Role of The Conserved NDR/LATS Kinase Orb6 in Controlling Cell Growth and Stress Adaptation

Adaptation to the nutritional environment is critical for all cells. RAS GTPase is a highly conserved GTP binding protein with crucial functions for cell growth and differentiation in response to environmental conditions. Inappropriate activation of human RAS GTPase has a causal role in cancer, congenital malformations, and neurodevelopmental disorders. Our lab previously found that the conserved NDR kinase Orb6 regulates the mRNA and protein levels of Ras1 guanine nucleotide exchange factor (GEF) Efc25, by controlling the coalescence of the mRNA-binding protein Sts5 into ribonucleoprotein (RNP) granules. However, it is still unclear whether the Orb6-Sts5 axis regulates RAS GTPase activity in response to the nutritional environment. Further, the molecular mechanisms by which Orb6 kinase controls Sts5 RNP coalescence are not characterized. Fission yeast Schizosaccharomyces pombe serves as a powerful model organism to study signaling pathways regulating nutrient sensing, cell growth, and cell adaptation due to its well-defined cell shape and growth pattern under nutrient variations. Using a phospho-specific antibody and an analog-sensitive orb6-as2 strain, Orb6 kinase activity and the effect of Orb6 kinase inhibition can be quantified. Here, I describe a novel mechanism tethering RAS GTPase to nutrient availability in fission yeast. The conserved NDR kinase Orb6 responds to nutritional cues and regulates Ras1 GTPase activity. Under nutrient-rich conditions, Orb6 kinase increases the protein levels of a Ras1 GTPase activator, the guanine nucleotide exchange factor Efc25, by phosphorylating Serine 86 of Sts5, a protein bound to efc25 mRNA. Under nutrient-depleted condition, Orb6 kinase activity decreases, leading to Sts5 RNP coalescence and decrease in Ras1 activity. By manipulating the extent of Orb6-mediated Sts5 coalescence into RNP granules, one can modulate Efc25 protein levels, Ras1 GTPase activity and as a result, cell growth and cell survival. Moreover, I find that Orb6 kinase activity is down-regulated in response to a broader range of stress stimuli, in a manner that is partially dependent on the conserved p38 MAPK Sty1. The conserved NDR/LATS kinase Orb6 plays a crucial role in promoting cell adaptation, balancing the opposing demands of promoting cell growth or extending chronological lifespan under nutrient fluctuation. My findings highlight the conserved kinase as a crucial enzyme in modulating cell growth and cell survival under favorable or adverse conditions, and in regulating cell adaptation in general stress response