Alkyl glucosides as effective solubilizing agents for bovine rhodopsin. A comparison with several commonly used detergents

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

Functional Nanocomposites Based on Fibrous Clays

This chapter is focused on functional nanocomposites based on the use of the microfibrous clays sepiolite and palygorskite as efficient fillers for diverse types of polymer matrices, from typical thermoplastics to biopolymers. The main features that govern the interaction between the silicates and the polymer matrix are discussed. The introduction addresses the structural and textural features of the fibrous silicates, as well as the possible synthetic approaches to increase the compatibility of these nanofillers with the polymeric matrix. Additionally, these clays can be easily functionalized through their surface silanol groups based on chemical reactions or by anchoring of nanoparticles. This allows for the preparation of a wide variety of functional polymer–clay nanocomposites. Thereafter, some relevant examples of nanocomposites derived from conventional polymers are reported, as well as of those based on polymers that exhibit electrical conductivity. Lastly, selected works employing sepiolite or palygorskite as fillers in polymeric matrixes of natural origin are discussed, showing the wide application of these resulting nanocomposites as bioplastics, as well as in biomedicine, environmental remediation and the development of sensor devices.

Recent biotechnological progress in enzymatic synthesis of glycosides

Glycosylation is one of the most important post-modification processes of small molecules and enables the parent molecule to have increased solubility, stability, and bioactivity. Enzyme-based glycosylation has achieved significant progress due to advances in protein engineering, DNA recombinant techniques, exploitation of biosynthetic gene clusters of natural products, and computer-based modeling programs. Our report summarizes glycosylation data that have been published within the past five years to provide an overall review of current progress. We also present the future trends and perspectives for glycosylation.

The bilayer enhances rhodopsin kinetic stability in bovine rod outer segment disk membranes

Rhodopsin is a kinetically stable protein constituting >90% of rod outer segment disk membrane protein. To investigate the bilayer contribution to rhodopsin kinetic stability, disk membranes were systematically disrupted by octyl-β-D-glucopyranoside. Rhodopsin kinetic stability was examined under subsolubilizing (rhodopsin in a bilayer environment perturbed by octyl-β-D-glucopyranoside) and under fully solubilizing conditions (rhodopsin in a micelle with cosolubilized phospholipids). As determined by DSC, rhodopsin exhibited a scan-rate-dependent irreversible endothermic transition at all stages of solubilization. The transition temperature (T(m)) decreased in the subsolubilizing stage. However, once the rhodopsin was in a micelle environment there was little change of the T(m) as the phospholipid/rhodopsin ratio in the mixed micelles decreased during the fully solubilized stage. Rhodopsin thermal denaturation is consistent with the two-state irreversible model at all stages of solubilization. The activation energy of denaturation (E(act)) was calculated from the scan rate dependence of the T(m) and from the rate of rhodopsin thermal bleaching at all stages of solubilization. The E(act) as determined by both techniques decreased in the subsolubilizing stage, but remained constant once fully solubilized. These results indicate the bilayer structure increases the E(act) to rhodopsin denaturation.

Phospholipid-sepiolite biomimetic interfaces for the immobilization of enzymes

Biomimetic interfaces based on phosphatidylcholine (PC) assembled to the natural silicate sepiolite were prepared for the stable immobilization of the urease and cholesterol oxidase enzymes. This is an important issue in practical advanced applications such as biocatalysis or biosensing. The supported lipid bilayer (BL-PC), prepared from PC adsorption, was used for immobilization of enzymes and the resulting biomimetic systems were compared to several other supported layers including a lipid monolayer (ML-PC), a mixed phosphatidylcholine/octyl-galactoside layer (PC-OGal), a cetyltrimethylammonium monolayer (CTA), and also to the bare sepiolite surface. Interfacial characteristics of these layers were investigated with a focus on layer packing density, hydrophilicity/hydrophobicity, and surface charge, which are being considered as key points for enzyme immobilization and stabilization of their biological activity. Cytoplasmic urease and membrane-bound cholesterol oxidase, which served as model enzymes, were immobilized on the different PC-based hybrid materials to probe their biomimetic character. Enzymatic activity was assessed by cyclic voltammetry and UV-vis spectrophotometry. The resulting enzyme/bio-organoclay hybrids were applied as active phase of a voltammetric urea biosensor and cholesterol bioreactor, respectively. Urease supported on sepiolite/BL-PC proved to maintain its enzymatic activity over several months while immobilized cholesterol oxidase demonstrated high reusability as biocatalyst. The results emphasize the good preservation of bioactivity due to the accommodation of the enzymatic system within the biomimetic lipid interface on sepiolite.

