A Novel Severe Acute Respiratory Syndrome Coronavirus Protein, U274, Is Transported to the Cell Surface and Undergoes Endocytosis

Fourteen ORFs have been identified in the severe acute respiratory syndrome-associated coronavirus (SARS-CoV) genome. ORF 3a of SARS-CoV codes for a recently identified transmembrane protein, but its function remains unknown. In this study we confirmed the 3a protein expression and investigated its localization at the surface of SARS-CoV-infected or 3a-cDNA-transfected cells. Our experiments showed that recombinant 3a protein can form a homotetramer complex through interprotein disulfide bridges in 3a-cDNA-transfected cells, providing a clue to ion channel function. The putative ion channel activity of this protein was assessed in 3a-complement RNA-injected Xenopus oocytes by two-electrode voltage clamp. The results suggest that 3a protein forms a potassium sensitive channel, which can be efficiently inhibited by barium. After FRhK-4 cells were transfected with an siRNA, which is known to suppress 3a expression, followed by infection with SARS-CoV, the released virus was significantly decreased, whereas the replication of the virus in the infected cells was not changed. Our observation suggests that SARS-CoV ORF 3a functions as an ion channel that may promote virus release. This finding will help to explain the highly pathogenic nature of SARS-CoV and to develop new strategies for treatment of SARS infection. caused alarm all over the world. The newly discovered human coronavirus named SARS-associated coronavirus (SARS-CoV) was identified as the causative agent for this disease (1, 2). SARSCoV has a large single-positive-strand RNA genome that contains 14 ORFs. Some of these ORFs encode viral structural proteins, such as spike protein, membrane protein, small envelope protein, and nucleocapsid protein, as well as viral replicase and protease (3). Those proteins play important roles in viral infection and replication. However, functions for other ORFs are not clear. Therefore, identification and characterization of new functional proteins from the ORFs will be helpful for understanding the pathogenesis of SARS-CoV. Up to now there are still no effective drugs or vaccines against SARS-CoV. The identification of new viral proteins and the elucidation of their functions will provide potential targets for design of drugs or vaccines against SARS.Our previous work has revealed that ORF 3a of SARS-CoV is such a viral protein (4). Since then, other publications have concurred in this observation and have shown that it is a structural protein (5-8). ORF 3a is located between the S and E protein loci and encodes a protein of 274 aa. The only available information based on proteomics and immunoblotting suggests that 3a protein is structural in nature, but its localization, topology, and biological function have not been identified.A computed biology analysis of the amino acid sequence of the 3a protein revealed that it has low similarity with any other known protein. Its C-terminal region shares Ϸ50% similarity to Plasmodium calcium pump protein and to the Shewanella outer-membrane porin. Interestingly, comparison of ORFs ...

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“…2C). Our finding, similar to former report (9), defines a rough topology of 3a protein on the cell membrane.…”

Section: Confirmation Of 3a Proteinsupporting

confidence: 81%

Severe acute respiratory syndrome-associated coronavirus 3a protein forms an ion channel and modulates virus release

Zheng

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

291

395

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“…The largest of these group-specific proteins is termed 3a and the 3a protein has also been shown to be expressed in SARS-CoV infected cells [4,5] and could be detected in tissues obtained from SARS patients [5][6][7]. Antibodies against 3a have also been detected in different cohorts of SARS patients [8][9][10][11].…”

Section: Introductionmentioning

confidence: 90%

“…The 3a protein consists of 274 amino acids (aa) and contains three putative transmembrane domains and it is expressed on the cell surface [4,12]. The topology of 3a on the cell surface was determined experimentally: its first 34 aa, i.e.…”

Section: Introductionmentioning

confidence: 99%

Amino acids 15–28 in the ectodomain of SARS coronavirus 3a protein induces neutralizing antibodies

2006

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A synthetic peptide corresponding to amino acids (aa) 15-28 of the severe acute respiratory syndrome coronavirus (SARS-CoV) 3a protein was used to raise polyclonal antibodies in rabbits. This anti-3a N-terminal antibody could detect 3a protein in infected cells, as did an anti-3a C-terminal antibody previously described. The latter targeted the C-terminal cytoplasmic domain of 3a (aa 134-274). The anti-3a N-terminal antibody could detect intracellular 3a as well as 3a expressed on the cell surface. Interestingly, only the anti-3a N-terminal antibody can inhibit SARS-CoV propagation in Vero E6 culture although the binding affinity of the anti-3a N-terminal antibody was lower than the anti-3a C-terminal antibody.

