Presynaptic potassium channels

The past year has witnessed some significant improvements in our understanding of the molecular diversity, subunit composition, and functional properties of K+ channels in heterologous expression systems. Immunocytochemical studies have yielded important information on the localization of K+ channel proteins to synaptic terminals in mammalian brain. Although a coherent picture of native presynaptic K+ channels' function in the mammalian central nervous system is not yet available, it may emerge from improvements in patch-clamp techniques and new applications of targeted knock-out technologies.

Mechanism of action of the epileptogenic drug pentylenetetrazol on a cloned neuronal potassium channel

The action of the epileptogenic agent pentylenetetrazol (PTZ) on a cloned potassium channel of the rat brain was studied. The Kv1.1 channel was expressed in oocytes of Xenopus laevis and potassium currents were investigated in outside-out and inside-out membrane patches. The results show that PTZ increased the multi-channel potassium currents at strongly negative potentials and decreased them at potentials positive to -35 mV both in outside-out and inside-out membrane patches. The extent and manner of PTZ action, the concentration dependence as well as the onset and time course of the PTZ effect were the same both in outside-out and inside-out membrane patches. The single-channel potassium currents showed an increase in open probability and frequency of opening and a decrease in close time at -50 mV and vice versa at 0 mV with application of PTZ. The amplitude of single-channel current, the open time and the latency to the first channel opening remained almost unchanged under PTZ. The results indicate that PTZ acts via the cell membrane and influences the membrane-associated part of the potassium channel. Thereby, PTZ accelerates the transition from the inactivated to the open state of the channel at strongly negative potentials and reduces it at slightly negative and positive potentials. This mechanism may be the basis for a gate function which is in favour of the development of epileptic discharges.

Distribution of high-conductance Ca(2+)-activated K+ channels in rat brain: targeting to axons and nerve terminals

Tissue expression and distribution of the high-conductance Ca(2+)-activated K+ channel Slo was investigated in rat brain by immunocytochemistry, in situ hybridization, and radioligand binding using the novel high-affinity (Kd 22 pM) ligand [3H]iberiotoxin-D19C ([3H]IbTX-D19C), which is an analog of the selective maxi-K peptidyl blocker IbTX. A sequence-directed antibody directed against Slo revealed the expression of a 125 kDa polypeptide in rat brain by Western blotting and precipitated the specifically bound [3H]IbTX-D19C in solubilized brain membranes. Slo immunoreactivity was highly concentrated in terminal areas of prominent fiber tracts: the substantia nigra pars reticulata, globus pallidus, olfactory system, interpeduncular nucleus, hippocampal formation including mossy fibers and perforant path terminals, medial forebrain bundle and pyramidal tract, as well as cerebellar Purkinje cells. In situ hybridization indicated high levels of Slo mRNA in the neocortex, olfactory system, habenula, striatum, granule and pyramidal cell layer of the hippocampus, and Purkinje cells. The distribution of Slo protein was confirmed in microdissected brain areas by Western blotting and radioligand-binding studies. The latter studies also established the pharmacological profile of neuronal Slo channels. The expression pattern of Slo is consistent with its targeting into a presynaptic compartment, which implies an important role in neural transmission.

Kv beta 1 subunit binding specific for shaker-related potassium channel alpha subunits

Voltage-activated potassium (Kv) channels from mammalian brain are hetero-oligomers containing alpha and beta subunits. Coexpression of Kv1 alpha and Kv beta 1 subunits confers rapid A-type inactivation on noninactivating potassium channels (delayed rectifiers) in expression systems in vitro. We have delineated a Kv1.5 aminoterminal region of up to 90 amino acids (residues 112-201) that is sufficient for interactions of Kv1.5 alpha and Kv beta 1 subunits. Within this region of the Kv1.5 amino terminus (residues 193-201), a Kv beta 1 interaction site necessary for Kv beta 1-mediated rapid inactivation of Kv1.5 currents was detected. This interaction site motif (FYE/QLGE/DEAM/L) is found exclusively in the Shaker-related subfamily (Kv1). The results show that hetero-oligomerization between alpha and Kv beta 1 subunits is restricted to Shaker-related potassium channel alpha subunits.

