Endosome maturation links PI3Kα signaling to lysosome repopulation during basal autophagy.

Autophagy depends on the repopulation of lysosomes to degrade intracellular components and recycle nutrients. How cells co-ordinate lysosome repopulation during basal autophagy, which occurs constitutively under nutrient-rich conditions, is unknown. Here, we identify an endosome-dependent phosphoinositide pathway that links PI3Kα signaling to lysosome repopulation during basal autophagy. We show that PI3Kα-derived PI(3)P generated by INPP4B on late endosomes was required for basal but not starvation-induced autophagic degradation. PI(3)P signals were maintained as late endosomes matured into endolysosomes, and served as the substrate for the 5-kinase, PIKfyve, to generate PI(3,5)P 2 . The SNX-BAR protein, SNX2, was recruited to endolysosomes by PI(3,5)P 2 and promoted lysosome reformation. Inhibition of INPP4B/PIKfyve-dependent lysosome reformation reduced autophagic clearance of protein aggregates during proteotoxic stress leading to increased cytotoxicity. Therefore under nutrient-rich conditions, PI3Kα, INPP4B, and PIKfyve sequentially contribute to basal autophagic degradation and protection from proteotoxic stress via PI(3,5)P 2 -dependent lysosome reformation from endolysosomes. These findings reveal that endosome maturation couples PI3Kα signaling to lysosome reformation during basal autophagy.