Structure of the voltage-dependent potassium channel is highly conserved from Drosophila to vertebrate central nervous systems

Fluorescent proteins and genetically encoded biosensors

The genetically encoded fluorescent sensors convert chemical and physical signals into light. They are powerful tools for the visualisation of physiological processes in living cells and freely moving animals. The fluorescent protein is the reporter module of a genetically encoded biosensor. In this study, we first review the history of the fluorescent protein in full emission spectra on a structural basis. Then, we discuss the design of the genetically encoded biosensor. Finally, we briefly review several major types of genetically encoded biosensors that are currently widely used based on their design and molecular targets, which may be useful for the future design of fluorescent biosensors.

Ion channel profiling of the Lymnaea stagnalis ganglia via transcriptome analysis

BACKGROUND

The pond snail Lymnaea stagnalis (L. stagnalis) has been widely used as a model organism in neurobiology, ecotoxicology, and parasitology due to the relative simplicity of its central nervous system (CNS). However, its usefulness is restricted by a limited availability of transcriptome data. While sequence information for the L. stagnalis CNS transcripts has been obtained from EST libraries and a de novo RNA-seq assembly, the quality of these assemblies is limited by a combination of low coverage of EST libraries, the fragmented nature of de novo assemblies, and lack of reference genome.

RESULTS

In this study, taking advantage of the recent availability of a preliminary L. stagnalis genome, we generated an RNA-seq library from the adult L. stagnalis CNS, using a combination of genome-guided and de novo assembly programs to identify 17,832 protein-coding L. stagnalis transcripts. We combined our library with existing resources to produce a transcript set with greater sequence length, completeness, and diversity than previously available ones. Using our assembly and functional domain analysis, we profiled L. stagnalis CNS transcripts encoding ion channels and ionotropic receptors, which are key proteins for CNS function, and compared their sequences to other vertebrate and invertebrate model organisms. Interestingly, L. stagnalis transcripts encoding numerous putative Ca2+ channels showed the most sequence similarity to those of Mus musculus, Danio rerio, Xenopus tropicalis, Drosophila melanogaster, and Caenorhabditis elegans, suggesting that many calcium channel-related signaling pathways may be evolutionarily conserved.

CONCLUSIONS

Our study provides the most thorough characterization to date of the L. stagnalis transcriptome and provides insights into differences between vertebrates and invertebrates in CNS transcript diversity, according to function and protein class. Furthermore, this study provides a complete characterization of the ion channels of Lymnaea stagnalis, opening new avenues for future research on fundamental neurobiological processes in this model system.

Kv1.1 channel subunits in the control of neurocardiac function

Voltage-gated Kv1.1 potassium channel α-subunits are broadly expressed in the nervous system where they act as critical regulators of neuronal excitability. Mutations in the KCNA1 gene, which encodes Kv1.1, are associated with the neurological diseases episodic ataxia and epilepsy. Studies in mouse models have shown that Kv1.1 is important for neural control of the heart and that Kcna1 deletion leads to cardiac dysfunction that appears to be brain-driven. Traditionally, KCNA1 was not believed to be expressed in the heart. However, recent studies have revealed that Kv1.1 subunits are not only present in cardiomyocytes, but that they also make an important heart-intrinsic functional contribution to outward K⁺ currents and action potential repolarization. This review recounts the winding history of discovery of KCNA1 gene expression and neurocardiac function from fruit flies to mammals and from brain to heart and looks at some of the salient questions that remain to be answered regarding emerging cardiac roles of Kv1.1.

Molecular determinants of beta-adrenergic signaling to voltage-gated K+ channels in the cerebral circulation

Voltage-gated K⁺ (K_v ) channels are major determinants of membrane potential in vascular smooth muscle cells (VSMCs) and regulate the diameter of small cerebral arteries and arterioles. However, the intracellular structures that govern the expression and function of vascular K_v channels are poorly understood. Scaffolding proteins including postsynaptic density 95 (PSD95) recently were identified in rat cerebral VSMCs. Primarily characterized in neurons, the PSD95 scaffold has more than 50 known binding partners, and it can mediate macromolecular signaling between cell-surface receptors and ion channels. In cerebral arteries, Shaker-type K_v 1 channels appear to associate with the PSD95 molecular scaffold, and PSD95 is required for the normal expression and vasodilator influence of members of this K⁺ channel gene family. Furthermore, recent findings suggest that the β1-subtype adrenergic receptor is expressed in cerebral VSMCs and forms a functional vasodilator complex with K_v 1 channels on the PSD95 scaffold. Activation of β1-subtype adrenergic receptors in VSMCs enables protein kinase A-dependent phosphorylation and opening of K_v 1 channels in the PSD95 complex; the subsequent K⁺ efflux mediates membrane hyperpolarization and vasodilation of small cerebral arteries. Early evidence from other studies suggests that other families of K_v channels and scaffolding proteins are expressed in VSMCs. Future investigations into these macromolecular complexes that modulate the expression and function of K_v channels may reveal unknown signaling cascades that regulate VSMC excitability and provide novel targets for ion channel-based medications to optimize vascular tone.

An annotated CNS transcriptome of the medicinal leech, Hirudo verbana: De novo sequencing to characterize genes associated with nervous system activity

The medicinal leech is one of the most venerated model systems for the study of fundamental nervous system principles, ranging from single-cell excitability to complex sensorimotor integration. Yet, molecular analyses have yet to be extensively applied to complement the rich history of electrophysiological study that this animal has received. Here, we generated the first de novo transcriptome assembly from the entire central nervous system of Hirudo verbana, with the goal of providing a molecular resource, as well as to lay the foundation for a comprehensive discovery of genes fundamentally important for neural function. Our assembly generated 107,704 contigs from over 900 million raw reads. Of these 107,704 contigs, 39,047 (36%) were annotated using NCBI's validated RefSeq sequence database. From this annotated central nervous system transcriptome, we began the process of curating genes related to nervous system function by identifying and characterizing 126 unique ion channel, receptor, transporter, and enzyme contigs. Additionally, we generated sequence counts to estimate the relative abundance of each identified ion channel and receptor contig in the transcriptome through Kallisto mapping. This transcriptome will serve as a valuable community resource for studies investigating the molecular underpinnings of neural function in leech and provide a reference for comparative analyses.

Transcript analysis and comparative evaluation of shaker and slowmo gene homologues from the European corn borer, Ostrinia nubilalis

The movement and dispersal of larval Lepidoptera impact their survival and distribution within the natural landscape. Homologues of the Drosophila behaviour-linked genes shaker (shkr) and slowmo (slmo) were identified from Ostrinia nubilalis (Lepidoptera: Crambidae). Onshkr was isolated as a 1610-nucleotide (nt) constitutively expressed transcript encoding a membrane-localized 469-amino-acid (aa) protein with a conserved tetramerization domain and the six-domain architecture necessary for the molecule to fold into an active K(+) channel. Three expressed splice variants of 682, 970 and 1604 nt were identified for the Onslmo gene, and encode predicted 141 and 228 aa proteins with a conserved protein of relevant evolutionary and lymphoid interest (PRELI) domain that may function in mitochondrial protein sorting and perinuclear protein localization. Onshkr and Onslmo protein sequences aligned within monophyletic lepidopteran groups.

