Gcn2
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GCN2 (general control nonderepressible 2) is a serine/threonine-protein kinase that senses amino acid deficiency through binding to uncharged transfer RNA (
tRNA Transfer ribonucleic acid (tRNA), formerly referred to as soluble ribonucleic acid (sRNA), is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes). In a cell, it provides the physical link between the gene ...
). It plays a key role in modulating amino acid metabolism as a response to nutrient deprivation.


Introduction

GCN2 is the only known eukaryotic initiation factor 2α kinase (eIF2α) in ''
Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungal microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have be ...
''. It inactivates eIF2α by phosphorylation at Serine 51 under conditions of amino acid deprivation, resulting in repression of general protein synthesis whilst allowing selected mRNA, such as the transcription factor GCN4, to be translated due to regions upstream of the coding sequence. Elevated levels of GCN4 stimulate the expression of amino acid biosynthetic genes, which code for enzymes required to synthesize all 20 major amino acids.


Structure

Protein kinase GCN2 is a multidomain protein and its C-terminus contains a region homologous to histidyl-tRNA synthetase (HisRS) next to the kinase catalytic moiety. This HisRS-like region forms a dimer and dimerization is required for GCN2 function. The crucial contribution to GCN2 function is the promotion of tRNA binding and the stimulation of the kinase domain via physical interaction. Binding of uncharged tRNA to this synthetase-like domain induces a conformational change in which the GCN2 domains rotate 180° normal to the dimerization surface and thereby transpose from their antiparallel to a parallel orientation. Subsequently, GCN2 is activated. GCN2 activation results from a conformation that facilitates ATP binding, leading to autophosphorylation of an
activation loop In molecular biology, an intrinsically disordered protein (IDP) is a protein that lacks a fixed or ordered three-dimensional structure, typically in the absence of its macromolecular interaction partners, such as other proteins or RNA. IDPs ran ...
which leads to maximal GCN2 kinase activity.


