Interaction of phosphatidylserine synthase from E. coli with lipid bilayers: coupled plasmon-waveguide resonance spectroscopy studies

Organization and function of anionic phospholipids in bacteria

In addition to playing a central role as a permeability barrier for controlling the diffusion of molecules and ions in and out of bacterial cells, phospholipid (PL) membranes regulate the spatial and temporal position and function of membrane proteins that play an essential role in a variety of cellular functions. Based on the very large number of membrane-associated proteins encoded in genomes, an understanding of the role of PLs may be central to understanding bacterial cell biology. This area of microbiology has received considerable attention over the past two decades, and the local enrichment of anionic PLs has emerged as a candidate mechanism for biomolecular organization in bacterial cells. In this review, we summarize the current understanding of anionic PLs in bacteria, including their biosynthesis, subcellular localization, and physiological relevance, discuss evidence and mechanisms for enriching anionic PLs in membranes, and conclude with an assessment of future directions for this area of bacterial biochemistry, biophysics, and cell biology.

An in vivo ratio control mechanism for phospholipid homeostasis: evidence from lipidomic studies

While it is widely accepted that the lipid composition of eukaryotic membranes is under homeostatic control, the mechanisms through which cells sense lipid composition are still the subject of debate. It has been postulated that membrane curvature elastic energy is the membrane property that is regulated by cells, and that lipid composition is maintained by a ratio control function derived from the concentrations of type II and type 0 lipids, weighted appropriately. We assess this proposal by seeking a signature of ratio control in quantified lipid composition data obtained by electrospray ionization mass spectrometry from over 40 independent asynchronous cell populations. Our approach revealed the existence of a universal 'pivot' lipid, which marks the boundary between type 0 lipids and type II lipids, and which is invariant between different cell types or cells grown under different conditions. The presence of such a pivot species is a distinctive signature of the operation in vivo, in human cell lines, of a control function that is consistent with the hypothesis that membrane elastic energy is homeostatically controlled.

Is there a preferential interaction between cholesterol and tryptophan residues in membrane proteins?

Recently, several indications have been found that suggest a preferential interaction between cholesterol and tryptophan residues located near the membrane-water interface. The aim of this study was to investigate by direct methods how tryptophan and cholesterol interact with each other and what the possible consequences are for membrane organization. For this purpose, we used cholesterol-containing model membranes of dimyristoylphosphatidylcholine (DMPC) in which a transmembrane model peptide with flanking tryptophans [acetyl-GWW(LA)8LWWA-amide], called WALP23, was incorporated to mimic interfacial tryptophans of membrane proteins. These model systems were studied with two complementary methods. (1) Steady-state and time-resolved Förster resonance energy transfer (FRET) experiments employing the fluorescent cholesterol analogue dehydroergosterol (DHE) in combination with a competition experiment with cholesterol were used to obtain information about the distribution of cholesterol in the bilayer in the presence of WALP23. The results were consistent with a random distribution of cholesterol which indicates that cholesterol and interfacial tryptophans are not preferentially located next to each other in these bilayer systems. (2) Solid-state 2H NMR experiments employing either deuterated cholesterol or indole ring-deuterated WALP23 peptides were performed to study the orientation and dynamics of both molecules. The results showed that the quadrupolar splittings of labeled cholesterol were not affected by an interaction with tryptophan-flanked peptides and, vice versa, that the quadrupolar splittings of labeled indole rings in WALP23 are not significantly influenced by addition of cholesterol to the bilayer. Therefore, both NMR and fluorescence spectroscopy results independently show that, at least in the model systems studied here, there is no evidence for a preferential interaction between cholesterol and tryptophans located at the bilayer interface.

