Individual subunits contribute independently to slow gating of bovine EAG potassium channels

State-independent inhibition of the oncogenic Kv10.1 channel by desethylamiodarone, a comparison with amiodarone

Kv10.1 is a voltage-dependent K channel whose ectopic expression is associated with several human cancers. Additionally, Kv10.1 has structure-function properties which are not yet well understood. We are using drugs of clinical importance in an attempt to gain insight on the relationship between pharmacology and characteristic functional properties of this channel. Herein, we report the interaction of desethylamiodarone (desAd), the active metabolic product of the antiarrhythmic amiodarone with Kv10.1: desAd binds to both closed and open channels, with most inhibition taking place from the open state, with affinity ~ 5 times smaller than that of amiodarone. Current inhibition by desAd and amiodarone is not synergistic. Upon repolarization desAd becomes trapped in Kv10.1 and thereafter dissociates slowly from closed-and-blocked channels. The addition of the Cole-Moore shift plus desAd open-pore-block time courses yields an increasing phase on the steady-state inhibition curve (H∞) at hyperpolarized holding potentials. In contrast to amiodarone, desAd does not inhibit the Kv10.1 Cole-Moore shift, suggesting that a relevant hydrophobic interaction between amiodarone and Kv10.1 participates in the inhibition of the Cole-Moore shift, which is lost with desAd.

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.

Structural, biochemical, and functional characterization of the cyclic nucleotide binding homology domain from the mouse EAG1 potassium channel

KCNH channels are voltage-gated potassium channels with important physiological functions. In these channels, a C-terminal cytoplasmic region, known as the cyclic nucleotide binding homology (CNB-homology) domain displays strong sequence similarity to cyclic nucleotide binding (CNB) domains. However, the isolated domain does not bind cyclic nucleotides. Here, we report the X-ray structure of the CNB-homology domain from the mouse EAG1 channel. Through comparison with the recently determined structure of the CNB-homology domain from the zebrafish ELK (eag-like K(+)) channel and the CNB domains from the MlotiK1 and HCN (hyperpolarization-activated cyclic nucleotide-gated) potassium channels, we establish the structural features of CNB-homology domains that explain the low affinity for cyclic nucleotides. Our structure establishes that the "self-liganded" conformation, where two residues of the C-terminus of the domain are bound in an equivalent position to cyclic nucleotides in CNB domains, is a conserved feature of CNB-homology domains. Importantly, we provide biochemical evidence that suggests that there is also an unliganded conformation where the C-terminus of the domain peels away from its bound position. A functional characterization of this unliganded conformation reveals a role of the CNB-homology domain in channel gating.

Human ether-à-go-go gene potassium channels are regulated by EGFR tyrosine kinase

Human ether á-go-go gene potassium channels (hEAG1 or Kv10.1) are expressed in brain and various human cancers and play a role in neuronal excitement and tumor progression. However, the functional regulation of hEAG channels by signal transduction is not fully understood. The present study was therefore designed to investigate whether hEAG1 channels are regulated by protein tyrosine kinases (PTKs) in HEK 293 cells stably expressing hEAG1 gene using whole-cell patch voltage-clamp, immunoprecipitation, Western blot, and mutagenesis approaches. We found that the selective epidermal growth factor receptor (EGFR) kinase inhibitor AG556 (10 μM), but not the platelet growth factor receptor (PDGFR) kinase inhibitor AG1295 (10 μM) or the Src-family inhibitor PP2 (10 μM), can inhibit hEAG1 current, and the inhibitory effect can be reversed by the protein tyrosine phosphatase (PTP) inhibitor orthovanadate. Immunoprecipitation and Western blot analysis revealed that tyrosine phosphorylation level of hEAG1 channels was reduced by AG556, and the reduction was significantly countered by orthovanadate. The hEAG1 mutants Y90A, Y344A and Y485A, but not Y376A and Y479A, exhibited reduced response to AG556. Interestingly, the inhibition effect of AG556 was lost in triple mutant hEAG1 channels at Y90, Y344, and Y485 with alanine. These results demonstrate for the first time that hEAG1 channel activity is regulated by EGFR kinase at the tyrosine residues Tyr90, Try344, and Try485. This effect is likely involved in regulating neuronal activity and/or tumor growth.

