Images of purified Shaker potassium channels

Bladder hyperactivity and increased excitability of bladder afferent neurons associated with reduced expression of Kv1.4 alpha-subunit in rats with cystitis

Hyperexcitability of C-fiber bladder afferent pathways has been proposed to contribute to urinary frequency and bladder pain in chronic bladder inflammation including interstitial cystitis. However, the detailed mechanisms inducing afferent hyperexcitability after bladder inflammation are not fully understood. Thus, we investigated changes in the properties of bladder afferent neurons in rats with bladder inflammation induced by intravesical application of hydrochloric acid. Eight days after the treatment, bladder function and bladder sensation were analyzed using cystometry and an electrodiagnostic device of sensory function (Neurometer), respectively. Whole cell patch-clamp recordings and immunohistochemical staining were also performed in dissociated bladder afferent neurons identified by a retrograde tracing dye, Fast Blue, injected into the bladder wall. Cystitis rats showed urinary frequency that was inhibited by pretreatment with capsaicin and bladder hyperalgesia mediated by C-fibers. Capsaicin-sensitive bladder afferent neurons from sham rats exhibited high thresholds for spike activation and a phasic firing pattern, whereas those from cystitis rats showed lower thresholds for spike activation and a tonic firing pattern. Transient A-type K(+) current density in capsaicin-sensitive bladder afferent neurons was significantly smaller in cystitis rats than in sham rats, although sustained delayed-rectifier K(+) current density was not altered after cystitis. The expression of voltage-gated K(+) Kv1.4 alpha-subunits, which can form A-type K(+) channels, was reduced in bladder afferent neurons from cystitis rats. These data suggest that bladder inflammation increases bladder afferent neuron excitability by decreasing expression of Kv1.4 alpha-subunits. Similar changes in capsaicin-sensitive C-fiber afferent terminals may contribute to bladder hyperactivity and hyperalgesia due to acid-induced bladder inflammation.

Genetic isolation of transport signals directing cell surface expression

Membrane proteins represent approximately 30% of the proteome in both prokaryotes and eukaryotes. The spatial localization of membrane-bound proteins is often determined by specific sequence motifs that may be regulated in response to physiological changes, such as protein interactions and receptor signalling. Identification of signalling motifs is therefore important for understanding membrane protein expression, function and transport mechanisms. We report a genetic isolation of novel motifs that confer surface expression. Further characterization showed that SWTY, one class of these isolated motifs with homology to previously reported forward transport motifs, has the ability to both override the RKR endoplasmic reticulum localization signal and potentiate steady-state surface expression. The genetically isolated SWTY motif is functionally interchangeable with a known motif in cardiac potassium channels and an identified motif in an HIV coreceptor, and operates by recruiting 14-3-3 proteins. This study expands the repertoire of and enables a screening method for membrane trafficking signals.

Fluorescence measurements reveal stoichiometry of K+ channels formed by modulatory and delayed rectifier alpha-subunits

Modulatory alpha-subunits, which comprise one-fourth of all voltagegated K(+) channel (Kv) alpha-subunits, do not assemble into homomeric channels, but selectively associate with delayed rectifier Kv2 subunits to form heteromeric channels of unknown stoichiometry. Their distinct expression patterns and unique functional properties have made these channels candidate molecular correlates for a broad set of native K(+) currents. Here, we combine FRET and electrophysiological measurements to determine the stoichiometry and geometry of heteromeric channels composed of the delayed rectifier Kv2.1 subunit and the modulatory Kv9.3 alpha-subunit. Kv channel alpha-subunits were fused with GFP variants, and heteromerization of different combinations of tagged and untagged alpha-subunits was studied. FRET, evaluated by acceptor photobleaching, was only observed upon formation of functional channels. Our results, obtained from two independent experimental paradigms, suggest the formation of heteromeric Kv2.1/Kv9.3 channels of fixed stoichiometry consisting of three Kv2.1 subunits and one Kv9.3 subunit. Strikingly, despite this uneven stoichiometry, we find that heteromeric Kv2.1/Kv9.3 channels maintain a pseudosymmetric arrangement of subunits around the central pore.

Homo- and heteromeric assembly of TRPV channel subunits

The vanilloid receptor-related TRP channels (TRPV1-6) mediate thermosensation, pain perception and epithelial Ca(2+) entry. As the specificity of TRPV channel heteromerization and determinants governing the assembly of TRPV subunits were largely elusive, we investigated the TRPV homo- and heteromultimerization. To analyze the assembly of TRPV subunits in living cells, we generated fluorescent fusion proteins or FLAG-tagged TRPV channel subunits. The interaction between TRPV subunits was assessed by analysis of the subcellular colocalization, fluorescence resonance energy transfer and coimmunoprecipitation. Our results demonstrate that TRPV channel subunits do not combine arbitrarily. With the exception of TRPV5 and TRPV6, TRPV channel subunits preferentially assemble into homomeric complexes. Truncation of TRPV1, expression of cytosolic termini of TRPV1 or TRPV4 and construction of chimeric TRPV channel subunits revealed that the specificity and the affinity of the subunit interaction is synergistically provided by interaction modules located in the transmembrane domains and in the cytosolic termini. The relative contribution of intramolecularly linked interaction modules presumably controls the overall affinity and the specificity of TRPV channel assembly.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

Structure of cation channels, revealed by single particle electron microscopy

A large barrier in the way to obtaining high-resolution structures of eukaryotic ion channels remains the expression and purification of sufficient amounts of channel protein to carry out crystallization trials. In the absence of crystals, the main available source of structural information has been electron microscopy (EM), which is well suited to the visualization of isolated macromolecular complexes and their conformational changes. The recently published EM structures outline native conformations of eukaryotic cation channels that until now have eluded crystallization. According to these results, homo-tetrameric K(+) channels have a distinct two-layer architecture with connectors conjoining the two layers, while the pseudo-tetrameric Ca(2+) or Na(+) channels are more globular and have flexible surface loops, which makes the identification of subunits complicated. Subunits can be identified using atomic structure docking into the EM maps, labeling, or deletion studies.

