Eukaryotic translation initiation factor binding proteins

Kahvejian, A., Svitkin, Y. V., Sukarieh, R., M Boutchou, M. N., and Sonenberg, N. (2005). Mammalian poly(A)-binding protein is a eukaryotic translation initiation factor, which acts via multiple mechanisms. Genes Dev. 19, 104—113. 

A. J. Waskiewicz, et al. Phosphorylation of the cap-binding protein eukaryotic translation initiation factor 4E by protein kinase Mnkl in vivo. Mol Cell Biol, 19, 1871-1880, 1999. 

AKT also regulates several mitogenic functions, such as cell growth, cell division, cell proliferation, and protein synihesis. Phosphorylation of the mammalian target of rampamycin (mTOR) by AKT results in inactivation of eukaryotic translation initiation factor 4E binding proteins (4E-BPs) to promote mRNA translation on ribosome (Dowling... 

Eukaryotes utilize many more initiation factors than do prokaryotes, and their interplay is much more intricate. The prefix elF denotes a eukaryotic initiation factor. For example, eIF-4E is a protein that binds directly to the 7-inethylguanosine cap (p. 846), whereas eIF-2, in association with GTP, delivers the met-tRNA to the ribosome. The difference in initiation mechanism between prokaryotes and eukaryotes is, in part, a conseciuence of the ence in RNA processing. The 5 end of mRNA is readily available to ribosomes immediately after transcription in prokaryotes. In contrast, pre-mRNA must be processed and transported to the cytoplasm in eukaryotes before translation is initialed. The 5 cap provides an easily recognizable starting point. In addition, the complexity of eukaryotic translation initiation provides another mechan ism for regulation of gene expression that we shall explore further in Chapter 31. 

The eukaryotic protein-synthesizing machinery begins translation of most cellular mRNAs within about 100 nucleotides of the 5 capped end as just described. However, some cellular mRNAs contain an internal ribosome entry site (IRES) located far downstream of the 5 end. In addition, translation of some viral mRNAs, which lack a 5 cap, is initiated at IRESs by the host-cell machinery of infected eukaryotic cells. Some of the same translation initiation factors that assist in ribosome scanning from a 5 cap are required for locating an internal AUG start codon, but exactly how an IRES is recognized is less clear. Recent results indicate that some IRESs fold into an RNA structure that binds to a third site on the ribosome, the E site, thereby positioning a nearby internal AUG start codon in the P site. 

Many more protein factors are involved in eukaryotic initiation some systems contain more than 10 initiation factors. Particular features of translation initiation are also different in the higher organisms. Most notably, prokaryotic ribosomes can initiate internally on an mRNA (even on circular RNAs), while in eukaryotes a preinitiation complex binds to the 5 -end of the mRNA and then progresses to an initiation complex. Eukaryotic mRNAs are capped at their 5 -end with a 7-methylguanosine triphosphate structure, and one of the eukaryotic initiation factors binds this capped end. The preinitiation complex then moves along the mRNA and initiates translation at the first AUG codon it comes to. Consistent with this scanning mechanism is the observation that eukaryotic mRNAs do not contain Shine-Dalgarno-like sequences. 

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. 

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.

The first phase of translation, initiation, involves several steps. First, two proteins, initiation factors IF-1 and IF-3, bind to the 30 S subunit (1). Another factor, IF-2, binds as a complex with GTP (2). This allows the subunit to associate with the mRNA and makes it possible for a special tRNA to bind to the start codon (3). In prokaryotes, this starter tRNA carries the substituted amino acid N-formylmethionine (fMet). In eukaryotes, it carries an unsubstituted methionine. Finally, the 50 S subunit binds to the above complex (4). During steps 3 and 4, the initiation factors are released again, and the GTP bound to IF-2 is hydrolyzed to GDP and Pj. 

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