An NAADP-gated Two-pore Channel Targeted to the Plasma Membrane Uncouples Triggering from Amplifying Ca2+ Signals

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a ubiquitous messenger proposed to stimulate Ca2+ release from acidic organelles via two-pore channels (TPCs). It has been difficult to resolve this trigger event from its amplification via endoplasmic reticulum Ca2+ stores, fuelling speculation that archetypal intracellular Ca2+ channels are the primary targets of NAADP. Here, we redirect TPC2 from lysosomes to the plasma membrane and show that NAADP evokes Ca2+ influx independent of ryanodine receptors and that it activates a Ca2+-permeable channel whose conductance is reduced by mutation of a residue within a putative pore. We there fore uncouple TPC2 from amplification pathways and prove that it is a pore-forming subunit of an NAADP-gated Ca2+ channel.