Stability of the neurotensin receptor NTS1 free in detergent solution and immobilized to affinity resin

Effect of Ligands and Transducers on the Neurotensin Receptor 1 Conformational Ensemble

Using a discrete, intracellular ¹⁹F nuclear magnetic resonance (NMR) probe on transmembrane helix 6 of the neurotensin receptor 1 (NTS1), we aim to understand how ligands and transducers modulate the receptor's structural ensemble in a solution. For apo NTS1, ¹⁹F NMR spectra reveal an ensemble of at least three conformational substates (one inactive and two active-like) in equilibrium that exchange on the millisecond to second timescale. Dynamic NMR experiments reveal that these substates follow a linear three-site exchange process that is both thermodynamically and kinetically remodeled by orthosteric ligands. As previously observed in other G protein-coupled receptors (GPCRs), the full agonist is insufficient to completely stabilize the active-like state. The inactive substate is abolished upon coupling to β-arrestin-1 (βArr1) or the C-terminal helix of Gα_q, which comprises ≳60% of the GPCR/G protein interface surface area. Whereas βArr1 exclusively selects for pre-existing active-like substates, the Gα_q peptide induces a new substate. Both transducer molecules promote substantial line broadening of active-like states, suggesting contributions from additional microsecond to millisecond exchange processes. Together, our study suggests that (i) the NTS1 allosteric activation mechanism may be alternatively dominated by induced fit or conformational selection depending on the coupled transducer, and (ii) the available static structures do not represent the entire conformational ensemble observed in a solution.

Detergent-free extraction of a functional low-expressing GPCR from a human cell line

Dopamine receptors (DRs) are class A G-Protein Coupled Receptors (GPCRs) prevalent in the central nervous system (CNS). These receptors mediate physiological functions ranging from voluntary movement and reward recognition to hormonal regulation and hypertension. Drugs targeting dopaminergic neurotransmission have been employed to treat several neurological and psychiatric disorders, including Parkinson's disease, schizophrenia, Huntington's disease, attention deficit hyperactivity disorder (ADHD), and Tourette's syndrome. In vivo, incorporation of GPCRs into lipid membranes is known to be key to their biological function and, by inference, maintenance of their tertiary structure. A further significant challenge in the structural and biochemical characterization of human DRs is their low levels of expression in mammalian cells. Thus, the purification and enrichment of DRs whilst retaining their structural integrity and function is highly desirable for biophysical studies. A promising new approach is the use of styrene-maleic acid (SMA) copolymer to solubilize GPCRs directly in their native environment, to produce polymer-assembled Lipodisqs (LQs). We have developed a novel methodology to yield detergent-free D1-containing Lipodisqs directly from HEK293f cells expressing wild-type human dopamine receptor 1 (D1). We demonstrate that D1 in the Lipodisq retains activity comparable to that in the native environment and report, for the first time, the affinity constant for the interaction of the peptide neurotransmitter neurotensin (NT) with D1, in the native state.

Functional Reconstitution of HlyB, a Type I Secretion ABC Transporter, in Saposin-A Nanoparticles

Type I secretion systems (T1SS) are ubiquitous transport machineries in Gram-negative bacteria. They comprise a relatively simple assembly of three membrane-localised proteins: an inner-membrane complex composed of an ABC transporter and a membrane fusion protein, and a TolC-like outer membrane component. T1SS transport a wide variety of substrates with broad functional diversity. The ABC transporter hemolysin B (HlyB), for example, is part of the hemolysin A-T1SS in Escherichia coli. In contrast to canonical ABC transporters, an accessory domain, a C39 peptidase-like domain (CLD), is located at the N-terminus of HlyB and is essential for secretion. In this study, we have established an optimised purification protocol for HlyB and the subsequent reconstitution employing the saposin-nanoparticle system. We point out the negative influence of free detergent on the basal ATPase activity of HlyB, studied the influence of a lysolipid or lipid matrix on activity and present functional studies with the full-length substrate proHlyA in its folded and unfolded states, which both have a stimulatory effect on the ATPase activity.

