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Open Access Research article

A putative diacidic motif in the SARS-CoV ORF6 protein influences its subcellular localization and suppression of expression of co-transfected expression constructs

Vithiagaran Gunalan123, Ali Mirazimi12 and Yee-Joo Tan4*

Author Affiliations

1 Center for Microbiological Preparedness, Swedish Institute for Infectious Disease Control, SE-171 82 Solna, Sweden

2 Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Nobels Väg 16, SE-17177 Stockholm, Sweden

3 Institute of Molecular and Cell Biology, 61 Biopolis Drive, A*STAR (Agency for Science, Technology and Research), Singapore 138673, Singapore

4 Department of Microbiology, Yong Loo Lin School of Medicine, National University Health System, National University of Singapore, MD4, 5 Science Drive 2, Singapore 117597, Singapore

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BMC Research Notes 2011, 4:446  doi:10.1186/1756-0500-4-446

Published: 25 October 2011

Abstract

Background

The ORF6 protein is one of the eight accessory proteins of the severe acute respiratory syndrome coronavirus (SARS-CoV). Numerous properties of ORF6 have been documented and this study focuses on two of these, namely, its ability to suppress the expression of co-transfected expression constructs and its subcellular localization to vesicular structures.

Results

Using a transient transfection system, ORF6's ability to suppress the expression of co-transfected expression constructs was measured in a quantitative manner. While ORF6 does not have a global effect on protein synthesis, quantitative real-time PCR revealed that it down-regulated the mRNA level of the co-transfected myc-nsp8 gene. Furthermore, alanine substitution of a diacidic cluster motif (aa53-56) in the ORF6 gene caused a reduction in the suppression of expression of co-transfected myc-nsp8 gene. Our previous study revealed that ORF6 localized to vesicular structures in SARS-CoV infected Vero E6 cells. Here, ORF6 was observed to be localized to similar vesicular structures in Vero E6 cells which have been transiently transfected with a mammalian expression plasmid encoding for untagged ORF6. ORF6 showed partial colocalization with cellular proteins CD63 and Lamp1, suggesting that the vesicular structures may be a subpopulation of endosomal/lysosomal vesicles. The alanine substitution of the diacidic cluster motif also altered the subcellular localization of the ORF6 protein, indicating a potential relationship between the subcellular localization of the ORF6 protein and its ability to suppress the expression of co-transfected expression constructs.

Conclusions

By combining quantitative real-time PCR and transient transfection system, a simple and safe method is established to measure ORF6's ability to suppress the expression of co-transfected myc-nsp8. In addition, immunofluorescence analysis revealed that the subcellular localization of ORF6 when expressed on its own is similar to that observed in SARS-CoV infected cells. Through the use of these two assays, a putative diacidic motif in the ORF6 protein was found to influence its subcellular localization and ability to suppress the expression of co-transfected expression constructs.