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WDR91 and SORF1, members of the WD repeat-containing protein 91 family, control phosphoinositide conversion by inhibiting phosphatidylinositol 3-kinase activity on endosomes, which promotes endosome maturation. Here, we report the crystal structure of the human WDR91 WD40 domain complexed with Rab7 that has an unusual interface at the C-terminus of the Rab7 switch II region. WDR91 is highly selective for Rab7 among the tested GTPases. A LIS1 homology (LisH) motif within the WDR91 N-terminal domain (NTD) mediates self-association and may contribute partly to the augmented interaction between full-length WDR91 and Rab7. Both the Rab7 binding site and the LisH motif are indispensable for WDR91 function in endocytic trafficking. For the WDR91 orthologue SORF1 lacking the C-terminal WD40 domain, a C-terminal amphipathic helix (AH) mediates strong interactions with liposomes containing acidic lipids. During evolution the human WDR91 ancestor gene might have acquired a WD40 domain to replace the AH for endosomal membrane targeting. [Display omitted] • The crystal structure of the human WDR91 WD40 domain with bound Rab7-GTP was determined • WDR91 N-terminal domain (NTD) contains a LisH self-association motif • LisH contributes to the augmented interaction between full-length WDR91 and Rab7 • The C. elegans WDR91 orthologue SORF1 uses an amphipathic helix for lipid binding Proteins from the WDR91 family control phosphoinositide conversion required for endosome maturation. By combining structural, biochemical, and cellular data, Ma et al. provide evidence that two representative members of this family WDR91 and SORF1 either use the Rab7-binding WD40 domain or an amphipathic helix for membrane targeting. [ABSTRACT FROM AUTHOR]

• Ameloblastin (Ambn) is multifunctional in its facilitation of enamel formation. • Human amelogenesis imperfecta cases and animal models emphasize importance of Ambn. • Its functions include ameloblast cell signaling, polarization, and matrix adhesion. • A conserved amphipathic helix motif mediates direct Ambn-cell membrane interaction. • The motif is part of a multitargeting domain mediating multiple Ambn interactions. • These various interactions could enable Ambn's cell polarization and adhesion roles. Extracellular matrix proteins play crucial roles in the formation of mineralized tissues like bone and teeth via multifunctional mechanisms. In tooth enamel, ameloblastin (Ambn) is one such multifunctional extracellular matrix protein implicated in cell signaling and polarity, cell adhesion to the developing enamel matrix, and stabilization of prismatic enamel morphology. To provide a perspective for Ambn structure and function, we begin this review by describing dental enamel and enamel formation (amelogenesis) followed by a description of enamel extracellular matrix. We then summarize the established domains and motifs in Ambn protein, human amelogenesis imperfecta cases, and genetically engineered mouse models involving mutated or null Ambn. We subsequently delineate in silico, in vitro , and in vivo evidence for the amphipathic helix in Ambn as a proposed cell-matrix adhesive and then more recent in vitro evidence for the multitargeting domain as the basis for dynamic interactions of Ambn with itself, amelogenin, and membranes. The multitargeting domain facilitates tuning between Ambn-membrane interactions and self/co-assembly and supports a likely overall role for Ambn as a matricellular protein. We anticipate that this review will enhance the understanding of multifunctional matrix proteins by consolidating diverse mechanisms through which Ambn contributes to enamel extracellular matrix mineralization. [ABSTRACT FROM AUTHOR]

Lipid droplets (LDs) are the main organelles for lipid storage, and their surfaces contain unique proteins with diverse functions, including those that facilitate the deposition and mobilization of LD lipids. Among organelles, LDs have an unusual structure with an organic, hydrophobic oil phase covered by a phospholipid monolayer. The unique properties of LD monolayer surfaces require proteins to localize to LDs by distinct mechanisms. Here we review the two pathways known to mediate direct LD protein localization: the CYTOLD pathway mediates protein targeting from the cy tosol to LD s, and the ERTOLD pathway functions in protein targeting from the e ndoplasmic r eticulum to LD s. We describe the emerging principles for each targeting pathway in animal cells and highlight open questions in the field. Lipid droplet (LD) organelles contain specific proteins, including metabolic enzymes, that differ among cell types. Proteins targeting the monolayer surface of lipid droplets do not follow the same principles as proteins targeting bilayer membranes. The targeting of proteins to lipid droplet surfaces is governed by the biophysical features of their unique oil–water interface. LD protein targeting occurs via two major pathways: CYTOLD and ERTOLD targeting. Elucidating the principles driving these protein targeting pathways has implications for further understanding physiology and metabolic diseases. [ABSTRACT FROM AUTHOR]

