Parvovirus-Induced Depletion of Cyclin B1 Prevents Mitotic Entry of Infected Cells

Parvoviruses halt cell cycle progression following initiation of their replication during S-phase and continue to replicate their genomes for extended periods of time in arrested cells. The parvovirus minute virus of mice (MVM) induces a DNA damage response that is required for viral replication and induction of the S/G2 cell cycle block. However, p21 and Chk1, major effectors typically associated with S-phase and G2-phase cell cycle arrest in response to diverse DNA damage stimuli, are either down-regulated, or inactivated, respectively, during MVM infection. This suggested that parvoviruses can modulate cell cycle progression by another mechanism. In this work we show that the MVM-induced, p21- and Chk1-independent, cell cycle block proceeds via a two-step process unlike that seen in response to other DNA-damaging agents or virus infections. MVM infection induced Chk2 activation early in infection which led to a transient S-phase block associated with proteasome-mediated CDC25A degradation. This step was necessary for efficient viral replication; however, Chk2 activation and CDC25A loss were not sufficient to keep infected cells in the sustained G2-arrested state which characterizes this infection. Rather, although the phosphorylation of CDK1 that normally inhibits entry into mitosis was lost, the MVM induced DDR resulted first in a targeted mis-localization and then significant depletion of cyclin B1, thus directly inhibiting cyclin B1-CDK1 complex function and preventing mitotic entry. MVM infection thus uses a novel strategy to ensure a pseudo S-phase, pre-mitotic, nuclear environment for sustained viral replication. Author Summary DNA viruses induce cellular DNA damage responses that can present a block to infection that must be overcome, or alternatively, can be utilized to viral advantage. Parvoviruses, the only known viruses of vertebrates that contain single-stranded linear DNA genomes, induce a robust DNA damage response (DDR) that features a cell cycle arrest that facilitates their replication. We show that the autonomous parvovirus MVM-induced cell cycle arrest is caused by a novel two-step mechanism that ensures a pseudo S phase, pre-mitotic, nuclear environment for sustained viral replication. A feature of this arrest is virally-induced depletion of the critical cell cycle regulator cyclin B1. Parvoviruses are important infectious agents that infect many vertebrate species including humans, and our study makes an important contribution to how these viruses achieve productive infection in host cells.