Effect of Depolarization on Binding Kinetics of Scorpion α-Toxin Highlights Conformational Changes of Rat Brain Sodium Channels

Abstract

Binding of scorpion α-toxins to receptor site 3 on voltage-gated sodium channels inhibits sodium current inactivation and is voltage-dependent. To reveal the direct effect of depolarization, we analyzed binding kinetics of the α-toxin Lqh-II (from Leiurus quinquestriatus hebraeus) to rat brain synaptosomes and effects on rat brain II (rBII) channels expressed in mammalian cells. Our results indicated that the 33-fold decrease in toxin affinity for depolarized (0 mV, 90 mM [K⁺]_out, K_d = 5.85 ± 0.5 nM) versus polarized (−55 mV, 5 mM [K⁺]_out, K_d = 0.18 ± 0.04 nM) synaptosomes at steady state results from a 48-fold reduction in the association rate (k_on at 5 mM [K⁺] = (12.0 ± 4) × 10⁶ M^-1 s^-1 and (0.25 ± 0.03) × 10⁶ M^-1 s^-1 at 90 mM [K⁺]_out) with nearly no change in the dissociation rate. Electrophysiological analyses of rBII channels expressed in mammalian cells revealed that approximately 75% and 40% of rBII occupied fast- and slow-inactivated states, respectively, at resting membrane potential of synaptosomes (−55 mV), and Lqh-II markedly increased the steady-state fast and slow inactivation. To mimic electrophysiological conditions we induced fast depolarization of toxin-bound synaptosomes, which generated a biphasic unbinding of Lqh-II from toxin−receptor complexes. The first fast off rate closely resembled values determined electrophysiologically for rBII in mammalian cells. The second off rate was similar to the voltage-independent steady-state value, attributed to binding to the slow-inactivated channel states. Thus, the Lqh-II voltage-dependent affinity highlights two independent mechanisms representing conformational changes of sodium channels associated with transitions among electrically visible and invisible inactivated states.