Green fluorescent proteins (GFPs) for measuring voltage

Measuring signal transduction in large numbers of cells, with high spatial and temporal resolution, is a fundamental problem for studying information processing in the nervous system. To address this problem, a family of detectors that are chimeras between signal transduction proteins and fluorescent proteins has been designed. The prototype sensor is a genetically encoded probe that can be used to measure transmembrane voltage in single cells. This article describes a modified green fluorescent protein (GFP), fused to a voltage-sensitive K(+) channel, so that voltage-dependent rearrangements in the K(+) channel induce changes in the fluorescence of GFP. The probe has a maximal fractional fluorescence change of 5.1%, making it comparable to some of the best organic voltage-sensitive dyes. Moreover, the fluorescent signal is expanded in time in a manner that makes the signal 30-fold easier to detect than a traditional linear dye. DNA-encoded sensors have the advantage that they may be introduced into an organism noninvasively and targeted to specific brain regions, cell types, and subcellular compartments.

A missense mutation in the Kv1.1 voltage-gated potassium channel-encoding gene KCNA1 is linked to human autosomal dominant hypomagnesemia

Primary hypomagnesemia is a heterogeneous group of disorders characterized by renal or intestinal magnesium (Mg2+) wasting, resulting in tetany, cardiac arrhythmias, and seizures. The kidney plays an essential role in maintaining blood Mg2+ levels, with a prominent function for the Mg2+-transporting channel transient receptor potential cation channel, subfamily M, member 6 (TRPM6) in the distal convoluted tubule (DCT). In the DCT, Mg2+ reabsorption is an active transport process primarily driven by the negative potential across the luminal membrane. Here, we studied a family with isolated autosomal dominant hypomagnesemia and used a positional cloning approach to identify an N255D mutation in KCNA1, a gene encoding the voltage-gated potassium (K+) channel Kv1.1. Kv1.1 was found to be expressed in the kidney, where it colocalized with TRPM6 along the luminal membrane of the DCT. Upon overexpression in a human kidney cell line, patch clamp analysis revealed that the KCNA1 N255D mutation resulted in a nonfunctional channel, with a dominant negative effect on wild-type Kv1.1 channel function. These data suggest that Kv1.1 is a renal K+ channel that establishes a favorable luminal membrane potential in DCT cells to control TRPM6-mediated Mg2+ reabsorption.

Manipulation of the potassium channel Kv1.1 and its effect on neuronal excitability in rat sensory neurons

Potassium channels play a critical role in regulating many aspects of action potential (AP) firing. To establish the contribution of the voltage-dependent potassium channel Kv1.1 in regulating excitability, we used the selective blocker dendrotoxin-K (DTX-K) and small interfering RNA (siRNA) targeted to Kv1.1 to determine their effects on AP firing in small-diameter capsaicin-sensitive sensory neurons. A 5-min exposure to 10 nM DTX-K suppressed the total potassium current (I(K)) measured at +40 mV by about 33%. DTX-K produced a twofold increase in the number of APs evoked by a ramp of depolarizing current. Associated with increased firing was a decrease in firing threshold and rheobase. DTX-K did not alter the resting membrane potential or the AP duration. A 48-h treatment with siRNA targeted to Kv1.1 reduced the expression of this channel protein by about 60% as measured in Western blots. After treatment with siRNA, I(K) was no longer sensitive to DTX-K, indicating a loss of functional protein. Similarly, after siRNA treatment exposure to DTX-K had no effect on the number of evoked APs, firing threshold, or rheobase. However, after siRNA treatment, the firing threshold had values similar to those obtained after acute exposure to DTX-K, suggesting that the loss of Kv1.1 plays a critical role in setting this parameter of excitability. These results demonstrate that Kv1.1 plays an important role in limiting AP firing and that siRNA may be a useful approach to establish the role of specific ion channels in the absence of selective antagonists.

Transient hippocampal down-regulation of Kv1.1 subunit mRNA during associative learning in rats

Voltage-gated potassium channels (Kv) are critically involved in learning and memory processes. It is not known, however, whether the expression of the Kv1.1 subunit, constituting Kv1 channels, can be specifically regulated in brain areas important for learning and memory processing. Radioactive in situ hybridization was used to evaluate the content of Kv1.1 alpha-subunit mRNA in the olfactory bulb, ventral, and dorsal hippocampus at different stages of an odor-discrimination associative task in rats. Naive, conditioned, and pseudoconditioned animals were sacrificed at different times either prior to a two-odor significance learning or after odor discrimination was established. Important decreases of Kv1.1 mRNA levels were transiently observed in the ventral hippocampus before successful learning when compared with the pseudoconditioned group. Moreover, temporal group analysis showed significant labeling alterations in the hippocampus of conditioned and pseudoconditioned groups throughout the training. Finally, Kv1.1 mRNA levels in the hippocampus were positively correlated with odor-reward association learning in rats that were beginning to discriminate between odors. These findings indicate that the Kv1.1 subunit is transiently down-regulated in the early stages of learning and suggest that Kv1 channel expression regulation is critical for the modification of neuronal substrates underlying new information acquisition.

Developmental regulation and adult maintenance of potassium channel proteins (Kv1.1 and Kv1.2) in the cochlear nucleus of the rat

The development and maintenance of the adult expression and distribution of Kv 1.1 and Kv 1.2, two voltage-dependent potassium channel subunits, were investigated in the anteroventral cochlear nucleus (AVCN) of the rat. Both Kv 1.1 and Kv 1.2 were found in AVCN neuronal cell bodies at birth, as detected by in situ hybridization and immunocytochemistry. However, Kv 1.1 and Kv 1.2 were not seen in axons until the end of the third postnatal week. From postnatal day 21 through adulthood, labeling for both potassium channels was in axonal processes, whereas the number of cell bodies labeled for Kv 1.1 decreased and there were no cell bodies labeled for Kv 1.2. Therefore, these two potassium channel proteins are targeted to their final subcellular destinations in axons well after hearing onset. Once the adult distribution pattern of Kv 1.1 and Kv 1.2 is attained, its maintenance does not depend on signals from auditory nerve synapses. Eliminating auditory nerve input to the cochlear nucleus by means of bilateral cochleotomy did not change Kv 1.1 or Kv 1.2 expression or distribution, as seen by in situ hybridization, immunocytochemistry and Western blot. Thus, normal excitatory synaptic input in adult animals is not a requirement to regulate the expression and cellular and subcellular distribution of these potassium channel proteins.

