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Molecular and Cellular Biology, August 1999, p. 5811-5822, Vol. 19, No. 8
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Multiple Roles of Ligand in Transforming the Dioxin
Receptor to an Active Basic Helix-Loop-Helix/PAS Transcription
Factor Complex with the Nuclear Protein Arnt
Michael J.
Lees and
Murray L.
Whitelaw*
Department of Biochemistry, University of
Adelaide, Adelaide 5005, South Australia, Australia
Received 2 November 1998/Returned for modification 7 December
1998/Accepted 28 April 1999
The dioxin receptor is a ligand-activated transcription factor
belonging to an emerging class of basic helix-loop-helix/PAS proteins
which show interaction with the molecular chaperone hsp90 in their
latent states and require heterodimerization with a general cofactor,
Arnt, to form active DNA binding complexes. Upon binding of polycyclic
aromatic hydrocarbons typified by dioxin, the dioxin receptor
translocates from the cytoplasm to the nucleus to allow interaction
with Arnt. Here we have bypassed the nuclear translocation step by
creating a cell line which expresses a constitutively nuclear dioxin
receptor, which we find remains in a latent form, demonstrating that
ligand has functional roles beyond initiating nuclear import of the
receptor. Treatment of the nuclear receptor with dioxin induces
dimerization with Arnt to form an active transcription factor complex,
while in stark contrast, treatment with the hsp90 ligand geldanamycin
results in rapid degradation of the receptor. Inhibition of degradation
by a proteasome inhibitor allowed geldanamycin to transform the nuclear
dioxin receptor to a heterodimer with Arnt (DR-Arnt). Our results
indicate that unchaperoned dioxin receptor is extremely labile and is
consistent with a concerted nuclear mechanism for receptor activation
whereby hsp90 is released from the ligand-bound dioxin receptor
concomitant with Arnt dimerization. Strikingly, artificial
transformation of the receptor by geldanamycin provided a DR-Arnt
complex capable of binding DNA but incapable of stimulating
transcription. Limited proteolysis of DR-Arnt heterodimers indicated
different conformations for dioxin versus geldanamycin-transformed receptors. Our studies of intracellular dioxin receptor transformation indicate that ligand plays multiple mechanistic roles during receptor activation, being important for nuclear translocation, transformation to an Arnt heterodimer, and maintenance of a structural integrity key
for transcriptional activation.
*
Corresponding author. Mailing address: Biochemistry,
University of Adelaide, Adelaide 5005, South Australia, Australia.
Phone: 61 (8) 8303-4724. Fax: 61 (8) 8303-4348. E-mail:
murray.whitelaw{at}adelaide.edu.au.
Molecular and Cellular Biology, August 1999, p. 5811-5822, Vol. 19, No. 8
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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