Binding of NAP-22, a calmodulin-binding neuronal protein, to raft-like domains in model membranes

Identification of genome-wide SNP-SNP and SNP-clinical Boolean interactions in age-related macular degeneration

We propose here a methodology to uncover modularities in the network of SNP-SNP interactions most associated with disease. We start by computing all possible Boolean binary SNP interactions across the whole genome. By constructing a weighted graph of the most relevant interactions and via a combinatorial optimization approach, we find the most highly interconnected SNPs. We show that the method can be easily extended to find SNP/environment interactions. Using a modestly sized GWAS dataset of age-related macular degeneration (AMD), we identify a group of only 19 SNPs, which include those in previously reported regions associated to AMD. We also uncover a larger set of loci pointing to a matrix of key processes and functions that are affected. The proposed integrative methodology extends and overlaps traditional statistical analysis in a natural way. Combinatorial optimization techniques allow us to find the kernel of the most central interactions, complementing current methods of GWAS analysis and also enhancing the search for gene-environment interaction.

Immunoelectron microscopic study of BASP1 and MARCKS location in the early and late rat spermatids

Immunoelectron microscopy was used to locate the proteins BASP1 and MARCKS in the post-meiotic spermatids of male rat testis. It was shown that in early spermatids, BASP1 and MARCKS accumulate in chromatoid bodies, which are characteristic organelles for these cells. During spermatogenesis, while the spermatid nucleus is still active, the chromatoid body periodically moves to the cell nucleus and absorbs the precursors of definite mRNAs and small RNAs. mRNAs are preserved in the chromatoid body until the corresponding proteins are needed, but their "fresh" mRNA cannot be formed due to the nucleus inactivation. The chromatoid body (0.5-1.5μm in diameter) has a cloud-like fibrous appearance with many fairly round cavities. In the chromatoid body, BASP1 and MARCKS are distributed mainly around the cavities and at periphery. Based on the known functions of BASP1 and MARCKS in neurons, it is conceivable that these proteins participate in non-random movements of the chromatoid body to the nucleus and in Ca(2+)-calmodulin enrichment. In late spermatids, BASP1 and MARCKS are located in the outer dense fiber layer belonging to a metabolically active spermatozoon region, the tail mid-piece. In spermatozoa, as in chromatoid body, BASP1 and MARCKS may bind Ca(2+)-calmodulin and therefore contribute to the activation of calcium-dependent biochemical processes.

Subcellular and regional location of "brain" proteins BASP1 and MARCKS in kidney and testis

Proteins BASP1 and MARCKS are abundant in axonal endings of neurons. Similarly to brain-specific protein GAP-43, BASP1 and MARCKS are reversibly bound to the plasma membrane. These proteins control both actin polymerization and actin cytoskeleton binding to the membrane. Performing these functions, BASP1 and MARCKS take part in growth cone guidance during development and in neurotransmitter secretion in adults. These activities predetermine the pivotal role of BASP1 and MARCKS in learning and memory. BASP1 and MARCKS were also found in non-nerve tissues, in particular, in the kidney and testis. Evidently, the physiological roles of these proteins differ in different tissues. Correspondingly, their intracellular location and activities may not be similar to those in neurons. In this paper, we analyze subcellular fractions (cytoplasm and nuclei) of rat kidney and testis with the purpose of determining the intracellular location of BASP1 and MARCKS. Western blots demonstrated that in these tissues, as in the brain, both proteins are present in the cytoplasm of the cell. According to our immunohistochemical study, BASP1 and MARCKS are specifically distributed in the tissues studied. In kidney, both proteins are present in cells located in glomeruli. In the testicular tubules, BASP1 is mainly expressed at the late stage of spermatogenesis (in spermatids) and is preserved in mature spermatozoa, while MARCKS appears equally during all stages of spermatogenesis. MARCKS is not found in mature spermatozoa. The results indicate that study of functions of BASP1 and MARCKS in the kidney and in the reproduction system holds much promise.

