Functional interaction between the bovine papillomavirus virus type 1 replicative helicase E1 and cyclin E-Cdk2

We have found that the replicative helicase El of bovine papillomavirus type 1 (BPV-1) interacts with a key cell cycle regulator of S phase, the cyclin E-Cdk2 kinase. The El helicase, which interacts with cyclin E and not with Cdk2, presents the highest affinity for catalytically active kinase complexes. In addition, El, cyclin E, and Cdk2 expressed in Xenopus egg extracts are quantitatively coimmunoprecipitated from crude extracts by either anti-Cdk2 or anti-E1 antibodies. El protein is also a substrate of the cyclin E-Cdk2 kinase in vitro. Using the viral components required for in vitro BPV-1 replication and free-membrane cytosol from Xenopus eggs, we show that efficient replication of BPV plasmids is dependent on the addition of El-cyclin E-Cdk2 complexes. Thus, the BPV initiator of replication and cyclin E-Cdk2 are likely to function together as a protein complex which may be the key to the cell cycle regulation of papillomavirus replication.