Maedi-visna virus and caprine arthritis encephalitis virus genomes encode a Vpr-like but no Tat protein

Abstract

A small open reading frame (ORF) in maedi-visna virus (MVV) and caprine arthritis encephalitis virus (CAEV) was initially named "tat" by analogy with a similarly placed ORF in the primate lentiviruses. The encoded "Tat" protein was ascribed the function of up regulation of the viral transcription from the long terminal repeat (LTR) promoter, but we have recently reported that MVV and CAEV Tat proteins lack trans-activation function activity under physiological conditions (S. Villet, C. Faure, B. Bouzar, G. Verdien, Y. Chebloune, and C. Legras, Virology 307:317-327, 2003). In the present work, we show that MVV Tat localizes to the nucleus of transfected cells, probably through the action of a nuclear localization signal in its C-terminal portion. We also show that, unlike the human immunodeficiency virus (HIV) Tat protein, MVV Tat was not secreted into the medium by transfected human or caprine cells in the absence of cell lysis but that, like the primate accessory protein Vpr, MVV and CAEV Tat proteins were incorporated into viral particles. In addition, analysis of the primary protein structures showed that small-ruminant lentivirus (SRLV) Tat proteins are more similar to the HIV type 1 (HIV-1) Vpr protein than to HIV-1 Tat. We also demonstrate a functional similarity between the SRLV Tat proteins and the HIV-1 Vpr product in the induction of a specific G(2) arrest of the cell cycle in MVV Tat-transfected cells, which increases the G(2)/G(1) ratio 2.8-fold. Together, these data strongly suggest that the tat ORF in the SRLV genomes does not code for a regulatory transactivator of the LTR but, rather, for a Vpr-like accessory protein.

FIG. 1.

Localization of wild-type and mutant MVV Tat-GFP fusion proteins. The TIGEF cell line was transfected with pGFP (A), pGFP-Tat (B), pGFP-5′ (C), and pGFP-3′ (D) plasmid constructs; 48 h posttransfection, the cells were observed by fluorescence microscopy. Magnification, ×400.

FIG. 2.

Release of the Tat protein in the supernatant of Tat-expressing cells. The human HeLa (A) or goat TIGEF (B) cell lines were transfected with the pTat-Flag-MVV and pTat-Flag-HIV plasmid constructs. At 48 h posttransfection, proteins from supernatants (lanes 2, 4, and 6) and cell lysates (lanes 1, 3, and 5) were immunoprecipitated using an anti-FLAG monoclonal antibody. Immunoblots of immunoprecipitated fractions from mock-transfected (lanes 1 and 2), pTat-Flag-MVV-transfected (lanes 3 and 4), and pTat-Flag-HIV-transfected (lanes 5 and 6) cells were performed with a specific monoclonal antibody directed against the FLAG epitope.

FIG. 3.

MVV and CAEV Tat proteins are associated with viral particles. TIGEF were inoculated (MOI, 1) with MVV (A) or CAEV (B and C) or were mock infected. At 24 h postinoculation, the cells were transfected with pTat-Flag-MVV (A) and pTat-Flag-CO (B and C) plasmids. At 48 h posttransfection, the supernatants were collected and virions were harvested from pellets by ultracentrifugation; cell lysates were then obtained following treatment with the lysing solution. (A and B) Proteins from the supernatants (lanes 1 and 3) and cell lysates (lanes 2 and 4) were analyzed by immunoblot assay by using an antibody directed against the FLAG epitope (diluted 10,000 times). Lanes 1 and 2, cells infected with MVV (A) or CAEV (B); lanes 3 and 4, mock-infected cells. (C) Pellets of virions were resuspended and loaded on the top of a linear sucrose gradient. After 16 h of ultracentrifugation, 12 fractions were collected and proteins were extracted and then analyzed by immunoblot assay with an anti-p25 Gag antibody to reveal the major CAEV capsid protein (diluted 1,000 times) or an anti-FLAG antibody (diluted 10,000 times) to reveal the CAEV Tat protein.

FIG. 4.

G₂ arrest of cell cycle in Tat-expressing cells. TIGEF were transfected with plasmid pGFP as a negative control or with plasmid pGFP-Tat. Cells were harvested 72 and 120 h posttransfection, and the DNA was stained with propidium iodide. Cells were analyzed by flow cytometry of more than 10,000 events with Cell Quest analysis software. Histograms of the flow cytometry analysis of the DNA content in transfected TIGEF harvested 72 h posttransfection are shown in panel A. The ordinate indicates the number of cells, and the abscissa indicates the DNA content. (B) The ratios of the percentages of cells in G₂/G₁ phases of the cell cycle are shown. Values shown at the top of each bar represent the mean value of results of at least three independent transfection experiments.

FIG. 5.

Comparison of the primary structures of the SRLV and HIV-1 Vpr and Tat proteins. The putative domains of MVV Tat, HIV-1 Tat, and HIV-1 Vpr are shown as stained boxes. The HIV-1 Tat protein contains an N-terminal proline-rich domain (Pro), a cysteine-rich domain (Cys), a core, a basic domain (basic), and a glutamine-rich domain (Gln). The HIV-1 Vpr protein contains an N-terminal acidic domain (acidic), two leucine-rich domains (Leu), and a basic domain (basic). MVV Tat protein contains an N-terminal acidic domain (acidic), a leucine-rich domain, and a cysteine-rich domain.