Development of an in vitro translation system from wheat germ that is dependent upon the addition of eukaryotic initiation factor 2.
Explore the source record for details and available documents.
Ammonium Sulfate↗
Biomedical subjects
J M Ravel
Publications and source records attributed to J M Ravel.
mRNA binding properties of wheat germ protein synthesis initiation factor 2.
The binding of protein synthesis initiation factor (eIF-) 2 to mRNA was measured by retention of the mRNA.eIF-2 complexes on nitrocellulose filters. The binding of eIF-2 to mRNA was inhibited by GDP and GTP; the inhibition by GTP was enhanced by the presence of Met-tRNA(i). In addition, the formation of eIF-2.GDP binary complex and eIF-2.GTP.Met-tRNA(i) ternary complex was inhibited by mRNA. These data indicate that mRNA binds to a site on eIF-2 that is the same or overlaps the site to which guanine nucleotides bind eIF-2. This finding strongly suggests that inhibition of ternary complex formation by mRNA may be the result of competition between mRNA and GTP and not competition between mRNA and Met-tRNA(i).
DNA↗
Determination of the amounts of protein synthesis initiation factors required for translation of rabbit alpha-globin and beta-globin mRNAs.
alpha-Globin mRNA was translated at 60-80% of the rate of beta-globin mRNA in a rabbit reticulocyte lysate, a wheat germ S30 and in a wheat germ partially purified system. The concentrations of the initiation factors (eIF-) required to obtain half-maximal rate of translation (C0.5) of these mRNAs were determined in the partially purified system from wheat germ. The C0.5s of eIF-3, eIF-2 and eIF-4F were found to be about the same. The C0.5s of eIF-4C and eIF-4A for alpha-globin mRNA were only slightly higher (1.5-fold) than those of beta-globin mRNA. The C0.5 for eIF-4B was 2.5-fold higher for alpha-globin mRNA. Addition of saturating amounts of these initiation factors to either the S-30 system or the partially purified system did not increase the rate of translation of alpha-globin mRNA. These results indicate that the difference in the rate of translation of alpha- and beta-globin mRNAs is not due solely to the amounts of factors required.
Animals↗
Characterization of wheat germ protein synthesis initiation factor eIF-4C and comparison of eIF-4C from wheat germ and rabbit reticulocytes.
Eukaryotic protein synthesis initiation factor (eIF)-4C was purified from wheat germ and the molecular weight was calculated to be approximately 19,000 by SDS-polyacrylamide gel electrophoresis. A similar molecular weight was determined by gel filtration chromatography indicating that wheat germ eIF-4C is functional as a single polypeptide chain. An efficient in vitro translation system dependent upon the addition of eIF-4C was developed. This system was used to determine the concentrations of eIF-4C required for the half-maximal rate of translation of satellite tobacco necrosis virus RNA, alfalfa mosaic virus RNA 4, and barley alpha-amylase mRNA. No significant differences in the concentrations of eIF-4C required for the translation of these mRNAs were observed, although differences were noted for eIF-4A and eIF-4F. This finding suggests that eIF-4C is not involved in the binding of mRNA to 40 S ribosomal subunits. In heterologous assays, rabbit reticulocyte eIF-4C was as active as wheat germ eIF-4C in the wheat germ eIF-4C-dependent system. In addition, wheat germ eIF-4C substituted for rabbit reticulocyte eIF-4C in in vitro assay systems from rabbit reticulocytes. These results indicate that eIF-4C from wheat and rabbit contain conserved functional domains.
Animals↗
The 5' and 3' untranslated regions of satellite tobacco necrosis virus RNA affect translational efficiency and dependence on a 5' cap structure.
Satellite tobacco necrosis virus RNA (STNV RNA) is a naturally uncapped viral RNA that contains 1239 nucleotides: 29 in the 5' untranslated region (UTR), 591 in the coding region and 619 in the 3' UTR. Mutations were made in the 5' and 3' UTRs, and the effects of these mutations on translational efficiency and cap independence were measured in an in vitro translation system from wheat germ. Removal of the first 12 nucleotides or 10 changes in the nucleotide sequence of the 5' UTR reduced translational efficiency approximately 3-fold; capping of these 5' mutant mRNAs restored their translational efficiencies. Truncation of the 3' UTR to nucleotide 627 or 700, or deletion of nucleotides 627-737, reduced translational efficiency more than 20-fold; capping of these 3' mutant mRNAs restored their translational efficiencies. These modifications in the 3' UTR increased the concentration of initiation factor 4F required for translation. Chimeric mRNAs were constructed which contained the coding region of rabbit alpha-globin mRNA and either the 5' UTR, 3' UTR, or both the 5' and 3' UTRs of STNV RNA. Both the 5' and 3' UTRs of STNV RNA were necessary to obtain cap-independent translation. These findings indicate that interaction between 5' UTR and the region between nucleotides 627 and 737 in the 3' UTR are required for cap-independent translation.
