Transfer of the scorpion toxin receptor to an insensitive potassium channel

A single conserved basic residue in the potassium channel filter region controls KCNQ1 insensitivity toward scorpion toxins

Although many studies concerning the sensitivity mechanism of scorpion toxin-potassium channel interactions have been reported, few have explored the biochemical insensitivity mechanisms of potassium channel receptors toward natural scorpion toxin peptides, such as the KCNQ1 channel. Here, by sequence alignment analyses of the human KCNQ1 channel and scorpion potassium channel MmKv2, which is completely insensitive to scorpion toxins, we proposed that the insensitivity mechanism of KCNQ1 toward natural scorpion toxins might involve two functional regions, the turret and filter regions. Based on this observation, a series of KCNQ1 mutants were constructed to study molecular mechanisms of the KCNQ1 channel insensitivity toward natural scorpion toxins. Electrophysiological studies of chimera channels showed that the channel filter region controls KCNQ1 insensitivity toward the classical scorpion toxin ChTX. Interestingly, further residue mutant experiments showed that a single basic residue in the filter region determined the insensitivity of KCNQ1 channels toward scorpion toxins. Our present work showed that amino acid residue diversification at common sites controls the sensitivity and insensitivity of potassium channels toward scorpion toxins. The unique insensitivity mechanism of KCNQ1 toward natural scorpion toxins will accelerate the rational design of potent peptide inhibitors toward this channel.

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Insensitivity mechanism of KCNQ1 towards scorpion toxins was still unclear.

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A single basic residue in the KCNQ1 filter region controls its insensitivity.

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Amino acid residue diversification controls KCNQ1 sensitivity and insensitivity.

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Our work will accelerate rational design of KCNQ1 peptide inhibitors.

Targeting the ion channel Kv1.3 with scorpion venom peptides engineered for potency, selectivity, and half-life

Ion channels are an attractive class of drug targets, but progress in developing inhibitors for therapeutic use has been limited largely due to challenges in identifying subtype selective small molecules. Animal venoms provide an alternative source of ion channel modulators, and the venoms of several species, such as scorpions, spiders and snails, are known to be rich sources of ion channel modulating peptides. Importantly, these peptides often bind to hyper-variable extracellular loops, creating the potential for subtype selectivity rarely achieved with small molecules. We have engineered scorpion venom peptides and incorporated them in fusion proteins to generate highly potent and selective Kv1.3 inhibitors with long in vivo half-lives. Kv1.3 has been reported to play a role in human T cell activation, and therefore, these Kv1.3 inhibitor fusion proteins may have potential for the treatment of autoimmune diseases. Our results support an emerging approach to generating subtype selective therapeutic ion channel inhibitors.

Different types of toxins targeting TRPV1 in pain

The transient receptor potential vanilloid 1(TRPV1) channels are members of the transient receptor potential (TRP) superfamily. Members of this family are expressed in primary sensory neurons and are best known for their role in nociception and sensory transmission. Multiple painful stimuli can activate these channels. In this review, we discussed the mechanisms of different types of venoms that target TRPV1, such as scorpion venom, botulinum neurotoxin, spider toxin, ciguatera fish poisoning (CFP) and neurotoxic shellfish poisoning (NSP). Some of these toxins activate TRPV1; however, some do not. Regardless of TRPV1 inhibition or activation, they occur through different pathways. For example, BoNT/A decreases TRPV1 expression levels by blocking TRPV1 trafficking to the plasma membrane, although the exact mechanism is still under debate. Vanillotoxins from tarantula (Psalmopoeus cambridgei) are proposed to activate TRPV1 via interaction with a region of TRPV1 that is homologous to voltage-dependent ion channels. Here, we offer a description of the present state of knowledge for this complex subject.

Structure of a pore-blocking toxin in complex with a eukaryotic voltage-dependent K(+) channel

Pore-blocking toxins inhibit voltage-dependent K⁺ channels (K_v channels) by plugging the ion-conduction pathway. We have solved the crystal structure of paddle chimera, a K_v channel in complex with charybdotoxin (CTX), a pore-blocking toxin. The toxin binds to the extracellular pore entryway without producing discernable alteration of the selectivity filter structure and is oriented to project its Lys27 into the pore. The most extracellular K⁺ binding site (S1) is devoid of K⁺ electron-density when wild-type CTX is bound, but K⁺ density is present to some extent in a Lys27Met mutant. In crystals with Cs⁺ replacing K⁺, S1 electron-density is present even in the presence of Lys27, a finding compatible with the differential effects of Cs⁺ vs K⁺ on CTX affinity for the channel. Together, these results show that CTX binds to a K⁺ channel in a lock and key manner and interacts directly with conducting ions inside the selectivity filter.

DOI:http://dx.doi.org/10.7554/eLife.00594.001

The deadly toxins produced by many creatures, including spiders, snakes, and scorpions, work by blocking the ion channels that are essential for the normal operation of many different types of cells. Ion channels are proteins and, as their name suggests, they allow ions—usually sodium, potassium, or calcium ions—to move in and out of cells. They are especially important for cells that generate or respond to electrical signals, such as neurons and the cells in heart muscle.

Ion channels are located in the lipid membranes that surround all cells, and the ions enter or leave the cell via a pore that runs through the channel protein. They can be opened and closed (or ‘gated’) in different ways: some ion channels open and close in response to voltages, whereas others are gated by biomolecules, such as neurotransmitters, that bind to them.

Now, Banerjee et al. have used x-ray crystallography to study the structure of the complex that is formed when charybdotoxin (CTX), a toxin that is found in scorpion venom, blocks a voltage-gated potassium channel. Previous studies have shown that CTX binds to the channel on the extracellular side of the pore. Banerjee et al. show that the toxin fits into the entrance to the channel like a key into a lock, which means the toxin is preformed to fit the shape of the channel.

The potassium ion channel is made up of four subunits, and the pore contains four ion-binding sites that form a ‘selectivity filter’: it is this filter that ensures that only potassium ions can pass through the channel when it is open. When CTX binds to the channel, a lysine residue poised at a critical position on the toxin is so close to the outermost ion-binding site that it prevents potassium ions binding to the site. The structure determined by Banerjee et al. explains many previous findings, including the fact that ions entering the pore from inside the cell can disrupt the binding between the toxin and the ion channel protein. It remains to be seen if the toxins that target the pore of other types of ion channels work in the same way.

DOI:http://dx.doi.org/10.7554/eLife.00594.002

Chemical derivatization and purification of peptide-toxins for probing ion channel complexes

Ion channels function as multi-protein complexes made up of ion-conducting α-subunits and regulatory β-subunits. To detect, identify, and quantitate the regulatory β-subunits in functioning K(+) channel complexes, we have chemically derivatized peptide-toxins that specifically react with strategically placed cysteine residues in the channel complex. Two protein labeling approaches have been developed to derivatize the peptide-toxin, charybdotoxin, with hydrophilic and hydrophobic bismaleimides, and other molecular probes. Using these cysteine-reactive peptide-toxins, we have specifically targeted KCNQ1-KCNE1 K(+) channel complexes expressed in both Xenopus oocytes and mammalian cells. The modular design of the reagents should permit this approach to be applied to the many ion channel complexes involved in electrical excitability as well as salt and water homoeostasis.

