Divalent cations slow activation of EAG family K+ channels through direct binding to S4

Kv12-encoded K+ channels drive the day-night switch in the repetitive firing rates of SCN neurons

Considerable evidence suggests that day-night rhythms in the functional expression of subthreshold potassium (K+) channels regulate daily oscillations in the spontaneous firing rates of neurons in the suprachiasmatic nucleus (SCN), the master circadian pacemaker in mammals. The K+ conductance(s) driving these daily rhythms in the repetitive firing rates of SCN neurons, however, have not been identified. To test the hypothesis that subthreshold Kv12.1/Kv12.2-encoded K+ channels play a role, we obtained current-clamp recordings from SCN neurons in slices prepared from adult mice harboring targeted disruptions in the Kcnh8 (Kv12.1-/-) or Kcnh3 (Kv12.2-/-) locus. We found that mean nighttime repetitive firing rates were higher in Kv12.1-/- and Kv12.2-/- than in wild type (WT), SCN neurons. In marked contrast, mean daytime repetitive firing rates were similar in Kv12.1-/-, Kv12.2-/-, and WT SCN neurons, and the day-night difference in mean repetitive firing rates, a hallmark feature of WT SCN neurons, was eliminated in Kv12.1-/- and Kv12.2-/- SCN neurons. Similar results were obtained with in vivo shRNA-mediated acute knockdown of Kv12.1 or Kv12.2 in adult SCN neurons. Voltage-clamp experiments revealed that Kv12-encoded current densities in WT SCN neurons are higher at night than during the day. In addition, the pharmacological block of Kv12-encoded currents increased the mean repetitive firing rate of nighttime, but not daytime, in WT SCN neurons. Dynamic clamp-mediated subtraction of modeled Kv12-encoded currents also selectively increased the mean repetitive firing rates of nighttime WT SCN neurons. Despite the elimination of the nighttime decrease in the mean repetitive firing rates of SCN neurons, however, locomotor (wheel-running) activity remained rhythmic in Kv12.1-/-, Kv12.2-/-, and Kv12.1-targeted shRNA-expressing, and Kv12.2-targeted shRNA-expressing animals.

Zinc activation of OTOP proton channels identifies structural elements of the gating apparatus

Otopetrin proteins (OTOPs) form proton-selective ion channels that are expressed in diverse cell types where they mediate detection of acids or regulation of pH. In vertebrates there are three family members: OTOP1 is required for formation of otoconia in the vestibular system and it forms the receptor for sour taste, while the functions of OTOP2 and OTOP3 are not yet known. Importantly, the gating mechanisms of any of the OTOP channels are not well understood. Here, we show that zinc (Zn²⁺⁾, as well as other transition metals including copper (Cu²⁺), potently activates murine OTOP3 (mOTOP3). Zn²⁺ pre-exposure increases the magnitude of mOTOP3 currents to a subsequent acid stimulus by as much as 10-fold. In contrast, mOTOP2 currents are insensitive to activation by Zn²⁺. Swapping the extracellular tm 11-12 linker between mOTOP3 and mOTOP2 was sufficient to eliminate Zn²⁺ activation of mOTOP3 and confer Zn²⁺ activation on mOTOP2. Mutation to alanine of H531 and E535 within the tm 11-12 linker and H234 and E238 within the 5-6 linker reduced or eliminated activation of mOTOP3 by Zn²⁺, indicating that these residues likely contribute to the Zn²⁺ activating site. Kinetic modeling of the data is consistent with Zn²⁺ stabilizing the opn2+en state of the channel, competing with H⁺ for activation of the channels. These results establish the tm 11-12 and tm 5-6 linkers as part of the gating apparatus of OTOP channels and a target for drug discovery. Zn²⁺ is an essential micronutrient and its activation of OTOP channels will undoubtedly have important physiological sequelae.