Regulation of K+ channel activities in plants: from physiological to molecular aspects

Plant voltage-gated channels belonging to the Shaker family participate in sustained K+ transport processes at the cell and whole plant levels, such as K+ uptake from the soil solution, long-distance K+ transport in the xylem and phloem, and K+ fluxes in guard cells during stomatal movements. The attention here is focused on the regulation of these transport systems by protein-protein interactions. Clues to the identity of the regulatory mechanisms have been provided by electrophysiological approaches in planta or in heterologous systems, and through analogies with their animal counterparts. It has been shown that, like their animal homologues, plant voltage-gated channels can assemble as homo- or heterotetramers associating polypeptides encoded by different Shaker genes, and that they can bind auxiliary subunits homologous to those identified in mammals. Furthermore, several regulatory processes (involving, for example, protein kinases and phosphatases, G proteins, 14-3-3s, or syntaxins) might be common to plant and animal Shakers. However, the molecular identification of plant channel partners is still at its beginning. This paper reviews current knowledge on plant K+ channel regulation at the physiological and molecular levels, in the light of the corresponding knowledge in animal cells, and discusses perspectives for the deciphering of regulatory networks in the future.

Structural characterisation of neuronal voltage-sensitive K+ channels heterologously expressed in Pichia pastoris

Neuronal voltage-dependent K(+) channels of the delayed rectifier type consist of multiple Kv alpha subunit variants, which assemble as hetero- or homotetramers, together with four Kv beta auxiliary subunits. Direct structural information on these proteins has not been forthcoming due to the difficulty in isolating the native K(+) channels. We have overexpressed the subunit genes in the yeast Pichia pastoris. The Kv1.2 subunit expressed alone is shown to fold into a native conformation as determined by high-affinity binding of 125I-labelled alpha-dendrotoxin, while co-expressed Kv1.2 and Kv beta 2 subunits co-assembled to form native-like oligomers. Sites of post-translational modifications causing apparent heterogeneity on SDS-PAGE were identified by site-directed mutagenesis. Engineering to include affinity tags and scale-up of production by fermentation allowed routine purification of milligram quantities of homo- and heteroligomeric channels. Single-particle electron microscopy of the purified channels was used to generate a 3D volume to 2.1 nm resolution. Protein domains were assigned by fitting crystal structures of related bacterial proteins. Addition of exogenous lipid followed by detergent dialysis produced well-ordered 2D crystals that exhibited mostly p12(1) symmetry. Projection maps of negatively stained crystals show the constituent molecules to be 4-fold symmetric, as expected for the octameric K(+) channel complex.

Redox-sensitive extracellular gates formed by auxiliary beta subunits of calcium-activated potassium channels

An important step to understanding ion channels is identifying the structural components that act as the gates to ion movement. Here we describe a new channel gating mechanism, produced by the beta3 auxiliary subunits of Ca2+-activated, large-conductance BK-type K+ channels when expressed with their pore-forming alpha subunits. BK beta subunits have a cysteine-rich extracellular segment connecting two transmembrane segments, with small cytosolic N and C termini. The extracellular segments of the beta3 subunits form gates to block ion permeation, providing a mechanism by which current can be rapidly diminished upon cellular repolarization. Furthermore, this gating mechanism is abolished by reduction of extracellular disulfide linkages, suggesting that endogenous mechanisms may regulate this gating behavior. The results indicate that auxiliary beta subunits of BK channels reside sufficiently close to the ion permeation pathway defined by the alpha subunits to influence or block access of small molecules to the permeation pathway.

Voltage-gated K+ channel from mammalian brain: 3D structure at 18A of the complete (alpha)4(beta)4 complex

Voltage-sensitive K(+) channels (Kv) serve numerous important roles, e.g. in the control of neuron excitability and the patterns of synaptic activity. Here, we use electron microscopy (EM) and single particle analysis to obtain the first, complete structure of Kv1 channels, purified from rat brain, which contain four transmembrane channel-forming alpha-subunits and four cytoplasmically-associated beta-subunits. The 18A resolution structure reveals an asymmetric, dumb-bell-shaped complex with 4-fold symmetry, a length of 140A and variable width. By fitting published X-ray data for recombinant components to our EM map, the modulatory (beta)(4) was assigned to the innermost 105A end, the N-terminal (T1)(4) domain of the alpha-subunit to the central 50A moiety and the pore-containing portion to the 125A membrane part. At this resolution, the selectivity filter could not be localised. Direct contact of the membrane component with the central (T1)(4) domain occurs only via peripheral connectors, permitting communication between the channel and beta-subunits for coupling of responses to changes in excitability and metabolic status of neurons.

Calnexin co-expression and the use of weaker promoters increase the expression of correctly assembled Shaker potassium channel in insect cells

Voltage-gated potassium channels control the membrane potential of excitable cells. To understand their function, knowledge of their structure is essential. However, these channels are scarce in natural sources, and overexpression is necessary to generate material for structural studies. We have compared functional expression of the Drosophila Shaker H4 potassium channel in stable insect cell lines and in baculovirus-infected insect cells, using three different baculovirus promoters. Stable insect cell lines expressed correctly assembled channel, which was glycosylated and found predominantly at, or close to, the cell surface. In comparison, the majority of baculovirus-overexpressed Shaker was intracellular and incorrectly assembled. The proportion of functional Shaker increased, however, if the weaker basic protein promoter was used rather than the stronger p10 or polyhedrin promoters. In addition, co-expression of the molecular chaperone, calnexin, increased the quantity of correctly assembled channel protein, suggesting that calnexin can be used to increase the efficiency of channel expression in insect cells.

Expression, solubilization, and biochemical characterization of the tight junction transmembrane protein claudin-4

The tight junction tetraspan protein claudin-4 creates a charge-selective pore in the paracellular pathway across epithelia. The structure of the pore is unknown, but is presumed to result from transcellular adhesive contacts between claudin's extracellular loops. Here we report the expression of claudin-4 by baculovirus infection of Sf9 cells and describe the biochemical analysis suggesting it has a hexameric quaternary configuration. We show the detergent perfluoro-octanoic acid is able to maintain oligomeric claudin species. Sucrose velocity centrifugation and laser light scattering are also used to investigate the oligomeric state of claudin-4. In contrast to proteins of similar topology, such as gap junction family connexins, the oligomeric state of claudins appears more dynamic. These data suggest the structural organization of claudins in tight junction pores is unique.

