THE CARBOXYL-TERMINUS OF THE HUMAN FOAMY VIRUS GAG PROTEIN CONTAINS SEPARABLE NUCLEIC-ACID BINDING AND NUCLEAR TRANSPORT DOMAINS

The Gag protein of human foamy virus (HFV) lacks Cys-His boxes present in the nucleocapsid (NC) domains of other retroviruses; instead it co ntains three glycine-arginine-rich motifs (GR boxes), We have expresse d the carboxyl end of HFV Gag containing the GR boxes (the NC domain e quivalent) and analyzed its nucleic acid binding properties. Our resul ts 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 filte r binding assays. Analysis of a mutant containing a heterologous seque nce in place of GR box I indicates that this motif is required for nuc leic acid binding and for viral replication. A mutant in GR hox II sti ll binds to RNA and DNA in vitro but virus containing this mutation do es not replicate and no nuclear staining of the Gag protein is found i n transfected cells. Surprisingly, a revertant from this mutant that c ompletely lacks GR box II and exhibits very little nuclear transport o f Gag can readily replicate in tissue culture. This finding thus provi des a direct evidence that although the sequences in GR box II can ser ve as a nuclear transport signal, they are not required for HFV replic ation and it is unlikely that nuclear localization of Gag protein play s any critical role during viral infection. Taken together, our result s suggest that the Gag protein of HFV may be more analogous to the cor e protein of the hepatitis B virus family than to conventional retrovi ral Gag protein.