1
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Roman R, Feranchak AP, Troetsch M, Dunkelberg JC, Kilic G, Schlenker T, Schaack J, Fitz JG. Molecular characterization of volume-sensitive SK(Ca) channels in human liver cell lines. Am J Physiol Gastrointest Liver Physiol 2002; 282:G116-22. [PMID: 11751164 DOI: 10.1152/ajpgi.2002.282.1.g116] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
In human liver, Ca(2+)-dependent changes in membrane K(+) permeability play a central role in coordinating functional interactions between membrane transport, metabolism, and cell volume. On the basis of the observation that K(+) conductance is partially sensitive to the bee venom toxin apamin, we aimed to assess whether small-conductance Ca(2+)-sensitive K(+) (SK(Ca)) channels are expressed endogenously and contribute to volume-sensitive K(+) efflux and cell volume regulation. We isolated a full-length 2,140-bp cDNA (hSK2) highly homologous to rat brain rSK2 cDNA, including the putative apamin-sensitive pore domain, from a human liver cDNA library. Identical cDNAs were isolated from primary human hepatocytes, human HuH-7 hepatoma cells, and human Mz-ChA-1 cholangiocarcinoma cells. Transduction of Chinese hamster ovary cells with a recombinant adenovirus encoding the hSK2-green fluorescent protein fusion construct resulted in expression of functional apamin-sensitive K(+) channels. In Mz-ChA-1 cells, hypotonic (15% less sodium glutamate) exposure increased K(+) current density (1.9 +/- 0.2 to 37.5 +/- 7.1 pA/pF; P < 0.001). Apamin (10-100 nM) inhibited K(+) current activation and cell volume recovery from swelling. Apamin-sensitive SK(Ca) channels are functionally expressed in liver and biliary epithelia and likely contribute to volume-sensitive changes in membrane K(+) permeability. Accordingly, the hSK2 protein is a potential target for pharmacological modulation of liver transport and metabolism through effects on membrane K(+) permeability.
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Affiliation(s)
- Richard Roman
- Department of Medicine, School of Medicine, University of Colorado Health Sciences Center, Denver, Colorado 80262, USA
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2
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Auguste P, Hugues M, Lazdunski M. Polypeptide constitution of receptors for apamin, a neurotoxin which blocks a class of Ca2+
-activated K+
channels. FEBS Lett 2001. [DOI: 10.1016/0014-5793(89)80451-x] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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3
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Wadsworth JD, Torelli S, Doorty KB, Strong PN. Structural diversity among subtypes of small-conductance Ca2+-activated potassium channels. Arch Biochem Biophys 1997; 346:151-60. [PMID: 9328295 DOI: 10.1006/abbi.1997.0280] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
125I-Apamin and photolabile derivatives of the toxin have been used to investigate the binding properties and subunit composition of small conductance Ca2+-activated potassium channels (SK(Ca) channels) expressed on plasma membranes from rat brain, rabbit liver, or rat pheochromocytoma (PC12) cells. On all preparations, 125I-apamin recognized single classes of acceptor binding sites with similar high affinity (Kd approximately 3-6 pM). Gallamine, however, was found to readily discriminate between 125I-apamin acceptors present in these preparations, showing a maximal approx nine-fold difference in affinity for acceptors expressed by rabbit liver or PC12 cells. Affinity-labeling patterns revealed the expression of different hetero-oligomeric combinations of high (86 or 59 kDa) and low (33 or 30 kDa) molecular mass 125I-apamin-binding polypeptides, consistent with pharmacological differences. Alternative expression of either 86- or 59-kDa polypeptides appeared to be the most important factor influencing gallamine's affinity for SK(Ca) channel subtypes. Both high- and low-molecular-mass polypeptides are integral membrane proteins, the latter being glycosylated in a tissue-specific manner.
