Mutations in the pore regions of the yeast K+ channel YKC1 affect gating by extracellular K+

Secrets of the fungus-specific potassium channel TOK family

Several families of potassium (K⁺) channels are found in membranes of all eukaryotes, underlining the importance of K⁺ uptake and redistribution within and between cells and organs. Among them, TOK (tandem-pore outward-rectifying K⁺) channels consist of eight transmembrane domains and two pore domains per subunit organized in dimers. These channels were originally studied in yeast, but recent identifications and characterizations in filamentous fungi shed new light on this fungus-specific K⁺ channel family. Although their actual function in vivo is often puzzling, recent works indicate a role in cellular K⁺ homeostasis and even suggest a role in plant-fungus symbioses. This review aims at synthesizing the current knowledge on fungal TOK channels and discussing their potential role in yeasts and filamentous fungi.

CHX14 is a plasma membrane K-efflux transporter that regulates K(+) redistribution in Arabidopsis thaliana

Potassium (K(+) ) is essential for plant growth and development, yet the molecular identity of many K(+) transporters remains elusive. Here we characterized cation/H(+) exchanger (CHX) 14 as a plasma membrane K(+) transporter. CHX14 expression was induced by elevated K(+) and histochemical analysis of CHX14 promoter::GUS transgenic plants indicated that CHX14 was expressed in xylem parenchyma of root and shoot vascular tissues of seedlings. CHX14 knockout (chx14) and CHX14 overexpression seedlings displayed different growth phenotypes during K(+) stress as compared with wild-type seedlings. Roots of mutant seedlings displayed higher K(+) uptake rates than wild-type roots. CHX14 expression in yeast cells deficient in K(+) uptake renders the mutant cells more sensitive to deficiencies of K(+) in the medium. CHX14 mediates K(+) efflux in yeast cells loaded with high K(+) . Uptake experiments using (86) Rb(+) as a tracer for K(+) with both yeast and plant mutants demonstrated that CHX14 expression in yeast and in planta mediated low-affinity K(+) efflux. Functional green fluorescent protein (GFP)-tagged versions of CHX14 were localized to both the yeast and plant plasma membranes. Taken together, we suggest that CHX14 is a plasma membrane K(+) efflux transporter involved in K(+) homeostasis and K(+) recirculation.

Complex interactions among residues within pore region determine the K+ dependence of a KAT1-type potassium channel AmKAT1

KAT1-type channels mediate K(+) influx into guard cells that enables stomatal opening. In this study, a KAT1-type channel AmKAT1 was cloned from the xerophyte Ammopiptanthus mongolicus. In contrast to most KAT1-type channels, its activation is strongly dependent on external K(+) concentration, so it can be used as a model to explore the mechanism for the K(+) -dependent gating of KAT1-type channels. Domain swapping between AmKAT1 and KAT1 reveals that the S5-pore-S6 region controls the K(+) dependence of AmKAT1, and residue substitutions show that multiple residues within the S5-Pore linker and Pore are involved in its K(+) -dependent gating. Importantly, complex interactions occur among these residues, and it is these interactions that determine its K(+) dependence. Finally, we analyzed the potential mechanism for the K(+) dependence of AmKAT1, which could originate from the requirement of K(+) occupancy in the selectivity filter to maintain its conductive conformation. These results provide new insights into the molecular basis of the K(+) -dependent gating of KAT1-type channels.

