Functional characterisation of a nicotinic acetylcholine receptor α subunit from the brown dog tick, Rhipicephalus sanguineus

Ticks and tick-borne diseases have a major impact on human and animal health worldwide. Current control strategies rely heavily on the use of chemical acaricides, most of which target the CNS and with increasing resistance, new drugs are urgently needed. Nicotinic acetylcholine receptors (nAChRs) are targets of highly successful insecticides. We isolated a full-length nAChR α subunit from a normalised cDNA library from the synganglion (brain) of the brown dog tick, Rhipicephalus sanguineus. Phylogenetic analysis has shown this R. sanguineus nAChR to be most similar to the insect α1 nAChR group and has been named Rsanα1. Rsanα1 is distributed in multiple tick tissues and is present across all life-stages. When expressed in Xenopus laevis oocytes Rsanα1 failed to function as a homomer, with and without the addition of either Caenorhabditis elegans resistance-to-cholinesterase (RIC)-3 or X. laevis RIC-3. When co-expressed with chicken β2 nAChR, Rsanα1 evoked concentration-dependent, inward currents in response to acetylcholine (ACh) and showed sensitivity to nicotine (100 μM) and choline (100 μM). Rsanα1/β2 was insensitive to both imidacloprid (100 μM) and spinosad (100 μM). The unreliable expression of Rsanα1 in vitro suggests that additional subunits or chaperone proteins may be required for more robust expression. This study enhances our understanding of nAChRs in arachnids and may provide a basis for further studies on the interaction of compounds with the tick nAChR as part of a discovery process for novel acaricides.     

Binding-gating coupling in a nondesensitizing α7 nicotinic receptor: A single channel pharmacological study

The highly conserved αLys145 has been suggested to play an important role in the early steps of activation of the nicotinic acetylcholine receptor (nAChR) by acetylcholine. Both macroscopic and single channel currents were recorded in the slowly desensitizing mutants L248T- and K145A-L248T-α7 receptors expressed in Xenopus oocytes. On ACh-evoked currents, substitution of Lys145 by alanine showed the same effects that in wild type receptors: moderately decreased gating function and a more-than-expected loss of ACh potency, thus validating the experimental model. Single channel analysis quantitatively agreed with macroscopic data and revealed that impaired gating function in the double mutant α7K145A/L248T is the consequence of a slower opening rate, β. Several nicotinic agonists were also studied, showing important features. Particularly, dimethylphenylpiperazinium (DMPP), acting as an antagonist in α7K145A, became a full agonist in α7K145A/L248T. Single channel analysis of DMPP-evoked currents showed effects of Lys145 removal similar to those observed with ACh. Data suggest that α7Lys145 facilitates the early steps of channel activation. Moreover, the slowly desensitizing mutant α7L248T could be an interesting tool for the study of channel activation in α7 receptors. Nevertheless, its extensively altered pharmacology precludes the simple extrapolation of pharmacological data obtained in singly mutated α7 receptors.     

Automating Single Subunit Counting of Membrane Proteins in Mammalian Cells

Elucidating subunit stoichiometry of neurotransmitter receptors is preferably carried out in a mammalian expression system where the rules of native protein assembly are strictly obeyed. Although successful in Xenopus oocytes, single subunit counting, manually counting photobleaching steps of GFP-tagged subunits, has been hindered in mammalian cells by high background fluorescence, poor control of expression, and low GFP maturation efficiency. Here, we present a fully automated single-molecule fluorescence counting method that separates tagged proteins on the plasma membrane from background fluorescence and contaminant proteins in the cytosol or the endoplasmic reticulum and determines the protein stoichiometry. Lower GFP maturation rates observed in cells cultured at 37 °C were partly offset using a monomeric version of superfolder GFP. We were able to correctly identify the stoichiometry of GluK2 and α1 glycine receptors. Our approach permits the elucidation of stoichiometry for a wide variety of plasma membrane proteins in mammalian cells with any commercially available TIRF microscope.     

