Two mechanisms of K(+)-dependent potentiation in Kv2.1 potassium channels

Elevation of external [K(+)] potentiates outward K(+) current through several voltage-gated K(+) channels. This increase in current magnitude is paradoxical in that it occurs despite a significant decrease in driving force. We have investigated the mechanisms involved in K(+)-dependent current potentiation in the Kv2.1 K(+) channel. With holding potentials of -120 to -150 mV, which completely removed channels from the voltage-sensitive inactivated state, elevation of external [K(+)] up to 10 mM produced a concentration-dependent increase in outward current magnitude. In the absence of inactivation, currents were maximally potentiated by 38%. At more positive holding potentials, which produced steady-state inactivation, K(+)-dependent potentiation was enhanced. The additional K(+)-dependent potentiation (above 38%) at more positive holding potentials was precisely equal to a K(+)-dependent reduction in steady-state inactivation. Mutation of two lysine residues in the outer vestibule of Kv2.1 (K356 and K382), to smaller, uncharged residues (glycine and valine, respectively), completely abolished K(+)-dependent potentiation that was not associated with inactivation. These mutations did not influence steady-state inactivation or the K(+)-dependent potentiation due to reduction in steady-state inactivation. These results demonstrate that K(+)-dependent potentiation can be completely accounted for by two independent mechanisms: one that involved the outer vestibule lysines and one that involved K(+)-dependent removal of channels from the inactivated state. Previous studies demonstrated that the outer vestibule of Kv2.1 can be in at least two conformations, depending on the occupancy of the selectivity filter by K(+) (Immke, D., M. Wood, L. Kiss, and S. J. Korn. 1999. J. Gen. Physiol. 113:819-836; Immke, D., and S. J. Korn. 2000. J. Gen. Physiol. 115:509-518). This change in conformation was functionally defined by a change in TEA sensitivity. Similar to the K(+)-dependent change in TEA sensitivity, the lysine-dependent potentiation depended primarily (>90%) on Lys-356 and was enhanced by lowering initial K(+) occupancy of the pore. Furthermore, the K(+)-dependent changes in current magnitude and TEA sensitivity were highly correlated. These results suggest that the previously described K(+)-dependent change in outer vestibule conformation underlies the lysine-sensitive, K(+)-dependent potentiation mechanism.     

Influence of permeant ions on voltage sensor function in the Kv2.1 potassium channel

We previously demonstrated that the outer vestibule of activated Kv2.1 potassium channels can be in one of two conformations, and that K(+) occupancy of a specific selectivity filter site determines which conformation the outer vestibule is in. These different outer vestibule conformations result in different sensitivities to internal and external TEA, different inactivation rates, and different macroscopic conductances. The [K(+)]-dependent switch in outer vestibule conformation is also associated with a change in rate of channel activation. In this paper, we examined the mechanism by which changes in [K(+)] modulate the rate of channel activation. Elevation of symmetrical [K(+)] or [Rb(+)] from 0 to 3 mM doubled the rate of on-gating charge movement (Q(on)), measured at 0 mV. Cs(+) produced an identical effect, but required 40-fold higher concentrations. All three permeant ions occupied the selectivity filter over the 0.03-3 mM range, so simple occupancy of the selectivity filter was not sufficient to produce the change in Q(on). However, for each of these permeant ions, the speeding of Q(on) occurred with the same concentration dependence as the switch between outer vestibule conformations. Neutralization of an amino acid (K356) in the outer vestibule, which abolishes the modulation of channel pharmacology and ionic currents by the K(+)-dependent reorientation of the outer vestibule, also abolished the K(+)-dependence of Q(on). Together, the data indicate that the K(+)-dependent reorientation in the outer vestibule was responsible for the change in Q(on). Moreover, similar [K(+)]-dependence and effects of mutagenesis indicate that the K(+)-dependent change in rate of Q(on) can account for the modulation of ionic current activation rate. Simple kinetic analysis suggested that K(+) reduced an energy barrier for voltage sensor movement. These results provide strong evidence for a direct functional interaction, which is modulated by permeant ions acting at the selectivity filter, between the outer vestibule of the Kv2.1 potassium channel and the voltage sensor.     

Potassium-dependent changes in the conformation of the Kv2.1 potassium channel pore.

