Maltose-neopentyl glycol (MNG) amphiphiles for solubilization, stabilization and crystallization of membrane proteins

Human F-ATP synthase as a drug target

Practical and conceptual barriers have kept human F-ATP synthase out of reach as a target for the treatment of human diseases. Although this situation has persisted for decades, it may change in the near future. In this review the principal functionalities of human F-ATP synthase--proton motive force / ATP interconversion, membrane bending and mitochondrial permeability transition--are surveyed in the context of their respective potential for pharmaceutical intervention. Further, the technical requirements necessary to allow drug designs that are effective at the multiple levels of functionality and modality of human F-ATP synthase are discussed. The structure-based development of gastric proton pump inhibitors is used to exemplify what might be feasible for human F-ATP synthase. And finally, four structural regions of the human F-ATP synthase are examined as potential sites for the development of structure based drug development.

Human mitochondrial carriers of the SLC25 family function as monomers exchanging substrates with a ping-pong kinetic mechanism

Members of the SLC25 mitochondrial carrier family link cytosolic and mitochondrial metabolism and support cellular maintenance and growth by transporting compounds across the mitochondrial inner membrane. Their monomeric or dimeric state and kinetic mechanism have been a matter of long-standing debate. It is believed by some that they exist as homodimers and transport substrates with a sequential kinetic mechanism, forming a ternary complex where both exchanged substrates are bound simultaneously. Some studies, in contrast, have provided evidence indicating that the mitochondrial ADP/ATP carrier (SLC25A4) functions as a monomer, has a single substrate binding site, and operates with a ping-pong kinetic mechanism, whereby ADP is imported before ATP is exported. Here we reanalyze the oligomeric state and kinetic properties of the human mitochondrial citrate carrier (SLC25A1), dicarboxylate carrier (SLC25A10), oxoglutarate carrier (SLC25A11), and aspartate/glutamate carrier (SLC25A13), all previously reported to be dimers with a sequential kinetic mechanism. We demonstrate that they are monomers, except for dimeric SLC25A13, and operate with a ping-pong kinetic mechanism in which the substrate import and export steps occur consecutively. These observations are consistent with a common transport mechanism, based on a functional monomer, in which a single central substrate-binding site is alternately accessible.

NDT-C11 as a Viable Novel Detergent for Single Particle Cryo-EM

Single particle cryo electron microscopy (cryo-EM) is now the major method for the determination of integral membrane protein structure. For the success of a given project the type of membrane mimetic used for extraction from the native cell membrane, purification to homogeneity and finally cryo-grid vitrification is crucial. Although small molecule amphiphiles - detergents - are the most widely used membrane mimetic, specific tailoring of detergent structure for single particle cryo-EM is rare and the demand for effective detergents not satisfied. Here, we compare the popular detergent lauryl maltose-neopentyl glycol (LMNG) with the novel detergent neopentyl glycol-derived triglucoside-C11 (NDT-C11) in its behavior as free detergent and when bound to two types of multisubunit membrane protein complexes - cyanobacterial photosystem I (PSI) and mammalian F-ATP synthase. We conclude that NDT-C11 has high potential to become a very useful detergent for single particle cryo-EM of integral membrane proteins.

Detergent Chemistry Modulates the Transgression of Planetary Boundaries including Antimicrobial Resistance and Drug Discovery

Detergent chemistry enables applications in the world today while harming safe operating spaces that humanity needs for survival. Aim of this review is to support a holistic thought process in the design of detergent chemistry. We harness the planetary boundary concept as a framework for literature survey to identify progresses and knowledge gaps in context with detergent chemistry and five planetary boundaries that are currently transgressed, i.e., climate, freshwater, land system, novel entities, biosphere integrity. Our survey unveils the status of three critical challenges to be addressed in the years to come, including (i) the implementation of a holistically, climate-friendly detergent industry; (ii) the alignment of materialistic and social aspects in creating technical solutions by means of sustainable chemistry; (iii) the development of detergents that serve the purpose of applications but do not harm the biosphere in their role as novel entities. Specifically, medically relevant case reports revealed that even the most sophisticated detergent design cannot sufficiently accelerate drug discovery to outperform the antibiotic resistance development that detergents simultaneously promote as novel entities. Safe operating spaces that humanity needs for its survival may be secured by directing future efforts beyond sustainable chemistry, resource efficiency, and net zero emission targets.

Advances in Membrane Mimetic Systems for Manipulation and Analysis of Membrane Proteins: Detergents, Polymers, Lipids and Scaffolds

Extracting membrane proteins from the hydrophobic environment of the biological membrane, in a physiologically relevant and stable state, suitable for downstream analysis remains a challenge. The traditional route to membrane protein extraction has been to use detergents and the last 15 years or so have seen a veritable explosion in the development of novel detergents with improved properties, making them more suitable for individual proteins and specific applications. There have also been significant advances in the development of encapsulation of membrane proteins in lipid based nanodiscs, either directly from the native membrane using polymers allowing effective capture of the protein and protein-associated membrane lipids, or via reconstitution of detergent extracted and purified protein into nanodiscs of defined lipid composition. All of these advances have been successfully applied to the study of membrane proteins via a range of techniques and there have been some spectacular membrane protein structures solved. In addition, the first detailed structural and biophysical analyses of membrane proteins retained within a biological membrane have been reported. Here we summarise and review the recent advances with respect to these new agents and systems for membrane protein extraction, reconstitution and analysis.

Purification and characterization of Cdr1, the drug-efflux pump conferring azole resistance in Candida species

Candida albicans and C. glabrata express exporters of the ATP-binding cassette (ABC) superfamily and address them to their plasma membrane to expel azole antifungals, which cancels out their action and allows the yeast to become multidrug resistant (MDR). In a way to understand this mechanism of defense, we describe the purification and characterization of Cdr1, the membrane ABC exporter mainly responsible for such phenotype in both species. Cdr1 proteins were functionally expressed in the baker yeast, tagged at their C-terminal end with either a His-tag for the glabrata version, cgCdr1-His, or a green fluorescent protein (GFP) preceded by a proteolytic cleavage site for the albicans version, caCdr1-P-GFP. A membrane Cdr1-enriched fraction was then prepared to assay several detergents and stabilizers, probing their level of extraction and the ATPase activity of the proteins as a functional marker. Immobilized metal-affinity and size-exclusion chromatographies (IMAC, SEC) were then carried out to isolate homogenous samples. Overall, our data show that although topologically and phylogenetically close, both proteins display quite distinct behaviors during the extraction and purification steps, and qualify cgCdr1 as a good candidate to characterize this type of proteins for developing future inhibitors of their azole antifungal efflux activity.

