Rhodopsin Activity Varies in Proteoliposomes Prepared by Different Techniques

Single Vesicle Fluorescence-Bleaching Assay for Multi-Parameter Analysis of Proteoliposomes by Total Internal Reflection Fluorescence Microscopy

Reconstitution of membrane proteins into model membranes is an essential approach for their functional analysis under chemically defined conditions. Established model-membrane systems used in ensemble average measurements are limited by sample heterogeneity and insufficient knowledge of lipid and protein content at the single vesicle level, which limits quantitative analysis of vesicle properties and prevents their correlation with protein activity. Here, we describe a versatile total internal reflection fluorescence microscopy-based bleaching protocol that permits parallel analysis of multiple parameters (physical size, tightness, unilamellarity, membrane protein content, and orientation) of individual proteoliposomes prepared with fluorescently tagged membrane proteins and lipid markers. The approach makes use of commercially available fluorophores including the commonly used nitrobenzoxadiazole dye and may be applied to deduce functional molecular characteristics of many types of reconstituted fluorescently tagged membrane proteins.

Single Proteoliposome High-Content Analysis Reveals Differences in the Homo-Oligomerization of GPCRs

G-protein-coupled receptors (GPCRs) control vital cellular signaling pathways. GPCR oligomerization is proposed to increase signaling diversity. However, many reports have arrived at disparate conclusions regarding the existence, stability, and stoichiometry of GPCR oligomers, partly because of cellular complexity and ensemble averaging of intrareconstitution heterogeneities that complicate the interpretation of oligomerization data. To overcome these limitations, we exploited fluorescence-microscopy-based high-content analysis of single proteoliposomes. This allowed multidimensional quantification of intrinsic monomer-monomer interactions of three class A GPCRs (β₂-adrenergic receptor, cannabinoid receptor type 1, and opsin). Using a billion-fold less protein than conventional assays, we quantified oligomer stoichiometries, association constants, and the influence of two ligands and membrane curvature on oligomerization, revealing key similarities and differences for three GPCRs with decidedly different physiological functions. The assays introduced here will assist with the quantitative experimental observation of oligomerization for transmembrane proteins in general.

Fluorescence Microscopy of Single Liposomes with Incorporated Pigment-Proteins

Reconstitution of transmembrane proteins into liposomes is a widely used method to study their behavior under conditions closely resembling the natural ones. However, this approach does not allow precise control of the liposome size, reconstitution efficiency, and the actual protein-to-lipid ratio in the formed proteoliposomes, which might be critical for some applications and/or interpretation of data acquired during the spectroscopic measurements. Here, we present a novel strategy employing methods of proteoliposome preparation, fluorescent labeling, purification, and surface immobilization that allow us to quantify these properties using fluorescence microscopy at the single-liposome level and for the first time apply it to study photosynthetic pigment-protein complexes LHCII. We show that LHCII proteoliposome samples, even after purification with a density gradient, always contain a fraction of nonreconstituted protein and are extremely heterogeneous in both protein density and liposome sizes. This strategy enables quantitative analysis of the reconstitution efficiency of different protocols and precise fluorescence spectroscopic study of various transmembrane proteins in a controlled nativelike environment.

