부산대학교 도서관

Yeast cells maintain an intricate network of nutrient signaling pathways enabling them to integrate information on the availability of different nutrients and adjust their metabolism and growth accordingly. Cells that are no longer capable of integrating this information, or that are unable to make the necessary adaptations, will cease growth and eventually die. Here, we studied the molecular basis underlying the synthetic lethality caused by loss of the protein kinase Sch9, a key player in amino acid signaling and proximal effector of the conserved growth-regulatory TORC1 complex, when combined with either loss of the cyclin-dependent kinase (CDK) Pho85 or loss of its inhibitor Pho81, which both have pivotal roles in phosphate sensing and cell cycle regulation. We demonstrate that it is specifically the CDK-cyclin pair Pho85-Pho80 or the partially redundant CDK-cyclin pairs Pho85-Pcl6/Pcl7 that become essential for growth when Sch9 is absent. Interestingly, the respective three CDK-cyclin pairs regulate the activity and distribution of the phosphatidylinositol-3 phosphate 5-kinase Fab1 on endosomes and vacuoles, where it generates phosphatidylinositol-3,5 bisphosphate that serves to recruit both TORC1 and its substrate Sch9. In addition, Pho85-Pho80 directly phosphorylates Sch9 at Ser726, and to a lesser extent at Thr723, thereby priming Sch9 for its subsequent phosphorylation and activation by TORC1. The TORC1-Sch9 signaling branch therefore integrates Pho85-mediated information at different levels. In this context, we also discovered that loss of the transcription factor Pho4 rescued the synthetic lethality caused by loss of Pho85 and Sch9, indicating that both signaling pathways also converge on Pho4, which appears to be wired to a feedback loop involving the high-affinity phosphate transporter Pho84 that fine-tunes Sch9-mediated responses. Author summary: Cells possess different signaling pathways that sense and signal the availability of nutrients. Crosstalk between these pathways is essential to integrate the incoming signals and allow cells to make appropriate adaptations to sustain their metabolism and proliferation. In this study, we deciphered the crosstalk between two well-known nutrient-responsive pathways in yeast, namely the PHO pathway that signals the availability of phosphate via the cyclin-dependent protein kinase Pho85, and the TORC1 signaling pathway that communicates information on the availability of free amino acids via its downstream effector kinase Sch9. We show that Pho85 facilitates the TORC1-dependent activation of Sch9 through two different mechanisms. By interfering with the biosynthesis of the lipid phosphatidylinositol-3,5 bisphosphate, Pho85 controls the recruitment of Sch9 at the vacuolar membrane, thereby bringing this effector in close proximity to TORC1. In addition, Pho85 also directly phosphorylates Sch9, which primes the latter for its subsequent phosphorylation and activation by TORC1. Conversely, we provide evidence that the TORC1-Sch9 axis gives feedback to the PHO pathway by restraining the nuclear translocation of the transcription factor Pho4 that controls the expression of genes encoding proteins required to maintain phosphate homeostasis. [ABSTRACT FROM AUTHOR]