Altered Closed State Inactivation Gating in Kv4.2 Channels Results in Developmental and Epileptic Encephalopathies in Human Patients

Kv4.2 subunits, encoded by KCND2, serve as the pore-forming components of voltage-gated, inactivating I_SA K⁺ channels expressed in the brain. I_SA channels inactivate without opening in response to subthreshold excitatory input, temporarily increasing neuronal excitability, the back propagation of action potentials, and Ca²⁺ influx into dendrites, thereby regulating mechanisms of spike timing-dependent synaptic plasticity. As previously described, a de novo variant in Kv4.2, p.Val404Met, is associated with an infant-onset developmental and epileptic encephalopathy (DEE) in monozygotic twin boys. The p.Val404Met variant enhances inactivation directly from closed states, but dramatically impairs inactivation after channel opening. We now report the identification of a closely related, novel, de novo variant in Kv4.2, p.Val402Leu, in a boy with an early-onset pharmacoresistant epilepsy that evolved to an epileptic aphasia syndrome (Continuous Spike Wave during Sleep Syndrome). Like p.Val404Met, the p.Val402Leu variant increases the rate of inactivation from closed states, but significantly slows inactivation after the pore opens. Although quantitatively the p.Val402Leu mutation alters channel kinetics less dramatically than p.Val404Met, our results strongly support the conclusion that p.Val402Leu and p.Val404Met cause the clinical features seen in the affected individuals and underscore the importance of closed state inactivation in I_SA channels in normal brain development and function. This article is protected by copyright. All rights reserved.

Infant and adult SCA13 mutations differentially affect Purkinje cell excitability, maturation, and viability in vivo

Mutations in KCNC3, which encodes the Kv3.3 K⁺ channel, cause spinocerebellar ataxia 13 (SCA13). SCA13 exists in distinct forms with onset in infancy or adulthood. Using zebrafish, we tested the hypothesis that infant- and adult-onset mutations differentially affect the excitability and viability of Purkinje cells in vivo during cerebellar development. An infant-onset mutation dramatically and transiently increased Purkinje cell excitability, stunted process extension, impaired dendritic branching and synaptogenesis, and caused rapid cell death during cerebellar development. Reducing excitability increased early Purkinje cell survival. In contrast, an adult-onset mutation did not significantly alter basal tonic firing in Purkinje cells, but reduced excitability during evoked high frequency spiking. Purkinje cells expressing the adult-onset mutation matured normally and did not degenerate during cerebellar development. Our results suggest that differential changes in the excitability of cerebellar neurons contribute to the distinct ages of onset and timing of cerebellar degeneration in infant- and adult-onset SCA13.

Spinocerebellar ataxia type 19/22 mutations alter heterocomplex Kv4.3 channel function and gating in a dominant manner

The dominantly inherited cerebellar ataxias are a heterogeneous group of neurodegenerative disorders caused by Purkinje cell loss in the cerebellum. Recently, we identified loss-of-function mutations in the KCND3 gene as the cause of spinocerebellar ataxia type 19/22 (SCA19/22), revealing a previously unknown role for the voltage-gated potassium channel, Kv4.3, in Purkinje cell survival. However, how mutant Kv4.3 affects wild-type Kv4.3 channel functioning remains unknown. We provide evidence that SCA19/22-mutant Kv4.3 exerts a dominant negative effect on the trafficking and surface expression of wild-type Kv4.3 in the absence of its regulatory subunit, KChIP2. Notably, this dominant negative effect can be rescued by the presence of KChIP2. We also found that all SCA19/22-mutant subunits either suppress wild-type Kv4.3 current amplitude or alter channel gating in a dominant manner. Our findings suggest that altered Kv4.3 channel localization and/or functioning resulting from SCA19/22 mutations may lead to Purkinje cell loss, neurodegeneration and ataxia.

Rapid development of Purkinje cell excitability, functional cerebellar circuit, and afferent sensory input to cerebellum in zebrafish

The zebrafish has significant advantages for studying the morphological development of the brain. However, little is known about the functional development of the zebrafish brain. We used patch clamp electrophysiology in live animals to investigate the emergence of excitability in cerebellar Purkinje cells, functional maturation of the cerebellar circuit, and establishment of sensory input to the cerebellum. Purkinje cells are born at 3 days post-fertilization (dpf). By 4 dpf, Purkinje cells spontaneously fired action potentials in an irregular pattern. By 5 dpf, the frequency and regularity of tonic firing had increased significantly and most cells fired complex spikes in response to climbing fiber activation. Our data suggest that, as in mammals, Purkinje cells are initially innervated by multiple climbing fibers that are winnowed to a single input. To probe the development of functional sensory input to the cerebellum, we investigated the response of Purkinje cells to a visual stimulus consisting of a rapid change in light intensity. At 4 dpf, sudden darkness increased the rate of tonic firing, suggesting that afferent pathways carrying visual information are already active by this stage. By 5 dpf, visual stimuli also activated climbing fibers, increasing the frequency of complex spiking. Our results indicate that the electrical properties of zebrafish and mammalian Purkinje cells are highly conserved and suggest that the same ion channels, Nav1.6 and Kv3.3, underlie spontaneous pacemaking activity. Interestingly, functional development of the cerebellum is temporally correlated with the emergence of complex, visually-guided behaviors such as prey capture. Because of the rapid formation of an electrically-active cerebellum, optical transparency, and ease of genetic manipulation, the zebrafish has great potential for functionally mapping cerebellar afferent and efferent pathways and for investigating cerebellar control of motor behavior.

Exome sequencing identifies de novo gain of function missense mutation in KCND2 in identical twins with autism and seizures that slows potassium channel inactivation

Numerous studies and case reports show comorbidity of autism and epilepsy, suggesting some common molecular underpinnings of the two phenotypes. However, the relationship between the two, on the molecular level, remains unclear. Here, whole exome sequencing was performed on a family with identical twins affected with autism and severe, intractable seizures. A de novo variant was identified in the KCND2 gene, which encodes the Kv4.2 potassium channel. Kv4.2 is a major pore-forming subunit in somatodendritic subthreshold A-type potassium current (ISA) channels. The de novo mutation p.Val404Met is novel and occurs at a highly conserved residue within the C-terminal end of the transmembrane helix S6 region of the ion permeation pathway. Functional analysis revealed the likely pathogenicity of the variant in that the p.Val404Met mutant construct showed significantly slowed inactivation, either by itself or after equimolar coexpression with the wild-type Kv4.2 channel construct consistent with a dominant effect. Further, the effect of the mutation on closed-state inactivation was evident in the presence of auxiliary subunits that associate with Kv4 subunits to form ISA channels in vivo. Discovery of a functionally relevant novel de novo variant, coupled with physiological evidence that the mutant protein disrupts potassium current inactivation, strongly supports KCND2 as the causal gene for epilepsy in this family. Interaction of KCND2 with other genes implicated in autism and the role of KCND2 in synaptic plasticity provide suggestive evidence of an etiological role in autism.