Inferring the lifetime of endosomal protein complexes by fluorescence recovery after photobleaching

Cellular signal transduction is dynamic, with signaling proteins continually associating and dissociating into and from protein complexes. Here we present a fluorescence recovery after photobleaching technique to determine the lifetime of protein complexes on intracellular vesicles. We use Bayesian inference based on a model that includes the diffusion of cytosolic proteins and their interaction with membrane-bound receptors. Our analysis is general: we incorporate prior information on protein diffusion, measurement error in determining fluorescence intensities, corrections for photobleaching, and variation in the concentration of receptors between vesicles. We apply our method to the complexes formed on endosomes by G-protein-coupled receptors and the protein β-arrestin. The lifetime of these complexes determines the recycling rate of the receptors. We find in mammalian cells that the bradykinin type 2 receptor and β-arrestin2 complex has a lifetime of ∼2 min, while the angiotensin II type 1A receptor and β-arrestin2 complex has a lifetime of ∼6 min. As well as allowing quantitative comparisons between experiments, our method provides in vivo parameters for systems biology simulations of signaling networks.     

Pharmacological and molecular studies on the interaction of varenicline with different nicotinic acetylcholine receptor subtypes. Potential mechanism underlying partial agonism at human α4β2 and α3β4 subtypes

To determine the structural components underlying differences in affinity, potency, and selectivity of varenicline for several human (h) nicotinic acetylcholine receptors (nAChRs), functional and structural experiments were performed. The Ca^2 + influx results established that: (a) varenicline activates (μM range) nAChR subtypes with the following rank sequence: hα7 > hα4β4 > hα4β2 > hα3β4 >>> hα1β1γδ; (b) varenicline binds to nAChR subtypes with the following affinity order (nM range): hα4β2 ~ hα4β4 > hα3β4 > hα7 >>> Torpedo α1β1γδ. The molecular docking results indicating that more hydrogen bond interactions are apparent for α4-containing nAChRs in comparison to other nAChRs may explain the observed higher affinity; and that (c) varenicline is a full agonist at hα7 (101%) and hα4β4 (93%), and a partial agonist at hα4β2 (20%) and hα3β4 (45%), relative to (±)-epibatidine. The allosteric sites found at the extracellular domain (EXD) of hα3β4 and hα4β2 nAChRs could explain the partial agonistic activity of varenicline on these nAChR subtypes. Molecular dynamics simulations show that the interaction of varenicline to each allosteric site decreases the capping of Loop C at the hα4β2 nAChR, suggesting that these allosteric interactions limit the initial step in the gating process. In conclusion, we propose that in addition to hα4β2 nAChRs, hα4β4 nAChRs can be considered as potential targets for the clinical activity of varenicline, and that the allosteric interactions at the hα3β4- and hα4β2-EXDs are alternative mechanisms underlying partial agonism at these nAChRs.     

Single-channel study of the binding-gating coupling in the slowly desensitizing chimeric α7-5HT3A receptor

We have studied the role of the highly conserved residue αLysine145 in the early steps of activation by acetylcholine of the nicotinic acetylcholine receptor (nAChR). Both macroscopic and single-channel currents were recorded in the slowly desensitizing chimeric mutant receptor α7V201-5HT3A/R432Q/R436D/R440A, made of α7 nAChRs and serotonin receptors of subtype 3A (ch1), and its corresponding mutant K145A (ch1/K145A) expressed in Xenopus oocytes. Mutant ch1/K145A receptors had a reduced gating function similar to that produced by the same mutation in the wild type receptor α7. The mutated receptor has reduced opening rate constants, β, and increased closing rate constants, α.     