19F-NMR studies of the impact of different detergents and nanodiscs on the A2A adenosine receptor

For the A2A adenosine receptor (A2AAR), a class A G-protein-coupled receptor (GPCR), reconstituted in n-dodecyl-β-D-maltoside (DDM)/‌‌‌‌‌cholesteryl hemisuccinate (CHS) mixed micelles, previous 19F-NMR studies revealed the presence of multiple simultaneously populated conformational states. Here, we study the influence of a different detergent, lauryl maltose neopentyl glycol (LMNG) in mixed micelles with CHS, and of lipid bilayer nanodiscs on these conformational equilibria. The populations of locally different substates are pronouncedly different in DDM/‌‌‌‌‌CHS and LMNG/‌‌‌‌‌CHS micelles, whereas the A2AAR conformational manifold in LMNG/‌‌‌‌‌CHS micelles is closely similar to that in the lipid bilayer nanodiscs. Considering that nanodiscs represent a closer match of the natural lipid bilayer membrane, these observations support that LMNG/‌‌‌‌‌CHS micelles are a good choice for reconstitution trials of class A GPCRs for NMR studies in solution.

Mobile barrier mechanisms for Na+-coupled symport in an MFS sugar transporter

While many 3D structures of cation-coupled transporters have been determined, the mechanistic details governing the obligatory coupling and functional regulations still remain elusive. The bacterial melibiose transporter (MelB) is a prototype of major facilitator superfamily transporters. With a conformation-selective nanobody, we determined a low-sugar affinity inward-facing Na+-bound cryoEM structure. The available outward-facing sugar-bound structures showed that the N- and C-terminal residues of the inner barrier contribute to the sugar selectivity. The inward-open conformation shows that the sugar selectivity pocket is also broken when the inner barrier is broken. Isothermal titration calorimetry measurements revealed that this inward-facing conformation trapped by this nanobody exhibited a greatly decreased sugar-binding affinity, suggesting the mechanisms for substrate intracellular release and accumulation. While the inner/outer barrier shift directly regulates the sugar-binding affinity, it has little or no effect on the cation binding, which is supported by molecular dynamics simulations. Furthermore, the hydron/deuterium exchange mass spectrometry analyses allowed us to identify dynamic regions; some regions are involved in the functionally important inner barrier-specific salt-bridge network, which indicates their critical roles in the barrier switching mechanisms for transport. These complementary results provided structural and dynamic insights into the mobile barrier mechanism for cation-coupled symport.

Rational Approach to Improve Detergent Efficacy for Membrane Protein Stabilization

Membrane protein structures are essential for the molecular understanding of diverse cellular processes and drug discovery. Detergents are not only widely used to extract membrane proteins from membranes but also utilized to preserve native protein structures in aqueous solution. However, micelles formed by conventional detergents are suboptimal for membrane protein stabilization, necessitating the development of novel amphiphilic molecules with enhanced protein stabilization efficacy. In this study, we prepared two sets of tandem malonate-derived glucoside (TMG) variants, both of which were designed to increase the alkyl chain density in micelle interiors. The alkyl chain density was modulated either by reducing the spacer length (TMG-Ms) or by introducing an additional alkyl chain between the two alkyl chains of the original TMGs (TMG-Ps). When evaluated with a few membrane proteins including a G protein-coupled receptor, TMG-P10,8 was found to be substantially more efficient at extracting membrane proteins and also effective at preserving protein integrity in the long term compared to the previously described TMG-A13. This result reveals that inserting an additional alkyl chain between the two existing alkyl chains is an effective way to optimize detergent properties for membrane protein study. This new biochemical tool and the design principle described have the potential to facilitate membrane protein structure determination.

Large-scale column-free purification of bovine F-ATP synthase

Mammalian F-ATP synthase is central to mitochondrial bioenergetics and is present in the inner mitochondrial membrane in a dynamic oligomeric state of higher oligomers, tetramers, dimers, and monomers. In vitro investigations of mammalian F-ATP synthase are often limited by the ability to purify the oligomeric forms present in vivo at a quantity, stability, and purity that meets the demand of the planned experiment. We developed a purification approach for the isolation of bovine F-ATP synthase from heart muscle mitochondria that uses a combination of buffer conditions favoring inhibitor factor 1 binding and sucrose density gradient ultracentrifugation to yield stable complexes at high purity in the milligram range. By tuning the glyco-diosgenin to lauryl maltose neopentyl glycol ratio in a final gradient, fractions that are either enriched in tetrameric or monomeric F-ATP synthase can be obtained. It is expected that this large-scale column-free purification strategy broadens the spectrum of in vitro investigation on mammalian F-ATP synthase.

Challenges of Protein-Protein Docking of the Membrane Proteins

Despite the recent advances in the determination of high-resolution membrane protein (MP) structures, the structural and functional characterization of MPs remains extremely challenging, mainly due to the hydrophobic nature, low abundance, poor expression, purification, and crystallization difficulties associated with MPs. Whereby the major challenges/hurdles for MP structure determination are associated with the expression, purification, and crystallization procedures. Although there have been significant advances in the experimental determination of MP structures, only a limited number of MP structures (approximately less than 1% of all) are available in the Protein Data Bank (PDB). Therefore, the structures of a large number of MPs still remain unresolved, which leads to the availability of widely unplumbed structural and functional information related to MPs. As a result, recent developments in the drug discovery realm and the significant biological contemplation have led to the development of several novel, low-cost, and time-efficient computational methods that overcome the limitations of experimental approaches, supplement experiments, and provide alternatives for the characterization of MPs. Whereby the fine tuning and optimizations of these computational approaches remains an ongoing endeavor.Computational methods offer a potential way for the elucidation of structural features and the augmentation of currently available MP information. However, the use of computational modeling can be extremely challenging for MPs mainly due to insufficient knowledge of (or gaps in) atomic structures of MPs. Despite the availability of numerous in silico methods for 3D structure determination the applicability of these methods to MPs remains relatively low since all methods are not well-suited or adequate for MPs. However, sophisticated methods for MP structure predictions are constantly being developed and updated to integrate the modifications required for MPs. Currently, different computational methods for (1) MP structure prediction, (2) stability analysis of MPs through molecular dynamics simulations, (3) modeling of MP complexes through docking, (4) prediction of interactions between MPs, and (5) MP interactions with its soluble partner are extensively used. Towards this end, MP docking is widely used. It is notable that the MP docking methods yet few in number might show greater potential in terms of filling the knowledge gap. In this chapter, MP docking methods and associated challenges have been reviewed to improve the applicability, accuracy, and the ability to model macromolecular complexes.

