Yeast Ypk protein kinase acts at the downstream of TOR2 pathway.
Yoshiaki Kamada, Yoshinori Ohsumi
Dept. of Cell Biology, Natl. Inst. for Basic Biology, Myodaiji-cho, Okazaki, 444-8585, Japan
In the budding yeast Saccharomyces cerevisiae, TOR protein plays various roles in response to cellular nutrient conditions. TOR regulates transcription, translation, organization of actin cytoskelton, autophagy etc. However, in the budding yeast how TOR itself is regulated or how TOR transmits its signal to the downstream is still unclear. To identify a novel factor acting around TOR protein, we screened multicopy suppressor of tor2 mutant. As the result, we obtained YPK2. YPK2 is a member of AGC protein kinase family, which includes mammalian p70 S6K, which is known as a direct target of mammalian TOR (mTOR). The YPK2 cloned as a suppressor had N-terminally truncated, and, to our surprise, the full length YPK2 gene did not rescue the growth defect of the tor2 mutant. We found well-conserved motif-like sequence (we called this motif as yeast TOS-motif) near the truncated region. Point mutation in this motif conferred the full length YPK2 on ability to suppress tor2 mutant. We also performed protein kinase assay of Ypk2, and Ypk2 activity is largely impaired in tor2 mutant. These results suggest that YPK1/2 receive TOR2 signalling via their yeast TOS-motif, and that they play an important role in TOR2 pathway.
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