Translation initiation factor, eIF

Naranda, T., MacMillan, S. E., and Hershey, J. W. B. (1994). Purified yeast translational initiation factor eIF-3 is an RNA-binding protein complex that contains the PRT1 protein. J. Biol. Chem. 269, 32286—32292. 

Mader, S., Lee, H., Pause, A., and Sonenberg, N. (1995). The translation initiation factor eIF-4E binds to a common motif shared by the translation factor eIF-4gamma and the translational repressors 4E-binding proteins. Mol. Cell. Biol. 15, 4990-4997. 

Pause, A., Methot, N., Svitkin, Y., Merrick, W. C., and Sonenberg, N. (1994). Dominant negative mutants of mammalian translation initiation factor eIF-4A define a critical role for eIF-4F in cap-dependent and cap-independent initiation of translation. EMBO J. 13, 1205-1215. 

It has been shown that expression of translation initiation factors eIF-4E and eIF-2alpha is increased in neoplastic cells of Hodgkin lymphoma, but not in surrounding lymphocytes. An increase in eIF-4E expression may lead to constitutively high expression of NF-kB. H/RSCs have high expression of c-FEIP, which protects cells from apoptosis.Tissue inhibitor of metal-loproteinases (TIMP-1 and TIMP-2) are proteins with proteinase inhibition and cytokine properties. TIMP-1... 

Rosenwald IB, Koifman L, Savas L, et al. Expression of the translation initiation factors eIF-4E and eIE-2 is frequently increased in neoplastic cells of Hodgkin lymphoma. Hum Pathol. 2008 39 910-916. 

In our laboratory, we have designed oligonucleotides to bind to 5 -cap structures and reagents to specifically cleave the unique 5 -cap structure (123). These studies demonstrated that 5 -cap-targeted oligonucleotides were capable of inhibiting the binding of the translation initiation factor eIF-4a (124). 

Type I (IFN-a/P) and type II (IFN-y) IFNs are major lines of defense against viral infection. IFNs mediate direct antiviral effector mechanisms that inhibit multiple steps of viral replication (Samuel 1991 Vilcek and Sen 1996). For example, 2, 5 -oligoadenylate synthetase (2, 5 -OAS) activates ribonuclease L, which degrades mRNA and limits the accumulation of viral transcripts. Protein kinase R blocks translation of viral transcripts by phosphorylating translation initiation factor eIF-2. Mx proteins block influenza, vesicular stomatitis virus, and herpes simplex virus replication by an unknown mechanism. 

Cooper HL, Park MH, Folk JE et al (1983) Identification of the hypusine-containing protein hy+ as translation initiation factor eIF-4D. Proc Natl Acad Sci USA 80 1854-1857 Doerfel LK, Wohlgemuth I, Kothe C et al (2013) EF-P is essential for rapid synthesis of proteins containing consecutive prohne residues. Science 339 85-88 Gerner EW, Mamont PS, Bernhardt A et al (1986) Post-translational modification of the protein-synthesis initiation factor eIF-4D by spermidine in rat hepatoma cells. Biochem J 239 379-386... 

Two initiation factors, eIF-3 and elF-lA, bind to the newly dissociated 40S ribosomal subunit. This delays its reassociation with the 60S subunit and allows other translation initiation factors to associate with the 40S subunit. 

De Benedetti, A., Joshi-Barve, S., Rinker-Schaeffer, C., and Rhoads, R. E. (1991). Expression of antisense RNA against initiation factor eIF-4E mRNA in HeLa cells results in lengthened cell division times, diminished translation rates, and reduced levels of both eIF-4E and the p220 component of eIF-4F. Mol. Cell Biol. 11, 5435-5445. 

Gribskov, M. (1992). Translational initiation factors IF-1 and eIF-2 alpha share an RNA-binding motif with prokaryotic ribosomal protein SI and polynucleotide phosphoryl-ase. Gene 119, 107-111. 

The de novo synthesis of proteins can be varied in response to external stimuli, such as hormones or heat stress. The regulation of protein biosynthesis ocems primarily via phosphorylation of translation initiation factors. The regulatory points in eucaryotes are, above all, the translation factors eIF-2 and elF-4. 

The vulnerable point for insulin-mediated regulation of translation is the initiation factor eIF-4E. This factor binds specifically to the 5 -cap structure of mRNA and is part of a larger complex, termed eIF-4F. A further component of eIF-4F is the eIF-4A protein, which possesses helicase activity. The binding of eIF-4E to the cap structure is necessary for the association of the 40S subunit with the 5 -end of the mRNA and for... 

