Cap-dependent translation Clausole campione

Cap-dependent translation. Translation initiation can be subdivided into four steps: 1) binding of the specific initiator Met-tRNA, to the small ribosomal subunit (40S), 2) binding of the formed complex to the cap structure at the 5’ end of mRNA, 3) scanning of the 5’untranslated region (5’UTR) of the mRNA and start codon recognition, 4) joining of the large ribosomal subunit (60S) to generate a translation competent ribosome (80S). As physiological conditions favor the association of 40S and 60S ribosomal subunit to form complete 80S ribosomes, but only free ribosomal subunits can initiate translation, it is important that post termination ribosomes dissociate (▇▇▇▇▇▇ and Hentze 2003). The eukaryotic initiation factors eIF3, eIF1, eIF1A and eIF6 are thought to promote this dissociation in eukaryotes (▇▇▇▇▇▇ and Pestova 2007). As reported in Figure 1, the first step in 43S preinitiation complex formation is the assembly of a ternary complex, consisting of eIF2, methionyl tRNA (met-tRNA) and GTP. Its assembly is stimulated by the guanine nt exchange factor (GEF) eIF2B. GTP is hydrolyzed after recognition of the AUG start codon producing eIF2 bound to GTP (▇▇▇▇▇▇▇▇▇ and ▇▇▇▇▇▇▇▇▇▇ 2007). eIF2B promotes GDP-GTP exchange to regenerate active eIF2. Binding of the ternary complex to the 40S ribosomal subunit is supported by eIF1, eIF1A and eIF3 in mammalian cells (▇▇▇▇▇▇ and ▇▇▇▇▇▇ 2003; ▇▇▇▇▇▇ and ▇▇▇▇▇▇▇ 2007). The 43S preinitiation complex is ready to bind to the 5’ end of the mRNA. Figure 1. Cap-mediated translation initiation (▇▇▇▇▇▇▇ and ▇▇▇▇▇▇ 2004) The eIF4F complex bounds to the 5’m7GpppN cap structure and promotes the recruitment of the 43S preinitiation complex to the 5’end of the mRNA generating a complex called 48S. eIF4F is composed of the cap-binding protein eIF4E, the scaffold protein eIF4G and the ATP- dependent helicase eIF4A that, assisted by eIF4B, unwinds secondary structures in the 5’UTR of the mRNA. The binding of the preinitiation complex to the mRNA involves the cooperative activities of eIF4F, eIF3, eIF4B and PABP. PABP was identified as a protein that associated with ▇▇▇▇▇ tail at the 3’UTR of the mRNA. The PABP-eIF4G interaction is thought to promote a circularization of the mRNA molecule forming a closed loop. This circularization provides a possible framework by which 3’UTR-binding proteins can regulate translation initiation (▇▇▇▇▇▇▇ and ▇▇▇▇▇▇ 2004). Once assembled near the 5’ end of the mRNA, the 48S complex scan along the mRNA to find the AUG starts codon...