Myxoma virus M-T5 protects infected cells from the stress of cell cycle arrest through its interaction with host cell cullin-1

The myxoma virus (MV) M-T5 gene encodes an ankyrin repeat protein that is important for virus replication in cells from several species. Insight was gained into the molecular mechanisms underlying the role of M-T5 as a host range determinant when the cell cycle regulatory protein cullin-1 (cul-1) was identified as a cellular binding partner of M-T5 and found to colocalize with the protein in both nuclear and cytosolic compartments. Consistent with this interaction, infection with wild-type MV (vMyxlac) or a deletion mutant lacking M-T5 (vMyxT5KO) differentially altered cell cycle progression in a panel of permissive and nonpermissive cells. Cells infected with vMyxlac transitioned rapidly out of the G₀G₁ phase and preferentially accumulated at the G₂/M checkpoint, whereas infection with vMyxT5KO impeded progression through the cell cycle, resulting in a greater percentage of cells retained at G₀/G₁. Levels of the cul-1 substrate, p27/Kip-1, were selectively increased in cells infected with vMyxT5KO compared to vMyxlac, concurrent with decreased phosphorylation of p27/Kip-1 at Thr187 and decreased ubiquitination. Compared to cells infected with vMyxlac, cell death was increased in vMyxT5KO-infected cells following treatment with diverse stimuli known to induce cell cycle arrest, including infection itself, serum deprivation, and exposure to proteasome inhibitors or double-stranded RNA. Moreover, infection with vMyxlac, but not vMyxT5KO, was sufficient to overcome the G₀/G₁ arrest induced by these stimuli. These findings suggest that M-T5 regulates cell cycle progression at the G₀/G ₁ checkpoint, thereby protecting infected cells from diverse innate host antiviral responses normally triggered by G₀/G₁ cell cycle arrest.