EFFECTS OF VIF MUTATIONS ON CELL-FREE INFECTIVITY AND REPLICATION OF SIMIAN IMMUNODEFICIENCY VIRUS

To investigate the function of the Vif protein of the simian immunodef iciency virus (SIV), mutations were introduced into the SIV(mac)239 vi f gene without affecting the reading frames of other overlapping genes . The phenotypes of these mutant viruses were examined with respect to viral replication and the expression and processing of viral proteins . Transfection of vif-mutant proviral DNA into established T cell line s resulted in a significant delay in the onset of virus replication co mpared to that seen with the wild-type provirus. The efficiency of rep lication of the vif-mutant virus was dependent on cell type. MT-4 cell s were permissive for replication of the vif mutant, while replication in CEMx174 cells was severely restricted. Little or no virus replicat ion was observed following cell-free infection of the CEMx174 cell lin e and macaque peripheral blood mononuclear cells (PBMC). These results indicate that the requirement for vif during the replication of SIV(m ac)239 is dependent on eel type, as has been observed for HIV-1. Follo wing cell-free infection, mutant viruses containing combined deletions in vif and the other regulatory genes (vpx, vpr, and nef) displayed r eplication kinetics similar to that of viruses containing the deletion of vif alone. Viral protein expression and processing in MT-4 cells o f vif-deleted viruses were indistinguishable from those of the wildtyp e virus. The effects of two different point mutations in vif were exam ined. One point mutant in vif reverted to the genetic sequence of the wild-type virus within 2 weeks. A second point mutant that did not sho w genetic reversion displayed a similar kinetics of virus replication to that of the vif deletion mutant. These data show that impaired repl ication of vif mutant viruses results from mutation of the vif gene ra ther than the deletion of cis-acting elements, and also indicate that a strong selection exists against vif-defective viruses.