A marine snail neurotoxin shares with scorpion toxins a convergent mechanism of blockade on the pore of voltage-gated K channels

kappa-Conotoxin-PVIIA (kappa-PVIIA) belongs to a family of peptides derived from a hunting marine snail that tar gets to a wide variety of ion channel s and receptors. kappa-PVIIA is a small, structurally constrained, 27-resid ue peptide that inhibits voltage-gated K channels. Three disulfide bonds sh ape a characteristic four-loop folding. The spatial localization of positiv ely charged residues in K-PVIIA exhibits strong structural mimicry to that of charybdotoxin, a scorpion toxin that occludes the pore of K channels. Me studied the mechanism by which this peptide inhibits Shaker, K channels ex pressed in Xenopus oocytes with the N-type inactivation removed. Chronicall y applied to whole oocytes or outside-out patches, kappa-PVIIA inhibition a ppears as a voltage-dependent relaxation in response to the depolarizing pu lse used to activate the channels. At any applied voltage, the relaxation r are depended linearly on the toxin concentration, indicating a bimolecular stoichiometry. Time constants and voltage dependence of the current relaxat ion produced by chronic applications agreed with that of rapid applications to open channels. Effective valence of the voltage dependence, z delta, is similar to 0.55 and resides primarily in the rare of dissociation from the channel, while the association rate is voltage independent with a magnitud e of 10(7)-10(8) M-1 s(-1), consistent with diffusion-limited binding. Comp atible with a purely competitive interaction for a site in the external ves tibule, tetraethylammonium, a well-known Ii-pore blocker, reduced kappa-PVI IA's association rate only. Removal of internal K+ reduced, but did not eli minate, the effective valence of the toxin dissociation rate to a value <0. 3. This trans-pore effect suggests that: (a) as in the alpha-KTx, a positiv ely changed side chain, possibly a Lys, interacts electrostatically with io ns residing inside the Shaker pore, and (b) a part of the toxin occupies an externally accessible K+ binding site, decreasing the degree of pore occup ancy by permeant ions, We conclude that, although evolutionarily distant to scorpion toxins, kappa-PVIIA shares with them a remarkably similar mechani sm of inhibition of K channels.