Quantitative analysis of severe acute respiratory syndrome (SARS)-associated coronavirus-infected cells using proteomic approaches: implications for cellular responses to virus infection

Quantitative proteomics using stable isotope labeling with amino acids in cell culture reveals changes in the cytoplasmic, nuclear, and nucleolar proteomes in Vero cells infected with the coronavirus infectious bronchitis virus

Virus-host interactions involve complex interplay between viral and host factors, rendering them an ideal target for proteomic analysis. Here we detail a high throughput quantitative proteomics analysis of Vero cells infected with the coronavirus infectious bronchitis virus (IBV), a positive strand RNA virus that replicates in the cytoplasm. Stable isotope labeling with amino acids in cell culture (SILAC) was used in conjunction with LC-MS/MS to identify and quantify 1830 cellular and two viral proteins from IBV-infected cells. Fractionation of cells into cytoplasmic, nuclear, and nucleolar extracts was used to reduce sample complexity and provide information on the trafficking of proteins between the different compartments. Each fraction showed a proportion of proteins exhibiting >or=2-fold changes in abundance. Ingenuity Pathway Analysis revealed that proteins that changed in response to infection could be grouped into different functional categories. These included proteins regulated by NF-kappaB- and AP-1-dependent pathways and proteins involved in the cytoskeleton and molecular motors. A luciferase-based reporter gene assay was used to validate the up-regulation of AP-1- and NF-kappaB-dependent transcription in IBV-infected cells and confirmed using immunofluorescence. Immunofluorescence was used to validate changes in the subcellular localization of vimentin and myosin VI in IBV-infected cells. The proteomics analysis also confirmed the presence of the viral nucleocapsid protein as localizing in the cytoplasm, nucleus, and nucleolus and the viral membrane protein in the cytoplasmic fraction. This research is the first application of SILAC to study total host cell proteome changes in response to positive sense RNA virus infection and illustrates the versatility of this technique as applied to infectious disease research.

Quantitative proteomics analysis reveals BAG3 as a potential target to suppress severe acute respiratory syndrome coronavirus replication

The discovery of a novel coronavirus (CoV) as the causative agent of severe acute respiratory syndrome (SARS) has highlighted the need for a better understanding of CoV replication. The replication of SARS-CoV is highly dependent on host cell factors. However, relatively little is known about the cellular proteome changes that occur during SARS-CoV replication. Recently, we developed a cell line expressing a SARS-CoV subgenomic replicon and used it to screen inhibitors of SARS-CoV replication. To identify host proteins important for SARS-CoV RNA replication, the protein profiles of the SARS-CoV replicon cells and parental BHK21 cells were compared using a quantitative proteomic strategy termed "stable-isotope labeling by amino acids in cell culture-mass spectrometry" (SILAC-MS). Our results revealed that, among the 1,081 host proteins quantified in both forward and reverse SILAC measurements, 74 had significantly altered levels of expression. Of these, significantly upregulated BCL2-associated athanogene 3 (BAG3) was selected for further functional studies. BAG3 is involved in a wide variety of cellular processes, including cell survival, cellular stress response, proliferation, migration, and apoptosis. Our results show that inhibition of BAG3 expression by RNA interference led to significant suppression of SARS-CoV replication, suggesting the possibility that upregulation of BAG3 may be part of the machinery that SARS-CoV relies on for replication. By correlating the proteomic data with these functional studies, the findings of this study provide important information for understanding SARS-CoV replication.

Differential proteome analysis of host cells infected with porcine circovirus type 2

Porcine circovirus type 2 (PCV2) is the primary causative agent of postweaning multisystemic wasting syndrome, which is an emerging swine immunosuppressive disease. To uncover cellular protein responses in PCV2-infected PK-15 cells, the comprehensive proteome profiles were analyzed utilizing two-dimensional gel electrophoresis (2-DE) coupled with MALDI-TOF/TOF identification. Multiple comparisons of 2-DE revealed that the majority of changes in protein expression occurred at 48-96 h after PCV2 infection. A total of 34 host-encoded proteins, including 15 up-regulated and 19 down-regulated proteins, were identified by MALDI-TOF/TOF analysis. According to cellular function, the differential expression proteins could be sorted into several groups: cytoskeleton proteins, stress response, macromolecular biosynthesis, energy metabolism, ubiquitin-proteasome pathway, signal transduction, gene regulation. Western blot analysis demonstrated the changes of alpha tubulin, beta actin, and cytokeratin 8 during infection. Colocalization and coimmunoprecipitation analyses confirmed that the cellular alpha tubulin interacts with the Cap protein of PCV2 in the infected PK-15 cells. These identified cellular constituents have important implications for understanding the host interactions with PCV2 and brings us a step closer to defining the cellular requirements for the underlying mechanism of PCV2 replication and pathogenesis.

