KCNQ2/3 current behaves like the square of PH probe FRET. (A) In four cells, single exponentials were fitted to simultaneously acquired KCNQ2/3 current and PH probe FRETr during VSP activation (2 s of +100 mV). Measurements from the same cell are connected by a line. (B) In five cells, single exponentials were fitted to simultaneously acquired KCNQ2/3 current and PH probe FRETr during recovery after VSP activation. KCNQ2/3 current was measured as tail current amplitude. (C) VSP effect on KCNQ current with voltage protocol as in Fig. 3 B. Recovery of KCNQ2/3 current at −20 mV was fitted with a double exponential: y = a − b*exp(−c*t) + d*exp(−f*t). (Inset) Summary of time constants from 31 cells. Time constant of the positive term (pos.) is 1/f, and that of the negative term (neg.) is 1/c. (D) Illustration of the consequence of squaring an exponential of the form y = 1 − exp(−t/τ). (E) Plot of KCNQ2/3 current at −20 mV (black) versus PH probe FRETr at the same time during recovery after VSP activation in the cell depicted in Fig. 2. Similar observations were made for three other cells. Red curve corresponds to the equation given. (F) Averaged KCNQ2/3 current at −20 mV versus averaged FRETr at the same time after M₁R activation, measured in separate cells (data from Figs. 5 D and 7 B in Jensen et al., 2009). (G) Illustration of the dependence of FRETr (approximated by PH_PIP₂; see Fig. S3) and KCNQ current on PIP₂ concentration as predicted by the model outlined in Fig. 7 and Tables I and II: K_d of PH probe is 2,000 µm⁻² for PIP₂ and 0.1 µM for IP₃ (0.16 µM IP₃); K_d of KCNQ is 2,000 µm⁻² for PIP₂. KCNQ current = (KCNQ_PIP₂)².