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

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

Voltage-gated sodium channels are the molecular targets of several groups of toxins that bind to specific receptor sites and modify their function. The well-know a-scorpion toxins bind in a voltage-dependent way to site 3, which is partially formed by the extracellular loop connecting S3 and S4 segments of domain IV, slowing down fast inactivation. While the movement of S4-DIV seems to be required for inactivation, its importance 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-toxin purified from the Brazilian scorpion Tityus serrulatus , on native sodium channels of GH3 cells and rat skeletal muscle sodium channels expressed in Xenopus laevis oocytes. We have observed an increase of the slow component of fast inactivation in the presence of Ts3 in 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 which derives from the movement of an equivalent electrical charge of 0,54 e . The recovery from 0 fast inactivation was significantly faster in Ts3-bound channels and there was no significant voltage dependence. Gating currents recorded from Xenopus oocytes became significantly faster 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 fluorescence measurements of domain IV suggested that this immobilization was not complete. We also observed an allosteric effect of Ts3 at the movement of S4-DI. We propose a kinetic model in which the bound Ts3 partially blocks the movement of S4-DIV. Whereas the remaining movement seems to be enough for the normal activation, a normal fast inactivation requires the full movement to take place.

ASSUNTO(S)

bioquímica teses. imunologia teses.

Documentos Relacionados