Protein synthesis eukaryotic initiation factors

Meyer, L. J., Brown-Leudi, M., Corbett, S., Tolan, D. R., and Hershey, J. W. B., 1981, The purification and characterization of multiple forms of protein synthesis eukaryotic initiation factors 2, 3 and 5 from rabbit reticulocytes, J. Biol. Chem. 256 351. 

Rhoads RE. Regulation of eukaryotic protein synthesis by initiation factors. J Biol Chem 268 3017-3020, 1993. 

Initiation of protein synthesis requires that an mRNA molecule be selected for translation by a ribosome. Once the mRNA binds to the ribosome, the latter finds the correct reading frame on the mRNA, and translation begins. This process involves tRNA, rRNA, mRNA, and at least ten eukaryotic initiation factors (elFs), some of which have multiple (three to eight) subunits. Also involved are GTP, ATP, and amino acids. Initiation can be divided into four steps (1) dissociation of the ribosome into its 40S and 60S subunits (2) binding of a ternary complex consisting of met-tRNAf GTP, and eIF-2 to the 40S ribosome to form a preinitiation complex (3) binding of mRNA to the 40S preinitiation complex to form a 43S initiation complex and (4) combination of the 43S initiation complex with the 60S ribosomal subunit to form the SOS initiation complex. 

The catalytic activities of the fortified wheat germ cell-free systems supplemented with each fraction were investigated (Fig. 2). As shown in Fig. 2, only 0 - 40 % ammonium sulfate fraction showed an enhancement in DHFR protein synthesis. This enhancement of protein experimental results and the fact that the various eukaryotic initiation factors are contained in synthesis was also confirmed by SDS-PAGE and autoradiography (Fig. 3). From the above 0-40 % ammonium sulfate fraction [5, 6], it can be concluded that the amount of initiation factors in a conventionally prepared wheat germ cell-fi extract is deficient for the translation of DHFR with internal ribosome entry site. Therefore, it needs to supplement a wheat germ cell-free extract with the fraction containing the limited initiation factors for the efficient protein translation, and this fortified cell-free system can be easily made by simple... 

Figure 7.5 Model of ferritin (and erythroid a-aminolaevulinate synthase) translation/ribosome binding regulation by IRP. In (a), with IRP not bound to the IRE (1) binding of the 43S preinitiation complex (consisting of the small ribosomal 40S subunit, GTP and Met-tRNAMet) to the mRNA is assisted by initiation factors associated with this complex, as well as additional eukaryotic initiation factors (elFs) that interact with the mRNA to facilitate 43S association. Subsequently (2), the 43S preinitiation complex moves along the 5 -UTR towards the AUG initiator codon, (3) GTP is hydrolysed, initiation factors are released and assembly of the 80S ribosome occurs. Protein synthesis from the open reading frame (ORF) can now proceed. In (b) With IRP bound to the IRE, access of the 43S preinitiation complex to the mRNA is sterically blocked. From Gray and Hentze, 1994, by permission of Oxford University Press.

Konieczny, A., and Safer, B. (1983). Purification of the eukaryotic initiation factor 2-eukaryotic initiation factor 2B complex and characterization of its guanine nucleotide exchange activity during protein synthesis initiation. J. Biol. Chem. 258, 3402—3408. 

Matts, R. L., Levin, D. H., and London, I. M. (1983). Effect of phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 on the function of reversing factor in the initiation of protein synthesis. Proc. Natl. Acad. Sci. USA 80, 2559—2563. 

Figure 12-2. Formation of the initiation complex for protein synthesis. Several eukaryotic initiation factors (elFs) ensure proper assembly at each step. The initiator Met-tRNA is bound in the peptidyl site of the SOS complex with its anticodon (black stripes) base paired to the AUG start codon (gray box) of the mRNA.

Many antibiotics, which inhibit protein synthesis, do not bind to ribosomes but block any of a variety of vital chemical processes needed for growth. Among them are pseudomonic acid, which inhibits isoleucyl-tRNA synthetase from many gram-positive bacteria.1111/VV Rapamycin, best known as an immunosuppressant (Box 9-F), inhibits phosphoinositide-3-kinase and also phosphorylation of the cap-binding protein 4G, a component of the eukaryotic initiation factor complex (Fig. 29-11 ).ww The bacterial enzyme peptide deformylase, which is absent from the human body, has been suggested as a target for design of synthetic antibiotics. 01... 

