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

Kv2 channels contribute to neuronal activity within the vagal afferent-nTS reflex arc

Diversity in the functional expression of ion channels contributes to the unique patterns of activity generated in visceral sensory A-type myelinated neurons versus C-type unmyelinated neurons in response to their natural stimuli. In the present study, Kv2 channels were identified as underlying a previously uncharacterized delayed rectifying potassium current expressed in both A- and C-type nodose ganglion neurons. Kv2.1 and 2.2 appear confined to the soma and initial segment of these sensory neurons; however, neither was identified in their central presynaptic terminals projecting onto relay neurons in the nucleus of the solitary tract (nTS). Kv2.1 and Kv2.2 were also not detected in the peripheral axons and sensory terminals in the aortic arch. Functionally, in nodose neuron somas, Kv2 currents exhibited frequency-dependent current inactivation and contributed to action potential repolarization in C-type neurons but not A-type neurons. Within the nTS, the block of Kv2 currents does not influence afferent presynaptic calcium influx or glutamate release in response to afferent activation, supporting our immunohistochemical observations. On the other hand, Kv2 channels contribute to membrane hyperpolarization and limit action potential discharge rate in second-order neurons. Together, these data demonstrate that Kv2 channels influence neuronal discharge within the vagal afferent-nTS circuit and indicate they may play a significant role in viscerosensory reflex function.NEW & NOTEWORTHY We demonstrate the expression and function of the voltage-gated delayed rectifier potassium channel Kv2 in vagal nodose neurons. Within sensory neurons, Kv2 channels limit the width of the broader C-type but not narrow A-type action potential. Within the nucleus of the solitary tract (nTS), the location of the vagal terminal field, Kv2 does not influence glutamate release. However, Kv2 limits the action potential discharge of nTS relay neurons. These data suggest a critical role for Kv2 in the vagal-nTS reflex arc.

The Selectivity Filter Is Involved in the U-Type Inactivation Process of Kv2.1 and Kv3.1 Channels

Voltage-gated potassium (Kv) channels display several types of inactivation processes, including N-, C-, and U-types. C-type inactivation is attributed to a nonconductive conformation of the selectivity filter (SF). It has been proposed that the activation gate and the channel's SF are allosterically coupled because the conformational changes of the former affect the structure of the latter and vice versa. The second threonine of the SF signature sequence (e.g., TTVGYG) has been proven to be essential for this allosteric coupling. To further study the role of the SF in U-type inactivation, we substituted the second threonine of the TTVGYG sequence by an alanine in the hKv2.1 and hKv3.1 channels, which are known to display U-type inactivation. Both hKv2.1-T377A and hKv3.1-T400A yielded channels that were resistant to inactivation, and as a result, they displayed noninactivating currents upon channel opening; i.e., hKv2.1-T377A and hKv3.1-T400A remained fully conductive upon prolonged moderate depolarizations, whereas in wild-type hKv2.1 and hKv3.1, the current amplitude typically reduces because of U-type inactivation. Interestingly, increasing the extracellular K⁺ concentration increased the macroscopic current amplitude of both hKv2.1-T377A and hKv3.1-T400A, which is similar to the response of the homologous T to A mutation in Shaker and hKv1.5 channels that display C-type inactivation. Our data support an important role for the second threonine of the SF signature sequence in the U-type inactivation gating of hKv2.1 and hKv3.1.

Dynamic all-optical drug screening on cardiac voltage-gated ion channels

Voltage-gated ion channels (VGCs) are prime targets for the pharmaceutical industry, but drug profiling on VGCs is challenging, since drug interactions are confined to specific conformational channel states mediated by changes in transmembrane potential. Here we combined various optogenetic tools to develop dynamic, high-throughput drug profiling assays with defined light-step protocols to interrogate VGC states on a millisecond timescale. We show that such light-induced electrophysiology (LiEp) yields high-quality pharmacological data with exceptional screening windows for drugs acting on the major cardiac VGCs, including hNav1.5, hKv1.5 and hERG. LiEp-based screening remained robust when using a variety of optogenetic actuators (ChR2, ChR2(H134R), CatCh, ChR2-EYFP-βArchT) and different types of organic (RH421, Di-4-ANBDQPQ, BeRST1) or genetic voltage sensors (QuasAr1). The tractability of LiEp allows a versatile and precise alternative to state-of-the-art VGC drug screening platforms such as automated electrophysiology or FLIPR readers.