KCNE4 is an inhibitory subunit to Kv1.1 and Kv1.3 potassium channels

Kv1 potassium channels are widely distributed in mammalian tissues and are involved in a variety of functions from controlling the firing rate of neurons to maturation of T-lymphocytes. Here we show that the newly described KCNE4 beta-subunit has a drastic inhibitory effect on currents generated by Kv1.1 and Kv1.3 potassium channels. The inhibition is found on channels expressed heterologously in both Xenopus oocytes and mammalian HEK293 cells. mKCNE4 does not inhibit Kv1.2, Kv1.4, Kv1.5, or Kv4.3 homomeric complexes, but it does significantly reduce current through Kv1.1/Kv1.2 and Kv1.2/Kv1.3 heteromeric complexes. Confocal microscopy and Western blotting reveal that Kv1.1 is present at the cell surface together with KCNE4. Real-time RT-PCR shows a relatively high presence of mKCNE4 mRNA in several organs, including uterus, kidney, lung, intestine, and in embryo, whereas a much lower mRNA level is detected in the heart and in five different parts of the brain. Having the broad distribution of Kv1 channels in mind, the demonstrated inhibitory property of KCNE4-subunits could locally and/or transiently have a dramatic influence on cellular excitability and on setting resting membrane potentials.

A novel conotoxin inhibiting vertebrate voltage-sensitive potassium channels

Toxins from cone snail (Conus species) venoms are multiple disulfide bonded peptides. Based on their pharmacological target (ion channels, receptors) and their disulfide pattern, they have been classified into several toxin families and superfamilies. Here, we report a new conotoxin, which is the first member of a structurally new superfamily of Conus peptides and the first conotoxin affecting vertebrate K+ channels. The new toxin, designated conotoxin ViTx, has been isolated from the venom of Conus virgo and comprises a single chain of 35 amino acids cross-linked by four disulfide bridges. Its amino acid sequence (SRCFPPGIYCTSYLPCCWGICCSTCRNVCHLRIGK) was partially determined by Edman degradation and deduced from the nucleotide sequence of the toxin cDNA. Nucleic acid sequencing also revealed a prepropeptide comprising 67 amino acid residues and demonstrated a posttranslational modification of the protein by releasing a six-residue peptide from the C-terminal. Voltage clamp studies on various ion channels indicated that the toxin inhibits the vertebrate K+ channels Kv1.1 and Kv1.3 but not Kv1.2. The chemically synthesized product exhibited the same physiological activity and identical molecular mass (3933.7 Da) as the native toxin.

Analysis of phosphorylation-dependent modulation of Kv1.1 potassium channels

The voltage-gated potassium channel Kv1.1 contains phosphorylation sites for protein kinase A (PKA) and protein kinase C (PKC). To study Kv1.1 protein expression and cellular distribution in regard to its level of phosphorylation, the effects of PKA and PKC activation on Kv1.1 were investigated in HEK 293 cells stably transfected with Kv1.1 (HEK 293/1). Without kinase activation, HEK 293/1 cells carry unphosphorylated Kv1.1 protein in the plasma membranes, whereas large amounts of phosphorylated and unphosphorylated Kv1.1 protein were located intracellularly. Activation of PKA resulted in phosphorylation of intracellular Kv1.1 protein, followed by a rapid translocation of Kv1.1 into the plasma membrane. Patch-clamp analysis revealed an increase in current amplitude upon PKA activation and demonstrated differences in the voltage dependence of current activation between unphosphorylated and phosphorylated Kv1.1 channels. In contrast to PKA, even prolonged activation of PKC did not lead to direct phosphorylation of Kv1.1, but induced Kv1.1 protein synthesis. Thus, protein kinases have direct and indirect effects on the functional expression of voltage-gated potassium channels. Our data suggest that the synergistic action of protein kinases may play an important role in the fine-tuning of Kv channel function.

Heterogeneous Expression Patterns of Mammalian Potassium Channel Genes in Developing and Adult Rat Brain

Voltage-gated K+ channels in the mammalian brain are functionally heterogeneous. Mechanisms which may underlie heterogeneity are the expression of multiple K+ channel subunit genes, alternative splicing and the formation of heteromultimers from different subunits. To examine the molecular basis of regional and cell-specific K+ channel expression in rat brain in situ hybridization techniques were used. The transcript distribution patterns of 11 cloned mammalian K+ channel genes encoding both slow- and fast-inactivating K+ channels from four different gene families were examined at different stages of development. The results show that each subunit-specific messenger RNA (mRNA) is independently expressed and is characterized by an individual expression pattern. In the hippocampal formation transcripts of RCK2, RCK3, RCK4, RCK5, Raw3 and rat Shal genes are heterogeneously expressed and regulated during postnatal development. RCK1, Raw1, Raw2 and DRK1 mRNAs, on the other hand, are present in the hippocampus throughout postnatal life. The expression patterns of the 11 genes partially overlap, suggesting the formation of different heteromultimeric K+ channel complexes.

Purification, characterization and biosynthesis of parabutoxin 3, a component of Parabuthus transvaalicus venom

A novel peptidyl inhibitor of voltage-gated K+ channels, named parabutoxin 3 (PBTx3), has been purified to homogeneity from the venom of Parabuthus transvaalicus. This scorpion toxin contains 37 residues, has a mass of 4274 Da and displays 41% identity with charybdotoxin (ChTx, also called 'alpha-KTx1.1'). PBTx3 is the tenth member (called 'alpha-KTx1.10') of subfamily 1 of K+ channel-blocking peptides known thus far. Electrophysiological experiments using Xenopus laevis oocytes indicate that PBTx3 is an inhibitor of Kv1 channels (Kv1.1, Kv1.2, Kv1.3), but has no detectable effects on Kir-type and ERG-type channels. The dissociation constants (Kd) for Kv1.1, Kv1.2 and Kv1.3 channels are, respectively, 79 microm, 547 nm and 492 nm. A synthetic gene encoding a PBTx3 homologue was designed and expressed as a fusion protein with the maltose-binding protein (MBP) in Escherichia coli. The recombinant protein was purified from the bacterial periplasm compartment using an amylose affinity resin column, followed by a gel filtration purification step and cleavage by factor Xa (fXa) to release the recombinant toxin peptide (rPBTx3). After final purification and refolding, rPBTx3 was shown to be identical to the native PBTx3 with respect to HPLC retention time, mass spectrometric analysis and functional properties. The three-dimensional structure of PBTx3 is proposed by homology modelling to contain a double-stranded antiparallel beta sheet and a single alpha-helix, connected by three disulfide bridges. The scaffold of PBTx3 is homologous to most other alpha-KTx scorpion toxins.