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.

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

Voltage-gated K+ channels share a common voltage sensor domain (VSD) consisting of four transmembrane helices, including a highly mobile S4 helix that contains the major gating charges. Activation of ether-a-go-go (EAG) family K+ channels is sensitive to external divalent cations. We show here that divalent cations slow the activation rate of two EAG family channels (Kv12.1 and Kv10.2) by forming a bridge between a residue in the S4 helix and acidic residues in S2. Histidine 328 in the S4 of Kv12.1 favors binding of Zn2+ and Cd2+, whereas the homologous residue Serine 321 in Kv10.2 contributes to effects of Mg2+ and Ni2+. This novel finding provides structural constraints for the position of transmembrane VSD helices in closed, ion-bound EAG family channels. Homology models of Kv12.1 and Kv10.2 VSD structures based on a closed-state model of the Shaker family K+ channel Kv1.2 match these constraints. Our results suggest close conformational conservation between closed EAG and Shaker family channels, despite large differences in voltage sensitivity, activation rates, and activation thresholds.

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

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

In vivo analysis of a gain-of-function mutation in the Drosophila eag-encoded K+ channel

Neuronal Na+ and K+ channels elicit currents in opposing directions and thus have opposing effects on neuronal excitability. Mutations in genes encoding Na+ or K+ channels often interact genetically, leading to either phenotypic suppression or enhancement for genes with opposing or similar effects on excitability, respectively. For example, the effects of mutations in Shaker (Sh), which encodes a K+ channel subunit, are suppressed by loss-of-function mutations in the Na+ channel structural gene para, but enhanced by loss-of-function mutations in a second K+ channel encoded by eag. Here we identify two novel mutations that suppress the effects of a Sh mutation on behavior and neuronal excitability. We used recombination mapping to localize both mutations to the eag locus, and we used sequence analysis to determine that both mutations are caused by a single amino acid substitution (G297E) in the S2-S3 linker of Eag. Because these novel eag mutations confer opposite phenotypes to eag loss-of-function mutations, we suggest that eag(G297E) causes an eag gain-of-function phenotype. We hypothesize that the G297E substitution may cause premature, prolonged, or constitutive opening of the Eag channels by favoring the "unlocked" state of the channel.

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+.

Subunit stoichiometry of the Kir1.1 channel in proton-dependent gating

Kir1.1 channel regulates membrane potential and K+ secretion in renal tubular cells. This channel is gated by intracellular protons, in which a lysine residue (Lys80) plays a critical role. Mutation of the Lys80 to a methionine (K80M) disrupts pH-dependent channel gating. To understand how an individual subunit in a tetrameric channel is involved in pH-dependent channel gating, we performed these studies by introducing K80M-disrupted subunits to tandem tetrameric channels. The pH sensitivity was studied in whole-cell voltage clamp and inside-out patches. Homomeric tetramers of the wild-type (wt) and K80M-disrupted channels showed a pH sensitivity almost identical to that of their monomeric counterparts. In heteromeric tetramers and dimers, pH sensitivity was a function of the number of wt subunits. Recruitment of the first single wt subunit shifts the pK(a) greatly, whereas additions of any extra wt subunit had smaller effects. Single-channel analysis revealed that the tetrameric channel with two or more wt subunits showed one substate conductance at approximately 40% of the full conductance, suggesting that four subunits act as two pairs. However, three and four substates of conductance were seen in the tetrameric wt-3K80M and 4K80M channels. Acidic pH increased long-time closures when there were two or more wt subunits. Disruption of more than two subunits led to flicking activity with appearance of a new opening event and loss of the long period of closures. Interestingly, the channel with two wt subunits at diagonal and adjacent configurations showed the same pH sensitivity, substate conductance, and long-time closure. These results thus suggest that one functional subunit is sufficient to act in the pH-dependent gating of the Kir1.1 channel, the channel sensitivity to pH increases with additional subunits, the full pH sensitivity requires contributions of all four subunits, and two subunits may be coordinated in functional dimers of either trans or cis configuration.

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.