Voltage-sensor transitions of the inward-rectifying K+ channel KAT1 indicate a latching mechanism biased by hydration within the voltage sensor

The Kv-like (potassium voltage-dependent) K(+) channels at the plasma membrane, including the inward-rectifying KAT1 K(+) channel of Arabidopsis (Arabidopsis thaliana), are important targets for manipulating K(+) homeostasis in plants. Gating modification, especially, has been identified as a promising means by which to engineer plants with improved characteristics in mineral and water use. Understanding plant K(+) channel gating poses several challenges, despite many similarities to that of mammalian Kv and Shaker channel models. We have used site-directed mutagenesis to explore residues that are thought to form two electrostatic countercharge centers on either side of a conserved phenylalanine (Phe) residue within the S2 and S3 α-helices of the voltage sensor domain (VSD) of Kv channels. Consistent with molecular dynamic simulations of KAT1, we show that the voltage dependence of the channel gate is highly sensitive to manipulations affecting these residues. Mutations of the central Phe residue favored the closed KAT1 channel, whereas mutations affecting the countercharge centers favored the open channel. Modeling of the macroscopic current kinetics also highlighted a substantial difference between the two sets of mutations. We interpret these findings in the context of the effects on hydration of amino acid residues within the VSD and with an inherent bias of the VSD, when hydrated around a central Phe residue, to the closed state of the channel.

Clustering of the K+ channel GORK of Arabidopsis parallels its gating by extracellular K+

GORK is the only outward-rectifying Kv-like K(+) channel expressed in guard cells. Its activity is tightly regulated to facilitate K(+) efflux for stomatal closure and is elevated in ABA in parallel with suppression of the activity of the inward-rectifying K(+) channel KAT1. Whereas the population of KAT1 is subject to regulated traffic to and from the plasma membrane, nothing is known about GORK, its distribution and traffic in vivo. We have used transformations with fluorescently-tagged GORK to explore its characteristics in tobacco epidermis and Arabidopsis guard cells. These studies showed that GORK assembles in puncta that reversibly dissociated as a function of the external K(+) concentration. Puncta dissociation parallelled the gating dependence of GORK, the speed of response consistent with the rapidity of channel gating response to changes in the external ionic conditions. Dissociation was also suppressed by the K(+) channel blocker Ba(2+) . By contrast, confocal and protein biochemical analysis failed to uncover substantial exo- and endocytotic traffic of the channel. Gating of GORK is displaced to more positive voltages with external K(+) , a characteristic that ensures the channel facilitates only K(+) efflux regardless of the external cation concentration. GORK conductance is also enhanced by external K(+) above 1 mm. We suggest that GORK clustering in puncta is related to its gating and conductance, and reflects associated conformational changes and (de)stabilisation of the channel protein, possibly as a platform for transmission and coordination of channel gating in response to external K(+) .

Selective mobility and sensitivity to SNAREs is exhibited by the Arabidopsis KAT1 K+ channel at the plasma membrane

Recent findings indicate that proteins in the SNARE superfamily are essential for cell signaling, in addition to facilitating vesicle traffic in plant cell homeostasis, growth, and development. We previously identified SNAREs SYP121/Syr1 from tobacco (Nicotiana tabacum) and the Arabidopsis thaliana homolog SYP121 associated with abscisic acid and drought stress. Disrupting tobacco SYP121 function by expressing a dominant-negative Sp2 fragment had severe effects on growth, development, and traffic to the plasma membrane, and it blocked K(+) and Cl(-) channel responses to abscisic acid in guard cells. These observations raise questions about SNARE control in exocytosis and endocytosis of ion channel proteins and their organization within the plane of the membrane. We have used a dual, in vivo tagging strategy with a photoactivatable green fluorescent protein and externally exposed hemagglutinin epitopes to monitor the distribution and trafficking dynamics of the KAT1 K(+) channel transiently expressed in tobacco leaves. KAT1 is localized to the plasma membrane within positionally stable microdomains of approximately 0.5 microm in diameter; delivery of the K(+) channel, but not of the PMA2 H(+)-ATPase, to the plasma membrane is suppressed by Sp2 fragments of tobacco and Arabidopsis SYP121, and Sp2 expression leads to profound changes in KAT1 distribution and mobility within the plane of the plasma membrane. These results offer direct evidence for SNARE-mediated traffic of the K(+) channel and a role in its distribution within subdomains of the plasma membrane, and they implicate a role for SNAREs in positional anchoring of the K(+) channel protein.

