Mechanism Of Membrane Interaction Research Articles

Assembly-Induced Membrane Selectivity of Artificial Model Peptides through Entropy-Enthalpy Competition.

Peptide design and drug development offer a promising solution for combating serious diseases or infections. In this study, using an AI-human negotiation approach, we have designed a class of minimal model peptides against tuberculosis (TB), among which K7W6 exhibits potent efficacy attributed to its assembly-induced function. Comprising lysine and tryptophan with an amphiphilic α-helical structure, the K7W6 sequence exhibits robust activity against various infectious bacteria causing TB (including clinically isolated and drug-resistant strains) both in vitro and in vivo. Moreover, it synergistically enhances the effectiveness of the first-line antibiotic rifampicin while displaying low potential for inducing drug resistance and minimal toxicity toward mammalian cells. Biophysical experiments and simulations elucidate that K7W6's exceptional performance can be ascribed to its highly selective and efficient membrane permeabilization activity induced by its distinctive self-assembly behavior. Additionally, these assemblies regulate the interplay between enthalpy and entropy during K7W6-membrane interaction, leading to the peptide's two-step mechanism of membrane interaction. These findings provide valuable insights into rational design principles for developing advanced peptide-based drugs while uncovering the functional role played by assembly.

Membrane-Active Peptides and Their Potential Biomedical Application.

Membrane-active peptides (MAPs) possess unique properties that make them valuable tools for studying membrane structure and function and promising candidates for therapeutic applications. This review paper provides an overview of the fundamental aspects of MAPs, focusing on their membrane interaction mechanisms and potential applications. MAPs exhibit various structural features, including amphipathic structures and specific amino acid residues, enabling selective interaction with multiple membranes. Their mechanisms of action involve disrupting lipid bilayers through different pathways, depending on peptide properties and membrane composition. The therapeutic potential of MAPs is significant. They have demonstrated antimicrobial activity against bacteria and fungi, making them promising alternatives to conventional antibiotics. MAPs can selectively target cancer cells and induce apoptosis, opening new avenues in cancer therapeutics. Additionally, MAPs serve as drug delivery vectors, facilitating the transport of therapeutic cargoes across cell membranes. They represent a fascinating class of biomolecules with significant potential in basic research and clinical applications. Understanding their mechanisms of action and designing peptides with enhanced selectivity and efficacy will further expand their utility in diverse fields. Exploring MAPs holds promise for developing novel therapeutic strategies against infections, cancer, and drug delivery challenges.

Characterization of membrane-interaction mechanisms of proteins using vacuum-ultraviolet circular dichroism spectroscopy.

Protein-membrane interactions play an important role in various biological phenomena, such as material transport, demyelinating diseases, and antimicrobial activity. We combined vacuum-ultraviolet circular dichroism (VUVCD) spectroscopy with theoretical (e.g., molecular dynamics and neural networks) and polarization experimental (e.g., linear dichroism and fluorescence anisotropy) methods to characterize the membrane interaction mechanisms of three soluble proteins (or peptides). α1 -Acid glycoprotein has the drug-binding ability, but the combination of VUVCD and neural-network method revealed that the membrane interaction causes the extension of helix in the N-terminal region, which reduces the binding ability. Myelin basic protein (MBP) is an essential component of the myelin sheath with a multi-layered structure. Molecular dynamics simulations using a VUVCD-guided system showed that MBP forms two amphiphilic and three non-amphiphilic helices as membrane interaction sites. These multivalent interactions may allow MBP to interact with two opposing membrane leaflets, contributing to the formation of a multi-layered myelin structure. The antimicrobial peptide magainin 2 interacts with the bacterial membrane, causing damage to its structure. VUVCD analysis revealed that the M2 peptides assemble in the membrane and turn into oligomers with a β-strand structure. Linear dichroism and fluorescence anisotropy suggested that the oligomers are inserted into the hydrophobic core of the membrane, disrupting the bacterial membrane. Overall, our findings demonstrate that VUVCD and its combination with theoretical and polarization experimental methods pave the way for unraveling the molecular mechanisms of biological phenomena related to protein-membrane interactions.

