Human RPB5, a subunit shared by eukaryotic nuclear RNA polymerases, binds human hepatitis B virus X protein and may play a role in X transactivation.

Abstract

The X gene of human hepatitis B virus encodes the polypeptide HBx which transactivates viral and host genes through a variety of cis‐acting enhancer elements present in RNA polymerases I, II and III promoters. To better understand the mechanism of X transactivation, we cloned cDNAs of proteins that bind HBx. Here we demonstrate that one of these cDNAs is a full‐length cDNA of human RPB5, a subunit shared by RNA polymerases. The HBx transactivation domain and the central region of human RPB5 were necessary for the specific binding of the two proteins as shown by: (i) in vitro assays using deletion mutants of fusion proteins; (ii) in vivo assays which detect associated proteins by co‐immunoprecipitation of the non‐fused proteins from transfected HepG2 cells. Over‐expressed HBx seemed to associate with assembled forms of endogenous human RPB5 in HBx‐transfected cells, since the endogenous RPB5 co‐immunoprecipitated with HBx. The HBx binding region of human RPB5 by itself stimulated chloramphenicol acetyltransferase activities from several different reporters having X‐responsive element(s). Our results support the idea that the interaction of HBx and human RPB5 can facilitate HBx transactivation and that human RPB5 has a domain which can communicate with transcriptional regulators.

Activated ras oncogene collaborates with HBx gene of hepatitis B virus to transform cells by suppressing HBx-mediated apoptosis

“…HBx does not directly bind to DNA and may stimulate transcription by interacting with transcription factors or with the basal transcription machinery of host RNA polymerase II and III (Cheong et al, 1995;Haviv et al, 1998;Maguire et al, 1991;Qadri et al, 1996;Williams and Andrisani, 1995). Besides its nuclear function as a transcriptional transactivator, several studies indicated that HBx in¯uences cellular signaling pathways in the cytoplasm as well, which is a function consistent with the predominant cytoplasmic localization of HBx in vivo and in most experimental systems (Benn and Schneider, 1994;Natoli et al, 1994).…”

UVDDB p127-binding modulates activities and intracellular distribution of Hepatitis B virus X protein

“…Similarly, the cellular pathways targeted by the viral protein to achieve transcriptional transactivation are a matter of debate. Indeed, in several studies, X has been reported to directly act on the transcriptional machinery (Cheong et al, 1995;Haviv et al, 1996;Lin et al, 1998;Maguire et al, 1991). In others, X appears to act far more upstream, by modulating the signal transduction cascade in the cytoplasm (Benn et al, 1996;Chirillo et al, 1996;Henkler et al, 1998;Kekule et al, 1993;Klein and Schneider, 1997;Lee and Yun, 1998;Su and Schneider, 1996).…”

p53-independent apoptotic effects of the hepatitis B virus HBx protein in vivo and in vitro

“…The 16.5 kDa HBx protein is a multifunctional transactivator which up-regulates a variety of viral and cellular genes through protein ± protein interactions. In the nucleus, HBx may function as a co-activator by interacting with elements of the transcription machinery (Cheong et al, 1995;Haviv et al, 1998;Qadri et al, 1995), and from cytoplasmic locations by activating Ras/Raf signaling and NF-kB (Benn and Schneider, 1994;Cross et al, 1993;Natoli et al, 1994;Su and Schneider, 1996).…”