Optimization and 13CH3 methionine labeling of a signaling competent neurotensin receptor 1 variant for NMR studies

Neurotensin is a 13-residue peptide that acts as a neuromodulator of classical neurotransmitters such as dopamine and glutamate in the mammalian central nervous system, mainly by activating the G protein-coupled receptor (GPCR), neurotensin receptor 1 (NTS₁). Agonist binding to GPCRs shifts the conformational equilibrium of the transmembrane helices towards distinct, thermodynamically favorable conformations that favor effector protein interactions and promotes cell signaling. The introduction of site specific labels for NMR spectroscopy has proven useful for investigating this dynamic process, but the low expression levels and poor stability of GPCRs is a hindrance to solution NMR experiments. Several thermostabilized mutants of NTS₁ have been engineered to circumvent this, with the crystal structures of four of these published. The conformational dynamics of NTS₁ however, has not been thoroughly investigated with NMR. It is generally accepted that stabilized GPCRs exhibit attenuated signaling, thus we thoroughly characterized the signaling characteristics of several thermostabilized NTS₁ variants to identify an optimal variant for protein NMR studies. A variant termed enNTS₁ exhibited the best combination of signaling capability and stability upon solubilization with detergents. enNTS₁ was subsequently labeled with ¹³CH₃-methionine in E. coli and purified to homogeneity in the absence of bound ligands. Using solution NMR spectroscopy we observed several well dispersed ¹³CH₃-methionine resonances, many of which exhibited chemical shift changes upon the addition of the high affinity agonist peptide, NT8-13. Thus, enNTS₁ represents a novel tool for investigating ligand induced conformational changes in NTS₁ to gain insights into the molecular mechanisms underlying neurotensin signaling.

Towards monitoring conformational changes of the GPCR neurotensin receptor 1 by single-molecule FRET

Neurotensin receptor 1 (NTSR1) is a G protein-coupled receptor that is important for signaling in the brain and the gut. Its agonist ligand neurotensin (NTS), a 13-amino-acid peptide, binds with nanomolar affinity from the extracellular side to NTSR1 and induces conformational changes that trigger intracellular signaling processes. Our goal is to monitor the conformational dynamics of single fluorescently labeled NTSR1. For this, we fused the fluorescent protein mNeonGreen to the C terminus of NTSR1, purified the receptor fusion protein from E. coli membranes, and reconstituted NTSR1 into liposomes with E. coli polar lipids. Using single-molecule anisotropy measurements, NTSR1 was found to be monomeric in liposomes, with a small fraction being dimeric and oligomeric, showing homoFRET. Similar results were obtained for NTSR1 in detergent solution. Furthermore, we demonstrated agonist binding to NTSR1 by time-resolved single-molecule Förster resonance energy transfer (smFRET), using neurotensin labeled with the fluorophore ATTO594.

Functional characterization of p7 viroporin from hepatitis C virus produced in a cell-free expression system

Using a cell-free expression system we produced the p7 viroporin embedded into a lipid bilayer in a single-step manner. The protein quality was assessed using different methods. We examined the channel forming activity of p7 and verified its inhibition by 5-(N,N-Hexamethylene) amiloride (HMA). Fourier transformed infrared spectroscopy (FTIR) experiments further showed that when p7 was inserted into synthetic liposomes, the protein displayed a native-like conformation similar to p7 obtained from other sources. Photoactivable amino acid analogs used for p7 protein synthesis enabled oligomerization state analysis in liposomes by cross-linking. Therefore, these findings emphasize the quality of the cell-free produced p7 proteoliposomes which can benefit the field of the hepatitis C virus (HCV) protein production and characterization and also provide tools for the development of new inhibitors to reinforce our therapeutic arsenal against HCV.

