Spin-label saturation-transfer electron spin resonance detection of transient association of rhodopsin in reconstituted membranes

Probing the Dynamics of Yersinia Adhesin A (YadA) in Outer Membranes Hints at Requirements for β-Barrel Membrane Insertion

The vast majority of cells are protected and functionalized by a dense surface layer of glycans, proteoglycans, and glycolipids. This surface represents an underexplored space in structural biology that is exceedingly challenging to recreate in vitro. Here, we investigate β-barrel protein dynamics within an asymmetric outer membrane environment, with the trimeric autotransporter Yersinia adhesin A (YadA) as an example. Magic-angle spinning NMR relaxation data and a model-free approach reveal increased mobility in the second half of strand β2 after the conserved G72, which is responsible for membrane insertion and autotransport, and in the subsequent loop toward β3. In contrast, the protomer-protomer interaction sites (β1i-β4i-1) are rigid. Intriguingly, the mobility in the β-strand section following G72 is substantially elevated in the outer membrane and less so in the detergent environment of microcrystals. A possible source is revealed by molecular dynamics simulations that show the formation of a salt bridge involving E79 and R76 in competition with a dynamic interplay of calcium binding by E79 and the phosphate groups of the lipids. An estimation of overall barrel motion in the outer membrane and detergent-containing crystals yields values of around 41 ns for both. The global motion of YadA in the outer membrane has a stronger rotational component orthogonal to the symmetry axis of the trimeric porin than in the detergent-containing crystal. In summary, our investigation shows that the mobility in the second half of β2 and the loop to β3 required for membrane insertion and autotransport is maintained in the final folded form of YadA.

Transient, nano-scale, liquid-like molecular assemblies coming of age

This review examines the dynamic mechanisms underlying cellular signaling, communication, and adhesion via transient, nano-scale, liquid-like molecular assemblies on the plasma membrane (PM). Traditional views posit that stable, solid-like molecular complexes perform these functions. However, advanced imaging reveals that many signaling and scaffolding proteins only briefly reside in these molecular complexes and that micron-scale protein assemblies on the PM, including cell adhesion structures and synapses, are likely made of archipelagoes of nanoliquid protein islands. Borrowing the concept of liquid-liquid phase separation to form micron-scale biocondensates, we propose that these nano-scale oligomers and assemblies are enabled by multiple weak but specific molecular interactions often involving intrinsically disordered regions. The signals from individual nanoliquid signaling complexes would occur as pulses. Single-molecule imaging emerges as a crucial technique for characterizing these transient nanoliquid assemblies on the PM, suggesting a shift toward a model where the fluidity of interactions underpins signal regulation and integration.

Advantages and potential limitations of applying AFM kymograph analysis to pharmaceutically relevant membrane proteins in lipid bilayers

Membrane proteins play critical roles in disease and in the disposition of many pharmaceuticals. A prime example is P-glycoprotein (Pgp) which moves a diverse range of drugs across membranes and out of the cell before a therapeutic payload can be delivered. Conventional structural biology methods have provided a valuable framework for comprehending the complex conformational changes underlying Pgp function, which also includes ATPase activity, but the lack of real-time information hinders understanding. Atomic force microscopy (AFM) is a single-molecule technique that is well-suited for studying active membrane proteins in bilayers and is poised to advance the field beyond static snapshots. After verifying Pgp activity in surface-support bilayers, we used kymograph analysis in conjunction with AFM imaging and simulations to study structural transitions at the 100 ms timescale. Though kymographs are frequently employed to boost temporal resolution, the limitations of the method have not been well characterized, especially for sparse non-crystalline distributions of pharmaceutically relevant membrane proteins like Pgp. Common experimental challenges are analyzed, including protein orientation, instrument noise, and drift. Surprisingly, a lateral drift of 75% of the protein dimension leads to only a 12% probability of erroneous state transition detection; average dwell time error achieves a maximum value of 6%. Rotational drift of proteins like Pgp, with azimuthally-dependent maximum heights, can lead to artifactual transitions. Torsional constraints can alleviate this potential pitfall. Confidence in detected transitions can be increased by adding conformation-altering ligands such as non-hydrolysable analogs. Overall, the data indicate that AFM kymographs are a viable method to access conformational dynamics for Pgp, but generalizations of the method should be made with caution.

Polymer brush-induced depletion interactions and clustering of membrane proteins

We investigate the effect of mobile polymer brushes on proteins embedded in biological membranes by employing both Asakura-Oosawa type of theoretical model and coarse-grained molecular dynamics simulations. The brush polymer-induced depletion attraction between proteins changes non-monotonically with the size of brush. The depletion interaction, which is determined by the ratio of the protein size to the grafting distance between brush polymers, increases linearly with the brush size as long as the polymer brush height is shorter than the protein size. When the brush height exceeds the protein size, however, the depletion attraction among proteins is slightly reduced. We also explore the possibility of the brush polymer-induced assembly of a large protein cluster, which can be related to one of many molecular mechanisms underlying recent experimental observations of integrin nanocluster formation and signaling.

Defining raft domains in the plasma membrane

Many plasma membrane (PM) functions depend on the cholesterol concentration in the PM in strikingly nonlinear, cooperative ways: fully functional in the presence of physiological cholesterol levels (35~45 mol%), and nonfunctional below 25 mol% cholesterol; namely, still in the presence of high concentrations of cholesterol. This suggests the involvement of cholesterol-based complexes/domains formed cooperatively. In this review, by examining the results obtained by using fluorescent lipid analogs and avoiding the trap of circular logic, often found in the raft literature, we point out the fundamental similarities of liquid-ordered (Lo)-phase domains in giant unilamellar vesicles, Lo-phase-like domains formed at lower temperatures in giant PM vesicles, and detergent-resistant membranes: these domains are formed by cooperative interactions of cholesterol, saturated acyl chains, and unsaturated acyl chains, in the presence of >25 mol% cholesterol. The literature contains evidence, indicating that the domains formed by the same basic cooperative molecular interactions exist and play essential roles in signal transduction in the PM. Therefore, as a working definition, we propose that raft domains in the PM are liquid-like molecular complexes/domains formed by cooperative interactions of cholesterol with saturated acyl chains as well as unsaturated acyl chains, due to saturated acyl chains' weak multiple accommodating interactions with cholesterol and cholesterol's low miscibility with unsaturated acyl chains and TM proteins. Molecules move within raft domains and exchange with those in the bulk PM. We provide a logically established collection of fluorescent lipid probes that preferentially partition into raft and non-raft domains, as defined here, in the PM.

Supramolecular organization of rhodopsin in rod photoreceptor cell membranes

Rhodopsin is the light receptor in rod photoreceptor cells that initiates scotopic vision. Studies on the light receptor span well over a century, yet questions about the organization of rhodopsin within the photoreceptor cell membrane still persist and a consensus view on the topic is still elusive. Rhodopsin has been intensely studied for quite some time, and there is a wealth of information to draw from to formulate an organizational picture of the receptor in native membranes. Early experimental evidence in apparent support for a monomeric arrangement of rhodopsin in rod photoreceptor cell membranes is contrasted and reconciled with more recent visual evidence in support of a supramolecular organization of rhodopsin. What is known so far about the determinants of forming a supramolecular structure and possible functional roles for such an organization are also discussed. Many details are still missing on the structural and functional properties of the supramolecular organization of rhodopsin in rod photoreceptor cell membranes. The emerging picture presented here can serve as a springboard towards a more in-depth understanding of the topic.