Greasing their way: lipid modifications determine protein association with membrane rafts

Increasing evidence suggests that biological membranes can be laterally subdivided into domains enriched in specific lipid and protein components and that these domains may be involved in the regulation of a number of vital cellular processes. An example is membrane rafts, which are lipid-mediated domains dependent on preferential association between sterols and sphingolipids and inclusive of a specific subset of membrane proteins. While the lipid and protein composition of rafts has been extensively characterized, the structural details determining protein partitioning to these domains remain unresolved. Here, we review evidence suggesting that post-translation modification by saturated lipids recruits both peripheral and transmembrane proteins to rafts, while short, unsaturated, and/or branched hydrocarbon chains prevent raft association. The most widely studied group of raft-associated proteins are glycophosphatidylinositol-anchored proteins (GPI-AP), and we review a variety of evidence supporting raft-association of these saturated lipid-anchored extracellular peripheral proteins. For transmembrane and intracellular peripheral proteins, S-acylation with saturated fatty acids mediates raft partitioning, and the dynamic nature of this modification presents an exciting possibility of enzymatically regulated raft association. The other common lipid modifications, that is, prenylation and myristoylation, are discussed in light of their likely role in targeting proteins to nonraft membrane regions. Finally, although the association between raft affinity and lipid modification is well-characterized, we discuss several open questions regarding regulation and remodeling of these post-translational modifications as well as their role in transbilayer coupling of membrane domains.

Mutational screening of BASP1 and transcribed processed pseudogene TPPsig-BASP1 in patients with Möbius syndrome

Möbius syndrome is a rare disorder primarily characterized by congenital facial palsy, frequently accompanied by ocular abduction anomalies and occasionally associated with orofacial, limb and musculoskeletal malformations. Abnormal development of cranial nerves V through XII underlines the disease pathogenesis. Although a genetic etiology for Möbius syndrome was proposed, molecular genetic studies to identify the causative gene(s) are scarce. In this study, we selected two candidate genes. One is BASP1 residing in a human chromosome 5p15.1-p15.2, syntenic to mouse chromosome 15qA2-qB2, to which a mouse model with facial nerve anomalies was mapped. The other is transcribed processed pseudogene TPPsig-BASP1, which is located on chromosome 13q flanking the putative locus for Möbius syndrome and might be involved in the regulation of the transcripts encoded by BASP1. Mutation analyses in nineteen patients excluded these genes as being candidates for Möbius syndrome.

Proteomic analysis of membrane microdomain-associated proteins in the dorsolateral prefrontal cortex in schizophrenia and bipolar disorder reveals alterations in LAMP, STXBP1 and BASP1 protein expression

The dorsolateral prefrontal cortex (dlpfc) is strongly implicated in the pathogenesis of schizophrenia (SCZ) and bipolar disorder (BPD) and, within this region, abnormalities in glutamatergic neurotransmission and synaptic function have been described. Proteins associated with these functions are enriched in membrane microdomains (MM). In the current study, we used two complementary proteomic methods, two-dimensional difference gel electrophoresis and one-dimensional sodium dodecyl sulphate polyacrylamide gel electrophoresis followed by reverse phase-liquid chromatography-tandem mass spectrometry (RP-LC-MS/MS) (gel separation liquid chromatography-tandem mass spectrometry (GeLC-MS/MS)) to assess protein expression in MM in pooled samples of dlpfc from SCZ, BPD and control cases (n=10 per group) from the Stanley Foundation Brain series. We identified 16 proteins altered in one/both disorders using proteomic methods. We selected three proteins with roles in synaptic function (syntaxin-binding protein 1 (STXBP1), brain abundant membrane-attached signal protein 1 (BASP1) and limbic system-associated membrane protein (LAMP)) for validation by western blotting. This revealed significantly increased expression of these proteins in SCZ (STXBP1 (24% difference; P<0.001), BASP1 (40% difference; P<0.05) and LAMP (22% difference; P<0.01)) and BPD (STXBP1 (31% difference; P<0.001), BASP1 (23% difference; P<0.01) and LAMP (20% difference; P<0.01)) in the Stanley brain series (n=20 per group). Further validation in dlpfc from the Harvard brain subseries (n=10 per group) confirmed increased protein expression in SCZ of STXBP1 (18% difference; P<0.0001), BASP1 (14% difference; P<0.0001) but not LAMP (20% difference; P=0.14). No significant differences in STXBP1, BASP1 or LAMP protein expression in BPD dlpfc were observed. This study, through proteomic assessments of MM in dlpfc and validation in two brain series, strongly implicates LAMP, STXBP1 and BASP1 in SCZ and supports the view of a neuritic and synaptic dysfunction in the neuropathology of SCZ.

