TPC2: what do you actually lose when you take a lysosomal protein out of its lysosome?
TPC2 native binders.
Its two endogenous gating cues work in completely different ways. PI(3,5)P₂ is a phosphoinositide essentially restricted to endolysosomal membranes. It binds TPC2 directly, coordinated by K203/K204/K207 in the S4-S5 linker, and unlike voltage-gated TPC1, TPC2 can be opened by that lipid alone (She et al., eLife 2019). So this is not a lipid that tunes the channel. It opens it, and only one compartment supplies it.
NAADP doesn’t bind TPC2 at all. It works through accessory proteins that confer NAADP sensitivity on the complex. And the two routes give different ions: PI(3,5)P₂ renders the channel Na⁺-selective, NAADP renders it Ca²⁺-permeable. Prof. Dr. Christian Grimm group at LMU Munich made that separable with biased agonists (Gerndt et al., eLife 2020).
It isn’t only soluble messengers either. TPC2 carries a Rab binding site in its N terminus and is a Rab effector, with Rab7a, the canonical late endosome and lysosome Rab, acting as an enhancer of its activity. One of its regulators is itself a marker of the compartment it sits in.
The result we keep coming back to: work from Sandeep patel group at UCL with Sean Sweeney at York (Gregori et al., J Cell Biol 2025). Human TPC2 expressed in Drosophila, which have no TPCs, disrupts dopaminergic vision and movement. But the same channel rerouted to the plasma membrane, still demonstrably active, does nothing. Neither does TPC1, targeted to different endolysosomal compartments. Nor does a Rab-binding mutant. The pore has to be open, it has to be open on the lysosome, and it has to be able to engage its Rab.
Same protein. Same conductance. Wrong membrane, not the same biology.
So TPC2 is less a protein than an assembly: a pore, a compartment-specific lipid, soluble partners, a Rab, and a membrane that supplies or recruits most of it. A reconstituted channel still tells you something real. But a synthetic bilayer doesn’t just change the conditions around the target, it removes a gate, and you don’t always know which one.
That’s the bet behind what we do at ORIA Bioscience: working hard everyday to keep the organelle in its best integrity and functionality, for you to measure the target in the system that actually gates it. With LYSO-Prep™ that already covers much of the lysosomal ion transport machinery in native membranes: TMEM175, TRPML1, CLC-7, and TMEM165.
TPC2 is now officially available.
