Anionic lipids stimulate Sec-independent insertion of a membrane protein lacking charged amino acid side chains

Cryo-EM structure of cell-free synthesized human histamine 2 receptor/Gs complex in nanodisc environment

Here we describe the cryo-electron microscopy structure of the human histamine 2 receptor (H2R) in an active conformation with bound histamine and in complex with Gs heterotrimeric protein at an overall resolution of 3.4 Å. The complex was generated by cotranslational insertion of the receptor into preformed nanodisc membranes using cell-free synthesis in E. coli lysates. Structural comparison with the inactive conformation of H2R and the inactive and Gq-coupled active state of H1R together with structure-guided functional experiments reveal molecular insights into the specificity of ligand binding and G protein coupling for this receptor family. We demonstrate lipid-modulated folding of cell-free synthesized H2R, its agonist-dependent internalization and its interaction with endogenously synthesized H1R and H2R in HEK293 cells by applying a recently developed nanotransfer technique.

Biochemical characterization of cell-free synthesized human β1 adrenergic receptor cotranslationally inserted into nanodiscs

Cell-free expression enables direct cotranslational insertion of G protein coupled receptors (GPCRs) and other membrane proteins into the defined membrane environments of nanodiscs. This technique avoids GPCR contacts with detergents and allows rapid identification of lipid effects on GPCR function as well as fast screening of receptor derivatives. Critical steps of conventional GPCR preparation from cellular membranes followed by detergent-based reconstitution into nanodisc membranes are thus eliminated. We report the efficient cotranslational insertion of full-length human β₁-adrenergic receptor and of a truncated derivative into preformed nanodisc membranes. Their biochemical characterization revealed significant differences in lipid requirements, dimer formation and ligand binding activity. The truncated receptor showed a higher affinity to most tested ligands, in particular in presence of choline-containing lipids. However, introducing the naturally occurring G389R polymorphism in the full-length receptor resulted into an increased affinity to the antagonists alprenolol and carvedilol. Receptor quality was generally improved by coexpression with the agonist isoproterenol and the percentage of the ligand binding active fraction was twofold increased. Specific coupling of full-length and truncated human receptors in nanodisc membranes to Mini-Gα_s protein as well as to purified G_s heterotrimer could be demonstrated and homogeneity of purified GPCR/G_s protein complexes in nanodiscs was demonstrated by negative stain single particle analysis.

Matrix-assisted ionization Fourier transform mass spectrometry for the analysis of lipids

RATIONALE

Assessing the utility of vacuum matrix-assisted ionization (MAI) for the direct and rapid analysis of lipids in complex samples with emphasis on bacterial taxonomy.

METHODS

Matrix-assisted ionization Fourier transform mass spectrometry (MAI-FTMS) was used to characterize polar and non-polar lipids in mixtures.

RESULTS

For non-polar lipid triacylglycerols (TAGs), MAI-FTMS produced lipid-specific ions for eight different edible oils and allowed these oils to be identified based on their MAI-FTMS profiles. For polar lipids from bacteria, MAI-FTMS of crude lipid extracts allowed taxonomic identification of eight blind-coded samples based on taxonomy-specific phospholipid profiles. MAI produced results comparable and complementary to benchmark MALDI and ESI methods currently used for characterization of polar and non-polar lipids in the same mixtures.

CONCLUSIONS

The newly developed MAI technique is a rapid, simple and complementary method for the characterization of polar and non-polar lipids in complex mixtures.

How bilayer properties influence membrane protein folding

The question of how proteins manage to organize into a unique three-dimensional structure has been a major field of study since the first protein structures were determined. For membrane proteins, the question is made more complex because, unlike water-soluble proteins, the solvent is not homogenous or even unique. Each cell and organelle has a distinct lipid composition that can change in response to environmental stimuli. Thus, the study of membrane protein folding requires not only understanding how the unfolded chain navigates its way to the folded state, but also how changes in bilayer properties can affect that search. Here we review what we know so far about the impact of lipid composition on bilayer physical properties and how those properties can affect folding. A better understanding of the lipid bilayer and its effects on membrane protein folding is not only important for a theoretical understanding of the folding process, but can also have a practical impact on our ability to work with and design membrane proteins.