Proof of oligomeric state of frog rhodopsin: visualization of dimer and oligomers on gels after BN- and HRCN-PAGE using antibodies to rhodopsin and by retinylopsin fluorescence

Staining by antibodies to rhodopsin (Rh) and fluorescence of N-retinylopsin (RO) have shown that digitonin (DIG)- , dodecyl-β-D-maltoside (DM)- , and sodium dodecyl sulfate (SDS)-solubilized frog Rh after BN- and HRCN-PAGE is situated in the gradient gel in the state of dimer with a slight content of higher oligomers (trimer, tetramer, etc.). With increasing detergent harshness (DIG < DM < SDS), the proportion of higher oligomers in extracts becomes more prominent. Formation of RO in rod outer segments (ROS) in the presence of 0.7 M NaBH(3)CN at pH 5.0 occurs only when Rh is simultaneously photolyzed during reduction. Dithiothreitol at the concentration of 0.005 M failed to induce RO production. Formation of a stable C-N bond between all-trans-retinal and opsin in RO is accompanied by decrease in the dimer share and increase in the share of the higher oligomers due to secondary dissociation-aggregation of solubilized opsin. The position of the Rh dimer in relation to the anode during both native electrophoreses is determined not only by its molecular mass, but probably also depends on unfolding degree (or form): the harsher the detergent, the closer to the anode the dimer is located. Treatment of ROS by agents modifying the cholesterol component of lipid membrane (MβCD, filipin III, nystatin, saponin) did not change the character of Rh oligomerization, thus showing that integrity of the cholesterol component of photoreceptor membrane is not a crucial factor for oligomerization of opsin. It is supposed that the dimer-oligomer "portrait" of frog Rh, which has been found by two methods of native electrophoresis in three detergents with different degree of harshness, corresponds to a physiological state of this protein in native photoreceptor membrane.

Lipid and peptide dynamics in membranes upon insertion of n-alkyl-beta-D-glucopyranosides

The effect of nonionic detergents of the n-alkyl-beta-D-glucopyranoside class on the ordering of lipid bilayers and the dynamics of membrane-embedded peptides were investigated with 2H- and 31P-NMR. 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was selectively deuterated at methylene segments C-2, C-7, and C-16 of the two fatty acyl chains. Two trans-membrane helices, WALP-19 and glycophorin A(71-98), were synthesized with Ala-d3 in the central region of the alpha-helix. n-Alkyl-beta-D-glucopyranosides with alkyl chains with 6, 7, 8, and 10 carbon atoms were added at increasing concentrations to the lipid membrane. The bilayer structure is retained up to a detergent/lipid molar ratio of 1:1. The insertion of the detergents leads to a selective disordering of the lipids. The headgroup region remains largely unaffected; the fatty acyl chain segments parallel to the detergent alkyl chain are only modestly disordered (10-20%), whereas lipid segments beyond the methyl terminus of the detergent show a decrease of up to 50%. The change in the bilayer order profile corresponds to an increase in bilayer entropy. Insertion of detergents into the lipid bilayers is completely entropy-driven. The entropy change accompanying lipid disorder is equivalent in magnitude to the hydrophobic effect. Ala-d3 deuterated WALP-19 and GlycA(71-97) were incorporated into bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine at a peptide/lipid molar ratio of 1:100 and measured above the 1,2-dimyristoyl-sn-glycero-3-phosphocholine gel/liquid-crystal phase transition. Well-resolved 2H-NMR quadrupole splittings were observed for the two trans-membrane helices, revealing a rapid rotation of the CD3 methyl rotor superimposed on an additional rotation of the whole peptide around the bilayer normal. The presence of detergent fluidizes the membrane and produces magnetic alignment of bilayer domains but does not produce essential changes in the peptide conformation or dynamics.

Combining atomic force and fluorescence microscopy for analysis of quantum-dot labeled protein-DNA complexes

Atomic force microscopy (AFM) and fluorescence microscopy are widely used for the study of protein-DNA interactions. While AFM excels in its ability to elucidate structural detail and spatial arrangement, it lacks the ability to distinguish between similarly sized objects in a complex system. This information is readily accessible to optical imaging techniques via site-specific fluorescent labels, which enable the direct detection and identification of multiple components simultaneously. Here, we show how the utilization of semiconductor quantum dots (QDs), serving as contrast agents for both AFM topography and fluorescence imaging, facilitates the combination of both imaging techniques, and with the addition of a flow based DNA extension method for sample deposition, results in a powerful tool for the study of protein-DNA complexes. We demonstrate the inherent advantages of this novel combination of techniques by imaging individual RNA polymerases (RNAP) on T7 genomic DNA.

Lighting up individual DNA binding proteins with quantum dots

The ability to determine the precise loci and occupancy of DNA-binding proteins is instrumental to our understanding of cellular processes like gene expression and regulation. We propose a single-molecule approach for the direct visualization of proteins bound to their template DNA. Fluorescent quantum dots (QD) are used to label proteins bound to DNA, allowing multicolor, nanometer-resolution localization. Protein-DNA complexes are linearly extended and imaged to determine the precise location of the protein binding sites. The method is demonstrated by detecting individual QD-labeled T7-RNA polymerases on the T7 bacteriophage genome. This work demonstrates the potential of this approach to precisely read protein binding position or, alternatively, "write" such information on extended DNA with QDs via sequence-specific molecular recognition.

