The Unusual Transmembrane Partition of the Hexameric Channel of the Hepatitis C Virus

Cholesterol Binds in a Reversed Orientation to TCRβ-TM in Which Its OH Group is Localized to the Center of the Lipid Bilayer

T cell receptor (TCR) signaling in response to antigen recognition is essential for the adaptive immune response. Cholesterol keeps TCRs in the resting conformation and mediates TCR clustering by directly binding to the transmembrane domain of the TCRβ subunit (TCRβ-TM), while cholesterol sulfate (CS) displaces cholesterol from TCRβ. However, the atomic interaction of cholesterol or CS with TCRβ remains elusive. Here, we determined the cholesterol and CS binding site of TCRβ-TM in phospholipid bilayers using solution nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulation. Cholesterol binds to the transmembrane residues within a CARC-like cholesterol recognition motif. Surprisingly, the polar OH group of cholesterol is placed in the hydrophobic center of the lipid bilayer stabilized by its polar interaction with K154 of TCRβ-TM. An aromatic interaction with Y158 and hydrophobic interactions with V160 and L161 stabilize this reverse orientation. CS binds to the same site, explaining how it competes with cholesterol. Site-directed mutagenesis of the CARC-like motif disrupted the cholesterol/CS binding to TCRβ-TM, validating the NMR and MD results.

Understanding molecular mechanisms of biologics drug delivery and stability from NMR spectroscopy

Protein therapeutics carry inherent limitations of membrane impermeability and structural instability, despite their predominant role in the modern pharmaceutical market. Effective formulations are needed to overcome physiological and physicochemical barriers, respectively, for improving bioavailability and stability. Knowledge of membrane affinity, cellular internalization, encapsulation, and release of drug-loaded carrier vehicles uncover the structural basis for designing and optimizing biopharmaceuticals with enhanced delivery efficiency and therapeutic efficacy. Understanding stabilizing and destabilizing interactions between protein drugs and formulation excipients provide fundamental mechanisms for ensuring the stability and quality of biological products. This article reviews the molecular studies of biologics using solution and solid-state NMR spectroscopy on structural attributes pivotal to drug delivery and stability. In-depth investigation of the structure-function relationship of drug delivery systems based on cell-penetrating peptides, lipid nanoparticles and polymeric colloidal, and biophysical and biochemical stability of peptide, protein, monoclonal antibody, and vaccine, as the integrative efforts on drug product design, will be elaborated.

An amphipathic Bax core dimer forms part of the apoptotic pore wall in the mitochondrial␣membrane

Bax proteins form pores in the mitochondrial outer membrane to initiate apoptosis. This might involve their embedding in the cytosolic leaflet of the lipid bilayer, thus generating tension to induce a lipid pore with radially arranged lipids forming the wall. Alternatively, Bax proteins might comprise part of the pore wall. However, there is no unambiguous structural evidence for either hypothesis. Using NMR, we determined a high-resolution structure of the Bax core region, revealing a dimer with the nonpolar surface covering the lipid bilayer edge and the polar surface exposed to water. The dimer tilts from the bilayer normal, not only maximizing nonpolar interactions with lipid tails but also creating polar interactions between charged residues and lipid heads. Structure-guided mutations demonstrate the importance of both types of protein-lipid interactions in Bax pore assembly and core dimer configuration. Therefore, the Bax core dimer forms part of the proteolipid pore wall to permeabilize mitochondria.

Inhibitor Development against p7 Channel in Hepatitis C Virus

Hepatitis C Virus (HCV) is the key cause of chronic and severe liver diseases. The recent direct-acting antiviral agents have shown the clinical success on HCV-related diseases, but the rapid HCV mutations of the virus highlight the sustaining necessity to develop new drugs. p7, the viroporin protein from HCV, has been sought after as a potential anti-HCV drug target. Several classes of compounds, such as amantadine and rimantadine have been testified for p7 inhibition. However, the efficacies of these compounds are not high. Here, we screened some novel p7 inhibitors with amantadine scaffold for the inhibitor development. The dissociation constant (Kd) of 42 ARD-series compounds were determined by nuclear magnetic resonance (NMR) titrations. The efficacies of the two best inhibitors, ARD87 and ARD112, were further confirmed using viral production assay. The binding mode analysis and binding stability for the strongest inhibitor were deciphered by molecular dynamics (MD) simulation. These ARD-series compounds together with 49 previously published compounds were further analyzed by molecular docking. Key pharmacophores were identified among the structure-similar compounds. Our studies suggest that different functional groups are highly correlated with the efficacy for inhibiting p7 of HCV, in which hydrophobic interactions are the dominant forces for the inhibition potency. Our findings provide guiding principles for designing higher affinity inhibitors of p7 as potential anti-HCV drug candidates.

