Alpha-dendrotoxin acceptor from bovine brain is a K+ channel protein. Evidence from the N-terminal sequence of its larger subunit

Ancillary subunits associated with voltage-dependent K+ channels

Since the first discovery of Kvbeta-subunits more than 15 years ago, many more ancillary Kv channel subunits were characterized, for example, KChIPs, KCNEs, and BKbeta-subunits. The ancillary subunits are often integral parts of native Kv channels, which, therefore, are mostly multiprotein complexes composed of voltage-sensing and pore-forming Kvalpha-subunits and of ancillary or beta-subunits. Apparently, Kv channels need the ancillary subunits to fulfill their many different cell physiological roles. This is reflected by the large structural diversity observed with ancillary subunit structures. They range from proteins with transmembrane segments and extracellular domains to purely cytoplasmic proteins. Ancillary subunits modulate Kv channel gating but can also have a great impact on channel assembly, on channel trafficking to and from the cellular surface, and on targeting Kv channels to different cellular compartments. The importance of the role of accessory subunits is further emphasized by the number of mutations that are associated in both humans and animals with diseases like hypertension, epilepsy, arrhythmogenesis, periodic paralysis, and hypothyroidism. Interestingly, several ancillary subunits have in vitro enzymatic activity; for example, Kvbeta-subunits are oxidoreductases, or modulate enzymatic activity, i.e., KChIP3 modulates presenilin activity. Thus different modes of beta-subunit association and of functional impact on Kv channels can be delineated, making it difficult to extract common principles underlying Kvalpha- and beta-subunit interactions. We critically review present knowledge on the physiological role of ancillary Kv channel subunits and their effects on Kv channel properties.

Understanding the molecular mechanism underlying the presynaptic toxicity of secreted phospholipases A2

An important group of toxins, whose action at the molecular level is still a matter of debate, is secreted phospholipases A(2) (sPLA(2)s) endowed with presynaptic or beta-neurotoxicity. The current belief is that these beta-neurotoxins (beta-ntxs) exert their toxicity primarily due to their extracellular enzymatic action on the plasma membrane of motoneurons at the neuromuscular junction. However, the discovery of several extra- and intracellular proteins, with high binding affinity for snake venom beta-ntxs, has raised the question as to whether this explanation is adequate to account for all the observed phenomena in the process of presynaptic toxicity. The purpose of this review is to critically examine the various published studies, including the most recent results on internalization of a beta-ntx into motor nerve terminals, in order to contribute to a better understanding of the molecular mechanism of beta-neurotoxicity. As a result, we propose that presynaptic neurotoxicity of sPLA(2)s is a result of both extra- and intracellular actions of beta-ntxs, involving enzymatic activity as well as interaction of the toxins with intracellular proteins affecting the cycling of synaptic vesicles in the axon terminals of vertebrate motoneurons.

Structure–function relationships and mechanism of anticoagulant phospholipase A2 enzymes from snake venoms

Phospholipase A(2) (PLA(2)) enzymes from snake venom are toxic and induce a wide spectrum of pharmacological effects, despite similarity in primary, secondary and tertiary structures and common catalytic properties. Thus, the structure-function relationships and the mechanism of this group of small proteins are subtle, complex and intriguing challenges. This review, taking the PLA(2) enzymes from spitting cobra (Naja nigricollis) venom as examples, describes the mechanism of anticoagulant effects. The strongly anticoagulant CM-IV inhibits both the extrinsic tenase and prothrombinase complexes, whereas the weakly anticoagulant PLA(2) enzymes (CM-I and CM-II) inhibit only the extrinsic tenase complex. CM-IV binds to factor Xa and interferes in its interaction with factor Va and the formation of prothrombinase complex. In contrast, CM-I and CM-II do not affect the formation of prothrombinase complex. In addition, CM-IV inhibits the extrinsic tenase complex by a combination of enzymatic and nonenzymatic mechanisms, while CM-I and CM-II inhibit by only enzymatic mechanism. These functional differences explain the disparity in the anticoagulant potency of N. nigricollis PLA(2) enzymes. Similarly, human secretory enzyme binds to factor Xa and inhibits the prothrombinase complex. We predicted the anticoagulant region of PLA(2) enzymes using a systematic and direct comparison of amino acid sequences. This region between 54 and 77 residues is basic in the strongly anticoagulant PLA(2) enzymes and neutral or negatively charged in weakly and non-anticoagulant enzymes. The prediction is validated independently by us and others using both site directed mutagenesis and synthetic peptides. Thus, strongly anticoagulant CM-IV binds to factor Xa (its target protein) through the specific anticoagulant site on its surface. In contrast, weakly anticoagulant enzymes, which lack the anticoagulant region fail to bind specifically to the target protein, factor Xa in the coagulation cascade. Thus, these studies strongly support the target model which suggests that protein-protein interaction rather than protein-phospholipid interaction determines the pharmacological specificity of PLA(2) enzymes.

Auxiliary subunits of Shaker-type potassium channels

Voltage-gated potassium channels are important determinants of membrane excitability. This family of ion channels is composed of several classes of proteins, including the pore-forming Kvalpha subunits and the recently identified auxiliary Kvbeta subunits. A combination of a large number of genes that encode various alpha subunits and beta subunits and the selective formation of alpha-alpha and alpha-beta heteromultimeric channels provides rich molecular diversity that allows for regulated functional heterogeneity in both excitable tissues and nonexcitable tissues. Because the Kvbeta subunits can either upregulate or downregulate potassium currents, depending on the specific subunit combination, it is essential to understand their function at the molecular level. Furthermore, targeted changes of the Kvbeta expression or disruption of certain alpha-beta interactions could serve as a molecular basis for designing drugs and therapy to regulate excitability clinically.

Excitement ahead: structure, function and mechanism of snake venom phospholipase A2 enzymes

Venom phospholipase A2 (PLA2) enzymes share similarity in structure and catalytic function with mammalian enzymes. However, in contrast to mammalian enzymes, many are toxic and induce a wide spectrum of pharmacological effects. Thus structure-function relationship of this group of small proteins is subtle, but complex puzzle to protein biochemists, molecular biologists, toxinologists, pharmacologists and physiologists. This review describes the present status of our understanding of their structure, function and mechanism. It was proposed that their unique ability to 'target' themselves to a specific organ or tissue is due to their high affinity binding to specific proteins which act as receptors (more precisely, acceptors). This specific binding of PLA2 is conferred by the presence of a 'pharmacological site' on its surface which is independent of the catalytic site. The high affinity interaction of PLA2 with its acceptor (or target protein) is probably due to the complementarity, in terms of charges, hydrophobicity and van der Waal's contact surfaces, between the pharmacological site and the binding site on the surface of the acceptor protein. Upon binding to the target, the PLA2 can induce its pharmacological effects by mechanisms either dependent on or independent of its catalytic activity. Because of the unprecedented wide spectrum of specific targeting to various tissues and organs, identification of the pharmacological sites has potential for exploitation in development of novel systems useful for 'delivering' specific proteins to a particular target tissue or organ. Thus research in this field will provide a lot of exciting opportunities.