Developmental nicotine exposure alters potassium currents in hypoglossal motoneurons of neonatal rat

We previously showed that nicotine exposure in utero and after birth via breast milk [developmental nicotine exposure (DNE)] is associated with many changes in the structure and function of hypoglossal motoneurons (XIIMNs), including a reduction in the size of the dendritic arbor and an increase in cell excitability. Interestingly, the elevated excitability was associated with a reduction in the expression of glutamate receptors on the cell body. Together, these observations are consistent with a homeostatic compensation aimed at restoring cell excitability. Compensation for increased cell excitability could also occur by changing potassium conductance, which plays a critical role in regulating resting potential, spike threshold, and repetitive spiking behavior. Here we test the hypothesis that the previously observed increase in the excitability of XIIMNs from DNE animals is associated with an increase in whole cell potassium currents. Potassium currents were measured in XIIMNs in brain stem slices derived from DNE and control rat pups ranging in age from 0 to 4 days by whole cell patch-clamp electrophysiology. All currents were measured after blockade of action potential-dependent synaptic transmission with tetrodotoxin. Compared with control cells, XIIMNs from DNE animals showed significantly larger transient and sustained potassium currents, but this was observed only under conditions of increased cell and network excitability, which we evoked by raising extracellular potassium from 3 to 9 mM. These observations suggest that the larger potassium currents in nicotine-exposed neurons are an important homeostatic compensation that prevents "runaway" excitability under stressful conditions, when neurons are receiving elevated excitatory synaptic input.NEW & NOTEWORTHY Developmental nicotine exposure is associated with increased cell excitability, which is often accompanied by compensatory changes aimed at normalizing excitability. Here we show that whole cell potassium currents are also increased in hypoglossal motoneurons from nicotine-exposed neonatal rats under conditions of increased cell and network excitability. This is consistent with a compensatory response aimed at preventing instability under conditions in which excitatory synaptic input is high and is compatible with the concept of homeostatic plasticity.

Depolarization increases phosphatidylinositol (PI) 4,5-bisphosphate level and KCNQ currents through PI 4-kinase mechanisms

A growing body of evidence shows that membrane phosphatidylinositol 4,5-bisphosphates (PtdIns(4,5)P(2), PIP(2)) play an important role in cell signaling. The presence of PIP(2) is fundamentally important for maintaining the functions of a large number of ion channels and transporters, and for other cell processes such as vesicle trafficking, mobility, and endo- and exocytosis. PIP(2) levels in the membrane are dynamically modulated, which is an important signaling mechanism for modulation of PIP(2)-dependent cellular processes. In this study, we describe a novel mechanism of membrane PIP(2) modulation. Membrane depolarization induces an elevation in membrane PIP(2), and subsequently increases functions of PIP(2)-sensitive KCNQ potassium channels expressed in Xenopus oocytes. Further evidence suggests that the depolarization-induced elevation of membrane PIP(2) occurs through increased activity of PI4 kinase. With increased recognition of the importance of PIP(2) in cell function, the effect of membrane depolarization in PIP(2) metabolism is destined to have important physiological implications.

The Arabidopsis vacuolar malate channel is a member of the ALMT family

In plants, malate is a central metabolite and fulfills a large number of functions. Vacuolar malate may reach very high concentrations and fluctuate rapidly, whereas cytosolic malate is kept at a constant level allowing optimal metabolism. Recently, a vacuolar malate transporter (Arabidopsis thaliana tonoplast dicarboxylate transporter, AttDT) was identified that did not correspond to the well-characterized vacuolar malate channel. We therefore hypothesized that a member of the aluminum-activated malate transporter (ALMT) gene family could code for a vacuolar malate channel. Using GFP fusion constructs, we could show that AtALMT9 (A. thaliana ALMT9) is targeted to the vacuole. Promoter-GUS fusion constructs demonstrated that this gene is expressed in all organs, but is cell-type specific as GUS activity in leaves was detected nearly exclusively in mesophyll cells. Patch-clamp analysis of an Atalmt9 T-DNA insertion mutant exhibited strongly reduced vacuolar malate channel activity. In order to functionally characterize AtALMT9 as a malate channel, we heterologously expressed this gene in tobacco and in oocytes. Overexpression of AtALMT9-GFP in Nicotiana benthamiana leaves strongly enhanced the malate current densities across the mesophyll tonoplasts. Functional expression of AtALMT9 in Xenopus oocytes induced anion currents, which were clearly distinguishable from endogenous oocyte currents. Our results demonstrate that AtALMT9 is a vacuolar malate channel. Deletion mutants for AtALMT9 exhibit only slightly reduced malate content in mesophyll protoplasts and no visible phenotype, indicating that AttDT and the residual malate channel activity are sufficient to sustain the transport activity necessary to regulate the cytosolic malate homeostasis.