Postsynaptic synucleins mediate endocannabinoid signaling

Endocannabinoids are among the most powerful modulators of synaptic transmission throughout the nervous system, and yet little is understood about the release of endocannabinoids from postsynaptic compartments. Here we report an unexpected finding that endocannabinoid release requires synucleins, key contributors to Parkinson’s disease. We show that endocannabinoids are released postsynaptically by a synuclein-dependent and SNARE-dependent mechanism. Specifically, we found that synuclein deletion blocks endocannabinoid-dependent synaptic plasticity; this block is reversed by postsynaptic expression of wild-type but not of mutant α-synuclein. Whole-cell recordings and direct optical monitoring of endocannabinoid signaling suggest that the synuclein deletion specifically blocks endocannabinoid release. Given the presynaptic role of synucleins in regulating vesicle lifecycle, we hypothesize that endocannabinoids are released via a membrane interaction mechanism. Consistent with this hypothesis, postsynaptic expression of tetanus toxin light chain, which cleaves synaptobrevin SNAREs, also blocks endocannabinoid-dependent signaling. The unexpected finding that endocannabinoids are released via a synuclein-dependent mechanism is consistent with a general function of synucleins in membrane trafficking and adds a piece to the longstanding puzzle of how neurons release endocannabinoids to induce synaptic plasticity.

Discovery of structural and functional transition sites for membrane-penetrating activity of sheep myeloid antimicrobial peptide-18

Cathelicidin antimicrobial peptides have an extended and/or unstructured conformation in aqueous solutions but fold into ordered conformations, such as the α-helical structure, when interacting with cellular membranes. These structural transitions can be directly correlated to their antimicrobial activity and its underlying mechanisms. SMAP-18, the N-terminal segment (residues 1–18) of sheep cathelicidin (SMAP-29), is known to kill microorganisms by translocating across membranes and interacting with their nucleic acids. The amino acid sequence of SMAP-18 contains three Gly residues (at positions 2, 7, and 13) that significantly affect the flexibility of its peptide structure. This study investigated the role of Gly residues in the structure, membrane interaction, membrane translocation, and antimicrobial mechanisms of SMAP-18. Five analogs were designed and synthesized through Gly → Ala substitution (i.e., G2A, G7A, G13A, G7,13A, and G2,7,13A); these substitutions altered the helical content of SMAP-18 peptides. We found that G7,13A and G2,7,13A changed their mode of action, with circular dichroism and nuclear magnetic resonance studies revealing that these analogs changed the structure of SMAP-18 from a random coil to an α-helical structure. The results of this experiment suggest that the Gly residues at positions 7 and 13 in SMAP-18 are the structural and functional determinants that control its three-dimensional structure, strain-specific activity, and antimicrobial mechanism of action. These results provide valuable information for the design of novel peptide-based antibiotics.

Anti-Cancer Peptides: Status and Future Prospects.

The dramatic rise in cancer incidence, alongside treatment deficiencies, has elevated cancer to the second-leading cause of death globally. The increasing morbidity and mortality of this disease can be traced back to a number of causes, including treatment-related side effects, drug resistance, inadequate curative treatment and tumor relapse. Recently, anti-cancer bioactive peptides (ACPs) have emerged as a potential therapeutic choice within the pharmaceutical arsenal due to their high penetration, specificity and fewer side effects. In this contribution, we present a general overview of the literature concerning the conformational structures, modes of action and membrane interaction mechanisms of ACPs, as well as provide recent examples of their successful employment as targeting ligands in cancer treatment. The use of ACPs as a diagnostic tool is summarized, and their advantages in these applications are highlighted. This review expounds on the main approaches for peptide synthesis along with their reconstruction and modification needed to enhance their therapeutic effect. Computational approaches that could predict therapeutic efficacy and suggest ACP candidates for experimental studies are discussed. Future research prospects in this rapidly expanding area are also offered.

Abstract 2918: Comprehensive survey of KRas4B membrane interaction and orientation