The voltage-gated K+ channel, Kv2.1, conducts Na+ in the absence of K+. External tetraethylammonium (TEAo) blocks K+ currents through Kv2.1 with an IC50 of 5 mM, but is completely without effect in the absence of K+. TEAo block can be titrated back upon addition of low [K+]. This suggested that the Kv2.1 pore undergoes a cation-dependent conformational rearrangement in the external vestibule. Individual mutation of lysine (Lys) 356 and 382 in the outer vestibule, to a glycine and a valine, respectively, increased TEAo potency for block of K+ currents by a half log unit. Mutation of Lys 356, which is located at the outer edge of the external vestibule, significantly restored TEAo block in the absence of K+ (IC50 = 21 mM). In contrast, mutation of Lys 382, which is located in the outer vestibule near the TEA binding site, resulted in very weak (extrapolated IC50 = approximately 265 mM) TEAo block in the absence of K+. These data suggest that the cation-dependent alteration in pore conformation that resulted in loss of TEA potency extended to the outer edge of the external vestibule, and primarily involved a repositioning of Lys 356 or a nearby amino acid in the conduction pathway. Block by internal TEA also completely disappeared in the absence of K+, and could be titrated back with low [K+]. Both internal and external TEA potencies were increased by the same low [K+] (30-100 microM) that blocked Na+ currents through the channel. In addition, experiments that combined block by internal and external TEA indicated that the site of K+ action was between the internal and external TEA binding sites. These data indicate that a K+-dependent conformational change also occurs internal to the selectivity filter, and that both internal and external conformational rearrangements resulted from differences in K+ occupancy of the selectivity filter. Kv2.1 inactivation rate was K+ dependent and correlated with TEAo potency; as [K+] was raised, TEAo became more potent and inactivation became faster. Both TEAo potency and inactivation rate saturated at the same [K+]. These results suggest that the rate of slow inactivation in Kv2.1 was influenced by the conformational rearrangements, either internal to the selectivity filter or near the outer edge of the external vestibule, that were associated with differences in TEA potency.     

Pharmacology and Surface Electrostatics of the K Channel Outer Pore Vestibule

In spite of a generally well-conserved outer vestibule and pore structure, there is considerable diversity in the pharmacology of K channels. We have investigated the role of specific outer vestibule charged residues in the pharmacology of K channels using tetraethylammonium (TEA) and a trivalent TEA analog, gallamine. Similar to Shaker K channels, gallamine block of Kv3.1 channels was more sensitive to solution ionic strength than was TEA block, a result consistent with a contribution from an electrostatic potential near the blocking site. In contrast, TEA block of another type of K channel (Kv2.1) was insensitive to solution ionic strength and these channels were resistant to block by gallamine. Neutralizing either of two lysine residues in the outer vestibule of these Kv2.1 channels conferred ionic strength sensitivity to TEA block. Kv2.1 channels with both lysines neutralized were sensitive to block by gallamine, and the ionic strength dependence of this block was greater than that for TEA. These results demonstrate that Kv3.1 (like Shaker) channels contain negatively charged residues in the outer vestibule of the pore that influence quaternary ammonium pharmacology. The presence of specific lysine residues in wild-type Kv2.1 channels produces an outer vestibule with little or no net charge, with important consequences for quaternary ammonium block. Neutralizing these key lysines results in a negatively charged vestibule with pharmacological properties approaching those of other types of K channels.     

Influence of pore residues on permeation properties in the Kv2.1 potassium channel. Evidence for a selective functional interaction of K+ with the outer vestibule

