Human immunodeficiency virus type 1 Vpr protein is incorporated into the virion in significantly smaller amounts than Gag and is phosphorylated in infected cells

Viral protein R (Vpr) of human immunodeficiency virus type 1 (HIV-1) is a s mall accessory protein involved in the nuclear import of viral DNA and the growth arrest of host cells. Several studies have demonstrated that a signi ficant amount of Vpr is incorporated into the virus particle via interactio n with the p6 domain of Gag, and it is generally assumed that Vpr is packag ed in equimolar ratio to Gag. We have quantitated the relative amount of Vp r in purified virions following [S-35]cysteine labeling of infected MT-4 ce lls, as well as by quantitative immunoblotting and found that Vpr is presen t in a molar ratio of approximately 1:7 compared to capsid. Analysis of iso lated core particles showed that Vpr is associated with the mature viral co re, despite quantitative loss of p6 from core preparations. Metabolic label ing of infected cells with ortho[P-32]phosphate revealed that a small fract ion of Vpr is phosphorylated in virions and infected cells.