Abstract Three RAS genes encode four Ras proteins, HRas, NRas, and KRas splice variants, KRas4A and KRas4B. In human cancers, KRAS oncogene is highly expressed (85%) as compared to other RAS oncogenes, NRAS (11%) and HRAS (4%). Oncogenic Ras leads to aberrant cell proliferation signaling, primarily through the MAPK (Raf/MEK/ERK) pathway. At the plasma membrane, nanoclustered Ras proteins promote Raf activation, mediating the downstream signaling pathway. Among the Ras isoforms, KRas4B in the membrane has been studied extensively due to its higher level of mutations in cancer. Although a number of experimental studies have focused on KRas4B subcellular localization and their nanocluster formation at the signaling platform, the mechanistic details of the membrane anchorage and the membrane orientation of KRas4B at atomic resolution are still unknown. Here, comprehensive computational studies were performed for KRas4B at the anionic lipid bilayers, composed of the phospholipids, PS and PA, and the phosphoinositides, PIP2, PI4P, and PI5P. Wild-type and two oncogenic mutants, G12D and G12V KRas4B in the GTP- and GDP-bound states were considered. The lysine rich HVR was post-translationally modified with the farnesyl and methyl groups. Previous studies showed that wild-type KRas4B-GDP interacts weakly and nonspecifically with anionic phospholipids, while oncogenic mutations alter selective anionic lipid binding toward anionic lipids, PA, PI3P, PI4P, and PI5P. Our studies indicate that the membrane interaction of KRas4B is highly sensitive depending on the types of nucleotides, mutational states, and anionic lipids enriched the lipid bilayer. While the GTP-bound spontaneously inserts its prenyl group into the hydrophobic core of the bilayer, the occurrence of spontaneous insertion is reduced in the GDP-bound form. The distance of GTP-bound catalytic domain from the bilayer surface is longer than the GDP-bound form. The HVR is sequestered by the GDP-bound catalytic domain, which causes the catalytic domain to be occluded and increases the membrane interaction by the catalytic domain. In contrast, the GTP-bound catalytic domain releases its HVR and liberates from the membrane interaction. The occlusion of catalytic domain is less prevalent in the GTP-bound state. KRas4BG12D/G12V-GTP exhibits the active-state orientation, exposing its effector binding site for recruiting the effectors. While PA and PIP2 support the active-state orientation, the PIP4 enriched bilayer prevents the spontaneous prenyl insertion resulting in the occlusion of catalytic domain. Our results underscore the importance of detailed structural mechanisms of KRas4B membrane interaction and orientation, elucidating vital mechanistic role of KRas4B, a key membrane-anchored protein in cancer. Funded by Frederick National Laboratory for Cancer Research, National Institutes of Health, under contract HHSN261201500003I. Citation Format: Hyunbum Jang, Mingzhen Zhang, Vadim Gaponenko, Ruth Nussinov. Comprehensive survey of KRas4B membrane interaction and orientation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2918.

Characterisation of cell membrane interaction mechanisms of antimicrobial peptides by electrical bilayer recording

Many antimicrobial peptides (AMPs) are cationic host defence peptides (HDPs) that interact with microbial membranes. This ability may lead to implementation of AMPs as therapeutics to overcome the wide-spread antibiotic resistance problem as the affected bacteria may not be able to recover from membrane lysis types of attack. AMP interactions with lipid bilayer membranes are typically explained through three mechanisms, i.e., barrel-stave pore, toroidal pore and carpet models. Electrical bilayer recording is a relatively simple and sensitive technique that is able to capture the nanoscale perturbations caused by the AMPs in the bilayer membranes. Molecular-level understanding of the behaviour of AMPs in relation to lipid bilayers mimicking bacterial and human cell membranes is essential for their development as novel therapeutic agents that are capable of targeted action against disease causing micro-organisms. The effects of four AMPs (aurein 1.2, caerin 1.1, citropin 1.1 and maculatin 1.1 from the skin secretions of Australian tree frogs) and the toxin melittin (found in the venom of honeybees) on two different phospholipid membranes were studied using the electrical bilayer recording technique. Bilayers composed of zwitterionic (DPhPC) and anionic (DPhPC/POPG) lipids were used to mimic the charge of eukaryotic and prokaryotic cell membranes, respectively, so as to determine the corresponding interaction mechanisms for different concentrations of the peptide. Analysis of the dataset corresponding to the four frog AMPs, as well as the resulting dataset corresponding to the bee toxin, confirms the proposed peptide-bilayer interaction models in existing publications and demonstrates the importance of using appropriate bilayer compositions and peptide concentrations for AMP studies.

