title: Biochemistry and functions of hepatitis B virus X protein.
abstract: Hepatitis B virus X gene codes for a small basic cytoplasmic protein and is able to transactivate viral and cellular genes, although X protein exhibits no DNA-binding activity. The mechanism of transactivation by X protein has been suggested to be via protein-protein interaction(s). X protein had amino acid sequences homologous to the functionally essential domain of Kunitz-type serine protease inhibitors, and these sequences were indispensable for transactivation function. X protein activated X-gene transcription itself and an X-responsive element were localized in their minimal promoter. Furthermore, tumor suppressor gene product p53, but not mutant p53, repressed X-gene transcription from the minimal promoter, indicating that X protein disrupts the function of normal p53, which represses transcription of X gene or cellular gene. Data suggest that inhibition of a hepatic serine protease by X protein leads to eliminate the suppressor effect of p53 on the basic transcription machinery in nucleus.
authors.affiliation: Department of Gene Research, Cancer Institute, Tokyo, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Koike
authors.forename: S
authors.initials: S
authors.lastname: Takada
mesh_terms: D000595:Amino Acid Sequence; D015967:Gene Expression Regulation, Viral; D016158:Genes, p53; D006515:Hepatitis B virus / Q000737:chemistry / Q000235:genetics; D008969:Molecular Sequence Data; D009154:Mutation; D011401:Promoter Regions, Genetic; D015842:Serine Proteinase Inhibitors / Q000737:chemistry / Q000502:physiology; D015534:Trans-Activators / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D014158:Transcription, Genetic; D054334:Viral Regulatory and Accessory Proteins
chemical_list: D015842:Serine Proteinase Inhibitors; D015534:Trans-Activators; D054334:Viral Regulatory and Accessory Proteins; C057628:hepatitis B virus X protein