Native subunit composition of two insect nicotinic receptor subtypes with differing affinities for the insecticide imidacloprid

Neonicotinoid insecticides, such as imidacloprid, are selective agonists of insect nicotinic acetylcholine receptors (nAChRs) and are used extensively to control a variety of insect pest species. The brown planthopper (Nilaparvata lugens), an insect pest of rice crops throughout Asia, is an important target species for control with neonicotinoid insecticides such as imidacloprid. Studies with nAChRs purified from N. lugens have identified two [³H]imidacloprid binding sites with different affinities (K_d = 3.5 ± 0.6 pM and 1.5 ± 0.2 nM). Co-immunoprecipitation studies with native preparations of N. lugens nAChRs, using subunit-selective antisera, have demonstrated the co-assembly of Nlα1, Nlα2 and Nlβ1 subunits into one receptor complex and of Nlα3, Nlα8 and Nlβ1 into another. Immunodepletion of Nlα1 or Nlα2 subunits resulted in the selective loss of the lower affinity imidacloprid binding site, whereas immunodepletion of Nlα3 or Nlα8 caused the selective loss of the high-affinity site. Immunodepletion of Nlβ1 resulted in a complete absence of specific imidacloprid binding. In contrast, immunodepletion with antibodies selective for other N. lugens nAChR subunits (Nlα4, Nlα6, Nlα7 and Nlβ2) had no significant effect on imidacloprid binding. Taken together, these data suggest that nAChRs containing Nlα1, Nlα2 and Nlβ1 constitute the lower affinity binding site, whereas nAChRs containing Nlα3, Nlα8 and Nlβ1 constitute the higher affinity binding site for imidacloprid in N. lugens.     

Elucidating the Aggregation Number of Dopamine-Induced α-Synuclein Oligomeric Assemblies

Conventional methods to determine the aggregation number, that is, the number of monomers per oligomer, struggle to yield reliable results for large protein aggregates, such as amyloid oligomers. We have previously demonstrated the use of a combination of single-molecule photobleaching and substoichiometric fluorescent labeling to determine the aggregation number of oligomers of human α-synuclein, implicated in Parkinson’s disease. We show here that this approach is capable of accurately resolving mixtures of multiple distinct molecular species present in the same sample of dopamine-induced α-synuclein oligomers, and that we can determine the respective aggregation numbers of each species from a single histogram of bleaching steps. We found two distinct species with aggregation numbers of 15–19 monomers and 34–38 monomers. These results show that this single-molecule approach allows for the systematic study of the aggregation numbers of complex supramolecular assemblies formed under different aggregation conditions.     

In silico characterization of cytisinoids docked into an acetylcholine binding protein

Homology models of nicotinic acetylcholine receptors (nAChRs) suggest that subtype specificity is due to non-conserved residues in the complementary subunit of the ligand-binding pocket. Cytisine and its derivatives generally show a strong preference for heteromeric α4β21 nAChRs over the homomeric α7 subtype, and the structural modifications studied do not cause large changes in their nAChR subtype selectivity. In the present work we docked cytisine, N-methylcytisine, and several pyridone ring-substituted cytisinoids into the crystallographic structure of the Lymnaea stagnalis acetylcholine binding protein (AChBP) co-crystallized with nicotine (1UW6). The graphical analysis of the best poses showed that cytisinoids have weak interactions with the side chains of the non-conserved amino acids in the complementary subunit justifying the use of PDB 1UWB as a surrogate for nAChR. Furthermore, we found a high correlation (R² = 0.96) between the experimental pIC₅₀ values at α4β21 nAChR and docking energy (S) of the best cytisinoid poses within the AChBP. Due to the quality of the correlation we suggest that this equation might be used as a predictive model to propose new cytisine-derived nAChRs ligands. Our docking results also suggest that further structural modifications of these cytisinoids will not greatly alter their α4β21/α7 selectivity.     

