Direct and energy-transfer photolabelling of brain muscarinic acetylcholine receptors

Functional characterization and potential applications for enhanced green fluorescent protein- and epitope-fused human M1 muscarinic receptors

Four recombinant human M1 (hM1) muscarinic acetylcholine receptors (mAChRs) combining several modifications were designed and overexpressed in HEK293 cells. Three different fluorescent chimera were obtained through fusion of the receptor N terminus with enhanced green fluorescent protein (EGFP), potential glycosylation sites and a large part of the third intracellular (i3) loop were deleted, a hexahistidine tag sequence was introduced at the receptor C terminus, and, finally, a FLAG epitope was either fused at the receptor N terminus or inserted into its shortened i3 loop. High expression levels and ligand binding properties similar to those of the wild-type hM1 receptor together with confocal microscopy imaging demonstrated that the recombinant proteins were correctly folded and targeted to the plasma membrane, provided that a signal peptide was added to the N-terminal domain of the fusion proteins. Their functional properties were examined through McN-A-343-evoked Ca2+ release. Despite the numerous modifications introduced within the hM1 sequence, all receptors retained nearly normal abilities (EC50 values) to mediate the Ca2+ response, although reduced amplitudes (Emax values) were obtained for the i3-shortened constructs. Owing to the bright intrinsic fluorescence of the EGFP-fused receptors, their detection, quantitation, and visualization as well as the selection of cells with highest expression were straightforward. Moreover, the presence of the different epitopes was confirmed by immunocytochemistry. Altogether, this work demonstrates that these EGFP- and epitope-fused hM1 receptors are valuable tools for further functional, biochemical, and structural studies of muscarinic receptors.

Pharmacological and structural integrity of muscarinic M2 acetylcholine receptors produced in Sf9 insect cells

Muscarinic acetylcholine receptors (human m2 subtype), expressed in Sf9 cells, using the baculovirus system, were purified and found to display the expected ligand binding properties, whether membrane-bound or affinity-purified. The purified recombinant receptors were specifically photolabelled with p-N,N-[3H]dimethylamino and p-N,N-[3H]dibutylamino benzene diazonium derivatives. Electrophoretic patterns for covalent radioactive incorporation of the probes were essentially similar to those for [3H]propylbenzilylcholine mustard-labelled receptor sites but were dependent on the infection time of Sf9 cells. Pharmacological properties of the recombinant receptors being unaltered did not reflect structural integrity of the protein as substantial proteolytic fragmentation was detected at a prolonged infection time, i.e., at the highest level of expression. Selection of overexpression conditions, as illustrated here for muscarinic receptors, thus requires not only pharmacological controls, but also analysis of the covalently labelled protein under strongly dissociating conditions.

Covalent labeling of muscarinic acetylcholine receptors by tritiated aryldiazonium photoprobes

p-dimethylamino (A) and p-dibutylamino (B) benzenediazonium salts, previously characterized as efficient labels of membrane-bound and solubilized muscarinic receptor sites, are endowed with overall interesting photochemical and alkylating properties that allow their use as structural probes of the muscarinic ligand binding domain to be considered. Under reversible binding conditions, these antagonists display no binding selectivity towards the 5 muscarinic acetylcholine receptor (mAChR) subtypes. They were used here, in a tritiated form, as photoaffinity labels of purified muscarinic receptors from porcine striatum, and their irreversible binding was assessed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. When irradiated under energy transfer conditions, [3H]A and [3H]B were both found to covalently label purified muscarinic receptor sites in a light-dependent and atropine-protectable manner. The electrophoretic migration properties of the alkylated sites were similar to those of [3H]propylbenzilylcholine mustard (PrBCM)-labeled mAChRs. Specific radioactive incorporation showed a clear dependency on probe concentration. Labeling efficiency was rather high, with up to 30% and even 60% of the receptor population being photolabeled by [3H]A and [3H]B, respectively. These two photoactivatable ligands have proven to be powerful tools for the structural analysis of other cholinergic targets (acetylcholinesterase and the nicotinic acetylcholine receptor) by allowing the characterization of a number of different residues belonging to their acetylcholine-binding domain. Altogether, these results reinforce the interest of our site-directed labeling approach because [3H]A- and [3H]B-alkylated mAChRs may now be considered as suitable materials to investigate the muscarinic receptor-binding pocket through peptide mapping, sequence analyses, and identification of radiolabeled amino acid residues.

Photochemical labeling of membrane-associated and channel-forming domains of proteins directed by energy transfer

Singlet-singlet energy transfer reactions from excited tryptophan residues to photoactivatable probes possessing a suitable chromophore, generate reactive species in the vicinity of the protein, leading to its covalent labeling. This delayed labeling process can be used to map the membrane-surrounded regions of proteins with improved efficiency when it is applied with appropriate photoactivatable phospholipids. The same principle could also be applied to the labeling of channel-forming transmembrane domains of ion channels, provided that suitable photoactivatable permeant ions were available. Both applications will be discussed with regard to their potential and feasibility.

m-Sulfonate benzene diazonium chloride: a powerful affinity label for the gamma-aminobutyric acid binding site from rat brain

m-Sulfonate benzene diazonium chloride (MSBD) was used to affinity-label the gamma-aminobutyric acid (GABA) binding site from rat brain membranes. To assess the irreversibility of the labeling reaction, we used an efficient ligand dissociation procedure combined to a rapid [3H]muscimol binding assay, both steps being performed on filter-adsorbed membranes. Inactivation of specific [3H]-muscimol binding sites by MSBD and its prevention by GABA were both time- and concentration-dependent. The time course of MSBD labeling was shortened as the pH of the incubation medium was increased from 6.2 to 8. These data suggest that MSBD can efficiently label the GABA binding site through alkylation of a residue having an apparent dissociation constant around neutrality.