R25 is an intracellular membrane receptor for a snake venom secretory phospholipase A(2)

Ammodytoxin is a presynaptically neurotoxic (beta-neurotoxic) snake venom secretory phospholipase A(2) (sPLA(2)). We detected a 25 kDa protein which binds the toxin with very high affinity (R25) in porcine cerebral cortex. Here we show that R25 is an integral membrane protein with intracellular localisation. It is the first sPLA(2) receptor known to date that localises to intracellular membranes. Centrifugation on sucrose gradients was used to fractionate porcine cerebral cortex. The subcellular composition of the fractions was determined by following the distribution of organelle-specific markers. The distribution of R25 in the fractions matched the distribution of the mitochondrial marker succinate dehydrogenase, but not the markers for plasma membrane, lysosomes, endoplasmic reticulum, synaptic and secretory vesicles. R25 most likely resides in mitochondria, which are known to be targets for sPLA(2) neurotoxins in the nerve ending and are potentially implicated in the process of beta-neurotoxicity.

Structural characterisation of neuronal voltage-sensitive K+ channels heterologously expressed in Pichia pastoris

Neuronal voltage-dependent K(+) channels of the delayed rectifier type consist of multiple Kv alpha subunit variants, which assemble as hetero- or homotetramers, together with four Kv beta auxiliary subunits. Direct structural information on these proteins has not been forthcoming due to the difficulty in isolating the native K(+) channels. We have overexpressed the subunit genes in the yeast Pichia pastoris. The Kv1.2 subunit expressed alone is shown to fold into a native conformation as determined by high-affinity binding of 125I-labelled alpha-dendrotoxin, while co-expressed Kv1.2 and Kv beta 2 subunits co-assembled to form native-like oligomers. Sites of post-translational modifications causing apparent heterogeneity on SDS-PAGE were identified by site-directed mutagenesis. Engineering to include affinity tags and scale-up of production by fermentation allowed routine purification of milligram quantities of homo- and heteroligomeric channels. Single-particle electron microscopy of the purified channels was used to generate a 3D volume to 2.1 nm resolution. Protein domains were assigned by fitting crystal structures of related bacterial proteins. Addition of exogenous lipid followed by detergent dialysis produced well-ordered 2D crystals that exhibited mostly p12(1) symmetry. Projection maps of negatively stained crystals show the constituent molecules to be 4-fold symmetric, as expected for the octameric K(+) channel complex.

The neurotoxic phospholipase A2 associates, through a non-phosphorylated binding motif, with 14-3-3 protein gamma and epsilon isoforms

Two novel acceptors for ammodytoxin C, a presynaptically neurotoxic phospholipase A(2) from snake venom, have been purified from porcine cerebral cortex by a toxin-affinity-based procedure. Using tandem mass spectrometry, the isolated acceptors were identified as 14-3-3 gamma and epsilon isoforms, highly conserved cytoplasmic proteins involved in the regulation of numerous physiological processes. The interaction between ammodytoxin C and 14-3-3 proteins is direct and not mediated by calmodulin, a high-affinity acceptor for both ammodytoxin C and 14-3-3 proteins, as demonstrated in pull-down experiments and by surface plasmon resonance. The latter technique gave an apparent dissociation constant of 1.0+/-0.2 microM for the interaction between chip-immobilized 14-3-3 and ammodytoxin C. 14-3-3 usually interacts with proteins through specific phospho-Ser/Thr motifs. Ammodytoxin C is not a phospho-protein, therefore the interaction must occur through a non-phosphorylated binding site, most probably the KEESEK sequence at its C-terminal end. The interaction we describe suggests an explanation for the pathophysiological effects evoked by some secreted phospholipases A(2), such as the inhibition of protein phosphorylation, of terminal ion currents, and of neurotransmission, as well as the initiation of neuronal cell death, all processes regulated by 14-3-3 proteins.

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.

Dendrotoxin sensitive potassium channels modulate GABA but not glutamate release in the rat entorhinal cortex in vitro

We have previously shown that the anticonvulsant drug, phenytoin, increases the frequency and amplitude of spontaneous inhibitory postsynaptic currents at GABA synapses on principal neurones in the rat entorhinal cortex. This effect is similar to that seen at other GABA synapses following blockade of voltage-gated potassium channels (Kv1.1, 1.2 and 1.6) with alpha-dendrotoxin. In the present study we examined whether dendrotoxins can alter GABA release at synapses in the entorhinal cortex. We recorded spontaneous inhibitory postsynaptic currents using whole cell voltage clamp techniques in slices of rat entorhinal cortex in vitro. alpha-Dendrotoxin evoked an increase in frequency and amplitude of spontaneous inhibitory postsynaptic currents, an effect that was blocked by prior perfusion with tetrodotoxin. The effect of the toxin did not occlude the increase in spontaneous inhibitory postsynaptic currents seen with phenytoin. Indeed, the effect of the two drugs together was, at least, additive on GABA release. Perfusion with the specific Kv1.1 blocker, dendrotoxin-K had no effect on GABA release. In addition, alpha-dendrotoxin had no effect on frequency or amplitude of spontaneous excitatory postsynaptic currents at glutamate synapses on entorhinal cortex neurones. We conclude that K-channels containing the Kv1.2 and/or 1.6 subunits modulate the release of GABA, but not glutamate in the entorhinal cortex. The modulation of GABA release by phenytoin is unlikely to be due to an effect on these channels.

