Detergent-free purification of ABC (ATP-binding-cassette) transporters

Studying GPCR Pharmacology in Membrane Microdomains: Fluorescence Correlation Spectroscopy Comes of Age

G protein-coupled receptors (GPCRs) are organised within the cell membrane into highly ordered macromolecular complexes along with other receptors and signalling proteins. Understanding how heterogeneity in these complexes affects the pharmacology and functional response of these receptors is crucial for developing new and more selective ligands. Fluorescence correlation spectroscopy (FCS) and related techniques such as photon counting histogram (PCH) analysis and image-based FCS can be used to interrogate the properties of GPCRs in these membrane microdomains, as well as their interaction with fluorescent ligands. FCS analyses fluorescence fluctuations within a small-defined excitation volume to yield information about their movement, concentration and molecular brightness (aggregation). These techniques can be used on live cells with single-molecule sensitivity and high spatial resolution. Once the preserve of specialist equipment, FCS techniques can now be applied using standard confocal microscopes. This review describes how FCS and related techniques have revealed novel insights into GPCR biology.

The effectiveness of styrene-maleic acid (SMA) copolymers for solubilisation of integral membrane proteins from SMA-accessible and SMA-resistant membranes

Solubilisation of biological lipid bilayer membranes for analysis of their protein complement has traditionally been carried out using detergents, but there is increasing interest in the use of amphiphilic copolymers such as styrene maleic acid (SMA) for the solubilisation, purification and characterisation of integral membrane proteins in the form of protein/lipid nanodiscs. Here we survey the effectiveness of various commercially-available formulations of the SMA copolymer in solubilising Rhodobacter sphaeroides reaction centres (RCs) from photosynthetic membranes. We find that formulations of SMA with a 2:1 or 3:1 ratio of styrene to maleic acid are almost as effective as detergent in solubilising RCs, with the best solubilisation by short chain variants (<30kDa weight average molecular weight). The effectiveness of 10kDa 2:1 and 3:1 formulations of SMA to solubilise RCs gradually declined when genetically-encoded coiled-coil bundles were used to artificially tether normally monomeric RCs into dimeric, trimeric and tetrameric multimers. The ability of SMA to solubilise reaction centre-light harvesting 1 (RC-LH1) complexes from densely packed and highly ordered photosynthetic membranes was uniformly low, but could be increased through a variety of treatments to increase the lipid:protein ratio. However, proteins isolated from such membranes comprised clusters of complexes in small membrane patches rather than individual proteins. We conclude that short-chain 2:1 and 3:1 formulations of SMA are the most effective in solubilising integral membrane proteins, but that solubilisation efficiencies are strongly influenced by the size of the target protein and the density of packing of proteins in the membrane.

Thermodynamics of nanodisc formation mediated by styrene/maleic acid (2:1) copolymer

Styrene/maleic acid copolymers (SMA) have recently attracted great interest for in vitro studies of membrane proteins, as they self-insert into and fragment biological membranes to form polymer-bounded nanodiscs that provide a native-like lipid-bilayer environment. SMA copolymers are available in different styrene/maleic acid ratios and chain lengths and, thus, possess different charge densities, hydrophobicities, and solubilisation properties. Here, we studied the equilibrium solubilisation properties of the most commonly used copolymer, SMA(2:1), by monitoring the formation of nanodiscs from phospholipid vesicles using 31P nuclear magnetic resonance spectroscopy, dynamic light scattering, and differential scanning calorimetry. Comparison of SMA(2:1) phase diagrams with those of SMA(3:1) and diisobutylene/maleic acid (DIBMA) revealed that, on a mass concentration scale, SMA(2:1) is the most efficient membrane solubiliser, despite its relatively mild effects on the thermotropic phase behaviour of solubilised lipids. In contrast with previous kinetic studies, our equilibrium experiments demonstrate that the solubilisation of phospholipid bilayers by SMA(2:1) is most efficient at moderately alkaline pH values. This pH dependence was also observed for the solubilisation of native Escherichia coli membranes, for which SMA(2:1) again turned out to be the most powerful solubiliser in terms of the total amounts of membrane proteins extracted.

