Observing monomer - dimer transitions of neurotensin receptors 1 in single SMALPs by homoFRET and in an ABELtrap

TL;DR

This study investigates the oligomerization states of neurotensin receptor 1 in living cells using single-molecule spectroscopy within SMALPs, revealing a significant presence of dimers prior to ligand binding.

Contribution

It introduces a novel approach combining SMALPs, homoFRET, and ABELtrap to analyze GPCR oligomerization in living cell membranes.

Findings

Detection of monomer and dimer populations of NTSR1 in SMALPs.

Extended observation of single SMALPs in solution using ABELtrap.

Identification of pre-existing receptor dimers before neurotensin binding.

Abstract

G protein-coupled receptors (GPCRs) are a large superfamily of membrane proteins that are activated by extracellular small molecules or photons. Neurotensin receptor 1 (NTSR1) is a GPCR that is activated by neurotensin, i.e. a 13 amino acid peptide. Binding of neurotensin induces conformational changes in the receptor that trigger the intracellular signaling processes. While recent single-molecule studies have reported a dynamic monomer - dimer equilibrium of NTSR1 in vitro, a biophysical characterization of the oligomerization status of NTSR1 in living mammalian cells is complicated. Here we report on the oligomerization state of the human NTSR1 tagged with mRuby3 by dissolving the plasma membranes of living HEK293T cells into 10 nm-sized soluble lipid nanoparticles by addition of styrene-maleic acid copolymers (SMALPs). Single SMALPs were analyzed one after another in solution by multi-parameter single molecule spectroscopy including brightness, fluorescence lifetime and anisotropy for homoFRET. Brightness analysis was improved using single SMALP detection in a confocal ABELtrap for extended observation times in solution. A bimodal brightness distribution indicated a significant fraction of dimeric NTSR1 in SMALPs or in the plasma membrane, respectively, before addition of neurotensin.