Lipid-binding Proteins in Membrane Digestion, Antigen Presentation, and Antimicrobial Defense

SPP-5 affects larval arrest via insulin signaling pathway in Caenorhabditis elegans

Diapause is an endocrine-mediated metabolic and growth arrest state in response to unfavorable external environments. The nematode Caenorhabditis elegans can enter diapause/arrest during embryonic, larval, or adult stages when subjected to detrimental external environments. Larval stage 1 (L1) arrest happens when animals hatch without food. Previous work has shown that the insulin pathway plays a prominent role in regulating L1 arrest. However, the downstream signal molecular mechanisms and biomarkers are still missing. In this study, we showed that SaPosin-like Protein family member SPP-5 is significantly upregulated during L1 arrest, suggesting that it could act as an L1 arrest biomarker. Using RNA interference we demonstrated that spp-5  knockdown accelerated larval development, while the overexpression resulted in L1 arrest. Consistently, SPP-5 level was significantly up-regulated in the L1 arrest daf-2(e1370) mutants, and spp-5(RNAi) suppressed the daf-2(e1370) induced L1 arrest. These results suggest that SPP-5 can serve as an L1 arrest biomarker and promote the arrest probably via the insulin signaling pathway.

Neuronal Ganglioside and Glycosphingolipid (GSL) Metabolism and Disease : Cascades of Secondary Metabolic Errors Can Generate Complex Pathologies (in LSDs)

Glycosphingolipids (GSLs) are a diverse group of membrane components occurring mainly on the surfaces of mammalian cells. They and their metabolites have a role in intercellular communication, serving as versatile biochemical signals (Kaltner et al, Biochem J 476(18):2623-2655, 2019) and in many cellular pathways. Anionic GSLs, the sialic acid containing gangliosides (GGs), are essential constituents of neuronal cell surfaces, whereas anionic sulfatides are key components of myelin and myelin forming oligodendrocytes. The stepwise biosynthetic pathways of GSLs occur at and lead along the membranes of organellar surfaces of the secretory pathway. After formation of the hydrophobic ceramide membrane anchor of GSLs at the ER, membrane-spanning glycosyltransferases (GTs) of the Golgi and Trans-Golgi network generate cell type-specific GSL patterns for cellular surfaces. GSLs of the cellular plasma membrane can reach intra-lysosomal, i.e. luminal, vesicles (ILVs) by endocytic pathways for degradation. Soluble glycoproteins, the glycosidases, lipid binding and transfer proteins and acid ceramidase are needed for the lysosomal catabolism of GSLs at ILV-membrane surfaces. Inherited mutations triggering a functional loss of glycosylated lysosomal hydrolases and lipid binding proteins involved in GSL degradation cause a primary lysosomal accumulation of their non-degradable GSL substrates in lysosomal storage diseases (LSDs). Lipid binding proteins, the SAPs, and the various lipids of the ILV-membranes regulate GSL catabolism, but also primary storage compounds such as sphingomyelin (SM), cholesterol (Chol.), or chondroitin sulfate can effectively inhibit catabolic lysosomal pathways of GSLs. This causes cascades of metabolic errors, accumulating secondary lysosomal GSL- and GG- storage that can trigger a complex pathology (Breiden and Sandhoff, Int J Mol Sci 21(7):2566, 2020).

M05B5.4 (Lysosomal phospholipase A2) promotes disintegration of autophagic vesicles to maintain C. elegans development

The autophagosome has two lipid bilayer membranes. The outer membrane fuses with the lysosome, while the inner membrane is degraded to release autophagic contents for degradation. It remains unclear how the inner vesicle of the autophagosome (called the autophagic vesicle) is disintegrated after autophagosome-lysosome fusion. Here, we identified C. elegans LPLA-2/M05B5.4 as a key enzyme that degrades membranous material in lysosomes. LPLA-2 is homologous to human PLA2G15, a lysosomal phospholipase A2 family protein that catalyzes cleavage of membrane phospholipids. We found that loss of LPLA-2 causes accumulation of large membrane whorls in enlarged lysosomes and both phenotypes are suppressed by blocking macroautophagy/autophagy. Moreover, autophagic vesicles persisted in enlarged lysosomes in PLA2G15 knockdown cells and lpla-2(lf) mutants, which suggests that the breakdown of the inner autophagosomal membrane in lysosomes is impaired. lpla-2(lf) mutants exhibit severe defects in both embryonic and larval development. Our data suggest that disintegration of the inner autophagosomal membrane by LPLA-2 promotes the release and subsequent degradation of autophagic contents in lysosomes, which is essential for C. elegans development.

Lysosomal Glycosphingolipid Storage Diseases

Glycosphingolipids are cell-type-specific components of the outer leaflet of mammalian plasma membranes. Gangliosides, sialic acid–containing glycosphingolipids, are especially enriched on neuronal surfaces. As amphi-philic molecules, they comprise a hydrophilic oligosaccharide chain attached to a hydrophobic membrane anchor, ceramide. Whereas glycosphingolipid formation is catalyzed by membrane-bound enzymes along the secretory pathway, degradation takes place at the surface of intralysosomal vesicles of late endosomes and lysosomes catalyzed in a stepwise fashion by soluble hydrolases and assisted by small lipid-binding glycoproteins. Inherited defects of lysosomal hydrolases or lipid-binding proteins cause the accumulation of undegradable material in lysosomal storage diseases (GM1 and GM2 gangliosidosis; Fabry, Gaucher, and Krabbe diseases; and metachromatic leukodystrophy). The catabolic processes are strongly modified by the lipid composition of the substrate-carrying membranes, and the pathological accumulation of primary storage compounds can trigger an accumulation of secondary storage compounds (e.g., small glycosphingolipids and cholesterol in Niemann-Pick disease).

Lysosomal phospholipase A2

Lysosomal phospholipase A2 (PLA2G15) is a ubiquitous enzyme uniquely characterized by a subcellular localization to the lysosome and late endosome. PLA2G15 has an acidic pH optimum, is calcium independent, and acts as a transacylase in the presence of N-acetyl-sphingosine as an acceptor. Recent studies aided by the delineation of the crystal structure of PLA2G15 have clarified further the catalytic mechanism, sn-1 versus sn-2 specificity, and the basis whereby cationic amphiphilic drugs inhibit its activity. PLA2G15 has recently been shown to hydrolyze short chain oxidized phospholipids which access the catalytic site directly based on their aqueous solubility. Studies on the PLA2G15 null mouse suggest a role for the enzyme in the catabolism of pulmonary surfactant. PLA2G15 may also have a role in host defense and in the processing of lipid antigens for presentation by CD1 proteins. This article is part of a Special Issue entitled Novel functions of phospholipase A2 Guest Editors: Makoto Murakami and Gerard Lambeau.

