Insertion of lipidated Ras proteins into lipid monolayers studied by infrared reflection absorption spectroscopy (IRRAS)

Interfacial behavior of Proteinase K enzyme at air-saline subphase

This study investigates the interfacial behavior of the proteinase K enzyme at air-water interface. Adsorption of enzyme on the surface was induced using saline subphase. The surface packing and stability of the enzyme was investigated using of surface pressure-area (π-A) and surface potential-area (ΔV-A) isotherms. Proteinase K enzyme forms film at air-aqueous interface and demonstrates good stability as shown through compression-decompression cycle experiments. To characterize the surface assembly morphology of the interfacial enzymes UV-vis and fluorescence spectroscopic techniques were used. The data revealed that the enzyme Langmuir monolayer has good homogeneity with no evidence of aggregates during compression. The secondary structure of the enzyme at interface was determined to be α-helix using p-polarized infrared-reflection absorption spectroscopy. This was confirmed through Circular dichroism spectra of the enzyme Langmuir-Blodgett (LB) film which showed that the major conformation present were α-helices.

Chemical approaches for investigating site-specific protein S-fatty acylation

Protein S-fatty acylation or S-palmitoylation is a reversible and regulated lipid post-translational modification (PTM) in eukaryotes. Loss-of-function mutagenesis studies have suggested important roles for protein S-fatty acylation in many fundamental biological pathways in development, neurobiology, and immunity that are also associated with human diseases. However, the hydrophobicity and reversibility of this PTM have made site-specific gain-of-function studies more challenging to investigate. In this review, we summarize recent chemical biology approaches and methods that have enabled site-specific gain-of-function studies of protein S-fatty acylation and the investigation of the mechanisms and significance of this PTM in eukaryotic biology.

Measuring protein insertion areas in lipid monolayers by fluorescence correlation spectroscopy

Membrane insertion of protein domains is an important step in many membrane remodeling processes, for example, in vesicular transport. The membrane area taken up by the protein insertion influences the protein binding affinity as well as the mechanical stress induced in the membrane and thereby its curvature. To our knowledge, this is the first optical measurement of this quantity on a system in equilibrium with direct determination of the number of inserted protein and no further assumptions concerning the binding thermodynamics. Whereas macroscopic total area changes in lipid monolayers are typically measured on a Langmuir film balance, finding the number of inserted proteins without perturbing the system and quantitating any small area changes has posed a challenge. Here, we address both issues by performing two-color fluorescence correlation spectroscopy directly on the monolayer. With a fraction of the protein being fluorescently labeled, the number of inserted proteins is determined in situ without resorting to invasive techniques such as collecting the monolayer by aspiration. The second color channel is exploited to monitor a small fraction of labeled lipids to determine the total area increase. Here, we use this method to determine the insertion area per molecule of Sar1, a protein of the COPII complex, which is involved in transport vesicle formation. Sar1 has an N-terminal amphipathic helix, which is responsible for membrane binding and curvature generation. An insertion area of (3.4 ± 0.8) nm² was obtained for Sar1 in monolayers from a lipid mixture typically used in COPII reconstitution experiments, in good agreement with the expected insertion area of the Sar1 amphipathic helix. By using the two-color approach, determining insertion areas relies only on local fluorescence measurements. No macroscopic area measurements are needed, giving the method the potential to also be applied to laterally heterogeneous monolayers and bilayers.

Kinetics of interaction between antimicrobial peptide nisin and Langmuir monolayers of DPPC and DPPG molecules

We have studied the kinetics of the interaction between antimicrobial peptide nisin and Langmuir monolayers of phospholipids DPPC and DPPG at the air-water interface using the surface manometry technique. The charge on the nisin and the lipid molecules is controlled by varying the pH of the subphase, and the interactions between them are studied by measuring the surface pressure of the lipid monolayer as a function of time after injecting the nisin in the subphase. A model based on the diffusion of particles under the influence of a constant force is developed to obtain an analytical expression for surface pressure as a function of time. The expression was found to fit well with the experimental data. The average hydrodynamic radius and the translational diffusion constant of the nisin molecules are calculated from the fit parameters for the different subphase pH solutions.

