The (52-96) C-terminal domain of Vpr stimulates HIV-1 IN-mediated homologous strand transfer of mini-viral DNA

Viral integrase (IN) and Vpr are both components of the human immunodeficiency virus type 1 (HIV-1) pre-integration complex. To investigate whether these proteins interact within this complex, we investigated the effects of Vpr and its subdomains on IN activity in vitro. When a 21mer oligonucleotide was used as a donor and acceptor, both Vpr and its C-terminal DNA-binding domain [(52-96)Vpr] inhibited the integration reaction, whereas the (1-51)Vpr domain did not affect IN activity. Steady-state fluorescence anisotropy showed that both full-length and (52-96)Vpr bind to the short oligonucleotide, thereby extending previous observations with long DNA. The concentrations of the two proteins required to inhibit IN activity were consistent with their affinities for the oligonucleotide. The use of a 492 bp mini-viral substrate confirmed that Vpr can inhibit the IN-mediated reaction. However, the activity of (52-96)Vpr differed notably since it stimulated specifically integration events involving two homologous mini-viral DNAs. Order of addition experiments indicated that the stimulation was maximal when IN, (50-96)Vpr and the mini-viral DNA were allowed to form a complex. Furthermore, in the presence of (50-96)Vpr, the binding of IN to the mini-viral DNA was dramatically enhanced. Taken together, these data suggest that (52-96)Vpr stimulates the formation of a specific complex between IN and the mini-viral DNA.