Emerging concepts of ganglioside metabolism

Gangliosides (GGs) are sialic acid-containing glycosphingolipids (GSLs) and major membrane components enriched on cellular surfaces. Biosynthesis of mammalian GGs starts at the cytosolic leaflet of endoplasmic reticulum (ER) membranes with the formation of their hydrophobic ceramide anchors. After intracellular ceramide transfer to Golgi and trans-Golgi network (TGN) membranes, anabolism of GGs, as well as of other GSLs, is catalyzed by membrane-spanning glycosyltransferases (GTs) along the secretory pathway. Combined activity of only a few promiscuous GTs allows for the formation of cell-type-specific glycolipid patterns. Following an exocytotic vesicle flow to the cellular plasma membranes, GGs can be modified by metabolic reactions at or near the cellular surface. For degradation, GGs are endocytosed to reach late endosomes and lysosomes. Whereas membrane-spanning enzymes of the secretory pathway catalyze GSL and GG formation, a cooperation of soluble glycosidases, lipases and lipid-binding cofactors, namely the sphingolipid activator proteins (SAPs), act as the main players of GG and GSL catabolism at intralysosomal luminal vesicles (ILVs).

The Caenorhabditis elegans Proteome Response to Naturally Associated Microbiome Members of the Genus Ochrobactrum

The nematode Caenorhabditis elegans interacts with a variety of bacteria as it feeds on microbes, and a number of these both associate and persist within the worm's intestine. Host-microbe interactions in C. elegans have been analyzed primarily at the transcriptome level with the host response often been monitored after challenge with pathogens. We assessed the proteome of C. elegans after growth on bacteria capable of colonizing its gut, via a comparative analysis of the nematode exposed to two naturally associated Ochrobactrum spp. (MYb71, MYb237) versus C. elegans grown on Escherichia coli OP50. A total of 4677 C. elegans proteins were identified, 3941 quantified. Significant alterations in protein abundances were observed for 122 proteins, 48 higher and 74 lower in abundance. We observed an increase in abundance of proteins potentially regulated via host signaling pathways, in addition to proteins involved in processing of foreign entities (e.g., lipase, proteases, glutathione metabolism). Decreased in abundance were proteins involved in both degradation and biosynthesis of amino acids, and enzymes associated with the degradation of peptidoglycan (lysozymes). The protein level differences between C. elegans grown on native microbiome members compared to the laboratory food bacterium may help to identify molecular processes involved in host-microbe interactions.

The Saposin-Like Protein AplD Displays Pore-Forming Activity and Participates in Defense Against Bacterial Infection During a Multicellular Stage of Dictyostelium discoideum

Due to their archaic life style and microbivor behavior, amoebae may represent a source of antimicrobial peptides and proteins. The amoebic protozoon Dictyostelium discoideum has been a model organism in cell biology for decades and has recently also been used for research on host-pathogen interactions and the evolution of innate immunity. In the genome of D. discoideum, genes can be identified that potentially allow the synthesis of a variety of antimicrobial proteins. However, at the protein level only very few antimicrobial proteins have been characterized that may interact directly with bacteria and help in fighting infection of D. discoideum with potential pathogens. Here, we focus on a large group of gene products that structurally belong to the saposin-like protein (SAPLIP) family and which members we named provisionally Apls (amoebapore-like peptides) according to their similarity to a comprehensively studied antimicrobial and cytotoxic pore-forming protein of the protozoan parasite Entamoeba histolytica. We focused on AplD because it is the only Apl gene that is reported to be primarily transcribed further during the multicellular stages such as the mobile slug stage. Upon knock-out (KO) of the gene, aplD⁻ slugs became highly vulnerable to virulent Klebsiella pneumoniae. AplD⁻ slugs harbored bacterial clumps in their interior and were unable to slough off the pathogen in their slime sheath. Re-expression of AplD in aplD⁻ slugs rescued the susceptibility toward K. pneumoniae. The purified recombinant protein rAplD formed pores in liposomes and was also capable of permeabilizing the membrane of live Bacillus megaterium. We propose that the multifarious Apl family of D. discoideum comprises antimicrobial effector polypeptides that are instrumental to interact with bacteria and their phospholipid membranes. The variety of its members would allow a complementary and synergistic action against a variety of microbes, which the amoeba encounters in its environment.

Ganglioside Metabolism and Its Inherited Diseases

Gangliosides are sialic acid containing glycosphingolipids, which are abundant in mammalian brain tissue. Several fatal human diseases are caused by defects in glycolipid metabolism. Defects in their degradation lead to an accumulation of metabolites upstream of the defective reactions, whereas defects in their biosynthesis lead to diverse problems in a large number of organs.Gangliosides are primarily positioned with their ceramide anchor in the neuronal plasma membrane and the glycan head group exposed on the cell surface. Their biosynthesis starts in the endoplasmic reticulum with the formation of the ceramide anchor, followed by sequential glycosylation reactions, mainly at the luminal surface of Golgi and TGN membranes, a combinatorial process, which is catalyzed by often promiscuous membrane-bound glycosyltransferases.Thereafter, the gangliosides are transported to the plasma membrane by exocytotic membrane flow. After endocytosis, they are degraded within the endolysosomal compartments by a complex machinery of degrading enzymes, lipid-binding activator proteins, and negatively charged lipids.

How α-Helical Motifs Form Functionally Diverse Lipid-Binding Compartments

Lipids are produced site-specifically in cells and then distributed nonrandomly among membranes via vesicular and nonvesicular trafficking mechanisms. The latter involves soluble amphitropic proteins extracting specific lipids from source membranes to function as molecular solubilizers that envelope their insoluble cargo before transporting it to destination sites. Lipid-binding and lipid transfer structural motifs range from multi-β-strand barrels, to β-sheet cups and baskets covered by α-helical lids, to multi-α-helical bundles and layers. Here, we focus on how α-helical proteins use amphipathic helical layering and bundling to form modular lipid-binding compartments and discuss the functional consequences. Preformed compartments generally rely on intramolecular disulfide bridging to maintain conformation (e.g., albumins, nonspecific lipid transfer proteins, saposins, nematode polyprotein allergens/antigens). Insights into nonpreformed hydrophobic compartments that expand and adapt to accommodate a lipid occupant are few and provided mostly by the three-layer, α-helical ligand-binding domain of nuclear receptors. The simple but elegant and nearly ubiquitous two-layer, α-helical glycolipid transfer protein (GLTP)-fold now further advances understanding.