Surfactant Charge Modulates Structure and Stability of Lipase-Embedded Monolayers at Marine-Relevant Aerosol Surfaces

Lipases, as well as other enzymes, are present and active within the sea surface microlayer (SSML). Upon bubble bursting, lipases partition into sea spray aerosol (SSA) along with surface-active molecules such as lipids. Lipases are likely to be embedded in the lipid monolayer at the SSA surface and thus have the potential to influence SSA interfacial structure and chemistry. Elucidating the structure of the lipid monolayer at SSA interfaces and how this structure is altered upon interaction with a protein system like lipase is of interest, given the importance of how aerosols interact with sunlight, influence cloud formation, and provide surfaces for chemical reactions. Herein, we report an integrated experimental and computational study of Burkholderia cepacia lipase (BCL) embedded in a lipid monolayer and highlight the important role of electrostatic, rather than hydrophobic, interactions as a driver for monolayer stability. Specifically, we combine Langmuir film experiments and molecular dynamics (MD) simulations to examine the detailed interactions between the zwitterionic dipalmitoylphosphatidylcholine (DPPC) monolayer and BCL. Upon insertion of BCL from the underlying subphase into the lipid monolayer, it is shown that BCL permeates and largely disorders the monolayer while strongly interacting with zwitterionic DPPC molecules, as experimentally observed by Langmuir adsorption curves and infrared reflectance absorbance spectroscopy. Explicitly solvated, all-atom MD is then used to provide insights into inter- and intramolecular interactions that drive these observations, with specific attention to the formation of salt bridges or ionic-bonding interactions. We show that after insertion into the DPPC monolayer, lipase is maintained at high surface pressures and in large BCL concentrations by forming a salt-bridge-stabilized lipase-DPPC complex. In comparison, when embedded in an anionic monolayer at low surface pressures, BCL preferentially forms intramolecular salt bridges, reducing its total favorable interactions with the surfactant and partitioning out of the monolayer shortly after injection. Overall, this study shows that the structure and dynamics of lipase-embedded SSA surfaces vary based on surface charge and pressure and that these variations have the potential to differentially modulate the properties of marine aerosols.

Binding of the GTPase Sar1 to a Lipid Membrane Monolayer: Insertion and Orientation Studied by Infrared Reflection⁻Absorption Spectroscopy

Membrane-interacting proteins are polyphilic polymers that engage in dynamic protein–protein and protein–lipid interactions while undergoing changes in conformation, orientation and binding interfaces. Predicting the sites of interactions between such polypeptides and phospholipid membranes is still a challenge. One example is the small eukaryotic GTPase Sar1, which functions in phospholipid bilayer remodeling and vesicle formation as part of the multimeric coat protein complex (COPII). The membrane interaction of Sar1 is strongly dependent on its N-terminal 23 amino acids. By monolayer adsorption experiments and infrared reflection-absorption spectroscopy (IRRAS), we elucidate the role of lipids in inducing the amphipathicity of this N-terminal stretch, which inserts into the monolayer as an amphipathic helix (AH). The AH inserting angle is determined and is consistent with the philicities and spatial distribution of the amino acid monomers. Using an advanced method of IRRAS data evaluation, the orientation of Sar1 with respect to the lipid layer prior to the recruitment of further COPII proteins is determined. The result indicates that only a slight reorientation of the membrane-bound Sar1 is needed to allow coat assembly. The time-course of the IRRAS analysis corroborates a role of slow GTP hydrolysis in Sar1 desorption from the membrane.

Characterization of Surface-Active Biofilm Protein BslA in Self-Assembling Langmuir Monolayer at the Air-Water Interface

Biofilm is an extracellular matrix of bacteria and serves as a protective shield of bacterial communities. It is crucial for microbial growth and one of the leading causes of human chronic infections as well. However, the structures and molecular mechanism of biofilm formation remain largely unknown. Here, we examined a protein, BslA, expressed in the biofilms of Bacillus subtilis. We characterized the Langmuir monolayers of BslA at the air/water interface. Using techniques in surface chemistry and spectroscopy, we found that BslA forms a stable and robust Langmuir monolayer at the air/water interface. Our results show that the BslA Langmuir monolayer underwent two-stage elasticity in the solid state phase upon mechanical compression: one is possibly due to the intermolecular interaction and the other is likely due to both the intermolecular compulsion and the intramolecular distortion. The Langmuir monolayer of BslA shows abrupt changes in rigidities and elasticities at ∼25 mN/m. This surface pressure is close to the one at which BlsA saturates the air/water interface as a self-assembled film without mechanical compression, corresponding to a mean molecular area of ∼700 Ų per molecule. Based on the results of surface UV-visible spectroscopy and infrared reflective-absorption spectroscopy, we propose that the BslA Langmuir monolayer carries intermolecular elasticity before ∼25 mN/m and both intermolecular and intramolecular elasticity after ∼25 mN/m. These results provide valuable insights into the understanding of biofilm-associated protein under high mechanical force, shedding light on further investigation of biofilm structure and functionalities.

How to gather useful and valuable information from protein binding measurements using Langmuir lipid monolayers

This review presents data on the influence of various experimental parameters on the binding of proteins onto Langmuir lipid monolayers. The users of the Langmuir methodology are often unaware of the importance of choosing appropriate experimental conditions to validate the data acquired with this method. The protein Retinitis pigmentosa 2 (RP2) has been used throughout this review to illustrate the influence of these experimental parameters on the data gathered with Langmuir monolayers. The methods detailed in this review include the determination of protein binding parameters from the measurement of adsorption isotherms, infrared spectra of the protein in solution and in monolayers, ellipsometric isotherms and fluorescence micrographs.

