a family of acetylcholine-gated chloride channel subunits in caenorhabditis elegans

A Family of Acetylcholine-gated Chloride Channel Subunits inCaenorhabditis elegans*

Received for publication, November 8, 2004, and in revised form, December 2, 2004Published, JBC Papers in Press, December 3, 2004, DOI 10.1074/jbc.M412644200

Igor Putrenko, Mahvash Zakikhani, and Joseph A. Dent‡From the Department of Biology, McGill University, Montreal, Quebec H3A 1B1, Canada

The genome of the nematode Caenorhabditis elegansencodes a surprisingly large and diverse superfamily ofgenes encoding Cys loop ligand-gated ion channels.Here we report the first cloning, expression, and phar-macological characterization of members of a family ofanion-selective acetylcholine receptor subunits. Twosubunits, ACC-1 and ACC-2, form homomeric channelsfor which acetylcholine and arecoline, but not nicotine,are efficient agonists. These channels are blocked byD-tubocurarine but not by !-bungarotoxin. We provideevidence that two additional subunits, ACC-3 andACC-4, interact with ACC-1 and ACC-2. The acetylcho-line-binding domain of these channels appears to havediverged substantially from the acetylcholine-bindingdomain of nicotinic receptors.

Fast (ionotropic) cholinergic neurotransmission is generallymediated by nicotinic acetylcholine (ACh)1 receptors (nAChRs).These are cation-selective channels and hence mediate excita-tory neurotransmission. However, electrophysiological evi-dence of ionotropic, ACh-gated chloride channels in molluscssuggests the existence of fast inhibitory cholinergic neurotrans-mission as well (1–3). The ACh-gated chloride channels inAplysia neurons respond to several agonists and antagonists ofnAChRs, indicating that, like the nAChRs, they may belong tothe superfamily of Cys loop ligand-gated ion channel (LGIC)subunits. Otherwise, little is known about the molecular na-ture of the receptors that mediate fast inhibitory cholinergicneurotransmission, whether this type of neurotransmission iswidespread in the animal kingdom, or how it evolved.

The Cys loop LGICs are encoded by a large and diverse genesuperfamily. These channels are pentameric and can be ho-momers or heteromers consisting of as many as four differentsubunits, each encoded by a different gene (4). Subunits of theCys loop LGIC superfamily share a topology that consists of alarge extracellular ligand-binding domain and four transmem-brane domains that form the ion-selective pore (4–6). In ver-

tebrates, the LGIC superfamily consists of two families of cat-ion-selective channels, the nicotinic ACh receptors and the5-hydroxytryptamine type 3 receptors, and two families of an-ion channels, the GABAA receptors and the glycine receptors(7). The repertoire of invertebrate LGICs is larger, including, inaddition to homologues of vertebrate channels, histamine-gated chloride channels (8, 9), a GABA-gated cation channel(10), a serotonin-gated anion channel (11), several glutamate-gated anion channels (12–16), and a divergent choline-gatednAChR (17). Thus, the LGIC channel structure appears flexibleenough to accommodate diverse ligands and ligand/ion selec-tivity pairings.

Although no genes encoding ACh-gated chloride channelshave been previously identified, it is likely that many inverte-brate receptors with unusual properties remain to be charac-terized. The genomes of Caenorhabditis elegans and Drosoph-ila melanogaster reveal numerous predicted Cys loop LGICsthat do not obviously belong to any family of known ion orligand specificity (18–21). C. elegans in particular encodes !70LGIC subunit genes, of which fewer than 20 have been char-acterized pharmacologically (19). The function of such a largeand diverse LGIC superfamily in a single species is unclear.

To better understand the constraints on Cys loop LGICstructure and evolution and to identify new modes of neuro-transmission, we have characterized several members of anovel family of channel subunits from C. elegans. These formACh-gated chloride channels exhibiting an unusual pharma-cology that appears to reflect a unique ACh-binding site.

