Universal translation initiation factor IF2/eIF5B

Role of aIF5B in archaeal translation initiation

In eukaryotes and in archaea late steps of translation initiation involve the two initiation factors e/aIF5B and e/aIF1A. In eukaryotes, the role of eIF5B in ribosomal subunit joining is established and structural data showing eIF5B bound to the full ribosome were obtained. To achieve its function, eIF5B collaborates with eIF1A. However, structural data illustrating how these two factors interact on the small ribosomal subunit have long been awaited. The role of the archaeal counterparts, aIF5B and aIF1A, remains to be extensively addressed. Here, we study the late steps of Pyrococcus abyssi translation initiation. Using in vitro reconstituted initiation complexes and light scattering, we show that aIF5B bound to GTP accelerates subunit joining without the need for GTP hydrolysis. We report the crystallographic structures of aIF5B bound to GDP and GTP and analyze domain movements associated to these two nucleotide states. Finally, we present the cryo-EM structure of an initiation complex containing 30S bound to mRNA, Met-tRNA_i^Met, aIF5B and aIF1A at 2.7 Å resolution. Structural data shows how archaeal 5B and 1A factors cooperate to induce a conformation of the initiator tRNA favorable to subunit joining. Archaeal and eukaryotic features of late steps of translation initiation are discussed.

Structural basis for the transition from translation initiation to elongation by an 80S-eIF5B complex

Recognition of a start codon by the initiator aminoacyl-tRNA determines the reading frame of messenger RNA (mRNA) translation by the ribosome. In eukaryotes, the GTPase eIF5B collaborates in the correct positioning of the initiator Met-tRNA_i^Met on the ribosome in the later stages of translation initiation, gating entrance into elongation. Leveraging the long residence time of eIF5B on the ribosome recently identified by single-molecule fluorescence measurements, we determine the cryoEM structure of the naturally long-lived ribosome complex with eIF5B and Met-tRNA_i^Met immediately before transition into elongation. The structure uncovers an unexpected, eukaryotic specific and dynamic fidelity checkpoint implemented by eIF5B in concert with components of the large ribosomal subunit.

Disparate Phenotypes Resulting from Mutations of a Single Histidine in Switch II of Geobacillus stearothermophilus Translation Initiation Factor IF2

The conserved Histidine 301 in switch II of Geobacillus stearothermophilus IF2 G2 domain was substituted with Ser, Gln, Arg, Leu and Tyr to generate mutants displaying different phenotypes. Overexpression of IF2H301S, IF2H301L and IF2H301Y in cells expressing wtIF2, unlike IF2H301Q and IF2H301R, caused a dominant lethal phenotype, inhibiting in vivo translation and drastically reducing cell viability. All mutants bound GTP but, except for IF2H301Q, were inactive in ribosome-dependent GTPase for different reasons. All mutants promoted 30S initiation complex (30S IC) formation with wild type (wt) efficiency but upon 30S IC association with the 50S subunit, the fMet-tRNA reacted with puromycin to different extents depending upon the IF2 mutant present in the complex (wtIF2 ≥ to IF2H301Q > IF2H301R >>> IF2H301S, IF2H301L and IF2H301Y) whereas only fMet-tRNA 30S-bound with IF2H301Q retained some ability to form initiation dipeptide fMet-Phe. Unlike wtIF2, all mutants, regardless of their ability to hydrolyze GTP, displayed higher affinity for the ribosome and failed to dissociate from the ribosomes upon 50S docking to 30S IC. We conclude that different amino acids substitutions of His301 cause different structural alterations of the factor, resulting in disparate phenotypes with no direct correlation existing between GTPase inactivation and IF2 failure to dissociate from ribosomes.

Four translation initiation pathways employed by the leaderless mRNA in eukaryotes

mRNAs lacking 5′ untranslated regions (leaderless mRNAs) are molecular relics of an ancient translation initiation pathway. Nevertheless, they still represent a significant portion of transcriptome in some taxons, including a number of eukaryotic species. In bacteria and archaea, the leaderless mRNAs can bind non-dissociated 70 S ribosomes and initiate translation without protein initiation factors involved. Here we use the Fleeting mRNA Transfection technique (FLERT) to show that translation of a leaderless reporter mRNA is resistant to conditions when eIF2 and eIF4F, two key eukaryotic translation initiation factors, are inactivated in mammalian cells. We report an unconventional translation initiation pathway utilized by the leaderless mRNA in vitro, in addition to the previously described 80S-, eIF2-, or eIF2D-mediated modes. This mechanism is a bacterial-like eIF5B/IF2-assisted initiation that has only been reported for hepatitis C virus-like internal ribosome entry sites (IRESs). Therefore, the leaderless mRNA is able to take any of four different translation initiation pathways in eukaryotes.