Solubilization of human cells by the styrene-maleic acid copolymer: Insights from fluorescence microscopy

Extracting membrane proteins from biological membranes by styrene-maleic acid copolymers (SMAs) in the form of nanodiscs has developed into a powerful tool in membrane research. However, the mode of action of membrane (protein) solubilization in a cellular context is still poorly understood and potential specificity for cellular compartments has not been investigated. Here, we use fluorescence microscopy to visualize the process of SMA solubilization of human cells, exemplified by the immortalized human HeLa cell line. Using fluorescent protein fusion constructs that mark distinct subcellular compartments, we found that SMA solubilizes membranes in a concentration-dependent multi-stage process. While all major intracellular compartments were affected without a strong preference, plasma membrane solubilization was found to be generally slower than the solubilization of organelle membranes. Interestingly, some plasma membrane-localized proteins were more resistant against solubilization than others, which might be explained by their presence in specific membrane domains with differing properties. Our results support the general applicability of SMA for the isolation of membrane proteins from different types of (sub)cellular membranes.

Polymer-encased nanodiscs with improved buffer compatibility

Styrene-maleic acid copolymers allow for solubilization and reconstitution of membrane proteins into nanodiscs. These polymer-encased nanodiscs are promising platforms for studies of membrane proteins in a near-physiologic environment without the use of detergents. However, current styrene-maleic acid copolymers display severe limitations in terms of buffer compatibility and ensued flexibility for various applications. Here, we present a new family of styrene-maleic acid copolymers that do not aggregate at low pH or in the presence of polyvalent cations, and can be used to solubilize membrane proteins and produce nanodiscs of controlled sizes.

Membrane proteins: is the future disc shaped?

The use of styrene maleic acid lipid particles (SMALPs) for the purification of membrane proteins (MPs) is a rapidly developing technology. The amphiphilic copolymer of styrene and maleic acid (SMA) disrupts biological membranes and can extract membrane proteins in nanodiscs of approximately 10 nm diameter. These discs contain SMA, protein and membrane lipids. There is evidence that MPs in SMALPs retain their native structures and functions, in some cases with enhanced thermal stability. In addition, the method is compatible with biological buffers and a wide variety of biophysical and structural analysis techniques. The use of SMALPs to solubilize and stabilize MPs offers a new approach in our attempts to understand, and influence, the structure and function of MPs and biological membranes. In this review, we critically assess progress with this method, address some of the associated technical challenges, and discuss opportunities for exploiting SMA and SMALPs to expand our understanding of MP biology.

Overcoming bottlenecks in the membrane protein structural biology pipeline

Membrane proteins account for a third of the eukaryotic proteome, but are greatly under-represented in the Protein Data Bank. Unfortunately, recent technological advances in X-ray crystallography and EM cannot account for the poor solubility and stability of membrane protein samples. A limitation of conventional detergent-based methods is that detergent molecules destabilize membrane proteins, leading to their aggregation. The use of orthologues, mutants and fusion tags has helped improve protein stability, but at the expense of not working with the sequence of interest. Novel detergents such as glucose neopentyl glycol (GNG), maltose neopentyl glycol (MNG) and calixarene-based detergents can improve protein stability without compromising their solubilizing properties. Styrene maleic acid lipid particles (SMALPs) focus on retaining the native lipid bilayer of a membrane protein during purification and biophysical analysis. Overcoming bottlenecks in the membrane protein structural biology pipeline, primarily by maintaining protein stability, will facilitate the elucidation of many more membrane protein structures in the near future.

Solubilization of Membrane Proteins into Functional Lipid-Bilayer Nanodiscs Using a Diisobutylene/Maleic Acid Copolymer

Once removed from their natural environment, membrane proteins depend on membrane-mimetic systems to retain their native structures and functions. To this end, lipid-bilayer nanodiscs that are bounded by scaffold proteins or amphiphilic polymers such as styrene/maleic acid (SMA) copolymers have been introduced as alternatives to detergent micelles and liposomes for in vitro membrane-protein research. Herein, we show that an alternating diisobutylene/maleic acid (DIBMA) copolymer shows equal performance to SMA in solubilizing phospholipids, stabilizes an integral membrane enzyme in functional bilayer nanodiscs, and extracts proteins of various sizes directly from cellular membranes. Unlike aromatic SMA, aliphatic DIBMA has only a mild effect on lipid acyl-chain order, does not interfere with optical spectroscopy in the far-UV range, and does not precipitate in the presence of low millimolar concentrations of divalent cations.