Function


Regulation of translation

GCN2 inhibits general translation by phosphorylation of eIF-2α at serine 51 within 15 min of amino acid deprivation, which then subsequently increases the affinity for the guanine exchange factor
eIF2B eIF2B is a protein complex found in eukaryotes. It is the guanine nucleotide exchange factor for the eukaryotic initiation factor 2 and therefore converts the inactive eIF2- GDP to the active eIF2- GTP. This activation is hindered by phosphorylat ...
to sequester eIF-2α leading to reduced formation of the ternary complex (TC) consisting of
eIF2 Eukaryotic Initiation Factor 2 (eIF2) is a eukaryotic initiation factor. It is required for most forms of eukaryotic translation initiation. eIF2 mediates the binding of tRNAiMet to the ribosome in a GTP-dependent manner. eIF2 is a heterotrimer ...
, GTP and the initiator Met-tRNA required for translation initiation.
eIF2 Eukaryotic Initiation Factor 2 (eIF2) is a eukaryotic initiation factor. It is required for most forms of eukaryotic translation initiation. eIF2 mediates the binding of tRNAiMet to the ribosome in a GTP-dependent manner. eIF2 is a heterotrimer ...
containing a phosphorylated alpha subunit shows an increased affinity for its only GEF,
eIF2B eIF2B is a protein complex found in eukaryotes. It is the guanine nucleotide exchange factor for the eukaryotic initiation factor 2 and therefore converts the inactive eIF2- GDP to the active eIF2- GTP. This activation is hindered by phosphorylat ...
, but
eIF2B eIF2B is a protein complex found in eukaryotes. It is the guanine nucleotide exchange factor for the eukaryotic initiation factor 2 and therefore converts the inactive eIF2- GDP to the active eIF2- GTP. This activation is hindered by phosphorylat ...
is only able to exchange
GDP Gross domestic product (GDP) is a monetary measure of the total market value of all the final goods and services produced and rendered in a specific time period by a country or countries. GDP is often used to measure the economic performance o ...
with GTP from unphosphorylated
eIF2 Eukaryotic Initiation Factor 2 (eIF2) is a eukaryotic initiation factor. It is required for most forms of eukaryotic translation initiation. eIF2 mediates the binding of tRNAiMet to the ribosome in a GTP-dependent manner. eIF2 is a heterotrimer ...
. So the recycling of
eIF2 Eukaryotic Initiation Factor 2 (eIF2) is a eukaryotic initiation factor. It is required for most forms of eukaryotic translation initiation. eIF2 mediates the binding of tRNAiMet to the ribosome in a GTP-dependent manner. eIF2 is a heterotrimer ...
, needed for TC formation, is inhibited by phosphorylation of eIF-2α, which in the end leads to a reduction of global translation rates. An opposing effect of the reduced availability of TC is the induction of
GCN4 Gcn4 is a transcription factor and a “master regulator” for gene expression which regulates close to one tenth of the yeast genome. In a study by Razaghi et al, amino acid starvation activated the transcription factor Gcn4p, resulting in tran ...
expression by translational regulation. Four short ORF's exist in the leader of the GCN4 mRNA. 40S Ribosomal Subunits scanning the
mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...
from 5' have TC bound and translate the first upstream open reading frame (uORF). Under non-starving condition there is enough ternary complex that the subunits rebind it before they reach uORF 4. Translation is again initiated, uORF2,3 or 4 translated and the 40S Subunits subsequently dissociate from GCN4 mRNA. Under starving conditions there is less TC present. Some of the 40S Subunits are not able to rebind TC before they reach uORF 4 but eventually rebind TC before reaching GCN4 coding sequence. Therefore, the reduction in TC formation resulting from GCN2 activation by amino acid starvation leads to the induction of GCN4 translation. GCN4 is the primary regulator in response to amino acid starvation, termed general amino acid control (GAAC). It acts as a transcription factor and activates several genes required for amino acid synthesis. Recently GCN2 has also been implicated in directing eating behavior in mammals by phosphorylating eIF-2α in the anterior
Piriform cortex The piriform cortex, or pyriform cortex, is a region in the brain, part of the rhinencephalon situated in the cerebrum. The function of the piriform cortex relates to the sense of smell. Structure The piriform cortex is part of the rhinencephal ...
(APC) of the brain. The molecular mechanisms governing this function are not yet known, but a basic zipper transcription factor called
ATF4 Activating transcription factor 4 (tax-responsive enhancer element B67), also known as ATF4, is a protein that in humans is encoded by the ''ATF4'' gene. Function This gene encodes a transcription factor that was originally identified as a w ...
is a possible candidate. ATF4 is related to GCN4.


Cell cycle control

GCN2 also regulates the
cell cycle The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...
by delaying entry into S phase upon ultraviolet (UV) radiation and exposure to methyl methanesulfonate ( MMS). Thereby the cell prevents passing the G1 checkpoint and starting DNA replication when the DNA is damaged. It has been hypothesized, that UV induces
nitric oxide synthase Nitric oxide synthases (NOSs) are a family of enzymes catalyzing the production of nitric oxide (NO) from L-arginine. NO is an important cellular signaling molecule. It helps modulate vascular tone, insulin secretion, airway tone, and peristal ...
activation and NO. production, which leads to the activation of GCN2 and that the cell cycle regulation by GCN2 is independent of eIF2α phosphorylation. Although the causal relationship between GCN2 and cell cycle delay is still under debate, it was suggested that the formation of the
pre-replication complex A pre-replication complex (pre-RC) is a protein complex that forms at the origin of replication during the initiation step of DNA replication. Formation of the pre-RC is required for DNA replication to occur. Complete and faithful replication of ...
is deferred by GCN2 upon UV-irradiation.


Lipid metabolism

The absence of essential amino acids causes a downregulation of key components of the lipid synthesis such as the
fatty acid synthase Fatty acid synthase (FAS) is an enzyme that in humans is encoded by the ''FASN'' gene. Fatty acid synthase is a multi-enzyme protein that catalyzes fatty acid synthesis. It is not a single enzyme but a whole enzymatic system composed of two ide ...
. Following leucine-deprivation in mammals, GCN2 decreases the expression of lipogenic genes via SREBP-1c. SREBP-1c actions upon genes regulating fatty-acid and triglyceride synthesis and is reduced by leucine deprivation in the liver in a GCN2-dependent manner.


