Lipid environment of acetylcholine receptor from Torpedo californica

Influence of lipid bilayer on the structure of the muscle-type nicotinic acetylcholine receptor

The muscle-type nicotinic acetylcholine receptor is a transmitter-gated ion channel residing in the plasma membrane of electrocytes and striated muscle cells. It is present predominantly at synaptic junctions, where it effects rapid depolarization of the postsynaptic membrane in response to acetylcholine released into the synaptic cleft. Previously, cryo-EM of intact membrane from Torpedo revealed that the lipid bilayer surrounding the junctional receptor has a uniquely asymmetric and ordered structure, due to a high concentration of cholesterol. It is now shown that this special lipid environment influences the transmembrane (TM) folding of the protein. All five submembrane MX helices of the membrane-intact junctional receptor align parallel to the surface of the cholesterol-ordered lipids in the inner leaflet of the bilayer; also, the TM helices in the outer leaflet are splayed apart. However in the structure obtained from the same protein after extraction and incorporation in nanodiscs, the MX helices do not align to a planar surface, and the TM helices arrange compactly in the outer leaflet. Realignment of the MX helices of the nanodisc-solved structure to a planar surface converts their adjoining TM helices into an obligatory splayed configuration, characteristic of the junctional receptor. Thus, the form of the receptor sustained by the special lipid environment of the synaptic junction is the one that mediates fast synaptic transmission; whereas, the nanodisc-embedded protein may be like the extrajunctional form, existing in a disordered lipid environment.

Carlos Gutiérrez-Merino: Synergy of Theory and Experimentation in Biological Membrane Research

Professor Carlos Gutiérrez-Merino, a prominent scientist working in the complex realm of biological membranes, has made significant theoretical and experimental contributions to the field. Contemporaneous with the development of the fluid-mosaic model of Singer and Nicolson, the Förster resonance energy transfer (FRET) approach has become an invaluable tool for studying molecular interactions in membranes, providing structural insights on a scale of 1-10 nm and remaining important alongside evolving perspectives on membrane structures. In the last few decades, Gutiérrez-Merino's work has covered multiple facets in the field of FRET, with his contributions producing significant advances in quantitative membrane biology. His more recent experimental work expanded the ground concepts of FRET to high-resolution cell imaging. Commencing in the late 1980s, a series of collaborations between Gutiérrez-Merino and the authors involved research visits and joint investigations focused on the nicotinic acetylcholine receptor and its relation to membrane lipids, fostering a lasting friendship.

Structure and function meet at the nicotinic acetylcholine receptor-lipid interface

The nicotinic acetylcholine receptor (nAChR) is a transmembrane protein that mediates fast intercellular communication in response to the endogenous neurotransmitter acetylcholine. It is the best characterized and archetypal molecule in the superfamily of pentameric ligand-gated ion channels (pLGICs). As a typical transmembrane macromolecule, it interacts extensively with its vicinal lipid microenvironment. Experimental evidence provides a wealth of information on receptor-lipid crosstalk: the nAChR exerts influence on its immediate membrane environment and conversely, the lipid moiety modulates ligand binding, affinity state transitions and gating of ion translocation functions of the receptor protein. Recent cryogenic electron microscopy (cryo-EM) studies have unveiled the occurrence of sites for phospholipids and cholesterol on the lipid-exposed regions of neuronal and electroplax nAChRs, confirming early spectroscopic and affinity labeling studies demonstrating the close contact of lipid molecules with the receptor transmembrane segments. This new data provides structural support to the postulated "lipid sensor" ability displayed by the outer ring of M4 transmembrane domains and their modulatory role on nAChR function, as we postulated a decade ago. Borrowing from the best characterized nAChR, the electroplax (muscle-type) receptor, and exploiting new structural information on the neuronal nAChR, it is now possible to achieve an improved depiction of these sites. In combination with site-directed mutagenesis, single-channel electrophysiology, and molecular dynamics studies, the new structural information delivers a more comprehensive portrayal of these lipid-sensitive loci, providing mechanistic explanations for their ability to modulate nAChR properties and raising the possibility of targetting them in disease.

Xenopus Oocytes as a Powerful Cellular Model to Study Foreign Fully-Processed Membrane Proteins

The use of Xenopus oocytes in electrophysiological and biophysical research constitutes a long and successful story, providing major advances to the knowledge of the function and modulation of membrane proteins, mostly receptors, ion channels, and transporters. Earlier reports showed that these cells are capable of correctly expressing heterologous proteins after injecting the corresponding mRNA or cDNA. More recently, the Xenopus oocyte has become an outstanding host-cell model to carry out detailed studies on the function of fully-processed foreign membrane proteins after their microtransplantation to the oocyte. This review focused on the latter overall process of transplanting foreign membrane proteins to the oocyte after injecting plasma membranes or purified and reconstituted proteins. This experimental approach allows for the study of both the function of mature proteins, with their native stoichiometry and post-translational modifications, and their putative modulation by surrounding lipids, mostly when the protein is purified and reconstituted in lipid matrices of defined composition. Remarkably, this methodology enables functional microtransplantation to the oocyte of membrane receptors, ion channels, and transporters from different sources including human post-mortem tissue banks. Despite the large progress achieved over the last decades on the structure, function, and modulation of neuroreceptors and ion channels in healthy and pathological tissues, many unanswered questions remain and, most likely, Xenopus oocytes will continue to help provide valuable responses.

Structure of a cholinergic cell membrane

While it has long been established that cell membranes are complex assemblies of proteins and bilayer-forming lipids, the inherent mobility and wide-ranging heterogeneity of the lipids have limited our ability to understand cell-membrane structure at a molecular level. Consequently, little is yet known about the protein-lipid and lipid-lipid interplay that exists in situ. The present study exploits the regular architecture of a cholinergic cell membrane to determine how the phospholipid and cholesterol organization is influenced by the protein surfaces and by differences in cholesterol concentration between the two leaflets. Lipids in the leaflet containing the most cholesterol form an ordered sterol-hydrocarbon “skin.” This hitherto unobserved hydrophobic-core structure has far-reaching implications in terms of how cholesterol-rich membranes are constructed and function.

Cell membranes are complex assemblies of proteins and lipids making transient or long-term associations that have yet to be characterized at a molecular level. Here, cryo-electron microscopy is applied to determine how phospholipids and cholesterol arrange between neighboring proteins (nicotinic acetylcholine receptors) of Torpedo cholinergic membrane. The lipids exhibit distinct properties in the two leaflets of the bilayer, influenced by the protein surfaces and by differences in cholesterol concentration. In the outer leaflet, the lipids show no consistent motif away from the protein surfaces, in keeping with their assumed fluidity. In the inner leaflet, where the cholesterol concentration is higher, the lipids organize into extensive close-packed linear arrays. These arrays are built from the sterol groups of cholesterol and the initial saturated portions of the phospholipid hydrocarbon chains. Together, they create an ordered ∼7 Å-thick “skin” within the hydrophobic core of the bilayer. The packing of lipids in the arrays appears to bear a close relationship to the linear cholesterol arrays that form crystalline monolayers at the air-water interface.

