Analysis of the vif gene of feline immunodeficiency virus.

We have examined FIV vif function in the context of molecular clone pF34 (GenBank Accession No. M25729). Rabbit antisera directed against the translation product of the vif gene identified a 29-kDa protein in tissue culture cells infected with FIV-pF34; this protein was not present in cultures of uninfected cells. Thus, the vif gene of this virus strain is expressed in infected cells. Three mutations were made in distinct regions of the vif gene of molecular clone FIV-pF34: (i) deletion of 223 bp from the central portion of the gene, (ii) site-directed mutation of a conserved N-terminal basic region, and (iii) site-directed mutation of a conserved C-terminal motif. FIV proviruses containing each of these mutations were assessed for replication following transfection into two feline adherent cell types, CrFK and G355-5. Reverse transcriptase and p24gag antigen assays of supernatants from transfected cultures revealed that all three vif mutants produced very little cell-free virus or viral protein in both cell types. Results of immunocytochemical staining of these cultures indicated that all three mutants expressed low levels of cell-associated FIV p24gag. These findings suggest that each of the three regions mutated in vif is critical for function. Our observations are consistent with studies showing marked attenuation of HIV-1 vif mutants in certain cell types. We conclude that the vif gene is a critical determinant of FIV-pF34 replication and infectivity in CrFK and G355-5 cells.