The human immunodeficiency virus type 1 (HIV-1) Vif protein is necessary at the time of viral particle formation yet functionally manifests its effect after virions enter target cells. This suggests that Vif either acts on another viral protein or is itself incorporated into particles. In this study, we have examined the latter possibility. We confirm our previous observation that Vif is incorporated into human immunodeficiency virus type 1 virions at a ratio of approximately 1 molecule of Vif for every 75 to 220 molecules of p24, or 7 to 20 molecules per virion. Furthermore, we demonstrate that the relative concentration of Vif is much lower in particles than in infected cells, whereas the opposite is observed for the main virus components. The viral envelope, Nef, Vpr, Vpu, protease, reverse transcriptase, integrase, nucleocapsid, and p6gag proteins as well as the viral genomic RNA are dispensable for Vif packaging. Furthermore, mutating several highly conserved residues (H-108, C-114, C-133, L-145, and Q-146) or deleting the C-terminal 18 amino acids of Vif, either of which severely impairs Vif function, does not abolish its incorporation into virions. Finally, Vif can be packaged into murine leukemia virus particles. On the basis of these data, we conclude that the specificity of Vif incorporation into virions remains an open question
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