Schizosaccharomyces pombe AGC family kinase Gad8p forms a conserved signaling module with TOR and PDK1-like kinases

Abstract

The TOR protein is a phosphoinositide kinase‐related kinase widely conserved among eukaryotes. Fission yeast tor1 encodes an ortholog of TOR, which is required for sexual development and growth under stressed conditions. We isolated gad8, which encodes a Ser/Thr kinase of the AGC family, as a high‐copy suppressor of the sterility of a tor1 mutant. Disruption of gad8 caused phenotypes similar to those of tor1 disruption. Gad8p was less phosphorylated and its kinase activity was undetectable in tor1Δ cells. Three amino acid residues corresponding to conserved phosphorylation sites in the AGC family kinases, namely Thr387 in the activation loop, Ser527 in the turn motif and Ser546 in the hydrophobic motif, were important for the kinase activity of Gad8p. Tor1p was responsible for the phosphorylation of Ser527 and Ser546, whereas Ksg1p, a PDK1‐like kinase, appeared to phosphorylate Thr387 directly. Altogether, Tor1p, Ksg1p and Gad8p appear to constitute a signaling module for sexual development and growth under stressed conditions in fission yeast, which resembles the mTOR–PDK1–S6K1 system in mammals and may represent a basic signaling module ubiquitous in eukaryotes.

Fission yeast Ryh1 GTPase activates TOR Complex 2 in response to glucose

“…Among those is the fission yeast Schizosaccharomyces pombe, a genetically amenable eukaryote with nonessential TORC2, which eases mutational analysis of this particular TOR complex. [14][15][16][17][18][19][20][21] Fission yeast TORC2 is composed of the catalytic subunit Tor1 kinase and the regulatory subunits named Ste20, Sin1, Wat1 and Bit61. 17,19,22 The key downstream target of TORC2 in fission yeast is the AGCfamily kinase Gad8, as evidenced by the observation that the gad8 deletion mutant phenocopies the TORC2-defective mutants.…”

“…17,19,22 The key downstream target of TORC2 in fission yeast is the AGCfamily kinase Gad8, as evidenced by the observation that the gad8 deletion mutant phenocopies the TORC2-defective mutants. 16,19 Like Akt activation by mammalian TORC2, Gad8 is phosphorylated within its hydrophobic motif by TORC2, resulting in increased Gad8 kinase activity essential for its cellular functions. 16,19 On the other hand, a major upstream regulator of fission yeast TORC2 has been found as the Rab small GTPase Ryh1.…”

“…16,19 Like Akt activation by mammalian TORC2, Gad8 is phosphorylated within its hydrophobic motif by TORC2, resulting in increased Gad8 kinase activity essential for its cellular functions. 16,19 On the other hand, a major upstream regulator of fission yeast TORC2 has been found as the Rab small GTPase Ryh1. 21 The GTP-bound, active form of Ryh1 physically interacts with TORC2 and promotes TORC2-dependent phosphorylation/activation of the Gad8 kinase.…”

“…22,[32][33][34] In this organism, TORC2 is also implicated in cellular nutritional responses, including the G1 arrest upon nitrogen starvation and the mitotic control in response to glucose. 16,19,35 Little is known, however, about the physiological stimuli that regulate fission yeast TORC2, whereas insulin and growth factors activate mammalian TORC2 (mTORC2) in a PI3K-dependent manner. 7,8,[36][37][38] In this report, we show that TORC2 in fission yeast is responsive to glucose and that the functional TORC2-Gad8 pathway is essential for cell proliferation in glucose-limited environment.…”

Roles of Pdk1p, a Fission Yeast Protein Related to Phosphoinositide-dependent Protein Kinase, in the Regulation of Mitosis and Cytokinesis

“…According to our results obtained from the sequence analysis, S. pombe Pdk1p is most related to Pkh1p and does not contain detectable PH domains. There is an additional PDK1-like protein in fission yeast, termed Ksg1, and its function is required for meiosis and growth (Niederberger and Schweingruber, 1999;Matsuo et al, 2003;Tang and McLeod, 2004), whereas Pdk1p is important for aspects of mitosis and cytokinesis.…”