No description available.

Wir untersuchen, welchen Einfluss die Verankerung von langen, hydrophilen Polymeren in Lipidmembranen auf deren elastische Eigenschaften ausübt. Theoretisch werden zwei Grenzbereiche für die spontane Krümmung der Membran erwartet: <br /> i) bei kleinen Oberflächenkonzentrationen des Polymers (Pilzgebiet) sollte die spontane Krümmung linear von der Oberflächendichte des verankerten Polymers abhängen;<br /> ii) bei hoher Bedeckung (Bürstengebiet) sollte die Abhängigkeit quadratisch sein. Wir versuchen, Vorhersagen für das Bürstengebiet zu prüfen, indem wir die morphologischen Veränderungen beobachten, die bei Riesen (Giant)- Vesikeln hervorgerufen werden. <br /> <br /> Als lange Polymere verwenden wir fluoreszenzmarkierte &#955;-Phage DNA Moleküle, die durch eine Biotin-Avidin-Biotin Verbindung an biotinhaltigen Lipidvesikeln befestigt sind. Wir kontrollieren die Oberflächenkonzentration der Anker durch Variation der Menge an biotinhaltigem Lipid in der Membran. Die Menge der an der Membran verankerten DNA wird durch Fluoreszenzmessungen quantifiziert. Änderungen in den elastischen Eigenschaften der Membran bei Anbindung der DNA, werden über eine Analyse der Vesikel-Fluktuationen kontrolliert. Die spontane Krümmung der Membran steigt mit der Oberflächenbeladung. Bei höheren Verankerungen bilden die Vesikel Knospen (budding). Die Größe der Knospen kann ebenfalls zur Bestimmung der Krümmung der Membran verwendet werden. Der Einfluss auf die Biegesteifigkeit ist Thema weiterer Untersuchungen. / We study the effect on the elastic properties of lipid membranes induced by anchoring of long hydrophilic polymers. Theoretically, two limiting regimes for the membrane spontaneous curvature are expected : <br /> i) at low surface polymer concentration (mushroom regime) the spontaneous curvature should scale linearly with the surface density of anchored polymers; <br /> ii) at high coverage (brush regime) the dependence should be quadratic. We attempt to test the predictions for the brush regime by monitoring the morphological changes induced on giant vesicles.<br /> <br /> As long polymers we use fluorescently labeled &#955;-phage DNA molecules which are attached to biotinylated lipid vesicles with a biotin-avidin-biotin linkage. By varying the amount of biotinylated lipid in the membrane we control the surface concentration of the anchors. The amount of anchored DNA to the membrane is quantified with fluorescence measurements. Changes in the elastic properties of the membrane as DNA grafts to it are monitored via analysis of the vesicle fluctuations. The spontaneous curvature of the membrane increases as a function of the surface coverage. At higher grafting concentrations the vesicles bud. The size of the buds can also be used to assess the membrane curvature. The effect on the bending stiffness is a subject of further investigation.

No description available.

Eukaryotic cells are characterised by membranes with varied and dynamic compositions and shapes. Consequently, membrane-binding proteins are tuned to recognise and modify these membrane states to perform their functions. To study the curvature sensitivity of proteins, I have developed a single-particle assay using NanoSight technology that tracks the Brownian motion of particles to measure their size. I optimised this system to track fluorescently labelled lipid-binding domains bound to liposomes of different sizes moving freely in solution. The comparison of the size distribution of the total liposomes with the fluorescently labelled population allowed me to determine their curvature preferences. To validate the method I tested proteins from the Bin/Amphiphysin/Rvs (BAR) superfamily, which are inherently curved and have known curvature preferences. My method was capable of recapitulating the behaviour of BAR domains with different curvature preferences. I then expanded the range of targets and showed that this assay is also capable of detecting curvature preferences for a variety of other lipid-binding domain families. As such, I identified AKT PH domain as a new curvature-sensing domain. Finally, using the ENTH domain of Epsin1 that causes vesicle budding, I demonstrated that this method can also be used to study membrane remodelling. Trafficking involves generation and sensing of membrane curvature combined with recognition of specific cargo. Endophilin consists of a curvature-sensitive BAR domain followed by an SH3 (Src-homology 3) domain and has recently been identified in a clathrin-independent endocytosis pathway, FEME (fast endophilin-mediated endocytosis), involved in the uptake of cell surface receptors. Endophilin recognises ligands via its SH3 domain, binding G-protein coupled receptors (GPCRs) directly in their intracellular loop 3 and receptor tyrosine kinase (RTKs) via adaptor proteins. However, a specific recognition motif has not been identified yet. Here, using a combination of biophysical approaches and NMR spectroscopy, I characterised the Endophilin binding motif of ALIX (ALG-2-interacting protein X) adaptor protein and of the GPCR $\alpha$2A adrenergic receptor. Comparison of SH3-peptide models resulted in a putative Endophilin recognition site.

