A novel pharmatope tag inserted into the beta4 subunit confers allosteric modulation to neuronal nicotinic receptors

Chemical Synthesis of a Functional Fluorescent-Tagged α-Bungarotoxin

α-bungarotoxin is a large, 74 amino acid toxin containing five disulphide bridges, initially identified in the venom of Bungarus multicinctus snake. Like most large toxins, chemical synthesis of α-bungarotoxin is challenging, explaining why all previous reports use purified or recombinant α-bungarotoxin. However, only chemical synthesis allows easy insertion of non-natural amino acids or new chemical functionalities. Herein, we describe a procedure for the chemical synthesis of a fluorescent-tagged α-bungarotoxin. The full-length peptide was designed to include an alkyne function at the amino-terminus through the addition of a pentynoic acid linker. Chemical synthesis of α-bungarotoxin requires hydrazide-based coupling of three peptide fragments in successive steps. After completion of the oxidative folding, an azide-modified Cy5 fluorophore was coupled by click chemistry onto the toxin. Next, we determined the efficacy of the fluorescent-tagged α-bungarotoxin to block acetylcholine (ACh)-mediated currents in response to muscle nicotinic receptor activation in TE671 cells. Using automated patch-clamp recordings, we demonstrate that fluorescent synthetic α-bungarotoxin has the expected nanomolar affinity for the nicotinic receptor. The blocking effect of fluorescent α-bungarotoxin could be displaced by incubation with a 20-mer peptide mimicking the α-bungarotoxin binding site. In addition, TE671 cells could be labelled with fluorescent toxin, as witnessed by confocal microscopy, and this labelling was partially displaced by the 20-mer competitive peptide. We thus demonstrate that synthetic fluorescent-tagged α-bungarotoxin preserves excellent properties for binding onto muscle nicotinic receptors.

SecA drives transmembrane insertion of RodZ, an unusual single-span membrane protein

The transmembrane (TM) helices of most type II single-span membrane proteins (S-SMPs) of Escherichia coli occur near the N-terminus, where the cell's targeting mechanisms can readily identify it as it emerges from the ribosome. However, the TM helices of a few S-SMPs, such as RodZ, occur a hundred or more residues downstream from the N-terminus, which raises fundamental questions about targeting and assembly. Because of RodZ's novelty and potential usefulness for understanding TM helix insertion in vivo, we examined its membrane targeting and assembly. We used RodZ constructs containing immunotags before the TM domain to assess membrane insertion using proteinase K digestion. We confirmed the N(in)-C(out) (type II) topology of RodZ and established the absence of a targeting signal other than the TM domain. RodZ was not inserted into the membrane under SecA depletion conditions or in the presence of sodium azide, which is known to inhibit SecA. Insertion failed when the TM proton gradient was abolished with Carbonyl cyanide m-chlorophenyl hydrazone. Insertion also failed when RodZ was expressed in SecE-depleted E. coli, indicating that the SecYEG translocon is required for RodZ assembly. Protease accessibility assays of RodZ in other E. coli depletion strains revealed that insertion is independent of SecB, YidC, and SecD/F. Insertion was found to be only weakly dependent on the signal recognition particle pathway: insertion was weakly dependent on the Ffh but independent of FtsY. We conclude that membrane insertion of RodZ requires only the SecYEG translocon, the SecA ATPase motor, and the TM proton motive force.

A role of the sulfonylurea receptor 1 in endocytic trafficking of ATP-sensitive potassium channels

The ATP-sensitive potassium (K(ATP) ) channel consisting of sulfonylurea receptor 1 (SUR1) and inward-rectifier potassium channel 6.2 (Kir6.2) has a well-established role in insulin secretion. Mutations in either subunit can lead to disease due to aberrant channel gating, altered channel density at the cell surface or a combination of both. Endocytic trafficking of channels at the plasma membrane is one way to influence surface channel numbers. It has been previously reported that channel endocytosis is dependent on a tyrosine-based motif in Kir6.2, while SUR1 alone is unable to internalize. In this study, we followed endocytic trafficking of surface channels in real time by live-cell imaging of channel subunits tagged with an extracellular minimal α-bungarotoxin-binding peptide labeled with a fluorescent dye. We show that SUR1 undergoes endocytosis independent of Kir6.2. Moreover, mutations in the putative endocytosis motif of Kir6.2, Y330C, Y330A and F333I are unable to prevent channel endocytosis. These findings challenge the notion that Kir6.2 bears the sole endocytic signal for K(ATP) channels and support a role of SUR1 in this trafficking process.

