IDENTIFICATION OF DOMAINS REQUIRED FOR TRANSCRIPTIONAL ACTIVATION ANDPROTEIN DIMERIZATION IN THE HUMAN PAPILLOMAVIRUS TYPE-16 E7 PROTEIN

To analyse the potential of the E7 oncogene of HPV-16 to activate tran scription, we constructed hybrid proteins containing various portions of the HPV-16 E7 protein fused to the DNA binding region of the bacter ial LexA repressor. We found that full length HPV-16 E7 is capable to mediate activation of two different reporter genes, which carry LexA b inding sites in their promoters. In contrast, E7 from HPV-11, a low-ri sk type papillomavirus, was unable to activate transcription, when ana lysed in the same assay. Mutations in the transforming domains of HPV- 16 E7 did not affect the ability of the protein to activate transcript ion, indicating that it represents a novel function of the oncoprotein , which is not sensitive to any known inactivating mutations. Analysis of E7 subdomains revealed that the N-terminal part of HPV-16 E7 retai ns the capacity to activate transcription. A second trans-activation d omain is located in the C-terminal part of E7; however, in the context of the full length E7 protein this activity is blocked by an adjacent domain. These results reveal a second pathway for transcriptional act ivation by HPV-16 E7, independent of its interaction with pRB-E2F comp lexes. Using the E7-LexA hybrid proteins, it is shown that E7 can form homodimers and this property involves a zinc finger structure in the C-terminal part of the protein, partially overlapping with the domain that negatively regulates transcriptional activation by E7.