Structure of the cytoplasmic beta subunit-T1 assembly of voltage-dependent K+ channels

The structure of the cytoplasmic assembly of voltage-dependent K+ channels was solved by x-ray crystallography at 2.1 angstrom resolution. The assembly includes the cytoplasmic (T1) domain of the integral membrane alpha subunit together with the oxidoreductase beta subunit in a fourfold symmetric T1(4)beta4 complex. An electrophysiological assay showed that this complex is oriented with four T1 domains facing the transmembrane pore and four beta subunits facing the cytoplasm. The transmembrane pore communicates with the cytoplasm through lateral, negatively charged openings above the T1(4)beta4 complex. The inactivation peptides of voltage-dependent K(+) channels reach their site of action by entering these openings.

Diverse expression of delayed rectifier K+ channel subtype transcripts in several types of smooth muscles of the rat

Diverse expression of voltage dependent K+ (Kv) channels was examined in smooth muscles (SMs); carotid artery (CA), mesenteric artery (MA), urinary bladder (UB), and vas deferens (VD) of the rat, using RT-PCR based analyses. Among eight Kv channel subtypes examined (Kv 1.1, Kv 1.2, Kv 1.5, Kv 1.6, Kv 2.1, Kv 2.2, Kv 3.1, and Kv 3.2), expression of three delayed rectifier Kv (KD) channel (Kv 1.2, Kv 1.5, and Kv 2.1) transcripts was observed in these SMs. To determine precisely the expression levels of the transcripts encoding K(D) subtypes, those of three K(D) subtypes (Kv 1.2, Kv 1.5, and Kv 2.1) were determined by competitive PCR. In vascular SM tissues, CA and MA, Kv 1.2 and Kv 1.5 transcripts were expressed at relatively high levels, whereas in visceral SM tissues, UB and VD, Kv 2.1 transcripts were expressed at the relatively high levels. These results suggest that the diverse expression of K(D) subtypes is, at least in part, responsible for differences in electrical excitability and also for the variation of the electrophysiological and pharmacological phenotypes as tonic and phasic SMs.

Developmental changes in the expression of Shaker- and Shab-related K(+) channels in neurons of the rat trigeminal ganglion

We have investigated properties of voltage-gated K(+) channels in neurons of the pre- and postnatal rat trigeminal ganglion (TG). To correlate functional data with information on gene expression of Shaker- and Shab-related channels in these pseudo-unipolar neurons, the patch-clamp technique was combined with the single-cell reverse transcription-polymerase chain reaction (RT-PCR). A majority (80%) of prenatal TG neurons possessed only sustained delayed rectifier currents with half-maximal current inactivation at -30 mV. In the postnatal cells, steady-state inactivation of sustained currents occurred at more negative voltages (half-maximal inactivation at -58 mV). About 65% of the postnatal cells displayed a transient outward component in addition to the sustained currents. With increasing age, the sensitivity of sustained currents to 4-aminopyridine (4-AP) decreased significantly. The Shaker channel toxins, alpha-dendrotoxin and agitoxin-2 (50 and 10 nM), were much less effective. Discrimination between both stages with tetraethylammonium chloride (5 mM) was not possible since the currents were reduced generally by about 50%. After recording, the cell content was harvested and single-cell RT-PCR was performed to compare K(+) current properties and mRNA expression within the same cell. Most cells simultaneously expressed several different Shaker- and Shab-like transcripts. At postnatal day 14, the frequency of cells carrying transcripts encoding Kv1.1 decreased. Detailed analysis revealed a higher 4-AP sensitivity of TG neurons expressing Kv1.1 transcripts.

Sensory activity affects sensory axon development in C. elegans

The simple nervous system of the nematode C. elegans consists of 302 neurons with highly reproducible morphologies, suggesting a hard-wired program of axon guidance. Surprisingly, we show here that sensory activity shapes sensory axon morphology in C. elegans. A class of mutants with deformed sensory cilia at their dendrite endings have extra axon branches, suggesting that sensory deprivation disrupts axon outgrowth. Mutations that alter calcium channels or membrane potential cause similar defects. Cell-specific perturbations of sensory activity can cause cell-autonomous changes in axon morphology. Although the sensory axons initially reach their targets in the embryo, the mutations that alter sensory activity cause extra axon growth late in development. Thus, perturbations of activity affect the maintenance of sensory axon morphology after an initial pattern of innervation is established. This system provides a genetically tractable model for identifying molecular mechanisms linking neuronal activity to nervous system structure.

Mutations in the S4 region isolate the final voltage-dependent cooperative step in potassium channel activation

Charged residues in the S4 transmembrane segment play a key role in determining the sensitivity of voltage-gated ion channels to changes in voltage across the cell membrane. However, cooperative interactions between subunits also affect the voltage dependence of channel opening, and these interactions can be altered by making substitutions at uncharged residues in the S4 region. We have studied the activation of two mutant Shaker channels that have different S4 amino acid sequences, ILT (V369I, I372L, and S376T) and Shaw S4 (the S4 of Drosophila Shaw substituted into Shaker), and yet have very similar ionic current properties. Both mutations affect cooperativity, making a cooperative transition in the activation pathway rate limiting and shifting it to very positive voltages, but analysis of gating and ionic current recordings reveals that the ILT and Shaw S4 mutant channels have different activation pathways. Analysis of gating currents suggests that the dominant effect of the ILT mutation is to make the final cooperative transition to the open state of the channel rate limiting in an activation pathway that otherwise resembles that of Shaker. The charge movement associated with the final gating transition in ILT activation can be measured as an isolated component of charge movement in the voltage range of channel opening and accounts for 13% ( approximately 1.8 e0) of the total charge moved in the ILT activation pathway. The remainder of the ILT gating charge (87%) moves at negative voltages, where channels do not open, and confirms the presence of Shaker-like conformational changes between closed states in the activation pathway. In contrast to ILT, the activation pathway of Shaw S4 seems to involve a single cooperative charge-moving step between a closed and an open state. We cannot detect any voltage-dependent transitions between closed states for Shaw S4. Restoring basic residues that are missing in Shaw S4 (R1, R2, and K7) rescues charge movement between closed states in the activation pathway, but does not alter the voltage dependence of the rate-limiting transition in activation.

Memory and long-term potentiation (LTP) dissociated: normal spatial memory despite CA1 LTP elimination with Kv1.4 antisense

Long-term potentiation (LTP) in the hippocampal slice preparation has been proposed as an in vitro model for long-term memory. However, correlation of LTP with memory in living animals has been difficult to demonstrate. Furthermore, in the last few years evidence has accumulated that dissociate the two. Because potassium channels might determine the weight of synapses in networks, we studied the role of Kv1.4, a presynaptic A-type voltage-dependent K+ channel, in both memory and LTP. Reverse transcription-PCR and Western blot analysis with specific antibodies showed that antisense oligodeoxyribonucleotide to Kv1.4 microinjected intraventricularly into rat brains obstructed hippocampal Kv1.4 mRNA, "knocking down" the protein in the hippocampus. This antisense knockdown had no effect on rat spatial maze learning, memory, or exploratory behavior, but eliminated both early- and late-phase LTP and reduced paired-pulse facilitation (a presynaptic effect) in CA1 pyramidal neurons without affecting dentate gyrus LTP. This presynaptic Kv1.4 knockdown together with previous postsynaptic Kv1.1 knockdown demonstrates that CA1 LTP is neither necessary nor sufficient for rat spatial memory.