Membrane Protein Structure Determination and Characterisation by Solution and Solid-State NMR

Biological membranes define the interface of life and its basic unit, the cell. Membrane proteins play key roles in membrane functions, yet their structure and mechanisms remain poorly understood. Breakthroughs in crystallography and electron microscopy have invigorated structural analysis while failing to characterise key functional interactions with lipids, small molecules and membrane modulators, as well as their conformational polymorphism and dynamics. NMR is uniquely suited to resolving atomic environments within complex molecular assemblies and reporting on membrane organisation, protein structure, lipid and polysaccharide composition, conformational variations and molecular interactions. The main challenge in membrane protein studies at the atomic level remains the need for a membrane environment to support their fold. NMR studies in membrane mimetics and membranes of increasing complexity offer close to native environments for structural and molecular studies of membrane proteins. Solution NMR inherits high resolution from small molecule analysis, providing insights from detergent solubilised proteins and small molecular assemblies. Solid-state NMR achieves high resolution in membrane samples through fast sample spinning or sample alignment. Recent developments in dynamic nuclear polarisation NMR allow signal enhancement by orders of magnitude opening new opportunities for expanding the applications of NMR to studies of native membranes and whole cells.

Structural basis of transmembrane coupling of the HIV-1 envelope glycoprotein

The prefusion conformation of HIV-1 envelope protein (Env) is recognized by most broadly neutralizing antibodies (bnAbs). Studies showed that alterations of its membrane-related components, including the transmembrane domain (TMD) and cytoplasmic tail (CT), can reshape the antigenic structure of the Env ectodomain. Using nuclear magnetic resonance (NMR) spectroscopy, we determine the structure of an Env segment encompassing the TMD and a large portion of the CT in bicelles. The structure reveals that the CT folds into amphipathic helices that wrap around the C-terminal end of the TMD, thereby forming a support baseplate for the rest of Env. NMR dynamics measurements provide evidences of dynamic coupling across the TMD between the ectodomain and CT. Pseudovirus-based neutralization assays suggest that CT-TMD interaction preferentially affects antigenic structure near the apex of the Env trimer. These results explain why the CT can modulate the Env antigenic properties and may facilitate HIV-1 Env-based vaccine design.

Comprehensive Multi-Omics Analysis Reveals Aberrant Metabolism of Epstein-Barr-Virus-Associated Gastric Carcinoma

The metabolic landscape of Epstein–Barr-virus-associated gastric cancer (EBVaGC) remains to be elucidated. In this study, we used transcriptomics, metabolomics, and lipidomics to comprehensively investigate aberrant metabolism in EBVaGC. Specifically, we conducted gene expression analyses using microarray-based data from gastric adenocarcinoma epithelial cell lines and tissue samples from patients with clinically advanced gastric carcinoma. We also conducted complementary metabolomics and lipidomics using various mass spectrometry platforms. We found a significant downregulation of genes related to metabolic pathways, especially the metabolism of amino acids, lipids, and carbohydrates. The effect of dysregulated metabolic genes was confirmed in a survival analysis of 3951 gastric cancer patients. We found 57 upregulated metabolites and 31 metabolites that were downregulated in EBVaGC compared with EBV-negative gastric cancer. Sixty-nine lipids, mainly ether-linked phospholipids and triacylglycerols, were downregulated, whereas 45 lipids, mainly phospholipids, were upregulated. In total, 15 metabolisms related to polar metabolites and 15 lipid-associated pathways were involved in alteration of metabolites by EBV in gastric cancer. In this work, we have described the metabolic landscape of EBVaGC at the multi-omics level. These findings could help elucidate the mechanism of EBVaGC oncogenesis.

