2013 08-05 activating uncoupled nachrs supp info · thin layer chromatography of lipid extracts...
TRANSCRIPT
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SUPPLEMENTARY INFORMATION
A novel mechanism for activating uncoupled nicotinic acetylcholine receptors
Corrie J.B. daCosta¶†, Lopamudra Dey¶, J.P. Daniel Therien, and John E. Baenziger* Department of Biochemistry, Microbiology, and Immunology, 451 Smyth Rd., Ottawa, ON, K1H 8M5; Tel: (613) 562‐5800 x8222; Fax: (613) 562‐5440 ¶These authors contributed equally to this work †Present Address: Receptor Biology Laboratory, Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, Minnesota 55905, USA.
SUPPLEMENTARY RESULTS
Includes: Supplementary Figures 1‐10 with captions
Nature Chemical Biology: doi:10.1038/NChemBio.1338
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Supplementary Figure 1 | The uncoupled nAChR. (a) The nAChR in reconstituted membranes adopts coupled (resting, R; open, O; and desensitized, D conformations) and uncoupled (U) conformations. (b) The uncoupled POPC‐nAChR does not transition to the desensitized state upon agonist binding. Ethidium bromide fluorescence emission intensity plotted in arbitrary units (au) as a function of time after addition of i) native‐nAChR, ii) PC/PA/Chol‐nAChR, or iii) POPC‐nAChR. At the indicated time points, the noted nAChR membranes (~60 μM BTX binding sites), 500 μM Carb, and 500 μM dibucaine were added. Dibucaine is an open channel blocker that binds competitively with ethidium. (c) The fraction of peptide hydrogens that remain in the protiated versus deuterated form is plotted as a function of time after exposure to 2H2O for PC‐nAChR (●) and PC/PA/Chol‐nAChR (■) both in the presence (filled symbols) and absence (open symbols) of Carb.
Nature Chemical Biology: doi:10.1038/NChemBio.1338
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Supplementary Figure 2 | Effect of lipid acyl chain length on the stability of detergent solubilized nAChR and its incorporation into lipid bilayers. (a) nAChR protein yields from affinity columns run in parallel in the presence of 1% cholate and the noted lipid to assess the effects of each lipid on the stability of the nAChR in the solubilized state. Percentage yields are relative to columns run in the presence of soybean asolectin. All values are ±standard deviation, n=2 experiments. (b) Incorporation of the nAChR into di14:1PC, di18:1PC, di22:1PC, and asolectin membranes was determined using step sucrose‐density gradients. nAChR‐free liposomes, reconstituted nAChR membranes (lipid:nAChR ratio roughly 500:1 (mol:mol)), and low lipid:nAChR aggregates settle at the interfaces between 0%/20%, 20%/35%, and 35%/70% sucrose, respectively (daCosta et al. J. Biol. Chem. (2009) 284, 17,819‐25). (c) Representative thin layer chromatography of lipid extracts from a di22:1PC reconstitution. TLC includes the lipid standards diacylglycerol (DAG), cholesterol (Chol), phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS). Mock refers to a mock lipid extraction performed in the absence of reconstituted nAChR membranes.
di14:1PC
di18:1PC
di22:1PC
Asolectin
9.9 ± 4.4
93.5 ± 5.6
100.0 ± 0.0
99.7 ± 0.5
Prot
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d (%
)
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90
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di14:1
PC
di16:1
PC
di18:1
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di20:1
PC
di22:1
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di24:1
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di14:1PC di18:1PC di22:1PC Asolectin
Membrane % Incorporationa
% of total protein at the 20-35% interface± standard deviation, n = 2
a
b
b
Nature Chemical Biology: doi:10.1038/NChemBio.1338
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Nature Chemical Biology: doi:10.1038/NChemBio.1338
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Supplementary Figure 4 | Infrared difference spectroscopy detects the structural changes induced in the nAChR upon ligand binding. (a) FTIR spectra of the nAChR recorded in the resting (top trace) and Carb‐desensitized (middle trace) states are visually identical because the structures of and local environments surrounding most residues in the protein are unaffected by the conformational change. Subtle vibrational shifts upon Carb‐induced desensitization can be detected by digitally subtracting the two spectra (lower trace) ‐ note the different absorption scale. (b) To obtain sufficiently high fidelity spectra for calculating spectral differences, films of the nAChR are deposited on a germanium internal reflection element. Two spectra in the ligand‐free and then one spectrum in the ligand‐bound states are recorded while alternately flowing buffer without and then with the ligand of interest (in this case, Carb), past the nAChR film. The spectrum recorded in the presence of Carb minus the second spectrum recorded in the absence of Carb is referred to as a Carb‐difference spectrum. The difference between the two spectra recorded in the absence of Carb is referred to as a control difference spectrum (see Supplementary Fig. 6b). The ability to remotely and repetitively trigger the conformational transition allows for the acquisition of highly reproducible infrared spectra from which subtle spectral differences can be accurately assessed. To achieve sufficient signal to noise, difference spectra are typically the average of between >50 individual difference experiments.