Chemical control of metabolically-engineered voltage-gated K+ channels

Metabolic oligosaccharide engineering is a powerful approach for installing unnatural glycans with unique functional groups into the glycocalyx of living cells and animals. Using this approach, we showed that K(+) channel complexes decorated with thiol-containing sialic acids were irreversibly inhibited with scorpion toxins bearing a pendant maleimide group. Irreversible inhibition required a glycosylated K(+) channel subunit and was completely reversible with mild reductant when the tether connecting the toxin to the maleimide contained a disulfide bond. Cleavage of the disulfide bond not only restored function, but delivered a biotin moiety to the modified K(+) channel subunit, providing a novel approach for preferentially labeling wild type K(+) channel complexes functioning in cells.

Arrangement of Kv1 alpha subunits dictates sensitivity to tetraethylammonium

Shaker-related Kv1 channels contain four channel-forming α subunits. Subfamily member Kv1.1 often occurs oligomerized with Kv1.2 α subunits in synaptic membranes, and so information was sought on the influence of their positions within tetramers on the channels’ properties. Kv1.1 and 1.2 α genes were tandem linked in various arrangements, followed by expression as single-chain proteins in mammalian cells. As some concatenations reported previously seemed not to reliably position Kv1 subunits in their assemblies, the identity of expressed channels was methodically evaluated. Surface protein, isolated by biotinylation of intact transiently transfected HEK-293 cells, gave Kv1.1/1.2 reactivity on immunoblots with electrophoretic mobilities corresponding to full-length concatenated tetramers. There was no evidence of protein degradation, indicating that concatemers were delivered intact to the plasmalemma. Constructs with like genes adjacent (Kv1.1-1.1-1.2-1.2 or Kv1.2-1.2-1.1-1.1) yielded delayed-rectifying, voltage-dependent K⁺ currents with activation parameters and inactivation kinetics slightly different from the diagonally positioned genes (Kv1.1-1.2-1.1-1.2 or 1.2–1.1-1.2-1.1). Pore-blocking petidergic toxins, α dendrotoxin, agitoxin-1, tityustoxin-Kα, and kaliotoxin, were unable to distinguish between the adjacent and diagonal concatamers. Unprecedentedly, external application of the pore-blocker tetraethylammonium (TEA) differentially inhibited the adjacent versus diagonal subunit arrangements, with diagonal constructs having enhanced susceptibility. Concatenation did not directly alter the sensitivities of homomeric Kv1.1 or 1.2 channels to TEA or the toxins. TEA inhibition of currents generated by channels made up from dimers (Kv1.1-1.2 and/or Kv1.2-1.1) was similar to the adjacently arranged constructs. These collective findings indicate that assembly of α subunits can be directed by this optimized concatenation, and that subunit arrangement in heteromeric Kv channels affects TEA affinity.

A bivalent tarantula toxin activates the capsaicin receptor, TRPV1, by targeting the outer pore domain

Toxins have evolved to target regions of membrane ion channels that underlie ligand binding, gating, or ion permeation, and have thus served as invaluable tools for probing channel structure and function. Here, we describe a peptide toxin from the Earth Tiger tarantula that selectively and irreversibly activates the capsaicin- and heat-sensitive channel, TRPV1. This high-avidity interaction derives from a unique tandem repeat structure of the toxin that endows it with an antibody-like bivalency. The "double-knot" toxin traps TRPV1 in the open state by interacting with residues in the presumptive pore-forming region of the channel, highlighting the importance of conformational changes in the outer pore region of TRP channels during activation.

Sea anemone toxins affecting potassium channels

The great diversity of K(+) channels and their wide distribution in many tissues are associated with important functions in cardiac and neuronal excitability that are now better understood thanks to the discovery of animal toxins. During the past few decades, sea anemones have provided a variety of toxins acting on voltage-sensitive sodium and, more recently, potassium channels. Currently there are three major structural groups of sea anemone K(+) channel (SAK) toxins that have been characterized. Radioligand binding and electrophysiological experiments revealed that each group contains peptides displaying selective activities for different subfamilies of K(+) channels. Short (35-37 amino acids) peptides in the group I display pore blocking effects on Kv1 channels. Molecular interactions of SAK-I toxins, important for activity and binding on Kv1 channels, implicate a spot of three conserved amino acid residues (Ser, Lys, Tyr) surrounded by other less conserved residues. Long (58-59 amino acids) SAK-II peptides display both enzymatic and K(+) channel inhibitory activities. Medium size (42-43 amino acid) SAK-III peptides are gating modifiers which interact either with cardiac HERG or Kv3 channels by altering their voltage-dependent properties. SAK-III toxins bind to the S3C region in the outer vestibule of Kv channels. Sea anemones have proven to be a rich source of pharmacological tools, and some of the SAK toxins are now useful drugs for the diagnosis and treatment of autoimmune diseases.

Tarantula toxins interacting with voltage sensors in potassium channels

Voltage-activated ion channels open and close in response to changes in membrane voltage, a process that is crucial for electrical signaling in the nervous system. The venom from many poisonous creatures contains a diverse array of small protein toxins that bind to voltage-activated channels and modify the gating mechanism. Hanatoxin and a growing number of related tarantula toxins have been shown to inhibit activation of voltage-activated potassium (K(v)) channels by interacting with their voltage-sensing domains. This review summarizes our current understanding of the mechanism by which these toxins alter gating, the location of the toxin receptor within K(v) channels and the disposition of this receptor with respect to the lipid membrane. The conservation of tarantula toxin receptors among voltage-activated ion channels will also be discussed.

Toxin-induced conformational changes in a potassium channel revealed by solid-state NMR

The active site of potassium (K+) channels catalyses the transport of K+ ions across the plasma membrane--similar to the catalytic function of the active site of an enzyme--and is inhibited by toxins from scorpion venom. On the basis of the conserved structures of K+ pore regions and scorpion toxins, detailed structures for the K+ channel-scorpion toxin binding interface have been proposed. In these models and in previous solution-state nuclear magnetic resonance (NMR) studies using detergent-solubilized membrane proteins, scorpion toxins were docked to the extracellular entrance of the K+ channel pore assuming rigid, preformed binding sites. Using high-resolution solid-state NMR spectroscopy, here we show that high-affinity binding of the scorpion toxin kaliotoxin to a chimaeric K+ channel (KcsA-Kv1.3) is associated with significant structural rearrangements in both molecules. Our approach involves a combined analysis of chemical shifts and proton-proton distances and demonstrates that solid-state NMR is a sensitive method for analysing the structure of a membrane protein-inhibitor complex. We propose that structural flexibility of the K+ channel and the toxin represents an important determinant for the high specificity of toxin-K+ channel interactions.

A structural interpretation of voltage-gated potassium channel inactivation

After channel activation, and in some cases with sub-threshold depolarizing stimuli, Kv channels undergo a time-dependent loss of conductivity by a family of mechanisms termed inactivation. To date, all identified inactivation mechanisms underlying loss of conduction in Kv channels appear to be distinct from deactivation, i.e. closure of the voltage-operated activation gate by changes in transmembrane voltage. Instead, Kv channel inactivation entails entry of channels into a stable, non-conducting state, and thereby functionally reduces the availability of channels for opening. That is, if a channel has inactivated, some time must expire after repolarization of the membrane voltage to allow the channel to recover and become available to open again. Dramatic differences between Kv channel types in the time course of inactivation and recovery underlie various roles in regulating cellular excitability and repolarization of action potentials. Therefore, the range of inactivation mechanisms exhibited by different Kv channels provides important physiological means by which the duration of action potentials in many excitable tissues can be regulated at different frequencies and potentials. In this review, we provide a detailed discussion of recent work characterizing structural and functional aspects of Kv channel gating, and attempt to reconcile these recent results with classical experimental work carried out throughout the 1990s that identified and characterized the basic mechanisms and properties of Kv channel inactivation. We identify and discuss numerous gaps in our understanding of inactivation, and review them in the light of new structural insights into channel gating.