Highlights • Autophagy involves complicated membrane dynamics that are regulated by Atg proteins. • Most Atg proteins lack transmembrane helices and bind membranes using membrane-binding domains and motifs. • Atg1 complex recruits Atg9 vesicles as an initial membrane source through protein-protein interaction. • Autophagy-specific phosphatidylinositol 3-kinase complex binds membranes using amphipathic helices and an aromatic loop. • Atg2-Atg18 and Atg20-Atg24 complexes target to membranes via binding to phosphatidylinositol 3-phosphate. • Atg conjugation system binds membranes via Atg3 and Atg5, and covalently conjugates Atg8 to phosphatidylethanolamine. Abstract Autophagy is an intracellular degradation system conserved among eukaryotes that mediates the degradation of various biomolecules and organelles. During autophagy, a double membrane-bound organelle termed an autophagosome is synthesized de novo and delivers targets from the cytoplasm to the lysosomes for degradation. Autophagosome formation involves complex and dynamic membrane rearrangements, which are regulated by dozens of autophagy-related (Atg) proteins. In this review, we summarize our current knowledge of membrane-binding domains and motifs in Atg proteins and discuss their roles in autophagy. [ABSTRACT FROM AUTHOR]

Abstract Membrane curvature remodeling induced by amphipathic helices (AHs) is essential in many biological processes. Here we studied a model amphipathic peptide, M2AH, derived from influenza A M2. We are interested in how M2AH may promote membrane curvature by altering membrane physical properties. We used atomic force microscopy (AFM) to examine changes in membrane topographic and mechanical properties. We used electron paramagnetic resonance (EPR) spectroscopy to explore changes in lipid chain mobility and chain orientational order. We found that M2AH perturbed lipid bilayers by generating nanoscale pits. The structural data are consistent with lateral expansion of lipid chain packing, resulting in a mechanically weaker bilayer. Our EPR spectroscopy showed that M2AH reduced lipid chain mobility and had a minimal effect on lipid chain orientational order. The EPR data are consistent with the surface-bound state of M2AH that acts as a chain mobility inhibitor. By comparing results from different lipid bilayers, we found that cholesterol enhanced the activity of M2AH in inducing bilayer pits and altering lipid chain mobility. The results were explained by considering specific M2AH-cholesterol recognition and/or cholesterol-induced expansion of interlipid distance. Both AFM and EPR experiments revealed a modest effect of anionic lipids. This highlights that membrane interaction of M2AH is mainly driven by hydrophobic forces. Lastly, we found that phosphatidylethanolamine (PE) lipids inhibited the activity of M2AH. We explained our data by considering interlipid hydrogen-bonding that can stabilize bilayer organization. Our results of lipid-dependent membrane modulations are likely relevant to M2AH-induced membrane restructuring. Graphical abstract Unlabelled Image Highlights • Amphipathic helices are important membrane curvature modulators. • M2AH perturbs lipid bilayer by forming nanoscale pits. • M2AH alters lipid chain mobility and chain orientational order. • Lipid species determine membrane modulations caused by M2AH. [ABSTRACT FROM AUTHOR]

Annexins are a multigene family of proteins involved in aggregation and fusion processes of biological membranes. One of its best-known members is annexin A2 (or p36), capable of binding to acidic phospholipids in a calcium-dependent manner, as occurs with other members of the same family. In its heterotetrameric form, especially with protein S100A10 (p11), annexin A2 has been involved as a determinant factor in innumerable biological processes like tumor development or anticoagulation. However, the subcellular coexistence of different pools of the protein, in which the monomeric form of annexin A2 is growing in functional relevance, is to date poorly described. In this work we present an exhaustive structural and functional characterization of monomeric human annexin A2 by using different recombinant mutants. The important role of the amphipathic N-terminal α-helix in membrane binding and aggregation has been analyzed. We have also studied the potential implication of lateral “antiparallel” protein dimers in membrane aggregation. In contrast to what was previously suggested, formation of these dimers negatively regulate aggregation. We have also confirmed the essential role of three lysine residues located in the convex surface of the molecule in calcium-free and calcium-dependent membrane binding and aggregation. Finally, we propose models for annexin A2-mediated vesicle aggregation mechanisms. [ABSTRACT FROM AUTHOR]

Bacterial phospholipid N -methyltransferases (Pmts) catalyze the formation of phosphatidylcholine (PC) via successive N -methylation of phosphatidylethanolamine (PE). They are classified into Sinorhizobium -type and Rhodobacter -type enzymes. The Sinorhizobium -type PmtA protein from the plant pathogen Agrobacterium tumefaciens is recruited to anionic lipids in the cytoplasmic membrane via two amphipathic helices called αA and αF. Besides its enzymatic activity, PmtA is able to remodel membranes mediated by the αA domain. According to the Heliquest program, αA- and αF-like amphipathic helices are also present in other Sinorhizobium - and Rhodobacter -type Pmt enzymes suggesting a conserved architecture of α-helical membrane-binding regions in these methyltransferases. As representatives of the two Pmt families, we investigated the membrane binding and remodeling capacity of Bradyrhizobium japonicum PmtA ( Sinorhizobium -type) and PmtX1 ( Rhodobacter -type), which act cooperatively to produce PC in consecutive methylation steps. We found that the αA regions in both enzymes bind anionic lipids similar to αA of A. tumefaciens PmtA. Membrane binding of PmtX1 αA is enhanced by its substrate monomethyl-PE indicating a substrate-controlled membrane association. The αA regions of all investigated enzymes remodel spherical liposomes into tubular filaments suggesting a conserved membrane-remodeling capacity of bacterial Pmts. Based on these results we propose that the molecular details of membrane-binding and remodeling are conserved among bacterial Pmts. [ABSTRACT FROM AUTHOR]