Rotational Diffusion of Membrane Proteins in Crowded Membranes

Membrane proteins travel along cellular membranes and reorient themselves to form functional oligomers and protein-lipid complexes. Following the Saffman-Delbrück model, protein radius sets the rate of this diffusive motion. However, it is unclear how this model, derived for ideal and dilute membranes, performs under crowded conditions of cellular membranes. Here, we study the rotational motion of membrane proteins using molecular dynamics simulations of coarse-grained membranes and 2-dimensional Lennard-Jones fluids with varying levels of crowding. We find that the Saffman-Delbrück model captures the size-dependency of rotational diffusion under dilute conditions where protein-protein interactions are negligible, whereas stronger scaling laws arise under crowding. Together with our recent work on lateral diffusion, our results reshape the description of protein dynamics in native membrane environments: The translational and rotational motions of proteins with small transmembrane domains are rapid, whereas larger proteins or protein complexes display substantially slower dynamics.

Affinity of rhodopsin to raft enables the aligned oligomer formation from dimers: Coarse-grained molecular dynamics simulation of disk membranes

The visual photopigment protein rhodopsin (Rh) is a typical G protein-coupled receptor (GPCR) that initiates the phototransduction cascade in retinal disk membrane of rod-photoreceptor cells. Rh molecule has a tendency to form dimer, and the dimer tends to form rows, which is suggested to heighten phototransduction efficiency in single-photon regime. In addition, the dimerization confers Rh an affinity for lipid raft, i.e. raftophilicity. However, the mechanism by which Rh-dimer raftophilicity contributes to the organization of the higher order structure remains unknown. In this study, we performed coarse-grained molecular dynamics simulations of a disk membrane model containing unsaturated lipids, saturated lipids with cholesterol, and Rh-dimers. We described the Rh-dimers by two-dimensional particle populations where the palmitoyl moieties of each Rh exhibits raftophilicity. We simulated the structuring of Rh in a disk for two types of Rh-dimer, i.e., the most and second most stable Rh dimers, which exposes the raftophilic regions at the dimerization-interface (H1/H8 dimer) and two edges away from the interface (H4/H5 dimer), respectively. Our simulations revealed that only the H1/H8 dimer could form a row structure. A small number of raftophilic lipids recruited to and intercalated in a narrow space between H1/H8 dimers stabilize the side-by-side interaction between dimers in a row. Our results implicate that the nano-sized lipid raft domains act as a “glue” to organize the long row structures of Rh-dimers.

Raftophilic rhodopsin-clusters offer stochastic platforms for G protein signalling in retinal discs

Rhodopsin is a G protein-coupled receptor (GPCR) that initiates the phototransduction cascade in retinal disc membrane. Recent studies have suggested that rhodopsin forms highly ordered rows of dimers responsible for single-photon detection by rod photoreceptors. Dimerization is also known to confer to rhodopsin a high affinity for ordered lipids (raftophilicity). However, the role of rhodopsin organization and its raftophilicity in phototransduction remains obscure, owing to the lack of direct observation of rhodopsin dynamics and distribution in native discs. Here, we explore the single-molecule and semi-multimolecule behaviour of rhodopsin in native discs. Rhodopsin forms transient meso-scale clusters, even in darkness, which are loosely confined to the disc centre. Cognate G protein transducin co-distributes with rhodopsin, and exhibits lateral translocation to the disc periphery upon activation. We demonstrate that rhodopsin offers inherently distributed and stochastic platforms for G protein signalling by self-organizing raftophilic clusters, which continually repeat generation/extinction in the disc membrane.

Influenza A M2 Channel Clustering at High Protein/Lipid Ratios: Viral Budding Implications

Protein dynamics in crowded environments is important for understanding protein functions in vivo and is especially relevant for membrane proteins because of the roles of protein-protein interactions in membrane protein functions and their regulation. Here, using solid-state NMR spectroscopy in combination with coarse-grained molecular dynamics simulations, we report that the rotational correlation time for the transmembrane domain of the influenza A M2 proton channel in lipid bilayers increases dramatically at an elevated protein/lipid ratio. This increase is attributable to persistent protein-protein interactions, thus revealing for the first time, to the best of our knowledge, extensive cluster formation of the M2 tetrameric channel. Such clustering appears to have direct biological relevance during budding of the nascent influenza virus, which does not use the endosomal sorting complexes required for transport machinery. Indeed, initial coarse-grained molecular dynamics simulations of the longer M2 construct known as the conductance domain suggest clustering-induced membrane curvature formation.

Autobiography of James S. Hyde

The papers, book chapters, reviews, and patents by James S. Hyde in the bibliography of this document have been separated into EPR and MRI sections, and within each section by topics. Within each topic, publications are listed chronologically. A brief summary is provided for each patent listed. A few publications and patents that do not fit this schema have been omitted. This list of publications is preceded by a scientific autobiography that focuses on selected topics that are judged to have been of most scientific importance. References to many of the publications and patents in the bibliography are made in the autobiography.

Cation-Selective Channel Regulated by Anions According to Their Hofmeister Ranking

Specificity of small ions, the Hofmeister ranking, is long-known and has many applications including medicine. Yet it evades consistent theoretical description. Here we study the effect of Hofmeister anions on gramicidin A channels in lipid membranes. Counterintuitively, we find that conductance of this perfectly cation-selective channel increases about two-fold in the H2 PO4- Collapse

Key Words

• interface phenomena
• ion channels
• ion specificity
• membranes
• nanopores

Collapse

MESH Headings

• Anions/metabolism
• Bacillus/metabolism
• Cations/metabolism
• Gramicidin/metabolism
• Ion Transport
• Kinetics
• Lipid Bilayers/metabolism
• Thermodynamics
• Unilamellar Liposomes/metabolism

Collapse

Grants

• Z01 HD000072-04 Intramural NIH HHS

Collapse

Affiliation(s)

Philip A Gurnev Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bldg. 9, Rm 1 N-124B, Bethesda, MD, 20892, USA
Torri C Roark Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bldg. 9, Rm 1 N-124B, Bethesda, MD, 20892, USA Department of Physics, Indiana University Purdue University Indianapolis, Indianapolis, IN, 46202, USA
Horia I Petrache Department of Physics, Indiana University Purdue University Indianapolis, Indianapolis, IN, 46202, USA
Alexander J Sodt Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bldg. 9, Rm 1 N-124B, Bethesda, MD, 20892, USA
Sergey M Bezrukov Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bldg. 9, Rm 1 N-124B, Bethesda, MD, 20892, USA

Collapse

Collaborators Collapse

Organization and dynamics of SNARE proteins in the presynaptic membrane

Our view of the lateral organization of lipids and proteins in the plasma membrane has evolved substantially in the last few decades. It is widely accepted that many, if not all, plasma membrane proteins and lipids are organized in specific domains. These domains vary widely in size, composition, and stability, and they represent platforms governing diverse cell functions. The presynaptic plasma membrane is a well-studied example of a membrane which undergoes rearrangements, especially during exo- and endocytosis. Many proteins and lipids involved in presynaptic function are known, and major efforts have been made to understand their spatial organization and dynamics. Here, we focus on the mechanisms underlying the organization of SNAREs, the key proteins of the fusion machinery, in distinct domains, and we discuss the functional significance of these clusters.