Pseudechetoxin binds to the pore turret of cyclic nucleotide-gated ion channels

Peptide toxins are invaluable tools for studying the structure and physiology of ion channels. Pseudechetoxin (PsTx) is the first known peptide toxin that targets cyclic nucleotide–gated (CNG) ion channels, which play a critical role in sensory transduction in the visual and olfactory systems. PsTx inhibited channel currents at low nM concentrations when applied to the extracellular face of membrane patches expressing olfactory CNGA2 subunits. Surprisingly, 500 nM PsTx did not inhibit currents through channels formed by the CNGA3 subunit from cone photoreceptors. We have exploited this difference to identify the PsTx-binding site on the extracellular face of CNG channels. Studies using chimeric channels revealed that transplantation of the pore domain from CNGA2 was sufficient to confer high affinity PsTx binding upon a CNGA3 background. To further define the binding site, reciprocal mutations were made at 10 nonidentical amino acid residues in this region. We found that two residues in CNGA2, D316 and Y321, were essential for high-affinity inhibition by PsTx. Furthermore, replacement of both residues was required to confer high-affinity PsTx inhibition upon CNGA3. Several other residues, including E325, also form favorable interactions with PsTx. In the CNGA2-E325K mutant, PsTx affinity was reduced by ∼5-fold to 120 nM. An electrostatic interaction with D316 does not appear to be the primary determinant of PsTx affinity, as modification of the D316C mutant with a negatively charged methanethiosulfonate reagent did not restore high affinity inhibition. The residues involved in PsTx binding are found within the pore turret and helix, in similar positions to residues that form the receptor for pore-blocking toxins in voltage-gated potassium channels. Furthermore, biophysical properties of PsTx block, including an unfavorable interaction with permeant ions, also suggest that it acts as a pore blocker. In summary, PsTx seems to occlude the entrance to the pore by forming high-affinity contacts with the pore turret, which may be larger than that found in the KcsA structure.

Molecular basis of α-KTx specificity

Potassium channel inhibitor peptides from scorpion venom, alpha-KTx, have greatly advanced our understanding of potassium channel structure and function, Because of their high affinity interaction with the outer pore, alpha-KTx's have aided, in identification of amino acids lining the pore and of proteins constituting functional channels. The alpha-KTx's display a large range of affinities for different potassium channels with differences in binding free energy exceeding approximately 8 kcal/mol. These differences in affinities are the foundation of alpha-KTx specificity and have aided in revealing the physiological and patho-physiological roles of potassium channels. The alpha-KTx subfamilies 1-3, display gross differences in specificity for maxi-K vs. KV channels. However, many potassium channels are largely untouched by alpha-KTx's. Differences in toxin binding free energy provide a quantitative framework for defining specificity. As a practical criterion for specificity a minimum binding free energy difference of 2.72 kcal/mol is proposed. Binding free energy differences for wild-type and mutant toxins and channels can point to amino acids underlying specificity and to unique features of potassium channel outer pores. Known 3D structures of potassium channels in combination with CLUSTALW sequence alignment of over 60 potassium channels reveal significant variation in alpha-KTx binding domains. Structure-based homology models of potassium channels complexed with alpha-KTxs, in combination with measurements of toxin binding free energy, will further our understanding of the molecular basis of alpha-KTx specificity.

Comparison of sea anemone and scorpion toxins binding to Kv1 channels: an example of convergent evolution

Comparison of data from functional mapping carried out on scorpion and sea anemones toxins blocking currents through voltage-gated potassium channels revealed that, despite their different 3D structures, the binding cores of these toxins displayed some similarities. Further molecular modeling studies suggested that these similarities reflect the use by these toxins of a common binding mode to exert their blocking function. Therefore, scorpion and sea anemone toxins offer an example of mechanistic convergent evolution.

Ito channels are octomeric complexes with four subunits of each Kv4.2 and K+ channel-interacting protein 2

Mammalian voltage-gated K+ channels are assemblies of pore-forming alpha-subunits and modulating beta-subunits. To operate correctly, Kv4 alpha-subunits in the heart and central nervous system require recently identified beta-subunits of the neuronal calcium sensing protein family called K+ channel-interacting proteins (KChIPs). Here, Kv4.2.KChIP2 channels are purified, integrity of isolated complexes confirmed, molar ratio of the subunits determined, and subunit valence established. A complex has 4 subunits of each type, a stoichiometry expected for other channels employing neuronal calcium sensing beta-subunits.

Protein surface recognition by rational design: nanomolar ligands for potassium channels

The rational design and development of four-fold symmetrical ligands for potassium channels is described.

Charybdotoxin Binding in the IKs Pore Demonstrates Two MinK Subunits in Each Channel Complex

I(Ks) voltage-gated K(+) channels contain four pore-forming KCNQ1 subunits and MinK accessory subunits in a number that has been controversial. Here, I(Ks) channels assembled naturally by monomer subunits are compared to those with linked subunits that force defined stoichiometries. Two strategies that exploit charybdotoxin (CTX)-sensitive subunit variants are applied. First, CTX on rate, off rate, and equilibrium affinity are found to be the same for channels of monomers and those with a fixed 2:4 MinK:KCNQ1 valence. Second, 3H-CTX and an antibody are used to directly quantify channels and MinK subunits, respectively, showing 1.97 +/- 0.07 MinK per I(Ks) channel. Additional MinK subunits do not enter channels of monomeric subunits or those with fixed 2:4 valence. We conclude that two MinK subunits are necessary, sufficient, and the norm in I(Ks) channels. This stoichiometry is expected for other K(+) channels that contain MinK or MinK-related peptides (MiRPs).

Defining the conductance of the closed state in a voltage-gated K+ channel

The opening and closing of the ion conduction pathway in ion channels underlies the generation and propagation of electrical signals in biological systems. Although electrophysiological approaches to measuring the flow of ions in the open state have contributed profoundly to our understanding of ion permeation and gating, it remains unclear how much the ion-throughput rate decreases upon closure of the ion conduction pore. To address this fundamental question, we expressed the Shaker Kv channel at high levels and then measured macroscopic K+ currents at negative membrane voltages and counted the number of channels by quantifying the translocation of gating charge. Our results show that the conductance of the closed state is between 0 and 0.16 fS, or at least 100,000 times lower than for the open state of the channel, indicating that the flow of ions is very tightly regulated in this class of K+ channels.

