Yeast perspectives on HIV-1 VPR.

Increasing evidence suggests that HIV-1 viral protein R (Vpr) plays an important role in viral pathogenesis, as its functions are being linked to viral activation, suppression of human immune functions and depletion of human CD4 lymphocytes, which are the major clinical manifestation of AIDS. In vitro, Vpr shows multiple activities both in mammalian and yeast cells, which include nuclear transport, induction of cell cycle G2 arrest, morphological changes and cell death. The occurrence of these activities in yeast indicates that Vpr interacts with highly conserved cellular processes to cause these effects and allows Vpr activities to be studied in these genetically well characterized organisms. Studies of Vpr in fission yeast (Schizosaccharomyces pombe) and budding yeast (Saccharomyces cerevisiae) have helped to establish these major conclusions. 1) Vpr induces G2 arrest through inhibitory phosphorylation of the cyclin-dependent kinase by a pathway in which protein phosphatase 2A plays an important role. 2) Vpr fulfills its essential role in the nuclear transport of the viral pre-integration complex by binding to a novel site on importin ?. 3) Vpr induces apoptosis by directly permeabilizing the mitochondrial membrane. 4) Vpr also appears to kill cells by mitochondrial-independent mechanisms. 5) G2 arrest and cell death induced by Vpr are two independent functions, and 6) amino acid residues of Vpr at position 29, 33 and 71 are important sites for maintaining the overall structure of Vpr. Future studies of Vpr in yeast are expected to make additional contributions to understanding the mechanisms of Vpr activities and may also help address the importance of these activities during the course of a HIV-1 infection.