View source for EIF2

{{lowercase title}}
'''Eukaryotic Initiation Factor 2''' ('''eIF2''') is a [[eukaryotic initiation factor]]. It is required for most forms of eukaryotic [[Translation (biology)|translation]] initiation. eIF2 mediates the binding of [[tRNA]]<sub>i</sub><sup>Met</sup> to the ribosome in a [[Guanosine triphosphate|GTP]]-dependent manner. eIF2 is a heterotrimer consisting of an [[EIF2S1|alpha]] (also called subunit 1, EIF2S1), a [[EIF2S2|beta]] (subunit 2, EIF2S2), and a [[EIF2S3|gamma]] (subunit 3, EIF2S3) subunit.

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

==Function==
[[Image:Eukaryotic initiation.png |thumb|right |400px |The process of initiation of translation in eukaryotes with eIF2 in light-green. Other factors are shown, too.]]
eIF2 is an essential factor for protein synthesis that forms a ternary complex (TC) with [[Guanosine triphosphate|GTP]] and the initiator [[Methionine|Met]]-[[tRNA]]<sub>i</sub><sup>Met</sup>. After its formation, the TC binds the [[Eukaryotic small ribosomal subunit (40S)|40S ribosomal subunit]] to form the [[43S preinitiation complex]] (43S PIC). 43S PIC assembly is believed to be stimulated by the [[Eukaryotic initiation factor|initiation factors]] [[eIF1]], [[eIF1AX|eIF1A]], and the [[eIF3]] complex according to ''in vitro'' experiments. The 43S PIC then binds [[messenger RNA|mRNA]] that has previously been unwound by the [[eIF4F]] complex. The 43S PIC and the eIF4F 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]], or GAP) 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]] (GEF) [[eIF2B]],<ref>eIF2B consists of the sub-units [[EIF2B1]], [[EIF2B2]], [[EIF2B3]], [[EIF2B4]], [[EIF2B5]]</ref> the GDP in eIF2 is exchanged for a GTP and the ternary complex reforms for a new round of translation initiation.
<ref name="pmid10216940">
{{cite journal
 |author=Kimball SR
 |year=1999
 |title=Eukaryotic initiation factor eIF2
 |journal=[[Int. J. Biochem. Cell Biol.]]
 |volume=31 |issue=1 |pages=25–9
 |doi=10.1016/S1357-2725(98)00128-9
 |pmid=10216940
}}</ref><ref name="pmid2687263">
{{cite journal
 |author=Hershey JW
 |year=1989
 |title=Protein phosphorylation controls translation rates
 |url=http://www.jbc.org/content/264/35/20823.full.pdf
 |journal=[[J. Biol. Chem.]]
 |volume=264 |issue=35 |pages=20823–6
 |doi=10.1016/S0021-9258(19)30005-5
 |pmid=2687263
|doi-access=free
 }}</ref><ref name="pmid16153175">
{{cite journal
 |author=Hinnebusch AG
 |year=2005
 |title=Translational regulation of GCN4 and the general amino acid control of yeast
 |journal=[[Annu. Rev. Microbiol.]]
 |volume=59 |pages=407–50
 |doi=10.1146/annurev.micro.59.031805.133833
 |pmid=16153175
}}</ref>

==Structure==
eIF2 is a heterotrimer of a total molar mass of 126 [[Dalton (unit)|kDa]] that is composed of the three sub-units: [[EIF2S1|α]] (sub-unit 1), [[EIF2S2|β]] (sub-unit 2), and [[EIF2S3|γ]] (sub-unit 3). 
The sequences of all three sub-units are highly conserved (pairwise amino acid identities for each sub-unit range from 47 to 72% when comparing the proteins of ''[[Homo sapiens]]'' and ''[[Saccharomyces cerevisiae]]'').
{| class="wikitable" style="text-align:center"
|+Table 1: sub-units of eIF2<ref name="pmid2687263"/><ref name="pmid15507151">
{{cite journal
 |vauthors=Kimball SR, Jefferson LS
 |year=2004
 |title=Amino acids as regulators of gene expression
 |journal=[[Nutr. Metab.]]
 |volume=1 |issue=1 |pages=3
 |doi=10.1186/1743-7075-1-3
 |pmc=524028
 |pmid=15507151
|doi-access=free
 }}</ref>
|-
! width="150" |sub-unit
! width="150" |[[EIF2S1|Alpha]]
! width="275" |[[EIF2S2|Beta]]
! width="200" |[[EIF2S3|Gamma]]
|-
| Molecular Weight / kDa
| 36
| 38
| 52
|-
| Similarity
| eIF2-alpha family<br/>{{InterPro|IPR011488|l=n}}
| eIF2-beta / eIF5 family<br/>{{InterPro|IPR002735|l=n}}
| GTP-binding elongation factor family (and others)<br/>{{plainlink|https://www.ebi.ac.uk/interpro/protein/P41091|P41091}}<!-- dissect it yourself -->
|-
| Interactions
| 
| Binding of eIF5, eIF2B and RNA
| Binding of GTP and RNA
|}