Spinocerebellar ataxia type 13 mutation that is associated with disease onset in infancy disrupts axonal pathfinding during neuronal development

Spinocerebellar ataxia type 13 (SCA13) is an autosomal dominant disease caused by mutations in the Kv3.3 voltage-gated potassium (K⁺) channel. SCA13 exists in two forms: infant onset is characterized by severe cerebellar atrophy, persistent motor deficits and intellectual disability, whereas adult onset is characterized by progressive ataxia and progressive cerebellar degeneration. To test the hypothesis that infant- and adult-onset mutations have differential effects on neuronal development that contribute to the age at which SCA13 emerges, we expressed wild-type Kv3.3 or infant- or adult-onset mutant proteins in motor neurons in the zebrafish spinal cord. We characterized the development of CaP (caudal primary) motor neurons at ∼36 and ∼48 hours post-fertilization using confocal microscopy and 3D digital reconstruction. Exogenous expression of wild-type Kv3.3 had no significant effect on CaP development. In contrast, CaP neurons expressing the infant-onset mutation made frequent pathfinding errors, sending long, abnormal axon collaterals into muscle territories that are normally innervated exclusively by RoP (rostral primary) or MiP (middle primary) motor neurons. This phenotype might be directly relevant to infant-onset SCA13 because interaction with inappropriate synaptic partners might trigger cell death during brain development. Importantly, pathfinding errors were not detected in CaP neurons expressing the adult-onset mutation. However, the adult-onset mutation tended to increase the complexity of the distal axonal arbor. From these results, we speculate that infant-onset SCA13 is associated with marked changes in the development of Kv3.3-expressing cerebellar neurons, reducing their health and viability early in life and resulting in the withered cerebellum seen in affected children.

Altered Kv3.3 channel gating in early-onset spinocerebellar ataxia type 13

Mutations in Kv3.3 cause spinocerebellar ataxia type 13 (SCA13). Depending on the causative mutation, SCA13 is either a neurodevelopmental disorder that is evident in infancy or a progressive neurodegenerative disease that emerges during adulthood. Previous studies did not clarify the relationship between these distinct clinical phenotypes and the effects of SCA13 mutations on Kv3.3 function. The F448L mutation alters channel gating and causes early-onset SCA13. R420H and R423H suppress Kv3 current amplitude by a dominant negative mechanism. However, R420H results in the adult form of the disease whereas R423H produces the early-onset, neurodevelopmental form with significant clinical overlap with F448L. Since individuals with SCA13 have one wild type and one mutant allele of the Kv3.3 gene, we analysed the properties of tetrameric channels formed by mixtures of wild type and mutant subunits. We report that one R420H subunit and at least one R423H subunit can co-assemble with the wild type protein to form active channels. The functional properties of channels containing R420H and wild type subunits strongly resemble those of wild type alone. In contrast, channels containing R423H and wild type subunits show significantly altered gating, including a hyperpolarized shift in the voltage dependence of activation, slower activation, and modestly slower deactivation. Notably, these effects resemble the modified gating seen in channels containing a mixture of F448L and wild type subunits, although the F448L subunit slows deactivation more dramatically than the R423H subunit. Our results suggest that the clinical severity of R423H reflects its dual dominant negative and dominant gain of function effects. However, as shown by R420H, reducing current amplitude without altering gating does not result in infant onset disease. Therefore, our data strongly suggest that changes in Kv3.3 gating contribute significantly to an early age of onset in SCA13.

R1 in the Shaker S4 occupies the gating charge transfer center in the resting state

During voltage-dependent activation in Shaker channels, four arginine residues in the S4 segment (R1-R4) cross the transmembrane electric field. It has been proposed that R1-R4 movement is facilitated by a "gating charge transfer center" comprising a phenylalanine (F290) in S2 plus two acidic residues, one each in S2 and S3. According to this proposal, R1 occupies the charge transfer center in the resting state, defined as the conformation in which S4 is maximally retracted toward the cytoplasm. However, other evidence suggests that R1 is located extracellular to the charge transfer center, near I287 in S2, in the resting state. To investigate the resting position of R1, we mutated I287 to histidine (I287H), paired it with histidine mutations of key voltage sensor residues, and determined the effect of extracellular Zn(2+) on channel activity. In I287H+R1H, Zn(2+) generated a slow component of activation with a maximum amplitude (A(slow,max)) of ∼56%, indicating that only a fraction of voltage sensors can bind Zn(2+) at a holding potential of -80 mV. A(slow,max) decreased after applying either depolarizing or hyperpolarizing prepulses from -80 mV. The decline of A(slow,max) after negative prepulses indicates that R1 moves inward to abolish ion binding, going beyond the point where reorientation of the I287H and R1H side chains would reestablish a binding site. These data support the proposal that R1 occupies the charge transfer center upon hyperpolarization. Consistent with this, pairing I287H with A359H in the S3-S4 loop generated a Zn(2+)-binding site. At saturating concentrations, A(slow,max) reached 100%, indicating that Zn(2+) traps the I287H+A359H voltage sensor in an absorbing conformation. Transferring I287H+A359H into a mutant background that stabilizes the resting state significantly enhanced Zn(2+) binding at -80 mV. Our results strongly support the conclusion that R1 occupies the gating charge transfer center in the resting conformation.

Frequency of KCNC3 DNA variants as causes of spinocerebellar ataxia 13 (SCA13)

Background

Gain-of function or dominant-negative mutations in the voltage-gated potassium channel KCNC3 (Kv3.3) were recently identified as a cause of autosomal dominant spinocerebellar ataxia. Our objective was to describe the frequency of mutations associated with KCNC3 in a large cohort of index patients with sporadic or familial ataxia presenting to three US ataxia clinics at academic medical centers.

Methodology

DNA sequence analysis of the coding region of the KCNC3 gene was performed in 327 index cases with ataxia. Analysis of channel function was performed by expression of DNA variants in Xenopus oocytes.

Principal Findings

Sequence analysis revealed two non-synonymous substitutions in exon 2 and five intronic changes, which were not predicted to alter splicing. We identified another pedigree with the p.Arg423His mutation in the highly conserved S4 domain of this channel. This family had an early-onset of disease and associated seizures in one individual. The second coding change, p.Gly263Asp, subtly altered biophysical properties of the channel, but was unlikely to be disease-associated as it occurred in an individual with an expansion of the CAG repeat in the CACNA1A calcium channel.

Conclusions

Mutations in KCNC3 are a rare cause of spinocerebellar ataxia with a frequency of less than 1%. The p.Arg423His mutation is recurrent in different populations and associated with early onset. In contrast to previous p.Arg423His mutation carriers, we now observed seizures and mild mental retardation in one individual. This study confirms the wide phenotypic spectrum in SCA13.

