GCN2 (Generally Uncontrollable 2) is a serine/threonine protein kinase that detects amino acid deficiency by binding to uncharged transfer RNA (tRNA).  GCN2 is the only eukaryotic initiation factor 2α kinase (eIF2α) known in Saccharomyces cerevisiae and plays a key role in regulating amino acid metabolism. It inactivates eIF2α by phosphorylation at serine 51 under amino acid deprivation conditions, thereby inhibiting the synthesis of general proteins, and at the same time, the selected mRNA (such as GCN4) is translated due to the upstream region of the coding sequence.  Increased GCN4 levels stimulate the expression of amino acid biosynthetic genes, which encode the enzymes needed to synthesize all 20 major amino acids.

Figure 1. Serine/threonine-protein kinase GCN2.

Regulations

In amino acid-rich cells, GCN2 remains inactivated by phosphorylation of serine at position 577, which is thought to depend on TORC1 activity. Inactivation of TORC1 by rapamycin affects GCN2 and at least partially affects the dephosphorylation of serine 577. The second stimulating input of GCN2 is applied by the GCN1/GCN20 complex. GCN1/GCN20 shows structural similarity to eEF3, which is an important factor for tRNA binding to ribosomes. The GCN1/GCN20 complex physically interacts with GCN2 by binding to its N-terminus. It is believed that GCN1/GCN20 promotes the transfer of tRNA from the ribosomal A site to the HisRS-like domain of GCN2. A third way to regulate this protein is through the conserved protein IMPACT, which acts as a GCN2 inhibitor in yeast, nematodes, and mammals.

Functions

GCN2 inhibits general translation by phosphorylating eIF-2α at serine 51 within 15 minutes after amino acid deprivation, and subsequently increases the affinity of guanine exchange factor eIF2B for chelated eIF-2α, thereby reducing the need for translation initiation The sequences of eIF2, GTP and the starting Met-tRNA. EIF2, which contains a phosphorylated alpha subunit, has enhanced affinity with its only GEF eIF2B, but eIF2B can only exchange GPH for GDP with unphosphorylated eIF2. Therefore, the cycle of eIF2 required for TC formation is inhibited by eIF-2α phosphorylation, which ultimately leads to a decrease in the overall translation rate. The opposite effect of reduced TC availability is the induction of GCN4 expression through translational regulation. There are four short ORFs in the GCN4 mRNA leader. The 40S ribosomal subunit from the 5 'mRNA was scanned to bind to TC and translated the first upstream open reading frame (uORF). Under non-starvation conditions, there are enough ternary complexes to allow the subunits to recombine before reaching uORF 4. Translation was initiated again, and uORF2, 3, or 4 was translated, and the 40S subunit was then separated from GCN4 mRNA. Under hungry conditions, TC content is even lower. Some 40S subunits were unable to recombine TC before reaching uORF 4, but eventually recombined TC before reaching the GCN4 coding sequence. Therefore, GCN2 activation caused by amino acid starvation leads to a reduction in TC formation leading to the induction of GCN4 translation. GCN4 is a major regulator of amino acid starvation, called the General Amino Acid Control (GAAC). It functions as a transcription factor and activates several genes required for amino acid synthesis. Recently, GCN2 also guided mammalian eating behaviors by phosphorylating eIF-2α in the anterior piriform cortex (APC) of the brain. The molecular mechanism controlling this function is unclear, but a basic zipper transcription factor called ATF4 is a possible candidate.

Reference

1. Zaborske JM; et al. Genome-wide analysis of tRNA charging and activation of the eIF2 kinase Gcn2p. J Biol Chem. 2009, 284 (37): 25254-67.