Apolipoprotein B (apoB) is a large amphipathic protein that is the structural scaffold for the formation of several classes of lipoproteins involved in lipid transport throughout the body. The goal of the present study was to identify specific domains in the apoB sequence that contribute to its lipid binding properties. A sequence analysis algorithm was developed to identify stretches of hydrophobic amino acids devoid of charged amino acids, which are referred to as hydrophobic cluster domains (HCDs). This analysis identified 78 HCDs in apoB with hydrophobic stretches ranging from 6 to 26 residues. Each HCD was analyzed in silico for secondary structure and lipid binding properties, and a subset was synthesized for experimental evaluation. One HCD peptide, B38, showed high affinity binding to both isolated HDL and LDL, and could exchange between lipoproteins. All-atom molecular dynamics simulations indicate that B38 inserts 3.7 Å below the phosphate plane of the bilayer. B38 forms an unusual α-helix with a broad hydrophobic face and polar serine and threonine residues on the opposite face. Based on this structure, we hypothesized that B38 could efflux cholesterol from cells. B38 showed a 12-fold greater activity than the 5A peptide, a bihelical Class A amphipathic helix (EC 50 of 0.2658 vs. 3.188 μM; p < 0.0001), in promoting cholesterol efflux from ABCA1 expressing BHK-1 cells. In conclusion, we have identified novel domains within apoB that contribute to its lipid biding properties. Additionally, we have discovered a unique amphipathic helix design for efficient ABCA1-specific cholesterol efflux. [ABSTRACT FROM AUTHOR]

An evolutionarily conserved feature of cellular organelles is the distinct phospholipid composition of their bounding membranes, which is essential to their identity and function. Within eukaryotic cells, two major lipid territories can be discerned, one centered on the endoplasmic reticulum and characterized by membranes with lipid packing defects, the other comprising plasma-membrane-derived organelles and characterized by membrane charge. We discuss how this cellular lipid organization is maintained, how lipid flux is regulated, and how perturbations in cellular lipid homeostasis can lead to disease. [ABSTRACT FROM AUTHOR]

Our studies presented in this report focus on the behavior of NA-CATH, an α-helical cathelicidin antimicrobial peptide, originally discovered in the Naja atra snake. It has demonstrated high potency against gram-positive and gram-negative bacteria with minimal hemolysis. Here we examine the kinetics, behaviors and potential mechanisms of the peptide in the presence of membrane liposome, modeling Escherichia coli , whose membrane exhibits distinct lipid phases. To understand NA-CATH interactions, the role of lipid phases is critical. We test three different lipid compositions to detangle the effect of phase on NA-CATH's activity using a series of leakage experiments. From these studies, we observe that NA-CATH changes from membrane disruption to pore-based lysing, depending on phases and lipid composition. This behavior also plays a major role in its kinetics. [ABSTRACT FROM AUTHOR]

The amphipathic helical (AH) membrane binding motif is recognized as a major device for lipid compositional sensing [1] . We explore the function and mechanism of sensing by the lipid biosynthetic enzyme, CTP:phosphocholine cytidylyltransferase (CCT). As the regulatory enzyme in phosphatidylcholine (PC) synthesis, CCT contributes to membrane PC homeostasis. CCT directly binds and inserts into the surface of bilayers that are deficient in PC and therefore enriched in lipids that enhance surface charge and/or create lipid packing voids. These two membrane physical properties induce the folding of the CCT M domain into a ≥ 60 residue AH. Membrane binding activates catalysis by a mechanism that has been partially deciphered. We review the evidence for CCT compositional sensing, and the membrane and protein determinants for lipid selective membrane-interactions. We consider the factors that promote the binding of CCT isoforms to the membranes of the ER, nuclear envelope, or lipid droplets, but exclude CCT from other organelles and the plasma membrane. The CCT sensing mechanism is compared with several other proteins that use an AH motif for membrane compositional sensing. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon. [ABSTRACT FROM AUTHOR]

Background Hemocyanins are respiratory proteins with multiple functions. In diverse crustaceans hemocyanins can release histidine-rich antimicrobial peptides in response to microbial challenge. In penaeid shrimp, strictly antifungal peptides are released from the C-terminus of hemocyanins. Methods The three-dimensional structure of the antifungal peptide PvHCt from Litopenaeus vannamei was determined by NMR. Its mechanism of action against the shrimp pathogen Fusarium oxysporum was investigated using immunochemistry, fluorescence and transmission electron microscopy. Results PvHCt folded into an amphipathic α-helix in membrane-mimicking media and displayed a random conformation in aqueous environment. In contact with F. oxysporum , PvHCt bound massively to the surface of fungal hyphae without being imported into the cytoplasm. At minimal inhibitory concentrations, PvHCt made the fungal membrane permeable to SYTOX-green and fluorescent dextran beads of 4 kDa. Higher size beads could not enter the cytoplasm. Therefore, PvHCt likely creates local damages to the fungal membrane. While the fungal cell wall appeared preserved, gradual degeneration of the cytoplasm most often resulting in cell lysis was observed in fungal spores and hyphae. In the remaining fungal cells, PvHCt induced a protective response by the formation of daughter hyphae. Conclusion The massive accumulation of PvHCt at the surface of fungal hyphae and subsequent insertion into the plasma membrane disrupt its integrity as a permeability barrier, leading to disruption of internal homeostasis and fungal death. General significance The histidine-rich antimicrobial peptide PvHCt derived from shrimp hemocyanin is a strictly antifungal peptide, which adopts an amphipathic α-helical structure, and selectively binds to and permeabilizes fungal cells. [ABSTRACT FROM AUTHOR]

