The carboxyl terminus of the human foamy virus Gag protein contains separable nucleic acid binding and nuclear transport domains

Abstract

The Gag protein of human foamy virus (HFV) lacks Cys-His boxes present in the nucleocapsid (NC) domains of other retroviruses; instead it contains three glycine-arginine-rich motifs (GR boxes). We have expressed the carboxyl end of HFV Gag containing the GR boxes (the NC domain equivalent) and analyzed its nucleic acid binding properties. Our results show that the NC domain of HFV Gag binds with high affinity to both RNA and DNA, in a sequence-independent manner, as determined by filter binding assays. Analysis of a mutant containing a heterologous sequence in place of GR box I indicates that this motif is required for nucleic acid binding and for viral replication. A mutant in GR box II still binds to RNA and DNA in vitro, but virus containing this mutation does not replicate and no nuclear staining of the Gag protein is found in transfected cells. Surprisingly, a revertant from this mutant that completely lacks GR box II and exhibits very little nuclear transport of Gag can readily replicate in tissue culture. This finding thus provides a direct evidence that although the sequences in GR box II can serve as a nuclear transport signal, they are not required for HFV replication and it is unlikely that nuclear localization of Gag protein plays any critical role during viral infection. Taken together, our results suggest that the Gag protein of HFV may be more analogous to the core protein of the hepatitis B virus family than to conventional retroviral Gag protein.

Chromatin Tethering of Incoming Foamy Virus by the Structural Gag Protein

“…The CBS of PFV Gag is partially transcomplemented by that of KSHV-LANA Collectively our present data and previous reports led us to propose that incoming PFV Gag binds the viral genome through the GRI domain (28) and host chromosomes through the CBS (Figure 1), located in close proximity to Residues 536-544 are shown with the indicated alanine substitutions below. The derived mutants were named Gag GYN, Gag LRP and Gag RTY.…”

Genetic Characterization of Simian Foamy Viruses Infecting Humans

“…The YXXL motif, important for correct particle assembly (43), was also totally conserved. The three GR boxes involved in distinct phases of viral replication (49,64,78) were present in all isolates. The viral protease-dependent cleavage site RAVN/TVTQ, which allows the generation of the p68 Gag subunit from the full Gag (57), which is essential for subsequent correct particle formation (19,80), displayed high homology among all isolates.…”

HTLV-3/4 and simian foamy retroviruses in humans: Discovery, epidemiology, cross-species transmission and molecular virology

“…Unlike Gag proteins from orthoretroviruses which are cleaved and processed into capsid, matrix and nucleoprotein, FV Gag proteins are only partially cleaved at the C-terminus generating two species: a long and a shorter proteins that only differ by 4 kDa, both being incorporated in the virion (Enssle et al, 1997). They contain three Glycine and arginine-rich regions (the GR boxes) important for the correct FV assembly and the nuclear localization of Gag (Lee and Linial, 2008;Yu et al, 1996b), the role of which is still not elucidated.…”