Beta-bungarotoxin is a potent inducer of apoptosis in cultured rat neurons by receptor-mediated internalization

The neurotoxic phospholipase A(2), beta-bungarotoxin (beta-BuTx), is a component of the snake venom from the Taiwanese banded krait Bungarus multicinctus. beta-BuTx affects presynaptic nerve terminal function of the neuromuscular junction and induces widespread neuronal cell death throughout the mammalian and avian CNS. To analyse the initial events of beta-BuTx-mediated cell death, the toxin was applied to cultured rat hippocampal neurons where it induced neuronal cell death in a concentration-dependent manner (EC(50) approximately equal to 5 x 10(-13) M) within 24 h. Fluorescence labelled beta-BuTx was completely incorporated by neurons within < 10 min. Binding and uptake of beta-BuTx, as well as induction of cell death, were efficiently antagonized by preincubation with dendrotoxin I, a blocker of voltage-gated potassium channels devoid of phospholipase activity. Binding of beta-BuTx was selective for neurofilament-positive cells. As evident from intense annexin-V and TUNEL stainings, application of beta-BuTx induced apoptotic cell death exclusively in neurons, leaving astrocytes unaffected. No evidence was obtained for any contribution of either caspases or calpains to beta-BuTx-induced apoptosis, consistent with the inability of the inhibitors Z-Asp-DCB and calpeptin, respectively, to protect neurons from beta-BuTx-induced cell death. These observations indicate that induction of cell death by beta-BuTx comprises several successive phases: (i) binding to neuronal potassium channels is the initial event, followed by (ii) internalization and (iii) induction of apoptotic cell death via a caspase-independent pathway.

A high affinity acceptor for phospholipase A2 with neurotoxic activity is a calmodulin

One of the high affinity binding proteins for ammodytoxin C, a snake venom presynaptically neurotoxic phospholipase A(2), has been purified from porcine cerebral cortex and characterized. After extraction from the membranes, the toxin-binding protein was isolated in a homogenous form using wheat germ lectin-Sepharose, Q-Sepharose, and ammodytoxin-CH-Sepharose chromatography. The specific binding of (125)I-ammodytoxin C to the isolated acceptor was inhibited to different extents by some neurotoxic phospholipases A(2), ammodytoxins, bee venom phospholipase A(2), agkistrodotoxin, and crotoxin; but not by nontoxic phospholipases A(2), ammodytin I(2), porcine pancreatic phospholipase A(2), and human type IIA phospholipase A(2); suggesting the significance of the acceptor in the mechanism of phospholipase A(2) neurotoxicity. The isolated acceptor was identified as calmodulin by tandem mass spectrometry. Since calmodulin is generally considered as an intracellular protein, the identity of this acceptor supports the view that secretory phospholipase A(2) neurotoxins have to be internalized to exert their toxic effect. Moreover, since ammodytoxin is known to block synaptic transmission, its interaction with calmodulin as an acceptor may constitute a valuable probe for further investigation of the role of the latter in this Ca(2+)-regulated process.

Twenty years of dendrotoxins

Dendrotoxins are small proteins that were isolated 20 years ago from mamba (Dendroaspis) snake venoms (Harvey, A.L., Karlsson, E., 1980. Dendrotoxin from the venom of the green mamba, Dendroaspis angusticeps: a neurotoxin that enhances acetylcholine release at neuromuscular junctions. Naunyn-Schmiedebergs Arch. Pharmacol. 312, 1-6.). Subsequently, a family of related proteins was found in mamba venoms and shown to be homologous to Kunitz-type serine protease inhibitors, such as aprotinin. The dendrotoxins contain 57-60 amino acid residues cross-linked by three disulphide bridges. The dendrotoxins have little or no anti-protease activity, but they were demonstrated to block particular subtypes of voltage-dependent potassium channels in neurons. Studies with cloned K(+) channels indicate that alpha-dendrotoxin from green mamba Dendroaspis angusticeps blocks Kv1.1, Kv1.2 and Kv1.6 channels in the nanomolar range, whereas toxin K from the black mamba Dendroaspis polylepis preferentially blocks Kv1.1 channels. Structural analogues of dendrotoxins have helped to define the molecular recognition properties of different types of K(+) channels, and radiolabelled dendrotoxins have also been useful in helping to discover toxins from other sources that bind to K(+) channels. Because dendrotoxins are useful markers of subtypes of K(+) channels in vivo, dendrotoxins have become widely used as probes for studying the function of K(+) channels in physiology and pathophysiology.

What does beta-bungarotoxin do at the neuromuscular junction?

beta-Bungarotoxin from the Taiwan banded krait, Bungarus multicinctus is a basic protein (pI=9.5), with a molecular weight of 21,800 consisting of two different polypeptide subunits. A phospholipase A(2) subunit named the A-chain and a non-phospholipase A(2) subunit named the B-chain, which is homologous to Kunitz protease inhibitors. The A-chain and the B-chain are covalently linked by one disulphide bridge. On mouse hemi-diaphragm nerve-muscle preparations, partially paralysed by lowering the external Ca(2+) concentration, beta-bungarotoxin classically produces triphasic changes in the contraction responses to indirect nerve stimulation. The initial transient inhibition of twitches (phase 1) is followed by a prolonged facilitatory phase (phase 2) and finally a blocking phase (phase 3). These changes in twitch tension are mimicked, to some extent, by similar changes to end plate potential amplitude and miniature end plate potential frequency. The first and second phases are phospholipase-independent and are thought to be due to the B-chain (a dendrotoxin mimetic) binding to or near to voltage-dependent potassium channels. The last phase (phase 3) is phospholipase dependent and is probably due to phospholipase A(2)-mediated destruction of membrane phospholipids in motor nerve terminals.

Neuronal receptors for phospholipases A(2 )and beta-neurotoxicity

Some phospholipases A(2) interrupt neuromuscular communication by blocking the release of neurotransmitter into the synaptic cleft. Despite numerous studies, the molecular mechanism of their action is still largely obscure. In this review the best-characterized receptors for beta-neurotoxins are presented. We propose a model which could be useful in investigating the apparent inconsistency between the observed heterogeneity in the neuronal binding of beta-neurotoxins and the very similar pathomorphological and electrophysiological effects which they produce in the intoxicated tissue. We assume that beta-neurotoxins enter the nerve ending to exert their toxic effect. The model involves different pathways for phospholipase A(2) neurotoxins to reach the site of action inside the neuron, their respective extra- and intracellular neuronal receptors being key features of the pathway. Once in the nerve cell, beta-neurotoxins impair the function of the synaptic vesicles by phospholipid hydrolysis of the inner leaflet of the vesicle bilayer. The proportion of the products of the phospholipid hydrolysis, lysophospholipids and phospholipids in the membrane, has been demonstrated to be very important for the shaping of the membrane, affecting its fusogenic properties. Due to the same final step in the action of beta-neurotoxins, phospholipid hydrolysis, the consequences of their poisoning are practically identical.