EHD2 restrains dynamics of caveolae by an ATP-dependent, membrane-bound, open conformation

The EH-domain-containing protein 2 (EHD2) is a dynamin-related ATPase that confines caveolae to the cell surface by restricting the scission and subsequent endocytosis of these membrane pits. For this, EHD2 is thought to first bind to the membrane, then to oligomerize, and finally to detach, in a stringently regulated mechanistic cycle. It is still unclear how ATP is used in this process and whether membrane binding is coupled to conformational changes in the protein. Here, we show that the regulatory N-terminal residues and the EH domain keep the EHD2 dimer in an autoinhibited conformation in solution. By significantly advancing the use of infrared reflection-absorption spectroscopy, we demonstrate that EHD2 adopts an open conformation by tilting the helical domains upon membrane binding. We show that ATP binding enables partial insertion of EHD2 into the membrane, where G-domain-mediated oligomerization occurs. ATP hydrolysis is related to detachment of EHD2 from the membrane. Finally, we demonstrate that the regulation of EHD2 oligomerization in a membrane-bound state is crucial to restrict caveolae dynamics in cells.

Binding of Short Cationic Peptides (KX)4K to Negatively Charged DPPG Monolayers: Competition between Electrostatic and Hydrophobic Interactions

The influence of the peptide sequence on the binding of short cationic peptides composed of five lysines alternating with uncharged amino acids within the series (KX)4K to negatively charged monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) was investigated by adsorption experiments in combination with epifluorescence microscopy. To evaluate the impact of electrostatic and hydrophobic contributions, different uncharged amino acids X with increasing hydrophobicity, where X = G (glycine), A (alanine), Abu (α-aminobutyric acid), V (valine), or L (leucine) were introduced into the peptide sequence to tune the peptide hydrophobicity. The adsorption kinetics of these peptides to a DPPG monolayer always showed two superimposed processes, one leading to an increase and another to a decrease of the surface pressure Π. Thus, the plots of the change in Π after peptide binding vs initial surface pressure of the monolayer showed an unusual behavior with maxima and negative changes in Π at high initial Π values. Epifluorescence microscopy confirmed that electrostatic binding of the peptides with a concomitant decrease in Π leads to a condensation of the lipid monolayer and the formation of liquid-condensed (LC) domains even at Π values where the monolayer is supposedly in the liquid-expanded (LE) state. An increase in hydrophobicity of the amino acid X was found to counteract the condensation and an increase in Π upon peptide binding is observed at low Π values, also concomitant with the formation of LC-domains. Compression of monolayers after peptide adsorption at low surface pressure for 4 h leads to a change of the isotherms compared to pure DPPG isotherms. The phase transition of DPPG from LE to LC state is smeared out or is shifted to higher surface pressure. Considerable changes in the shapes of LC-domains were observed after peptide binding. Growth of the LC-domains was hindered in most cases and regular domain patterns were formed. Binding of (KL)4K leads to a decrease in line tension and the formation of extended filaments protruding from initially circular domains.

Mechanisms of membrane binding of small GTPase K-Ras4B farnesylated hypervariable region

K-Ras4B belongs to a family of small GTPases that regulates cell growth, differentiation and survival. K-ras is frequently mutated in cancer. K-Ras4B association with the plasma membrane through its farnesylated and positively charged C-terminal hypervariable region (HVR) is critical to its oncogenic function. However, the structural mechanisms of membrane association are not fully understood. Here, using confocal microscopy, surface plasmon resonance, and molecular dynamics simulations, we observed that K-Ras4B can be distributed in rigid and loosely packed membrane domains. Its membrane binding domain interaction with phospholipids is driven by membrane fluidity. The farnesyl group spontaneously inserts into the disordered lipid microdomains, whereas the rigid microdomains restrict the farnesyl group penetration. We speculate that the resulting farnesyl protrusion toward the cell interior allows oligomerization of the K-Ras4B membrane binding domain in rigid microdomains. Unlike other Ras isoforms, K-Ras4B HVR contains a single farnesyl modification and positively charged polylysine sequence. The high positive charge not only modulates specific HVR binding to anionic phospholipids but farnesyl membrane orientation. Phosphorylation of Ser-181 prohibits spontaneous farnesyl membrane insertion. The mechanism illuminates the roles of HVR modifications in K-Ras4B targeting microdomains of the plasma membrane and suggests an additional function for HVR in regulation of Ras signaling.