Transmembrane Domain Recognition during Membrane Protein Biogenesis and Quality Control

One-fourth of eukaryotic genes code for integral membrane proteins, nearly all of which are inserted and assembled at the endoplasmic reticulum (ER). The defining feature of membrane proteins is one or more transmembrane domains (TMDs). During membrane protein biogenesis, TMDs are selectively recognized, shielded, and chaperoned into the lipid bilayer, where they often assemble with other TMDs. If maturation fails, exposed TMDs serve as a cue for engagement of degradation pathways. Thus, TMD-recognition factors in the cytosol and ER are essential for membrane protein biogenesis and quality control. Here, we discuss the growing assortment of cytosolic and membrane-embedded TMD-recognition factors, the pathways within which they operate, and mechanistic principles of recognition.

Liposome-chaperoned cell-free synthesis for the design of proteoliposomes: Implications for therapeutic delivery

Cell-free (CF) protein synthesis has emerged as a powerful technique platform for efficient protein production in vitro. Liposomes have been widely studied as therapeutic carriers due to their biocompatibility, biodegradability, low toxicity, flexible surface manipulation, easy preparation, and higher cargo encapsulation capability. However, rapid immune clearance, insufficient targeting capacity, and poor cytoplasmic delivery efficiency substantially restrict their clinical application. The incorporation of functional membrane proteins (MPs) or peptides allows the transfer of biological properties to liposomes and imparts them with improved circulation, increased targeting, and efficient intracellular delivery. Liposome-chaperoned CF synthesis enables production of proteoliposomes in one-step reaction, which not only substantially simplifies the production procedure but also keeps protein functionality intact. Building off these observations, proteoliposomes with integrated MPs represent an excellent candidate for therapeutic delivery. In this review, we describe recent advances in CF synthesis with emphasis on detailing key factors for improving CF expression efficiency. Furthermore, we provide insights into strategies for rational design of proteoliposomal nanodelivery systems via CF synthesis.

STATEMENT OF SIGNIFICANCE

Liposome-chaperoned CF synthesis has emerged as a powerful approach for the design of recombinant proteoliposomes in one-step reaction. The incorporation of bioactive MPs or peptides into liposomes via CF synthesis can facilitate the development of proteoliposomal nanodelivery systems with improved circulation, increased targeting, and enhanced cellular delivery capacity. Moreover, by adapting lessons learned from natural delivery vehicles, novel bio-inspired proteoliposomes with enhanced delivery properties could be produced in CF systems. In this review, we first give an overview of CF synthesis with focus on enhancing protein expression in liposome-chaperoned CF systems. Furthermore, we intend to provide insight into harnessing CF-synthesized proteoliposomes for efficient therapeutic delivery.

Immunoblotting with Peptide Antibodies: Differential Immunoreactivities Caused by Certain Amino Acid Substitutions in a Short Peptide and Possible Effects of Differential Refolding of the Peptide on a Nitrocellulose or PVDF Membrane

Immunodetection using antibodies, e.g., Western blotting, is generally utilized to measure the amount of a certain protein in a protein mixture. For valid interpretation of results observed by immunodetection, strict attention must be paid to the factors affecting the immunoreactivities of the antibodies. We here describe the step-by-step procedures to demonstrate that substitution of certain amino acids in a peptide can cause remarkable differences in its immunoreactivity with antibodies against epitope tags in the immobilized peptide. Refolding of the peptide on the membrane in a way that masks the epitope to different degrees was the possible reason for their distinct immunoreactivities with the antibodies. The results in this chapter suggest that we need to interpret carefully the experimental results involving immunodetection.

Reconstitution of mitochondrial presequence translocase into proteoliposomes

The isolation and functional reconstitution of large membrane protein complexes is an important step towards the biochemical characterization of such sophisticated molecular machines. Reconstitution is a multistep process that requires the mild solubilization of membrane protein complexes from native membrane preparations, the purification of the complexes from protein-detergent solutions, and their incorporation into artificial phospholipid vesicles through controlled detergent removal. The major challenge is to preserve the integrity and catalytic activity of the often fragile membrane protein assemblies during the entire procedure. Here we describe the protocols for a particularly intricate example, the functional reconstitution of the mitochondrial presequence translocase (TIM23 complex). This highly versatile and dynamic protein complex is the main protein translocation machinery of the inner mitochondrial membrane and mediates the import of precursor proteins with N-terminal presequences from the cytosol.

