p300/CBP Trascriptional co-activators: A Common Cellular Target of HPV E6 and E7 Oncoproteins

E6 and E7 are the two major oncoproteins encoded by the human papillomaviruses (HPV). During HPV infection they need to play multiple roles, interfering with several cellular pathways in order to create a favourable environment for viral replication, by neutralising the cellular surveillance controls that are turned on as a result of unscheduled DNA synthesis. These requirements are more difficult for the high-risk HPV types, which begin to replicate their genomes in differentiating cells, and as a result, the activities of the E6 and E7 proteins have more pronounced effects on their cellular target proteins. In this thesis, the data demonstrating that the p300 transcriptional co-activator is a common target of E6 and E7 proteins, will be presented. From these studies, it seems likely that both viral proteins, although binding within the same domains of p300, target apparently different functions of p300. In the first part of the thesis, the interaction between the E6 proteins from different PV types has been analyzed. All PV E6 tested (both human and animal in origin) showed similar levels of interaction with p300, suggesting that this interaction plays a fundamental role in the PV life cycle and is a common requirement for viral replication. However, only high-risk mucosal HPV E6s were able to restore the function of an Ad E1a mutant, deficient in p300 binding and transformation in BRK transformation assays. It can be speculated that transcriptional inhibition of p300 is a prerequisite for cellular transformation mediated by high-risk mucosal E6s, hence only high-risk E6s were able to repress p300 transcriptional activity, while low-risk E6s appeared to have only marginal effects (PATEL et al. 1999; ZIMMERMANN et al. 1999). In the second part of the study, the interaction between and p300 was also investigated. Both low- and high-risk E7 proteins showed significant levels of interaction with p300. As a consequence of this interaction, the high-risk E7 protein was able to block the p300 co-activation function. There are at least two mechanisms involved in this repression: first - inhibition of protein-protein interactions and second - inhibition of HAT activity of p300. Whilst the first one allows the competition between the E7 protein and different cellular proteins for the limiting amounts of p300 within cell, the second allows E7 to manipulate the acetylation state of histones, which may lead to the subsequent alteration of chromatin structure and perturbation in gene expression. In addition, may also block the acetylation of other target proteins of p300. Interaction of both E6 and E7 with a common cellular target, the p300 transcriptional co-activator, may thus provide a means for interfering with the control of key biological processes, such as cell cycle progression, differentiation and apoptosis.