The recent development of techniques for recording currents through single ionic channels has led to the identification of a K+-specific channel that is activated by cytoplasmic Ca2+. The channel has complex properties, being activated by depolarizing voltages and having a voltage-sensitivity that is modulated by cytoplasmic Ca2+ levels. The conduction behaviour of the channel is also unusual, its high ionic selectivity being displayed simultaneously with a very high unitary conductance. Very little is known about the biochemistry of this channel, largely due to the lack of a suitable ligand for use as a biochemical probe for the channel. We describe here a protein inhibitor of single Ca2+-activated K+ channels of mammalian skeletal muscle. This inhibitor, a minor component of the venom of the Israeli scorpion, Leiurus quinquestriatus, reversibly blocks the large Ca2+-activated K+ channel in a simple biomolecular reaction. We have partially purified the active component, a basic protein of relative molecular mass (Mr) approximately 7,000.

Voltage-sensitive sodium channels and calcium channels are homologous proteins with distinctly different selectivity for permeation of inorganic cations. This difference in function is specified by amino acid residues located within P-region segments that link presumed transmembrane elements S5 and S6 in each of four repetitive Domains I, II, III, and IV. By analyzing the selective permeability of Na+, K+, and Ca2+ in various mutants of the mu 1 rat muscle sodium channel, the results in this paper support the concept that a conserved motif of four residues contributed by each of the Domains I-IV, termed the DEKA locus in sodium channels and the EEEE locus in calcium channels, determines the ionic selectivity of these channels. Furthermore, the results indicate that the Lys residue in Domain III of the sodium channel is the critical determinant that specifies both the impermeability of Ca2+ and the selective permeability of Na+ over K+. We propose that the alkylammonium ion of the Lys(III) residue acts as an endogenous cation within the ion binding site/selectivity filter of the sodium channel to tune the kinetics and affinity of inorganic cation binding within the pore in a manner analogous to ion-ion interactions that occur in the process of multi-ion channel conduction.

The gating kinetics of a Cat'-activated K' channel from adult rat muscle plasma membrane are studied in artificial planar bilayers . Analysis of single-channel fluctuations distinguishes two Ca 2 '-and voltage-dependent processes : (a) short-lived channel closure (<1 ms) events appearing in a bursting pattern ; (b) opening and closing events ranging from one to several hundred milliseconds in duration . The latter process is studied independently of the first and is denoted as the primary gating mode .

Peptide toxins with high affinity, divergent pharmacological functions, and isoform-specific selectivity are powerful tools for investigating the structure-function relationships of voltagegated sodium channels (VGSCs). Although a number of interesting inhibitors have been reported from tarantula venoms, little is known about the mechanism for their interaction with VGSCs. We show that huwentoxin-IV (HWTX-IV), a 35-residue peptide from tarantula Ornithoctonus huwena venom, preferentially inhibits neuronal VGSC subtypes rNav1.2, rNav1.3, and hNav1.7 compared with muscle subtypes rNav1.4 and hNav1.5. Of the five VGSCs examined, hNav1.7 was most sensitive to HWTX-IV (IC 50 ϳ 26 nM). Following application of 1 M HWTX-IV, hNav1.7 currents could only be elicited with extreme depolarizations (>؉100 mV). Recovery of hNav1.7 channels from HWTX-IV inhibition could be induced by extreme depolarizations or moderate depolarizations lasting several minutes. Site-directed mutagenesis analysis indicated that the toxin docked at neurotoxin receptor site 4 located at the extracellular S3-S4 linker of domain II. Mutations E818Q and D816N in hNav1.7 decreased toxin affinity for hNav1.7 by ϳ300-fold, whereas the reverse mutations in rNav1.4 (N655D/ Q657E) and the corresponding mutations in hNav1.5 (R812D/ S814E) greatly increased the sensitivity of the muscle VGSCs to HWTX-IV. Our data identify a novel mechanism for sodium channel inhibition by tarantula toxins involving binding to neurotoxin receptor site 4. In contrast to scorpion ␤-toxins that trap the IIS4 voltage sensor in an outward configuration, we propose that HWTX-IV traps the voltage sensor of domain II in the inward, closed configuration.

