Bovine papillomavirus E1 protein binds specifically DNA polymerase alpha but not replication protein A

Extracts prepared from either mouse cells or monkey cells were examined for the ability to support in vitro bovine papillomavirus type 1 (BPV1) DNA replication, and they were used in parallel as a source of host replication proteins for affinity chromatography. DNA synthesis exhibited an absolute requirement for BPV1 E1 protein. In contrast to previous observations, we found that low levels of E1 were highly efficient in initiating DNA replication in the absence of the BPV1 transcription factor E2. Surprisingly, COS-1 cell extract allowed a high rate of BPV1 DNA replication, supporting an efficient production of mature circular DNA molecules, whereas in mouse cell extracts, the replication products mostly consisted of replicative intermediates. Submitting the extracts to affinity chromatography allowed specific binding of DNA polymerase alpha-primase to E1 protein, up to a total depletion of the extract, regardless of the origin of the cell extract. Furthermore, replication protein A was not retained on E1 affinity columns, even when E2 was complexed with E1. These data confirm that the interactions between E1 and DNA polymerase alpha-primase do not exhibit cell-type specificity, as had already been suggested by data from in vivo and in vitro replication assays, but they imply that other cellular proteins may affect the level of E1-dependent replication.