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Journal of Virology, December 2009, p. 12483-12498, Vol. 83, No. 23
0022-538X/09/$08.00+0     doi:10.1128/JVI.01747-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Jaagsiekte Sheep Retrovirus Encodes a Regulatory Factor, Rej, Required for Synthesis of Gag Protein{triangledown}

Andrew Hofacre, Takayuki Nitta, and Hung Fan*

Department of Molecular Biology and Biochemistry and Cancer Research Institute, University of California, Irvine, California 92697

Received 18 August 2008/ Accepted 14 September 2009

Retroviruses express Gag and Pol proteins by translation of unspliced genome-length viral RNA. For some retroviruses, transport of unspliced viral RNA to the cytoplasm is mediated by small regulatory proteins such as human immunodeficiency virus Rev, while other retroviruses contain constitutive transport elements in their RNAs that allow transport without splicing. In this study, we found that the betaretrovirus Jaagsiekte sheep retrovirus (JSRV) encodes within the env gene a trans-acting factor (Rej) necessary for the synthesis of Gag protein from unspliced viral RNA. Deletion of env sequences from a JSRV proviral expression plasmid (pTN3) abolished its ability to produce Gag polyprotein in transfected 293T cells, and Gag synthesis could be restored by cotransfection of an env expression plasmid ({Delta}GP). Deletion analysis localized the complementing activity (Rej) to the putative Env signal peptide, and a signal peptide expression construct showed Rej activity. Two other betaretroviruses, mouse mammary tumor virus (MMTV) and human endogenous retrovirus type K, encode analogous factors (Rem and Rec, respectively) that are encoded from doubly spliced env mRNAs. Reverse transcriptase-PCR cloning and sequencing identified alternate internal splicing events in the 5' end of JSRV env that could signify analogous doubly spliced Rej mRNAs, and cDNA clones expressing two of them also showed Rej activity. The predicted Rej proteins contain motifs similar to those found in MMTV Rem and other analogous retroviral regulatory proteins. Interestingly, in most cell lines, JSRV expression plasmids with Rej deleted showed normal transport of unspliced JSRV RNA to the cytoplasm; however, in 293T cells Rej modestly enhanced export of unspliced viral RNA (2.8-fold). Metabolic labeling experiments with [35S]methionine indicated that JSRV Rej is required for the synthesis of viral Gag polyprotein. Thus, in most cell lines, the predominant function of Rej is to facilitate translation of unspliced viral mRNA.

* Corresponding author. Mailing address: Cancer Research Institute, Department of Molecular Biology and Biochemistry, University of California, Irvine, Irvine, CA 92697-3905. Phone: (949) 824-5554. Fax: (949) 824-4023. E-mail: hyfan{at}uci.edu

{triangledown} Published ahead of print on 23 September 2009.

Journal of Virology, December 2009, p. 12483-12498, Vol. 83, No. 23
0022-538X/09/$08.00+0     doi:10.1128/JVI.01747-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Nitta, T., Hofacre, A., Hull, S., Fan, H. (2009). Identification and Mutational Analysis of a Rej Response Element in Jaagsiekte Sheep Retrovirus RNA. J. Virol. 83: 12499-12511 [Abstract] [Full Text]  


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