Membrane stretch accelerates activation and slow inactivation in Shaker channels with S3-S4 linker deletions

At low P(open)(V) Shaker exhibits pronounced stretch-activation. Possible explanations for Shaker's sensitivity to tension include 1) Shaker channels are sufficiently distensible that stretch produces novel channel states and 2) Shaker channels expand in the plane of the membrane during voltage gating. For channels expressed in oocytes, we compared effects of patch stretch on Shaker and mutants that retain their voltage-gating ability but activate sluggishly because all or most of the S3-S4 linker has been deleted. Deletants had 10, 5, or 0 amino acid (aa) linkers, whereas wild-type is 31 aa. In deletants, though activation is exceptionally slow, slow inactivation is exceptionally quick; the resulting kinetic match was a bonus that allowed effects of stretch to be followed simultaneously in both processes. With the intact linker, an approximately 3 orders of magnitude mismatch in the two processes makes this impracticable. Standard stretch stimuli increased the rates and extent of activation by about the same degree in wild type and deletants, with effects especially pronounced near the foot of G(V). In deletants (where slow inactivation is strongly coupled to activation) stretch also accelerated slow inactivation. Maximum conductances were unaffected by stretch in all variants. In ramp clamp dose experiments, near-lytic patch stretch acted, for all variants, like a approximately 10 mV hyperpolarizing shift. These results suggested that, whether basal rates were high (wild type) or low (deletants), stretch acted by facilitating voltage-dependent activation. Channel activity was therefore simulated with/without "tension," tension being simulated via rate changes at voltage-dependent closed-closed transitions that might involve in-plane expansion (explanation 2). Simulated Delta P(open) arising from approximately 2 kT of "mechanical gating energy" mimicked experimental effects seen with comfortably sub-lytic stretch.

Loss of the potassium channel beta-subunit gene, KCNAB2, is associated with epilepsy in patients with 1p36 deletion syndrome

PURPOSE

Clinical features associated with chromosome 1p36 deletion include characteristic craniofacial abnormalities, mental retardation, and epilepsy. The presence and severity of specific phenotypic features are likely to be correlated with loss of a distinct complement of genes in each patient. We hypothesize that hemizygous deletion of one, or a few, critical gene(s) controlling neuronal excitability is associated with the epilepsy phenotype. Because ion channels are important determinants of seizure susceptibility and the voltage-gated K(+) channel beta-subunit gene, KCNAB2, has been localized to 1p36, we propose that deletion of this gene may be associated with the epilepsy phenotype.

METHODS

Twenty-four patients were evaluated by fluorescence in situ hybridization with a probe containing KCNAB2. Clinical details were obtained by neurologic examination and EEG.

RESULTS

Nine patients are deleted for the KCNAB2 locus, and eight (89%) of these have epilepsy or epileptiform activity on EEG. The majority of patients have a severe seizure phenotype, including infantile spasms. In contrast, of those not deleted for KCNAB2, only 27% have chronic seizures, and none had infantile spasms.

CONCLUSIONS

Lack of the beta subunit would be predicted to reduce K(+) channel-mediated membrane repolarization and increase neuronal excitability, suggesting a possible relation between loss of this gene and the development of seizures. Because some patients with seizures were not deleted for KCNAB2, there may be additional genes within 1p36 that contribute to epilepsy in this syndrome. Hemizygosity of this gene in a majority of monosomy 1p36 syndrome patients with epilepsy suggests that haploinsufficiency for KCNAB2 is a significant risk factor for epilepsy.

A central role for the T1 domain in voltage-gated potassium channel formation and function

To interpret the recent atomic structures of the Kv (voltage-dependent potassium) channel T1 domain in a functional context, we must understand both how the T1 domain is integrated into the full-length functional channel protein and what functional roles the T1 domain governs. The T1 domain clearly plays a role in restricting Kv channel subunit heteromultimerization. However, the importance of T1 tetramerization for the assembly and retention of quarternary structure within full-length channels has remained controversial. Here we describe a set of mutations that disrupt both T1 assembly and the formation of functional channels and show that these mutations produce elevated levels of the subunit monomer that becomes subject to degradation within the cell. In addition, our experiments reveal that the T1 domain lends stability to the full-length channel structure, because channels lacking the T1 containing N terminus are more easily denatured to monomers. The integration of the T1 domain ultrastructure into the full-length channel was probed by proteolytic mapping with immobilized trypsin. Trypsin cleavage yields an N-terminal fragment that is further digested to a tetrameric domain, which remains reactive with antisera to T1, and that is similar in size to the T1 domain used for crystallographic studies. The trypsin-sensitive linkages retaining the T1 domain are cleaved somewhat slowly over hours. Therefore, they seem to be intermediate in trypsin resistance between the rapidly cleaved extracellular linker between the first and second transmembrane domains, and the highly resistant T1 core, and are likely to be partially structured or contain dynamic structure. Our experiments suggest that tetrameric atomic models obtained for the T1 domain do reflect a structure that the T1 domain sequence forms early in channel assembly to drive subunit protein tetramerization and that this structure is retained as an integrated stabilizing structural element within the full-length functional channel.

Three-dimensional structure of a voltage-gated potassium channel at 2.5 nm resolution

BACKGROUND

The voltage-gated potassium channel Shaker from Drosophila consists of a tetramer of identical subunits, each containing six transmembrane segments. The atomic structure of a bacterial homolog, the potassium channel KcsA, is much smaller than Shaker. It does not have a voltage sensor and other important domains like the N-terminal tetramerization (T1) domain. The structure of these additional elements has to be studied in the more complex voltage-gated channels.

RESULTS

We determined the three-dimensional structure of the entire Shaker channel at 2.5 nm resolution using electron microscopy. The four-fold symmetric structure shows a large and a small domain linked by thin 2 nm long connectors. To interpret the structure, we used the crystal structures of the isolated T1 domain and the KcsA channel. A unique density assignment was made based on the symmetry and dimensions of the crystal structures and domains, identifying the smaller domain as the cytoplasmic mass of Shaker containing T1 and the larger domain as embedded in the membrane.

CONCLUSIONS

The two-domain architecture of the Shaker channel is consistent with the recently proposed "hanging gondola" model for the T1 domain, putting the T1 domain at a distance from the membrane domain but attached to it by thin connectors. The space between the two domains is sufficient to permit cytoplasmic access of ions and the N-terminal inactivation domain to the pore region. A hanging gondola architecture has also been observed in the nicotinic acetylcholine receptor and the KcsA structure, suggesting that it is a common element of ion channels.