Solution NMR: A powerful tool for structural and functional studies of membrane proteins in reconstituted environments

A third of the genes in prokaryotic and eukaryotic genomes encode membrane proteins that are either essential for signal transduction and solute transport or function as scaffold structures. Unlike many of their soluble counterparts, the overall structural and functional organization of membrane proteins is sparingly understood. Recent advances in X-ray crystallography, cryo-EM, and nuclear magnetic resonance (NMR) are closing this gap by enabling an in-depth view of these ever-elusive proteins at atomic resolution. Despite substantial technological advancements, however, the overall proportion of membrane protein entries in the Protein Data Bank (PDB) remains <4%. This paucity is mainly attributed to difficulties associated with their expression and purification, propensity to form large multisubunit complexes, and challenges pertinent to identification of an ideal detergent, lipid, or detergent/lipid mixture that closely mimic their native environment. NMR is a powerful technique to obtain atomic-resolution and dynamic details of a protein in solution. This is accomplished through an assortment of isotopic labeling schemes designed to acquire multiple spectra that facilitate deduction of the final protein structure. In this review, we discuss current approaches and technological developments in the determination of membrane protein structures by solution NMR and highlight recent structural and mechanistic insights gained with this technique. We also discuss strategies for overcoming size limitations in NMR applications, and we explore a plethora of membrane mimetics available for the structural and mechanistic understanding of these essential cellular proteins.

Critical Effect of the Detergent:Protein Ratio on the Formation of the Hepatitis C Virus p7 Channel

The p7 protein encoded by the hepatitis C virus forms a cation-selective viroporin in the membrane. One of the most intriguing findings about the p7 viroporin is its unique hexameric structure in dodecylphosphocholine (DPC) micelles determined by nuclear magnetic resonance (NMR), but the hexameric structure was recently challenged by another NMR study of p7, also in DPC detergent, which claimed that the p7 in this detergent is monomeric. Here, we show that p7 oligomerization is highly sensitive to the detergent:protein ratio used in protein reconstitution and that the 40-fold difference in this ratio between the two studies was the cause of their different conclusions. In addition, we have performed extensive measurements of interchain paramagnetic relaxation enhancements (PREs) for p7 hexamers reconstituted in DPC micelles and in 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dihexanoyl-sn-glycero-3-phosphocholine bicelles. In both cases, interchain PREs are overall consistent with the hexameric structure determined in micelles. Our data validate the overall architecture of the p7 hexamer while highlighting the importance of the detergent:protein ratio in membrane protein sample preparation.

Structure determination protocol for transmembrane domain oligomers

The transmembrane (TM) anchors of cell surface proteins have been one of the 'blind spots' in structural biology because they are generally very hydrophobic, sometimes dynamic, and thus difficult targets for structural characterization. A plethora of examples show these membrane anchors are not merely anchors but can multimerize specifically to activate signaling receptors on the cell surface or to stabilize envelope proteins in viruses. Through a series of studies of the TM domains (TMDs) of immune receptors and viral membrane proteins, we have established a robust protocol for determining atomic-resolution structures of TM oligomers by NMR in bicelles that closely mimic a lipid bilayer. Our protocol overcomes hurdles typically encountered by structural biology techniques such as X-ray crystallography and cryo-electron microscopy (cryo-EM) when studying small TMDs. Here, we provide the details of the protocol, covering five major technical aspects: (i) a general method for producing isotopically labeled TM or membrane-proximal (MP) protein fragments that involves expression of the protein (which is fused to TrpLE) into inclusion bodies and releasing the target protein by cyanogen bromide (CNBr) cleavage; (ii) determination of the oligomeric state of TMDs in bicelles; (iii) detection of intermolecular contacts using nuclear Overhauser effect (NOE) experiments; (iv) structure determination; and (v) paramagnetic probe titration (PPT) to characterize the membrane partition of the TM oligomers. This protocol is broadly applicable for filling structural gaps of many type I/II membrane proteins. The procedures may take 3-6 months to complete, depending on the complexity and stability of the protein sample.

Hepatitis C virus sequence divergence preserves p7 viroporin structural and dynamic features

The hepatitis C virus (HCV) viroporin p7 oligomerizes to form ion channels, which are required for the assembly and secretion of infectious viruses. The 63-amino acid p7 monomer has two putative transmembrane domains connected by a cytosolic loop, and has both N- and C- termini exposed to the endoplasmic reticulum (ER) lumen. NMR studies have indicated differences between p7 structures of distantly related HCV genotypes. A critical question is whether these differences arise from the high sequence variation between the different isolates and if so, how the divergent structures can support similar biological functions. Here, we present a side-by-side characterization of p7 derived from genotype 1b (isolate J4) in the detergent 6-cyclohexyl-1-hexylphosphocholine (Cyclofos-6) and p7 derived from genotype 5a (isolate EUH1480) in n-dodecylphosphocholine (DPC). The 5a isolate p7 in conditions previously associated with a disputed oligomeric form exhibits secondary structure, dynamics, and solvent accessibility broadly like those of the monomeric 1b isolate p7. The largest differences occur at the start of the second transmembrane domain, which is destabilized in the 5a isolate. The results show a broad consensus among the p7 variants that have been studied under a range of different conditions and indicate that distantly related HCVs preserve key features of structure and dynamics.