Expression, purification and in vitro functional reconstitution of the chemokine receptor CCR1

Chemokine receptors are a specific class of G-protein-coupled receptors (GPCRs) that control cell migration associated with routine immune surveillance, inflammation and development. In addition to their roles in normal physiology, these receptors and their ligands are involved in a large number of inflammatory diseases, cancer and AIDS, making them prime therapeutic targets in the pharmaceutical industry. Like other GPCRs, a significant obstacle in determining structures and characterizing mechanisms of activation has been the difficulty in obtaining high levels of pure, functional receptor. Here we describe a systematic effort to express the chemokine receptor CCR1 in mammalian cells, and to purify and reconstitute it in functional form. The highest expression levels were obtained using an inducible HEK293 system. The receptor was purified using a combination of N- (StrepII or Hemagglutinin) and C-terminal (His8) affinity tags. Function was assessed by ligand binding using a novel fluorescence polarization assay with fluorescein-labeled chemokine. A strict dependence of function on the detergent composition was observed, as solubilization of CCR1 in n-dodecyl-beta-D-maltopyranoside/cholesteryl hemisuccinate yielded functional receptor with a K(d) of 21 nM for the chemokine CCL14, whereas it was non-functional in phosphocholine detergents. Differences in function were observed despite the fact that both these detergent types maintained the receptor in a state characterized by monomers and small oligomers, but not large aggregates. While optimization is still warranted, yields of approximately 0.1-0.2mg of pure functional receptor per 10(9) cells will permit biophysical studies of this medically important receptor.

Robust cross-links in molluscan adhesive gels: testing for contributions from hydrophobic and electrostatic interactions

The cross-linking interactions that provide cohesive strength to molluscan adhesive gels were investigated. Metal-based interactions have been shown to play an important role in the glue of the slug Arion subfuscus (Draparnaud), but other types of interactions may also contribute to the glue's strength and their role has not been investigated. This study shows that treatments that normally disrupt hydrophobic or electrostatic interactions have little to no effect on the slug glue. High salt concentrations and non-ionic detergent do not affect the solubility of the proteins in the glue or the ability of the glue proteins to stiffen gels. In contrast, metal chelation markedly disrupts the gel. Experiments with gel filtration chromatography identify a 40 kDa protein that is a central component of the cross-links in the glue. This 40 kDa protein forms robust macromolecular aggregations that are stable even in the presence of high concentrations of salt, non-ionic detergent, urea or metal chelators. Metal chelation during glue secretion, however, may block some of these cross-links. Such robust, non-specific interactions in an aqueous environment are highly unusual for hydrogels and reflect an intriguing cross-linking mechanism.

Recombination reaction of rhodopsin in situ studied by photoconversion of “indicator yellow”

We measured the kinetics of recombination of 11-cis-retinal with opsin in intact frog rod outer segment (ROS). The rhodopsin in ROS was bleached and allowed to decay to "indicator yellow," a photoproduct where all-trans-retinal is partly free, and partly bound to non-specific amino groups of disk membranes. By briefly illuminating the "indicator yellow" by an intense 465 or 380-nm flash, we then photoconverted all-trans-retinal to (mostly) the 11-cis- form thus introducing into ROS a certain amount of cis-chromophore. The recombination of cis-retinal with opsin and the formation of rhodopsin were followed by fast single-cell microspectrophotometry. Regeneration proceeded with a time constant of approximately 3.5 min; up to 27% of bleached visual pigment was restored. The regenerated pigment consisted of 91% rhodopsin (11-cis-chromophore) and 9% of presumably isorhodopsin (9-cis-chromophore). The recombination of 11-cis-retinal with opsin inside the ROS proceeds substantially faster than rhodopsin regeneration in the intact eye and, hence, is not the rate-limiting step in the visual cycle.

Coating of poly(dimethylsiloxane) with n-dodecyl-beta-D-maltoside to minimize nonspecific protein adsorption

Poly(dimethylsiloxane)(PDMS) surface is coated with n-dodecyl-beta-D-maltoside, which reduces the nonspecifically adsorbed protein on the PDMS surface to the single molecule level.

Structural studies on rhodopsin

Bovine rhodopsin is the prototypical G protein coupled receptor (GPCR). It was the first GPCR to be obtained in quantity and studied in detail. It is also the first GPCR for which detailed three dimensional structural information has been obtained. Reviewed here are the experiments leading up to the high resolution structure determination of rhodopsin and the most recent structural information on the activation and stability of this integral membrane protein.

Dietary n-3 FA modulate long and very long chain FA content, rhodopsin content, and rhodopsin phosphorylation in rat rod outer segment after light exposure

