Molecular determinants of external barium block in <i>Shaker</i> potassium channels

Hurst, Raymond S; Toro, Ligia; Stefani, Enrico

doi:10.1016/0014-5793(96)00516-9

Cited by 32 publications

(39 citation statements)

References 33 publications

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“…In Shaker K + channels, residues in the signature sequence and around it have been found to affect block by external Ba 2+ ions (Hurst et al 1996;Harris et al 1998). Notably, the threonine at position 441 in Shaker, which is located at the corresponding position to T141 in Kir2.1, also affected the affinity for Ba 2+ block.…”

Section: Discussionmentioning

confidence: 99%

Mechanism of Ba²⁺ block of a mouse inwardly rectifying K⁺ channel: differential contribution by two discrete residues

Alagem

Dvir

Reuveny

2001

The Journal of Physiology

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Inwardly rectifying potassium (Kir) channels are involved in many physiological processes, such as setting the excitability state of nerve and muscle, potassium secretion and hormone release. They act by allowing the flux of potassium ions near the potassium equilibrium potential, thus keeping the resting membrane potential hyperpolarized. The inward rectification is attributed to a voltage-dependent block of the channel pore by intracellular magnesium and polyamines (Fakler et al. 1995;Lopatin et al. 1995).Inorganic cations have been widely used to probe the permeation and gating mechanisms of potassium channels (Hille, 1992 (Standen & Stanfield, 1978, 1980 Ohmori, 1978;Biermans et al. 1987;Harvey & Ten Eick, 1989; Shioya et al. 1993; Reuveny et al. 1996; Sabirov et al. 1997a; Shieh et al. 1998;Doring et al. 1998;Dart et al. 1998). The interaction of divalent cations with Kir channels is thought to occur via two distinct binding sites; a shallow site that barely senses the membrane electric field, and a deeper one located approximately half-way within the membrane electrical field. Channel block by Mg 2+ and Ca 2+ ions was found to occur through the shallow site, whereas the block by Ba 2+ and Sr 2+ ions is mediated through the deeper one (Standen & Stanfield, 1978; Shioya et al. 1993; Reuveny et al. 1996; Sabirov et al. 1997b; Shieh et al. 1998). For all divalent cations, a single ion suffices to block the channel. In most cases of deepsite blockers, the block follows first-order kinetics, taking several seconds to reach a steady-state (Standen & Stanfield, 1978; Shieh et al. 1998). An exception is the G-protein-coupled inwardly rectifying potassium channel family, where part of the Ba 2+ block reaches steady state in an unmeasurably short time (Carmeliet & Mubagwa, 1986).Since the recent cloning of many Kir channels, some progress has been made in understanding the molecular mechanisms involved in the channel block by divalent cations. Sabirov et al. (1997b) showed that a highly conserved arginine residue at position 148 in Kir2.1/IRK1 forms a barrier for external cations. Mutating R148 to histidine allowed Mg 2+ and Ca 2+(shallow blockers) to bind more deeply within the electric field. Block by Ba 2+ and Cs + became more rapid, while the affinity and voltage dependence of the block remained unchanged. Additional information related to the role of this conserved arginine in channel block came from a unique member of the Kir channel family, Kir7.1. This channel has a methionine at the position corresponding to that of the conserved arginine. This methionine was 1. The block of the IRK1/Kir2.1 inwardly rectifying K + channel by a Ba 2+ ion is highly voltage dependent, where the ion binds approximately half-way within the membrane electrical field. The mechanism by which two distinct mutations, E125N and T141A, affect Ba 2+ block of Kir2.1 was investigated using heterologous expression in Xenopus oocytes.2. Analysis of the blocking kinetics showed that E125 and T141 affect the entry and binding of Ba 2+ t...

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Section: Discussionmentioning

confidence: 99%

Mechanism of Ba²⁺ block of a mouse inwardly rectifying K⁺ channel: differential contribution by two discrete residues

Alagem

Dvir

Reuveny

2001

The Journal of Physiology

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“…However, it may also act as a competitive potassium channel blocker (Hurst et al 1996). In MN5, barium clearly reduces the amplitude of calcium-independent I(A) as demonstrated by addition of barium to cadmium containing external solution.…”

Section: Barium Blocks Calcium-independent Potassium Channels In Mn5mentioning

confidence: 99%

“…Barium acts as a competitive blocker for a variety of potassium channels (Hurst et al 1996). In addition, barium does not activate calcium-dependent potassium currents.…”

Section: Barium Blocks a Large Portion Of Phrixotoxin-2-sensitive Outmentioning

confidence: 99%

ShakerandShalMediate Transient Calcium-Independent Potassium Current in a Drosophila Flight Motoneuron

Ryglewski

Duch

2009

Journal of Neurophysiology

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Ryglewski S, Duch C. Shaker and Shal mediate transient calciumindependent potassium current in a Drosophila flight motoneuron. J Neurophysiol 102: 3673-3688, 2009. First published October 14, 2009 doi:10.1152/jn.00693.2009. Ionic currents underlie the firing patterns, excitability, and synaptic integration of neurons. Despite complete sequence information in multiple species, our knowledge about ion channel function in central neurons remains incomplete. This study analyzes the potassium currents of an identified Drosophila flight motoneuron, MN5, in situ. MN5 exhibits four different potassium currents, two fast-activating transient ones and two sustained ones, one of each is calcium activated. Pharmacological and genetic manipulations unravel the specific contributions of Shaker and Shal to the calcium independent transient A-type potassium currents. ␣-dendrotoxin (Shaker specific) and phrixotoxin-2 (Shal specific) block different portions of the transient calcium independent A-type potassium current. Following targeted expression of a Shaker dominant negative transgene in MN5, the remaining A-type potassium current is ␣-dendrotoxin insensitive. In Shal RNAi knock down the remaining A-type potassium current is phrixotoxin-2 insensitive. Additionally, barium blocks calcium-activated potassium currents but also a large portion of phrixotoxin-2-sensitive A-type currents. Targeted knock down of Shaker or Shal channels each cause identical reduction in total potassium current amplitude as acute application of ␣-dendrotoxin or phrixotoxin-2, respectively. This shows that the knock downs do not cause upregulation of potassium channels underlying other A-type channels during development. Immunocytochemistry and targeted expression of modified GFP-tagged Shaker channels with intact targeting sequence in MN5 indicate predominant axonal localization. These data can now be used to investigate the roles of Shaker and Shal for motoneuron intrinsic properties, synaptic integration, and spiking output during behavior by targeted genetic manipulations.