Cholesterol-induced conformational changes in the oxytocin receptor

Recent studies suggest that cholesterol binding is widespread among GPCRs (G-protein-coupled receptors). In the present study, we analysed putative cholesterol-induced changes in the OTR [OT (oxytocin) receptor], a prototype of cholesterol-interacting GPCRs. For this purpose, we have created recombinant OTRs that are able to bind two small-sized fluorescence-labelled ligands simultaneously. An OTR antagonist was chosen as one of the ligands. To create a second ligand-binding site, a small-sized α-BTB (bungarotoxin binding) site was inserted at the N-terminus or within the third extracellular loop of the OTR. All receptor constructs were functionally active and bound both ligands with high affinity in the nanomolar range. Measurements of the quenching behaviour, fluorescence anisotropy and energy transfer of both receptor-bound ligands were performed to monitor receptor states at various cholesterol concentrations. The quenching studies suggested no major changes in the molecular environment of the fluorophores in response to cholesterol. The fluorescence anisotropy data indicated that cholesterol affects the dynamics or orientation of the antagonist. The energy transfer efficiency between both ligands clearly increased with increasing cholesterol. Overall, cholesterol induced both a changed orientation and a decreased distance of the receptor-bound ligands, suggesting a more compact receptor state in association with cholesterol.

Fluorophore assisted light inactivation (FALI) of recombinant 5-HT₃A receptor constitutive internalization and function

Fluorescent proteins and molecules are now widely used to tag and visualize proteins resulting in an improved understanding of protein trafficking, localization, and function. In addition, fluorescent tags have also been used to inactivate protein function in a spatially and temporally-defined manner, using a technique known as fluorophore-assisted light inactivation (FALI) or chromophore-assisted light inactivation (CALI). In this study we tagged the serotonin₃ A subunit with the α-bungarotoxin binding sequence (BBS) and subsequently labeled 5-HT₃A/BBS receptors with fluorescently conjugated α-bungarotoxin in live cells. We show that 5-HT₃A/BBS receptors are constitutively internalized in the absence of an agonist and internalization as well as receptor function are inhibited by fluorescence. The fluorescence-induced disruption of function and internalization was reduced with oxygen radical scavengers suggesting the involvement of reactive oxygen species, implicating the FALI process. Furthermore, these data suggest that intense illumination during live-cell microscopy may result in inadvertent FALI and inhibition of protein trafficking.

A new subfamily of conotoxins belonging to the A-superfamily

Two novel conotoxins from vermivorous cone snails Conus pulicarius and Conus tessulatus, designated as Pu14.1 and ts14a, were identified by cDNA cloning and peptide purification, respectively. The signal sequence of Pu14.1 is identical to that of α-conotoxins, while its predicted mature peptide, pu14a, shares high sequence similarity with ts14a, with only one residue different in their first intercysteine loop, which contains 10 residues and is rich in proline. Both pu14a and ts14a contain four separate cysteines in framework 14 (C-C-C-C). Peptide pu14a was chemically synthesized, air oxidized, and the connectivity of its two disulfide bonds was determined to be C1-C3, C2-C4, which is the same as found in α-conotoxins. The synthetic pu14a induced a sleeping symptom in mice and was toxic to freshwater goldfish upon intramuscular injection. Using the Xenopus oocyte heterologous expression system, 1μM of pu14a demonstrated to inhibit the rat neuronal α3β2-containing as well as the mouse neuromuscular α1β1γδ subtypes of nicotinic acetylcholine receptors, and then rapidly dissociated from the receptors. However, this toxin had no inhibitory effect on potassium channels in mouse superior cervical ganglion neurons. According to the identical signal sequence to α-conotoxins, the unique cysteine framework and molecular target of pu14a, we propose that pu14a and ts14a may represent a novel subfamily in the A-superfamily, designated as α1-conotoxins.