Computational structural biology of opioid receptors

The publication of high-resolution structures for all of the opioid receptor subfamilies has unveiled exciting opportunities for mechanistic insight into the molecular mechanisms underlying the biology of nociception, reward, and higher cognitive functions, as well as promises for progress in several clinical areas such as pain management, physiological dependence, addiction, and mood disorders. To turn this promise into novel and improved therapeutic entities, however, this information needs to be supplemented with research strategies that explore the dynamic behavior of the proteins and their interactions with other receptors and ligands in their physiological environment.Here we describe state-of-the-art molecular dynamics computational protocols, based on all-atom and coarse-grained modeling techniques, designed to estimate crucial thermodynamic and kinetic parameters describing the binding of small-molecule ligands and the formation of supramolecular complexes.

The nanoscale organization of the B lymphocyte membrane

The fluid mosaic model of Singer and Nicolson correctly predicted that the plasma membrane (PM) forms a lipid bi-layer containing many integral trans-membrane proteins. This model also suggested that most of these proteins were randomly dispersed and freely diffusing moieties. Initially, this view of a dynamic and rather unorganized membrane was supported by early observations of the cell surfaces using the light microscope. However, recent studies on the PM below the diffraction limit of visible light (~250nm) revealed that, at nanoscale dimensions, membranes are highly organized and compartmentalized structures. Lymphocytes are particularly useful to study this nanoscale membrane organization because they grow as single cells and are not permanently engaged in cell:cell contacts within a tissue that can influence membrane organization. In this review, we describe the methods that can be used to better study the protein:protein interaction and nanoscale organization of lymphocyte membrane proteins, with a focus on the B cell antigen receptor (BCR). Furthermore, we discuss the factors that may generate and maintain these membrane structures.

Beyond standard molecular dynamics: investigating the molecular mechanisms of G protein-coupled receptors with enhanced molecular dynamics methods

The majority of biological processes mediated by G Protein-Coupled Receptors (GPCRs) take place on timescales that are not conveniently accessible to standard molecular dynamics (MD) approaches, notwithstanding the current availability of specialized parallel computer architectures, and efficient simulation algorithms. Enhanced MD-based methods have started to assume an important role in the study of the rugged energy landscape of GPCRs by providing mechanistic details of complex receptor processes such as ligand recognition, activation, and oligomerization. We provide here an overview of these methods in their most recent application to the field.

Update of the 1972 Singer-Nicolson Fluid-Mosaic Model of Membrane Structure

The Fluid-Mosaic Membrane Model of cell membrane structure was based on thermodynamic principals and the available data on component lateral mobility within the membrane plane [Singer SJ, Nicolson GL. The Fluid Mosaic Model of the structure of cell membranes. Science 1972; 175: 720-731]. After more than forty years the model remains relevant for describing the basic nano-scale structures of a variety of biological membranes. More recent information, however, has shown the importance of specialized membrane domains, such as lipid rafts and protein complexes, in describing the macrostructure and dynamics of biological membranes. In addition, membrane-associated cytoskeletal structures and extracellular matrix also play roles in limiting the mobility and range of motion of membrane components and add new layers of complexity and hierarchy to the original model. An updated Fluid-Mosaic Membrane Model is described, where more emphasis has been placed on the mosaic nature of cellular membranes where protein and lipid components are more crowded and limited in their movements in the membrane plane by lipid-lipid, protein-protein and lipid-protein interactions as well as cell-matrix, cell-cell and cytoskeletal interactions. These interactions are important in restraining membrane components and maintaining the unique mosaic organization of cell membranes into functional, dynamic domains.

Application of ESR spin label oximetry in food science

Lipid oxidation attributed to the presence of oxygen has long been a focal area for food science research due in early years mainly to its broad impact on the quality and shelf stability. The need to effectively strategize interventions to detect and eventually eliminate lipid oxidation in food remains as evidence on nutritional and health implications continue to accumulate. Electron spin resonance (ESR) spin label oximetry has been shown capable of detecting dissolved oxygen concentration in both liquid and gaseous phases based on the collision between oxygen and stable free radicals. This review aimed to summarize not just the principles and rationale of ESR spin label oximetry but also the wide spectrum of ESR spin label oximetry applications to date. The feasibility to identify in very early stage oxygen generation and consumption offers a promising tool for controlling lipid oxidation in food and biological systems.

Electron paramagnetic resonance studies of functionally active, nitroxide spin-labeled peptide analogues of the C-terminus of a G-protein alpha subunit

The C-terminal tail of the transducin alpha subunit, Gtalpha(340-350), is known to bind and stabilize the active conformation of rhodopsin upon photoactivation (R*). Five spin-labeled analogues of Gtalpha(340-350) demonstrated native-like activity in their ability to bind and stabilize R*. The spin-label 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) was employed at interior sites within the peptide, whereas a Proxyl (3-carboxyl-2,2,5,5-tetramethyl-pyrrolidinyloxy) spin-label was employed at the amino terminus of the peptide. Upon binding to R*, the electron paramagnetic resonance spectrum of TOAC(343)-Gtalpha(340-350) revealed greater immobilization of the nitroxide when compared to that of the N-terminally modified Proxyl-Gtalpha(340-350) analogue. A doubly labeled Proxyl/TOAC(348)-Gtalpha(340-350) was examined by DEER spectrocopy to determine the distribution of distances between the two nitroxides in the peptides when in solution and when bound to R*. TOAC and Proxyl spin-labels in this GPCR-G-protein alpha-peptide system provide unique biophysical probes that can be used to explore the structure and conformational changes at the rhodopsin-G-protein interface.

Hierarchical organization of the plasma membrane: investigations by single-molecule tracking vs. fluorescence correlation spectroscopy

Single-molecule tracking and fluorescence correlation spectroscopy (FCS) applied to the plasma membrane in living cells have allowed a number of unprecedented observations, thus fostering a new basic understanding of molecular diffusion, interaction, and signal transduction in the plasma membrane. It is becoming clear that the plasma membrane is a heterogeneous entity, containing diverse structures on nano-meso-scales (2-200 nm) with a variety of lifetimes, where certain membrane molecules stay together for limited durations. Molecular interactions occur in the time-dependent inhomogeneous two-dimensional liquid of the plasma membrane, which might be a key for plasma membrane functions.