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Affiliation(s)
- J D Wadsworth
- Department of Paediatrics and Neonatal Medicine, Royal Postgraduate Medical School, Hammersmith Hospital, London, United Kingdom
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4
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Doorty KB, Bevan S, Wadsworth JD, Strong PN. A novel small conductance Ca2+-activated K+ channel blocker from Oxyuranus scutellatus taipan venom. Re-evaluation of taicatoxin as a selective Ca2+ channel probe. J Biol Chem 1997; 272:19925-30. [PMID: 9242659 DOI: 10.1074/jbc.272.32.19925] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Taicatoxin, isolated from the venom of the Australian taipan snake Oxyuranus scutellatus, has been previously regarded as a specific blocker of high threshold Ca2+ channels in heart. Here we show that taicatoxin (in contrast to a range of other Ca2+ channel blockers) interacts with apamin-sensitive, small conductance, Ca2+-activated potassium channels on both chromaffin cells and in the brain. Taicatoxin displays high affinity recognition of 125I-apamin acceptor-binding sites, present on rat synaptosomal membranes (Ki = 1.45 +/- 0.22 nM) and also specifically blocks affinity-labeling of a 33-kDa 125I-apamin-binding polypeptide on rat brain membranes. Taicatoxin (50 nM) completely blocks apamin-sensitive after-hyperpolarizing slow tail K+ currents generated in rat chromaffin cells (mean block 97 +/- 3%, n = 12) while only partially reducing total voltage-dependent Ca2+ currents (mean block 12 +/- 4%, n = 6). In view of these findings, the use of taicatoxin as a specific ligand for Ca2+ channels should now be reconsidered.
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Affiliation(s)
- K B Doorty
- Neuromuscular Unit, Department of Paediatrics and Neonatal Medicine, Royal Postgraduate Medical School, Hammersmith Hospital, London W12 0NN, United Kingdom
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5
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Wadsworth JD, Doorty KB, Ganellin CR, Strong PN. Photolabile derivatives of 125I-apamin: defining the structural criteria required for labeling high and low molecular mass polypeptides associated with small conductance Ca(2+)-activated K+ channels. Biochemistry 1996; 35:7917-27. [PMID: 8672494 DOI: 10.1021/bi9602371] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
The structure of apamin-sensitive Ca(2+)-activated K+ channels has been investigated using high-affinity, photolabile azidoaryl derivatives of 125I-[alpha-formyl-Cys1]apamin and 125I-[epsilon-formyl-Lys4]-apamin. Labeling patterns suggest that similar structural constraints are required for labeling analogous polypeptides associated with distinct channel subtypes. When photoprobes are coupled at the epsilon-amino-Lys4 position of apamin, comparable low molecular mass (approximately 30 kDa) polypeptides are efficiently labeled on either brain or liver plasma membranes, irrespective of the structure of the photoprobe. However, when photoprobes are coupled at the alpha-amino-Cys1 position of apamin, the pattern of labeling on both brain and liver plasma membranes varies, depending upon the length of the spacer arm incorporated into the photoprobe. Spacer arms of approximately 8-9 A efficiently label only high molecular mass polypeptides (86, 59 kDa), accompanied by weak, variable labeling of a 44-kDa component. A shorter spacer arm (5.7 A) results in feeble labeling of 86- and 59-kDa polypeptides and barely detectable labeling of 44- and approximately 30-kDa polypeptides. In contrast, a long spacer arm (12.8 A) efficiently labels only approximately 30-kDa polypeptides. These findings point to close similarities in the topography of the 125I-apamin binding site present on pharmacologically distinct subtypes of apamin-sensitive Ca2+-activated K+ channels and indicates that heterooligomeric association of high and low molecular mass polypeptide subunits may be a general structural feature of members belonging to this family of K+ channels.