Cell-Free Synthesis of Bunyavirales Proteins in View of Their Structural Characterization by Nuclear Magnetic Resonance

The Rift Valley fever virus is one of the bunyaviruses on the WHO's priority list of pathogens that may cause future pandemics. A better understanding of disease progression and viral pathogenesis is urgently needed to develop treatments. The non-structural proteins NSs and NSm of human pathogenic bunyaviruses represent promising therapeutic targets, as they are often key virulence factors. However, their function is still poorly understood, and their structure is yet unknown, mainly because no successful production of these highly complex proteins has been reported. Here we propose a powerful combination of wheat germ cell-free protein synthesis and NMR to study the structure of these proteins and in particular detail cell-free synthesis and lipid reconstitution methods that can be applied to complex membrane proteins.

Specific protonation of acidic residues confers K+ selectivity to the gastric proton pump

The gastric proton pump (H+,K+-ATPase) transports a proton into the stomach lumen for every K+ ion exchanged in the opposite direction. In the lumen-facing state of the pump (E2), the pump selectively binds K+ despite the presence of a 10-fold higher concentration of Na+. The molecular basis for the ion selectivity of the pump is unknown. Using molecular dynamics simulations, free energy calculations, and Na+ and K+-dependent ATPase activity assays, we demonstrate that the K+ selectivity of the pump depends upon the simultaneous protonation of the acidic residues E343 and E795 in the ion-binding site. We also show that when E936 is protonated, the pump becomes Na+ sensitive. The protonation-mimetic mutant E936Q exhibits weak Na+-activated ATPase activity. A 2.5-Å resolution cryo-EM structure of the E936Q mutant in the K+-occluded E2-Pi form shows, however, no significant structural difference compared with wildtype except less-than-ideal coordination of K+ in the mutant. The selectivity toward a specific ion correlates with a more rigid and less fluctuating ion-binding site. Despite being exposed to a pH of 1, the fundamental principle driving the K+ ion selectivity of H+,K+-ATPase is similar to that of Na+,K+-ATPase: the ionization states of the acidic residues in the ion-binding sites determine ion selectivity. Unlike the Na+,K+-ATPase, however, protonation of an ion-binding glutamate residue (E936) confers Na+ sensitivity.

Trends in the Diversification of the Detergentome

Detergents are amphiphilic molecules that serve as enabling steps for today's world applications. The increasing diversity of the detergentome is key to applications enabled by detergent science. Regardless of the application, the optimal design of detergents is determined empirically, which leads to failed preparations, and raising costs. To facilitate project planning, here we review synthesis strategies that drive the diversification of the detergentome. Synthesis strategies relevant for industrial and academic applications include linear, modular, combinatorial, bio-based, and metric-assisted detergent synthesis. Scopes and limitations of individual synthesis strategies in context with industrial product development and academic research are discussed. Furthermore, when designing detergents, the selection of molecular building blocks, i. e., head, linker, tail, is as important as the employed synthesis strategy. To facilitate the design of safe-to-use and tailor-made detergents, we provide an overview of established head, linker, and tail groups and highlight selected scopes and limitations for applications. It becomes apparent that most recent contributions to the increasing chemical diversity of detergent building blocks originate from the development of detergents for membrane protein studies. The overview of synthesis strategies and molecular blocks will bring us closer to the ability to predictably design and synthesize optimal detergents for challenging future applications.

Mobile barrier mechanisms for Na+-coupled symport in an MFS sugar transporter

While many 3D structures of cation-coupled transporters have been determined, the mechanistic details governing the obligatory coupling and functional regulations still remain elusive. The bacterial melibiose transporter (MelB) is a prototype of the Na+-coupled major facilitator superfamily transporters. With a conformational nanobody (Nb), we determined a low-sugar affinity inward-facing Na+-bound cryoEM structure. Collectively with the available outward-facing sugar-bound structures, both the outer and inner barriers were localized. The N- and C-terminal residues of the inner barrier contribute to the sugar selectivity pocket. When the inner barrier is broken as shown in the inward-open conformation, the sugar selectivity pocket is also broken. The binding assays by isothermal titration calorimetry revealed that this inward-facing conformation trapped by the conformation-selective Nb exhibited a greatly decreased sugar-binding affinity, suggesting the mechanisms for the substrate intracellular release and accumulation. While the inner/outer barrier shift directly regulates the sugar-binding affinity, it has little or no effect on the cation binding, which is also supported by molecular dynamics simulations. Furthermore, the use of this Nb in combination with the hydron/deuterium exchange mass spectrometry allowed us to identify dynamic regions; some regions are involved in the functionally important inner barrier-specific salt-bridge network, which indicates their critical roles in the barrier switching mechanisms for transport. These complementary results provided structural and dynamic insights into the mobile barrier mechanism for cation-coupled symport.

Melamine-cored glucosides for membrane protein solubilization and stabilization: importance of water-mediated intermolecular hydrogen bonding in detergent performance

Membrane proteins play essential roles in a number of biological processes, and their structures are important in elucidating such processes at the molecular level and also for rational drug design and development. Membrane protein structure determination is notoriously challenging compared to that of soluble proteins, due largely to the inherent instability of their structures in non-lipid environments. Micelles formed by conventional detergents have been widely used for membrane protein manipulation, but they are suboptimal for long-term stability of membrane proteins, making downstream characterization difficult. Hence, there is an unmet need for the development of new amphipathic agents with enhanced efficacy for membrane protein stabilization. In this study, we designed and synthesized a set of glucoside amphiphiles with a melamine core, denoted melamine-cored glucosides (MGs). When evaluated with four membrane proteins (two transporters and two G protein-coupled receptors), MG-C11 conferred notably enhanced stability compared to the commonly used detergents, DDM and LMNG. These promising findings are mainly attributed to a unique feature of the MGs, i.e., the ability to form dynamic water-mediated hydrogen-bond networks between detergent molecules, as supported by molecular dynamics simulations. Thus, MG-C11 is the first example of a non-peptide amphiphile capable of forming intermolecular hydrogen bonds within a protein-detergent complex environment. Detergent micelles formed via a hydrogen-bond network could represent the next generation of highly effective membrane-mimetic systems useful for membrane protein structural studies.

Molecular Mechanisms behind Conformational Transitions of the Influenza Virus Hemagglutinin Membrane Anchor

Membrane fusion is a fundamental process that is exploited by enveloped viruses to enter host cells. In the case of the influenza virus, fusion is facilitated by the trimeric viral hemagglutinin protein (HA). So far, major focus has been put on its N-terminal fusion peptides, which are directly responsible for fusion initiation. A growing body of evidence points also to a significant functional role of the HA C-terminal domain, which however remains incompletely understood. Our computational study aimed to elucidate the structural and functional interdependencies within the HA C-terminal region encompassing the transmembrane domain (TMD) and the cytoplasmic tail (CT). In particular, we were interested in the conformational shift of the TMD in response to varying cholesterol concentration in the viral membrane and in its modulation by the presence of CT. Using free-energy calculations based on atomistic molecular dynamics simulations, we characterized transitions between straight and tilted metastable TMD configurations under varying conditions. We found that the presence of CT is essential for achieving a stable, highly tilted TMD configuration. As we demonstrate, such a configuration of HA membrane anchor likely supports the tilting motion of its ectodomain, which needs to be executed during membrane fusion. This finding highlights the functional role of, so far, the relatively overlooked CT region.