The Kv2.1 potassium channel contains a lysine in the outer vestibule (position 356) that markedly reduces open channel sensitivity to changes in external [K(+)]. To investigate the mechanism underlying this effect, we examined the influence of this outer vestibule lysine on three measures of K(+) and Na(+) permeation. Permeability ratio measurements, measurements of the lowest [K(+)] required for interaction with the selectivity filter, and measurements of macroscopic K(+) and Na(+) conductance, were all consistent with the same conclusion: that the outer vestibule lysine in Kv2.1 interferes with the ability of K(+) to enter or exit the extracellular side of the selectivity filter. In contrast to its influence on K(+) permeation properties, Lys 356 appeared to be without effect on Na(+) permeation. This suggests that Lys 356 limited K(+) flux by interfering with a selective K(+) binding site. Combined with permeation studies, results from additional mutagenesis near the external entrance to the selectivity filter indicated that this site was located external to, and independent from, the selectivity filter. Protonation of a naturally occurring histidine in the same outer vestibule location in the Kv1.5 potassium channel produced similar effects on K(+) permeation properties. Together, these results indicate that a selective, functional K(+) binding site (e.g., local energy minimum) exists in the outer vestibule of voltage-gated K(+) channels. We suggest that this site is the location of K(+) hydration/dehydration postulated to exist based on the structural studies of KcsA. Finally, neutralization of position 356 enhanced outward K(+) current magnitude, but did not influence the ability of internal K(+) to enter the pore. These data indicate that in Kv2.1, exit of K(+) from the selectivity filter, rather than entry of internal K(+) into the channel, limits outward current magnitude. We discuss the implications of these findings in relation to the structural basis of channel conductance in different K(+) channels.     

External tetraethylammonium as a molecular caliper for sensing the shape of the outer vestibule of potassium channels

External tetraethylammonium (TEA+) blocked currents through Kv1.1 channels in a voltage-independent manner between 0 and 100 mV. Lowering extracellular pH (pHo) increased the Kd for TEA+ block. A histidine at position 355 in the Kv1.1 channel protein (homologous to Shaker 425) was responsible for this pH-dependent reduction of TEA+ sensitivity, since the TEA+ effect became independent of pHo after chemical modification of the Kv1.1 channel at H355 and in the H355G and H355K mutant Kv1.1 channels. The Kd values for TEA+ block of the two mutant channels (0.34 +/- 0.06 mM, n = 7 and 0.84 +/- 0. 09 mM, n = 13, respectively) were as expected for a vestibule containing either no or a total of four positive charges at position 355. In addition, the pH-dependent TEA+ effect in the wt Kv1.1 channel was sensitive to the ionic strength of the solution. All our observations are consistent with the idea that lowering pHo increased protonation of H355. This increase in positive charge at H355 will repel TEA+ electrostatically, resulting in a reduction of the effective [TEA+]o at the receptor site. From this reduction we can estimate the distance between TEA+ and each of the four histidines at position 355 to be approximately 10 A, assuming fourfold symmetry of the channel and assuming that TEA+ binds in the central axis of the pore. This determination of the dimensions of the outer vestibule of Kv1.1 channels confirms and extends earlier reports on K+ channels using crystal structure data as well as peptide toxin/channel interactions and points out a striking similarity between vestibules of Kv1.1 and KcsA channels.     

Anorexic effect of K+ channel blockade in mesenteric arterial smooth muscle and intestinal epithelial cells.

Activity of voltage-gated K+ (Kv) channels controls membrane potential (E(m)). Membrane depolarization due to blockade of K+ channels in mesenteric artery smooth muscle cells (MASMC) should increase cytoplasmic free Ca2+ concentration ([Ca2+]cyt) and cause vasoconstriction, which may subsequently reduce the mesenteric blood flow and inhibit the transportation of absorbed nutrients to the liver and adipose tissue. In this study, we characterized and compared the electrophysiological properties and molecular identities of Kv channels and examined the role of Kv channel function in regulating E(m) in MASMC and intestinal epithelial cells (IEC). MASMC and IEC functionally expressed multiple Kv channel alpha- and beta-subunits (Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5, Kv2.1, Kv4.3, and Kv9.3, as well as Kvbeta1.1, Kvbeta2.1, and Kvbeta3), but only MASMC expressed voltage-dependent Ca2+ channels. The current density and the activation and inactivation kinetics of whole cell Kv currents were similar in MASMC and IEC. Extracellular application of 4-aminopyridine (4-AP), a Kv-channel blocker, reduced whole cell Kv currents and caused E(m) depolarization in both MASMC and IEC. The 4-AP-induced E(m) depolarization increased [Ca2+]cyt in MASMC and caused mesenteric vasoconstriction. Furthermore, ingestion of 4-AP significantly reduced the weight gain in rats. These results suggest that MASMC and IEC express multiple Kv channel alpha- and beta-subunits. The function of these Kv channels plays an important role in controlling E(m). The membrane depolarization-mediated increase in [Ca2+]cyt in MASMC and mesenteric vasoconstriction may inhibit transportation of absorbed nutrients via mesenteric circulation and limit weight gain.     