Determining structure and action mechanism of LBF14 by molecular simulation

The recently discovered, membrane–active peptide LBF14 contains several non–proteinogenic amino acids and is able to transform vesicles into tubule networks. The exact membrane interaction mechanism and detailed secondary structure are yet to be determined. We performed molecular dynamics simulations of LBF14 and let it fold de novo into its ensemble of native secondary structures. Histidine protonation state effects on secondary structure were investigated. An MD simulation of the peptide with a lipid bilayer was performed. Simulation results were compared to circular dichroism and electron paramagnetic resonance data of previous studies. LBF14 contains a conserved helical section in an otherwise random structure. Helical stability is influenced by histidine protonation. The peptide localized to the polar layer of the membrane, consistent with experimental results. While the overall secondary structure is unaffected by membrane interaction, Ramachandran plot analysis yielded two distinct peptide conformations during membrane interaction. This conformational change was accompanied by residue repositioning within the membrane. LBF14 only affected the local order in the membrane, and had no measurable effect on pressure. The simulation results are consistent with the previously proposed membrane interaction mechanism of LBF14 and can additionally explain the local interaction mechanism. Communicated by Ramaswamy H. Sarma

Understanding the Structural Basis of Epha2‐dimerization, ‐membrane Interactions and Its Implications in Cancer Progression

EphA2 plays a critical role in cellular growth, differentiation and motility. In line with EphA2 mRNA expression in multiple tissues and organs, its overexpression is reported in several different cancer types, and even in cancer-derived cell lines. EphA2 overexpression may have significance and could be used as a biomarker in the clinical management of cancer. Besides this, the differential EphA2 expression in normal versus cancer cells makes it a key therapeutic target. In EphA2, ligand binding regulates the monomer-dimer equilibrium through stabilization of the dimeric state by inducing a conformational change in the extracellular domain. Thus, dimerization is a key regulatory step in the activity and signaling process of EphA2. In EphA2, the juxtamembrane (JM) region follows the TM domain at one end and connects to a catalytic domain at the other. The JM domain functions in synergy with the TM domain for signal transduction. The JM domain has several basic residues (K/R) that are closely positioned to the membrane surface. Other signaling molecules specifically PIP2 and PIP3 utilize these basic amino acids for binding and occlude the nearby region from phosphorylation. Therefore, studying the structural mechanism of EphA2 dimerization and membrane interactions will help us better understand the activity and signaling of this receptor in different cancer types. Importantly, working with membrane proteins is extremely challenging. However, the recent developments in molecular dynamics simulation have provided a powerful tool to study membrane proteins and their interactions. Here, we modeled the relevant domains of the EphA2 receptor and studying its structural mechanism of activation for signaling by harnessing the power of advanced in-silico techniques. Our objective is to understand the structural basis of EphA2 dimerization and the role of the transmembrane region in signal transduction of EphA2 and also the characterization of protein-lipid bilayer interaction and their role in EphA2 regulation.

Antibacterial mechanism of brevilaterin B: an amphiphilic lipopeptide targeting the membrane of Listeria monocytogenes.

Antimicrobial peptides (AMPs) are recognized as promising safe alternatives to antibiotics for its low drug-resistance. Brevilaterin B, a newly discovered antimicrobial lipopeptide produced by Brevibacillus laterosporus S62-9, exhibits efficient antibacterial activity on Listeria monocytogenes with a minimum inhibitory concentration of 1μgmL-1. The present research aimed to investigate the antibacterial mechanism of brevilaterin B against Listeria monocytogenes. Brevilaterin B caused membrane depolarization and the breakup of the cytomembrane as measured by 3,3-dipropylthiadicarbocyanine iodide and transmission electron microscopy, respectively. Using 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (7:3) as a model membrane, results proved that brevilaterin B could bind to liposomes, integrate into the lipid bilayer, and consequently increase the permeability of liposomes to calcein. The secondary structure of brevilaterin B also changed from an unstructured coil to a mainly β-sheet conformation as measured by circular dichroism. Brevilaterin B exhibits antibacterial activity by a membrane interaction mechanism, which provides a theoretical basis for using brevilaterin B as a promising natural and effective antimicrobial agent against pathogenic bacteria. KEY POINTS: • Brevilaterin B exhibited antibacterial activity against Listeria monocytogenes. • Brevilaterin B exhibited membrane interaction mechanism. • Brevilaterin B showed conformational change when interacted with liposome.

