Cell cycle G2 arrest induced by HIV-1 Vpr in fission yeast (Schizosaccharomyces pombe) is independent of cell death and early genes in the DNA damage checkpoint

HIV-1 Vpr induces cell cycle G2 arrest, morphological changes and cell death in human and fission yeast cells. The cellular targets for G2 arrest were expected to be the inhibitory phosphorylation sites of Cdc2, as G2 arrest correlates with hyperphosphorylation and decreased activity of Cdc2 in both human and fission yeast cells. In this study, we present direct evidence of genetic suppression of Vpr-induced G2 arrest by cdc2 mutations. Mutations in cdc2 (cdc2-1w and cdc2-3w) reduce the ability of Vpr to induce G2 arrest. A strain with a mutation changing the Tyr15 of Cdc2 to the non-phosphorylated Phe (Y15F) eliminated Vpr-induced G2 arrest indicating that Tyr15 of Cdc2 is the sole target for induction of G2 arrest by Vpr. Although the G2 arrest induced by DNA damage also proceeds through phosphorylation of Tyr15, the rad1, rad3, rad9 and rad17 mutations, which eliminate the G2 checkpoint for DNA damage, did not block the G2 arrest induced by Vpr. Furthermore, Vpr expression did not alter sensitivity of these rad mutants to UV radiation. Thus, the pathways for the induction of G2 arrest by DNA damage and Vpr are not identical. Interestingly, Vpr still induces cell death and morphological changes in the Y15F Cdc2 strain indicating that G2 arrest is not required for morphological changes and cell death. This conclusion was further supported by the observation that mutations in Vpr, which have lost their ability to induce G2 arrest, retained the ability to kill cells.