Structural and functional characterization of human potassium channel subunit beta 1 (KCNA1B)

Voltage-activated Shaker-related potassium channels (kv1) consist of alpha and beta subunits. We have analysed the structure of the human KCNA1B (hKv beta 1) gene. KCNA1B is > 250 kb in size and encodes at least three Kv beta 1 splice variants. The Kv beta 1 open reading frame is divided into 14 exons. In contrast, genes coding for family members of KCNA (Kv 1 alpha) subunits are markedly smaller and have intronless open reading frames. The expression of Kv 1 alpha and Kv beta mRNA was compared in Northern blots of poly(A+) RNA isolated from various human brain tissues. The results suggest an intricate and cell-specific regulation of Kv 1 alpha and Kv beta mRNA synthesis such that distinct combinations of alpha and beta subunits would occur in different nuclei of the brain. The splice variants hKv beta 1.1 and hKv beta 1.2 were functionally characterized in coexpression studies with hKv 1.5 alpha subunits in 293 cells. It is shown that the confer rapid inactivation on hKv 1.5 channels with different potencies. This may be due to differences in their amino terminal sequences and/or inactivating domains. It is also shown that the amino terminal Kv beta 1.1 and Kv 1.4 alpha inactivating domains compete with each other, probably for the binding to the same receptor site(s) on Kv 1 alpha-subunits.

Molecular and functional characterization of a rat brain Kv beta 3 potassium channel subunit

A novel potassium channel beta-subunit (Kv beta 3) was cloned from rat brain being the third member of a Kv beta subunit gene family. It is a protein of 403 amino acid residues with a 68% amino acid sequence homology to Kv beta 1.1. Kv beta 3 is primarily expressed in rat brain having a distribution distinct to those of Kv beta 1.1 and Kv beta 2. This subunit also has a long N-terminal structure and induces inactivation in N-terminal deleted Kv1.4 but not in other members of the Kv1 channel family. Similarly to Kv beta 1.1, the Kv beta 3-induced inactivation is regulated by the intracellular redox potential.

Immunohistochemical localization of five members of the Kv1 channel subunits: contrasting subcellular locations and neuron-specific co-localizations in rat brain

A large variety of potassium channels is involved in regulating integration and transmission of electrical signals in the nervous system. Different types of neurons, therefore, require specific patterns of potassium channel subunits expression and specific regulation of subunit coassembly into heteromultimeric channels, as well as subunit-specific sorting and segregation. This was investigated by studying in detail the expression of six different alpha-subunits of voltage-gated potassium channels in the rat hippocampus, cerebellum, olfactory bulb and spinal cord, combining in situ hybridization and immunocytochemistry. Specific polyclonal antibodies were prepared for five alpha-subunits (Kv1.1, Kv1.2, Kv1.3 Kv1.4, Kv1.6) of the Shaker-related subfamily of rat Kv channels, which encode delayed-rectifier type and rapidly inactivating A-type potassium channels. Their distribution was compared to that of an A-type potassium channel (Kv3.4), belonging to the Shaw-related subfamily of rat Kv channels. Our results show that these Kv channel alpha-subunits are differentially expressed in rat brain neurons. We did not observe in various neurons a stereotypical distribution of Kv channel alpha-subunits to dendritic and axonal compartments, but a complex differential subcellular subunit distribution. The different Kv channel subunits are targeted either to presynaptic or to postsynaptic domains, depending on neuronal cell type. Thus, distinct combinations of Kv1 alpha-subunits are co-localized in different neurons. The implications of these findings are that both differential expression and assembly as well as subcellular targeting of Kv channel alpha-subunits may contribute to Kv channel diversity and thereby to presynaptic and postsynaptic membrane excitability.

A vector for the synthesis of cRNAs encoding Myc epitope-tagged proteins in Xenopus laevis oocytes