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). What is important to consider is that 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|zinc finger motif]] that was shown to play a role in both ternary complex and 43S preinitiation complex formation. There are also two guanine nucleotide-binding sequences that 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 that has been shown by [[X-ray crystallography]]. A zinc knuckle motif is able to bind one Zn<sup>2+</sup> cation.<ref name="pmid16153175"/><ref name="pmid14688270">
{{cite journal
 |vauthors=Roll-Mecak A, Alone P, Cao C, Dever TE, Burley SK
 |year=2004
 |title=X-ray structure of translation initiation factor eIF2gamma: implications for tRNA and eIF2alpha binding
 |journal=[[J. Biol. Chem.]]
 |volume=279 |issue=11 |pages=10634–42
 |doi=10.1074/jbc.M310418200
 |pmid=14688270
|doi-access=free
 }}</ref><ref name="pmid15341733">
{{cite journal
 |vauthors=Ito T, Marintchev A, Wagner G
 |year=2004
 |title=Solution structure of human initiation factor eIF2alpha reveals homology to the elongation factor eEF1B
 |journal=[[Structure (journal)|Structure]]
 |volume=12 |issue=9 |pages=1693–704
 |doi=10.1016/j.str.2004.07.010
 |pmid=15341733
|doi-access=free
 }}</ref> It is related to some elongation factors like [[EF-Tu]].<ref>{{cite journal |last1=Schmitt |first1=E |last2=Blanquet |first2=S |last3=Mechulam |first3=Y |title=The large subunit of initiation factor aIF2 is a close structural homologue of elongation factors. |journal=The EMBO Journal |date=2 April 2002 |volume=21 |issue=7 |pages=1821–32 |doi=10.1093/emboj/21.7.1821 |pmid=11927566|pmc=125960 }}</ref>

==Regulation==
[[Image:EIF2regulation.jpg |thumb |right |300px |Regulation of translation initiation via phosphorylation of Ser51 in eIF2's α-subunit.<ref name="pmid11042214">
{{cite journal
 |vauthors=Nika J, Rippel S, Hannig EM
 |year=2001
 |title=Biochemical analysis of the eIF2beta gamma complex reveals a structural function for eIF2alpha in catalyzed nucleotide exchange
 |journal=[[J. Biol. Chem.]]
 |volume=276 |issue=2 |pages=1051–6
 |doi=10.1074/jbc.M007398200
 |pmid=11042214
|doi-access=free
 }}</ref>]]

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/threonine-specific protein kinase|serine kinases]] that phosphorylate [[serine]] 51. Those kinases act as a result of stress such as amino acid deprivation ([[EIF2AK4|GCN2]]), ER stress ([[EIF2AK3|PERK]]), the presence of dsRNA ([[Protein kinase R|PKR]]) heme deficiency ([[Heme-regulated inhibitor kinase|HRI]]), or [[interferon]].<ref>
{{Cite journal
 |vauthors=Samuel CE
 |year=1979
 |title=Mechanism of interferon action: Phosphorylation of protein synthesis initiation factor eIF-2 in interferon-treated human cells by a ribosome-associated kinase processing site specificity similar to hemin-regulated rabbit reticulocyte kinase
 |journal=[[Proc Natl Acad Sci USA]]
 |volume=76 |issue=2 |pages=600–4
 |bibcode=1979PNAS...76..600S
 |doi=10.1073/pnas.76.2.600
 |pmid=284384
 |pmc=382996
 |doi-access=free
 }}</ref> Once phosphorylated, eIF2 shows increased affinity for eIF2B, its GEF. However, eIF2B is able to exchange GDP for GTP only if eIF2 is in its unphosphorylated state. Phosphorylated eIF2, however, 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. As a consequence, translation comes to a halt since initiation is no longer possible without any available ternary complex. Furthermore, low concentration of ternary complex allows the expression of GCN4 (starved condition), which, in turn, results in increased activation of amino acid synthesis genes<ref name="pmid10216940"/><ref name="pmid2687263"/><ref name="pmid16153175"/><ref name="pmid11042214"/><ref name="test">
{{cite journal
 |vauthors=Hope A, Struhl K
 |year=1987
 |title=GCN4, a eukaryotic transcriptional activator protein, binds as a dimer to target DNA
 |journal=[[The EMBO Journal]]
 |volume=6 |issue=9 |pages=2781–2784
 |pmc=553703
 |pmid=3678204
|doi=10.1002/j.1460-2075.1987.tb02573.x
 }}</ref>

