Tx2-6, uma toxina da aranha Phoneutria nigriventer que modifica os canais de Na+ sensíveis à voltagem

Abstract

Many organisms produce polypeptides with toxic activity against other organisms. The voltage-gated sodium channels are the main targets of many toxins studied so far. Two specific binding sites of polypeptide toxins have been particularly characterized on the sodium channels. Toxins which bind to the site 3 cause persistent activation of sodium channel by inhibiting the inactivation. Toxins which bind to the site 4 shift the voltage dependence of activation to more negative potentials. The toxin Tx2-6, obtained from PhTx2 fraction of the Brazilian Phoneutria nigriventer spider venom, is a basic peptide that features 48 amino acids, rich in cysteines. This peptide is very toxic to mice and reproduces the predominant symptoms produced by the whole venom. The purpose of this study is to characterize the effect of Tx2-6 on the sodium channels of frog skeletal muscle, using the modified technique of loose patch clamp as a method for the study of the macroscopic sodium currents. Our results show that the Tx2-6 modifies the kinetics of sodium channels in a dose-dependent manner. The Tx2-6 at 1 ìm prolongs the inactivation time constant and shifts the voltage dependence of both activation and steady-state inactivation to more negative values ( -7.4 and -10 mV, respectively). The toxin also decreases the potential for reversal in 10 mV and reduces the sodium current peak by 48%. However, the toxin showed no effect on recovery from inactivation or the deactivation of the sodium current. Together, these complex changes can explain the toxic effect of Tx2-6. This toxin shows little primary sequence identity with toxins that acts on site 3 and 4 of the sodium channels.