Interaction of the neurotoxic and nontoxic secretory phospholipases A2 with the crotoxin inhibitor from Crotalus serum

Crotalus durissus terrificus snakes possess a protein in their blood, named crotoxin inhibitor from Crotalus serum (CICS), which protects them against crotoxin, the main toxin of their venom. CICS neutralizes the lethal potency of crotoxin and inhibits its phospholipase A2 (PLA2) activity. The aim of the present study is to investigate the specificity of CICS towards snake venom neurotoxic PLA2s (beta-neurotoxins) and nontoxic mammalian PLA2s. This investigation shows that CICS does not affect the enzymatic activity of pancreatic and nonpancreatic PLA2s, bee venom PLA2 and Elapidae beta-neurotoxins but strongly inhibits the PLA2 activity of Viperidae beta-neurotoxins. Surface plasmon resonance and PAGE studies further demonstrated that CICS makes complexes with monomeric and multimeric Viperidae beta-neurotoxins but does not interact with nontoxic PLA2s. In the case of dimeric beta-neurotoxins from Viperidae venoms (crotoxin, Mojave toxin and CbICbII), which are made by the noncovalent association of a PLA2 with a nonenzymatic subunit, CICS does not react with the noncatalytic subunit, instead it binds tightly to the PLA2 subunit and induces the dissociation of the heterocomplex. In vitro assays performed with Torpedo synaptosomes showed a protective action of CICS against Viperidae beta-neurotoxins but not against other PLA2 neurotoxins, on primary and evoked liberation of acetylcholine. In conclusion, CICS is a specific PLA2 inhibitor of the beta-neurotoxins from the Viperidae family.

Identification and purification of a novel receptor for secretory phospholipase A(2) in porcine cerebral cortex

A specific phospholipase A(2) receptor from porcine cerebral cortex has been characterized (K(d) = 145 nM, B(max) = 0.4 pmol/mg membrane protein) by using a radioiodinated derivative of ammodytoxin C (AtxC), a snake venom presynaptically neurotoxic group IIA phospholipase A(2). After the receptor was solubilized in a ligand-binding form, it was approximately 14,000-fold enriched by chromatography on wheat germ lectin-Sepharose and AtxC-Affi-Gel 10. The receptor is a single chain glycoprotein with an apparent molecular mass of 180 kDa and binds toxic and non-toxic phospholipases A(2) of either group I or II. It also recognizes conjugates of bovine serum albumin with mannose, N-acetylglucosamine, and galactose. In its molecular mass and pharmacological profile, the AtxC receptor resembles the M-type receptor for secretory phospholipases A(2) from rabbit skeletal muscle (a C-type multilectin, homologous to macrophage mannose receptor), yet in terms of relative abundance in brain and antigenicity, these two receptors are completely different. A further AtxC receptor of approximately 200 kDa discovered in porcine liver was, however, recognized by anti-rabbit M-type phospholipase A(2) receptor antibodies. There are, therefore, two immunologically distinct secretory phospholipase A(2) receptors of about 200 kDa in the same species. Although the liver receptor is related to the M-type secretory phospholipase A(2) receptors, the brain receptor is not and belongs to a novel group of secretory phospholipase A(2) receptors.

Subunit composition of Kv1 channels in human CNS

The alpha subunits of Shaker-related K+ channels (Kv1.X) show characteristic distributions in mammalian brain and restricted coassembly. Despite the functional importance of these voltage-sensitive K+ channels and involvement in a number of diseases, little progress has been achieved in deciphering the subunit composition of the (alpha)4(beta)4 oligomers occurring in human CNS. Thus, the association of alpha and beta subunits was investigated in cerebral grey and white matter and spinal cord from autopsy samples. Immunoblotting established the presence of Kv1.1, 1.2, and 1.4 in all the tissues, with varying abundance. Sequential immunoprecipitations identified the subunits coassembled. A putative tetramer of Kv1.3/1.4/1.1/1.2 was found in grey matter. Both cerebral white matter and spinal cord contained the heterooligomers Kv1.1/1.4 and Kv1.1/1.2, similar to grey matter, but both lacked Kv1.3 and the Kv1.4/1.2 combination. An apparent Kv1.4 homooligomer was detected in all the samples, whereas only the brain tissue possessed a putative Kv1.2 homomer. In grey matter, Kvbeta2.1 was coassociated with the Kv1.1/1.2 combination and Kv1.2 homooligomer. In white matter, Kvbeta2.1 was associated with Kv1.2 only, whereas Kvbeta1.1 coprecipitated with all the alpha subunits present. This represents the first description of Kv1 subunit complexes in the human CNS and demonstrates regional variations, indicative of functional specialisation.

Crocalbin: a new calcium-binding protein that is also a binding protein for crotoxin, a neurotoxic phospholipase A2

Utilizing Marathon-ready cDNA library and a gene-specific primer corresponding to a partial amino acid sequence determined previously, the complete nucleotide sequence for the cDNA of crocalbin, which binds crotoxin (a phospholipase A2) and Ca2+, was obtained by polymerase chain reaction. The open reading frame of the cDNA encodes a novel polypeptide of 315 amino acid residues, including a signal sequence of 19 residues. This protein contains six potential Ca(2+)-binding domains, one N-glycosylation site, and a large amount of acidic amino acid residues. The ability to bind Ca2+ has been ascertained by calcium overlay experiment. Evidenced by sequence similarity in addition, it is concluded that crocalbin is a new member of the reticulocalbin family of calcium-binding proteins.

Identification of a new high-affinity binding protein for neurotoxic phospholipases A2

Ammodytoxin C is a neurotoxic phospholipase A2 which blocks the release of neurotransmitter from the nerve terminal. Using a radioiodinated derivative of the toxin, we located its specific high-affinity binding site in the demyelinated P2 fraction of porcine cerebral cortex (Kd = 15 nM; Bmax = 1.5 pmol/mg membrane protein). In cross-linking experiments on a membrane preparation, 125I-ammodytoxin C labeled a protein of 25 kDa. The formation of a specific adduct was not inhibited by nontoxic phospholipases A2 or even by neurotoxic phospholipases A2 which have practically identical pathophysiological activities to ammodytoxin C: agkistrodotoxin, Oxyuranus scutellatus 2 phospholipase A2, taipoxin, beta-bungarotoxin, notexin, and crotoxin. 125I-ammodytoxin C specific cross-linking was inhibited, however, by mannosylated BSA, suggesting the presence of a carbohydrate-recognition domain in the acceptor structure. According to the pharmacological and structural properties, the ammodytoxin acceptor from porcine cerebral cortex differs from other so far identified as phospholipase A2 acceptors and represents a new type of a high-affinity binding protein for neurotoxic phospholipases A2.

