Regulation of PHO81, a cyclin-dependent kinase inhibitor in
Saccharomyces cerevisiae .
Janine Knight, Lawrence Bergman
Molecular and Cellular Biology, MCP Hahnemann University, 2900 Queen
Lane, Philadelphia, PA 19129, USA
In yeast, the repressible acid phosphatase PHO5 is
transcriptionally repressed in high phosphate growth media and de-repressed in low phosphate. The Pho80p-Pho85p cyclin-cyclin dependent
kinase (CDK) complex acts as a negative regulator to block PHO5
expression when phosphate levels are high. The PHO81 gene encodes
an inhibitor of the Pho80p-PHO85p CDK complex. In high phosphate, Pho81p
is inactive, resulting in an active CDK complex and thus no expression
of PHO5. In low phosphate, Pho81p is an active inhibitor of the
Pho80p-Pho85p CDK complex, resulting in expression of the PHO5.
However, the mechanism of regulation of Pho81p activity is not known.
Recent studies in our laboratory have shown that Pho81p is
phosphorylated by the Pho80p-Pho85p CDK complex in-vitro. PHO81
encodes a 134 kD protein with 9 potential sites for phosphorylation by a
CDK complex. To determine the significance of phosphorylation, we have
used site-directed mutagenesis to alter these sites. The resulting
mutants were introduced into a yeast strain containing a deletion of the
PHO81 gene and the effect of the mutation on PHO5
expression was assayed. Results suggest that phosphorylation of
particular residues within Pho81p is crucial for its activity as a
cyclin-dependent kinase inhibitor. Current work is focused on
determining exactly how phosphorylation at these sites affects Pho81p
function. These studies will provide insight into regulation of cyclin-dependent kinase inhibitor activity.
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