Even though specific differences distinguish the initiation process in eukaryotes and prokaryotes, three things must be accomplished to initiate protein synthesis in all systems (1) The small ribosomal subunit must bind the initiator tRNA (2) the appropriate initiating codon on mRNA must be located and (3) the large ribosomal subunit must associate with the complex of the small subunit, the initiating tRNA, and mRNA. Nonribosomal proteins, known as initiation factors (IFs), participate in each of these three processes. IFs interact transiently with a ribosome during initiation and thus differ from ribosomal proteins, which remain continuously associated with the same ribosome. 

Initiation of protein synthesis in eukaryotes requires at least nine distinct eukaryotic initiation factors (elFs) (see Table 1) compared to the three initiation factors (IFs) in prokaryotes (see Topic H2). 

A list of key differences between prokaryotes and eukaryotes with respect to protein synthesis is shown in Table 9-1. These include the existence of multiple eukaryotic initiation factors that facilitate the assembly of the riboso-mal protein synthetic machinery, whereas there are only three for prokaryotes. An initiation site on bacterial mRNA consists of the AUG initiation codon preceded with a gap of approximately 10 bases by the Shine-Dalgamo polypurine hexamer, whereas the 5 Cap (a 7-methylguanylate residue in a 5 —>5 triphosphate linkage) acts as an initiation signal in eukaryotes. In prokaryotes, the first or A-terminal amino acid is a formyl-methionine (fMet), but in eukaryotes it is usually a simple methionine. Additionally, the size and nature of the prokaryotic ribosomes are quite different from the eukaryotic ribosomes. 

Synthesis of the protein. Initiation involves formation of a complex containing the initial methionyl-tRNA bound to the AUG start codon of the mRNA and to the P site of the ribosome It requires GTP and proteins known as eukaryotic initiation factors (elFs). 

Fig. 15.8. Initiation of protein synthesis. P site = peptidyl site on the ribosome A site = aminoacyl site on the ribosome (The A and P sites or portions of them are indicated by dashed lines) elF = eukaryotic initiation factor.

Maskin is a cytoplasmic polyadenylation element-binding protein-associated factor. Dormant state of maternal mRNAs in immature oocytes is maintained by an abortive interaction of this protein with the eukaryotic initiation factors 4E and 4G. Phosphorylation of maskin promotes the dissociation of this interaction, thereby allows the dormant mRNAs to be translated actively. Aurora phosphorylation of maskin is reported to be involved in protein synthesis in maturing clam and Xenopus oocytes and in centrosome-dep>endent microtubule assembly at mitosis (Kinoshita et al. 2005 Pascreau et al. 2005). 

Roles of Eukaryotic Initiation Factor 2 Ancillary Factors in the Regulation of Eukaryotic Protein Synthesis Initiation Naba K. Gupta... 

LEVIN, D. and LONDON, I.M. Regulation of protein synthesis Activation by double-stranded RNA of a protein kinase that phosphorylates eukaryotic initiation factor 2. Proc. Natl. 

Jacobsen, H., Epstein, D. A., Friedmann, R. A., Safer, B., and Torrence, P. F., 1983, Double-stranded RNA-dependent phosphorylation of protein PI and eukaryotic initiation factor 2a does not correlate with protein synthesis inhibition in a cell-free system from interferon-treated mouse L cells, Proc. Natl. Acad. Sci. USA 80 41. 

Kaempfer, R., 1984, Differential gene expression by messenger RNA competition for eukaryotic initiation factor 2, in Protein Synthesis Translation and Post-Translational Events (A. K. Abraham, T. S. Eikhom, and I. F. Pryme, eds.), pp. 57-76, The Humana Press, Clifton, New Jersey. 

Inhibition of HeLa cell protein synthesis following poliovirus infection correlates with the proteolysis of a 220,000 dalton polypeptide associated with eukaryotic initiation factor 3 and a cap binding protein complex, J. Biol. Chem. 257 14806. 

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