Neural circuit activity in freely behaving zebrafish (Danio rerio)

Examining neuronal network activity in freely behaving animals is advantageous for probing the function of the vertebrate central nervous system. Here, we describe a simple, robust technique for monitoring the activity of neural circuits in unfettered, freely behaving zebrafish (Danio rerio). Zebrafish respond to unexpected tactile stimuli with short- or long-latency escape behaviors, which are mediated by distinct neural circuits. Using dipole electrodes immersed in the aquarium, we measured electric field potentials generated in muscle during short- and long-latency escapes. We found that activation of the underlying neural circuits produced unique field potential signatures that are easily recognized and can be repeatedly monitored. In conjunction with behavioral analysis, we used this technique to track changes in the pattern of circuit activation during the first week of development in animals whose trigeminal sensory neurons were unilaterally ablated. One day post-ablation, the frequency of short- and long-latency responses was significantly lower on the ablated side than on the intact side. Three days post-ablation, a significant fraction of escapes evoked by stimuli on the ablated side was improperly executed, with the animal turning towards rather than away from the stimulus. However, the overall response rate remained low. Seven days post-ablation, the frequency of escapes increased dramatically and the percentage of improperly executed escapes declined. Our results demonstrate that trigeminal ablation results in rapid reconfiguration of the escape circuitry, with reinnervation by new sensory neurons and adaptive changes in behavior. This technique is valuable for probing the activity, development, plasticity and regeneration of neural circuits under natural conditions.

Functional effects of spinocerebellar ataxia type 13 mutations are conserved in zebrafish Kv3.3 channels

Background

The zebrafish has been suggested as a model system for studying human diseases that affect nervous system function and motor output. However, few of the ion channels that control neuronal activity in zebrafish have been characterized. Here, we have identified zebrafish orthologs of voltage-dependent Kv3 (KCNC) K⁺ channels. Kv3 channels have specialized gating properties that facilitate high-frequency, repetitive firing in fast-spiking neurons. Mutations in human Kv3.3 cause spinocerebellar ataxia type 13 (SCA13), an autosomal dominant genetic disease that exists in distinct neurodevelopmental and neurodegenerative forms. To assess the potential usefulness of the zebrafish as a model system for SCA13, we have characterized the functional properties of zebrafish Kv3.3 channels with and without mutations analogous to those that cause SCA13.

Results

The zebrafish genome (release Zv8) contains six Kv3 family members including two Kv3.1 genes (kcnc1a and kcnc1b), one Kv3.2 gene (kcnc2), two Kv3.3 genes (kcnc3a and kcnc3b), and one Kv3.4 gene (kcnc4). Both Kv3.3 genes are expressed during early development. Zebrafish Kv3.3 channels exhibit strong functional and structural homology with mammalian Kv3.3 channels. Zebrafish Kv3.3 activates over a depolarized voltage range and deactivates rapidly. An amino-terminal extension mediates fast, N-type inactivation. The kcnc3a gene is alternatively spliced, generating variant carboxyl-terminal sequences. The R335H mutation in the S4 transmembrane segment, analogous to the SCA13 mutation R420H, eliminates functional expression. When co-expressed with wild type, R335H subunits suppress Kv3.3 activity by a dominant negative mechanism. The F363L mutation in the S5 transmembrane segment, analogous to the SCA13 mutation F448L, alters channel gating. F363L shifts the voltage range for activation in the hyperpolarized direction and dramatically slows deactivation.

Conclusions

The functional properties of zebrafish Kv3.3 channels are consistent with a role in facilitating fast, repetitive firing of action potentials in neurons. The functional effects of SCA13 mutations are well conserved between human and zebrafish Kv3.3 channels. The high degree of homology between human and zebrafish Kv3.3 channels suggests that the zebrafish will be a useful model system for studying pathogenic mechanisms in SCA13.

Transfer of ion binding site from ether-a-go-go to Shaker: Mg2+ binds to resting state to modulate channel opening

In ether-à-go-go (eag) K(+) channels, extracellular divalent cations bind to the resting voltage sensor and thereby slow activation. Two eag-specific acidic residues in S2 and S3b coordinate the bound ion. Residues located at analogous positions are approximately 4 A apart in the x-ray structure of a Kv1.2/Kv2.1 chimera crystallized in the absence of a membrane potential. It is unknown whether these residues remain in proximity in Kv1 channels at negative voltages when the voltage sensor domain is in its resting conformation. To address this issue, we mutated Shaker residues I287 and F324, which correspond to the binding site residues in eag, to aspartate and recorded ionic and gating currents in the presence and absence of extracellular Mg(2+). In I287D+F324D, Mg(2+) significantly increased the delay before ionic current activation and slowed channel opening with no readily detectable effect on closing. Because the delay before Shaker opening reflects the initial phase of voltage-dependent activation, the results indicate that Mg(2+) binds to the voltage sensor in the resting conformation. Supporting this conclusion, Mg(2+) shifted the voltage dependence and slowed the kinetics of gating charge movement. Both the I287D and F324D mutations were required to modulate channel function. In contrast, E283, a highly conserved residue in S2, was not required for Mg(2+) binding. Ion binding affected activation by shielding the negatively charged side chains of I287D and F324D. These results show that the engineered divalent cation binding site in Shaker strongly resembles the naturally occurring site in eag. Our data provide a novel, short-range structural constraint for the resting conformation of the Shaker voltage sensor and are valuable for evaluating existing models for the resting state and voltage-dependent conformational changes that occur during activation. Comparing our data to the chimera x-ray structure, we conclude that residues in S2 and S3b remain in proximity throughout voltage-dependent activation.

KCNC3: phenotype, mutations, channel biophysics-a study of 260 familial ataxia patients

We recently identified KCNC3, encoding the Kv3.3 voltage-gated potassium channel, as the gene mutated in SCA13. One g.10684G>A (p.Arg420His) mutation caused late-onset ataxia resulting in a nonfunctional channel subunit with dominant-negative properties. A French early-onset pedigree with mild mental retardation segregated a g.10767T>C (p.Phe448Leu) mutation. This mutation changed the relative stability of the channel's open conformation. Coding exons were amplified and sequenced in 260 autosomal-dominant ataxia index cases of European descent. Functional analyses were performed using expression in Xenopus oocytes. The previously identified p.Arg420His mutation occurred in three families with late-onset ataxia. A novel mutation g.10693G>A (p.Arg423His) was identified in two families with early-onset. In one pedigree, a novel g.10522G>A (p.Arg366His) sequence variant was seen in one index case but did not segregate with affected status in the respective family. In a heterologous expression system, the p.Arg423His mutation exhibited dominant-negative properties. The p.Arg420His mutation, which results in a nonfunctional channel subunit, was recurrent and associated with late-onset progressive ataxia. In two families the p.Arg423His mutation was associated with early-onset slow-progressive ataxia. Despite a phenotype reminiscent of the p.Phe448Leu mutation, segregating in a large early-onset French pedigree, the p.Arg423His mutation resulted in a nonfunctional subunit with a strong dominant-negative effect.

