Towards a model of GCN2 activation

Abstract

Cells must be able to sense and adapt to their surroundings to thrive in a dynamic environment. Key to adapting to a low nutrient environment is the Integrated Stress Response (ISR), a short-lived pathway that allows cells to either regain cellular homeostasis or facilitate apoptosis during periods of stress. Central to the ISR is the protein kinase General Control Non-depressible 2 (GCN2), which is responsible for sensing starvation. Upon amino acid deficiency, GCN2 is activated and initiates the ISR by phosphorylating the translation initiation factor eIF2α, stalling protein translation, and activating the transcription factor ATF4, which in turn up-regulates autophagy and biosynthesis pathways. A key outstanding question is how GCN2 is activated from an autoinhibited state. Until recently, a model of activation focussed on the increase of deacylated tRNA associated with amino acid starvation, with deacylated tRNA binding directly to GCN2 and releasing autoinhibition. However, in vivo experiments have pointed towards an alternative, deacylated-tRNA-independent mechanism of activation. Here, we review the various factors that may facilitate GCN2 activation, including recent research showing that the P-stalk complex, a ribosome-associated heteropentameric protein complex, is a potent activator of GCN2.

Metabolic shift underlies recovery in reversible infantile respiratory chain deficiency

“…In support of our findings, increased CK, low carnitine, slightly increased acyl-carnitines, increased TCA cycle metabolites and accumulation of lipids, glycogen and abnormal mitochondria have been reported previously in RIRCD patients (Boczonadi et al, 2015). FGF21 has been recently described to drive the integrated mitochondrial stress response and activate a cascade of events in mitochondrial myopathy patients and mice, thus leading to a distinct metabolic remodelling (AlJohani, Khan et al, 2019, Forsstrom et al, 2019, Lehtonen, Forsstrom et al, 2016, von Holstein-Rathlou, BonDurant et al, 2016). Here we show, that FGF21 is associated with the activation of an integrated stress response (increased GCN2, phospho EIF2α, ATF4 and ATF5) (Pakos-Zebrucka, Koryga et al, 2016) in RIRCD, concomitant with an increase in fatty acid oxidation and TCA cycle associated enzymes, thus, leading to an alternative energy source (bypassing complex IV and V) which, most likely to compensate for the lack of ATP.…”

“…We observed a significant increase in GCN2, a serine/threonine kinase activated by amino-acid deprivation and S 51 eIF2α, a downstream target of GCN2 (Figure 2E-F), in affected muscle. When phosphorylated, eIF2α stalls capdependent mRNA translation and initiates transcriptional stress response via ATF4, which was also increased in RIRCD muscle (Figure 2E-F) concomitant with stimulating the expression of amino acid biosynthetic and transporter genes (Masson, 2019).…”

UV damage induces G3BP1-dependent stress granule formation that is not driven by translation arrest via mTOR inhibition

“…GCN2 possesses a histidyl-tRNA synthetase-like (HisRS) domain close to the c-terminus that is involved in autoinhibitory interactions in the inactive GCN2 dimer. Binding of “uncharged” deacylated tRNAs by HisRS domain causes structural rearrangements in the GCN2 dimer interface allowing for autophosphorylation and activation of the kinase (11, 49). Another, tRNA-independent mechanism of GCN2 activation by the P-stalk of ribosomes stalled during elongation have been described, which can complement tRNA-dependent activation when amino acids are depleted (33, 50).…”

The integrated stress response in pulmonary disease

“…Since eIF2α phosphorylation integrates signals from diverse sources, its downstream pathway has been named the “integrated stress response” (ISR) ( figure 1 ). Four stress-sensing kinases trigger the ISR [ 4 ]: protein kinase R (PKR) responds to cytosolic double-stranded (ds)RNA [ 5 ]; PKR-like endoplasmic reticulum kinase (PERK) detects endoplasmic reticulum (ER) stress [ 6 , 7 ]; heme-regulated inhibitor (HRI) responds to iron deficiency and oxidative stress [ 8 ]; while general control non-depressible (GCN)2) is activated during amino acid starvation [ 9 , 10 ].…”