Membrane-bound conformation of M13 major coat protein: a structure validation through FRET-derived constraints

M13 major coat protein, a 50-amino-acid-long protein, was incorporated into DOPC/DOPG (80/20 molar ratio) unilamellar vesicles. Over 60% of all amino acid residues was replaced with cysteine residues, and the single cysteine mutants were labeled with the fluorescent label I-AEDANS. The coat protein has a single tryptophan residue that is used as a donor in fluorescence (or Förster) resonance energy transfer (FRET) experiments, using AEDANS-labeled cysteines as acceptors. Based on FRET-derived constraints, a straight alpha-helix is proposed as the membrane-bound conformation of the coat protein. Different models were tested to represent the molecular conformations of the donor and acceptor moieties. The best model was used to make a quantitative comparison of the FRET data to the structures of M13 coat protein and related coat proteins in the Protein Data Bank. This shows that the membrane-bound conformation of the coat protein is similar to the structure of the coat protein in the bacteriophage that was obtained from x-ray diffraction. Coat protein embedded in stacked, oriented bilayers and in micelles turns out to be strongly affected by the environmental stress of these membrane-mimicking environments. Our findings emphasize the need to study membrane proteins in a suitable environment, such as in fully hydrated unilamellar vesicles. Although larger proteins than M13 major coat protein may be able to handle environmental stress in a different way, any membrane protein with water exposed parts in the C or N termini and hydrophilic loop regions should be treated with care.

Plasmon resonance methods in GPCR signaling and other membrane events

The existence of surface guided electromagnetic waves has been theoretically predicted from Maxwell's equations and investigated during the first decades of the 20th century. However, it is only since the late 1960's that they have attracted the interest of surface physicists and earned the moniker of "surface plasmon". With the advent of commercially available instruments and well established theories, the technique has been used to study a wide variety of biochemical and biotechnological phenomena. Spectral response of the resonance condition serves as a sensitive indicator of the optical properties of thin films immobilized within a wavelength of the surface. This enhanced surface sensitivity has provided a boon to the surface sciences, and fosters collaboration between surface chemistry, physics and the ongoing biological and biotechnological revolution. Since then, techniques based on surface plasmons such as Surface Plasmon Resonance (SPR), SPR Imaging, Plasmon Waveguide Resonance (PWR) and others, have been increasingly used to determine the affinity and kinetics of a wide variety of real time molecular interactions such as protein-protein, lipid-protein and ligand-protein, without the need for a molecular tag or label. The physical-chemical methodologies used to immobilize membranes at the surface of these optical devices are reviewed, pointing out advantages and limitations of each method. The paper serves to summarize both historical and more recent developments of these technologies for investigating structure-function aspects of these molecular interactions, and regulation of specific events in signal transduction by G-protein coupled receptors (GPCRs).

Lipid bilayer topology of the transmembrane alpha-helix of M13 Major coat protein and bilayer polarity profile by site-directed fluorescence spectroscopy

This article presents a new formalism to perform a quantitative fluorescence analysis using the Stokes shift of AEDANS-labeled cysteine mutants of M13 major coat protein incorporated in lipid bilayers. This site-directed fluorescence spectroscopy approach enables us to obtain the topology of the bilayer-embedded transmembrane alpha-helix from the orientation and tilt angles, and relative bilayer location. Both in pure dioleoylphosphatidylcholine and dioleoylphosphatidylcholine/dioleoylphosphatidylglycerol (4:1 mol/mol) bilayers, which have a similar bilayer thickness, the tilt angle of the transmembrane helix of the coat protein turns out to be 23 degrees +/- 4. Upon decreasing the hydrophobic thickness on going from dieicosenoylphosphatidylcholine to dimyristoylphosphatidylcholine, the tilt angle and orientation angle of the transmembrane alpha-helix change. The protein responds to an increase of hydrophobic stress by increasing the tilt angle so as to keep much of its hydrophobic part inside the bilayer. At the same time, the transmembrane helix rotates at its long axis so as to optimize the hydrophobic and electrostatic interactions of the C-terminal phenylalanines and lysines, respectively. The increase of tilt angle cannot completely keep the hydrophobic protein section within the bilayer, but the C-terminal part remains anchored at the acyl-chain/glycerol backbone interface at the cost of the N-terminal section. In addition, our analysis results in the profile of the dielectric constant of the hydrophobic domain of the bilayer. For all phospholipid bilayers studied the profile has a concave shape, with a value of the dielectric constant of 4.0 in the center of the bilayer. The dielectric constant increases on approaching the headgroup region with a value of 12.4 at the acyl-chain/glycerol backbone interface for the various phosphatidylcholines with different chain lengths. For dioleoylphosphatidylcholine/dioleoylphosphatidylglycerol (4:1 mol/mol) bilayers the value of the dielectric constant at the acyl-chain/glycerol backbone interface is 18.6. In conclusion, the consistency of our analysis shows that the applied cysteine-scanning mutagenesis method with AEDANS labeling of a helical transmembrane protein in combination with a quantitative formalism offers a reliable description of the lipid bilayer topology of the protein and bilayer properties. This also indicates that the spacer link between the protein and AEDANS label is long enough to monitor the local polarity of the lipid environment and not that of the amino-acid residues of the protein, and short enough to have the topology of the protein imposing on the fluorescence properties of the AEDANS label.