Voltage-dependent conformational changes of KVAP S4 segment in bacterial membrane environment

The nature and magnitude of voltage sensor conformational changes during ion channel activation are controversial. We have analyzed the topology of the K(V)AP voltage sensor domain in the absence and presence of a hyperpolarized voltage using native, right-side out membrane vesicles from E. coli. This approach does not disrupt the normal membrane environment of the channel protein and does not involve detergent solubilization. We found that voltage-dependent conformational changes are focused in the N-terminal half of the K(V)AP S4 segment, in excellent agreement with results obtained with Shaker. Homologous residues in the K(V)AP and Shaker S4 segments are transferred from the extracellular to the intracellular compartment upon hyperpolarization. Taken together with X-ray structures indicating that the K(V)AP S4 segment is outwardly displaced at 0 mV compared to S4 in a mammalian Shaker channel, our results are consistent with the idea that S4 moves further during voltage-dependent activation in K(V)AP than in Shaker.

Differences between ion binding to eag and HERG voltage sensors contribute to differential regulation of activation and deactivation gating

HERG (KCNH2) and ether-à-go-go (eag) (KCNH1) are members of the same subfamily of voltage-gated K+ channels. In eag, voltage-dependent activation is significantly slowed by extracellular divalent cations. To exert this effect, ions bind to a site located between transmembrane segments S2 and S3 in the voltage sensor domain where they interact with acidic residues that are conserved only among members of the eag subfamily. In HERG channels, extracellular divalent ions significantly accelerate deactivation. To investigate the ionbinding site in HERG, acidic residues in S2 and S3 were neutralized singly or in pairs to alanine, and the functional effects of extracellular Mg(2+) were characterized in Xenopus oocytes. To modulate deactivation kinetics in HERG, divalent cations interact with eag subfamily-specific acidic residues (D460 and D509) and also with an acidic residue in S2 (D456) that is widely conserved in the voltage-gated channel superfamily. In contrast, the analogous widely-conserved residue does not contribute to the ion-binding site that modulates activation kinetics in eag. We propose that structural differences between the ion-binding sites in the eag and HERG voltage sensors contribute to the differential regulation of activation and deactivation gating in these channels. A previously proposed model for S4 conformational changes during voltagedependent activation can account for the differential regulation of gating seen in eag and HERG.

Distance measurements reveal a common topology of prokaryotic voltage-gated ion channels in the lipid bilayer

Voltage-dependent ion channels are fundamental to the physiology of excitable cells because they underlie the generation and propagation of the action potential and excitation-contraction coupling. To understand how ion channels work, it is important to determine their structures in different conformations in a membrane environment. The validity of the crystal structure for the prokaryotic K(+) channel, K(V)AP, has been questioned based on discrepancies with biophysical data from functional eukaryotic channels, underlining the need for independent structural data under native conditions. We investigated the structural organization of two prokaryotic voltage-gated channels, NaChBac and K(V)AP, in liposomes by using luminescence resonance energy transfer. We describe here a transmembrane packing representation of the voltage sensor and pore domains of the prokaryotic Na channel, NaChBac. We find that NaChBac and K(V)AP share a common arrangement in which the structures of the Na and K selective pores and voltage-sensor domains are conserved. The packing arrangement of the voltage-sensing region as determined by luminescence resonance energy transfer differs significantly from that of the K(V)AP crystal structure, but resembles that of the eukaryotic K(V)1.2 crystal structure. However, the voltage-sensor domain in prokaryotic channels is closer to the pore domain than in the K(V)1.2 structure. Our results indicate that prokaryotic and eukaryotic channels that share similar functional properties have similar helix arrangements, with differences arising likely from the later introduction of additional structural elements.

Mutations in voltage-gated potassium channel KCNC3 cause degenerative and developmental central nervous system phenotypes

Potassium channel mutations have been described in episodic neurological diseases. We report that K+ channel mutations cause disease phenotypes with neurodevelopmental and neurodegenerative features. In a Filipino adult-onset ataxia pedigree, the causative gene maps to 19q13, overlapping the SCA13 disease locus described in a French pedigree with childhood-onset ataxia and cognitive delay. This region contains KCNC3 (also known as Kv3.3), encoding a voltage-gated Shaw channel with enriched cerebellar expression. Sequencing revealed two missense mutations, both of which alter KCNC3 function in Xenopus laevis expression systems. KCNC3(R420H), located in the voltage-sensing domain, had no channel activity when expressed alone and had a dominant-negative effect when co-expressed with the wild-type channel. KCNC3(F448L) shifted the activation curve in the negative direction and slowed channel closing. Thus, KCNC3(R420H) and KCNC3(F448L) are expected to change the output characteristics of fast-spiking cerebellar neurons, in which KCNC channels confer capacity for high-frequency firing. Our results establish a role for KCNC3 in phenotypes ranging from developmental disorders to adult-onset neurodegeneration and suggest voltage-gated K+ channels as candidates for additional neurodegenerative diseases.

CACNA1A mutations causing episodic and progressive ataxia alter channel trafficking and kinetics

BACKGROUND

CACNA1A encodes CaV2.1, the pore-forming subunit of P/Q-type voltage-gated calcium channel complexes. Mutations in CACNA1A cause a wide range of neurologic disturbances variably associated with cerebellar degeneration. Functional studies to date focus on electrophysiologic defects that do not adequately explain the phenotypic findings.

OBJECTIVE

To investigate whether some missense mutations might interfere with protein folding and trafficking, eventually leading to protein aggregation and neuronal injury.

METHODS

The authors studied the functional consequences of two pore missense mutations, C287Y and G293R, in two families with EA2, one newly discovered and the other previously reported. Both mutations caused episodic and interictal ataxia. The biophysical properties of mutant and wild type calcium channels were examined by whole-cell patch-clamp recordings in transfected COS-7 cells. The plasma membrane targeting was visualized by confocal fluorescence imaging on CaV2.1 tagged with green fluorescent protein.

RESULTS

The mutant channels exhibited a marked reduction in current expression and deficiencies in plasma membrane targeting.

CONCLUSIONS

In addition to altered channel function, the deficiency in protein misfolding and trafficking associated with the C287Y and G293R mutants may contribute to the slowly progressive cerebellar ataxia.

