Browsing by Author "Dey, Souvik"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Crystal structures of GCN2 protein kinase C-terminal domains suggest regulatory differences in yeast and mammals
    (ASBMB, 2014-04-09) He, Hongzhen; Singh, Isha; Wek, Sheree A.; Dey, Souvik; Baird, Thomas D.; Wek, Ronald C.; Georgiadis, Millie M.; Department of Biochemistry & Molecular Biology, IU School of Medicine
    In response to amino acid starvation, GCN2 phosphorylation of eIF2 leads to repression of general translation and initiation of gene reprogramming that facilitates adaptation to nutrient stress. GCN2 is a multidomain protein with key regulatory domains that directly monitor uncharged tRNAs which accumulate during nutrient limitation, leading to activation of this eIF2 kinase and translational control. A critical feature of regulation of this stress response kinase is its C-terminal domain (CTD). Here, we present high resolution crystal structures of murine and yeast CTDs, which guide a functional analysis of the mammalian GCN2. Despite low sequence identity, both yeast and mammalian CTDs share a core subunit structure and an unusual interdigitated dimeric form, albeit with significant differences. Disruption of the dimeric form of murine CTD led to loss of translational control by GCN2, suggesting that dimerization is critical for function as is true for yeast GCN2. However, although both CTDs bind single- and double-stranded RNA, murine GCN2 does not appear to stably associate with the ribosome, whereas yeast GCN2 does. This finding suggests that there are key regulatory differences between yeast and mammalian CTDs, which is consistent with structural differences
  • Thumbnail Image
    Item
    Transcriptional regulation of ATF4 is critical for controlling the Integrated Stress Response during eIF2 phosphorylation
    (2012-05) Dey, Souvik; Wek, Ronald C.; Edenberg, Howard J.; Gallagher, Patricia; Turchi, John J.
    In response to different environmental stresses, phosphorylation of eIF2 (eIF2P) represses global translation coincident with preferential translation of ATF4. ATF4 is a transcriptional activator of the integrated stress response, a program of gene expression involved in metabolism, nutrient uptake, anti-oxidation, and the activation of additional transcription factors, such as CHOP/GADD153, that can induce apoptosis. Although eIF2P elicits translational control in response to many different stress arrangements, there are selected stresses, such as exposure to UV irradiation, that do not increase ATF4 expression despite robust eIF2P. In this study we addressed the underlying mechanism for variable expression of ATF4 in response to eIF2P during different stress conditions and the biological significance of omission of enhanced ATF4 function. We show that in addition to translational control, ATF4 expression is subject to transcriptional regulation. Stress conditions such as endoplasmic reticulum stress induce both transcription and translation of ATF4, which together enhance expression of ATF4 and its target genes in response to eIF2P. By contrast, UV irradiation represses ATF4 transcription, which diminishes ATF4 mRNA available for translation during eIF2∼P. eIF2P enhances cell survival in response to UV irradiation. However, forced expression of ATF4 and its target gene CHOP leads to increased sensitivity to UV irradiation. In this study, we also show that C/EBPβ is a transcriptional repressor of ATF4 during UV stress. C/EBPβ binds to critical elements in the ATF4 promoter resulting in its transcriptional repression. The LIP isoform of C/EBPβ, but not the LAP version is regulated following UV exposure and directly represses ATF4 transcription. Loss of the LIP isoform results in increased ATF4 mRNA levels in response to UV irradiation, and subsequent recovery of ATF4 translation, leading to enhanced expression of its target genes. Together these results illustrate how eIF2P and translational control, combined with transcription factors regulated by alternative signaling pathways, can direct programs of gene expression that are specifically tailored to each environmental stress.