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Molecular and Cellular Biology, September 1999, p. 6029-6040, Vol. 19, No. 9
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Functional Analysis of the Yeast Glc7-Binding
Protein Reg1 Identifies a Protein Phosphatase Type 1-Binding Motif as
Essential for Repression of ADH2 Expression
Kenneth M.
Dombek,*
Valentina
Voronkova,
Alexa
Raney, and
Elton T.
Young
Department of Biochemistry, University of
Washington, Seattle, Washington 98195-7350
Received 22 March 1999/Returned for modification 13 May
1999/Accepted 17 June 1999
In Saccharomyces cerevisiae, the protein phosphatase
type 1 (PP1)-binding protein Reg1 is required to maintain complete
repression of ADH2 expression during growth on glucose.
Surprisingly, however, mutant forms of the yeast PP1 homologue Glc7,
which are unable to repress expression of another glucose-regulated
gene, SUC2, fully repressed ADH2. Constitutive
ADH2 expression in reg1 mutant cells did
require Snf1 protein kinase activity like constitutive SUC2
expression and was inhibited by unregulated cyclic AMP-dependent protein kinase activity like ADH2 expression in derepressed
cells. To further elucidate the functional role of Reg1 in repressing ADH2 expression, deletions scanning the entire length of
the protein were analyzed. Only the central region of the protein
containing the putative PP1-binding sequence RHIHF was found to be
indispensable for repression. Introduction of the I466M F468A
substitutions into this sequence rendered Reg1 almost nonfunctional.
Deletion of the central region or the double substitution prevented
Reg1 from significantly interacting with Glc7 in two-hybrid analyses. Previous experimental evidence had indicated that Reg1 might target Glc7 to nuclear substrates such as the Snf1 kinase complex. Subcellular localization of a fully functional Reg1-green fluorescent protein fusion, however, indicated that Reg1 is cytoplasmic and excluded from
the nucleus independently of the carbon source. When the level of Adr1
was modestly elevated, ADH2 expression was no longer fully
repressed in glc7 mutant cells, providing the first direct evidence that Glc7 can repress ADH2 expression. These
results suggest that the Reg1-Glc7 phosphatase is a cytoplasmic
component of the machinery responsible for returning Snf1 kinase
activity to its basal level and reestablishing glucose repression. This implies that the activated form of the Snf1 kinase complex must cycle
between the nucleus and the cytoplasm.
*
Corresponding author. Mailing address: Department of
Biochemistry, Box 357350, University of Washington, Seattle, WA
98195-7350. Phone: (206) 543-6035. Fax: (206) 685-9144. E-mail:
kmd{at}u.washington.edu.
Molecular and Cellular Biology, September 1999, p. 6029-6040, Vol. 19, No. 9
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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