Overcoming challenges of HERG potassium channel liability through rational design: Eag1 inhibitors for cancer treatment

Two decades of research have proven the relevance of ion channel expression for tumor progression in virtually every indication, and it has become clear that inhibition of specific ion channels will eventually become part of the oncology therapeutic arsenal. However, ion channels play relevant roles in all aspects of physiology, and specificity for the tumor tissue remains a challenge to avoid undesired effects. Eag1 (K_V 10.1) is a voltage-gated potassium channel whose expression is very restricted in healthy tissues outside of the brain, while it is overexpressed in 70% of human tumors. Inhibition of Eag1 reduces tumor growth, but the search for potent inhibitors for tumor therapy suffers from the structural similarities with the cardiac HERG channel, a major off-target. Existing inhibitors show low specificity between the two channels, and screenings for Eag1 binders are prone to enrichment in compounds that also bind HERG. Rational drug design requires knowledge of the structure of the target and the understanding of structure-function relationships. Recent studies have shown subtle structural differences between Eag1 and HERG channels with profound functional impact. Thus, although both targets' structure is likely too similar to identify leads that exclusively bind to one of the channels, the structural information combined with the new knowledge of the functional relevance of particular residues or areas suggests the possibility of selective targeting of Eag1 in cancer therapies. Further development of selective Eag1 inhibitors can lead to first-in-class compounds for the treatment of different cancers.

External Cd2+ and protons activate the hyperpolarization-gated K+ channel KAT1 at the voltage sensor

The functionally diverse cyclic nucleotide binding domain (CNBD) superfamily of cation channels contains both depolarization-gated (e.g., metazoan EAG family K+ channels) and hyperpolarization-gated channels (e.g., metazoan HCN pacemaker cation channels and the plant K+ channel KAT1). In both types of CNBD channels, the S4 transmembrane helix of the voltage sensor domain (VSD) moves outward in response to depolarization. This movement opens depolarization-gated channels and closes hyperpolarization-gated channels. External divalent cations and protons prevent or slow movement of S4 by binding to a cluster of acidic charges on the S2 and S3 transmembrane domains of the VSD and therefore inhibit activation of EAG family channels. However, a similar divalent ion/proton binding pocket has not been described for hyperpolarization-gated CNBD family channels. We examined the effects of external Cd2+ and protons on Arabidopsisthaliana KAT1 expressed in Xenopus oocytes and found that these ions strongly potentiate voltage activation. Cd2+ at 300 µM depolarizes the V50 of KAT1 by 150 mV, while acidification from pH 7.0 to 4.0 depolarizes the V50 by 49 mV. Regulation of KAT1 by Cd2+ is state dependent and consistent with Cd2+ binding to an S4-down state of the VSD. Neutralization of a conserved acidic charge in the S2 helix in KAT1 (D95N) eliminates Cd2+ and pH sensitivity. Conversely, introduction of acidic residues into KAT1 at additional S2 and S3 cluster positions that are charged in EAG family channels (N99D and Q149E in KAT1) decreases Cd2+ sensitivity and increases proton potentiation. These results suggest that KAT1, and presumably other hyperpolarization-gated plant CNBD channels, can open from an S4-down VSD conformation homologous to the divalent/proton-inhibited conformation of EAG family K+ channels.

Voltage vs. Ligand I: Structural basis of the intrinsic flexibility of S3 segment and its significance in ion channel activation

We systematically predict the internal flexibility of the S3 segment, one of the most mobile elements in the voltage-sensor domain. By analyzing the primary amino acid sequences of V-sensor containing proteins, including Hv1, TPC channels and the voltage-sensing phosphatases, we established correlations between the local flexibility and modes of activation for different members of the VGIC superfamily. Taking advantage of the structural information available, we also assessed structural aspects to understand the role played by the flexibility of S3 during the gating of the pore. We found that S3 flexibility is mainly determined by two specific regions: (1) a short NxxD motif in the N-half portion of the helix (S3a), and (2) a short sequence at the beginning of the so-called paddle motif where the segment has a kink that, in some cases, divide S3 into two distinct helices (S3a and S3b). A good correlation between the flexibility of S3 and the reported sensitivity to temperature and mechanical stretch was found. Thus, if the channel exhibits high sensitivity to heat or membrane stretch, local S3 flexibility is low. On the other hand, high flexibility of S3 is preferentially associated to channels showing poor heat and mechanical sensitivities. In contrast, we did not find any apparent correlation between S3 flexibility and voltage or ligand dependence. Overall, our results provide valuable insights into the dynamics of channel-gating and its modulation.