External K+ modulates the activity of the Arabidopsis potassium channel SKOR via an unusual mechanism

Plant outward-rectifying K+ channels mediate K+ efflux from guard cells during stomatal closure and from root cells into the xylem for root-shoot allocation of potassium (K). Intriguingly, the gating of these channels depends on the extracellular K+ concentration, although the ions carrying the current are derived from inside the cell. This K+ dependence confers a sensitivity to the extracellular K+ concentration ([K+]) that ensures that the channels mediate K+ efflux only, regardless of the [K+] prevailing outside. We investigated the mechanism of K+-dependent gating of the K+ channel SKOR of Arabidopsis by site-directed mutagenesis. Mutations affecting the intrinsic K+ dependence of gating were found to cluster in the pore and within the sixth transmembrane helix (S6), identifying an 'S6 gating domain' deep within the membrane. Mapping the SKOR sequence to the crystal structure of the voltage-dependent K+ channel KvAP from Aeropyrum pernix suggested interaction between the S6 gating domain and the base of the pore helix, a prediction supported by mutations at this site. These results offer a unique insight into the molecular basis for a physiologically important K+-sensory process in plants.

Interactive domains between pore loops of the yeast K+ channel TOK1 associate with extracellular K+ sensitivity

Gating of the outward-rectifying K+ channel TOK1 of Saccharomyces cerevisiae is controlled by membrane voltage and extracellular K+ concentration. Previous studies identified two kinetically distinct effects of K+, and site-mutagenic analysis associated these K+-dependencies with domains of the extracellular turrets of the channel protein. We have mapped the TOK1 pore domains to extant K+ channel crystal structures to target additional residues contributing to TOK1 gating. Leu270, located in the first pore domain of TOK1, was found to be critical for gating and its K+ sensitivity. Analysis of amino acid substitutions indicated that spatial position of the polypeptide backbone is a primary factor determining gating sensitivity to K+. The strongest effects, with L270Y, L270F and L270W, led to more than a 30-fold decrease in apparent K+ affinity and an inversion in the apparent K+-dependence of voltage-dependent gating compared with the wild-type current. A partial rescue of wild-type gating was obtained on substitution in the second pore domain with the double mutant L270D/A428Y. These, and additional results, demarcate extracellular domains that are associated with the K+-sensitivity of TOK1 and they offer primary evidence for a synergy in gating between the two pore domains of TOK1, demonstrating an unexpected degree of long-distance interaction across the mouth of the K+ channel.

Regulation of the fast vacuolar channel by cytosolic and vacuolar potassium

At resting cytosolic Ca(2)(+), passive K(+) conductance of a higher plant tonoplast is likely dominated by fast vacuolar (FV) channels. This patch-clamp study describes K(+)-sensing behavior of FV channels in Beta vulgaris taproot vacuoles. Variation of K(+) between 10 and 400 mM had little effect on the FV channel conductance, but a pronounced one on the open probability. Shift of the voltage dependence by cytosolic K(+) could be explained by screening of the negative surface charge with a density sigma = 0.25 e(-)/nm(2). Vacuolar K(+) had a specific effect on the FV channel gating at negative potentials without significant effect on closed-open transitions at positive ones. Due to K(+) effects at either membrane side, the potential at which the FV channel has minimal activity was always situated at approximately 50 mV below the potassium equilibrium potential, E(K(+)). At tonoplast potentials below or equal to E(K(+)), the FV channel open probability was almost independent on the cytosolic K(+) but varied in a proportion to the vacuolar K(+). Therefore, the release of K(+) from the vacuole via FV channels could be controlled by the vacuolar K(+) in a feedback manner; the more K(+) is lost the lower will be the transport rate.