Monotopic proteins represent a specialized group of membrane proteins in that they are engaged in biochemical events taking place at the membrane interface. In particular, the monotopic lipid-synthesizing enzymes are able to synthesize amphiphilic lipid products by catalyzing two biochemically distinct molecules (substrates) at the membrane interface. Thus, from an evolutionary point of view, anchoring into the membrane interface enables monotopic enzymes to confer sensitivity to a changing environment by regulating their activities in the lipid biosynthetic pathways in order to maintain a certain membrane homeostasis. We are focused on a plant lipid-synthesizing enzyme DGD2 involved in phosphate shortage stress, and analyzed the potentially important lipid anchoring segments of it, by a set of biochemical and biophysical approaches. A mechanism was proposed to explain how DGD2 adjusts its activity to maintain a proper membrane. In addition, a multivariate-based bioinformatics approach was used to predict the lipid-binding segments for GT-B fold monotopic enzymes. In contrast, a soluble protein Myr1 from yeast, implicated in vesicular traffic, was also proposed to be a membrane stress sensor as it is able to exert different binding properties to stressed membranes, which is probably due to the presence of strongly plus-charged clusters in the protein. Moreover, a bacterial monotopic enzyme MGS was found to be able to induce massive amounts of intracellular vesicles in Escherichia coli cells. The mechanisms involve several steps: binding, bilayer lateral expansion, stimulation of lipid synthesis, and membrane bending. Proteolytic and mutant studies indicate that plus-charged residues and the scaffold-like structure of MGS are crucial for the vesiculation process. Hence, a number of features are involved governing the behaviour of monotopic membrane proteins at the lipid bilayer interface. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 5: Manuscript.

La communication entre les différentes organelles se fait par l’intermédiaire du trafic vésiculaire, un processus qui nécessite un remodelage continu des membranes. Les vésicules fortement courbées bourgeonnent d'un compartiment donneur et fusionnent avec un compartiment accepteur. Les protéines impliquées dans le bourgeonnement et fusion des vésicules ont été largement étudiées. Récemment, la découverte de détecteurs de courbure membranaire a révélé que le trafic membranaire pourrait être régulé à un niveau supplémentaire, par la détection de la forme de la membrane. Le premier détecteur de courbure membranaire identifié était le motif ALPS (Amphipathic Lipid Packing Sensor), qui a été trouvé dans un certain nombre de protéines de la voie sécrétoire précoce et l'enveloppe nucléaire. La protéine d’arrimage GMAP-210 localisé au niveau du cis-Golgi, est composée d’une longue superhélice (coiled-coil) et d’un motif ALPS à l'extrême N-terminale. Il a été démontré in vitro, que ce motif se replie et forme une hélice amphipathique capable de se fixer sur des petits liposomes. Toutefois, l'identité des vésicules, reconnues par ce détecteur de courbure dans la cellule, reste inconnue. α-Synucléine est une autre protéine qui se lie préférentiellement à des membranes très courbées. Cette protéine localisée sur les vésicules synaptiques, est impliquée dans la régulation du taux de vésicules au niveau des terminaisons nerveuses pré-synaptiques. Connue pour son rôle central dans le développement de la maladie de Parkinson, α-synucléine contient une région non structurée en solution, mais qui forme une hélice amphipathique au contact de petits liposomes in vitro. Les hélices amphipathiques formées par le motif ALPS et α-synucléine sont très différentes aussi bien sur le plan chimique que sur le plan conformationel. Le motif ALPS possède une face hydrophobe bien développée, mais un coté polair pauvre avec très peu de résidus chargés. α-Synucléine, en revanche, a un côté hydrophobe modéré, et une face polaire zwitterionique riche en résidus chargés. L'objectif principal du projet était de comparer les propriétés de liaison aux membranaires in vivo et in vitro de ces deux hélices amphipathiques de structure opposée. L’expression de ces deux sondes chez la levure, favorise l'accumulation de structures vésiculaires de propriétés différentes. L'extrémité N-terminale de la protéine GMAP-210 contenant son motif ALPS (GMAPN) co-localisé spécifiquement avec des marqueurs de la voie sécrétoire précoce, alors une sonde contenant une portion de l’hélice amphipathique d’α-synucléine co-localise avec des marqueurs endocytiques et post-Golgiens. La mutagenèse du motif ALPS et l'inversion de la séquence de ALPS dans GMAPN confirment que ce détecteur de courbure membranaire se fixe spécifiquement aux vésicules via des interactions directes protéines-lipides, plutôt que les interactions