Retinal dynamics during light activation of rhodopsin revealed by solid-state NMR spectroscopy

Rhodopsin is a canonical member of class A of the G protein-coupled receptors (GPCRs) that are implicated in many of the drug interventions in humans and are of great pharmaceutical interest. The molecular mechanism of rhodopsin activation remains unknown as atomistic structural information for the active metarhodopsin II state is currently lacking. Solid-state (2)H NMR constitutes a powerful approach to study atomic-level dynamics of membrane proteins. In the present application, we describe how information is obtained about interactions of the retinal cofactor with rhodopsin that change with light activation of the photoreceptor. The retinal methyl groups play an important role in rhodopsin function by directing conformational changes upon transition into the active state. Site-specific (2)H labels have been introduced into the methyl groups of retinal and solid-state (2)H NMR methods applied to obtain order parameters and correlation times that quantify the mobility of the cofactor in the inactive dark state, as well as the cryotrapped metarhodopsin I and metarhodopsin II states. Analysis of the angular-dependent (2)H NMR line shapes for selectively deuterated methyl groups of rhodopsin in aligned membranes enables determination of the average ligand conformation within the binding pocket. The relaxation data suggest that the beta-ionone ring is not expelled from its hydrophobic pocket in the transition from the pre-activated metarhodopsin I to the active metarhodopsin II state. Rather, the major structural changes of the retinal cofactor occur already at the metarhodopsin I state in the activation process. The metarhodopsin I to metarhodopsin II transition involves mainly conformational changes of the protein within the membrane lipid bilayer rather than the ligand. The dynamics of the retinylidene methyl groups upon isomerization are explained by an activation mechanism involving cooperative rearrangements of extracellular loop E2 together with transmembrane helices H5 and H6. These activating movements are triggered by steric clashes of the isomerized all-trans retinal with the beta4 strand of the E2 loop and the side chains of Glu(122) and Trp(265) within the binding pocket. The solid-state (2)H NMR data are discussed with regard to the pathway of the energy flow in the receptor activation mechanism.

Retinal conformation and dynamics in activation of rhodopsin illuminated by solid-state H NMR spectroscopy

Solid-state NMR spectroscopy gives a powerful avenue for investigating G protein-coupled receptors and other integral membrane proteins in a native-like environment. This article reviews the use of solid-state (2)H NMR to study the retinal cofactor of rhodopsin in the dark state as well as the meta I and meta II photointermediates. Site-specific (2)H NMR labels have been introduced into three regions (methyl groups) of retinal that are crucially important for the photochemical function of rhodopsin. Despite its phenomenal stability (2)H NMR spectroscopy indicates retinal undergoes rapid fluctuations within the protein binding cavity. The spectral lineshapes reveal the methyl groups spin rapidly about their three-fold (C(3)) axes with an order parameter for the off-axial motion of SC(3) approximately 0.9. For the dark state, the (2)H NMR structure of 11-cis-retinal manifests torsional twisting of both the polyene chain and the beta-ionone ring due to steric interactions of the ligand and the protein. Retinal is accommodated within the rhodopsin binding pocket with a negative pretwist about the C11=C12 double bond. Conformational distortion explains its rapid photochemistry and reveals the trajectory of the 11-cis to trans isomerization. In addition, (2)H NMR has been applied to study the retinylidene dynamics in the dark and light-activated states. Upon isomerization there are drastic changes in the mobility of all three methyl groups. The relaxation data support an activation mechanism whereby the beta-ionone ring of retinal stays in nearly the same environment, without a large displacement of the ligand. Interactions of the beta-ionone ring and the retinylidene Schiff base with the protein transmit the force of the retinal isomerization. Solid-state (2)H NMR thus provides information about the flow of energy that triggers changes in hydrogen-bonding networks and helix movements in the activation mechanism of the photoreceptor.

Solid-state 2H NMR spectroscopy of retinal proteins in aligned membranes

Solid-state 2H NMR spectroscopy gives a powerful avenue to investigating the structures of ligands and cofactors bound to integral membrane proteins. For bacteriorhodopsin (bR) and rhodopsin, retinal was site-specifically labeled by deuteration of the methyl groups followed by regeneration of the apoprotein. 2H NMR studies of aligned membrane samples were conducted under conditions where rotational and translational diffusion of the protein were absent on the NMR time scale. The theoretical lineshape treatment involved a static axial distribution of rotating C-C2H3 groups about the local membrane frame, together with the static axial distribution of the local normal relative to the average normal. Simulation of solid-state 2H NMR lineshapes gave both the methyl group orientations and the alignment disorder (mosaic spread) of the membrane stack. The methyl bond orientations provided the angular restraints for structural analysis. In the case of bR the retinal chromophore is nearly planar in the dark- and all-trans light-adapted states, as well upon isomerization to 13-cis in the M state. The C13-methyl group at the "business end" of the chromophore changes its orientation to the membrane upon photon absorption, moving towards W182 and thus driving the proton pump in energy conservation. Moreover, rhodopsin was studied as a prototype for G protein-coupled receptors (GPCRs) implicated in many biological responses in humans. In contrast to bR, the retinal chromophore of rhodopsin has an 11-cis conformation and is highly twisted in the dark state. Three sites of interaction affect the torsional deformation of retinal, viz. the protonated Schiff base with its carboxylate counterion; the C9-methyl group of the polyene; and the beta-ionone ring within its hydrophobic pocket. For rhodopsin, the strain energy and dynamics of retinal as established by 2H NMR are implicated in substituent control of activation. Retinal is locked in a conformation that is twisted in the direction of the photoisomerization, which explains the dark stability of rhodopsin and allows for ultra-fast isomerization upon absorption of a photon. Torsional strain is relaxed in the meta I state that precedes subsequent receptor activation. Comparison of the two retinal proteins using solid-state 2H NMR is thus illuminating in terms of their different biological functions.

G Protein-Coupled Receptors Self-Assemble in Dynamics Simulations of Model Bilayers

Many integral membrane proteins assemble to form oligomeric structures in biological membranes. In particular, seven-transmembrane helical G protein-coupled receptors (GPCRs) appear to self-assemble constitutively in membranes, but the mechanism and physiological role of this assembly are unknown. We developed and employed coarse-grain molecular dynamics (CGMD) models to investigate the molecular basis of how the physicochemical properties of the phospholipid bilayer membrane affect self-assembly of visual rhodopsin, a prototypical GPCR. The CGMD method is a mesoscopic simulation technique in which groups of atoms are mapped to particles on the basis of a four-to-one rule. This systematic reduction of the degrees of freedom allows for computationally efficient calculation of the structure and dynamics of molecular assemblies for larger time and length scales than accessible to atomistic models, providing here an unprecedented view of spontaneous protein assembly in biomembranes. Systems with up to 16 rhodopsin molecules at a protein-to-lipid ratio of 1:100 were simulated for time scales of up to 8 micros. The results obtained for four different phospholipid environments showed that localized adaptation of the membrane bilayer to the presence of receptors is reproducibly most pronounced near transmembrane helices 2, 4, and 7. This local membrane deformation appears to be a key factor defining the rate, extent, and orientational preference of protein-protein association. The implications of our findings are discussed within a framework of a generalized mechanism of membrane protein self-assembly.