Deep learning driven de novo drug design based on gastric proton pump structures

Existing drugs often suffer in their effectiveness due to detrimental side effects, low binding affinity or pharmacokinetic problems. This may be overcome by the development of distinct compounds. Here, we exploit the rich structural basis of drug-bound gastric proton pump to develop compounds with strong inhibitory potency, employing a combinatorial approach utilizing deep generative models for de novo drug design with organic synthesis and cryo-EM structural analysis. Candidate compounds that satisfy pharmacophores defined in the drug-bound proton pump structures, were designed in silico utilizing our deep generative models, a workflow termed Deep Quartet. Several candidates were synthesized and screened according to their inhibition potencies in vitro, and their binding poses were in turn identified by cryo-EM. Structures reaching up to 2.10 Å resolution allowed us to evaluate and re-design compound structures, heralding the most potent compound in this study, DQ-18 (N-methyl-4-((2-(benzyloxy)-5-chlorobenzyl)oxy)benzylamine), which shows a Ki value of 47.6 nM. Further high-resolution cryo-EM analysis at 2.08 Å resolution unambiguously determined the DQ-18 binding pose. Our integrated approach offers a framework for structure-based de novo drug development based on the desired pharmacophores within the protein structure.

Identity, structure, and function of the mitochondrial permeability transition pore: controversies, consensus, recent advances, and future directions

The mitochondrial permeability transition (mPT) describes a Ca2+-dependent and cyclophilin D (CypD)-facilitated increase of inner mitochondrial membrane permeability that allows diffusion of molecules up to 1.5 kDa in size. It is mediated by a non-selective channel, the mitochondrial permeability transition pore (mPTP). Sustained mPTP opening causes mitochondrial swelling, which ruptures the outer mitochondrial membrane leading to subsequent apoptotic and necrotic cell death, and is implicated in a range of pathologies. However, transient mPTP opening at various sub-conductance states may contribute several physiological roles such as alterations in mitochondrial bioenergetics and rapid Ca2+ efflux. Since its discovery decades ago, intensive efforts have been made to identify the exact pore-forming structure of the mPT. Both the adenine nucleotide translocase (ANT) and, more recently, the mitochondrial F1FO (F)-ATP synthase dimers, monomers or c-subunit ring alone have been implicated. Here we share the insights of several key investigators with different perspectives who have pioneered mPT research. We critically assess proposed models for the molecular identity of the mPTP and the mechanisms underlying its opposing roles in the life and death of cells. We provide in-depth insights into current controversies, seeking to achieve a degree of consensus that will stimulate future innovative research into the nature and role of the mPTP.

Adenosine A2A Receptor (A2AAR) Ligand Screening Using the 19F-NMR Probe FPPA

The binding affinity of G protein-coupled receptor (GPCR) ligands is customarily measured by radio-ligand competition experiments. As an alternative approach, ¹⁹F nuclear magnetic resonance spectroscopy (¹⁹F-NMR) is used for the screening of small-molecule lead compounds in drug discovery; the two methods are complementary in that the measurements are performed with widely different experimental conditions. Here, we used the structure of the A_2A adenosine receptor (A_2AAR) complex with V-2006 (3-(4-amino-3-methylbenzyl)-7-(furan-2-yl)-3H-[1,2,3]triazolo[4,5-d]pyrimidin-5-amine) as the basis for the design of a fluorine-containing probe molecule, FPPA (4-(furan-2-yl)-7-(4-(trifluoromethyl)benzyl)-7H-pyrrolo[2,3-d]pyramidin-2-amine), for binding studies with A_2AAR. A protocol of experimental conditions for drug screening and measurements of drug binding affinities using 1D ¹⁹F-NMR observation of FPPA is validated with studies of known A_2AAR ligands. ¹⁹F-NMR with FPPA is thus found to be a robust approach for the discovery of ligands with new core structures, which will expand the libraries of A_2AAR-targeting drug candidates.

Role of surfactants in electron cryo-microscopy film preparation

Single-particle electron cryo-microscopy (cryo-EM) has become an effective and straightforward approach to determine the structure of membrane proteins. However, obtaining cryo-EM grids of sufficient quality for high-resolution structural analysis remains a major bottleneck. One of the difficulties arises from the presence of detergents, which often leads to a lack of control of the ice thickness. Amphipathic polymers such as amphipols (APols) are detergent substitutes, which have proven to be valuable tools for cryo-EM studies. In this work, we investigate the physico-chemical behavior of APol- and detergent-containing solutions and show a correlation with the properties of vitreous thin films in cryo-EM grids. This study provides new insight on the potential of APols, allowing a better control of ice thickness while limiting protein adsorption at the air-water interface, as shown with the full-length mouse serotonin 5-HT3A receptor whose structure has been solved in APol. These findings may speed up the process of grid optimization to obtain high-resolution structures of membrane proteins.

Considerations in production of the prokaryotic ZIP family transporters for structural and functional studies

Zinc ions play essential roles as components of enzymes and many other important biomolecules, and are associated with numerous diseases. The uptake of Zn2+ and other metal ions require a widely distributed transporter protein family called Zrt/Irt-like Proteins (ZIP family), the majority members of which tend to have eight transmembrane helices with both N- and C- termini located on the extracellular or periplasmic side. Their small sizes and dynamic conformations bring many difficulties in their production for structural studies either by crystallography or Cryo-EM. Here, we summarize the problems that may encounter at the various steps of processing the ZIP proteins from gene to structural and functional studies, and provide some solutions and examples from our and other labs for the cloning, expression, purification, stability screening, metal ion transport assays and structural studies of prokaryotic ZIP family transporters using Escherichia coli as a heterologous host.