Druggable Lipid Binding Sites in Pentameric Ligand-Gated Ion Channels and Transient Receptor Potential Channels

Lipids modulate the function of many ion channels, possibly through direct lipid-protein interactions. The recent outpouring of ion channel structures by cryo-EM has revealed many lipid binding sites. Whether these sites mediate lipid modulation of ion channel function is not firmly established in most cases. However, it is intriguing that many of these lipid binding sites are also known sites for other allosteric modulators or drugs, supporting the notion that lipids act as endogenous allosteric modulators through these sites. Here, we review such lipid-drug binding sites, focusing on pentameric ligand-gated ion channels and transient receptor potential channels. Notable examples include sites for phospholipids and sterols that are shared by anesthetics and vanilloids. We discuss some implications of lipid binding at these sites including the possibility that lipids can alter drug potency or that understanding protein-lipid interactions can guide drug design. Structures are only the first step toward understanding the mechanism of lipid modulation at these sites. Looking forward, we identify knowledge gaps in the field and approaches to address them. These include defining the effects of lipids on channel function in reconstituted systems using asymmetric membranes and measuring lipid binding affinities at specific sites using native mass spectrometry, fluorescence binding assays, and computational approaches.

Lipid Membrane State Change by Catalytic Protonation and the Implications for Synaptic Transmission

In cholinergic synapses, the neurotransmitter acetylcholine (ACh) is rapidly hydrolyzed by esterases to choline and acetic acid (AH). It is believed that this reaction serves the purpose of deactivating ACh once it has exerted its effect on a receptor protein (AChR). The protons liberated in this reaction, however, may by themselves excite the postsynaptic membrane. Herein, we investigated the response of cell membrane models made from phosphatidylcholine (PC), phosphatidylserine (PS) and phosphatidic acid (PA) to ACh in the presence and absence of acetylcholinesterase (AChE). Without a catalyst, there were no significant effects of ACh on the membrane state (lateral pressure change ≤0.5 mN/m). In contrast, strong responses were observed in membranes made from PS and PA when ACh was applied in presence of AChE (>5 mN/m). Control experiments demonstrated that this effect was due to the protonation of lipid headgroups, which is maximal at the pK (for PS: pKCOOH≈5.0; for PA: pKHPO4-≈8.5). These findings are physiologically relevant, because both of these lipids are present in postsynaptic membranes. Furthermore, we discussed evidence which suggests that AChR assembles a lipid-protein interface that is proton-sensitive in the vicinity of pH 7.5. Such a membrane could be excited by hydrolysis of micromolar amounts of ACh. Based on these results, we proposed that cholinergic transmission is due to postsynaptic membrane protonation. Our model will be falsified if cholinergic membranes do not respond to acidification.

Peimine, an Anti-Inflammatory Compound from Chinese Herbal Extracts, Modulates Muscle-Type Nicotinic Receptors

Fritillaria bulbs are used in Traditional Chinese Medicine to treat several illnesses. Peimine (Pm), an anti-inflammatory compound from Fritillaria, is known to inhibit some voltage-dependent ion channels and muscarinic receptors, but its interaction with ligand-gated ion channels remains unexplored. We have studied if Pm affects nicotinic acetylcholine receptors (nAChRs), since they play broad functional roles, both in the nervous system and non-neuronal tissues. Muscle-type nAChRs were incorporated to Xenopus oocytes and the action of Pm on the membrane currents elicited by ACh (I_AChs) was assessed. Functional studies were combined with virtual docking and molecular dynamics assays. Co-application of ACh and Pm reversibly blocked I_ACh, with an IC₅₀ in the low micromolar range. Pm inhibited nAChR by: (i) open-channel blockade, evidenced by the voltage-dependent inhibition of I_Ach, (ii) enhancement of nAChR desensitization, revealed by both an accelerated I_ACh decay and a decelerated I_ACh deactivation, and (iii) resting-nAChR blockade, deduced from the I_ACh inhibition elicited by Pm when applied before ACh superfusion. In good concordance, virtual docking and molecular dynamics assays demonstrated that Pm binds to different sites at the nAChR, mostly at the transmembrane domain. Thus, Pm from Fritillaria bulbs, considered therapeutic herbs, targets nAChRs with high affinity, which might account for its anti-inflammatory actions.

Protein-lipid architecture of a cholinergic postsynaptic membrane

The cholinergic postsynaptic membrane is an acetyl-choline receptor-rich membrane mediating fast chemical communication at the nerve-muscle synapse. Here, cryo-EM is used to examine the protein-lipid architecture of this membrane in tubular vesicles obtained from the (muscle-derived) electric organ of the Torpedo ray. As reported earlier, the helical arrangement of the protein component of the vesicles facilitates image averaging and enables us to determine how cholesterol and phospho-lipid molecules are distributed in the surrounding matrix, using headgroup size as a means to discriminate between the two kinds of lipid. It is shown that cholesterol segregates preferentially around the receptors in both leaflets of the lipid bilayer, interacting robustly with specific transmembrane sites and creating a network of bridging microdomains. Cholesterol interactions with the receptor are apparently essential for stabilizing and maintaining its physiological architecture, since the transmembrane structure contracts, involving displacements of the helices at the outer membrane surface by ∼2 Å (1-3 Å), when this lipid is extracted. The microdomains may promote cooperativity between neighbouring receptors, leading to an enhanced postsynaptic response.

Boundary lipids of the nicotinic acetylcholine receptor: Spontaneous partitioning via coarse-grained molecular dynamics simulation

Reconstituted nicotinic acetylcholine receptors (nAChRs) exhibit significant gain-of-function upon addition of cholesterol to reconstitution mixtures, and cholesterol affects the organization of nAChRs within domain-forming membranes, but whether nAChR partitions to cholesterol-rich liquid-ordered ("raft" or l_o) domains or cholesterol-poor liquid-disordered (l_do) domains is unknown. We use coarse-grained molecular dynamics simulations to observe spontaneous interactions of cholesterol, saturated lipids, and polyunsaturated (PUFA) lipids with nAChRs. In binary Dipalmitoylphosphatidylcholine:Cholesterol (DPPC:CHOL) mixtures, both CHOL and DPPC acyl chains were observed spontaneously entering deep "non-annular" cavities in the nAChR TMD, particularly at the subunit interface and the β subunit center, facilitated by the low amino acid density in the cryo-EM structure of nAChR in a native membrane. Cholesterol was highly enriched in the annulus around the TMD, but this effect extended over (at most) 5-10 Å. In domain-forming ternary mixtures containing PUFAs, the presence of a single receptor did not significantly affect the likelihood of domain formation. nAChR partitioned to any cholesterol-poor l_do domain that was present, regardless of whether the l_do or l_o domain lipids had PC or PE headgroups. Enrichment of PUFAs among boundary lipids was positively correlated with their propensity for demixing from cholesterol-rich phases. Long n-3 chains (tested here with Docosahexaenoic Acid, DHA) were highly enriched in annular and non-annular embedded sites, partially displacing cholesterol and completely displacing DPPC, and occupying sites even deeper within the bundle. Shorter n-6 chains were far less effective at displacing cholesterol from non-annular sites.

