Reconstituted Human TPC1 Is a Proton-Permeable Ion Channel and Is Activated by NAADP or Ca2+.

NAADP potently triggers Ca(2+) release from acidic lysosomal and endolysosomal Ca(2+) stores. Human two-pore channels (TPC1 and TPC2), which are located on these stores, are involved in this process, but there is controversy over whether TPC1 and TPC2 constitute the Ca(2+) release channels. We therefore examined the single-channel properties of human TPC1 after reconstitution into bilayers of controlled composition. We found that TPC1 was permeable not only to Ca(2+) but also to monovalent cations and that permeability to protons was the highest (relative permeability sequence: H(+) >> K(+) > Na(+) >/= Ca(2+)). NAADP or Ca(2+) activated TPC1, and the presence of one of these ligands was required forchannel activation. The endolysosome-located lipid phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2] had no effect on TPC1 open probability but significantly increased the relative permeability of Na(+) to Ca(2+) and of H(+)to Ca(2+). Furthermore, our data showed that, although both TPC1 and TPC2 are stimulated by NAADP, these channels differ in ion selectivity and modulation by Ca(2+) and pH. We propose that NAADP triggers H(+) release from lysosomes and endolysomes through activation of TPC1, but that the Ca(2+)-releasing ability ofTPC1 will depend on the ionic composition of the acidic stores and may be influenced by other regulators that affect TPC1 ion permeation.