Biomarkers of lung-related diseases: current knowledge by proteomic approaches

The lung epithelial surface is one of the vital barriers or sensors in the body responding to the external atmosphere and thereby always subjecting to direct toxicological exposure, stress, stimulus, or infection. Due to its relatively higher sensitivity in response to toxicants, the use of lung epithelial cell culture and lung tissue from animal models or patients has facilitated our learning to lung physiopathology and toxicopharmacology. The recent advancement of proteomics has made it possible to investigate the cellular response at a global level. In this review, the potential applications of proteomic approach in studying lung-related diseases and biomarker discovery will be discussed.

Proteomics analysis of the nucleolus in adenovirus-infected cells

Adenoviruses replicate primarily in the host cell nucleus, and it is well established that adenovirus infection affects the structure and function of host cell nucleoli in addition to coding for a number of nucleolar targeted viral proteins. Here we used unbiased proteomics methods, including high throughput mass spectrometry coupled with stable isotope labeling by amino acids in cell culture (SILAC) and traditional two-dimensional gel electrophoresis, to identify quantitative changes in the protein composition of the nucleolus during adenovirus infection. Two-dimensional gel analysis revealed changes in six proteins. By contrast, SILAC-based approaches identified 351 proteins with 24 proteins showing at least a 2-fold change after infection. Of those, four were previously reported to have aberrant localization and/or functional relevance during adenovirus infection. In total, 15 proteins identified as changing in amount by proteomics methods were examined in infected cells using confocal microscopy. Eleven of these proteins showed altered patterns of localization in adenovirus-infected cells. Comparing our data with the effects of actinomycin D on the nucleolar proteome revealed that adenovirus infection apparently specifically targets a relatively small subset of nucleolar antigens at the time point examined.

Application of proteomics methods for pathogen discovery

Proteomics have been used widely to study proteins in complex materials such as cells, body fluids, tissues, and organisms. Application of advance proteomic techniques for the characterization of disease-specific proteins may provide information for the detection of potential infectious agents. In this report, two proteomics techniques, a two-dimensional differential gel electrophoresis (2D-DIGE) and a one-dimensional gel electrophoresis and one-dimensional liquid chromatography coupled with mass spectrometry (GeLC-MS/MS), were applied for investigating viral proteins from cultured cells inoculated with a clinical sample. The 2D-DIGE method identified five viral proteins of vaccinia virus that are only present in infected cells, these results are in agreement with findings determined by genome based methods. The GeLC-MS/MS method identified eight vaccinia virus proteins out of 428 proteins detected in the sample. These results demonstrate that proteomic techniques can be used effectively for the detection of infectious agents. Given that the methods are capable of applying to proteins without a prior knowledge of the pathogen present, proteomics has a potential of being developed as a molecular tool for pathogen discovery, and disease diagnosis of emerging infectious diseases and for bioterrorism defense.

Analyses of the spleen proteome of chickens infected with Marek's disease virus

Marek's disease virus (MDV), which causes a lymphoproliferative disease in chickens, is known to induce host responses leading to protection against disease in a manner dependent on genetic background of chickens and virulence of the virus. In the present study, changes in the spleen proteome at 7, 14 and 21 days post-infection in response to MDV infection were studied using two-dimensional polyacrylamide gel electrophoresis. Differentially expressed proteins were identified using one-dimensional liquid chromatography electrospray ionization tandem mass spectrometry (1D LC ESI MS/MS). Comparative analysis of multiple gels revealed that the majority of changes had occurred at early stages of the disease. In total, 61 protein spots representing 48 host proteins were detected as either quantitatively (false discovery rate (FDR)<or=0.05 and fold change>or=2) or qualitatively differentially expressed at least once during different sampling points. Overall, the proteins identified in the present study are involved in a variety of cellular processes such as the antigen processing and presentation, ubiquitin-proteasome protein degradation (UPP), formation of the cytoskeleton, cellular metabolism, signal transduction and regulation of translation. Notably, early stages of the disease were characterized by changes in the UPP, and antigen presentation. Furthermore, changes indicative of active cell proliferation as well as apoptosis together with significant changes in cytoskeletal components that were observed throughout the experimental period suggested the complexity of the pathogenesis. The present findings provide a basis for further studies aimed at elucidation of the role of these proteins in MDV interactions with its host.

