Bovine papilloma virus-transformed cells contain multiple E2 proteins.

Genetic evidence suggests that the bovine papilloma virus type 1 (BPV) E2 open reading frame may encode at least two gene products involved in the regulation of viral gene expression. One, which is probably the full-length product, trans-activates transcription via an enhancer in the viral regulatory region. A second, containing sequences from the 3' end of the open reading frame, inhibits the trans-activating activity of the first product. We now report the identification and initial characterization of three E2-encoded proteins, with mobilities corresponding to 48, 31, and 28 kDa in cells transformed by the wild-type BPV. Pulse-chase experiments indicated that the 48-kDa protein had the longest half-life (40 min), but there was no indication that one species was the precursor of another. The 48-kDa species corresponds to the full-length trans-activating protein. The two smaller species contain only carboxyl-terminal determinants, and either or both could represent inhibitory E2 proteins. Subcellular fractionation localized all three E2 proteins to the nucleus. Consistent with the low rate of viral transcription in BPV-transformed cells, the 31-kDa presumptive repressor species was more abundant than the 48-kDa species.