Rhodopsin/lipid hydrophobic matching-rhodopsin oligomerization and function

Lipid composition of the membrane and rhodopsin packing density strongly modulate the early steps of the visual response of photoreceptor membranes. In this study, lipid-order and bovine rhodopsin function in proteoliposomes composed of the sn-1 chain perdeuterated lipids 14:0d27-14:1-PC, 16:0d31-16:1-PC, 18:0d35-18:1-PC, or 20:0d39-20:1-PC at rhodopsin/lipid molar ratios from 1:70 to 1:1000 (mol/mol) were investigated. Clear evidence for matching of hydrophobic regions on rhodopsin transmembrane helices and hydrophobic thickness of lipid bilayers was observed from (2)H nuclear magnetic resonance order parameter measurements at low rhodopsin concentrations. Thin bilayers stretched to match the length of transmembrane helices observed as increase of sn-1 chain order, while thicker bilayers were compressed near the protein. A quantitative analysis of lipid-order parameter changes suggested that the protein adjusts its conformation to bilayer hydrophobic thickness as well, which confirmed our earlier circular-dichroism measurements. Changes in lipid order parameters upon rhodopsin incorporation vanished for bilayers with a hydrophobic thickness of 27 ± 1 Å, suggesting that this is the bilayer thickness at which rhodopsin packs in bilayers at the lowest membrane perturbation. The lipid-order parameter studies also indicated that a hydrophobic mismatch between rhodopsin and lipids triggers rhodopsin oligomerization with increasing rhodopsin concentrations. Both hydrophobic mismatch and rhodopsin oligomerization result in substantial shifts of the equilibrium between the photointermediates metarhodopsin I and metarhodopsin II; increasing bilayer thickness favors formation of metarhodopsin II while oligomerization favors metarhodopsin I. The results highlight the importance of hydrophobic matching for rhodopsin structure, oligomerization, and function.

Nanoscale high-content analysis using compositional heterogeneities of single proteoliposomes

Proteoliposome reconstitution is a standard method to stabilize purified transmembrane proteins in membranes for structural and functional assays. Here we quantified intrareconstitution heterogeneities in single proteoliposomes using fluorescence microscopy. Our results suggest that compositional heterogeneities can severely skew ensemble-average proteoliposome measurements but also enable ultraminiaturized high-content screens. We took advantage of this screening capability to map the oligomerization energy of the β2-adrenergic receptor using ∼10(9)-fold less protein than conventional assays.

Different phosphorylation rates among vertebrate cone visual pigments with different spectral sensitivities

Cone photoreceptor subtypes having different spectral sensitivities exhibit different recovery kinetics in their photoresponses in some vertebrates. Phosphorylation by G protein-coupled receptor kinase (GRK) is essential for the rapid inactivation of light-activated visual pigment, which is the rate-limiting step of the cone photoresponse recovery in salamander. In this study we compared the rate of light-dependent phosphorylation by GRK7 of carp green- and blue-sensitive cone visual pigments. Blue pigment was phosphorylated significantly less effectively than green pigment, suggesting that the difference in the pigment phosphorylation rate is responsible for the difference in photoresponse kinetics among cone photoreceptor subtypes.

Rhodopsin-lipid interactions studied by NMR

The biophysical properties of the lipid matrix are known to influence function of integral membrane proteins. We report on a sample preparation method for reconstitution of membrane proteins which uses porous anodic aluminum oxide (AAO) filters with 200-nm-wide pores of high density. The substrate permits formation of tubular, single membranes that line the inner surface of pores. One square centimeter of filter with a thickness of 60μm yields on the order of 500cm(2) of solid-supported single bilayer surface, sufficient for NMR studies. The tubular bilayers are free of detergent, fully hydrated, and accessible for ligands from one side of the membrane. The use of AAO filters greatly improves reproducibility of the reconstitution process such that the influence of protein on lipid order parameters can be studied with high resolution. As an example, results for the G protein-coupled receptor of class A, bovine rhodopsin, are shown. By (2)H NMR order parameter measurements, it is detected that rhodopsin insertion elastically deforms membranes near the protein. Furthermore, by (1)H saturation-transfer NMR under conditions of magic angle spinning, we demonstrate detection of preferences in interactions of rhodopsin with particular lipid species. It is assumed that function of integral membrane proteins depends on both protein-induced elastic deformations of the lipid matrix and preferences for interaction of the protein with particular lipid species in the first layer of lipids surrounding the protein.

