Replication of bovine papillomavirus type 1 (BPV-1) DNA has been shown to require two viral proteins known to interact in a molecular complex: E2, a transcription activator, and E1, another nuclear phosphoprotein, which binds to the replication origin and for which helicase/ATPase activities have previously been reported. Here we characterize the BPV-1 E1 ATPase activity. In contrast to Seo (, 90, 702–706, 1993), we were able to detect this activity in the absence of nucleic acid in partially purified preparations of either E1 protein or of E1–E2 protein complex. Measurements of specific activity and kinetic parameters gave similar values for preparations of various kinds. ATPase activity was quantitatively retained by immunoprecipitates obtained by using anti-E1 or, in the case of E1–E2 complex, anti-E2 antibodies. Significantly, preparations of bacterially expressed glutathione -transferase-E1 fusion protein exhibited levels of DNA-independent ATPase activity comparable to those of baculovirus-expressed E1. The presence of nucleic acids of various types, including stoichiometric amounts of a BPV-1 DNA fragment containing E1 and E2 binding sites, did not grossly affect E1 ATPase activity, the most notable effect being a 2-fold stimulation by unspecific ssDNA. Altogether, our results indicate that BPV-1 E1 possesses an intrinsic ATPase activity which does not depend on the presence of nucleic acid; moreover, they render unlikely any modulation of E1 ATPase activity due to binding either E2 protein or target DNA sequences, or as a result of protein phosphorylation.


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