Infection with the hepatitis B virus has been identified as one of the major causes of liver cancer. A large body of experimental work points to a central role for the virally encoded protein HBx in this form of carcinogensis. HBx is expressed in HBV-infected liver cells and interacts with a wide range of cellular proteins, thereby interfering in cellular processes including cell signaling, cycle regulation and apoptosis. In order to identify possible new targets of the HBx protein, we performed a yeast two-hybrid screen using a truncated protein mini-HBx(18-142) as the bait. In addition to known interacting partners, such as RXR and UVDDB 1, we identified several new candidates including the human transcriptional regulatory protein p120E4F, which has been implicated in the regulation of mitosis and the cell cycle. In vitro pull down experiments confirmed the interaction and transcription activation assays in the yeast demonstrated that HBx protein was able to repress GAL4AD-p120E4F-dependent activation of a reporter gene under the control of E4F binding sites found in the adenovirus E4 promoter and the HBV enhancer II region. We also showed that the cysteine residues in HBx are necessary for its interaction with UVDDB1 but not for the interaction with RXR or p120E4F. The possible functional relevance of the interaction between HBx and E4F proteins is discussed in the contexts of cellular transformation and host-virus co-evolution. (c) 2005 Elsevier B.V. All rights reserved.11513142