| The yeast vacuole is known to undergo changes in size and number in response to hyperosmotic shock and changes in the cell cycle, in a process that remains poorly defined. In this work, we demonstrate that chemical or genetic perturbations of ER homeostasis that result in ER stress, or the accumulation of unfolded or misfolded proteins that overwhelms the functional capacity of the ER, initiates vacuolar fragmentation. We find that vacuoles fragment independently of known ER stress signaling pathways, but instead rely on the Target of Rapamycin signaling network (TOR) and its downstream components, Sch9 and Tap42/Sit4. We explore the relationship between ER stress and TORC1, and find evidence that they function in parallel pathways to influence vacuolar morphology. Furthermore, we perform an unbiased, in-vivo , genome-wide visual screen that identifies 315 genes required for efficient vacuolar fragmentation in response to ER stress. We identify known fission factors, validating this approach, as well as novel factors, demonstrating the specificity of this response. Furthermore, we show that ER stress alters distribution of the vacuolar ATPase (V-ATPase) assembly factor, Vph2, in a TORC1-dependent manner. Interestingly, we find that rescue of vacuolar acidification does not restore the defects in vacuolar fragmentation identified in Vph2 mutants, raising the possibility of a different role of the V-ATPase in vacuolar fragmentation. |