Proteomic alteration of PK-15 cells after infection by classical swine fever virus

Viral infections usually result in alterations in the host cell proteome which determine the fate of the infected cells and the progress of pathogenesis. To uncover cellular protein responses in classical swine fever virus-infected PK-15 cells, a proteomic analysis was conducted using 2D PAGE followed by MALDI-TOF-MS/MS identification. Altered expression of 35 protein spots in infected cells at 48 h p.i. were identified in 2D gels, with 21 of these being characterized by MALDI-TOF-MS/MS, including 16 upregulated proteins and 5 down-regulated proteins. Western-blot analysis confirmed the up-regulation of annexin 2 and down-regulation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The altered proteins could be sorted into 7 groups according to cellular function: cytoskeleton, energy metabolism, replication/transcription and translation processes, protein processing, antioxidative stress proteins, heat shock proteins and signal transduction. The altered expression of these proteins provides a response profile of PK-15 host cells to CSFV infection. Further study of these altered proteins may facilitate understanding the mechanisms of CSFV infection and pathogenesis.

Identification of replication-competent HSV-1 Cgal+ strain signaling targets in human hepatoma cells by functional organelle proteomics

In the present work, we have attempted a comprehensive analysis of cytosolic and microsomal proteomes to elucidate the signaling pathways impaired in human hepatoma (Huh7) cells upon herpes simplex virus type 1 (HSV-1; Cgal(+)) infection. Using a combination of differential in-gel electrophoresis and nano liquid chromatography/tandem mass spectrometry, 18 spots corresponding to 16 unique deregulated cellular proteins were unambiguously identified, which were involved in the regulation of essential processes such as apoptosis, mRNA processing, cellular structure and integrity, signal transduction, and endoplasmic-reticulum-associated degradation pathway. Based on our proteomic data and additional functional studies target proteins were identified indicating a late activation of apoptotic pathways in Huh7 cells upon HSV-1 Cgal(+) infection. Additionally to changes on RuvB-like 2 and Bif-1, down-regulation of Erlin-2 suggests stimulation of Ca(2+)-dependent apoptosis. Moreover, activation of the mitochondrial apoptotic pathway results from a time-dependent multi-factorial impairment as inferred from the stepwise characterization of constitutive pro- and anti-apoptotic factors. Activation of serine-threonine protein phosphatase 2A (PP2A) was also found in Huh7 cells upon HSV-1 Cgal(+) infection. In addition, PP2A activation paralleled dephosphorylation and inactivation of downstream mitogen-activated protein (MAP) kinase pathway (MEK(1/2), ERK(1/2)) critical to cell survival and activation of proapoptotic Bad by dephosphorylation of Ser-112. Taken together, our results provide novel molecular information that contributes to define in detail the apoptotic mechanisms triggered by HSV-1 Cgal(+) in the host cell and lead to the implication of PP2A in the transduction of cell death signals and cell survival pathway arrest.

Proteomics analysis of differential expression of cellular proteins in response to avian H9N2 virus infection in human cells

We present the first proteomic analysis on the cellular responses to avian influenza virus (H9N2) infection in a human cell line in different time courses in order to search for target proteins for viral pathogenesis/adaptation studies. By using 2-DE coupled with MALDI-TOF MS and nano-ESI-MS/MS, we identified a set of differentially expressed cellular proteins, including cytoplasmic actin, cytokeratin, prohibitin, enoyl-CoA hydratase, peptide-prolyl cis-trans isomerase A (PPIase A), chloride intracellular channel protein 1, pyruvate dehydrogenase E1 component subunit beta, adenine phosphoribosyltransferase, guanine nucleotide-binding protein subunit beta, nucleoside diphosphate kinase A, elongation factor 1-beta and splicing factor, arginine/serine rich 1. The most significant changes in different time courses were found in cytoplasmic actin and cytokeratin, both of which constituted the major components of cytoskeleton network in the cells. The obtained data suggested a possible role of the cytoskeleton during avian influenza virus infection of mammalian cells, which might help for better understanding of the dynamics of avian influenza virus and host interaction in mammalian cell setting.