EXPERIMENTAL PROCEDURES

Cloning ACC cDNAs—Poly(A") RNA was purified from adult worms(Bristol N2 strain). First strand cDNA was synthesized with oligo(dT)primer using the avian myeloblastosis virus reverse transcriptasesystem (Invitrogen Canada Inc., Burlington, Ontario, Canada). Theopen reading frame of ACC-1, -2, and -4, as predicted in Wormbase(available on the World Wide Web at www.wormbase.org/), wasamplified by PCR using the following primers: ACC-1, 5#-GGGGACA-AGTTTGTACAAAAAAGCAGGCTCATATGAGTCATCCGGGTTGGA-TTAT-3# and 5#-GGGGACCACTTTGTACAAGAAAGCTGGGTCTAGA-TTAAGGTTGATCAATATTCACA-3#; ACC-2, 5#-GGGGACAAGTTTGT-ACAAAAAAGCAGGCTCATATGATATTTACTCTTTTATCAACACTG-CCT-3# and 5#-GGGGACCACTTTGTACAAGAAAGCTGGGTCTAGAT-TATCCGTCAACTCGATT-3#; ACC-4, 5#-GGGGTACCATATGCGACTA-ATCATATTAGTAATCT-3# and 5#-GCTCTAGATTAGATAGTTCTAAC-CAATAGTTTTCC-3#. PCR products were subcloned either into pDON-R201 (ACC-1 and ACC-2) via recombination reaction using the Gate-way Cloning Technology kit (Invitrogen) or into pBluescript (Strata-gene Inc., La Jolla, CA) using the KpnI and XbaI sites (ACC-4). ACC-3was first amplified from cDNA using a primer to the SL-1 transplicedleader sequence 5#- GGGGACAAGTTTGTACAAAAAAGCAGGCTGGA-TCCTTTAATTACCCAAGTTTGAG-3# and a 3# primer corresponding tothe end of the putative open reading frame, 5#-GGGGACCACTTTGT-ACAAGAAAGCTGGGTCTGCAGTCATGTGTTAACAGTAAGGTAAT-AT-3#. The resulting PCR product was cloned into pDONR and se-quenced. A new 5#-primer corresponding to the 5#-end of the open read-ing frame was used with the previous 3#-primer to amplify the openreading frame from the cloned ACC-3 cDNA. Three cDNA clones of each

* This work was supported by a Research Partnership betweenNatural Sciences and Engineering Research Council of Canada, Agri-culture and Agrifoods Canada, and Crompton Co. (Guelph, Ontario,Canada). The costs of publication of this article were defrayed in part bythe payment of page charges. This article must therefore be herebymarked “advertisement” in accordance with 18 U.S.C. Section 1734solely to indicate this fact.

The nucleotide sequence(s) reported in this paper has been submittedto the GenBankTM/EBI Data Bank with accession number(s) AY849796.

‡ To whom correspondence should be addressed: Dept. of Biology,McGill University, 1205 Dr. Penfield Ave., Montreal, Quebec H3A 1B1,Canada. Tel.: 514-398-3724; Fax: 514-398-5069; E-mail: [email protected].

1 The abbreviations used are: ACh, acetylcholine; nAChR, nicotinicacetylcholine receptor; LGIC, ligand-gated ion channel; C6, hexametho-nium; d!e, dihydro-!-erythroidine; "-BT, "-bungarotoxin; d-TC,D-tubocurarine.

THE JOURNAL OF BIOLOGICAL CHEMISTRY Vol. 280, No. 8, Issue of February 25, pp. 6392–6398, 2005© 2005 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in U.S.A.

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gene were sequenced to determine the true open reading frames and tofind possible mutations resulting from reverse transcription-PCR.Nonsilent mutations were fixed either by overlap extension PCR (22) orby splicing together mutation-free cDNA fragments using convenientrestriction sites.

Sequence Analysis—Amino acid sequences were aligned using theClustalW program (available on the World Wide Web at clustalw.genome.ad.jp). Transmembrane domains were predicted using theTMHMM method based on a hidden Markov model (23). SignalP 2.0,NetGlyc 1.0, and NetPhos 2.0 programs based on artificial neuronalnetworks were used to predict signal peptide sequences (24). All of theabove mentioned prediction programs are available on the World WideWeb at www.cbs.dtu.dk/services.

Expression in Xenopus Oocytes and Electrophysiology—cDNAs weresubcloned into the pT7N expression vector (25). The pT7NcDNA con-structs were linearized with SalI (ACC-2) or BamHI (ACC-1, -3, and -4),and capped cRNAs were transcribed using the MEGAscript Kit (Am-bion, Austin, TX). Synthesized cRNAs were recovered by LiCl precipi-tation and resuspended in nuclease-free H2O at a final concentration of1 #g/ml.