Animals↗
Identification of an isozyme form of protein synthesis initiation factor 4F in plants.
We showed previously that wheat germ extracts contain two forms of protein synthesis initiation factor 4F that have very similar functional properties (Browning, K. S., Lax, S. R., and Ravel, J. M. (1987) J. Biol. Chem. 262, 11228-11232). One form, designated eIF-4F, is a complex containing two subunits, p220 and p26. The other form, designated eIF-(iso)4F, is a complex containing two subunits, p82 and p28, which are antigenically distinct from the subunits of eIF-4F. Both the p26 subunit of eIF-4F and the p28 subunit of eIF-(iso)4F are m7G cap-binding proteins. In this investigation, affinity-purified antibodies to the p220 and p26 subunits of wheat germ eIF-4F and to the p82 and p28 subunits of wheat germ eIF-(iso)4F were used to determine if isozyme forms of eIF-4F are present in maize and cauliflower. Extracts from wheat germ, maize root tips, and cauliflower inflorescences were partially purified by adsorption on m7GTP-Sepharose and elution with m7GTP (MGS eluate). Analysis by sodium dodecyl sulfate gel electrophoresis and immunoblotting with antibodies to the subunits of the wheat germ factors showed that the MGS eluate from maize contains polypeptides that react with antibodies to the p82 and p28 subunits of wheat eIF-(iso)4F, as well as polypeptides that react with antibodies to the p220 and p26 subunits of wheat eIF-4F. The MGS eluate from cauliflower also contains polypeptides that reacted with antibodies to the subunits of wheat eIF-(iso)4F. These results indicate that both maize and cauliflower contain the isozyme form of eIF-4F. In addition, it was found that the factors in the MGS eluate from maize support polypeptide synthesis in a system from wheat deficient in eIF-4F and eIF-(iso)4F, whereas the factors in the MGS eluate from cauliflower support polypeptide synthesis only to a small extent.
Brassica↗
Hypoxia enhances phosphorylation of eukaryotic initiation factor 4A in maize root tips.
We have identified two isoforms of initiation factor 4A (eIF-4A) in maize root tips, with distinct isoelectric points and similar molecular mass (approximately 50 kDa). Both isoforms of maize eIF-4A cross-react with antibodies raised against wheat germ eIF-4A, and one of the maize proteins (higher pI isoform) comigrates with purified wheat germ eIF-4A on two-dimensional gels. The two maize eIF-4As were indistinguishable by comparative peptide fingerprint analysis, which also showed a very strong similarity between eIF-4A in maize roots and wheat germ. Maize eIF-4As copurify with eIF-4F and eIF-(iso)4F on a 7-methyl-GTP-Sepharose affinity column, indicating that they are part of the 5'-cap-binding complex. Two-dimensional gel electrophoresis and immunoblotting of proteins from 32P-labeled maize root tips revealed that the lower pI isoform of eIF-4A is phosphorylated. Two-dimensional phosphopeptide maps of trypsin-digested eIF-4A contained one principal phosphorylated fragment; phosphoamino acid analysis indicated phosphorylation of threonine. In oxygenated maize root tips, the ratio of phosphorylated to nonphosphorylated eIF-4A is approximately 0.2. This ratio increases to approximately 1 within 20 min following the onset of hypoxia, due to interconversion between the two maize eIF-4A isoforms. The hypoxia-induced phosphorylation of eIF-4A is discussed with respect to metabolic responses, and the translational control of gene expression, in hypoxic plant tissues.