Uncovering the lipidic basis for the preparation of functional nicotinic acetylcholine receptor detergent complexes for structural studies

This study compares the lipid composition, including individual phospholipid molecular species of solubilized nAChR detergent complexes (nAChR-DCs) with those of the bulk lipids from their source, Torpedo californica (Tc) electric tissue. This lipidomic analysis revealed seventy-seven (77) phospholipid species in the Tc tissue. Analysis of affinity-purified nAChR-DCs prepared with C-12 to C-16 phospholipid analog detergents alkylphosphocholine (FC) and lysofoscholine (LFC) demonstrated that nAChR-DCs prepared with FC12, LFC14, and LFC16 contained >60 phospholipids/nAChR, which was more than twice of those prepared with FC14, FC16, and LFC12. Significantly, all the nAChR-DCs lacked ethanolamine and anionic phospholipids, contained only four cholesterol molecules, and a limited number of phospholipid molecular species per nAChR. Upon incorporation into oocytes, FC12 produce significant functionality, whereas LFC14 and LFC16 nAChR-DCs displayed an increased functionality as compared to the crude Tc membrane. All three nAChR-DCs displayed different degrees of alterations in macroscopic activation and desensitization kinetics.

Heterogeneous Inhibition in Macroscopic Current Responses of Four Nicotinic Acetylcholine Receptor Subtypes by Cholesterol Enrichment

The nicotinic acetylcholine receptor (nAChR), located in the cell membranes of neurons and muscle cells, mediates the transmission of nerve impulses across cholinergic synapses. In addition, the nAChR is also found in the electric organs of electric rays (e.g., the genus Torpedo). Cholesterol, which is a key lipid for maintaining the correct functionality of membrane proteins, has been found to alter the nAChR function. We were thus interested to probe the changes in the functionality of different nAChRs expressed in a model membrane with modified cholesterol to phospholipid ratios (C/P). In this study, we examined the effect of increasing the C/P ratio in Xenopus laevis oocytes expressing the neuronal α7, α4β2, muscle-type, and Torpedo californica nAChRs in their macroscopic current responses. Using the two-electrode voltage clamp technique, it was found that the neuronal α7 and Torpedo nAChRs are significantly more sensitive to small increases in C/P than the muscle-type nAChR. The peak current versus C/P profiles during enrichment display different behaviors; α7 and Torpedo nAChRs display a hyperbolic decay with two clear components, whereas muscle-type and α4β2 nAChRs display simple monophasic decays with different slopes. This study clearly illustrates that a physiologically relevant increase in membrane cholesterol concentration produces a remarkable reduction in the macroscopic current responses of the neuronal α7 and Torpedo nAChRs functionality, whereas the muscle nAChR appears to be the most resistant to cholesterol inhibition among all four nAChR subtypes. Overall, the present study demonstrates differential profiles for cholesterol inhibition among the different types of nAChR to physiological cholesterol increments in the plasmatic membrane. This is the first study to report a cross-correlation analysis of cholesterol sensitivity among different nAChR subtypes in a model membrane.

Sphingolipid/cholesterol regulation of neurotransmitter receptor conformation and function

Like all other monomeric or multimeric transmembrane proteins, receptors for neurotransmitters are surrounded by a shell of lipids which form an interfacial boundary between the protein and the bulk membrane. Among these lipids, cholesterol and sphingolipids have attracted much attention because of their well-known propensity to segregate into ordered platform domains commonly referred to as lipid rafts. In this review we present a critical analysis of the molecular mechanisms involved in the interaction of cholesterol/sphingolipids with neurotransmitter receptors, in particular acetylcholine and serotonin receptors, chosen as representative members of ligand-gated ion channels and G protein-coupled receptors. Cholesterol and sphingolipids interact with these receptors through typical binding sites located in both the transmembrane helices and the extracellular loops. By altering the conformation of the receptors ("chaperone-like" effect), these lipids can regulate neurotransmitter binding, signal transducing functions, and, in the case of multimeric receptors, subunit assembly and subsequent receptor trafficking to the cell surface. Several sphingolipids (especially gangliosides) also exhibit low/moderate affinity for neurotransmitters. We suggest that such lipids could facilitate (i) the attachment of neurotransmitters to the post-synaptic membrane and in some cases (ii) their subsequent delivery to specific protein receptors. Overall, various experimental approaches provide converging evidence that the biological functions of neurotransmitters and their receptors are highly dependent upon sphingolipids and cholesterol, which are active partners of synaptic transmission. Several decades of research have been necessary to untangle the skein of a complex network of molecular interactions between neurotransmitters, their receptors, cholesterol and sphingolipids. This sophisticated crosstalk between all four distinctive partners may allow a fine biochemical tuning of synaptic transmission.

U18666A, a cholesterol-inhibition agent, modulates human neuronal nicotinic acetylcholine receptors heterologously expressed in SH-EP1 cell line

In this study, we evaluate the effects of (3beta)-3-[2-(diethylamino)ethoxy]androst-5-en-17-one dihydrochloride (U18666A), a cholesterol synthesis/transporter inhibitor, on selected human neuronal nicotinic acetylcholine receptors (nAChRs) heterologously expressed in the SH-EP1 cell line using whole-cell patch-clamp recordings. The results indicate that with 2-min pretreatment, U18666A inhibited different nAChR subtypes with a rank-order of potency (IC(50) of whole-cell peak current): alpha4beta2 (8.0 +/- 3.0 nM) > alpha3beta2 (1.7 +/- 0.4 microM) > alpha4beta4 (26 +/- 7.2 microM) > alpha7 (> 100 microM), suggesting this compound is more selective to alpha4beta2-nAChRs. Thus, the pharmacological profiles and mechanisms of U18666A acting on alpha4beta2-nAChRs were investigated in detail. U18666A suppresses both peak and steady state components of whole-cell currents mediated by human alpha4beta2-nAChRs in response to nicotine. In nicotine-induced concentration-response curves, U18666A reduces nicotine-induced current at maximally effective agonist concentrations without influencing nicotine's EC(50) value, suggesting a non-competitive inhibition. U18666A-induced inhibition of nAChR function is concentration-, voltage-, and use-dependent, suggesting an open channel block. Taken into consideration of approximately 10 000-fold enhancement of the potency of U18666A after 2-min pre-treatment, this compound also likely inhibits alpha4beta2-nAChRs through a close channel block. In addition, the U18666A-induced inhibition in alpha4beta2-nAChRs is not mediated by either increased receptor endocytosis or altered cell cholesterol. These data indicate that U18666A is a potent antagonist of alpha4beta2-nAChRs and may be useful as a tool in the functional characterization and pharmacological profiling of nAChRs, as well as a potential candidate for smoking cessation.