Molecular determinants for high-affinity block of human EAG potassium channels by antiarrhythmic agents

Undesired block of human ERG1 potassium channels is the basis for cardiac side effects of many different types of drugs. Therefore, it is important to know exactly why some drugs particularly bind to these channels with high affinity. Upon expression in mammalian cells and Xenopus laevis oocytes, we investigated the inhibition of the closely related hEAG1 and hEAG2 channels by agents that have previously been reported to block hERG1 channels. Clofilium inhibited hEAG1 and hERG1 with the same potency, whereas hEAG2 was about 150-fold less sensitive to this antiarrhythmic agent. The molecular determinants for this difference are residues Ser436 and Val437 in the inner cavity of the pore and Ala453, which is located in S6 (i.e., remote from the inner cavity). A modeling approach that allowed for partial conformational relaxation of hEAG model structures upon ligand docking suggests that high-affinity block of ether à go-go channels is mediated by an anchoring of the clofilium alkane tail between S6 and the pore helices. In qualitative agreement with experiments, the mutations of hEAG1 residues Ser436 and Val437 to the corresponding larger hEAG2 residues (Thr432, Ile433) resulted in reduced sterical fit between the ligand and the binding cavity. The model is further supported by functional assays involving (+)-N-[1'-(6-cyano-1,2,3,4-tetrahydro-2(R)-naphthalenyl)-3,4-dihydro-4(R)-hydroxyspiro(2H-1-benzopyran-2,4'-piperidin)-6-yl]methanesulfonamide monohydrochloride (MK-499), terfenadine, quinidine, and tetrabutylammonium that are differentially affected by mutations in the pore pocket.

Conformational switch between slow and fast gating modes: allosteric regulation of voltage sensor mobility in the EAG K+ channel

Voltage-gated EAG K+ channels switch between fast and slow gating modes in a Mg2+-dependent manner by an unknown mechanism. We analyzed molecular motions in and around the voltage-sensing S4 in bEAG1. Using accessibility and perturbation analyses, we found that activation increases both the charge occupancy and volume of S4 side chains in the gating canal. Fluorescence measurements suggest that mode switching is due to a motion of the S2/S3 side of the gating canal. We propose that when S4 is in the resting state and its thin end is in the gating canal, a conformational rearrangement of S2/S3 narrows the canal around S4, forming the Mg2+ binding site. Binding of Mg2+ is proposed to stabilize this conformation and to slow opening of the gate by impeding S4's voltage-sensing outward motion.

Functional distinction of human EAG1 and EAG2 potassium channels

Human ether à go-go potassium channel 2 (hEAG2) was cloned and its properties were compared with the previously characterized isoform hEAG1. In the Xenopus oocyte expression system the time course of activation was about four times slower and the voltage required for half-maximal subunit activation was about 10 mV greater for hEAG2 channels. However, its voltage dependence was smaller and, therefore, hEAG2 channels start to open at more negative voltages than hEAG1. Coexpression of both isoforms and kinetic analysis of the resulting currents indicated that they can form heteromeric channel complexes in which the slow activation phenotype of hEAG2 is dominant. Upon expression in mammalian cells, quinidine blocked hEAG1 channels (IC(50) 1.4 microM) more potently than hEAG2 channels (IC(50) 152 microM), thus providing a useful tool for the functional distinction between hEAG1 and hEAG2 potassium channels.

Differential expression of genes encoding subthreshold-operating voltage-gated K+ channels in brain

The members of the three subfamilies (eag, erg, and elk) of the ether-a-go-go (EAG) family of potassium channel pore-forming subunits express currents that, like the M-current (I(M)), could have considerable influence on the subthreshold properties of neuronal membranes, and hence the control of excitability. A nonradioactive in situ hybridization (NR-ISH) study of the distribution of the transcripts encoding the eight known EAG family subunits in rat brain was performed to identify neuronal populations in which the physiological roles of EAG channels could be studied. These distributions were compared with those of the mRNAs encoding the components of the classical M-current (Kcnq2 and Kcnq3). NR-ISH was combined with immunohistochemistry to specific neuronal markers to help identify expressing neurons. The results show that each EAG subunit has a specific pattern of expression in rat brain. EAG and Kcnq transcripts are prominent in several types of excitatory neurons in the cortex and hippocampus; however, only one of these channel components (erg1) was consistently expressed in inhibitory interneurons in these areas. Some neuronal populations express more than one product of the same subfamily, suggesting that the subunits may form heteromeric channels in these neurons. Many neurons expressed multiple EAG family and Kcnq transcripts, such as CA1 pyramidal neurons, which contained Kcnq2, Kcnq3, eag1, erg1, erg3, elk2, and elk3. This indicates that the subthreshold current in many neurons may be complex, containing different components mediated by a number of channels with distinct properties and neuromodulatory responses.