Naja atra cathelicidin (NA-CATH) is a 34-amino acid highly cationic peptide identified in Chinese cobras to possess potent toxicity against gram-negative and gram-positive bacteria and low toxicity against host cells. Here, we report the NMR solution structure of the full-length NA-CATH peptide and its interaction with liposomes. The structure shows a well-defined α-helix between residues Phe3 to Lys23, on which one surface is lined by the side-chains of one arginine and 11 lysine residues, while the other side is populated by hydrophobic residues. The last eleven amino acids, which are predominately aromatic and hydrophobic in nature, have no defined structure. NMR data reveal that these residues do not interact with the hydrophobic residues of the helix, indicating that the C-terminal residues have random conformations. Fluorescence requenching experiments, in which liposomes serve as a mimic of the bacterial membranes, result in fluorophore leakage that is consistent with a membrane thinning or transient pore formation mechanism. NMR titration studies of the peptide–liposome interaction reveal that the peptide is in fast exchange with the liposome, consistent with the fluorescent studies. These data indicate that full length NA-CATH possesses a helical segment and unstructured C-terminal tail that disrupts the bilayer to induce leakage and lysing. [ABSTRACT FROM AUTHOR]

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα. [ABSTRACT FROM AUTHOR]

Dengue virus (DENV) infection is a growing public health threat with more than one-third of the world's population at risk. Non-structural protein 4A (NS4A), one of the least characterized viral proteins, is a highly hydrophobic transmembrane protein thought to induce the membrane alterations that harbor the viral replication complex. The NS4A N-terminal (amino acids 1–48), has been proposed to contain an amphipathic α-helix (AH). Mutations (L6E; M10E) designed to reduce the amphipathic character of the predicted AH, abolished viral replication and reduced NS4A oligomerization. Nuclear magnetic resonance (NMR) spectroscopy was used to characterize the N-terminal cytoplasmic region (amino acids 1–48) of both wild type and mutant NS4A in the presence of SDS micelles. Binding of the two N-terminal NS4A peptides to liposomes was studied as a function of membrane curvature and lipid composition. The NS4A N-terminal was found to contain two AHs separated by a non-helical linker. The above mentioned mutations did not significantly affect the helical secondary structure of this domain. However, they reduced the affinity of the N-terminal NS4A domain for lipid membranes. Binding of wild type NS4A(1–48) to liposomes is highly dependent on membrane curvature. [ABSTRACT FROM AUTHOR]

Over the last decade, compelling evidence has linked the development of Alzheimer's disease (AD) to defective intracellular trafficking of the amyloid precursor protein (APP). Faulty APP trafficking results in an overproduction of Aβ peptides, which is generally agreed to be the primary cause of AD-related pathogenesis. LR11 (SorLA), a type I transmembrane sorting receptor, has emerged as a key regulator of APP trafficking and processing. It directly interacts with APP and diverts it away from amyloidogenic processing. The 54-residue cytosolic domain of LR11 is essential for its proper intracellular localization and trafficking which, in turn, determines the fate of APP. Here, we have found a surprising membrane-proximal amphipathic helix in the cytosolic domain of LR11. Moreover, a peptide corresponding to this region folds into an α-helical structure in the presence of liposomes and transforms liposomes to small vesicles and tubule-like particles. We postulate that this amphipathic helix may contribute to the dynamic remodeling of membrane structure and facilitate LR11 intracellular transport. This article is part of a Special Issue entitled: NMR Spectroscopy for Atomistic Views of Biomembranes and Cell Surfaces. Guest Editors: Lynette Cegelski and David P.Weliky. [ABSTRACT FROM AUTHOR]

Amphiphysin and endophilin are two members of the N-BAR protein family. We have reported membrane interactions of the helix 0 of endophilin (H0-Endo). Here we investigate membrane modulations caused by the helix 0 of amphiphysin (H0-Amph). Electron paramagnetic resonance (EPR) spectroscopy was used to explore membrane properties. H0-Amph was found to reduce lipid mobility, make the membrane interior more polar, and decrease lipid chain orientational order. The EPR data also showed that for anionic membranes, H0-Endo acted as a more potent modulator. For instance, at peptide-to-lipid (P/L) ratio of 1/20, the peak-to-peak splitting was increased by 0.27 G and 1.89 G by H0-Amph and H0-Endo, respectively. Similarly, H0-Endo caused a larger change in the bilayer polarity than H0-Amph (30% versus 12% at P/L = 1/20). At P/L = 1/50, the chain orientational order was decreased by 26% and 66% by H0-Amph and H0-Endo, respectively. The different capabilities were explained by considering hydrophobicity score distributions. We employed atomic force microscopy to investigate membrane structural changes. Both peptides caused the formation of micron-sized holes. Interestingly, only H0-Amph induced membrane fusion as evidenced by the formation of high-rise regions. Lastly, experiments of giant unilamellar vesicles showed that H0-Amph and H0-Endo generated thin tubules and miniscule vesicles, respectively. Together, our studies showed that both helices are effective in altering membrane properties; the observed changes might be important for membrane curvature induction. Importantly, comparisons between the two peptides revealed that the degree of membrane remodeling is dependent on the sequence of the N-terminal helix of the N-BAR protein family. [Display omitted] • Helix 0 is thought to bind membranes and change membrane curvature. • Helix 0 modifies lipid chain mobility and order, and bilayer polarity. • Helix 0 induces micron-sized holes in planar membranes. • Helix 0 generates tubules extending from giant unilamellar vesicles. • Membrane modulations are dependent on the sequence of helix 0. [ABSTRACT FROM AUTHOR]