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Affiliation(s)
- J D Wadsworth
- Department of Paediatrics and Neonatal Medicine, Royal Postgraduate Medical School, Hammersmith Hospital, London, U.K
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6
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Dunn PM, Benton DC, Campos Rosa J, Ganellin CR, Jenkinson DH. Discrimination between subtypes of apamin-sensitive Ca(2+)-activated K+ channels by gallamine and a novel bis-quaternary quinolinium cyclophane, UCL 1530. Br J Pharmacol 1996; 117:35-42. [PMID: 8825340 PMCID: PMC1909372 DOI: 10.1111/j.1476-5381.1996.tb15151.x] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
1. Gallamine, dequalinium and a novel bis-quaternary cyclophane, UCL 1530 (8,19-diaza-3(1,4),5(1,4)-dibenzena-1 (1,4),7(1,4)-diquinolina-cyclononadecanephanedium) were tested for their ability to block actions mediated by the small conductance, apamin-sensitive Ca(2+)-activated K+ (SKCa) channels in rat cultured sympathetic neurones and guinea-pig isolated hepatocytes. 2. SKCa channel block was assessed in sympathetic neurones by the reduction in the slow afterhyperpolarization (AHP) that follows an action potential, and in hepatocytes by the inhibition of the SKCa mediated net loss of K+ that results from the application of angiotensin II. 3. The order of potency for inhibition of the AHP in sympathetic neurones was UCL 1530 > dequalinium > gallamine, with IC50 values of 0.08 +/- 0.02, 0.60 +/- 0.05 and 68.0 +/- 8.4 microM respectively, giving an equi-effective molar ratio between gallamine and UCL 1530 of 850. 4. The same three compounds inhibited angiotensin II-evoked K+ loss from guinea-pig hepatocytes in the order dequalinium > UCL 1530 > gallamine, with an equi-effective molar ratio for gallamine to UCL 1530 of 5.8, 150 fold less than in sympathetic neurones. 5. Dequalinium and UCL 1530 were as effective on guinea-pig as on rat sympathetic neurones. 6. UCL 1530 at 1 microM had no effect on the voltage-activated Ca2+ current in rat sympathetic neurones, but inhibited the hyperpolarization produced by direct elevation of cytosolic Ca2+. 7. Direct activation of SKCa channels by raising cytosolic Ca2+ in hepatocytes evoked an outward current which was reduced by the three blockers, with dequalinium being the most potent. 8. These results provide evidence that the SKCa channels present in guinea-pig hepatocytes and rat cultured sympathetic neurones are different, and that this is not attributable to species variation. UCL 1530 and gallamine should be useful tools for the investigation of subtypes of apamin-sensitive K+ channels.
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Affiliation(s)
- P M Dunn
- Department of Pharmacology, University College London
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7
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Gmachl M, Kreil G. The precursors of the bee venom constituents apamin and MCD peptide are encoded by two genes in tandem which share the same 3'-exon. J Biol Chem 1995; 270:12704-8. [PMID: 7759523 DOI: 10.1074/jbc.270.21.12704] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
From a cDNA library prepared from venom glands of worker bees, clones encoding the precursors of apamin and MCD peptide have been isolated. The cDNAs are similar at the 5'-ends and identical in their 3'-regions. Analysis of the corresponding genes has revealed the existence of six exons separated by introns rich in A + T. Starting from the 5'-end, these exons are arranged in the following order: three exons of the mast cell-degranulating (MCD) peptide precursor, two exons of the gene for the apamin precursor, and finally a 3'-exon present in both cDNAs. This suggests that the bulk of the apamin gene resides in the third intron of the MCD peptide gene. Using inverse polymerase chain reaction, a segment of genomic DNA upstream of the first exon of the MCD precursor gene was obtained. The sequence of this segment shows 81% identity to the DNA sequence preceding the first exon of the apamin gene and both contain a putative TATA box. We thus propose that the mRNA encoding the apamin precursor originates from a primary transcript which starts in the third intron of the MCD peptide gene. Both cDNAs encode unusually small precursors comprising only 46 amino acids in case of apamin and 50 in the case of the MCD peptide.
