Effects of Zn²⁺ on channels containing I287H paired with histidine mutations in the S3–S4 loop. (A; left) I127H+I360H double mutant. (Middle) I360H single mutant. (B; left) I127H+L361H double mutant. (Middle) L361H single mutant. Currents were evoked by pulsing from −80 to +60 mV in the absence (black) or presence of 10 µM Zn²⁺ (red). Representative current traces have been scaled to the same amplitude and overlaid. At right, the box plots show τ_act values measured at +60 mV in the indicated concentrations of Zn²⁺ for (A) I287H+I360H and the I360H single-mutant channel or (B) I287H+L361H and the L361H single-mutant channel. Double-mutant current traces were fitted with the sum of two exponential functions (blue boxes, τ_fast; red boxes, τ_slow). Single-mutant current traces were fitted with a single-exponential function (black boxes, τ_act). Mean values of τ_act, τ_fast, or τ_slow that differed significantly from no Zn²⁺ are indicated: *, P < 0.05; †, P < 0.01; ‡, P < 0.005; §, P < 0.0005 (n = 3–10).