Vibrational spectroscopy for probing molecular-level interactions in organic films mimicking biointerfaces

Investigation into nanostructured organic films has served many purposes, including the design of functionalized surfaces that may be applied in biomedical devices and tissue engineering and for studying physiological processes depending on the interaction with cell membranes. Of particular relevance are Langmuir monolayers, Langmuir-Blodgett (LB) and layer-by-layer (LbL) films used to simulate biological interfaces. In this review, we shall focus on the use of vibrational spectroscopy methods to probe molecular-level interactions at biomimetic interfaces, with special emphasis on three surface-specific techniques, namely sum frequency generation (SFG), polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) and surface-enhanced Raman scattering (SERS). The two types of systems selected for exemplifying the potential of the methods are the cell membrane models and the functionalized surfaces with biomolecules. Examples will be given on how SFG and PM-IRRAS can be combined to determine the effects from biomolecules on cell membrane models, which include determination of the orientation and preservation of secondary structure. Crucial information for the action of biomolecules on model membranes has also been obtained with PM-IRRAS, as is the case of chitosan removing proteins from the membrane. SERS will be shown as promising for enabling detection limits down to the single-molecule level. The strengths and limitations of these methods will also be discussed, in addition to the prospects for the near future.

H-Ras forms dimers on membrane surfaces via a protein-protein interface

The lipid-anchored small GTPase Ras is an important signaling node in mammalian cells. A number of observations suggest that Ras is laterally organized within the cell membrane, and this may play a regulatory role in its activation. Lipid anchors composed of palmitoyl and farnesyl moieties in H-, N-, and K-Ras are widely suspected to be responsible for guiding protein organization in membranes. Here, we report that H-Ras forms a dimer on membrane surfaces through a protein-protein binding interface. A Y64A point mutation in the switch II region, known to prevent Son of sevenless and PI3K effector interactions, abolishes dimer formation. This suggests that the switch II region, near the nucleotide binding cleft, is either part of, or allosterically coupled to, the dimer interface. By tethering H-Ras to bilayers via a membrane-miscible lipid tail, we show that dimer formation is mediated by protein interactions and does not require lipid anchor clustering. We quantitatively characterize H-Ras dimerization in supported membranes using a combination of fluorescence correlation spectroscopy, photon counting histogram analysis, time-resolved fluorescence anisotropy, single-molecule tracking, and step photobleaching analysis. The 2D dimerization Kd is measured to be ∼1 × 10(3) molecules/µm(2), and no higher-order oligomers were observed. Dimerization only occurs on the membrane surface; H-Ras is strictly monomeric at comparable densities in solution. Analysis of a number of H-Ras constructs, including key changes to the lipidation pattern of the hypervariable region, suggest that dimerization is a general property of native H-Ras on membrane surfaces.

Peptide and protein binding to lipid monolayers studied by FT-IRRA spectroscopy

Lipid monolayers at the air-water interface represent half of a lipid bilayer and are therefore suitable model systems for studying the binding of peripheral proteins and polypeptides as well as proteins containing hydrophobic membrane anchors to membrane interfaces. Infrared reflection-absorption spectroscopy (IRRAS) of these monolayer films at the air-water interface provides information on the state of the lipid monolayers as well as on the conformational and orientational order of the film constituents. We will review shortly the experimental set-up and the possibilities for obtaining structural information before several applications of the method to lipid-protein monolayers will be described. We will focus on examples where the analysis of the protein and peptide bands for pure monolayers of these compounds are combined with experiments where the same compounds are bound to lipid monolayers. Combination of these experiments leads to detailed information about the conformational properties and the orientation of the molecules at the air-water interface in contrast to being bound to the lipid-water interface. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

The role of G-domain orientation and nucleotide state on the Ras isoform-specific membrane interaction

Ras proteins are proto-oncogenes that function as molecular switches linking extracellular stimuli with an overlapping but distinctive range of biological outcomes. Although modulatable interactions between the membrane and the Ras C-terminal hypervariable region (HVR) harbouring the membrane anchor motifs enable signalling specificity to be determined by their location, it is becoming clear that the spatial orientation of different Ras proteins is also crucial for their functions. To reveal the orientation of the G-domain at membranes, we conducted an extensive study on different Ras isoforms anchored to model raft membranes. The results show that the G-domain mediates the Ras-membrane interaction by inducing different sets of preferred orientations in the active and inactive states with largely parallel orientation relative to the membrane of most of the helices. The distinct locations of the different isoforms, exposing them to different effectors and regulators, coupled with different G-domain-membrane orientation, suggests synergy between this type of recognition motif and the specificity conferred by the HVR, thereby validating the concept of isoform specificity in Ras.