Deletion of the K(V)1.1 potassium channel causes epilepsy in mice

Mice lacking the voltage-gated potassium channel alpha subunit, K(V)1.1, display frequent spontaneous seizures throughout adult life. In hippocampal slices from homozygous K(V)1.1 null animals, intrinsic passive properties of CA3 pyramidal cells are normal. However, antidromic action potentials are recruited at lower thresholds in K(V)1.1 null slices. Furthermore, in a subset of slices, mossy fiber stimulation triggers synaptically mediated long-latency epileptiform burst discharges. These data indicate that loss of K(V)1.1 from its normal localization in axons and terminals of the CA3 region results in increased excitability in the CA3 recurrent axon collateral system, perhaps contributing to the limbic and tonic-clonic components of the observed epileptic phenotype. Axonal action potential conduction was altered as well in the sciatic nerve--a deficit potentially related to the pathophysiology of episodic ataxia/myokymia, a disease associated with missense mutations of the human K(V)1.1 gene.

Role of the S4 in cooperativity of voltage-dependent potassium channel activation

Charged residues in the S4 transmembrane segment of voltage-gated cation channels play a key role in opening channels in response to changes in voltage across the cell membrane. However, the molecular mechanism of channel activation is not well understood. To learn more about the role of the S4 in channel gating, we constructed chimeras in which S4 segments from several divergent potassium channels, Shab, Shal, Shaw, and Kv3.2, were inserted into a Shaker potassium channel background. These S4 donor channels have distinctly different voltage-dependent gating properties and S4 amino acid sequences. None of the S4 chimeras have the gating behavior of their respective S4 donor channels. The conductance-voltage relations of all S4 chimeras are shifted to more positive voltages and the slopes are decreased. There is no consistent correlation between the nominal charge content of the S4 and the slope of the conductance-voltage relation, suggesting that the mutations introduced by the S4 chimeras may alter cooperative interactions in the gating process. We compared the gating behavior of the Shaw S4 chimera with its parent channels, Shaker and Shaw, in detail. The Shaw S4 substitution alters activation gating profoundly without introducing obvious changes in other channel functions. Analysis of the voltage-dependent gating kinetics suggests that the dominant effect of the Shaw S4 substitution is to alter a single cooperative transition late in the activation pathway, making it rate limiting. This interpretation is supported further by studies of channels assembled from tandem heterodimer constructs with both Shaker and Shaw S4 subunits. Activation gating in the heterodimer channels can be predicted from the properties of the homotetrameric channels only if it is assumed that the mutations alter a cooperative transition in the activation pathway rather than independent transitions.

Comparison and characterization of the venoms of three Parabuthus scorpion species occurring in southern Africa

Parabuthus transvaalicus, P. granulatus, and P. villosus are three medically important scorpion species occurring in southern Africa which can cause severe envenoming among people. In contrast to many other genera, no data is available on the venom composition of scorpions belonging to the genus Parabuthus. Here we have investigated the components which may contribute to the venomous potential. The constancy of venom composition within each of the three species and between the three species was investigated by means of gel filtration chromatography. The venoms of the three species each were characterized by a constant and typical elution pattern, resulting in a 'gel filtration fingerprint' which allows distinction between each species. It appears that certain components in the venoms are common to either all three species, or to two of the three species. This points to a clear interspecies relationship within the genus. We also describe the isolation and characterization of some of the polypeptide toxins present in the venoms of P. villosus, P. transvaalicus and P. granulatus by means of reversed phase chromatography and screening of the toxic components on voltage-activated potassium and sodium channels. Our results confirm that toxins which inhibit potassium channels and alter sodium channel gating are present in the venoms studied.

The 1997 Stevenson Award Lecture. Cardiac K+channel gating: cloned delayed rectifier mechanisms and drug modulation

K+ channels are ubiquitous membrane proteins, which have a central role in the control of cell excitability. In the heart, voltage-gated delayed rectifier K+ channels, like Kv1.5, determine repolarization and the cardiac action potential plateau duration. Here we review the broader properties of cloned voltage-gated K+ channels with specific reference to the hKv1.5 channel in heart. We discuss the basic structural components of K+ channels such as the pore, voltage sensor, and fast inactivation, all of which have been extensively studied. Slow, or C-type, inactivation and the structural features that control pore opening are less well understood, although recent studies have given new insight into these problems. Information about channel transitions that occur prior to opening is provided by gating currents, which reflect charge-carrying transitions between kinetic closed states. By studying modulation of the gating properties of K+ channels by cations and with drugs, we can make a more complete interpretation of the state dependence of drug and ion interactions with the channel. In this way we can uncover the detailed mechanisms of action of K+ channel blockers such as tetraethylammonium ions and 4-aminopyridine, and antiarrhythmic agents such as nifedipine and quinidine.Key words: potassium channel, Kv1.5, channel gating, inactivation, pore region, gating currents.

Isolation, characterization, and mapping of two human potassium channels

Two novel human genes encoding putative potassium channels, kH1 and kH2, were identified from a human fetal brain cDNA library. Sequence analysis showed that kH1 and kH2 are homologous to rat IK8 and rat K13, respectively. The kH1 encodes a polypeptide of 495 amino acids, which shares 88% and 95% identity to IK8 at the nucleotide and amino acid level, respectively. The kH2 encodes a polypeptide of 515 amino acids with 86% and 92% identity to K13 at the nucleotide and amino acid level, respectively. Northern blot studies revealed that one mRNA species, approximately 5kb, of the kH1 was expressed abundantly in tissues examined, including the heart, skeletal muscle, and less abundant in the brain, liver, kidney, and pancreas. Interestingly, an alternative spliced form of 2.4 kb mRNA species of the kH1 was also found in the brain. Unlike kH1, 2.4 kb of kH2 was expressed predominantly in the brain, placenta, and the skeletal muscle where it shared a differently spliced form of the kH2 mRNA, approximately 2.0 kb. Fluorescence in situ hybridization localized kH1 to the human chromosome 2p25 and kH2 to the human chromosome 20q13.