Nature Chemical Biology: doi:10.1038/NChemBio.1338
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Supplementary Figure 5 | Infrared difference spectra provide a spectral fingerprint of nAChR‐agonist physical interactions. The majority of peaks in the Carb‐difference spectrum reflect shifts in the vibrations of binding site residues that occur when they interact physically with the agonist. (a) Structures of the agonists Carb and tetramethylamine (TMA). (b) A schematic of the nAChR agonist binding site showing the “anionic sub‐site”, which is formed by aromatic π‐electrons (blue) that interact with the quaternary amine of Carb or TMA, and the “esterophilic sub‐site” (green), which interacts with the ester carbonyl of Carb. (c) Carb‐difference (top trace), TMA‐difference (middle trace), and Carb‐minus‐TMA (lower trace) difference spectra. The Carb‐difference spectrum exhibits a vibration due to the Carb C=O stretching vibration (asterisk), but this vibration is absent in the TMA‐difference spectrum (TMA lacks the C=O functional group). Both Carb‐ and TMA‐difference spectra exhibit vibrational shifts that result from the resting‐to‐desensitized conformational change (light shading). They also exhibit prominent negative and positive peaks near 1620 and 1515 cm‐1 due to vibrational shifts of anionic sub‐site aromatic residues that occur upon binding the quaternary amine of Carb or TMA. A Carb‐minus‐TMA difference spectrum is obtained when a spectrum recorded in the presence of TMA is subtracted from a spectrum recorded subsequently in the presence of Carb. The Carb‐minus‐TMA difference lacks peaks due to the resting‐to‐desensitized conformational change, as the desensitized conformation is stabilized in the presence of both Carb and TMA. Difference peaks due to the formation of quaternary amine‐aromatic interactions are also absent because the same amine‐aromatic interactions occur in the presence of both agonists. The Carb‐minus‐TMA difference exhibits two positive/negative couples (labeled, +/‐) due to the vibrational shifts of an esterophilic sub‐site residue. The negative peaks reflect the vibrations of this residue in the presence of TMA (i.e. no interactions with TMA), while the positive peaks reflect the vibrations of this same residue upon formation of physical interactions with the ester carbonyl of Carb.
Nature Chemical Biology: doi:10.1038/NChemBio.1338
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Supplementary Figure 6 | Conformationally sensitive bands in Carb‐difference spectra. (a) Carb‐difference spectra recorded from aso‐nAChR (trace i) and PC/PA/Chol‐nAChR (trace ii) both exhibit amide I and II band intensity near 1655 and 1545 cm‐1 that reflects structural changes in the polypeptide backbone upon transition from the resting to the desensitized state. Carb‐difference spectra recorded while the nAChR is maintained continuously in the desensitized state by the local anesthetic, dibucaine, do not exhibit intensity at these marker frequencies (trace iii). PC‐nAChR does not exhibit intensity at the same marker frequencies and thus also does not undergo the resting‐to‐desensitized conformational transition, despite Carb binding (trace iv). Note that the appearance of a vibration due to bound Carb (1724 cm‐1) shows that Carb binds to PC‐nAChR. (b) Comparision of the Carb‐difference and control difference spectra recorded from aso‐nAChR (trace i), di22:1PC‐nAChR (trace ii), and di18:1PC‐nAChR (trace iii). The control difference spectra are shown in grey.
