Early lipid-mediated responses to hyperosmotic stress at the yeast vacuole

In yeast, early adaptation to hyperosmotic stress involves organelle-based mechanisms, including synthesis of phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂) in the endolysosomal system. This low-level signaling lipid drives vacuolar fragmentation and activates the V-ATPase, which acidifies the vacuole and promotes salt sequestration. Under NaCl stress, PI(3,5)P₂ rapidly accumulates, triggering increased V-ATPase activity and vacuolar remodeling; these responses are impaired by deficient PI(3,5)P₂ synthesis. We visualized movements of a GFP fusion protein with the cytosolic domain of V-ATPase subunit Vph1 (Vph1NT-GFP) in a microfluidic system during salt stress. Upon NaCl addition, Vph1NT-GFP rapidly relocalizes to a region adjacent to the vacuole in a PI(3,5)P₂-dependent manner. The intensity and duration of this response depend on salt concentration, but the response is diminished by 30-45 min, even if salt is readded. Vph1NT-GFP returns to the same location upon repeated salt challenge, suggesting that PI(3,5)P₂ synthesis occurs at a localized domain/contact site that may be endosomal. When the high osmolarity glycerol pathway, which coordinates long-term transcriptional changes, is disrupted, Vph1NT-GFP recruitment is significantly extended. This underscores the integration of lipid signaling and transcriptional regulation in osmoadaptation. These findings suggest activation of endolysosomal targets by PI(3,5)P₂ synthesis provides immediate protection that primes cells for longer-term survival strategies.