Membrane contact sites are regions of close apposition between organelles that facilitate information transfer.Here, we Chase Rack reveal an essential role for Ca2+ derived from the endo-lysosomal system in maintaining contact between endosomes and the endoplasmic reticulum (ER).Antagonizing action of the Ca2+-mobilizing messenger NAADP, inhibiting its target endo-lysosomal ion channel, TPC1, and buffering local Ca2+ fluxes all clustered and enlarged late endosomes/lysosomes.We show that TPC1 localizes to ER-endosome contact sites and is required for their formation.Reducing NAADP-dependent contacts delayed EGF receptor de-phosphorylation consistent with close apposition of endocytosed receptors with the ER-localized phosphatase PTP1B.
In accord, downstream Chef Knives MAP kinase activation and mobilization of ER Ca2+ stores by EGF were exaggerated upon NAADP blockade.Membrane contact sites between endosomes and the ER thus emerge as Ca2+-dependent hubs for signaling.