The proneurotrophin receptor sortilin is required for Mycobacterium tuberculosis control by macrophages

Sorting of luminal and membrane proteins into phagosomes is critical for the immune function of this organelle. However, little is known about the mechanisms that contribute to the spatiotemporal regulation of this process. Here, we investigated the role of the proneurotrophin receptor sortilin during phagosome maturation and mycobacterial killing. We show that this receptor is acquired by mycobacteria-containing phagosomes via interactions with the adaptor proteins AP-1 and GGAs. Interestingly, the phagosomal association of sortilin is critical for the delivery of acid sphingomyelinase (ASMase) and required for efficient phagosome maturation. Macrophages from Sort1(-/-) mice are less efficient in restricting the growth of Mycobacterium bovis BCG and M. tuberculosis. In vivo, Sort1(-/-) mice showed a substantial increase in cellular infiltration of neutrophils in their lungs and higher bacterial burden after infection with M. tuberculosis. Altogether, sortilin defines a pathway required for optimal intracellular mycobacteria control and lung inflammation in vivo.

Neuronal sphingolipidoses: Membrane lipids and sphingolipid activator proteins regulate lysosomal sphingolipid catabolism

Glycosphingolipids and sphingolipids of cellular plasma membranes (PMs) reach luminal intra-lysosomal vesicles (LVs) for degradation mainly by pathways of endocytosis. After a sorting and maturation process (e.g. degradation of sphingomyelin (SM) and secretion of cholesterol), sphingolipids of the LVs are digested by soluble enzymes with the help of activator (lipid binding and transfer) proteins. Inherited defects of lipid-cleaving enzymes and lipid binding and transfer proteins cause manifold and fatal, often neurodegenerative diseases. The review summarizes recent findings on the regulation of sphingolipid catabolism and cholesterol secretion from the endosomal compartment by lipid modifiers, an essential stimulation by anionic membrane lipids and an inhibition of crucial steps by cholesterol and SM. Reconstitution experiments in the presence of all proteins needed, hydrolase and activator proteins, reveal an up to 10-fold increase of ganglioside catabolism just by the incorporation of anionic lipids into the ganglioside carrying membranes, whereas an additional incorporation of cholesterol inhibits GM2 catabolism substantially. It is suggested that lipid and other low molecular modifiers affect the genotype-phenotype relationship observed in patients with lysosomal diseases.

Sphingolipid transfer proteins defined by the GLTP-fold

Glycolipid transfer proteins (GLTPs) originally were identified as small (~24 kDa), soluble, amphitropic proteins that specifically accelerate the intermembrane transfer of glycolipids. GLTPs and related homologs now are known to adopt a unique, helically dominated, two-layer 'sandwich' architecture defined as the GLTP-fold that provides the structural underpinning for the eukaryotic GLTP superfamily. Recent advances now provide exquisite insights into structural features responsible for lipid headgroup selectivity as well as the adaptability of the hydrophobic compartment for accommodating hydrocarbon chains of differing length and unsaturation. A new understanding of the structural versatility and evolutionary premium placed on the GLTP motif has emerged. Human GLTP-motifs have evolved to function not only as glucosylceramide binding/transferring domains for phosphoinositol 4-phosphate adaptor protein-2 during glycosphingolipid biosynthesis but also as selective binding/transfer proteins for ceramide-1-phosphate. The latter, known as ceramide-1-phosphate transfer protein, recently has been shown to form GLTP-fold while critically regulating Group-IV cytoplasmic phospholipase A2 activity and pro-inflammatory eicosanoid production.

Macrophage defense mechanisms against intracellular bacteria

Macrophages and neutrophils play a decisive role in host responses to intracellular bacteria including the agent of tuberculosis (TB), Mycobacterium tuberculosis as they represent the forefront of innate immune defense against bacterial invaders. At the same time, these phagocytes are also primary targets of intracellular bacteria to be abused as host cells. Their efficacy to contain and eliminate intracellular M. tuberculosis decides whether a patient initially becomes infected or not. However, when the infection becomes chronic or even latent (as in the case of TB) despite development of specific immune activation, phagocytes have also important effector functions. Macrophages have evolved a myriad of defense strategies to combat infection with intracellular bacteria such as M. tuberculosis. These include induction of toxic anti-microbial effectors such as nitric oxide and reactive oxygen intermediates, the stimulation of microbe intoxication mechanisms via acidification or metal accumulation in the phagolysosome, the restriction of the microbe's access to essential nutrients such as iron, fatty acids, or amino acids, the production of anti-microbial peptides and cytokines, along with induction of autophagy and efferocytosis to eliminate the pathogen. On the other hand, M. tuberculosis, as a prime example of a well-adapted facultative intracellular bacterium, has learned during evolution to counter-balance the host's immune defense strategies to secure survival or multiplication within this otherwise hostile environment. This review provides an overview of innate immune defense of macrophages directed against intracellular bacteria with a focus on M. tuberculosis. Gaining more insights and knowledge into this complex network of host-pathogen interaction will identify novel target sites of intervention to successfully clear infection at a time of rapidly emerging multi-resistance of M. tuberculosis against conventional antibiotics.

Mechanistic insights into the lipid interaction of an ancient saposin-like protein

The members of the expanding family of saposin-like proteins (SAPLIPs) have various biological functions in plants, animals, and humans. In addition to a similar protein backbone, these proteins have in common the fact that they interact with lipid membranes. According to their phylogenetic position, it has long been thought that amoeboid protozoans produce archetypes of SAPLIPs and that these are lytic proteins that can perforate membranes of prokaryotic and eukaryotic target cells. Here, we show that an amoebic SAPLIP from Entamoeba invadens does not form lytic pores in membranes but displays several characteristics that are known from human saposins. The protein named invaposin changes the conformation from a closed to an open form in the presence of lipid membranes, acts in a pH-dependent manner, selectively binds anionic lipids, aggregates lipid vesicles of the preferred composition, and dimerizes upon acidification. Our data indicate that the principal features of the lipid-binding saposins evolved long before the appearance of the vertebrate lineage and push the origin of saposins even deeper down the phylogenetic tree to unicellular organisms.