Optical detection of rate-determining ion-modulated conformational changes of the ether-à-go-go K+ channel voltage sensor

In voltage-dependent ether-à-go-go (eag) K+ channels, the process of activation is modulated by Mg2+ and other divalent cations, which bind to a site in the voltage sensor and slow channel opening. Previous analysis of eag ionic and gating currents indicated that Mg2+ has a much larger effect on ionic than gating current kinetics. From this, we hypothesized that ion binding modulates voltage sensor conformational changes that are poorly represented in gating current recordings. We have now tested this proposal by using a combined electrophysiological and optical approach. We find that a fluorescent probe attached near S4 in the voltage sensor reports on two phases of the activation process. One component of the optical signal corresponds to the main charge-moving conformational changes of the voltage sensor. This is the phase of activation that is well represented in gating current recordings. Another component of the optical signal reflects voltage sensor conformational changes that occur at more hyperpolarized potentials. These transitions, which are rate-determining for activation and highly modulated by Mg2+, have not been detected in gating current recordings. Our results demonstrate that the eag voltage sensor undergoes conformational changes that have gone undetected in electrical measurements. These transitions account for the time course of eag activation in the presence and absence of extracellular Mg2+.

Binding site in eag voltage sensor accommodates a variety of ions and is accessible in closed channel

In ether-a-go-go K+ channels, voltage-dependent activation is modulated by ion binding to a site located in an extracellular-facing crevice between transmembrane segments S2 and S3 in the voltage sensor. We find that acidic residues D278 in S2 and D327 in S3 are able to coordinate a variety of divalent cations, including Mg2+, Mn2+, and Ni2+, which have qualitatively similar functional effects, but different half-maximal effective concentrations. Our data indicate that ions binding to individual voltage sensors in the tetrameric channel act without cooperativity to modulate activation gating. We have taken advantage of the unique phenotype of Ni2+ in the D274A channel, which contains a mutation of a nonbinding site residue, to demonstrate that ions can access the binding site from the extracellular solution when the voltage sensor is in the resting conformation. Our results are difficult to reconcile with the x-ray structure of the KvAP K+ channel, in which the binding site residues are widely separated, and with the hydrophobic paddle model for voltage-dependent activation, in which the voltage sensor domain, including the S3-S4 loop, is near the cytoplasmic side of the membrane in the closed channel.

Voltage sensor mutations differentially target misfolded K+ channel subunits to proteasomal and non-proteasomal disposal pathways

In Shaker K(+) channels, formation of an electrostatic interaction between two charged residues, D316 and K374 in transmembrane segments S3 and S4, respectively, is a key step in voltage sensor biogenesis. Mutations D316K and K374E disrupt formation of the voltage sensor and lead to endoplasmic reticulum retention. We have now investigated the fates of these misfolded proteins. Both are significantly less stable than the wild-type protein. D316K is degraded by cytoplasmic proteasomes, whereas K374E is degraded by a lactacystin-insensitive, non-proteasomal pathway. Our results suggest that the D316K and K374E proteins are misfolded in recognizably different ways, an observation with implications for voltage sensor biogenesis.

BK channels: the spring between sensor and gate

K+ channels contain two main functional domains, an ion-selective pore and a sensor that determines whether the cytoplasmic pore gate is open or closed. In this issue of Neuron, Niu et al. provide compelling evidence that the link between sensor and gate is a remarkably simple mechanical spring.

Transient calnexin interaction confers long-term stability on folded K+ channel protein in the ER

We recently showed that an unglycosylated form of the Shaker potassium channel protein is retained in the endoplasmic reticulum (ER) and degraded by proteasomes in mammalian cells despite apparently normal folding and assembly. These results suggest that channel proteins with a native structure can be substrates for ER-associated degradation. We have now tested this hypothesis using the wild-type Shaker protein. Wild-type Shaker is degraded by cytoplasmic proteasomes when it is trapped in the ER and prevented from interacting with calnexin. Neither condition alone is sufficient to destabilize the protein. Proteasomal degradation of the wild-type protein is abolished when ER mannosidase I trimming of the core glycan is inhibited. Our results indicate that transient interaction with calnexin provides long-term protection from ER-associated degradation.

Critical assessment of a proposed model of Shaker

Detailed three-dimensional structures at atomic resolution are essential to understand how voltage-activated K(+) channels function. The X-ray crystallographic structure of the KvAP channel has offered the first view at atomic resolution of the molecular architecture of a voltage-activated K(+) channel. In the crystal, the voltage sensors are bound by monoclonal Fab fragments, which apparently induce a non-native conformation of the tetrameric channel. Thus, despite this significant advance our knowledge of the native conformation of a Kv channel in a membrane remains incomplete. Numerous results from different experimental approaches provide very specific constraints on the structure of K(+) channels in functional conformations. These results can be used to go further in trying to picture the native conformation of voltage-gated K(+) channels. However, the direct translation of all the available information into three-dimensional models is not straightforward and many questions about the structure of voltage-activated K(+) channels are still unanswered. Our aim in this review is to summarize the most important pieces of information currently available and to provide a critical assessment of the model of Shaker recently proposed by Lainé et al.

Atomic proximity between S4 segment and pore domain in Shaker potassium channels

A recently proposed model for voltage-dependent activation in K+ channels, largely influenced by the KvAP X-ray structure, suggests that S4 is located at the periphery of the channel and moves through the lipid bilayer upon depolarization. To investigate the physical distance between S4 and the pore domain in functional channels in a native membrane environment, we engineered pairs of cysteines, one each in S4 and the pore of Shaker channels, and identified two instances of spontaneous intersubunit disulfide bond formation, between R362C/A419C and R362C/F416C. After reduction, these cysteine pairs bound Cd2+ with high affinity, verifying that the residues are in atomic proximity. Molecular modeling based on the MthK structure revealed a single position for S4 that was consistent with our results and many other experimental constraints. The model predicts that S4 is located in the groove between pore domains from different subunits, rather than at the periphery of the protein.

Structural basis of two-stage voltage-dependent activation in K+ channels

The structure of the voltage sensor and the detailed physical basis of voltage-dependent activation in ion channels have not been determined. We now have identified conserved molecular rearrangements underlying two major voltage-dependent conformational changes during activation of divergent K(+) channels, ether-à-go-go (eag) and Shaker. Two conserved arginines of the S4 voltage sensor move sequentially into an extracellular gating pocket, where they interact with an acidic residue in S2. In eag, these transitions are modulated by a divalent ion that binds in the gating pocket. Conservation of key molecular details in the activation mechanism confirms that voltage sensors in divergent K(+) channels share a common structure. Molecular modeling reveals that structural constraints derived from eag and Shaker specify the unique packing arrangement of transmembrane segments S2, S3, and S4 within the voltage sensor.