Animal virus schemes for translation dominance

Viruses have adapted a broad range of unique mechanisms to modulate the cellular translational machinery to ensure viral translation at the expense of cellular protein synthesis. Many of these promote virus-specific translation by use of molecular tags on viral mRNA such as internal ribosome entry sites (IRES) and genome-linked viral proteins (VPg) that bind translation machinery components in unusual ways and promote RNA circularization. This review describes recent advances in understanding some of the mechanisms in which animal virus mRNAs gain an advantage over cellular transcripts, including new structural and biochemical insights into IRES function and novel proteins that function as alternate met-tRNA_i^met carriers in translation initiation. Comparisons between animal and plant virus mechanisms that promote translation of viral mRNAs are discussed.

Archaeal translation initiation factor aIF2 can substitute for eukaryotic eIF2 in ribosomal scanning during mammalian 48S complex formation

Heterotrimeric translation initiation factor (IF) a/eIF2 (archaeal/eukaryotic IF 2) is present in both Eukarya and Archaea. Despite strong structural similarity between a/eIF2 orthologs from the two domains of life, their functional relationship is obscure. Here, we show that aIF2 from Sulfolobus solfataricus can substitute for its mammalian counterpart in the reconstitution of eukaryotic 48S initiation complexes from purified components. aIF2 is able to correctly place the initiator Met-tRNA(i) into the P-site of the 40S ribosomal subunit and accompany the entire set of eukaryotic translation IFs in the process of cap-dependent scanning and AUG codon selection. However, it seems to be unable to participate in the following step of ribosomal subunit joining. In accordance with this, aIF2 inhibits rather than stimulates protein synthesis in mammalian cell-free system. The ability of recombinant aIF2 protein to direct ribosomal scanning suggests that some archaeal mRNAs may utilize this mechanism during translation initiation.

Activation of initiation factor 2 by ligands and mutations for rapid docking of ribosomal subunits

We previously identified mutations in the GTPase initiation factor 2 (IF2), located outside its tRNA-binding domain, compensating strongly (A-type) or weakly (B-type) for initiator tRNA formylation deficiency. We show here that rapid docking of 30S with 50S subunits in initiation of translation depends on switching 30S subunit-bound IF2 from its inactive to active form. Activation of wild-type IF2 requires GTP and formylated initiator tRNA (fMet-tRNA(i)). In contrast, extensive activation of A-type IF2 occurs with only GTP or with GDP and fMet-tRNA(i), implying a passive role for initiator tRNA as activator of IF2 in subunit docking. The theory of conditional switching of GTPases quantitatively accounts for all our experimental data. We find that GTP, GDP, fMet-tRNA(i) and A-type mutations multiplicatively increase the equilibrium ratio, K, between active and inactive forms of IF2 from a value of 4 × 10(-4) for wild-type apo-IF2 by factors of 300, 8, 80 and 20, respectively. Functional characterization of the A-type mutations provides keys to structural interpretation of conditional switching of IF2 and other multidomain GTPases.

GTP-independent tRNA delivery to the ribosomal P-site by a novel eukaryotic translation factor

During translation, aminoacyl-tRNAs are delivered to the ribosome by specialized GTPases called translation factors. Here, we report the tRNA binding to the P-site of 40 S ribosomes by a novel GTP-independent factor eIF2D isolated from mammalian cells. The binding of tRNA(i)(Met) occurs after the AUG codon finds its position in the P-site of 40 S ribosomes, the situation that takes place during initiation complex formation on the hepatitis C virus internal ribosome entry site or on some other specific RNAs (leaderless mRNA and A-rich mRNAs with relaxed scanning dependence). Its activity in tRNA binding with 40 S subunits does not require the presence of the aminoacyl moiety. Moreover, the factor possesses the unique ability to deliver non-Met (elongator) tRNAs into the P-site of the 40 S subunit. The corresponding gene is found in all eukaryotes and includes an SUI1 domain present also in translation initiation factor eIF1. The versatility of translation initiation strategies in eukaryotes is discussed.