We describe a plasmid, pNKS2-myc, designed for convenient in-frame fusion of an antibody-specific epitope sequence to the N terminus of a desired cDNA and subsequent synthesis of transcripts that direct the synthesis of the tagged polypeptide in Xenopus laevis (Xl) oocytes. pNKS2-myc contains an SP6 promoter, followed by the translation initiation sequence of the Na,K-pump beta 3 subunit of Xl and the sequence encoding an epitope derived from the human c-myc proto-oncogene product. Appropriate restriction sites allow one to insert virtually any desired cDNA fragment directly behind the epitope-specific sequence and before a long poly(A) tail. After linearization with EcoRI or NotI, polyadenylated cRNA can be synthesized that is efficiently translated in Xl oocytes. The utility of pNKS2-myc is demonstrated by cloning cDNAs coding for Na,K-pump subunits into this vector and injecting the corresponding cRNAs into oocytes. The tagged mouse beta 1 and beta 2 subunit isoforms could be purified from detergent extracts of these cells by immunoprecipitation with a generally available monoclonal antibody (mAb) to the tag, 9E10, as well as with specific mAb that recognize individual beta subunit isoforms. Under native conditions, endogenous and coexpressed exogenous alpha 1 subunits (the catalytic subunit of the Na,K-pump) were co-precipitated, indicating that the N-terminal addition of the decapeptide epitope has no adverse effect on the folding of beta subunits nor on their assembly with alpha subunits. Furthermore, the Myc-specific mAb likewise precipitated a Myc-tagged Na,K-pump alpha 1 subunit together with any of the co-synthesized beta subunits.

Effects of lead on cloned voltage-operated neuronal potassium channels

The action of lead (Pb2+) on cloned voltage-operated potassium channels of the rat brain was investigated in oocytes of Xenopus laevis. Pb2+ was found to decrease the potassium currents. This effect was due to a shift of the current-voltage relation in a positive direction (up to 30 mV). The Pb2+ effect appeared at a threshold concentration of about 0.1 mumol/l and was maximal at a concentration of about 30 mumol/l. At a potential of -30 mV, the concentration needed for a 50% reduction of the potassium current was 1.0 mumol/l. The depressant effect of Pb2+ was obtained with all potassium channels tested (Kv1.1, Kv1.2, Kv1.4, Kv2.1, Kv3.4). It was minimal for the Kv2.1 channel and maximal for the Kv1.1 channel at potentials negative to 0 mV. An effect comparable with that of Pb2+ could not be induced by the application of magnesium or calcium. The external application of Pb2+ led to a decrease of potassium currents in outside-out but not in inside-out membrane patches. Overall, Pb2+ had a significant effect on the potassium channels which may contribute to the mechanisms of Pb2+ neurotoxicity.

minibrain: a new protein kinase family involved in postembryonic neurogenesis in Drosophila

The development of the adult central nervous system of Drosophila requires a precise and reproducible pattern of neuroblast proliferation during postembryonic neurogenesis. We show here that mutations in the minibrain (mnb) gene cause an abnormal spacing of neuroblasts in the outer proliferation center (opc) of larval brain, with the implication that mnb opc neuroblasts produce less neuronal progeny than do wild type. As a consequence, the adult mnb brain exhibits a specific and marked size reduction of the optic lobes and central brain hemispheres. The insufficient number of distinct neurons in mnb brains is correlated with specific abnormalities in visual and olfactory behavior. The mnb gene encodes a novel, cell type-specific serine-threonine protein kinase family that is expressed and required in distinct neuroblast proliferation centers during postembryonic neurogenesis. The mnb kinases share extensive sequence similarities with kinases involved in the regulation of cell division.

Physical and genetic localization of a Shab subfamily potassium channel (KCNB1) gene to chromosomal region 20q13.2

A human delayed rectifier K+ channel gene has been localized to the long arm of human chromosome 20q13.2 by fluorescence in situ hybridization of genomic P1 clones from this locus. A polymorphic (GA) microsatellite repeat was identified in one of the P1 clones. The new SSR marker (D20S436) was genotyped in four CEPH pedigrees. Two-point linkage analysis indicated linkage of this marker to a PCR marker, D20S109, with a maximum lod score of 9.32 at theta = 0.001. The assignment of this K+ channel gene to 20q13.2 eliminates it as a candidate for the gene associated with benign familial neonatal convulsions (BFNC), which has been localized to 20q13.3. Genetically, the K+ channel gene maps more than 30 cM proximal to the BFNC locus.

