HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 VIRAL-PROTEIN-R (VPR) ARRESTS CELLS IN THE G(2) PHASE OF THE CELL-CYCLE BY INHIBITING P34(CDC2) ACTIVITY

The Vpr accessory gene product of human immunodeficiency virus types 1 and 2 and simian immunodeficiency virus is believed to play a role in permitting entry of the viral core into the nucleus of nondividing ce lls. A second role for Vpr was recently suggested by Rogel et al, (M. E. Rogel, L. I. Wu, and M. Emerman, J. Virol, 69:882-888, 1995), who s howed that Vpr prevents the establishment in vitro of chronically infe cted HIV producer cell lines, apparently by causing infected cells to arrest in the G(2)/M phase of the cell cycle. In cycling cells, progre ssion from G(2) to M phase is driven by activation of the p34(cdc2)/cy clin B complex, an event caused; in part, by dephosphorylation of two regulatory amino acids of p34(cdc)2 (Thr-14 and Tyr-15). We show here that Vpr arrests the cell cycle in G(2) by preventing the activation o f the p34(cdc2)/cyclin B complex. Vpr expression in cells caused p34(c dc2) to remain in the phosphorylated, inactive state, p34(cdc2)/cyclin B complexes immunoprecipitated from cells expressing Vpr were almost completely inactive in a histone H1 kinase assay. Coexpression of a co nstitutively active mutant p34(cdc2) molecule with Vpr relieved the G( 2) arrest. These findings strongly suggest that Vpr arrests cells in G (2) by preventing the activation of the p34(cdc2)/cyclin B complex tha t is required for entry into M phase. In vivo, Vpr might, by preventin g p34(cdc2) activation, delay or prevent apoptosis of infected cells. This would increase the amount of virus each infected cell produced.