Damage sensing recruitment of a lipid phosphatase couples lysosomal membrane repair to proteostatic adaptation
Restoration of organellar membrane integrity is critical for maintaining cellular homeostasis. Lysosomal membrane damage activates local repair machineries and global stress responses, but how signaling lipid metabolism is engaged by damage sensors to support and mechanistically link these processes remains poorly understood. Here we show that the phosphoinositide 3-phosphatase MTMR14 is recruited to damaged lysosomes through calcium-dependent binding to sphingomyelin. At these sites, MTMR14 promotes local PI(3)P hydrolysis and supports PI(4)P accumulation, thereby facilitating formation of ER-lysosome contact sites associated with membrane repair, without affecting ESCRT recruitment. MTMR14-dependent lipid remodelling causes reduced mTORC1 signalling and a decrease in global protein synthesis, consistent with an acute proteostatic adaptation to lysosomal injury. Cells lacking MTMR14 display impaired damage-induced lipid remodelling, altered repair-associated structures, sustained protein synthesis, and increased sensitivity to lysosomal injury, all of which can be mitigated by mTORC1/S6K inhibition. Our findings identify damage-sensing recruitment of MTMR14 and local PI(3)P turnover on damaged lysosomes as a phosphoinositide module that promotes lysosomal membrane integrity and homeostasis while functionally linking nutrient signalling to proteostasis under membrane stress.