Probing membrane order and topography in supported lipid bilayers by combined polarized total internal reflection fluorescence-atomic force microscopy

Determining the local structure, dynamics, and conformational requirements for protein-protein and protein-lipid interactions in membranes is critical to understanding biological processes ranging from signaling to the translocating and membranolytic action of antimicrobial peptides. We report here the application of a combined polarized total internal reflection fluorescence microscopy-in situ atomic force microscopy platform. This platform's ability to image membrane orientational order was demonstrated on DOPC/DSPC/cholesterol model membranes containing the fluorescent membrane probe, DiI-C(20) or BODIPY-PC. Spatially resolved order parameters and fluorophore tilt angles extracted from the polarized total internal reflection fluorescence microscopy images were in good agreement with the topographical details resolved by in situ atomic force microscopy, portending use of this technique for high-resolution characterization of membrane domain structures and peptide-membrane interactions.

Lipid domain morphologies in phosphatidylcholine-ceramide monolayers

In cells, one of the main roles of ceramide-enriched membrane domains is to recruit or exclude intracellular signaling molecules and receptors, thereby facilitating signal transduction cascades. Accordingly, in model membranes, even low contents of ceramide segregate into lateral domains. The impact of the N-acyl chain on this segregation and on the morphology of the domains remains to be explored. Using Langmuir monolayers, we have systematically studied binary mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and ceramide (2:1, molar ratio) and varied the N-acyl chain length of ceramide from 2 to 24 carbon atoms (Cer2 to Cer24). Fluid Cer2, Cer6, and Cer8/DMPC mixtures were miscible at all surface pressures. Longer ceramides, however, formed surface pressure-dependent immiscible mixtures with DMPC. The domain morphology under fluorescence microscopy after including a trace amount of fluorescent NBD-phosphatidylcholine into DMPC/Cer mixtures was found to be very sensitive to the N-acyl chain length. Shorter ceramides (Cer10-Cer14) formed flower-like (seaweed) domains, whereas longer ceramides (N-acyl chain length>14 carbon atoms) formed round and regular domains. We attribute the formation of the flower patterns to diffusive morphological instabilities during domain growth.

Characterizing the chemical complexity of patterned biomimetic membranes

Biomembranes are complex, heterogeneous, dynamic systems playing essential roles in numerous processes such as cell signaling and membrane trafficking. Model membranes provide simpler platforms for studying biomembrane dynamics under well-controlled environments. Here we present a modified polymer lift-off approach to introduce chemical complexity into biomimetic membranes by constructing domains of one lipid composition (here, didodecylphosphatidylcholine) that are surrounded by a different lipid composition (e.g., dipentadecylphosphatidylcholine), which we refer to as patterned backfilled samples. Fluorescence microscopy and correlation spectroscopy were used to characterize this patterning approach. We observe two types of domain populations: one with diffuse boundaries and a minor fraction with sharp edges. Lipids within the diffuse domains in patterned backfilled samples undergo anomalous diffusion, which results from nonideally mixed clusters of gel phase lipid within the fluid domains. No lateral diffusion was observed within the minor population of domains with well-defined borders. These results suggest that, while membrane patterning by a variety of approaches is useful for biophysical and biosensor applications, a thorough and systematic characterization of the resulting biomimetic membrane, and its unpatterned counterpart, is essential.

Overview of membrane rafts

Transient lateral microdomains (rafts) in cell membranes have been postulated to perform a number of important functions in normal cells, and are also thought to be critically involved in several pathological conditions. However, there are still a number of fundamental unanswered questions concerning the composition, size, dynamics, and stability of membrane rafts. These questions are currently being addressed by a number of sophisticated biophysical, biochemical, and computational methodologies.

Phase behavior and the partitioning of caveolin-1 scaffolding domain peptides in model lipid bilayers

The membrane binding and model lipid raft interaction of synthetic peptides derived from the caveolin scaffolding domain (CSD) of the protein caveolin-1 have been investigated. CSD peptides bind preferentially to liquid-disordered domains in model lipid bilayers composed of cholesterol and an equimolar ratio of dioleoylphosphatidylcholine (DOPC) and brain sphingomyelin. Three caveolin-1 peptides were studied: the scaffolding domain (residues 83-101), a water-insoluble construct containing residues 89-101, and a water-soluble construct containing residues 89-101. Confocal and fluorescence microscopy investigation shows that the caveolin-1 peptides bind to the more fluid cholesterol-poor phase. The binding of the water-soluble peptide to lipid bilayers was measured using fluorescence correlation spectroscopy (FCS). We measured molar partition coefficients of 10(4) M(-1) between the soluble peptide and phase-separated lipid bilayers and 10(3) M(-1) between the soluble peptide and bilayers with a single liquid phase. Partial phase diagrams for our phase-separating lipid mixture with added caveolin-1 peptides were measured using fluorescence microscopy. The water-soluble peptide did not change the phase morphology or the miscibility transition in giant unilamellar vesicles (GUVs); however, the water-insoluble and full-length CSD peptides lowered the liquid-liquid melting temperature.