Electrophoresis, Gel, Two-Dimensional↗
The absence of a m7G cap on beta-globin mRNA and alfalfa mosaic virus RNA 4 increases the amounts of initiation factor 4F required for translation.
beta-Globin mRNA and alfalfa mosaic virus (AMV) RNA 4, two naturally capped mRNAs, and satellite tobacco necrosis virus (STNV) RNA, a naturally uncapped mRNA, were prepared by in vitro transcription with and without a 5' m7G cap structure (m7G(5')ppp(5')N). The translation of the capped and uncapped forms of these mRNAs was measured in a crude S30 system and a partially purified system from wheat germ. In the S30 system the uncapped forms of beta-globin mRNA and AMV RNA 4 are much less active (greater than or equal to 10%) than their capped forms, whereas the uncapped and capped forms of STNV RNA are equally active. The low activity of uncapped beta-globin mRNA and AMV RNA 4 in the S30 system is due, in part, to inactivation of the uncapped mRNAs in this system. Additional studies, carried out in the partially purified system in which very little inactivation of the mRNAs occurs, show that the uncapped and capped forms of beta-globin mRNA or AMV RNA 4 differ markedly with respect to the amount of eukaryotic initiation factor (eIF)-4F required for translation. For beta-globin mRNA the absence of the 5' cap structure increases the concentration of eIF-4F required for half-maximal translation about 6-fold (from 10 to 60 nM) and for AMV RNA 4 it increases the concentration of eIF-4F about 12-fold (from 5 to 60 nM). The concentrations of eIF-3, eIF-4A, and eIF-4B required for half-maximal translation of the uncapped forms of beta-globin mRNA and AMV RNA 4 are either the same or only slightly higher (1.5- to 2-fold) than the concentrations required for the capped forms. With STNV RNA the concentration of eIF-4F required for half-maximal translation of either uncapped or capped STNV RNA is 3 nM, and the concentrations of eIF-3, eIF-4A, and eIF-4B required for the two forms are also the same. The translation of the capped and uncapped forms of beta-globin mRNA and AMV RNA 4 is inhibited strongly by low concentrations of m7GTP in the partially purified system containing low concentrations of eIF-4F. Under the same conditions, the translation of capped or uncapped STNV RNA is inhibited only slightly by m7GTP. These findings suggest the possibility that the mechanism by which eIF-4F interacts and initiates translation with naturally uncapped mRNAs may not be identical to the mechanism by which eIF-4F interacts and initiates translation of naturally capped mRNAs.
Animals↗
Determination of the amounts of the protein synthesis initiation and elongation factors in wheat germ.
Previous work by Browning et al. (Browning, K. S., Lax, S. R., Humphreys, J., Ravel, J. M., Jobling, S. A., and Gehrke, L. (1988) J. Biol. Chem. 263, 9630-9634) indicated that wheat germ extracts do not contain sufficient amounts of some of the protein synthesis initiation factors to obtain optimal translation of all mRNAs. In this investigation, a quantitative enzyme-linked immunosorbent assay was used to determine the amounts of eukaryotic initiation factors (eIF) 2, 3, 4A, 4F, and (iso)4F as well as the amounts of 40 S ribosomal subunits and elongation factors (EF) 1 alpha and 2 present in wheat germ extracts. EF-1 alpha is present in the highest amount (approximately 5% of the total protein), and eIF-4F is present in the lowest amount (approximately 0.03% of the total protein). The micromolar amounts of the factors and ribosomes are as follows: EF-1 alpha, 34; EF-2, 5.2; eIF-2, 1.5; eIF-3, 0.7; eIF-4A, 3.0, eIF-4F, 0.09; eIF-(iso)4F, 0.8; and 40 S ribosomal subunits, 3.2. The molar ratios of the factors to 40 S ribosomal subunits are approximately 11:1 for EF-1 alpha, 1.6:1 for EF-2, 0.45:1 for eIF-2, 0.2:1 for eIF-3, 0.9:1 for eIF-4A, 0.03:1 for eIF-4F, and 0.25:1 for eIF-(iso)4F. These findings strongly suggest that the concentrations of the initiation factors, particularly those factors required for the binding of mRNA to ribosomes, may play a major role in regulating the translation of mRNAs within the cell.
Antibodies↗
Translation initiation factors that function as RNA helicases from mammals, plants and yeast.