Structural organization of the voltage sensor in voltage-dependent potassium channels

The structural organization of the voltage sensor in K+ channels has been investigated by second site suppressor analysis in Shaker and by identification of a metal ion binding site in ether-à-go-go (eag). In Shaker, two groups of interacting charged residues have been identified. K374 in the S4 segment interacts with E293 in S2 and D316 in S3, whereas E283 in S2 interacts with R368 and R371, two voltage-sensing residues in S4. Interactions of E283 with R368 and R371 are voltage dependent. The results suggest that E283 is located in a water-filled pocket near the extracellular surface of the protein. During voltage-dependent activation of Shaker channels, R368 and R371 move into this pocket and come into proximity with E283. In eag channels, extracellular Mg2+ directly modulates the activation process by binding to two acidic residues that are located in an analogous pocket. These acidic residues are found only in eag family members, accounting for the specificity of Mg2+ modulation to that family. These compatible results from Shaker and eag suggest a model for the packing and conformational changes of transmembrane segments in the voltage sensor of K+ channels.

Loss-of-function EA2 mutations are associated with impaired neuromuscular transmission

OBJECTIVE

To examine the functional consequences of episodic ataxia type 2 (EA2)-causing nonsense and missense mutations in vitro and to characterize the basis of fluctuating weakness in patients with E2A.

BACKGROUND

Mutations in CACNA1A encoding the Ca(v)2.1 calcium channel subunit cause EA2 through incompletely understood mechanisms. Although the Ca(v)2.1 subunit is important for neurotransmission at the neuromuscular junction, weakness has not been considered a feature of EA2.

METHODS

The disease-causing mutations in three unrelated patients with EA2 and fluctuating weakness were identified by mutation screening and sequencing. Mutant constructs harboring mutations R1281X, F1406C, R1549X were transfected into COS7 cells and expressed for patch clamp studies. Single-fiber electromyography (SFEMG) was performed in patients to examine synaptic transmission at the neuromuscular junction.

RESULTS

Functional studies in COS7 cells of nonsense and missense EA2 mutants demonstrated markedly decreased current densities compared with wild type. SFEMG demonstrated jitter and blocking in these patients with EA2, compared with normal subjects and three patients with SCA-6.

CONCLUSION

EA2-causing missense and nonsense mutations in CACNA1A produced mutant channels with diminished whole cell calcium channel activity in vitro due to loss of function. Altered biophysical properties or reduced efficiency of plasma membrane targeting of mutant channels may contribute to abnormal neuromuscular transmission, manifesting as myasthenic syndrome.

Glycosylation increases potassium channel stability and surface expression in mammalian cells

N-linked glycosylation is not required for the cell surface expression of functional Shaker potassium channels in Xenopus oocytes (Santacruz-Toloza, L., Huang, Y., John, S. A., and Papazian, D. M. (1994) Biochemistry 33, 5607-5613). We have now investigated whether glycosylation increases the stability, cell surface expression, and proper folding of Shaker protein expressed in mammalian cells. The turnover rates of wild-type protein and an unglycosylated mutant (N259Q,N263Q) were compared in pulse-chase experiments. The wild-type protein was stable, showing little degradation after 48 h. In contrast, the unglycosylated mutant was rapidly degraded (t(1/2) = approximately 18 h). Lactacystin slowed the degradation of the mutant protein, implicating cytoplasmic proteasomes in its turnover. Rapid lactacystin-sensitive degradation could be conferred on wild-type Shaker by a glycosylation inhibitor. Expression of the unglycosylated mutant on the cell surface, assessed using immunofluorescence microscopy and biotinylation, was dramatically reduced compared with wild type. Folding and assembly were analyzed by oxidizing intersubunit disulfide bonds, which provides a fortuitous hallmark of the native structure. Surprisingly, formation of disulfide-bonded adducts was quantitatively similar in the wild-type and unglycosylated mutant proteins. Our results indicate that glycosylation increases the stability and cell surface expression of Shaker protein but has little effect on acquisition of the native structure.

Mg(2+) modulates voltage-dependent activation in ether-à-go-go potassium channels by binding between transmembrane segments S2 and S3

Extracellular Mg(2+) directly modulates voltage-dependent activation in ether-à-go-go (eag) potassium channels, slowing the kinetics of ionic and gating currents (Tang, C.-Y., F. Bezanilla, and D.M. Papazian. 2000. J. Gen. Physiol. 115:319-337). To exert its effect, Mg(2+) presumably binds to a site in or near the eag voltage sensor. We have tested the hypothesis that acidic residues unique to eag family members, located in transmembrane segments S2 and S3, contribute to the Mg(2+)-binding site. Two eag-specific acidic residues and three acidic residues found in the S2 and S3 segments of all voltage-dependent K(+) channels were individually mutated in Drosophila eag, mutant channels were expressed in Xenopus oocytes, and the effect of Mg(2+) on ionic current kinetics was measured using a two electrode voltage clamp. Neutralization of eag-specific residues D278 in S2 and D327 in S3 eliminated Mg(2+)-sensitivity and mimicked the slowing of activation kinetics caused by Mg(2+) binding to the wild-type channel. These results suggest that Mg(2+) modulates activation kinetics in wild-type eag by screening the negatively charged side chains of D278 and D327. Therefore, these residues are likely to coordinate the bound ion. In contrast, neutralization of the widely conserved residues D284 in S2 and D319 in S3 preserved the fast kinetics seen in wild-type eag in the absence of Mg(2+), indicating that D284 and D319 do not mediate the slowing of activation caused by Mg(2+) binding. Mutations at D284 affected the eag gating pathway, shifting the voltage dependence of Mg(2+)-sensitive, rate limiting transitions in the hyperpolarized direction. Another widely conserved residue, D274 in S2, is not required for Mg(2+) sensitivity but is in the vicinity of the binding site. We conclude that Mg(2+) binds in a water-filled pocket between S2 and S3 and thereby modulates voltage-dependent gating. The identification of this site constrains the packing of transmembrane segments in the voltage sensor of K(+) channels, and suggests a molecular mechanism by which extracellular cations modulate eag activation kinetics.

Extracellular Mg(2+) modulates slow gating transitions and the opening of Drosophila ether-à-Go-Go potassium channels

We have characterized the effects of prepulse hyperpolarization and extracellular Mg(2+) on the ionic and gating currents of the Drosophila ether-à-go-go K(+) channel (eag). Hyperpolarizing prepulses significantly slowed channel opening elicited by a subsequent depolarization, revealing rate-limiting transitions for activation of the ionic currents. Extracellular Mg(2+) dramatically slowed activation of eag ionic currents evoked with or without prepulse hyperpolarization and regulated the kinetics of channel opening from a nearby closed state(s). These results suggest that Mg(2+) modulates voltage-dependent gating and pore opening in eag channels. To investigate the mechanism of this modulation, eag gating currents were recorded using the cut-open oocyte voltage clamp. Prepulse hyperpolarization and extracellular Mg(2+) slowed the time course of ON gating currents. These kinetic changes resembled the results at the ionic current level, but were much smaller in magnitude, suggesting that prepulse hyperpolarization and Mg(2+) modulate gating transitions that occur slowly and/or move relatively little gating charge. To determine whether quantitatively different effects on ionic and gating currents could be obtained from a sequential activation pathway, computer simulations were performed. Simulations using a sequential model for activation reproduced the key features of eag ionic and gating currents and their modulation by prepulse hyperpolarization and extracellular Mg(2+). We have also identified mutations in the S3-S4 loop that modify or eliminate the regulation of eag gating by prepulse hyperpolarization and Mg(2+), indicating an important role for this region in the voltage-dependent activation of eag.

