Ambient pH signaling regulates nuclear localization of the Aspergillus nidulans PacC transcription factor

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 beta-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.