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“…In vitro studies have also shown the 3a protein to interact with the viral envelope, membrane and spike proteins (Tan et al, 2004b;Tan, 2005) and the cellular protein caveolin-1 (Padhan et al, 2007). A deletion of orf3a was shown to reduce virus titres, but not to eliminate virus replication (Yount et al, 2005), and convalescent sera from SARS patients have antibodies to the 3a protein (Tan et al, 2004a), suggesting that it is expressed during virus infection of the host.…”

Section: Introductionmentioning

confidence: 99%

“…In addition to the prototypic coronavirus genes, the SARS-CoV genome also contains nine unique open reading frames (ORFs) (Marra et al, 2003). Of these, orf3a is the largest and encodes a protein of 274 aa, variously called ORF3A (Ito et al, 2005), X1 (Rota et al, 2003) or U274 (Tan et al, 2004b). The 3a protein has been predicted to contain an N-terminal signal peptide followed by three transmembrane domains and a C-terminal cytoplasmic domain of approximately 150 aa (Zeng et al, 2004).…”

Section: Introductionmentioning

confidence: 99%

Severe acute respiratory syndrome coronavirus 3a protein activates the mitochondrial death pathway through p38 MAP kinase activation

Padhan

Minakshi

Towheed

et al. 2008

Journal of General Virology

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The molecular mechanisms governing severe acute respiratory syndrome coronavirus-induced pathology are not fully understood. Virus infection and some individual viral proteins, including the 3a protein, induce apoptosis. However, the cellular targets leading to 3a protein-mediated apoptosis have not been fully characterized. This study showed that the 3a protein modulates the mitochondrial death pathway in two possible ways. Activation of caspase-8 through extrinsic signal(s) caused Bid activation. In the intrinsic pathway, there was activation of caspase-9 and cytochrome c release from the mitochondria. This was the result of increased Bax oligomerization and higher levels of p53 in 3a protein-expressing cells, which depended on the activation of p38 MAP kinase (MAPK) in these cells. For p38 activation and apoptosis induction, the 3a cytoplasmic domain was sufficient. In direct Annexin V staining assays, the 3a protein-expressing cells showed increased apoptosis that was attenuated with the p38 MAPK inhibitor SB203580. A block in nuclear translocation of the STAT3 transcription factor in cells expressing the 3a protein was also observed. These results have been used to present a model of 3a-mediated apoptosis. INTRODUCTIONThe aetiological agent for severe acute respiratory syndrome (SARS) was identified as a novel coronavirus (SARS-CoV) (Peiris et al., 2003). SARS-CoV has a polyadenylated, positive-sense RNA genome of approximately 30 kb (Marra et al., 2003). In addition to the prototypic coronavirus genes, the SARS-CoV genome also contains nine unique open reading frames (ORFs) (Marra et al., 2003). Of these, orf3a is the largest and encodes a protein of 274 aa, variously called ORF3A (Ito et al., 2005), X1 (Rota et al., 2003) or U274 (Tan et al., 2004b). The 3a protein has been predicted to contain an N-terminal signal peptide followed by three transmembrane domains and a C-terminal cytoplasmic domain of approximately 150 aa (Zeng et al., 2004).The 3a protein is associated with virus particles produced following infection of Vero E6 or Caco-2 cells (Ito et al., 2005;Shen et al., 2005) and can assemble into virus-like particles when co-expressed with the membrane and envelope proteins in insect cells (Shen et al., 2005). In vitro studies have also shown the 3a protein to interact with the viral envelope, membrane and spike proteins (Tan et al., 2004b;Tan, 2005) and the cellular protein caveolin-1 (Padhan et al., 2007). A deletion of orf3a was shown to reduce virus titres, but not to eliminate virus replication (Yount et al., 2005), and convalescent sera from SARS patients have antibodies to the 3a protein (Tan et al., 2004a), suggesting that it is expressed during virus infection of the host. The 3a protein was shown to upregulate the expression of fibrinogen in A549 lung epithelial cells and to possess an ionchannel activity selective for monovalent cations (Lu et al., 2006). Ectopic expression of the 3a protein has been shown to induce apoptosis in Vero E6 cells through activation of caspase-8, chromatin c...

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A Novel Severe Acute Respiratory Syndrome Coronavirus Protein, U274, Is Transported to the Cell Surface and Undergoes Endocytosis

Cited by 154 publications

References 58 publications

Severe acute respiratory syndrome-associated coronavirus 3a protein forms an ion channel and modulates virus release

Severe acute respiratory syndrome-associated coronavirus 3a protein forms an ion channel and modulates virus release

Amino acids 15–28 in the ectodomain of SARS coronavirus 3a protein induces neutralizing antibodies

Severe acute respiratory syndrome coronavirus 3a protein activates the mitochondrial death pathway through p38 MAP kinase activation

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