A previous study has shown that the long and very long chain FA (VLCFA) content of the rat retina responds to changes in dietary n-6/n-3 ratio of the fat fed (1). The present study tested whether similar changes in these FA are associated with alterations in rhodopsin content and rhodopsin phosphorylation after light treatment. Weanling rats were fed diets containing 20% (w/w, 40% energy) fat with either high (4.8%, w/w) or low (1.2%, w/w) n-3 FA. After 6 wk of feeding, half of the animals in each group were exposed to light for 48 h at 350 lx or were kept in complete darkness. In the rod outer segment, the high n-3 diet treatment increased the level of 20:5n-3 and 22:6n-3 and reduced the levels of 20:4n-6 and 24:4n-6 in PC, PE, and PS. After the feeding of a high n-3 FA diet, total n-3 pentaenoic VLCFA from C24 to C34 increased in PC, whereas the n-6 tetra- and pentaenoic VLCFA decreased. No changes occurred in n-3 hexaenoic VLCFA regardless of the level of 22:6n-3 in the diet. After light exposure, animals fed a high n-3 FA diet showed reduction in 22:6n-3 as well as in n-6 and n-3 VLCFA in PC. FFA and TG fractions contained increased levels of both 20:4n-6 and 22:6n-3 after light exposure. Dark-adapted rhodopsin content and rhodopsin phosphorylation in the rod outer segment of rats fed the low n-3 FA diet were higher than in animals fed a high n-3 FA diet. After light exposure, animals fed the low n-3 FA diet lost more rhodopsin compared to animals fed the high n-3 FA diet, resulting in less phosphorylation of rhodopsin. Results indicate that the FA composition, rhodopsin content, and phosphorylation in visual cells is influenced by the dietary n-3 FA fed as well as by light exposure. The results also imply that 22:6n-3 may not be the precursor for synthesis of hexaenoic VLCFA.

The Dispersion Properties of Precipitated Calcium Carbonate Suspensions Adsorbed with Alkyl Polyglycoside in Aqueous Medium

The zeta potentials and dispersion properties of precipitated calcium carbonate suspensions adsorbed with alkyl polyglycosides in aqueous medium were investigated. Within the investigated pH ranges, the adsorption curves of alkyl polyglycosides on calcium carbonates show sigmoidal shapes, and the zeta potential decreases as the amount of adsorption increases. At positively charged surfaces of low pH, the adsorption amounts were greater than those at negatively charged surfaces, indicating that alkyl polyglycosides were negatively charged in aqueous solutions. At low concentrations of alkyl polyglycosides, the dispersion stabilities of suspensions were very poor and showed no linearity with zeta potentials over the entire range of pHs, which may be attributed to the onset of hydrophobic interaction between particles due to the adsorption of surfactant molecules. This destabilization continued until monolayer coverage by the surfactant layer was complete. Based on the classical DLVO theory, there may be a strong hydrophobic interaction between particles. Beyond monolayer adsorption, the dispersion stability increases, probably by the formation of hemimicelle or admicelle. Therefore, it is believed that ionization of alkyl polyglycosides and admicelles of surfactants on particle surface plays a key role in the stability of dispersions and the abrupt increase in adsorption. Copyright 2000 Academic Press.

Differential rhodopsin regeneration in photoreceptor membranes is correlated with variations in membrane properties

Rhodopsin, the major transmembrane protein in both the plasma membrane and the disk membranes of photoreceptor rod outer segments (ROS) forms the apo-protein opsin upon the absorption of light. In vivo the regeneration of rhodopsin is necessary for subsequent receptor activation and for adaptation, in vitro this regeneration can be followed after the addition of 11-cis retinal. In this study we investigated the ability of bleached rhodopsin to regenerate in the compositionally different membrane environments found in photoreceptor rod cells. When 11-cis retinal was added to bleached ROS plasma membrane preparations, rhodopsin did not regenerate within the same time course or to the same extent as bleached rhodopsin in disk membranes. Over 80% of the rhodopsin in newly formed disks regenerated within 90 minutes while only 40% regenerated in older disks. Since disk membrane cholesterol content increases as disks are displaced from the base to the apical tip of the outer segment, we looked at the affect of membrane cholesterol content on the regeneration process. Enrichment or depletion of disk membrane cholesterol did not alter the % rhodopsin that regenerated. Bulk membrane properties measured with a sterol analog, cholestatrienol and a fatty acid analog, cis parinaric acid, showed a more ordered, less "fluid", lipid environment within plasma membrane relative to the disks. Collectively these results show that the same membrane receptor, rhodopsin, functions differently as monitored by regeneration in the different lipid environments within photoreceptor rod cells. These differences may be due to the bulk properties of the various membranes.

Adsorption of n -Dodecyl-beta-d -maltoside on Solids

Adsorption of a typical sugar-based surfactant, n -dodecyl-beta-d -maltoside (DM), on hydrophilic solids, silica, alumina, titania, and hematite, and a hydrophobic solid, graphite, was studied. Effects of salts and pH on the adsorption on alumina as well as the electrokinetic potential of the particles after surfactant adsorption were studied to determine the adsorption mechanisms. Hydrophobicity and settling rate were measured to explore the surfactant conformation on the particle surfaces. For hydrophilic solids, DM was found to adsorb strongly on alumina, titania, and hematite but weakly on silica. While hydrogen bonding is postulated to be the major driving force for the adsorption on hydrophilic solids, for hydrophobic solid, the adsorption is mainly due to the hydrophobic interactions. The different behaviors of surfactant on hydrophilic and hydrophobic solids were attributed to the different interactions between surfactant and solids. Also, the surfactant is estimated to form a bilayer on alumina while on graphite it forms a monolayer. The surface hydrophobicity and stability of the solids are discussed in terms of the adsorbed monolayer/bilayer formation on the particles.