Influence of pore residues on permeation properties in the Kv2.1 potassium channel. Evidence for a selective functional interaction of K+ with the outer vestibule

The Kv2.1 potassium channel contains a lysine in the outer vestibule (position 356) that markedly reduces open channel sensitivity to changes in external [K(+)]. To investigate the mechanism underlying this effect, we examined the influence of this outer vestibule lysine on three measures of K(+) and Na(+) permeation. Permeability ratio measurements, measurements of the lowest [K(+)] required for interaction with the selectivity filter, and measurements of macroscopic K(+) and Na(+) conductance, were all consistent with the same conclusion: that the outer vestibule lysine in Kv2.1 interferes with the ability of K(+) to enter or exit the extracellular side of the selectivity filter. In contrast to its influence on K(+) permeation properties, Lys 356 appeared to be without effect on Na(+) permeation. This suggests that Lys 356 limited K(+) flux by interfering with a selective K(+) binding site. Combined with permeation studies, results from additional mutagenesis near the external entrance to the selectivity filter indicated that this site was located external to, and independent from, the selectivity filter. Protonation of a naturally occurring histidine in the same outer vestibule location in the Kv1.5 potassium channel produced similar effects on K(+) permeation properties. Together, these results indicate that a selective, functional K(+) binding site (e.g., local energy minimum) exists in the outer vestibule of voltage-gated K(+) channels. We suggest that this site is the location of K(+) hydration/dehydration postulated to exist based on the structural studies of KcsA. Finally, neutralization of position 356 enhanced outward K(+) current magnitude, but did not influence the ability of internal K(+) to enter the pore. These data indicate that in Kv2.1, exit of K(+) from the selectivity filter, rather than entry of internal K(+) into the channel, limits outward current magnitude. We discuss the implications of these findings in relation to the structural basis of channel conductance in different K(+) channels.

Effect of external pH on activation of the Kv1.5 potassium channel

We studied the mechanism by which external acidification from pH 7.3 to 6.8 reduced current magnitude in the Kv1.5 potassium channel. At physiological external [K(+)], a shift in the voltage-dependence of activation was entirely responsible for the acidification-induced decrease in Kv1.5 current magnitude (pK = 7.15). Elevation of external [Ca(2+)] or [Mg(2+)] identically shifted activation curves to the right and identically shifted the pH-sensitivity of the activation curves to more acidic values. Similar observations were made with the Kv2.1 K(+) channel, except that the pK for the activation shift was out of the physiological range. These data are consistent with a mechanism by which acidification shifted activation via modification of a local surface potential. Elimination of eight positive charges within the outer vestibule of the conduction pathway had no effect on the voltage-dependence of activation at pH 7.3 or higher, which suggested that sites exposed to the conduction pathway within the outer vestibule did not directly contribute to the relevant local surface potential. However, mutations at position 487 (within the conduction pathway) displaced the pK of the pH-sensitive shift in activation, such that the sensitivity of Kv1.5 current to physiologically relevant changes in pH was reduced or eliminated. These results suggest that, among voltage-gated K(+) channels, activation in Kv1.5 is uniquely sensitive to physiologically relevant changes in pH because the pK for the sites that contribute to the local surface potential effect is near pH 7. Moreover, the pK for the activation shift depends not only on the nature of the sites involved but also on structural orientation conferred, in part, by at least one residue within the conduction pathway.

Control of outer vestibule dynamics and current magnitude in the Kv2.1 potassium channel

In Kv2.1 potassium channels, changes in external [K+] modulate current magnitude as a result of a K+-dependent interconversion between two outer vestibule conformations. Previous evidence indicated that outer vestibule conformation (and thus current magnitude) is regulated by the occupancy of a selectivity filter binding site by K+. In this paper, we used the change in current magnitude as an assay to study how the interconversion between outer vestibule conformations is controlled. With 100 mM internal K+, rapid elevation of external [K+] from 0 to 10 mM while channels were activated produced no change in current magnitude (outer vestibule conformation did not change). When channels were subsequently closed and reopened in the presence of elevated [K+], current magnitude was increased (outer vestibule conformation had changed). When channels were activated in the presence of low internal [K+], or when K+ flow into conducting channels was transiently interrupted by an internal channel blocker, increasing external [K+] during activation did increase current magnitude (channel conformation did change). These data indicate that, when channels are in the activated state under physiological conditions, the outer vestibule conformation remains fixed despite changes in external [K+]. In contrast, when channel occupancy is lowered, (by channel closing, an internal blocker or low internal [K+]), the outer vestibule can interconvert between the two conformations. We discuss evidence that the ability of the outer vestibule conformation to change is regulated by the occupancy of a nonselectivity filter site by K+. Independent of the outer vestibule-based potentiation mechanism, Kv2.1 was remarkably insensitive to K+-dependent processes that influence current magnitude (current magnitude changed by <7% at membrane potentials between -20 and 30 mV). Replacement of two outer vestibule lysines in Kv2.1 by smaller neutral amino acids made current magnitude dramatically more sensitive to the reduction in K+ driving force (current magnitude changed by as much as 40%). When combined, these outer vestibule properties (fixed conformation during activation and the presence of lysines) all but prevent variation in Kv2.1 current magnitude when [K+] changes during activation. Moreover, the insensitivity of Kv2.1 current magnitude to changes in K+ driving force promotes a more uniform modulation of current over a wide range of membrane potentials by the K+-dependent regulation of outer vestibule conformation.

Structure of the BgK-Kv1.1 complex based on distance restraints identified by double mutant cycles. Molecular basis for convergent evolution of Kv1 channel blockers

A structural model of BgK, a sea anemone toxin, complexed with the S5-S6 region of Kv1.1, a voltage-gated potassium channel, was determined by flexible docking under distance restraints identified by a double mutant cycles approach. This structure provides the molecular basis for identifying the major determinants of the BgK-Kv1.1 channel interactions involving the BgK dyad residues Lys(25) and Tyr(26). These interactions are (i) electrostatic interactions between the extremity of Lys(25) side chain and carbonyl oxygen atoms of residues from the channel selectivity filter that may be strengthened by solvent exclusion provided by (ii) hydrophobic interactions involving BgK residues Tyr(26) and Phe(6) and Kv1.1 residue Tyr(379) whose side chain protrudes in the channel vestibule. In other Kv1 channel-BgK complexes, these interactions are likely to be conserved, implicating both conserved and variable residues from the channels. The data suggest that the conservation in sea anemone and scorpion potassium channel blockers of a functional dyad composed of a lysine, and a hydrophobic residue reflects their use of convergent binding solutions based on a crucial interplay between these important conserved interactions.

K(V)alpha1 channels in murine arterioles: differential cellular expression and regulation of diameter

The primary objectives of this study were to reveal cell-specific expression patterns and functions of voltage-gated K(+) channel (K(V)alpha1) subunits in precapillary arterioles of the murine cerebral circulation. K(V)alpha1 were detected using peptide-specific antibodies in immunofluorescence and Western blotting assays. K(V)1.2 was localized almost exclusively to endothelial cells, whereas K(V)1.5 was discretely localized to the nerves and nerve terminals that innervate the arterioles. K(V)1.5 also localized specifically to arteriolar nerves in human pial membrane. K(V)1.5 was notable for its absence from smooth muscle cells. K(V)1.3, K(V)1.4, and K(V)1.6 were localized to endothelial and smooth muscle cells, although K(V)1.4 had a low expression level. K(V)1.1 was not expressed. Therefore, we show that different cell types of pial arterioles have distinct physiological expression profiles of K(V)alpha1, conferring the possibility of differential modulation by extracellular and second messengers. Furthermore, we show recombinant agitoxin-2 and margatoxin are potent vasoconstrictors, suggesting that K(V)alpha1 subunits have a major function in determining arteriolar resistance to blood flow.

