Figure 5: Modulation of hTPC1 by luminal calcium.

(a) Current recordings obtained before (top), during (middle) and after (bottom) perfusion of 90 nM PI(3,5)P₂. Main voltage pulse from −80 to +90 mV, step +10 mV; holding voltage: −70 mV, tail voltage at −50 mV. Internal pipette (luminal side) filled with standard solution plus 2 mM EGTA resulting in a free calcium concentration of 1 μM. (b) Normalized conductances vs voltage at different Ca²⁺ concentration in the pipette. Data were obtained by the starting value of tail currents at −50 mV. Applied main voltage pulse from −80 to +90 mV. Continuous lines represented fitting of the data by a Boltzmann equation. Data from 4 different vacuoles for each luminal calcium concentration. (c) Half activation voltage and slope from the Boltzmann fit of B vs luminal free calcium concentration. (d) Relaxation time constants versus voltage at different luminal calcium concentration. For the sake of clarity, data at 50 μM luminal calcium were omitted. They were reported in Supplemental Fig. 3. Continuous lines in c and d were obtained from a global fit of data in b and d by the four state model shown in Supplemental Fig. 3 and described in Supplemental Appendix. (e) allosteric mathematical model for hTPC1 modulation by both cytosolic and luminal Ca²⁺.