Moraxella catarrhalis expresses a cardiolipin synthase that impacts adherence to human epithelial cells

The major phospholipid constituents of Moraxella catarrhalis membranes are phosphatidylglycerol, phosphatidylethanolamine, and cardiolipin (CL). However, very little is known regarding the synthesis and function of these phospholipids in M. catarrhalis. In this study, we discovered that M. catarrhalis expresses a cardiolipin synthase (CLS), termed MclS, that is responsible for the synthesis of CL within the bacterium. The nucleotide sequence of mclS is highly conserved among M. catarrhalis isolates and is predicted to encode a protein with significant amino acid similarity to the recently characterized YmdC/ClsC protein of Escherichia coli. Isogenic mclS mutant strains were generated in M. catarrhalis isolates O35E, O12E, and McGHS1 and contained no observable levels of CL. Site-directed mutagenesis of a highly conserved HKD motif of MclS also resulted in a CL-deficient strain. Moraxella catarrhalis, which depends on adherence to epithelial cells for colonization of the human host, displays significantly reduced levels of adherence to HEp-2 and A549 cell lines in the mclS mutant strains compared to wild-type bacteria. The reduction in adherence appears to be attributed to the absence of CL. These findings mark the first instance in which a CLS has been related to a virulence-associated trait.

Signal recognition particle: an essential protein-targeting machine

The signal recognition particle (SRP) and its receptor compose a universally conserved and essential cellular machinery that couples the synthesis of nascent proteins to their proper membrane localization. The past decade has witnessed an explosion in in-depth mechanistic investigations of this targeting machine at increasingly higher resolutions. In this review, we summarize recent work that elucidates how the SRP and SRP receptor interact with the cargo protein and the target membrane, respectively, and how these interactions are coupled to a novel GTPase cycle in the SRP·SRP receptor complex to provide the driving force and enhance the fidelity of this fundamental cellular pathway. We also discuss emerging frontiers in which important questions remain to be addressed.

Another Piece of the Membrane Puzzle: Extending Slipids Further

To be able to model complex biological membranes in a more realistic manner, the force field Slipids (Stockholm lipids) has been extended to include parameters for sphingomyelin (SM), phosphatidylglycerol (PG), phosphatidylserine (PS) lipids, and cholesterol. Since the parametrization scheme was faithful to the scheme used in previous editions of Slipids, all parameters are consistent and fully compatible. The results of careful validation of a number of key structural properties for one and two component lipid bilayers are in excellent agreement with experiments. Potentials of mean force for transferring water across binary mixtures of lipids and cholesterol were also computed in order to compare water permeability rates to experiments. In agreement with experimental and simulation studies, it was found that the permeability and partitioning of water is affected by cholesterol in lipid bilayers made of saturated lipids to the largest extent. With the extensions of Slipids presented here, it is now possible to study complex systems containing many different lipids and proteins in a fully atomistic resolution in the isothermic-isobaric (NPT) ensemble, which is the proper ensemble for membrane simulations.

The cell-free integration of a polytopic mitochondrial membrane protein into liposomes occurs cotranslationally and in a lipid-dependent manner

The ADP/ATP Carrier (AAC) is the most abundant transporter of the mitochondrial inner membrane. The central role that this transporter plays in cellular energy production highlights the importance of understanding its structure, function, and the basis of its pathologies. As a means of preparing proteoliposomes for the study of membrane proteins, several groups have explored the use of cell-free translation systems to facilitate membrane protein integration directly into preformed unilamellar vesicles without the use of surfactants. Using AAC as a model, we report for the first time the detergent-free reconstitution of a mitochondrial inner membrane protein into liposomes using a wheat germ-based in vitro translation system. Using a host of independent approaches, we demonstrate the efficient integration of AAC into vesicles with an inner membrane-mimetic lipid composition and, more importantly, that the integrated AAC is functionally active in transport. By adding liposomes at different stages of the translation reaction, we show that this direct integration is obligatorily cotranslational, and by synthesizing stable ribosome-bound nascent chain intermediates, we show that the nascent AAC polypeptide interacts with lipid vesicles while ribosome-bound. Finally, we show that the presence of the phospholipid cardiolipin in the liposomes specifically enhances AAC translation rate as well as the efficiency of vesicle association and integration. In light of these results, the possible mechanisms of liposome-assisted membrane protein integration during cell-free translation are discussed with respect to the mode of integration and the role of specific lipids.