Two mechanisms of K(+)-dependent potentiation in Kv2.1 potassium channels

Elevation of external [K(+)] potentiates outward K(+) current through several voltage-gated K(+) channels. This increase in current magnitude is paradoxical in that it occurs despite a significant decrease in driving force. We have investigated the mechanisms involved in K(+)-dependent current potentiation in the Kv2.1 K(+) channel. With holding potentials of -120 to -150 mV, which completely removed channels from the voltage-sensitive inactivated state, elevation of external [K(+)] up to 10 mM produced a concentration-dependent increase in outward current magnitude. In the absence of inactivation, currents were maximally potentiated by 38%. At more positive holding potentials, which produced steady-state inactivation, K(+)-dependent potentiation was enhanced. The additional K(+)-dependent potentiation (above 38%) at more positive holding potentials was precisely equal to a K(+)-dependent reduction in steady-state inactivation. Mutation of two lysine residues in the outer vestibule of Kv2.1 (K356 and K382), to smaller, uncharged residues (glycine and valine, respectively), completely abolished K(+)-dependent potentiation that was not associated with inactivation. These mutations did not influence steady-state inactivation or the K(+)-dependent potentiation due to reduction in steady-state inactivation. These results demonstrate that K(+)-dependent potentiation can be completely accounted for by two independent mechanisms: one that involved the outer vestibule lysines and one that involved K(+)-dependent removal of channels from the inactivated state. Previous studies demonstrated that the outer vestibule of Kv2.1 can be in at least two conformations, depending on the occupancy of the selectivity filter by K(+) (Immke, D., M. Wood, L. Kiss, and S. J. Korn. 1999. J. Gen. Physiol. 113:819-836; Immke, D., and S. J. Korn. 2000. J. Gen. Physiol. 115:509-518). This change in conformation was functionally defined by a change in TEA sensitivity. Similar to the K(+)-dependent change in TEA sensitivity, the lysine-dependent potentiation depended primarily (>90%) on Lys-356 and was enhanced by lowering initial K(+) occupancy of the pore. Furthermore, the K(+)-dependent changes in current magnitude and TEA sensitivity were highly correlated. These results suggest that the previously described K(+)-dependent change in outer vestibule conformation underlies the lysine-sensitive, K(+)-dependent potentiation mechanism.

Generating a high affinity scorpion toxin receptor in KcsA-Kv1.3 chimeric potassium channels

The crystal structure of the bacterial K(+) channel, KcsA (Doyle, D. A., Morais, C. J., Pfuetzner, R. A., Kuo, A., Gulbis, J. M., Cohen, S. L., Chait, B. T., and MacKinnon, R. (1998) Science 280, 69-77), and subsequent mutagenesis have revealed a high structural conservation from bacteria to human (MacKinnon, R., Cohen, S. L., Kuo, A., Lee, A., and Chait, B. T. (1998) Science 280, 106-109). We have explored this conservation by swapping subregions of the M1-M2 linker of KcsA with those of the S5-S6 linker of the human Kv-channel Kv1.3. The chimeric K(+) channel constructs were expressed in Escherichia coli, and their multimeric state was analyzed after purification. We used two scorpion toxins, kaliotoxin and hongotoxin 1, which bind specifically to Kv1.3, to analyze the pharmacological properties of the KcsA-Kv1.3 chimeras. The results demonstrate that the high affinity scorpion toxin receptor of Kv1.3 could be transferred to KcsA. Our biochemical studies with purified KcsA-Kv1.3 chimeras provide direct chemical evidence that a tetrameric channel structure is necessary for forming a functional scorpion toxin receptor. We have obtained KcsA-Kv1.3 chimeras with kaliotoxin affinities (IC(50) values of approximately 4 pm) like native Kv1.3 channels. Furthermore, we show that a subregion of the S5-S6 linker may be an important determinant of the pharmacological profile of K(+) channels. Using available structural information on KcsA and kaliotoxin, we have developed a structural model for the complex between KcsA-Kv1.3 chimeras and kaliotoxin to aid future pharmacological studies of K(+) channels.

A point mutation in the maxi-K clone dSlo forms a high affinity site for charybdotoxin

This work investigated the interaction of CTX with two cloned analogues of the maxi-K channel, dSlo and hSlo. dSlo has been reported to be CTX insensitive. Single channel analysis revealed that dSlo was weakly blocked by the toxin, with a very high K(D) of 5.8 microM. The hSlo channels bound wild-type, recombinant CTX with high affinity and in a bimolecular fashion, and displayed a half-blocking concentration (K(D)) of 36 nM. A glutamate residue was substituted for the wild-type threonine at position 290 in dSlo. The mutant channel was expressed in COS-7 cells and reconstituted into lipid bilayers for single channel analysis. The mutant channel bound wild-type, recombinant CTX with high affinity and in a bimolecular fashion, and displayed a half-blocking concentration (K(D)) of 23 nM. Changing just one residue from threonine to glutamate at position 290 in dSlo changed the affinity of the channel's CTX-receptor over 100-fold. Kinetic analysis revealed that this large increase in affinity was due to decreasing the dissociation rate of the toxin. These results suggest that a CTX receptor does exist in the dSlo channel mouth and that the threonine at position 290 destabilizes the toxin on the binding site.

Extracellular Surface Charges in Voltage-Gated Ion Channels

A large number of charged amino acids are in the extracellularly located parts of the voltage-gated ion channels. Recent findings suggest that these surface charges contribute to the channel functions in the sensing of voltage, the binding of substances, and the sensing of H(+) concentration.

Mapping the functional anatomy of BgK on Kv1.1, Kv1.2, and Kv1.3. Clues to design analogs with enhanced selectivity

BgK is a peptide from the sea anemone Bunodosoma granulifera, which blocks Kv1.1, Kv1.2, and Kv1.3 potassium channels. Using 25 analogs substituted at a single position by an alanine residue, we performed the complete mapping of the BgK binding sites for the three Kv1 channels. These binding sites included three common residues (Ser-23, Lys-25, and Tyr-26) and a variable set of additional residues depending on the particular channel. Shortening the side chain of Lys-25 by taking out the four methylene groups dramatically decreased the BgK affinity to all Kv1 channels tested. However, the analog K25Orn displayed increased potency on Kv1.2, which makes this peptide a selective blocker for Kv1.2 (K(D) 50- and 300-fold lower than for Kv1.1 and Kv1.3, respectively). BgK analogs with enhanced selectivity could also be made by substituting residues that are differentially involved in the binding to some of the three Kv1 channels. For example, the analog F6A was found to be >500-fold more potent for Kv1.1 than for Kv1.2 and Kv1.3. These results provide new information about the mechanisms by which a channel blocker distinguishes individual channels among closely related isoforms and give clues for designing analogs with enhanced selectivity.

