Feline immunodeficiency virus Orf-A localizes to the nucleus and induces cell cycle arrest

Feline immunodeficiency virus (FIV) gene orf-A, also designated orf-2, encodes a 77 amino acid accessory protein reported to be critical for efficient viral replication in vitro and in vivo and previously implicated to encode a Tat protein for FIV. However, recent studies have shown Orf-A to be important in the late steps of the FIV life cycle involved in virion formation and in early steps involved in virus infectivity (J. Virol. 77 (2003) 8819). The present study reports that expression of a GFP-Orf-A fusion protein in both primate and feline cell lines results in nuclear localization of this FIV accessory protein. Moreover, a nuclear localization signal (NLS) critical for nuclear import was mapped to amino acid residues 43 through 53 of Orf-A. Lastly, transient expression of GFP-Orf-A in cells induced an arrest at the second gap (G ₂) of the cell cycle. Our findings reveal that Orf-A is a nuclear protein that expresses properties similar to those reported for human immunodeficiency virus-1 (HIV-1)-encoded Vpr.