Do small headgroups of phosphatidylethanolamine and phosphatidic acid lead to a similar folding pattern of the K(+) channel?

Phospholipid headgroups act as major determinants in proper folding of oligomeric membrane proteins. The K(+)-channel KcsA is the most popular model protein among these complexes. The presence of zwitterionic nonbilayer lipid phosphatidylethanolamine (PE) is crucial for efficient tetramerization and stabilization of KcsA in a lipid bilayer. In this study, the influence of PE on KcsA folding properties was analyzed by tryptophan fluorescence and acrylamide quenching experiments and compared with the effect of anionic phosphatidic acid (PA). The preliminary studies suggest that the small size and hydrogen bonding capability of the PE headgroup influences KcsA folding via a mechanism quite similar to that observed for anionic PA.

The potassium channel KcsA: a model protein in studying membrane protein oligomerization and stability of oligomeric assembly?

Many membrane proteins are functional as stable oligomers. An understanding of the conditions that elicit and enhance oligomerization is important in many therapeutics. In this regard, protein-protein and protein-lipid interactions play crucial roles in the assembly and stability of oligomeric complexes. Recent years have seen a rapid increase in the mechanistic information on the importance of cytoplasmic termini in determining subunit assembly and stability of oligomeric complexes. In addition, the role of specific protein-lipid interaction between anionic phospholipids and "hot spots" on the protein surface has also become evident in stabilizing oligomeric assemblies. This review focuses on several contemporary developments of membrane proteins that stabilize oligomers by taking the potassium channel KcsA as an exemplary ion channel.

On the role of anionic lipids in charged protein interactions with membranes

We investigate the role of anionic lipids in the binding to, and subsequent movement of charged protein groups in lipid membranes, to help understand the role of membrane composition in all membrane-active protein sequences. We demonstrate a small effect of phosphatidylglycerol (PG) lipids on the ability of an arginine (Arg) side chain to bind to, and cross a lipid membrane, despite possessing a neutralizing charge. We observe similar membrane deformations in lipid bilayers composed of phosphatidylcholine (PC) and PC/PG mixtures, with comparable numbers of water and lipid head groups pulled into the bilayer hydrocarbon core, and prohibitively large ~20 kcal/mol barriers for Arg transfer across each bilayer, dropping by just 2-3 kcal/mol due to the binding of PG lipids. We explore the causes of this small effect of introducing PG lipids and offer an explanation in terms of the limited membrane interaction for the choline groups of PC lipids bound to the translocating ion. Our calculations reveal a surprising lack of preference for Arg binding to PG lipids themselves, but a small increase in interfacial binding affinity for lipid bilayers containing PG lipids. These results help to explain the nature of competitive lipid binding to charged protein sequences, with implications for a wide range of membrane binding domains and cell perturbing peptides.

Lipid activation of the signal recognition particle receptor provides spatial coordination of protein targeting

Phospholipid binding leads to accelerated assembly of the bacterial SRP receptor FtsY and SRP, allowing cargo proteins to be delivered to target membranes more efficiently.

The signal recognition particle (SRP) and SRP receptor comprise the major cellular machinery that mediates the cotranslational targeting of proteins to cellular membranes. It remains unclear how the delivery of cargos to the target membrane is spatially coordinated. We show here that phospholipid binding drives important conformational rearrangements that activate the bacterial SRP receptor FtsY and the SRP–FtsY complex. This leads to accelerated SRP–FtsY complex assembly, and allows the SRP–FtsY complex to more efficiently unload cargo proteins. Likewise, formation of an active SRP–FtsY GTPase complex exposes FtsY’s lipid-binding helix and enables stable membrane association of the targeting complex. Thus, membrane binding, complex assembly with SRP, and cargo unloading are inextricably linked to each other via conformational changes in FtsY. These allosteric communications allow the membrane delivery of cargo proteins to be efficiently coupled to their subsequent unloading and translocation, thus providing spatial coordination during protein targeting.

