Figure 4.

β4 subunit dramatically slows down CTx and IbTx association and dissociation. Currents in outside-out patches were evoked by repetitive pulses as in Fig. 3. Currents were measured at the end of the pulse and plotted as a function of time; arrows mark the time of toxin application and washout. To attain toxin block in a reasonable time, CTx concentrations were 3 nM for α (a) and α + β1 (b), and 500 nM for α + β4 (c). IbTx concentrations were 10 nM for α (d), 100 nM for α + β1 (e), and 1 μM for α + β4 (f). k_on, k_off, and K_d values are given in M⁻¹s⁻¹, s⁻¹, and nM, respectively, and were obtained as described in Experimental Procedures. Gray lines are fits to data. (g) Mean k_on values (M⁻¹s⁻¹) ± SD (error bars) for CTx and IbTx block in α, α + β1, and α + β4 channels. CTx: k_on-α = 2 ± 1 × 10⁷ (n = 7), k_on-α+β1 = 1.9 ± 1.8 × 10⁶ (n = 4), and k_on-α+β4 = 8 ± 7 × 10⁴ (n = 10); IbTx: k_on-α = 2 ± 0.7 × 10⁶ (n = 4), k_on-α+β1 = 1.1 ± 0.5 × 10⁵ (n = 3), and k_on-α+β4 = 4 ± 2 × 10³ (n = 4). n = number of experiments. For both toxins, values for α + β1 and α + β4 were significantly different from those of α subunit (P < 0.005). In two out of seven cases, IbTx block of α + β4 channels had even slower on-rates (not included in mean value).