Characterization of the Stomatin Domain Involved in Homo-oligomerization and Lipid Raft Association

The cytoplasmically oriented monotopic integral membrane protein stomatin forms high-order oligomers and associates with lipid rafts. To characterize the domains that are involved in oligomerization and detergent-resistant membrane (DRM) association, we expressed truncation and point mutants of stomatin and analyzed their size and buoyancy by ultracentrifugation methods. A small C-terminal region of stomatin that is largely hydrophobic, Ser-Thr-Ile-Val-Phe-Pro-Leu-Pro-Ile (residues 264-272), proved to be crucial for oligomerization, whereas the N-terminal domain (residues 1-20) and the last 12 C-terminal amino acids (residues 276-287) were not essential. The introduction of alanine substitutions in the region 264-272 resulted in the appearance of monomers. Remarkably, only three of these residues, Ile-Val-Phe (residues 266-268), were found to be indispensable for the DRM association. Interestingly, the exchange of Pro-269 and to some extent the residues 270-272, which are essential for oligomerization, did not affect the DRM association of stomatin. This suggests that the formation of oligomers is not necessary for the association of stomatin with DRMs. Internal deletions near the membrane anchoring domain resulted in the formation of intermediate size oligomers suggesting a conformational interdependence of large parts of the C-terminal region. Fluorescence recovery after photobleaching analysis of the tagged, monomeric, non-DRM mutant ST-(1-262)-green fluorescent protein and wild type stomatin StomGFP showed a significantly higher lateral mobility of the truncation mutant in the plasma membrane suggesting a membrane interaction of the respective C-terminal region also in vivo.

ROLES OF BILAYER MATERIAL PROPERTIES IN FUNCTION AND DISTRIBUTION OF MEMBRANE PROTEINS

Structural, compositional, and material (elastic) properties of lipid bilayers exert strong influences on the interactions of water-soluble proteins and peptides with membranes, the distribution of transmembrane proteins in the plane of the membrane, and the function of specific membrane channels. Theoretical and experimental studies show that the binding of either cytoplasmic proteins or extracellular peptides to membranes is regulated by the presence of charged lipids and that the sorting of transmembrane proteins into or out of membrane microdomains (rafts) depends on several factors, including bilayer material properties governed by the presence of cholesterol. Recent studies have also shown that bilayer material properties modify the permeability of membrane pores, formed either by protein channels or by cell-lytic peptides.

Detergent-free caveolae proteome suggests an interaction with ER and mitochondria

Recent proteomic studies of detergent resistant membrane fractions have begun to characterize the protein composition of caveolae and lipid rafts. The methods used in most of these studies, however, are not able to distinguish between plasma membrane and internal membrane lipid domains. Here we used a non-detergent method for obtaining fractions enriched in caveolae derived from the plasma membrane of multiple cell types. Unexpectedly, the proteins in the caveolae proteome suggest these lipid domains may interact with elements of ER and mitochondria. A comparison of the partial proteome we obtained with other published reports identifies 26 proteins that are candidate marker proteins for identifying caveolae in multiple cell types.

Partitioning of membrane molecules between raft and non-raft domains: Insights from model-membrane studies

The special physical and functional properties ascribed to lipid rafts in biological membranes reflect their distinctive organization and composition, properties that are hypothesized to rest in part on the differential partitioning of various membrane components between liquid-ordered and liquid-disordered lipid environments. This review describes the principles and findings of recently developed methods to monitor the partitioning of membrane proteins and lipids between liquid-ordered and liquid-disordered domains in model membranes, and how these approaches can aid in elucidating the properties of rafts in biological membranes.