Regulation of ion channel expression by cytoplasmic subunits

Neuronal and cardiac voltage-gated ion channels contain auxiliary subunits that can profoundly affect the gating of the pore-forming and voltage-sensing alpha subunits. Recent studies on the structurally similar cytoplasmic beta subunits of Ca2+ and K+ channels reveal that these subunits can also exert profound effects on the expression of the integral membrane protein channel components. The mechanisms by which these effects occur are now being elucidated through a combined approach that employs biophysical, pharmacological, cell biological and biochemical techniques.

Specific antibodies to the external vestibule of voltage-gated potassium channels block current

Using delayed-rectifier potassium channels as examples, we have designed two specific blockers by generating specific antipeptide antibodies to epitopes in the external vestibules of two channel proteins, Kv1.2 and Kv3.1. These antibodies reduced whole-cell Kv1.2 or Kv3.1 currents in transfected cells and the effect was blocked by the corresponding peptide antigen, but not by control peptides. A control antibody had little effect on Kv1.2 currents and the Kv1.2 blocker antibody had limited effect on other related potassium currents. Furthermore, the Kv1.2 blocking antibody inhibited dendrotoxin binding to Kv1.2 channel proteins in transfected cells. Moreover, using the Kv1.2 blocker antibody, we determined the presence and relative contribution of endogenous Kv1.2 to the overall endogenous K+ currents in NG108 neuronal cells. This guided design of specific channel blockers will facilitate future physiological studies on ion channel functions.

Voltage-activated potassium channels in mammalian neurons and their block by novel pharmacological agents

1. Electrophysiological studies have shown that a number of different types of potassium (K) channel currents exist in mammalian neurons. Among them are the voltage-gated K channel-currents which have been classified as fast-inactivating A-type currents (KA) and slowly inactivating delayed-rectifier type currents (KDR). 2. Two major molecular superfamilies of K channel have been identified; the KIR superfamily and the Shaker-related superfamily with a number of different pore-forming alpha-subunits in each superfamily. 3. Within the Shaker-related superfamily are the KV family, comprising of at least 18 different alpha-subunits that almost certainly underlie classically defined KA and KDR currents. However, the relationship between each of these cloned alpha-subunits and native voltage-gated K currents remains, for the most part, to be established. 4. Classical pharmacological blockers of voltage-gated K channels such as tetraethylammonium ions (TEA), 4-aminopyridine (4-AP), and certain toxins lack selectivity between different native channel currents and between different cloned K channel currents. 5. A number of other agents block neuronal voltage-gated K channels. All of these compounds are used primarily for other actions they possess. They include organic calcium (Ca) channel blockers, divalent and trivalent metal ions and certain calcium signalling agents such as caffeine. 6. A number of clinically active tricyclic compounds such as imipramine, amitriptyline, and chlorpromazine are also potent inhibitors of neuronal voltage-gated K channels. These compounds are weak bases and it appears that their uncharged form is required for activity. These compounds may provide a useful starting point for the rational design of novel selective K channel blocking agents.

Kvbeta2 inhibits the Kvbeta1-mediated inactivation of K+ channels in transfected mammalian cells

Cloned auxiliary beta-subunits (e.g. Kvbeta1) modulate the kinetic properties of the pore-forming alpha-subunits of a subset of Shaker-like potassium channels. Coexpression of the alpha-subunit and Kvbeta2, however, induces little change in channel properties. Since more than one beta-subunit has been found in individual K+ channel complexes and expression patterns of different beta-subunits overlap in vivo, it is important to test the possible physical and/or functional interaction(s) between different beta-subunits. In this report, we show that both Kvbeta2 and Kvbeta1 recognize the same region on the pore-forming alpha-subunits of the Kv1 Shaker-like potassium channels. In the absence of alpha-subunits the Kvbeta2 polypeptide interacts with additional beta-subunit(s) to form either a homomultimer with Kvbeta2 or a heteromultimer with Kvbeta1. When coexpressing alpha-subunits and Kvbeta1 in the presence of Kvbeta2, we find that Kvbeta2 is capable of inhibiting the Kvbeta1-mediated inactivation. Using deletion analysis, we have localized the minimal interaction region that is sufficient for Kvbeta2 to associate with both alpha-subunits and Kvbeta1. This mapped minimal interaction region is necessary and sufficient for inhibiting the Kvbeta1-mediated inactivation, consistent with the notion that the inhibitory activity of Kvbeta2 results from the coassembly of Kvbeta2 with compatible alpha-subunits and possibly with Kvbeta1. Together, these results provide biochemical evidence that Kvbeta2 may profoundly alter the inactivation activity of another beta-subunit by either differential subunit assembly or by competing for binding sites on alpha-subunits, which indicates that Kvbeta2 is capable of serving as an important determinant in regulating the kinetic properties of K+ currents.

Purification, visualization, and biophysical characterization of Kv1.3 tetramers

The voltage-gated K+ channel of T-lymphocytes, Kv1.3, was heterologously expressed in African Green Monkey kidney cells (CV-1) using a vaccinia virus/T7 hybrid expression system; each infected cell exhibited 10(4) to 5 x 10(5) functional channels on the cell surface. The protein, solubilized with detergent (3-[cholamidopropyl)dimethylammonio]-1-propanesulfonic acid or cholate), was purified to near-homogeneity by a single nickel-chelate chromatography step. The Kv1.3 protein expressed in vaccinia virus-infected cells and its purified counterpart are both modified by a approximately 2-kDa core-sugar moiety, most likely at a conserved N-glycosylation site in the external S1-S2 loop; absence of the sugar does not alter the biophysical properties of the channel nor does it affect expression levels. Purified Kv1.3 has an estimated size of approximately 64 kDa in denaturing SDS-polyacrylamide electrophoresis gels, consistent with its predicted size based on the amino acid sequence. By sucrose gradient sedimentation, purified Kv1.3 is seen primarily as a single peak with an approximate mass of 270 kDa, compatible with its being a homotetrameric complex of the approximately 64-kDa subunits. When reconstituted in the presence of lipid and visualized by negative-staining electron microscopy, the purified Kv1.3 protein forms small crystalline domains consisting of tetramers with dimensions of approximately 65 x 65 A. The center of each tetramer contains a stained depression which may represent the ion conduction pathway. Functional reconstitution of the Kv1.3 protein into lipid bilayers produces voltage-dependent K+-selective currents that can be blocked by two high affinity peptide antagonists of Kv1.3, margatoxin and stichodactylatoxin.