The role of extramembranous cytoplasmic termini in assembly and stability of the tetrameric K(+)-channel KcsA

Membrane-active alcohol 2,2,2-trifluoroethanol has been proven to be an attractive tool in the investigation of the intrinsic stability of integral membrane protein complexes by taking K⁺-channel KcsA as a suitable and representative ion channel. In the present study, the roles of both cytoplasmic N and C termini in channel assembly and stability of KcsA were determined. The N terminus (1–18 residues) slightly increased tetramer stability via electrostatic interactions in the presence of 30 mol.% acidic phosphatidylglycerol (PG) in phosphatidylcholine lipid bilayer. Furthermore, the N terminus was found to be potentially required for efficient channel (re)assembly. In contrast, truncation of the C terminus (125–160 residues) greatly facilitated channel reversibility from either a partially or a completely unfolded state, and this domain was substantially involved in stabilizing the tetramer in either the presence or absence of PG in lipid bilayer. These studies provide new insights into how extramembranous parts play their crucial roles in the assembly and stability of integral membrane protein complexes.

The role of phosphatidic acid and cardiolipin in stability of the tetrameric assembly of potassium channel KcsA

In this study, the roles of two anionic phospholipids-phosphatidic acid (PA), which is an important signaling molecule, and cardiolipin (CL), which plays a crucial role in the bioenergetics of the cell-in stabilizing the oligomeric structure of potassium channel KcsA were determined. The stability of KcsA was drastically increased as a function of PA or CL content (mol%) in phosphatidylcholine (PC) bilayers. Deletion of the membrane-associated N terminus significantly reduced channel stability at high levels of PA content; however, the intrinsic stability of this protein was marginally affected in the presence of CL. These studies indicate that the electrostatic-hydrogen bond switch between PA and N terminus, involving basic residues, is much stronger than the stabilizing effect of CL. Furthermore, the unique properties of the PA headgroup alter protein assembly and folding properties differently from the CL headgroup, and both lipids stabilize the tetrameric assembly via their specific interaction on the extra- or the intracellular side of KcsA.

Substitution of certain amino acids in a short peptide causes a significant difference in their immunoreactivities with antibodies against different epitopes: evidence for possible folding of the peptide on a nitrocellulose or PVDF membrane

Substitution of amino acids in a peptide caused remarkable differences in its immunoreactivities with antibodies against 3 epitopes in the immobilized peptide. The observed differences in immunoreactivities among the peptides were not due to the differences in efficiencies of their transfer onto nitrocellulose or PVDF membranes. Rather, possible folding of the peptide on the membrane was considered to be the reason for their distinct immunoreactivities with the antibodies.

Phosphatidylglycerol lipids enhance folding of an alpha helical membrane protein

Membrane lipids are increasingly being recognised as active participants in biological events. The precise roles that individual lipids or global properties of the lipid bilayer play in the folding of membrane proteins remain to be elucidated, Here, we find a significant effect of phosphatidylglycerol (PG) on the folding of a trimeric alpha helical membrane protein from Escherichia coli diacylglycerol kinase. Both the rate and the yield of folding are increased by increasing the amount of PG in lipid vesicles. Moreover, there is a direct correlation between the increase in yield and the increase in rate; thus, folding becomes more efficient in terms of speed and productivity. This effect of PG seems to be a specific requirement for this lipid, rather than a charge effect. We also find an effect of single-chain lyso lipids in decreasing the rate and yield of folding. We compare this to our previous work in which lyso lipids increased the rate and yield of another membrane protein, bacteriorhodopsin. The contrasting effect of lyso lipids on the two proteins can be explained by the different folding reaction mechanisms and key folding steps involved. Our findings provide information on the lipid determinants of membrane protein folding.

YidC as an essential and multifunctional component in membrane protein assembly

Membrane proteins fulfill a number of vital functions in prokaryotic and eukaryotic cells. They are often organized in multicomponent complexes, folded within the membrane bilayer and interacting with the cytoplasmic and periplasmic or external soluble compartments. For the biogenesis of integral membrane proteins, the essential biochemical steps are (1) the insertion and topogenesis of the transmembrane protein segments into the lipid bilayer, (2) the three-dimensional folding of the translocated hydrophilic domains, and (3) the assembly into multimeric complexes. Intensive research has elucidated the basic mechanisms of membrane protein insertion in the homologous translocation machineries of different cellular systems. Whereas the Sec translocation system is found in the endoplasmic reticulum of eukaryotic cells and in the prokaryotic plasma membrane, the YidC-Oxa1 membrane insertase is present in prokaryotic and organellar membranes. This review focuses on the discoveries of the YidC system in bacterial as well as the Oxa1/Alb3 protein family of eukaryotic cells and will particularly emphasize evolutionary aspects.