Protein-promoted membrane domains

The current notion of biological membranes encompasses a very complex structure, made of dynamically changing compartments or domains where different membrane components partition. These domains have been related to important cellular functions such as membrane sorting, signal transduction, membrane fusion, neuronal maturation, and protein activation. Many reviews have dealt with membrane domains where lipid-lipid interactions direct their formation, especially in the case of raft domains, so in this review we considered domains induced by integral membrane proteins. The nature of the interactions involved and the different mechanisms through which membrane proteins segregate lipid domains are presented, in particular with regard to those induced by the nAChR. It may be concluded that coupling of favourable lipid-lipid and lipid-protein interactions is a general condition for this phenomenon to occur.

Structural basis for lipid modulation of nicotinic acetylcholine receptor function

The nicotinic acetylcholine receptor (AChR) is the archetype molecule in the superfamily of ligand-gated ion channels (LGIC). Members of this superfamily mediate fast intercellular communication in response to endogenous neurotransmitters. This review is focused on the structural and functional crosstalk between the AChR and lipids in the membrane microenvironment, and the modulation exerted by the latter on ligand binding and ion translocation. Experimental approaches using Laurdan extrinsic fluorescence and Förster-type resonance energy transfer (FRET) that led to the characterization of the polarity and molecular dynamics of the liquid-ordered phase AChR-vicinal lipids and the bulk membrane lipids, and the asymmetry of the AChR-rich membrane are reviewed first. The topological relationship between protein and lipid moieties and the changes in physical properties induced by exogenous lipids are discussed next. This background information lays the basis for understanding the occurrence of lipid sites in the AChR transmembrane region, and the selectivity of the protein-lipid interactions. Changes in FRET efficiency induced by fatty acids, phospholipid and cholesterol (Chol), led to the identification of discrete sites for these lipids on the AChR protein, and electron-spin resonance (ESR) spectroscopy has recently facilitated determination of the stoichiometry and selectivity for the AChR of the shell lipid. The influence of lipids on AChR function is discussed next. Combined single-channel and site-directed mutagenesis data fostered the recognition of lipid-sensitive residues in the transmembrane region, dissecting their contribution to ligand binding and channel gating, opening and closing. Experimental evidence supports the notion that the interface between the protein moiety and the adjacent lipid shell is the locus of a variety of pharmacologically relevant processes, including the action of steroids and other lipids.

Identifying the cholesterol binding domain in the nicotinic acetylcholine receptor with [125I]azido-cholesterol

A novel photoreactive analog of cholesterol, 3alpha-(4-azido-3-[125I]iodosalicylic)-cholest-5-ene ([125I]azido-cholesterol), was used to label both native acetylcholine receptor (AChR)-rich membranes from Torpedo californica and affinity-purified Torpedo AChRs reconstituted into lipid vesicles. In both cases all four AChR subunits incorporated [125I]azido-cholesterol on an equal molar basis and neither the pattern nor the extent of labeling was affected by the presence of the agonist carbamylcholine. Labeled regions in each of the AChR subunits were initially mapped by Staphylococcus aureus V8 protease digestion to large fragments which contain the AChR transmembrane segments. Sites of [125I]azido-cholesterol incorporation were further mapped by exhaustive tryptic digestion of the V8 protease subunit fragments alphaV8-20 (alphaSer-173-Glu-338), alphaV8-10 (alphaAsn-339-Gly-439), and gammaV8-14 (gammaLeu-373-Pro-489). The digests were separated by reverse-phase high-performance liquid chromatography and labeled peptides identified by amino-terminal sequence analysis. [125I]Azido-cholesterol labeling was localized to peptides that contain almost exclusively the alpha-M4, alpha-M1 and gamma-M4 membrane spanning segments. These results establish that the binding domain for cholesterol is at the lipid-protein interface of the AChR.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Dynamic properties of Newcastle Disease Virus envelope and their relations with viral hemagglutinin-neuraminidase membrane glycoprotein

The lipid composition of Newcastle Disease Virus (NDV) Clone-30 strain shows a low lipid/protein ratio, a high cholesterol/phospholipid molar ratio, and major phospholipids being qualitatively different to other NDV strains. The major fatty acyl constituents are palmitic, stearic, oleic, and linoleic acids; cerebrosides, sulfatides and two kinds of gangliosides are also found in the NDV membrane. It is reported for the first time in NDV that phospholipid classes are asymmetrically distributed over the two leaflets of the membrane: 60 +/- 4.5% of the phosphatidylcholine and 70 +/- 5.0% of the sphingomyelin are in the outer monolayer. Intact viral membranes and reconstituted NDV envelopes showed similar dynamic properties. Hemagglutinin-neuraminidase (HN) and fusion (F) proteins of NDV membrane affect the lipid thermotropic behaviour in reconstituted proteoliposomes made up of a single class of phospholipids. It is shown that the lipid composition is more important than the bulk membrane fluidity/order for both sialidase (neuraminidase) and hemagglutinating HN activities. Sialidase and hemagglutinating activities requires the presence of definite phospholipids (phosphatidylethanolamine) in its environment.

Papulacandin B resistance in budding and fission yeasts: isolation and characterization of a gene involved in (1,3)beta-D-glucan synthesis in Saccharomyces cerevisiae

Papulacandin B, an antifungal agent that interferes with the synthesis of yeast cell wall (1,3)beta-D-glucan, was used to isolate resistant mutants in Schizosaccharomyces pombe and Saccharomyces cerevisiae. The resistance to papulacandin B always segregated as a recessive character that defines a single complementation group in both yeasts (pbr1+ and PBR1, respectively). Determination of several kinetic parameters of (1,3)beta-D-glucan synthase activity revealed no differences between S. pombe wild-type and pbr1 mutant strains except in the 50% inhibitory concentration for papulacandin B of the synthases (about a 50-fold increase in mutant activity). Inactivation of the synthase activity of both yeasts after in vivo treatment with the antifungal agent showed that mutant synthases were more resistant than the corresponding wild-type ones. Detergent dissociation of the S. pombe synthase into soluble and particulate fractions and subsequent reconstitution indicated that the resistance character of pbr1 mutants resides in the particulate fraction of the enzyme. Cloning and sequencing of PBR1 from S. cerevisiae revealed a gene identical to others recently reported (FKS1, ETG1, CWH53, and CND1). Its disruption leads to reduced levels of both (1,3)beta-D-glucan synthase activity and the alkali-insoluble cell wall fraction. Transformants containing the PBR1 gene reverse the defect in (1,3)beta-D-glucan synthase. It is concluded that Pbr1p is probably part of the (1,3)beta-D-glucan synthase complex.

