Viral protein R (Vpr) of human immunodeficiency virus type 1 induces G2 arrest in cells from distantly related eukaryotes including human and fission yeast through inhibitory phosphorylation of tyrosine 15 (Tyr15) on Cdc2. Since the DNA damage and DNA replication checkpoints also induce G2 arrest through phosphorylation of Tyr15, it seemed possible that Vpr induces G2 arrest through the checkpoint pathways. However, Vpr does not use either the early or the late checkpoint genes that are required for G2 arrest in response to DNA damage or inhibition of DNA synthesis indicating that Vpr induces G2 arrest by an alternative pathway. It was found that protein phosphatase 2A (PP2A) plays an important role in the induction of G2 arrest by Vpr since mutations in genes coding for a regulatory or catalytic subunit of PP2A reduce Vpr-induced G2 arrest. Vpr was also found to upregulate PP2A, supporting a model in which Vpr activates the PP2A holoenzyme to induce G2 arrest. PP2A is known to interact genetically in fission yeast with the Wee1 kinase and Cdc25 phosphatase that act on Tyr15 of Cdc2. Both Wee1 and Cdc25 play a role in Vpr-induced G2 arrest since a wee1 deletion reduces Vpr-induced G2 arrest and a direct in vivo assay shows that Vpr inhibits Cdc25. Additional support for both Wee1 and Cdc25 playing a role in Vpr-induced G2 arrest comes from a genetic screen, which identified genes whose overexpression affects Vpr-induced G2 arrest. For this genetic screen, a strain was constructed in which cell killing by Vpr was nearly eliminated while the effect of Vpr on the cell cycle was clearly indicated by an increase in cell length. Overexpression of the wos2 gene, an inhibitor of Wee1, suppresses Vpr-induced G2 arrest while overexpression of rad25, an inhibitor of Cdc25, enhances Vpr-induced G2 arrest. These two genes may be part of the uncharacterized pathway for Vpr-induced G2 arrest in which Vpr upregulates PP2A to activate Wee1 and inhibit Cdc25.
Copyright 2001 Academic Press.