Proton-Detected Solid-State NMR Spectroscopy of a Zinc Diffusion Facilitator Protein in Native Nanodiscs

The structure, dynamics, and function of membrane proteins are intimately linked to the properties of the membrane environment in which the proteins are embedded. For structural and biophysical characterization, membrane proteins generally need to be extracted from the membrane and reconstituted in a suitable membrane-mimicking environment. Ensuring functional and structural integrity in these environments is often a major concern. The styrene/maleic acid co-polymer has recently been shown to be able to extract lipid/membrane protein patches directly from native membranes to form nanosize discoidal proteolipid particles, also referred to as native nanodiscs. In this work, we show that high-resolution solid-state NMR spectra can be obtained from an integral membrane protein in native nanodiscs, as exemplified by the 2×34 kDa bacterial cation diffusion facilitator CzcD.

Characterization of an archaeal photoreceptor/transducer complex from Natronomonas pharaonis assembled within styrene–maleic acid lipid particles

The archaeal receptor/transducer complex NpSRII/NpHtrII retains its integrity upon reconstitution in styrene–maleic acid lipid particles.

Solubilization of lipids and lipid phases by the styrene-maleic acid copolymer

A promising tool in membrane research is the use of the styrene-maleic acid (SMA) copolymer to solubilize membranes in the form of nanodiscs. Since membranes are heterogeneous in composition, it is important to know whether SMA thereby has a preference for solubilization of either specific types of lipids or specific bilayer phases. Here, we investigated this by performing partial solubilization of model membranes and analyzing the lipid composition of the solubilized fraction. We found that SMA displays no significant lipid preference in homogeneous binary lipid mixtures in the fluid phase, even when using lipids that by themselves show very different solubilization kinetics. By contrast, in heterogeneous phase-separated bilayers, SMA was found to have a strong preference for solubilization of lipids in the fluid phase as compared to those in either a gel phase or a liquid-ordered phase. Together the results suggest that (1) SMA is a reliable tool to characterize native interactions between membrane constituents, (2) any solubilization preference of SMA is not due to properties of individual lipids but rather due to properties of the membrane or membrane domains in which these lipids reside and (3) exploiting SMA resistance rather than detergent resistance may be an attractive approach for the isolation of ordered domains from biological membranes.

Membrane protein extraction and purification using styrene–maleic acid (SMA) copolymer: effect of variations in polymer structure

The use of styrene–maleic acid (SMA) copolymers to extract and purify transmembrane proteins, while retaining their native bilayer environment, overcomes many of the disadvantages associated with conventional detergent-based procedures. This approach has huge potential for the future of membrane protein structural and functional studies. In this investigation, we have systematically tested a range of commercially available SMA polymers, varying in both the ratio of styrene and maleic acid and in total size, for the ability to extract, purify and stabilise transmembrane proteins. Three different membrane proteins (BmrA, LeuT and ZipA), which vary in size and shape, were used. Our results show that several polymers, can be used to extract membrane proteins, comparably to conventional detergents. A styrene:maleic acid ratio of either 2:1 or 3:1, combined with a relatively small average molecular mass (7.5–10 kDa), is optimal for membrane extraction, and this appears to be independent of the protein size, shape or expression system. A subset of polymers were taken forward for purification, functional and stability tests. Following a one-step affinity purification, SMA 2000 was found to be the best choice for yield, purity and function. However, the other polymers offer subtle differences in size and sensitivity to divalent cations that may be useful for a variety of downstream applications.

