For a long time, superoxide generation by an NADPH oxidase was considered as an oddity only found in professional phagocytes. Over the last years, six homologs of the cytochrome subunit of the phag...

/pdf/the-nox-family-of-ros-generating-nadph-oxidases-physiology-zjc88zvh59.pdf

The NOX Family of ROS-Generating NADPH Oxidases: Physiology and Pathophysiology

The Cationminus signpi Interaction.

Excitability. Excitability of cell membranes is crucial for signaling in many types of cell. Excitation in the physiological sense means that the cell membrane potential undergoes characteristic changes which, in most cases, go in the depolarizing direction. Single depolarization from the resting potential to potentials near 0 mV has generally been called an action potential. A schematic representation of a neuronal action potential is given in Fig. 12.1 A. The action potential is triggered when the membrane potential, which was at the resting level, depolarizes and reaches the threshold of excitation. This depolarization, which triggers the action potential, is generated by depolarizing synaptic currents, or depolarizing current coming from a membrane region that is already excited (propagation of an action potential), or by pacemaker currents mediated by pacemaker channels, or by current injected externally by an electrode. The duration of different types of action potential varies from seconds to less than 1 ms.

Ion Channels in Excitable Membranes

https://www.cell.com/cell/pdf/0092-8674(95)90340-2.pdf

A mechanistic link between an inherited and an acquird cardiac arrthytmia: HERG encodes the IKr potassium channel

Voltage-dependent potassium ion (K+) channels (Kv channels) conduct K+ ions across the cell membrane in response to changes in the membrane voltage, thereby regulating neuronal excitability by modulating the shape and frequency of action potentials. Here we report the crystal structure, at a resolution of 2.9 angstroms, of a mammalian Kv channel, Kv1.2, which is a member of the Shaker K+ channel family. This structure is in complex with an oxido-reductase beta subunit of the kind that can regulate mammalian Kv channels in their native cell environment. The activation gate of the pore is open. Large side portals communicate between the pore and the cytoplasm. Electrostatic properties of the side portals and positions of the T1 domain and beta subunit are consistent with electrophysiological studies of inactivation gating and with the possibility of K+ channel regulation by the beta subunit.

http://science.sciencemag.org/content/sci/309/5736/897.full.pdf

Crystal Structure of a Mammalian Voltage-Dependent Shaker Family K + Channel

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, pore-forming α-subunits and a peripheral β-subunit. We have cloned a β-subunit (Kvβ1) that is specifically expressed in the rat nervous system. Association of Kvβ1 with α-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 Kvβ1 amino terminus.

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

THE sodium channel, one of the family of structurally homologous voltage-gated ion channels1, differs from other members, such as the calcium and the potassium channels, in its high selectivity for Na+. This selectivity presumably reflects a distinct structure of its ion-conducting pore. We have recently identified two clusters of predominantly negatively charged amino-acid residues, located at equivalent positions in the four internal repeats of the sodium channel as the main determinants of sensitivity to the blockers tetrodotoxin and saxitoxin2. All site-directed mutations reducing net negative charge at these positions also caused a marked decrease in single-channel conductance2. Thus these two amino-acid clusters probably form part of the extracellular mouth and/or the pore wall of the sodium channel. We report here the effects on ion selectivity of replacing lysine at position 1,422 in repeat III and/or alanine at position 1,714 in repeat IV of rat sodium channel II (ref. 3), each located in one of the two clusters, by glutamic acid, which ccurs at the equivalent positions in calcium channels. These amino-acid substitutions, unlike other substitutions in the adjacent regions, alter ion-selection properties of the sodium channel to resemble those of calcium channels. This result indicates that lysine 1,422 and alanine 1,714 are critical in deter mining the ion selectivity of the sodium channel, suggesting that these residues constitute part of the selectivity filter of the channel.

Calcium channel characteristics conferred on the sodium channel by single mutations.

After removal of N-type inactivation in Shaker K channels another inactivation process remains (C-type inactivation) The C-type inactivation time course is reversibly slowed when external [K+] increases The effect of K+ is mimicked by Rb+ and, with less potency, by the less permeant ions Na+, Cs+, and NH4+ These results, which can be explained by the foot-in-the-door model of gating, could reflect the variable interaction of cations with amino acids in the ion-conducting pore Mutations at position 449 (near the outer mouth of the pore) produce drastic changes in C-type inactivation kinetics and in its interaction with monovalent cations Replacement of threonine in the wild-type by glutamic acid or lysine leads to a hundred-fold acceleration of inactivation (time constant approximately 25 ms) In contrast, placing valine at this position results in channels that do not inactivate in 45 s Moreover, high K+, besides slowing down the inactivation kinetics, produces an increase in current amplitude despite a concomitant decrease in K+ driving force This second effect, which is larger in mutants with faster inactivation kinetics, is caused by an increase in the number of channels that open on depolarization Thus, C-type inactivation is a process influenced by the ionic composition of the external milieu which strongly depends on the amino acid at position 449 in the pore region These findings may help to explain the variability in inactivation kinetics observed in the various types of K channels

Effects of external cations and mutations in the pore region on C-type inactivation of Shaker potassium channels.

MODULATION of neuronal excitability by regulation of K+ channels potentially plays a part in short-term memory1 but has not yet been studied at the molecular level Regulation of K+ channels by protein phosphorylation2–5 and oxygen6 has been described for various tissues and cell types; regulation of fast-inactivating K+ channels mediating IK(A) currents has not yet been described Functional expression of cloned mammalian K+ channels7–13 has provided a tool for studying their regulation at the molecular level We report here that fast-inactivating K+ currents mediated by cloned K+ channel subunits derived from mammalian brain expressed in Xenopus oocytes are regulated by the reducing agent glutathione This type of regulation may have a role in vivo to link metabolism to excitability and to regulate excitability in specific membrane areas of mammalian neurons

Regulation of fast inactivation of cloned mammalian IK(A) channels by cysteine oxidation.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2891%2981159-6

Stefan H. Heinemann

Papers

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

Calcium channel characteristics conferred on the sodium channel by single mutations.

Effects of external cations and mutations in the pore region on C-type inactivation of Shaker potassium channels.

Regulation of fast inactivation of cloned mammalian IK(A) channels by cysteine oxidation.

Mapping the site of block by tetrodotoxin and saxitoxin of sodium channel II.