Chemical synthesis and characterization of two α4/7-conotoxins

α-Conotoxins are small disulfide-constrained peptides that act as potent and selective antagonists on specific subtypes of nicotinic acetylcholine receptors (nAChRs). We previously cloned two α-conotoxins, Mr1.1 from the molluscivorous Conus marmoreus and Lp1.4 from the vermivorous Conus leopardus. Both of them have the typical 4/7-type framework of the subfamily of α-conotoxins that act on neuronal nAChRs. In this work, we chemically synthesized these two toxins and characterized their functional properties. The synthetic Mr1.1 could primarily inhibit acetylcholine (ACh)-evoked currents reversibly in the oocyte-expressed rat α7 nAChR, whereas Lp1.4 was an unexpected specific blocker of the mouse fetal muscle α1β1γδ receptor. Although their inhibition affinities were relatively low, their unique receptor recognition profiles make them valuable tools for toxin-receptor interaction studies. Mr1.1 could also suppress the inflammatory response to pain in vivo, suggesting that it should be further investigated with respect to its molecular role in analgesia and its mechanism or therapeutic target for the treatment of pain.

K(V)4.2 channels tagged in the S1-S2 loop for alpha-bungarotoxin binding provide a new tool for studies of channel expression and localization

We report the first successful insertion of an engineered, high-affinity alpha-bungarotoxin (Bgtx) binding site into a voltage-gated ion channel, K(V)4.2, using a short, intra-protein embedded sequence (GGWRYYESSLEPYPDGG), derived from a previously described mimotope peptide, HAP. A major benefit to this approach is the ability to live-image the distribution and fate of functional channels on the plasma membrane surface. The Bgtx binding sequence was introduced into the putative extracellular loop between the S1 and S2 transmembrane domains of K(V)4.2. Following co-expression with KChIP3 in tsA201 cells, S1-S2 HAP-tagged channels express at levels comparable to wild-type K(V)4.2, and their activation and inactivation kinetics are minimally altered under most conditions. Binding assays, as well as live staining of surface-expressed K(V)4.2 channels with fluorescent-Bgtx, readily demonstrate specific binding of Bgtx to HAP-tagged K(V)4.2 expressed on the surface of tsA201 cells. Similar live-imaging results were obtained with HAP-tagged K(V)4.2 transfected into hippocampal neurons in primary culture suggesting applicability for future in vivo studies. Furthermore, the activation kinetics of S1-S2-tagged K(V)4.2 channels are minimally affected by the binding of Bgtx, suggesting a limited role if any for the S1-S2 loop in voltage sensing or gating associated conformational changes. Successful functional insertion of the HAP sequence into the S1-S2 linker of K(V)4.2 suggests that other related channels may similarly be amenable to this tagging strategy.

Role of loop 9 on the function of neuronal nicotinic receptors

We have studied the role of loop 9 in the function of neuronal nicotinic receptors. By systematically mutating the residues in the loop we have determined that the most important amino acids determining the coupling of binding to gating are the ones closer to the transmembrane region. Single mutations at location E173 in homomeric alpha7 receptors destroyed their function by completely abolishing the current while preserving the expression at the membrane. In contrast, heteromeric receptor alpha3beta4 with the same mutations retained some function. We conclude that loop 9 has a different role in the function of homomeric and heteromeric receptors.

Engineering neuronal nicotinic acetylcholine receptors with functional sensitivity to alpha-bungarotoxin: a novel alpha3-knock-in mouse

We report here the construction of a novel knock-in mouse expressing chimeric alpha3 nicotinic acetylcholine receptor (nAChR) subunits with pharmacological sensitivity to alpha-bungarotoxin (alphaBTX). Sensitivity was generated by substituting five amino acids in the loop C (beta9-beta10) region of the mouse alpha3 subunit with the corresponding residues from the alpha1 subunit of the muscle type receptor from Torpedo californica. To demonstrate the utility of the underlying concept, expressed alpha3[5] subunits were characterized in the superior cervical ganglia (SCG) of homozygous knock-in mice, where the synaptic architecture of postsynaptic alpha3-containing nAChR clusters could now, for the first time, be directly visualized and interrogated by live-staining with rhodamine-conjugated alphaBTX. Consistent with the postsynaptic localization of ganglionic nAChRs, the alphaBTX-labeled puncta colocalized with a marker for synaptic varicosities. Following in vivo deafferentation, these puncta persisted but with significant changes in intensity and distribution that varied with the length of the recovery period. Compound action potentials and excitatory postsynaptic potentials recorded from SCG of mice homozygous for alpha3[5] were abolished by 100 nmalphaBTX, even in an alpha7 null background, demonstrating that synaptic throughput in the SCG is completely dependent on the alpha3-subunit. In addition, we observed that the genetic background of various inbred and outbred mouse lines greatly affects the functional expression of alpha3[5]-nAChRs, suggesting a powerful new approach for exploring the molecular mechanisms underlying receptor assembly and trafficking. As alphaBTX-sensitive sequences can be readily introduced into other nicotinic receptor subunits normally insensitive to alphaBTX, the findings described here should be applicable to many other receptors.