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“…In Shaker, mutation of GYGD to GYGN results in a nonconducting channel that retains its gating properties. 14 Inclusion in the tetramer of 1 or 2 monomers containing D/T, D/N, or D/C mutations yields a conducting channel. [15][16][17] Mutation of 2 monomers to D378T in Kv2.1 resulted in a channel with reduced selectivity for K ϩ over Na ϩ and strong outward rectification.…”

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confidence: 99%

Novel Gain-of-Function Mechanism in K ⁺ Channel–Related Long-QT Syndrome:

Lees‐Miller

Duan

Teng

et al. 2000

Circulation Research

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Abstract-The N629D mutation, adjacent to the GFG signature sequence of the HERG1 A K ϩ channel, causes long-QT syndrome (LQTS). Expression of N629D in Xenopus oocytes produces a rapidly activating, noninactivating current. N629D is nonselective among monovalent cations; permeation of K ϩ was similar to that of Na ϩ or Cs ϩ . During repolarization to potentials between Ϫ30 and Ϫ70 mV, N629D manifested an inward tail current, which was abolished by replacement of extracellular Na ϩ (Na ϩ e ) with extracellular N-methyl-D-glucamine (NMG e ). Because LQTS occurs in heterozygous patients, we coexpressed N629D and wild type (WT) at equimolar concentrations. Heteromultimer formation was demonstrated by analyzing the response to 0 [K ϩ ] e . The outward time-dependent current was nearly eliminated for WT at 0 [K ϩ ] e , whereas no reduction was observed for homomultimeric N629D or for the equimolar coexpressed current. To assess physiological significance, dofetilide-sensitive currents were recorded during application of simulated action potential clamps. During phase 3 repolarization, WT manifested outward currents, whereas homomultimeric N629D manifested inward depolarizing currents. During coexpression studies, variable phenotypes were observed ranging from a reduction in outward repolarizing current to net inward depolarizing current during phase 3. In summary, N629D replaces the WT outward repolarizing tail current with an inward depolarizing sodium current, which is expected to delay later stages of repolarization and contribute to arrhythmogenesis. Thus, the consequences of N629D resemble the pathophysiology seen in LQT3 Na ϩ channel mutations and may be considered the first LQTS K Key Words: long-QT syndrome Ⅲ HERG1 Ⅲ K ϩ channel Ⅲ gain of function F amilial long-QT syndrome (LQTS) results from defects in sodium and potassium ion channels that cause prolongation of cardiac repolarization and arrhythmias. 1 LQT2 is associated with mutations of the human ether-a-go-go-related gene, HERG1. [2][3][4][5][6][7][8][9][10] The HERG1 primary transcript is alternatively processed, giving rise to at least 3 functional mRNAs, HERG1 A, HERG1 AЈ, and HERG1 B, encoding proteins with distinct physiological properties. 11,12 There are several mechanisms by which individual mutations in HERG1 produce LQTS. 5-9 Some exert a dominant phenotype through loss of repolarizing current. These include mutations that either cause defects in intracellular transport or result in channels that do not open. Alternatively, V630L forms heterotetramers with wild type (WT) that have reduced open probability due to a negative shift in voltage dependence of inactivation. Abnormally fast deactivation mutations caused by mutations in the N terminus of HERG1 A result in reduction of outward current during phase 3 repolarization and thus LQTS. 10 The recently described N629D mutation 9 is of particular interest, as it alters the pore selectivity signature sequence from GFGN to GFGD. 13 In most K ϩ channels, including the extensively studied Shaker chann...

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Molecular determinants of external barium block in Shaker potassium channels

Cited by 32 publications

References 33 publications

Mechanism of Ba²⁺ block of a mouse inwardly rectifying K⁺ channel: differential contribution by two discrete residues

Mechanism of Ba²⁺ block of a mouse inwardly rectifying K⁺ channel: differential contribution by two discrete residues

ShakerandShalMediate Transient Calcium-Independent Potassium Current in a Drosophila Flight Motoneuron

Novel Gain-of-Function Mechanism in K ⁺ Channel–Related Long-QT Syndrome:

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Molecular determinants of external barium block in Shaker potassium channels

Cited by 32 publications

References 33 publications

Mechanism of Ba2+ block of a mouse inwardly rectifying K+ channel: differential contribution by two discrete residues

Mechanism of Ba2+ block of a mouse inwardly rectifying K+ channel: differential contribution by two discrete residues

ShakerandShalMediate Transient Calcium-Independent Potassium Current in a Drosophila Flight Motoneuron

Novel Gain-of-Function Mechanism in K + Channel–Related Long-QT Syndrome:

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Mechanism of Ba²⁺ block of a mouse inwardly rectifying K⁺ channel: differential contribution by two discrete residues

Mechanism of Ba²⁺ block of a mouse inwardly rectifying K⁺ channel: differential contribution by two discrete residues

Novel Gain-of-Function Mechanism in K ⁺ Channel–Related Long-QT Syndrome: