Correlation of phospholipid structure with functional effects on the nicotinic acetylcholine receptor. A modulatory role for phosphatidic acid

Preferential electrostatic interactions of phosphatidic acid with arginines

Phosphatidic acid (PA) is an anionic lipid that preferentially interacts with proteins in a diverse set of cellular processes such as transport, apoptosis, and neurotransmission. One such interaction is that of the PA lipids with the proteins of voltage-sensitive ion channels. In comparison to several other similarly charged anionic lipids, PA lipids exhibit much stronger interactions. Intrigued and motivated by this finding, we sought out to gain deeper understanding into the electrostatic interactions of anionic lipids with charged proteins. Using the voltage sensor domain (VSD) of the KvAP channel as a model system, we performed long-timescale atomistic simulations to analyze the interactions of POPA, POPG, and POPI lipids with arginines (ARGs). Our simulations reveal two mechanisms. First, POPA is able to interact not only with surface ARGs but is able to snorkel and interact with a buried arginine. POPG and POPI lipids on the other hand show weak interactions even with both the surface and buried ARGs. Second, deprotonated POPA with -2 charge is able to break the salt-bridge connection between VSD protein segments and establish its own electrostatic bond with the ARG. Based on these findings, we propose a headgroup size hypothesis for preferential solvation of proteins by charged lipids. These findings may be valuable in understanding how PA lipids could be modulating kinematics of transmembrane proteins in cellular membranes.

Modulation of a rapid neurotransmitter receptor-ion channel by membrane lipids

Membrane lipids modulate the proteins embedded in the bilayer matrix by two non-exclusive mechanisms: direct or indirect. The latter comprise those effects mediated by the physicochemical state of the membrane bilayer, whereas direct modulation entails the more specific regulatory effects transduced via recognition sites on the target membrane protein. The nicotinic acetylcholine receptor (nAChR), the paradigm member of the pentameric ligand-gated ion channel (pLGIC) superfamily of rapid neurotransmitter receptors, is modulated by both mechanisms. Reciprocally, the nAChR protein exerts influence on its surrounding interstitial lipids. Folding, conformational equilibria, ligand binding, ion permeation, topography, and diffusion of the nAChR are modulated by membrane lipids. The knowledge gained from biophysical studies of this prototypic membrane protein can be applied to other neurotransmitter receptors and most other integral membrane proteins.

Fluorescence Studies of Nicotinic Acetylcholine Receptor and Its Associated Lipid Milieu: The Influence of Erwin London's Methodological Approaches

Erwin London dedicated considerable effort to understanding lipid interactions with membrane-resident proteins and how these interactions shaped the formation and maintenance of lipid phases and domains. In this endeavor, he developed ad hoc techniques that greatly contributed to advancements in the field. We have employed and/or modified/extended some of his methodological approaches and applied them to investigate lipid interaction with the nicotinic acetylcholine receptor (nAChR) protein, the paradigm member of the superfamily of rapid pentameric ligand-gated ion channels (pLGIC). Our experimental systems ranged from purified receptor protein reconstituted into synthetic lipid membranes having known effects on receptor function, to cellular systems subjected to modification of their lipid content, e.g., varying cholesterol levels. We have often employed fluorescence techniques, including fluorescence quenching of diphenylhexatriene (DPH) extrinsic fluorescence and of nAChR intrinsic fluorescence by nitroxide spin-labeled phospholipids, DPH anisotropy, excimer formation of pyrene-phosphatidylcholine, and Förster resonance energy transfer (FRET) from the protein moiety to the extrinsic probes Laurdan, DPH, or pyrene-phospholipid to characterize various biophysical properties of lipid-receptor interactions. Some of these strategies are revisited in this review. Special attention is devoted to the anionic phospholipid phosphatidic acid (PA), which stabilizes the functional resting form of the nAChR. The receptor protein was shown to organize its PA-containing immediate microenvironment into microdomains with high lateral packing density and rigidity. PA and cholesterol appear to compete for the same binding sites on the nAChR protein.

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.

Sequential purification and characterization of Torpedo californica nAChR-DC supplemented with CHS for high-resolution crystallization studies

Over the past 10 years we have been developing a multi-attribute analytical platform that allows for the preparation of milligram amounts of functional, high-pure, and stable Torpedo (muscle-type) nAChR detergent complexes for crystallization purpose. In the present work, we have been able to significantly improve and optimize the purity and yield of nicotinic acetylcholine receptors in detergent complexes (nAChR-DC) without compromising stability and functionality. We implemented new methods in the process, such as analysis and rapid production of samples for future crystallization preparations. Native nAChR was extracted from the electric organ of Torpedo californica using the lipid-like detergent LysoFos Choline 16 (LFC-16), followed by three consecutive steps of chromatography purification. We evaluated the effect of cholesteryl hemisuccinate (CHS) supplementation during the affinity purification steps of nAChR-LFC-16 in terms of receptor secondary structure, stability and functionality. CHS produced significant changes in the degree of β-secondary structure, these changes compromise the diffusion of the nAChR-LFC-16 in lipid cubic phase. The behavior was reversed by Methyl-β-Cyclodextrin treatment. Also, CHS decreased acetylcholine evoked currents of Xenopus leavis oocyte injected with nAChR-LFC-16 in a concentration-dependent manner. Methyl-β-Cyclodextrin treatment do not reverse functionality, however column delipidation produced a functional protein similar to nAChR-LFC-16 without CHS treatment.

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.