Agonist-induced displacement of quinacrine from its binding site on the nicotinic acetylcholine receptor: plausible agonist membrane partitioning mechanism

It was previously demonstrated that high concentrations of cholinergic agonists such as acetylcholine (ACh), carbamylcholine (CCh), suberyldicholine (SubCh) and spin-labelled acetylcholine (SL-ACh) displaced quinacrine from its high-affinity binding site located at the lipid-protein interface of the nicotinic acetylcholine receptor (AChR) (Anas, H. R. and Johnson, D. A. (1995) Biochemistry, 34, 1589-1595). In order to account for the agonist self-inhibitory binding site which overlaps, at least partially, with the quinacrine binding site, we determined the partition coefficient (Kp) of these agonists relative to the local anaesthetic tetracaine in AChR native membranes from Torpedo californica electric organ by examining (1) the ability of tetracaine and SL-ACh to quench membrane-partitioned 1-pyrenedecanoic acid (C10-Py) monomer fluorescence, and (2) the ability of ACh, CCh and SubCh to induce an increase in the excimer/monomer ratio of C10-Py-labelled AChR membrane fluorescence. To further assess the differences in agonist accessibility to the quinacrine binding site, we calculated the agonist concentration in the lipid membrane (CM) at an external agonist concentration high enough to inhibit 50% of quinacrine binding (IC50), which in turn was obtained by agonist back titration of AChR-bound quinacrine. Initial experiments established that high agonist concentrations do not affect either transmembrane proton concentration equilibria (pH) of AChR membrane suspension or AChR-bound quinacrine fluorescence spectra. The agonist membrane partitioning experiments indicated relatively small (< or = 20) Kp values relative to tetracaine. These values follow the order: SL-ACh>SubCh>>CCh-ACh. A direct correlation was observed between Kp and the apparent inhibition constant (Ki) for agonists to displace AChR-bound quinacrine. Particularly, agonist with high KpS such as SL-ACh and SubCh showed low Ki values, and this relationship was opposite for CCh and ACh. The calculated CM values indicated significant (between 7 and 54 mM) agonist accessibility to lipid membrane. By themselves, these results support the conjecture that agonist self-inhibition seems to be mediated by the quinacrine binding site via a membrane approach mechanism. The existence of an agonist self-inhibitory binding site, not located in the channel lumen would indicate an allosteric mechanism of ion channel inhibition; however, we can not discard that the process of agonist self-inhibition can also be mediated by a steric blockage of the ion channel.

Lipid modulation of nicotinic acetylcholine receptor function: the role of membrane lipid composition and fluidity

The effects of membrane lipid composition and fluidity on AChR ion channel function were studied after reconstituting the receptor with sphingomyelin, phosphatidylcholines with different degrees of unsaturation, or different neutral lipids. AChR ion flux activity was shown to be retained in some membranes of both high and low fluidity, as measured by the steady-state anisotropy of the membrane probes diphenylhexatriene and trimethylammonium diphenylhexatriene. The results suggest that lipid composition is more important than bulk membrane fluidity in determining AChR ion channel function.

Thermal stability of Torpedo californica acetylcholine receptor in a cholesterol lipid environment

Controlled heating of acetylcholine receptor (AChR) vesicles inactivates the alpha-bungarotoxin (alpha-Bgtx) binding sites with a T50 (temperature at which 50% of the initial capacity to bind alpha-Bgtx remains) of 60 +/- 0.2 degrees C. The same value was obtained for receptor reconstituted in lipid vesicles from Torpedo electroplax where the % mol composition of cholesterol to phospholipid was 30. However, when the reconstitution was carried out in dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidic acid (DOPA) vesicles (3:1 molar ratio), T50 of the curves decreased to 56 +/- 0.2 degrees C and no carbamylcholine stimulated 22Na+ flux was detected. Inclusion of cholesterol in the DOPC-DOPA vesicles increased the toxin binding site stability. The maximal T50 of the toxin binding curves was 63 +/- 0.1 degrees C when the % mol cholesterol/mol DOPC:DOPA in the vesicles was 33. Under these conditions AChR was able to translocate ions, a property that was lost upon heating at 46 degrees C. Preincubation of AChR in the presence of d-tubocurarine, tetracaine or procaine did not affect T50 values of toxin binding. However, a slight increment in thermal stability was found when the receptor was preincubated in the presence of carbamylcholine. The results show that cholesterol requirements for protecting against thermal inactivation of toxin binding and ion gating properties are different and the carbamylcholine-bound receptor may have a different conformation.

Transverse distance between the membrane and the agonist binding sites on the Torpedo acetylcholine receptor: a fluorescence study

Fluorescence dipolar resonance energy transfer between a receptor-bound fluorescent agonist, dansyl-C6-choline, and two membrane-partitioned fluorescent probes, C18-rhodamine and C12-eosin, was used to measure the transverse distance between the acetylcholine (ACh) binding sites on the intact Torpedo nicotinic acetylcholine receptor (nAChR) and the surface of the lipid membrane. Control experiments demonstrated that: (1) dansyl-C6-choline binds to cobra-alpha-toxin sensitive sites on the nAChR with a KD approximately 20 nM, (2) the quantum yield of dansyl-C6-choline increases 3.1-fold upon binding, and (3) the receptor-bound dansyl-C6-choline fluorescence is stable for at least 2 h. The calculated transverse distances between receptor-bound dansyl-C6-choline and the membrane-partitioned acceptors, C12-eosin and C18-rhodamine, were 31 and 39 A, respectively. Therefore, given the dimensions of the extracellular domain of the receptor, the ACh binding sites are located significantly below (approximately 25 A) the extracellular apex of the nAChR. These results are in agreement with the recent proposed location for the ACh binding sites in a pocket within each of the two alpha-subunits, approximately 30 A above the membrane surface (Unwin, N. (1993) J. Mol. Biol. 229: 1101-1124).

Labeling of the nicotinic acetylcholine receptor by a photoactivatable steroid probe: effects of cholesterol and cholinergic ligands

A photoactivatable steroid, p-azidophenacyl 3 alpha-hydroxy-5 beta-cholan-24- ate (APL), has been synthesized and used instead of cholesterol to functionally reconstitute purified acetylcholine receptor (AcChR) into vesicles made of asolectin phospholipids. Upon irradiation, the extent of AcChR photolabeling by APL is directly proportional to the amount of APL incorporated into the reconstituted vesicles and the maximum stoichiometry observed corresponds to approx. 50 mol of APL bound per mol of AcChR. Furthermore, all four subunits of the AcChR become labeled by APL and the observed labeling pattern resembles the 2:1:1:1 stoichiometry characteristic of these subunits within the AcChR complex. The presence of either cholesterol or neutral lipids from asolectin in the reconstituted bilayer decreases both, the incorporation of APl into the vesicles and the covalent labeling of the AcChR upon irradiation, without altering the stoichiometry of labeling in AcChR subunits stated above. This suggests that the potential interaction sites for the photoactivatable probe in the reconstituted AcChR are mostly those normally occupied by the natural neutral lipids. Carbamylcholine, a cholinergic agonist, also reduces the extent of APL photolabeling of the AcChR in a dose-dependent manner but, in contrast to the effects of cholesterol, the presence of carbamylcholine alters the stoichiometry of labeling in the AcChR subunits. This, along with the observation that such a decrease in the extent of APL photolabeling caused by carbamylcholine can be blocked by preincubation with alpha-bungarotoxin, suggest that AcChR desensitization induced by prolonged exposure to cholinergic agonists encompasses a rearrangement of transmembrane portions of the AcChR protein, which can be sensed by the photoactivatable probe. Conversely, presence of (+)-tubocurarine, a competitive cholinergic antagonist, has no effects on altering either the extent of APL photolabeling of the AcChR or the distribution of the labeling among AcChR subunits.

