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  • 1
    Electronic Resource
    Electronic Resource
    The large conductance Ca2+-activated potassium channel (pSlo) of the cockroach Periplaneta americana: structure, localization in neurons and electrophysiology (2003)
    Oxford, UK : Blackwell Science, Ltd
    European journal of neuroscience 17 (2003), S. 0 
    Details
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Voltage-activated, Ca2+-sensitive K+ channels (BK or maxi K,Ca channels) play a major role in the control of neuronal excitability. We have cloned pSlo, the BK channel α subunit of the cockroach Periplaneta americana. The amino acid sequence of pSlo shows 88% identity to dSlo from Drosophila. There are five alternatively spliced positions in pSlo showing differential expression in various tissues. A pSlo-specific antibody prominently stained the octopaminergic dorsal unpaired median (DUM) neurons and peptidergic midline neurons in Periplaneta abdominal ganglia. HEK293 cells expressing pSlo exhibit K+ channels of 170 pS conductance. They have a tendency for brief closures, exhibit subconductance states and show slight inward rectification. Activation kinetics and voltage dependence are controlled by cytoplasmic [Ca2+]. In contrast to dSlo, pSlo channels are sensitive to charybdotoxin and iberiotoxin. Mutagenesis at two positions (E254 and Q285) changed blocking efficacy of charybdotoxin. In contrast to pSlo expressed in HEK293 cells, native IbTx-sensitive K,Ca currents in DUM and in peptidergic neurons, exhibited rapid, partial inactivation. The fast component of the K,Ca current partly accounts for the repolarization and the early after-hyperpolarization of the action potential. By means of Ca2+-induced repolarization, BK channels may reduce the risk of Ca2+ overload in cockroach neurons. Interestingly, the neurons expressing pSlo were also found to express taurine, a messenger that is likely to limit overexcitation by an autocrine mechanism in mammalian central neurons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1460-9568.2003.02550.x
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  • 2
    Electronic Resource
    Electronic Resource
    Temperature and pressure dependence of Shaker K+ channel N- and C-type inactivation (1997)
    Springer
    European biophysics journal 26 (1997), S. 433-445 
    Details
    ISSN: 1432-1017
    Keywords: Key words Potassium channel ; Inactivation ; Hydrostatic pressure ; Thermodynamics ; Site-directed mutagenesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract Shaker B potassium channels undergo rapid N-type and slow C-type inactivation. While N-type inactivation is supposed to be mediated by occlusion of the pore by the N-terminal protein structure, the molecular mechanisms leading to C-type inactivation are less well understood. Considering N-type inactivation as a model for a protein conformational transition, we investigated inactivation of heterologously expressed Shaker B potassium channels and mutants thereof, showing various degrees of C-type inactivation, under high hydrostatic (oil) pressure. In addition to the derived apparent activation and reaction volumes (ΔV), experiments at various temperatures yielded estimates for enthalpic (ΔH) and entropic (TΔS) contributions. N-type inactivation was accelerated by increasing temperature and slowed by high hydrostatic pressure yielding at equilibrium ΔH = 76 kJ/mole, TΔS = 82 kJ/mole, and ΔV = 0.18 nm3 indicating that the transition to the N-type inactivated state is accompanied by an increase in volume and a decrease in order. N-terminally deleted ShΔ6–46 constructs with additional mutations at either position 449 or 463 were used to investigate C-type inactivation. In particular at high temperatures, inactivation occurred in two phases indicating more than one process. At equilibrium the following values were estimated for the major inactivation component of mutant ShΔ6–46 T449A: ΔH = –64 kJ/mole, TΔS = –60 kJ/mole, and ΔV = –0.25 nm3, indicating that the C-type inactivated state occupies a smaller volume and is more ordered than the noninactivated state. Thus, hydrostatic pressure affects N- and C-type inactivation in opposite ways.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002490050098
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  • 3
    Electronic Resource
    Electronic Resource
    Pore properties of rat brain II sodium channels mutated in the selectivity filter domain (1996)
    Springer
    European biophysics journal 25 (1996), S. 75-91 
    Details
    ISSN: 1432-1017