Oocytes were harvested from mature female Xenopus laevis accord-ing to standard procedures (26). Oocytes were maintained at 20 °C inND96 solution (96 mM NaCl, 2 mM KCl, 1.8 mM CaCl2, 1 mM MgCl2, and5 mM Hepes, pH 7.5) supplemented with 100 mg/ml gentamycin and 550mg/ml pyruvate. Oocytes were injected with 40 nl of cRNA using theNanoject system (Drummond Scientific, Broomall, PA) and incubatedfor 2 days before measurements were taken.

Two-electrode voltage clamp recordings were performed using theAxoClamp 2B amplifier (Axon Instruments, Foster City, CA). Oocyteswere perfused in an RC-12 recording chamber (Warner Instrument Inc.,Hamden, CT) or a Maltese Cross chamber (ALA Scientific Instruments,Westbury, NY). Data were acquired at 1 kHz using Clampex software(Axon Instruments, Foster City, CA). All drugs were obtained fromSigma. Dose-response curves for agonists were generated by applyingincreasing concentrations of drug followed by 3–9-min washes. To de-termine the EC50 and Hill coefficient, dose-response curves (shown assmooth curves in graphs) were fitted using the Hill equation as follows,

f$I% $ $Imax&I' ! /$EC50 ! % &I' ! %% % Imin (Eq. 1)

where I represents the normalized response, Imax is the maximal re-sponse elicited by a saturating concentration of agonist, EC50 is theconcentration of agonist inducing half-maximal response, n is the Hillcoefficient, and Imin is the normalized response at lowest agonistconcentration.

Oocytes were preincubated with antagonist for 1 min (10 min for"-BT) prior to co-application of the antagonist with either 1 #M or 10 #M

ACh for oocytes expressing ACC-1 or ACC-2, respectively. The ampli-tude of response to co-application of ACh and antagonist was normal-ized to that of the response to ACh alone.

Because the ACC-1 and ACC-2 channels inactivate slowly in thepresence of ACh, I-V curves were measured using voltage ramps of 4mV/s in the presence of either 2 or 20 #M ACh for oocytes expressingACC-1 or ACC-2, respectively. For ion substitution experiments, so-dium gluconate or arginine-Cl were substituted for NaCl in the ND96solution. The current amplitude was normalized to the peak response innormal ND96 at (80 mV.

RESULTS

Cloning of Members of the ACC Subunit Family—A neigh-bor-joining tree of predicted C. elegans LGIC subunit genesrevealed the presence of a distinct clade of 16 LGIC geneswhose only characterized member was the serotonin-gatedchloride channel subunit MOD-1 (11). Members of this cladehave no orthologs in Drosophila or vertebrate genomes (Fig.1A, data not shown). The newly identified clade appeared tobreak down into three subgroups that we reasoned mightcorrespond to families of channels with distinct ligand spec-ificities. We isolated and sequenced four cDNAs from onesubgroup: ACC-1 (also known as F58G6.4), ACC-2 (C53D6.3),ACC-3 (F55D10.5), and ACC-4 (T27E9.9). Open readingframes of the ACC-1 (1401 bp), ACC-2 (1338 bp), and ACC-4(1227 bp) cDNAs corresponded to those annotated in thegenome data base. The ACC-3 cDNA (1554 bp) wastranspliced with an SL1 leader sequence and differed from

the predicted open reading frame in the first and sevenththrough ninth exons. All typical features attributed to LGICsubunits, such as a signal sequence, an N-terminal extracel-lular domain with the Cys loop, four transmembrane domains(M1–M4), and a large cytoplasmic loop located between M3and M4, were recognizable in the amino acid sequences ofthese proteins (Fig. 1B). The ACC-1 and ACC-2 putativeproteins are 40% identical to each other at the amino acidlevel and exhibit 31 and 29% identity to MOD-1, respectively.In contrast, ACC-2 shows 23% identity to UNC-49B, aC. elegans GABA-gated chloride channel subunit (27), and isonly 16% identical to UNC-38, a C. elegans nAChR subunit.