Graphical analysis of mass and anisotropy changes observed by plasmon-waveguide resonance spectroscopy can provide useful insights into membrane protein function

Plasmon-waveguide resonance spectroscopy is a recently developed optical method that allows characterization of mass and structural changes in two-dimensionally ordered thin films (e.g., proteolipid membranes) deposited onto a sensor surface. Full analysis of these systems involves fitting theoretical curves (obtained using Maxwell's equations) to experimental spectra measured using s- and p-polarized excitation. This allows values to be obtained for refractive indices and optical extinction coefficients in these two directions, as well as a value for film thickness, thereby providing information about mass density and anisotropy changes. This is a time-consuming process that works well for simple systems in which only a single conformational event occurs, but cannot distinguish between events involving multiple conformations that proceed either sequentially or in a parallel series of events. This article describes a graphical method that can distinguish between mass density and anisotropy changes in a simpler, more rapid procedure, even for processes that proceed via multiple conformational events. This involves measurement of plasmon-waveguide resonance spectral shifts obtained upon molecular interactions occurring in deposited films with both s- and p-polarized excitation, and transforming these from an (s-p) coordinate system into a (mass-structure) coordinate system. This procedure is illustrated by data obtained upon the binding of a small peptide, penetratin, to solid-supported lipid bilayer membranes.

Plasmon-waveguide resonance spectroscopy applied to three potential drug targets: cyclooxygenase-2, hepatitis C virus RNA polymerase and integrin αVβ3

Plasmon-waveguide resonance (PWR) spectroscopy has been used to study the interactions between ligands that correspond to inhibitors, activators or substrates and three integral membrane proteins representing potential drug targets; cyclooxygenases 1 and 2 (COX-1 and -2), integrin alphaVbeta3, and hepatitis C virus RNA polymerase. The proteins were incorporated into an egg phosphatidylcholine bilayer deposited onto the surface of the PWR resonator, and changes in the amplitude and position of the PWR spectra due to mass density increases and conformational transitions have been used to characterize the kinetics and binding affinities corresponding to these interactions. Although the partition of COX-2 into the bilayer was not as efficient as was the case with the other two proteins, sufficient protein could be incorporated to allow ligand binding to be observed. It was also possible to incorporate COX-1 into a lipid bilayer by adding a suspension of microsomal membrane fragments containing this enzyme to a preformed bilayer, and to observe binding of an inhibitory ligand. The interactions between integrin alphaVbeta3 and two ligands with different in vivo efficacies could be distinguished by both spectral measurements and binding kinetics. In the case of the RNA polymerase, the kinetics of PWR spectral changes upon adding a substrate solution to an enzyme-template complex were consistent with those obtained from direct measurements of enzymatic turnover. These experiments demonstrate the utility of PWR spectroscopy to provide novel information regarding drug interactions with membrane proteins in a lipid environment and to distinguish conformational changes induced by binding of various drug molecules.

Phosphatidylethanolamine enhances rhodopsin photoactivation and transducin binding in a solid supported lipid bilayer as determined using plasmon-waveguide resonance spectroscopy

