MORG1 limits mTORC1 signaling by inhibiting Rag GTPases

The MAP kinase scaffold MORG1 shapes cell death in unresolved ER stress in Arabidopsis

Governed by the unfolded protein response (UPR), the ability to counteract endoplasmic reticulum (ER) stress is critical for maintaining cellular homeostasis under adverse conditions. Unresolved ER stress leads to cell death through mechanisms that are yet not completely known. To identify key UPR effectors involved in unresolved ER stress, we performed an ethyl methanesulfonate (EMS) suppressor screen on the Arabidopsis bzip28/60 mutant, which is impaired in activating cytoprotective UPR pathways. This screen identified MAP kinase organizer 1 (MORG1), a conserved MAP kinase scaffold protein, as a previously uncharacterized regulator of ER stress tolerance. The coffin1 mutant, which carries a mutation in MORG1 , exhibited enhanced resilience to ER stress by partially restoring UPR gene expression and promoting growth under stress conditions. Mechanistically, we found that MORG1 modulates MPK6-dependent phosphorylation of the stress-responsive transcription factor WRKY8. Loss of WRKY8 phenocopied the coffin1 mutant, highlighting WRKY8's role as a key repressor in the UPR. Together, these findings reveal a MORG1-MPK6-WRKY8 signaling axis that fine-tunes UPR gene expression, providing new insights into ER stress regulation and strategies for improving stress tolerance in multicellular eukaryotes.

Nutrient sensing of mTORC1 signaling in cancer and aging

The mechanistic target of rapamycin complex 1 (mTORC1) is indispensable for preserving cellular and organismal homeostasis by balancing the anabolic and catabolic processes in response to various environmental cues, such as nutrients, growth factors, energy status, oxygen levels, and stress. Dysregulation of mTORC1 signaling is associated with the progression of many types of human disorders including cancer, age-related diseases, neurodegenerative disorders, and metabolic diseases. The way mTORC1 senses various upstream signals and converts them into specific downstream responses remains a crucial question with significant impacts for our perception of the related physiological and pathological process. In this review, we discuss the recent molecular and functional insights into the nutrient sensing of the mTORC1 signaling pathway, along with the emerging role of deregulating nutrient-mTORC1 signaling in cancer and age-related disorders.

WDR83/MORG1 inhibits RRAG GTPase-MTORC1 signaling to facilitate basal autophagy

Macroautophagy/autophagy is a conserved lysosomal degradation process composed of both selective and nonselective degradation pathways. The latter occurs upon nutrient depletion. Selective autophagy exerts quality control of damaged organelles and macromolecules and is going on also under nutrient-replete conditions. Proper regulation of autophagy is vital for cellular homeostasis and prevention of disease. During nutrient availability, autophagy is inhibited by the MTORC1 signaling pathway. However, selective, basal autophagy occurs continuously. How the MTORC1 pathway is fine-tuned to facilitate basal constitutive autophagy is unclear. Recently, we identified the WD-domain repeat protein WDR83/MORG1 as a negative regulator of MTORC1 signaling allowing basal, selective autophagy. WDR83 interacts with both the Ragulator and active RRAG GTPases to prevent recruitment of the MTORC1 complex to the lysosome. Consequently, WDR83 depletion leads to hyperactivation of the MTORC1 pathway and a strong decrease in basal autophagy. As a consequence of WDR83 depletion cell proliferation and migration increase and low levels of WDR83 mRNA are correlated with poor prognosis for several cancers.