Saturation and microsecond gating of current indicate depletion-induced instability of the MaxiK selectivity filter

Patch clamp experiments on single MaxiK channels expressed in HEK293 cells were performed with a high temporal resolution (50-kHz filter) in symmetrical solutions with 50, 150, or 400 mM KCl and 2.5 mM CaCl(2) and 2.5 mM MgCl(2). At membrane potentials >+100 mV, the single-channel current showed a negative slope resistance, concomitantly with a flickery block, which was not influenced by Ca(2+) or Mg(2+). The analysis of the amplitude histograms by beta distributions revealed that current in this voltage range was reduced by two effects: rate limitation at the cytosolic side of the pore and gating with rate constants 10-20-fold higher than the cutoff frequency of the filter (i.e., dwell times in the microsecond range). The data were analyzed in terms of a model that postulates a coupling between both effects; if the voltage over the selectivity filter withdraws ions from the cavity at a higher rate than that of refilling from the cytosol, the selectivity filter becomes instable because of ion depletion, and current is interrupted by the resulting flickering. The fit of the IV curves revealed a characteristic voltage of 35 mV. In contrast, the voltage dependence of the gating factor R, i.e., the ratio between true and apparent single-channel current, could be fitted by exponentials with a characteristic voltage of 60 mV, suggesting that only part of the transmembrane potential is felt by the flux through the selectivity filter.

Control of single channel conductance in the outer vestibule of the Kv2.1 potassium channel

Current magnitude in Kv2.1 potassium channels is modulated by external [K+]. In contrast to behavior expected from the change in electrochemical driving force, outward current through Kv2.1 channels becomes larger when extracellular [K+] is increased within the physiological range. The mechanism that underlies this unusual property involves the opening of Kv2.1 channels into one of two different outer vestibule conformations, which are defined by their sensitivity to TEA. Channels that open into a TEA-sensitive conformation generate larger macroscopic currents, whereas channels that open into a TEA-insensitive conformation generate smaller macroscopic currents. At higher [K+], more channels open into the TEA-sensitive conformation. In this manuscript, we examined the mechanism by which the conformational change produced a change in current magnitude. We started by testing the simplest hypothesis: that each pharmacologically defined channel conformation produces a different single channel conductance, one smaller and one larger, and that the [K+]-dependent change in current magnitude reflects the [K+]-dependent change in the percentage of channels that open into each of the two conformations. Using single channel and macroscopic recordings, as well as hidden Markov modeling, we were able to quantitatively account for [K+]-dependent regulation of macroscopic current with this model. Combined with previously published work, these results support a model whereby an outer vestibule lysine interferes with K+ flux through the channel, and that the [K+]-dependent change in orientation of this lysine alters single channel conductance by changing the level of this interference. Moreover, these results provide an experimental example of single channel conductance being modulated at the outer end of the conduction pathway by a mechanism that involves channel activation into open states with different outer vestibule conformations.

Potassium channels

Potassium channels are integral membrane proteins that selectively transport K+ across the cell membrane. They are present in all mammalian cells and have a wide variety of roles in both excitable and nonexcitable cells. The phenotypic diversity required to accomplish their various roles is created by differences in conductance, the timecourse and mechanisms of different gating events, and the interaction of channels with a variety of accessory proteins. Through the integration of biophysical, molecular, structural, and theoretical studies, significant progress has been made toward understanding the structural basis of K+ channel function, and diseases associated with K+ channel dysfunction.