Octyl-beta-D-glucopyranoside partitioning into lipid bilayers: thermodynamics of binding and structural changes of the bilayer

The interaction of the nonionic detergent octyl-beta-D-glucopyranoside (OG) with lipid bilayers was studied with high-sensitivity isothermal titration calorimetry (ITC) and solid-state 2H-NMR spectroscopy. The transfer of OG from the aqueous phase to lipid bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) can be investigated by employing detergent at concentrations below the critical micellar concentration; it can be defined by a surface partition equilibrium with a partition coefficient of K = 120 +/- 10 M-1, a molar binding enthalpy of delta H degrees D = 1.3 +/- 0.15 kcal/mol, and a free energy of binding of delta G degrees D = -5.2 kcal/mol. The heat of transfer is temperature dependent, with a molar heat capacity of delta CP = -75 cal K-1 mol-1. The large heat capacity and the near-zero delta H are typical for a hydrophobic binding equilibrium. The partition constant K decreased to approximately 100 M-1 for POPC membranes mixed with either negatively charged lipids or cholesterol, but was independent of membrane curvature. In contrast, a much larger variation was observed in the partition enthalpy. delta H degrees D increased by about 50% for large vesicles and by 75% for membranes containing 50 mol% cholesterol. Structural changes in the lipid bilayer were investigated with solid-state 2H-NMR. POPC was selectively deuterated at the headgroup segments and at different positions of the fatty acyl chains, and the measurement of the quadrupolar splittings provided information on the conformation and the order of the bilayer membrane. Addition of OG had almost no influence on the lipid headgroup region, even at concentrations close to bilayer disruption. In contrast, the fluctuations of fatty acyl chain segments located in the inner part of the bilayer increased strongly with increasing OG concentration. The 2H-NMR results demonstrate that the headgroup region is the most stable structural element of the lipid membrane, remaining intact until the disordering of the chains reaches a critical limit. The perturbing effect of OG is thus different from that of another nonionic detergent, octaethyleneglycol mono-n-dodecylether (C12E8), which produces a general disordering at all levels of the lipid bilayer. The OG-POPC interaction was also investigated with POPC monolayers, using a Langmuir trough. In the absence of lipid, the measurement of the Gibbs adsorption isotherm for pure OG solutions yielded an OG surface area of AS = 51 +/- 3 A2. On the other hand, the insertion area AI of OG in a POPC monolayer was determined by a monolayer expansion technique as AI = 58 +/- 10 A2. The similar area requirements with AS approximately AI indicate an almost complete insertion of OG into the lipid monolayer. The OG partition constant for a POPC monolayer at 32 mN/m was Kp approximately 320 M-1 and thus was larger than that for a POPC bilayer.

Solubilization of the overexpressed integral membrane protein alkane monooxygenase of the recombinant Escherichia coli W3110[pGEc47]

The integral membrane-bound alkane monooxygenase (AlkB) from Pseudomonas oleovorans has been overexpressed in the recombinant Escherichia coli strain W3110[pGEc47] and expression levels of 10 to 15% relative to the total cell protein were reached. The amount of phospholipids in induced cells is about 3-fold higher compared to the wild-type and AlkB has been shown to be located in small membrane vesicles. We present here a study on the solubilization of these AlkB containing membrane vesicles by different detergents with special emphasis on structural requirements for a surfactant preserving the activity of AlkB. Moreover, the effects of the detergents used on the complete alkane hydroxylase system was studied.

Fluorescence studies of the location and membrane accessibility of the palmitoylation sites of rhodopsin

Fluorescent fatty acid labels have been incorporated into the palmitoylation sites of rhodopsin and used to probe the membrane accessibility and location of these sites. The fluorescence properties of anthroyloxy and pyrenyl fatty acids bound to rhodopsin were investigated in a reconstituted vesicle system. Collisional quenching of fluorescence by stearic acid (DSA) labeled with doxyls in the 16, 12, and 5 positions was used to determine the membrane accessibility and disposition of the modifying fatty acids. To properly determine the membrane concentration of these quenchers, the dependence of the Stern-Volmer parameters on both quencher and vesicle concentration was determined. An analysis of these dependences provided a correction for partitioning of the quencher between the aqueous phase and the membrane. After this correction, the relative effectiveness of doxyl quenchers was 16-DSA > 12-DSA > 5-DSA. Parallel studies on free anthroyloxy and pyrenyl fatty acids incorporated into the reconstituted system showed the same dependence on quencher position. These results indicate that the labels at the palmitoylation sites of rhodopsin are situated in the membrane much as a free fatty acid. This anchoring of the palmitates in the membrane results in the formation of a fourth cytoplasmic loop.

Room temperature trapping of rhodopsin photointermediates

By suspending bovine rhodopsin in trehalose-water glass films, it is possible to trap photostates in the light-activation process. Because of the unusually high vitrification temperature of trehalose-water mixtures, this trapping can be accomplished at room temperature. This allows for a facile investigation of the spectroscopic properties of rhodopsin's photointermediates. Depending on experimental conditions, it is possible to trap photolysis products that have visible absorbance spectra closely resembling the two different photointermediates, metarhodopsin I and metarhodopsin II. When rhodopsin is maintained in the native rod outer segment membrane, the photolysis product has the spectral properties of metarhodopsin I. Upon detergent solubilization, the photolysis product closely resembles metarhodopsin II. Ultraviolet circular dichroism spectra show that the metarhodopsin I product had no change in secondary structure compared with unbleached rhodopsin. The metarhodopsin II product did show a significant decrease in alpha-helical content. Resonance energy transfer was measured from extrinsic probes located on each of the cytoplasmic cysteine residues to the retinal in the trapped photoproducts. It is seen that these distances are the same for rhodopsin and metarhodopsin I while metarhodopsin II shows considerably shorter distances. Metarhodopsin II is intimately associated with the signal transduction process, and the present results suggest that large structural changes have occurred in the transition to this state. These results demonstrate the utility of room temperature trapping of photostates in trehalose-water glasses.