Effect of fluoxetine on a neuronal, voltage-dependent potassium channel (Kv1.1)

1. Fluoxetine (Prozac) is widely used as an antidepressant drug and is assumed to be a selective 5-hydroxytryptamine (5-HT) reuptake inhibitor (SSRI). Claims that its beneficial psychotropic effects extend beyond those in treatment of depression have drawn clinical and popular attention to this compound, raising the question of whether there is anything exceptional about the supposed selective actions. 2. We have used the voltage clamp technique to study the effect of fluoxetine on a neuronal, voltage-dependent potassium (K+) channel (RCK1; Kv1.1), expressed in p6nopus laevis oocytes. This channel subunit is abundantly expressed in the central nervous system and K+ channels containing this subunit are involved in the repolarization process of many types of neurones. 3. Blockade of the K+ currents by fluoxetine was found to be use- and dose-dependent. Wash-out of this compound could not be achieved. Fluoxetine did not affect the ion selectivity of this K+ channel, as the reversal potential was unaltered. 4. Slowing of both activation and deactivation kinetics of the channel by fluoxetine was observed, including tail current crossover upon repolarization. 5. Hodgkin-Huxley type of models and more generalized Markov chain models were used to fit the kinetics of the data. Based upon a Markov kinetic scheme, our data can be interpreted to mean that blockade of fluoxetine consists of two components: a voltage-independent occurring in the last closed, but available state of the channel, and a voltage-dependent occurring in the open state. 6. This study describes the first biophysical working model for the mechanism of action of fluoxetine on a neuronal, voltage-dependent K+ channel, RCK1. Although this channel is not very potently blocked by fluoxetine when expressed in oocytes, this study may help us to understand some of the clinical symptoms seen with elevated serum concentrations of this SSRI.

A conserved arginine residue in the pore region of an inward rectifier K channel (IRK1) as an external barrier for cationic blockers

The number, sign, and distribution of charged residues in the pore-forming H5 domain for inward-rectifying K channels (IRK1) are different from the otherwise homologous H5 domains of other voltage-gated K channels. We have mutated Arg148, which is perfectly conserved in all inward rectifiers, to His in the H5 of IRK1 (Kir2. 1). Channel activity was lost by the mutation, but coexpression of the mutant (R148H) along with the wild-type (WT) mRNA revealed populations of channels with reduced single-channel conductances. Long-lasting and flickery sublevels were detected exclusively for the coexpressed channels. These findings indicated that the mutant subunit formed hetero-oligomers with the WT subunit. The permeability ratio was altered by the mutation, while the selectivity sequence (K+ > Rb+ > NH4+ >> Na+) was preserved. The coexpression made the IRK1 channel more sensitive to extracellular block by Mg2+ and Ca2+, and turned this blockade from a voltage-independent to a -dependent process. The sensitivity of the mutant channels to Mg2+ was enhanced at higher pH and by an increased ratio of mutant:WT mRNA, suggesting that the charge on the Arg site controlled the sensitivity. The blocking rate of open channel blockers, such as Cs+ and Ba2+, was facilitated by coexpression without significant change in the steady state block. Evaluation of the electrical distance to the binding site for Mg2+ or Ca2+ and that to the barrier peak for block by Cs+ or Ba2+ suggest that Arg148 is located between the external blocking site for Mg2+ or Ca2+ and the deeper blocking site for Cs+ or Ba2+ in the IRK1 channel. It is concluded that Arg148 serves as a barrier to cationic blockers, keeping Mg2+ and Ca2+ out from the electric field of the membrane.

Alternative splicing in the pore-forming region of shaker potassium channels

We have cloned cDNAs for the shaker potassium channel gene from the spiny lobster Panulirus interruptus. As previously found in Drosophila, there is alternative splicing at the 5' and 3' ends of the coding region. However, in Panulirus shaker, alternative splicing also occurs within the pore-forming region of the protein. Three different splice variants were found within the P region, two of which bestow unique electrophysiological characteristics to channel function. Pore I and pore II variants differ in voltage dependence for activation, kinetics of inactivation, current rectification, and drug resistance. The pore 0 variant lacks a P region exon and does not produce a functional channel. This is the first example of alternative splicing within the pore-forming region of a voltage-dependent ion channel. We used a recently identified potassium channel blocker, kappa-conotoxin PVIIA, to study the physiological role of the two pore forms. The toxin selectively blocked one pore form, whereas the other form, heteromers between the two pore forms, and Panulirus shal were not blocked. When it was tested in the Panulirus stomatogastric ganglion, the toxin produced no effects on transient K+ currents or synaptic transmission between neurons.

A genetically encoded optical probe of membrane voltage

Measuring electrical activity in large numbers of cells with high spatial and temporal resolution is a fundamental problem for the study of neural development and information processing. To address this problem, we have constructed a novel, genetically encoded probe that can be used to measure transmembrane voltage in single cells. We fused a modified green fluorescent protein (GFP) into a voltage-sensitive K+ channel so that voltage-dependent rearrangements in the K+ channel would induce changes in the fluorescence of GFP. The probe has a maximal fractional fluorescence change of 5.1%, making it comparable to some of the best organic voltage-sensitive dyes. Moreover, the fluorescent signal is expanded in time in a way that makes the signal 30-fold easier to detect. A voltage sensor encoded into DNA has the advantage that it may be introduced into an organism noninvasively and targeted to specific developmental stages, brain regions, cell types, and subcellular compartments.

Secondary structure, stability and tetramerisation of recombinant K(V)1.1 potassium channel cytoplasmic N-terminal fragment

The recombinant N-terminal fragment (amino acids 14-162) of a tetrameric voltage-gated potassium channel (K(V)1.1) has been studied using spectroscopic techniques. Evidence is presented that it forms a tetramer in aqueous solution, whereas when solubilised in 1% Triton X-100 it remains monomeric. The secondary structure content of both monomeric and tetrameric K(V)1.1 N-terminal fragment has been estimated from FTIR and CD spectroscopy to be 20-25% alpha-helix, 20-25% beta-sheet, 20% turns and 30-40% random coil. Solubilisation of the protein in detergent is shown by hydrogen-deuterium exchange analysis to alter tertiary structure rather than secondary structure and this may be the determining factor in tetramerisation ability. Using molecular modelling we propose a supersecondary structure consisting of two structural domains.

Blockade of clomipramine and amitriptyline analgesia by an antisense oligonucleotide to mKv1.1, a mouse Shaker-like K+ channel

The effect of an antisense oligonucleotide to the K+ channel coding mKv1.1 mRNA on antinociception induced by the tricyclic antidepressants, clomipramine (20-35 mg kg(-1) s.c.) and amitriptyline (10-25 mg kg(-1) s.c.), was investigated in the mouse hot-plate test. Antisense oligonucleotide (0.5-1.0-2.0-3.0 nmol per i.c.v. injection) produced a dose-dependent inhibition of clomipramine and amitriptyline antinociception 72 h after the last i.c.v. injection. The sensitivity to both analgesic drugs returned 7 days after antisense oligonucleotide injection, indicating the absence of irreversible damage or toxicity. Treatment with a degenerated oligonucleotide did not modify the clomipramine- and amitriptyline-induced antinociception in comparison with that in naive (unpretreated controls), vector and saline i.c.v.-injected mice. A quantitative reverse transcription-polymerase chain reaction (RT-PCR) study demonstrated a reduction in mRNA levels only in the antisense oligonucleotide treated group. Antisense oligonucleotide, degenerated oligonucleotide or vector pretreatment, in the range of doses used, did not produce any behavioural impairment as revealed by the mouse rotarod and hole-board tests. The present results indicate that modulation of the mKv1.1 K+ channel plays an important role in the central analgesia induced by the tricyclic antidepressants, clomipramine and amitriptyline.

