Mecanismo molecular de ação da Ts3 em canais de sódio dependentes devoltagem

Abstract

Voltage-gated sodium channels are the molecular targets of several groups of toxinsthat bind to specific receptor sites and modify their function. The well-know a-scorpiontoxins bind in a voltage-dependent way to site 3, which is partially formed by theextracellular loop connecting S3 and S4 segments of domain IV, slowing down fastinactivation. While the movement of S4-DIV seems to be required for inactivation, itsimportance for the activation is still a matter of dispute. Several evidences suggest that S4-DIV has an important role in the voltage-dependent association of the site 3 toxins,however a molecular mechanism is still unknown. We studied the effects of Ts3, an a-toxinpurified from the Brazilian scorpionTityus serrulatus, on native sodium channels of GH3cells and rat skeletal muscle sodium channels expressed inXenopuslaevisoocytes. Wehave observed an increase of the slow component of fast inactivation in the presence of Ts3in both systems. That effect was reverted when strong depolarizing pulses were applied.We have observed that the activation energy to remove the toxin is 27 kJ/mol, part of whichderives from the movement of an equivalent electrical charge of 0,54e. The recovery from0fast inactivation was significantly faster in Ts3-bound channels and there was no significantvoltage dependence. Gating currents recorded fromXenopusoocytes became significantlyfaster in the presence of Ts3 and the total charge was reduced by 30 % in these conditions,suggesting that S4-DIV was immobilized. However, site-specific fluorescencemeasurements of domain IV suggested that this immobilization was not complete. We alsoobserved an allosteric effect of Ts3 at the movement of S4-DI. We propose a kinetic modelin which the bound Ts3 partially blocks the movement of S4-DIV. Whereas the remainingmovement seems to be enough for the normal activation, a normal fast inactivation requiresthe full movement to take place.