Modeling Neuronal Nicotinic and GABA Receptors: Important Interface Salt-Links and Protein Dynamics

Protein motions in the Cys-loop ligand-gated ion receptors that govern the gating mechanism are still not well understood. The details as to how motions in the ligand-binding domain are translated to the transmembrane domain and how subunit rotations are linked to bring about the cooperative movements involved in gating are under investigation. Homology models of the α4β2 nicotinic acetylcholine (nACh) and β2α1γ2 GABA receptors were constructed based on the torpedo neuromuscular-like nicotinic receptor structure. The template constructed for the full electron microscopy structure must be considered more reliable for structure-function studies due to the preservation of the E45-R209 salt-link. Many other salt-links are seen to transiently form, including switching off of the E45-R209 link, within a network of potential salt-links at the binding domain to the transmembrane domain interface region. Several potentially important intersubunit salt-links form in both the nAChR and GABAR structures during the simulation and appear conserved across many subunit combinations, such as the salt-link between α4.E262 and β2.K255 in nAChR (β2.E262 and α1.K263 in GABAR), at the top of the pore-lining M2 helices, and the intersubunit link of R210 on the M1-linker to E168 on the β8-sheet of the adjacent subunit in the GABA receptor (E175-K46 being the structurally equivalent link in the nAChR, with reversed polarity). A network of other salt-links may be vital for transmitting the cooperative gating motions between subunits that become biased upon ligand binding. The changes seen in the simulations suggest that this network of salt-links helps to set limits and specific states for the conformational changes involved in gating of the receptor. We hope that these hypotheses will be tested experimentally in the near future.     

A potential interaction between the SARS-CoV-2 spike protein and nicotinic acetylcholine receptors

Changeux et al. (Changeux et al. C. R. Biol. 343:33–39.) recently suggested that the SARS-CoV-2 spike protein may interact with nicotinic acetylcholine receptors (nAChRs) and that such interactions may be involved in pathology and infectivity. This hypothesis is based on the fact that the SARS-CoV-2 spike protein contains a sequence motif similar to known nAChR antagonists. Here, we use molecular simulations of validated atomically detailed structures of nAChRs and of the spike to investigate the possible binding of the Y674-R685 region of the spike to nAChRs. We examine the binding of the Y674-R685 loop to three nAChRs, namely the human α4β2 and α7 subtypes and the muscle-like αβγδ receptor from Tetronarce californica. Our results predict that Y674-R685 has affinity for nAChRs. The region of the spike responsible for binding contains a PRRA motif, a four-residue insertion not found in other SARS-like coronaviruses. The conformational behavior of the bound Y674-R685 is highly dependent on the receptor subtype; it adopts extended conformations in the α4β2 and α7 complexes but is more compact when bound to the muscle-like receptor. In the α4β2 and αβγδ complexes, the interaction of Y674-R685 with the receptors forces the loop C region to adopt an open conformation, similar to other known nAChR antagonists. In contrast, in the α7 complex, Y674-R685 penetrates deeply into the binding pocket in which it forms interactions with the residues lining the aromatic box, namely with TrpB, TyrC1, and TyrC2. Estimates of binding energy suggest that Y674-R685 forms stable complexes with all three nAChR subtypes. Analyses of simulations of the glycosylated spike show that the Y674-R685 region is accessible for binding. We suggest a potential binding orientation of the spike protein with nAChRs, in which they are in a nonparallel arrangement to one another.     

3D-QSAR and QSSR studies of 3,8-diazabicyclo[4.2.0]octane derivatives as neuronal nicotinic acetylcholine receptors by comparative molecular field analysis (CoMFA)

High subtype selectivity (α4β2 over α2β3) of neuronal nicotinic acetylcholine receptor (nAChR) agonists is critical for the rational design of less toxic drugs used for the treatment of neurodegenerative and psychiatric diseases. Here, three CoMFA models of pEC₅₀(α4β2), pEC₅₀(α2β3) and p[EC₅₀(α4β2)/EC₅₀(α2β3)] (pEC₅₀(α4β2)pEC₅₀(α2β3)) were developed to study the quantitative structure–activity relationship (QSAR) and quantitative structure–selectivity relationship (QSSR) of the 3,8-diazabicyclo[4.2.0]octane derivatives as nAChRs agonists. The parameters of the three models were 0.584, 0.792, and 0.599 for cross-validated r² (r²_CV), 0.924, 0.935 and 0.875 for conventional r². Analyses indicated that both the steric and electrostatic factors should be considered in the rational design of more active and selective nAChR agonists.     