Anomalous Oligomerization Behavior of E. coli Aquaporin Z in Detergent and in Nanodiscs

Aquaporins are tetrameric integral membrane proteins that act as water channels, and can also permeabilize membranes to other solutes. The monomer appears to be the functional form despite all aquaporins being organized as tetramers, which therefore must provide a clear functional advantage. In addition to this quaternary organization, some aquaporins can act as adhesion molecules in membrane junctions, when tetramers located in opposing membranes interact via their extracellular domains. These stacked forms have been observed in a range of aquaporins, whether using lipidic membrane environments, in electron crystallography, or using detergent micelles, in single-particle cryo-electron microscopy (cryo-EM). In the latter technique, structural studies can be performed when the aquaporin is reconstituted into nanodiscs of lipids that are surrounded by a protein scaffold. During attempts to study E. coli Aquaporin Z (AqpZ), we have found that in some conditions these nanodiscs tend to form filaments that appear to be either thicker head-to-tail or thinner side-to-side stacks of nanodiscs. Nanodisc oligomerization was observed using orthogonal analytical techniques analytical ultra-centrifugation and mass photometry, although the nature of the oligomers (head-to-tail or side-to-side) could not be determined. Using the latter technique, the AqpZ tetramer itself formed oligomers of increasing size when solubilized only in detergent, which is consistent with multiple stacking of AqpZ tetramers. We observed images consistent with both of these filaments in negative staining EM conditions, but only thicker filaments in cryo-EM conditions. We hypothesize that the apparent nanodisc side-to-side arrangement that can only be visualized in negative staining conditions is related to artifacts due to the sample preparation. Filaments of any kind were not observed in EM when nanodiscs did not contain AqpZ, or after addition of detergent into the nanodisc cryo-EM preparation, at concentrations that did not disrupt nanodisc formation. To our knowledge, these filaments have not been observed in nanodiscs preparations of other membrane proteins. AqpZ, like other aquaporins has a charge asymmetry between the cytoplasmic (more positive) and the extracellular sides, which may explain the likely head-to-tail stacking observed, both in nanodisc preparations and also in detergent micelles.

Tris(hydroxymethyl)aminomethane Linker-Bearing Triazine-Based Triglucosides for Solubilization and Stabilization of Membrane Proteins

High-resolution membrane protein structures are essential for a fundamental understanding of the molecular basis of diverse cellular processes and for drug discovery. Detergents are widely used to extract membrane-spanning proteins from membranes and maintain them in a functional state for downstream characterization. Due to limited long-term stability of membrane proteins encapsulated in conventional detergents, development of novel agents is required to facilitate membrane protein structural study. In the current study, we designed and synthesized tris(hydroxymethyl)aminomethane linker-bearing triazine-based triglucosides (TTGs) for solubilization and stabilization of membrane proteins. When these glucoside detergents were evaluated for four membrane proteins including two G protein-coupled receptors, a few TTGs including TTG-C10 and TTG-C11 displayed markedly enhanced behaviors toward membrane protein stability relative to two maltoside detergents [DDM (n-dodecyl-β-d-maltoside) and LMNG (lauryl maltose neopentyl glycol)]. This is a notable feature of the TTGs as glucoside detergents tend to be inferior to maltoside detergents at stabilizing membrane proteins. The favorable behavior of the TTGs for membrane protein stability is likely due to the high hydrophobicity of the lipophilic groups, an optimal range of hydrophilic-lipophilic balance, and the absence of cis-trans isomerism.

Rapid and Highly Stable Membrane Reconstitution by LAiR Enables the Study of Physiological Integral Membrane Protein Functions

Functional reintegration into lipid environments represents a major challenge for in vitro investigation of integral membrane proteins (IMPs). Here, we report a new approach, termed LMNG Auto-insertion Reintegration (LAiR), for reintegration of IMPs into lipid bilayers within minutes. The resulting proteoliposomes displayed an unprecedented capability to maintain proton gradients and long-term stability. LAiR allowed for monitoring catalysis of a membrane-bound, physiologically relevant polyisoprenoid quinone substrate by Escherichia coli cytochromes bo ₃ (cbo ₃) and bd (cbd) under control of the proton motive force. LAiR also facilitated bulk-phase detection and physiological assessment of the "proton leak" in cbo ₃, a controversial catalytic state that previously was only approachable at the single-molecule level. LAiR maintained the multisubunit integrity and higher-order oligomeric states of the delicate mammalian F-ATP synthase. Given that LAiR can be applied to both liposomes and planar membrane bilayers and is compatible with IMPs and lipids from prokaryotic and eukaryotic sources, we anticipate LAiR to be applied broadly across basic research, pharmaceutical applications, and biotechnology.

Travel light: Essential packing for membrane proteins with an active lifestyle

We review the considerable progress during the recent decade in the endeavours of designing, optimising, and utilising carrier particle systems for structural and functional studies of membrane proteins in near-native environments. New and improved systems are constantly emerging, novel studies push the perceived limits of a given carrier system, and specific carrier systems consolidate and entrench themselves as the system of choice for particular classes of target membrane protein systems. This review covers the most frequently used carrier systems for such studies and emphasises similarities and differences between these systems as well as current trends and future directions for the field. Particular interest is devoted to the biophysical properties and membrane mimicking ability of each system and the manner in which this may impact an embedded membrane protein and an eventual structural or functional study.

The non-Newtonian behavior of detergents during concentration is increased by macromolecules, in trans, and results in their over-concentration

Concentration of pure membrane proteins in detergent solution results in detergent concentration, albeit in unknown amounts. This phenomenon is observed in every lab working on membrane proteins, but has seldom been investigated. In this study, we explored the behavior of detergents mixed with membrane proteins during the step of sample concentration using centrifugal devices. We show that detergent over-concentrate with the presence of polymers, typically membrane or soluble proteins but also polysaccharides. The over-concentration of detergents depends on centrifugal force applied to the device. With the use of a specific dye, we observed the formation of a mesh on the concentrator device. Importantly, reducing the centrifugal speed allows to reduce the concentration of detergents when mixed to macromolecules, as tested with 3 different membrane proteins. All together, these results highlight the non-Newtonian behavior of detergents and provides a solid framework to investigators to improve drastically biochemical and structural studies of membrane proteins.

Synthesis, Self-Assembly Properties, and Degradation Characterization of a Nonionic Photocleavable Azo-Sulfide Surfactant Family

We report the synthesis and characterization of a new family of maltose-derived nonionic surfactants that contain a photocleavable azo-sulfide linker (mAzo). The self-assembly properties of these surfactants were investigated using surface tension measurements to determine the critical micelle concentration (CMC), dynamic light scattering (DLS) to reveal the hydrodynamic radius of their self-assemblies, and transmission electron microscopy (TEM) to elucidate the micelle morphology. Ultraviolet-visible (UV-visible) spectroscopy confirmed the rapid photodegradation of these surfactants, but surface tension measurements of the surfactant solutions before and after degradation showed unusual degradation products. The photodegradation process was further studied using online liquid chromatography coupled with mass spectrometry (LC-MS),which revealed that these surfactants can form another photo-stable surfactant post-degradation. Finally, traditionally challenging proteins from heart tissue were solubilized using the mAzo surfactants to demonstrate their potential in biological applications.