The cryo-EM structure of the acid activatable pore-forming immune effector Macrophage-expressed gene 1

Macrophage-expressed gene 1 (MPEG1/Perforin-2) is a perforin-like protein that functions within the phagolysosome to damage engulfed microbes. MPEG1 is thought to form pores in target membranes, however, its mode of action remains unknown. We use cryo-Electron Microscopy (cryo-EM) to determine the 2.4 Å structure of a hexadecameric assembly of MPEG1 that displays the expected features of a soluble prepore complex. We further discover that MPEG1 prepore-like assemblies can be induced to perforate membranes through acidification, such as would occur within maturing phagolysosomes. We next solve the 3.6 Å cryo-EM structure of MPEG1 in complex with liposomes. These data reveal that a multi-vesicular body of 12 kDa (MVB12)-associated β-prism (MABP) domain binds membranes such that the pore-forming machinery of MPEG1 is oriented away from the bound membrane. This unexpected mechanism of membrane interaction suggests that MPEG1 remains bound to the phagolysosome membrane while simultaneously forming pores in engulfed bacterial targets.

Structure of Membrane-Bound Huntingtin Exon 1 Reveals Membrane Interaction and Aggregation Mechanisms

Huntington's disease is caused by a polyQ expansion in the first exon of huntingtin (Httex1). Membrane interaction of huntingtin is of physiological and pathological relevance. Using electron paramagnetic resonance and Overhauser dynamic nuclear polarization, we find that the N-terminal residues 3-13 of wild-type Httex1(Q25) form a membrane-bound, amphipathic α helix. This helix is positioned in the interfacial region, where it is sensitive to membrane curvature and electrostatic interactions with head-group charges. Residues 14-22, which contain the first five residues of the polyQ region, are in a transition region that remains in the interfacial region without taking up a stable, α-helical structure. The remaining C-terminal portion is solvent exposed. The phosphomimetic S13D/S16D mutations, which are known to protect from toxicity, inhibit membrane binding and attenuate membrane-mediated aggregation of mutant Httex1(Q46) due to electrostatic repulsion. Targeting the N-terminal membrane anchor using post-translational modifications or specific binders could be a potential means to reduce aggregation and toxicity invivo.

Effects of membrane lipid composition on Mycobacterium tuberculosis EsxA membrane insertion: A dual play of fluidity and charge

As a key virulence factor of Mycobacterium tuberculosis, EsxA or 6-kDa early secreted antigenic target (ESAT-6) has been implicated in phagosome rupture and mycobacterial translocation from the phagosome to the cytosol within macrophages. Our previous studies have shown that EsxA permeabilizes liposomal membrane at acidic pH and a membrane-permeabilization defective mutant Q5K attenuates mycobacterial cytosolic translocation and virulence in macrophages. To further probe the mechanism of EsxA membrane permeabilization, here we characterized the effects of various lipid compositions, including biologically relevant phagosome-mimicking lipids and lipid rafts, on the structural stability and membrane insertion of EsxA WT and Q5K. We have found a complex dual play of membrane fluidity and charge in regulating EsxA membrane insertion. Moreover, Q5K affects the membrane insertion through a structure- and lipid composition-independent mechanism. The results of this study provide a novel insights into the mechanism of EsxA membrane interaction.

The disordered plant dehydrin Lti30 protects the membrane during water-related stress by cross-linking lipids

Dehydrins are intrinsically disordered proteins, generally expressed in plants as a response to embryogenesis and water-related stress. Their suggested functions are in membrane stabilization and cell protection. All dehydrins contain at least one copy of the highly conserved K-segment, proposed to be a membrane-binding motif. The dehydrin Lti30 (Arabidopsis thaliana) is up-regulated during cold and drought stress conditions and comprises six K-segments, each with two adjacent histidines. Lti30 interacts with the membrane electrostatically via pH-dependent protonation of the histidines. In this work, we seek a molecular understanding of the membrane interaction mechanism of Lti30 by determining the diffusion and molecular organization of Lti30 on model membrane systems by imaging total internal reflection- fluorescence correlation spectroscopy (ITIR-FCS) and molecular dynamics (MD) simulations. The dependence of the diffusion coefficient explored by ITIR-FCS together with MD simulations yields insights into Lti30 binding, domain partitioning, and aggregation. The effect of Lti30 on membrane lipid diffusion was studied on fluorescently labeled supported lipid bilayers of different lipid compositions at mechanistically important pH conditions. In parallel, we compared the mode of diffusion for short individual K-segment peptides. The results indicate that Lti30 binds the lipid bilayer via electrostatics, which restricts the mobility of lipids and bound protein molecules. At low pH, Lti30 binding induced lipid microdomain formation as well as protein aggregation, which could be correlated with one another. Moreover, at physiological pH, Lti30 forms nanoscale aggregates when proximal to the membrane suggesting that Lti30 may protect the cell by "cross-linking" the membrane lipids.