ACC-1 and -2 Form ACh-gated Chloride Channels—To de-termine the ligand specificity of the ACC subunits, we ex-pressed ACC cRNAs in Xenopus oocytes and voltage-clampedthe oocytes at (80 mV. ACC-1- and ACC-2-injected oocytesexhibited an ACh-elicited inward current with maximal mag-nitude varying from 0.8 to 3.4 #A (Fig. 2, A and C). TheACC-1-dependent current showed almost no desensitizationeven at saturating ACh concentrations, whereas the ACC-2-de-pendent current desensitized. ACC-1 responded to ACh with ahalf-effector concentration (EC50) of 0.26 ) 0.01 #M and anestimated Hill coefficient of 1.26 ) 0.04 (Fig. 2B), whereasACC-2 was much less sensitive, responding with an EC50 of9.54 ) 0.11 #M and a Hill coefficient of 2.64 ) 0.08 (Fig. 2D).Oocytes injected with ACC-3 cRNAs exhibited a weak response(10–30 nA) to 1 mM ACh, and those injected with ACC-4showed no response. An ACh-induced current was not detectedin distilled H2O-injected or noninjected oocytes. Oocytes ex-pressing ACC cRNAs did not respond to 1 mM GABA, gluta-mate, glycine, histamine, or dopamine (not shown). ACC-2-injected oocytes responded slightly to 1 mM serotonin (*2% ofthe maximal ACh response) and to octopamine (!1%), but noresponse to these compounds was observed in oocytes express-ing other ACC subunits.

ACC-1, -2, and -3 share a proline-alanine motif of the secondtransmembrane domain (corresponding to the intermediatering of nAChRs) that has been shown to confer anion selectivityin vertebrate GABA and glycine receptors (28–30) (Fig. 1B). Todetermine whether ACC-1 and ACC-2 are also chloride chan-nels, we generated I-V curves. The reversal potentials in ND96for ACC-1 or ACC-2 homomeric channels were (18.7 ) 1.4 mVand (18 ) 1.6 mV, respectively, consistent with the equilib-rium potential for chloride (Fig. 3, A and B) (31). When thenonpermeant anion gluconate was substituted for chloride (7.6mM final external chloride), there was a positive shift in thereversal potential of 65.7 mV (ACC-1) and 68.6 mV (ACC-2). Incontrast, when arginine was substituted for sodium, the shift inreversal potential was negligible (Ereversal + (17.2 ) 1.7 mV,ACC-1, and (17.0 ) 1, ACC-2) (Fig. 3, B and C). We observedshifts of 59.8 ) 4.9 and 54.4 ) 3.7 mV in the reversal potentialfor a 10-fold change in chloride concentration for ACC-1 andACC-2, respectively (Fig. 3C). This is in agreement with thetheoretical shift of 58 mV predicted by the Nernst equation forchloride-selective channels.

The Pharmacology of the ACC Channels Reflects a UniqueLigand-binding Site—The sequence identity between the ACCsand nAChRs in their extracellular ligand-binding domains isrelatively low. More specifically, the six ligand-binding loops(A–F), as predicted from lineups with nAChRs, are not wellconserved between nAChRs and the ACCs (Fig. 1B). Therefore,we predicted that the ACCs would have a unique pharmaco-logical profile. We tested agonists and antagonists of vertebratenAChRs on the ACC receptors. The classical nAChR agonistnicotine at 1 mM was a poor agonist of the ACC channels (Fig.4A). At 0.5 mM, nicotine was a partial antagonist of ACC-1 but

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FIG. 1. The ACC family of LGICs. A, a neighbor joining tree of the LGIC subunit superfamily polypeptides in C. elegans (ce_) withacetylcholine-binding protein (AChBP), selected rat (v_), and Drosophila (d_) LGIC subunits included for comparison. The numbers at branchpoints are bootstrap values. B, lineup of ACC-1 to -4 with the serotonin-gated chloride channel MOD-1 and the rat "9 nicotinic acetylcholinereceptor. The four predicted transmembrane domains (M1–M4) are underlined. The filled triangle marks the Cys loop cysteines. The two daggersmark the proline-alanine motif that determines anion selectivity. The lowercase letters (a–f) indicate the position of the six loops that form theligand-binding site of nAChRs. The shading indicates degree of conservation.