Detecting microdomains in intact cell membranes

Current models for cellular plasma membranes focus on spatial heterogeneity and how this heterogeneity relates to cell function. In particular, putative lipid raft membrane domains have been postulated to exist based in large part on the results that a significant fraction of the membrane is detergent insoluble and that molecules facilitating key membrane processes like signal transduction are often found in the detergent-resistant membrane fraction. Yet, the in vivo existence of lipid rafts remains extremely controversial because, despite being sought for more than a decade, evidence for their presence in intact cell membranes is inconclusive. In this review, a variety of experimental techniques that have been or might be used to look for lipid microdomains in intact cell membranes are described. Experimental results are highlighted and the strengths and limitations of different techniques for microdomain identification and characterization are assessed.

Induction of raft-like domains by a myristoylated NAP-22 peptide and its Tyr mutant

The N-terminally myristoylated, 19-amino acid peptide, corresponding to the amino terminus of the neuronal protein NAP-22 (NAP-22 peptide) is a naturally occurring peptide that had been shown by fluorescence to cause the sequestering of a Bodipy-labeled PtdIns(4,5)P2 in a cholesterol-dependent manner. The present work, using differential scanning calorimetry (DSC), extends the observation that formation of a PtdIns(4,5)P2-rich domain is cholesterol dependent and shows that it also leads to the formation of a cholesterol-depleted domain. The PtdIns(4,5)P2 used in the present work is extracted from natural sources and does not contain any label and has the native acyl chain composition. Peptide-induced formation of a cholesterol-depleted domain is abolished when the sole aromatic amino acid, Tyr11 is replaced with a Leu. Despite this, the modified peptide can still sequester PtdIns(4,5)P2 into domains, probably because of the presence of a cluster of cationic residues in the peptide. Cholesterol and PtdIns(4,5)P2 also modulate the insertion of the peptide into the bilayer as revealed by 1H NOESY MAS/NMR. The intensity of cross peaks between the aromatic protons of the Tyr residue and the protons of the lipid indicate that in the presence of cholesterol there is a change in the nature of the interaction of the peptide with the membrane. These results have important implications for the mechanism by which NAP-22 affects actin reorganization in neurons.

Do proteins facilitate the formation of cholesterol-rich domains?

Both biological and model membranes can exhibit the formation of domains. A brief review of some of the diverse methodologies used to identify the presence of domains in membranes is given. Some of these domains are enriched in cholesterol. The segregation of lipids into cholesterol-rich domains can occur in both pure lipid systems as well as membranes containing peptides and proteins. Peptides and proteins can promote the formation of cholesterol-rich domains not only by preferentially interacting with cholesterol and being sequestered into these regions of the membrane, but also indirectly as a consequence of being excluded from cholesterol-rich domains. The redistribution of components is dictated by the thermodynamics of the system. The formation of domains in a biological membrane is a consequence of all of the intermolecular interactions including those among lipid molecules as well as between lipids and proteins.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Mass spectrometric analysis of the glycosphingolipid-enriched microdomains of rat natural killer cells

Glycosphingolipid-enriched microdomains (GEM) are membrane entities that concentrate glycosylphosphatiolylinositol(GPI)-anchored, acylated and membrane proteins important for immune receptor signaling. Using rat leukemic cell line RNK-16 we have initiated proteomic studies of microdomains in natural killer (NK) cells. Isolated plasma membranes were treated with Brij 58, or Nonidet-P40, or sodium carbonate. Extracts were separated by sucrose density gradient centrifugation into very light membrane, medium light membrane and heavy fractions, and a complete protein profile was analyzed by tandem mass spectrometry. Up to 250 proteins were unambiguously identified in each analyzed fraction. The first study of the proteome of NK cell GEM revealed several new aspects including identification of molecules not expected to be expressed in rat NK cells (e.g., NAP-22) or associated with GEM (e.g., NKR-P1, CD45, CD2). Moreover, it provided clear data consolidating controversial views concerning the occurrence of major histcompatibility complex glycoproteins and RT6.1/CD73/CD38 complex in NK cells. Our results also identified a large number of receptors as candidates for future functional studies.