Phylogenetic conservation of protein-lipid motifs in pentameric ligand-gated ion channels

Using the crosstalk between the nicotinic acetylcholine receptor (nAChR) and its lipid microenvironment as a paradigm, this short overview analyzes the occurrence of structural motifs which appear not only to be conserved within the nAChR family and contemporary eukaryotic members of the pentameric ligand-gated ion channel (pLGIC) superfamily, but also extend to prokaryotic homologues found in bacteria. The evolutionarily conserved design is manifested in: 1) the concentric three-ring architecture of the transmembrane region, 2) the occurrence in this region of distinct lipid consensus motifs in prokaryotic and eukaryotic pLGIC and 3) the key participation of the outer TM4 ring in conveying the influence of the lipid membrane environment to the middle TM1-TM3 ring and this, in turn, to the inner TM2 channel-lining ring, which determines the ion selectivity of the channel. The preservation of these constant structural-functional features throughout such a long phylogenetic span likely points to the successful gain-of-function conferred by their early acquisition. This article is part of a Special Issue entitled: Lipid-protein interactions.

pH-dependent solution structure and activity of a reduced form of the host-defense peptide myticin C (Myt C) from the mussel Mytilus galloprovincialis

Myticin C (Myt C) is a highly variable host-defense peptide (HDP) associated to the immune response in the mediterranean mussel (Mytilus galloprovincialis), which has shown to be active across species due to its strong antiviral activity against a fish rhabdovirus found in fish cells overexpressing this HDP. However, the potential antimicrobial properties of any synthetic analogue of Myt C has not yet been analysed. Thus, in this work we have synthesised the sequence of the mature peptide of Myt C variant c and analysed the structure activity relationships of its reduced (non-oxidized) form (red-MytCc). In contrast to results previously reported for oxidized isoforms of mussel myticins, red-MytCc was not active against bacteria at physiological pH and showed a moderate antiviral activity against the viral haemorrhagic septicaemia (VHS) rhabdovirus. However, its chemotactic properties remained active. Structure/function studies in neutral and acid environments by means of infrared spectroscopy indicated that the structure of red-MytCc is pH dependent, with acid media increasing its alpha-helical content. Furthermore, red-MytCc was able to efficiently aggregate artificial phospholipid membranes at low pH, as well as to inhibit the Escherichia coli growth, suggesting that this activity is attributable to its more structured form in an acidic environment. All together, these results highlight the dynamic and environmentally sensitive behavior of red-Myt C in solution, and provide important insights into Myt C structure/activity relationships and the requirements to exert its antimicrobial/immunomodulatory activities. On the other hand, the pH-dependent direct antimicrobial activity of Myt C suggests that this HDP may be a suitable template for the development of antimicrobial agents that would function selectively in specific pH environments, which are sorely needed in this "antibiotic-resistance era".

Lipids in autophagy: constituents, signaling molecules and cargo with relevance to disease

The balance between protein and lipid biosynthesis and their eventual degradation is a critical component of cellular health. Autophagy, the catabolic process by which cytoplasmic material becomes degraded in lysosomes, can be induced by various physiological stimuli to maintain cellular homeostasis. Autophagy was for a long time considered a non-selective bulk process, but recent data have shown that unwanted components such as aberrant protein aggregates, dysfunctional organelles and invading pathogens can be selectively eliminated by autophagy. Recently, also intracellular lipid droplets were described as specific autophagic cargo, indicating that autophagy plays a role in lipid metabolism and storage (Singh et al., 2009 [1]). Moreover, over the past several years, it has become increasingly evident that lipids and lipid-modifying enzymes play important roles in the autophagy process itself, both at the level of regulation of autophagy and as membrane constituents required for formation of autophagic vesicles. In this review, we will discuss the interplay between lipids and autophagy, as well as the role of lipid-binding proteins in autophagy. We also comment on the possible implications of this mutual interaction in the context of disease. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

Role of phosphatidic acid in plant galactolipid synthesis

Phosphatidic acid (PA) is a precursor metabolite for phosphoglycerolipids and also for galactoglycerolipids, which are essential lipids for formation of plant membranes. PA has in addition a main regulatory role in a number of developmental processes notably in the response of the plant to environmental stresses. We review here the different pools of PA dispatched at different locations in the plant cell and how these pools are modified in different growth conditions, particularly during plastid membrane biogenesis and when the plant is exposed to phosphate deprivation. We analyze how these modifications can affect galactolipid synthesis by tuning the activity of MGD1 enzyme allowing a coupling of phospho- and galactolipid metabolisms. Some mechanisms are considered to explain how physicochemical properties of PA allow this lipid to act as a central internal sensor in plant physiology.

Lipid signaling and homeostasis: PA- is better than PA-H, but what about those PIPs?

Although cellular membranes are composed of hundreds of distinct lipid species, the lipid composition is maintained within a narrow range. The regulatory circuit responsible for this homeostasis in yeast depends on a membrane-bound transcriptional repressor that translocates to the nucleus in response to the abundance of its lipid ligand on the membrane. Feedback control in this system is provided because the lipid ligand is also an end product of the activity of the transcription factor. This basic design is also evident in higher eukaryotes such as Drosophila and mammals, but with important differences in the lipid being sensed, the composition of the sensors, and the fine-tuning of the response. New work indicates that regulation of intracellular pH levels in yeast by glucose availability may fine-tune the binding of the repressor to its lipid ligand, providing a mechanism that connects phospholipid metabolism to nutrient sensing. The importance of pH effects in this pathway raises the possibility that additional lipid-signaling pathways may be regulated by the protonation state of the lipid or its effector.

Phosphatidic acid is a pH biosensor that links membrane biogenesis to metabolism

Recognition of lipids by proteins is important for their targeting and activation in many signaling pathways, but the mechanisms that regulate such interactions are largely unknown. Here, we found that binding of proteins to the ubiquitous signaling lipid phosphatidic acid (PA) depended on intracellular pH and the protonation state of its phosphate headgroup. In yeast, a rapid decrease in intracellular pH in response to glucose starvation regulated binding of PA to a transcription factor, Opi1, that coordinately repressed phospholipid metabolic genes. This enabled coupling of membrane biogenesis to nutrient availability.

Cations mediate interactions between the nicotinic acetylcholine receptor and anionic lipids

Interactions between the nicotinic acetylcholine receptor (nAChR) and phosphatidic acid (PA) are bidirectional in that membranes containing PA are effective at stabilizing an agonist-responsive nAChR, whereas incorporation of the nAChR into the same membranes leads to a substantial increase in lipid lateral packing density. A previous study suggested that the ability of PA to adopt a dianionic ionization state is key. We monitored the ionization state of PA in both reconstituted and protein-free membranes. In model membranes composed of PA and 3:2 (mol/mol) phosphatidylcholine (PC)/PA, the monoanionic-to-dianionic transition of PA was detected with a pKa of 8.7 and 6.5, respectively. In the reconstituted 3:2 PC/PA membranes, however, PA was stabilized in a monoanionic state at pH values up to 10. Although dianionic PA does not play a role in nAChR function, we found that both the stabilization of monoanionic PA and the concentration of other cations at the bilayer surface can account for changes in bilayer physical properties that are observed upon incorporation of the nAChR into 3:2 PC/PA membranes. A nAChR-induced concentration of cations at the bilayer surface likely mediates interactions between the nAChR and the anionic lipids in its membrane environment.

Phosphatidyl alcohols: effect of head group size on domain forming properties and interactions with sterols

In this study, we have examined the membrane properties and sterol interactions of phosphatidyl alcohols varying in the size of the alcohol head group coupled to the sn-3-linked phosphate. Phosphatidyl alcohols of interest were dipalmitoyl derivatives with methanol (DPPMe), ethanol (DPPEt), propanol (DPPPr), or butanol (DPPBu) head groups. The Phosphatidyl alcohols are biologically relevant, because they can be formed in membranes by the phospholipase D reaction in the presence of alcohol. The melting behavior of pure phosphatidyl alcohols and mixtures with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or cholesterol was assessed using high sensitivity differential scanning calorimetry (DSC). DPPMe had the highest melting temperature ( approximately 49 degrees C), whereas the other phosphatidyl alcohols had similar melting temperatures as DPPC ( approximately 40-41 degrees C). All phosphatidyl alcohols, except DPPMe, also showed good miscibility with DPPC. The effects of cholesterol on the melting behavior and membrane order in multilamellar bilayer vesicles were assessed using steady-state anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) and DSC. The ordering effect of cholesterol in the fluid phase was lower for all phosphatidyl alcohols as compared to DPPC and decreased with increasing head group size. The formation of ordered domains containing the phosphatidyl alcohols in complex bilayer membranes was determined using fluorescence quenching of DPH or the sterol analogue cholesta-5,7,(11)-trien-3-beta-ol (CTL). The phosphatidyl alcohols did not appear to form sterol-enriched ordered domains, whereas DPPMe, DPPEt appeared to form ordered domains in the temperature window examined (10-50 degrees C). The partitioning of CTL into bilayer membranes containing phosphatidyl alcohols was to a small extent increased for DPPMe and DPPEt, but in general, sterol interactions were weak or unfavorable for the phosphatidyl alcohols. Our results show that the biophysical and sterol interacting properties of phosphatidyl alcohols, having identical acyl chain structures, are markedly dependent on the size of the head group.

Anionic lipids allosterically modulate multiple nicotinic acetylcholine receptor conformational equilibria

Anionic lipids influence the ability of the nicotinic acetylcholine receptor to gate open in response to neurotransmitter binding, but the underlying mechanisms are poorly understood. We show here that anionic lipids with relatively small headgroups, and thus the greatest ability to influence lipid packing/bilayer physical properties, are the most effective at stabilizing an agonist-activatable receptor. The differing abilities of anionic lipids to stabilize an activatable receptor stem from differing abilities to preferentially favor resting over both uncoupled and desensitized conformations. Anionic lipids thus modulate multiple acetylcholine receptor conformational equilibria. Our data suggest that both lipids and membrane physical properties act as classic allosteric modulators influencing function by interacting with and thus preferentially stabilizing different native acetylcholine receptor conformational states.

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.

Nicotinic acetylcholine receptor induces lateral segregation of phosphatidic acid and phosphatidylcholine in reconstituted membranes

Purified nicotinic acetylcholine receptor (AChR) protein was reconstituted into synthetic lipid membranes having known effects on receptor function in the presence and absence of cholesterol (Chol). The phase behavior of a lipid system (DPPC/DOPC) possessing a known lipid phase profile and favoring nonfunctional, desensitized AChR was compared with that of a lipid system (POPA/POPC) containing the anionic phospholipid phosphatidic acid (PA), which stabilizes the functional resting form of the AChR. Fluorescence quenching of diphenylhexatriene (DPH) extrinsic fluorescence and AChR intrinsic fluorescence by a nitroxide spin-labeled phospholipid showed that the AChR diminishes the degree of DPH quenching and promotes DPPC lateral segregation into an ordered lipid domain, an effect that was potentiated by Chol. Fluorescence anisotropy of the probe DPH increased in the presence of AChR or Chol and also made apparent shifts to higher values in the transition temperature of the lipid system in the presence of Chol and/or AChR. The values were highest when both Chol and AChR were present, further reinforcing the view that their effect on lipid segregation is additive. These results can be accounted for by the increase in the size of quencher-free, ordered lipid domains induced by AChR and/or Chol. Pyrene phosphatidylcholine (PyPC) excimer (E) formation was strongly reduced owing to the restricted diffusion of the probe induced by the AChR protein. The analysis of Forster energy transfer (FRET) from the protein to DPH further indicates that AChR partitions preferentially into these ordered lipid microdomains, enriched in saturated lipid (DPPC or POPA), which segregate from liquid phase-enriched DOPC or POPC domains. Taken together, the results suggest that the AChR organizes its immediate microenvironment in the form of microdomains with higher lateral packing density and rigidity. The relative size of such microdomains depends not only on the phospholipid polar headgroup and fatty acyl chain saturation but also on AChR protein-lipid interactions. Additional evidence suggests a possible competition between Chol and POPA for the same binding sites on the AChR protein.

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.

Segregation of phosphatidic acid-rich domains in reconstituted acetylcholine receptor membranes

Purified Acetylcholine Receptor (AcChR) from Torpedo has been reconstituted at low (approximately 1:3500) and high (approximately 1:560) protein to phospholipid molar ratios into vesicles containing egg phosphatidylcholine, cholesterol, and different dimyristoyl phospholipids (dimyristoyl phosphatidylcholine, phosphatidylserine, phosphatidylglycerol and phosphatidic acid) as probes to explore the effects of the protein on phospholipid organization by differential scanning calorimetry, infrared, and fluorescence spectroscopy. All the experimental results indicate that the presence of the AcChR protein, even at the lower protein to phospholipid molar ratio, directs lateral phase separation of the monoanionic phosphoryl form of the phosphatidic acid probe, causing the formation of specific phosphatidic acid-rich lipid domains that become segregated from the bulk lipids and whose extent (phosphatidic acid sequestered into the domain, out of the total population in the vesicle) is protein-dependent. Furthermore, fluorescence energy transfer using the protein tryptophan residues as energy donors and the fluorescence probes trans-parinaric acid or diphenylhexatriene as acceptors, establishes that the AcChR is included in the domain. Other dimyristoyl phospholipid probes (phosphatidylcholine, phosphatidylserine, phosphatidylglycerol) under identical conditions could not mimic the protein-induced domain formation observed with the phosphatidic acid probe and result in ideal mixing of all lipid components in the reconstituted vesicles. Likewise, in the absence of protein, all the phospholipid probes, including phosphatidic acid, exhibit ideal mixing behavior. Since phosphatidic acid and cholesterol have been implicated in functional modulation of the reconstituted AcChR, it is suggested that such a specific modulatory role could be mediated by domain segregation of the relevant lipid classes.

Unique effects of different fatty acid species on the physical properties of the torpedo acetylcholine receptor membrane

To study the effects produced by free fatty acids (FFA) on the biophysical properties of Torpedo marmorata nicotinic acetylcholine receptor-rich native membranes and to investigate the topology of their binding site(s), fluorescence measurements were carried out using the fluorescent probe Laurdan (6-dodecanoyl-2-(dimethylamino) naphthalene) and ADIFAB, an Acrylodan-derivatized intestinal fatty acid-binding protein. The generalized polarization (GP) of the former probe was used to learn about the physical state of the membrane upon FFA binding. Saturated FFA induced a slight increase in GP, whereas cis-unsaturated fatty acids decreased GP. Double bond isomerism could also be distinguished; oleic acid (18:1cis) induced a net disordering effect, whereas elaidic acid (18:1trans) produced no changes in GP. The changes in the efficiency of the Förster energy transfer from the protein to Laurdan brought about by addition of FFA, together with the distances involved in this process, indicate that all FFA studied share a common site at the lipid-protein interface. However, despite being located at the same site, each class of FFA differs in its effect on the physical properties of the membrane. These data lead us to suggest that it is the direct action of FFA at the lipid-protein interface, displacing essential lipids from their sites rather than changes in bulk properties such as membrane fluidity that accounts for the effect of FFA on the acetylcholine receptor membrane.

Lipid-protein interactions at the nicotinic acetylcholine receptor. A functional coupling between nicotinic receptors and phosphatidic acid-containing lipid bilayers

The structural and functional properties of reconstituted nicotinic acetylcholine receptor membranes composed of phosphatidyl choline either with or without cholesterol and/or phosphatidic acid have been examined to test the hypothesis that receptor conformational equilibria are modulated by the physical properties of the surrounding lipid environment. Spectroscopic and chemical labeling data indicate that the receptor in phosphatidylcholine alone is stabilized in a desensitized-like state, whereas the presence of either cholesterol or phosphatidic acid favors a resting-like conformation. Membranes that effectively stabilize a resting-like state exhibit a relatively large proportion of non-hydrogen-bonded lipid ester carbonyls, suggesting a relatively tight packing of the lipid head groups and thus a well ordered membrane. Functional reconstituted membranes also exhibit gel-to-liquid crystal phase transition temperatures that are higher than those of nonfunctional reconstituted membranes composed of phosphatidylcholine alone. Significantly, incorporation of the receptor into phosphatidic acid-containing membranes leads to a dramatic increase in both the lateral packing densities and the gel-to-liquid crystal phase transition temperatures of the reconstituted lipid bilayers. These results suggest a functional link between the nicotinic acetylcholine receptor and the physical properties of phosphatidic acid-containing membranes that could underlie the mechanism by which this lipid preferentially enhances receptor function.

Probing the effects of membrane cholesterol in the Torpedo californica acetylcholine receptor and the novel lipid-exposed mutation alpha C418W in Xenopus oocytes

The effects of cholesterol on the ion-channel function of the Torpedo acetylcholine receptor (nAChR) and the novel lipid-exposed gain in function alpha C418W mutation have been investigated in Xenopus laevis oocytes. We found conditions to increase the cholesterol/phospholipid (C/P) molar ratio on the plasma membrane of Xenopus oocytes from 0.5 to 0.87, without significant physical damage or change in morphology to the oocytes. In addition, we developed conditions to deplete endogenous cholesterol from oocytes using a methyl-beta-cyclodextrin incubation procedure without causing membrane instability of the cells. Methyl-beta-cyclodextrin was also used to examine the reversibility of the inhibitory effect of cholesterol on AChR function. Depletion of 43% of endogenous cholesterol from oocytes (C/P = 0.3) did not show any significant change in macroscopic response of the wild type, whereas in the alpha C418W mutant the same cholesterol depletion caused a dramatic gain-in-function response of this lipid-exposed mutation in addition to the increased response caused by the mutation itself. Increasing the C/P ratio to 0.87 caused an inhibition of the macroscopic response of the Torpedo wild type of about 52%, whereas the alpha C418W mutation showed an 81% inhibition compared with the responses of control oocytes. The wild type receptor did not recover from this inhibition when the excess cholesterol was depleted to near normal C/P ratios; however, the alpha C418W mutant displayed 63% of the original current, which indicates that the inhibition of this lipid-exposed mutant was significantly reversed. The ability of the alpha C418W mutation to recover from the inhibition caused by cholesterol enrichment suggests that the interaction of cholesterol with this lipid-exposed mutation is significantly different from that of the wild type. The present data demonstrate that a single lipid-exposed position in the AChR could alter the modulatory effect of cholesterol on AChR function.

Effect of membrane lipid composition on the conformational equilibria of the nicotinic acetylcholine receptor

The effects of cholesterol (Chol) and an anionic lipid, dioleoylphosphatidic acid (DOPA) on the conformational equilibria of the nicotinic acetylcholine receptor (nAChR) have been investigated using Fourier transform infrared difference spectroscopy. The difference between spectra recorded in the presence and absence of agonist from the nAChR reconstituted into 3:1:1 egg phosphatidylcholine (EPC)/DOPA/Chol membranes exhibits positive and negative bands that serve as markers of the structural changes associated with the resting to desensitized conformational change. These markers are absent in similar difference spectra recorded from the nAChR reconstituted into EPC membranes lacking both Chol and DOPA, indicating that the nAChR cannot undergo conformational change in response to agonist binding. When low levels of either Chol or DOPA up to 25 mol % of the total lipid are included in the EPC membranes, the markers suggest the predominant stabilization of a conformation that is a structural intermediate between the resting and desensitized states. At higher levels of either Chol or DOPA, the nAChR is stabilized in a conformation that is capable of undergoing agonist-induced desensitization, although DOPA appears to be required for the nAChR to adopt a conformation fully equivalent to that found in native and 3:1:1 EPC/DOPA/Chol membranes. The ability of these two structurally diverse lipids, as well as others (Ryan, S. E., Demers, C. N., Chew, J. P., Baenziger, J. E. (1996) J. Biol. Chem. 271, 24590-24597), to modulate the functional state of the nAChR suggests that lipids act on the nAChR via an indirect effect on some physical property of the lipid bilayer. The data also suggest that anionic lipids are essential to stabilize a fully functional nAChR. We propose that membrane fluidity modulates the relative populations of nAChRs in the resting and desensitized states but that subtle structural changes in the presence of anionic lipids are essential for full activity.

Internal dynamics of the nicotinic acetylcholine receptor in reconstituted membranes

The structure and 1H/2H exchange kinetics of affinity-purified nAChR reconstituted into egg phosphatidylcholine membranes with increasing levels of either dioleoylphosphatidic acid (DOPA) or cholesterol (Chol) have been examined using infrared spectroscopy. All spectra of the reconstituted nAChR membranes recorded after 72 h in 2H2O exhibit comparable amide I band shapes, suggesting a similar secondary structure for the nAChR in each lipid environment. Increasing levels of either DOPA or Chol, however, lead to an increasing intensity of the amide II band, indicating a decreasing proportion of nAChR peptide hydrogens that have exchanged for deuterium. Spectra recorded as a function of time after exposure of the nAChR to 2H2O show that the presence of either lipid slows down the 1H/2H exchange of those peptide hydrogens that normally exchange on the minutes to hours time scale. The slowing of peptide 1H/2H exchange correlates with both an increasing ability of the nAChR to undergo agonist-induced conformational change [Baenziger, J. E., Morris, M.-L., Darsaut, T. E., and Ryan, S. E. (1999) in preparation] and possibly a decreasing membrane fluidity. Our data suggest that lipid composition dependent changes in nAChR peptide 1H/2H exchange kinetics reflect altered internal dynamics of the nAChR. Lipids may influence protein function by changing the internal dynamics of integral membrane proteins.

Lysophosphatidylcholine modulates catalytically important motions of the Ca-ATPase phosphorylation domain

Catalytically important motions of the Ca-ATPase, modulated by the physical properties of surrounding membrane phospholipids, have been suggested to be rate-limiting under physiological conditions. To identify the nature of the structural coupling between the Ca-ATPase and membrane phospholipids, we have investigated the functional and structural effects resulting from the incorporation of the lysophospholipid 1-myristoyl-2-hydroxy-sn-glycerol-3-phosphocholine (LPC) into native sarcoplasmic reticulum (SR) membranes. Nonsolubilizing concentrations of LPC abolish changes in fluorescence signals associated with either intrinsic or extrinsic chromophores that monitor normal conformational transitions accompanying calcium activation of the Ca-ATPase. There are corresponding decreases in the rates of calcium transport coupled to ATP hydrolysis, suggesting that LPC may increase conformational barriers associated with catalytic function. Fluorescence anisotropy measurements of the lipid analogue 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) partitioned into SR membranes indicate that LPC does not significantly modify lipid acyl chain rotational dynamics, suggesting differences in headgroup conformation between LPC and diacylglycerol phosphatidylcholines. Complementary measurements using phosphorescence anisotropy of erythrosin isothiocyanate at Lys464 on the Ca-ATPase provide a measure of the dynamic structure of the phosphorylation domain, and indicate that LPC restricts the amplitude of rotational motion. These results suggest a structural linkage between the cytosolic phosphorylation domain and the conformation of membrane phospholipid headgroups. Thus, changes in membrane phospholipid composition can modulate membrane surface properties and affect catalytically important motions of the Ca-ATPase in a manner that suggests a role for LPC generated during signal transduction.

Agonist binding and affinity state transitions in reconstituted nicotinic acetylcholine receptors revealed by single and sequential mixing stopped-flow fluorescence spectroscopies

The affinity state of nicotinic acetylcholine receptors (nAcChoRs) reconstituted into either dioleoylphosphatidylcholine (DOPC) or a mixture of dioleoylphosphatidylcholine, dioleoylphosphatidic acid, and cholesterol (DOPC/DOPA/cholesterol) has been determined using single and sequential mixing stopped-flow fluorescence spectroscopies. These techniques have millisecond temporal resolution, permitting low- and high-affinity conformational states of the nAcChoR to be resolved following mixing with the fluorescent partial agonist Dns-C6-Cho from their characteristic Dns-C6-Cho dissociation rates. Our studies reveal that prior to agonist-induced affinity state conversion, nAcChoRs reconstituted into either DOPC or DOPC/DOPA/cholesterol are predominantly in a conformational state that has a low affinity for agonist. Prolonged exposure to Dns-C6-Cho converts nearly all DOPC/DOPA/cholesterol-reconstituted nAcChoRs to the high-affinity state. In contrast, Dns-C6-Cho converts only half of all DOPC-reconstituted nAcChoRs to the high-affinity state. The other half persists in a low-affinity state characterized by a Kd for Dns-C6-Cho of 0.61+/-0.07 microM. This Kd is similar to that previously reported for Dns-C6-Cho binding to low-affinity, resting-state nAcChoRs in native membranes. However, affinity state conversion of DOPC-reconstituted nAcChoRs may be facilitated by re-reconstituting them into bilayers composed of DOPC/DOPA/cholesterol. These results indicate that the lipid bilayer composition modulates nAcChoR agonist-induced affinity state transitions.

Quantitative single-cell-reverse transcription-PCR demonstrates that A-current magnitude varies as a linear function of shal gene expression in identified stomatogastric neurons

Different Shaker family alpha-subunit genes generate distinct voltage-dependent K+ currents when expressed in heterologous expression systems. Thus it generally is believed that diverse neuronal K+ current phenotypes arise, in part, from differences in Shaker family gene expression among neurons. It is difficult to evaluate the extent to which differential Shaker family gene expression contributes to endogenous K+ current diversity, because the specific Shaker family gene or genes responsible for a given K+ current are still unknown for nearly all adult neurons. In this paper we explore the role of differential Shaker family gene expression in creating transient K+ current (IA) diversity in the 14-neuron pyloric network of the spiny lobster, Panulirus interruptus. We used two-electrode voltage clamp to characterize the somatic IA in each of the six different cell types of the pyloric network. The size, voltage-dependent properties, and kinetic properties of the somatic IA vary significantly among pyloric neurons such that the somatic IA is unique in each pyloric cell type. Comparing these currents with the IAs obtained from oocytes injected with Panulirus shaker and shal cRNA (lobster Ishaker and lobster Ishal, respectively) reveals that the pyloric cell IAs more closely resemble lobster Ishal than lobster Ishaker. Using a novel, quantitative single-cell-reverse transcription-PCR method to count the number of shal transcripts in individual identified pyloric neurons, we found that the size of the somatic IA varies linearly with the number of endogenous shal transcripts. These data suggest that the shal gene contributes substantially to the peak somatic IA in all neurons of the pyloric network.

Quantitative single-cell-reverse transcription-PCR demonstrates that A-current magnitude varies as a linear function of shal gene expression in identified stomatogastric neurons

Different Shaker family alpha-subunit genes generate distinct voltage-dependent K+ currents when expressed in heterologous expression systems. Thus it generally is believed that diverse neuronal K+ current phenotypes arise, in part, from differences in Shaker family gene expression among neurons. It is difficult to evaluate the extent to which differential Shaker family gene expression contributes to endogenous K+ current diversity, because the specific Shaker family gene or genes responsible for a given K+ current are still unknown for nearly all adult neurons. In this paper we explore the role of differential Shaker family gene expression in creating transient K+ current (IA) diversity in the 14-neuron pyloric network of the spiny lobster, Panulirus interruptus. We used two-electrode voltage clamp to characterize the somatic IA in each of the six different cell types of the pyloric network. The size, voltage-dependent properties, and kinetic properties of the somatic IA vary significantly among pyloric neurons such that the somatic IA is unique in each pyloric cell type. Comparing these currents with the IAs obtained from oocytes injected with Panulirus shaker and shal cRNA (lobster Ishaker and lobster Ishal, respectively) reveals that the pyloric cell IAs more closely resemble lobster Ishal than lobster Ishaker. Using a novel, quantitative single-cell-reverse transcription-PCR method to count the number of shal transcripts in individual identified pyloric neurons, we found that the size of the somatic IA varies linearly with the number of endogenous shal transcripts. These data suggest that the shal gene contributes substantially to the peak somatic IA in all neurons of the pyloric network.

Interactions of the nicotinic acetylcholine receptor transmembrane segments with the lipid bilayer in native receptor-rich membranes

Proper ion channel function of the nicotinic acetylcholine receptor (nAChR) requires the interaction of the protein with distinct lipid species present in the receptor's membrane microenvironment. Two classes of lipid binding sites present at the protein-membrane interface have been postulated: annular binding sites primarily occupied by phospholipids and non-annular binding sites mainly occupied by cholesterol [Jones & McNamee (1988) Biochemistry 27, 2364-2374]. We investigated the binding of these lipids to the nAChR and potential dynamics of these interactions during events associated with signal transduction by electron spin resonance spectroscopy (ESR) using spin-labeled analogues of phospholipids, androstane, and stearic acid. Protein-lipid interactions were characterized in receptor-rich membranes prepared from Torpedo californica electric tissue preserving the native lipid environment of the nAChR. We found a strong preference of the receptor for the phosphatidylserine (PS) analogue as compared to the other probes. Up to 57% of PS were perturbed by the membrane protein, while the fraction of motionally restricted lipid for the other analogues was on the order of 30%. After removal of the extramembrane portions of the membrane-bound receptor, we observed a loss of binding sites for the spin-labeled analogue of androstane and for stearic acid, but not for phospholipids and sphingomyelin analogues. Our results demonstrate the existence of topologically distinct lipid binding sites for different lipid species. In the case of cholesterol, extramembrane portions of the receptor are involved, whereas the transmembrane segments meet the requirements for the binding of phospholipids. Tyrosine phosphorylation of the nAChR did not affect protein-lipid interactions in samples of intact nAChR. Similarly, no significant changes were observed in the presence of carbamoylcholine at concentrations that caused rapid and quantitative desensitization of the nAChR.

Tryptophan substitutions at the lipid-exposed transmembrane segment M4 of Torpedo californica acetylcholine receptor govern channel gating

Our previous amino acid substitutions at the postulated lipid-exposed transmembrane segment M4 of the Torpedo californica acetylcholine receptor (AChR) focused on the alpha C418 position. A tryptophan substitution on the alpha C418 produced a 3-fold increase in normalized macroscopic response to acetylcholine in voltage-clamped Xenopus laevis oocytes (Lee et al., 1994). This result was explained by a 23-fold decrease in the closing rate constant measured from single-channel analysis (Ortiz-Miranda et al., 1996). In this study, we introduce more tryptophan substitutions at different positions of this postulated lipid-exposed segment M4 in order to examine functional consequences at the single-channel level. From a series of amino acid substitutions at alpha G421, only phenylalanine and tryptophan produced a substantial increase in the open time constant. The lack of response from a tyrosine substitution at the alpha G421 suggests that the side chain volume is not the main structural element responsible for the effect of tryptophan on the stabilization of the open state of the channel. Three multiple mutants, alpha C418W/G421A, alpha C418W/G421W, and alpha C418W/beta C447W, were constructed in order to establish the correlation between the number of lipid-exposed tryptophans and the channel open time constant. The alpha C418W/G421A double mutant demonstrated that when both previous mutations are combined the open time constant was increased 1.5-fold relative to the alpha C418W. When the two mutants (alpha C418W and alpha G421W) were combined in a single mutation, a functional receptor was expressed and the open time constant of the new double mutant increased to 33.4 ms, an 80-fold increase relative to wild type. Estimations of free energy changes calculated from the rate constant for the opening transition suggest that each tryptophan contributes to the stabilization of the open state of the channel by about 0.8 kcal/mol, and the effect of tryptophan substitutions on the free energy is additive. This result suggests that in the channel gating mechanism of the AChR, each subunit contributes independently to the energy barrier between the open and closed state. At selected positions within the postulated lipid surface of the AChR, tryptophan substitutions could establish hydrophobic and perhaps dipole interactions that may play a dramatic role in the channel gating mechanism.

Structural effects of neutral and anionic lipids on the nicotinic acetylcholine receptor. An infrared difference spectroscopy study

The effects of both neutral and anionic lipids on the structure of the nicotinic acetylcholine receptor (nAChR) have been probed using infrared difference spectroscopy. The difference between infrared spectra of the nAChR recorded using the attenuated total reflectance technique in the presence and absence of the neurotransmitter analog, carbamylcholine, exhibits a complex pattern of positive and negative bands that provides a spectral map of the structural changes that occur in the nAChR upon ligand binding and subsequent desensitization. This spectral map is essentially identical in difference spectra recorded from native, native alkaline-extracted, and affinity-purified nAChR reconstituted into either soybean asolectin or egg phosphatidylcholine membranes containing both neutral and anionic lipids. This result suggests both a similar structure of the nAChR and a similar resting to desensitized conformational change in each membrane environment. In contrast, difference spectra recorded from the nAChR reconstituted into egg phosphatidylcholine membranes lacking neutral and/or anionic lipids all exhibit an essentially identical pattern of band intensity variations, which is similar to the pattern of variations observed in difference spectra recorded in the continuous presence of the desensitizing local anesthetic, dibucaine. The difference spectra suggest that the main effect of both neutral and anionic lipids in a reconstituted egg phosphatidylcholine membrane is to help stabilize the nAChR in a resting conformation. In the absence of neutral and/or anionic lipids, the nAChR is converted into an alternate conformation that appears to be analogous to the local anesthetic-induced desensitized state. Significantly, the proportion of receptors found in the resting versus the putative desensitized state appears to be dependent upon the final lipid composition of the reconstituted membrane. A lipid-dependent modulation of the equilibrium between a channel-active resting and channel-inactive desensitized state may account for the modulations of nAChR activity that are observed in different lipid membranes.

Interaction between ion channel-inactivating peptides and anionic phospholipid vesicles as model targets

Studies of rapid (N-type) inactivation induced by different synthetic inactivating peptides in several voltage-dependent cation channels have concluded that the channel inactivation "entrance" (or "receptor" site for the inactivating peptide) consists of a hydrophobic vestibule within the internal mouth of the channel, separated from the cytoplasm by a region with a negative surface potential. These protein domains are conformed from alternative sequences in the different channels and thus are relatively unrestricted in terms of primary structure. We are reporting here on the interaction between the inactivating peptide of the Shaker B K+ channel (ShB peptide) or the noninactivating ShB-L7E mutant with anionic phospholipid vesicles, a model target that, as the channel's inactivation "entrance," contains a hydrophobic domain (the vesicle bilayer) separated from the aqueous media by a negatively charged vesicle surface. When challenged by the anionic phospholipid vesicles, the inactivating ShB peptide 1) binds to the vesicle surface with a relatively high affinity, 2) readily adopts a strongly hydrogen-bonded beta-structure, likely an intramolecular beta "hairpin," and 3) becomes inserted into the hydrophobic bilayer by its folded N-terminal portion, leaving its positively charged C-terminal end exposed to the extravesicular aqueous medium. Similar experiments carried out with the noninactivating, L7E-ShB mutant peptide show that this peptide 1) binds also to the anionic vesicles, although with a lower affinity than does the ShB peptide, 2) adopts only occasionally the characteristic beta-structure, and 3) has completely lost the ability to traverse the anionic interphase at the vesicle surface and to insert into the hydrophobic vesicle bilayer. Because the negatively charged surface and the hydrophobic domains in the model target may partly imitate those conformed at the inactivation "entrance" of the channel proteins, we propose that channel inactivation likely includes molecular events similar to those observed in the interaction of the ShB peptide with the phospholipid vesicles, i.e., binding of the peptide to the region of negative surface potential, folding of the bound peptide as a beta-structure, and its insertion into the channel's hydrophobic vestibule. Likewise, we relate the lack of channel inactivation seen with the mutant ShB-L7E peptide to the lack of ability shown by this peptide to cross through the anionic interphase and insert into the hydrophobic domains of the model vesicle target.