Agonist activation and alpha-bungarotoxin inhibition of wild type and mutant alpha7 nicotinic acetylcholine receptors

The properties of wild type and mutant rat nicotinic alpha7 receptors expressed in Xenopus oocytes were investigated using electrophysiology and site-directed mutagenesis. When compared at individual agonist concentrations, neither the normalised nicotinic, nor acetylcholine, responses of the wild type receptors were significantly different from the corresponding responses obtained from a first extracellular domain mutant, phenylalanine(189)tyrosine (P0.05). The dissociation constants (K(D)) of the wild type (4.7 nM) and Phe(189)Tyr mutant (5.2 nM) receptors for alpha-bungarotoxin were estimated by an electrophysiological approach. The similarity of the results suggests that the mutation did not lead to a widespread disruption of structure-function relationships, although a slight change in nicotine sensitivity may have occurred. In contrast, the mutations (Tyr(190)Gln, first extracellular domain), (Glu(261)Ala, M2 region) severely compromised receptor function. An additional mutation was made in a negatively charged motif of the second extracellular domain which is conserved in homomeric nicotinic receptors. This mutation, Asp(268)Ala, also caused a loss of function. Thus the structure-function relationships in nicotinic alpha7 receptors have parallels with heteromeric nicotinic receptors, but there may also be some marked differences.

Voltage sensitivity and gating charge in Shaker and Shab family potassium channels

The members of the voltage-dependent potassium channel family subserve a variety of functions and are expected to have voltage sensors with different sensitivities. The Shaker channel of Drosophila, which underlies a transient potassium current, has a high voltage sensitivity that is conferred by a large gating charge movement, approximately 13 elementary charges. A Shaker subunit's primary voltage-sensing (S4) region has seven positively charged residues. The Shab channel and its homologue Kv2.1 both carry a delayed-rectifier current, and their subunits have only five positively charged residues in S4; they would be expected to have smaller gating-charge movements and voltage sensitivities. We have characterized the gating currents and single-channel behavior of Shab channels and have estimated the charge movement in Shaker, Shab, and their rat homologues Kv1.1 and Kv2.1 by measuring the voltage dependence of open probability at very negative voltages and comparing this with the charge-voltage relationships. We find that Shab has a relatively small gating charge, approximately 7.5 e(o). Surprisingly, the corresponding mammalian delayed rectifier Kv2.1, which has the same complement of charged residues in the S2, S3, and S4 segments, has a gating charge of 12.5 e(o), essentially equal to that of Shaker and Kv1.1. Evidence for very strong coupling between charge movement and channel opening is seen in two channel types, with the probability of voltage-independent channel openings measured to be below 10(-9) in Shaker and below 4 x 10(-8) in Kv2.1.

Binding of correolide to K(v)1 family potassium channels. Mapping the domains of high affinity interaction

Correolide, a novel nortriterpene natural product, potently inhibits the voltage-gated potassium channel, K(v)1.3, and [(3)H]dihydrocorreolide (diTC) binds with high affinity (K(d) approximately 10 nM) to membranes from Chinese hamster ovary cells that express K(v)1.3 (Felix, J. P., Bugianesi, R. M., Schmalhofer, W. A., Borris, R., Goetz, M. A., Hensens, O. D., Bao, J.-M., Kayser, F. , Parsons, W. H., Rupprecht, K., Garcia, M. L., Kaczorowski, G. J., and Slaughter, R. S. (1999) Biochemistry 38, 4922-4930). Mutagenesis studies were used to localize the diTC binding site and to design a high affinity receptor in the diTC-insensitive channel, K(v)3.2. Transferring the pore from K(v)1.3 to K(v)3.2 produces a chimera that binds peptidyl inhibitors of K(v)1.3 with high affinity, but not diTC. Transfer of the S(5) region of K(v)1.3 to K(v)3.2 reconstitutes diTC binding at 4-fold lower affinity as compared with K(v)1.3, whereas transfer of the entire S(5)-S(6) domain results in a normal K(v)1.3 phenotype. Substitutions in S(5)-S(6) of K(v)1.3 with nonconserved residues from K(v)3.2 has identified two positions in S(5) and one in S(6) that cause significant alterations in diTC binding. High affinity diTC binding can be conferred to K(v)3.2 after substitution of these three residues with the corresponding amino acids found in K(v)1.3. These results suggest that lack of sensitivity of K(v)3.2 to diTC is a consequence of the presence of Phe(382) and Ile(387) in S(5), and Met(458) in S(6). Inspection of K(v)1.1-1.6 channels indicates that they all possess identical S(5) and S(6) domains. As expected, diTC binds with high affinity (K(d) values 7-21 nM) to each of these homotetrameric channels. However, the kinetics of binding are fastest with K(v)1.3 and K(v)1.4, suggesting that conformations associated with C-type inactivation will facilitate entry and exit of diTC at its binding site. Taken together, these findings identify K(v)1 channel regions necessary for high affinity diTC binding, as well as, reveal a channel conformation that markedly influences the rate of binding of this ligand.

Pharmacology of voltage-gated and calcium-activated potassium channels

Several important new findings have furthered the development of voltage-gated and calcium-activated potassium channel pharmacology. The molecular constituents of several members of these large ion channel families were identified. Small-molecule modulators of some of these channels were reported, including correolide, the first potent, small-molecule, natural product inhibitor of the Shaker family of voltage-gated potassium channels to be disclosed. The initial crystal structure of a bacterial potassium channel was determined; this work gives a physical basis for understanding the mechanisms of ion selectivity and ion conduction. With the recent molecular characterization of a potassium channel structure and the discovery of new templates for channel modulatory agents, the ability to rationally identify and develop potassium channel agonists and antagonists may become a reality in the near future.

Potassium-dependent changes in the conformation of the Kv2.1 potassium channel pore

The voltage-gated K+ channel, Kv2.1, conducts Na+ in the absence of K+. External tetraethylammonium (TEAo) blocks K+ currents through Kv2.1 with an IC50 of 5 mM, but is completely without effect in the absence of K+. TEAo block can be titrated back upon addition of low [K+]. This suggested that the Kv2.1 pore undergoes a cation-dependent conformational rearrangement in the external vestibule. Individual mutation of lysine (Lys) 356 and 382 in the outer vestibule, to a glycine and a valine, respectively, increased TEAo potency for block of K+ currents by a half log unit. Mutation of Lys 356, which is located at the outer edge of the external vestibule, significantly restored TEAo block in the absence of K+ (IC50 = 21 mM). In contrast, mutation of Lys 382, which is located in the outer vestibule near the TEA binding site, resulted in very weak (extrapolated IC50 = approximately 265 mM) TEAo block in the absence of K+. These data suggest that the cation-dependent alteration in pore conformation that resulted in loss of TEA potency extended to the outer edge of the external vestibule, and primarily involved a repositioning of Lys 356 or a nearby amino acid in the conduction pathway. Block by internal TEA also completely disappeared in the absence of K+, and could be titrated back with low [K+]. Both internal and external TEA potencies were increased by the same low [K+] (30-100 microM) that blocked Na+ currents through the channel. In addition, experiments that combined block by internal and external TEA indicated that the site of K+ action was between the internal and external TEA binding sites. These data indicate that a K+-dependent conformational change also occurs internal to the selectivity filter, and that both internal and external conformational rearrangements resulted from differences in K+ occupancy of the selectivity filter. Kv2.1 inactivation rate was K+ dependent and correlated with TEAo potency; as [K+] was raised, TEAo became more potent and inactivation became faster. Both TEAo potency and inactivation rate saturated at the same [K+]. These results suggest that the rate of slow inactivation in Kv2.1 was influenced by the conformational rearrangements, either internal to the selectivity filter or near the outer edge of the external vestibule, that were associated with differences in TEA potency.