Human islet amyloid polypeptide at the air-aqueous interface: a Langmuir monolayer approach

Human islet amyloid polypeptide (hIAPP) is the source of the major component of the amyloid deposits found in the islets of Langerhans of around 95 per cent type 2 diabetic patients. The formation of aggregates and mature fibrils is thought to be responsible for the dysfunction and death of the insulin-producing pancreatic β-cells. Investigation on the conformation, orientation and self-assembly of the hIAPP at time zero could be beneficial for our understanding of its stability and aggregation process. To obtain these insights, the hIAPP at time zero was studied at the air-aqueous interface using the Langmuir monolayer technique. The properties of the hIAPP Langmuir monolayer at the air-aqueous interface on a NaCl subphase with pH 2.0, 5.6 and 9.0 were examined by surface pressure- and potential-area isotherms, UV-Vis absorption, fluorescence spectroscopy and Brewster angle microscopy. The conformational and orientational changes of the hIAPP Langmuir monolayer under different surface pressures were characterized by p-polarized infrared-reflection absorption spectroscopy, and the results did not show any prominent changes of conformation or orientation. The predominant secondary structure of the hIAPP at the air-aqueous interface was α-helix conformation, with a parallel orientation to the interface during compression. These results showed that the hIAPP Langmuir monolayer at the air-aqueous interface was stable, and no aggregate or domain of the hIAPP at the air-aqueous interface was observed during the time of experiments.

Negatively charged phospholipids trigger the interaction of a bacterial Tat substrate precursor protein with lipid monolayers

Folded proteins can be translocated across biological membranes via the Tat machinery. It has been shown in vitro that these Tat substrates can interact with membranes prior to translocation. Here we report a monolayer and infrared reflection-absorption spectroscopic (IRRAS) study of the initial states of this membrane interaction, the binding to a lipid monolayer at the air/water interface serving as a model for half of a biological membrane. Using the model Tat substrate HiPIP (high potential iron-sulfur protein) from Allochromatium vinosum, we found that the precursor preferentially interacts with monolayers of negatively charged phospholipids. The signal peptide is essential for the interaction of the precursor protein with the monolayer because the mature HiPIP protein showed no interaction with the lipid monolayer. However, the individual signal peptide interacted differently with the monolayer compared to the complete precursor protein. IRRA spectroscopy indicated that the individual signal peptide forms mainly aggregated β-sheet structures. This β-sheet formation did not occur for the signal peptide when being part of the full length precursor. In this case it adopted an α-helical structure upon membrane insertion. The importance of the signal peptide and the mature domain for the membrane interaction is discussed in terms of current ideas of Tat substrate-membrane interactions.

Membrane-mediated induction and sorting of K-Ras microdomain signaling platforms

The K-Ras4B GTPase is a major oncoprotein whose signaling activity depends on its correct localization to negatively charged subcellular membranes and nanoclustering in membrane microdomains. Selective localization and clustering are mediated by the polybasic farnesylated C-terminus of K-Ras4B, but the mechanisms and molecular determinants involved are largely unknown. In a combined chemical biological and biophysical approach we investigated the partitioning of semisynthetic fully functional lipidated K-Ras4B proteins into heterogeneous anionic model membranes and membranes composed of viral lipid extracts. Independent of GDP/GTP-loading, K-Ras4B is preferentially localized in liquid-disordered (l(d)) lipid domains and forms new protein-containing fluid domains that are recruiting multivalent acidic lipids by an effective, electrostatic lipid sorting mechanism. In addition, GDP-GTP exchange and, thereby, Ras activation results in a higher concentration of activated K-Ras4B in the nanoscale signaling platforms. Conversely, palmitoylated and farnesylated N-Ras proteins partition into the l(d) phase and concentrate at the l(d)/l(o) phase boundary of heterogeneous membranes. Next to the lipid anchor system, the results reveal an involvement of the G-domain in the membrane interaction process by determining minor but yet significant structural reorientations of the GDP/GTP-K-Ras4B proteins at lipid interfaces. A molecular mechanism for isoform-specific Ras signaling from separate membrane microdomains is postulated from the results of this study.

Influence of the lipid anchor motif of N-ras on the interaction with lipid membranes: a surface plasmon resonance study