Outer pore residues control the H(+) and K(+) sensitivity of the Arabidopsis potassium channel AKT3

The Arabidopsis phloem channel AKT3 is the founder of a subfamily of shaker-like plant potassium channels characterized by weak rectification, Ca(2+) block, proton inhibition, and, as shown in this study, K(+) sensitivity. In contrast to inward-rectifying, acid-activated K(+) channels of the KAT1 family, extracellular acidification decreases AKT3 currents at the macroscopic and single-channel levels. Here, we show that two distinct sites within the outer mouth of the K(+)-conducting pore provide the molecular basis for the pH sensitivity of this phloem channel. After generation of mutant channels and functional expression in Xenopus oocytes, we identified the His residue His-228, which is proximal to the K(+) selectivity filter (GYGD) and the distal Ser residue Ser-271, to be involved in proton susceptibility. Mutations of these sites, H228D and S271E, drastically reduced the H(+) and K(+) sensitivity of AKT3. Although in K(+)-free bath solutions outward K(+) currents were abolished completely in wild-type AKT3, S271E as well as the AKT3-HDSE double mutant still mediated K(+) efflux. We conclude that the pH- and K(+)-dependent properties of the AKT3 channel involve residues in the outer mouth of the pore. Both properties, H(+) and K(+) sensitivity, allow the fine-tuning of the phloem channel and thus seem to represent important elements in the control of membrane potential and sugar loading.

KCNKØ: opening and closing the 2-P-domain potassium leak channel entails "C-type" gating of the outer pore

Essential to nerve and muscle function, little is known about how potassium leak channels operate. KCNKØ opens and closes in a kinase-dependent fashion. Here, the transition is shown to correspond to changes in the outer aspect of the ion conduction pore. Voltage-gated potassium (VGK) channels open and close via an internal gate; however, they also have an outer pore gate that produces "C-type" inactivation. While KCNKØ does not inactivate, KCNKØ and VGK channels respond in like manner to outer pore blockers, potassium, mutations, and chemical modifiers. Structural relatedness is confirmed: VGK residues that come close during C-type gating predict KCNKØ sites that crosslink (after mutation to cysteine) to yield channels controlled by reduction and oxidization. We conclude that similar outer pore gates mediate KCNKØ opening and closing and VGK channel C-type inactivation despite their divergent structures and physiological roles.

Immunity to K1 killer toxin: internal TOK1 blockade

K1 killer strains of Saccharomyces cerevisiae harbor RNA viruses that mediate secretion of K1, a protein toxin that kills virus-free cells. Recently, external K1 toxin was shown to directly activate TOK1 channels in the plasma membranes of sensitive yeast cells, leading to excess potassium flux and cell death. Here, a mechanism by which killer cells resist their own toxin is shown: internal toxin inhibits TOK1 channels and suppresses activation by external toxin.

Block of Kcnk3 by protons. Evidence that 2-P-domain potassium channel subunits function as homodimers

KCNK subunits have two pore-forming P domains and four predicted transmembrane segments. To assess the number of subunits in each pore, we studied external proton block of Kcnk3, a subunit prominent in rodent heart and brain. Consistent with a pore-blocking mechanism, inhibition was dependent on voltage, potassium concentration, and a histidine in the first P domain (P1H). Thus, at pH 6.8 with 20 mm potassium half the current passed by P1H channels was blocked (apparently via two sites approximately 10% into the electrical field) whereas channels with an asparagine substitution (P1N) were fully active. Furthermore, pore blockade by barium was sensitive to pH in P1H but not P1N channels. Although linking two Kcnk3 subunits in tandem to produce P1H-P1H and P1N-P1N channels bearing four P domains did not alter these attributes, the mixed tandems P1H-P1N and P1N-P1H were half-blocked at pH approximately 6.4, apparently via a single site. This implicates a dimeric structure for Kcnk3 channels with two (and only two) P1 domains in each pore and argues that P2 domains also contribute to pore formation.