Hydrophobic photolabeling studies identify the lipid-protein interface of the 5-HT3A receptor

A HEK-293 cell line that stably expresses mouse 5-HT(3A)Rs containing a C-terminal extension that confers high-affinity binding of alpha-bungarotoxin (alphaBgTx) was established (alphaBgTx-5-HT(3A)Rs) and used to purify alphaBgTx-5-HT(3A)Rs in a lipid environment for use in structural studies using photoaffinity labeling. alphaBgTx-5-HT(3A)Rs were expressed robustly (60 pmol of [(3)H]BRL-43694 binding sites (approximately 3 microg of receptor) per milligram of protein) and displayed the same functional properties as wild-type receptors (serotonin EC(50) = 5.3 +/- 0.04 microM). While [(125)I]alphaBgTx bound to the alphaBgTx-5-HT(3A)Rs with high affinity (K(d) = 11 nM), application of nonradioactive alphaBgTx (up to 300 microM) had no effect on serotonin-induced current responses. alphaBgTx-5-HT(3A)Rs were purified on an alphaBgTx-derivatized affinity column from detergent extracts in milligram quantities and at approximately 25% purity. The hydrophobic photolabel 3-trifluoromethyl-3-(m-[(125)I]iodophenyl)diazirine ([(125)I]TID) was used to identify the amino acids at the lipid-protein interface of purified and lipid-reconstituted alphaBgTx-5-HT(3A)Rs. [(125)I]TID photoincorporation into the alphaBgTx-5-HT(3A)R subunit was initially mapped to subunit proteolytic fragments of 8 kDa, containing the M4 transmembrane segment and approximately 60% of incorporated (125)I, and 17 kDa, containing the M1-M3 transmembrane segments. Within the M4 segment, [(125)I]TID labeled Ser(451), equivalent to the [(125)I]TID-labeled residue Thr(422) at the lipid-exposed face of the Torpedo nicotinic acetylcholine receptor (nAChR) alpha1M4 alpha-helix. These results provide a first definition of the surface of the 5-HT(3A)R M4 helix that is exposed to lipid and establish that this surface is equivalent to the surface exposed to lipid in the Torpedo nAChR.

Characterization of a novel alpha4/4-conotoxin, Qc1.2, from vermivorous Conus quercinus

As part of continuing studies of the identification of gene organization and cloning of novel alpha-conotoxins, the first alpha4/4-conotoxin identified in a vermivorous Conus species, designated Qc1.2, was originally obtained by cDNA and genomic DNA cloning from Conus quercinus collected in the South China Sea. The predicted mature toxin of Qc1.2 contains 14 amino acid residues with two disulfide bonds (I-III, II-IV connectivity) in a native globular configuration. The mature peptide of Qc1.2 is supposed to contain an N-terminal post-translationally processed pyroglutamate residue and a free carboxyl C-terminus. This peptide was chemically synthesized and refolded for further characterization of its functional properties. The synthetic Qc1.2 has two interconvertible conformations in aqueous solution, which may be due to the cis-trans isomerization of the two successive Pro residues in its first Cys loop. Using the Xenopus oocyte heterologous expression system, Qc1.2 was shown to selectively inhibit both rat neuronal alpha3beta2 and alpha3beta4 subtypes of nicotinic acetylcholine receptors with low potency. A block of about 63% and 37% of the ACh-evoked currents was observed, respectively, and the toxin dissociated rapidly from the receptors. Compared with other characterized alpha-conotoxin members, the unusual structural features in Qc1.2 that confer to its receptor recognition profile are addressed.