Structural analysis and dynamics of retinal chromophore in dark and meta I states of rhodopsin from 2H NMR of aligned membranes

Rhodopsin is a prototype for G protein-coupled receptors (GPCRs) that are implicated in many biological responses in humans. A site-directed (2)H NMR approach was used for structural analysis of retinal within its binding cavity in the dark and pre-activated meta I states. Retinal was labeled with (2)H at the C5, C9, or C13 methyl groups by total synthesis, and was used to regenerate the opsin apoprotein. Solid-state (2)H NMR spectra were acquired for aligned membranes in the low-temperature lipid gel phase versus the tilt angle to the magnetic field. Data reduction assumed a static uniaxial distribution, and gave the retinylidene methyl bond orientations plus the alignment disorder (mosaic spread). The dark-state (2)H NMR structure of 11-cis-retinal shows torsional twisting of the polyene chain and the beta-ionone ring. The ligand undergoes restricted motion, as evinced by order parameters of approximately 0.9 for the spinning C-C(2)H(3) groups, with off-axial fluctuations of approximately 15 degrees . Retinal is accommodated within the rhodopsin binding pocket with a negative pre-twist about the C11=C12 double bond that explains its rapid photochemistry and the trajectory of 11-cis to trans isomerization. In the cryo-trapped meta I state, the (2)H NMR structure shows a reduction of the polyene strain, while torsional twisting of the beta-ionone ring is maintained. Distortion of the retinal conformation is interpreted through substituent control of receptor activation. Steric hindrance between trans retinal and Trp265 can trigger formation of the subsequent activated meta II state. Our results are pertinent to quantum and molecular mechanics simulations of ligands bound to GPCRs, and illustrate how (2)H NMR can be applied to study their biological mechanisms of action.

Curvature and hydrophobic forces drive oligomerization and modulate activity of rhodopsin in membranes

G protein-coupled receptors (GPCRs) are essential components of cellular signaling pathways. They are the targets of many current pharmaceuticals and are postulated to dimerize or oligomerize in cellular membranes in conjunction with their functional mechanisms. We demonstrate using fluorescence resonance energy transfer how association of rhodopsin occurs by long-range lipid-protein interactions due to geometrical forces, yielding greater receptor crowding. Constitutive association of rhodopsin is promoted by a reduction in membrane thickness (hydrophobic mismatch), but also by an increase in protein/lipid molar ratio, showing the importance of interactions extending well beyond a single annulus of boundary lipids. The fluorescence data correlate with the pK(a) for the MI-to-MII transition of rhodopsin, where deprotonation of the retinylidene Schiff base occurs in conjunction with helical movements leading to activation of the photoreceptor. A more dispersed membrane environment optimizes formation of the MII conformation that results in visual function. A flexible surface model explains both the dispersal and activation of rhodopsin in terms of bilayer curvature deformation (strain) and hydrophobic solvation energy. The bilayer stress is related to the lateral pressure profile in terms of the spontaneous curvature and associated bending rigidity. Transduction of the strain energy (frustration) of the bilayer drives protein oligomerization and conformational changes in a coupled manner. Our findings illuminate the physical principles of membrane protein association due to chemically nonspecific interactions in fluid lipid bilayers. Moreover, they yield a conceptual framework for understanding how the tightly regulated lipid compositions of cellular membranes influence their protein-mediated functions.

Conformational states and dynamics of rhodopsin in micelles and bilayers

Nitroxide sensors were placed in rhodopsin at sites 140, 227, 250, and 316 to monitor the dynamics and conformation of the receptor at the cytoplasmic surface in solutions of dodecyl maltoside (DM), digitonin, and phospholipid bilayers of two compositions. The EPR spectra reveal a remarkable similarity of rhodopsin structure and the activating conformational change in DM and bilayers, the hallmark of which is an outward tilt of transmembrane helix VI. This conformational change is blocked in solutions of digitonin, although changes in optical absorbance accompany activation, showing that absorbance and structural changes are not necessarily coupled. In DM and bilayers, the receptor is apparently in equilibrium between conformational substates whose populations are modulated by activation. Despite the general similarity in the two environments, the receptor conformations have increased flexibility in DM relative to bilayers. For the activated receptor in DM and bilayers, a pH-dependent conformational equilibrium is identified that may correspond to the optically characterized MII(a)()-MII(b)() equilibrium. No specific effects of headgroup composition on receptor conformation in lipid bilayers were found.

Toward understanding the dynamics of membrane-raft-based molecular interactions

The cell membrane is a 2-dimensional non-ideal liquid containing dynamic structures on various time-space scales, and the raft domain is one of them. Existing literature supports the concept that raft dynamics may be important for its formation and function: the raft function may be supported by stimulation-induced raft association/coalescence and recruitment of various raftophilic molecules to coalesced rafts, and, importantly, they both may happen transiently. Thus, one must always consider the limited association time of a raft or a raftophilic molecule with another raft, even when one interprets the results of static experiments, such as immunofluorescence and pull-down assays. Critical considerations on the chemical fixation mechanism and immunocolocalization data suggest that the temporary nature of raft-based molecular interactions may explain why colocalization results are sensitive to subtle variations in experimental conditions employed in different laboratories.

Membrane Switch Hypothesis. 2. Domain Structure of Phagocytes in Horses with Recurrent Airway Obstruction

The mechanism of recurrent airway obstruction (RAO) in horses was investigated by measuring the membrane domain structure and oxy-redoxy activity in phagocytes isolated from bronchoalveolar lavage fluid (BAL) and from the blood of healthy and RAO horses by electron paramagnetic resonance (EPR). Differences in the activity of intracellular antioxidant enzymes CAT, GPx, and SOD measured in phagocytes of RAO horses in comparison to healthy horses showed that the phagocytes were affected by oxidative stress. In comparison with polymorphonuclear leukocytes (phagocytes) from the blood of healthy horses the reduction mechanisms in BAL were faster and coincided with the merging of disordered membrane domains, while in horses with RAO the reduction and membrane domain structure remained unchanged. We assume that the merging of lipid domains observed in phagocytes from BAL of healthy horses could promote cluster formation of membrane proteins or ligands, which could trigger the activation process in phagocytes of healthy horses and consequently the physiological response that probably did not happen in phagocytes of RAO horses.

Paradigm shift of the plasma membrane concept from the two-dimensional continuum fluid to the partitioned fluid: high-speed single-molecule tracking of membrane molecules

Recent advancements in single-molecule tracking methods with nanometer-level precision now allow researchers to observe the movement, recruitment, and activation of single molecules in the plasma membrane in living cells. In particular, on the basis of the observations by high-speed single-particle tracking at a frame rate of 40,000 frames s(1), the partitioning of the fluid plasma membrane into submicron compartments throughout the cell membrane and the hop diffusion of virtually all the molecules have been proposed. This could explain why the diffusion coefficients in the plasma membrane are considerably smaller than those in artificial membranes, and why the diffusion coefficient is reduced upon molecular complex formation (oligomerization-induced trapping). In this review, we first describe the high-speed single-molecule tracking methods, and then we critically review a new model of a partitioned fluid plasma membrane and the involvement of the actin-based membrane-skeleton "fences" and anchored-transmembrane protein "pickets" in the formation of compartment boundaries.