Role of cholesterol in the formation and nature of lipid rafts in planar and spherical model membranes

Sterols play a crucial regulatory and structural role in the lateral organization of eukaryotic cell membranes. Cholesterol has been connected to the possible formation of ordered lipid domains (rafts) in mammalian cell membranes. Lipid rafts are composed of lipids in the liquid-ordered (l(o)) phase and are surrounded with lipids in the liquid-disordered (l(d)) phase. Cholesterol and sphingomyelin are thought to be the principal components of lipid rafts in cell and model membranes. We have used fluorescence microscopy and fluorescence recovery after photobleaching in planar supported lipid bilayers composed of porcine brain phosphatidylcholine (bPC), porcine brain sphingomyelin (bSM), and cholesterol to map the composition-dependence of l(d)/l(o) phase coexistence. Cholesterol decreases the fluidity of bPC bilayers, but disrupts the highly ordered gel phase of bSM, leading to a more fluid membrane. When mixed with bPC/bSM (1:1) or bPC/bSM (2:1), cholesterol induces the formation of l(o) phase domains. The fraction of the membrane in the l(o) phase was found to be directly proportional to the cholesterol concentration in both phospholipid mixtures, which implies that a significant fraction of bPC cosegregates into l(o) phase domains. Images reveal a percolation threshold, i.e., the point where rafts become connected and fluid domains disconnected, when 45-50% of the total membrane is converted to the l(o) phase. This happens between 20 and 25 mol % cholesterol in 1:1 bPC/bSM bilayers and between 25 and 30 mol % cholesterol in 2:1 bPC/bSM bilayers at room temperature, and at approximately 35 mol % cholesterol in 1:1 bPC/bSM bilayers at 37 degrees C. Area fractions of l(o) phase lipids obtained in multilamellar liposomes by a fluorescence resonance energy transfer method confirm and support the results obtained in planar lipid bilayers.

Nonequilibrium behavior in supported lipid membranes containing cholesterol

We investigate lateral organization of lipid domains in vesicles versus supported membranes and monolayers. The lipid mixtures used are predominantly DOPC/DPPC/Chol and DOPC/BSM/Chol, which have been previously shown to produce coexisting liquid phases in vesicles and monolayers. In a monolayer at an air-water interface, these lipids have miscibility transition pressures of approximately 12-15 mN/m, which can rise to 32 mN/m if the monolayer is exposed to air. Lipid monolayers can be transferred by Langmuir-Schäfer deposition onto either silanized glass or existing Langmuir-Blodgett supported monolayers. Micron-scale domains are present in the transferred lipids only if they were present in the original monolayer before deposition. This result is valid for transfers at 32 mN/m and also at lower pressures. Domains transferred to glass supports differ from liquid domains in vesicles because they are static, do not align in registration across leaflets, and do not reappear after temperature is cycled. Similar static domains are found for vesicles ruptured onto glass surfaces. Although supported membranes on glass capture some aspects of vesicles in equilibrium (e.g., gel-liquid transition temperatures and diffusion rates of individual lipids), the collective behavior of lipids in large liquid domains is poorly reproduced.

Miscibility of ternary mixtures of phospholipids and cholesterol in monolayers, and application to bilayer systems

We investigate miscibility transitions of two different ternary lipid mixtures, DOPC/DPPC/Chol and POPC/PSM/Chol. In vesicles, both of these mixtures of an unsaturated lipid, a saturated lipid, and cholesterol form micron-scale domains of immiscible liquid phases for only a limited range of compositions. In contrast, in monolayers, both of these mixtures produce two distinct regions of immiscible liquid phases that span all compositions studied, the alpha-region at low cholesterol and the beta-region at high cholesterol. In other words, we find only limited overlap in miscibility phase behavior of monolayers and bilayers for the lipids studied. For vesicles at 25 degrees C, the miscibility phase boundary spans portions of both the monolayer alpha-region and beta-region. Within the monolayer beta-region, domains persist to high pressures, yet within the alpha-region, miscibility phase transition pressures always fall below 15 mN/m, far below the bilayer equivalent pressure of 32 mN/m. Approximately equivalent phase behavior is observed for monolayers of DOPC/DPPC/Chol and for monolayers of POPC/PSM/Chol. As expected, pressure-area isotherms of our ternary lipid mixtures yield smaller molecular area and compressibility for monolayers containing more saturated acyl chains and cholesterol. All monolayer experiments were conducted under argon. We show that exposure of unsaturated lipids to air causes monolayer surface pressures to decrease rapidly and miscibility transition pressures to increase rapidly.