Isolation of yeast complex IV in native lipid nanodiscs

We used the amphipathic styrene maleic acid (SMA) co-polymer to extract cytochrome c oxidase (CytcO) in its native lipid environment from S. cerevisiae mitochondria. Native nanodiscs containing one CytcO per disc were purified using affinity chromatography. The longest cross-sections of the native nanodiscs were 11nm×14nm. Based on this size we estimated that each CytcO was surrounded by ~100 phospholipids. The native nanodiscs contained the same major phospholipids as those found in the mitochondrial inner membrane. Even though CytcO forms a supercomplex with cytochrome bc₁ in the mitochondrial membrane, cyt. bc₁ was not found in the native nanodiscs. Yet, the loosely-bound Respiratory SuperComplex factors were found to associate with the isolated CytcO. The native nanodiscs displayed an O₂-reduction activity of ~130 electrons CytcO^-1s^-1 and the kinetics of the reaction of the fully reduced CytcO with O₂ was essentially the same as that observed with CytcO in mitochondrial membranes. The kinetics of CO-ligand binding to the CytcO catalytic site was similar in the native nanodiscs and the mitochondrial membranes. We also found that excess SMA reversibly inhibited the catalytic activity of the mitochondrial CytcO, presumably by interfering with cyt. c binding. These data point to the importance of removing excess SMA after extraction of the membrane protein. Taken together, our data shows the high potential of using SMA-extracted CytcO for functional and structural studies.

Influence of lipid bilayer properties on nanodisc formation mediated by styrene/maleic acid copolymers

Copolymers of styrene and maleic acid (SMA) have gained great attention as alternatives to conventional detergents, as they offer decisive advantages for studying membrane proteins and lipids in vitro. These polymers self-insert into artificial and biological membranes and, at sufficiently high concentrations, solubilise them into disc-shaped nanostructures containing a lipid bilayer core surrounded by a polymer belt. We have used (31)P nuclear magnetic resonance spectroscopy and dynamic light scattering to systematically study the solubilisation of vesicles composed of saturated or unsaturated phospholipids by an SMA copolymer with a 3 : 1 styrene/maleic acid molar ratio at different temperatures. Solubilisation was thermodynamically rationalised in terms of a three-stage model that treats various lipid/polymer aggregates as pseudophases. The solubilising capacity of SMA(3 : 1) towards a saturated lipid is higher in the gel than in the liquid-crystalline state of the membrane even though solubilisation is slower. Although the solubilisation of mixed fluid membranes is non-selective, the presence of a non-bilayer phospholipid lowers the threshold at which the membrane becomes saturated with SMA(3 : 1) but raises the polymer concentration required for complete solubilisation. Both of these trends can be explained by considering the vesicle-to-nanodisc transfer free energies of the lipid and the polymer. On the basis of the phase diagrams thus obtained, re-association of polymer-solubilised lipids with vesicles is possible under mild conditions, which has implications for the reconstitution of proteins and lipids from nanodiscs into vesicular membranes. Finally, the phase diagrams provide evidence for the absence of free SMA(3 : 1) in vesicular lipid suspensions.

Artificial membranes for membrane protein purification, functionality and structure studies

Membrane proteins represent one of the most important targets for pharmaceutical companies. Unfortunately, technical limitations have long been a major hindrance in our understanding of the function and structure of such proteins. Recent years have seen the refinement of classical approaches and the emergence of new technologies that have resulted in a significant step forward in the field of membrane protein research. This review summarizes some of the current techniques used for studying membrane proteins, with overall advantages and drawbacks for each method.

A method for detergent-free isolation of membrane proteins in their local lipid environment

Despite the great importance of membrane proteins, structural and functional studies of these proteins present major challenges. A significant hurdle is the extraction of the functional protein from its natural lipid membrane. Traditionally achieved with detergents, purification procedures can be costly and time consuming. A critical flaw with detergent approaches is the removal of the protein from the native lipid environment required to maintain functionally stable protein. This protocol describes the preparation of styrene maleic acid (SMA) co-polymer to extract membrane proteins from prokaryotic and eukaryotic expression systems. Successful isolation of membrane proteins into SMA lipid particles (SMALPs) allows the proteins to remain with native lipid, surrounded by SMA. We detail procedures for obtaining 25 g of SMA (4 d); explain the preparation of protein-containing SMALPs using membranes isolated from Escherichia coli (2 d) and control protein-free SMALPS using E. coli polar lipid extract (1-2 h); investigate SMALP protein purity by SDS-PAGE analysis and estimate protein concentration (4 h); and detail biophysical methods such as circular dichroism (CD) spectroscopy and sedimentation velocity analytical ultracentrifugation (svAUC) to undertake initial structural studies to characterize SMALPs (∼2 d). Together, these methods provide a practical tool kit for those wanting to use SMALPs to study membrane proteins.