Accessory protein facilitated CFTR-CFTR interaction, a molecular mechanism to potentiate the chloride channel activity

The cystic fibrosis transmembrane conductance regulator (CFTR) gene encodes a chloride channel protein that belongs to the superfamily of ATP binding cassette (ABC) transporters. Phosphorylation by protein kinase A in the presence of ATP activates the CFTR-mediated chloride conductance of the apical membranes. We have identified a novel hydrophilic CFTR binding protein, CAP70, which is also concentrated on the apical surfaces. CAP70 consists of four PDZ domains, three of which are capable of binding to the CFTR C terminus. Linking at least two CFTR molecules via cytoplasmic C-terminal binding by either multivalent CAP70 or a bivalent monoclonal antibody potentiates the CFTR chloride channel activity. Thus, the CFTR channel can be switched to a more active conducting state via a modification of intermolecular CFTR-CFTR contact that is enhanced by an accessory protein.

Reconstructing voltage sensor-pore interaction from a fluorescence scan of a voltage-gated K+ channel

X-ray crystallography has made considerable recent progress in providing static structures of ion channels. Here we describe a complementary method-systematic fluorescence scanning-that reveals the structural dynamics of a channel. Local protein motion was measured from changes in the fluorescent intensity of a fluorophore attached at one of 37 positions in the pore domain and in the S4 voltage sensor of the Shaker K+ channel. The local rearrangements that accompany activation and slow inactivation were mapped onto the homologous structure of the KcsA channel and onto models of S4. The results place clear constraints on S4 location, voltage-dependent movement, and the mechanism of coupling of S4 motion to the operation of the slow inactivation gate in the pore domain.

Localization and molecular determinants of the Hanatoxin receptors on the voltage-sensing domains of a K(+) channel

Hanatoxin inhibits voltage-gated K(+) channels by modifying the energetics of activation. We studied the molecular determinants and physical location of the Hanatoxin receptors on the drk1 voltage-gated K(+) channel. First, we made multiple substitutions at three previously identified positions in the COOH terminus of S3 to examine whether these residues interact intimately with the toxin. We also examined a region encompassing S1-S3 using alanine-scanning mutagenesis to identify additional determinants of the toxin receptors. Finally, guided by the structure of the KcsA K(+) channel, we explored whether the toxin interacts with the peripheral extracellular surface of the pore domain in the drk1 K(+) channel. Our results argue for an intimate interaction between the toxin and the COOH terminus of S3 and suggest that the Hanatoxin receptors are confined within the voltage-sensing domains of the channel, at least 20-25 A away from the central pore axis.

A localized interaction surface for voltage-sensing domains on the pore domain of a K+ channel

Voltage-gated K+ channels contain a central pore domain and four surrounding voltage-sensing domains. How and where changes in the structure of the voltage-sensing domains couple to the pore domain so as to gate ion conduction is not understood. The crystal structure of KcsA, a bacterial K+ channel homologous to the pore domain of voltage-gated K+ channels, provides a starting point for addressing this question. Guided by this structure, we used tryptophan-scanning mutagenesis on the transmembrane shell of the pore domain in the Shaker voltage-gated K+ channel to localize potential protein-protein and protein-lipid interfaces. Some mutants cause only minor changes in gating and when mapped onto the KcsA structure cluster away from the interface between pore domain subunits. In contrast, mutants producing large changes in gating tend to cluster near this interface. These results imply that voltage-sensing domains interact with localized regions near the interface between adjacent pore domain subunits.

Internalization of the Kv1.4 potassium channel is suppressed by clustering interactions with PSD-95

The contribution of voltage-dependent ion channels to nerve function depends upon their cell-surface distributions. Nevertheless, the mechanisms underlying channel localization are poorly understood. Two phenomena appear particularly important: the clustering of channels by membrane-associated guanylate kinases (MAGUKs), such as PSD-95, and the regional stabilization of cell-surface proteins by differential suppression of endocytosis. Could these phenomena be related? To test this possibility we examined the effect of PSD-95 on the internalization rate of Kv1.4 K(+) channels in transfected HEK293 cells using cell-surface biotinylation assays. When expressed alone Kv1.4 was internalized with a half-life of 87 min, but, in the presence of PSD-95, Kv1.4 internalization was completely suppressed. Immunochemistry and electrophysiology showed PSD-95 had little effect on total or cell-surface levels of Kv1.4 or on current amplitude, activation, or inactivation kinetics. Clustering was necessary and sufficient to suppress Kv1.4 internalization since C35S-PSD-95, a mutant reported to bind but not cluster Kv1.4, (confirmed by imaging cells co-expressing a functional, GFP-variant-tagged Kv1.4) restored and, surprisingly, enhanced the rate of Kv1.4 internalization (t((1)/(2)) = 16 min). These data argue PSD-95-mediated clustering suppresses Kv1.4 internalization and suggest a fundamentally new role for PSD-95, and perhaps other MAGUKs, orchestrating the stabilization of channels at the cell-surface.

Role of individual surface charges of voltage-gated K channels

Fixed charges on the extracellular surface of voltage-gated ion channels influence the gating. In previous studies of cloned voltage-gated K channels, we found evidence that the functional surface charges are located on the peptide loop between the fifth transmembrane segment and the pore region (the S5-P loop). In the present study, we determine the role of individual charges of the S5-P loop by correlating primary structure with experimentally calculated surface potentials of the previously investigated channels. The results suggest that contributions to the surface potential at the voltage sensor of the different residues varies in an oscillating pattern, with the first residue of the N-terminal end of the S5-P loop, an absolutely conserved glutamate, contributing most. An analysis yields estimates of the distance between the residues and the voltage sensor, the first N-terminal residue being located at a distance of 5-6 A. To explain the results, a structural hypothesis, comprising an alpha-helical N-terminal end of the S5-P loop, is presented.

Images of oligomeric Kv beta 2, a modulatory subunit of potassium channels

The Shaker type voltage-gated potassium (K+) channel consists of four pore-forming Kv alpha subunits. The channel expression and kinetic properties can be modulated by auxiliary hydrophilic Kv beta subunits via formation of heteromultimeric Kv alpha-Kv beta complexes. Because each (Kv alpha)4 could recruit more than one Kv beta subunit and different Kv beta subunits could potentially interact, the stoichiometry of alpha-beta and beta-beta complexes is therefore critical for understanding the functional regulation of Shaker type potassium channels. We expressed and purified Kv beta 2 subunit in Sf9 insect cells. The purified Kv beta 2, examined by atomic force and electron microscopy techniques, is found predominately as a square-shaped tetrameric complex with side dimensions of 100 x 100 A2 and height of 51 A. Thus, Kv beta 2 is capable of forming a tetramer in the absence of pore-forming alpha subunits. The center of the Kv beta 2 complex was observed to be the most heavily stained region, suggesting that this region could be part of an extended tubular structure connecting the inner mouth of the ion permeation pathway to the cytoplasmic environment.

