EIF2

Eukaryotic Initiation Factor 2 (eIF2) is a eukaryotic initiation factor. It is required in the initiation of translation. In this fundamental process of life, the ribosome builds proteins according to the information encoded on the mRNA. eIF2 mediates the binding of tRNAmet to the ribosome in a GTP-dependent manner. eIF2 is a heterotrimer consisting of an alpha (also called subunit 1), a beta (subunit 2), and a gamma (subunit 3) subunit.

Once the initiation is completed, eIF2 is released from the ribosome bound to GDP as an inactive binary complex. To participate in another round of translation initiation, this GDP must be exchanged for GTP.

Function
eIF2 is an essential factor for protein synthesis that forms a ternary complex (TC) with GTP and the initiator Met-tRNA. After its formation the TC binds the 40S ribosomal subunit to form the 43S preinitiation complex (PIC). PIC-assembly is believed to be stimulated by the initiation factors eIF1, eIF2A and the eIF3 complex according to in vitro experiments. The 43S PIC then binds mRNA that has previously been unwound by the eIF4s. The 43S PIC and the eIF4 proteins form a new 48S complex on the mRNA which starts searching along the mRNA for the start codon (AUG). Upon base pairing of the AUG-codon with the Met-tRNA, eIF5 (which is a GTPase activating protein) is recruited to the complex and induces eIF2 to hydrolyse its GTP. This causes eIF2-GDP to be released from this 48S complex and translation begins after recruitment of the 60S ribosomal subunit and formation of the 80S initiation complex. Finally, with the help of the Guanine nucleotide exchange factor eIF2B, the GDP in eIF2 is exchanged for a GTP and the ternary complex reforms for a new round of translation initiation.

Structure
eIF2 is a heterotrimer of a total molar mass of 126 kDa that is composed of the three subunits α (subunit 1), β (subunit 2), and γ (subunit 3). The sequences of all three subunits are highly conserved (pairwise amino acid identities for each subunit range from 47 – 72 % when comparing the proteins of Homo sapiens and Saccharomyces cerevisiae).

The α-subunit contains the main target for phosphorylation, a serine at position 51. It also contains a S1 motif domain, which is a potential RNA binding-site. Therefore the α-subunit can be considered the regulatory subunit of the trimer.

The β-subunit contains multiple phosphorylation sites (residues 2, 13, 67, 218). More importantly there are also three lysine clusters in the N-terminal domain (NTD) which are important for the interaction with eIF2B. Moreover the sequence of the protein comprises a zinc finger motif which was shown to play a role in both ternary complex and 43S preinitiation complex formation. There are also two guanine nucleotide binding sequences which have not been shown to be involved in the regulation of eIF2 activity. The β-subunit is also believed to interact with both tRNA and mRNA.

The γ-subunit comprises three guanine nucleotide binding sites and is known to be the main docking site for GTP/GDP. It also contains a tRNA binding cavity which has been shown by X-ray crystallography. A zinc knuckle motif is able to bind one Zn2+ cation.

Regulation
eIF2 activity is regulated by a mechanism involving both guanine nucleotide exchange and phosphorylation. Phosphorylation takes place at the α-subunit which is a target for a number of serine kinases that phosphorylate serine 51. Those kinases act as a result of stress such as amino acid deprivation (GCN2), ER stress (PERK), the presence of dsRNA (PKR) or Heme deficiency (HRI). Once phosphorylated, eIF2 shows increased affinity for its Guanine nucleotide exchange factor eIF2B. However, eIF2B is only able to exchange GDP for GTP if eIF2 is in its unphosphorylated state. Phosphorylated eIF2, though, due to its stronger binding acts as an inhibitor of its own GEF (eIF2B). Since the cellular concentration of eIF2B is much lower than that of eIF2, even a small amount of phosphorylated eIF2 can completely abolish eIF2B activity by sequestration. Without the GEF, eIF2 can no longer be returned to its active (GTP-bound) state. Consequently translation comes to a halt since initiation is no longer possible without any available ternary complex.

Disease
Since eIF2 is essential for translation initiation and therefore protein synthesis, defects in eIF2 are lethal. The protein is highly conserved among evolutionary remote species - indicating a large impact of mutations on cell viability. Therefore no diseases directly related to mutations in eIF2 can be observed. However there are many illnesses caused by down-regulation of eIF2 through its upstream kinases. For example, increased concentrations of active PKR and inactive (phosphorylated) eIF2 were found in patients suffering from neurodegenerative diseases such as Alzheimer’s, Parkinson’s and Huntington’s disease. There is also one proven example of a disease related to the GEF eIF2B. Mutations in all of the five subunits of eIF2B could be linked with leukoencephalopathy, an illness that causes the brain’s white matter to disappear. It is still not fully understood why only brain cells seem to be affected by these defects. Potentially reduced levels of unstable regulatory proteins might play a role in the development of the diseases mentioned.