Nature Chemical Biology: doi:10.1038/NChemBio.1338
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Supplementary Figure 7 | Kinteics of ethidium binding to the nAChR. (a) Fluorescence emission intensity of ethidium bromide recorded as a function of time after addition of aso‐nAChR ii) without (grey line) or iii) with (black line) 30 min prior incubation with Carb: In i) the traces recorded with and without prior incubation with Carb are superimposed, normalizing the spectra to highlight the different rates of ethidium binding under the two conditions. (b) Curve fitting of the ethidium binding data.
Nature Chemical Biology: doi:10.1038/NChemBio.1338
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Supplementary Figure 8 | Rapid ethidium binding after the addition of Carb is governed by a rapid transition to the open state. The fluorescence emission of ethidium was monitored with PC/PA/Chol‐nAChR. Trace i, at the indicated times ~50 nM nAChR, 500 μM Carb, and 500 μM dibucaine (Dib) were added to a 0.3 μM solution of ethidium. Trace ii, nAChR was preincubated with 500 μM Carb before adding to the ethidium solution. Trace iii, the nAChR was added to a solution of both ethidium and Carb. The latter shows that the rapid binding upon Carb addition (trace i) is not due to a mixing phenomenon, but is governed by agonist‐induced conformational transitions. Data from daCosta et al., J. Biol. Chem. (2009) 284, 33841‐33849.
Nature Chemical Biology: doi:10.1038/NChemBio.1338
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Supplementary Figure 9 | Effects of membrane physical environment on nAChR conformational transitions. (a) The deconvolved lipid ester carbonyl vibration showing components due to hydrogen bonded (~1730 cm‐1) and non‐hydrogen bonded (~1740 cm‐1) lipid C=O from i) aso‐nAChR, ii) PC/PA‐nAChR, iii) PC/PG‐nAChR, iv) PC/Chol‐nAChR, v) di22:1PC‐nAChR, vi) di18:1PC‐nAChR, vii) di14:1PC‐nAChR, and viii) POPC‐nAChR. All spectra recorded at 22.5 oC. (b) No ethidium binding shows that POPC‐nAChR is locked in a low‐affinity ethidium binding uncoupled conformation at temperatures close to or well above the gel‐to‐liquid crystal phase transition of the reconstituted bilayer (‐1.8 oC). (c) The nAChR in PC/PA membranes, with PA containing palmitoyloleoyl (POPA) or dioleoyl (DOPA) acyl chains is stabilized in a resting conformation that undergoes robust conformational transitions at varying temperatures above and below the gel to liquid crystal phase transition temperatures of the two bilayers (PC/POPA‐nAChR, +23.7 oC; PC/DOPA‐nAChR, +0.5 oC (data from daCosta et al. J. Biol. Chem. (2002) 277, 201‐208)). The deconvolved lipid ester carbonyl (left panel) and Carb‐difference (right panel) recorded from i) PC/POPA‐nAChR at 22.5 oC, PC/DOPA‐nAChR at 22.5 oC, iii) PC/POPA‐nAChR at 15 oC, and iv) PC/POPA‐nAChR at 30 oC.
Nature Chemical Biology: doi:10.1038/NChemBio.1338
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Supplementary Figure 10 | The lipid exposed transemembrane α‐helices of the nAChR and KcsA. (a) Comparison of the nAChR M4 α‐helix sequences with those of the lipid‐exposed transmembrane α‐helices of KcsA. Solid boxes denote the transmembrane α‐helices. Dashed boxes highlight regions that are not observed in the nAChR structure. (b) The transmembrane domain structures of the nAChR (pdb code 2BG9) and KcsA (pdb code 1BL8). For the nAChR, the γ‐subunit is in the foreground with the αγ and αδ subunits on the left and right, respectively. Transmembrane α‐helices are shown as transparent ribbon diagrams. Lipid exposed aromatic (yellow), positive (blue), and negative (red) residues on the nAChR M4 α‐helices and on the KcsA inner and outer α‐helices are highlighted as spheres. Individual subunits of both the nAChR (α‐subunit) and KcsA are shown below, with two residues lining the channel axis in each subunit shown as tan sticks.
Nature Chemical Biology: doi:10.1038/NChemBio.1338