Phosphorylation of a K+ channel alpha subunit modulates the inactivation conferred by a beta subunit. Involvement of cytoskeleton

Voltage-gated K+ channels isolated from mammalian brain are composed of alpha and beta subunits. Interaction between coexpressed Kv1.1 (alpha) and Kvbeta1.1 (beta) subunits confers rapid inactivation on the delayed rectifier-type current that is observed when alpha subunits are expressed alone. Integrating electrophysiological and biochemical analyses, we show that the inactivation of the alphabeta current is not complete even when alpha is saturated with beta, and the alphabeta current has an inherent sustained component, indistinguishable from a pure alpha current. We further show that basal and protein kinase A-induced phosphorylations at Ser-446 of the alpha protein increase the extent, but not the rate, of inactivation of the alphabeta channel, without affecting the association between alpha and beta. In addition, the extent of inactivation is increased by agents that lead to microfilament depolymerization. The effects of phosphorylation and of microfilament depolymerization are not additive. Taken together, we suggest that phosphorylation, via a mechanism that involves the interaction of the alphabeta channel with microfilaments, enhances the extent of inactivation of the channel. Furthermore, phosphorylation at Ser-446 also increases current amplitudes of the alphabeta channel as was shown before for the alpha channel. Thus, phosphorylation enhances in concert inactivation and current amplitudes, thereby leading to a substantial increase in A-type activity.

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.

Molecular properties of voltage-gated K+ channels

Subfamilies of voltage-activated K+ channels (Kv1-4) contribute to controlling neuron excitability and the underlying functional parameters. Genes encoding the multiple alpha subunits from each of these protein groups have been cloned, expressed and the resultant distinct K+ currents characterized. The predicted amino acid sequences showed that each alpha subunit contains six putative membrane-spanning alpha-helical segments (S1-6), with one (S4) being deemed responsible for the channels' voltage sensing. Additionally, there is an H5 region, of incompletely defined structure, that traverses the membrane and forms the ion pore; residues therein responsible for K+ selectively have been identified. Susceptibility of certain K+ currents produced by the Shaker-related subfamily (Kv1) to inhibition by alpha-dendrotoxin has allowed purification of authentic K+ channels from mammalian brain. These are large (M(r) approximately 400 kD), octomeric sialoglycoproteins composed of alpha and beta subunits in a stoichiometry of (alpha)4(beta)4, with subtypes being created by combinations of subunit isoforms. Subsequent cloning of the genes for beta 1, beta 2 and beta 3 subunits revealed novel sequences for these hydrophilic proteins that are postulated to be associated with the alpha subunits on the inner side of the membrane. Coexpression of beta 1 and Kv1.4 subunits demonstrated that this auxiliary beta protein accelerates the inactivation of the K+ current, a striking effect mediate by an N-terminal moiety. Models are presented that indicate the functional domains pinpointed in the channel proteins.

Ultrastructural localization of a voltage-gated K+ channel alpha subunit (KV 1.2) in the rat cerebellum

A highly specific monoclonal antibody and pre-embedding immunocytochemistry were employed to examine the distribution of the K+ channel, alpha subunit K(V)1.2 in the rat cerebellum. At the light microscopic level, the heaviest immunoreactivity was seen in the basket cell pinceau at the base of Purkinje cells, with lighter staining of basket and Golgi cell bodies and a punctate pattern in the granule cell and molecular layers. Electron microscopy was performed to identify the ultrastructural location of K(V)1.2 alpha subunit in these labelled structures. This revealed that the labelling of the pinceau was confined to the preterminal axonal plexus, the area immediately around the Purkinje axon initial segment being relatively devoid of staining. Basket cell parent axons were not immunostained, but gave rise to heavily stained fine processes. Immunoreactivity was also seen in myelinated axons in the granule cell layer and in the medial cerebellar nucleus, the staining being most concentrated at the juxtaparanodal regions of the axons. An unusual pattern of staining was seen in some mossy fibre terminals, with staining restricted to fine protuberances of mossy fibre glomeruli. Structures contacted by these protuberances included adjoining glial processes. Immunostaining was absent from Purkinje cell bodies, dendrites, their axon initial segments and their terminals in the medial cerebellar nucleus. In this study, the alpha subunit K(V)1.2 was localized to a number of different cell types in the cerebellum. Each neuronal type displays a distinct subcellular distribution of the subunit.

Beta subunits promote K+ channel surface expression through effects early in biosynthesis

Voltage-gated K+ channels are protein complexes composed of ion-conducting integral membrane alpha subunits and cytoplasmic beta subunits. Here, we show that, in transfected mammalian cells, the predominant beta subunit isoform in brain, Kv beta 2, associates with the Kv1.2 alpha subunit early in channel biosynthesis and that Kv beta 2 exerts multiple chaperone-like effects on associated Kv1.2 including promotion of cotranslational N-linked glycosylation of the nascent Kv1.2 polypeptide, increased stability of Kv beta 2/Kv1.2 complexes, and increased efficiency of cell surface expression of Kv1.2. Taken together, these results indicate that while some cytoplasmic K+ channel beta subunits affect the inactivation kinetics of alpha subunits, a more general, and perhaps more fundamental, role is to mediate the biosynthetic maturation and surface expression of voltage-gated K+ channel complexes. These findings provide a molecular basis for recent genetic studies indicating that beta subunits are key determinants of neuronal excitability.

NAB domain is essential for the subunit assembly of both alpha-alpha and alpha-beta complexes of shaker-like potassium channels

There are at least five subfamilies of Shaker-like K+ channels. The diverse function of K+ channels are thought to be further modulated by hydrophilic beta subunits. Here we report that Kv beta 1 inactivates RCK4 and Shaker B K+ channels of the Kv1 subfamily, but not Shal2 of the Kv4 subfamily. This correlates the subfamily-specific bindings of Kv beta 1 to the cytoplasmic N-terminal domains of Kv1 alpha subunits. We map the Kv beta 1-binding site to a region overlapping NABKv1, a domain that specifies different Kv1 alpha subunits to form heterotetramers. Using chimeric alpha subunits, we demonstrate that NABKv1 is essential for the Kv beta 1-mediated inactivation. These results suggest that Kv beta 1 modulates a subset of K+ channels through the specific assembly of alpha-beta complexes and reveal the dual function of the NAB domain in mediating the assembly of both alpha-alpha and alpha-beta complexes.