ELISA for human serum leucine-rich alpha-2-glycoprotein-1 employing cytochrome c as the capturing ligand

Leucine-rich alpha-2-glycoprotein-1 (LRG) is a serum glycoprotein of unknown function that has shown promise based on qualitative assessments as a biomarker for certain diseases including microbial infections and cancer. However, the lack of a quantitative assay for LRG has limited its application. Here an indirect enzyme-linked immunosorbent assay (ELISA) for quantifying LRG in human serum is described in which cytochrome c is employed as the capturing ligand and a monoclonal antibody specific for LRG is used to detect the captured glycoprotein. Application of this assay in quantifying LRG in various patients' sera is demonstrated. The concentration of LRG in sera of control subjects as determined by this assay is approximately 50 microg/ml. Consistent with expectations from published reports, LRG was found to be significantly elevated in the sera of some patients with a bacterial infection (toxic shock syndrome, TSS). LRG was only slightly elevated in patients infected with the human immunodeficiency virus as compared to uninfected control subjects, while normal levels of LRG were observed in patients with non-infectious diseases (inflammatory arthritis and neurological disorders, primarily Parkinson's disease). Although LRG and C-reactive protein (CRP) are both produced by the liver and are classified as acute-phase proteins, there was no significant correlation between the levels of LRG and CRP in the sera of the patients. Thus, LRG and CRP measurements are non-redundant and indicate different physiological contexts. The ELISA described in this report should be useful to further assess serum LRG as a biomarker for clinical diagnostics.

Viral proteomics: global evaluation of viruses and their interaction with the host

Viruses constantly adapt to and modulate the host environment during replication and propagation. Both DNA and RNA viruses encode multifunctional proteins that interact with and modify host cell proteins. While viral genomes were the first complete sequences known, the corresponding proteomes are only now elucidated, with some surprising results. Even more daunting is the task to globally monitor the impact of viral infection on the proteome of the host cell and many technical hurdles must still be overcome in order to facilitate robust and reproducible measurements. Further complicating the picture is the dynamic nature of proteins, including post-translational modifications, enzymatic cleavage and activation or destruction by proteolytic events. Nevertheless, several promising studies have been published using high-throughput methods directly measuring protein abundance. Particularly, quantitative or semiquantitative mass spectrometry-based analysis of viral and cellular proteomes are now being used to characterize viruses and their host interaction. In addition, the full set of interactions between viral and host proteins, the interactome, is beginning to emerge, with often unexpected interactions that need to be carefully validated. In this review, we will discuss two major areas of viral proteomics: first, virion proteomics (such as the protein characterization of viral particles) and second, proteoviromics, including the viral protein interactomics and the quantitative analysis of host cell proteome during viral infection.

Proteomics analysis of host cells infected with infectious bursal disease virus

The effect of infectious bursal disease virus (IBDV) infection on cellular protein expression is essential for viral pathogenesis. To characterize the cellular response to IBDV infection, the differential proteomes of chicken embryo fibroblasts, with and without IBDV infection, were analyzed at different time points with two-dimensional gel electrophoresis (2-DE) followed by MALDI-TOF/TOF identification. Comparative analysis of multiple 2-DE gels revealed that the majority of protein expression changes appeared at 48 and 96 h after IBDV infection. Mass spectrometry identified 51 altered cellular proteins, including 13 up-regulated proteins and 38 down-regulated proteins 12-96 h after infection. Notably 2-DE analysis revealed that IBDV infection induced the increased expression of polyubiquitin, apolipoprotein A-I, heat shock 27-kDa protein 1, actins, tubulins, eukaryotic translation initiation factor 4A isoform 2, acidic ribosomal phosphoprotein, and ribosomal protein SA isoform 2. In addition, IBDV infection considerably suppressed those cellular proteins involved in ubiquitin-mediated protein degradation, energy metabolism, intermediate filaments, host translational apparatus, and signal transduction. Moreover 38 corresponding genes of the differentially expressed proteins were quantitated by real time RT-PCR to examine the transcriptional profiles between infected and uninfected chicken embryo fibroblasts. Western blot further confirmed the inhibition of Rho protein GDP dissociation inhibitor expression and the induction of polyubiquitin during IBDV infection. Subcellular distribution analysis of the cytoskeletal proteins vimentin and beta-tubulin clearly demonstrated that IBDV infection induced the disruption of the vimentin network and microtubules late in IBDV infection. Thus, this work effectively provides useful dynamic protein-related information to facilitate further investigation of the underlying mechanism of IBDV infection and pathogenesis.