Functional expression of a rat homologue of the voltage gated either á go-go potassium channel reveals differences in selectivity and activation kinetics between the Drosophila channel and its mammalian counterpart

We have cloned a mammalian (rat) homologue of Drosophila ether á go-go (eag) cDNA, which encodes a distinct type of voltage activated potassium (K) channel. The derived Drosophila and rat eag polypeptides share > 670 amino acids, with a sequence identity of 61%, exhibiting a high degree of similarity at the N-terminus, the hydrophobic core including the pore forming P region and a potential cyclic nucleotide binding site. Rat eag mRNA is specifically expressed in the central nervous system. In the Xenopus oocyte expression system rat eag mRNA gives rise to voltage activated K channels which have distinct properties in comparison with Drosophila eag channels and other voltage activated K channels. Thus, the rat eag channel further extends the known diversity of K channels. Most notably, the kinetics of rat eag channel activation depend strongly on holding membrane potential. Hyperpolarization slows down the kinetics of activation; conversely depolarization accelerates the kinetics of activation. This novel K channel property may have important implications in neural signal transduction allowing neurons to tune their repolarizing properties in response to membrane hyperpolarization.

Potassium currents in epilepsy: effects of the epileptogenic agent pentylenetetrazol on a cloned potassium channel

The effect of the epileptogenic agent pentylenetetrazol (PTZ) on the cloned rat brain potassium channel Kv1.1 (labelled also RCK1) was investigated in the Xenopus laevis oocyte expression system. The Kv1.1 channel was affected by PTZ in a voltage-dependent manner. PTZ increased the potassium currents at more negative potentials and decreased them at more positive potentials. At a potential of -50 mV the potassium currents were increased by 0.97 and at -20 mV decreased by 0.21 of control value with 100 mmol/l PTZ. The potential at which the inversion from increase to decrease occurred was -33 mV. The inactivation characteristic of the current was shifted to more negative potentials by PTZ. The PTZ effect was obtained at a threshold concentration of 1 mmol/l and increased with rising PTZ concentrations. After removal of the tissues covering the oocyte membrane, the PTZ effect was augmented; with a concentration of 10 mmol/l PTZ the potassium currents at 0 mV were decreased by 0.04 in oocytes with covering tissues and by 0.27 of control value in oocytes without covering tissues. Under current-clamp conditions, PTZ decreased small depolarizations and increased larger depolarizations. This effect of PTZ represents a 'discriminatory function' that may contribute to epileptogenesis in nervous tissues.

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

Structural and functional diversity of voltage-gated Kv1-type potassium channels in rat brain is enhanced by the association of two different types of subunits, the membrane-bound, poreforming alpha-subunits and a peripheral beta-subunit. We have cloned a beta-subunit (Kv beta 1) that is specifically expressed in the rat nervous system. Association of Kv beta 1 with alpha-subunits confers rapid A-type inactivation on non-inactivating Kv1 channels (delayed rectifiers) in expression systems in vitro. This effect is mediated by an inactivating ball domain in the Kv beta 1 amino terminus.

Gating and conductance properties of a human delayed rectifier K+ channel expressed in frog oocytes

1. The human delayed rectifier K+ channel h-DRK1, a homologue to the DRK1 channel in the rat, was expressed in Xenopus oocytes. Single-channel currents were measured in micropatches; macroscopic currents were measured either in macropatches, giant patches, or whole oocytes. 2. Macroscopic currents activated at -20 mV and more positive. The instantaneous current-voltage relationship rectified outwardly to a higher degree than predicted by the Goldman-Hodgkin-Katz equation. 3. With the giant patch technique, ionic and putative on- and off-gating currents were recorded simultaneously. The large ratio of the moved gating charges to the amplitude of the ionic current indicated that less than 1% of the gating channels actually opened. 4. The single-channel conductance between 0 and +80 mV was calculated to be 9.4 pS. The channels opened with sublevels which appeared either independently of the fully open level as separate openings, in conjunction with the opening to and closing from the fully open level, or by starting from and ending at the fully open level. 5. The channels opened with two voltage-independent open time constants in the range 1-10 ms (filter 1 kHz). The burst open probability was fitted monoexponentially with time constants in the range of tens of milliseconds. 6. Assuming a sequential Markovian model with four independent voltage-controlled transitions, fit of the steady-state open probability of macroscopic currents showed two components of activation differing in their half-maximal value. 7. The fit of time courses of cumulative first latency and ensemble-averaged currents in single-channel patches suggested that even a single channel may operate with the two different components of activation. 8. It is concluded that h-DRK1 channels considerably rectify in an outward direction and that an apparently flat voltage dependence of activation may be explained by the overlap of two different components.