Ras GTPases play a crucial role in signal transduction cascades involved in cell differentiation and proliferation, and membrane binding is essential for their proper function. To determine the influence of the nature of the lipid anchor motif and the difference between the active (GTP) and inactive (GDP) forms of N-Ras on partitioning and localization in the lipid membrane, five different N-Ras constructs with different lipid anchors and nucleotide loading (Far/Far (GDP), HD/Far (GDP), HD/HD (GDP), Far (GDP), and HD/Far (GppNHp)) were synthesized. Using the surface plasmon resonance technique, we were able to follow the insertion and dissociation process of the lipidated proteins into and out of model membranes consisting of pure liquid-ordered (l(o)) or liquid-disordered (l(d)) phase and a heterogeneous two-phase mixture, i.e., a raft mixture with l(o) + l(d) phase coexistence. In addition, we examined the influence of negatively charged headgroups and stored curvature elastic stress on the binding properties of the lipidated N-Ras proteins. In most cases, significant differences were found for the various anchor motifs. In general, N-Ras proteins insert preferentially into a fluidlike, rather than a rigid, ordered lipid bilayer environment. Electrostatic interactions with lipid headgroups or stored curvature elastic stress of the membrane seem to have no drastic effect on the binding and dissociation processes of the lipidated proteins. The monofarnesylated N-Ras exhibits generally the highest association rate and fastest dissociation process in fluidlike membranes. Double lipidation, especially including farnesylation, of the protein leads to drastically reduced initial binding rates but strong final association. The change in the nucleotide loading of the natural N-Ras HD/Far induces a slightly different binding and dissociation kinetics, as well as stability of association, and seems to influence the tendency to segregate laterally in the membrane plane. The GDP-bound inactive form of N-Ras with an HD/Far anchor shows stronger membrane association, which might be due to a more pronounced tendency to self-assemble in the membrane matrix than is seen with the active GTP-bound form.

Ranaspumin-2: structure and function of a surfactant protein from the foam nests of a tropical frog

Ranaspumin-2 (Rsn-2) is a monomeric, 11 kDa surfactant protein identified as one of the major foam nest components of the túngara frog (Engystomops pustulosus), with an amino acid sequence unlike any other protein described so far. We report here on its structure in solution as determined by high-resolution NMR analysis, together with investigations of its conformation and packing at the air-water interface using a combination of infrared and neutron reflectivity techniques. Despite the lack of any significant sequence similarity, Rsn-2 in solution adopts a compact globular fold characteristic of the cystatin family, comprising a single helix over a four-stranded sheet, in a motif not previously associated with surfactant activity. The NMR structure of Rsn-2 shows no obvious amphiphilicity that might be anticipated for a surfactant protein. This suggests that it must undergo a significant conformational change when incorporated into the air-water interface that may involve a hinge-bending, clamshell opening of the separate helix and sheet segments to expose hydrophobic faces to air while maintaining the highly polar surfaces in contact with the underlying water layer. This model is supported by direct observation of the relative orientations of secondary structure elements at the interface by infrared reflection absorption spectroscopy, and by protein packing densities determined from neutron reflectivity profiles.

Secondary structure of lipidated Ras bound to a lipid bilayer

Ras proteins are small guanine nucleotide binding proteins that regulate many cellular processes, including growth control. They undergo distinct post-translational lipid modifications that are required for appropriate targeting to membranes. This, in turn, is critical for Ras biological function. However, most in vitro studies have been conducted on nonlipidated truncated forms of Ras proteins. Here, for the first time, attenuated total reflectance-FTIR studies of lipid-modified membrane-bound N-Ras are performed, and compared with nonlipidated truncated Ras in solution. For these studies, lipidated N-Ras was prepared by linking a farnesylated and hexadecylated N-Ras lipopeptide to a truncated N-Ras protein (residues 1-181). It was then bound to a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer tethered on an attenuated total reflectance crystal. The structurally sensitive amide I absorbance band in the IR was detected and analysed to determine the secondary structure of the protein. The NMR three-dimensional structure of truncated Ras was used to calibrate the contributions of the different secondary structural elements to the amide I absorbance band of truncated Ras. Using this novel approach, the correct decomposition was selected from several possible solutions. The same parameter set was then used for the membrane-bound lipidated Ras, and provided a reliable decomposition for the membrane-bound form in comparison with truncated Ras. This comparison indicates that the secondary structure of membrane-bound Ras is similar to that determined for the nonlipidated truncated Ras protein for the highly conserved G-domain. This result validates the multitude of investigations of truncated Ras without anchor in vitro. The novel attenuated total reflectance approach opens the way for detailed studies of the interaction network of the membrane-bound Ras protein.

Membrane binding of lipidated Ras peptides and proteins--the structural point of view

Biological membranes are interesting interfaces, at which important biological processes occur. In addition to integral membrane proteins, a number of proteins bind to the membrane surface and associate with it. Posttranslational lipid modification is one important mechanism, by which soluble molecules develop a propensity towards the membrane and reversibly bind to it. Membrane binding by insertion of hydrophobic lipid moieties is relevant for up to 10% of all cellular proteins. A particular interesting lipid-modified protein is the small GTPase Ras, which plays a key role in cellular signal transduction. Until recently, the structural basis for membrane binding of Ras was not well-defined. However, with the advent of new synthesis techniques and the advancement of several biophysical methods, a number of structural and dynamical features about membrane binding of Ras proteins have been revealed. This review will summarize the chemical biology of Ras and discuss in more detail the biophysical and structural features of the membrane bound C-terminus of the protein.