Encapsulated membrane proteins: A simplified system for molecular simulation

Over the past 50years there has been considerable progress in our understanding of biomolecular interactions at an atomic level. This in turn has allowed molecular simulation methods employing full atomistic modelling at ever larger scales to develop. However, some challenging areas still remain where there is either a lack of atomic resolution structures or where the simulation system is inherently complex. An area where both challenges are present is that of membranes containing membrane proteins. In this review we analyse a new practical approach to membrane protein study that offers a potential new route to high resolution structures and the possibility to simplify simulations. These new approaches collectively recognise that preservation of the interaction between the membrane protein and the lipid bilayer is often essential to maintain structure and function. The new methods preserve these interactions by producing nano-scale disc shaped particles that include bilayer and the chosen protein. Currently two approaches lead in this area: the MSP system that relies on peptides to stabilise the discs, and SMALPs where an amphipathic styrene maleic acid copolymer is used. Both methods greatly enable protein production and hence have the potential to accelerate atomic resolution structure determination as well as providing a simplified format for simulations of membrane protein dynamics. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

The styrene-maleic acid copolymer: a versatile tool in membrane research

A new and promising tool in membrane research is the detergent-free solubilization of membrane proteins by styrene-maleic acid copolymers (SMAs). These amphipathic molecules are able to solubilize lipid bilayers in the form of nanodiscs that are bounded by the polymer. Thus, membrane proteins can be directly extracted from cells in a water-soluble form while conserving a patch of native membrane around them. In this review article, we briefly discuss current methods of membrane protein solubilization and stabilization. We then zoom in on SMAs, describe their physico-chemical properties, and discuss their membrane-solubilizing effect. This is followed by an overview of studies in which SMA has been used to isolate and investigate membrane proteins. Finally, potential future applications of the methodology are discussed for structural and functional studies on membrane proteins in a near-native environment and for characterizing protein-lipid and protein-protein interactions.

Detergent-Free Membrane Protein Purification

Membrane proteins are localized within a lipid bilayer; in order to purify them for functional and structural studies the first step must involve solubilizing or extracting the protein from these lipids. To date this has been achieved using detergents which disrupt the bilayer and bind to the protein in the transmembrane region. However finding conditions for optimal extraction, without destabilizing protein structure, is time consuming and expensive. Here we present a recently-developed method using a styrene-maleic acid (SMA) co-polymer instead of detergents. The SMA co-polymer extracts membrane proteins in a small disc of lipid bilayer which can be used for affinity chromatography purification, thus enabling the purification of membrane proteins while maintaining their native lipid bilayer environment.

Nanoparticle self-assembly in mixtures of phospholipids with styrene/maleic acid copolymers or fluorinated surfactants

Self-assembling nanostructures in aqueous mixtures of bilayer-forming lipids and micelle-forming surfactants are relevant to in vitro studies on biological and synthetic membranes and membrane proteins. Considerable efforts are currently underway to replace conventional detergents by milder alternatives such as styrene/maleic acid (SMA) copolymers and fluorinated surfactants. However, these compounds and their nanosized assemblies remain poorly understood as regards their interactions with lipid membranes, particularly, the thermodynamics of membrane partitioning and solubilisation. Using (19)F and (31)P nuclear magnetic resonance spectroscopy, static and dynamic light scattering, and isothermal titration calorimetry, we have systematically investigated the aggregational state of a zwitterionic bilayer-forming phospholipid upon exposure to an SMA polymer with a styrene/maleic acid ratio of 3 : 1 or to a fluorinated octyl phosphocholine derivative called F(6)OPC. The lipid interactions of SMA(3 : 1) and F(6)OPC can be thermodynamically conceptualised within the framework of a three-stage model that treats bilayer vesicles, discoidal or micellar nanostructures, and the aqueous solution as distinct pseudophases. The exceptional solubilising power of SMA(3 : 1) is reflected in very low membrane-saturating and solubilising polymer/lipid molar ratios of 0.10 and 0.15, respectively. Although F(6)OPC saturates bilayers at an even lower molar ratio of 0.031, this nondetergent does not solubilise lipids even at >1000-fold molar excess, thus highlighting fundamental differences between these two types of mild membrane-mimetic systems. We rationalise these findings in terms of a new classification of surfactants based on bilayer-to-micelle transfer free energies and discuss practical implications for membrane-protein research.