Mutational and biophysical analysis suggests that an intracellular COOH-terminal domain of the large conductance Ca 2؉ -activated K ؉ channel (BK channel) contains Ca 2؉ -binding site(s) that are allosterically coupled to channel opening. However the structural basis of Ca 2؉ binding to BK channels is unknown. To pursue this question, we overexpressed the COOH-terminal 280 residues of the Drosophila slowpoke BK channel (Dslo-C280) as a FLAG-and His 6-tagged protein in Escherichia coli. We purified Dslo-C280 in soluble form and used a 45 Ca 2؉ -overlay protein blot assay to detect Ca 2؉ binding. Dslo-C280 exhibits specific binding of 45 Ca 2؉ in comparison with various control proteins and known EF-hand Ca 2؉ -binding proteins. A mutation (D5N5) of Dslo-C280, in which five consecutive Asp residues of the ''Ca-bowl'' motif are changed to Asn, reduces 45 Ca 2؉ -binding activity by 56%. By electrophysiological assay, the corresponding D5N5 mutant of the Drosophila BK channel expressed in HEK293 cells exhibits lower Ca 2؉ sensitivity for activation and a shift of Ϸ؉80 mV in the midpoint voltage for activation. This effect is associated with a decrease in the Hill coefficient (N) for activation by Ca 2؉ and a reduction in apparent Ca 2؉ affinity, suggesting the loss of one Ca 2؉ -binding site per monomer. These results demonstrate a functional correlation between Ca 2؉ binding to a specific region of the BK protein and Ca 2؉ -dependent activation, thus providing a biochemical approach to study this process.

A Ca-activated, K-selective channel from plasma membrane of rat skeletal muscle was studied in artificial lipid bilayers formed from either phosphatidylethanolamine (PE) or phosphatidylserine (PS). In PE, the single-channel conductance exhibited a complex dependence on symmetrical K+ concentration that could not be described by simple Michaelis-Menten saturation. At low K+ concentrations the channel conductance was higher in PS membranes, but approached the same conductance observed in PE above 0.4 m KCl. At the same Ca2+ concentration and voltage, the probability of channel opening was significantly greater in PS than PE. The differences in the conduction and gating, observed in the two lipids, can be explained by the negative surface charge of PS compared to the neutral PE membrane. Model calculations of the expected concentrations of K+ and Ca2+ at various distances from a PS membrane surface, using Gouy-Chapman-Stern theory, suggest that the K+-conduction and Ca2+-activation sites sense a similar fraction of the surface potential, equivalent to the local electrostatic potential at a distance of 9 A from the surface.

Recent evidence indicates that ionic selectivity in voltage-gated Na+ channels is mediated by a small number of residues in P-region segments that link transmembrane elements S5 and S6 in each of four homologous domains denoted I, II, III, and IV. Important determinants for this function appear to be a set of conserved charged residues in the first three homologous domains, Asp(I), Glu(II), and Lys(III), located in a region of the pore called the DEKA locus. In this study, we examined several Ala-substitution mutations of these residues for alterations in ionic selectivity, inhibition of macroscopic current by external Ca2+ and H+, and molecular sieving behavior using a series of organic cations ranging in size from ammonium to tetraethylammonium. Whole-cell recording of wild-type and mutant channels of the rat muscle μ1 Na+ channel stably expressed in HEK293 cells was used to compare macroscopic current–voltage behavior in the presence of various external cations and an intracellular reference solution containing Cs+ and very low Ca2+. In particular, we tested the hypothesis that the Lys residue in domain III of the DEKA locus is responsible for restricting the permeation of large organic cations. Mutation of Lys(III) to Ala largely eliminated selectivity among the group IA monovalent alkali cations (Li+, Na+, K+, Rb+, Cs+) and permitted inward current of group IIA divalent cations (Mg2+, Ca2+, Sr2+, Ba2+). This same mutation also resulted in the acquisition of permeability to many large organic cations such as methylammonium, tetramethylammonium, and tetraethylammonium, all of which are impermeant in the native channel. The results lead to the conclusion that charged residues of the DEKA locus play an important role in molecular sieving behavior of the Na+ channel pore, a function that has been previously attributed to a hypothetical region of the channel called the “selectivity filter.” A detailed examination of individual contributions of the Asp(I), Glu(II), and Lys(III) residues and the dependence on molecular size suggests that relative permeability of organic cations is a complex function of the size, charge, and polarity of these residues and cation substrates. As judged by effects on macroscopic conductance, charged residues of the DEKA locus also appear to play a role in the mechanisms of block by external Ca2+ and H+, but are not essential for the positive shift in activation voltage that is produced by these ions.

Single Na' channels from rat skeletal muscle plasma membrane vesicles were inserted into planar lipid bilayers formed from neutral phospholipids and were observed in the presence of batrachotoxin . The batrachotoxinmodified channel activates in the voltage range -120 to -80 mV and remains open almost all the time at voltages positive to -60 mV . Low levels of tetrodotoxin (TTX) induce slow fluctuations .of channel current, which represent the binding and dissociation of single TTX molecules to single channels . The rates of association and dissociation of TTX are both voltage dependent, and the association rate is competitively inhibited by Na'. This inhibition is observed only when Na' is increased on the TTX binding side of the channel . The results suggest that the TTX receptor site is located at the channel's outer mouth, and that the Na' competition site is not located deeply within the channel's conduction pathway.