The positive allosteric modulator morantel binds at noncanonical subunit interfaces of neuronal nicotinic acetylcholine receptors

We are interested in the positive allosteric modulation of neuronal nicotinic acetylcholine (ACh) receptors and have recently shown that the anthelmintic compound morantel potentiates by enhancing channel gating of the alpha3beta2 subtype. Based on the demonstration that morantel-elicited currents were inhibited by the classic ACh competitor dihydro-beta-erythroidine in a noncompetitive manner and that morantel still potentiates at saturating concentrations of agonist (Wu et al., 2008), we hypothesized that morantel binds at the noncanonical beta2(+)/alpha3(-) subunit interface. In the present study, we created seven cysteine-substituted subunits by site-directed mutagenesis, choosing residues in the putative morantel binding site with the aid of structural homology models. We coexpressed the mutant subunits and their respective wild-type partners in Xenopus oocytes and characterized the morantel potentiation of ACh-evoked currents, as well as morantel-evoked currents, before and after treatment with a variety of methanethiosulfonate (MTS)-based compounds, using voltage-clamp recordings. The properties of four of the seven mutants, two residues on each side of the interface, were changed by MTS treatments. Coapplication with ACh enhanced the extent of MTS modification for alpha3A106Cbeta2 and alpha3beta2S192C receptors. The activities of two mutants, alpha3T115Cbeta2 and alpha3beta2T150C, were dramatically altered by MTS modification. For alpha3beta2T150C, while peak current amplitudes were reduced, potentiation was enhanced. For alpha3T115Cbeta2, both current amplitudes and potentiation were reduced. MTS modification and morantel were mutually inhibitory: MTS treatment decreased morantel-evoked currents and morantel decreased the rate of MTS modification. We conclude that the four residues showing MTS effects contribute to the morantel binding site.

Proteomic analysis of an alpha7 nicotinic acetylcholine receptor interactome

The alpha7 nicotinic acetylcholine receptor (nAChR) is well established as the principal high-affinity alpha-bungarotoxin-binding protein in the mammalian brain. We isolated carbachol-sensitive alpha-bungarotoxin-binding complexes from total mouse brain tissue by affinity immobilization followed by selective elution, and these proteins were fractionated by SDS-PAGE. The proteins in subdivided gel lane segments were tryptically digested, and the resulting peptides were analyzed by standard mass spectrometry. We identified 55 proteins in wild-type samples that were not present in comparable brain samples from alpha7 nAChR knockout mice that had been processed in a parallel fashion. Many of these 55 proteins are novel proteomic candidates for interaction partners of the alpha7 nAChR, and many are associated with multiple signaling pathways that may be implicated in alpha7 function in the central nervous system. The newly identified potential protein interactions, together with the general methodology that we introduce for alpha-bungarotoxin-binding protein complexes, form a new platform for many interesting follow-up studies aimed at elucidating the physiological role of neuronal alpha7 nAChRs.

alpha4/7-conotoxin Lp1.1 is a novel antagonist of neuronal nicotinic acetylcholine receptors

Cone snails comprise approximately 700 species of venomous molluscs which have evolved the ability to generate multiple toxins with varied and exquisite selectivity. alpha-Conotoxin is a powerful tool for defining the composition and function of nicotinic acetylcholine receptors which play a crucial role in excitatory neurotransmission and are important targets for drugs and insecticides. An alpha4/7 conotoxin, Lp1.1, originally identified by cDNA and genomic DNA cloning from Conus leopardus, was found devoid of the highly conserved Pro residue in the first intercysteine loop. To further study this toxin, alpha-Lp1.1 was chemically synthesized and refolded into its globular disulfide isomer. The synthetic Lp1.1 induced seizure and paralysis on freshwater goldfish and selectively reversibly inhibited ACh-evoked currents in Xenopus oocytes expressing rat alpha3beta2 and alpha6alpha3beta2 nAChRs. Comparing the distinct primary structure with other functionally related alpha-conotoxins could indicate structural features in Lp1.1 that may be associated with its unique receptor recognition profile.