ABC transporter research: going strong 40 years on

In most organisms, ABC transporters constitute one of the largest families of membrane proteins. In humans, their functions are diverse and underpin numerous key physiological processes, as well as being causative factors in a number of clinically relevant pathologies. Advances in our understanding of these diseases have come about through combinations of genetic and protein biochemical investigations of these transporters and the power of in vitro and in vivo investigations is helping to develop genotype–phenotype understanding. However, the importance of ABC transporter research goes far beyond human biology; microbial ABC transporters are of great interest in terms of understanding virulence and drug resistance and industrial biotechnology researchers are exploring the potential of prokaryotic ABC exporters to increase the capacity of synthetic biology systems. Plant ABC transporters play important roles in transport of hormones, xenobiotics, metals and secondary metabolites, pathogen responses and numerous aspects of development, all of which are important in the global food security area. For 3 days in Chester, this Biochemical Society Focused Meeting brought together researchers with diverse experimental approaches and with different fundamental questions, all of which are linked by the commonality of ABC transporters.

Characterizing diverse orthologues of the cystic fibrosis transmembrane conductance regulator protein for structural studies

As an ion channel, the cystic fibrosis transmembrane conductance regulator (CFTR) protein occupies a unique niche within the ABC family. Orthologues of CFTR are extant throughout the animal kingdom from sharks to platypods to sheep, where the osmoregulatory function of the protein has been applied to differing lifestyles and diverse organ systems. In humans, loss-of-function mutations to CFTR cause the disease cystic fibrosis, which is a significant health burden in populations of white European descent. Orthologue screening has proved fruitful in the pursuit of high-resolution structural data for several membrane proteins, and we have applied some of the princples developed in previous studies to the expression and purification of CFTR. We have overexpressed this protein, along with evolutionarily diverse orthologues, in Saccharomyces cerevisiae and developed a purification to isolate it in quantities sufficient for structural and functional studies.

The synthesis of recombinant membrane proteins in yeast for structural studies

Historically, recombinant membrane protein production has been a major challenge meaning that many fewer membrane protein structures have been published than those of soluble proteins. However, there has been a recent, almost exponential increase in the number of membrane protein structures being deposited in the Protein Data Bank. This suggests that empirical methods are now available that can ensure the required protein supply for these difficult targets. This review focuses on methods that are available for protein production in yeast, which is an important source of recombinant eukaryotic membrane proteins. We provide an overview of approaches to optimize the expression plasmid, host cell and culture conditions, as well as the extraction and purification of functional protein for crystallization trials in preparation for structural studies.

Aquaporin-Based Biomimetic Polymeric Membranes: Approaches and Challenges

In recent years, aquaporin biomimetic membranes (ABMs) for water separation have gained considerable interest. Although the first ABMs are commercially available, there are still many challenges associated with further ABM development. Here, we discuss the interplay of the main components of ABMs: aquaporin proteins (AQPs), block copolymers for AQP reconstitution, and polymer-based supporting structures. First, we briefly cover challenges and review recent developments in understanding the interplay between AQP and block copolymers. Second, we review some experimental characterization methods for investigating AQP incorporation including freeze-fracture transmission electron microscopy, fluorescence correlation spectroscopy, stopped-flow light scattering, and small-angle X-ray scattering. Third, we focus on recent efforts in embedding reconstituted AQPs in membrane designs that are based on conventional thin film interfacial polymerization techniques. Finally, we describe some new developments in interfacial polymerization using polyhedral oligomeric silsesquioxane cages for increasing the physical and chemical durability of thin film composite membranes.