Subcellular distribution of N-ethylmaleimide-sensitive and -insensitive phosphatidic acid phosphohydrolase in rat brain

The dephosphorylation of phosphatidic acid by phosphatidic acid phosphohydrolase (PAP) is important in both cell-signalling and in glycerolipid metabolism. However, these roles are apparently performed by two different enzymes, which can be distinguished by their sensitivity in vitro to N-ethylmaleimide (NEM). Both of these enzymes are present in rat brain as well as a wide range of other rat tissues. However, the quantity and specific activity of each enzyme varies considerably between different tissues, as does the ratio of the two enzymes in each tissue. Tissues rich in glycerolipids are abundant in NEM-sensitive PAP, whereas there is no obvious pattern to the distribution of the NEM-insensitive enzyme in the different tissues tested. Studies on brain cortex, which is relatively rich in both forms of PAP, indicate that the NEM-insensitive PAP is located in the synaptosomes, and the NEM-sensitive enzyme present in the cytosol and microsomes. The NEM-sensitive PAP can also be translocated from the cytosol to the microsomes by oleate. When assayed against a range of phosphatidic acids, NEM-sensitive PAP showed a preference for phosphatidic acids with short acyl chains and for those containing arachidonate, whereas NEM-insensitive PAP had a preference for short and unsaturated acyl chains. The two isozymes also had different activity profiles against these substrates suggesting that they are in fact different enzymes. The implications for these results on the putative roles of the two forms of PAP are discussed.

Mutations in the M4 domain of Torpedo californica acetylcholine receptor dramatically alter ion channel function

Site-directed mutagenesis was used to mutate alpha Cys418 and beta Cys447 in the M4 domain of Torpedo californica acetylcholine receptor expressed in Xenopus laevis oocytes. The M4 region is a transmembrane domain thought to be located at the lipid-protein interface. By whole-cell voltage clamp analysis, mutation of both alpha subunits to alpha Trp418 increased maximal channel activity approximately threefold, increased the desensitization rate compared with wild-type receptor, and shifted the EC50 for acetylcholine from 32 microM to 13 microM. Patch measurements of single-channel currents revealed that the alpha Trp418 increased channel open times approximately 28-fold at 13 degrees C with no effect on channel conductance. All of our measured functional changes in the alpha Trp418 mutant are consistent with a simple kinetic model of the acetylcholine receptor in which only the channel closing rate is altered by the mutation. Our results show that changes in protein structure at the putative lipid-protein interface can dramatically affect receptor function.

Protein-lipid interactions and Torpedo californica nicotinic acetylcholine receptor function. 2. Membrane fluidity and ligand-mediated alteration in the accessibility of gamma subunit cysteine residues to cholesterol

Fluorescence-quenching and energy-transfer measurements were carried out to further characterize lipid-protein interactions involving the nicotinic acetylcholine receptor (AChR) from Torpedo californica in reconstituted membranes. To assess the fluidity of the receptor microenvironment, cis- and trans-parinaric acids were used to take advantage of the preferential partitioning behavior of the trans isomer for the gel phase. A relatively higher extent of energy transfer from the intrinsic tryptophan fluorescence of AChR in dielaidoylphosphatidylcholine bilayers to cis-parinaric acid in both the gel and the fluid phase suggests that the AChR is surrounded by a relatively fluid annulus of lipids. The ability of AChR to accommodate and interact with specific lipids such as cholesterol and fatty acids in the vicinity of pyrene-labeled cysteine residues in the membranous domain and/or the membrane-water interface region of the gamma subunit was assessed. Pyrene-labeled AChR prepared in (6,7-dibromostearoyl)phosphatidylcholine showed a 25% decrease in fluorescence as sites accessible to phospholipids were occupied; subsequent addition of dibromocholesterol hemisuccinate (DiBrCHS) caused further quenching by about 25%. This result is consistent with the presence of sites accessible to cholesterol, but not accessible to phospholipids, in the vicinity of the cysteine-bound pyrene in the membranous domain of the AChR. Quenching by DiBrCHS was sensitive to the presence of an AChR activator (carbamylcholine) but not a competitive antagonist (alpha-bungarotoxin). The Stern-Volmer quenching constant was 0.123 in the absence of added ligands and 0.167 and 0.134 in the presence of carbamylcholine and alpha-bungarotoxin, respectively, corresponding to accessibilities of 65%, 90%, and 70%.

FTIR analysis of nicotinic acetylcholine receptor secondary structure in reconstituted membranes

Using Fourier-transform infrared resonance spectroscopy, we examined the structure of the purified Torpedo californica nicotinic acetylcholine receptor in reconstituted dioleoylphosphatidylcholine membranes in H2O and D2O. Using the amide-I band, we calculated the secondary structure of nAChR in H2O to be approx. 19% alpha-helix, 42% beta-structure, 24% turns and 15% unordered. The secondary structure content in D2O was estimated to be 14% alpha-helix, 37% beta-structure, 29% turns and 20% unordered. In the presence of phosphatidic acid the beta-structure content in D2O increased significantly from 37% to 42%. This suggests that an ionic interaction between negatively-charged lipid head groups and positively-charged peptide side chains may stabilize a beta-structure conformation that is necessary for receptor function. The inclusion of cholesterol in the reconstituted membranes significantly increased the alpha-helix content from 14% to 17%. These results support the hypothesis that cholesterol may induce a transmembrane region to undergo a unordered-to-helix transition which is necessary to maintain the integrity of the ion channel. Additionally, we found that nAChR did not undergo major secondary structure changes when subjected to conditions that induce desensitization. This is consistent with the view that the mechanism of desensitization consists of small quaternary rearrangements of the subunits rather than large changes in receptor secondary structure.