Molecular dynamics and interactions for creation of stimulation-induced stabilized rafts from small unstable steady-state rafts

We have evaluated the sizes and lifetimes of rafts in the plasma membrane from the existing literature, with a special attention paid to their intrinsically broad distributions and the limited time and space scales that are covered by the observation methods used for these studies. Distinguishing the rafts in the steady state (reserve rafts) from those after stimulation or unintentional crosslinking of raft molecules (stabilized receptor-cluster rafts) is critically important. In resting cells, the rafts appear small and unstable, and the consensus now is that their sizes are smaller than the optical diffraction limit (250 nm). Upon stimulation, the raft-preferring receptors are clustered, inducing larger, stabilized rafts, probably by coalescing small, unstable rafts or cholesterol-glycosphingolipid complexes in the receptor clusters. This receptor-cluster-induced conversion of raft types may be caused by suppression of alkyl chain isomerization and the lipid lateral diffusion in the cluster, with the aid of exclusion of cholesterol from the bulk domain and the boundary region of the majority of transmembrane proteins. We critically inspected the possible analogy to the boundary lipid concept. Finally, we propose a hypothesis for the coupling of GPI-anchored receptor signals with lipid-anchored signaling molecules in the inner-leaflet raft.

Functional characterization of rhodopsin monomers and dimers in detergents

Rhodopsin (Rho) is a G protein-coupled receptor that initiates phototransduction in rod photoreceptors. High expression levels of Rho in the disc membranes of rod outer segments and the propensity of Rho to form higher oligomeric structures are evident from atomic force microscopy, transmission electron microscopy, and chemical cross-linking experiments. To explore the structural and functional properties of Rho in n-dodecyl-beta-maltoside, frequently used to purify heterologously expressed Rho and its mutants, we used gel filtration techniques, blue native gel electrophoresis, and functional assays. Here, we show that in micelles containing n-dodecyl-beta-maltoside at concentrations greater than 3 mM, Rho is present as a single monomer per detergent micelle. In contrast, in 12 mM 3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate (CHAPS), micelles contain mostly dimeric Rho. The cognate G protein transducin (Gt) appears to have a preference for binding to the Rho dimer, and the complexes fall apart in the presence of guanosine 5'-3-O-(thio)triphosphate. Cross-linked Rho dimers release the chromophore at a slower rate than monomers and are much more resistant to heat denaturation. Both Rho(*) monomers and dimers are capable of activating Gt, and both of them are phosphorylated by Rho kinase. Rho expressed in HEK293 cells is also readily cross-linked by a bifunctional reagent. These studies provide an explanation of how detergent influences the oligomer-dimermonomer equilibrium of Rho and describe the functional characterization of Rho monomers and dimers in detergent.

Hydrophobic coupling of lipid bilayer energetics to channel function

The hydrophobic coupling between membrane-spanning proteins and the lipid bilayer core causes the bilayer thickness to vary locally as proteins and other "defects" are embedded in the bilayer. These bilayer deformations incur an energetic cost that, in principle, could couple membrane proteins to each other, causing them to associate in the plane of the membrane and thereby coupling them functionally. We demonstrate the existence of such bilayer-mediated coupling at the single-molecule level using single-barreled as well as double-barreled gramicidin channels in which two gramicidin subunits are covalently linked by a water-soluble, flexible linker. When a covalently attached pair of gramicidin subunits associates with a second attached pair to form a double-barreled channel, the lifetime of both channels in the assembly increases from hundreds of milliseconds to a hundred seconds--and the conductance of each channel in the side-by-side pair is almost 10% higher than the conductance of the corresponding single-barreled channels. The double-barreled channels are stabilized some 100,000-fold relative to their single-barreled counterparts. This stabilization arises from: first, the local increase in monomer concentration around a single-barreled channel formed by two covalently linked gramicidins, which increases the rate of double-barreled channel formation; and second, from the increased lifetime of the double-barreled channels. The latter result suggests that the two barrels of the construct associate laterally. The underlying cause for this lateral association most likely is the bilayer deformation energy associated with channel formation. More generally, the results suggest that the mechanical properties of the host bilayer may cause the kinetics of membrane protein conformational transitions to depend on the conformational states of the neighboring proteins.

Dynamics of raft molecules in the cell and artificial membranes: approaches by pulse EPR spin labeling and single molecule optical microscopy

Lipid rafts in the plasma membrane, domains rich in cholesterol and sphingolipids, have been implicated in a number of important membrane functions. Detergent insolubility has been used to define membrane "rafts" biochemically. However, such an approach does not directly contribute to the understanding of the size and the lifetime of rafts, dynamics of the raft-constituent molecules, and the function of rafts in the membrane in situ. To address these issues, we have developed pulse EPR spin labeling and single molecule tracking optical techniques for studies of rafts in both artificial and cell membranes. In this review, we summarize our results and perspectives obtained by using these methods. We emphasize the importance of clearly distinguishing small/unstable rafts (lifetime shorter than a millisecond) in unstimulated cells and stabilized rafts induced by liganded and oligomerized (GPI-anchored) receptor molecules (core receptor rafts, lifetime over a few minutes). We propose that these stabilized rafts further induce temporal, greater rafts (signaling rafts, lifetime on the order of a second) for signaling by coalescing other small/unstable rafts, including those in the inner leaflet of the membrane, each containing perhaps one molecule of the downstream effector molecules. At variance with the general view, we emphasize the importance of cholesterol segregation from the liquid-crystalline unsaturated bulk-phase membrane for formation of the rafts, rather than the affinity of cholesterol and saturated alkyl chains. In the binary mixture of cholesterol and an unsaturated phospholipid, cholesterol is segregated out from the bulk unsaturated liquid-crystalline phase, forming cholesterol-enriched domains or clustered cholesterol domains, probably due to the lateral nonconformability between the rigid planar transfused ring structure of cholesterol and the rigid bend of the unsaturated alkyl chain at C9-C10. However, such cholesterol-rich domains are small, perhaps consisting of only several cholesterol molecules, and are short-lived, on the order of 1-100 ns. We speculate that these cholesterol-enriched domains may be stabilized by the presence of saturated alkyl chains of sphingomyelin or glycosphingolipids, and also by clustered raft proteins. In the influenza viral membrane, one of the simplest forms of a biological membrane, the lifetime of a protein and cholesterol-rich domain was evaluated to be on the order of 100 micro, again showing the short lifetime of rafts in an unstimulated state. Finally, we propose a thermal Lego model for rafts as the basic building blocks for signaling pathways in the plasma membrane.

Pulse EPR detection of lipid exchange between protein-rich raft and bulk domains in the membrane: methodology development and its application to studies of influenza viral membrane

A pulse saturation-recovery electron paramagnetic resonance (EPR) method has been developed that allows estimation of the exchange rates of a spin-labeled lipid between the bulk domain and the protein-rich membrane domain, in which the rate of collision between the spin label and molecular oxygen is reduced (slow-oxygen transport domain, or SLOT domain). It is based on the measurements of saturation-recovery signals of a lipid spin label as a function of concentrations of both molecular oxygen and the spin label. Influenza viral membrane, one of the simplest paradigms for the study of biomembranes, showed the presence of two membrane domains with slow and fast collision rates with oxygen (a 16-fold difference) at 30 degrees C. The outbound rate from and the inbound rate into the SLOT domain (or possibly the rate of the domain disintegration and formation) were estimated to be 7.7 x 10(4) and 4.6 x 10(4) s(-1), (15 micros residency time), respectively, indicating that the SLOT domain is highly dynamic and that the entire SLOT domain represents about one-third of the membrane area. Because the oxygen transport rate in the SLOT domain is a factor of two smaller than that in purple membrane, where bacteriorhodopsin is aggregated, we propose that the SLOT domain in the viral membrane is the cholesterol-rich raft domain stabilized by the trimers of hemagglutinin and/or the tetramers of neuraminidase.

