Hepatitis B virus X protein (HBX) was studied for its capacity to form
a specific complex with the retinoblastoma tumour suppressor protein
(pRB), and for its effect on the expression of pRB. HEX was synthesize
d by in vitro transcription and translation in the presence of [S-35]m
ethionine. The synthesized HEX was assayed for its binding to a glutat
hione-S-transferase (GST)-pRB fusion protein bound to Sepharose beads.
The in vivo binding was investigated by a co-immunoprecipitation and
Western blot analysis of the cell extract from a CMV-HBX-transfected h
epatoblastoma cell line, Hep G2 cells. These experiments demonstrated
that HEX was unable to form a detectable complex with pRB. However, th
e level of pRB increased considerably in Hep G2 cells transfected with
CMV-HBX clone. The alteration of pRB expression by HEX could be a mec
hanism, contributing to the development of hepatocellular carcinoma (H
CC) in human.