Primary structure of a beta subunit of alpha-dendrotoxin-sensitive K+ channels from bovine brain

Voltage-dependent cation channels are large heterooligomeric proteins. Heterologous expression of cDNAs encoding the alpha subunits alone of K+, Na+, or Ca2+ channels produces functional multimeric proteins; however, coexpression of those for the latter two with their auxiliary proteins causes dramatic changes in the resultant membrane currents. Fast-activating, voltage-sensitive K+ channels from brain contain four alpha and beta subunits, tightly associated in a 400-kDa complex; although molecular details of the alpha-subunit proteins have been determined, little is known about the beta-subunit constituent. Proteolytic fragments of a beta subunit from bovine alpha-dendrotoxin-sensitive neuronal K+ channels yielded nine different sequences. In the polymerase chain reaction, primers corresponding to two of these peptides amplified a 329-base-pair fragment in a lambda gt10 cDNA library from bovine brain; a full-length clone subsequently isolated encodes a protein of 367 amino acids (M(r) approximately 40,983). It shows no significant homology with any known protein. Unlike the channels' alpha subunits, the hydropathy profile of this sequence failed to reveal transmembrane domains. Several consensus phosphorylation motifs are apparent and, accordingly, the beta subunit could be phosphorylated in the intact K+ channels. These results, including the absence of a leader sequence and N-glycosylation, are consistent with the beta subunit being firmly associated on the inside of the membrane with alpha subunits, as speculated in a simplified model of these authentic K+ channels. Importantly, this first primary structure of a K(+)-channel beta subunit indicates that none of the cloned auxiliary proteins of voltage-dependent cation channels, unlike their alpha subunits, belong to a super-family of genes.

Antibodies specific for distinct Kv subunits unveil a heterooligomeric basis for subtypes of alpha-dendrotoxin-sensitive K+ channels in bovine brain

The authentic subunit compositions of neuronal K+ channels purified from bovine brain were analyzed using a monoclonal antibody (mAb 5), reactive exclusively with the Kv1.2 subunit of the latter and polyclonal antibodies specific for fusion proteins containing C-terminal regions of four mammalian Kv proteins. Western blotting of the K+ channels isolated from several brain regions, employing the selective blocker alpha-dendrotoxin (alpha-DTX), revealed the presence in each of four different Kvs. Variable amounts of Kv1.1 and 1.4 subunits were observed in the K+ channels purified from cerebellum, corpus striatum, hippocampus, cerebral cortex, and brain stem; on the other hand, contents of Kv1.6 and 1.2 subunits appeared uniform throughout. Each Kv-specific antibody precipitated a different proportion (anti-Kv1.2 > 1.1 >> 1.6 > 1.4) of the channels detectable with radioiodinated alpha-DTX in every brain region, consistent with a widespread distribution of these oligomeric subtypes. Such heterooligomeric combinations were further documented by the lack of additivity upon their precipitation with a mixture of antibodies to Kv1.1 and Kv1.2; moreover, cross-blotting of the multimers precipitated by mAb 5 showed that they contain all four Kv proteins. Collectively, these findings demonstrate that subtypes of alpha-DTX-susceptible K+ channels are prevalent throughout mammalian brain which are composed of different Kv proteins assembled in complexes, shown previously to also contain auxiliary beta-subunits [Parcej, D. N., Scott, V. E. S., & Dolly, J.O. (1992) Biochemistry 31, 11084-11088].

Implication of frequenin in the facilitation of transmitter release in Drosophila

1. We have investigated the possible role of frequenin in the modulation of synaptic facilitation at the larval Drosophila neuromuscular junctions. Excitatory junctional currents (EJCs) and presynaptic nerve terminal currents were recorded by external electrodes in normal larvae and in transgenic larvae carrying an extra insertion of the frequenin cDNA. 2. Motor nerve stimulation by twin pulses or trains of stimuli provoked EJC facilitation which was about three times higher in transgenic larvae compared to controls. Unconditioned EJCs revealed, however, similar quantal content and Ca2+ sensitivity in both Drosophila strains. 3. Differences between normal and transgenic Drosophila in the quantal content of the facilitated EJC do not depend on differences in the duration of the repolarization phase of the presynaptic action potential. 4. Perfusion of tetrodotoxin or of low-Na+ solutions abolished the enhancement of the EJC facilitation observed in the transformants. These treatments only slightly affected the facilitation of normal junctions. 5. These results suggest that (i) internal Na+ accumulation can enhance facilitation of transmitter release in Drosophila neuromuscular junctions overexpressing frequenin, and (ii) this effect possibly depends on a modulation of the activity of the Na(+)-Ca2+ exchanger by frequenin.