Characterization of Ca(2+)-activated 86Rb+ fluxes in rat C6 glioma cells: a system for identifying novel IKCa-channel toxins

1. The pharmacological characteristics of a putative Ca2+ activated K+ channel (IKCa channel) in rat glioma C6 cells were studied in the presence of the Ca2+ ionophore, ionomycin and various K+ channel blockers, 86Rb+ being used as a radioisotopic tracer for K+. 2. The resting 86Rb+ influx into C6 cells was 318 +/- 20 pmol s-1. The threshold for ionomycin activation of 86Rb+ influx was approx. 100 nM. At ionomycin concentrations above the activation threshold, the initial rate of 86Rb+ influx was proportional to ionophore concentration. Ionomycin-activated 86Rb+ flux was saturable (EC50 = 0.62 +/- 0.03 microM) and was not inhibited by ouabain. 3. Intracellular Ca2+ increased within 30 s from a basal level of 42 +/- 2 nM to 233 +/- 17 nM, after addition of 2 microM ionomycin. During this period, intracellular pH fell from 7.03 +/- 0.04 to 6.87 +/- 0.03 and the cell hyperpolarized from -34 +/- 10 mV to -76 +/- 2 mV. 4. Single channel conductance measurements on inside-out patches in physiological K+ solutions identified a 14 +/- 3 pS CA(2+)-activated K+ current between -25 mV and +50 mV. In symmetrical (100 mM) K+, the single channel conductance was 26 pS. 5. Externally applied quinine (IC50 = 0.12 +/- 0.34 mM) and tetraethylammonium chloride (IC50 = 10 +/- 1.9 mM) inhibited 86Rb+ influx into C6 cells in a concentration-dependent manner. Charybdotoxin (IC50 = 0.5 +/- 0.02 nM) and iberiotoxin (IC50 = 800 +/- 150 nM), as well as the crude venoms from the scorpions Leiurus quinquestriatus and Mesobuthus tamulus, also inhibited 86Rb+ influx. In contrast, apamin and toxin I had no inhibitory effects on 86Rb+ flux. A screen of fractions from cation exchange h.p.l.c. of Mesob. tamulus venom revealed the presence of at least four charybdotoxin-like peptides. One of these was iberiotoxin; the other three are novel toxins. 6. The ionomycin-activated 86Rb+ influx into rat C6 glioma cells has proved to be a valuable pharmacological assay for the screening of toxins and crude venoms which modify intermediate conductance, Ca2+ activated K+ channel activity.

Generation and characterization of subtype-specific monoclonal antibodies to K+ channel alpha- and beta-subunit polypeptides

Molecular characterization of mammalian voltage-sensitive K+ channel genes and their expression became possible with the cloning of the Shaker locus of Drosophila. However, analysis of the expression patterns and subunit composition of native K+ channel protein complexes requires immunological probes specific for the individual K+ channel gene products expressed in excitable tissue. Here, we describe the generation and characterization of monoclonal antibodies (mAbs) against eight distinct mammalian K+ channel polypeptides; the Kv1.1, Kv1.2, Kv1.4, Kv1.5 and Kv1.6 Shaker-related alpha-subunits, the Kv2.1 Shab-related alpha-subunit, and the Kv beta 1 and Kv beta 2 beta-subunits. We characterized the subtype-specificity of these mAbs against native K+ channels in mammalian brain and against recombinant K+ channels expressed in transfected mammalian cells. In addition, we used these mAbs to investigate the cellular and subcellular distribution of the corresponding polypeptides in rat cerebral cortex, as well as their expression levels across brain regions.

Ammodytoxin A acceptor in bovine brain synaptic membranes

Ammodytoxin A, the presynaptic neurotoxin from Vipera ammodytes ammodytes venom, was found to bind specifically and with high affinity to bovine cortex synaptic membrane preparation. The detected ammodytoxin A high-affinity binding was characterized by equilibrium binding analysis which revealed a single high-affinity binding site with Kd 4.13 nM and Bmax 6.67 pmoles/mg of membrane protein. 125I-ammodytoxin A was covalently cross-linked to its neuronal acceptor using a chemical cross-linking technique. As revealed by subsequent SDS-PAGE analysis and autoradiography, 125I-ammodytoxin A specifically attached to membrane components with apparent mol. wts 53,000-56,000. Besides by the native ammodytoxin A, the binding of radioiodinated ammodytoxin A to the neuronal acceptor was highly attenuated, also by other two iso-neurotoxins from V. a. ammodytes venom, ammodytoxins B and C, and neurotoxin crotoxin B from the venom of the South American rattlesnake (Crotalus durissus terrificus). Vipera berus berus phospholipase A2 was a weaker inhibitor, whereas nontoxic phospholipase A2, ammodytoxin I2 and myotoxic phospholipase A2 homologue, ammodytin L, both from V. a. ammodytes venom as well, were very weak inhibitors. No inhibitory effect on 125I-ammodytoxin A specific binding at all was, however, obtained with alpha-dendrotoxin, beta-bungarotoxin and crotoxin A, respectively. Treatment of synaptic membranes with proteinase K and Staphylococcus aureus V-8 proteinase, a combination of PNGase F and neuroaminidase, heat or acid lowered the 125I-ammodytoxin A specific binding to various extents but never completely abolished it. The ammodytoxin A binding site in bovine synaptic membranes is thus most likely a combination of membrane glycoprotein acceptor and membrane phospholipids. As ammodytoxin A reduced the second negative component of the perineural waveform, measured on mouse triangularis sterni preparation, which is very likely a result of an inhibition of a fraction of the terminal K+ currents, the ammodytoxin A acceptor could well be connected with K+ channels.

