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Journal of Virology, May 2005, p. 6142-6151, Vol. 79, No. 10
0022-538X/05/$08.00+0     doi:10.1128/JVI.79.10.6142-6151.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Infectious Bronchitis Virus 3a Protein Localizes to a Novel Domain of the Smooth Endoplasmic Reticulum

Department of Cell Biology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

Received 5 November 2004/ Accepted 14 January 2005

All coronaviruses possess small open reading frames (ORFs) between structural genes that have been hypothesized to play important roles in pathogenesis. Infectious bronchitis virus (IBV) ORF 3a is one such gene. It is highly conserved among group 3 coronaviruses, suggesting that it has an important function in infection. IBV 3a protein is expressed in infected cells but is not detected in virions. Sequence analysis predicted that IBV 3a was a membrane protein; however, only a fraction behaved like an integral membrane protein. Microscopy and immunoprecipitation studies demonstrated that IBV 3a localized to the cytoplasm in a diffuse pattern as well as in sharp puncta in both infected and transfected cells. These puncta did not overlap cellular organelles or other punctate structures. Confocal microscopy demonstrated that IBV 3a puncta lined up along smooth endoplasmic reticulum (ER) tubules and, in a significant number of instances, were partially surrounded by these tubules. Our results suggest that IBV 3a is partially targeted to a novel domain of the smooth ER.

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