The inactivation behaviour of voltage-gated K-channels may be determined by association of alpha- and beta-subunits

Voltage-gated K-channels of the Shaker related subfamily have two subunits, membrane integrated alpha- and peripheral beta-subunits. alpha-Subunits may assemble as tetramers and form in in vitro expression systems functional K-channels. beta-Subunits cannot from channels by themselves. Like for alpha-subunits, the rat nervous system apparently expresses a family of beta-subunit proteins. We have demonstrated that one rat K-channel beta-subunit, Kv beta 1, contains an inactivating domain. Upon association of alpha- and Kv beta 1-subunits, delayed-rectifier type K-channels are converted to rapidly inactivating A-type K-channels. The beta-subunit inactivation domain acts via a ball and chain type mechanism previously proposed for N-type inactivation of alpha-subunits. The association of alpha- and beta-subunits endows the nervous system with an unprecedented flexibility and diversity of K-channels which may play an important role in the regulation of nervous excitability.

Chromosomal mapping in the mouse of eight K(+)-channel genes representing the four Shaker-like subfamilies Shaker, Shab, Shaw, and Shal

The four Shaker-like subfamilies of Shaker-, Shab-, Shaw-, and Shal-related K+ channels in mammals have been defined on the basis of their sequence homologies to the corresponding Drosophila genes. Using interspecific backcrosses between Mus musculus and Mus spretus, we have chromosomally mapped in the mouse the Shaker-related K(+)-channel genes Kcna1, Kcna2, Kcna4, Kcna5, and Kcna6; the Shab-related gene Kcnb1; the Shaw-related gene Kcnc4; and the Shal-related gene Kcnd2. The following localizations were determined: Chr 2, cen-Acra-Kcna4-Pax-6-a-Pck-1-Kras-3-Kcn b1 (corresponding human Chrs 11p and 20q, respectively); Chr 3, cen-Hao-2-(Kcna2, Kcnc4)-Amy-1 (human Chr 1); and Chr 6, cen-Cola-2-Met-Kcnd2-Cpa-Tcrb-adr/Clc-1-Hox-1.1-Myk - 103-Raf-1-(Tpi-1, Kcna1, Kcna5, Kcna6) (human Chrs 7q and 12p, respectively). Thus, there is a cluster of at least three Shaker-related K(+)-channel genes on distal mouse Chr 6 and a cluster on Chr 2 that at least consists of one Shaker-related and one Shaw-related gene. The three other K(+)-channel genes are not linked to each other. The map positions of the different types of K(+)-channel genes in the mouse are discussed in relation to those of their homologs in man and to hereditary diseases of mouse and man that might involve K+ channels.

A site accessible to extracellular TEA+ and K+ influences intracellular Mg2+ block of cloned potassium channels

The members of the RCK family of cloned voltage-dependent K+ channels are quite homologous in primary structure, but they are highly diverse in functional properties. RCK4 channels differ from RCK1 and RCK2 channels in inactivation and permeation properties, the sensitivity to external TEA, and to current modulation by external K+ ions. Here we show several other interesting differences: While RCK1 and RCK2 are blocked in a voltage and concentration dependent manner by internal Mg2+ ions, RCK4 is only weakly blocked at very high potentials. The single-channel current-voltage relations of RCK4 are rather linear while RCK2 exhibits an inwardly rectifying single-channel current in symmetrical K+ solutions. The deactivation of the channels, measured by tail current protocols, is faster in RCK4 by a factor of two compared with RCK2. In a search for the structural motif responsible for these differences, point mutants creating homology between RCK2 and RCK4 in the pore region were tested. The single-point mutant K533Y in the background of RCK4 conferred the properties of Mg2+ block, tail current kinetics, and inward ion permeation of RCK2 to RCK4. This mutant was previously shown to be responsible for the alterations in external TEA sensitivity and channel regulation by external K+ ions. Thus, this residue is expected to be located at the external side of the pore entrance. The data are consistent with the idea that the mutation alters the channel occupancy by K+ and thereby indirectly affects internal Mg2+ block and channel closing.