Human Papillomavirus Type 16 E1/E4-Induced G sub(2) Arrest Is Associated with Cytoplasmic Retention of Active Cdk1/Cyclin B1 Complexes

Human papillomavirus type 16 (HPV16) can cause cervical cancer. Expression of the viral E1/E4 protein is lost during malignant progression, but in premalignant lesions, E1/E4 is abundant in cells supporting viral DNA amplification. Expression of 16E1/E4 in cell culture causes G sub(2) cell cycle arrest. Here we show that unlike many other G sub(2) arrest mechanisms, 16E1/E4 does not inhibit the kinase activity of the Cdk1/cyclin B1 complex. Instead, 16E1 super(∧ )E4 uses a novel mechanism in which it sequesters Cdk1/cyclin B1 onto the cytokeratin network. This prevents the accumulation of active Cdk1/cyclin B1 complexes in the nucleus and hence prevents mitosis. A mutant 16E1/E4 (T22A, T23A) which does not bind cyclin B1 or alter its intracellular location fails to induce G sub(2) arrest. The significance of these results is highlighted by the observation that in lesions induced by HPV16, there is evidence for Cdk1/cyclin B1 activity on the keratins of 16E1/E4-expressing cells. We hypothesize that E1/E4-induced G sub(2) arrest may play a role in creating an environment optimal for viral DNA replication and that loss of E1/E4 expression may contribute to malignant progression.