Role of eIF2α Kinases in Translational Control and Adaptation to Cellular Stress

Abstract

A central mechanism regulating translation initiation in response to environmental stress involves phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α). Phosphorylation of eIF2α causes inhibition of global translation, which conserves energy and facilitates reprogramming of gene expression and signaling pathways that help to restore protein homeostasis. Coincident with repression of protein synthesis, many gene transcripts involved in the stress response are not affected or are even preferentially translated in response to increased eIF2α phosphorylation by mechanisms involving upstream open reading frames (uORFs). This review highlights the mechanisms regulating eIF2α kinases, the role that uORFs play in translational control, and the impact that alteration of eIF2α phosphorylation by gene mutations or small molecule inhibitors can have on health and disease.

Figure 1.

Phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α) regulates global and gene-specific translation. The eIF2α kinases general control nonderepressible 2 (GCN2) and protein kinase R (PKR)-like endoplasmic reticulum (ER) kinase (PERK) are activated by nutritional stress or perturbations in the ER, respectively. Type 1 protein phosphatase complex (PP1c) combines with CReP to dephosphorylate eIF2α during basal conditions and GADD34 in feedback control of the integrated stress response (ISR). Phosphorylation of eIF2α reduces global translation initiation coincident with preferential translation of ATF4, encoding a basic zipper (bZIP) transcriptional activator that dimerizes with other transcript factors to regulate transcription of ISR genes that function in adaptation to stress.

Figure 2.

Phosphorylation of the α subunit of eukaryotic initiation factor 2 (P-eIF2α) enhances translation of multiple integrated stress response (ISR) genes by mechanisms involving upstream open reading frames (uORFs). P-eIF2α reduces global protein synthesis concurrent with preferential translation genes involved in diverse cellular functions. Preferential translation of ATF4, CHOP, GADD34, EPRS, and CDKN1A involves uORFs as described in the text. IBTKα (Baird et al. 2014; Willy et al. 2017), BiP (Starck et al. 2016), BACE1 (O’Connor et al. 2008), PKCη (Raveh-Amit et al. 2009), SLC35A4 (Andreev et al. 2015; Sidrauski et al. 2015), and CAT1 (Yaman et al. 2003) have also been reported to be preferentially translated directly or indirectly by P-eIF2α during stress.

Figure 3.

Upstream open reading frames (uORFs) can have different functions in preferential translation in the integrated stress response (ISR). The uORFs and their function are highlighted for the indicated gene transcripts. The 5′-leader of the messenger RNAs (mRNAs) is indicated as a solid line. The coding sequences (CDSs) are indicated by the bar on the far right of each transcript, with uORFs indicated by the light gray bars. Scanning and elongating ribosomes are indicated by the ovals, with small and large ribosomal subunits. Arrows indicate ribosome bypass, reinitiation or termination, and release.

Figure 4.

The integrated stress response (ISR) features different translational control mechanisms with upstream open reading frames (uORFs). (A) Illustration of the mechanism of ATF4 delayed translation reintiation that functions to enhance ATF4 synthesis on phosphorylation of the α subunit of eukaryotic initiation factor 2 (P-eIF2α) and stress. In nonstressed conditions, there are low levels of P-eIF2α and abundant eIF2•GTP (guanosine triphosphate). Following translation of uORF1 (green bar), ribosomes (ovals indicated by large and small subunits) rapidly reacquire new eIF2•GTP•Met-tRNA_i^Met and reinitiate at the inhibitory uORF2 (red bar), which overlaps out-of-frame with the ATF4 coding sequence (CDS) (blue bar). Therefore, there are low levels of ATF4 and its target genes in the absence of stress. In response to stress, enhanced P-eIF2α and low eIF2•GTP delay reinitiation, allowing ribosomes to proceed through uORF2, and instead translate the ATF4 CDS. (B) Translation of GADD34 involves a fraction of the translating ribosome scanning through an inhibitory uORF2 (red bar) in response to P-eIF2α and stress. The uORF1 (gray bar), which overlaps out-of-frame uORF2, is not well translated and is a modest dampener in the translation of GADD34. (C) Expression of CReP involves a fraction of ribosomes translating uORF2 (red bar) and reinitiating at the CReP CDS independent P-eIF2α and stress. Therefore, synthesis of CReP is largely constitutive regardless of stress conditions. The CReP ORF1 (gray bar) functions to lower translation of the CReP CDS only modestly. (D) Substitution of the Pro-Pro-Gly-stop codons and nine nucleotides 3′- of the GADD34 uORF2 for the corresponding uORF2 regions in the CReP transcript (indicated by red portion of the uORF and messenger RNA [mRNA]) leads to lowered translation of the CReP hybrid that is preferentially translated in response to stress and P-eIF2α induction (Young et al. 2015).