Fourier transform infrared study of the rod outer segment disk and plasma membranes of vertebrate retina

Phospholipid composition and structure of disk and plasma membranes purified from bovine rod outer segments (ROS) are examined using Fourier transform infrared spectroscopy. Vibrational data indicate that both disk and plasma membranes lack sphingophospholipids, in contrast to the lens membranes. The hydrocarbon chains of the disk lipids are unsaturated by a factor of 5 over the acyl chains of the plasma lipids. The plasma lipids with 3-fold higher cholesterol and 5-fold higher saturation melt at a higher temperature (26 degrees C) than the disk lipids which melt at 16 degrees C. The transition temperature decreases by more than 20 degrees C in going from disk lipids to disk membrane, indicating a large drop in the enthalpy of the ROS membrane-matrix, presumably due to enhanced rhodopsin-lipid interaction. The lipid composition predisposes the disk and plasma membranes to be fluid and structurally disordered (about 84%) around physiological temperature. The fluid phospholipid environment of the disk membrane (i.e., just a few degrees above subzero temperatures) is considered to be vital for the ROS photoreceptor function. The amide I band profile of rhodopsin indicates an extensive alpha-helical (53%) peptide chain, with little beta-sheet (21%) and beta-turns (18%) in ROS membranes. This structure and/or conformation is conserved between 0-60 degrees C even though disk and plasma lipids undergo a phase change. The H-D exchange data indicate that as much as 84% of the peptide residues of ROS membranes in partially bleached retinas is accessible to D2O solvent after 1 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of depalmitoylation on physicochemical properties of rhodopsin

In an effort to determine the functionality of palmitoylation in rhodopsin, a number of physicochemical properties of depalmitoylated rhodopsin were monitored. Approximately 70% of the rhodopsin was depalmitoylated in rod outer segments by a mild hydroxylamine treatment that resulted in minimal bleaching of rhodopsin. Subsequent purification by affinity chromatography could be used to remove hydroxylamine-bleached rhodopsin. Parallel physical studies were performed on both purified, detergent-solubilized rhodopsin and rhodopsin in rod outer segments. No effect was seen on the rate of metarhodopsin II formation for depalmitoylated rhodopsin. A small effect was seen in the biphasic behavior of the rate of retinal regeneration. The circular dichroism spectrum of depalmitoylated, purified rhodopsin was virtually identical to that of the native protein. These results suggest that depalmitoylation does not greatly affect the conformational structure of rhodopsin. Circular dichroism at 222 nm was used to monitor the thermal denaturation of depalmitoylated and native rhodopsin. A small but significant decrease in the in rod outer segments. In both cases, the van't Hoff parameters showed an increase in positive enthalpy for denaturation relative to the native state. This is largely counterbalanced by an increase in positive entropy relative to the native states. The circular dichroism of the "denatured" state showed a high alpha-helix content. Depalmitoylated rhodopsin had a lower helix content than native protein in this high-temperature state. The changes in the thermodynamics upon depalmitoylation were attributed to structural changes in the denatured state.

Rhodopsin mobility, structure, and lipid-protein interaction in squid photoreceptor membranes

Treatment of outer segment membranes from Loligo forbesi with endoprotease-V8 from Staphylococcus aureus results in cleavage of the C-terminal extension of the squid rhodopsin, with accompanying reduction of the apparent molecular weight from 47,000 to 36,000 on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Negative-stain electron microscopy of the intact membranes shows that small clusters of the rhodopsin C-termini form structures extending from the membrane surface and that these are absent after protease treatment. Fourier transform infrared spectra of the amide I band of the protein indicate that removal of the C-terminal extension increases the relative alpha-helical content of squid rhodopsin to a level comparable to that for bovine rhodopsin in disk membranes, and to an extent which suggests that the alpha-helical structure lies mainly in the M(r) 36,000 (transmembrane) section of the protein. Saturation-transfer electron spin resonance (ESR) spectroscopy of the spin-labeled protein reveals that the rotational diffusion of squid rhodopsin in outer segment membranes that have been extensively washed with urea to remove peripheral proteins is much slower than that of bovine rhodopsin in rod outer segment disk membranes. This reduction in rotational mobility is also found with purified squid rhodopsin reconstituted in egg phosphatidylcholine and in urea-washed outer segment membranes which have been treated with endoprotease-V8 to remove the C-terminal extension of squid rhodopsin. In the latter case, the saturation-transfer ESR spectra are virtually identical to those of the non-proteolyzed membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptide hormone-membrane interactions: the aggregational and conformational state of lipo-gastrin derivatives and their receptor binding affinity

