Protein synthesis in eukaryotic organisms requires cooperation among a large number of polypeptides including ribosomal proteins, modification enzymes and ribosome associated translation factors. The initiation phase of protein synthesis requires a set of eIFs (eukaryotic translation initiation factors) which are comprised of multiple polypeptide subunits. eIF2 (eukaryotic initiation factor-2) is a GTP-binding protein that escorts the initiation specific form of met-tRNA onto the ribosome. Important functions of eIF2 include delivery of charged initiator methionyl-tRNA to the ribosome as well as a role in identifying the translational start site.A variety of stimuli modulate eIF2 activity which in turn regulates mRNA translation. Four distinct eIF2-α kinases phosphorylate eIF2α and regulate protein synthesis in response to various environmental stress. These are HRI, PKR - the interferon inducible dsRNA dependent kinase, PEK and GCN2. Whereas HRI and PKR are restricted to mammalian cells, GCN2 and PEK are widely distributed in eukaryotes. PKR normally is expressed at low levels and is inactive in the cell. IFN-γ causes its levels to increase and the same viral dsRNA that induced the synthesis of IFN is used to convert inactive PKR to the active form. Once activated, PKR phosphorylates the protein eIF2α which further leads to translation inhibition. PEK is an ER transmembrane protein kinase that phosphorylates eIF2α in response to ER stress. Phosphorylation of eIF2α reduces the formation of translation initiation complexes, which leads to reduced recognition of AUG initiation codons and therefore general translational attenuation. GCN2 stimulates the translation of transcriptional activator GCN4 by phosphorylating eIF2α in response to amino acid starvation.
The guanine nucleotide exchange factor of eIF2, namely eIF2B, is consists of five different subunits (α-ε). The ε-subunit of eIF2B is the catalytic one and is phosphorylated in vivo at several different sites. One of these is a target for GSK3. Phosphorylation of eIF2B by GSK3 inhibits its activity. GSK3 is switched off in response to insulin and growth factors via a signaling pathway that involves PI3K and PKB. This provides a signaling pathway through which insulin and other stimuli can activate eIF2B and thus turn on an important step in translation initiation. Signaling via the classical MAPK cascade is involved in the activation of eIF2B by some growth factors. eIF2 transfers bound initiator met-tRNA to the 40S ribosomal subunit-mRNA complex.
During initiation, eIF2 forms a complex with GTP and initiator methionyl-tRNAi (met-tRNAi) and this ternary complex subsequently binds to the 40S ribosomal subunit to form the 43S preinitiation complex. Through the action of a translation initiation factor complex referred to as eIF4F, which is comprised of eIF4A, eIF4E and eIF4G, mRNA is bound to the 43S preinitiation complex resulting in formation of the 48S preinitiation complex. Then the GDP bound to eIF2 is hydrolyzed in an eIF5-mediated process and initiation factors are released from the ribosome. Before binding Met-tRNAi and reforming the ternary complex, the GDP bound to eIF2 must be exchanged for GTP, a reaction that is catalyzed by the guanine nucleotide exchange factor, eIF2B. eIF2 in its phosphorylated form is very strongly bound by eIF2B. Since eIF2B is present in limiting quantities, phosphorylation of only a small amount of eIF2 results in total inhibition of translation by sequestration of eIF2B. The cellular stress response protein GADD34 binds to PP1 and attenuates translational elongation of key transcriptional factors through dephosphorylation of eIF2-α. Phosphorylation of eIF2-α terminates global protein translation and induces apoptosis.