To maximize the probability of survival, unicellular organisms, such as yeast, have elaborate mechanisms to sense stressful environments and modulate their growth and metabolism accordingly. Although cells in multicellular organisms are exposed to less environmental fluctuation, the stress response mechanisms are conserved also in animals and play pivotal roles in cellular homeostasis. Among those mechanisms are intracellular signaling pathways known as the stress-activated protein kinase (SAPK) cascade and the TOR (target of rapamycin) pathways. They are implicated in the pathology of numerous human diseases, including cancers, inflammatory conditions and metabolic syndromes, where cellular homeostasis is disturbed by genetic disorders, infection and other etiologic agents. Therefore, elucidating the regulation and interplay of these signaling pathways have broad implications for improving human health. The long-term objective of the research described in this proposal is to understand at a molecular level the crosstalk between the SAPK cascade and the TOR complex 2 (TORC2) signaling pathway, both of which control cellular response to stressful environments. The studies will utilize the genetically amenable model system provided by the fission yeast Schizosaccharomyces pombe, which has SAPK and TORC2 pathways highly homologous to those in humans. Furthermore, in both fission yeast and humans, an essential TORC2 component, Sin1, has been found to interact with SAPK, possibly serving as a molecular link between the two signaling pathways. The three specific aims of the proposal focus on protein-protein interactions that mediate signaling within and between the SAPK and TORC2 pathways. The first is to understand how two MAPK kinase kinases (MAPKKKs) in the SAPK cascade form a heteromer complex and how the MAPKKK complex is regulated upon stress. The second aim is to determine how the interaction of SAPK with the Sin1 protein modulates TORC2 activity. The third aim is to discover how proteins interacting with Sin1 regulate TORC2 in response to extracellular stimuli. It is anticipated that the findings from these studies will serve as an invaluable paradigm for the investigation of the human SAPK and TORC2 pathways, facilitating understanding of their interplay in clinical contexts. PUBLIC HEALTH RELEVANCE: Cells have evolutionarily conserved mechanisms to monitor the environment and modulate their growth and metabolism accordingly. This research utilizes fission yeast as a genetically amenable model to study the regulation and interaction of such mechanisms, which are implicated in a number of human diseases including deregulated cell proliferation in cancer and inflammatory responses in arthritis and asthma.