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Affiliation(s)
- M Gmachl
- Institute of Molecular Biology, Austrian Academy of Sciences, Salzburg
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8
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Wadsworth J, Doorty K, Strong P. Comparable 30-kDa apamin binding polypeptides may fulfill equivalent roles within putative subtypes of small conductance Ca(2+)-activated K+ channels. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)32416-x] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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9
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Sokol PT, Hu W, Yi L, Toral J, Chandra M, Ziai MR. Cloning of an apamin binding protein of vascular smooth muscle. JOURNAL OF PROTEIN CHEMISTRY 1994; 13:117-28. [PMID: 7912073 DOI: 10.1007/bf01891999] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The receptor for the bee venom derived neurotoxin, apamin, is widely believed to be an integral component of the small conductance calcium-activated potassium channel in many excitable cells. By affinity chromatography on immobilized apamin, a 78 kD apamin binding protein of the bovine brain synaptosomes was isolated. Antibodies were elicited against this protein and used to clone a cDNA from a porcine vascular smooth muscle expression library. This gene (Kcal 1.8) codes for a 438 amino protein with four potential transmembrane domains, one putative calcium binding site, a protein kinase C phosphorylation site, and a leucine zipper motif. Kcal 1.8 encoded protein has no significant sequence homologies with any known ion channels or receptors. Kcal 1.8 is likely to encode a protein associated with the small conductance calcium-activated potassium channel in vascular smooth muscle.
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MESH Headings
- Amino Acid Sequence
- Animals
- Antibodies, Monoclonal
- Apamin/metabolism
- Binding Sites
- Blotting, Northern
- Blotting, Western
- Brain/metabolism
- Calcium/metabolism
- Cattle
- Cell Line
- Chromatography, Affinity
- Cloning, Molecular/methods
- Electrophoresis, Polyacrylamide Gel
- Gene Library
- Humans
- Leucine Zippers
- Melanoma
- Molecular Sequence Data
- Molecular Weight
- Muscle, Smooth, Vascular/metabolism
- Neuroblastoma
- Poly A/analysis
- Poly A/biosynthesis
- Potassium Channels/biosynthesis
- Potassium Channels/chemistry
- Potassium Channels/isolation & purification
- Protein Structure, Secondary
- RNA, Messenger/analysis
- RNA, Messenger/biosynthesis
- Recombinant Proteins/biosynthesis
- Recombinant Proteins/chemistry
- Swine
- Synaptosomes/metabolism
- Tumor Cells, Cultured
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Affiliation(s)
- P T Sokol
- Department of Molecular Pharmacology, American Cyanamid Company, Lederle Laboratories, Pearl River, New York 10965
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10
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Affiliation(s)
- K S Atwal
- Bristol-Myers Squibb Pharmaceutical Research Institute, Princeton, New Jersey 08543-4000
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11
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Pácha J, Vorlicek J, Teisinger J. Identification of apamin binding sites in rat intestinal mucosa. Life Sci 1992; 51:423-9. [PMID: 1321938 DOI: 10.1016/0024-3205(92)90410-q] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Apamin, a specific blocker of one class of Ca(2+)-activated K+ channes, was used to detect the apamin receptors associated with K+ channels in the mucosa of the rat jejunum and colon. Two receptor sites for 125I-apamin have been identified. These sites differed in their affinity for apamin (jejunum: KD1 = 1.1 nM and KD2 = 170 nM; colon: KD1 = 0.5 nM and KD2 = 1.1 nM and KD2 = 140 nM) and the maximum number of sites (jejunum: B(max1) = 111 and B(max2) = 4030; colon: B(max1) = 187 and B(max2) = 7550 fmol/mg of protein). 125I-apamin binding was stimulated by K+ ions with K0.5 = 1.0 mM and inhibited by the neuromuscular blocker tubocurarine (KI = 50 microM). We interpret these data to demonstrate that the high-affinity, low-capacity binding sites reflect the existence of apamin-sensitive K+ channels in the intestinal mucosa.
