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Mol. Cell. Biol., Apr 1996, 1824-1831, Vol 16, No. 4
S Malik and SK Karathanasis
The orphan nuclear receptor hepatocyte nuclear factor 4 (HNF-4) is required
for development and maintenance of the liver phenotype. HNF-4 activates
several hepatocyte-specific genes, including the gene encoding
apolipoprotein AI (apoAI), the major protein component of plasma
high-density lipoprotein. The apoAI gene is activated by HNF-4 through a
nuclear receptor binding element (site A) located in its liver-specific
enhancer. To decipher the mechanism whereby HNF-4 enhances apoAI gene
transcription, we have reconstituted its activity in a cell-free system.
Functional HNF-4 was purified to homogeneity from a bacterial expression
system. In in vitro transcription assays employing nuclear extract from
HeLa cells, which do not contain HNF-4, recombinant HNF-4 stimulated
transcription from basal promoters linked to site A. Activation by HNF-4
did not exhibit a ligand requirement, but phosphorylation of HNF-4 in the
in vitro transcription system was observed. The activation function of
HNF-4 was localized to a domain displaying strong homology to the conserved
AF-2 region of nuclear receptors. Dissection of the transcription cycle
revealed that HNF-4 activated transcription by facilitating assembly of a
preinitiation complex intermediate consisting of TBP, the TATA box-binding
protein component of TFIID and TFIID, via direct physical interactions with
TFIIB. However, recruitment of TFIIB by HNF-4 was not sufficient for
activation, since HNF-4 deletion derivatives lacking AF-2 bound TFIIB. On
the basis of these results, HNF-4 appears to activate transcription at two
distinct levels. The first step involves AF-2-independent recruitment of
TFIIB to the promoter complex; the second step is AF-2 dependent and
entails entry of preinitiation complex components acting downstream of
TFIIB.
Copyright © 1996, American Society for Microbiology
TFIIB-directed transcriptional activation by the orphan nuclear receptor hepatocyte nuclear factor 4
Department of Cardiovascular Molecular Biology, Lederle Laboratories, Pearl River, New York 10965, USA.
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