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

Evidence for dimerization of dimers in K+ channel assembly

Voltage-gated K+ channels are tetrameric, but how the four subunits assemble is not known. We analyzed inactivation kinetics and peak current levels elicited for a variety of wild-type and mutant Kv1.3 subunits, expressed singly, in combination, and as tandem constructs, to show that 1) the dominant pathway involves a dimerization of dimers, and 2) dimer-dimer interaction may involve interaction sites that differ from those involved in monomer-monomer association. Moreover, using nondenaturing gel electrophoresis, we detected dimers and tetramers, but not trimers, in the translation reaction of Kv1.3 monomers.

Subunit folding and assembly steps are interspersed during Shaker potassium channel biogenesis

In the voltage-dependent Shaker K+ channel, distinct regions of the protein form the voltage sensor, contribute to the permeation pathway, and recognize compatible subunits for assembly. To investigate channel biogenesis, we disrupted the formation of these discrete functional domains with mutations, including an amino-terminal deletion, Delta97-196, which is likely to disrupt subunit oligomerization; D316K and K374E, which prevent proper folding of the voltage sensor; and E418K and C462K, which are likely to disrupt pore formation. We determined whether these mutant subunits undergo three previously identified assembly events as follows: 1) tetramerization of Shaker subunits, 2) assembly of Shaker (alpha) and cytoplasmic beta subunits, and 3) association of the amino and carboxyl termini of adjacent Shaker subunits. Delta97-196 subunits failed to establish any of these quaternary interactions. The Delta97-196 deletion also prevented formation of the pore. The other mutant subunits assembled into tetramers and associated with the beta subunit but did not establish proximity between the amino and carboxyl termini of adjacent subunits. The results indicate that oligomerization mediated by the amino terminus is required for subsequent pore formation and either precedes or is independent of folding of the voltage sensor. In contrast, the amino and carboxyl termini of adjacent subunits associate late during biogenesis. Because subunits with folding defects oligomerize, we conclude that Shaker channels need not assemble from pre-folded monomers. Furthermore, association with native subunits can weakly promote the proper folding of some mutant subunits, suggesting that steps of folding and assembly alternate during channel biogenesis.

Intermediate conductances during deactivation of heteromultimeric Shaker potassium channels

A previous study of the T442S mutant Shaker channel revealed activation-coupled subconductance levels that apparently represent kinetic intermediates in channel activation (Zheng, J., and F.J. Sigworth. 1997. J. Gen. Physiol. 110:101-117). We have now extended the study to heteromultimeric channels consisting of various numbers of mutant subunits as well as channels without mutant subunits, all in the background of a chimeric Shaker channel having increased conductance. It has been found that activation-coupled sublevels exist in all these channel types, and are traversed in at least 80% of all deactivation time courses. In symmetric K+ solutions, the currents in the two sublevels have a linear voltage dependence, being 23-44% and 54-70% of the fully open conductance. Sublevels in different channel types share similar voltage dependence of the mean lifetime and similar ion selectivity properties. However, the mean lifetime of each current level depends approximately geometrically on the number of mutant subunits in the channel, becoming shorter in channels having fewer mutant subunits. Each mutant subunit appears to stabilize all of the conducting states by approximately 0.5 kcal/mol. Consistent with previous results in the mutant channel, sublevels in channels with two or no mutant subunits also showed ion selectivities that differ from that of the fully open level, having relatively higher K+ than Rb+ conductances. A model is presented in which Shaker channels have two coupled activation gates, one associated with the selectivity filter and a second associated with the S6 helix bundle.

Two-dimensional crystallization and projection structure of KcsA potassium channel

Potassium channels are integral membrane proteins that play a crucial role in regulating diverse cell functions in both electrically excitable and non-excitable cells. Molecular cloning has revealed a diverse family of genes that encode these proteins, and a variety of experimental strategies have defined functional domains. We have cloned, over-expressed and purified the KcsA potassium channel to homogeneity and reconstituted this channel protein with phospholipids to form two-dimensional crystals. The crystals belong to plane group p4 and have unit cell dimensions of a=b=48 A. A projection map at 6 A resolution has been obtained by electron crystallography. The map shows that the protein is a homotetramer, having a low-density region on the 4-fold axis that is the site of the ion conduction pathway. Each monomer contains density features that are consistent with the molecular model of a truncated form of KcsA recently determined by X-ray crystallography.

Shaker and ether-à-go-go K+ channel subunits fail to coassemble in Xenopus oocytes

Members of different voltage-gated K+ channel subfamilies usually do not form heteromultimers. However, coassembly between Shaker and ether-à-go-go (eag) subunits, members of two distinct K+ channel subfamilies, was suggested by genetic and functional studies (Zhong and Wu. 1991. Science. 252: 1562-1564; Chen, M.-L., T. Hoshi, and C.-F. Wu. 1996. Neuron. 17:535-542). We investigated whether Shaker and eag form heteromultimers in Xenopus laevis oocytes using electrophysiological and biochemical approaches. Coexpression of Shaker and eag subunits produced K+ currents that were virtually identical to the sum of separate Shaker and eag currents, with no change in the kinetics of Shaker inactivation. According to the results of dominant negative and reciprocal coimmunoprecipitation experiments, the Shaker and eag proteins do not interact. We conclude that Shaker and eag do not coassemble to form heteromultimers in Xenopus oocytes.

The sodium channel has four domains surrounding a central pore

The voltage-gated sodium channel generates the action potential. This 300-kDa protein has four homologous regions, which are also homologous to the voltage-sensitive tetrameric potassium channel. We isolated sodium channels from Electrophorus electricus electroplax by detergent solubilization and immunoaffinity chromatography and studied their structure by electron microscopy of negatively stained specimens. Different projections were aligned, classified, and averaged. In side view, the channel protein exhibits the shape of a truncated cone, 14 nm in height. One end has a diameter of 12 nm and is asymmetric, while the other is more symmetric and has a diameter of 7-10 nm. In top views, the sodium channel appears to consist of four domains of different size and to have a stain-filled pore in the center.