    Keywords: Key words Sodium channel ; Permeation ; Selectivity ; Oocyte expression ; Mutagenesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract Ion selectivity of voltage-activated sodium channels is determined by amino-acid residues in the pore regions of all four homologous repeats. The major determinants are the residues DEKA (for repeats I-IV) which form a putative ring structure in the pore; the homologous structure in Ca-channels consists of EEEE. By combining site-directed mutagenesis of a non-inactivating form of the rat brain sodium channel II with electrophysiological methods, we attempted to quantify the importance of charge, size, and side-chain position of the amino-acid residues within this ring structure on channel properties such as monovalent cation selectivity, single-channel conductance, permeation and selectivity of divalent cations, and channel block by extracellular Ca2+ and tetrodotoxin (TTX). In all mutant channels tested, even those with the same net charge in the ring structure as the wild type, the selectivity for Na+ and Li+ over K+, Rb+, Cs+, and NH4 + was significantly reduced. The changes in charge did not correlate in a simple fashion with the single-channel conductances. Permeation of divalent ions (Ca2+, Ba2+, Sr2+, Mg2+, Mn2+) was introduced by some of the mutations. The IC50 values for the Ca2+ block of Na+ currents decreased exponentially with increasing net negative charge of the selectivity ring. The sensitivity towards channel block by TTX was reduced in all investigated mutants. Mutations in repeat IV are an exception as they caused smaller effects on all investigated channel properties compared with the other repeats.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002490050020
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  • 4
    Electronic Resource
    Electronic Resource
    Modification of C-type inactivating Shaker potassium channels by chloramine-T (1996)
    Springer
    Pflügers Archiv 431 (1996), S. 483-493 
    Details
    ISSN: 1432-2013
    Keywords: Key words Potassium channel ; Oxidation ; C-type inactivation ; Oocyte expression ; Channel rundown
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Shaker potassium channels undergo a slow C-type inactivation which can be hastened dramatically by single-point mutations in or near the pore region. We found that the oxidizing agent chloramine-T (Chl-T) causes an irreversible loss of current for those mutants which show C-type inactivation. For several mutants at position T449, which show a wide spectrum of inactivation time constants, the time constant of current rundown induced by Chl-T correlated with the speed of inactivation. Rundown was accelerated when the channels were in the inactivated state but rundown also occurred when channels were not opened or inactivated. Apparently, only those channels which can undergo C-type inactivation are accessible to Chl-T. In order to gain information about the target amino-acid residue for the action of Chl-T and the structural rearrangements occurring during C-type inactivation, several mutant channel proteins were compared with respect to their response to Chl-T. Since Chl-T can oxidize cysteine and methionine residues, we mutated the possible targets in and close to the pore region, namely C462 to A, and M440 and M448 to I. While the residues M440 and C462 were not important for channel rundown, mutation of M448 to I made the channels more resistant to Chl-T by about one order of magnitude. While inactivation was accelerated upon application of Chl-T in most mutants, mutation of M448 to I aboli-shed this effect on the time course of inactivation, indicating that M448 is one of the target residues for Chl-T.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050026
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  • 5
    Electronic Resource
    Electronic Resource
    N-Terminal deletions of rKv1.4 channels affect the voltage dependence of channel availability (1999)
    Springer
    Pflügers Archiv 438 (1999), S. 141-146 
    Details
    ISSN: 1432-2013
    Keywords: Key words Activation ; Inactivation ; Mutagenesis ; Potassium channels
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  Rat Kv1.4 potassium channels undergo rapid inactivation, which is mediated by the N-terminal structure of the polypeptide. This inactivation can be removed by N-terminal deletion of about 20 residues. However, more substantial deletion (e.g. 37 residues) restores inactivation suggesting a second inactivating domain [Kondoh et al. J Biol Chem 272:19333–19338, 1997]. Here we provide evidence that this inactivation shares all properties with N-type inactivation. Pore mutations, which are supposed to affect C-type inactivation, have no effect. In addition, the redox regulation of inactivation, which is typical for Kv1.4 channels, can be conferred to the inactivation of the deleted constructs by incorporation of an N-terminal cysteine residue. The most remarkable feature of this secondary inactivation is the existence of two components in the steady-state voltage dependence of inactivation. For mutant rKv1.4Δ2–37 about 90% of the channels only activate when the holding membrane potential is more negative than about –120 mV; the remaining 10% show the typical threshold at –60 mV. Mutagenesis of the truncated channel affected the relative amplitudes of these two components, but not the voltage dependence. The results suggest that the secondary ball structures of rKv1.4 channels interact with the protein structures responsible for activation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050891
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