Ribosome binding to eukaryotic mRNAs requires the concerted action of three eukaryotic initiation factors: eIF-4A, eIF-4B and eIF-4F as well as the hydrolysis of ATP. These initiation factors are implicated in the unwinding of mRNA 5' secondary structure and have been isolated from mammals, yeast and wheat germ. We used an RNA unwinding assay to compare the activities of these factors from the different species. We also measured the inter-species interchangeability of these factors in the unwinding reaction. In mammals, it has been previously shown that a combination of rabbit reticulocyte eIF-4F and -4B or eIF-4A and -4B were active in the RNA unwinding assay. In wheat germ, the combination of eIF-4A and eIF-4F resulted in RNA unwinding in a reaction that was stimulated by eIF-4B. Mammalian eIF-4A was able to substitute in this system. We also show that yeast eIF-4A is able to effectively substitute for mammalian eIF-4A in duplex RNA unwinding in combination with mammalian eIF-4B, while wheat-germ eIF-4A was only partially able to substitute. Taken together, these results suggest that initiation factor requirements for RNA unwinding are largely similar in mammals, yeast and plants.
Animals↗
Evidence that the 59-kDa protein synthesis initiation factor from wheat germ is functionally similar to the 80-kDa initiation factor 4B from mammalian cells.
The results of this investigation show that the 59-kDa protein synthesis initiation factor from wheat germ, designated eukaryotic initiation factor (eIF)-4G by Browning et al. (Browning, K.S., Maia, D.M., Lax, S.R., and Ravel, J.M. (1987) J. Biol. Chem. 262, 539-541), cross-links to the 5'-terminal cap of oxidized mRNA in the presence of eIF-4A, eIF-4F, and ATP, stimulates the RNA-dependent ATPase activities of eIF-4A and a mixture of eIF-4A and eIF-4F, and stimulates the unwinding activities of eIF-4A, eIF-4F, and a mixture of eIF-4A and eIF-4F. These findings strongly suggest that the 59-kDa factor from wheat germ is the functional equivalent of the 80-kDa protein synthesis initiation factor, eIF-4B, from mammalian cells. Recent reports indicate that the wheat germ initiation factor which contains two subunits of 80 and 28 kDa and which was given the designation "eIF-4B" by Lax et al. (Lax, S.R., Lauer, S.J., Browning, K. S., and Ravel, J.M. (1986) Methods Enzymol. 118, 109-128) is an isozyme form of eIF-4F and not the functional equivalent of mammalian eIF-4B. On the basis of functional characteristics we propose that the designation for the wheat germ factor containing the 80- and 28-kDa polypeptides be changed from eIF-4B to eIF-(iso)4F and the designation for the 59-kDa factor be changed from eIF-4G to eIF-4B.
Adenosine Triphosphate↗
Properties of the subunits of wheat germ initiation factor 3.
Wheat germ initiation factor 3 (eukaryotic initiation factor 3, eIF-3) contains ten non-identical subunits (p116, p107, p87, p83, p56, p45, p41, p36, p34 and p28). Monoclonal antibodies to all except two of the subunits (p41 and p28) were obtained. None of the monoclonal antibodies react with more than one subunit, and only monoclonal antibodies to p36 inhibit the ability of eIF-3 to support initiation of polypeptide synthesis. Two of the subunits (p116 and p107) are highly basic polypeptides (pI greater than or equal to 8); five (p87, p56, p45, p34 and p28) are acidic polypeptides (pI = 5.4-6.1); and three (p83, p41 and p36) appear to exist in more than one isoelectric form. Eight of the subunits of eIF-3 are iodinated rapidly in vitro; the highest incorporation is into p56 and the lowest incorporation is into p28. No incorporation into p41 or p28 is observed. When eIF-3 is treated with N-[3H]ethylmaleimide, approx. 30 alkyl groups per eIF-3 are incorporated, and the eIF-3 is inactivated. No incorporation into p83 or p28 is observed; incorporation of the alkyl groups into the other eight subunits occurs at different rates. The rate of inactivation of eIF-3 by N-ethylmaleimide is slower than the overall rate of incorporation of alkyl groups. eIF-3 is stable between pH 5.5 and 10. Below pH 5.5, eIF-3 is inactivated and precipitation of protein occurs. Partial dissociation of the subunits and inactivation of eIF-3 is obtained by treatment with 2 M urea. Attempts to reassociate the subunits into an active particle were unsuccessful.
Alkylation↗
Evidence that the 5'-untranslated leader of mRNA affects the requirement for wheat germ initiation factors 4A, 4F, and 4G.