Flash photolysis studies have shown that the membrane lipid environment strongly influences the ability of rhodopsin to form the key metarhodopsin II intermediate. Here we have used plasmon-waveguide resonance (PWR) spectroscopy, an optical method sensitive to both mass and conformation, to probe the effects of lipid composition on conformational changes of rhodopsin induced by light and due to binding and activation of transducin (G(t)). Octylglucoside-solubilized rhodopsin was incorporated by detergent dilution into solid-supported bilayers composed either of egg phosphatidylcholine or various mixtures of a nonlamellar-forming lipid (dioleoylphosphatidylethanolamine; DOPE) together with a lamellar-forming lipid (dioleoylphosphatidylcholine; DOPC). Light-induced proteolipid conformational changes as a function of pH correlated well with previous flash photolysis studies, indicating that the PWR spectral shifts monitored metarhodopsin II formation. The magnitude of these effects, and hence the extent of the conformational transition, was found to be proportional to the DOPE content. Our data are consistent with previous suggestions that lipids having a negative spontaneous curvature favor elongation of rhodopsin during the activation process. In addition, measurements of the G(t)/rhodopsin interaction in a DOPC/DOPE (25:75) bilayer at pH 5 demonstrated that light activation increased the affinity for G(t) from 64 nM to 0.7 nM, whereas G(t) affinity for dark-adapted rhodopsin was unchanged. By contrast, in DOPC bilayers the affinity of G(t) for light-activated rhodopsin was only 18 nM at pH 5. Moreover exchange of GDP for GTP gamma S was also monitored by PWR spectroscopy. Only the light-activated receptor was able to induce this exchange which was unaffected by DOPE incorporation. These findings demonstrate that nonbilayer-forming lipids can alter functionally linked conformational changes of G-protein-coupled receptors in membranes, as well as their interactions with downstream effector proteins.

Lipid dependence and activity control of phosphatidylserine synthase fromEscherichia coli

The activity of phosphatidylserine synthase from Escherichia coli depends significantly on the nature and level of the lipids in the matrix, at which the enzyme is operating. To elucidate the role of anionic lipids in the regulation of PtdSer synthase, its activity was studied in mixed micelles containing phosphatidylglycerol (one charge) or diphosphatidylglycerol (two charges), the two main anionic membrane lipids in E. coli. Membrane association and activity of PtdSer synthase were increased by the two lipids, indicating their essential role in the positive regulation mechanism of the phosphatidylethanolamine level in the E. coli membrane.

Effects of Sample Thickness on the Optical Properties of Surface Plasmon-Coupled Emission

In recent reports, we demonstrated coupling of excited fluorophores with thin silver or gold films resulting in directional surface plasmon-coupled emission (SPCE) through the silver film and into the glass substrate. In the present report, we describe the spectral and spatial properties of SPCE from sulforhomamine 101 in polyvinyl alcohol (PVA) films of various thicknesses on 50-nm silver films. The PVA thickness varied from about 30 to 750 nm. In thin PVA films with a thickness less than 160 nm, SPCE occurred at a single angle in the glass substrate and displayed only p polarization. As the PVA thickness increased to 300 nm, we observed SPCE at two angles, with different s or p polarization for each angle. For PVA films from 500 to 750 nm thick, we observed SPCE at three or four angles, with alternating s and p polarizations. The multiple rings of SPCE and the unusual s-polarized emission are consistent with the expected waveguide modes in the silver-PVA composite film. However, in contrast to our expectations, the average lifetimes of SPCE were not substantially changed from the PVA films. The observation of SPCE at multiple angles and with different polarization opens new opportunities for the use of SPCE to study anisotropic systems or to develop unique sensing devices.

Plasmon-waveguide resonance studies of ligand binding to the human beta 2-adrenergic receptor

Plasmon-waveguide resonance (PWR) spectroscopy is an optical technique that can be used to probe the molecular interactions occurring within anisotropic proteolipid membranes in real time without requiring molecular labeling. This method directly monitors mass density, conformation, and molecular orientation changes occurring in such systems and allows determination of protein-ligand binding constants and binding kinetics. In the present study, PWR has been used to monitor the incorporation of the human beta(2)-adrenergic receptor into a solid-supported egg phosphatidylcholine lipid bilayer and to follow the binding of full agonists (isoproterenol, epinephrine), a partial agonist (dobutamine), an antagonist (alprenolol), and an inverse agonist (ICI-118,551) to the receptor. The combination of differences in binding kinetics and the PWR spectral changes point to the occurrence of multiple conformations that are characteristic of the type of ligand, reflecting differences in the receptor structural states produced by the binding process. These results provide new evidence for the conformational heterogeneity of the liganded states formed by the beta(2)-adrenergic receptor.

