Widely Diverse Structures Fit the Same Site
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Structural model for ␥-aminobutyric acid receptor noncompetitive antagonist binding: Widely diverse structures fit the same site Ligong Chen*, Kathleen A. Durkin†, and John E. Casida*‡ *Environmental Chemistry and Toxicology Laboratory, Department of Environmental Science, Policy, and Management and †Molecular Graphics Facility, College of Chemistry, University of California, Berkeley, CA 94720 Contributed by John E. Casida, January 13, 2006 Several major insecticides, including ␣-endosulfan, lindane, and block chloride flux so the target is referred to as the GABA fipronil, and the botanical picrotoxinin are noncompetitive antag- receptor NCA-binding site. Vertebrate GABA receptors consist of onists (NCAs) for the GABA receptor. We showed earlier that ␣, , ␥, , and other subunits in various combinations, for example,  human 3 homopentameric GABAA receptor recognizes all of the ␣12␥2 as a heteropentamer and 1 as a homopentamer (13–15). important GABAergic insecticides and reproduces the high insec- The molecular localization of the NCA site defined here (Fig. 1B) ticide sensitivity and structure-activity relationships of the native was first indicated by mutagenesis studies (16) as A2Ј (17–20), T6Ј insect receptor. Despite large structural diversity, the NCAs are (21, 22), and L9Ј (23, 24) in the cytoplasmic half of the transmem- proposed to fit a single binding site in the chloride channel lumen brane 2 domain of the channel (Fig. 2). Drosophila resistant to lined by five transmembrane 2 segments. This hypothesis is ex- dieldrin (RDL) have a mutation conferring GABA receptor in- amined with the  homopentamer by mutagenesis, pore structure 3 sensitivity identified as A2ЈS (17). The NCA target of the GABAA studies, NCA binding, and molecular modeling. The 15 amino acids receptor requires a  subunit, and a 3 homopentamer is sufficient in the cytoplasmic half of the pore were mutated to cysteine, for binding (9, 26). Importantly, the 3 subunit from human brain, serine, or other residue for 22 mutants overall. Localization of when expressed in insect Sf9 cells, assembles to form a receptor ,A-1C, A2C, T6C, and L9C (index numbers for the transmembrane sensitive to all of the important GABAergic insecticides (9) and 2 region) in the channel lumen was established by disulfide surprisingly, reproduces the insecticide sensitivity and structure- 3 cross-linking. Binding of two NCA radioligands [ H]1-(4-ethynyl- activity relationships of the native insect receptor (27). Studies of 3 phenyl)-4-n-propyl-2,6,7-trioxabicyclo[2.2.2]octane and [ H] 3,3- the GABA receptor NCA site are therefore simplified by using this bis-trifluoromethyl-bicyclo[2,2,1]heptane-2,2-dicarbonitrile was dra- highly sensitive 3 homopentamer, an approach verified by showing matically reduced with 8 of the 15 mutated positions, focusing here that Cys and Ser or Phe mutations in 3 at each of the 2Ј,6Ј, attention on A2,T6, and L9 as proposed binding sites, consistent Ј  and 9 positions greatly reduce or destroy NCA radioligand binding. with earlier mutagenesis studies. The cytoplasmic half of the 3 This study tested the hypothesis that insecticides and convul- homopentamer pore was modeled as an ␣-helix. The six NCAs sants of many chemical types act at the same GABA receptor site listed above plus t-butylbicyclophosphorothionate fit the 2 to 9 in the same way to initiate insecticidal action and mammalian pore region forming hydrogen bonds with the T6 hydroxyl and toxicity. The goal was to characterize the GABA receptor–NCA ,hydrophobic interactions with A2,T6, and L9 alkyl substituents interaction by using the human GABA receptor recombinant thereby blocking the channel. Thus, widely diverse NCA structures A  homopentamer as a model. The first step was to prepare Cys fit the same GABA receptor  subunit site with important impli- 3 and other mutations to scan the cytoplasmic half of M2 and the cations for insecticide cross-resistance and selective toxicity be- flanking region (Ϫ4Ј to 10Ј), overall 22 mutants involving 15 tween insects and mammals. positions. The mutants were used to identify Cys residues  ͉ ͉ ͉ ͉ undergoing disulfide cross-linking as a guide to channel pore 3 homopentamer transmembrane 2 insecticide disulfide trapping 3 3 receptor model structure (28). Next, [ H]EBOB and [ H]BIDN were used to identify positions where mutation altered binding (6, 8). Finally, modeling of the NCA-binding domain (29, 30) was applied to the est insect control in the past 60 years was achieved, in part,  homopentamer to determine whether the wide diversity of Ͼ ϫ 9 3 Pby application of 3 billion (3 10 ) pounds of polychlo- NCAs could fit the same site. rocycloalkane insecticides, including cyclodienes (e.g., ␣-en- dosulfan and dieldrin), lindane and its isomers, and others, which