Trans-channel interactions in batrachotoxin-modified skeletal muscle sodium channels: voltage-dependent block by cytoplasmic amines, and the influence of mu-conotoxin GIIIA derivatives and permeant ions

Orientation of μ-conotoxin PIIIA in a sodium channel vestibule, based on voltage dependence of its binding

Mutant cycle analysis has been used in previous studies to constrain possible docking orientations for various toxins. As an independent test of the bound orientation of μ-conotoxin PIIIA, a selectively targeted sodium channel pore blocker, we determined the contributions to binding voltage dependence of specific residues on the surface of the toxin. A change in the "apparent valence" (zδ) of the block, which is associated with a change of a specific toxin charge, reflects a change in the charge movement within the transmembrane electric field as the toxin binds. Toxin derivatives with charge-conserving mutations (R12K, R14K, and K17R) showed zδ values similar to those of wild type (0.61 ± 0.01, mean ± S.E.M.). Charge-changing mutations produced a range of responses. Neutralizing substitutions for Arg14 and Lys17 showed the largest reductions in zδ values, to 0.18 ± 0.06 and 0.20 ± 0.06, respectively, whereas unit charge-changing substitutions for Arg12, Ser13, and Arg20 gave intermediate values (0.24 ± 0.07, 0.33 ± 0.04, and 0.32 ± 0.05), which suggests that each of these residues contributes to the dependence of binding on the transmembrane voltage. Two mutations, R2A and G6K, yielded no significant change in zδ. These observations suggest that the toxin binds with Arg2 and Gly6 facing the extracellular solution, and Arg14 and Lys17 positioned most deeply in the pore. In this study, we used molecular dynamics to simulate toxin docking and performed Poisson-Boltzmann calculations to estimate the changes in local electrostatic potential when individual charges were substituted on the toxin's surface. Consideration of two limiting possibilities suggests that most of the charge movement associated with toxin binding reflects sodium redistribution within the narrow part of the pore.

The tetrodotoxin receptor of voltage-gated sodium channels--perspectives from interactions with micro-conotoxins

Neurotoxin receptor site 1, in the outer vestibule of the conducting pore of voltage-gated sodium channels (VGSCs), was first functionally defined by its ability to bind the guanidinium-containing agents, tetrodotoxin (TTX) and saxitoxin (STX). Subsequent studies showed that peptide micro-conotoxins competed for binding at site 1. All of these natural inhibitors block single sodium channels in an all-or-none manner on binding. With the discovery of an increasing variety of micro-conotoxins, and the synthesis of numerous derivatives, observed interactions between the channel and these different ligands have become more complex. Certain micro-conotoxin derivatives block single-channel currents partially, rather than completely, thus enabling the demonstration of interactions between the bound toxin and the channel's voltage sensor. Most recently, the relatively small micro-conotoxin KIIIA (16 amino acids) and its variants have been shown to bind simultaneously with TTX and exhibit both synergistic and antagonistic interactions with TTX. These interactions raise new pharmacological possibilities and place new constraints on the possible structures of the bound complexes of VGSCs with these toxins.

Trans-channel interactions in batrachotoxin-modified rat skeletal muscle sodium channels: kinetic analysis of mutual inhibition between mu-conotoxin GIIIA derivatives and amine blockers

R13X derivatives of mu-conotoxin GIIIA bind externally to single sodium channels and block current incompletely with mean "blocked" durations of several seconds. We studied interactions between two classes of blockers (mu-conotoxins and amines) by steady state, kinetic analysis of block of BTX-modified Na channels in planar bilayers. The amines cause all-or-none block at a site internal to the selectivity filter. TPrA and DEA block single Na channels with very different kinetics. TPrA induces discrete, all-or-none, blocked events (mean blocked durations, approximately 100 ms), whereas DEA produces a concentration-dependent reduction of the apparent single channel amplitude ("fast" block). These distinct modes of action allow simultaneous evaluation of block by TPrA and DEA, showing a classical, competitive interaction between them. The apparent affinity of TPrA decreases with increasing [DEA], based on a decrease in the association rate for TPrA. When an R13X mu-conotoxin derivative and one of the amines are applied simultaneously on opposite sides of the membrane, a mutually inhibitory interaction is observed. Dissociation constants, at +50 mV, for TPrA ( approximately 4 mM) and DEA ( approximately 30 mM) increase by approximately 20%-50% when R13E (nominal net charge, +4) or R13Q (+5) is bound. Analysis of the slow blocking kinetics for the two toxin derivatives showed comparable decreases in affinity of the mu-conotoxins in the presence of an amine. Although this mutual inhibition seems to be qualitatively consistent with an electrostatic interaction across the selectivity filter, quantitative considerations raise questions about the mechanistic details of the interaction.