K+ activation of kir3.1/kir3.4 and kv1.4 K+ channels is regulated by extracellular charges

K+ activates many inward rectifier and voltage-gated K+ channels. In each case, an increase in K+ current through the channel can occur despite a reduced driving force. We have investigated the molecular mechanism of K+ activation of the inward rectifier K+ channel, Kir3.1/Kir3.4, and the voltage-gated K+ channel, Kv1.4. In the Kir3.1/Kir3.4 channel, mutation of an extracellular arginine residue, R155, in the Kir3.4 subunit markedly reduced K+ activation of the channel. The same mutation also abolished Mg2+ block of the channel. Mutation of the equivalent residue in Kv1.4 (K532) abolished K+ activation as well as C-type inactivation of the Kv1.4 channel. Thus, whereas C-type inactivation is a collapse of the selectivity filter, K+ activation could be an opening of the selectivity filter. K+ activation of the Kv1.4 channel was enhanced by acidic pH. Mutation of an extracellular histidine residue, H508, that mediates the inhibitory effect of protons on Kv1.4 current, abolished both K+ activation and the enhancement of K+ activation at acidic pH. These results suggest that the extracellular positive charges in both the Kir3.1/Kir3.4 and the Kv1.4 channels act as "guards" and regulate access of K+ to the selectivity filter and, thus, the open probability of the selectivity filter. Furthermore, these data suggest that, at acidic pH, protonation of H508 inhibits current through the Kv1.4 channel by decreasing K+ access to the selectivity filter, thus favoring the collapse of the selectivity filter.

Potassium-dependent slow inactivation of Kir1.1 (ROMK) channels

The ROMK (Kir1.1) family of epithelial K channels can be inactivated by a combination of low internal pH and low external K, such that alkalization does not reopen the channels unless external K is elevated. Previous work suggested that this inactivation results from an allosteric interaction between an inner pH gate and an outer K sensor, and could be described by a simple three-state kinetic model. In the present study, we report that a sustained depolarization slowly inactivated (half-time = 10-15 min) ROMK channels that had been engineered for increased affinity to internal polyamines. Furthermore, this inactivation occurred at external [K] < or =1 mM in ROMK mutants whose inner pH gate was constitutively open (ROMK2-K61M mutation). Both pH and voltage inactivation depended on external K in a manner reminiscent of C-type inactivation, but having a much slower time course. Replacement of ROMK extracellular loop residues by Kir2.1 homologous residues attenuated or abolished this inactivation. These results are consistent with the hypothesis that there are (at least) two separate closure processes in these channels: an inner pH-regulated gate, and an outer (inactivation) gate, where the latter is modulated by both voltage and external [K].

Mechanisms underlying modulation of neuronal KCNQ2/KCNQ3 potassium channels by extracellular protons

Changes in extracellular pH occur during both physiological neuronal activity and pathological conditions such as epilepsy and stroke. Such pH changes are known to exert profound effects on neuronal activity and survival. Heteromeric KCNQ2/3 potassium channels constitute a potential target for modulation by H⁺ ions as they are expressed widely within the CNS and have been proposed to underlie the M-current, an important determinant of excitability in neuronal cells. Whole-cell and single-channel recordings demonstrated a modulation of heterologously expressed KCNQ2/3 channels by extracellular H⁺ ions. KCNQ2/3 current was inhibited by H⁺ ions with an IC₅₀ of 52 nM (pH 7.3) at −60 mV, rising to 2 μM (pH 5.7) at −10 mV. Neuronal M-current exhibited a similar sensitivity. Extracellular H⁺ ions affected two distinct properties of KCNQ2/3 current: the maximum current attainable upon depolarization (I_max) and the voltage dependence of steady-state activation. Reduction of I_max was antagonized by extracellular K⁺ ions and affected by mutations within the outer-pore turret, indicating an outer-pore based process. This reduction of I_max was shown to be due primarily to a decrease in the maximum open-probability of single KCNQ2/3 channels. Single-channel open times were shortened by acidosis (pH 5.9), while closed times were increased. Acidosis also recruited a longer-lasting closed state, and caused a switch of single-channel activity from the full-conductance state (∼8 pS) to a subconductance state (∼5 pS). A depolarizing shift in the activation curve of macroscopic KCNQ2/3 currents and single KCNQ2/3 channels was caused by acidosis, while alkalosis caused a hyperpolarizing shift. Activation and deactivation kinetics were slowed by acidosis, indicating specific effects of H⁺ ions on elements involved in gating. Contrasting modulation of homomeric KCNQ2 and KCNQ3 currents revealed that high sensitivity to H⁺ ions was conferred by the KCNQ3 subunit.