The relationship between the chemical structure, physicochemical properties, and mucosal adjuvanticity of sugar-based surfactants

The relationship between the chemical structure, physicochemical properties, and mucosal adjuvanticity of sugar-based surfactants (SBSs) has not been sufficiently elucidated. Thus, in the present study, we systematically analyzed 11 SBSs for mucosal adjuvanticity. Ovalbumin (OVA)-specific antibody titers were measured in mice immunized intranasally with OVA plus SBS. We found that four SBSs (trehalose monododecanoate, sucrose monododecanoate, n-dodecyl-α-d-maltopyranoside, and n-dodecyl-β-d-maltopyranoside) exhibited the most potent adjuvanticity. We identified the following associations between chemical structure and adjuvanticity: 1) OVA-specific antibody titer increased with an increasing number of carbon atoms in the alkyl chain; 2) the adjuvanticity was not affected by the type of sugar or bond between the sugar and alkyl chain; and 3) SBSs with rigid structures exhibited less adjuvanticity. The relationship between physicochemical properties and adjuvanticity was as follows: 1) SBSs exhibited adjuvanticity above the critical micelle concentration and 2) in the SBSs with potent adjuvanticity, the diameter of the SBS-OVA complex was 70-75 nm. Our study indicates evidence for the direct involvement of chemical structure and physicochemical properties in determining adjuvanticity in SBSs.

Strategies for the Purification of Membrane Proteins

Membrane proteins account for approximately 30% of the coding regions of all sequenced genomes, and they play crucial roles in many fundamental cell processes. However, there are relatively few membrane proteins with known three-dimensional structures. This is likely due to technical challenges associated with membrane protein extraction, solubilization, and purification. Membrane proteins are classified based on the level of interaction with membrane lipid bilayers, with peripheral membrane proteins associating non-covalently with the membrane, and integral membrane proteins associating more strongly by means of hydrophobic interactions. Generally speaking, peripheral membrane proteins can be purified by milder techniques than integral membrane proteins, with the latter's extraction requiring phospholipid bilayer disruption using detergents or organic solvents. In this chapter, important considerations for membrane protein purification are addressed, with a focus on the initial stages of membrane protein solubilization, where problems are most frequently encountered. Protocols are outlined for the extraction of peripheral membrane proteins, solubilization of integral membrane proteins, and sample clean-up and concentration.

Specific Xray diffraction patterns of membrane proteins caused by secondary structure collinearity

Diffraction anisotropy is a phenomenon that impacts more specifically membrane proteins, compared to soluble ones, but the reasons for this discrepancy remained unclear. Often, it is referred to a difference in resolution limits between highest and lowest diffraction limits as a signature for anisotropy. We show in this article that there is no single correlation between anisotropy and difference in resolution limits, with notably a substantial number of structures displaying various anisotropy with no difference in resolution limits. We further investigated diffraction intensity profiles, and observed a peak centred on 4.9 Å resolution more predominant in membrane proteins. Since this peak is in the region corresponding to secondary structures, we investigated the influence of secondary structure ratio. We showed that secondary structure content has little influence on this profile, while secondary structure collinearity in membrane proteins correlate with a stronger peak. Finally, we could further show that the presence of this peak is linked to higher diffraction anisotropy. These results bring to light a specific diffraction of membrane protein crystals, which calls for a specific handling by crystallographic software. It also brings an explanation for investigators struggling with their anisotropic data.

New insights into the structure and function of class B1 GPCRs

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors. Class B1 GPCRs constitute a subfamily of 15 receptors that characteristically contain large extracellular domains (ECDs) and respond to long polypeptide hormones. Class B1 GPCRs are critical regulators of homeostasis, and as such, many are important drug targets. While most transmembrane proteins, including GPCRs, are recalcitrant to crystallization, recent advances in electron cryo-microscopy (cryo-EM) have facilitated a rapid expansion of the structural understanding of membrane proteins. As a testament to this success, structures for all the class B1 receptors bound to G proteins have been determined by cryo-EM in the past five years. Further advances in cryo-EM have uncovered dynamics of these receptors, ligands, and signalling partners. Here, we examine the recent structural underpinnings of the class B1 GPCRs with an emphasis on structure-function relationships.

Transition-Linker Containing Detergents for Membrane Protein Studies

It is a pressing need, but still challenging to explore the structure and function of membrane proteins (MPs). One of the main obstacles is the limited availability of matched detergents for the handling of specific MPs. We describe herein the design of new detergents by incorporation of a transition linker between the hydrophilic head and the hydrophobic tail. This design allows a gradual change of hydrophobicity between the outside and inside of micelles, in contrast to the abrupt switch in conventional detergents. Notably, many of these detergents assembled into micelles in while retaining low critical micelle concentrations. Meanwhile, thermal stabilizing evaluation identified superior detergents for representative MPs, including G protein-coupled receptors and a transporter protein. Among them, further improved the NMR study of MPs. We anticipate these that results will encourage future detergent expansion through new remodeling on the traditional detergent scaffold.

19F NMR: A promising tool for dynamic conformational studies of G protein-coupled receptors

Advances in X-ray crystallography and cryoelectron microscopy enabled unprecedented insights into the activation processes of G protein-coupled receptors (GPCRs). However, these static receptor structures provide limited information about dynamics and conformational transitions that play pivotal roles in mediating signaling diversity through the multifaceted interactions between ligands, receptors, and transducers. Developing NMR approaches to probe the dynamics of conformational transitions will push the frontier of receptor science toward a more comprehensive understanding of these signaling processes. Although much progress has been made during the last decades, it remains challenging to delineate receptor conformational states and interrogate the functions of the individual states at a quantitative level. Here we cover the progress of ¹⁹F NMR applications in GPCR conformational and dynamic studies during the past 20 years. Current challenges and limitations of ¹⁹F NMR for studying GPCR dynamics are also discussed, along with experimental strategies that will drive this field forward.

Detergents and alternatives in cryo-EM studies of membrane proteins

Structure determination of membrane proteins has been a long-standing challenge to understand the molecular basis of life processes. Detergents are widely used to study the structure and function of membrane proteins by various experimental methods, and the application of membrane mimetics is also a prevalent trend in the field of cryo-EM analysis. This review focuses on the widely-used detergents and corresponding properties and structures, and also discusses the growing interests in membrane mimetic systems used in cryo-EM studies, providing insights into the role of detergent alternatives in structure determination.