The efficiency of translation of alfalfa mosaic virus (AMV) RNA 4, barley alpha-amylase (B alpha A) mRNA, and two chimeric mRNAs, AMV 4-B alpha A and B alpha A-AMV 4 (in which the 5' leader sequences of the two mRNAs were interchanged), was measured in an S30 extract from wheat germ and a fractionated system from wheat germ in which translation could be made dependent upon initiation factor (eIF) 3, 4A, 4F, or 4G. In the S30 system, AMV RNA 4 and the chimeric mRNA AMV 4-B alpha A are translated much more efficiently than B alpha A mRNA and the chimeric mRNA B alpha A-AMV 4. When the S30 system was supplemented with high amounts of purified eIF-3, eIF-4A, eIF-4F, and eIF-4G, B alpha A and B alpha A-AMV 4 mRNAs were translated as efficiently as AMV RNA 4 and AMV 4-B alpha A mRNA. These findings indicated that the mRNAs containing the B alpha A leader sequence required higher amounts of one or more of the initiation factors (eIF-3, eIF-4A, eIF-4F, and eIF-4G) for efficient translation. Determination of the amounts of the initiation factors required for translation in the fractionated system showed that AMV RNA 4 required 2-4-fold lower amounts of eIF-3, eIF-4A, eIF-4F, and eIF-4G than did B alpha A mRNA. Replacement of the B alpha A leader sequence with that of AMV RNA 4 decreased the amounts of eIF-4A, eIF-4G, and eIF-3 required, but did not affect the amount of eIF-4F required. Replacement of the AMV RNA 4 leader sequence with that of B alpha A mRNA increased the amounts of eIF-4F, eIF-4G, and eIF-3 required, but did not affect the amount of eIF-4A required. These data strongly suggest that the amounts of the factors required are affected not only by the 5' leader itself but also by interactions between the 5' leader and a region(s) of the mRNA 3' to the initiation codon.
Capsid↗
Evidence that the requirements for ATP and wheat germ initiation factors 4A and 4F are affected by a region of satellite tobacco necrosis virus RNA that is 3' to the ribosomal binding site.
A cDNA containing the complete genome of satellite tobacco necrosis virus (STNV) RNA was constructed and cloned into a plasmid vector containing the T7 polymerase promotor. A second clone containing the first 54 nucleotides from the 5' end, which includes the ribosome binding site, was also constructed. RNAs were transcribed from these plasmids (pSTNV1239 and pSTNV54) and tested for their ability to bind to wheat germ 40 S ribosomal subunits in the presence of wheat germ initiation factors eIF-4A, eIF-4F, eIF-4G, eIF-3, eIF-2, Met-tRNA, ATP, and guanosine 5'-(beta, gamma-imino)triphosphate (GMP-PNP). Maximal binding of the STNV RNA transcribed from pSTNV1239 is obtained only in the presence of all the initiation factors and ATP. In contrast, close to maximal binding of STNV RNA transcribed from pSTNV54 is obtained in the absence of eIF-4A, eIF-4F, eIF-4G, and ATP. A series of deletion clones from the 3' end of the STNV cDNA was prepared, and the requirements for binding to 40 S ribosomal subunits were determined. STNV RNAs containing more than 134 nucleotides from the 5' end require eIF-4A, eIF-4F, eIF-4G, and ATP for maximal binding to 40 S ribosomal subunits, whereas STNV RNAs containing 86 nucleotides or less no longer require ATP and these factors. These findings indicate that a region 3' to the initiation codon affects the requirements for eIF-4A, eIF-4F, eIF-4G, and ATP.
Adenosine Triphosphate↗
Initiation factors that bind mRNA. A comparison of mammalian factors with wheat germ factors.
Three mammalian eukaryotic initiation factors (eIF) are required for the ATP-dependent binding of mRNA to the 40 S ribosomal subunit. These three factors, eIF-4A, eIF-4B, and eIF-4F, have also been isolated from wheat germ. Three assays were used to measure the ability of the wheat germ factors to interact with and/or substitute for the mammalian factors. Two assay systems were used to measure partial reactions involving the interaction of the three factors, ATP, and mRNA: 1) RNA-dependent ATP hydrolysis and 2) cross-linking of the factors to the 5' cap of oxidized mRNA. A third assay system was used to measure the ability of the factors to support initiation of protein synthesis. The results of the ATP hydrolysis and cross-linking experiments indicate that the wheat germ factors can interact with or substitute for the mammalian factors. Wheat germ eIF-4A appears to be functionally equivalent to mammalian eIF-4A. Wheat germ eIF-4B and eIF-4F appear to be isozymes possessing functions similar to mammalian eIF-4F. Wheat germ eIF-4B does not appear to be a functional equivalent to the mammalian eIF-4B. In a complete translation system from wheat germ, mammalian factors partially substitute for wheat germ factors, whereas the wheat germ factors are ineffective in the mammalian system.
