A Retrospective on eIF2A-and Not the Alpha Subunit of eIF2

Abstract

Initiation of protein synthesis in eukaryotes is a complex process requiring more than 12 different initiation factors, comprising over 30 polypeptide chains. The functions of many of these factors have been established in great detail; however, the precise role of some of them and their mechanism of action is still not well understood. Eukaryotic initiation factor 2A (eIF2A) is a single chain 65 kDa protein that was initially believed to serve as the functional homologue of prokaryotic IF2, since eIF2A and IF2 catalyze biochemically similar reactions, i.e., they stimulate initiator Met-tRNA_i binding to the small ribosomal subunit. However, subsequent identification of a heterotrimeric 126 kDa factor, eIF2 (α,β,γ) showed that this factor, and not eIF2A, was primarily responsible for the binding of Met-tRNA_i to 40S subunit in eukaryotes. It was found however, that eIF2A can promote recruitment of Met-tRNA_i to 40S/mRNA complexes under conditions of inhibition of eIF2 activity (eIF2α-phosphorylation), or its absence. eIF2A does not function in major steps in the initiation process, but is suggested to act at some minor/alternative initiation events such as re-initiation, internal initiation, or non-AUG initiation, important for translational control of specific mRNAs. This review summarizes our current understanding of the eIF2A structure and function.

Keywords: alternative initiation; eIF2A; non-AUG initiation; stress response; translational initiation.

Figure 1

Overview of the major stages of prokaryotic (left) and eukaryotic cap-dependent translation (right) initiation pathways.

Figure 2

The structural model of the yeast eIF2•GTP•Met-tRNA_i ternary complex adapted from the structure of the yeast 48S complex (PDB 3JAP [65]) with the 40S ribosome and other factors omitted. The eIF2α subunit is in yellow (Ser51 is in magenta; Van der Waals radii of the side chain atoms are shown), the eIF2β subunit is in blue and the eIF2γ subunit is in red. Met-tRNA_i is in cyan. Arrow points to GTP (in gray).

Figure 3

Representative sequence alignments of eIF2A proteins from yeast (Saccharomyces cerevisiae), zebrafish, (Danio rerio), mouse (Mus musculus), human (Homo sapiens), pig (Sus scrofa), chicken (Gallus gallus) and alligator (Alligator mississippiensis).

Figure 4

Predicted structure of the human eIF2A protein, showing top view (left) and side view (right) of the β-propeller domain. N-terminal domain (NTD) (residues 1-415) folds into 9-bladed β-propeller. The C-terminal domain (CTD) is less structured and may consist of two to three smaller subdomains (with very C-terminal fragment residues 533-585 represented by a helix bundle). Residues 505–520 (in magenta on the right image) represent a potential PEST motif. Homology modeling was done using Phyre2 Protein Fold Recognition Server.

Figure 5

eIF2A initiation pathway (only the stage leading to the assembly of 48S complex is shown).