Pore stoichiometry of a voltage-gated chloride channel

Ion channels allow ions to pass through cell membranes by forming aqueous permeation pathways (pores). In contrast to most known ion channels, which have single pores, a chloride channel belonging to the CIC family (Torpedo CIC-0) has functional features that suggest that it has a unique 'double-barrelled' architecture in which each of two subunits forms an independent pore. This model is based on single-channel recordings of CIC-0 that has two equally spaced and independently gated conductance states. Other CIC isoforms do not behave in this way, raising doubts about the applicability of the model to all CIC channels. Here we determine the pore stoichiometry of another CIC isoform, human CIC-1, by chemically modifying cysteines that have been substituted for other amino acids located within the CIC ion-selectivity filter. The CIC-1 channel can be rendered completely susceptible to block by methanethiosulphonate reagents when only one of the two subunits contains substituted cysteines. Thiol side chains placed at corresponding positions in both subunits can form intersubunit disulphide bridges and coordinate Cd2+, indicating that the pore-forming regions from each subunit line the same conduction pathway. We conclude that human CIC-1 has a single functional pore.

Recombinant expression, purification and characterization of Kch, a putative Escherichia coli potassium channel protein

The Escherichia coli gene kch, similar in primary structure to eukaryotic voltage-gated potassium channels, was cloned and overexpressed in E. coli. The protein was solubilized from the plasma membrane with dodecylmaltopyranoside, lauryldimethylamine oxide or N-laurylsarcosine and was purified in milligram amounts by imidazole elution from a nickel-chelate column. The molecular mass of the purified protein in a number of detergents with 12 carbon atom chains suggests that rKch forms primarily tetramers of the 50 kDa monomers. CD spectroscopy of the purified protein indicates a significant alpha-helical content that is preserved upon addition of SDS.

Crystal structure of the tetramerization domain of the Shaker potassium channel

Voltage-dependent, ion-selective channels such as Na+, Ca2+ and K+ channel proteins function as tetrameric assemblies of identical or similar subunits. The clustering of four subunits is thought to create an aqueous pore centred at the four-fold symmetry axis. The highly conserved, amino-terminal cytoplasmic domain (approximately 130 amino acids) immediately preceding the first putative transmembrane helix S1 is designated T1. It is known to confer specificity for tetramer formation, so the heteromeric assembly of K+-channel subunits is an important mechanism for the observed channel diversity. We have determined the crystal structure of the T1 domain of a Shaker potassium channel at 1.55 A resolution. The structure reveals that four identical subunits are arranged in a four-fold symmetry surrounding a centrally located pore about 20 A in length. Subfamily-specific assembly is provided primarily by polar interactions encoded in a conserved set of amino acids at its tetramerization interface. Most highly conserved amino acids in the T1 domain of all known potassium channels are found in the core of the protein, indicating a common structural framework for the tetramer assembly.

Inactivating peptide of the Shaker B potassium channel: conformational preferences inferred from studies on simple model systems

Previous studies on the interaction between the inactivating peptide of the Shaker B K+ channel (ShB peptide, H2N-MAAVAGLYGLGEDRQHRKKQ) and anionic phospholipid vesicles, used as model targets, have shown that the ShB peptide: (i) binds to the vesicle surface with high affinity; (ii) readily adopts a strongly hydrogen-bonded beta-structure; and (iii) becomes inserted into the hydrophobic bilayer. We now report fluorescence studies showing that the vesicle-inserted ShB peptide is in a monomeric form and, therefore, the observed beta-structure must be intramolecularly hydrogen-bonded to produce a beta-hairpin conformation. Also, additional freeze-fracture and accessibility-to-trypsin studies, which aimed to estimate how deeply and in which orientation the folded monomeric peptide inserts into the model target, have allowed us to build structural models for the target-inserted peptide. In such models, the peptide has been folded near G6 to configure a long beta-hairpin modelled to produce an internal cancellation of net charges in the stretch comprising amino acids 1-16. As to the positively charged C-terminal portion of the ShB peptide (RKKQ), this has been modelled to be in parallel with the anionic membrane surface to facilitate electrostatic interactions. Since the negatively charged surface and the hydrophobic domains in the model vesicle target may partly imitate those present at the inactivation 'entrance' in the channel protein [Kukuljan, M., Labarca, P. and Latorre, R. (1995) Am. J. Physiol. Cell Physiol. 268, C535-C556], we believe that the structural models postulated here for the vesicle-inserted peptide could help to understand how the ShB peptide associates with the channel during inactivation and why mutations at specific sites in the ShB peptide sequence, such as that in the ShB-L7E peptide, result in non-inactivating peptide variants.

A novel junction-like membrane complex in the optic nerve astrocyte of the Japanese macaque with a possible relation to a potassium ion channel

BACKGROUND

A new type of junction-like membrane complex (JMC) was detected between adjacent astrocytes in the optic nerve of Japanese macaque (macaca fuscata). This membrane complex morphologically resembled a cell junction, but a possible role for potassium ion channels could not be denied based on freeze-fracture replica observation. We attempted to determine the chemical nature and function of the novel JMC.

METHODS

Using an electron microscope, we observed JMCs in the optic nerve astrocyte. In addition, we observed them using a freeze-fracture replica and immunohistochemistry with connexin 43, a gap junction specific protein. Furthermore, immunolocalization of an inwardly rectifying potassium ion channel, K(AB)-2 (Kir4.1), was studied with a confocal laser-scanning microscope, and an electron microscope using a newly developed pre-embedding method.

RESULTS

These JMCs were abundant around the blood vessel in the area just behind the lamina cribrosa. At JMCs the inner leaflet was thicker than the outer leaflet and electron-dense materials were packed in the intercellular space. Freeze-fracture replica observation revealed orthogonal arrays of particles, probably at the place of JMCs, that have been considered a potassium ion channel. No connexin 43 immunoreactivity was detected in JMCs, while K(AB)-2 was mostly localized on either side of the opposing cell membranes of JMC.

CONCLUSIONS

These JMCs do not seem to be a simple junction, but relate to a potassium ion channel. The area just behind the lamina cribrosa may be important in terms of conductance of the optic nerve impulse.