Expression and Subcellular Distribution of GFP-Tagged Human Tetraspanin Proteins in Saccharomyces cerevisiae

Tetraspanins are integral membrane proteins that function as organizers of multimolecular complexes and modulate function of associated proteins. Mammalian genomes encode approximately 30 different members of this family and remotely related eukaryotic species also contain conserved tetraspanin homologs. Tetraspanins are involved in a number of fundamental processes such as regulation of cell migration, fusion, immunity and signaling. Moreover, they are implied in numerous pathological states including mental disorders, infectious diseases or cancer. Despite the great interest in tetraspanins, the structural and biochemical basis of their activity is still largely unknown. A major bottleneck lies in the difficulty of obtaining stable and homogeneous protein samples in large quantities. Here we report expression screening of 15 members of the human tetraspanin superfamily and successful protocols for the production in S. cerevisiae of a subset of tetraspanins involved in human cancer development. We have demonstrated the subcellular localization of overexpressed tetraspanin-green fluorescent protein fusion proteins in S. cerevisiae and found that despite being mislocalized, the fusion proteins are not degraded. The recombinantly produced tetraspanins are dispersed within the endoplasmic reticulum membranes or localized in granule-like structures in yeast cells. The recombinantly produced tetraspanins can be extracted from the membrane fraction and purified with detergents or the poly (styrene-co-maleic acid) polymer technique for use in further biochemical or biophysical studies.

G-protein coupled receptor solubilization and purification for biophysical analysis and functional studies, in the total absence of detergent

G-protein coupled receptors (GPCRs) constitute the largest class of membrane proteins and are a major drug target. A serious obstacle to studying GPCR structure/function characteristics is the requirement to extract the receptors from their native environment in the plasma membrane, coupled with the inherent instability of GPCRs in the detergents required for their solubilization. In the present study, we report the first solubilization and purification of a functional GPCR [human adenosine A_2A receptor (A_2AR)], in the total absence of detergent at any stage, by exploiting spontaneous encapsulation by styrene maleic acid (SMA) co-polymer direct from the membrane into a nanoscale SMA lipid particle (SMALP). Furthermore, the A_2AR–SMALP, generated from yeast (Pichia pastoris) or mammalian cells, exhibited increased thermostability (∼5°C) compared with detergent [DDM (n-dodecyl-β-D-maltopyranoside)]-solubilized A_2AR controls. The A_2AR–SMALP was also stable when stored for prolonged periods at 4°C and was resistant to multiple freeze-thaw cycles, in marked contrast with the detergent-solubilized receptor. These properties establish the potential for using GPCR–SMALP in receptor-based drug discovery assays. Moreover, in contrast with nanodiscs stabilized by scaffold proteins, the non-proteinaceous nature of the SMA polymer allowed unobscured biophysical characterization of the embedded receptor. Consequently, CD spectroscopy was used to relate changes in secondary structure to loss of ligand binding ([³H]ZM241385) capability. SMALP-solubilization of GPCRs, retaining the annular lipid environment, will enable a wide range of therapeutic targets to be prepared in native-like state to aid drug discovery and understanding of GPCR molecular mechanisms.

It is universally acknowledged that exposing cell-surface receptors to detergent is detrimental. We have used a polymer to extract the receptor and surrounding lipid as a nanoparticle that provides a novel solution compatible with purification and receptor-based drug discovery assays.

Molecular model for the solubilization of membranes into nanodisks by styrene maleic Acid copolymers

A recent discovery in membrane research is the ability of styrene-maleic acid (SMA) copolymers to solubilize membranes in the form of nanodisks allowing extraction and purification of membrane proteins from their native environment in a single detergent-free step. This has important implications for membrane research because it allows isolation as well as characterization of proteins and lipids in a near-native environment. Here, we aimed to unravel the molecular mode of action of SMA copolymers by performing systematic studies using model membranes of varying compositions and employing complementary biophysical approaches. We found that the SMA copolymer is a highly efficient membrane-solubilizing agent and that lipid bilayer properties such as fluidity, thickness, lateral pressure profile, and charge density all play distinct roles in the kinetics of solubilization. More specifically, relatively thin membranes, decreased lateral chain pressure, low charge density at the membrane surface, and increased salt concentration promote the speed and yield of vesicle solubilization. Experiments using a native membrane lipid extract showed that the SMA copolymer does not discriminate between different lipids and thus retains the native lipid composition in the solubilized particles. A model is proposed for the mode of action of SMA copolymers in which membrane solubilization is mainly driven by the hydrophobic effect and is further favored by physical properties of the polymer such as its relatively small cross-sectional area and rigid pendant groups. These results may be helpful for development of novel applications for this new type of solubilizing agent, and for optimization of the SMA technology for solubilization of the wide variety of cell membranes found in nature.