Activation of vascular smooth muscle K+ channels by endothelium-derived relaxing factors

1. Endothelium-derived relaxing factors (EDRF), including nitric oxide (NO), prostacyclin (PGI2) and an as yet uncharacterized endothelium-derived hyperpolarizing factor (EDHF), are now recognized to induce relaxation of vascular smooth muscle, in part via the activation of K+ channels. 2. Experiments using selective K+ channel blockers, including iberiotoxin (IbTX), glibenclamide, apamin and 4-aminopyridine (4-AP) to inhibit endothelium-induced relaxation suggest that more than one type of K+ channel may be involved, depending on the species and tissue, including: (i) large conductance Ca(2+)-activated (BKCa) channels; (ii) ATP-sensitive (KATP) channels; (iii) small conductance Ca(2+)-activated (SKCa) channels; and (iv) voltage-gated (Kv) K+ channels. 3. Recent observations suggest a role for Kv channels in some vessels based on a sensitivity of NO- and PGI2-mediated relaxations to 4-AP, as well as a complete suppression of EDHF-dependent relaxation by a combination of charybdotoxin (ChTX) and apamin but not IbTX and apamin. 4. The molecular identity of the K+ channels affected by EDRF is not well characterized. Recently, findings indicate that the pore-forming alpha-subunit tetramers of vascular smooth muscle BKCa channels are due to the expression of the so-called Slo channel gene. The identities of the KATP, SKCa and Kv channels involved in endothelium-dependent vasodilation are not known. 5. The component of whole-cell Kv current affected by PGI2 may be due to slowly inactivating, 4-AP-sensitive, 15 pS delayed-rectifier K+ channels (KDR); the activity of these channels in vascular myocytes is increased by forskolin and protein kinase A (PKA) and rabbit portal vein Kv1.5 pore-forming alpha-subunits, which appear to be a component of native KDR current and possess consensus phosphorylation sequences for PKA.

Contribution of the selectivity filter to inactivation in potassium channels

Voltage-gated K+ channels exhibit a slow inactivation process, which becomes an important influence on the rate of action potential repolarization during prolonged or repetitive depolarization. During slow inactivation, the outer mouth of the permeation pathway undergoes a conformational change. We report here that during the slow inactivation process, the channel progresses through at least three permeation states; from the initial open state that is highly selective for K+, the channel enters a state that is less permeable to K+ and more permeable to Na+, and then proceeds to a state that is non-conducting. Similar results were obtained in three different voltage-gated K+ channels: Kv2.1, a channel derived from Shaker (Shaker Delta A463C), and a chimeric channel derived from Kv2.1 and Kv1.3 that displays classical C-type inactivation. The change in selectivity displayed both voltage- and time-dependent properties of slow inactivation and was observed with K+ on either side of the channel. Elevation of internal [K+] inhibited Na+ conduction through the inactivating channel in a concentration-dependent manner. These results indicate that the change in selectivity filter function is an integral part of the slow inactivation mechanism, and argue against the hypothesis that the inactivation gate is independent from the selectivity filter. Thus, these data suggest that the selectivity filter is itself the inactivation gate.

Heteromultimeric potassium channels formed by members of the Kv2 subfamily

Four alpha-subunits are thought to coassemble and form a voltage-dependent potassium (Kv) channel. Kv alpha-subunits belong to one of four major subfamilies (Kv1, Kv2, Kv3, Kv4). Within a subfamily up to eight different genetic isotypes exist (e.g., Kv1.1, Kv1.2). Different isotypes within the Kv1 or Kv3 subfamily coassemble. It is not known, however, whether the only two members of the vertebrate Kv2 subfamily identified thus far, Kv2.1 and Kv2.2, heteromultimerize. This might account for the lack of detection of heteromultimeric Kv2 channels in situ despite the coexpression of Kv2.1 and Kv2.2 mRNAs within the same cell. To probe whether Kv2 isotypes can form heteromultimers, we developed a dominant-negative mutant Kv2.2 subunit to act as a molecular poison of Kv2 subunit-containing channels. The dominant-negative Kv2.2 suppresses formation of functional channels when it is coexpressed in oocytes with either wild-type Kv2.2 or Kv2.1 subunits. These results indicate that Kv2.1 and Kv2.2 subunits are capable of heteromultimerization. Thus, in native cells either Kv2.1 and Kv2.2 subunits are targeted at an early stage to different biosynthetic compartments or heteromultimerization otherwise is inhibited.

Functional architectures of animal toxins: a clue to drug design?

Toxic proteins are produced by a diversity of venomous animals from various phyla. They are often of small size, possess a large density of disulfide bonds and exert multiple functions directed toward a variety of molecular targets, including a diversity of enzymes and ion channels. The aim of this brief and non-exhaustive review is three-fold. First, the structural context associated with the functional diversity of animal toxins is presented. Among various situations, it is shown that toxins with a similar fold can exert different functions and that toxins with unrelated folds can exert similar functions. Second, the functional sites of some animal toxins are presented. Their comparison shed light on how (i) distinct functions can be exerted by similarly folded toxins and (ii) similar functions can be shared by structurally distinct toxins. Third, it is shown that part of the functional site of foreign proteins can be grafted on an animal toxin scaffold, opening new perspectives in the domain of protein engineering.

Delineation of the functional site of alpha-dendrotoxin. The functional topographies of dendrotoxins are different but share a conserved core with those of other Kv1 potassium channel-blocking toxins

We identified the residues that are important for the binding of alpha-dendrotoxin (alphaDTX) to Kv1 potassium channels on rat brain synaptosomal membranes, using a mutational approach based on site-directed mutagenesis and chemical synthesis. Twenty-six of its 59 residues were individually substituted by alanine. Substitutions of Lys5 and Leu9 decreased affinity more than 1000-fold, and substitutions of Arg3, Arg4, Leu6, and Ile8 by 5-30-fold. Substitution of Lys5 by norleucine or ornithine also greatly altered the binding properties of alphaDTX. All of these analogs displayed similar circular dichroism spectra as compared with the wild-type alphaDTX, indicating that none of these substitutions affect the overall conformation of the toxin. Substitutions of Ser38 and Arg46 also reduced the affinity of the toxin but, in addition, modified its dichroic properties, suggesting that these two residues play a structural role. The other residues were excluded from the recognition site because their substitutions caused no significant affinity change. Thus, the functional site of alphaDTX includes six major binding residues, all located in its N-terminal region, with Lys5 and Leu9 being the most important. Comparison of the functional site of alphaDTX with that of DTX-K, another dendrotoxin (Smith, L. A., Reid, P. F., Wang, F. C., Parcej, D. N., Schmidt, J. J., Olson, M. A., and Dolly, J. O. (1997) Biochemistry 36, 7690-7696), reveals that they only share the predominant lysine and probably a leucine residue; the additional functional residues differ from one toxin to the other. Comparison of the functional site of alphaDTX with those of structurally unrelated potassium channel-blocking toxins from venomous invertebrates revealed the common presence of a protruding key lysine with a close important hydrophobic residue (Leu, Tyr, or Phe) and few additional residues. Therefore, irrespective of their phylogenetic origin, all of these toxins may have undergone a functional convergence. The functional site of alphaDTX is topographically unrelated to the "antiprotease site" of the structurally analogous bovine pancreatic trypsin inhibitor.