Cytosolic increased labile Zn2+ contributes to arrhythmogenic action potentials in left ventricular cardiomyocytes through protein thiol oxidation and cellular ATP depletion

Intracellular labile (free) Zn²⁺-level ([Zn²⁺]_i) is low and increases markedly under pathophysiological conditions in cardiomyocytes. High [Zn²⁺]_i is associated with alterations in excitability and ionic-conductances while exact mechanisms are not clarified yet. Therefore, we examined the elevated-[Zn²⁺]_i on some sarcolemmal ionic-mechanisms, which can mediate cardiomyocyte dysfunction. High-[Zn²⁺]_i induced significant changes in action potential (AP) parameters, including depolarization in resting membrane-potential and prolongations in AP-repolarizing phases. We detected also the time-dependent effects such as induction of spontaneous APs at the time of ≥ 3 min following [Zn²⁺]_i increases, a manner of cellular ATP dependent and reversible with disulfide-reducing agent dithiothreitol, DTT. High-[Zn²⁺]_i induced inhibitions in voltage-dependent K⁺-channel currents, such as transient outward K⁺-currents, I_to, steady-state currents, I_ss and inward-rectifier K⁺-currents, I_K1, reversible with DTT seemed to be responsible from the prolongations in APs. We, for the first time, demonstrated that lowering cellular ATP level induced significant decreaeses in both I_ss and I_K1, while no effect on I_to. However, the increased-[Zn²⁺]_i could induce marked activation in ATP-sensitive K⁺-channel currents, I_KATP, depending on low cellular ATP and thiol-oxidation levels of these channels. The mRNA levels of Kv4.3, Kv1.4 and Kv2.1 were depressed markedly with increased-[Zn²⁺]_i with no change in mRNA level of Kv4.2, while the mRNA level of I_KATP subunit, SUR2A was increased significantly with increased-[Zn²⁺]_i, being reversible with DTT. Overall we demonstrated that high-[Zn²⁺]_i, even if nanomolar levels, alters cardiac function via prolonged APs of cardiomyocytes, at most, due to inhibitions in voltage-dependent K⁺-currents, although activation of I_KATP is playing cardioprotective role, through some biochemical changes in cellular ATP- and thiol-oxidation levels. It seems, a well-controlled [Zn²⁺]_i can be novel therapeutic target for cardiac complications under pathological conditions including oxidative stress.

Ether-à-go-go K+ channels: effective modulators of neuronal excitability

Mammalian ether-à-go-go (EAG) channels are voltage-gated K⁺ channels. They are encoded by the KCNH gene family and divided into three subfamilies, eag (Kv10), erg (eag-related gene; Kv11) and elk (eag-like; Kv12). All EAG channel subtypes are expressed in the brain where they effectively modulate neuronal excitability. This Topical Review describes the biophysical properties of each of the EAG channel subtypes, their function in neurons and the neurological diseases induced by EAG channel mutations. In contrast to the function of erg currents in the heart, where they contribute to repolarization of the cardiac action potential, erg currents in neurons are involved in the maintenance of the resting potential, setting of action potential threshold and frequency accommodation. They can even support high frequency firing by preventing a depolarization-induced Na⁺ channel block. EAG channels are modulated differentially, e.g. eag channels by intracellular Ca²⁺ , erg channels by extracellular K⁺ and GPCRs, and elk channels by changes in pH. So far, only currents mediated by erg channels have been recorded in neurons with the help of selective blockers. Neuronal eag and elk currents have not been isolated due to the lack of suitable channel blockers. However, findings in KO mice indicate a physiological role of eag1 currents in synaptic transmission and an involvement of elk2 currents in cognitive performance. Human eag1 and eag2 gain-of-function mutations underlie syndromes associated with epileptic seizures.

Chronic Alcohol, Intrinsic Excitability, and Potassium Channels: Neuroadaptations and Drinking Behavior

Neural mechanisms underlying alcohol use disorder remain elusive, and this lack of understanding has slowed the development of efficacious treatment strategies for reducing relapse rates and prolonging abstinence. While synaptic adaptations produced by chronic alcohol exposure have been extensively characterized in a variety of brain regions, changes in intrinsic excitability of critical projection neurons are understudied. Accumulating evidence suggests that prolonged alcohol drinking and alcohol dependence produce plasticity of intrinsic excitability as measured by changes in evoked action potential firing and after-hyperpolarization amplitude. In this chapter, we describe functional changes in cell firing of projection neurons after long-term alcohol exposure that occur across species and in multiple brain regions. Adaptations in calcium-activated (K_Ca2), voltage-dependent (K_V7), and G protein-coupled inwardly rectifying (K_ir3 or GIRK) potassium channels that regulate the evoked firing and after-hyperpolarization parallel functional changes in intrinsic excitability induced by chronic alcohol. Moreover, there are strong genetic links between alcohol-related behaviors and genes encoding K_Ca2, K_V7, and GIRK channels, and pharmacologically targeting these channels reduces alcohol consumption and alcohol-related behaviors. Together, these studies demonstrate that chronic alcohol drinking produces adaptations in K_Ca2, K_V7, and GIRK channels leading to impaired regulation of the after-hyperpolarization and aberrant cell firing. Correcting the deficit in the after-hyperpolarization with positive modulators of K_Ca2 and K_V7 channels and altering the GIRK channel binding pocket to block the access of alcohol represent a potentially highly effective pharmacological approach that can restore changes in intrinsic excitability and reduce alcohol consumption in affected individuals.