Mechanistic predictions of the influence of collagen-binding domain sequences on human LL37 interactions with model lipids using quartz crystal microbalance with dissipation.

Modifications of human-derived antimicrobial peptide LL37 with collagen binding domains (CBD-LL37) hold promise as alternatives to antibiotics due to their wider therapeutic ratio than unmodified LL37 when interacting with collagen substrates such as commercial wound dressings. However, CBD-LL37 lipid membrane interaction mechanisms (against both mammalian and bacterial lipids) are not well understood. Our goal was to develop a mechanistic explanation of how CBDs modulate peptide-lipid interactions leading to their observed bioactivities, in order to better understand their potential for clinical applications. The authors studied time- and concentration-dependent interactions of CBD-LL37 modified with collagenase (cCBD) and fibronectin (fCBD) CBDs, with zwitterionic and anionic supported lipid bilayers, in order to model mammalian erythrocytes and bacterial cells, respectively. Quartz crystal microbalance with dissipation monitoring (QCM-D) was used to characterize peptide-lipid interactions at concentrations in the immunomodulatory (0.5-1.0 μM), antimicrobial (1.0-5.0 μM), and cytotoxic (5.0-10.0 μM) ranges. Their prior work with zwitterionic membranes demonstrated that cCBD-LL37 formed transmembrane pores while fCBD-LL37 underwent surface adsorption. Our goal in this study is to better interpret these results, by investigating the data at a wider concentration range and for two types of lipids, and by applying the Voigt-Kelvin viscoelastic model to calculate thickness and density changes of the peptide-lipid films as a function of time and concentration, thus providing information to help build detailed mechanisms of peptide/bilayer interactions. For pore-forming cCBD-LL37 and unmodified LL37, they found that there was a relationship between layer thicknesses and pore formation, which was attributed to different peptide orientation changes influenced by bilayer charge prior to pore formation. Specifically, cCBD-LL37 at 0.5 and 1.0 μM demonstrated higher thicknesses on zwitterionic than anionic membranes, indicating that prior to insertion into zwitterionic membranes, it orients perpendicular to the surface, which was also consistent with the higher dissipation changes observed on zwitterionic membranes. fCBD-LL37 demonstrated a bilayer adsorption mechanism with a preference toward anionic lipids. Adsorption of fCBD-LL37 onto anionic lipids demonstrated a rapid first adsorption step that transitioned depending on the number of fCBD-LL37 molecules on the bilayer. For this peptide at higher concentrations, greater dissipation changes were observed than for fCBD-LL37 physically adsorbed onto surfaces without bilayers. This suggests that peptide-peptide interactions promoted by the fCBD domain dominated after saturation. The development of a structure-function relationship for cCBD-LL37 and fCBD-LL37 demonstrates promise for using QCM-D predictions to inform the rational design of novel, antimicrobial, and noncytotoxic CBD-LL37 for clinical applications.

The Molecular Mechanism and Structural Analysis of Membrane Interaction via Fera and C2 Domains in Ferlins Associated with Muscular Dystrophy and Cancer

The Molecular Mechanism and Structural Analysis of Membrane Interaction via Fera and C2 Domains in Ferlins Associated with Muscular Dystrophy and Cancer

Unveiling the Multifaceted Mechanisms of Antibacterial Activity of Buforin II and Frenatin 2.3S Peptides from Skin Micro-Organs of the Orinoco Lime Treefrog (Sphaenorhynchus lacteus).

Amphibian skin is a rich source of natural compounds with diverse antimicrobial and immune defense properties. Our previous studies showed that the frog skin secretions obtained by skin micro-organs from various species of Colombian anurans have antimicrobial activities against bacteria and viruses. We purified for the first time two antimicrobial peptides from the skin micro-organs of the Orinoco lime treefrog (Sphaenorhynchus lacteus) that correspond to Buforin II (BF2) and Frenatin 2.3S (F2.3S). Here, we have synthesized the two peptides and tested them against Gram-negative and Gram-positive bacteria, observing an effective bactericidal activity at micromolar concentrations. Evaluation of BF2 and F2.3S membrane destabilization activity on bacterial cell cultures and synthetic lipid bilayers reveals a distinct membrane interaction mechanism. BF2 agglutinates E. coli cells and synthetic vesicles, whereas F2.3S shows a high depolarization and membrane destabilization activities. Interestingly, we found that F2.3S is able to internalize within bacterial cells and can bind nucleic acids, as previously reported for BF2. Moreover, bacterial exposure to both peptides alters the expression profile of genes related to stress and resistance response. Overall, these results show the multifaceted mechanism of action of both antimicrobial peptides that can provide alternative tools in the fight against bacterial resistance.

