FIGURE 2.

TPC1 is activated by acidic luminal pH and luminal Ca²⁺. A and B, example activity traces of TPC1 expressed in HEK cells at cytoplasmic pH of 7.3 and luminal pH of 7.3 (A) and 5.0 (B) in the absence (left traces) or presence (right traces) of 1 μm NAADP and at a membrane potential of −60 mV. The indicated fragments are shown at an expanded time scale. C shows the mean ± S.E. of the indicated numbers of experiments with *** denoting p < 0.001. D, for determination of luminal Ca²⁺ dependence, recording conditions were as in Fig. 1, except that the luminal solution contained the indicated Ca²⁺ concentrations and TPC1 was activated by 500 nm cytoplasmic NAADP. The numbers next to symbols indicate the numbers of experiments, and the curve was fitted with a Hill equation with an apparent K_m of 180 μm.