Detergent-Free Isolation of Membrane Proteins and Strategies to Study Them in a Near-Native Membrane Environment

Atomic-resolution structural studies of membrane-associated proteins and peptides in a membrane environment are important to fully understand their biological function and the roles played by them in the pathology of many diseases. However, the complexity of the cell membrane has severely limited the application of commonly used biophysical and biochemical techniques. Recent advancements in NMR spectroscopy and cryoEM approaches and the development of novel membrane mimetics have overcome some of the major challenges in this area. For example, the development of a variety of lipid-nanodiscs has enabled stable reconstitution and structural and functional studies of membrane proteins. In particular, the ability of synthetic amphipathic polymers to isolate membrane proteins directly from the cell membrane, along with the associated membrane components such as lipids, without the use of a detergent, has opened new avenues to study the structure and function of membrane proteins using a variety of biophysical and biological approaches. This review article is focused on covering the various polymers and approaches developed and their applications for the functional reconstitution and structural investigation of membrane proteins. The unique advantages and limitations of the use of synthetic polymers are also discussed.

Structures of Multisubunit Membrane Complexes With the CRYO ARM 200

Progress in structural membrane biology has been significantly accelerated by the ongoing ‘Resolution Revolution’ in cryo-electron microscopy (cryo-EM). In particular, structure determination by single-particle analysis has evolved into the most powerful method for atomic model building of multisubunit membrane protein complexes. This has created an ever-increasing demand in cryo-EM machine time, which to satisfy is in need of new and affordable cryo-electron microscopes. Here, we review our experience in using the JEOL CRYO ARM 200 prototype for the structure determination by single-particle analysis of three different multisubunit membrane complexes: the Thermus thermophilus V-type ATPase V_O complex, the Thermosynechococcus elongatus photosystem I monomer and the flagellar motor lipopolysaccharide peptidoglycan ring (LP ring) from Salmonella enterica.

Native-like environments afford novel mechanistic insights into membrane proteins

Advances in cryogenic electron microscopy (cryo-EM) enabled routine near-atomic structure determination of membrane proteins, while nanodisc technology has provided a way to provide membrane proteins with a native or native-like lipid environment. After giving a brief history of membrane mimetics, we present example structures of membrane proteins in nanodiscs that revealed information not provided by structures obtained in detergent. We describe how the lipid environment surrounding the membrane protein can be custom designed during nanodisc assembly and how it can be modified after assembly to test functional hypotheses. Because nanodiscs most closely replicate the physiologic environment of membrane proteins and often afford novel mechanistic insights, we propose that nanodiscs ought to become the standard for structural studies on membrane proteins.

Foldable detergents for membrane protein stability

Detergents are widely used for membrane protein structural study. Many recently developed detergents contain multiple tail and head groups, which are typically connected via a small and branched linker. Due to their inherent compact structures, with small inter-alkyl chain distances, these detergents form micelles with high alkyl chain density in the interiors, a feature favorably associated with membrane protein stability. A recent study on tandem triazine maltosides (TZM) revealed a distinct trend; despite possession of an apparently large inter-alkyl chain distance, the TZM-Es were highly effective at stabilizing membrane proteins. Thanks to the incorporation of a flexible spacer between the two triazine rings in the linker region, these detergents are prone to folding into a compact architecture in micellar environments instead of adopting an extended conformation. Detergent foldability represents a new concept of novel detergent design with significant potential for future detergent development.

Development of 1,3-acetonedicarboxylate-derived glucoside amphiphiles (ACAs) for membrane protein study

Detergents are extensively used for membrane protein manipulation. Membrane proteins solubilized in conventional detergents are prone to denaturation and aggregation, rendering downstream characterization of these bio-macromolecules difficult. Although many amphiphiles have been developed to overcome the limited efficacy of conventional detergents for protein stabilization, only a handful of novel detergents have so far proved useful for membrane protein structural studies. Here, we introduce 1,3-acetonedicarboxylate-derived amphiphiles (ACAs) containing three glucose units and two alkyl chains as head and tail groups, respectively. The ACAs incorporate two different patterns of alkyl chain attachment to the core detergent unit, generating two sets of amphiphiles: ACA-As (asymmetrically alkylated) and ACA-Ss (symmetrically alkylated). The difference in the attachment pattern of the detergent alkyl chains resulted in minor variation in detergent properties such as micelle size, critical micelle concentration, and detergent behaviors toward membrane protein extraction and stabilization. In contrast, the impact of the detergent alkyl chain length on protein stability was marked. The two C11 variants (ACA-AC11 and ACA-SC11) were most effective at stabilizing the tested membrane proteins. The current study not only introduces new glucosides as tools for membrane protein study, but also provides detergent structure–property relationships important for future design of novel amphiphiles.

Newly developed amphiphiles, designated ACAs, are not only efficient at extracting G protein-coupled receptors from the membranes, but also conferred enhanced stability to the receptors compared to the gold standards (DDM and LMNG).

Ugi Reaction Mediated Detergent Assembly for Membrane Protein Studies

Despite the continuous efforts, the current repertoire of detergents is still far from sufficient for the biophysics studies of membrane proteins (MPs). Toward the rapid expansion of detergent diversity, we herein report a new strategy based on Ugi reaction mediated modular assembly. Structural varieties, including hydrophobic tails and hydrophilic heads, could be conveniently introduced from the multiple reaction components. New detergents then were comprehensively evaluated in the physical properties and preliminarily screened by the thermal stabilization for a transporter MsbA and a spectrum of G protein-coupled receptors (GPCRs). For the glucagon-like peptide-1 receptor (GLP-1R), a class B GPCR, detergent M-23-M finally stood out in a second evaluation for the maintenance of homogeneity and was further illustrated its application in the improvement of NMR study. Besides the promising utility in the MP study, the current results exhibit intriguing structural-physical relationship that would allow the guidance in the tuning of detergent properties in the future.

Foldable Detergents for Membrane Protein Study: Importance of Detergent Core Flexibility in Protein Stabilization

Membrane proteins are of biological and pharmaceutical significance. However, their structural study is extremely challenging mainly due to the fact that only a small number of chemical tools are suitable for stabilizing membrane proteins in solution. Detergents are widely used in membrane protein study, but conventional detergents are generally poor at stabilizing challenging membrane proteins such as G protein-coupled receptors and protein complexes. In the current study, we prepared tandem triazine-based maltosides (TZMs) with two amphiphilic triazine units connected by different diamine linkers, hydrazine (TZM-Hs) and 1,2-ethylenediamine (TZM-Es). These TZMs were consistently superior to a gold standard detergent (DDM) in terms of stabilizing a few membrane proteins. In addition, the TZM-Es containing a long linker showed more general protein stabilization efficacy with multiple membrane proteins than the TZM-Hs containing a short linker. This result indicates that introduction of the flexible1,2-ethylenediamine linker between two rigid triazine rings enables the TZM-Es to fold into favourable conformations in order to promote membrane protein stability. The novel concept of detergent foldability introduced in the current study has potential in rational detergent design and membrane protein applications.