The (2RS)-1,2-dipalmitoyl-3-mercaptoglycerol/-, (2RS)-1,2-dimyristoyl-3-mercaptoglycerol/-, and (2RS)-1-myristoyl-2-palmitoyl-3-mercaptoglycerol/maleoyl-bet a-alanyl- [Nle15]-human-gastrin-(2-17) adducts were prepared as lipo-gastrin derivatives of explicitly primary amphiphilic properties. As representative of this class of lipo-gastrins, the dimyristoyl derivative has been thoroughly characterized in its aggregational state since, among the three compounds, theoretically it should exhibit the lowest degree of lipid character. It aggregates in aqueous solution to form monodispersed unilamellar spherical vesicles with dislocation of the peptide moiety at the bilayer surface in predominantly unordered structure. The liposomes are remarkably stable toward solubilization with trifluoroethanol and toward vesicle to micelle transition with neutral and negatively charged surfactants even above their critical micellar concentrations. Asymmetric fusion with the detergent micelles induces polydispersion of the liposomes in terms of shape and size without affecting in significant manner the mode of display of the gastrin portions at the bilayer surface. Only the positively charged hexadecyltrimethylammonium hydroxide provokes the collapse of the vesicles into mixed micelles with concomitant altered dislocation of the gastrin-peptide in the new aggregational state. Despite the lipid properties of the gastrin derivatives, i.e., formation of liposomes, they retain remarkable receptor affinities (IC50 = 1.5 x 10(-9) M for myristoyl-palmitoyl-gastrin, IC50 = 2.0 x 10(-9) M for di-myristoyl-gastrin and IC50 = 3.1 x 10(-9) M for di-palmitoyl-gastrin vs IC50 = 2.8 x 10(-10) M for Nle15-gastrin). Since the displacement of radiolabeled Nle15-gastrin from rat pancreatic acinar cell line membrane preparations by both the parent gastrin hormone and the three lipo-gastrins occurs in parallel manner, the data support a mechanism of receptor occupancy via accumulation of the gastrins at the membrane surface and their two-dimensional diffusion to the target receptor. Thereby the differentiated decrease of affinity in function of fatty acid chain length has to be attributed to the energetically more or less favored transfer of the monomers from the donor vesicles to the acceptor membranes. Moreover, according to this model migration of the lipo-gastrins with their interdigitating di-fatty-acyl moieties should be delayed, again in lipid structure-dependent manner, in comparison to the parent gastrin molecule, which is free to float in the membrane interfacial phase.

Properties of HIV membrane reconstituted from its recombinant gp160 envelope glycoprotein

Human immunodeficiency virus (HIV) membrane has been reconstituted from the recombinant envelope glycoprotein precursor (gp160) by a detergent dialysis technique. Electron microscopy shows that gp160-virosomes are spherical vesicles with a mean diameter identical to that of viral particles. Enzyme-linked immunosorbent assay and immunogold labeling demonstrate efficient association of gp160 with lipid vesicles and proteolysis treatment reveals an asymmetric insertion with about 90% of glycoproteins having their gp120-moiety pointing outside. Glycoproteins are organized as dimers and tetramers and gp160 retains its ability to specifically bind CD4 receptor after reconstitution into virosome.

A new surfactant series, the N-alkylamino-1-deoxylactitols: application for extraction of 'op' opiate receptors from frog brain

A new series of surfactants, the N-alkylamino-1-deoxylactitols, was prepared and employed to extract 'op' opiate receptors from frog brain. These surfactants are both cheap and convenient to prepare. Receptors were reproducibly extracted in a good yield using N-nonylamino-1-deoxylactitol. This derivative, which was not denaturing during the extraction process, could thus be used instead of the more costly digitonin, whose rather variable purity affects yield.

Isolation of the mitochondrial benzodiazepine receptor: association with the voltage-dependent anion channel and the adenine nucleotide carrier

The mitochondrial benzodiazepine receptor (mBzR) has been solubilized with retention of reversible ligand binding, and the associated subunits were characterized. mBzR comprises immunologically distinct protein subunits of 18-, 30-, and 32-kDa. The 18-kDa protein is labeled by the isoquinoline carboxamide mBzR ligand [3H]PK14105, whereas the 30- and 32-kDa subunits are labeled by the benzodiazepine (Bz) ligands [3H]flunitrazepam and [3H]AHN-086. Selective antibodies and reagents identify the 32- and 30-kDa proteins as the voltage-dependent anion channel (VDAC) and the adenine nucleotide carrier (ADC), respectively. While isoquinoline carboxamide and Bz ligands target different subunits, they interact allosterically, as the binding of Bz and isoquinoline carboxamide ligands is mutually competitive at low nanomolar concentrations. Moreover, eosin-5-maleimide and mercuric chloride inhibit [3H]PK11195 binding to the intact receptor via sulfhydryl groups that are present in ADC. VDAC and ADC, outer and inner mitochondrial membrane channel proteins, respectively, together with the 18-kDa subunit, may comprise mBzR at functionally important transport sites at the junction of two mitochondrial membranes.