Metabolic and cellular bases of sphingolipidoses

Lysosomes are cellular stomachs. They degrade macromolecules and release their components as nutrients into the cytosol. Digestion of sphingolipids and other membrane lipids occurs at luminal intraendosomal vesicles and IMs (intraendosomal membranes). Sphingolipid and membrane digestion needs catabolic hydrolases with the help of lipid-binding proteins [SAPs (sphingolipid activator proteins)] and anionic lipids such as BMP [bis(monoacylglycero)phosphate]. Inherited defects of hydrolases or SAPs or uptake of cationic amphiphilic drugs cause lipid accumulation, eventually leading to death, especially in inherited sphingolipid storage diseases. IMs are formed during endocytosis and their lipid composition is adjusted for degradation. Their cholesterol content, which stabilizes membranes, decreases and the level of negatively charged BMP, which stimulates sphingolipid degradation, increases. At the level of late endosomes, cholesterol is transported out of the luminal vesicles preferentially by cholesterol-binding proteins, NPC (Niemann-Pick type C)-2 and NPC-1. Their defects lead to an endolysosomal accumulation of cholesterol and sphingolipids in Niemann-Pick type C disease. BMP and ceramide stimulate NPC-2-mediated cholesterol transfer, whereas sphingomyelin inhibits it. Anionic membrane lipids also activate sphingomyelin degradation by ASM (acid sphingomyelinase), facilitating cholesterol export by NPC-2. ASM is a non-specific phospholipase C and degrades more than 23 phospholipids. SAPs are membrane-perturbing proteins which solubilize lipids, facilitating glycolipid digestion by presenting them to soluble catabolic enzymes at acidic pH. High BMP and low cholesterol levels favour lipid extraction and membrane disintegration by saposin A and B. The simultaneous inherited defect of saposins A-D causes a severe membrane and sphingolipid storage disease, also disrupting the water permeability barrier of the skin.

Pore-forming toxins from pathogenic amoebae

Some amoeboid protozoans are facultative or obligate parasites in humans and bear an enormous cytotoxic potential that can result in severe destruction of host tissues and fatal diseases. Pathogenic amoebae produce soluble pore-forming polypeptides that bind to prokaryotic and eukaryotic target cell membranes and generate pores upon insertion and oligomerization. This review summerizes the current knowledge of such small protein toxins from amoebae, compares them with related proteins from other species, focuses on their three-dimensional structures, and gives insights into divergent activation mechanisms. The potential use of pore-forming toxins in biotechnology will be briefly outlined.

The role of sphingolipid metabolism in cutaneous permeability barrier formation

The epidermal permeability barrier of mammalian skin is localized in the stratum corneum. Corneocytes are embedded in an extracellular, highly ordered lipid matrix of hydrophobic lipids consisting of about 50% ceramides, 25% cholesterol and 15% long and very long chain fatty acids. The most important lipids for the epidermal barrier are ceramides. The scaffold of the lipid matrix is built of acylceramides, containing ω-hydroxylated very long chain fatty acids, acylated at the ω-position with linoleic acid. After glucosylation of the acylceramides at Golgi membranes and secretion, the linoleic acid residues are replaced by glutamate residues originating from proteins exposed on the surface of corneocytes. Removal of their glucosyl residues generates a hydrophobic surface on the corneocytes used as a template for the formation of extracellular lipid layers of the water permeability barrier. Misregulation or defects in the formation of extracellular ceramide structures disturb barrier function. Important anabolic steps are the synthesis of ultra long chain fatty acids, their ω-hydroxylation, and formation of ultra long chain ceramides and glucosylceramides. The main probarrier precursor lipids, glucosylceramides and sphingomyelins, are packed in lamellar bodies together with hydrolytic enzymes such as glucosylceramide-β-glucosidase and acid sphingomyelinase and secreted into the intercelullar space between the stratum corneum and stratum granulosum. Inherited defects in the extracellular hydrolytic processing of the probarrier acylglucosylceramides impair epidermal barrier formation and cause fatal diseases: such as prosaposin deficiency resulting in lack of lysosomal lipid binding and transfer proteins, or the symptomatic clinical picture of the "collodion baby" in the absence of glucocerebrosidase. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Loss of function of glucocerebrosidase GBA2 is responsible for motor neuron defects in hereditary spastic paraplegia

Spastic paraplegia 46 refers to a locus mapped to chromosome 9 that accounts for a complicated autosomal-recessive form of hereditary spastic paraplegia (HSP). With next-generation sequencing in three independent families, we identified four different mutations in GBA2 (three truncating variants and one missense variant), which were found to cosegregate with the disease and were absent in controls. GBA2 encodes a microsomal nonlysosomal glucosylceramidase that catalyzes the conversion of glucosylceramide to free glucose and ceramide and the hydrolysis of bile acid 3-O-glucosides. The missense variant was also found at the homozygous state in a simplex subject in whom no residual glucocerebrosidase activity of GBA2 could be evidenced in blood cells, opening the way to a possible measurement of this enzyme activity in clinical practice. The overall phenotype was a complex HSP with mental impairment, cataract, and hypogonadism in males associated with various degrees of corpus callosum and cerebellar atrophy on brain imaging. Antisense morpholino oligonucleotides targeting the zebrafish GBA2 orthologous gene led to abnormal motor behavior and axonal shortening/branching of motoneurons that were rescued by the human wild-type mRNA but not by applying the same mRNA containing the missense mutation. This study highlights the role of ceramide metabolism in HSP pathology.

