Abstract

Membrane fusion and fission events in intracellular trafficking are controlled by both intraluminal Ca2+ release and phosphoinositide (PIP) signalling. However, the molecular identities of the Ca2+ release channels and the target proteins of PIPs are elusive. In this paper, by direct patch-clamping of the endolysosomal membrane, we report that PI(3,5)P2, an endolysosome-specific PIP, binds and activates endolysosome-localized mucolipin transient receptor potential (TRPML) channels with specificity and potency. Both PI(3,5)P2-deficient cells and cells that lack TRPML1 exhibited enlarged endolysosomes/vacuoles and trafficking defects in the late endocytic pathway. We find that the enlarged vacuole phenotype observed in PI(3,5)P2-deficient mouse fibroblasts is suppressed by overexpression of TRPML1. Notably, this PI(3,5)P2-dependent regulation of TRPML1 is evolutionarily conserved. In budding yeast, hyperosmotic stress induces Ca2+ release from the vacuole. In this study, we show that this release requires both PI(3,5)P2 production and a yeast functional TRPML homologue. We propose that TRPMLs regulate membrane trafficking by transducing information regarding PI(3,5)P2 levels into changes in juxtaorganellar Ca2+, thereby triggering membrane fusion/fission events. Phosphoinositides activate intracellular ion channels to release Ca2+ from membrane-bound stores. This study demonstrates that Ca2+-permeable mucolipin TRP channels, TRPMLs, are activated by the phospholipid PI(3,5)P2in murine endolysosomes and yeast vacuoles.