POP2 protein
(Pop2p) of S. cerevisiae is thought to be a component of
protein complex which regulates transcription of glucose-repressible
genes. We found that the Pop2p was a phospho-protein and the
phosphorylation state was regulated by the environmental glucose. The
Pop2p is phosphorylated at the 97th threoneine residue within two
minutes by glucose removal and dephosphorylated within one minute by
readdtion of glucose. We think that the phosphorylation of the Pop2p
is one of the earliest events in the signal transduction pathway of
the glucose sensing in yeast (submitted). Here we present biochemical
and genetic evidences that Yak1p is a major kinase which
phosphorylates the Pop2p in the glucose depletion condition. Using a
synthetic peptide containing the phosphorylation site of Pop2p as a
substrate, we purified the Pop2p peptide kinase near homogeneity and
identified as Yak1p. YAK1 encodes a protein kinase which
antagonizes Ras-PKA pathway. The Pop2 peptide kinase activity in
vitro was abolished in the yak1 null mutant strain, and the
phosphorylation of Pop2p in the yak1 strain was not
observed. Pull-down experiment showed that Yak1p interacted with Bmh1p
(nuclear exclusion molecule) only in the presence of glucose. We
confirmed that GFP-Yak1p fusion protein rapidly localized to nuclei
upon glucose removal. Our results suggest that YAK1 kinase is a
component of the glucose-sensing pathway in yeast.
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