Production of caloxanthin 3'-β-D-glucoside, zeaxanthin 3,3'-β-D-diglucoside, and nostoxanthin in a recombinant Escherichia coli expressing system harboring seven carotenoid biosynthesis genes, including crtX and crtG

The aim of this study was to produce rare β-carotene-modified carotenoids possessing 2-O (-H or -glu) and/or 3-O (-H or -glu) functionalities in their β-ionone ring(s) using a recombinant Escherichia coli approach. This involved expressing seven carotenoid biosynthesis genes (crtE, crtB, crtI, crtY, crtZ, crtX and crtG). From the cells of the recombinant E. coli, caloxanthin (β,β-carotene-2,3,2′,3′-tetrol)-3′-β-d-glucose, zeaxanthin (β,β-carotene-3,3′-diol) 3,3′-β-d-diglucoside, and nostoxanthin (β,β-carotene-2,3,3′-triol) (rare carotenoids) were isolated and identified. Caloxanthin 3′-β-d-glucose displayed potent ¹O₂ quenching activity (IC₅₀ 19 μM).     

The influence of selected O-alkyl derivatives of cyclodextrins on the enzymatic decomposition of l-tryptophan by l-tryptophan indole-lyase

A series of O-alkyl derivatives of cyclodextrin: heksakis[2,3,6-tri-O-(2′-methoxyethyl)]-α-cyclodextrin; heksakis(2,3-di-O-methyl)-α-cyclodextrin; heptakis(2,3-di-O-methyl)-β-cyclodextrin; heksakis[2,3-di-O-methyl-6-O-(2′-methoxyethyl)]-α-cyclodextrin; heptakis[2,3-di-O-methyl-6-O-(2′-methoxyethyl)]-β-cyclodextrin; heksakis[2,3-di-O-(2′-methoxyethyl)]-α-cyclodextrin and heptakis[2,3-di-O-(2′-methoxyethyl)]-β-cyclodextrin have been synthesized. Purity and composition of the obtained substances were examined. The cyclodextrin derivatives listed above as well as (2-hydroxypropyl)-α-cyclodextrin and (2-hydroxypropyl)-β-cyclodextrin, the two commercially available ones, have been investigated as the additives in the course of enzymatic decomposition of l-tryptophan by l-tryptophan indole-lyase. It has been found that each of cyclodextrin derivatives causes the inhibition of enzymatic process, both competitive and non-competitive. The competitive inhibition is connected with the formation of inclusion complexes between cyclodextrins and l-tryptophan, related to the geometry of these complexes. The mechanism of the non-competitive inhibition is not so evident; it could be related to the formation of the cyclodextrin complexes on the surface of the enzyme, leading to the change in the flexibility of the enzyme molecule.     

Identification of N-terminal extracellular domain determinants in nicotinic acetylcholine receptor (nAChR) α6 subunits that influence effects of wild-type or mutant β3 subunits on function of α6β2*- or α6β4*-nAChR

Despite the apparent function of naturally expressed mammalian α6*-nicotinic acetylcholine receptors (α6*-nAChR; where * indicates the known or possible presence of additional subunits), their functional and heterologous expression has been difficult. Here, we report that coexpression with wild-type β3 subunits abolishes the small amount of function typically seen for all-human or all-mouse α6β4*-nAChR expressed in Xenopus oocytes. However, levels of function and agonist potencies are markedly increased, and there is atropine-sensitive blockade of spontaneous channel opening upon coexpression of α6 and β4 subunits with mutant β3 subunits harboring valine-to-serine mutations at 9'- or 13'-positions. There is no function when α6 and β2 subunits are expressed alone or in the presence of wild-type or mutant β3 subunits. Interestingly, hybrid nAChR containing mouse α6 and human (h) β4 subunits have function potentiated rather than suppressed by coexpression with wild-type hβ3 subunits and potentiated further upon coexpression with hβ3(V9'S) subunits. Studies using nAChR chimeric mouse/human α6 subunits indicated that residues involved in effects seen with hybrid nAChR are located in the α6 subunit N-terminal domain. More specifically, nAChR hα6 subunit residues Asn-143 and Met-145 are important for dominant-negative effects of nAChR hβ3 subunits on hα6hβ4-nAChR function. Asn-143 and additional residues in the N-terminal domain of nAChR hα6 subunits are involved in the gain-of-function effects of nAChR hβ3(V9'S) subunits on α6β2*-nAChR function. These studies illuminate the structural bases for effects of β3 subunits on α6*-nAChR function and suggest that unique subunit interfaces involving the complementary rather than the primary face of α6 subunits are involved.     

