Conditional mutations in the mitotic chromosome binding function of the bovine papillomavirus type 1 E2 protein.

The papillomavirus E2 protein is required for viral transcriptional regulation, DNA replication and genome segregation. We have previously shown that the E2 transactivator protein and BPV1 genomes are associated with mitotic chromosomes; E2 links the genomes to cellular chromosomes to ensure efficient segregation to daughter nuclei. The transactivation domain of the E2 protein is necessary and sufficient for association of the E2 protein with mitotic chromosomes. To determine which residues of this 200-amino-acid domain are important for chromosomal interaction, E2 proteins with amino acid substitutions in each conserved residue of the transactivation domain were tested for their ability to associate with mitotic chromosomes. Chromatin binding was assessed by using immunofluorescence on both spread and directly fixed mitotic chromosomes. E2 proteins defective in the transactivation and replication functions were unable to associate with chromosomes, and those that were competent in these functions were attached to mitotic chromosomes. However, several mutated proteins that were defective for chromosomal interaction could associate with chromosomes after treatment with agents that promote protein folding or when cells were incubated at lower temperatures. These results indicate that precise folding of the E2 transactivation domain is crucial for its interaction with mitotic chromosomes and that this association can be modulated.