Detergent optimized membrane protein reconstitution in liposomes for solid state NMR

For small helical membrane proteins, their structures are highly sensitive to their environment, and solid state NMR is a structural technique that can characterize these membrane proteins in native-like lipid bilayers and proteoliposomes. To date, a systematic method by which to evaluate the effect of the solubilizing detergent on proteoliposome preparations for solid state NMR of membrane proteins has not been presented in the literature. A set of experiments are presented aimed at determining the conditions most amenable to dialysis mediated reconstitution sample preparation. A membrane protein from M. tuberculosis is used to illustrate the method. The results show that a detergent that stabilizes the most protein is not always ideal and sometimes cannot be removed by dialysis. By focusing on the lipid and protein binding properties of the detergent, proteoliposome preparations can be readily produced, which provide double the signal-to-noise ratios for both the oriented sample and magic angle spinning solid state NMR. The method will allow more membrane protein drug targets to be structurally characterized in lipid bilayer environments.

Transient and stable expression of the neurotensin receptor NTS1: a comparison of the baculovirus-insect cell and the T-REx-293 expression systems

Nowadays, baculovirus-infected insect cells and tetracycline-inducible mammalian cell lines (T-REx-293) are intensively used for G protein-coupled receptor (GPCR) production for crystallography purposes. Here we constructed a suspension T-REx-293 cell line to stably express an engineered neurotensin receptor 1 (NTS1) mutant and we quantitatively compared this cell line with the transient baculovirus-insect cell system throughout a milligram-scale NTS1 expression and purification process. The two systems were comparable with respect to functional NTS1 expression levels and receptor binding affinity for the agonist [³H] neurotensin. However, NTS1 surface display on T-REx-293 cells determined by radio-ligand binding assays was 2.8 fold higher than that on insect cells. This work demonstrates two approaches for preparing milligram quantities of purified NTS1 suitable for structural studies and provides useful input to users in choosing and optimizing an appropriate expression host for other GPCRs.

[Handling G-protein-coupled receptors: expression, purification and in vitro stabilization]

Among the different classes of integral membrane proteins, G protein-coupled receptors (GPCR) constitute the largest family. They are involved in most essential physiological functions and particularly play a key role in cell-to-cell communication and sensory signal transduction. They represent targets for approximately 30% of currently marketed drugs. In order to better understand their functioning, define their tridimensional structure and develop novel selective and efficient therapeutic compounds, it is crucial to purify these proteins for a full characterization. However, this biochemical step is not trivial since GPCR are present in membranes at very low levels and they require detergents to be extracted from their natural lipid environment and be handled as functional proteins. No universal strategy for GPCR production, purification and stabilization is currently available; each single GPCR possesses a unique set of physicochemical characteristics, preference for some detergents upon solubilization and specific conditions for purification. During the last decade, major breakthroughs regarding overexpression, purification and above all GPCR stabilization, thanks to amphipols and nanodiscs, opened very exciting perspectives for structural and dynamic investigations of these membrane proteins. The aim of this chapter is to provide an overview of the different aspects of GPCR handling.

Assessment of a fully active class A G protein-coupled receptor isolated from in vitro folding

We provide a protocol for the preparation of fully active Y2 G protein-coupled receptors (GPCRs). Although a valuable target for pharmaceutical research, information about the structure and dynamics of these molecules remains limited due to the difficulty in obtaining sufficient amounts of homogeneous and fully active receptors for in vitro studies. Recombinant expression of GPCRs as inclusion bodies provides the highest protein yields at lowest costs. But this strategy can only successfully be applied if the subsequent in vitro folding results in a high yield of active receptors and if this fraction can be isolated from the nonactive receptors in a homogeneous form. Here, we followed that strategy to provide large quantities of the human neuropeptide Y receptor type 2 and determined the folding yield before and after ligand affinity chromatography using a radioligand binding assay. Directly after folding, we achieved a proportion of ~25% active receptor. This value could be increased to ~96% using ligand affinity chromatography. Thus, a very homogeneous sample of the Y2 receptor could be prepared that exhibited a K(D) value of 0.1 ± 0.05 nM for the binding of polypeptide Y, which represents one of the natural ligands of the Y2 receptor.