Molecular basis of drug interaction with L-type Ca2+ channels

Different types of voltage-gated Ca2+ channels exist in the plasma membrane of electrically excitable cells. By controlling depolarization-induced Ca2+ entry into cells they serve important physiological functions, such as excitation-contraction coupling, neurotransmitter and hormone secretion, and neuronal plasticity. Their function is fine-tuned by a variety of modulators, such as enzymes and G-proteins. Block of so-called L-type Ca2+ channels by drugs is exploited as a therapeutic principle to treat cardiovascular disorders, such as hypertension. More recently, block of so-called non-L-type Ca2+ channels was found to exert therapeutic effects in the treatment of severe pain and ischemic stroke. As the subunits of different Ca2+ channel types have been cloned, the modulatory sites for enzymes, G-proteins, and drugs can now be determined using molecular engineering and heterologous expression. Here we summarize recent work that has allowed us to determine the sites of action of L-type Ca2+ channel modulators. Together with previous biochemical, electrophysiological, and drug binding data these results provide exciting insight into the molecular pharmacology of this voltage-gated Ca2+ channel family.

Modulation of C-type inactivation by K+ at the potassium channel selectivity filter

With prolonged or repetitive activation, voltage-gated K+ channels undergo a slow (C-type) inactivation mechanism, which decreases current flow through the channel. Previous observations suggest that C-type inactivation results from a localized constriction in the outer mouth of the channel pore and that the rate of inactivation is controlled by the-rate at which K+ leaves an unidentified binding site in the pore. We have functionally identified two K+ binding sites in the conduction pathway of a chimeric K+ channel that conducts Na+ in the absence of K+. One site has a high affinity for K+ and contributes to the selectivity filter mechanism for K+ over Na+. Another site, external to the high-affinity site, has a lower affinity for K+ and is not involved in channel selectivity. Binding of K+ to the high-affinity binding site slowed inactivation. Binding of cations to the external low-affinity site did not slow inactivation directly but could slow it indirectly, apparently by trapping K+ at the high-affinity site. These data support a model whereby C-type inactivation involves a constriction at the selectivity filter, and the constriction cannot proceed when the selectivity filter is occupied by K+.

The interaction of Na+ and K+ in voltage-gated potassium channels. Evidence for cation binding sites of different affinity

Voltage-gated potassium (K+) channels are multi-ion pores. Recent studies suggest that, similar to calcium channels, competition between ionic species for intrapore binding sites may contribute to ionic selectivity in at least some K+ channels. Molecular studies suggest that a putative constricted region of the pore, which is presumably the site of selectivity, may be as short as one ionic diameter in length. Taken together, these results suggest that selectivity may occur at just a single binding site in the pore. We are studying a chimeric K+ channel that is highly selective for K+ over Na+ in physiological solutions, but conducts Na+ in the absence of K+. Na+ and K+ currents both display slow (C-type) inactivation, but had markedly different inactivation and deactivation kinetics; Na+ currents inactivated more rapidly and deactivated more slowly than K+ currents. Currents carried by 160 mM Na+ were inhibited by external K+ with an apparent IC50 <30 microM. K+ also altered both inactivation and deactivation kinetics of Na+ currents at these low concentrations. In the complementary experiment, currents carried by 3 mM K+ were inhibited by external Na+, with an apparent IC50 of approximately 100 mM. In contrast to the effects of low [K+] on Na+ current kinetics, Na+ did not affect K+ current kinetics, even at concentrations that inhibited K+ currents by 40-50%. These data suggest that Na+ block of K+ currents did not involve displacement of K+ from the high affinity site involved in gating kinetics. We present a model that describes the permeation pathway as a single high affinity, cation-selective binding site, flanked by low affinity, nonselective sites. This model quantitatively predicts the anomalous mole fraction behavior observed in two different K+ channels, differential K+ and Na+ conductance, and the concentration dependence of K+ block of Na+ currents and Na+ block of K+ currents. Based on our results, we hypothesize that the permeation pathway contains a single high affinity binding site, where selectivity and ionic modulation of gating occur.

A human intermediate conductance calcium-activated potassium channel

An intermediate conductance calcium-activated potassium channel, hIK1, was cloned from human pancreas. The predicted amino acid sequence is related to, but distinct from, the small conductance calcium-activated potassium channel subfamily, which is approximately 50% conserved. hIK1 mRNA was detected in peripheral tissues but not in brain. Expression of hIK1 in Xenopus oocytes gave rise to inwardly rectifying potassium currents, which were activated by submicromolar concentrations of intracellular calcium (K0.5 = 0.3 microM). Although the K0.5 for calcium was similar to that of small conductance calcium-activated potassium channels, the slope factor derived from the Hill equation was significantly reduced (1.7 vs. 3. 5). Single-channel current amplitudes reflected the macroscopic inward rectification and revealed a conductance level of 39 pS in the inward direction. hIK1 currents were reversibly blocked by charybdotoxin (Ki = 2.5 nM) and clotrimazole (Ki = 24.8 nM) but were minimally affected by apamin (100 nM), iberiotoxin (50 nM), or ketoconazole (10 microM). These biophysical and pharmacological properties are consistent with native intermediate conductance calcium-activated potassium channels, including the erythrocyte Gardos channel.

The pore-lining region of shaker voltage-gated potassium channels: comparison of beta-barrel and alpha-helix bundle models

Although there is a large body of site-directed mutagenesis data that identify the pore-lining sequence of the voltage-gated potassium channel, the structure of this region remains unknown. We have interpreted the available biochemical data as a set of topological and orientational restraints and employed these restraints to produce molecular models of the potassium channel pore region, H5. The H5 sequence has been modeled either as a tetramer of membrane-spanning beta-hairpins, thus producing an eight-stranded beta-barrel, or as a tetramer of incompletely membrane-spanning alpha-helical hairpins, thus producing an eight-staved alpha-helix bundle. In total, restraints-directed modeling has produced 40 different configurations of the beta-barrel model, each configuration comprising an ensemble of 20 structures, and 24 different configurations of the alpha-helix bundle model, each comprising an ensemble of 24 structures. Thus, over 1300 model structures for H5 have been generated. Configurations have been ranked on the basis of their predicted pore properties and on the extent of their agreement with the biochemical data. This ranking is employed to identify particular configurations of H5 that may be explored further as models of the pore-lining region of the voltage-gated potassium channel pore.

Hanatoxin modifies the gating of a voltage-dependent K+ channel through multiple binding sites

We studied the mechanism by which Hanatoxin (HaTx) inhibits the drk1 voltage-gated K+ channel. HaTx inhibits the K+ channel by shifting channel opening to more depolarized voltages. Channels opened by strong depolarization in the presence of HaTx deactivate much faster upon repolarization, indicating that toxin bound channels can open. Thus, HaTx inhibits the drk1 K+ channel, not by physically occluding the ion conduction pore, but by modifying channel gating. Occupancy of the channel by HaTx was studied using various strength depolarizations. The concentration dependence for equilibrium occupancy as well as the kinetics of onset and recovery from inhibition indicate that multiple HaTx molecules can simultaneously bind to a single K+ channel. These results are consistent with a simple model in which HaTx binds to the surface of the drk1 K+ channel at four equivalent sites and alters the energetics of channel gating.