Ras nanoclusters: molecular structure and assembly

H-, N- and K-ras4B are lipid-anchored, peripheral membrane guanine nucleotide binding proteins. Recent work has shown that Ras proteins are laterally segregated into non-overlapping, dynamic domains of the plasma membrane called nanoclusters. This lateral segregation is important to specify Ras interactions with membrane-associated proteins, effectors and scaffolding proteins and is critical for Ras signal transduction. Here we review biological, in vitro and structural data that provide insight into the molecular basis of how palmitoylated Ras proteins are anchored to the plasma membrane. We explore possible mechanisms for how the interactions of H-ras with a lipid bilayer may drive nanocluster formation.

H-ras protein in a bilayer: interaction and structure perturbation

Ras GTPases become functionally active when anchored to membranes by inserting their lipid modified side chains. Their role in cell division, development, and cancer has made them targets of extensive research efforts, yet the mechanism of membrane insertion and the structure of the resulting complex remain elusive. Recently, the structure of the full-length H-ras protein in a DMPC bilayer has been computationally characterized. Here, the atomic interactions between the H-ras membrane anchor and the DMPC bilayer are investigated in detail. We find that the palmitoylated cysteines and Met182 have dual contributions to membrane affinity: hydrogen bonding by their amides and van der Waals interaction by their hydrophobic side chains. The polar side chains help maintain the orientation of the anchor. Although the overall structure of the bilayer is similar to that of a pure DMPC, there are localized perturbations. These perturbations depend on the insertion depth and backbone localization of the anchor, which in turn is modulated by the catalytic domain and the linker. The pattern of anchor amide-DMPC phosphate/carbonyl hydrogen bonds and the flexibility of Palm184 are important in discriminating between different modes of ras-DMPC interactions. The results provide structural arguments in support of the proposed participation of ras in the organization of membrane nanoclusters.

Determination of the contribution of the myristoyl group and hydrophobic amino acids of recoverin on its dynamics of binding to lipid monolayers

It has been postulated that myristoylation of peripheral proteins would facilitate their binding to membranes. However, the exact involvement of this lipid modification in membrane binding is still a matter of debate. Proteins containing a Ca(2+)-myristoyl switch where the extrusion of their myristoyl group is dependent on calcium binding is best illustrated by the Ca(2+)-binding recoverin, which is present in retinal rod cells. The parameters responsible for the modulation of the membrane binding of recoverin are still largely unknown. This study was thus performed to determine the involvement of different parameters on recoverin membrane binding. We have used surface pressure measurements and PM-IRRAS spectroscopy to monitor the adsorption of myristoylated and nonmyristoylated recoverin onto phospholipid monolayers in the presence and absence of calcium. The adsorption curves have shown that the myristoyl group and hydrophobic residues of myristoylated recoverin strongly accelerate membrane binding in the presence of calcium. In the case of nonmyristoylated recoverin in the presence of calcium, hydrophobic residues alone are responsible for its much faster monolayer binding than myristoylated and nonmyristoylated recoverin in the absence of calcium. The infrared spectra revealed that myristoylated and nonmyristoylated recoverin behave very different upon adsorption onto phospholipid monolayers. Indeed, PM-IRRAS spectra indicated that the myristoyl group allows a proper orientation and organization as well as faster and stronger binding of myristoylated recoverin to lipid monolayers compared to nonmyristoylated recoverin. Simulations of the spectra have allowed us to postulate that nonmyristoylated recoverin changes conformation and becomes hydrated at large extents of adsorption as well as to estimate the orientation of myristoylated recoverin with respect to the monolayer plane. In addition, adsorption measurements and electrophoresis of trypsin-treated myristoylated recoverin in the presence of zinc or calcium demonstrated that recoverin has a different conformation but a similar extent of monolayer binding in the presence of such ions.

Organization, structure and activity of proteins in monolayers

Many different processes take place at the cell membrane interface. Indeed, for instance, ligands bind membrane proteins which in turn activate peripheral membrane proteins, some of which are enzymes whose action is also located at the membrane interface. Native cell membranes are difficult to use to gain information on the activity of individual proteins at the membrane interface because of the large number of different proteins involved in membranous processes. Model membrane systems, such as monolayers at the air-water interface, have thus been extensively used during the last 50 years to reconstitute proteins and to gain information on their organization, structure and activity in membranes. In the present paper, we review the recent work we have performed with membrane and peripheral proteins as well as enzymes in monolayers at the air-water interface. We show that the structure and orientation of gramicidin has been determined by combining different methods. Furthermore, we demonstrate that the secondary structure of rhodopsin and bacteriorhodopsin is indistinguishable from that in native membranes when appropriate conditions are used. We also show that the kinetics and extent of monolayer binding of myristoylated recoverin is much faster than that of the nonmyristoylated form and that this binding is highly favored by the presence polyunsaturated phospholipids. Moreover, we show that the use of fragments of RPE65 allow determine which region of this protein is most likely involved in membrane binding. Monomolecular films were also used to further understand the hydrolysis of organized phospholipids by phospholipases A2 and C.