Abstract 678: Raf-1 cysteine-rich domain (CRD) supports active orientations of KRas4B/Raf-1 at the membrane

Abstract Ras is a small GTPase, controlling signal transduction pathways and promoting cell proliferation and survival. In cancer, there are two independent pathways in tumor proliferation: one includes Raf/MEK/ERK (MAPK) and PI3K/Akt/mTOR. KRas4B is the most abundant oncogenic isoform. In the MAPK pathway, KRas4B preferentially recruits Raf-1 and activates it. The high-affinity interaction of Ras binding domain (RBD) of Raf with Ras was solved, but the relative position of Raf's cysteine-rich domain (CRD) in the Ras/Raf complex at the membrane and the key question of exactly how it affects Raf signaling are daunting. Here, we employ all-atom MD simulations to investigate membrane-anchored Raf-1 CRD and suggest a model of KRas4B/Raf-1 complex at the anionic lipid bilayer composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phosphoserine (DOPS) in mole ratio 4:1. Active Ras modulates Raf activation, with KRas4B preferentially activating Raf-1. We show that the Raf-1 CRD stably binds anionic lipid bilayers inserting a positively-charged loop into the amphipathic interface. The loop contains key basic residues, responsible for the membrane association. Importantly, when in complex with Ras/RBD, covalently-connected CRD presents the same membrane interaction mechanism, with CRD locating at the space between the RBD and membrane, suggesting that CRD is an intrinsic membrane binding domain of Raf kinase. To date, CRD's role was viewed in terms of stabilizing Raf-membrane interaction. Our observations argue for a key role of CRD that Raf-1 CRD not only offers an additional anchor for the KRas4B/Raf-1 complex, but by reducing the fluctuations of Ras/RBD it also increases the population of Ras/RBD at the membrane and enhances the already high affinity between Ras and RBD. In the KRas4B/RBD-CRD complex, Raf-1 CRD supports the active-state Ras orientation at the membrane, releasing the catalytic domain from the transiently confined orientation. This promotes MAPK signaling, the key Ras proliferative pathway, above other pathways. Even though in the absence of CRD the Ras/Raf-1 RBD interaction can elicit MAPK signaling, the enhanced stability of the complex at the membrane rendered by CRD can considerably intensify it, by helping in Raf's dimerization. Our data reveal these significant roles of CRD at the membrane in the Raf activation. Funded by Frederick National Laboratory for Cancer Research, National Institutes of Health, under contract HHSN261200800001E. Citation Format: Hyunbum Jang, Ruth Nussinov. Raf-1 cysteine-rich domain (CRD) supports active orientations of KRas4B/Raf-1 at the membrane [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 678.

Vibrio cholerae cytolysin: Multiple facets of the membrane interaction mechanism of a β‐barrel pore‐forming toxin

Vibrio cholerae cytolysin (VCC) is a membrane-damaging protein toxin with potent cytolytic/cytotoxic activity against wide range of eukaryotic cells. VCC is a β-barrel pore-forming toxin (β-PFT), and it inflicts damage to the target cell membranes by forming transmembrane heptameric β-barrel pores. To exert pore-forming activity, VCC must bind to the cell membranes in an efficient manner. Efficient interaction with the cell membranes is an essential pre-requisite to trigger subsequent structural/conformational and organizational changes in the toxin molecules leading toward formation of the transmembrane oligomeric β-barrel pores. Based on the large numbers of studies investigating the mode of action of VCC, it is now evident that VCC is capable of using multiple distinct mechanisms to recognize and bind to the membrane components and cell surface molecules. In this review article, we present an overview of our current understanding regarding the membrane interaction mechanisms of VCC, and their functional implications for the pore-forming activity of the toxin. © 2018 IUBMB Life, 70(4):260-266, 2018.