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Affiliation(s)
- J Pácha
- Institute of Physiology, Czechoslovak Academy of Sciences, Prague
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12
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Labbé-Jullié C, Granier C, Albericio F, Defendini ML, Ceard B, Rochat H, Van Rietschoten J. Binding and toxicity of apamin. Characterization of the active site. EUROPEAN JOURNAL OF BIOCHEMISTRY 1991; 196:639-45. [PMID: 2013287 DOI: 10.1111/j.1432-1033.1991.tb15860.x] [Citation(s) in RCA: 50] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
The structural features of apamin, a natural octadecapeptide from bee venom, enabling binding to its receptor and the expression of toxicity in mice, have been delineated by studying the effects on binding and toxicity of chemical modifications and amino acid substitutions in synthetic analogues. The results obtained indicate that the only hydrophobic residue, leucine at position 10, can be changed to alanine without a significant decrease in the specific activity. The need for a correct conformation has been established and also the importance of Gln-17 and the side chains of Arg-13 and Arg-14 (besides the charge effects). The interaction of apamin with its receptor, a calcium-activated potassium channel, is thus mediated by a precise topology around these three residues. Due to the ability to detect very low specific activities for some of the analogues, it has been shown that, individually, none of these interactions constitute an essential criteria for binding per se, but that their presence is necessary for the high specific activity of the toxin.
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Affiliation(s)
- C Labbé-Jullié
- Centre National de la Recherche Scientifique UA 1179, Faculté de Médecine Secteur Nord, Marseille, France
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13
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Leveque C, Marqueze B, Couraud F, Seagar M. Polypeptide components of the apamin receptor associated with a calcium activated potassium channel. FEBS Lett 1990; 275:185-9. [PMID: 2175714 DOI: 10.1016/0014-5793(90)81468-4] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Photoaffinity labeling of rat brain membranes with [125I]ANPAA-apamin incorporated radioactivity into polypeptides of 86 and 59 kDa and occasionally a more weakly labeled component of 45 kDa. These polypeptides were immunoprecipitated with anti-apamin antibodies and treated with glycosidases. Neither the 86 nor the 59 kDa polypeptide appeared to be N-glycosylated. Partial proteolytic mapping of affinity labeled polypeptides with chymotrypsin or V8 protease generated an identical pattern. These results suggest that the 59 and 45 kDa components are not additional subunits of an oligomeric protein but result from cleavage of the 86 kDa polypeptide.
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Affiliation(s)
- C Leveque
- Laboratoire de Biochimie, CNRS UA1179, Faculté de Médecine Nord, Marseille, France
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14
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15
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Dreyer F. Peptide Toxins and Potassium Channels. Rev Physiol Biochem Pharmacol 1990. [DOI: 10.1007/978-3-662-41884-0_3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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16
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Abstract
Many venom toxins interfere with ion channel function. Toxins, as specific, high affinity ligands, have played an important part in purifying and characterizing many ion channel proteins. Our knowledge of potassium ion channel structure is meager because until recently, no specific potassium channel toxins were known, or identified as such. This review summarizes the sudden explosion of research on potassium channel toxins that has occurred in recent years. Toxins are discussed in terms of their structure, physiological and pharmacological properties, and the characterization of toxin binding sites on different subtypes of potassium ion channels.
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Affiliation(s)
- P N Strong
- Jerry Lewis Muscle Research Centre, Department of Paediatrics and Neonatal Medicine, Royal Postgraduate Medical School, London, U.K
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17
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Marquèze B, Seagar MJ, Couraud F. Characterization of apamin-binding protein associated with a Ca2+ -activated K+ channel. JOURNAL OF PROTEIN CHEMISTRY 1989; 8:425-7. [PMID: 2551328 DOI: 10.1007/bf01674309] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Affiliation(s)
- B Marquèze
- Zentrum für Molekulare Biologie, Universität Heidelberg, FRG
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