Determination of the Oligomer Size of Amyloidogenic Protein β-Amyloid(1-40) by Single-Molecule Spectroscopy

Amyloid diseases are traditionally characterized by the appearance of inter- and intracellular fibrillar protein deposits, termed amyloid. Historically, these deposits have been thought to be the etiology of the disease. However, recent evidence suggests that small oligomers of the amyloidogenic protein/peptide are the origin of neurotoxicity. Although the importance of identifying the toxic oligomeric species is widely recognized, such identification is challenging because these oligomers are metastable, occur at low concentration, and are characterized by a high degree of heterogeneity. In this work, a fluorescently labeled β-amyloid(1-40) is used as a model amyloidogenic peptide to test the effectiveness of what we believe is a novel approach based on single-molecule spectroscopy. We find that by directly counting the photobleaching steps in the fluorescence, we can determine the number of subunits in individual β-amyloid(1-40) oligomers, which allows us to easily distinguish among different species in the mixtures. The results are further analyzed by comparison with Monte Carlo simulations to show that the variability seen in the size of photobleaching steps can be explained by assuming random dipole orientations for the chromophores in a given oligomer. In addition, by accounting for bias in the oligomer size distribution due to the need to subtract background noise, we can make the results more quantitative. Although the oligomer size determined in this work is limited to only small species, our single-molecule results are in good quantitative agreement with high-performance liquid chromatography gel filtration data and demonstrate that single-molecule spectroscopy can provide useful insights into the issues of heterogeneity and ultimately cellular toxicity in the study of amyloid diseases.     

How Fluorescent Tags Modify Oligomer Size Distributions of the Alzheimer Peptide

Within the complex aggregation process of amyloidogenic peptides into fibrils, early stages of aggregation play a central role and reveal fundamental properties of the underlying mechanism of aggregation. In particular, low-molecular-weight aggregates of the Alzheimer amyloid-β peptide (Aβ) have attracted increasing interest because of their role in cytotoxicity and neuronal apoptosis, typical of aggregation-related diseases. One of the main techniques used to characterize oligomeric stages is fluorescence spectroscopy. To this end, Aβ peptide chains are functionalized with fluorescent tags, often covalently bound to the disordered N-terminus region of the peptide, with the assumption that functionalization and presence of the fluorophore will not modify the process of self-assembly nor the final fibrillar structure. In this investigation, we systematically study the effects of four of the most commonly used fluorophores on the aggregation of Aβ (1–40). Time-resolved and single-molecule fluorescence spectroscopy have been chosen to monitor the oligomer populations at different fibrillation times, and transmission electron microscopy, atomic force microscopy and x-ray diffraction to investigate the structure of mature fibrils. Although the structures of the fibrils were only slightly affected by the fluorescent tags, the sizes of the detected oligomeric species varied significantly depending on the chosen fluorophore. In particular, we relate the presence of high-molecular-weight oligomers of Aβ (1–40) (as found for the fluorophores HiLyte 647 and Atto 655) to net-attractive, hydrophobic fluorophore-peptide interactions, which are weak in the case of HiLyte 488 and Atto 488. The latter leads for Aβ (1–40) to low-molecular-weight oligomers only, which is in contrast to Aβ (1–42). The disease-relevant peptide Aβ (1–42) displays high-molecular-weight oligomers even in the absence of significant attractive fluorophore-peptide interactions. Hence, our findings reveal the potentially high impact of the properties of fluorophores on transient aggregates, which needs to be included in the interpretation of experimental data of oligomers of fluorescently labeled peptides.     

