Organelle-specific subunit interactions of the vertebrate two-pore channel family.

The organellar targeting of two-pore channels (TPCs) and their capacity to associate as homo- and heterodimers may be critical to endolysosomal signaling. A more detailed understanding of the functional association of vertebrate TPC1-3 is therefore necessary. We report here that when stably expressed in HEK293 cells, human (h) TPC1 and chicken (c) TPC3 were specifically targeted to different subpopulations of endosomes, hTPC2 was specifically targeted to lysosomes, and rabbit (r) TPC3 was specifically targeted to both endosomes and lysosomes. Intracellular dialysis of NAADP evoked a Ca(2+) transient in HEK293 cells that stably overexpressed hTPC1, hTPC2, and rTPC3, but not in cells that stably expressed cTPC3. The Ca(2+) transients induced in cells that overexpressed endosome-targeted hTPC1 were abolished upon depletion of acidic Ca(2+) stores by bafilomycin A1, but remained unaffected following depletion of endoplasmic reticulum stores by thapsigargin. In contrast, Ca(2+) transients induced via lysosome-targeted hTPC2 and endolysosome-targeted rTPC3 were abolished by bafilomycin A1 and markedly attenuated by thapsigargin. NAADP induced marked Ca(2+) transients in HEK293 cells that stably coexpressed hTPC2 with hTPC1 or cTPC3, but failed to evoke any such response in cells that coexpressed interacting hTPC2 and rTPC3 subunits. We therefore conclude that 1) all three TPC subtypes may support Ca(2+) signaling from their designate acidic stores, and 2) lysosome-targeted (but not endosome-targeted) TPCs support coupling to the endoplasmic reticulum.