Acetylcholine-gated Chloride Channel Subunits in C. elegans6394 at University of British Colum



FIG. 2. ACC-1 and ACC-2 form ho-momeric acetylcholine-gated chan-nels. A and B, traces showing the re-sponse of ACC-1 and ACC-2 homomers toacetylcholine and arecoline. C and D, ag-onist dose-response curves to acetylcho-line (‚), arecoline (Œ), and atropine (").The number of data points is indicated inparentheses. E and F, inhibitor responsecurves. Shown is inhibition of the re-sponse to 1 and 10 #M ACh for ACC-1 andACC-2, respectively. E, strychnine; ‚, at-ropine in E and dihydro-!-erythroidine inF. n + 4 for all curves in E and F. Theerror bars represent S.E.




not ACC-2. Cytisine, another nAChR agonist, similarly actedas a weak agonist of ACC-2 channels and as an antagonist ofboth ACC-1 and -2. The highest potency antagonists had therank order: D-tubocurarine (d-TC) & strychnine , atropine ,dihydro-!-erythroidine (d!e) , hexamethonium (C6) forACC-1, and d-TC & strychnine & d!e , C6 for ACC-2 (Figs. 2(E and F) and 4B). At 1 mM, C6 acted simultaneously as anagonist and an antagonist of ACC-1. Interestingly, 20 #M d-TC,a competitive antagonist of nAChRs and 5-hydroxytryptaminetype 3 receptors, blocked ACC-1 and -2 completely. Weak acti-vation by nicotine and cytisine and block by strychnine, d-TC,and d!e are characteristic of the vertebrate "9 nAChR chan-nels (32, 33) and of the Aplysia ACh-gated chloride channel (3).However, unlike these channels, ACC-1 and -2 were notblocked by "-bungarotoxin ("-BT).

Although nicotinic agonists had little effect on ACC-1 and

ACC-2, arecoline, an agonist of metabotropic ACh receptors,evoked a current that was 82–89% of the maximal ACh-elicitedcurrent (Fig. 2, A and C). Arecoline evoked a slowly desensitiz-ing current in both the ACC-1- and ACC-2-expressing oocyteswith an EC50 of 4.7 ) 0.11 and 754 ) 22 #M, respectively (Fig.2, B and D). A rebound inward current was observed upon theremoval of arecoline in both types of receptors, an indication ofagonist-dependent open channel block (34, 35). We noted thatthe estimated Hill coefficients for arecoline of 2.66 ) 0.23 and1.59 ) 0.09 for the ACC-2- and ACC-1-expressing oocytes,respectively, are not significantly different from the Hill coef-ficients for the response to ACh, indicating a similar degree ofcooperativity of ACh and arecoline. Atropine, a nonselectiveantagonist of metabotropic ACh receptors and a competitiveantagonist of "9 nAChRs, activated the ACC-2 channel with anEC50 of 873 ) 63 #M with an estimated Hill coefficient of 0.98 )0.07 (Fig. 2D) but evoked only 43% of maximal ACh-elicitedcurrent. In contrast, we detected no atropine-evoked activation(up to 1 mM) of the ACC-1-expressing oocytes. Instead, atropineblocked the response of ACC-1 channels to 1 #M ACh with anIC50 of 23 #M (Fig. 2). Thus, the pharmacological profiles ofACC-1 and -2 support a distinct ACC ligand-binding site.

ACC-3 and ACC-4 May Form Obligate Heteromers—BecauseACC-3 and ACC-4 do not respond robustly to ACh as ho-momers, we considered the possibility that they form obligateheteromers with other ACh-gated chloride channel subunits.ACC-3 forms a functional heteromeric channel with ACC-1.Co-expression of ACC-1 " ACC-3 generated a channel thatexhibited a pronounced desensitization compared with theACC-1 homomer (Fig. 5A versus Fig. 2A). Moreover, the re-sponse of ACC-1 " ACC-3-expressing oocytes to ACh was morethan 200-fold less potent (EC50 + 39.6 ) 1.6 #M) than that of

FIG. 3. ACC-1 and ACC-2 are anion channels. A and B, I-V curvesof ACC-1 and ACC-2 channels in normal (E) external solution (96 mMsodium and 104 mM chloride), low (‚) sodium (0 mM), and low (Œ)chloride (7.6 mM) ND96. n + 4 for all curves in A and B. C, plot ofreversal potential versus external chloride concentration. Each pointrepresents a reversal potential of an oocyte expressing ACC-1 (Œ) orACC-2 (E). The line represents the theoretical relationship predicted bythe Nernst equation for a chloride-selective channel assuming 43 mMinternal chloride. The error bars represent S.E.

