[Show abstract][Hide abstract] ABSTRACT: Voltage-gated sodium channels are important in initiating and propagating nerve impulses in various tissues, including cardiac muscle, skeletal muscle, the brain, and the peripheral nerves. Hyperexcitability of these channels leads to such disorders as cardiac arrhythmias (Na(v)1.5), myotonias (Na(v)1.4), epilepsies (Na(v)1.2), and pain (Na(v)1.7). Thus, there is strong motivation to identify isoform-specific blockers and the molecular determinants underlying their selectivity among these channels. μ-Conotoxin KIIIA blocks rNa(v)1.2 (IC(50), 5 nM), rNa(v)1.4 (37 nM), and hNa(v)1.7 (97 nM), expressed in mammalian cells, with high affinity and a maximal block at saturating concentrations of 90 to 95%. Mutations of charged residues on both the toxin and channel modulate the maximal block and/or affinity of KIIIA. Two toxin substitutions, K7A and R10A, modulate the maximal block (52-70%). KIIIA-H12A and R14A were the only derivatives tested that altered Na(v) isoform specificity. KIIIA-R14A showed the highest affinity for Na(v)1.7, a channel involved in pain signaling. Wild-type KIIIA has a 2-fold higher affinity for Na(v)1.4 than for Na(v)1.7, which can be attributed to a missing outer vestibule charge in domain III of Na(v)1.7. Reciprocal mutations Na(v)1.4 D1241I and Na(v)1.7 I1410D remove the affinity differences between these two channels for wild-type KIIIA without affecting their affinities for KIIIA-R14A. KIIIA is the first μ-conotoxin to show enhanced activity as pH is lowered, apparently resulting from titration of the free N terminus. Removal of this free amino group reduced the pH sensitivity by 10-fold. Recognition of these molecular determinants of KIIIA block may facilitate further development of subtype-specific, sodium channel blockers to treat hyperexcitability disorders.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: The first μ-conotoxin studied, μCTX GIIIA, preferentially blocked voltage-gated skeletal muscle sodium channels, Na(v)1.4, while μCTX PIIIA was the first to show significant blocking action against neuronal voltage-gated sodium channels. PIIIA shares >60% sequence identity with the well-studied GIIIA, and both toxins preferentially block the skeletal muscle sodium channel isoform. Two important features of blocking by wild-type GIIIA are the toxin's high binding affinity and the completeness of block of a single channel by a bound toxin molecule. With GIIIA, neutral replacement of the critical residue, Arg-13, allows a residual single-channel current (∼30% of the unblocked, unitary amplitude) when the mutant toxin is bound to the channel and reduces the binding affinity of the toxin for Na(v)1.4 (∼100-fold) [Becker, S., et al. (1992) Biochemistry 31, 8229-8238]. The homologous residue in PIIIA, Arg-14, is also essential for completeness of block but less important in the toxin's binding affinity (∼55% residual current and ∼11-fold decrease in affinity when substituted with alanine or glutamine). The weakened dominance of this key arginine in PIIIA is also seen in the fact that there is not just one (R13 in GIIIA) but three basic residues (R12, R14, and K17) for which individual neutral replacement enables a substantial residual current through the bound channel. We suggest that, despite a high degree of sequence conservation between GIIIA and PIIIA, the weaker dependence of PIIIA's action on its key arginine and the presence of a nonconserved histidine near the C-terminus may contribute to the greater promiscuity of its interactions with different sodium channel isoforms.
[Show abstract][Hide abstract] ABSTRACT: The ClC protein family includes voltage-gated chloride channels and chloride/proton exchangers. In eukaryotes, ClC proteins regulate membrane potential of excitable cells, contribute to epithelial transport, and aid in lysosomal acidification. Although structure/function studies of ClC proteins have been aided greatly by the available crystal structures of a bacterial ClC chloride/proton exchanger, the availability of useful pharmacological tools, such as peptide toxin inhibitors, has lagged far behind that of their cation channel counterparts. Here we report the isolation, from Leiurus quinquestriatus hebraeus venom, of a peptide toxin inhibitor of the ClC-2 chloride channel. This toxin, GaTx2, inhibits ClC-2 channels with a voltage-dependent apparent K(D) of approximately 20 pm, making it the highest affinity inhibitor of any chloride channel. GaTx2 slows ClC-2 activation by increasing the latency to first opening by nearly 8-fold but is unable to inhibit open channels, suggesting that this toxin inhibits channel activation gating. Finally, GaTx2 specifically inhibits ClC-2 channels, showing no inhibitory effect on a battery of other major classes of chloride channels and voltage-gated potassium channels. GaTx2 is the first peptide toxin inhibitor of any ClC protein. The high affinity and specificity displayed by this toxin will make it a very powerful pharmacological tool to probe ClC-2 structure/function.
[Show abstract][Hide abstract] ABSTRACT: Peptide toxins from animal venom have been used for many years for the identification and study of cation-permeable ion channels. However, no peptide toxins have been identified that interact with known anion-selective channels, including cystic fibrosis transmembrane conductance regulator (CFTR), the protein defective in cystic fibrosis and a member of the ABC transporter superfamily. Here, we describe the identification and initial characterization of a novel 3.7-kDa peptide toxin, GaTx1, which is a potent and reversible inhibitor of CFTR, acting from the cytoplasmic side of the membrane. Thus, GaTx1 is the first peptide toxin identified that inhibits a chloride channel of known molecular identity. GaTx1 exhibited high specificity, showing no effect on a panel of nine transport proteins, including Cl(-) and K(+) channels, and ABC transporters. GaTx1-mediated inhibition of CFTR channel activity is strongly state-dependent; both potency and efficacy are reduced under conditions of elevated [ATP], suggesting that GaTx1 may function as a non-competitive inhibitor of ATP-dependent channel gating. This tool will allow the application of new quantitative approaches to study CFTR structure and function, particularly with respect to the conformational changes that underlie transitions between open and closed states.