Loss of the TOR Kinase Tor2 Mimics Nitrogen Starvation and Activates the Sexual Development Pathway in Fission Yeast

Abstract

Fission yeast has two TOR (target of rapamycin) kinases, namely Tor1 and Tor2. Tor1 is required for survival under stressed conditions, proper G 1 arrest, and sexual development. In contrast, Tor2 is essential for growth. To analyze the functions of Tor2, we constructed two temperature-sensitive tor2 mutants. Interestingly, at the restrictive temperature, these mutants mimicked nitrogen starvation by arresting the cell cycle in G 1 phase and initiating sexual development. Microarray analysis indicated that expression of nitrogen starvationresponsive genes was induced extensively when Tor2 function was suppressed, suggesting that Tor2 normally mediates a signal from the nitrogen source. As with mammalian and budding yeast TOR, we find that fission yeast TOR also forms multiprotein complexes analogous to TORC1 and TORC2. The raptor homologue, Mip1, likely forms a complex predominantly with Tor2, producing TORC1. The rictor/Avo3 homologue, Ste20, and the Avo1 homologue, Sin1, appear to form TORC2 mainly with Tor1 but may also bind Tor2. The Lst8 homologue, Wat1, binds to both Tor1 and Tor2. Our analysis shows, with respect to promotion of G 1 arrest and sexual development, that the loss of Tor1 (TORC2) and the loss of Tor2 (TORC1) exhibit opposite effects. This highlights an intriguing functional relationship among TOR kinase complexes in the fission yeast Schizosaccharomyces pombe.

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

“…On the other hand, TORC1 in S. pombe is stimulated by nitrogen sources such as ammonium, and its mutational inactivation induces cellular responses similar to those triggered by nitrogen starvation. 22,32 Thus, the TOR signaling pathways in fission yeast seem to mediate major nutritional input, nitrogen and carbon/energy.…”

“…43 Nitrogen starvation inhibits the TORC1 activity, and mutational inactivation of TORC1 provokes phenotypes similar to those of nitrogenstarved cells, including G1 arrest and sexual development. 18,22,32,44 To examine whether the TORC2 activity is also regulated in response to nitrogen availability, exponentially growing wild-type cells in the synthetic EMM medium were shifted to the same medium lacking the nitrogen source ammonium chloride (EMM-N). As reported previously, 34 the TORC1-dependent phosphorylation of the S6 kinase Psk1 diminished and became undetectable within 30 min (Fig.…”

“…[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. 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.…”

“…6,[23][24][25][26][27][28][29][30][31] Fission yeast TORC1 is activated in the presence of nitrogen and carbon sources, and phosphorylates the major S6 kinase Psk1. 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.…”

The TSC/Rheb/TOR Signaling Pathway in Fission Yeast and Mammalian Cells: Temperature Sensitive and Constitutive Active Mutants of TOR

“…[22][23][24][25] By using genetic screens in fission yeast, a variety of TOR mutants have been identified. [23][24][25][26] They are temperature sensitive mutants and constitutively active mutants. In this article, we will describe these novel mutants and discuss the significance of the findings.…”