protéines-protéines. Notre analyse a montré que ces détecteurs de courbure mammifères, exprimés dans la levure préservent leur capacité à cibler des vésicules spécifiques, vésicules de la voie sécrétoire précoce pour les motifs ALPS, et vésicules d’endocytose/post-Golgi pour α-synucléine. La composition membranaire de ces vésicules correspond à la composition des liposomes fixés par le motif ALPS et α-synucléine in vitro. Les propriétés biochimiques opposées du motif ALPS et α-synucléine, sont parfaitement adaptés à chacun de ces deux environnements membranaires dans la cellule. Le programme HeliQuest est conçu pour identifier des hélices amphipathiques capables de se lier sur les membranes, y compris les motifs ALPS. Un nouveau module conçu pour identifier les hélices amphipathiques avec des propriétés similaires à α-synucléine a été récemment élaboré. Les recherches effectuées dans les bases de données de protéines de levure et humaines ont permis d’identifier des hélices amphipathiques candidats qui ont des propriétés similaires à α-synucléine, dans de nombreuses protéines. Nous avons préparé un ensemble de sondes, dans lequel ces hélices sont insérées à la fin de la superhélice de GMAPN. Une première étude de leur co-localisation dans les cellules de levure avec un ensemble de marqueurs démontre une localisation spécifique, ce qui suggère que ces hélices peuvent avoir la capacité de cibler des membranes de manière spécifique. D'autres travaux seraient nécessaires pour confirmer ou pas si ces hélices amphipathiques font partie d'une nouvelle classe de détecteurs de courbure ayant les mêmes propriétés que α-synucléine. / Communication between membrane-bound organelles is mediated by vesicular trafficking, a process which requires continual membrane remodeling. Highly curved vesicles bud from a donor compartment through functioning of different coat protein complexes, and fuse with an acceptor compartment thanks to proteins of the membrane fusion machinery. The proteins involved in vesicle budding and fusion have been extensively studied. Recently, the discovery of membrane curvature sensors revealed that membrane trafficking could be regulated at an additional level, through detection of the shape of a membrane. The first membrane curvature sensor identified was the ALPS (Amphipathic Lipid Packing Sensor) motif, which has been found in a number of proteins that function in the early secretory pathway and nuclear envelope. One example is GMAP-210, a long coiled-coil tether localizing to cis-Golgi membranes, which has an ALPS motif at its extreme N-terminus. This ALPS motif was found to fold into an amphipathic helix and bind to small liposomes in vitro. However, the identity of the vesicles that this curvature sensor binds to in cells is not known. Another protein - α-synuclein - has also been reported to bind preferentially to highly curved membranes. This neuronal protein localizes to synaptic vesicles and is involved in maintaining the reserve pool of vesicles in pre-synaptic nerve terminals. α-Synuclein, known for its central role in the development of Parkinson’s disease, contains a region that is unstructured in solution, but forms an amphipathic helix upon binding to small liposomes in vitro. The chemistry and geometry of the amphipathic helices formed by ALPS motifs and α-synuclein are very different. The ALPS motif has a well-developed hydrophobic face but a poor polar side with few charged residues. α-Synuclein, in contrast, has a restrained hydrophobic side, and a zwitterionic polar face rich in charged residues. The main goal of the project was to compare the in vivo and in vitro membrane binding properties of these two amphipathic helices of opposite structure. When expressed in yeast cells, these two curvature sensors promoted the accumulation of vesicular structures possessing different characteristics. The N-terminus of GMAP-210 containing its ALPS motif (GMAPN) co-localized specifically with early secretory pathway markers, whereas a probe containing a portion of the amphipathic membrane-binding helix of α-synuclein co-localized with endocytic and post-Golgi markers. Mutagenesis of the ALPS motif and the inversion of the ALPS sequence in GMAPN support the conclusion that this membrane curvature sensor is targeted to specific vesicles in cells through direct protein-lipid, rather than protein-protein interactions. Our analysis has shown, remarkably, that mammalian curvature sensors expressed in yeast cells preserve their capacity to target specific vesicles, those of the early secretory pathway for ALPS motifs, and endocytic/post-Golgi vesicles for α-synuclein. The membrane composition of these vesicles corresponds to the preferred in vitro liposome binding properties of these membrane curvature sensors. The contrasting chemistries of ALPS motifs and α-synuclein are well adapted to