Proteomic analysis of host responses in HepG2 cells during dengue virus infection

Dengue virus infection remains a public health problem worldwide. However, its pathogenic mechanisms and pathophysiology are still poorly understood. We performed proteomic analysis to evaluate early host responses (as indicated by altered proteins) in human target cells during dengue virus infection. HepG2 cells were infected with dengue virus serotype 2 (DEN-2) at multiplicity of infection (MOI) of 0.1, 0.5, and 1.0. Quantitative analyses of DEN-2 infection and cell death at 12, 24, and 48 h postinfection showed that the MOI of 1.0 with 24 h postinfection duration was the optimal condition to evaluate early host responses, as this condition provided the high %Infection ( approximately 80%), while %Cell death ( approximately 20%) was comparable to that of the mock-control cells. Proteins derived from the mock-control and DEN-2-infected cells were resolved by 2-D PAGE ( n = 5 gels for each group) and visualized by SYPRO Ruby stain. Quantitative intensity analysis revealed 17 differentially expressed proteins, which were successfully identified by peptide mass fingerprinting. Most of these altered proteins were the key factors involved in transcription and translation processes. Further functional study on these altered proteins may lead to better understanding of the pathogenic mechanisms and host responses to dengue virus infection, and also to the identification of new therapeutic targets for dengue virus infection.

Modulation of the host cell proteome by the intracellular apicomplexan parasite Toxoplasma gondii

To investigate how intracellular parasites manipulate their host cell environment at the molecular level, we undertook a quantitative proteomic study of cells following infection with the apicomplexan parasite Toxoplasma gondii. Using conventional two-dimensional electrophoresis, difference gel electrophoresis (DIGE), and mass spectrometry, we identified host proteins that were consistently modulated in expression following infection. We detected modification of protein expression in key metabolic pathways, including glycolysis, lipid and sterol metabolism, mitosis, apoptosis, and structural-protein expression, suggestive of global reprogramming of cell metabolism by the parasite. Many of the differentially expressed proteins had not been previously implicated in the response to the parasite, while others provide important corroborative protein evidence for previously proposed hypotheses of pathogen-cell interactions. Significantly, over one-third of all modulated proteins were mitochondrial, and this was further investigated by DIGE analysis of a mitochondrion-enriched preparation from infected cells. Comparison of our proteomic data with previous transcriptional studies suggested that a complex relationship exits between transcription and protein expression that may be partly explained by posttranslational modifications of proteins and revealed the importance of investigating protein changes when interpreting transcriptional data. To investigate this further, we used phosphatase treatment and DIGE to demonstrate changes in the phosphorylation states of several key proteins following infection. Overall, our findings indicate that the host cell proteome responds in a dramatic way to T. gondii invasion, in terms of both protein expression changes and protein modifications, and reveal a complex and intimate molecular relationship between host and parasite.

Viral proteomics

Viruses have long been studied not only for their pathology and associated disease but also as model systems for molecular processes and as tools for identifying important cellular regulatory proteins and pathways. Recent advances in mass spectrometry methods coupled with the development of proteomic approaches have greatly facilitated the detection of virion components, protein interactions in infected cells, and virally induced changes in the cellular proteome, resulting in a more comprehensive understanding of viral infection. In addition, a rapidly increasing number of high-resolution structures for viral proteins have provided valuable information on the mechanism of action of these proteins as well as aided in the design and understanding of specific inhibitors that could be used in antiviral therapies. In this paper, we discuss proteomic studies conducted on all eukaryotic viruses and bacteriophages, covering virion composition, viral protein structures, virus-virus and virus-host protein interactions, and changes in the cellular proteome upon viral infection.