Incorporation of the nicotinic acetylcholine receptor into planar multilamellar films: characterization by fluorescence and Fourier transform infrared difference spectroscopy

A method for preparing thin, planar films of nicotinic acetylcholine receptor (nAChR) membranes that retain the ability to undergo the resting to desensitized state transition and that are suitable for spectroscopic studies has been developed. Native, alkaline-extracted nAChR membranes from Torpedo are dried under nitrogen on either a plastic microscope coverslip or a germanium internal reflection element (IRE) and then equilibrated with buffer. The drying procedure has no effect on the functional state of the nAChR as judged by a fluorescence assay using the probe ethidium bromide. The times required for an acetylcholine analogue (carbamylcholine), a local anesthetic (dibucaine), and a fluorescent probe (ethidium bromide) to penetrate films of varying degrees of thickness, interact with the receptor, and then to be washed from the films have been established. Under these conditions, the nAChR films can be repetitively cycled between the resting and desensitized states. Both fluorescence and infrared spectroscopy show that the films adhere strongly to either support even with buffer flowing continuously past the film surface. Fourier transform infrared difference spectra calculated from spectra recorded in the presence and absence of carbamylcholine show small, reproducible bands which reflect changes in nAChR structure upon desensitization.

Fatty acid composition of subcellular particles from sheep platelets and topological distribution of phosphatidylethanolamine fatty acids in the plasma membrane

The fatty acid composition of individual phospholipids in subcellular fractions of sheep platelets and the asymmetrical distribution of phosphatidylethanolamine (PE) fatty acyl chains across the plasma membrane were examined. The main fatty acids of total lipid extracts were oleic (18:1; 32-41%), linoleic (18:2, 10-17%), stearic (18:0; 13-15%), palmitic (16:0; 11-15%) and arachidonic (20:4; 8-12%) acids, with a saturated/unsaturated ratio of about 0.4. Each phospholipid class had a distinct fatty acid pattern. Sphingomyelin (SM) showed the highest degree of saturation (50%), with large proportions of behenic (22:0), 18:0 and 16:0 acids. The main fatty acid in PE, phosphatidylserine (PS) and phosphatidylcholine (PC) was 18:1n-9. Our findings suggest that fatty acids are asymmetrically distributed between the choline versus the non-choline phospholipids, and also between plasma membranes and intracellular membranes. The transbilayer distribution of PE fatty acids in plasma membranes from non-stimulated sheep platelets was investigated using trinitrobenzene-sulfonic acid (TNBS). A significant degree of asymmetry was found, which is a new observation in a non-polar cell. The PE molecules from the inner monolayer contained higher amounts of 18:2 and significantly less 18:1 and 20:5 than those found in the outer monolayer, although no major differences were detected in the transbilayer distribution of total unsaturated versus saturated PE acyl chains.

Verapamil prevents the effects of daunomycin on the thermotropic phase transition of model lipid bilayers

High-sensitivity differential scanning calorimetry and fluorescence-depolarization techniques were used to study how the presence of daunomycin and/or verapamil affect the thermotropic behaviour of dipalmitoyl phosphatidylcholine (DPPC) vesicles. Daunomycin, a potent anti-cancer agent, perturbs the thermodynamic parameters associated with the lipid phase transition: it decreases the enthalpy change, lowers the transition temperature and reduces the co-operative behavior of the phospholipid molecules. Verapamil, on the other hand, produces smaller alterations in the lipid phase transition. However, when daunomycin and verapamil are present simultaneously in the DPPC vesicles, it is observed that verapamil prevents, in a concentration-dependent manner, the alteration in the phospholipid phase transition expected from the presence of daunomycin in the bilayer. Furthermore, drug-binding studies suggest that the observed interference of verapamil in the daunomycin/phospholipid interaction occurs without a decrease in the amount of daunomycin bound to the lipid bilayer and without the formation of a daunomycin-verapamil complex. Because of the importance of drug-membrane interactions in anthracycline cytotoxicity, we speculate that the lipid bilayer of biological membranes may provide appropriate sites at which the presence of verapamil influences the activity of daunomycin.

The lipid bilayer of acetylcholine receptor clusters of cultured rat myotubes is organized into morphologically distinct domains

We have studied the composition and organization of the lipid bilayer at the large, substrate-associated clusters of acetylcholine receptors (AChR) that form in cultured rat myotubes. These clusters have a characteristic morphology consisting of alternating linear domains of AChR-rich and AChR-poor membrane, the latter involved in attaching the myotube to the substrate. We partially purified AChR clusters by extracting cultured rat myotubes with the cholesterol-specific detergent, saponin. The lipid bilayer of the cluster preparation was analyzed biochemically and the substructure of the bilayers was studied morphologically using the fluorescent probes, dansyl polymyxin B, and 3,3'-di(C12H25 and C18H37) indocarbocyanine iodide (C12- and C18-diI). Our results demonstrate that preparations of AChR clusters have a lipid composition biochemically similar to that of the surrounding plasma membrane. Morphologically, however, the lipid bilayer appears to be arranged into domains that resemble the interdigitating pattern seen for the AChR. This distinctive lipid organization is not due to the use of saponin to purify clusters, as we obtained similar results with clusters isolated by physically shearing myotube cultures. The domain-like organization of the bilayer at clusters is disrupted by treatments that disperse AChR clusters in intact myotubes or that remove peripheral membrane proteins from isolated clusters. This suggests that such proteins may contribute to the organization of the bilayer. Two additional factors may also contribute to the organization of the bilayer: physical constraints imposed by sites of substrate attachment and, to a lesser extent, "boundary" lipid associated with AChR.

Effect of local anaesthetics on steroid-nicotinic acetylcholine receptor interactions in native membranes of Torpedo marmorata electric organ

Interactions between steroids and the nicotinic acetylcholine receptor (AChR) have been studied in native membrane vesicles from Torpedo marmorata electric organ by electron spin resonance (ESR) and fluorescence techniques. ESR spectra of spin-labelled cholestane (CSL) revealed that this steroid probe was incorporated into the AChR-rich membrane vesicles in regions which were to a certain extent enriched preferentially in the steroid, both in the presence and in the absence of local anaesthetics. Since the nitroxide group present in CSL is also a paramagnetic quencher of the intrinsic protein fluorescence, this property was used to characterize the AChR-steroid interactions. The quenching induced by CSL was sensitive both to AChR concentration and to the action of cholinergic agonists. In competition experiments, the ability of CSL to quench the AChR intrinsic fluorescence was markedly inhibited by benzocaine, tetracaine and QX-222 (a quaternary trimethylammonium derivative of lidocaine), and was totally inhibited by procaine. The effectiveness of local anaesthetics in inhibiting CSL-induced quenching followed the order: procaine much greater than benzocaine approximately greater than tetracaine greater than QX-222. This inhibition effect was shown not to be charge-dependent. The data can be interpreted in terms of a model requiring specific association sites for local anaesthetics on the hydrophobic surface of the AChR which at least partially overlap with those for steroids.

