Integrated Stress Response Antibody Sampler Kit #56392

Protein kinase-like endoplasmic reticulum kinase (PERK) is a eukarotic initiation factor 2 alpha (eIF2α) kinase and transmembrane protein resident in the endoplasmic reticulum (ER) membrane that couples ER stress signals to translation inhibition. ER stress increases PERK activity, which then phosphorylates eIF2α to reduce translation (1-3). Phosphorylation of the eIF2α subunit is a well-documented mechanism of downregulating protein synthesis under a variety of stress conditions (16,17). Kinases activated by viral infection (PKR), endoplasmic reticulum stress (PERK/PEK), amino acid deprivation (GCN2), and hemin deficiency (HRI) can phosphorylate eIF2α (4,5). GCN2 is also required for UV light-induced translation inhibition, and in vivo phosphorylation of murine GCN2 at Thr898 is induced by both UV irradiation and by leucine deprivation (6).

Heme-regulated inhibitor (HRI), a heme-regulated eIF2α kinase, is a cytosolic protein kinase that is highly expressed in erythroid cells (7,8). Upon stress conditions such as heme deficiency, oxidative stress, osmotic shock, mitochondrial dysfunction, and heat shock, HRI is a key activator of the integrated stress response (ISR). Following activation, HRI is autophosphorylated, leading to eIF2α phosphorylation, which controls protein synthesis by inhibiting translational initiation (9-13).

Protein kinase R (PKR) is transcriptionally induced by interferon and activated by double-stranded RNA (dsRNA). PKR inhibits translation initiation by phosphorylating eIF2α and also controls the activation of several transcription factors, such as NF-κB, p53, and the Stats. In addition, PKR mediates apoptosis induced by various stimuli, including LPS, TNF-α, viral infection, and serum starvation (14,15).

Growth arrest and DNA damage-inducible protein (GADD34) is essential for cell stress responses. It is induced by stresses such as DNA damage and endoplasmic reticulum (ER) stress. GADD34 binds to type 1 protein phosphatase (PP1), a serine/threonine phosphatase, and promotes PP1 to dephosphorylate eIF2α (18,19). GADD34 expression is correlated with eIF2α dephosphorylation late in the stress response, suggesting a critical role for GADD34 in a negative feedback loop to reduce stress response signaling and shift cells from protein synthesis inhibition to stress-induced gene expression (20).

Activating transcription factor-4 (ATF-4), an ATF/cAMP-responsive element-binding protein family member, functions in the PERK and eIF2α ER stress-responsive pathway (21-23). ER stress represses the translation of the majority of mRNAs but selectively stimulates translation of certain mRNAs, including ATF-4 (22). Induced expression of ATF-4 increases the expression of genes critical for the recovery from ER stress (24).