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

Abstract

ABSTRACT 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 2 cell cycle arrest. Here we show that unlike many other G 2 arrest mechanisms, 16E1 ∧ E4 does not inhibit the kinase activity of the Cdk1/cyclin B1 complex. Instead, 16E1 ∧ 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 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 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.