KCNK2: reversible conversion of a hippocampal potassium leak into a voltage-dependent channel

Potassium leak channels are essential to neurophysiological function. Leaks suppress excitability through maintenance of resting membrane potential below the threshold for action potential firing. Conversely, voltage-dependent potassium channels permit excitation because they do not interfere with rise to threshold, and they actively promote recovery and rapid re-firing. Previously attributed to distinct transport pathways, we demonstrate here that phosphorylation of single, native hippocampal and cloned KCNK2 potassium channels produces reversible interconversion between leak and voltage-dependent phenotypes. The findings reveal a pathway for dynamic regulation of excitability.

KCNQ potassium channels: physiology, pathophysiology, and pharmacology

KCNQ genes encode a growing family of six transmembrane domains, single pore-loop, K(+) channel alpha-subunits that have a wide range of physiological correlates. KCNQ1 (KvLTQ1) is co-assembled with the product of the KCNE1 (minimal K(+)-channel protein) gene in the heart to form a cardiac-delayed rectifier-like K(+) current. Mutations in this channel can cause one form of inherited long QT syndrome (LQT1), as well as being associated with a form of deafness. KCNQ1 can also co-assemble with KCNE3, and may be the molecular correlate of the cyclic AMP-regulated K(+) current present in colonic crypt cells. KCNQ2 and KCNQ3 heteromultimers are thought to underlie the M-current; mutations in these genes may cause an inherited form of juvenile epilepsy. The KCNQ4 gene is thought to encode the molecular correlate of the I(K,n) in outer hair cells of the cochlea and I(K,L) in Type I hair cells of the vestibular apparatus, mutations in which lead to a form of inherited deafness. The recently identified KCNQ5 gene is expressed in brain and skeletal muscle, and can co-assemble with KCNQ3, suggesting it may also play a role in the M-current heterogeneity. This review will set this family of K(+) channels amongst the other known families. It will highlight the genes, physiology, pharmacology, and pathophysiology of this recently discovered, but important, family of K(+) channels.

Potassium leak channels and the KCNK family of two-P-domain subunits

With a bang, a new family of potassium channels has exploded into view. Although KCNK channels were discovered only five years ago, they already outnumber other channel types. KCNK channels are easy to identify because of their unique structure--they possess two preforming domains in each subunit. The new channels function in a most remarkable fashion: they are highly regulated, potassium-selective leak channels. Although leak currents are fundamental to the function of nerves and muscles, the molecular basis for this type of conductance had been a mystery. Here we review the discovery of KCNK channels, what has been learned about them and what lies ahead. Even though two-P-domain channels are widespread and essential, they were hidden from sight in plain view--our most basic questions remain to be answered.

Extracellular Ba2+ and voltage interact to gate Ca2+ channels at the plasma membrane of stomatal guard cells

Ca2+ channels at the plasma membrane of stomatal guard cells contribute to increases in cytosolic free [Ca2+] ([Ca2+](i)) that regulate K+ and Cl- channels for stomatal closure in higher-plant leaves. Under voltage clamp, the initial rate of increase in [Ca2+](i) in guard cells is sensitive to the extracellular divalent concentration, suggesting a close interaction between the permeant ion and channel gating. To test this idea, we recorded single-channel currents across the Vicia guard cell plasma membrane using Ba2+ as a charge carrying ion. Unlike other Ca2+ channels characterised to date, these channels activate at hyperpolarising voltages. We found that the open probability (P(o)) increased strongly with external Ba2+ concentration, consistent with a 4-fold cooperative action of Ba2+ in which its binding promoted channel opening in the steady state. Dwell time analyses indicated the presence of a single open state and at least three closed states of the channel, and showed that both hyperpolarising voltage and external Ba2+ concentration prolonged channel residence in the open state. Remarkably, increasing Ba2+ concentration also enhanced the sensitivity of the open channel to membrane voltage. We propose that Ba2+ binds at external sites distinct from the permeation pathway and that divalent binding directly influences the voltage gate.