Flavonoid glycosides of the black locust tree, Robinia pseudoacacia (Leguminosae)

Four flavone glycosides isolated from extracts of the leaves of Robinia pseudoacacia (Leguminosae) were characterised by spectroscopic and chemical methods as the 7-O-β-d-glucuronopyranosyl-(1 → 2)[α-l-rhamnopyranosyl-(1 → 6)]-β-d-glucopyranosides of acacetin (5,7-dihydroxy-4′-methoxyflavone), apigenin (5,7,4′-trihydroxyflavone), diosmetin (5,7,3′-trihydroxy-4′-methoxyflavone) and luteolin (5,7,3′,4′-tetrahydroxyflavone). Assignment of glycosidic ¹H and ¹³C resonances in their NMR spectra was facilitated by ²J_HC correlations detected using the H2BC (heteronuclear two-bond correlation) pulse sequence. Spectroscopic analysis of two known triglycosides, acacetin 7-O-β-d-glucopyranosyl-(1 → 2)[α-l-rhamnopyranosyl-(1 → 6)]-β-d-glucopyranoside (previously unrecorded from this species) and acacetin 7-O-β-d-xylopyranosyl-(1 → 2)[α-l-rhamnopyranosyl-(1 → 6)]-β-d-glucopyranoside (‘acacetin trioside’), enabled inconsistencies in the literature relating to these structures to be resolved. Comparison of the flavonoid chemistry of leaves and flowers of R. pseudoacacia using LC-UV and LC-MS showed that flavone 7-O-glycosides, particularly of acacetin, predominated in the former, whereas the latter comprised mainly flavonol 3,7-di-O-glycosides, including several examples new to this species. Tissue dependent differences in flavonoid chemistry were also evident from the glycosylation patterns of the compounds.     

Distinguishing between Protein Dynamics and Dye Photophysics in Single-Molecule FRET Experiments

Förster resonance energy transfer (FRET) efficiency distributions in single-molecule experiments contain both structural and dynamical information. Extraction of this information from these distributions requires a careful analysis of contributions from dye photophysics. To investigate how mechanisms other than FRET affect the distributions obtained by counting donor and acceptor photons, we have measured single-molecule fluorescence trajectories of a small α/β protein, i.e., protein GB1, undergoing two-state, folding/unfolding transitions. Alexa 488 donor and Alexa 594 acceptor dyes were attached to cysteines at positions 10 and 57 to yield two isomers—donor¹⁰/acceptor⁵⁷ and donor⁵⁷/acceptor¹⁰—which could not be separated in the purification. The protein was immobilized via binding of a histidine tag added to a linker sequence at the N-terminus to cupric ions embedded in a polyethylene-glycol-coated glass surface. The distribution of FRET efficiencies assembled from the trajectories is complex with widths for the individual peaks in large excess of that caused by shot noise. Most of this complexity can be explained by two interfering photophysical effects—a photoinduced red shift of the donor dye and differences in the quantum yield of the acceptor dye for the two isomers resulting from differences in quenching rate by the cupric ion. Measurements of steady-state polarization, calculation of the donor-acceptor cross-correlation function from photon trajectories, and comparison of the single molecule and ensemble kinetics all indicate that conformational distributions and dynamics do not contribute to the complexity.     

乙酰胆碱受体α4β2亚型在非洲爪蟾卵母细胞中的表达

通过建立乙酰胆碱受体α4β2亚型(α4β2 nAChR)在非洲爪蟾卵母细胞中的表达模型,以便以α4β2 nAChR为作用靶点进行药物筛选。将乙酰胆碱受体亚基基因α4、β2体外转录获得的cRNA,通过显微注射的方法注入非洲爪蟾卵母细胞,并对该受体的表达情况进行检测。结果显示,乙酰胆碱受体α4β2亚型在蛙卵细胞中获得了有效表达,记录到典型的ACh配体门控的离子通道内向电流。