Higher-Order Clustering of the Transmembrane Anchor of DR5 Drives Signaling

Receptor clustering on the cell membrane is critical in the signaling of many immunoreceptors, and this mechanism has previously been attributed to the extracellular and/or the intracellular interactions. Here, we report an unexpected finding that for death receptor 5 (DR5), a receptor in the tumor necrosis factor receptor superfamily, the transmembrane helix (TMH) alone in the receptor directly assembles a higher-order structure to drive signaling and that this structure is inhibited by the unliganded ectodomain. Nuclear magnetic resonance structure of the TMH in bicelles shows distinct trimerization and dimerization faces, allowing formation of dimer-trimer interaction networks. Single-TMH mutations that disrupt either trimerization or dimerization abolish ligand-induced receptor activation. Surprisingly, proteolytic removal of the DR5 ectodomain can fully activate downstream signaling in the absence of ligand. Our data suggest a receptor activation mechanism in which binding of ligand or antibodies to overcome the pre-ligand autoinhibition allows TMH clustering and thus signaling.

Synthetic Biology-Based Solution NMR Studies on Membrane Proteins in Lipid Environments

Although membrane proteins are in the focus of biochemical research for many decades the general knowledge of this important class is far behind soluble proteins. Despite several recent technical developments, the most challenging feature still is the generation of high-quality samples in environments suitable for the selected application. Reconstitution of membrane proteins into lipid bilayers will generate the most native-like environment and is therefore commonly desired. However, it poses tremendous problems to solution-state NMR analysis due to the dramatic increase in particle size resulting in high rotational correlation times. Nevertheless, a few promising strategies for the solution NMR analysis of membrane inserted proteins are emerging and will be discussed in this chapter. We focus on the generation of membrane protein samples in nanodisc membranes by cell-free systems and will describe the characteristic advantages of that platform in providing tailored protein expression and folding environments. We indicate frequent problems that have to be overcome in cell-free synthesis, nanodisc preparation, and customization for samples dedicated for solution-state NMR. Detailed instructions for sample preparation are given, and solution NMR approaches suitable for membrane proteins in bilayers are compiled. We further discuss the current strategies applied for signal detection from such difficult samples and describe the type of information that can be extracted from the various experiments. In summary, a comprehensive guideline for the analysis of membrane proteins in native-like membrane environments by solution-state NMR techniques will be provided.

Behavior of Most Widely Spread Lipids in Isotropic Bicelles

Isotropic bicelles are a widely used membrane mimetic for structural studies of membrane proteins and their transmembrane domains. Simple and cheap in preparation, they contain a patch of lipid bilayer that reproduces the native environment of membrane proteins. Despite the obvious power of bicelles in reproducing the various kinds of environments, the vast majority of structural studies employ the single lipid/detergent system. On the other hand, even if the alternative bicelle composition is used, the properties of mixtures are not characterized, and the mere presence of lipid bilayer and discoidal shape of bicelle particles is not confirmed. Here we present an extensive investigation of various bicellar mixtures and describe the behavior of bicelles with lipids other than classical DMPC, namely sphingomyelins (SM), phosphatidylethanolamines (PE), phosphatidylglycerols (PG), phosphatidylserines (PS), and cholesterol. These lipids are rarely used in modern structural biology, but can help a lot in understanding the influence of the membrane composition on the properties of both integral and peripheral membrane proteins. Additionally, the ability of diheptanoylphosphatidylcholine (DH7PC) to serve as a rim-forming agent was investigated. We followed the phase transitions as revealed by ³¹P NMR and size of particles measured by ¹H NMR diffusion as the criteria of the proper morphology and structure of bicelles. As an outcome, we state that SM exclusively, and PG/PS in mixtures with zwitterionic lipids can form small isotropic bicelles, which reproduce the key features of lipid behavior in bilayers. Mixtures, containing exclusively the anionic lipids, fail to reveal the lipid phase transition and do not follow the size predicted for the ideal bicelle particles. PE and DH7PC are the unwanted components of bicellar mixtures, and cholesterol can be added to bicelles, however, with certain precautions. In combination with our several most recent works, this study provides a practical guide for the preparation of small isotropic bicelles.