Recombinant cannabinoid type 2 receptor in liposome model activates g protein in response to anionic lipid constituents

Human cannabinoid type 2 (CB(2)) receptor expressed in Escherichia coli was purified and successfully reconstituted in the functional form into lipid bilayers composed of POPC, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS), and cholesteryl hemisuccinate (CHS). Reconstitution was performed by detergent removal from the protein/lipid/detergent mixed micelles either on an adsorbent column, or by rapid dilution to below the critical micelle concentration of detergent followed by removal of detergent monomers on a concentrator. Proteoliposomes prepared at a protein/phospholipid/CHS molar ratio of 1/620-650/210-220 are free of detergent as shown by (1)H NMR, have a homogeneous protein/lipid ratio shown by isopycnic gradient ultracentrifugation, and are small in size with a mean diameter of 150-200 nm as measured by dynamic light scattering. Functional integrity of the reconstituted receptor was confirmed by quantitative binding of (2)H-labeled agonist CP-55,940-d(6) measured by (2)H magic angle spinning NMR, as well as by activation of G protein. The efficiency of G protein activation by agonist-bound CB(2) receptor was affected by negative electric surface potentials of proteoliposomes controlled by the content of anionic CHS or POPS. The activation was highest at an anionic lipid content of about 50 mol %. There was no correlation between the efficiency of G protein activation and an increase of hydrocarbon chain order induced by CHS or cholesterol. The results suggest the importance of anionic lipids in regulating signal transduction by CB(2) receptor and other class A GPCR. The successful reconstitution of milligram quantities of pure, functional CB(2) receptor enables a wide variety of structural studies.

Rhodopsin in nanodiscs has native membrane-like photointermediates

Time-dependent studies of membrane protein function are hindered by extensive light scattering that impedes application of fast optical absorbance methods. Detergent solubilization reduces light scattering but strongly perturbs rhodopsin activation kinetics. Nanodiscs may be a better alternative if they can be shown to be free from the serious kinetic perturbations associated with detergent solubilization. To resolve this, we monitored absorbance changes due to photointermediates formed on the microsecond to hundred millisecond time scale after excitation of bovine rhodopsin nanodiscs and compared them to photointermediates that form in hypotonically washed native membranes as well as to those that form in lauryl maltoside suspensions at 15 and 30 °C over a pH range from 6.5 to 8.7. Time-resolved difference spectra were collected from 300 to 700 nm at a series of time delays after photoexcitation and globally fit to a sum of time-decaying exponential terms, and the photointermediates present were determined from the spectral coefficients of the exponential terms. At the temperatures and pHs studied, photointermediates formed after photoexcitation of rhodopsin in nanodiscs are extremely similar to those that form in native membrane, in particular displaying the normal forward shift of the Meta I(480) ⇄ Meta II equilibrium with increased temperature and reduced pH which occurs in native membrane but which is not observed in lauryl maltoside detergent suspensions. These results were obtained using the amount of rhodopsin in nanodiscs which is required for optical experiments with rhodopsin mutants. This work demonstrates that late, physiologically important rhodopsin photointermediates can be characterized in nanodiscs, which provide the superior optical properties of detergent without perturbing the activation sequence.

Reconstitution in liposome bilayers enhances nucleotide binding affinity and ATP-specificity of TrwB conjugative coupling protein

Bacterial conjugative systems code for an essential membrane protein that couples the relaxosome to the DNA transport apparatus, called type IV coupling protein (T4CP). TrwB is the T4CP of the conjugative plasmid R388. In earlier work we found that this protein, purified in the presence of detergents, binds preferentially purine nucleotides trisphosphate. In contrast a soluble truncated mutant TrwBΔN70 binds uniformly all nucleotides tested. In this work, TrwB has been successfully reconstituted into liposomes. The non-membranous portion of the protein is almost exclusively oriented towards the outside of the vesicles. Functional analysis of TrwB proteoliposomes demonstrates that when the protein is inserted into the lipid bilayer the affinity for adenine and guanine nucleotides is enhanced as compared to that of the protein purified in detergent or to the soluble deletion mutant, TrwBΔN70. The protein specificity for adenine nucleotides is also increased. No ATPase activity has been found in TrwB reconstituted in proteoliposomes. This result suggests that the N-terminal transmembrane segment of this T4CP interferes with its ATPase activity and can be taken to imply that the TrwB transmembrane domain plays a regulatory role in its biological activity.