Partitioning of NaPi cotransporter in cholesterol-, sphingomyelin-, and glycosphingolipid-enriched membrane domains modulates NaPi protein diffusion, clustering, and activity

In dietary potassium deficiency there is a decrease in the transport activity of the type IIa sodium/phosphate cotransporter protein (NaPi) despite an increase in its apical membrane abundance. This novel posttranslational regulation of NaPi activity is mediated by the increased glycosphingolipid content of the potassium-deficient apical membrane. However, the mechanisms by which these lipids modulate NaPi activity have not been determined. We determined if in potassium deficiency NaPi is increasingly partitioned in cholesterol-, sphingomyelin-, and glycosphingolipid-enriched microdomains of the apical membrane and if the increased presence of NaPi in these microdomains modulates its activity. By using a detergent-free density gradient flotation technique, we found that 80% of the apical membrane NaPi partitions into the low density cholesterol-, sphingomyelin-, and GM1-enriched fractions characterized as "lipid raft" fractions. In potassium deficiency, a higher proportion of NaPi was localized in the lipid raft fractions. By combining fluorescence correlation spectroscopy and photon counting histogram methods for control and potassium-deficient apical membranes reconstituted into giant unilamellar vesicles, we showed a 2-fold decrease in lateral diffusion of NaPi protein and a greater than 2-fold increase in size of protein aggregates/clusters in potassium deficiency. Our results indicate that NaPi protein is localized in membrane microdomains, that in potassium deficiency a larger proportion of NaPi protein is present in these microdomains, and that NaPi lateral diffusion is slowed down and NaPi aggregation/clustering is increased in potassium deficiency, both of which could be associated with the decreased Na/Pi cotransport activity in potassium deficiency.

Lipid rafts-protein association and the regulation of protein activity

Lipid rafts are membrane microdomains enriched in saturated phospholipids, sphingolipids, and cholesterol. They have a varied but distinct protein composition and have been implicated in diverse cellular processes including polarized traffic, signal transduction, endo- and exo-cytoses, entrance of obligate intracellular pathogens, and generation of pathological forms of proteins associated with Alzheimer's and prion diseases. Raft proteins can be permanently or temporarily associated to lipid rafts. Here, we review recent advances on the biochemical and cell biological characterization of rafts, and on the emerging concept of the temporary residency of proteins in rafts as a regulatory mechanism of their biological activity.

Motor, sensory and autonomic nerve terminals containing NAP-22 immunoreactivity in the rat muscle

Neuron-enriched acidic protein having a molecular mass of 22 kDa, NAP-22, is a Ca(2+)-dependent calmodulin-binding protein and is phosphorylated with protein kinase C (PKC). This protein is localized to the biological membrane via myristoylation and found in the membrane fraction of the brain and in the synaptic vesicle fraction. Recent studies showed that NAP-22 is localized in the membrane raft domain in a cholesterol-dependent manner and suggest a role for NAP-22 in maturation and/or maintenance of nerve terminals by controlling cholesterol-dependent membrane dynamics. The present study revealed the immunohistochemical distribution of NAP-22 in the peripheral nerves in rat muscles. In all examined muscles, nerve terminals in the motor endplates showed NAP-22 immunoreactivity associated with the membranes of synaptic vesicles and nerve terminals. In the muscle spindles, annulospiral endings, which made spirals around the intrafusal muscles, showed intense NAP-22 immunoreactivity. Autonomic nerve fibers around the intramuscular blood vessels also showed the immunoreactivity for NAP-22. NAP-22 immunoreactivity in these peripheral nerves was observed from birth to adulthood (100 days after birth). Though growth-associated protein-43 (GAP-43) immunoreactivity in these nerves was observed from birth, this immunoreactivity decreased from 20 days after birth. These findings suggest that NAP-22 is distributed and regulates functions in the motor, sensory and autonomic nerve terminals in the peripheral nervous system.

Lipid raft domains and protein networks in T-cell receptor signal transduction

Activation of the T-cell antigen receptor (TCR) is a key event in triggering the physiological responses of T lymphocytes to antigen. The earliest TCR-evoked signalling steps, such as tyrosine phosphorylations, ras activation and induction of Ca(2+) fluxes, are initiated in the T-cell plasma membrane. It has been implicated that cholesterol- and sphingolipid-rich membrane domains, termed lipid rafts, form platforms for the regulation and transduction of TCR signals at the plasma membrane; however, recent experiments have now differentiated distinct roles for lipid-raft-mediated and protein-mediated interactions in the formation of TCR signalling membrane domains.