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.

Cloning and functional expression of rat eag2, a new member of the ether-à-go-go family of potassium channels and comparison of its distribution with that of eag1

A second mammalian gene for the ether-à-go-go (eag) potassium channel has been cloned from the rat, and its predicted protein sequence is 70% identical to that of rat ether-à-go-go1 with a further 10% conservatively replaced residues. The rat eag2 mRNA was predominantly expressed in neural tissue and was not detected in adult skeletal, cardiac, or smooth muscle. Within the brain, its distribution overlaps that of rat ether-à-go-go1 in specific regions within the cortex and olfactory bulb, but was differentially distributed in other locations, being scanty within the cerebellum, and most notably present in the thalamus, inferior colliculus, and certain brainstem nuclei. Heterologous expression of rat eag2 in HEK-293 cells gave rise to a voltage-gated, noninactivating potassium current, active at the cells' resting potential and blocked by low nanomolar concentrations of cytosolic calcium. Thus, in neurones, this current is likely to impart a modulation in membrane conductance, which is sensitively responsive to resting internal calcium, and levels of electrical activity.

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.

Cloning of components of a novel subthreshold-activating K(+) channel with a unique pattern of expression in the cerebral cortex

Potassium channels that are open at very negative membrane potentials govern the subthreshold behavior of neurons. These channels contribute to the resting potential and help regulate the degree of excitability of a neuron by affecting the impact of synaptic inputs and the threshold for action potential generation. They can have large influences on cell behavior even when present at low concentrations because few conductances are active at these voltages. We report the identification of a new K(+) channel pore-forming subunit of the ether-à-go-go (Eag) family, named Eag2, that expresses voltage-gated K(+) channels that have significant activation at voltages around -100 mV. Eag2 expresses outward-rectifying, non-inactivating voltage-dependent K(+) currents resembling those of Eag1, including a strong dependence of activation kinetics on prepulse potential. However, Eag2 currents start activating at subthreshold potentials that are 40-50 mV more negative than those reported for Eag1. Because they activate at such negative voltages and do not inactivate, Eag2 channels will contribute sustained outward currents down to the most negative membrane potentials known in neurons. Although Eag2 mRNA levels in whole brain appear to be low, they are highly concentrated in a few neuronal populations, most prominently in layer IV of the cerebral cortex. This highly restricted pattern of cortical expression is unlike that of any other potassium channel cloned to date and may indicate specific roles for this channel in cortical processing. Layer IV neurons are the main recipient of the thalamocortical input. Given their functional properties and specific distribution, Eag2 channels may play roles in the regulation of the behavioral state-dependent entry of sensory information to the cerebral cortex.

Block of an ether-a-go-go-like K(+) channel by imipramine rescues egl-2 excitation defects in Caenorhabditis elegans

K(+) channels are key regulators of cellular excitability. Mutations that activate K(+) channels can lower cellular excitability, whereas those that inhibit K(+) channels may increase excitability. We show that the Caenorhabditis elegans egl-2 gene encodes an eag K(+) channel and that a gain-of-function mutation in egl-2 blocks excitation in neurons and muscles by causing the channel to open at inappropriately negative voltages. Tricyclic antidepressants reverse egl-2(gf) mutant phenotypes, suggesting that EGL-2 is a tricyclic target. We verified this by showing that EGL-2 currents are inhibited by imipramine. Similar inhibition is observed with the mouse homolog MEAG, suggesting that inhibition of EAG-like channels may mediate some clinical side effects of this class of antidepressants.