Abstract: Venom of Lachesana tarabaevi (Zodariidae, “ant spiders”) exhibits high insect toxicity and serves a rich source of potential insecticides. Five new peptide toxins active against insects were isolated from the venom by means of liquid chromatography and named latartoxins (LtTx). Complete amino acid sequences of LtTx (60-71 residues) were established by a combination of Edman degradation, mass spectrometry and selective proteolysis. Three toxins have eight cysteine residues that form four intramolecular disulfide bridges, and two other molecules contain an additional cystine; three LtTx are C-terminally amidated. Latartoxins can be allocated to two groups with members similar to CSTX and LSTX toxins from Cupiennius salei (Ctenidae) and Lycosa singoriensis (Lycosidae). The interesting feature of the new toxins is their modular organization: they contain an N-terminal cysteine-rich (knottin or ICK) region as in many neurotoxins from spider venoms and a C-terminal linear part alike some cytolytic peptides. The C-terminal fragment of one of the most abundant toxins LtTx-1a was synthesized and shown to possess membrane-binding activity. It was found to assume amphipathic α-helical conformation in membrane-mimicking environment and exert antimicrobial activity at micromolar concentrations. The tails endow latartoxins with the ability to bind and damage membranes; LtTx show cytolytic activity in fly larvae neuromuscular preparations. We suggest a membrane-dependent mode of action for latartoxins with their C-terminal linear modules acting as anchoring devices. [Copyright &y& Elsevier]

While natural antimicrobial peptides are potential therapeutic agents, their physicochemical properties and bioactivity generally need to be enhanced for clinical and commercial development. We have previously developed a cationic, amphipathic α-helical, 11-residue peptide (named herein GA-W2: FLGWLFKWASK-NH) with potent antimicrobial and hemolytic activity, which was derived from a 24-residue, natural antimicrobial peptide isolated from frog skin. Here, we attempted to optimize peptide bioactivity by a rational approach to sequence modification. Seven analogues were generated from GA-W2, and their activities were compared with that of a 12-residue peptide, omiganan, which is being developed for clinical and commercial applications. Most of the modifications reported here improved antimicrobial activity. Among them, the GA-K4AL (FAKWAFKWLKK-NH) peptide displayed the most potent antimicrobial activity with negligible hemolytic activity, superior to that of omiganan. The therapeutic index of GA-K4AL was improved more than 53- and more than 31-fold against Gram-negative and Gram-positive bacteria, respectively, compared to that of the starting peptide, GA-W2. Given its relatively shorter length and simpler amino acid composition, our sequence-optimized GA-K4AL peptide may thus be a potentially useful antimicrobial peptide agent. [ABSTRACT FROM AUTHOR]

Abstract: Hepatitis C Virus (HCV) is a leading cause of liver disease and represents a significant public health challenge. Treatments for this disease are inadequate and improved antiviral therapies are necessary. Several such antivirals are in development, most of which target the well-characterized NS3 protease or the NS5B polymerase. In contrast, the nonstructural 4B (NS4B) protein, though essential for HCV RNA replication, has been the subject of few pharmacological studies. One of the functions ascribed to this protein is the ability to form intracellular membrane-associated foci (MAF), which are believed to be related to the sites of viral replication. Here, we report the identification of a small molecule that inhibits HCV replication and disrupts the organization of these MAF. Genetic analysis links the compound''s mode of action to the NS4B gene product, and transient transfections of NS4B-GFP demonstrate that treatment with this compound can lead to the formation of novel elongated assemblies of NS4B. Furthermore, an in vitro dynamic light scattering assay provides evidence that the second amphipathic helix of NS4B may be the target of the drug. Our results demonstrate that this molecule represents a new potential class of HCV inhibitors and also provides us with a useful tool for studying the HCV life cycle. [Copyright &y& Elsevier]