A novel beta subunit increases rate of inactivation of specific voltage-gated potassium channel alpha subunits

Voltage-gated potassium channel beta subunits are cytoplasmic proteins that co-purify with the pore-forming alpha subunits. One of these subunits, Kv beta 1 from rat brain, was previously demonstrated to increase the rate of inactivation of Kv1.1 and Kv1.4 when co-expressed in Xenopus oocytes. We have cloned and characterized a novel voltage-gated K+ channel beta subunit. The cDNA, designated Kv beta 3, has a 408-amino acid open reading frame. It possesses a unique 79-amino acid N-terminal leader, but is identical with rat Kv beta 1 over the 329 C-terminal amino acids. The Kv beta 3 transcript was found in many tissues, but was most abundant in aorta and left ventricle of the heart. Co-expression of Kv beta 3 with K+ channel alpha subunits shows that this beta subunit can increase the rate of inactivation from 4- to 7-fold in a Kv1.4 or Shaker B channel. Kv beta 3 had no effect on Kv1.1, unlike Kv beta 1 which can increase rate of inactivation of this alpha subunit more than 100-fold. Other kinetic parameters were unaffected. This study shows that voltage-gated K+ channel beta subunits are present outside the central nervous system, and that at least one member of this family selectively modulates inactivation of K+ channel alpha subunits.

Conversion of bovine pancreatic phospholipase A2 at a single site into a competitor of neurotoxic phospholipases A2 by site-directed mutagenesis

A 45-kDa polypeptide preferentially present in neuronal membranes was previously identified as a subunit of a binding (or receptor) protein for several phospholipase A2 variants with neurotoxicity, including crotoxin, by chemical cross-linking experiments (Yen, C.-H., and Tzeng, M.-C. (1991) Biochemistry 30, 11473-11477). The binding of crotoxin to this receptor protein was completely suppressed by sufficient F22Y, a mutated bovine pancreatic phospholipase A2 generated by site-directed mutagenesis of Phe22 of the wild-type enzyme to Tyr. The IC50 of this inhibition was estimated to be 1 microM. In sharp contrast, the wild-type enzyme gave no effect even at 50 microM. This mutation resulted in only minor and localized structural perturbations with little effect on enzymatic activity. Other phospholipase A2 molecules capable of competing with crotoxin for this binding invariably have Tyr at this position. It was concluded that this Tyr residue is an important determinant for the binding of a number of phospholipase A2 variants to the 45-kDa receptor.

Blockade by dendrotoxin homologues of voltage-dependent K+ currents in cultured sensory neurones from neonatal rats

1. Homologues of dendrotoxin (Dtx) were isolated from the crude venom of Green and Black Mamba snakes and examined for K+ channel blocking activity in neonatal rat dorsal root ganglion cells (DRGs) by whole-cell patch clamp recording. 2. Outward potassium current activated by depolarization was composed of two major components: a slowly inactivating current (SIC, tau decay approximately 50 ms, 200 ms and 2s), and a non-inactivating current (NIC, tau decay > 2 min). Tail current analysis revealed two time constants of deactivation of total outward current, 3-12 ms and 50-150 ms (at -80 mV) which corresponded to SIC and NIC, respectively. 3. All the homologues (alpha-, beta-, gamma- and delta-Dtx and toxins I and K) blocked outward current activated by depolarization in a dose-dependent manner. The most potent in blocking total outward current was delta-Dtx (EC50 of 0.5 +/- 0.2 nM), although there were no statistically significant differences in potency between any of the homologues. 4. Qualitative differences in the nature of the block were noted between homologues. In particular, the block by delta-Dtx was time-dependent, whereas that by alpha-Dtx was not. 5. alpha-Dtx was a much better blocker of SIC (EC50 = 1.0 +/- 0.4 nM) than was delta-Dtx (EC50 = 17.6 +/- 5.8 nM). Furthermore, delta-Dtx was selective for NIC (EC50 +/- 0.24 +/- 0.03 nM) over SIC and reduced the slow component of tail currents (NIC), preferentially. On the other hand, a-Dtx did not significantly distinguish between SIC and NIC although tail current analysis showed that a-Dtxpreferentially reduced the fast component of tail currents (SIC).6. The results confirm, using direct electrophysiological methods, that homologues of dendrotoxins from Mamba snake venom block K+ channels in rat sensory neurones. Furthermore, a-Dtx and 6-Dtx distinguish between sub-types of K+ channels in these cells and may thus be useful pharmacological tools in other neuronal K+ channel studies.

Inactivation properties of voltage-gated K+ channels altered by presence of beta-subunit

Structural and functional diversity of voltage-gated Kv1-type potassium channels in rat brain is enhanced by the association of two different types of subunits, the membrane-bound, poreforming alpha-subunits and a peripheral beta-subunit. We have cloned a beta-subunit (Kv beta 1) that is specifically expressed in the rat nervous system. Association of Kv beta 1 with alpha-subunits confers rapid A-type inactivation on non-inactivating Kv1 channels (delayed rectifiers) in expression systems in vitro. This effect is mediated by an inactivating ball domain in the Kv beta 1 amino terminus.

Antibodies specific for distinct Kv subunits unveil a heterooligomeric basis for subtypes of alpha-dendrotoxin-sensitive K+ channels in bovine brain

The authentic subunit compositions of neuronal K+ channels purified from bovine brain were analyzed using a monoclonal antibody (mAb 5), reactive exclusively with the Kv1.2 subunit of the latter and polyclonal antibodies specific for fusion proteins containing C-terminal regions of four mammalian Kv proteins. Western blotting of the K+ channels isolated from several brain regions, employing the selective blocker alpha-dendrotoxin (alpha-DTX), revealed the presence in each of four different Kvs. Variable amounts of Kv1.1 and 1.4 subunits were observed in the K+ channels purified from cerebellum, corpus striatum, hippocampus, cerebral cortex, and brain stem; on the other hand, contents of Kv1.6 and 1.2 subunits appeared uniform throughout. Each Kv-specific antibody precipitated a different proportion (anti-Kv1.2 > 1.1 >> 1.6 > 1.4) of the channels detectable with radioiodinated alpha-DTX in every brain region, consistent with a widespread distribution of these oligomeric subtypes. Such heterooligomeric combinations were further documented by the lack of additivity upon their precipitation with a mixture of antibodies to Kv1.1 and Kv1.2; moreover, cross-blotting of the multimers precipitated by mAb 5 showed that they contain all four Kv proteins. Collectively, these findings demonstrate that subtypes of alpha-DTX-susceptible K+ channels are prevalent throughout mammalian brain which are composed of different Kv proteins assembled in complexes, shown previously to also contain auxiliary beta-subunits [Parcej, D. N., Scott, V. E. S., & Dolly, J.O. (1992) Biochemistry 31, 11084-11088].