each of these two major membrane environments in the cell. The HeliQuest algorithm is designed to search databases for membrane-binding amphipathic helices, including ALPS motifs. A new module designed to identify amphipathic helices with properties similar to α-synuclein has recently been developed. Searches of both yeast and human protein databases has identified candidate α-synuclein-like amphipathic helices in numerous proteins. We prepared a set of probes, in which these helices are displayed at the end of the GMAPN coiled-coil. An initial study of their co-localization in yeast cells with a set of organelle markers demonstrates specific localization patterns, suggesting that these helices may have specific membrane targeting capacities. Further work will explore the question of whether these amphipathic helices are part of a novel class of α-synuclein-like curvature sensors.

Endozytose erfordert hohe Membrankrümmung und führt zu Flächenänderungen der Membranhälften. Dies kann durch eine Oberflächendifferenz zwischen den Schichten initiiert werden, die durch geänderte Lipidzusammensetzungen hervorgerufen werden kann. Daher wurde die Hypothese aufgestellt, dass Lipid-Transporter zu Beginn der Endozytose für veränderte Flächenverhältnisse verantwortlich sind. Um den Einfluss veränderter Flächen auf Endozytose zu untersuchen, wurden die Oberflächenverhältnisse der Membran durch Zugabe von Phospholipiden verändert und anschließend Endozytose gemessen. Abhängig von der Sorte wurden die Lipide nur in die äußere Schicht eingebaut oder auch auf die innere Seite transportiert, wodurch die entsprechende Seite vergrößert wurde. Die Zugabe verschiedener Aminophospholipide, die auf die innere Membranseite transportiert werden, führte zu gesteigerter „bulk flow“ Endocytose in K562-Zellen. Darüber hinaus deuten die Ergebnisse darauf hin, dass Clathrin-vermittelte Endozytose von Hep2-Zellen ebenfalls stimuliert wurde. Umgekehrt hatte die Zugabe von Lipiden, die auf der äußeren Hälfte bleiben, reduzierte „bulk flow“- oder Clathrin-vermittelte Endozytose in verschiedenen Zelllinien zur Folge. Bemerkenswert ist, dass auch Clathrin-vermittelte Endozytose durch die Lipidzugabe beeinflusst wurde, obwohl gerade in diesem Weg viele Proteine involviert sind, die Krümmung induzieren können. Dies passt zu einem neuen Modell wie Lipidtransporter in Endozytose involviert sind. Durch den Transport von Lipiden und die zusätzliche Interaktion mit Endozytoseproteinen, könnten diese Transporter zwei Mechanismen zur Erzeugung von Krümmung miteinander verbinden: Membrankrümmung induziert durch eine Flächenasymmetrie zwischen den Membranhälften und durch Wechselwirkung mit Proteinen. Die Ergebnisse dieser Arbeit deuten darauf hin, dass die für Endozytose notwendige Krümmung durch die durch Lipidtransport induzierte Flächenasymmetrie der Membranschichten unterstützt wird. / Endocytic engulfment requires high local membrane curvature and causes significant area changes of the membrane leaflets. This can be initiated by differences between the surface areas of the two monolayers related to leaflet specific modulation of lipid composition. Thus, it was proposed that lipid translocators, pumping phospholipids from the outer to the inner leaflet, account for monolayer area asymmetry as an early step in endocytosis. To elucidate the influence of this asymmetry on endocytosis, surface area relation was altered by adding exogenous phospholipids to living cells and changes in endocytic activity were quantified. Depending on the lipid species, exogenous lipids were only incorporated into the outer layer or subsequently translocated across the plasma membrane thereby increasing either the outer or inner surface area. Addition of different analogues of aminophospholipids, which are translocated to the inner leaflet, led to an enhancement of bulk flow endocytosis in K562 cells. Moreover, our data indicate that clathrin-mediated endocytosis of Hep2 cells was stimulated as well. Inversely, addition of phospholipids, which remain on the outer layer, reduced bulk flow or clathrin-mediated endocytosis in various cell lines. Notably, also clathrin-mediated endocytosis was influenced by the addition of lipids, although many proteins noted for their ability to induce membrane curvature are known to be implicated in this pathway. This corroborates a recent model how aminophospholipid translocases are implicated in endocytosis. Upon translocating lipids and additionally interacting with endocytic accessory proteins, lipid translocators could integrate two processes to generate curvature: membrane bending based on monolayer area asymmetry and protein-related mechanisms. Collectively, findings in the present study suggest that curvature generation in endocytosis is supported by the induction of monolayer area asymmetry mediated by the translocation of lipids.