Inactivation of a voltage-dependent K+ channel by beta subunit. Modulation by a phosphorylation-dependent interaction between the distal C terminus of alpha subunit and cytoskeleton

Kv1.1/Kvbeta1.1 (alphabeta) K+ channel expressed in Xenopus oocytes was shown to have a fast inactivating current component. The fraction of this component (extent of inactivation) is increased by microfilament disruption induced by cytochalasins or by phosphorylation of the alpha subunit at Ser-446, which impairs the interaction of the channel with microfilaments. The relevant sites of interaction on the channel molecules have not been identified. Using a phosphorylation-deficient mutant of alpha, S446A, to ensure maximal basal interaction of the channel with the cytoskeleton, we show that one relevant site is the end of the C terminus of alpha. Truncation of the last six amino acids resulted in alphabeta channels with an extent of inactivation up to 2.5-fold larger and its further enhancement by cytochalasins being reduced 2-fold. The wild-type channels exhibited strong inactivation, which could not be markedly increased either by cytochalasins or by the C-terminal mutations, indicating that the interaction of the wild-type channels with microfilaments was minimal to begin with, presumably because of extensive basal phosphorylation. Since the C-terminal end of Kv1.1 was shown to participate in channel clustering via an interaction with members of the PSD-95 family of proteins, we propose that a similar interaction with an endogenous protein takes place, contributing to channel connection to the oocyte cytoskeleton. This is the first report to assign a modulatory role to such an interaction: together with the state of phosphorylation of the channel, it regulates the extent of inactivation conferred by the beta subunit.

Reversible antisense inhibition of Shaker-like Kv1.1 potassium channel expression impairs associative memory in mouse and rat

Long-term memory is thought to be subserved by functional remodeling of neuronal circuits. Changes in the weights of existing synapses in networks might depend on voltage-gated potassium currents. We therefore studied the physiological role of potassium channels in memory, concentrating on the Shaker-like Kv1.1, a late rectifying potassium channel that is highly localized within dendrites of hippocampal CA3 pyramidal and dentate gyrus granular cells. Repeated intracerebroventricular injection of antisense oligodeoxyribonucleotide to Kv1.1 reduces expression of its particular intracellular mRNA target, decreases late rectifying K+ current(s) in dentate granule cells, and impairs memory but not other motor or sensory behaviors, in two different learning paradigms, mouse passive avoidance and rat spatial memory. The latter, hippocampal-dependent memory loss occurred in the absence of long-term potentiation changes recorded both from the dentate gyrus or CA1. The specificity of the reversible antisense targeting of mRNA in adult animal brains may avoid irreversible developmental and genetic background effects that accompany transgenic "knockouts".

Effect of lanthanum on voltage-dependent gating of a cloned mammalian neuronal potassium channel

The effect of the trivalent cation lanthanum (La3+) on voltage-dependent gating of a cloned mammalian neuronal Kv1.1 potassium channel was studied under whole-cell voltage-clamp conditions in oocytes of Xenopus laevis. La3+ (100 microM) was found to decrease the potassium currents at all test potentials and to shift the midpoint of the fraction open channels/membrane voltage curve by approximately +20 mV. The opening and closing time constants of Kv1.1 channels were empirically fitted with a 4th power Hodgkin-Huxley formalism, or with mono- and multi-exponentials. It was found that La3+ slowed down the kinetics of activation, speeded up those of deactivation, and shifted the opening kinetics by approximately + 60 mV. Interestingly, all these parameters of channel gating were not affected equally by La3+. Furthermore, amplitudes of the inward tail currents evoked at potentials more negative than the potassium equilibrium potential (E(K+)) were more strongly inhibited by La3+ than those of the outward tail currents evoked at potentials more positive than E(K+). This suggests voltage-dependent block and binding of La3+ to the Kv1.1 channel protein. We conclude that these actions cannot be explained in terms of surface charge considerations alone. Our results provide evidence for a direct interaction with the potassium channel protein, shedding new light on the mechanism of action of this lanthanide.

A novel subunit for shal K+ channels radically alters activation and inactivation

Shal (Kv4) potassium channel genes encode classical subthreshold A-currents, and their regulation may be a key factor in determining neuronal firing frequency. The inactivation rate of Shal channels is increased by a presently unidentified class of proteins in both Drosophila and mammals. We have cloned a novel Shal channel subunit (jShalgamma1) from the jellyfish Polyorchis penicillatus that alters Shal currents from both invertebrates and vertebrates. When co-expressed with the conserved jellyfish Shal homolog jShal1, jShalgamma1 dramatically changes both the rate of inactivation and voltage range of activation and steady-state inactivation. jShalgamma1 provides fast inactivation by a classic N-type mechanism, which is independent of its effects on voltage dependence. jShalgamma1 forms functional channels only as a heteromultimer, and jShalgamma1 + jShal1 heteromultimers are functional only in a 2:2 subunit stoichiometry.

The N-terminal domain of a K+ channel beta subunit increases the rate of C-type inactivation from the cytoplasmic side of the channel

Voltage-gated K+ channels are complexes of membrane-bound, ion-conducting alpha and cytoplasmic ancillary (beta) subunits. The primary physiologic effect of coexpression of alpha and beta subunits is to increase the intrinsic rate of inactivation of the alpha subunit. For one beta subunit, Kv beta 1.1, inactivation is enhanced through an N-type mechanism. A second beta subunit, Kv beta 1.2, has been shown to increase inactivation, but through a distinct mechanism. Here we show that the degree of enhancement of Kv beta 1.2 inactivation is dependent on the amino acid composition in the pore mouth of the alpha subunit and the concentration of extracellular K+. Experimental conditions that promote C-type inactivation also enhance the stimulation of inactivation by Kv beta 1.2, showing that this beta subunit directly stimulates C-type inactivation. Chimeric constructs containing just the nonconserved N-terminal region of Kv beta 1.2 fused with an alpha subunit behave in a similar fashion to coexpressed Kv beta 1.2 and alpha subunit. This shows that it is the N-terminal domain of Kv beta 1.2 that mediates the increase in C-type inactivation from the cytoplasmic side of the pore. We propose a model whereby the N terminus of Kv beta 1.2 acts as a weakly binding "ball" domain that associates with the intracellular vestibule of the alpha subunit to effect a conformational change leading to enhancement of C-type inactivation.

