FRET imaging reveals that functional neurokinin-1 receptors are monomeric and reside in membrane microdomains of live cells

Abstract

The lateral organization of a prototypical G protein-coupled receptor, the neurokinin-1 receptor (NK1R), was investigated in living cells by fluorescence resonance energy transfer (FRET) microscopy, taking advantage of the recently developed acyl carrier protein (ACP) labeling technique. The NK1R was expressed as fusion protein with ACP to which small fluorophores were then covalently bound. Our approach allowed the recording of FRET images of receptors on living cells with unprecedented high signal-to-noise ratios and a subsequent unequivocal quantification of the FRET data owing to (i) the free choice of optimal fluorophores, (ii) the labeling of NK1Rs exclusively on the cell surface, and (iii) the precise control of the donor-acceptor molar ratio. Our single-cell FRET measurements exclude the presence of constitutive or ligandinduced homodimers or oligomers of NK1Rs. The strong dependence of FRET on the receptor concentration further reveals that NK1Rs tend to concentrate in microdomains, which are found to constitute Ϸ1% of the cell membrane and to be sensitive to cholesterol depletion.ACP labeling ͉ G protein-coupled receptor (GPCR) oligomerization G protein-coupled receptors (GPCRs) were for a long time presumed to be distributed in the plasma membrane exclusively in a monomeric form (1, 2), but recent reports have unveiled a more complex behavior; in particular, dimeric structures have been found for several GPCRs using biochemical and biophysical methods (3-9). Dimerization can occur between receptors of the same subtype (homodimerization) or of different subtypes (heterodimerization). Some GPCRs remain dimeric all of the time, whereas others cycle between monomeric and dimeric states in a ligand-regulated process (7). Although GPCR homodimerization seems to be important for receptor ontology and trafficking, heterodimerization might result in altered ligand selectivity and distinctive coupling to signal transduction pathways, providing an additional possibility for the fine tuning of cellular signaling.In addition to dimerization, the lateral distribution of GPCRs in cell membranes has been extensively debated recently. Several reports based on biochemical (10), plasmon-resonance spectroscopy (11), single-molecule microscopy (12), and fluorescence recovery after photobleaching experiments (13) propose that GPCRs are localized in microdomains, but a clear demonstration of the existence and nature of such microdomains in living cells remains elusive, in particular because the interpretation of biochemical data can be rather equivocal (14-17). Compartmentalization in form of microdomains was proposed to explain the efficiency of signal transduction at the low physiological surface concentrations of the signaling partners by their enrichment inside specialized signaling platforms (10, 18).Bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET) experiments have gained increasing interest to investigate these two central questions on GPCR signaling. (i) They can be p...

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“…However, other GPCRs also were found to associate as dimers and higher-order oligomers in living cells (see, e.g., refs. 6-8), although monomers prevail at low expression levels (9,10). Molecular modeling (11)(12)(13), FRET measurements (14)(15)(16), and biochemical approaches (5,17,18) are consistent with a model in which dimers or higher-order oligomers of rhodopsin, or class A GPCRs in general, constitute the functional unit for G protein activation (19)(20)(21).…”

ABCA1 dimer–monomer interconversion during HDL generation revealed by single-molecule imaging

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Regulated Norepinephrine Transporter Interaction with the Neurokinin-1 Receptor Establishes Transporter Subcellular Localization

“…Many GPCRs including NK1R mediate PKC signaling, and are known to redistribute signaling partner proteins when activated. NK1R is localized in lipid rafts (15,16) and activation of NK1R stimulates translocation of PKC into lipid rafts, where it interacts with the receptor and causes its desensitization (15). This raises the interesting possibility that NET localized in the rafts may be found in association with * This work was supported, in whole or in part, by National Institutes of Health NK1R.…”