Gating mechanisms of voltage-gated proton channels

Hv1 is a voltage-gated proton-selective channel that plays critical parts in host defense, sperm motility, and cancer progression. Hv1 contains a conserved voltage-sensor domain (VSD) that is shared by a large family of voltage-gated ion channels, but it lacks a pore domain. Voltage sensitivity and proton conductivity are conferred by a unitary VSD that consists of four transmembrane helices. The architecture of Hv1 differs from that of cation channels that form a pore in the center among multiple subunits (as in most cation channels) or homologous repeats (as in voltage-gated sodium and calcium channels). Hv1 forms a dimer in which a cytoplasmic coiled coil underpins the two protomers and forms a single, long helix that is contiguous with S4, the transmembrane voltage-sensing segment. The closed-state structure of Hv1 was recently solved using X-ray crystallography. In this article, we discuss the gating mechanism of Hv1 and focus on cooperativity within dimers and their sensitivity to metal ions.

Ether-à-go-go family voltage-gated K+ channels evolved in an ancestral metazoan and functionally diversified in a cnidarian-bilaterian ancestor

We examined the evolutionary origins of the ether-à-go-go (EAG) family of voltage-gated K(+) channels, which have a strong influence on the excitability of neurons. The bilaterian EAG family comprises three gene subfamilies (Eag, Erg and Elk) distinguished by sequence conservation and functional properties. Searches of genome sequence indicate that EAG channels are metazoan specific, appearing first in ctenophores. However, phylogenetic analysis including two EAG family channels from the ctenophore Mnemiopsis leidyi indicates that the diversification of the Eag, Erg and Elk gene subfamilies occurred in a cnidarian/bilaterian ancestor after divergence from ctenophores. Erg channel function is highly conserved between cnidarians and mammals. Here we show that Eag and Elk channels from the sea anemone Nematostella vectensis (NvEag and NvElk) also share high functional conservation with mammalian channels. NvEag, like bilaterian Eag channels, has rapid kinetics, whereas NvElk activates at extremely hyperpolarized voltages, which is characteristic of Elk channels. Potent inhibition of voltage activation by extracellular protons is conserved between mammalian and Nematostella EAG channels. However, characteristic inhibition of voltage activation by Mg(2+) in Eag channels and Ca(2+) in Erg channels is reduced in Nematostella because of mutation of a highly conserved aspartate residue in the voltage sensor. This mutation may preserve sub-threshold activation of Nematostella Eag and Erg channels in a high divalent cation environment. mRNA in situ hybridization of EAG channels in Nematostella suggests that they are differentially expressed in distinct cell types. Most notable is the expression of NvEag in cnidocytes, a cnidarian-specific stinging cell thought to be a neuronal subtype.

The direct modulatory activity of zinc toward ion channels

The divalent zinc ion is a cation that plays an indispensable role as a structural constituent of numerous proteins, including enzymes and transcription factors. Recently, it has been suggested that zinc also plays a dynamic role in extracellular and intracellular signaling as well. Ion channels are pore-forming proteins that control the flow of specific ions across the membrane, which is important to maintain ion gradients. In this review, we outline the modulatory effect of zinc on the activities of several ion channels through direct binding of zinc into histidine, cysteine, aspartate, and glutamate moieties of channel proteins. The binding of zinc to ion channels results in the activation or inhibition of the channel due to conformational changes. These novel aspects of ion-channel activity modulation by zinc provide new insights into the physiological regulation of ion channels.