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.

Control of ion channel expression for patch clamp recordings using an inducible expression system in mammalian cell lines

BACKGROUND

Many molecular studies of ion channel function rely on the ability to obtain high quality voltage clamp recordings using the patch clamp technique. For a variety of channel types studied in mammalian cell heterologous expression systems, the lack of experimenter control over expression levels severely hinders the ability to obtain a high percentage of cells with an expression level suitable for high quality recordings. Moreover, it has been nearly impossible to obtain expression levels in mammalian cells well suited for single channel recordings. We describe here the use of an inducible promoter system in a stably transfected mammalian cell line that produces nearly 100% success in obtaining ion channel expression levels suitable for either whole cell or single ion channel studies.

RESULTS

We used a tetracycline-regulated expression system to control K+ channel expression in a CHO (Chinese hamster ovary) cell line. Current magnitudes within a reasonably narrow range could be easily and reliably obtained for either macroscopic or single channel recordings. Macroscopic currents of 1-2 nA could be obtained in nearly 100% of cells tested. The desired expression level could be obtained within just 2 to 3 hours, and remained stable at room temperature. Very low expression levels of transfected channels could also be obtained, which resulted in a >70% success rate in the ability to record single channel currents from a patch. Moreover, at these low expression levels, it appeared that endogenous channels produced little or no contamination.

CONCLUSION

This approach to controlling ion channel expression is relatively simple, greatly enhances the speed and efficiency with which high quality macroscopic current data can be collected, and makes it possible to easily and reliably record single channel currents in a mammalian cell heterologous expression system. Whereas we demonstrate the ability of this system to control expression levels of voltage-gated K+ channels, it should be applicable to all other channel types that express well in mammalian expression systems.

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.

Control of outer vestibule dynamics and current magnitude in the Kv2.1 potassium channel

In Kv2.1 potassium channels, changes in external [K+] modulate current magnitude as a result of a K+-dependent interconversion between two outer vestibule conformations. Previous evidence indicated that outer vestibule conformation (and thus current magnitude) is regulated by the occupancy of a selectivity filter binding site by K+. In this paper, we used the change in current magnitude as an assay to study how the interconversion between outer vestibule conformations is controlled. With 100 mM internal K+, rapid elevation of external [K+] from 0 to 10 mM while channels were activated produced no change in current magnitude (outer vestibule conformation did not change). When channels were subsequently closed and reopened in the presence of elevated [K+], current magnitude was increased (outer vestibule conformation had changed). When channels were activated in the presence of low internal [K+], or when K+ flow into conducting channels was transiently interrupted by an internal channel blocker, increasing external [K+] during activation did increase current magnitude (channel conformation did change). These data indicate that, when channels are in the activated state under physiological conditions, the outer vestibule conformation remains fixed despite changes in external [K+]. In contrast, when channel occupancy is lowered, (by channel closing, an internal blocker or low internal [K+]), the outer vestibule can interconvert between the two conformations. We discuss evidence that the ability of the outer vestibule conformation to change is regulated by the occupancy of a nonselectivity filter site by K+. Independent of the outer vestibule-based potentiation mechanism, Kv2.1 was remarkably insensitive to K+-dependent processes that influence current magnitude (current magnitude changed by <7% at membrane potentials between -20 and 30 mV). Replacement of two outer vestibule lysines in Kv2.1 by smaller neutral amino acids made current magnitude dramatically more sensitive to the reduction in K+ driving force (current magnitude changed by as much as 40%). When combined, these outer vestibule properties (fixed conformation during activation and the presence of lysines) all but prevent variation in Kv2.1 current magnitude when [K+] changes during activation. Moreover, the insensitivity of Kv2.1 current magnitude to changes in K+ driving force promotes a more uniform modulation of current over a wide range of membrane potentials by the K+-dependent regulation of outer vestibule conformation.