Phosphatidylethanolamine and monoglucosyldiacylglycerol are interchangeable in supporting topogenesis and function of the polytopic membrane protein lactose permease

To determine the specific role lipids play in membrane protein topogenesis in vivo, the orientation with respect to the membrane bilayer of Escherichia coli lactose permease (LacY) transmembrane (TM) domains and their flanking extramembrane domains was compared after assembly in native membranes and membranes with genetically modified lipid content using the substituted cysteine accessibility method for determining TM domain mapping. LacY assembled in the absence of the major membrane lipid phosphatidylethanolamine (PE) does not carry out uphill transport of substrate and displays an inverted orientation for the N-terminal six-TM domain helical bundle (Bogdanov, M., Heacock, P. N., and Dowhan, W. (2002) EMBO J. 21, 2107-2116). Strikingly, the replacement of PE in vivo by the foreign lipid monoglucosyldiacylglycerol (MGlcDAG), synthesized by the Acholeplasma laidlawii MGlcDAG synthase, restored uphill transport and supported the wild type TM topology of the N-terminal helical bundle of LacY. An interchangeable role in defining membrane protein TM domain orientation and supporting function is played by the two most abundant lipids, PE and MGlcDAG, in gram-negative and gram-positive bacteria, respectively. Therefore, these structurally diverse lipids endow the membrane with similar properties necessary for the proper organization of protein domains in LacY that are highly sensitive to lipids as topological determinants.

Proton-decoupled 15N and 31P solid-state NMR investigations of the Pf3 coat protein in oriented phospholipid bilayers

The coat proteins of filamentous phage are first synthesized as transmembrane proteins and then assembled onto the extruding viral particles. We investigated the transmembrane conformation of the Pseudomonas aeruginosa Pf3 phage coat protein using proton-decoupled 15N and 31P solid-state NMR spectroscopy. The protein was either biochemically purified and uniformly labelled with 15N or synthesized chemically and labelled at specific sites. The proteins were then reconstituted into oriented phospholipid bilayers and the resulting samples analysed. The data suggest a model in which the protein adopts a tilted helix with an angle of approximately 30 degrees and an N-terminal 'swinging arm' at the membrane surface.

The role of lipids in membrane insertion and translocation of bacterial proteins

Phospholipids are essential building blocks of membranes and maintain the membrane permeability barrier of cells and organelles. They provide not only the bilayer matrix in which the functional membrane proteins reside, but they also can play direct roles in many essential cellular processes. In this review, we give an overview of the lipid involvement in protein translocation across and insertion into the Escherichia coli inner membrane. We describe the key and general roles that lipids play in these processes in conjunction with the protein components involved. We focus on the Sec-mediated insertion of leader peptidase. We describe as well the more direct roles that lipids play in insertion of the small coat proteins Pf3 and M13. Finally, we focus on the role of lipids in membrane assembly of oligomeric membrane proteins, using the potassium channel KcsA as model protein. In all cases, the anionic lipids and lipids with small headgroups play important roles in either determining the efficiency of the insertion and assembly process or contributing to the directionality of the insertion process.

Diversity and versatility of lipid-protein interactions revealed by molecular genetic approaches

The diversity in structures and physical properties of lipids provides a wide variety of possible interactions with proteins that affect their assembly, organization, and function either at the surface of or within membranes. Because lipids have no catalytic activity, it has been challenging to define many of their precise functions in vivo in molecular terms. Those processes responsive to lipids are attuned to the native lipid environment for optimal function, but evidence that lipids with similar properties or even detergents can sometimes partially replace the natural lipid environment has led to uncertainty as to the requirement for specific lipids. The development of strains of microorganisms in which membrane lipid composition can be genetically manipulated in viable cells has provided a set of reagents to probe lipid functions. These mutants have uncovered previously unrecognized roles for lipids and provided in vivo verification for putative functions described in vitro. In this review, we summarize how these reagent strains have provided new insight into the function of lipids. The role of specific lipids in membrane protein folding and topological organization is reviewed. The evidence is summarized for the involvement of anionic lipid-enriched domains in the organization of amphitropic proteins on the membrane surface into molecular machines involved in DNA replication and cell division.

