The vpr product of the human immunodeficiency virus type 1 (HIV-1) acts in trans to accelerate virus replication and cytopathic effect in T cells. Here it is shown that the HIV-1 viral particle contains multiple copies of the vpr protein. The vpr product is the first regulatory protein of HIV-1 to be found in the virus particle. This observation raises the possibility that vpr acts to facilitate the early steps of infection before de novo viral protein synthesis occurs. The gag, pol, and env

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... es of human immunodeficiency virus type 1 (HIV-1) encode the structural and replicative proteins that are incorporated into the virus particle. HIV-1 specifies at least six additional proteins which regulate viral replication (Fig. 1) . Two of these genes, tat and rev, are essential for virus replication. The remaining genes, nef, vif, vpu, and vpr, are not required for virus replication, although mutations in these genes alter the replication properties of the virus (3). vpr was recently demonstrated to accelerate the replication and the cytopathic effect of HIV-1 in CD4+ T cells (2, 5). vpr was also shown to specify a 15-kilodalton (kDa) protein that acts in trans to increase expression of viral proteins. vpr also stimulates expression of heterologous genes driven by the HIV-1 long terminal repeat (LTR) as well as other promoters (2). To determine whether the vpr protein is incorporated into the virus particle, CD4+ T cells were transfected with infectious proviruses isogenic except for the expression of vpr. Cells were transfected by the DEAE-dextran technique (8). The provirus that expresses the full-length vpr protein of 96 amino acids is designated HXBRU+. A provirus isogenic except for a frameshift mutation in vpr that is predicted to terminate the product at amino acid 40 was used as a control (HXBRU-) (2). Eight days posttransfection, both cultures were metabolically labeled with 100 ,uCi of [35S]cysteine per ml for 8 h as described elsewhere (9). Cells were collected by centrifugation, washed once with phosphate-buffered saline, and lysed with 750 [lI of a buffer which contained 0.05 M Tris hydro-