Structural insights into the dual strategy of recognition by peptidoglycan recognition protein, PGRP-S: structure of the ternary complex of PGRP-S with lipopolysaccharide and stearic acid

Peptidoglycan recognition proteins (PGRPs) are part of the innate immune system. The 19 kDa Short PGRP (PGRP-S) is one of the four mammalian PGRPs. The concentration of PGRP-S in camel (CPGRP-S) has been shown to increase considerably during mastitis. The structure of CPGRP-S consists of four protein molecules designated as A, B, C and D forming stable intermolecular contacts, A–B and C–D. The A–B and C–D interfaces are located on the opposite sides of the same monomer leading to the the formation of a linear chain with alternating A–B and C–D contacts. Two ligand binding sites, one at C–D contact and another at A–B contact have been observed. CPGRP-S binds to the components of bacterial cell wall molecules such as lipopolysaccharide (LPS), lipoteichoic acid (LTA), and peptidoglycan (PGN) from both Gram-positive and Gram-negative bacteria. It also binds to fatty acids including mycolic acid of the Mycobacterium tuberculosis (Mtb). Previous structural studies of binary complexes of CPGRP-S with LPS and stearic acid (SA) have shown that LPS binds to CPGRP-S at C–D contact (Site-1) while SA binds to it at the A–B contact (Site-2). The binding studies using surface plasmon resonance showed that LPS and SA bound to CPGRP-S in the presence of each other. The structure determination of the ternary complex showed that LPS and SA bound to CPGRP-S at Site-1 and Site-2 respectively. LPS formed 13 hydrogen bonds and 159 van der Waals contacts (distances ≤4.2 Å) while SA formed 56 van der Waals contacts. The ELISA test showed that increased levels of productions of pro-inflammatory cytokines TNF-α and IFN-γ due to LPS and SA decreased considerably upon the addition of CPGRP-S.

Ganglioside biochemistry

Gangliosides are sialic acid-containing glycosphingolipids. They occur especially on the cellular surfaces of neuronal cells, where they form a complex pattern, but are also found in many other cell types. The paper provides a general overview on their structures, occurrence, and metabolism. Key functional, biochemical, and pathobiochemical aspects are summarized.

Structure and function of a unique pore-forming protein from a pathogenic acanthamoeba

Human pathogens often produce soluble protein toxins that generate pores inside membranes, resulting in the death of target cells and tissue damage. In pathogenic amoebae, this has been exemplified with amoebapores of the enteric protozoan parasite Entamoeba histolytica. Here we characterize acanthaporin, to our knowledge the first pore-forming toxin to be described from acanthamoebae, which are free-living, bacteria-feeding, unicellular organisms that are opportunistic pathogens of increasing importance and cause severe and often fatal diseases. We isolated acanthaporin from extracts of virulent Acanthamoeba culbertsoni by tracking its pore-forming activity, molecularly cloned the gene of its precursor and recombinantly expressed the mature protein in bacteria. Acanthaporin was cytotoxic for human neuronal cells and exerted antimicrobial activity against a variety of bacterial strains by permeabilizing their membranes. The tertiary structures of acanthaporin's active monomeric form and inactive dimeric form, both solved by NMR spectroscopy, revealed a currently unknown protein fold and a pH-dependent trigger mechanism of activation.

SPP-3, a saposin-like protein of Caenorhabditis elegans, displays antimicrobial and pore-forming activity and is located in the intestine and in one head neuron

Caenopores belong to the saposin-like protein superfamily in Caenorhabditis elegans with 33 putative antimicrobial and pore-forming proteins. In this study, we analysed one selected member of this multifarious protein family, namely SPP-3, in detail, as its coding gene has been described to be inducible after bacterial challenge. The recombinant protein was antimicrobially active against a wide range of gram-negative and gram-positive bacteria and displayed membrane-permeabilizing and pH-dependent pore-forming activity. Promoter activity of the respective gene, spp-3, was localized to the intestine and the head neuron SDQR. While gene silencing had no apparent effect on the number of surviving Escherichia coli bacteria in the intestine, it increased the egg laying significantly. Accordingly, SPP-3 is a protein with antimicrobial activity that is presumably part of the redundant armamentarium of effector proteins in the worm's intestine, may help to protect neurons, and appears to be involved in regulating reproduction.

Distribution and characteristics of ABFs, cecropins, nemapores, and lysozymes in nematodes

Several groups of antimicrobial effector molecules have been identified in nematodes, but most studies have been limited to Caenorhabditis elegans and, to a lesser extent, Ascaris suum. Although these two species are not closely related, they are not representative of overall nematode diversity. This study utilized available sequence information to investigate whether four groups of antimicrobial effectors (defensin-like antibacterial factors [ABFs], cecropins, saposin domain-containing proteins, and lysozymes) are components of an archetypal nematode immune system or more narrowly restricted. Saposin domain-containing proteins (caenopores in C. elegans) and lysozymes were widely distributed and found in most taxa, but likely have digestive as well as defensive functions. ABFs were widely distributed in fewer taxa, suggesting selective loss in some lineages. In contrast, cecropins were identified in only three related species, suggesting acquisition of this effector molecule in their common ancestor.

NKT cell ligand recognition logic: molecular basis for a synaptic duet and transmission of inflammatory effectors

NKT cells that express the semi-invariant TCR are innate-like lymphocytes whose functions are regulated by self and foreign glycolipid ligands presented by the Ag-presenting, MHC class I-like molecule CD1d. Activation of NKT cells in vivo results in rapid release of copious amounts of effector cytokines and chemokines with which they regulate innate and adaptive immune responses to pathogens, certain types of cancers, and self-antigens. The nature of CD1d-restricted ligands, the manner in which they are recognized, and the unique effector functions of NKT cells suggest an immunoregulatory role for this T cell subset. Their ability to respond fast and our ability to steer NKT cell cytokine response to altered lipid ligands make them an important target for vaccine design and immunotherapies against autoimmune diseases. This review summarizes our current understanding of CD1d-restricted ligand recognition by NKT cells and how these innate-like lymphocytes regulate inflammation.

Insights into the membrane interactions of the saposin-like proteins Na-SLP-1 and Ac-SLP-1 from human and dog hookworm

Saposin-like proteins (SAPLIPs) from soil-transmitted helminths play pivotal roles in host-pathogen interactions and have a high potential as targets for vaccination against parasitic diseases. We have identified two non-orthologous SAPLIPs from human and dog hookworm, Na-SLP-1 and Ac-SLP-1, and solved their three-dimensional crystal structures. Both proteins share the property of membrane binding as monitored by liposome co-pelleting assays and monolayer adsorption. Neither SAPLIP displayed any significant haemolytic or bactericidal activity. Based on the structural information, as well as the results from monolayer adsorption, we propose models of membrane interactions for both SAPLIPs. Initial membrane contact of the monomeric Na-SLP-1 is most likely by electrostatic interactions between the membrane surface and a prominent basic surface patch. In case of the dimeric Ac-SLP-1, membrane interactions are most likely initiated by a unique tryptophan residue that has previously been implicated in membrane interactions in other SAPLIPs.

