Bringing GPCR Structural Biology to Medical Applications: Insights from Both V2 Vasopressin and Mu-Opioid Receptors

G-protein coupled receptors (GPCRs) are versatile signaling proteins that regulate key physiological processes in response to a wide variety of extracellular stimuli. The last decade has seen a revolution in the structural biology of clinically important GPCRs. Indeed, the improvement in molecular and biochemical methods to study GPCRs and their transducer complexes, together with advances in cryo-electron microscopy, NMR development, and progress in molecular dynamic simulations, have led to a better understanding of their regulation by ligands of different efficacy and bias. This has also renewed a great interest in GPCR drug discovery, such as finding biased ligands that can either promote or not promote specific regulations. In this review, we focus on two therapeutically relevant GPCR targets, the V2 vasopressin receptor (V2R) and the mu-opioid receptor (µOR), to shed light on the recent structural biology studies and show the impact of this integrative approach on the determination of new potential clinical effective compounds.

Integrated NMR and cryo-EM atomic-resolution structure determination of a half-megadalton enzyme complex

Atomic-resolution structure determination is crucial for understanding protein function. Cryo-EM and NMR spectroscopy both provide structural information, but currently cryo-EM does not routinely give access to atomic-level structural data, and, generally, NMR structure determination is restricted to small (<30 kDa) proteins. We introduce an integrated structure determination approach that simultaneously uses NMR and EM data to overcome the limits of each of these methods. The approach enables structure determination of the 468 kDa large dodecameric aminopeptidase TET2 to a precision and accuracy below 1 Å by combining secondary-structure information obtained from near-complete magic-angle-spinning NMR assignments of the 39 kDa-large subunits, distance restraints from backbone amides and ILV methyl groups, and a 4.1 Å resolution EM map. The resulting structure exceeds current standards of NMR and EM structure determination in terms of molecular weight and precision. Importantly, the approach is successful even in cases where only medium-resolution cryo-EM data are available.

NMR structure determination is challenging for proteins with a molecular weight above 30 kDa and atomic-resolution structure determination from cryo-EM data is currently not the rule. Here the authors describe an integrated structure determination approach that simultaneously uses NMR and EM data and allows them to determine the structure of the 468 kDa dodecameric aminopeptidase TET2 complex.

1H, 13C and 15N backbone chemical shift assignments of camelid single-domain antibodies against active state µ-opioid receptor

Nanobodies are single chain antibodies that have become a highly valuable and versatile tool for biomolecular and therapeutic research. One application field is the stabilization of active states of flexible proteins, among which G-protein coupled receptors represent a very important class of membrane proteins. Here we present the backbone and side-chain assignment of the ¹H, ¹³C and ¹⁵N resonances of Nb33 and Nb39, two nanobodies that recognize and stabilize the µ-opioid receptor to opioids in its active agonist-bound conformation. In addition, we present a comparison of their secondary structures as derived from NMR chemical shifts.

Mapping conformational heterogeneity of mitochondrial nucleotide transporter in uninhibited states

One of the less well understood aspects of membrane transporters is the dynamic coupling between conformational change and substrate transport. NMR approaches are used herein to investigate conformational heterogeneity of the GTP/GDP carrier (GGC) from yeast mitochondria. NMR residual dipolar coupling (RDC) analysis of GGC in a DNA-origami nanotube liquid crystal shows that several structured segments have different generalized degrees of order (GDO), thus indicating the presence of conformational heterogeneity. Complete GDO mapping reveals asymmetry between domains of the transporter and even within certain transmembrane helices. Nucleotide binding partially reduces local structural heterogeneity, and the substrate binds to multiple sites along the transport cavity. These observations suggest that mitochondrial carriers in the uninhibited states are intrinsically plastic and structural plasticity is asymmetrically distributed among the three homologous domains.

Solution nuclear magnetic resonance spectroscopy

Solution nuclear magnetic resonance (NMR) spectroscopy has come a long way in characterizing the structure and function of biological molecules since the first one-dimensional spectrum of protein was recorded about 30 years ago. To date (September 1, 2012), there are 9,521 solution NMR structures in the Protein Data Bank, compared to 74,009 determined by crystallographic methods. Unlike X-ray and electron microscopy (EM) methods, which are based on the concepts of Fourier optics and image reconstruction, structure determination by NMR involves measuring structural restraints and finding structural solutions that satisfy the restraints. Although the NMR approach is much less direct in a physical sense, it has proven itself over the years to be capable of de novo structure determination at high precision. Moreover, the method is highly versatile and can be used in a variety of ways for addressing mechanistic questions. NMR measurements of protein internal dynamics and protein-protein or protein-ligand interaction are directly relevant to function in vivo because the molecules are often in physiological buffer conditions. The method can also be applied to investigate protein-folding intermediates, conformational changes, as well as intrinsically unfolded proteins. Recently, along with X-ray and EM, solution NMR has entered a state of rapid growth for structural studies of membrane proteins, already demonstrating its feasibility in de novo structure determination of membrane-embedded ion channels and receptors. As the hardware advances rapidly, especially in cryogenic probes that have much higher sensitivity, the sample concentration required for solution NMR investigation is decreasing, hopefully soon to a concentration level at which nonspecific protein aggregation is no longer an issue. After three decades of improvement in spectrometer technology, NMR pulse experiments, isotope labeling schemes, and structure determination software, we believe that solution NMR will truly enter the production phase in the next decade to answer biological questions of high impact, and to become more versatile than ever in complementing X-ray and EM in investigating protein structure and function.

An efficient protocol for the complete incorporation of methyl-protonated alanine in perdeuterated protein

A strategy for the introduction of ((1)H,(13)C-methyl)-alanine into perdeuterated proteins is described. Specific protonation of alanine methyl groups to a level of 95% can be achieved by overexpressing proteins in M9/D(2)O based bacterial growth medium supplemented with 800 mg/l of 2-[(2)H], 3-[(13)C] L: -alanine. However, though simple, this approach results in undesired, non-specific background labeling due to isotope scrambling via different amino acid metabolic pathways. Following a careful analysis of known metabolic pathways we found that co-addition of perdeuterated forms of alpha-ketoisovalerate-d(7), succinate-d(4) and L: -isoleucine-d(10) with labeled L: -alanine, reduces undesired background labeling to <1%. When combined with recently developed methyl TROSY experiments, this methyl-specific labeling protocol permits the acquisition of excellent quality correlation spectra of alanine methyl groups in high molecular weight proteins. Our cost effective strategy offers a significant enhancement in the level of incorporation of methyl-labeled alanine in overexpressed proteins over previously reported methods.