PIP3 dynamics observed in membranes are responsible for the protruding edge formation in cancer and amoeboid cells. The mechanisms that maintain those PIP3 domains in three-dimensional space remain elusive, due to limitations in observation and analysis techniques. Recently, a strong relation between the cell geometry, the spatial confinement of the membrane, and the excitable signal transduction system has been revealed by Hörning and Shibata (2019) using a novel 3D spatiotemporal analysis methodology that enables the study of membrane signaling on the entire membrane (Hörning and Shibata, 2019). Here, using 3D spatial fluctuation and phase map analysis on actin polymerization inhibited Dictyostelium cells, we reveal a spatial asymmetry of PIP3 signaling on the membrane that is mediated by the contact perimeter of the plasma membrane—the spatial boundary around the cell-substrate adhered area on the plasma membrane. We show that the contact perimeter guides PIP3 waves and acts as a pinning site of PIP3 phase singularities, that is, the center point of spiral waves. The contact perimeter serves as a diffusion influencing boundary that is regulated by a cell size- and shape-dependent curvature. Our findings suggest an underlying mechanism that explains how local curvature can favor actin polymerization when PIP3 domains get pinned at the curved protrusive membrane edges in amoeboid cells.

Das Amyloid-beta Peptid (Ab) ist der Hauptbestandteil der extrazellulären Plaques bei der Alzheimerschen Krankheit. Das Ziel der vorliegenden Arbeit ist es, die Mechanismen der Wechselwirkungen des Ab mit der Plasmamembran und der nachfolgenden zellulären Aufnahme aufzuklären. Die Aggregation, die zelluläre Aufnahme und die Zytotoxizität von Ab42 wurden durch Verwendung von fluoreszenzmarkierten Ab42 in einem Neuroblastomzellkulturmodell untersucht. Sowohl bei Inkubation mit Monomeren als auch mit Aggregaten wurde in den Zellen Ab42 detektiert. Dabei binden Ab42 Monomere und kleine Aggregate zunächst an die Zellmembran. Allerdings erfolgt keine direkte Aufnahme von Monomeren in die Zelle. Erst nach Ausbildung von Aggregaten mit geordneter Sekundärstruktur wurde Ab42 in den endozytotischen Vesikel detektiert. Voraussetzung für den an der Membran ablaufenden Aggregationsprozess ist, dass die Monomere oberhalb einer kritischen Konzentration anwesend sind, um eine Bildung von beta-Faltblatt-Strukturen (bF) und entsprechenden Aggregaten zu ermöglichen. Ab42 Aggregate, die sich durch eine bF auszeichneten, benötigten keine kritische Schwellenkonzentration für die endozytotische Aufnahme. Eng mit der Aufnahme von Ab42 Aggregaten war die Veränderung des zellulären Metabolismus verbunden. Um die Wechselwirkung zwischen Ab und der Membrannäher zu charakterisieren, wurden Modellmembransystemen einschl. riesigen Membranvesikeln genutzt. Dabei wurde beobachtet, dass sowohl Ab42 als auch Ab40 Einstülpungen in der Membran induzieren können. Kleine Aggregate beider Isoformen, die noch keine bF aufweisen, interagierten bevorzugt mit der ungeordneten Lipidphase und induzierten dabei eine negative Membrankrümmung. Diese Beobachtungen legen den Schluss nahe, dass möglicherweise das Ab selbst den endozytotischen Prozess unterstützt oder diesen sogar einleiten könnte. Dies könnte auch auf eine mögliche physiologische Funktion von Ab Aggregaten, die nicht toxisch sind, hindeuten. / The accumulation of Amyloid beta peptide 1-42 (Ab42) in extracellular plaques is one of the pathological hallmarks of Alzheimer’s disease. Several studies have suggested that a cellular reuptake of Ab42 may be a crucial step in its cytotoxicity, but mechanisms of Ab-membrane interaction and subsequent cellular uptake are not yet understood. The first aim of the present study is to answer the question whether aggregate formation is a prerequisite or a consequence of Ab-membrane interaction and of