External protons destabilize the activated voltage sensor in hERG channels

Extracellular acidosis shifts hERG channel activation to more depolarized potentials and accelerates channel deactivation; however, the mechanisms underlying these effects are unclear. External divalent cations, e.g., Ca(2+) and Cd(2+), mimic these effects and coordinate within a metal ion binding pocket composed of three acidic residues in hERG: D456 and D460 in S2 and D509 in S3. A common mechanism may underlie divalent cation and proton effects on hERG gating. Using two-electrode voltage clamp, we show proton sensitivity of hERG channel activation (pKa = 5.6), but not deactivation, was greatly reduced in the presence of Cd(2+) (0.1 mM), suggesting a common binding site for the Cd(2+) and proton effect on activation and separable effects of protons on activation and deactivation. Mutational analysis confirmed that D509 plays a critical role in the pH dependence of activation, as shown previously, and that cooperative actions involving D456 and D460 are also required. Importantly, neutralization of all three acidic residues abolished the proton-induced shift of activation, suggesting that the metal ion binding pocket alone accounts for the effects of protons on hERG channel activation. Voltage-clamp fluorimetry measurements demonstrated that protons shifted the voltage dependence of S4 movement to more depolarized potentials. The data indicate a site and mechanism of action for protons on hERG activation gating; protonation of D456, D460 and D509 disrupts interactions between these residues and S4 gating charges to destabilize the activated configuration of S4.

External pH modulates EAG superfamily K+ channels through EAG-specific acidic residues in the voltage sensor

The Ether-a-go-go (EAG) superfamily of voltage-gated K⁺ channels consists of three functionally distinct gene families (Eag, Elk, and Erg) encoding a diverse set of low-threshold K⁺ currents that regulate excitability in neurons and muscle. Previous studies indicate that external acidification inhibits activation of three EAG superfamily K⁺ channels, Kv10.1 (Eag1), Kv11.1 (Erg1), and Kv12.1 (Elk1). We show here that Kv10.2, Kv12.2, and Kv12.3 are similarly inhibited by external protons, suggesting that high sensitivity to physiological pH changes is a general property of EAG superfamily channels. External acidification depolarizes the conductance–voltage (GV) curves of these channels, reducing low threshold activation. We explored the mechanism of this high pH sensitivity in Kv12.1, Kv10.2, and Kv11.1. We first examined the role of acidic voltage sensor residues that mediate divalent cation block of voltage activation in EAG superfamily channels because protons reduce the sensitivity of Kv12.1 to Zn²⁺. Low pH similarly reduces Mg²⁺ sensitivity of Kv10.1, and we found that the pH sensitivity of Kv11.1 was greatly attenuated at 1 mM Ca²⁺. Individual neutralizations of a pair of EAG-specific acidic residues that have previously been implicated in divalent block of diverse EAG superfamily channels greatly reduced the pH response in Kv12.1, Kv10.2, and Kv11.1. Our results therefore suggest a common mechanism for pH-sensitive voltage activation in EAG superfamily channels. The EAG-specific acidic residues may form the proton-binding site or alternatively are required to hold the voltage sensor in a pH-sensitive conformation. The high pH sensitivity of EAG superfamily channels suggests that they could contribute to pH-sensitive K⁺ currents observed in vivo.

Multistate structural modeling and voltage-clamp analysis of epilepsy/autism mutation Kv10.2-R327H demonstrate the role of this residue in stabilizing the channel closed state

Voltage-gated potassium channel Kv10.2 (KCNH5) is expressed in the nervous system, but its functions and involvement in human disease are poorly understood. We studied a human Kv10.2 channel mutation (R327H) recently identified in a child with epileptic encephalopathy and autistic features. Using multistate structural modeling, we demonstrate that the Arg327 residue in the S4 helix of voltage-sensing domain has strong ionic interactions with negatively charged residues within the S1-S3 helices in the resting (closed) and early-activation state but not in the late-activation and fully-activated (open) state. The R327H mutation weakens ionic interactions between residue 327 and these negatively charged residues, thus favoring channel opening. Voltage-clamp analysis showed a strong hyperpolarizing (∼70 mV) shift of voltage dependence of activation and an acceleration of activation. Our results demonstrate the critical role of the Arg327 residue in stabilizing the channel closed state and explicate for the first time the structural and functional change of a Kv10.2 channel mutation associated with neurological disease.

