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J Virol, March 1998, p. 1737-1743, Vol. 72, No. 3
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Cloning and Characterization of a Novel Hepatitis B
Virus x Binding Protein That Inhibits Viral Replication
Margherita
Melegari,
Pier
Paolo
Scaglioni, and
Jack R.
Wands*
Molecular Hepatology Laboratory, Massachusetts General
Hospital Cancer Center, and Harvard Medical School, Charlestown,
Massachusetts 02129
Received 29 July 1997/Accepted 21 November 1997
The hepatitis B virus and the mammalian hepadnavirus genomes encode
for a short open reading frame called x. Expression of the protein
product (HBx) appears necessary for establishment of natural infection.
However, in vitro studies have suggested a multifunctional role for HBx
as an indirect transcriptional transactivator of a variety of different
viral and cellular promoters. Indeed, HBx has no known direct DNA
binding properties but may interact with transcription factors as well
as activate intracellular signaling pathways associated with cell
growth. To further address the possible functional role of HBx in the
life cycle of hepatitis B virus, we performed an analysis using the
yeast two-hybrid system to screen a cDNA library derived from a
hepatocellular carcinoma cell line with a HBx fusion bait in an attempt
to identify cellular partners that may bind to and alter the biologic
properties of HBx. A HBx-interacting protein that specifically
complexes with the carboxy terminus of wild-type HBx was identified and
designated XIP. This 9.6-kDa protein is capable of binding to HBx in
vitro, and transient and stable expression in hepatocellular carcinoma cells abolishes the transactivation properties of HBx on luciferase constructs driven by AP-1 and endogenous hepatitis B virus
enhancer/promoter elements. Investigation of the role of XIP in
hepatitis B virus replication in differentiated hepatocellular
carcinoma cells revealed that XIP expression reduces wild-type
hepatitis B virus replication to levels observed following transfection
with an HBx-minus virus. In contrast, the replication levels of the
duck hepatitis B virus, a hepadnavirus that lacks the x open reading
frame, were unchanged in the context of XIP expression. We propose that
one of the physiologic functions of the cellular protein XIP is to
negatively regulate HBx activity and thus to alter the replication life
cycle of the virus.
*
Corresponding author. Mailing address: Molecular
Hepatology Laboratory, MGH Cancer Center, 149 13th St., 7th Floor,
Charlestown, MA 02129. Phone: (617) 726-5601. Fax: (617) 726-5609. E-mail: wands{at}helix.mgh.harvard.edu.
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