Regulation of ROMK by extracellular cations

The effect of external potassium (K) and cesium (Cs) on the inwardly rectifying K channel ROMK2 (K(ir)1.1b) was studied in Xenopus oocytes. Elevating external K from 1 to 10 mM increased whole-cell outward conductance by a factor of 3.4 +/- 0.4 in 15 min and by a factor of 5.7 +/- 0.9 in 30 min (n = 22). Replacing external Na by Cs blocked inward conductance but increased whole-cell conductance by a factor of 4.5 +/- 0.5 over a period of 40 min (n = 15). In addition to this slow increase in conductance, there was also a small, rapid increase in conductance that occurred as soon as ROMK was exposed to external cesium or 10 mM K. This rapid increase could be explained by the observed increase in ROMK single-channel conductance from 6.4 +/- 0.8 pS to 11.1 +/- 0.8 pS (10 mM K, n = 8) or 11.7 +/- 1.2 pS (Cs, n = 8). There was no effect of either 10 mM K or cesium on the high open probability (P(o) = 0.97 +/- 0.01; n = 12) of ROMK outward currents. In patch-clamp recordings, the number of active channels increased when the K concentration at the outside surface was raised from 1 to 50 mM K. In cell-attached patches, exposure to 50 mM external K produced one or more additional channels in 9/16 patches. No change in channel number was observed in patches continuously exposed to 50 mM external K. Hence, the slow increase in whole-cell conductance is interpreted as activation of pre-existing ROMK channels that had been inactivated by low external K. This type of time-dependent channel activation was not seen with IRK1 (K(ir)2.1) or in ROMK2 mutants in which any one of 6 residues, F129, Q133, E132, V121, L117, or K61, were replaced by their respective IRK1 homologs. These results are consistent with a model in which ROMK can exist in either an activated mode or an inactivated mode. Within the activated mode, individual channels undergo rapid transitions between open and closed states. High (10 mM) external K or Cs stabilizes the activated mode, and low external K stabilizes the inactivated mode. Mutation of a pH-sensing site (ROMK2-K61) prevents transitions from activated to inactivated modes. This is consistent with a direct effect of external K or Cs on the gating of ROMK by internal pH.

Proton block and voltage gating are potassium-dependent in the cardiac leak channel Kcnk3

Potassium leak conductances were recently revealed to exist as independent molecular entities. Here, the genomic structure, cardiac localization, and biophysical properties of a murine example are considered. Kcnk3 subunits have two pore-forming P domains and unique functional attributes. At steady state, Kcnk3 channels behave like open, potassium-selective, transmembrane holes that are inhibited by physiological levels of proton. With voltage steps, Kcnk3 channels open and close in two phases, one appears to be immediate and one is time-dependent (tau = approximately 5 ms). Both proton block and gating are potassium-sensitive; this produces an anomalous increase in outward flux as external potassium levels rise because of decreased proton block. Single Kcnk3 channels open across the physiological voltage range; hence they are "leak" conductances; however, they open only briefly and rarely even after exposure to agents that activate other potassium channels.

Molecular insights into the structure and function of plant K(+) transport mechanisms

Our understanding of plant potassium transport has increased in the past decade through the application of molecular biological techniques. In this review, recent work on inward and outward rectifying K(+) channels as well as high affinity K(+) transporters is described. Through the work on inward rectifying K(+) channels, we now have precise details on how the structure of these proteins determines functional characteristics such as ion conduction, pH sensitivity, selectivity and voltage sensing. The physiological function of inward rectifying K(+) channels in plants has been clarified through the analysis of expression patterns and mutational analysis. Two classes of outward rectifying K(+) channels have now been cloned from plants and their initial characterisation is reviewed. The physiological role of one class of outward rectifying K(+) channel has been demonstrated to be involved in long distance transport of K(+) from roots to shoots. The molecular structure and function of two classes of energised K(+) transporters are also reviewed. The first class is energised by Na(+) and shares structural similarities with K(+) transport mechanisms in bacteria and fungi. Structure-function studies suggest that it should be possible to increase the K(+) and Na(+) selectivity of these transporters, which will enhance the salt tolerance of higher plants. The second class of K(+) transporter is comprised of a large gene family and appears to have a dual affinity for K(+). A suite of molecular techniques, including gene cloning, oocyte expression, RNA localisation and gene inactivation, is now being used to fully characterise the biophysical and physiological function of plants K(+) transport mechanisms.

