The eIF2α kinases: their structures and functions

Abstract

Cell signaling in response to an array of diverse stress stimuli converges on the phosphorylation of the α-subunit of eukaryotic initiation factor 2 (eIF2). Phosphorylation of eIF2α on serine 51 results in a severe decline in de novo protein synthesis and is an important strategy in the cell's armory against stressful insults including viral infection, the accumulation of misfolded proteins, and starvation. The phosphorylation of eIF2α is carried out by a family of four kinases, PERK (PKR-like ER kinase), PKR (protein kinase double-stranded RNA-dependent), GCN2 (general control non-derepressible-2), and HRI (heme-regulated inhibitor). Each primarily responds to a distinct type of stress or stresses. Thus, while significant sequence similarity exists between the eIF2α kinases in their kinase domains, underlying their common role in phosphorylating eIF2α, additional unique features determine the regulation of these four proteins, that is, what signals activate them. This review will describe the structure of each eIF2α kinase and discuss how this is linked to their activation and function. In parallel to the general translational attenuation elicited by eIF2α kinase activation the translation of stress-induced mRNAs, most notably activating transcription factor 4 (ATF4) is enhanced and these set in motion cascades of gene expression constituting the integrated stress response (ISR), which seek to remediate stress and restore homeostasis. Depending on the cellular context and concurrent signaling pathways active, however, translational attenuation can also facilitate apoptosis. Accordingly, the role of the kinases in determining cell fate will also be discussed.

Fig. 1

Domain organization of the four mammalian eIF2α kinases. Polypeptides are represented as bars running from N- to C-terminal domains from left to right. Length in amino acids is of the human proteins. SP signal peptide, TM transmembrane domain, KD kinase domain, DS RBD double-stranded RNA binding domain, PKD pseudokinase domain, His/Rs histidyl-tRNA synthetase-related domain, RB ribosome binding. Domains involved in sensing stress signals/activation are in green. Kinase domains are in yellow. Other domains are colored blue. Domains are drawn to scale

Fig. 2

The activation of the eIF2α kinases and consequences of eIF2α phosphorylation. A large number of signals result in activation of the four eIF2α kinases, which then proceed to phosphorylate the α subunit of eIF2 blocking the recycling of GDP to GTP on the γ subunit of eIF2 by eIF2B. This has the effect of reducing levels of active eIF2-GTP, which leads to a block in general translation while concomitantly promoting enhanced translation of elements of the integrated stress response (ISR) such as ATF4. ATF4 upregulates numerous genes involved in amino acid homeostasis, redox metabolism, and apoptosis. For more details, see text. Not all activating signals for the eIF2α kinases are shown. Arrows denote activation or induction, while blunt lines indicate inhibition

Fig. 3

A simplified schematic depicting eIF2 status in unstressed cells and in response to eIF2 kinase activation. In the absence of eIF2α kinase activation levels of eIF2-GTP are replenished by the nucleotide exchange factor, eIF2B allowing frequent assembly of the 43S preinitiation complex and efficient recognition of translational start codons (not shown). In response to diverse stress stimuli eIF2α kinases phosphorylate eIF2; this prevents eIF2B from exchanging eIF2′s GDP for GTP and results in a fall in the levels of eIF2-GTP-Met-tRNA. This ensures that formation of the preinitiation complex is hampered and that start codons are recognized with a lower frequency. See text for more details. For the sake of clarity and simplicity, not all the subunits of the 43S preinitiation complex are depicted. The subunits are not drawn to scale