Number and stoichiometry of subunits in the native atrial G-protein-gated K+ channel, IKACh

The G-protein-regulated, inwardly rectifying K+ (GIRK) channels are critical for functions as diverse as heart rate modulation and neuronal post-synaptic inhibition. GIRK channels are distributed predominantly throughout the heart, brain, and pancreas. In recent years, GIRK channels have received a great deal of attention for their direct G-protein betagamma (Gbetagamma) regulation. Native cardiac IKACh is composed of GIRK1 and GIRK4 subunits (Krapivinsky, G., Gordon, E. A., Wickman, K. A., Velimirovic, B., Krapivinsky, L., and Clapham, D. E. (1995) Nature 374, 135-141). Here, we examine the quaternary structure of IKACh using a variety of complementary approaches. Complete cross-linking of purified atrial IKACh protein formed a single adduct with a total molecular weight that was most consistent with a tetramer. In addition, partial cross-linking of purified IKACh produced subsets of molecular weights consistent with monomers, dimers, trimers, and tetramers. Within the presumed protein dimers, GIRK1-GIRK1 and GIRK4-GIRK4 adducts were formed, indicating that the tetramer was composed of two GIRK1 and two GIRK4 subunits. This 1:1 GIRK1 to GIRK4 stoichiometry was confirmed by two independent means, including densitometry of both silver-stained and Western-blotted native atrial IKACh. Similar experimental results could potentially be obtained if GIRK1 and GIRK4 subunits assembled randomly as 2:2 and equally sized populations of 3:1 and 1:3 tetramers. We also show that GIRK subunits may form homotetramers in expression systems, although the evidence to date suggests that GIRK1 homotetramers are not functional. We conclude that the inwardly rectifying atrial K+ channel, IKACh, a prototypical GIRK channel, is a heterotetramer and is most likely composed of two GIRK1 subunits and two GIRK4 subunits.

Structural dynamics of the Streptomyces lividans K+ channel (SKC1): secondary structure characterization from FTIR spectroscopy

Fourier transform infrared (FTIR) spectroscopy was used to probe the secondary structure, orientation, and the kinetics of amide hydrogen-deuterium exchange (HX) of the small K+ channel from Streptomyces lividans. Frequency component analysis of the amide I band showed that SKC1 is composed of 44-46% alpha-helix, 21-24% beta-sheet, 10-12% turns and 18-20% unordered structures. The order parameter S of the helical component of SKC1 was between 0.60 and 0.69. Close to 80% of SKC1 amide protons exchange within approximately 3 h of D2O exposure, suggesting that the channel is largely accessible to solvent exchange. These results are consistent with a model of SKC1 in which helices slightly tilted from the membrane normal line the water-filled vestibules that flank the K+ selectivity filter.

Distinct functional stoichiometry of potassium channel beta subunits

Shaker-type potassium channels play important roles in determining the electrical excitability of cells. The native channel complex is thought to be formed by four pore-forming alpha subunits that provide four interaction sites for auxiliary modulatory Kvbeta subunits. Because Kvbeta subunits possess diverse modulatory activities including either up-regulation or down-regulation of potassium currents, differential assembly of the alpha-beta complex could give rise to diverse current properties. However, the detailed physical and functional stoichiometry of the alpha-beta complex remains unknown. Kvbeta1 subunits reduce potassium currents through inactivation, whereas Kvbeta2 subunits enhance potassium currents by inhibiting the Kvbeta1-mediated inactivation and at the same time by promoting the surface expression of certain potassium channels. In this report we show that Kvbeta1 and Kvbeta2 of the Shaker-type potassium channels display distinct functional stoichiometry to interact with the Kv1 alpha subunits, a subfamily of Shaker-type potassium channels. The interaction of Kvbeta1 subunits with alpha subunits is consistent with the alpha4betan model, where n equals 0, 1, 2, 3, or 4, depending upon the relative concentration of alpha and beta subunits. The alpha4betan stoichiometry allows for gradual changes of the Kvbeta1-mediated inactivation. In contrast, Kvbeta2 subunits self-associate to form oligomers and interact with the alpha subunits via alpha4beta4 stoichiometry, which permits effective multivalent associations with alpha subunits. Such distinct functional stoichiometry of Kvbeta1 and Kvbeta2 provides a molecular mechanism that is well suited to their contrasting activities of up-regulation or down-regulation of potassium currents.

Activation of Shaker potassium channels. III. An activation gating model for wild-type and V2 mutant channels

A functional kinetic model is developed to describe the activation gating process of the Shaker potassium channel. The modeling in this paper is constrained by measurements described in the preceding two papers, including macroscopic ionic and gating currents and single channel ionic currents. These data were obtained from the normally activating wild-type channel as well as a mutant channel V2, in which the leucine at position 382 has been mutated to a valine. Different classes of models that incorporate Shaker's symmetrical tetrameric structure are systematically examined. Many simple gating models are clearly inadequate, but a model that can account for all of the qualitative features of the data has the channel open after its four subunits undergo three transitions in sequence, and two final transitions that reflect the concerted action of the four subunits. In this model, which we call Scheme 3+2', the channel can also close to several states that are not part of the activation path. Channel opening involves a large total charge movement (10.8 e0), which is distributed among a large number of small steps each with rather small charge movements (between 0.6 and 1.05 e0). The final two transitions are different from earlier steps by having slow backward rates. These steps confer a cooperative mechanism of channel opening at Shaker's activation voltages. In the context of Scheme 3+2', significant effects of the V2 mutation are limited to the backward rates of the final two transitions, implying that L382 plays an important role in the conformational stability of the final two states.

Activation of Shaker potassium channels. II. Kinetics of the V2 mutant channel

This second of three papers, in which we functionally characterize activation gating in Shaker potassium channels, focuses on the properties of a mutant channel (called V2), in which the leucine at position 382 (in the Shaker B sequence) is mutated to valine. The general properties of V2's ionic and gating currents are consistent with changes in late gating transitions, in particular, with V2 disrupting the positively cooperative gating process of the normally activating wild type (WT) channel. An analysis of forward and backward rate constants, analogous to that used for WT in the previous paper, indicates that V2 causes little change in the rates for most of the transitions in the activation path, but causes large changes in the backward rates of the final two transitions. Single channel data indicate that the V2 mutation causes moderate changes in the rates of transitions to states that are not in the activation path, but little change in the rates from these states. V2's data also yield insights into the general properties of the activation gating process that could not be readily obtained from the WT channel, including evidence that intermediate transitions have rapid backward rates, and an estimate of a total charge 2 e0 for the final two transitions. Taken together, these data will help constrain an activation gating model in the third paper of this series, while also providing an explanation for V2's effects.

