Vif is viral infectivity factor and is important in the production of highly infectious virus. It is incorporated into the mature virus (7-100 molecules per virus) although incorporation into the virus seems non-specific.. After cell infection, Vif mutants show lower levels of viral DNA and Vif appears important in viral DNA synthesis. Vif mutants only show infectivity defects in certain cell types. It is suggested that permissive cells (those that do not need Vif for production of highly infectious virus) produce a factor that compensates for lack of Vif. The activity of Vif may be regulated by phosphorylation since mutation of serine 144 which can be phosphorylated leads to a viral infectivity defect. Electron microscopy shows that Vif mutants have very altered core structures and so Vif may be important in assembly and maturation. 

For further information see: Alan Frankel and John A. T. Young, HIV-1: Fifteen proteins and an RNA Annual Review of Biochemistry 67: 1-25, 1998