Membrane integration of E. coli model membrane proteins

The molecular events of membrane translocation and insertion have been investigated using a number of different model proteins. Each of these proteins has specific features that allow interaction with the membrane components which ensure that the proteins reach their specific local destination and final conformation. This review will give an overview on the best-characterized proteins studied in the bacterial system and emphasize the distinct aspects of the pathways.

Effect of the electrostatic charge on the mechanism inducing liposome solubilization: a kinetic study by synchrotron radiation SAXS

The anionic surfactant sodium dodecyl sulfate (SDS) was used to induce the initial steps of the solubilization of liposomes. The structural transformations as well as the kinetics associated with this initial period were studied by means of time-resolved small-angle X-ray scattering (SAXS) using a synchrotron radiation source. Neutral and electrically charged (anionic and cationic) liposomes were used to investigate the effect of the electrostatic charges on the kinetics of these initial steps. The mechanism that induces the solubilization process consisted of adsorption of surfactant on the bilayers and desorption of mixed micelles from the liposomes surface to the aqueous medium. In all cases the time needed for desorption of the first mixed micelles was shorter than that for complete adsorption of the surfactant on the liposomes surface. The present work demonstrates that adsorption of the SDS molecules on negatively charged liposomes was slower and release of mixed micelles from the surface of these liposomes was faster than for neutral liposomes. In contrast, in the case of positively charged liposomes, the adsorption and release processes were, respectively, faster and slower than those for neutral vesicles.

Escherichia coli YidC is a membrane insertase for Sec-independent proteins

YidC is a recently discovered bacterial membrane protein that is related to the mitochondrial Oxa1p and the Alb3 protein of chloroplasts. These proteins are required in the membrane integration process of newly synthesized proteins that do not require the classical Sec machinery. Here we demonstrate that YidC is sufficient for the membrane integration of a Sec-independent protein. Microgram amounts of the purified single-spanning Pf3 coat protein were efficiently inserted into proteoliposomes containing the purified YidC. A mutant Pf3 coat protein with an extended hydrophobic region was inserted independently of YidC into the membrane both in vivo and in vitro, but its insertion was accelerated by YidC. These results show that YidC can function separately from the Sec translocase to integrate membrane proteins into the lipid bilayer.

Reversible topological organization within a polytopic membrane protein is governed by a change in membrane phospholipid composition

Once inserted, transmembrane segments of polytopic membrane proteins are generally considered stably oriented due to the large free energy barrier to topological reorientation of adjacent extramembrane domains. However, the topology and function of the polytopic membrane protein lactose permease of Escherichia coli are dependent on the membrane phospholipid composition, revealing topological dynamics of transmembrane domains after stable membrane insertion (Bogdanov, M., Heacock, P. N., and Dowhan, W. (2002) EMBO J. 21, 2107-2116). In this study, we show that the high affinity phenylalanine permease PheP shares many similarities with lactose permease. PheP assembled in a mutant of E. coli lacking phosphatidylethanolamine (PE) exhibited significantly reduced active transport function and a complete inversion in topological orientation of the N terminus and adjoining transmembrane hairpin loop compared with PheP in a PE-containing strain. Introduction of PE following the assembly of PheP triggered a reorientation of the N terminus and adjacent hairpin to their native orientation associated with regain of wild-type transport function. The reversible orientation of these secondary transport proteins in response to a change in phospholipid composition might be a result of inherent conformational flexibility necessary for transport function or during protein assembly.

An alternative model of the twin arginine translocation system

The twin arginine translocation (Tat) system is a machinery which can translocate folded proteins across energy transducing membranes. Currently it is supposed that Tat substrates bind directly to Tat translocon components before a ApH-driven translocation occurs. In this review, an alternative model is presented which proposes that membrane integration could precede Tat-dependent translocation. This idea is mainly supported by the recent observations of Tat-independent membrane insertion of Tat substrates in vivo and in vitro. Membrane insertion may allow i) a quality control of the folded state by membrane bound proteases like FtsH, ii) the recognition of the membrane spanning signal peptide by Tat system components, and iii) a pulling mechanism of translocation. In some cases of folded Tat substrates, the membrane targeting process may require ATP-dependent N-terminal unfolding-steps.

