The SARS Coronavirus 3a protein causes endoplasmic reticulum stress and induces ligand-independent downregulation of the type 1 interferon receptor

• Published in: PloS one Vol. 4; no. 12; p. e8342
• Main Authors: Minakshi, Rinki, Padhan, Kartika, Rani, Manjusha, Khan, Nabab, Ahmad, Faizan, Jameel, Shahid
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 17.12.2009; Public Library of Science (PLoS)
• Subjects: Activating transcription factor 4; Activating transcription factor 6; Activation; Animals; Apoptosis; Biological response modifiers; Biotechnology; CCAAT/enhancer-binding protein; Cell cycle; Cell Line; Chaperones; Confocal; Coronaviridae; Coronaviruses; Cytomegalovirus; Cytometry; Down-Regulation; eIF-2 Kinase - metabolism; Endoplasmic reticulum; Endoplasmic Reticulum - metabolism; Endoplasmic Reticulum - pathology; Endoribonucleases - metabolism; Eukaryotic Initiation Factor-2 - metabolism; Flow cytometry; Genes; Genetic engineering; Genomes; Glucose; Golgi apparatus; Hepatitis; HIV; Homology; Human immunodeficiency virus; Humans; Immunity; Infectious Diseases; Infectious Diseases/Viral Infections; Initiation factor eIF-2α; Innate immunity; Inositol; Interdisciplinary aspects; Interferon; Kinases; Ligands; Lysosomes - metabolism; Microscopic analysis; Phosphatase; Phosphorylation; Protein folding; Protein Processing, Post-Translational; Protein synthesis; Proteins; Receptor, Interferon alpha-beta - metabolism; RNA; SARS coronavirus; Serine; Severe acute respiratory syndrome; Signaling; Stress; Stress (Physiology); Stress response; Stresses; Transcription activation; Transcription factors; Translocation; Ubiquitination; Unfolded Protein Response; Viral Proteins - metabolism; Virology; Virology/Effects of Virus Infection on Host Gene Expression; Viruses; India
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0008342

The Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) is reported to cause apoptosis of infected cells and several of its proteins including the 3a accessory protein, are pro-apoptotic. Since the 3a protein localizes to the endoplasmic reticulum (ER)-Golgi compartment, its role in causing ER stress was investigated in transiently transfected cells. Cells expressing the 3a proteins showed ER stress based on activation of genes for the ER chaperones GRP78 and GRP94. Since ER stress can cause differential modulation of the unfolded protein response (UPR), which includes the inositol-requiring enzyme 1 (IRE-1), activating transcription factor 6 (ATF6) and PKR-like ER kinase (PERK) pathways, these were individually tested in 3a-expressing cells. Only the PERK pathway was found to be activated in 3a-expressing cells based on (1) increased phosphorylation of eukaryotic initiation factor 2 alpha (eIF2alpha) and inhibitory effects of a dominant-negative form of eIF2alpha on GRP78 promoter activity, (2) increased translation of activating transcription factor 4 (ATF4) mRNA, and (3) ATF4-dependent activation of the C/EBP homologous protein (CHOP) gene promoter. Activation of PERK affects innate immunity by suppression of type 1 interferon (IFN) signaling. The 3a protein was found to induce serine phosphorylation within the IFN alpha-receptor subunit 1 (IFNAR1) degradation motif and to increase IFNAR1 ubiquitination. Confocal microscopic analysis showed increased translocation of IFNAR1 into the lysosomal compartment and flow cytometry showed reduced levels of IFNAR1 in 3a-expressing cells. These results provide further mechanistic details of the pro-apoptotic effects of the SARS-CoV 3a protein, and suggest a potential role for it in attenuating interferon responses and innate immunity.