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Molecular and Cellular Biology, March 2001, p. 1688-1699, Vol. 21, No. 5
Centro de Investigaciones Biológicas
CSIC, Madrid 28006, Spain
Received 10 August 2000/Returned for modification 11 October
2000/Accepted 7 December 2000
The Aspergillus nidulans zinc finger transcription
factor PacC is activated by proteolytic processing in response to
ambient alkaline pH. The pH-regulated step is the transition of
full-length PacC from a closed to an open, protease-accessible
conformation. Here we show that in the absence of ambient pH signaling,
the C-terminal negative-acting domain prevents the nuclear localization of full-length closed PacC. In contrast, the processed PacC form is
almost exclusively nuclear at any ambient pH. In the presence of
ambient pH signaling, the fraction of PacC that is in the open conformation but has not yet been processed localizes to the nucleus. Therefore, ambient alkaline pH leads to an increase in nuclear PacC by
promoting the proteolytic elimination of the negative-acting domain to
yield the processed form and by increasing the proportion of
full-length protein that is in the open conformation. These findings
explain why mutations resulting in commitment of PacC to processing
irrespective of ambient pH lead to permanent PacC activation and
alkalinity mimicry. A nuclear import signal that targets
Escherichia coli
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.5.1688-1699.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Ambient pH Signaling Regulates Nuclear Localization
of the Aspergillus nidulans PacC Transcription
Factor

-galactosidase to the nucleus has
been located to the PacC zinc finger region. A mutation abolishing DNA
binding does not prevent nuclear localization of the processed form,
showing that PacC processing does not lead to nuclear localization by
passive diffusion of the protein made possible by the reduction in
size, followed by retention in the nucleus after DNA binding.
*
Corresponding author. Mailing address: Centro de
Investigaciones Biológicas CSIC, Velázquez 144, Madrid
28006, Spain. Phone: 34 91 5644562, ext. 4358. Fax: 34 91 5627518. E-mail: penalva{at}cib.csic.es.
Present address: Zentrum für Molekulare Biologie der
Universität Heidelberg, 69120 Heidelberg, Germany.
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