Abstract: Pyruvate oxidase from Escherichia coli (EcPOX) is a thiamin diphosphate- (ThDP) and FAD-dependent peripheral membrane protein that carries out the irreversible oxidative decarboxylation of pyruvate to acetate and carbon dioxide. Concomitant two-electron reduction of the flavin cofactor was suggested to induce a structural rearrangement of the C-terminus triggering recruitment of the protein from the cytosol to the cell membrane, where the electrons are eventually transferred to final electron acceptor ubiquinone 8. Binding to the membrane, or alternatively, mild proteolytic digestion leads to a multifold enhancement of both the catalytic activity and substrate affinity. Recent X-ray crystallographic studies on EcPOX in the resting state and on a C-terminal truncation variant mimicking the membrane-bound activated form have fueled our understanding of the membrane-binding mechanism and concomitant catalytic activation. In the resting state, the auto-inhibitory C-terminal membrane anchor adopts a half-barrel/helix fold that occludes the active site. Upon activation, the C-terminus is expelled and becomes structurally flexible thereby freeing the active site. Circular dichroism spectroscopic analysis revealed the isolated C-terminus to be disordered, however, formation of a helical structure was observed in the presence of micelles. Limited proteolysis experiments indicate that activation of EcPOX involves at least two sequential structural transitions: the first occurring after binding of pyruvate to ThDP and the second after two-electron reduction of the flavin. [Copyright &y& Elsevier]

Abstract: The peptide 4F is known to have potent anti-atherogenic activity. 4F is an 18 residue peptide that has a sequence capable of forming a class A amphipathic helix. Several other class A amphipathic helical, 18 residue peptides with the same polar face but with increasing Phe residues on the nonpolar face have been synthesized with varying degrees of biological activity. In this work we compared the properties of the original 2F peptide, modeled on the consensus sequence of the amphipathic helical segments of the apolipoprotein A-I with the peptide 4F that has two Leu residues replaced with Phe. We demonstrate that the more biologically active 4F peptide has the greatest affinity for binding to several molecular species of oxidized lipids. Lipoprotein particles can be formed by solubilizing 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) with peptides. These solubilized lipoprotein particles extract oxidized lipid from liposomes of POPC containing 5 mol% of oxidized lipid. The peptides with the strongest anti-atherogenic activity interact most strongly with the oxidized lipid. The results show that there is a correlation between the biological potency of these peptides and their ability to interact with certain specific cytotoxic lipids, suggesting that this interaction may contribute favourably to their biological properties. [Copyright &y& Elsevier]

Abstract: Bacteriophage M13 major coat protein is extensively used as a biophysical, biochemical, and molecular biology reference system for studying membrane proteins. The protein has several elements that control its position and orientation in a lipid bilayer. The N-terminus is dominated by the presence of negatively charged amino acid residues (Glu2, Asp4, and Asp5), which will always try to extend into the aqueous phase and therefore act as a hydrophilic anchor. The amphipathic and the hydrophobic transmembrane part contain the most important hydrophobic anchoring elements. In addition there are specific aromatic and charged amino acid residues in these domains (Phe 11, Tyr21, Tyr24, Trp26, Phe42, Phe45, Lys40, Lys43, and Lys44) that fine-tune the association of the protein to the lipid bilayer. The interfacial Tyr residues are important recognition elements for precise protein positioning, a function that cannot be performed optimally by residues with an aliphatic character. The Trp26 anchor is not very strong: depending on the context, the tryptophan residue may move in or out of the membrane. On the other hand, Lys residues and Phe residues at the C-terminus of the protein act in a unique concerted action to strongly anchor the protein in the lipid bilayer. [Copyright &y& Elsevier]

Abstract: Background: Chylomicronemia syndrome can be caused by 2 autosomal recessive disorders — lipoprotein lipase (LPL) deficiency and apolipoprotein C-II (apo C-II) deficiency. Methods: We described 2 siblings with chylomicronemia syndrome of a consanguineous family. To determine the molecular basis of chylomicronemia syndrome in this family, we performed direct DNA sequencing of the LPL and APOC2 genes of the proband. Results: A novel homozygous mutation, Leu72Pro, in the APOC2 gene was found in both siblings whereas their parents were carriers. No LPL mutations were detected in the siblings. Apo C-II contains 3 amphipathic alpha helices; the C-terminal alpha helix is composed of residues 64 to 74. Substitution of residue 72 from a helix former leucine to a helix breaker, proline, is predicted to change the secondary structure of the C-terminal helix and subsequently alter the interaction between apo C-II and LPL. Conclusions: To our knowledge, Leu72Pro is the first missense mutation identified in the C-terminal of apo C-II. The result is consistent with the current biochemical and structural findings that the C-terminal helix of apo C-II is important for activation of LPL. [Copyright &y& Elsevier]

Neuropeptide Y (NPY) is one of the most abundant peptides in the central nervous system of mammals. It belongs to the best-conserved peptides in nature, i.e., the amino acid sequences of even evolutionary widely separated species are very similar to each other. Using porcine NPY, which differs from human NPY only at position 17 (a leucine residue exchanged for a methionine), labeled with a TOAC spin probe at the 2nd, 32nd, or 34th positions of the peptide backbone, the membrane binding and penetration of NPY was determined using EPR and NMR spectroscopy. The vesicular membranes were composed of phosphatidylcholine and phosphatidylserine at varying mixing ratios. From the analysis of the EPR line shapes, the spectral contributions of free, dimerized, and membrane bound NPY could be separated. This analysis was further supported by quenching experiments, which selected the contributions of the bound NPY fraction. The results of this study give rise to a model where the α-helical part of NPY (amino acids 13–36) penetrates the membrane interface. The unstructured N-terminal part (amino acids 1–12) extends into the aqueous phase with occasional contacts with the lipid headgroup region. Besides the mixing ratio of zwitterionic and negatively charged phospholipid species, the electrostatic peptide membrane interactions are influenced by the pH value, which determines the net charge of the peptide resulting in a modified membrane binding affinity. The results of these variations indicate that NPY binding to phospholipid membranes depends strongly on the electrostatic interactions. An estimation of the transfer energy of the peptide from aqueous solution to the membrane interface ΔG supports the preferential interaction of NPY with negatively charged membranes. [Copyright &y& Elsevier]