Identification of self-lipids presented by CD1c and CD1d proteins

The CD1 family consists of five proteins that are related to the peptide-presenting MHC class I family. T cells can recognize the presentation of both foreign and self-derived lipids on four CD1 family members. The identities of the self-lipids capable of stimulating autoreactive T cell responses remain elusive or controversial. Here, we employed mass spectrometry to analyze the lipid content of highly purified CD1c and CD1d protein samples. We report the identification of 11 novel self-lipids presented by CD1c and nine by CD1d. Rigorous controls provide strong evidence that the identified lipids were specifically loaded into the lipid-binding site of the CD1 molecules. The diverse but distinct population of lipids identified from each CD1 family member implies each present a different subset of self-lipids, and the enrichment of particular motifs indicates that the lipids that are presented by CD1 family members could be predicted. Finally, our results imply the CD1 system surveys the endoplasmic reticulum, Golgi apparatus, and/or secretory compartments, in addition to its well characterized surveillance of the endocytic and lysosomal compartments.

A mutation in the saposin C domain of the sphingolipid activator protein (Prosaposin) gene causes neurodegenerative disease in mice

Saposins A, B, C, and D are small amphiphatic glycoproteins that are encoded in tandem within a precursor protein (prosaposin, PSAP), and are required for in vivo degradation of sphingolipids. Humans with saposin C deficiency exhibit the clinical presentation of Gaucher-like disease. We generated two types of saposin C mutant mice, one carrying a homozygous missense mutation (C384S) in the saposin C domain of prosaposin (Sap-C(-/-)) and the other carrying the compound heterozygous mutation with a second null Psap allele (Psap(-/C384S)). During early life stages, both Sap-C(-/-) and Psap(-/C384S) mice grew normally; however, they developed progressive motor and behavioral deficits after 3 months of age and the majority of affected mice could scarcely move by about 15 months. They showed no signs of hepatosplenomegaly throughout their lives. No accumulation of glucosylceramide and glucosylsphingosine was detected in the brain or liver of both Sap-C(-/-) and Psap(-/C384S) mice. Neuropathological analyses revealed patterned loss of cerebellar Purkinje cells, widespread axonal spheroids filled with membrane-derived concentric or lamellar electron-dense bodies, and lipofuscin-like deposition in the neurons. Soap-bubble-like inclusion bodies were detected in the trigeminal ganglion cells and the vascular endothelial cells. Compound heterozygous Psap(-/C384S) mice showed qualitatively identical but faster progression of the neurological phenotypes than Sap-C(-/-) mice. These results suggest the in vivo role of saposin C in axonal membrane homeostasis, the disruption of which leads to neurodegeneration in lysosomal storage disease.

A modified lipid composition in Fabry disease leads to an intracellular block of the detergent-resistant membrane-associated dipeptidyl peptidase IV

Fabry disease is an X-linked lysosomal storage disorder that leads to abnormal accumulation of glycosphingolipids due to a deficiency of alpha-galactosidase A (AGAL). The consequences of these alterations on the targeting of membrane proteins are poorly understood. Glycosphingolipids are enriched in Triton-X-100- resistant lipid rafts [detergent-resistant membranes (DRMs)] and play an important role in the transport of several membrane-associated proteins. Here, we show that In fibroblasts of patients suffering from Fabry disease, the colocalization of AGAL with the lysosomal marker LAMP2 is decreased compared with wild-type fibroblasts concomitant with a reduced transport of AGAL to lysosomes. Furthermore, overall composition of membrane lipids in the patients' fibroblasts as well as in DRMs reveals a substantial increase in the concentration of glycolipids and a slight reduction of phosphatidylethanolamine (PE). The altered glycolipid composition in Fabry fibroblasts is associated with an intracellular accumulation and impaired trafficking of the Triton-X-100 DRM-associated membrane glycoprotein dipeptidyl peptidase IV (DPPIV) in transfected Fabry cells, whereas no effect could be observed on the targeting of aminopeptidase N (ApN) that is not associated with this type of DRM. We propose that changes in the lipid composition of cell membranes in Fabry disease disturb the ordered Triton X-100 DRMs and have implications on the trafficking and sorting of DRM-associated proteins and the overall protein-lipid interaction at the cell membrane. Possible consequences could be altered signalling at the cell surface triggered by DRM-associated proteins, with implications on gene regulation and subsequent protein expression.

Golgi-to-phagosome transport of acid sphingomyelinase and prosaposin is mediated by sortilin

Sortilin, also known as neurotensin receptor 3 (NTR3), is a transmembrane protein with a dual function. It acts as a receptor for neuromediators and growth factors at the plasma membrane, but it has also been implicated in binding and transport of some lysosomal proteins. However, the role of sortilin during phagosome maturation has not been investigated before. Here, we show that in macrophages, sortilin is mainly localized in the Golgi and transported to latex-bead phagosomes (LBPs). Using live-cell imaging and electron microscopy, we found that sortilin is delivered to LBPs in a manner that depends on its cytoplasmic tail. We also show that sortilin participates in the direct delivery of acid sphingomyelinase (ASM) and prosaposin (PS) to the phagosome, bypassing fusion with lysosomal compartments. Further analysis confirmed that ASM and PS are targeted to the phagosome by sortilin in a Brefeldin-A-sensitive pathway. Analysis of primary macrophages isolated from Sort1(-/-) mice indicated that the delivery of ASM and PS, but not pro-cathepsin D, to LBPs was severely impaired. We propose a pathway mediated by sortilin by which selected lysosomal proteins are transported to the phagosome along a Golgi-dependent route during the maturation of phagosomes.

Bis(monoacylglycero)phosphate and ganglioside GM1 spontaneously form small homogeneous vesicles at specific concentrations

The morphology and size of hydrated lipid dispersions of bis(monoacylglycero)phosphate (BMP) mixed with varying mole percentages of the ganglioside GM1 were investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Electron paramagnetic resonance (EPR) spectroscopy of these same mixtures, doped at 0.5 mol% with doxyl labeled lipids, was used to investigate acyl-chain packing. Results show that for 20-30% GM1, hydrated BMP:GM1 mixtures spontaneously form small spherical vesicles with diameters approximately 100 nm and a narrow size distribution profile. For other concentrations of GM1, hydrated dispersions with BMP have non-spherical shapes and heterogeneous size profiles, with average vesicle diameters>400 nm. All samples were prepared at pH 5.5 to mimic the lumen acidity of the late endosome where BMP is an essential component of intraendosomal vesicle budding, lipid sorting and trafficking. These findings indicate that GM1 and BMP under a limited concentration range spontaneously form small vesicles of homogeneous size in an energy independent manner without the need of protein templating. Because BMP is essential for intraendosomal vesicle formation, these results imply that lipid-lipid interactions may play a critical role in the endosomal process of lipid sorting and trafficking.