Adenosine Triphosphate↗
Identification of a Kinase in Wheat Germ that Phosphorylates the Large Subunit of Initiation Factor 4F.
A kinase has been isolated from wheat (Triticum aestivum) germ that phosphorylates the 220 kilodaltons (kD) subunit of wheat germ initiation factor (eIF) 4F, the 80 kD subunit of eIF-4B (an isozyme form of eIF-4F) and eIF-4G (the functional equivalent to mammalian eIF-4B). The kinase elutes from Sephacryl S-200 slightly in front of ovalbumin. The kinase phosphorylates casein and histone IIA to a small extent, but does not phosphorylate phosvitin. Of the wheat germ initiation factors, elongation factors, and small and large ribosomal subunits, only eIF-4F, eIF-4B, and eIF-4G are phosphorylated to a significant extent. The kinase phosphorylates eIF-4F to the extent of two phosphates per mole of the 220 kD subunit and phosphorylates eIF-4B to the extent of one phosphate per mole of the 80 kD subunit. The 26 kD subunit of eIF-4F and the 28 kD subunit of eIF-4B are not phosphorylated by the kinase. The kinase phosphorylates the 59 kD component of eIF-4G to the extent of 0.25 phosphate per mole of eIF-4G. Phosphorylation of eIF-4F and eIF-4B does not affect their ability to support the binding of mRNA to small ribosomal subunits in vitro.
Journal Article↗
Identification of two messenger RNA cap binding proteins in wheat germ. Evidence that the 28-kDa subunit of eIF-4B and the 26-kDa subunit of eIF-4F are antigenically distinct polypeptides.
Previous work has shown that eukaryotic initiation factor (eIF)-4B from wheat germ is a complex containing two subunits, 80 and 28 kDa, and eIF-4F from wheat germ is a complex containing two subunits, 220 and 26 kDa (Lax, S., Fritz, W., Browning, K., and Ravel, J. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 330-333). Here we show that both the 28-kDa subunit of eIF-4B and the 26-kDa subunit of eIF-4F cross-link to the 5' terminus of capped and oxidized satellite tobacco necrosis virus RNA in the absence of ATP and that the cross-linking of both polypeptides is inhibited by m7GDP. Several lines of evidence indicate that the 28-kDa and the 26-kDa cap binding proteins of eIF-4B and eIF-4F are antigenically distinct polypeptides. Rabbit polyclonal antibodies raised to intact eIF-4B or to the isolated 28-kDa subunit of eIF-4B react strongly with the 28-kDa subunit of eIF-4B on immunoblots, but show only a very weak reaction with the 26-kDa subunit of eIF-4F under the same conditions. In addition, a mouse monoclonal antibody was obtained that reacts strongly with the 26-kDa subunit of eIF-4F but does not react with the 28-kDa subunit of eIF-4B. Evidence is presented also which indicates that the higher molecular weight subunits of eIF-4B and eIF-4F are antigenically distinct. Rabbit polyclonal antibodies raised to intact eIF-4B or the isolated 80-kDa subunit inhibit eIF-4B-dependent polypeptide synthesis but do not inhibit eIF-4F-dependent polypeptide synthesis. Rabbit polyclonal antibodies raised to eIF-4F inhibit eIF-4F-dependent polypeptide synthesis but do not inhibit eIF-4B-dependent polypeptide synthesis.
Antibodies↗
Identification of a new protein synthesis initiation factor from wheat germ.
A previously unidentified factor has been isolated from wheat germ that stimulates globin mRNA-directed polypeptide synthesis in vitro. This factor is separated from eukaryotic initiation factor (eIF)-4B by chromatography on m7GTP-Sepharose. eIF-4B binds to m7GTP-Sepharose, whereas the stimulatory factor does not. Further purification of the factor yields a preparation that contains one major polypeptide with a molecular weight of approximately 59,000, This factor enhances the binding of globin mRNA to 40 S ribosomal subunits in the presence of eIF-2, eIF-3, eIF-4A, and either eIF-4B or eIF-4F and has been designated eIF-4G.
Globins↗