Effect of external pH on activation of the Kv1.5 potassium channel

We studied the mechanism by which external acidification from pH 7.3 to 6.8 reduced current magnitude in the Kv1.5 potassium channel. At physiological external [K(+)], a shift in the voltage-dependence of activation was entirely responsible for the acidification-induced decrease in Kv1.5 current magnitude (pK = 7.15). Elevation of external [Ca(2+)] or [Mg(2+)] identically shifted activation curves to the right and identically shifted the pH-sensitivity of the activation curves to more acidic values. Similar observations were made with the Kv2.1 K(+) channel, except that the pK for the activation shift was out of the physiological range. These data are consistent with a mechanism by which acidification shifted activation via modification of a local surface potential. Elimination of eight positive charges within the outer vestibule of the conduction pathway had no effect on the voltage-dependence of activation at pH 7.3 or higher, which suggested that sites exposed to the conduction pathway within the outer vestibule did not directly contribute to the relevant local surface potential. However, mutations at position 487 (within the conduction pathway) displaced the pK of the pH-sensitive shift in activation, such that the sensitivity of Kv1.5 current to physiologically relevant changes in pH was reduced or eliminated. These results suggest that, among voltage-gated K(+) channels, activation in Kv1.5 is uniquely sensitive to physiologically relevant changes in pH because the pK for the sites that contribute to the local surface potential effect is near pH 7. Moreover, the pK for the activation shift depends not only on the nature of the sites involved but also on structural orientation conferred, in part, by at least one residue within the conduction pathway.     

Molecular docking of the scorpion toxin Tc1 to the structural model of the voltage-gated potassium channel Kv1.1 from human Homo sapiens

In this study, structural model of the pore loop region of the voltage-gated potassium channel Kv1.1 from human Homo sapiens was constructed based on the crystallographic structure of KcsA by structural homology. The pore loop region of Kv1.1 exhibits similar folds as that of KcsA. The structural feature of the selectivity filter of Kv1.1 is nearly identical to that of KcsA, whereas most of the structural variations occur in the turret as well as in the inner and outer helices. Molecular docking experiments of the scorpion toxin Tc1 from Tityus cambridgei to the outer vestibule of KcsA as well as Kv1.1 were subsequently performed with various initial Tc1 orientations. Tc1 was found to form the most stable complexes with these two K+ channels when the side chain of Lys14 occupies the pore of the selectivity filter through electrostatic interaction. Tc1 binds preferentially towards Kv1.1 than KcsA due to stronger hydrophobic and electrostatic interactions formed between the toxin and the selectivity filter and outer vestibule of Kv1.1. Furthermore, surface complementarity of the outer vestibules of the channels to the Tc1 spatial conformations also plays an important role in stabilizing both the Tc1/KcsA and Tc1/Kv1.1 complexes.     

External pore collapse as an inactivation mechanism for Kv4.3 K+ channels

Kv4 channels are thought to lack a C-type inactivation mechanism (collapse of the external pore) and to inactivate as a result of a concerted action of cytoplasmic regions of the channel. To investigate whether Kv4 channels have outer pore conformational changes during the inactivation process, the inactivation properties of Kv4.3 were characterized in 0 mM and in 2 mM external K+ in whole-cell voltage-clamp experiments. Removal of external K+ increased the inactivation rates and favored cumulative inactivation by repetitive stimulation. The reduction in current amplitude during repetitive stimulation and the faster inactivation rates in 0 mM external K+ were not due to changes in the voltage dependence of channel opening or to internal K+ depletion. The extent of the collapse of the K+ conductance upon removal of external K+ was more pronounced in NMG+-than in Na+-containing solutions. The reduction in the current amplitude during cumulative inactivation by repetitive stimulation is not associated with kinetic changes, suggesting that it is due to a diminished number of functional channels with unchanged gating properties. These observations meet the criteria for a typical C-type inactivation, as removal of external K+ destabilizes the conducting state, leading to the collapse of the pore. A tentative model is presented, in which K+ bound to high-affinity K+-binding sites in the selectivity filter destabilizes an outer neighboring K+ modulatory site that is saturated at approximately 2 mM external K+. We conclude that Kv4 channels have a C-type inactivation mechanism and that previously reported alterations in the inactivation rates after N- and C- termini mutagenesis may arise from secondary changes in the electrostatic interactions between K+-binding sites in the selectivity filter and the neighboring K+-modulatory site, that would result in changes in its K+ occupancy.     

