Particle assembly and Vpr expression in human immunodeficiency virus type 1-infected cells demonstrated by immunoelectron microscopy.

The 96 amino acid viral protein R (Vpr) of human immunodeficiency virus type 1 (HIV-1) was detected during virus assembly in intracellular vacuoles and at the plasma membrane on peripheral blood mononuclear cells. In both immature and mature virus particles, Vpr was located immediately beneath the viral envelope, colocalizing with the core structural protein, Gag p24. Vpr was present in intracellular HIV-1 wild-type virions at 50% of the level found in extracellular HIV-1 particles. Cells infected with HIV-1 strains with C-terminal truncations of Vpr manifested a different pattern of Vpr expression. A mutant with an alteration of amino acids 79 to 85 exhibited a 23% reduction in total levels of Vpr expression, but a marked accumulation of Vpr in intracellular rather than extracellular virions. A mutant with the last 17 amino acids of Vpr deleted expressed only 10% of wild-type levels of Vpr. These observations indicate that Vpr is incorporated into virions from the cytoplasmic aspect of either the vacuolar or plasma membrane. Furthermore, the proportion of Vpr on intracellular compared to extracellular virions is affected by a specific locus within the protein.