Cholesterol dependent recruitment of di22:6-PC by a G protein-coupled receptor into lateral domains

Bovine rhodopsin was reconstituted into mixtures of didocosahexaenoylphosphatidylcholine (di22:6-PC), dipalmitoylphosphatidylcholine (di16:0-PC), sn-1-palmitoyl-sn-2-docosahexaenoylphosphatidylcholine (16:0, 22:6-PC) and cholesterol. Rhodopsin denaturation was examined by using high-sensitivity differential scanning calorimetry. The unfolding temperature was increased at lower levels of lipid unsaturation, but the highest temperature was detected for native disk membranes: di22:6-PC < 16:0,22:6-PC < di16:0,18:1-PC < native disks. The incorporation of 30 mol% of cholesterol resulted in 2-4 degrees C increase of denaturation temperature in all reconstituted systems examined. From the analysis of van't Hoff's and calorimetric enthalpies, it was concluded that the presence of cholesterol in di22:6-PC-containing bilayers induces a level of cooperativity in rhodopsin unfolding. Fluorescence resonance energy transfer (FRET), using lipids labeled at the headgroup with pyrene (Py) as donors and rhodopsin retinal group as acceptor of fluorescence, was used to study rhodopsin association with lipids. Higher FRET efficiencies detected for di22:6-PE-Py, compared to di16:0-PE-Py, in mixed di22:6-PC-di16:0-PC-cholesterol bilayers, indicate preferential segregation of rhodopsin with polyunsaturated lipids. The effective range of the rhodopsin-lipid interactions facilitating cluster formation exceeds two adjacent lipid layers. In similar mixed bilayers containing no cholesterol, cluster formation was absent at temperatures above lipid phase transition, indicating a crucial role of cholesterol in microdomain formation.

Hydrophobic mismatch between proteins and lipids in membranes

This review addresses the possible consequences of a mismatch in length between the hydrophobic part of membrane-spanning proteins and the hydrophobic bilayer thickness for membrane structure and function. Overviews are given first of the results of studies in defined model systems. These studies address effects of mismatch on protein activity, stability, orientation, aggregational state, localization, and conformation. With respect to the lipids, effects of mismatch are discussed on lipid chain order, phase transition temperature, lipid phase behavior, and microdomain formation. From these studies, it is concluded that hydrophobic mismatch can strongly affect protein and lipid organization, but that the precise consequences depend on the individual properties of the proteins and lipids. Examples of these properties include the propensity of lipids to form non-lamellar structures, the amino acid composition of the hydrophobic transmembrane segments of the proteins, the nature of the membrane anchoring residues, and the number of transmembrane helices. Finally, the effects of mismatch in biological membranes are discussed and its possible consequences for functional membrane processes, such as protein sorting, protein insertion, and regulation of bilayer thickness.

A single-residue deletion alters the lipid selectivity of a K+ channel-associated peptide in the beta-conformation: spin label electron spin resonance studies

Lipid-peptide interactions with the 27-residue peptide of sequence KLEALYILMVLGFFGFFTLGIMLSYIR reconstituted as beta-sheet assemblies in dimyristoylphosphatidylcholine bilayers have been studied by electron spin resonance (ESR) spectroscopy with spin-labeled lipids. The peptide corresponds to residues 42-68 of the IsK voltage-gated K+ channel protein and contains the single putative transmembrane span of this protein. Lipid-peptide interactions give rise to a second component in the ESR spectra of lipids spin-labeled on the 14C atom of the chain that corresponds to restriction of the lipid mobility by direct interaction with the peptide assemblies. From the dependence on the lipid/peptide ratio, the stoichiometry of lipid interaction is found to be about two phospholipids/peptide monomer. The sequence of selectivity for lipid association with the peptide assemblies is in the order phosphatidic acid > stearic acid = phosphatidylserine > phosphatidylglycerol = phosphatidylcholine. Comparison with previous data for a corresponding 26-residue mutant peptide with a single deletion of the apolar residue Leu2 (Horvath et al., 1995. Biochemistry 34:3893-3898), indicates a very similar mode of membrane incorporation for native and mutant peptides, but a strongly modified pattern and degree of specificity for the interaction with negatively charged lipids. The latter is interpreted in terms of the relative orientations of the charged amino acid side chains in the beta-sheet assemblies of the native and deletion-mutant peptides.

Molecular sorting of lipids by bacteriorhodopsin in dilauroylphosphatidylcholine/distearoylphosphatidylcholine lipid bilayers

A combined experimental and theoretical study is performed on binary dilauroylphosphatidylcholine/distearoylphosphatidylcholine (DLPC/DSPC) lipid bilayer membranes incorporating bacteriorhodopsin (BR). The system is designed to investigate the possibility that BR, via a hydrophobic matching principle related to the difference in lipid bilayer hydrophobic thickness and protein hydrophobic length, can perform molecular sorting of the lipids at the lipid-protein interface, leading to lipid specificity/selectivity that is controlled solely by physical factors. The study takes advantage of the strongly nonideal mixing behavior of the DLPC/DSPC mixture and the fact that the average lipid acyl-chain length is strongly dependent on temperature, particularly in the main phase transition region. The experiments are based on fluorescence energy transfer techniques using specifically designed lipid analogs that can probe the lipid-protein interface. The theoretical calculations exploit a microscopic molecular interaction model that embodies the hydrophobic matching as a key parameter. At low temperatures, in the gel-gel coexistence region, experimental and theoretical data consistently indicate that BR is associated with the short-chain lipid DLPC. At moderate temperatures, in the fluid-gel coexistence region, BR remains in the fluid phase, which is mainly composed of short-chain lipid DLPC, but is enriched at the interface between the fluid and gel domains. At high temperatures, in the fluid phase, BR stays in the mixed lipid phase, and the theoretical data suggest a preference of the protein for the long-chain DSPC molecules at the expense of the short-chain DLPC molecules. The combined results of the experiments and the calculations provide evidence that a molecular sorting principle is active because of hydrophobic matching and that BR exhibits physical lipid selectivity. The results are discussed in the general context of membrane organization and compartmentalization and in terms of nanometer-scale lipid-domain formation.

Depth dependence of the perturbing effect of placing a bulky group (oxazolidine ring spin labels) in the membrane on the membrane phase transition

Electron paramagnetic resonance (EPR) and differential scanning calorimetry (DSC) have been used to study the effect on the phase transition of dimyristoylphosphatidylcholine membranes of incorporating various stearic acid spin labels (SASL's) that contain the bulky oxazolidine ring at various positions along the stearyl chain. SASL's lowered the phase transition temperature and decreased the size of the cooperative unit, with the effects stronger in the order of 9- > 12- > 5- > 16-SASL > stearic acid (no label). Incorporation of stearic acid without the spin label slightly increases the phase transition temperature. Incorporation of 9-SASL (3 mol% of lipid) decreased the transition temperature by 1.8 degrees C and the cooperative unit to 1/5 of that without the spin label, while the effect of 16-SASL was slight. The effect on transition enthalpy was small. It is concluded that the perturbing effect of placing a bulky group on the alkyl chain on phase transition is through inducing packing defects in the gel-phase.