Photoaffinity labeling of the Torpedo californica nicotinic acetylcholine receptor with an aryl azide derivative of phosphatidylserine

A photoactivatable analogue of phosphatidylserine, 125I-labeled 4-azidosalicylic acid-phosphatidylserine (125I ASA-PS), was used to label both native acetylcholine receptor (AchR)-rich membranes from Torpedo californica and AchR membranes affinity purified from Torpedo reconstituted into asolectin (a crude soybean lipid extract) vesicles. The radioiodinated arylazido group attaches directly to the phospholipid head group and thus probes for regions of the AchR structure in contact with the negatively charged head group of phosphatidylserine. All four subunits of the AchR incorporated the label, with the alpha subunit incorporating approximately twice as much as each of the other subunits on a per mole basis. The regions of the AchR alpha subunit that incorporated 125I ASA-PS were mapped by Staphylococcus aureus V8 protease digestion. The majority of label incorporated into fragments representing a more complete digestion of the alpha subunit was localized to 11.7- and 10.1-kDa V8 cleavage fragments, both beginning at Asn-339 and of sufficient length to contain the hydrophobic regions M1, M2, and M3 was also significantly labeled. In contrast, V8 cleavage fragments representing roughly a third of the amino-terminal portion of the alpha subunit incorporated little or no detectable amount of probe.

The lipid environment of the nicotinic acetylcholine receptor in native and reconstituted membranes

Detailed knowledge of the membrane framework surrounding the nicotinic acetylcholine receptor (AChR) is key to an understanding of its structure, dynamics, and function. Recent theoretical models discuss the structural relationship between the AChR and the lipid bilayer. Independent experimental data on the composition, metabolism, and dynamics of the AChR lipid environment are analyzed in the first part of the review. The composition of the lipids in which the transmembrane AChR chains are inserted bears considerable resemblance among species, perhaps providing this evolutionarily conserved protein with an adequate milieu for its optimal functioning. The effects of lipids on the latter are discussed in the second part of the review. The third part focuses on the information gained on the dynamics of AChR and lipids in the membrane, a section that also covers the physical properties and interactions between the protein, its immediate annulus, and the bulk lipid bilayer.

Phospholipase A2 hydrolysis of membrane phospholipids causes structural alteration of the nicotinic acetylcholine receptor

Thermal perturbation techniques have been used to probe structural alteration of the nicotinic acetylcholine receptor as a function of perturbations of its native membrane environment. Differential scanning calorimetry and a technique involving heat inactivation of the alpha-bungarotoxin-binding sites on the receptor protein reveal that there is a profound destabilization of the acetylcholine receptor structure when receptor-containing membranes are exposed to phospholipase A2. The characteristic calorimetric transition assigned to irreversible denaturation of the receptor protein and the heat inactivation profile of alpha-bungarotoxin-binding sites are shifted to lower temperatures by approx. 7 and 5 C degrees, respectively, upon exposure to phospholipase A2 at a phospholipase/neurotoxin binding site molar ratio of about 1:100. The effects of phospholipase A2 on receptor structure can be (i) reversed by using bovine serum albumin as a scavenger of phospholipase hydrolysis products of membrane phospholipids, and (ii) stimulated by incorporation into the membranes of free, polyunsaturated fatty acids. In particular, linolenic acid (18:3(n-3] causes detectable destabilization of the alpha-bungarotoxin binding sites on the receptor at free fatty acid/receptor molar ratios as low as 10:1. Furthermore, alteration of receptor structure by added phospholipase occurs very rapidly, which is consistent with the observation of rapid in situ phospholipase A2 hydrolysis of membrane phospholipids, particularly highly unsaturated phosphatidylethanolamine and phosphatidylserine. Based on previously published data on the inhibition of acetylcholine receptor cation-gating activity caused by the presence of either phospholipase A2 or free fatty acids (Andreasen T.J. and McNamee M.G. (1980) Biochemistry 19, 4719), we interpret our data as indicative of a correlation between structural and functional alterations of the membrane-bound acetylcholine receptor induced by phospholipase A2 hydrolysis products.

Association of daunomycin to membrane domains studied by fluorescence resonance energy transfer

1,6-Diphenyl-1,3,5-hexatriene and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene are fluorophores used to explore different hydrophobic domains of membrane bilayers (Andrich, M.P. and Vanderkooi, J.M. (1976) Biochemistry 15, 1257-1265; Prendergast, F.G., Haugland, R.P. and Callahan, P.J. (1981) Biochemistry 20, 7333-7338). Fluorescence resonance energy transfer between these fluorophores, acting as energy donors, and the anthracycline, daunomycin, as the acceptor, was used to analyze the interaction of the drug with natural membranes, and its relative location within the membrane bilayer. The transfer process was demonstrated by: (1) emission fluorescence of the acceptor when the samples were excited at the excitation maximum of the donor (360 nm); and (2) progressive quenching of the energy donor (at 428 nm) when in the presence of increasing acceptor concentration. Also, the disruption of the energy transfer by solubilization of the membrane with Triton X-100 evidences a role for the membrane in providing the appropriate site(s) for energy transfer to occur. At moderately low daunomycin/membrane lipid ratios, the different efficiencies of resonance energy transfer between the two donors and daunomycin predicts a preferential, but not exclusive, location of the drug at membrane 'surface' domains, i.e., those regions of the bilayer explored by the 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene probe. In support of this observation, a large fraction (approx. 75%) of membrane-associated daunomycin was rapidly sequestered away from the membrane upon addition of excess DNA, which forms high-affinity complexes with daunomycin (Chaires, J.B., Dattagupta, n. and Crothers, D.M. (1982) Biochemistry 21, 3927-3932), thus acting as a drug 'sink'. Also, a large fraction of drug was accessible to fluorescence quenching by iodide, a collisional water-soluble quencher. On the other hand, a smaller population of the membrane-associated daunomycin was characterized by slow sequestering by the added DNA and inaccessibility to quenching by iodide. We conclude that the daunomycin, which is only slowly sequestered, is located deep within the hydrophobic domains of the bilayer, likely to be those probed by 1,6-diphenyl-1,3,5-hexatriene.