Caenopore-5: the three-dimensional structure of an antimicrobial protein from Caenorhabditis elegans

The caenopore-5 protein encoded by the spp-5 gene is one of the 33 caenopores identified in Caenorhabditis elegans and is a pore-forming peptide which plays an important role in the elimination of Escherichia coli ingested by the worm. Thus, caenopore-5 appears to contribute to the nutrition of the worm while simultaneously protecting the organism against pathogens. Here, three-dimensional heteronuclear NMR spectroscopy was used to solve the solution structure of caenopore-5. The NMR data revealed that two conformers of caenopore-5 exist in solution which differ by the isomerization of the peptide bond of Pro-81. The overall structure of the two caenopore-5 conformers consists of five amphiphatic helices connected by three disulfide bonds. The five helices are arranged in a folded leaf which is the characteristic signature of the SAPLIP family. The structure presented here is the first of an effector protein of the defensive system elucidated for the well-known model organism C. elegans.

Structural determination and tryptophan fluorescence of heterokaryon incompatibility C2 protein (HET-C2), a fungal glycolipid transfer protein (GLTP), provide novel insights into glycolipid specificity and membrane interaction by the GLTP fold

HET-C2 is a fungal protein that transfers glycosphingolipids between membranes and has limited sequence homology with human glycolipid transfer protein (GLTP). The human GLTP fold is unique among lipid binding/transfer proteins, defining the GLTP superfamily. Herein, GLTP fold formation by HET-C2, its glycolipid transfer specificity, and the functional role(s) of its two Trp residues have been investigated. X-ray diffraction (1.9 A) revealed a GLTP fold with all key sugar headgroup recognition residues (Asp(66), Asn(70), Lys(73), Trp(109), and His(147)) conserved and properly oriented for glycolipid binding. Far-UV CD showed secondary structure dominated by alpha-helices and a cooperative thermal unfolding transition of 49 degrees C, features consistent with a GLTP fold. Environmentally induced optical activity of Trp/Tyr/Phe (2:4:12) detected by near-UV CD was unaffected by membranes containing glycolipid but was slightly altered by membranes lacking glycolipid. Trp fluorescence was maximal at approximately 355 nm and accessible to aqueous quenchers, indicating free exposure to the aqueous milieu and consistent with surface localization of the two Trps. Interaction with membranes lacking glycolipid triggered significant decreases in Trp emission intensity but lesser than decreases induced by membranes containing glycolipid. Binding of glycolipid (confirmed by electrospray injection mass spectrometry) resulted in a blue-shifted emission wavelength maximum (approximately 6 nm) permitting determination of binding affinities. The unique positioning of Trp(208) at the HET-C2 C terminus revealed membrane-induced conformational changes that precede glycolipid uptake, whereas key differences in residues of the sugar headgroup recognition center accounted for altered glycolipid specificity and suggested evolutionary adaptation for the simpler glycosphingolipid compositions of filamentous fungi.

Biochemical basis of Fabry disease with emphasis on mitochondrial function and protein trafficking

Fabry disease, also known as Anderson-Fabry disease, is an X-linked lysosomal storage disorder. The clinical picture is highly variable and usually milder in females. It is a multisystemic disease involving many organs. Fabry disease is due to a deficiency of alpha-galactosidase A caused by different usually "private" mutations. Enzyme replacement therapy (ERT) has been established, other therapeutic options are at an experimental stage. Classically, mechanical deposition of storage material in blood vessels was believed to lead to decreased blood supply with consecutive organ dysfunction. Recently, however, many secondary biochemical processes have been discussed to be involved in the pathogenesis of Fabry disease. For example, compromised energy metabolism has been found both in vitro and in vivo, altered lipid composition of membranes can lead to abnormalities in trafficking and sorting of rafts-associated proteins. We discuss the role of these secondary phenomena in the pathogenesis of Fabry disease.

Screening for the drug-phospholipid interaction: correlation to phospholipidosis

Phospholipid bilayers represent a complex, anisotropic environment fundamentally different from bulk oil or octanol, for instance. Even "simple" drug association to phospholipid bilayers can only be fully understood if the slab-of-hydrocarbon approach is abandoned and the complex, anisotropic properties of lipid bilayers reflecting the chemical structures and organization of the constituent phospholipids are considered. The interactions of drugs with phospholipids are important in various processes, such as drug absorption, tissue distribution, and subcellular distribution. In addition, drug-lipid interactions may lead to changes in lipid-dependent protein activities, and further, to functional and morphological changes in cells, a prominent example being the phospholipidosis (PLD) induced by cationic amphiphilic drugs. Herein we briefly review drug-lipid interactions in general and the significance of these interactions in PLD in particular. We also focus on a potential causal connection between drug-induced PLD and steatohepatitis, which is induced by some cationic amphiphilic drugs.

Lysosomal degradation of membrane lipids

The constitutive degradation of membrane components takes place in the acidic compartments of a cell, the endosomes and lysosomes. Sites of lipid degradation are intralysosomal membranes that are formed in endosomes, where the lipid composition is adjusted for degradation. Cholesterol is sorted out of the inner membranes, their content in bis(monoacylglycero)phosphate increases, and, most likely, sphingomyelin is degraded to ceramide. Together with endosomal and lysosomal lipid-binding proteins, the Niemann-Pick disease, type C2-protein, the GM2-activator, and the saposins sap-A, -B, -C, and -D, a suitable membrane lipid composition is required for degradation of complex lipids by hydrolytic enzymes.

Ligand extraction properties of the GM2 activator protein and its interactions with lipid vesicles

The GM2 activator protein (GM2AP) is an accessory protein required for the enzymatic conversion of GM2 to GM3 by hydrolases in the lysosomal compartments of cells. Here, GM2AP interactions with lipid vesicles are investigated by sucrose-loaded vesicle sedimentation and gel filtration assays, and the effects of pH and lipid composition on membrane binding and lipid extraction are characterized. The sedimentation experiments allow for facile quantification of the percentage of protein in solution and on the bilayer surface, with detailed analysis of the protein:lipid complex that remains in solution. Optimum binding and ligand extraction is found for pH 4.8 where <15% of the protein remains surface associated regardless of the lipid composition. In addition to extracting GM2, we find that GM2AP readily extracts dansyl-headgroup-labeled lipids as well as other phospholipids from vesicles. The ability of GM2AP to extract dansyl-DHPE from vesicles is altered by pH and the specific ligand GM2. Although the unique endosomal lipid, bis(monoacylglycero)phosphate, is not required for ligand extraction, it does enhance the extraction efficiency of GM2 when cholesterol is present in the vesicles.

