Interaction with the p6 domain of the Gag precursor mediates incorporationinto virions of Vpr and Vpx proteins from primate lentiviruses

Vpr and Vpx proteins from human and simian immunodeficiency viruses (HIV an d SIV) are incorporated into virions in quantities equivalent to those of t he viral Gag proteins. We demonstrate here that Vpr and Vpx proteins from d istinct lineages of primate lentiviruses were able to bind to their respect ive Gag precursors. The capacity of HIV type 1 (HIV-1) Vpr mutants to bind to pr55(Gag) was correlated with their incorporation into virions. Molecula r analysis of these interactions revealed that they required the C-terminal p6 domain of the Gag precursors. While the signal for HIV-1 Vpr binding li es in the leucine triplet repeat region of the p6 domain reported to be ess ential for incorporation, SIVsm, Gag lacking the equivalent region still bo und to SIVsm Vpr and Vpx, indicating that the determinants for Gag binding are located upstream of this region of the p6 domain. Binding to Gag cleava ge products showed that HIV-1 Vpr interacted directly with the nucleocapsid protein (NC), whereas SIVsm Vpr and Vpx did not interact with NC but with the p6 protein, These results (i) reveal differences between HIV-1 and SIVs m for the p6 determinants required for Vpr and Vpx binding to Gag and (ii) suggest that HIV-1 Vpr and SIVsm Vpr and Vpx interact with distinct cleavag e products of the precursor following proteolytic processing in the virions .