Expression of Kv1.1 delayed rectifier potassium channels in Lec mutant Chinese hamster ovary cell lines reveals a role for sialidation in channel function

Kv1.1 potassium (K+) channels contain significant amounts of negatively charged sialic acids. To examine the role of sialidation in K+ channel function, Chinese hamster ovary cell lines deficient in glycosylation (Lec mutants) were transfected with rat brain Kv1.1 cDNA. The K+ channel was functionally expressed in all cell lines, but the voltage dependence of activation (V1/2) was shifted to more positive voltages and the activation kinetics were slower in the mutant cell lines compared with control. A similar positive shift in V1/2 was recorded in control cells expressing Kv1.1 following treatment with sialidase or by raising extracellular Ca2+. In contrast, these treatments had little or no effect on the Lec mutants, which indicates that channel sialic acids appear to be the negative surface charges sensitive to Ca2+. The data suggest that sialic acid addition modifies Kv1.1 channel function, possibly by influencing the local electric field detected by its voltage sensor, but that these carbohydrates are not required for cell surface expression.

Do voltage-gated Kv1.1 and inward rectifier Kir2.1 potassium channels form heteromultimers?

Possible heteromultimer formation between Kv- and Kir-type K+ channels was investigated, in connection with the known functional diversity of K+ channels in vivo. Voltage-clamp experiments were performed on Xenopus oocytes, either injected with concatenated Kir2.1-Kv1.1 mRNA, or co-injected with Kv1.1 and Kir2.1 mRNA. K+ currents could be approximated by the algebraic sum of the 2 K+ current types alone. The tandem construct did not show functional expression, although it could be detected by Western blotting. We conclude that Kv1.1 and Kir2.1 alpha-subunit proteins fail to assemble and do not contribute functional diversity to K+ channels.

Fast protein liquid chromatography for the purification of animal venoms

In this paper a methodology is presented for the purification of toxins acting on neuronal potassium channels. These neurotoxins are ideal tools for studying the physiological functioning of ion channels, for classifying them and for exploring them in several neuronal regions. Separation procedures leading to the identification of neurotoxins almost always include gel filtration chromatography as well as other chromatographic techniques such as ion exchange and reversed phase chromatography. The use of a Superdex 30 prep grade gel filtration column, which has the advantage of superior resolution in comparison with traditionally used gel filtration column, which has the advantage of superior resolution in comparison with traditionally used gel filtration columns such as a Sephadex G-50 column, is reported here. The biological assay incorporates the use of Xenopus laevis oocytes, which express potassium channels. Screening of the collected venom fractions was performed by means of the whole-cell voltage clamp technique. The combination of these techniques represents a fast and efficient identification procedure in the search for new and selective neurotoxins for cloned channels and receptors.

Strategy for rapid immobilization of prey by a fish-hunting marine snail

Some venomous animals capture prey with remarkable efficiency and speed. The purple cone, Conus purpurascens, uses two parallel physiological mechanisms requiring multiple neurotoxins to immobilize fish rapidly: neuromuscular block, and excitotoxic shock. The latter requires the newly characterized peptide kappa-conotoxin PVIIA, which inhibits the Shaker potassium channel 2-4, and beta-conotoxin PVIA5, which delays sodium-channel inactivation. Despite the extreme biochemical diversity in venoms, the number of effective strategic alternatives for prey capture are limited. How securely prey is initially tethered may strongly influence the venom strategy evolved by a predator.

Modulation by protein kinase C activation of rat brain delayed-rectifier K+ channel expressed in Xenopus oocytes

The modulation by protein kinase C (PKC) of the RCK1 K+ channel was investigated in Xenopus oocytes by integration of two-electrode voltage clamp, site-directed mutagenesis and SDS-PAGE analysis techniques. Upon application of beta-phorbol 12-myristate 13-acetate (PMA) the current was inhibited by 50-90%. No changes in the voltage sensitivity of the channel, changes in membrane surface area or selective elimination of RCK1 protein from the plasma membrane could be detected. The inhibition was mimicked by 1-oleoyl-2-acetyl-rac-glycerol (OAG) but not by alphaPMA, and was blocked by staurosporine and calphostin C. Upon deletion of most of the N-terminus a preceding enhancement of about 40% of the current was prominent in response to PKC activation. Its physiological significance is discussed. The N-terminus deletion eliminated 50% of the inhibition. However, phosphorylation of none of the ten classical PKC phosphorylation sites on the channel molecule could account, by itself or in combination with others, for the inhibition. Thus, our results show that PKC activation can modulate the channel conductance in a bimodal fashion. The N-terminus is involved in the inhibition, however, not via its direct phosphorylation.

Kv beta 1 subunit binding specific for shaker-related potassium channel alpha subunits

Voltage-activated potassium (Kv) channels from mammalian brain are hetero-oligomers containing alpha and beta subunits. Coexpression of Kv1 alpha and Kv beta 1 subunits confers rapid A-type inactivation on noninactivating potassium channels (delayed rectifiers) in expression systems in vitro. We have delineated a Kv1.5 aminoterminal region of up to 90 amino acids (residues 112-201) that is sufficient for interactions of Kv1.5 alpha and Kv beta 1 subunits. Within this region of the Kv1.5 amino terminus (residues 193-201), a Kv beta 1 interaction site necessary for Kv beta 1-mediated rapid inactivation of Kv1.5 currents was detected. This interaction site motif (FYE/QLGE/DEAM/L) is found exclusively in the Shaker-related subfamily (Kv1). The results show that hetero-oligomerization between alpha and Kv beta 1 subunits is restricted to Shaker-related potassium channel alpha subunits.

An improved fractionation and fast screening method for the identification of new and selective neurotoxins

Neurotoxins have highly specific actions on molecular targets, and thus offer an effective means of characterizing the growing number of identified ion channels and receptors in the nervous system. Separation procedures leading to the identification of neurotoxins almost always include gel filtration chromatography, combined with ion-exchange and/or reversed phase chromatography. We present here an improved fractionation method based on the use of a new Superdex 30 prep grade HiLoad 16/60 FPLC gel filtration column. This single-step gel filtration protocol results in a shortening of the purification process and allows a superior qualitative separation of (neuro-)peptides in crude venoms as compared to any other type of gel filtration column used thus far. Screening of the collected fractions for potential ion channel blocking properties was performed by means of the whole-cell voltage clamp technique. To increase both the amount and speed of expression in Xenopus laevis oocytes of cloned ion channels, we employed a high-expression vector, pGEMHE, wherein the cDNA encoding a neuronal voltage-dependent potassium channel (RCK1) was subcloned. The combination of these techniques represents a fast and efficient identification procedure in the quest for new and selective neurotoxins for cloned channels and receptors.