Mechanism of islet amyloid polypeptide fibrillation at lipid interfaces studied by infrared reflection absorption spectroscopy

Islet amyloid polypeptide (IAPP) is a pancreatic hormone and one of a number of proteins that are involved in the formation of amyloid deposits in the islets of Langerhans of type II diabetes mellitus patients. Though IAPP-membrane interactions are known to play a major role in the fibrillation process, the mechanism and the peptide's conformational changes involved are still largely unknown. To obtain new insights into the conformational dynamics of IAPP upon its aggregation at membrane interfaces and to relate these structures to its fibril formation, we studied the association of IAPP at various interfaces including neutral as well as charged phospholipids using infrared reflection absorption spectroscopy. The results obtained reveal that the interaction of human IAPP with the lipid interface is driven by the N-terminal part of the peptide and is largely driven by electrostatic interactions, as the protein is able to associate strongly with negatively charged lipids only. A two-step process is observed upon peptide binding, involving a conformational transition from a largely alpha-helical to a beta-sheet conformation, finally forming ordered fibrillar structures. As revealed by simulations of the infrared reflection absorption spectra and complementary atomic force microscopy studies, the fibrillar structures formed consist of parallel intermolecular beta-sheets lying parallel to the lipid interface but still contain a significant number of turn structures. We may assume that these dynamical conformational changes observed for negatively charged lipid interfaces play an important role as the first steps of IAPP-induced membrane damage in type II diabetes.

Langmuir-Blodgett films formed by continuously varying surface pressure. Characterization by IR spectroscopy and epifluorescence microscopy

Monolayer films of phospholipids at the air-water interface have been transferred to solid substrates under conditions of continuously varying surface pressure, an approach termed COVASP. The molecular and supramolecular properties of the film constituents have been characterized with two complementary techniques. IR spectroscopy was used to monitor chain conformation as a function of transfer surface pressure. Results were compared to those from Langmuir films determined directly at the A/W interface by IR reflection-absorption spectroscopy (IRRAS). The methylene stretching frequencies for both proteated and acyl chain perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC and DPPC-d62) in the transferred molecules indicate that the phospholipids retain at least, in part, their surface pressure-dependent chain-conformational order characteristics. The line widths of these modes are somewhat reduced, suggestive of slower rates of reorientational motion in the Langmuir-Blodgett (LB) films. Epifluorescence microscopy reveals a progressive condensation gradient, including nucleation and growth of probe-excluding condensed domains along the transfer line. DPPC condensation, observed along a single LB film, was qualitatively comparable to compression-driven condensation as observed in situ or in conventional LB films transferred at constant pressures. However, condensation along the compression isotherm in COVASP-LB films was reduced by 15-20% as compared to films equilibrated at different constant pressures, probably the result of kinetic differences in equilibration processes. As a preliminary demonstration of the utility of this new approach, the monolayer --> multilayer transition known to occur (Eur. Biophys. J. 2005, 34, 243) in a four-component model for pulmonary surfactant has been examined. IR parameters from both the lipid and the protein constituents of the film all indicate that the transition persists during the transfer process. This new approach for the study of transferred films will permit the efficient characterization of lipid-protein interactions and structural transitions occurring in pulmonary surfactant films subjected to dynamic compression.

Membrane binding of a lipidated N-Ras protein studied in lipid monolayers

The adsorption of doubly lipidated full-length N-Ras protein on 1,2-dipalmitoyl-sn-phosphatidylcholine (DPPC) monolayers was studied by lateral pressure analysis, grazing incidence X-ray diffraction (GIXD), and specular reflectivity (XR). N-Ras protein adsorbs to the DPPC monolayer (lateral pressure of 20 mN/m) from the subphase thereby increasing the lateral pressure in the monolayer by 4 mN/m. The protein insertion does not alter the tilt angle and structure of the lipid molecules at the air/water interface but influences the electron density profile of the monolayer. Further, electron density differences into the subphase were observed. The Fresnel normalized reflectivity could be reconstructed in the analysis using box models yielding electron density profiles of the DPPC monolayer in the absence and in the presence of N-Ras protein. The electron density profiles of the DPPC monolayer in the presence of Ras showed clear intensity variations in the headgroup/glycerol/upper chain region, the so-called interface region where previous bilayer studies had confirmed Ras binding.