FIG. 4. ACC pharmacology. A, mean activation of homomeric chan-nels expressed in oocytes as a percentage of the response to 10 #M(ACC-2; white bars) or 1 #M (ACC-1; black bars) ACh. B, mean percent-age inhibition of the response to ACh (concentrations as in A). n isindicated above each bar. The error bars represent S.E.




homomeric ACC-1 channels. The estimated Hill coefficient of0.78 ) 0.02 indicates that the ACC-1 " ACC-3 heteromer hasfewer ACh binding sites than the ACC-1 homomer (Fig. 5B).Coexpression of ACC-1 with ACC-4 did not change significantlythe maximal response, EC50 (0.36 ) 0.02) or Hill coefficient

(1.11 ) 0.07) relative to the ACC-1 homomer (Fig. 5B). Thus,there is no indication that ACC-4 interacts with ACC-1.

Oocytes coexpressing the ACC-2 cRNA with the ACC-3 orACC-4 cRNAs either exhibited a weak response of 50–60 nA to1 mM ACh or did not respond, respectively (Fig. 5C); nor didACC-2 " ACC-3- nor ACC-2 " ACC-4-expressing oocytes re-spond to 1 mM serotonin, GABA, glutamate, glycine, or hista-mine. Inhibition was specific for ACC-2, since ACC-3 andACC-4 did not inhibit expression of a glutamate-gated chloridechannel subunit, AVR-15, or expression of ACC-1 (Fig. 5C; seeabove). We interpreted this result as indicating that ACC-3 and-4 are able to assemble with ACC-2 in a heteromeric channeland interfere with its gating or trafficking.

DISCUSSION

We have identified a new family of Cys loop LGICs, thenematode ACh-gated chloride channels. We report the firstmolecular characterization of an anion-selective ACh recep-tor and show that a distinct class of Cys loop LGICs hasevolved to mediate inhibitory cholinergic neurotransmission.This is also the first evidence of ACh-gated chloride channelsin nematodes and suggests that fast inhibitory cholinergicneurotransmission is more widespread in the animal king-dom than previously suspected.

The ACh-gated chloride channel subunits in C. elegans be-long to the superfamily of Cys loop ligand-gated ion channels.As such, we would predict that the ACC channels are pentam-eric. We showed that both ACC-1 and ACC-2 form homomericchannels when expressed in Xenopus oocytes but also thatACC-1 interacts with ACC-3 and ACC-2 interacts with bothACC-3 and ACC-4. The interaction of ACC-1 with ACC-3 pro-duces a channel that could function in vivo, albeit one with alower sensitivity to ACh than the ACC-1 homomer. The inter-action of ACC-2 with ACC-3 and -4 is more problematic, sincethese subunits appear to inhibit ACC-2. We suspect that thesesubunits assemble into heteromers but require additional sub-units to form a functional channel. However, we cannot rule outthe possibility that, although capable of assembling into aheteromer, these subunits are prevented from doing so in vivoor that ACC-3 and -4 negatively regulate ACC-2. Determiningwhich subunits are co-expressed in vivo will help resolve thisissue. Finally, the ability of the ACC-3 and -4 to associate withACC-1 and -2 is consistent with the proposition that the ACCsconstitute a family of ACh-gated chloride channel subunits.

One of the most unusual features of these channels is theassociation of gating by ACh with anion selectivity. We canpoint to a clear structural motif that accounts for the anionselectivity. The M2 transmembrane domains line the pore ofthe channel and determine ion selectivity (30). At the cytoplas-mic end of this domain is a Pro-Ala-Arg motif that is found inmost anion channels in C. elegans. Similar motifs are found invertebrate channels, and site-directed mutagenesis of this mo-tif has confirmed its importance in determining anion selectiv-ity in GABAA and glycine receptors (28–30).

