Inhibition of single Shaker K channels by kappa-conotoxin-PVIIA

kappa-Conotoxin-PVIIA (kappa-PVIIA) is a 27-residue basic (+4) peptide from the venom of the predator snail Conus purpurascens. A single kappa-PVIIA molecule interrupts ion conduction by binding to the external mouth of Shaker K channels. The blockade of Shaker by kappa-PVIIA was studied at the single channel level in membrane patches from Xenopus oocytes. The amplitudes of blocked and closed events were undistinguishable, suggesting that the toxin interrupts ion conduction completely. Between -20 and 40 mV kappa-PVIIA increased the latency to the first opening by one order of magnitude in a concentration-independent fashion. Because kappa-PVIIA has higher affinity for the closed channels at high enough concentration to block >90% of the resting channels, the dissociation rate could be estimated from the analysis of the first latency. At 0 mV, the dissociation rate was 20 s(-1) and had an effective valence of 0.64. The apparent closing rate increased linearly with kappa-PVIIA] indicating an association rate of 56 microM(-1) s(-1). The toxin did not modify the fraction of null traces. This result suggests that the structural rearrangements in the external mouth contributing to the slow inactivation preserve the main geometrical features of the toxin-receptor interaction.