The role of the vpx, vpr, and nef genes in the replication of simian immunodeficiency virus (SIV) was investigated using point and deletion mutations in these genes. The effects on replication kinetics of single or combined mutants—vpx, vpr, vpx-vpr, vpx-nef, vpr-nef, and vpx-vpr-nef—in established lymphoid CEMx174 and MT-4 cells were negligible, except that the postinfection appearance of vpx-nef, vpr-nef, and vpx-vpr-nef progeny virus was slightly delayed in MT-4 cells. The vpx, but not the vpr, point mutation reverted to wild-type sequences within 12 days after infection, suggesting that stronger selection pressure for Vpx than for Vpr expression might exist in these established cell lines. In contrast to growth in the lymphoid cell lines, replication of vpx-deleted viruses in macaque peripheral blood mononuclear cells (PBMC) was severely impaired, indicating that Vpx is necessary for efficient replication in PBMC. In contrast, the vpr mutant exhibited different degrees of impairment depending on the donor animal used as a source of PBMC. A virus encoding a Vpx-Vpr fusion protein replicated in PBMC comparably to a vpr deletion mutant virus, whereas a frameshift deletion at the vpx-vpr junction of this mutant eliminated virus replication, suggesting that deletion of the C-terminal half of Vpx was partially compensated by the presence of the large Vpr portion in the fusion protein. Deletion of the nef gene did not affect SIVmac replication in PBMC. The Vpx and Vpr proteins expressed in COS-1 cells were detected in the extracellular medium and did not crossreact with Vpr- and Vpx-specific antisera, in spite of extensive amino acid similarity between these proteins. These studies indicate the importance of Vpx and Vpr in SIVmac infection and suggest that these proteins are antigenically and functionally distinct.
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