Solid-state NMR spectroscopy is being developed at a fast pace for the structural investigation of immobilized and non-crystalline biomolecules. These include proteins and peptides associated with phospholipid bilayers. In contrast to solution NMR spectroscopy, where complete or almost complete averaging leads to isotropic values, the anisotropic character of nuclear interactions is apparent in solid-state NMR spectra. In static samples the orientation dependence of chemical shift, dipolar or quadrupolar interactions, therefore, provides angular constraints when the polypeptides have been reconstituted into oriented membranes. Furthermore, solid-state NMR spectroscopy of aligned samples offers distinct advantages in allowing access to dynamic processes such as topological equilibria or rotational diffusion in membrane environments. Alternatively, magic angle sample spinning (MAS) results in highly resolved NMR spectra, provided that the sample is sufficiently homogenous. MAS spinning solid-state NMR spectra allow to measure distances and dihedral angles with high accuracy. The technique has recently been developed to selectively establish through-space and through-bond correlations between nuclei, similar to the approaches well-established in solution-NMR spectroscopy. [Copyright &y& Elsevier]

The peptide denoted K159 (30 residues) derives from the catalytic core (CC) sequence of HIV-1 integrase (IN, residues 147–175). In the crystal structure of CC, the corresponding segment belongs to the α4 helix (residues 148–168, including residues Glu 152, Lys 156 and Lys 159, crucial for enzyme activity and DNA recognition), a loop (residues 169–171) and a part of the α5 helix (171–175), involved in enzyme dimerization. We used the fluorescence and the circular dichroism (CD) properties in the near-UV of the aromatic side chain of a tyrosine residue added at the C-terminal end of K159 in order to analyze the behavior of the concentrated and diluted peptide in aqueous trifluoroethanol (TFE), in an attempt to connect the information obtainable at high (NMR), medium (CD) and low (fluorescence) concentrations of the peptide. Altogether, the C-terminal tyrosine residue provided indirect information on the global conformation of K159 and on the local orientation and environment of the residue. The propensity of TFE to stabilize α-helical conformations in peptides was confirmed in CD and fluorescence experiments at relatively high (20–160 μM) and low (2–16 μM) concentrations, respectively. At relatively high concentration, stabilization of the peptide into α-helical conformation favored its auto-association likely in parallel coiled-coil dimers, as pointed out in our previous work [Eur. J. Biochem. 253 (1998) 236]. This was further confirmed by ANS (1-anilinonaphtalene-8-sulfonic acid) analysis and fluorescence temperature coefficient measurement. With diluted K159, a Stern–Volmer analysis with positively and negatively charged quenchers indicated that, when the intermolecular interactions were absent, the tyrosine was in a positively charged environment, as if the peptide folded into a U-shaped conformation similar to that present in the crystal structure of the enzyme. [Copyright &y& Elsevier]

A comprehensive phylogenetic analysis of the core subunits of succinate:quinone oxidoreductases and quinol:fumarate oxidoreductases is performed, showing that the classification of the enzymes as type A to E based on the type of the membrane anchor fully correlates with the specific characteristics of the two core subunits. A special emphasis is given to the type E enzymes, which have an atypical association to the membrane, possibly involving anchor subunits with amphipathic helices. Furthermore, the redox properties of the SQR/QFR proteins are also reviewed, stressing out the recent observation of redox-Bohr effect upon haem reduction, observed for the Desulfovibrio gigas and Rhodothermus marinus enzymes, which indicates a direct protonation event at the haems or at a nearby residue. Finally, the possible contribution of these enzymes to the formation/dissipation of a transmembrane proton gradient is discussed, considering recent experimental and structural data. [Copyright &y& Elsevier]

The amphipathic helix 0 of endophilin (i.e., H0-Endo) is important to membrane binding, but its function of curvature generation remains controversial. We used electron paramagnetic resonance (EPR) spectroscopy to study effects of H0-Endo on membrane material properties. We found that H0-Endo reduced lipid chain mobility and increased bilayer polarity, i.e., making the bilayer interior more polar. Lipid-dependent examination revealed that anionic lipids augmented the effect of H0-Endo, while cholesterol had a minimal impact. Our EPR spectroscopy of magnetically aligned bicelles showed that as the peptide-to-lipid ratio increased, the lipid chain orientational order decreased gradually, followed by a sudden loss. We discuss an interfacial-bound model of the amphipathic H0-Endo to account for all EPR data. We used atomic force microscopy and fluorescence microscopy to explore membrane morphological changes. We found that H0-Endo caused the formation of micron-sized holes in mica-supported planar bilayers. Hole formation is likely caused by two competing forces – the adhesion force exerted by the substrate represses bilayer budging, whereas the line tension originating from peptide clustering has a tendency of destabilizing bilayer organization. In the absence of substrate influences, membrane curvature induction was manifested by generating small vesicles surrounding giant unilamellar vesicles. Our results of membrane perforation and vesiculation suggest that the functionality of H0-Endo is more than just coordinating membrane binding of endophilin. Unlabelled Image • Membrane curvature generation by helix 0 remains questionable. • Helix 0 modifies lipid chain mobility and order, and bilayer polarity. • Helix 0 induces transmembrane holes in planar membranes. • Helix 0 generates small vesicles surrounding giant unilamellar vesicles. • The effects of helix 0 depend on membrane lipid compositions. [ABSTRACT FROM AUTHOR]