Ca2+ channels at the plasma membrane of stomatal guard cells are activated by hyperpolarization and abscisic acid

In stomatal guard cells of higher-plant leaves, abscisic acid (ABA) evokes increases in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) by means of Ca(2+) entry from outside and release from intracellular stores. The mechanism(s) for Ca(2+) flux across the plasma membrane is poorly understood. Because [Ca(2+)](i) increases are voltage-sensitive, we suspected a Ca(2+) channel at the guard cell plasma membrane that activates on hyperpolarization and is regulated by ABA. We recorded single-channel currents across the Vicia guard cell plasma membrane using Ba(2+) as a charge-carrying ion. Both cell-attached and excised-patch measurements uncovered single-channel events with a maximum conductance of 12.8 +/- 0.4 pS and a high selectivity for Ba(2+) (and Ca(2+)) over K(+) and Cl(-). Unlike other Ca(2+) channels characterized to date, these channels rectified strongly toward negative voltages with an open probability (P(o)) that increased with [Ba(2+)] outside and decreased roughly 10-fold when [Ca(2+)](i) was raised from 200 nM to 2 microM. Adding 20 microM ABA increased P(o), initially by 63- to 260-fold; in both cell-attached and excised patches, it shifted the voltage sensitivity for channel activation, and evoked damped oscillations in P(o) with periods near 50 s. A similar, but delayed response was observed in 0.1 microM ABA. These results identify a Ca(2+)-selective channel that can account for Ca(2+) influx and increases in [Ca(2+)](i) triggered by voltage and ABA, and they imply a close physical coupling at the plasma membrane between ABA perception and Ca(2+) channel control.

Mutations in the yeast two pore K+ channel YKC1 identify functional differences between the pore domains

The K+ channel of Saccharomyces cerevisiae encoded by the YKC1 gene includes two pore-loop sequences that are thought to form the hydrophilic lining of the pore. Gating of the channel is promoted by membrane depolarisation and is regulated by the extracellular K+ concentration ([K+]o) both in the yeast and when expressed in Xenopus oocytes. Our previous work showed that substitutions of equivalent residues L293 and A428 within the pore-loops had qualitatively similar effects on both the [K+]o-sensitivity of channel gating and its voltage-dependence. Here, we report that mutations of equivalent residues N275 and N410, N-terminal from the K+ channel signature sequences of the two pores, have very different actions on channel gating and, in this case, are without effect on its voltage-sensitivity. The mutation N410D slowed current activation in a [K+]o-dependent manner and it accelerated deactivation, but without significant effect on the apparent affinity for K+. The N275D mutant, by contrast, had little effect on the [K+]o-sensitivity for activation and it greatly altered the. [K+]o-dependence of current deactivation. Neither mutant affected the voltage-dependence of the steady-state current nor the ability for other alkali cations to substitute for K+ in regulating gating. The double mutant N410D-N275D showed characteristics of N410D in the [K+]o-sensitivity of current activation and of N275D in the [K+]o-sensitivity of deactivation, suggesting that little interaction occurs between pore domains with mutations at these sites. The results indicate that the two pore domains are not functionally equivalent and they suggest that the regulation of gating by external K+ is mediated by K+ binding at two physically distinct sites with different actions.