding the methylated 5'-terminal sequence, m7GpppN, of eukaryotic messenger RNA

Ribosomal salt washes of Artemia salina embryos contain a protein(s) that binds [3H]m7GpppGpC and [3H]m7GpppGmpC, as measured by retention on nitrocellulose membrane filters. These oligonucleotides correspond in structure to the methylated 5'-terminal sequences (caps) present in many eukaryotic mRNAs. The cap binding protein does not bind the unmethylated counterparts of caps, e.g., [32P]GpppGpCp, or a derivative of m7GpppGmpC containing ring-opened m7G. None of the purified initiation factors IF-MP, IF-M2A, IF-M2B, IF-M3, or IF-MI binds the m7G-containing oligonucleotides.

Polypeptide chain initiation in eukaryotes: initiation factor requirements for translation of natural messengers

A protein-synthesizing system consisting of ribosomes and supernatant of undeveloped Artemia saline embryos is used for assay of mRNA translation and initiation factors. This system contains the components needed for chain elongation but has low levels of mRNA and initiation factors. Exogenous mRNA is readily translated upon addition of high-speed supernatant or ribosomal salt wash of developing embryos as a source of initiation factors (IF). This requirement can be largely satisfied by a mixture of the reticulocyte factors IF-MP, IF-M3, IF-M2A, and IF-M2B. A. salina IF-M1, which is present in undeveloped embryo supernatant, can be inactivated by A. salina IF-M1 antibody without affecting translation.

Effect of development on the translation of messenger RNA in Artemia salina embryos

Cell-free preparations from encapsulated Artemia salina embryos readily translate poly(U), but there is no endogenous protein synthesis or translation of added natural mRNA until development resumes upon incubation in saline. High-salt-washed 80S ribosomes and highspeed supernatant (cytosol) were prepared from undeveloped eggs and from eggs allowed to develop to a stage prior to hatching. Endogenous protein synthesis occurs only with ribosomes from developed embryos, whether with undeveloped or developed cytosol. This is mainly elongation of preformed polypeptide chains, for it is largely resistant to edeine, an inhibitor of chain initiation. Edeinesensitive translation of added natural mRNA occurs with both developed and undeveloped ribosomes but requires developed cytosol. Translation of brome mosaic virus RNA is not much, if at all, further stimulated by high-saltwash of developed ribosomes, but that of globin mRNA is markedly enhanced. Levels of the chain initiation factor EIF-1 are the same before and after development. These results are consistent with the view that resumption of developemtn is triggered by transcription, with ensuing translation of the resulting messengers to yield, among other proteins, mRNA-recognizing initiation factors of the IF-3 type which are partly free in the cytosol and partly ribosome-bound. The data also suggest that different factors may be involved in the translation of brome mosaic virus RNA and globin mRNA by this system.

Protein synthesis in the cotyledons of Pisum sativum L. Protein factors involved in the binding of phenylalanyl-transfer ribonucleic acid to ribosomes

1. Proteinaceous factors contained in a 0.5m-KCl extract of ribosomes from pea cotyledons form a ternary complex at 0 degrees C with [(14)C]phenylalanyl-tRNA and poly(U). The complex is measured by its quantitative retention on Millipore filters. 2. Complex-assembly is optimal at 5mm-Mg(2+) and is independent of GTP and ribosomes. 3. The addition of ribosomes is required to stabilize the complex at 34 degrees C. The complex binds to a puromycin-sensitive site on the ribosome. 4. Soluble factors from the 250000g supernatant of pea cotyledon form a Millipore-retainable complex dependent on GTP and ribosomes. 5. Complex-formation by soluble factors has a Mg(2+) optimum of 10-12mm and forms a puromycin-insensitive complex with ribosomes. 6. The function of the ribosomal protein factors and the supernatant fraction in initiation of protein synthesis is discussed.

Use of purified polysomes from rabbit reticulocytes in a specific test for initiation factors

Initiation factors, extracted from ribosomal particles by high salt, are usually tested for with the extracted ribosomes. However, it is theoretically possible that such tests may respond not only to factors (i.e., proteins that join and leave a ribosome during its cycle), but also to true ribosomal proteins that may have been extracted. We have eliminated this circularity by tests with purified polysomes, which have not been extracted with high salt; the polysomes are deficient in initiation factors because they have been largely separated from the native subunits. Tests with both systems, applied to extracts of different classes of particles, suggest that high salt does extract both initiation factors and essential ribosomal proteins. Extracts of native subunits contain both, and stimulate both systems; while, in the presence of these extracts, the extracts of polysomes further stimulate only the extracted ribosomes.

Conformational control of the interaction of eukaryotic elongation factors EF-1 and EF-2 with ribosomes

As in the case with prokaryotic systems, Artemia salina elongation factors EF-1 and EF-2 interact with a common site or with closely overlapping sites on the Artemia ribosome. This feature of ribosomal design must restrict interaction with the ribosome to only one of the factors at alternating steps of chain elongation. In support of this view we find that EF-1, but not EF-2, interacts with the post-translocation ribosome, whereas the reverse is true of the pre-translocation ribosome. Conformational changes probably account for the alternating selectivity of the translating ribosome for each elongation factor.