Biophysical Characterization of Membrane Proteins Embedded in Nanodiscs Using Fluorescence Correlation Spectroscopy

Proteins embedded in biological membranes perform essential functions in all organisms, serving as receptors, transporters, channels, cell adhesion molecules, and other supporting cellular roles. These membrane proteins comprise ~30% of all human proteins and are the targets of ~60% of FDA-approved drugs, yet their extensive characterization using established biochemical and biophysical methods has continued to be elusive due to challenges associated with the purification of these insoluble proteins. In response, the development of nanodisc techniques, such as nanolipoprotein particles (NLPs) and styrene maleic acid polymers (SMALPs), allowed membrane proteins to be expressed and isolated in solution as part of lipid bilayer rafts with defined, consistent nanometer sizes and compositions, thus enabling solution-based measurements. Fluorescence correlation spectroscopy (FCS) is a relatively simple yet powerful optical microscopy-based technique that yields quantitative biophysical information, such as diffusion kinetics and concentrations, about individual or interacting species in solution. Here, we first summarize current nanodisc techniques and FCS fundamentals. We then provide a focused review of studies that employed FCS in combination with nanodisc technology to investigate a handful of membrane proteins, including bacteriorhodopsin, bacterial division protein ZipA, bacterial membrane insertases SecYEG and YidC, Yersinia pestis type III secretion protein YopB, yeast cell wall stress sensor Wsc1, epidermal growth factor receptor (EGFR), ABC transporters, and several G protein-coupled receptors (GPCRs).

Glyco-steroidal amphiphiles (GSAs) for membrane protein structural study

Integral membrane proteins pose considerable challenges to high resolution structural analysis. Maintaining membrane proteins in their native state during protein isolation is essential for structural study of these bio-macromolecules. Detergents are the most commonly used amphiphilic compounds for stabilizing membrane proteins in solution outside a lipid bilayer. We previously introduced a glyco-diosgenin (GDN) detergent that was shown to be highly effective at stabilizing a wide range of membrane proteins. This steroidal detergent has additionally gained attention due to its compatibility with membrane protein structure study via cryo-EM. However, synthetic inconvenience limits widespread use of GDN in membrane protein study. To improve its synthetic accessibility and to further enhance detergent efficacy for protein stabilization, we designed a new class of glyco-steroid-based detergents using three steroid units: cholestanol, cholesterol and diosgenin. These new detergents were efficiently prepared and showed marked efficacy for protein stabilization in evaluation with a few model membrane proteins including two G protein-coupled receptors. Some new agents were not only superior to a gold standard detergent, DDM, but were also more effective than the original GDN at preserving protein integrity long term. These agents represent valuable alternatives to GDN, and are likely to facilitate structural determination of challenging membrane proteins.

Functional Overexpression of Membrane Proteins in E. coli: The Good, the Bad, and the Ugly

Overexpression of properly folded membrane proteins is a mandatory step for their functional and structural characterization. One of the most used expression systems for the production of proteins is Escherichia coli. Many advantageous strains combined with T7 expression systems have been developed over the years. Recently, we showed that the choice of the strain is critical for the functionality of membrane proteins, even when the proteins are successfully incorporated in the membrane (Mathieu et al. Sci Rep. 2019; 9(1):2654). Notably, the amount and/or activity of the T7-RNA polymerase, which drives the transcription of the genes of interest, may indirectly affect the folding and functionality of overexpressed membrane proteins. Moreover, we reported a general trend in which mild detergents mainly extract the population of active membrane proteins, whereas a harsher detergent like Fos-choline 12 could solubilize them irrespectively of their functionality. Based on these observations, we provide some guidelines to optimize the quality of membrane proteins overexpressed in E. coli.

Biophysical and functional properties of purified glucose-6-phosphatase catalytic subunit 1

Glucose-6-phosphatase catalytic subunit 1 (G6PC1) plays a critical role in hepatic glucose production during fasting by mediating the terminal step of the gluconeogenesis and glycogenolysis pathways. In concert with accessory transport proteins, this membrane-integrated enzyme catalyzes glucose production from glucose-6-phosphate (G6P) to support blood glucose homeostasis. Consistent with its metabolic function, dysregulation of G6PC1 gene expression contributes to diabetes, and mutations that impair phosphohydrolase activity form the clinical basis of glycogen storage disease type 1a. Despite its relevance to health and disease, a comprehensive view of G6PC1 structure and mechanism has been limited by the absence of expression and purification strategies that isolate the enzyme in a functional form. In this report, we apply a suite of biophysical and biochemical tools to fingerprint the in vitro attributes of catalytically active G6PC1 solubilized in lauryl maltose neopentyl glycol (LMNG) detergent micelles. When purified from Sf9 insect cell membranes, the glycosylated mouse ortholog (mG6PC1) recapitulated functional properties observed previously in intact hepatic microsomes and displayed the highest specific activity reported to date. Additionally, our results establish a direct correlation between the catalytic and structural stability of mG6PC1, which is underscored by the enhanced thermostability conferred by phosphatidylcholine and the cholesterol analog cholesteryl hemisuccinate. In contrast, the N96A variant, which blocks N-linked glycosylation, reduced thermostability. The methodologies described here overcome long-standing obstacles in the field and lay the necessary groundwork for a detailed analysis of the mechanistic structural biology of G6PC1 and its role in complex metabolic disorders.

Biophysical and functional characterization of the human TAS1R2 sweet taste receptor overexpressed in a HEK293S inducible cell line

Sweet taste perception is mediated by a heterodimeric receptor formed by the assembly of the TAS1R2 and TAS1R3 subunits. TAS1R2 and TAS1R3 are class C G-protein-coupled receptors whose members share a common topology, including a large extracellular N-terminal domain (NTD) linked to a seven transmembrane domain (TMD) by a cysteine-rich domain. TAS1R2-NTD contains the primary binding site for sweet compounds, including natural sugars and high-potency sweeteners, whereas the TAS1R2-TMD has been shown to bind a limited number of sweet tasting compounds. To understand the molecular mechanisms governing receptor–ligand interactions, we overexpressed the human TAS1R2 (hTAS1R2) in a stable tetracycline-inducible HEK293S cell line and purified the detergent-solubilized receptor. Circular dichroism spectroscopic studies revealed that hTAS1R2 was properly folded with evidence of secondary structures. Using size exclusion chromatography coupled to light scattering, we found that the hTAS1R2 subunit is a dimer. Ligand binding properties were quantified by intrinsic tryptophan fluorescence. Due to technical limitations, natural sugars have not been tested. However, we showed that hTAS1R2 is capable of binding high potency sweeteners with K_d values that are in agreement with physiological detection. This study offers a new experimental strategy to identify new sweeteners or taste modulators that act on the hTAS1R2 and is a prerequisite for structural query and biophysical studies.