Ionic mechanisms mediating the myogenic response in newborn porcine cerebral arteries

Mechanisms that underlie autoregulation in the newborn vasculature are unclear. Here we tested the hypothesis that in newborn porcine cerebral arteries intravascular pressure elevates wall tension, leading to an increase in intracellular calcium concentration ([Ca2+]i) and a constriction that is opposed by pressure-induced K+ channel activation. Incremental step (20 mmHg) elevations in intravascular pressure between 10 and 90 mmHg induced an immediate transient elevation in arterial wall [Ca2+]i and a short-lived constriction that was followed by a smaller steady-state [Ca2+]i elevation and sustained constriction. Pressures between 10 and 90 mmHg increased steady-state arterial wall [Ca2+]i between approximately 142 and 299 nM and myogenic (defined as passive-active) tension between 25 and 437 dyn/cm. The relationship between pressure and myogenic tension was strongly Ca2+ dependent until forced dilation. At low pressure, 60 mM K+ induced a steady-state elevation in arterial wall [Ca2+]i and a constriction. Nimodipine, a voltage-dependent Ca2+ channel blocker, and removal of extracellular Ca2+ similarly dilated arteries at low or high pressures. 4-Aminopyridine, a voltage-dependent K+ (Kv) channel blocker, induced significantly larger constrictions at high pressure, when compared with those at low pressure. Although selective Ca2+-activated K+ (KCa) channel blockers and intracellular Ca2+ release inhibitors induced only small constrictions at low and high pressures, a low concentration of caffeine (1 microM), a ryanodine-sensitive Ca2+ release (RyR) channel activator, increased KCa channel activity and induced dilation. These data suggest that in newborn cerebral arteries, intravascular pressure elevates wall tension, leading to voltage-dependent Ca2+ channel activation, an increase in wall [Ca2+]i and Ca2+-dependent constriction. In addition, pressure strongly activates Kv channels that opposes constriction but only weakly activates KCa channels.     

Unitary Ca2+ current through mammalian cardiac and amphibian skeletal muscle ryanodine receptor Channels under near-physiological ionic conditions

Ryanodine receptor (RyR) channels from mammalian cardiac and amphibian skeletal muscle were incorporated into planar lipid bilayers. Unitary Ca2+ currents in the SR lumen-to-cytosol direction were recorded at 0 mV in the presence of caffeine (to minimize gating fluctuations). Currents measured with 20 mM lumenal Ca2+ as exclusive charge carrier were 4.00 and 4.07 pA, respectively, and not significantly different. Currents recorded at 1-30 mM lumenal Ca2+ concentrations were attenuated by physiological [K+] (150 mM) and [Mg2+] (1 mM), in the same proportion (approximately 55%) in mammalian and amphibian channels. Two amplitudes, differing by approximately 35%, were found in amphibian channel studies, probably corresponding to alpha and beta RyR isoforms. In physiological [Mg2+], [K+], and lumenal [Ca2+] (1 mM), the Ca2+ current was just less than 0.5 pA. Comparison of this value with the Ca2+ flux underlying Ca2+ sparks suggests that sparks in mammalian cardiac and amphibian skeletal muscles are generated by opening of multiple RyR channels. Further, symmetric high concentrations of Mg2+ substantially reduced the current carried by 10 mM Ca2+ (approximately 40% at 10 mM Mg2+), suggesting that high Mg2+ may make sparks smaller by both inhibiting RyR gating and reducing unitary current.     