Ab endocytosis. We visualized aggregate formation of fluorescently labeled Ab42 by Förster resonance energy transfer and tracked its internalization by human neuroblastoma cells. Both monomeric and aggregated Ab42 entered the cells, however, monomer uptake faced a concentration threshold and occurred only at concentrations and time scales that allowed beta-sheet-rich (bS) aggregates to form. By uncoupling membrane binding from internalization, we found that Ab42 monomers as well as small aggregate species bound rapidly to the plasma membrane and formed bS aggregates. These structures were subsequently taken up and accumulated in endocytic vesicles. This process correlated with inhibition of cellular metabolism activities. Our data therefore imply that the formation of bS aggregates at the cell membrane is a prerequisite for Ab42 uptake and cytotoxicity. The second aim of the study is to investigate the Ab-membrane interaction in vitro by using giant unilamellar vesicles and giant plasma membrane vesicles as model membrane systems. We found that both Ab isoforms, Ab42 and Ab40, interacted with the liquid disordered phase of model membranes. Early aggregation intermediates, which did not yet bind to the amyloiddophilic dye Thioflavin T, induced negative membrane curvature. The ability of Ab to induce membrane deformation suggests that Ab may facilitate its own endocytosis. It also hints at a possible physiological function of non-toxic Ab aggregate species.

The interplay between lipids and membrane proteins is known to affect membrane protein topology and thus have significant effect (control) on their functions. In this PhD thesis, the influence of lipids on the membrane protein function was studied using three different membrane protein models. A monotopic membrane protein, monoglucosyldiacylglyecerol synthase (MGS) from Acholeplasma laidlawii is known to induce intracellular vesicles when expressed in Escherichia coli. The mechanism leading to this unusual phenomenon was investigated by various biochemical and biophysical techniques. The results indicated a doubling of lipid synthesis in the cell, which was triggered by the selective binding of MGS to anionic lipids. Multivariate data analysis revealed a good correlation with MGS production. Furthermore, preferential anionic lipid sequestering by MGS was shown to induce a different fatty acid modeling of E. coli membranes. The roles of specific lipid binding and the probable mechanism leading to intracellular vesicle formation were also investigated. As a second model, a MGS homolog from Synechocystis sp. PCC6803 was selected. MgdA is an integral membrane protein with multiple transmembrane helices and a unique membrane topology. The influence of different type of lipids on MgdA activity was tested with different membrane fractions of Synechocystis. Results indicated a very distinct profile compared to Acholeplasma laidlawii MGS. SQDG, an anionic lipid was found to be the species of the membrane that increased the MgdA activity 7-fold whereas two other lipids (PG and PE) had only minor effects on MgdA. Additionally, a working model of MgdA for the biosynthesis and flow of sugar lipids between Synechocystis membranes was proposed. The last model system was another integral membrane protein with a distinct structure but also a different function. The envelope stress sensor, CpxA and its interaction with E. coli membranes were studied. CpxA autophosphorylation activity was found to be positively regulated by phosphatidylethanolamine and negatively by anionic lipids. In contrast, phosphorylation of CpxR by CpxA revealed to be increased with PG but inhibited by CL. Non-bilayer lipids had a negative impact on CpxA phosphotransfer activity. Taken together, these studies provide a better understanding of the significance of the interplay of lipids and model membrane proteins discussed here.