Direct observation of G-protein binding to the human delta-opioid receptor using plasmon-waveguide resonance spectroscopy

Using a recently developed method (Salamon, Z., Macleod, H. A., and Tollin, G. (1997) Biophys. J. 73, 2791-2797), plasmon-waveguide resonance spectroscopy, we have been able, for the first time, to directly measure the binding between the human brain delta-opioid receptor (hDOR) and its G-protein effectors in real-time. We have found that the affinity of the G-proteins toward the receptor is highly dependent on the nature of the ligand pre-bound to the receptor. The highest affinity was observed when the receptor was bound to an agonist ( approximately 10 nm); the lowest when receptor was bound to an antagonist ( approximately 500 nm); and no binding at all was observed when the receptor was bound to an inverse agonist. We also have found direct evidence for the existence of an additional G-protein binding conformational state that corresponds to the unliganded receptor, which has a G-protein binding affinity of approximately 60 nm. Furthermore, GTP binding to the receptor.G-protein complex was only observed when the agonist was pre-bound. Similar studies were carried out using the individual G-protein subtypes for both the agonist and the unliganded receptor. Significant selectivity toward the different G-protein subtypes was observed. Thus, the unliganded receptor had highest affinity toward the Galphao (Kd approximately 20 nm) and lowest affinity toward the Galphai2 ( approximately 590 nm) subtypes, whereas the agonist-bound state had highest affinity for the Galphao and Galphai2 subtypes (Kd approximately 9 nm and approximately 7 nm, respectively). GTP binding was also highly selective, both with respect to ligand and G-protein subtype. We believe that this methodology provides a powerful new way of investigating transmembrane signaling.

Surface plasmon resonance spectroscopy: an emerging tool for the study of peptide-membrane interactions

The interactions between peptides and membranes mediate a wide variety of biological processes, and characterization of the molecular details of these interactions is central to our understanding of cellular events such as protein trafficking, cellular signaling and ion-channel formation. A wide variety of biophysical techniques have been combined with the use of model membrane systems to study peptide-membrane interactions, and have provided important information on the relationship between membrane-active peptide structure and their biological function. However, what has generally not been reported is a detailed analysis of the affinity of peptide for different membrane systems, which has largely been due to the difficulty in obtaining this information. To address this issue, surface plasmon resonance (SPR) spectroscopy has recently been applied to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. This article provides an overview of these recent applications that demonstrate the potential of SPR to enhance our molecular understanding of membrane-mediated peptide function.

Plasmon-waveguide resonance and impedance spectroscopy studies of the interaction between penetratin and supported lipid bilayer membranes

The interaction between the cell-penetrating peptide, penetratin, and solid-supported lipid bilayer membranes consisting of either egg phosphatidylcholine (PC) or a 75/25 mol% mixture of egg PC and palmitoyloleylphosphatidylglycerol has been studied by simultaneously measuring plasmon-waveguide resonance (PWR) spectra and impedance spectra of lipid-peptide mixtures. When penetratin was incorporated into an egg PC + palmitoyloleylphosphatidylglycerol bilayer, PWR measurements showed a hyperbolic increase in the average refractive index and the refractive index anisotropy, with no change in membrane thickness, over a concentration range between 0 and 2 micro M peptide. In the case of an egg PC bilayer, a biphasic dependence was observed, with a decrease in average refractive index and anisotropy and no thickness change occurring between 0 and 5 micro M peptide, and an increase in membrane thickness occurring between 5 and 15 micro M peptide with no further change in the refractive index parameters. For both membranes, the impedance spectroscopy measurements demonstrated that the electrical resistance was not altered by peptide incorporation, whereas a decrease in membrane capacitance occurred with the same concentration dependence as observed in the PWR experiments, although for the PC membrane no further changes in electrical properties were observed in the higher concentration range. A structural interpretation of these results is described, in which the peptide binds electrostatically within the headgroup region of the bilayer and influences the headgroup conformation, amount of bound water, and the lipid-packing density, without perturbing the hydrocarbon core of the bilayer.

Surface plasmon resonance spectroscopy in the study of membrane-mediated cell signalling

Peptide-membrane interactions contribute to many important biological processes such as cellular signaling, protein trafficking and ion-channel formation. During receptor-mediated signalling, activated intracellular signalling molecules are often recruited into receptor-induced signaling complexes at the cytoplasmic surface of the cell membrane. Such recruitment can depend upon protein-protein and protein-lipid interactions as well as protein acylation. A wide variety of biophysical techniques have been combined with the use of model membrane systems to study these interactions and have provided important information on the relationship between the structure of these proteins involved in cell signalling and their biological function. More recently, surface plasmon resonance (SPR) spectroscopy has also been applied to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. This article provides an overview of these recent applications, which demonstrate the potential of SPR to enhance our molecular understanding of membrane-mediated cellular signalling.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.

