FIGURE 2.

Kinetic analysis of block by mutant μ-CTX derivatives. (A) Left Panel: Time courses of onset and offset of R13A (30 μM) block of WT μ1 channels during toxin wash in (solid squares) and wash out (open circles). Normalized peak sodium currents elicited by depolarization to −10 mV from a holding potential of −100 mV were plotted versus time. Data were fitted with a mono-exponential function to estimate τ_on and τ_off for toxin binding and unbinding respectively. The equations used for deriving k_on and k_off from τ_on and τ_off have been previously described (Li et al., 2000). Right Panel: Representative records of μ1 currents elicited during wash in (top) and wash out (bottom) of R13A (30 μM). Sweeps (15 msec duration) shown were separated by 6-s intervals for clarity. (B) Logarithmic plot of the dissociation rate constants (k_off) versus the reciprocal of the association rate constants (k_on). The horizontal and vertical dotted lines respectively represent the levels of 1/k_on and k_off for the WT channels. All mutant toxins studied (except K11A) had substantial effects on both k_on and k_off (see text for details). The largest effects on k_on and k_off were found with R19A and R13A, respectively.