Kv3 channel assembly, trafficking and activity are regulated by zinc through different binding sites

Zinc, a divalent heavy metal ion and an essential mineral for life, regulates synaptic transmission and neuronal excitability via ion channels. However, its binding sites and regulatory mechanisms are poorly understood. Here, we report that Kv3 channel assembly, localization and activity are regulated by zinc through different binding sites. Local perfusion of zinc reversibly reduced spiking frequency of cultured neurons most likely by suppressing Kv3 channels. Indeed, zinc inhibited Kv3.1 channel activity and slowed activation kinetics, independent of its site in the N-terminal T1 domain. Biochemical assays surprisingly identified a novel zinc-binding site in the Kv3.1 C-terminus, critical for channel activity and axonal targeting, but not for the zinc inhibition. Finally, mutagenesis revealed an important role of the junction between the first transmembrane (TM) segment and the first extracellular loop in sensing zinc. Its mutant enabled fast spiking with relative resistance to the zinc inhibition. Therefore, our studies provide novel mechanistic insights into the multifaceted regulation of Kv3 channel activity and localization by divalent heavy metal ions.

Asymmetric divergence in structure and function of HCN channel duplicates in Ciona intestinalis

Hyperpolarization-activated Cyclic Nucleotide (HCN) channels are voltage-gated cation channels and are critical for regulation of membrane potential in electrically active cells. To understand the evolution of these channels at the molecular level, we cloned and examined two of three HCN homologs of the urochordate Ciona intestinalis (ciHCNa and ciHCNb). ciHCNa is like mammalian HCNs in that it possesses similar electrical function and undergoes N-glycosylation of a sequon near the pore. ciHCNb lacks the pore-associated N-glycosylation sequon and is predictably not N-glycosylated, and it also has an unusual gating phenotype in which the channel's voltage-sensitive gate appears to close incompletely. Together with previous findings, the data support an evolutionary trajectory in which an HCN ancestor underwent lineage-specific duplication in Ciona, to yield one HCN with most features that are conserved with the mammalian HCNs and another HCN that has been uniquely altered.

Intracellular linkers are involved in Mg2+-dependent modulation of the Eag potassium channel

Modulation of activation kinetics by divalent ions is one of the characteristic features of Eag channels. Here, we report that Mg(2+)-dependent deceleration of Eag channel activation is significantly attenuated by a G297E mutation, which exhibits a gain-of-function phenotype in Drosophila by suppressing the effect of shaker mutation on behavior and neuronal excitability. The G297 residue is located in the intracellular linker of transmembrane segments S2 and S3, and is thus not involved in direct binding of Mg(2+) ions. Moreover, mutation of the only positively charged residue in the other intracellular linker between S4 and S5 also results in a dramatic reduction of Mg(2+)-dependent modulation of Eag activation kinetics. Collectively, the two mutations in eag eliminate or even paradoxically reverse the effect of Mg(2+) on channel activation and inactivation kinetics. Together, these results suggest an important role of the intracellular linker regions in gating processes of Eag channels.

KCNE1 and KCNE3 beta-subunits regulate membrane surface expression of Kv12.2 K(+) channels in vitro and form a tripartite complex in vivo

Voltage-gated potassium channels that activate near the neuronal resting membrane potential are important regulators of excitation in the nervous system, but their functional diversity is still not well understood. For instance, Kv12.2 (ELK2, KCNH3) channels are highly expressed in the cerebral cortex and hippocampus, and although they are most likely to contribute to resting potassium conductance, surprisingly little is known about their function or regulation. Here we demonstrate that the auxiliary MinK (KCNE1) and MiRP2 (KCNE3) proteins are important regulators of Kv12.2 channel function. Reduction of endogenous KCNE1 or KCNE3 expression by siRNA silencing, significantly increased macroscopic Kv12.2 currents in Xenopus oocytes by around 4-fold. Interestingly, an almost 9-fold increase in Kv12.2 currents was observed with the dual injection of KCNE1 and KCNE3 siRNA, suggesting an additive effect. Consistent with these findings, over-expression of KCNE1 and/or KCNE3 suppressed Kv12.2 currents. Membrane surface biotinylation assays showed that surface expression of Kv12.2 was significantly increased by KCNE1 and KCNE3 siRNA, whereas total protein expression of Kv12.2 was not affected. KCNE1 and KCNE3 siRNA shifted the voltages for half-maximal activation to more hyperpolarized voltages, indicating that KCNE1 and KCNE3 may also inhibit activation gating of Kv12.2. Native co-immunoprecipitation assays from mouse brain membranes imply that KCNE1 and KCNE3 interact with Kv12.2 simultaneously in vivo, suggesting the existence of novel KCNE1-KCNE3-Kv12.2 channel tripartite complexes. Together these data indicate that KCNE1 and KCNE3 interact directly with Kv12.2 channels to regulate channel membrane trafficking.