==Disease==
Since eIF2 is essential for most forms of translation initiation and therefore protein synthesis, defects in eIF2 are often 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 with neurodegenerative diseases such as [[Alzheimer's disease|Alzheimer's]], [[Parkinson's disease|Parkinson's]], and [[Huntington's disease|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 are associated with Vanishing White Matter (VWM) disease, a genetic leukodystrophy which causes the brain's white matter to degenerate and disappear.<ref>{{Cite journal|last1=Knaap|first1=Marjo S. van der|last2=Pronk|first2=Jan C.|last3=Scheper|first3=Gert C.|date=2006-05-01|title=Vanishing white matter disease|url=https://www.thelancet.com/journals/laneur/article/PIIS1474-4422(06)70440-9/abstract|journal=The Lancet Neurology|language=English|volume=5|issue=5|pages=413–423|doi=10.1016/S1474-4422(06)70440-9|issn=1474-4422|pmid=16632312|s2cid=44301370|url-access=subscription}}</ref><ref>{{Cite journal|last1=Leegwater|first1=Peter A. J.|last2=Vermeulen|first2=Gerre|last3=Könst|first3=Andrea A. M.|last4=Naidu|first4=Sakkubai|last5=Mulders|first5=Joyce|last6=Visser|first6=Allerdien|last7=Kersbergen|first7=Paula|last8=Mobach|first8=Dragosh|last9=Fonds|first9=Dafna|last10=van Berkel|first10=Carola G. M.|last11=Lemmers|first11=Richard J. L. F.|date=December 2001|title=Subunits of the translation initiation factor eIF2B are mutant in leukoencephalopathy with vanishing white matter|url=https://www.nature.com/articles/ng764z|journal=Nature Genetics|language=en|volume=29|issue=4|pages=383–388|doi=10.1038/ng764|pmid=11704758|s2cid=20313523|issn=1546-1718|url-access=subscription}}</ref> 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.<ref name="pmid16153175"/><ref name="pmid17496426">
{{cite journal
 |vauthors=Chang RC, Yu MS, Lai CS
 |year=2006
 |title=Significance of molecular signaling for protein translation control in neurodegenerative diseases
 |journal=[[Neurosignals]]
 |volume=15 |issue=5 |pages=249–58
 |doi=10.1159/000102599
 |pmid=17496426
|doi-access=free
 }}</ref>

==See also==
* [[Eukaryotic initiation factors]]
* [[EIF-2 kinase|Kinases of eIF2]]
** [[Heme-regulated inhibitor kinase|HRI (Heme-regulated inhibitor kinase)]] or [[EIF2AK1]]
** [[Protein kinase R|PKR (Protein kinase R)]]
** [[EIF2AK3|PERK (PKR-like ER-localized eIF2α kinase)]]
** [[EIF2AK4|GCN2 (eukaryotic translation initiation factor 2 alpha kinase 4)]]
* [[eIF2A]]
* [[eIF2D]]
*[[Leukoencephalopathy with vanishing white matter]]

==References==
{{Reflist |2}}

==External links==
* {{MeshName |EIF-2}}
* [http://www.nature.com/nrmicro/journal/v5/n1/box/nrmicro1558_BX1.html Cap-dependent translation initiation] from Nature Reviews Microbiology. A good image and overview of the function of initiation factors

{{Initiation factors}}

{{DEFAULTSORT:Eif2}}
[[Category:Protein biosynthesis]]
[[Category:Gene expression]]


Regulation of protein synthesis in eukaryotes. Mode of action of eRF, an eIF-2-recycling factor from rabbit reticulocytes involved in GDP/GTP exchange.
Salimans M, Goumans H, Amesz H, Benne R, Voorma HO.Eur J Biochem. 1984 Nov