Voltage-dependent structural interactions in the Shaker K(+) channel

Using a strategy related to intragenic suppression, we previously obtained evidence for structural interactions in the voltage sensor of Shaker K(+) channels between residues E283 in S2 and R368 and R371 in S4 (Tiwari-Woodruff, S.K., C.T. Schulteis, A.F. Mock, and D. M. Papazian. 1997. Biophys. J. 72:1489-1500). Because R368 and R371 are involved in the conformational changes that accompany voltage-dependent activation, we tested the hypothesis that these S4 residues interact with E283 in S2 in a subset of the conformational states that make up the activation pathway in Shaker channels. First, the location of residue 283 at hyperpolarized and depolarized potentials was inferred by substituting a cysteine at that position and determining its reactivity with hydrophilic, sulfhydryl-specific probes. The results indicate that position 283 reacts with extracellularly applied sulfhydryl reagents with similar rates at both hyperpolarized and depolarized potentials. We conclude that E283 is located near the extracellular surface of the protein in both resting and activated conformations. Second, we studied the functional phenotypes of double charge reversal mutations between positions 283 and 368 and between 283 and 371 to gain insight into the conformations in which these positions approach each other most closely. We found that combining charge reversal mutations at positions 283 and 371 stabilized an activated conformation of the channel, and dramatically slowed transitions into and out of this state. In contrast, charge reversal mutations at positions 283 and 368 stabilized a closed conformation, which by virtue of the inferred position of 368 corresponds to a partially activated (intermediate) closed conformation. From these results, we propose a preliminary model for the rearrangement of structural interactions of the voltage sensor during activation of Shaker K(+) channels.

Calnexin associates with Shaker K+ channel protein but is not involved in quality control of subunit folding or assembly

Calnexin is part of an ER chaperone system that monitors and promotes the proper folding and assembly of glycosylated membrane proteins. To investigate the role of calnexin in the biogenesis of the voltage-dependent Shaker K+ channel, wild-type and mutant Shaker proteins were expressed in mammalian cells. Association with calnexin was assayed by coimmunoprecipitation. Calnexin interacted transiently with wild-type Shaker protein in the ER. In contrast, calnexin failed to associate with an unglycosylated Shaker mutant that makes active, cell surface channels. Therefore, glycosylation of Shaker protein is required for association with calnexin, but calnexin is not required for the proper folding and assembly of Shaker channels. We also investigated whether calnexin is involved in the ER retention of mutant Shaker proteins defective in subunit folding, assembly, or pore formation. Each of the mutant proteins associated transiently with calnexin during biogenesis. Calnexin dissociated from wild-type and mutant proteins with similar time courses. Thus, non-native Shaker proteins escape the folding sensor of the calnexin chaperone system. Furthermore, stable association with calnexin is not the mechanism by which these mutant proteins are retained in the ER. Our results indicate that calnexin is not involved in the quality control of subunit folding, assembly, or pore formation in Shaker K+ channels.

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.

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.

How Does an Ion Channel Sense Voltage?

Voltage-dependent ion channels underlie nerve and muscle excitability. Recent studies have increased our understanding of how voltage controls channel activity. Charged residues in transmembrane segments that contribute to the voltage sensor have been identified. During activation, voltage-sensing residues traverse a significant fraction of the transmembrane electric field, dramatically increasing the probability that the channel will enter an open, ion-conducting state.

Electrostatic interactions between transmembrane segments mediate folding of Shaker K+ channel subunits

In voltage-dependent Shaker K+ channels, charged residues E293 in transmembrane segment S2 and R365, R368, and R371 in S4 contribute significantly to the gating charge movement that accompanies activation. Using an intragenic suppression strategy, we have now probed for structural interaction between transmembrane segments S2, S3, and S4 in Shaker channels. Charge reversal mutations of E283 in S2 and K374 in S4 disrupt maturation of the protein. Maturation was specifically and efficiently rescued by second-site charge reversal mutations, indicating that electrostatic interactions exist between E283 in S2 and R368 and R371 in S4, and between K374 in S4 and E293 in S2 and D316 in S3. Rescued subunits were incorporated into functional channels, demonstrating that a native structure was restored. Our data indicate that K374 interacts with E293 and D316 within the same subunit. These electrostatic interactions mediate the proper folding of the protein and are likely to persist in the native structure. Our results raise the possibility that the S4 segment is tilted relative to S2 and S3 in the voltage-sensing domain of Shaker channels. Such an arrangement might provide solvent access to voltage-sensing residues, which we find to be highly tolerant of mutations.

Transfer of voltage independence from a rat olfactory channel to the Drosophila ether-à-go-go K+ channel

The S4 segment is an important part of the voltage sensor in voltage-gated ion channels. Cyclic nucleotide-gated channels, which are members of the superfamily of voltage-gated channels, have little inherent sensitivity to voltage despite the presence of an S4 segment. We made chimeras between a voltage-independent rat olfactory channel (rolf) and the voltage-dependent ether-à-go-go K+ channel (eag) to determine the basis of their divergent gating properties. We found that the rolf S4 segment can support a voltage-dependent mechanism of activation in eag, suggesting that rolf has a potentially functional voltage sensor that is silent during gating. In addition, we found that the S3-S4 loop of rolf increases the relative stability of the open conformation of eag, effectively converting eag into a voltage-independent channel. A single charged residue in the loop makes a significant contribution to the relative stabilization of the open stage in eag. Our data suggest that cyclic nucleotide-gated channels such as rolf contain a voltage sensor which, in the physiological voltage range, is stabilized in an activated conformation that is permissive for pore opening.

Potassium channel alpha and beta subunits assemble in the endoplasmic reticulum

We have characterized the maturation of Shaker K+ channel protein and the cellular site of assembly of pore-forming alpha and cytoplasmic beta subunits in a transfected mammalian cell line. Shaker protein is made as a partially glycosylated, immature precursor that is converted to a fully glycosylated, mature product. Shaker protein did not mature when transport from the endoplasmic reticulum (ER) to the Golgi apparatus was blocked. Consistent with this finding, only the immature form was sensitive to digestion with endoglycosidase H. These results indicate that the immature protein is core-glycosylated in the ER, whereas the oligosaccharides of the mature protein have been further processed in the Golgi compartment. After inhibiting ER-to-Golgi transport, the oligomeric state of Shaker subunits was assessed by cross-linking in intact cells or by solubilization and sucrose gradient sedimentation. The results indicate that Shaker subunits assemble with each other in the ER. When co-expressed, the Kvbeta2 subunit also associated with Shaker in the ER. Assembly with the beta2 subunit did not increase the rate or extent of Shaker protein maturation. Our results indicate that the biogenesis of Shaker K+ channels in vivo involves core glycosylation and subunit assembly in the ER, followed by efficient transfer to the Golgi apparatus where the oligosaccharides are modified.