Photoacoustic calorimetric study of the conversion of rhodopsin and isorhodopsin to lumirhodopsin

The enthalpy and volume changes for the conversion of rhodopsin and isorhodopsin to lumirhodopsin have been investigated by time-resolved photoacoustic calorimetry. The conversion of rhodopsin to lumirhodopsin is endothermic by 3.9 +/- 5.9 kcal/mol and is accompanied by an increase in volume of 29.1 +/- 0.8 mL/mol. The lumirhodopsins produced from rhodopsin and isorhodopsin are energetically equivalent.

The crystal structure and thermotropic liquid-crystal properties of N-n-undecyl-D-gluconamide

N-n-Undecyl-D-gluconamide, C17H35O6, crystallizes in space group P1, with one molecule in a unit cell a = 5.2267(6), b = 19.628(9), c = 4.7810(4) A, alpha = 93.23(2), beta = 95.60(1), gamma = 89.58(2) degrees, V = 487.35 A3, Dx = 1.19 g.cm-3. The crystal lattice is isostructural with N-n-heptyl-D-gluconamide having monolayer head-to-tail molecular packing. The molecules have a V-shaped conformation. The hydrogen bonding of the gluconamide moieties includes a four-link homodromic cycle. The transition to a smectic A liquid-crystal phase at 156.7 degrees is preceded by two crystal-to-crystal phase transitions at 77.2 degrees and 99.4 degrees. The long d-spacing of the intermediate crystal phase of 39 A, and the d-spacing of the liquid-crystal phase of 32 A, are consistent with a transition to a bilayer head-to-head molecular packing.

Virosomes reconstituted from human immunodeficiency virus proteins and lipids

Purified Human Immunodeficiency Virus (HIV) was solubilized in octylglucopyranoside. After centrifugation, the supernatant was added to lipid-detergent mixed micelles. Formation of virosomes occurred during overnight dialysis. Centrifugation on a continuous glycerol gradient showed that envelope glycoproteins (gp120 and gp41) and matrix protein p17 but not core protein p25 were associated to virosomes. Proteolytic treatment of virosomes indicates that gp120 is oriented toward the outside as in the virus particles, whereas p17 protein is anchored on both sides of the liposomal membrane. Virosomes are spherical vesicles with approximately the size of the virus as shown by electron microscopy.

Reconstitution of translocation-competent membrane vesicles from detergent-solubilized dog pancreas rough microsomes

Dog pancreas rough microsomes were solubilized in 1% octyl beta-glucoside, and membrane vesicles were reconstituted by slow 30-fold dilution with a buffer of low ionic strength. Asymmetric assembly of the membranes occurred during reconstitution since the vesicles formed contained ribosomes bound only to the vesicular outer surfaces. The reconstituted vesicles were similar in protein composition to native rough microsomes, although these vesicles were largely devoid of luminal-content proteins. These reconstituted vesicles could translocate and process nascent secretory (human placental lactogen) and membrane proteins (influenza hemagglutinin and rat liver ribophorin I) synthesized in cell-free translation systems programmed with the corresponding mRNAs. Signal cleavage and N-glycosylation only occurred when the reconstituted membranes were present during translation, providing evidence that the translocation apparatus was asymmetrically assembled into the reconstituted membranes. When a supernatant lacking ribosomes and particles greater than 50S from centrifuging the detergent-solubilized microsomes at high speed was used for reconstitution, smooth-surfaced membrane vesicles were obtained that, except for the absence of ribosomal proteins, were similar in protein composition to that of the reconstituted vesicles from total solubilized rough microsomes. The reconstituted smooth-surfaced vesicles, however, were totally inactive in cotranslational processing and translocation of nascent polypeptides. These findings suggest that ribosomes and/or large macromolecular complexes, not dissociated under our solubilization conditions, are essential for in vitro assembly of a functional translocation apparatus.

The lyotropic liquid crystal properties of n-octyl 1-O-beta-D-glucopyranoside and related n-alkyl pyranosides

X-ray diffraction patterns have been obtained for the lyotropic phases of n-octyl 1-O-beta-D-glucopyranoside and the related n-heptyl, n-nonyl and n-decyl compounds with water. The octyl compound exhibits all three liquid crystal phases and forms a micellar solution with increasing solvation, when the crystal come into contact with water at room temperature. The X-ray diffraction patterns show a one-dimensional lamellar phase with [dx] = 28.4 A, a three-dimensional face-centered cubic phase with [a] = 51.2 A, and a two-dimensional hexagonal phase with [a] = [b] = 36.7 A. The micellar solution has a distribution pattern with a maximum at [dx] = 33.8 A. Crystals of the heptyl, nonyl and decyl derivatives form only the lamellar phases and the micellar solution on contact with water at room temperature.

An analysis of the periodicity of conserved residues in sequence alignments of G-protein coupled receptors. Implications for the three-dimensional structure

Twenty-three sequences from the family of G-protein coupled receptors have been aligned according to the 'historical alignment' procedure of Feng and Doolittle. Fourier transform analysis of this reveals that parts of five of the seven putative membrane-spanning regions exhibit a periodicity of conserved/nonconserved residues which is compatible with the periodicity of the alpha-helix. This would place the conserved residues on one side of the helix, which may face the inside of the proposed seven membered helical bundle.