Proteomic analysis of up-regulated proteins in human promonocyte cells expressing severe acute respiratory syndrome coronavirus 3C-like protease

The pathogenesis of severe acute respiratory syndrome coronavirus (SARS CoV) is an important issue for treatment and prevention of SARS. Previously, SARS CoV 3C-like protease (3CLpro) has been demonstrated to induce apoptosis via the activation of caspase-3 and caspase-9 (Lin, C. W., Lin, K. H., Hsieh, T. H., Shiu, S. Y. et al., FEMS Immunol. Med. Microbiol. 2006, 46, 375-380). In this study, proteome analysis of the human promonocyte HL-CZ cells expressing SARS CoV 3CLpro was performed using 2-DE and nanoscale capillary LC/ESI quadrupole-TOF MS. Functional classification of identified up-regulated proteins indicated that protein metabolism and modification, particularly in the ubiquitin proteasome pathway, was the main biological process occurring in SARS CoV 3CLpro-expressing cells. Thirty-six percent of identified up-regulated proteins were located in the mitochondria, including apoptosis-inducing factor, ATP synthase beta chain and cytochrome c oxidase. Interestingly, heat shock cognate 71-kDa protein (HSP70), which antagonizes apoptosis-inducing factor was shown to down-regulate and had a 5.29-fold decrease. In addition, confocal image analysis has shown release of mitochondrial apoptogenic apoptosis-inducing factor and cytochrome c into the cytosol. Our results revealed that SARS CoV 3CLpro could be considered to induce mitochondrial-mediated apoptosis. The study provides system-level insights into the interaction of SARS CoV 3CLpro with host cells, which will be helpful in elucidating the molecular basis of SARS CoV pathogenesis.

Plasma proteomics of pancreatic cancer patients by multi-dimensional liquid chromatography and two-dimensional difference gel electrophoresis (2D-DIGE): up-regulation of leucine-rich alpha-2-glycoprotein in pancreatic cancer

We investigated the aberrant expression of plasma proteins in patients with pancreatic cancer. High-abundance plasma proteins (albumin, transferrin, haptoglobin, alpha-1-antitrypsin, IgG and IgA) were depleted by use of an immuno-affinity column, and low-abundance ones were separated into five fractions by anion-exchange chromatography. The fractionated plasma proteins were subjected to 2D-DIGE with highly sensitive fluorescent dyes. The quantitative protein expression profiles obtained by 2D-DIGE were compared between two plasma protein mixtures: one from five non-cancer bearing healthy donors and the other from five patients with pancreatic cancer. Among 1200 protein spots, we found that 33 protein spots were differently expressed between the two mixtures; 27 of these were up-regulated and six were down-regulated in cancer. Mass spectrometry and database searching allowed the identification of the proteins corresponding to the gel spots. Up-regulation of leucine-rich alpha-2-glycoprotein (LRG), which has not previously been implicated in pancreatic cancer, was observed. Western blotting with an anti-LRG antibody validated the up-regulation of LRG in an independent series of plasma samples from healthy controls, patients with chronic pancreatitis, and patients with pancreatic cancer. Our results demonstrate the application of a combination of multi-dimensional liquid chromatography with 2D-DIGE for plasma proteomics and suggest the clinical utility of LRG plasma level measurement.

Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV)

Severe acute respiratory syndrome (SARS) is caused by a coronavirus (CoV), SARSCoV. SARS-CoV belongs to the family Coronaviridae, which are enveloped RNA viruses in the order Nidovirales. Global research efforts are continuing to increase the understanding of the virus, the pathogenesis of the disease it causes (SARS), and the “heterogeneity of individual infectiousness” as well as shedding light on how to prepare for other emerging viral diseases. Promising drugs and vaccines have been identified. The milestones achieved have resulted from a truly international effort. Molecular studies dissected the adaptation of this virus as it jumped from an intermediary animal, the civet, to humans, thus providing valuable insights into processes of molecular emergence.

Modulation of the unfolded protein response by the severe acute respiratory syndrome coronavirus spike protein

Perturbation of the function of endoplasmic reticulum (ER) causes stress leading to the activation of cell signaling pathways known as the unfolded protein response (UPR). Severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) uses ER as a site for synthesis and processing of viral proteins. In this report, we demonstrate that infection with SARS-CoV induces the UPR in cultured cells. A comparison with M, E, and NSP6 proteins indicates that SARS-CoV spike (S) protein sufficiently induces transcriptional activation of several UPR effectors, including glucose-regulated protein 78 (GRP78), GRP94, and C/EBP homologous protein. A substantial amount of S protein accumulates in the ER. The expression of S protein exerts different effects on the three major signaling pathways of the UPR. Particularly, it induces GRP78/94 through PKR-like ER kinase but has no influence on activating transcription factor 6 or X box-binding protein 1. Taken together, our findings suggest that SARS-CoV S protein specifically modulates the UPR to facilitate viral replication.