Intersubunit interaction between amino- and carboxyl-terminal cysteine residues in tetrameric shaker K+ channels

Shaker potassium (K+) channels normally lack intrasubunit and intersubunit disulfide bonds. However, disulfide bonds are formed between Shaker subunits in intact cells exposed to oxidizing conditions. Upon electrophoresis under nonreducing conditions, intersubunit disulfide bond formation was detected by the presence of four high molecular weight adducts of Shaker protein. This result suggests that intracellular cysteine residues are in sufficiently close proximity in the native structure of the Shaker channel to form intersubunit disulfide bonds. To test this hypothesis, wild-type and mutant Shaker proteins were exposed to oxidizing conditions in intact cells. Intersubunit disulfide bond formation was eliminated upon serine substitution of either C96 in the amino terminal or C505 in the carboxyl terminal of the protein. In contrast, disulfide bond formation was not eliminated upon serine substitution of both C301 and C308 in the cytoplasmic loop between transmembrane segments S2 and S3. Exposure of Shaker-expressing cells to oxidizing conditions did not significantly alter the amplitude, kinetics, or voltage dependence of the Shaker current, demonstrating that the native tertiary and quaternary structures of the channel were maintained under oxidizing conditions. These results indicate that intersubunit disulfide bonds form between C96 and C505, providing evidence that the amino- and carboxyl-terminal regions of adjacent subunits are in proximity in the native structure of the channel. The disulfide-bonded adducts were found to represent a dimer, a trimer, and two forms of tetramer, one linear and one circular, containing one, two, three, or four disulfide bonds, respectively. These results provide a direct biochemical demonstration that Shaker K+ channels contain four pore-forming subunits.

Voltage-sensing residues in the S2 and S4 segments of the Shaker K+ channel

The activation of Shaker K+ channels is steeply voltage dependent. To determine whether conserved charged amino acids in putative transmembrane segments S2, S3, and S4 contribute to the gating charge of the channel, the total gating charge movement per channel was measured in channels containing neutralization mutations. Of eight residues tested, four contributed significantly to the gating charge: E293, an acidic residue in S2, and R365, R368, and R371, three basic residues in the S4 segment. The results indicate that these residues are a major component of the voltage sensor. Furthermore, the S4 segment is not solely responsible for gating charge movement in Shaker K+ channels.

Electrostatic interactions of S4 voltage sensor in Shaker K+ channel

The S4 segment comprises part of the voltage sensor in Shaker K+ channels. We have used a strategy similar to intragenic suppression, but without a genetic selection, to identify electrostatic interactions of the S4 segment that may be important in the mechanism of voltage-dependent activation. The S4 neutralization mutations K374Q and R377Q block maturation of the protein, suggesting that they prevent proper folding. K374Q is specifically and efficiently rescued by the second site mutations E293Q and D316N, located in putative transmembrane segments S2 and S3, respectively. These results suggest that K374, E293, and D316 form a network of strong, local, electrostatic interactions that stabilize the structure of the channel. Some other double mutant combinations result in inefficient suppression, identifying weak, presumably long-range electrostatic interactions. A simple structural hypothesis is proposed to account for the effects of the rescued double mutant combinations on the relative stabilities of open and closed channel conformations.

Conserved cysteine residues in the shaker K+ channel are not linked by a disulfide bond

Many voltage-activated K+ channels contain two conserved cysteine residues in putative transmembrane segments S2 and S6. It has been proposed that these cysteines form an intrasubunit disulfide bond [Guy, H.R., & Conti, F. (1990) Trends Neurosci. 13, 201-206]. This proposal was tested using site-directed mutagenesis followed by electrophysiological and biochemical analysis of the Shaker B K+ channel. Each Shaker B subunit contains seven cysteine residues, including the conserved residues C286 and C462 and a less conserved cysteine, C245. Each cysteine in the Shaker B protein can be mutated individually without eliminating functional activity, indicating that the protein does not contain a disulfide bond that is essential for protein folding or the assembly of active channels. To determine whether there is a nonessential disulfide bond, Shaker B protein was subjected to limited proteolysis. Fragments were analyzed by electrophoresis under reducing and nonreducing conditions followed by immunoblotting. The results indicate that the two conserved residues C286 and C462 do not form a disulfide bond with each other or with C245. In addition, the subunits are not linked by disulfide bonds. In HEK293T cells, Shaker B protein is first made as an incompletely glycosylated precursor that is converted to the fully glycosylated mature protein. Glycosylation occurs at two positions in the S1-S2 loop.

Glycosylation of shaker potassium channel protein in insect cell culture and in Xenopus oocytes

We have studied the glycosylation of Shaker K+ channel protein made in two expression systems: an insect cell culture line and amphibian oocytes. In both systems, two potential sites for N-linked glycosylation were modified. The modified sites were located between the first and second putative transmembrane segments, S1 and S2. Although the same sites appeared to be glycosylated in both systems, the fraction of protein glycosylated and the size, structure, or composition of the oligosaccharide chains added were quite different. The results indicate that the S1-S2 loop is extracellular, consistent with a cytoplasmic location for the N-terminus and a transmembrane disposition for hydrophobic segment S1. We have also shown that glycosylation occurs in two stages in oocytes, generating an immature and a mature form of Shaker protein. However, glycosylation is not required either for the assembly of functional channels or for their transport to the cell surface.

Purification and reconstitution of functional Shaker K+ channels assayed with a light-driven voltage-control system

Voltage-dependent potassium channels are integral membrane proteins that control the excitability of nerve and muscle. The cloning of genes for K+ channels has led to structure/function analysis using a combination of site-directed mutagenesis and electrophysiology. As a result, much has been learned about how these proteins work. A deeper understanding of their function will require detailed structural characterization, however. We now report the purification of Shaker K+ channels from an insect expression system using immunoaffinity methods. The purified channels have been reconstituted, assayed using a novel, light-driven, vesicular voltage-control system, and shown to be functional. This approach will enable us to compare and optimize methods for protein production and purification. Purification of active protein is a prerequisite for detailed structural analysis, since activity is the key indication that the structural integrity of the channel has been preserved during biochemical procedures. Thus, this work represents a first step toward the determination of the structure of Shaker K+ channels.