Apoptotic surface delivery of K+ channels

Apoptosis in cortical neurons requires efflux of cytoplasmic potassium mediated by a surge in Kv2.1 channel activity. Pharmacological blockade or molecular disruption of these channels in neurons prevents apoptotic cell death, while ectopic expression of Kv2.1 channels promotes apoptosis in non-neuronal cells. Here, we use a cysteine-containing mutant of Kv2.1 and a thiol-reactive covalent inhibitor to demonstrate that the increase in K+ current during apoptosis is due to de novo insertion of functional channels into the plasma membrane. Biotinylation experiments confirmed the delivery of additional Kv2.1 protein to the cell surface following an apoptotic stimulus. Finally, expression of botulinum neurotoxins that cleave syntaxin and synaptosome-associated protein of 25 kDa (SNAP-25) blocked upregulation of surface Kv2.1 channels in cortical neurons, suggesting that target soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins support proapoptotic delivery of K+ channels. These data indicate that trafficking of Kv2.1 channels to the plasma membrane causes the apoptotic surge in K+ current.     

Modulation of Kv2.1 channel gating and TEA sensitivity by distinct domains of SNAP-25

Distinct domains within the SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) proteins, STX1A (syntaxin 1A) and SNAP-25 (synaptosome-associated protein-25 kDa), regulate hormone secretion by their actions on the cell's exocytotic machinery, as well as voltage-gated Ca2+ and K+ channels. We examined the action of distinct domains within SNAP-25 on Kv2.1 (voltage gated K+ 2.1) channel gating. Dialysis of N-terminal SNAP-25 domains, S197 (SNAP-25(1-197)) and S180 (SNAP-25(1-180)), but not S206 (full-length SNAP-25(1-206)) increased the rate of Kv2.1 channel activation and slowed channel inactivation. Remarkably, these N-terminal SNAP-25 domains, acting on the Kv2.1 cytoplasmic N-terminus, potentiated the external TEA (tetraethylammonium)-mediated block of Kv2.1. To further examine whether these are effects of the channel pore domain, internal K+ was replaced with Na+ and external K+ was decreased from 4 to 1 mM, which decreased the IC50 of the TEA block from 6.8+/-0.9 mM to >100 mM. Under these conditions S180 completely restored TEA sensitivity (7.9+/-1.5 mM). SNAP-25 C-terminal domains, SNAP-25(198-206) and SNAP-25(181-197), had no effect on Kv2.1 gating kinetics. We conclude that different domains within SNAP-25 can form distinct complexes with Kv2.1 to execute a fine allosteric regulation of channel gating and the architecture of the outer pore structure in order to modulate cell excitability.     

Modeling ion permeation through batrachotoxin-modified Na+ channels from rat skeletal muscle with a multi-ion pore.

The mechanism of ion permeation through Na+ channels that have been modified by batrachotoxin (BTX) and inserted into planar bilayers has been generally described by models based on single-ion occupancy, with or without an influence of negative surface charge, depending on the tissue source. For native Na+ channels there is evidence suggestive of a multi-ion conduction mechanism. To explore the question of ion occupancy, we have reexamined permeation of Na+, Li+, and K+ through BTX-modified Na+ channels from rat skeletal muscle. Single-channel current-voltage (I-V) behavior was studied in neutral lipid bilayers in the presence of symmetrical Na+ concentrations ranging from 0.5 to 3,000 mM. The dependence of unitary current on the mole fraction of Na+ was also examined in symmetrical mixtures of Na(+)-Li+ and Na(+)-K+ at a constant total ionic strength of 206 and 2,006 mM. The dependence of unitary conductance on symmetrical Na+ concentration does not exhibit Michaelis-Menten behavior characteristic of single-ion occupancy but can be simulated by an Eyring-type model with three barriers and two sites (3B2S) that includes double occupancy and ion-ion repulsion. Best-fit energy barrier profiles for Na+, Li+, and K+ were obtained by nonlinear curve fitting of I-V data using the 3B2S model. The Na(+)-Li+ and Na(+)-K+ mole-fraction experiments do not exhibit an anomalous mole-fraction effect. However, the 3B2S model is able to account for the biphasic dependence of unitary conductance on symmetrical [Na+] that is suggestive of multiple occupancy and the monotonic dependence of unitary current on the mole fraction of Na+ that is compatible with single or multiple occupancy. The best-fit 3B2S barrier profiles also successfully predict bi-ionic reversal potentials for Na(+)-Li+ and Na(+)-K+ in both orientations across the channel. Our experimental and modeling results reconcile the dual personality of ion permeation through Na+ channels, which can display features of single or multiple occupancy under various conditions. To a first approximation, the 3B2S model developed for this channel does not require corrections for vestibule surface charge. However, if negative surface charges of the protein do influence conduction, the conductance behavior in the limit of low [Na+] does not correspond to a Gouy-Chapman model of planar surface charge.     