Evidence that the effects of phospholipids on the activity of the Ca(2+)-ATPase do not involve aggregation

The Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum, solubilized in monomeric from in C12E8, has been reconstituted by dialysis into sealed vesicles of dioleoyl phosphatidylcholine [di(C18:1)PC], dimyristoleoyl phosphatidylcholine [di(C14:1)PC], dinervonyl phosphatidylcholine [di(C24:1)PC] or dipalmitoyl phosphatidylcholine [di(C16:0)PC] in the gel phase, at a phospholipid/ATPase molar ratio of 10,000: 1. Cross-linking experiments show that ATPase molecules are present in these reconstituted vesicles as isolated monomeric species. ATPase activities for the reconstituted vesicles are about half of those for the ATPase reconstituted with the same lipid in unsealed membrane fragments, attributed to a close to random orientation for the ATPase molecules in the reconstituted vesicles. ATPase activities for the ATPase in reconstituted vesicles of di(C14:1)PC or di(C24:1)PC are less than in vesicles of di(C18:1)PC, and no activity could be detected for the ATPase in di(C16:0)PC in the gel phase. It is concluded that effects of lipids on the activity of the ATPase are independent of any changes in the state of aggregation of the ATPase. Inhibition of ATPase activity by spermine and by the hydrophilic domain of phospholamban are observed both for the unreconstituted ATPase and for the ATPase in reconstituted vesicles, so that inhibition is independent of any aggregation caused by these polycationic species. Stimulation of ATPase activity by jasmone is also observed both for the unreconstituted ATPase and for the ATPase in reconstituted vesicles, so that stimulation of the ATPase also does not follow from any change in the state of aggregation of the ATPase.

Modulation of rhodopsin function by properties of the membrane bilayer

A prevalent model for the function of rhodopsin centers on the metarhodopsin I (MI) to metarhodopsin II (MII) conformational transition as the triggering event for the visual process. Flash photolysis techniques enable one to determine the [MII]/[MI] ratio for rhodopsin in various recombinant membranes, and thus investigate the roles of the phospholipid head groups and the lipid acyl chains systematically. The results obtained to date clearly show that the pK for the acid-base MI-MII equilibrium of rhodopsin is modulated by the lipid environment. In bilayers of phosphatidylcholines the MI-MII equilibrium is shifted to the left; whereas in the native rod outer segment membranes it is shifted to the right, i.e., at neutral pH near physiological temperature. The lipid mixtures sufficient to yield full photochemical function of rhodopsin include a native-like head group composition, viz, comprising phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS), in combination with polyunsaturated docosahexaenoic acid (DHA; 22:6 omega 3) chains. Yet such a native-like lipid mixture is not necessary for the MI-MII conformational transition of rhodopsin; one can substitute other lipid compositions having similar properties. The MI-MII transition is favored by relatively small head groups which produce a condensed bilayer surface, viz, a comparatively small interfacial area as in the case of PE, together with bulky acyl chains such as DHA which prefer a relatively large cross sectional area. The resulting force imbalance across the layer gives rise to a curvature elastic stress of the lipid/water interface, such that the lipid mixtures yielding native-like behavior form reverse hexagonal (HII) phases at slightly higher temperatures. A relatively unstable membrane is needed: lipids tending to form the lamellar phase do not support full native-like photochemical function of rhodopsin. Thus chemically specific properties of the various lipids are not required, but rather average or material properties of the entire assembly, which may involve the curvature free energy of the membrane-lipid water interface. These findings reveal that the membrane lipid bilayer has a direct influence on the energetics of the conformational states of rhodopsin in visual excitation.

Compartmentalized structure of the plasma membrane for receptor movements as revealed by a nanometer-level motion analysis

Movements of transferrin and alpha 2-macroglobulin receptor molecules in the plasma membrane of cultured normal rat kidney (NRK) fibroblastic cells were investigated by video-enhanced contrast optical microscopy with 1.8 nm spatial precision and 33 ms temporal resolution by labeling the receptors with the ligand-coated nanometer-sized colloidal gold particles. For both receptor species, most of the movement trajectories are of the confined diffusion type, within domains of approximately 0.25 microns2 (500-700 nm in diagonal length). Movement within the domains is random with a diffusion coefficient approximately 10(-9) cm2/s, which is consistent with that expected for free Brownian diffusion of proteins in the plasma membrane. The receptor molecules move from one domain to one of the adjacent domains at an average frequency of 0.034 s-1 (the residence time within a domain approximately 29 s), indicating that the plasma membrane is compartmentalized for diffusion of membrane receptors and that long-range diffusion is the result of successive intercompartmental jumps. The macroscopic diffusion coefficients for these two receptor molecules calculated on the basis of the compartment size and the intercompartmental jump rate are approximately 2.4 x 10(-11) cm2/s, which is consistent with those determined by averaging the long-term movements of many particles. Partial destruction of the cytoskeleton decreased the confined diffusion mode, increased the simple diffusion mode, and induced the directed diffusion (transport) mode. These results suggest that the boundaries between compartments are made of dynamically fluctuating membrane skeletons (membrane-skeleton fence model).

Confined lateral diffusion of membrane receptors as studied by single particle tracking (nanovid microscopy). Effects of calcium-induced differentiation in cultured epithelial cells

The movements of E-cadherin, epidermal growth factor receptor, and transferrin receptor in the plasma membrane of a cultured mouse keratinocyte cell line were studied using both single particle tracking (SPT; nanovid microscopy) and fluorescence photobleaching recovery (FPR). In the SPT technique, the receptor molecules are labeled with 40 nm-phi colloidal gold particles, and their movements are followed by video-enhanced differential interference contrast microscopy at a temporal resolution of 33 ms and at a nanometer-level spatial precision. The trajectories of the receptor molecules obtained by SPT were analyzed by developing a method that is based on the plot of the mean-square displacement against time. Four characteristic types of motion were observed: (a) stationary mode, in which the microscopic diffusion coefficient is less than 4.6 x 10(-12) cm2/s; (b) simple Brownian diffusion mode; (c) directed diffusion mode, in which unidirectional movements are superimposed on random motion; and (d) confined diffusion mode, in which particles undergoing Brownian diffusion (microscopic diffusion coefficient between 4.6 x 10(-12) and 1 x 10(-9) cm2/s) are confined within a limited area, probably by the membrane-associated cytoskeleton network. Comparison of these data obtained by SPT with those obtained by FPR suggests that the plasma membrane is compartmentalized into many small domains 300-600 nm in diameter (0.04-0.24 microns2 in area), in which receptor molecules are confined in the time scale of 3-30 s, and that the long-range diffusion observed by FPR can occur by successive movements of the receptors to adjacent compartments. Calcium-induced differentiation decreases the sum of the percentages of molecules in the directed diffusion and the stationary modes outside of the cell-cell contact regions on the cell surface (which is proposed to be the percentage of E-cadherin bound to the cytoskeleton/membrane-skeleton), from approximately 60% to 8% (low- and high-calcium mediums, respectively).