The MAPK cascade also has direct effects upon protein synthesis, i.e., on the translation of mRNA messages. For example, insulin stimulates phosphorylation of proteins that regulate a translation initiation factor, a protein called eIF-4E (see Chapter 29). Phosphorylation of inhibitory proteins allows them to dissociate from the initiation factor so that protein synthesis can proceed 485/486... 

In the second step, the virus synthesizes an inhibitor that inactivates an initiation factor and by the mid-point of infection, most ribosomes are directed towards translating viral mENA. This two step process could explain the differential effect of such perturbations as exposure to high salt concentrations and histidinol treatment on cellular and viral mENA translation and the inactivation of initiation factor eIF-4B after infection of HeLa cells with poliovirus. 

The currently available evidence (1 5) suggested that the binding of host mRNAs to ribosomes was specifically inhibited in infected cells (2). Furthermore, viral mRNA had been shown to outcompete host mMA at the initiation step of protein synthesis (5), and vitro studies demontrated that picomavirus RNA was preferentially translated over host mRNA when both were present in saturating amoimts (6-7). This competition can be relieved by the addition of a specific initiation factor, eIF-4B, suggesting that the amount of factor available and possibly the higher affinity of viral mRNA for this factor determine the preferential translation of viral templates (8). 

Small RNAs may also be involved in regulating the translation of mRNA in eukaryotic cells. Of the stimulatory RNAs, the best characterized is a small RNA of about 160 nucleotides, which accumulates in cells after infection with adenovirus. This virus-associated RNA, VA-RNAi, which is required to maintain general protein synthesis, acts by inhibiting the phosphorylation of the alpha subunit of initiation factor eIF-2. 

The best studied case of viral inhibition of host cell protein synthesis is poliovirus infection of HeLa cells. Infected cells, and extracts prepared from these cells, are impaired in their ability to bind capped mRNAs to ribosomes, and thus fail to initiate translation. At least one multisubunit initiation factor (eIF-4F) which contains a documented cap-binding protein, and which is required for translation of most mRNAs, is functionally inactive and structurally disrupted following infection. The biochemical lesion underlying the dissociation of this factor is not yet understood, nor has its precise mechanism of action in uninfected cells been determined. Expression, but not extensive replication, of the infecting viral genome is required for inhibition to occur, but the molecular mediator of the inactivation remains to be identified. The fact that viral protein synthesis occurs under conditions of inhibition of cellular protein synthesis demands that viral mRNA initiate translation differently from its host cell. The discrimination apparently involves a lack of dependence on the cap structure, but additionally requires other sequence and/or structural features which permit initiation by a cap-independent mechanism (Brown et ai, 1982). Virtually nothing is known about how viral mRNA associates with ribosomes to enable it to bypass those steps in the usual initiation process which are inhibited following infection. 

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. 

Exposure of cells to interferon normally results in the induction of a protein kinase termed eIF-2a protein kinase. The enzyme, which is synthesized in a catalytically inactive form, is activated by exposure to dsRNA. The activated kinase then phosplorylates its substrate, i.e. eIF-2a, which is the smallest subunit of initiation factor 2 (eIF2). This, in turn, blocks construction of the smaller ribosomal subunit, thereby preventing translation of all viral (and cellular) mRNA (Figure 8.6). 

Fig. 1.57. Model of the regulation of translation by insulin. Insulin ( and other growth factors) activates the Akt kinase pathway (see ch. 10), whose final result is the phosphorylation of 4E-BPl, a regulatory protein of translation initiation. The 4E-BP1 protein inactivates the initation factor eIF-4E by complex formation. eIE-4E is required, together with the proteins eIE-4A and p220, for the binding of the 40S subunit of the ribosome to the cap structure of the mRNA. If the 4E-BP1 protein becomes phosphorylated as a result of insulin-mediated activation of the PI3 kinase/Akt kinase cascade, then eIE-4E is liberated from the inactive eIP-4E 4E-BPl complex and protein biosynthesis can begin.

Inactivation of eukaryotic translation factors by covalent modification is one of the few mechanisms known to regulate the rate of translation. Specific protein kinases have been identified that phosphorylate and inactive both eIF-1 and EF-2. The significance of the phosphorylation of EF-2 as a regulatory mechanism of the elongation rate is still not clear, but the phosphorylation of eIF-2 appears to be a general mechanism for controlling translation initiation in many cells. 

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