The human antimicrobial protein psoriasin acts by permeabilization of bacterial membranes

Psoriasin, a member of the S100 family of calcium-binding proteins (S100A7) is highly upregulated in the skin of psoriasis patients. As it has recently been found to exhibit antimicrobial activity, an important role of psoriasin in surface defence has been suggested. The similarity of the three-dimensional structures of psoriasin and amoebapore A, an ancient antimicrobial, pore-forming peptide from Entamoeba histolytica, intrigued us to investigate whether the human psoriasin is also able to permeabilize bacterial membranes. Here, we demonstrate that psoriasin exerts pore-forming activity at pH values below 6 demonstrating that disruption of microbial membranes is the basis of its antimicrobial activity at low pH. Furthermore, the killing activity of psoriasin shows pH-dependent target specificity. At neutral pH, the Gram-negative bacterium E. coli is killed apparently without compromising its membrane, whereas at low pH exclusively the Gram-positive bacterium B. megaterium is killed by permeabilization of its cytoplasmic membrane.

Principles of lysosomal membrane degradation: Cellular topology and biochemistry of lysosomal lipid degradation

Cellular membranes enter the lysosomal compartment by endocytosis, phagocytosis, or autophagy. Within the lysosomal compartment, membrane components of complex structure are degraded into their building blocks. These are able to leave the lysosome and can then be utilized for the resynthesis of complex molecules or can be further degraded. Constitutive degradation of membranes occurs on the surface of intra-endosomal and intra-lysosomal membrane structures. Many integral membrane proteins are sorted to the inner membranes of endosomes and lysosome after ubiquitinylation. In the lysosome, proteins are degraded by proteolytic enzymes, the cathepsins. Phospholipids originating from lipoproteins or cellular membranes are degraded by phospholipases. Water-soluble glycosidases sequentially cleave off the terminal carbohydrate residues of glycoproteins, glycosaminoglycans, and glycosphingolipids. For glycosphingolipids with short oligosaccharide chains, the additional presence of membrane-active lysosomal lipid-binding proteins is required. The presence of lipid-binding proteins overcomes the phase problem of water soluble enzymes and lipid substrates by transferring the substrate to the degrading enzyme or by solubilizing the internal membranes. The lipid composition of intra-lysosomal vesicles differs from that of the plasma membrane. To allow at least glycosphingolipid degradation by hydrolases and activator proteins, the cholesterol content of these intraorganellar membranes decreases during endocytosis and the concentration of bis(monoacylglycero)phosphate, a stimulator of sphingolipid degradation, increases. A considerable part of our current knowledge about mechanism and biochemistry of lysosomal lipid degradation is derived from a class of human diseases, the sphingolipidoses, which are caused by inherited defects within sphingolipid and glycosphingolipid catabolism.

Disparate proteins use similar architectures to damage membranes

Membrane disruption can efficiently alter cellular function; indeed, pore-forming toxins (PFTs) are well known as important bacterial virulence factors. However, recent data have revealed that structures similar to those found in PFTs are found in membrane active proteins across disparate phyla. Many similarities can be identified only at the 3D-structural level. Of note, domains found in membrane-attack complex proteins of complement and perforin (MACPF) resemble cholesterol-dependent cytolysins from Gram-positive bacteria, and the Bcl family of apoptosis regulators share similar architectures with Escherichia coli pore-forming colicins. These and other correlations provide considerable help in understanding the structural requirements for membrane binding and pore formation.

A dansyl fluorescence-based assay for monitoring kinetics of lipid extraction and transfer

Lipid transfer proteins have important roles in cellular biology, and fluorescence spectroscopy has found wide range use as a facile means for time-resolved monitoring of protein-lipid interactions. Here, we show how the fluorescence emission properties of dansyl-DHPE can be exploited to characterize lipid extraction and lipid transfer kinetics. The GM2 activator protein serves as an example of a lipid transfer protein where the ability to independently characterize lipid extraction from donor vesicles, formation of a protein:lipid complex in solution, and release of lipid from the complex to acceptor liposomes is crucial for full kinetic characterization of lipid transfer.

Synthetic polypeptide adsorption to Cu-IDA containing lipid films: a model for protein-membrane interactions

Adsorption of synthetic alanine-rich peptides to lipid monolayers was studied by X-ray and neutron reflectivity, grazing incidence X-ray diffraction (GIXD), and circular dichroic spectroscopy. The peptides contained histidine residues to drive adsorption to Langmuir monolayers of lipids with iminodiacetate headgroups loaded with Cu2+. Adsorption was found to be irreversible with respect to bulk peptide concentration. The peptides were partially helical in solution at room temperature, the temperature of the adsorption assays. Comparisons of the rate of binding and the structure of the adsorbed layer were made as a function of the number of histidines (from 0 to 2) and also as a function of the positioning of the histidines along the backbone. For peptides containing two histidines on the same side of the helical backbone, large differences were observed in the structure of the adsorbed layer as a function of the spacing of the histidines. With a spacing of 6 A, there was a substantial increase in helicity upon binding (from 17% to 31%), and the peptides adsorbed to a final density approaching that of a nearly completed monolayer of alpha-helices adsorbed side-on. The thickness of the adsorbed layer (17 +/- 2.5 A) was slightly greater than the diameter of alpha-helices, suggesting that the free, unstructured ends extended into solution. With a spacing of 30 A between histidines, a far weaker increase in helicity upon binding was observed (from 13% to 19%) and a much lower packing density resulted. The thickness of the adsorbed layer (10 +/- 4 A) was smaller, consistent with the ends being bound to the monolayer. Striking differences were observed in the interaction of the two types of peptide with the lipid membrane by GIXD, consistent with binding by two correlated sites only for the case of 6 A spacing. All these results are attributed to differences in spatial correlation between the histidines as a function of separation distance along the backbone for these partially helical peptides. Finally, control over orientation was demonstrated by placing a histidine on an end of the sequence, which resulted in adsorbed peptides oriented perpendicular to the membrane.