Oxidation of an engineered pore cysteine locks a voltage-gated K+ channel in a nonconducting state.

We report the use of cysteine-substituted mutants in conjunction with in situ oxidation to determine the physical proximity of a pair of engineered cysteines in the pore region of the voltage-gated K+ channel Kv2.1. We show that the newly introduced cysteine 1379C, located near the outer end of the narrow ion-conduction pathway, renders the K+ channel sensitive to oxidation by H2O2, but only if the native cysteine at position 394 in S6 remains in place. Conservative substitutions in S6 for cysteine 394 abolish H2O2 sensitivity in the Kv2.1 mutant 1379C. Comparative immunoblot analysis of wild-type and 1379C Kv2.1-expressing HEK293 cells demonstrates the presence of subunit dimers for 1379C, but not for wild-type Kv2.1. At the single-channel level, the probability of opening of 1379C channels, unlike wild-type, is reduced in the presence of H2O2; however, oxidation of 1379C does not alter unit current. These findings imply that cysteine 379, located near the outer end of the narrow ion-conduction pathway, participates in disulfide bridge formation, locking the channel in a nonconducting state from which it cannot undergo conformational transitions required for opening.     

Electrostatics in the cytoplasmic pore produce intrinsic inward rectification in kir2.1 channels

Inward rectifier K+ channels are important in regulating membrane excitability in many cell types. The physiological functions of these channels are related to their unique inward rectification, which has been attributed to voltage-dependent block. Here, we show that inward rectification can also be induced by neutral and positively charged residues at site 224 in the internal vestibule of tetrameric Kir2.1 channels. The order of extent of inward rectification is E224K mutant > E224G mutant > wild type in the absence of internal blockers. Mutating the glycines at the equivalent sites to lysines also rendered weak inward rectifier Kir1.1 channels more inwardly rectifying. Also, conjugating positively charged methanethiosulfonate to the cysteines at site 224 induced strong inward rectification, whereas negatively charged methanethiosulfonate alleviated inward rectification in the E224C mutant. These results suggest that charges at site 224 may control inward rectification in the Kir2.1 channel. In a D172N mutant, spermine interacting with E224 and E299 induced channel inhibition during depolarization but did not occlude the pore, further suggesting that a mechanism other than channel block is involved in the inward rectification of the Kir2.1 channel. In this and our previous studies we showed that the M2 bundle crossing and selectivity filter were not involved in the inward rectification induced by spermine interacting with E224 and E299. We propose that neutral and positively charged residues at site 224 increase a local energy barrier, which reduces K+ efflux more than K+ influx, thereby producing inward rectification.     

The Link between Ion Permeation and Inactivation Gating of Kv4 Potassium Channels

Kv4 potassium channels undergo rapid inactivation but do not seem to exhibit the classical N-type and C-type mechanisms present in other Kv channels. We have previously hypothesized that Kv4 channels preferentially inactivate from the preopen closed state, which involves regions of the channel that contribute to the internal vestibule of the pore. To further test this hypothesis, we have examined the effects of permeant ions on gating of three Kv4 channels (Kv4.1, Kv4.2, and Kv4.3) expressed in Xenopus oocytes. Rb⁺ is an excellent tool for this purpose because its prolonged residency time in the pore delays K⁺ channel closing. The data showed that, only when Rb⁺ carried the current, both channel closing and the development of macroscopic inactivation are slowed (1.5- to 4-fold, relative to the K⁺ current). Furthermore, macroscopic Rb⁺ currents were larger than K⁺ currents (1.2- to 3-fold) as the result of a more stable open state, which increases the maximum open probability. These results demonstrate that pore occupancy can influence inactivation gating in a manner that depends on how channel closing impacts inactivation from the preopen closed state. By examining possible changes in ionic selectivity and the influence of elevating the external K⁺ concentration, additional experiments did not support the presence of C-type inactivation in Kv4 channels.