Composition of lipids in elasmobranch electric organ and acetylcholine receptor membranes

The composition of phospholipids from electric organ and from membranes enriched in acetylcholine receptors (AChRs) is analyzed in three elasmobranch fish (Torpedo marmorata, Torpedo californica, and Discopyge tschudii). Irrespective of their purity, AChR-containing membranes are similar to electric organ in lipid and fatty acid composition. The following characteristics are common to the three species: (a) Choline, ethanolamine, and serine glycerophospholipids account for 80-90% of the phospholipids. (b) Their major fatty acid constituents are monoenes, saturates, and long-chain (n-3) polyenes (especially docosahexaenoate). (c) A large proportion of the ethanolamine glycerophospholipids (30-50%) is made up by plasmenylethanolamine, which contains fewer polyenes than phosphatidylethanolamine per mole of lipid. (d) Polyphosphoinositides represent 20-30% of the inositides of electric organ. (e) Phosphatidylinositol and phosphatidate have large proportions of 20- and 22-carbon polyenes. (f) Diphosphatidylglycerol and triacylglycerols are rich in oleate but also contain long-chain polyenes. (g) Sphingomyelin has monoenes and saturates ranging from 14 to 26 carbons. Species-related variations are observed (a) in the ratios between some phospholipid classes and subclasses and (b) in the relative abundance of the major polyunsaturated acyl chains of phospholipids. Despite these differences, the average unsaturation and length of fatty acids in major phospholipid classes are similar for the three species.

Equilibrium and dynamic structure of large, unilamellar, unsaturated acyl chain phosphatidylcholine vesicles. Higher order analysis of 1,6-diphenyl-1,3,5-hexatriene and 1-[4-(trimethylammonio)phenyl]- 6-phenyl-1,3,5-hexatriene anisotropy decay

Equilibrium and dynamic structural properties of minimally to highly unsaturated acyl chain, large, unilamellar phosphatidylcholine (PC) vesicles have been characterized by the dynamic fluorescence properties of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH). Fluorescence lifetimes and equilibrium and dynamic rotational properties of these probes were analyzed by limited-frequency phase-modulation fluorometry in egg PC, palmitoyloleoyl-PC (POPC), dioleoyl-PC (DOPC), palmitoylarachidonoyl-PC (PAPC), and palmitoyldocosahexaenoyl-PC (P-22:6-PC) vesicles over a temperature range from 5 to 37 degrees C. DPH equilibrium orientational distributions were derived according to a model permitting bimodal orientational distributions in which the parallel probability maximum was aligned parallel to the bilayer normal and the orthogonal probability maximum was oriented parallel to the plane of the bilayer. TMA-DPH orientational distributions were derived according to the same model except that all probability was constrained to the parallel orientation. TMA-DPH fluorescence lifetimes were much more sensitive than those of DPH to variations in acyl chain composition and temperature although the same qualitative behavior was generally observed with both probes. Greater acyl chain unsaturation and higher sample temperatures each gave rise to shorter lifetimes consistent with increased water penetrability into the bilayers. Equilibrium order of the hydrocarbon core (as probed by DPH) and of the interfacial and head group regions of the bilayers (as probed by TMA-DPH) was reduced by increasing levels of unsaturation and by higher sample temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)

Extraction of peripheral proteins is accompanied by selective depletion of certain glycerophospholipid classes and changes in the phosphorylation pattern of acetylcholine-receptor-rich-membrane proteins

The widely used alkaline treatment of acetylcholine-receptor (AChR)-rich membranes from Torpedo marmorata (electric fish) and Discopyge tschudii (a marine ray) results not only in the extraction of non-receptor peripheral proteins but also in that of glycerophospholipids (approximately 13%). Minor acidic phospholipids, notably phosphatidic acid and polyphosphoinositides, are particularly enriched in the NaOH extracts. When electrocytes or receptor-rich membranes are incubated with [32P]Pi or [gamma-32P]ATP, polyphosphoinositides accumulate most of the label (approximately 45% in D. tschudii; 96% in T. marmorata) and exhibit the highest specific radioactivity. Furthermore, more than 50% of these phosphorylated lipids are extracted by NaOH together with the peripheral membrane proteins. NaOH treatment also results in modification of the phosphorylation pattern of AChR membrane proteins. Phosphorylation decreases in the Mr-43,000 group of peripheral proteins and in the gamma-subunit of the receptor. The results indicate that polyphosphoinositides constitute a metabolically very active lipid pool in the postsynaptic membrane, and that a substantial proportion of these phospholipids are preferentially released from the membrane together with other acidic phospholipids upon peripheral-protein extraction. The conclusion is drawn that membranes submitted to the above treatments can no longer be considered equivalent to native ones in terms of their phospholipid composition and phosphorylation characteristics.

(1-Pyrene)sulfonyl azide: a fluorescent probe for measuring the transmembrane topology of acetylcholine receptor subunits

(1-Pyrene)sulfonyl azide (PySA), a fluorescent, lipophilic photolabel, was used as a probe for the transmembrane topology of the acetylcholine receptor (AchR) subunits. Photolabeling of native, alkaline-extracted, and reconstituted AchR membrane preparations resulted in the labeling of all the AchR subunits. However the reconstituted AchR membrane preparations incorporated twice as much PySA into each mole of the AchR complex. Photolabeling of all subunits of the AchR does not appear to alter the agonist concentration response of AchR-mediated cation translocation.

Rearrangement of aminophospholipids in bilayers from sheep platelet plasma membranes and platelet liposomes by increasing their cholesterol levels

Phospholipid orientation in platelet plasma membranes and other blood cells, such as erythrocytes, appears to be rather similar. The negatively charged phospholipids are almost exclusively located on the inner leaflet of the bilayer. No phosphatidylserine is present on the outer membrane bilayer. The results of the present study, using a specific reagent for amino groups, trinitrobenzenesulfanilic acid, showed that in sheep platelet plasma membranes enriched with free exogenous cholesterol, an alteration in the aminophospholipid topology occurs, with a portion of phosphatidylserine moving from the inner to the outer side. A progressive appearance of aminophospholipids in the outer membrane bilayer was also observed in artificial vesicles prepared with total lipids from sheep platelets supplemented with increased amounts of free cholesterol.

Lipid alterations in liver and kidney induced by normobaric hyperoxia: correlations with changes in microsomal membrane fluidity

The effects of normobaric hyperoxia on both microsomal membrane fluidity and mechanism of phospholipid synthesis in rabbit liver and kidney have been studied. Hyperoxia induces in both organs an impairment of de novo synthesis of glycerolipids which could be due to an inactivation of acyltransferase activities involved in the initial formation of phosphatidic acid. The ability to replace phospholipid fatty acids by reacylation mechanism decreases slightly in the hyperoxic kidney, while it does not change in the hyperoxic liver. Concerning the effect of high arterial pO2 on microsomal membrane fluidity, the hyperoxic liver shows a more fluid environment within the membrane core of microsomes; however, no difference was shown in that of microsomal membrane core of hyperoxic kidney. An insight into the lipid composition of microsomes indicates that liver microsomal membranes have lower cholesterol content and higher unsaturation degree of phospholipid fatty acids, whereas hyperoxic kidney microsomes become more saturated and did not show any difference in their cholesterol content. In both hyperoxic liver and kidney microsomes, phospholipid content decreases in agreement with the depression of phosphatidic acid biosynthesis. These results are discussed in relation to the values of microsomal membrane microviscosity obtained.