Results PHARMACOLOGY are now highly restricted or banned except for endosulfan and Mutagenesis and Protein Expression. The transfection efficiency of some uses of lindane (1–3). One of the replacement compounds each recombinant baculovirus was examined by PCR analysis. is the phenylpyrazole fipronil. All of these insecticides and the The nonrecombinant virus would give one 839-bp band of its botanical picrotoxinin (PTX) have widely diverse chemical polyhedrin region and the recombinant virus incorporating the structures but appear to act at the same nerve target. It is 1,425-bp  cDNA would appear at 2.3 kb. Each extracted therefore important to understand how these compounds work 3 recombinant virus gave only one 2.3-kb band (Fig. 3A), indicat- in mammals and insects, or how they do not work when resistant ing a recombination efficiency for the target gene of nearly 100% insect strains appear. for all mutants and the WT. Further, all PCR products from The GABA-gated chloride channel is the target for the insecti- cides and toxicants referred to above based on radioligand binding and electrophysiology studies (3–10). Important radioligands in Conflict of interest statement: No conflicts declared. these developments are [3H]dihydroPTX (4, 11), [35S]t- 3 Abbreviations: BIDN, 3,3-bis-trifluoromethyl-bicyclo[2,2,1]heptane-2,2-dicarbonitrile; butylbicyclophosphorothionate (TBPS) (5, 12), [ H]1-(4-ethynyl- Cu:phen, copper:phenanthroline; EBOB, 1-(4-ethynylphenyl)-4-n-propyl-2,6,7-trioxabicyclo- phenyl)-4-n-propyl-2,6,7-trioxabicyclo[2.2.2]octane (EBOB) (6), [2.2.2]octane; M2, transmembrane 2; NCA, noncompetitive antagonist; PTX, picrotoxinin; and [3H]3,3-bis-trifluoromethyl-bicyclo[2,2,1]heptane-2,2- RDL, resistant to dieldrin; TBPS, t-butylbicyclophosphorothionate. dicarbonitrile (BIDN) (8) (Fig. 1A). All of these compounds act in ‡To whom correspondence should be addressed: E-mail: [email protected]. mammals and insects as noncompetitive antagonists (NCAs) to © 2006 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0600370103 PNAS ͉ March 28, 2006 ͉ vol. 103 ͉ no. 13 ͉ 5185–5190 Downloaded by guest on September 25, 2021 Fig. 1. Structure-activity relationships of seven GABA receptor noncompetitive antagonists. (A) Structures of three important insecticides (lindane, fipronil, and ␣-endosulfan) and four radioligands (asterisk designates labeling position). The high potencies of each compound with the 3 homopentamer are indicated 3 3 by the 2.7 nM Kd for [ H]EBOB on direct binding and 0.47–59 nM IC50 values for the other compounds in displacing [ H]EBOB binding (9). (B) Models of four antagonists positioned as in A showing their proposed 3 homopentamer M2 binding sites in the channel lumen. A, L, and T refer to the side chains of the interacting 2Ј,6Ј, and 9Ј residues, respectively. virus extraction were sequenced and confirmed as the right cases of a molecular mass increase from 55 kDa to Ϸ130 kDa for mutations. the monomer and dimer, respectively, as detected by SDS͞ The expression levels of the WT and mutant 3 subunits were PAGE and immunoblotting (Fig. 4). Cys substituents at Ϫ1Ј,2Ј determined by Western blotting. The monoclonal anti--chain (weak), 6Ј, and 9Ј formed disulfide-linked dimers in the presence antibody recognized a very specific band at Ϸ55 kDa with similar but not in the absence of Cu:phen. Only trace amounts of the Ϫ1Ј intensity for membrane extracts of the WT and each mutant (Fig. and 9Ј monomers are left with Cu:phen indicating more exten- 3B). Equal protein transfer levels were determined by Ponceau sive reaction possibly due to higher flexibility at these positions. S staining. As exceptions, two mutants (L3ЈC and L3ЈF) were not Dimers were not detected under the same conditions for Cys expressed. substituents at 0Ј,1Ј,4Ј,5Ј,7Ј,8Ј, and 10Ј, although in some cases there were apparent losses in receptor levels on oxidation. Disulfide Cross-Linking Profiles. Oxidation of the Cys mutants with Disulfides at Ϫ1Ј,2Ј, and 9Ј were completely reversed with DTT, copper sulfate:1,10-phenanthroline (Cu:phen) resulted in four but the one at 6Ј was only partially reversed. Effect of Site-Specific Mutations on [3H]EBOB and [3H]BIDN Binding. Membranes (100 g of protein) from the WT were assayed with [3H]EBOB (1 nM) or [3H]BIDN (2.5 nM) by using incubations for 90 min at 25°C. Specific binding (n ϭ 10) was 2,458 Ϯ 250 dpm for [3H]EBOB and 1,253 Ϯ 100 dpm for [3H]BIDN with nonspecific binding of 496 Ϯ 45 and 225 Ϯ 16 dpm, respectively, i.e., 83–85% specific relative to total binding. Using the same Fig. 2. Alignment of the cytoplasmic half of the M2 and flanking sequences of various GABA receptor subunits. The species are human or rat for ␣, , and ␥, rat for , and Drosophila for WT and RDL. Index numbers for positioning in Fig. 3. Baculovirus transfection efficiencies and protein expression levels of M2 (25) are shown at the top.