pX, the HBV-encoded coactivator, suppresses the phenotypes of TBP and TAF(II)250 mutants

Hepatitis B virus (HBV) infects humans and causes a wide range of clinical manifestations, from acute hepatitis to hepatocellular carcinoma (HCC). The HBV genome contains multiple promoters with gene expression regulated predominantly by the cellular transcription initiation machinery. Accordingly, the HBV-encoded pX, the only known vital regulator, is a potent transcription coactivator. We investigated the relationship between pX and cellular coactivators. We show that pX restores wild-type activity to inactive TBP(AS) mutants with poor TAF(II)250 and activator-binding activity. This pX-mediated recovery, however, is not obtained with inactive TBP(AS) mutants in binding of other general transcription factors. Remarkably, ts13, a cell line temperature sensitive for TAF(II)250 function, exhibiting growth arrest and apoptosis at the restrictive temperature, is rescued partially by pX expression, thus generating a pX-dependent cell growth. Collectively, our results suggest that pX suppresses some of the phenotypes of TBP and TAF(II)250 mutations, implying that pX circumvents the need for a holo-TFIID complex for transcription activation to proceed.