Human immunodeficiency virus type 1 viral protein R (Vpr) arrests cells in the G₂ 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 cells. 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 showed that Vpr prevents the establishment in vitro of chronically infected HIV producer cell lines, apparently by causing infected cells to arrest in the G₂/M phase of the cell cycle. In cycling cells, progression from G₂ to M phase is driven by activation of the p34^cdc2/cyclin B complex, an event caused, in part, by dephosphorylation of two regulatory amino acids of p34^cdc2 (Thr-14 and Tyr-15). We show here that Vpr arrests the cell cycle in G₂ by preventing the activation of the p34^cdc2/cyclin B complex. Vpr expression in cells caused p34^cdc2 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 constitutively active mutant p34^cdc2 molecule with Vpr relieved the G₂ arrest. These findings strongly suggest that Vpr arrests cells in G₂ by preventing the activation of the p34^cdc2/cyclin B complex that is required for entry into M phase. In vivo, Vpr might, by preventing p34^cdc2 activation, delay or prevent apoptosis of infected cells. This would increase the amount of virus each infected cell produced.