The sensor protein KdpD inserts into the Escherichia coli membrane independent of the Sec translocase and YidC

KdpD is a sensor kinase protein in the inner membrane of Escherichia coli containing four transmembrane regions. The periplasmic loops connecting the transmembrane regions are intriguingly short and protease mapping allowed us to only follow the translocation of the second periplasmic loop. The results show that neither the Sec translocase nor the YidC protein are required for membrane insertion of the second loop of KdpD. To study the translocation of the first periplasmic loop a short HA epitope tag was genetically introduced into this region. The results show that also the first loop was translocated independently of YidC and the Sec translocase. We conclude that KdpD resembles a new class of membrane proteins that insert into the membrane without enzymatic assistance by the known translocases. When the second periplasmic loop was extended by an epitope tag to 27 amino acid residues, the membrane insertion of this loop of KdpD depended on SecE and YidC. To test whether the two periplasmic regions are translocated independently of each other, the KdpD protein was split between helix 2 and 3 into two approximately equal-sized fragments. Both constructed fragments, which contained KdpD-N (residues 1-448 of KdpD) and the KdpD-C (residues 444-894 of KdpD), readily inserted into the membrane. Similar to the epitope-tagged KdpD protein, only KdpD-C depended on the presence of the Sec translocase and YidC. This confirms that the four transmembrane helices of KdpD are inserted pairwise, each translocation event involving two transmembrane helices and a periplasmic loop.

Influence of lipids on membrane assembly and stability of the potassium channel KcsA

Recently we observed in an in vitro system that newly synthesized KcsA assembles efficiently into a tetramer in lipid vesicles [van Dalen et al. (2002) FEBS Lett. 511, 51-58]. Here we used this system to get insight into the importance of the lipid composition for KcsA membrane association and tetramerization and we compared this to the lipid dependency of the thermo-stability of the KcsA tetramer. It was found that a large amount of phosphatidylethanolamine (>40 mol%) and a lower amount of phosphatidylglycerol (approximately 20-30 mol%) were optimal for efficient KcsA membrane association and tetramerization. Strikingly, vesicles of the abundant and commonly used membrane lipid phosphatidylcholine did not support assembly, further demonstrating the importance of membrane lipid composition for KcsA assembly. The in vitro assembled KcsA tetramer showed similar thermo-stability in biological and pure lipid membranes, demonstrating that both tetramers are alike. In addition, we show that solubilization of the membrane with detergent reduces the thermo-stability of the tetramer. The highest KcsA tetramer stability was observed in intact bilayers in the presence of anionic lipids.

A polytopic membrane protein displays a reversible topology dependent on membrane lipid composition

To address the role of phospholipids in the topological organization of polytopic membrane proteins, the function and assembly of lactose permease (LacY) was studied in mutants of Escherichia coli lacking phosphatidylethanolamine (PE). PE is required for the proper conformation and active transport function of LacY. The N-terminal half of LacY assembled in PE-lacking cells adopts an inverted topology in which normally non-translocated domains are translocated and vice versa. Post-assembly synthesis of PE triggers a conformational change, resulting in a lipid-dependent recovery of normal conformation and topology of at least one LacY subdomain accompanied by restoration of active transport. These results demonstrate that membrane protein topology once attained can be changed in a reversible manner in response to alterations in phospholipid composition, and may be subject to post-assembly proofreading to correct misfolded structures.

Insertion and glycosylation of Pf3-derived membrane proteins in microsomes

To get insight into the insertion mechanism of small newly synthesized single-spanning membrane proteins, Pf3 coat protein mutants were constructed with potential glycosylation sites in the N-terminus. Some of these proteins, when synthesized in vitro in the presence of microsomes, became efficiently glycosylated, proving that they insert into the membrane and translocate their N-terminus to the lumenal side. Such Pf3 constructs also insert efficiently into Escherichia coli vesicles and even in pure lipid vesicles, suggesting a common mechanism, which might be spontaneous. Glycosylation was sensitive to changes in the amino acid sequence of the N-terminus, suggesting that it depends on the structure of the protein and/or its positioning with respect to the lipid-water interface.