Current and emerging commercial optical biosensors

The field of commercial optical biosensors is rapidly evolving, with new systems and detection methods being developed each year. This review outlines the currently available biosensor hardware and highlights unique features of each platform. Affinity-based biosensor technology, with its high sensitivity, wide versatility and high throughput, is playing a significant role in basic research, pharmaceutical development, and the food and environmental sciences. Likewise, the increasing popularity of biosensors is prompting manufacturers to develop new instrumentation for dedicated applications. We provide a preview of some of the emerging commercial systems that are dedicated to drug discovery, proteomics, clinical diagnostics and routine biomolecular interaction analysis.

Sequence properties of the 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii membranes. Recognition of a large group of lipid glycosyltransferases in eubacteria and archaea

Synthesis of the nonbilayer-prone alpha-monoglucosyldiacylglycerol (MGlcDAG) is crucial for bilayer packing properties and the lipid surface charge density in the membrane of Acholeplasma laidlawii. The gene for the responsible, membrane-bound glucosyltransferase (alMGS) (EC ) was sequenced and functionally cloned in Escherichia coli, yielding MGlcDAG in the recombinants. Similar amino acid sequences were encoded in the genomes of several Gram-positive bacteria (especially pathogens), thermophiles, archaea, and a few eukaryotes. All of these contained the typical EX(7)E catalytic motif of the CAZy family 4 of alpha-glycosyltransferases. The synthesis of MGlcDAG by a close sequence analog from Streptococcus pneumoniae (spMGS) was verified by polymerase chain reaction cloning, corroborating a connection between sequence and functional similarity for these proteins. However, alMGS and spMGS varied in dependence on anionic phospholipid activators phosphatidylglycerol and cardiolipin, suggesting certain regulatory differences. Fold predictions strongly indicated a similarity for alMGS (and spMGS) with the two-domain structure of the E. coli MurG cell envelope glycosyltransferase and several amphipathic membrane-binding segments in various proteins. On the basis of this structure, the alMGS sequence charge distribution, and anionic phospholipid dependence, a model for the bilayer surface binding and activity is proposed for this regulatory enzyme.

Optical anisotropy in lipid bilayer membranes: coupled plasmon-waveguide resonance measurements of molecular orientation, polarizability, and shape

The birefringence and linear dichroism of anisotropic thin films such as proteolipid membranes are related to molecular properties such as polarizability, shape, and orientation. Coupled plasmon-waveguide resonance (CPWR) spectroscopy is shown in the present work to provide a convenient means of evaluating these parameters in a single lipid bilayer. This is illustrated by using 1-10 mol % of an acyl chain chromophore-labeled phosphatidylcholine (PC) incorporated into a solid-supported PC bilayer deposited onto a hydrated silica surface. CPWR measurements were made of refractive index and extinction coefficient anisotropies with two exciting light wavelengths, one of which is absorbed by the chromophore and one of which is not. These results were used to calculate longitudinal and transverse molecular polarizabilities, the orientational order parameter and average angle between the longitudinal axis of the lipid molecule and the membrane normal, and the molecular shape factors of the lipid molecules. The values thereby obtained are in excellent agreement with parameters determined by other techniques, and provide a powerful tool for analyzing lipid-protein, protein-protein, and protein-ligand interactions in proteolipid films.

Plasmon resonance studies of agonist/antagonist binding to the human delta-opioid receptor: new structural insights into receptor-ligand interactions

Structural changes accompanying the binding of ligands to the cloned human delta-opioid receptor immobilized in a solid-supported lipid bilayer have been investigated using coupled plasmon-waveguide resonance spectroscopy. This highly sensitive technique directly monitors mass density, conformation, and molecular orientation changes occurring in anisotropic thin films and allows direct determination of binding constants. Although both agonist binding and antagonist binding to the receptor cause increases in molecular ordering within the proteolipid membrane, only agonist binding induces an increase in thickness and molecular packing density of the membrane. This is a consequence of mass movements perpendicular to the plane of the bilayer occurring within the lipid and receptor components. These results are consistent with models of receptor function that involve changes in the orientation of transmembrane helices.