Regulation

Gcn2 is held in its inactive state via several auto-inhibitory molecular interactions until exposed to an activating signal. Binding of uncharged tRNA to the synthetase-like domain results in allosteric re-arrangements. This leads to Gcn2 auto-phosphorylation at specific sites in the activation loop of the kinase domain. This phosphorylation then allows Gcn2 to efficiently phosphorylate eIF2α. In yeast cells, GCN2 is kept inactive via phosphorylation at serine 577, which is thought to depend on the activity of TORC1. Inactivation of TORC1 by Rapamycin affects GCN2 and at least partly by dephosphorylation of serine 577. This leads to activation of GCN2 even in amino acid replete cells, probably by increasing the affinity of GCN2 for uncharged tRNA, so that even basal levels permit tRNA binding. However, this phosphorylation site in Gcn2 is not conserved in fission yeast or in mammalian cells. Another stimulatory input to GCN2 is exerted by a complex of GCN1/GCN20. GCN1/GCN20 shows structural similarity to eEF3, a factor important in the binding of tRNA to
ribosome Ribosomes () are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (messenger RNA translation). Ribosomes link amino acids together in the order s ...
s. The GCN1/GCN20 complex physically interacts with GCN2 by binding to its N-terminus. It is thought that GCN1/GCN20 facilitates the transfer of tRNA from the ribosomal A site to the HisRS-like domain of GCN2. An additional mechanism of regulation of this protein is through the conserved protein IMPACT, that acts both in yeast, nematodes and mammals as an inhibitor of GCN2.


Homologues

There are also GCN2 homologues in ''
Neurospora crassa ''Neurospora crassa'' is a type of red bread mold of the phylum Ascomycota. The genus name, meaning 'nerve spore' in Greek, refers to the characteristic striations on the spores. The first published account of this fungus was from an infestatio ...
'', '' C. elegans'', ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...
'' and mice. Thus, GCN2 may be the most widespread and founding member of the eIF-2α kinase subfamily.


See also

* Eukaryotic initiation factors * The three subunits of eIF2: ** α –
EIF2S1 Eukaryotic translation initiation factor 2 subunit 1 (eIF2α) is a protein that in humans is encoded by the ''EIF2S1'' gene. Function The protein encoded by this gene is the alpha (α) subunit of the translation initiation factor eIF2 prote ...
** β –
EIF2S2 Eukaryotic translation initiation factor 2 subunit 2 (eIF2β) is a protein that in humans is encoded by the ''EIF2S2'' gene. Function Eukaryotic translation initiation factor 2 (eIF2) functions in the early steps of protein synthesis by forming ...
** γ –
EIF2S3 Eukaryotic translation initiation factor 2 subunit 3 (eIF2γ) is a protein that in humans is encoded by the ''EIF2S3'' gene. Function Eukaryotic translation initiation factor 2 (eIF2) functions in the early steps of protein synthesis by formin ...
* Kinases of eIF2 ** HRI (Heme-regulated inhibitor kinase) or
EIF2AK1 Eukaryotic translation initiation factor 2-alpha kinase 1 is an enzyme that in humans is encoded by the ''EIF2AK1'' gene. Function EIF2AK1 inhibits protein synthesis at the translation initiation level, in response to various stress conditions, ...
** PKR (Protein kinase R) ** PERK (PKR-like ER-localized eIF2α kinase) *
EIF2AK4 Eukaryotic translation initiation factor 2-alpha kinase 4 is an enzyme that in humans is encoded by the ''EIF2AK4'' gene. EIF2AK4 belongs to a family of kinases that phosphorylate the alpha subunit of eukaryotic translation initiation factor-2 (E ...


References

{{Reflist, 3 Proteins Saccharomyces cerevisiae genes