Activation of shaker potassium channels. I. Characterization of voltage-dependent transitions

The conformational changes associated with activation gating in Shaker potassium channels are functionally characterized in patch-clamp recordings made from Xenopus laevis oocytes expressing Shaker channels with fast inactivation removed. Estimates of the forward and backward rates for transitions are obtained by fitting exponentials to macroscopic ionic and gating current relaxations at voltage extremes, where we assume that transitions are unidirectional. The assignment of different rates is facilitated by using voltage protocols that incorporate prepulses to preload channels into different distributions of states, yielding test currents that reflect different subsets of transitions. These data yield direct estimates of the rate constants and partial charges associated with three forward and three backward transitions, as well as estimates of the partial charges associated with other transitions. The partial charges correspond to an average charge movement of 0.5 e0 during each transition in the activation process. This value implies that activation gating involves a large number of transitions to account for the total gating charge displacement of 13 e0. The characterization of the gating transitions here forms the basis for constraining a detailed gating model to be described in a subsequent paper of this series.

Regulation of mammalian Shaker-related K+ channels: evidence for non-conducting closed and non-conducting inactivated states

1. Using the whole-cell recording mode we have characterized two non-conducting states in mammalian Shaker-related voltage-gated K+ channels induced by the removal of extracellular potassium, K+o. 2. In the absence of K+o, current through Kv1.4 was almost completely abolished due to the presence of a charged lysine residue at position 533 at the entrance to the pore. Removal of K+o had a similar effect on current through Kv1.3 when the histidine at the homologous position (H404) was protonated (pH 6.0). Channels containing uncharged residues at the corresponding position (Kv1.1: Y; Kv1.2: V) did not exhibit this behaviour. 3. To characterize the nature of the interaction between Kv1.3 and K+o concentration ([K+]o), we replaced H404 with amino acids of different character, size and charge. Substitution of hydrophobic residues (A, V and L) either in all four subunits or in only two subunits in the tetramer made the channel insensitive to the removal of K+o, possibly by stabilizing the channel complex. Replacement of H404 with the charged residue arginine, or the polar residue asparagine, enhanced the sensitivity of the channel to 0 mM K+o, possibly by making the channel unstable in the absence of K+o. Mutation at a neighbouring position (400) had a similar effect. 4. The effect of removing K+o on current amplitude does not seem to be correlated with the rate of C-type inactivation since the slowly inactivating G380F mutant channel exhibited a similar [K+]o dependence as the wild-type Kv1.3 channel. 5. CP-339,818, a drug that recognizes only the inactivated conformation of Kv1.3, could not block current in the absence of K+o unless the channels were inactivated through depolarizing pulses. 6. We conclude that removal of K+o induces the Kv1.3 channel to transition to a non-conducting 'closed' state which can switch into a non-conducting 'inactivated' state upon depolarization.

Voltage-gated and inwardly rectifying potassium channels

This lecture is dedicated to Max Delbrück and Seymour Benzer. Max Delbrück was our graduate advisor. He introduced us to a variety of biophysical problems, and taught us ways of thinking about these problems by example. Potassium channels was one of the topics included in his journal club in the early seventies; Max also carefully considered the feasibility of purifying potassium channels then. It was in Seymour Benzer's laboratory that we began to look for Drosophila mutants that affect synaptic transmission at the larval neuromuscular junction. Shaker was the first behavioural mutant we tested that gave a robust phenotype, a phenotype that could be mimicked by treating wild-type preparations with a potassium channel blocker. This mutant fly has led us to our subsequent molecular studies of potassium channels. Since we settled in the University of California, San Francisco, and began to study neural development as well as potassium channels, we have settled into the pattern of each attending meetings and presenting our studies on one of these two areas so as to avoid both being away from home and our children at the same time. In following this pattern, I will be presenting the studies of potassium channels as part of our long-term collaboration. In this talk I will first briefly take you through the path that led us to the molecular studies of potassium channels and then discuss the diversity and modulation of these potassium channels at the molecular and physiological level.

Electrostatic binding of proteins to membranes. Theoretical predictions and experimental results with charybdotoxin and phospholipid vesicles

We previously applied the Poisson-Boltzmann equation to atomic models of phospholipid bilayers and basic peptides to calculate their electrostatic interactions from first principles (Ben-Tal, N., B. Honig, R. M. Peitzsch, G. Denisov, and S. McLaughlan. 1996. Binding of small basic peptides to membranes containing acidic lipids. Theoretical models and experimental results. Biophys. J. 71:561-575). Specifically, we calculated the molar partition coefficient, K (the reciprocal of the lipid concentration at which 1/2 the peptide is bound), of simple basic peptides (e.g., pentalysine) with phospholipid vesicles. The theoretical predictions agreed well with experimental measurements of the binding, but the agreement could have been fortuitous because the structure(s) of these flexible peptides is not known. Here we use the same theoretical approach to calculate the membrane binding of two small proteins of known structure: charybdotoxin (CTx) and iberiotoxin (IbTx); we also measure the binding of these proteins to phospholipid vesicles. The theoretical model describes accurately the dependence of K on the ionic strength and mol % acidic lipid in the membrane for both CTx (net charge +4) and IbTx (net charge +2). For example, the theory correctly predicts that the value of K for the binding of CTx to a membrane containing 33% acidic lipid should decrease by a factor of 10(5) when the salt concentration increases from 10 to 200 mM. We discuss the limitations of the theoretical approach and also consider a simple extension of the theory that incorporates nonpolar interactions.

The pore-lining region of shaker voltage-gated potassium channels: comparison of beta-barrel and alpha-helix bundle models

Although there is a large body of site-directed mutagenesis data that identify the pore-lining sequence of the voltage-gated potassium channel, the structure of this region remains unknown. We have interpreted the available biochemical data as a set of topological and orientational restraints and employed these restraints to produce molecular models of the potassium channel pore region, H5. The H5 sequence has been modeled either as a tetramer of membrane-spanning beta-hairpins, thus producing an eight-stranded beta-barrel, or as a tetramer of incompletely membrane-spanning alpha-helical hairpins, thus producing an eight-staved alpha-helix bundle. In total, restraints-directed modeling has produced 40 different configurations of the beta-barrel model, each configuration comprising an ensemble of 20 structures, and 24 different configurations of the alpha-helix bundle model, each comprising an ensemble of 24 structures. Thus, over 1300 model structures for H5 have been generated. Configurations have been ranked on the basis of their predicted pore properties and on the extent of their agreement with the biochemical data. This ranking is employed to identify particular configurations of H5 that may be explored further as models of the pore-lining region of the voltage-gated potassium channel pore.