Crystal structure of a myristoylated CAP-23/NAP-22 N-terminal domain complexed with Ca2+/calmodulin

A variety of viral and signal transduction proteins are known to be myristoylated. Although the role of myristoylation in protein-lipid interaction is well established, the involvement of myristoylation in protein-protein interactions is less well understood. CAP-23/NAP-22 is a brain-specific protein kinase C substrate protein that is involved in axon regeneration. Although the protein lacks any canonical calmodulin (CaM)-binding domain, it binds CaM with high affinity. The binding of CAP-23/NAP-22 to CaM is myristoylation dependent and the N-terminal myristoyl group is directly involved in the protein-protein interaction. Here we show the crystal structure of Ca2+-CaM bound to a myristoylated peptide corresponding to the N-terminal domain of CAP-23/NAP-22. The myristoyl moiety of the peptide goes through a hydrophobic tunnel created by the hydrophobic pockets in the N- and C-terminal domains of CaM. In addition to the myristoyl group, several amino-acid residues in the peptide are important for CaM binding. This is a novel mode of binding and is very different from the mechanism of binding in other CaM-target complexes.

Ceramide selectively displaces cholesterol from ordered lipid domains (rafts): implications for lipid raft structure and function

Ceramide is a membrane lipid involved in a number of crucial biological processes. Recent evidence suggests that ceramide is likely to reside and function within lipid rafts; ordered sphingolipid and cholesterol-rich lipid domains believed to exist within many eukaryotic cell membranes. Using lipid vesicles containing co-existing raft domains and disordered fluid domains, we find that natural and saturated synthetic ceramides displace sterols from rafts. Other raft lipids remain raft-associated in the presence of ceramide, showing displacement is relatively specific for sterols. Like cholesterol-containing rafts, ceramide-rich "rafts" remain in a highly ordered state. Comparison of the sterol-displacing abilities of natural ceramides with those of saturated diglycerides and an unsaturated ceramide demonstrates that tight lipid packing is critical for sterol displacement by ceramide. Based on these results, and the fact that cholesterol and ceramides both have small polar headgroups, we propose that ceramides and cholesterol compete for association with rafts because of a limited capacity of raft lipids with large headgroups to accommodate small headgroup lipids in a manner that prevents unfavorable contact between the hydrocarbon groups of the small headgroup lipids and the surrounding aqueous environment. Minimizing the exposure of cholesterol and ceramide to water may be a strong driving force for the association of other molecules with rafts. Furthermore, displacement of sterol from rafts by ceramide is very likely to have marked effects upon raft structure and function, altering liquid ordered properties as well as molecular composition. In this regard, certain previously observed physiological processes may be a result of displacement. In particular, a direct connection to the previously observed sphingomyelinase-induced displacement of cholesterol from plasma membranes in cells is proposed.

Quaternary structure of the neuronal protein NAP-22 in aqueous solution

NAP-22, a myristoylated, anionic protein, is a major protein component of the detergent-insoluble fraction of neurons. After extraction from the membrane, it is readily soluble in water. NAP-22 will partition only into membranes with specific lipid compositions. The lipid specificity is not expected for a monomeric myristoylated protein. We have studied the self-association of NAP-22 in solution. Sedimentation velocity experiments indicated that the protein is largely associated. The low concentration limiting s value is approximately 1.3 S, indicating a highly asymmetric monomer. In contrast, a nonmyristoylated form of the protein shows no evidence of oligomerization by velocity sedimentation and has an s value corresponding to the smallest component of NAP-22, but without the presence of higher oligomers. Sedimentation equilibrium runs indicate that there is a rapidly reversible equilibrium between monomeric and oligomeric forms of the protein followed by a slower, more irreversible association into larger aggregates. In situ atomic force microscopy of the protein deposited on mica from freshly prepared dilute solution revealed dimers on the mica surface. The values of the association constants obtained from the sedimentation equilibrium data suggest that the weight concentration of the monomer exceeds that of the dimer below a total protein concentration of 0.04 mg/ml. Since the concentration of NAP-22 in the neurons of the developing brain is approximately 0.6 mg/ml, if the protein were in solution, it would be in oligomeric form and bind specifically to cholesterol-rich domains. We demonstrate, using fluorescence resonance energy transfer, that at low concentrations, NAP-22 labeled with Texas Red binds equally well to liposomes of phosphatidylcholine either with or without the addition of 40 mol% cholesterol. Thus, oligomerization of NAP-22 contributes to its lipid selectivity during membrane binding.