Should I stay or should I go? Eukaryotic translation initiation factors 1 and 1A control start codon recognition

Abstract

Start codon selection is a key step in translation initiation as it sets the reading frame for decoding. Two eukaryotic initiation factors, eIF1 and eIF1A, are key actors in this process. Recent work has elucidated many details of the mechanisms these factors use to control start site selection. eIF1 prevents the irreversible GTP hydrolysis that commits the ribosome to initiation at a particular codon. eIF1A both promotes and inhibits commitment through the competing influences of its two unstructured termini. Both factors perform their tasks through a variety of interactions with other components of the initiation machinery, in many cases mediated by the unstructured regions of the two proteins.

FIGURE 1.

Model of eukaryotic translation initiation. The first step of translation initiation is the binding of a TC composed of eIF2, GTP, and Met-tRNA^Met_i to the 40 S ribosomal subunit forming the 43 S PIC. TC may bind the 40 S subunit as part of the MFC along with eIF1, eIF5, and eIF3. eIF1A also binds and facilitates MFC recruitment. The N- and C-terminal tails of eIF1 and eIF1A are represented in green and pink, respectively. Although their locations in the complex are not known, some proposed interactions are shown here. The PIC then binds mRNA with the help of eIF3, the eIF4F complex, eIF4B, and PABP. For clarity, these factors are not shown. The complex scans the mRNA for the start codon. During this time, GTP bound to eIF2 can be hydrolyzed with the help of eIF5, but this reaction is reversible because phosphate is not released. During the scanning process, the PIC is in an open state, which is in equilibrium with a closed state that is not capable of scanning but is able to investigate the codon in the P-site. In this figure, the closed state is represented by a lock, holding the mRNA in place. When an AUG codon is identified in the P-site, the equilibrium shifts toward the closed state. eIF1 dissociates from the 40 S ribosome, and the phosphate ion bound to eIF2 can now be released, making hydrolysis irreversible and committing the PIC to initiate translation at the codon currently in the P-site. After dissociation from the ribosome, eIF1 may remain bound to the PIC through an interaction with eIF3 (29). After eIF2·GDP and eIF5 dissociate, the 60 S subunit joins the 40 S subunit with the help of the GTPase eIF5B. The 80 S initiation complex is now ready to begin the elongation phase of translation. Factors in this figure are not drawn to scale to make the smaller factors visible.

FIGURE 2.

NMR structures of eIF1 and eIF1A. a, the solution structure of human eIF1 (36) is represented in rainbow coloring, from red at the C terminus to blue at the N terminus. The ensemble of structures consistent with the NMR data is overlaid to accentuate the unstructured nature of the N-terminal tail. b, the solution structure of human eIF1A (44) is represented as described for a. Note that both the N- and C-terminal regions of eIF1A are unstructured.