The LAH4 family of amphipathic peptides exhibits pronounced antimicrobial, cell penetrating and nucleic acid transfection activities. Furthermore, variants were designed with potent lentiviral transduction enhancement. When viewed along a helical wheel the four histidines are arranged to form an amphipathic structure. In order to optimize some of these biological activities the number of leucine and alanine residues exposed to the hydrophilic surface was systematically varied which resulted in the design of vectofusin a peptide with strong lentiviral transduction enhancement activities. Here the series of peptides with varying numbers of alanine or leucine residues, respectively, framed by the histidines was tested for their calcein release activity. Interestingly, the membrane pore formation and DNA transfection activities show a clear correlation with the hydrophilic angle. In contrast the membrane partitioning and the propensity to adopt helical conformations was hardly affected as long as the hydrophilic angle did not exceed a limiting value of 150°. • Histidine-rich peptides have potent antimicrobial, cell penetration and transduction activities. • Membrane pore formation correlates with the hydrophilic angle of the amphipathic helical peptides • Transfection activity follows hydrophilic angle and pore formation. [ABSTRACT FROM AUTHOR]

Membrane curvature sensing via helical protein domains, such as those identified in Amphiphysin and ArfGAP1, have been linked to a diverse range of cellular processes. However, these regions can vary significantly between different protein families and thus remain challenging to identify from sequence alone. Greater insight into the protein-lipid interactions that drive this behavior could lead to production of therapeutics that specifically target highly curved membranes. Here we demonstrate the curvature-dependence of membrane binding for an amphipathic helix (APH) in a plant reticulon, namely RTNLB13 from A. thaliana. We utilize solution-state nuclear magnetic resonance spectroscopy to establish the exact location of the APH and map the residues involved in protein-membrane interactions at atomic resolution. We find that the hydrophobic residues making up the membrane binding site are conserved throughout all A. thaliana reticulons. Our results also provide mechanistic insight that leads us to propose that membrane binding by this APH may act as a feedback element, only forming when ER tubules reach a critical size and adding stabilization to these structures without disrupting the bilayer. A shallow hydrophobic binding interface appears to be a feature shared more broadly across helical curvature sensors and would automatically restrict the penetration depth of these structures into the membrane. We also suggest this APH is highly tuned to the composition of the membrane in which it resides, and that this property may be universal in curvature sensors thus rationalizing the variety of mechanisms reported for these functional elements. Unlabelled Image • C-terminal amphipathic helix in plant reticulon only folds on curved membranes. • Folding does not require specific surface charge or lipid composition. • Helical residues involved in protein-membrane interaction mapped using NMR. • A shallow hydrophobic interface makes up the helix-membrane binding site. • Hydrophobic residues in binding site are conserved in all A. thaliana isoforms. [ABSTRACT FROM AUTHOR]

In mitochondria, CDP-diacylglycerol (CDP-DAG) is a crucial precursor for cardiolipin biosynthesis. Mitochondrial CDP-DAG is synthesized by the translocator assembly and maintenance protein 41 (Tam41) through an elusive process. Here we show that Tam41 adopts sequential catalytic mechanism, and report crystal structures of the bulk N-terminal region of Tam41 from Schizosaccharomyces pombe in the apo and CTP-bound state. The structure reveals that Tam41 contains a nucleotidyltransferase (NTase) domain and a winged helix domain. CTP binds to an "L"-shaped pocket sandwiched between the two domains. Rearrangement of a loop region near the active site is essential for opening the CTP-binding pocket. Docking of phosphatidic acid/CDP-DAG in the structure suggests a lipid entry/exit pathway connected to the "L"-shaped pocket. The C-terminal region of SpTam41 contains a positively charged amphipathic helix crucial for membrane association and participates in binding phospholipids. These results provide detailed insights into the mechanism of CDP-DAG biosynthesis in mitochondria. • Structure of Tam41 reveals an NTase domain fused with a winged helix domain • CTP binds to an "L"-shaped active-site pocket sandwiched between two domains • The C-terminal amphipathic helices mediate the association of Tam41 with membrane • A potential pathway connects the active site with the membrane for lipid entry/exit Jiao et al. report the structure of mitochondrial CDP-diacylglycerol synthase Tam41 and reveal the CTP-binding active site sandwiched between a nucleotidyltransferase domain and a winged helix domain. The C-terminal region of Tam41 contains an amphipathic helix involved in membrane association. Detailed insights into the pathway for lipid entry/exit are obtained. [ABSTRACT FROM AUTHOR]