Detergent-free isolation, characterization, and functional reconstitution of a tetrameric K+ channel: the power of native nanodiscs

A major obstacle in the study of membrane proteins is their solubilization in a stable and active conformation when using detergents. Here, we explored a detergent-free approach to isolating the tetrameric potassium channel KcsA directly from the membrane of Escherichia coli, using a styrene-maleic acid copolymer. This polymer self-inserts into membranes and is capable of extracting membrane patches in the form of nanosize discoidal proteolipid particles or "native nanodiscs." Using circular dichroism and tryptophan fluorescence spectroscopy, we show that the conformation of KcsA in native nanodiscs is very similar to that in detergent micelles, but that the thermal stability of the protein is higher in the nanodiscs. Furthermore, as a promising new application, we show that quantitative analysis of the co-isolated lipids in purified KcsA-containing nanodiscs allows determination of preferential lipid-protein interactions. Thin-layer chromatography experiments revealed an enrichment of the anionic lipids cardiolipin and phosphatidylglycerol, indicating their close proximity to the channel in biological membranes and supporting their functional relevance. Finally, we demonstrate that KcsA can be reconstituted into planar lipid bilayers directly from native nanodiscs, which enables functional characterization of the channel by electrophysiology without first depriving the protein of its native environment. Together, these findings highlight the potential of the use of native nanodiscs as a tool in the study of ion channels, and of membrane proteins in general.

The use of SMALPs as a novel membrane protein scaffold for structure study by negative stain electron microscopy

Despite the great progress recently made in resolving their structures, investigation of the structural biology of membrane proteins still presents major challenges. Even with new technical advances such as lipidic cubic phase crystallisation, obtaining well-ordered crystals remains a significant hurdle in membrane protein X-ray crystallographic studies. As an alternative, electron microscopy has been shown to be capable of resolving > 3.5 Å resolution detail in membrane proteins of modest (~ 300 kDa) size, without the need for crystals. However, the conventional use of detergents for either approach presents several issues, including the possible effects on structure of removing the proteins from their natural membrane environment. As an alternative, it has recently been demonstrated that membrane proteins can be effectively isolated, in the absence of detergents, using a styrene maleic acid co-polymer (SMA). This approach yields SMA lipid particles (SMALPs) in which the membrane proteins are surrounded by a small disk of lipid bilayer encircled by polymer. Here we use the Escherichia coli secondary transporter AcrB as a model membrane protein to demonstrate how a SMALP scaffold can be used to visualise membrane proteins, embedded in a near-native lipid environment, by negative stain electron microscopy, yielding structures at a modest resolution in a short (days) timeframe. Moreover, we show that AcrB within a SMALP scaffold is significantly more active than the equivalent DDM stabilised form. The advantages of SMALP scaffolds within electron microscopy are discussed and we conclude that they may prove to be an important tool in studying membrane protein structure and function.

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Maintaining membrane proteins in a native-like environment is difficult.

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SMALP scaffolds efficiently extract AcrB from the membrane.

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We show SMALP scaffolds to be a robust tool for rapid structural analysis by EM.

Bacterial reaction centers purified with styrene maleic acid copolymer retain native membrane functional properties and display enhanced stability

Integral membrane proteins often present daunting challenges for biophysical characterization, a fundamental issue being how to select a surfactant that will optimally preserve the individual structure and functional properties of a given membrane protein. Bacterial reaction centers offer a rare opportunity to compare the properties of an integral membrane protein in different artificial lipid/surfactant environments with those in the native bilayer. Here, we demonstrate that reaction centers purified using a styrene maleic acid copolymer remain associated with a complement of native lipids and do not display the modified functional properties that typically result from detergent solubilization. Direct comparisons show that reaction centers are more stable in this copolymer/lipid environment than in a detergent micelle or even in the native membrane, suggesting a promising new route to exploitation of such photovoltaic integral membrane proteins in device applications.