The other striking property of the ACCs is their unusualligand binding site. The sequences of ACC-1 and ACC-2 sub-units differ substantially from both nematode and vertebratenAChRs in their extracellular ligand-binding domains (Fig. 1B,data not shown). Photoaffinity labeling and mutagenesis stud-ies (36), confirmed by analysis of the crystal structure of ace-tylcholine-binding protein (37), have identified residues thatdefine the ACh binding site at the interface of two subunits.Residues on one side of the (")-subunit (the " subunit innAChRs) contribute loops A, B, and C. Residues on the otherside of the adjacent (()-subunit (' and ( in nAChRs) contributeloops D, E, and F, which form the complementary part of thebinding pocket. However, even the loops forming the ligand-

FIG. 5. ACC subunits interact. A, dose-response curves of co-ex-pressed channels. ACh is a less potent agonist of the ACC-1/ACC-3heteromer (‚) than of the ACC-1 homomer (!). Co-expression of ACC-4with ACC-1 (E) has no effect on the potency of ACh. n is indicated inparentheses. The error bars represent S.E. B, response of the co-ex-pressed ACC-1 and ACC-3 showing increased rate of desensitization ofthe heteromeric channel. C, mean maximal current response in oocytesexpressing subunit combinations. ACC-3 and ACC-4 inhibit expressionof ACC-2 but not expression of ACC-1 or the glutamate-gated chloridechannel formed by AVR-15B. n is indicated in parentheses.




binding pocket are not conserved between the ACCs and thenAChRs (Fig. 1B). Most notably, the adjacent cysteines of the Cloop, a hallmark of the ligand-binding nAChR " subunits, areabsent from the ACCs. Thus, ACCs may have evolved theability to bind ACh independently of the nAChRs.

The unusual pharmacological profiles of the ACC subunitssupport a unique acetylcholine-binding site. That nicotine,the defining agonist of nAChRs, and cytisine, a related ago-nist, are weak agonists and/or antagonists of ACC-1 and -2distinguishes the ACCs from most nicotinic receptors. Theweak nicotine response is not, however, unique. The C. el-egans levamisole receptors (nAChRs) are insensitive to nico-tine, and it has been shown that C. elegans has both nicotine-sensitive and -insensitive cation-selective ACh receptors (38).Moreover, nicotine and cytisine are antagonists of the verte-brate "9 nAChRs (33). More surprising is that arecoline is anefficient agonist of both ACC-1 and -2, although with sub-stantially lower affinity than ACh. Arecoline has been postu-lated to act on cation-selective nematode and insect LGICs, sothis sensitivity to arecoline may be a common feature ofinvertebrate ACh receptors (39, 40). Finally, "-BT, which isselective for vertebrate "7 and "9 nAChRs in the centralnervous system and the nAChRs of the neuromuscular junc-tion, does not block the ACC channels. "-BT binds primarilyto the C loop in nAChRs (41). Therefore, the lack of effect onACC channels presumably reflects the inability of "-BT tobind the ACC subunit’s divergent C loop sequences.

Are the Aplysia ACh-gated ion channels orthologs of theACCs? There are pharmacological similarities that suggestthey might be (3). One of the two Aplysia channels identifieddesensitized slowly, a characteristic of ACC-1 homomers andACC-1/ACC-3 heteromers. Nicotine and cytisine were also pooragonists of the slowly desensitizing Aplysia channel. Potenciesof the antagonists for nematode and mollusk receptors were inthe same range. However, Aplysia receptors appear to have anEC50 for ACh of ,100 #M. Perhaps most importantly, theAplysia channels were blocked by "-BT, indicating that theyshare a much greater similarity to the nicotinic receptors, atleast in the C loop, than do the ACC subunits. Ultimately,whether the Aplysia channels evolved independently will haveto be determined by sequence analysis.

Economically important antiparasitic nematocides target Cysloop LGICs. Levamisole is an agonist of a subset of nicotinic-typeacetylcholine receptors (42), and ivermectin activates glutamate-gated chloride channels (12). Both drug targets have propertiesthat are unique to nematodes, making it possible to create drugsthat are relatively ineffective agonists of homologous channels inthe vertebrate host. The levamisole receptors are most similar tothe vertebrate nicotine-sensitive nAChRs ("1–6) in sequence butare not nicotine-sensitive, whereas glutamate-gated chloridechannels are not found in vertebrates (38). Since ACCs are alsonot found in vertebrates, they are promising targets for thedevelopment of highly specific nematocides.

Acknowledgment—We thank E. Cooper for critical reading of themanuscript.

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a family of acetylcholine-gated chloride channel subunits in caenorhabditis elegans

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