Molecular determinants of the inhibition of human Kv1.5 potassium currents by external protons and Zn(2+)

Using human Kv1.5 channels expressed in HEK293 cells we assessed the ability of H+o to mimic the previously reported action of Zn(2+) to inhibit macroscopic hKv1.5 currents, and using site-directed mutagenesis, we addressed the mechanistic basis for the inhibitory effects of H(+)(o) and Zn(2+). As with Zn(2+), H(+)(o) caused a concentration-dependent, K(+)(o)-sensitive and reversible reduction of the maximum conductance (g(max)). With zero, 5 and 140 mM K(+)(o) the pK(H) for this decrease of g(max) was 6.8, 6.2 and 6.0, respectively. The concentration dependence of the block relief caused by increasing [K(+)](o) was well fitted by a non-competitive interaction between H(+)(o) and K(+)(o), for which the K(D) for the K(+) binding site was 0.5-1.0 mM. Additionally, gating current analysis in the non-conducting mutant hKv1.5 W472F showed that changing from pH 7.4 to pH 5.4 did not affect Q(max) and that charge immobilization, presumed to be due to C-type inactivation, was preserved at pH 5.4. Inhibition of hKv1.5 currents by H+o or Zn(2+) was substantially reduced by a mutation either in the channel turret (H463Q) or near the pore mouth (R487V). In light of the requirement for R487, the homologue of Shaker T449, as well as the block-relieving action of K(+)(o), we propose that H(+) or Zn(2+) binding to histidine residues in the pore turret stabilizes a channel conformation that is most likely an inactivated state.

Permeant ion-dependent changes in gating of Kir2.1 inward rectifier potassium channels

We studied the effect of monovalent thallium ion (Tl(+)) on the gating of single Kir2.1 channels, which open and close spontaneously at a constant membrane potential. In cell-attached recordings of single-channel inward current, changing the external permeant ion from K(+) to Tl(+) decreases the mean open-time by approximately 20-fold. Furthermore, the channel resides predominantly at a subconductance level, which results from a slow decay (tau = 2.7 ms at -100 mV) from the fully open level immediately following channel opening. Mutation of a pore-lining cysteine (C169) to valine abolishes the slow decay and subconductance level, and single-channel recordings from channels formed by tandem tetramers containing one to three C169V mutant subunits indicate that Tl(+) must interact with at least three C169 residues to induce these effects. However, the C169V mutation does not alter the single-channel closing kinetics of Tl(+) current. These results suggest that Tl(+) ions change the conformation of the ion conduction pathway during permeation and alter gating by two distinct mechanisms. First, they interact with the thiolate groups of C169 lining the cavity to induce conformational changes of the ion passageway, and thereby produce a slow decay of single-channel current and a dominant subconductance state. Second, they interact more strongly than K(+) with the main chain carbonyl oxygens lining the selectivity filter to destabilize the open state of the channel and, thus, alter the open/close kinetics of gating. In addition to altering gating, Tl(+) greatly diminishes Ba(2+) block. The unblocking rate of Ba(2+) is increased by >22-fold when the external permeant ion is switched from K(+) to Tl(+) regardless of the direction of Ba(2+) exit. This effect cannot be explained solely by ion-ion interactions, but is consistent with the notion that Tl(+) induces conformational changes in the selectivity filter.

Affinity and location of an internal K+ ion binding site in shaker K channels

We have examined the interaction between TEA and K+ ions in the pore of Shaker potassium channels. We found that the ability of external TEA to antagonize block of Shaker channels by internal TEA depended on internal K+ ions. In contrast, this antagonism was independent of external K+ concentrations between 0.2 and 40 mM. The external TEA antagonism of internal TEA block increased linearly with the concentration of internal K+ ions. In addition, block by external TEA was significantly enhanced by increases in the internal K+ concentration. These results suggested that external TEA ions do not directly antagonize internal TEA, but rather promote increased occupancy of an internal K+ site by inhibiting the emptying of that site to the external side of the pore. We found this mechanism to be quantitatively consistent with the results and revealed an intrinsic affinity of the site for K+ ions near 65 mM located approximately 7% into the membrane electric field from the internal end of the pore. We also found that the voltage dependence of block by internal TEA was influenced by internal K+ ions. The TEA site (at 0 internal K+) appeared to sense approximately 5% of the field from the internal end of the pore (essentially colocalized with the internal K+ site). These results lead to a refined picture of the number and location of ion binding sites at the inner end of the pore in Shaker K channels.