Two potent alpha3/5 conotoxins from piscivorous Conus achatinus

Every cone snail produces a mixture of different conotoxins and secretes them to immobilize their prey and predators. alpha3/5 Conotoxins, isolated from fish-hunting cone snails, target muscle nicotinic acetylcholine receptors. The structure and function of alpha3/5 conotoxin from the piscivorous Conus achatinus have not been studied. We synthesized two pentadecamer peptides, Ac1.1a and Ac1.1b, with appropriate disulfide bonding, based on cDNA sequences of alpha3/5 conotoxins from C. achatinus. Ac1.1a and Ac1.1b differ by only one amino acid residue. They have similar potency on blocking recombinant mouse muscle acetylcholine receptor expressed in Xenopus laevis oocytes, with IC50 values of 36 nM and 26 nM, respectively. For Ac1.1b, deletion of the first three N-terminal amino acids did not change its activity, indicating that the N-terminus is not involved in the interaction with its receptor. Furthermore, our experiments indicate that both toxins strongly prefer the alpha1-delta subunit interface instead of the alpha1-gamma binding site on the mouse muscle nicotinic acetylcholine receptor. These peptides provide additional tools for the study of the structure and function of nicotinic receptor.

Cytoplasmic regions adjacent to the M3 and M4 transmembrane segments influence expression and function of ?7 nicotinic acetylcholine receptors. A study with single amino acid mutants

We studied the role of the cytoplasmic regions adjacent to the M3 and M4 transmembrane segments of alpha7 nicotinic receptors in the expression of functional channels. For this purpose, a total of 50 amino acids were mutated throughout the mentioned regions. Mutants close to M3, from Arg294 to Leu321, showed slight modifications in the levels of alpha-bungarotoxin binding sites and acetylcholine-evoked currents. Exceptions were mutants located at two clusters (His296 to Pro300 and Ile312 to Trp316), which exhibited low expression levels. In addition, some mutants showed altered functional responses. Many mutants close to M4 showed increased receptor expression, especially the ones located at the hydrophobic face of a putative amphipathic helix. This effect seems to be the consequence of a combination of increased receptor biosynthesis, higher transport efficiency and delayed degradation, such that we postulate that elements in the amphipathic domain strongly influence receptor stability. Finally, some mutants in this region showed altered functional responses: elimination of positively charged residues (Arg424 and Arg426) increased currents, whereas the opposite was observed upon suppression of negatively charged ones (Glu430 and Glu432). These results suggest that the cytoplasmic regions close to M3 and M4 play important structural and functional roles.

Allosteric control of oligonucleotide hybridization by metal-induced cyclization

A bis(terpyridine) modified single-stranded DNA smoothly forms a stable cycle by Fe2+-assisted ring closure. A novel, complex type of allosteric behavior is observed with Zn2+, which reversibly off-regulates binding of a complementary oligonucleotide only in a narrow concentration range.

The cysteine-rich with EGF-like domains 2 (CRELD2) protein interacts with the large cytoplasmic domain of human neuronal nicotinic acetylcholine receptor alpha4 and beta2 subunits

Using a yeast two-hybrid screening we report the isolation of a novel human protein, hCRELD2beta, that interacts specifically with the large cytoplasmic regions of human nicotinic acetylcholine receptor (nAChR) alpha4 and beta2 subunits, both in yeast cells and in vitro. This interaction is not detected with nAChR alpha7 and alpha3 subunits. The hCRELD2 gene encodes for multiple transcripts, likely to produce multiple protein isoforms. A previously reported one has been renamed as CRELD2alpha. Isoforms alpha and beta are expressed in all tissues examined and have the same N-terminal and central regions but alternative C-terminal regions. Both isoforms interact with the alpha4 subunit. Within this subunit the interaction was localized to the N-terminal region of the large cytoplasmic loop. The CRELD2beta protein is present at the endoplasmic reticulum where colocalized with alpha4beta2 nAChRs upon cell transfection. Immunohistochemistry experiments demonstrated the presence of CRELD2 in the rat brain at sites where alpha4beta2 receptors have been previously detected. Labeling was restricted to neuronal perikarya. Finally, CRELD2 decreases the functional expression and impairs membrane transport of alpha4beta2 nAChRs in Xenopus leavis oocytes, without affecting alpha3beta4 and alpha7 nAChR expression. These results suggest that CRELD2 can act as a specific regulator of alpha4beta2 nAChR expression.