Orientation and dynamics of melittin in membranes of varying composition utilizing NBD fluorescence

Characterization of a novel MgtE homolog and its structural dynamics in membrane mimetics

Magnesium (Mg2+) is the most abundant divalent cation in the cell and is critical for numerous cellular processes. Despite its importance, the mechanisms of intracellular Mg2+ transport and its regulation are poorly understood. MgtE is the main Mg2+ transport system in almost half of bacterial species and is an ortholog of mammalian SLC41A1 transporters, which are implicated in neurodegenerative diseases and cancer. To date, only MgtE from Thermus thermophilus (MgtETT) has been extensively characterized, mostly in detergent micelles, and gating-related structural dynamics in biologically relevant membranes are scarce. The MgtE homolog from Bacillus firmus (MgtEBF) is unique since it lacks the entire Mg2+-sensing N-domain but has conserved structural motifs in the TM-domain for Mg2+ transport. In this work, we have successfully purified this novel homolog in a stable and functional form, and ColabFold structure prediction analysis suggests a homodimer. Further, microscale thermophoresis experiments show that MgtEBF binds Mg2+ and ATP, similar to MgtETT. Importantly, we show that, despite lacking the N-domain, MgtEBF mediates Mg2+ transport function in the presence of an inwardly directed Mg2+ gradient in reconstituted proteoliposomes. Furthermore, comparison of the organization and dynamics of Trp residues in the TM-domain of MgtEBF in membrane mimetics, in apo- and Mg2+-bound forms, suggests that the cytoplasmic binding of Mg2+ might involve modest gating-related conformational changes at the TM-domain. Overall, our results show that the gating-related structural dynamics (hydration dynamics, conformational heterogeneity) of the full-length MgtEBF is significantly changed in functionally pertinent membrane environment, emphasizing the importance of lipid-protein interactions in MgtE gating mechanisms.

Cyclohexylalanine-Containing α-Helical Amphipathic Peptide Targets Cardiolipin, Rescuing Mitochondrial Dysfunction in Kidney Injury

Mitochondrial dysfunction is linked to degenerative diseases, resulting from cardiolipin (CL)-induced disruption of cristae structure in the inner mitochondrial membrane (IMM); therefore, preserving cristae and preventing CL remodeling offer effective strategies to maintain mitochondrial function. To identify reactive oxygen species (ROS)-blocking agents against mitochondrial dysfunction, a library of cyclohexylamine-containing cell-penetrating α-helical amphipathic "bundle" peptides were screened. Among these, CMP3013 is selectively bound to abnormal mitochondria, preserving the cristae structure impaired by mitochondria-damaging agents. With a stronger affinity for CL compared with other IMM lipid components, CMP3013 exhibited high selectivity. Consequently, it protected cristae, reduced ROS production, and enhanced adenosine triphosphate (ATP) generation. In mouse models of acute kidney injury, a 1 mg/kg dose of CMP3013 demonstrated remarkable efficacy, highlighting its potential as a therapeutic agent for mitochondrial dysfunction-related disorders. Overall, CMP3013 represents a promising agent for mitigating mitochondrial dysfunction and associated diseases.

Cholesterol modulates the structural dynamics of the paddle motif loop of KvAP voltage sensor

KvAP is a prokaryotic Kv channel, which has been widely used as a model system to understand voltage- and lipid-dependent gating mechanisms. In phospholipid membranes, the KvAP-VSD adopts the activated/'Up' conformation, whereas the presence of non-phospholipids in membranes favours the structural transition to resting/'Down' state. The S3b-S4 paddle motif loop of KvAP-VSD is functionally important as this participates in protein-protein interactions and is the target for animal toxins. In this study, we have monitored the modulatory role of cholesterol - the physiologically-relevant non-phospholipid - on the organization and dynamics of the S3b-S4 loop of the isolated KvAP-VSD in membranes by site-directed fluorescence approaches using the environmental sensitivity of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-ethylenediamine (NBD) fluorescence. Our results show that cholesterol alters the dynamic nature (rotational and hydration dynamics) of S3b-S4 loop in a segmental fashion, i.e., the residues 110 to 114 and 115 to 117 behave differently in the presence of cholesterol, which is accompanied by considerable change in conformational heterogeneity. Further, quantitative depth measurements using the parallax quenching method reveal that the sensor loop is located at the shallow interfacial region of cholesterol-containing membranes, suggesting that the sensor loop organization is not directly correlated with S4 helix movement. Our results clearly show that cholesterol-induced changes in bilayer properties may not be the predominant factor for the sensor loop's altered structural dynamics, but can be attributed to the conformational change of the KvAP-VSD in cholesterol-containing membranes. Overall, these results are relevant for gating mechanisms, particularly the lipid-dependent gating, of Kv channels in membranes.

Topology of the SecA ATPase Bound to Large Unilamellar Vesicles

The soluble cytoplasmic ATPase motor protein SecA powers protein transport across the Escherichia coli inner membrane via the SecYEG translocon. Although dimeric in solution, SecA associates monomerically with SecYEG during secretion according to several crystallographic and cryo-EM structural studies. The steps SecA follows from its dimeric cytoplasmic state to its active SecYEG monomeric state are largely unknown. We have previously shown that dimeric SecA in solution dissociates into monomers upon electrostatic binding to negatively charged lipid vesicles formed from E. coli lipids. Here we address the question of the disposition of SecA on the membrane prior to binding to membrane embedded SecYEG. We mutated to cysteine, one at a time, 25 surface-exposed residues of a Cys-free SecA. To each of these we covalently linked the polarity-sensitive fluorophore NBD whose intensity and fluorescence wavelength-shift change upon vesicle binding report on the the local membrane polarity. We established from these measurements the disposition of SecA bound to the membrane in the absence of SecYEG. Our results confirmed that SecA is anchored in the membrane interface primarily by the positive charges of the N terminus domain. But we found that a region of the nucleotide binding domain II is also important for binding. Both domains are rich in positively charged residues, consistent with electrostatic interactions playing the major role in membrane binding. Selective replacement of positively charged residues in these domains with alanine resulted in weaker binding to the membrane, which allowed us to quantitate the relative importance of the domains in stabilizing SecA on membranes. Fluorescence quenchers inside the vesicles had little effect on NBD fluorescence, indicating that SecA does not penetrate significantly across the membrane. Overall, the topology of SecA on the membrane is consistent with the conformation of SecA observed in crystallographic and cryo-EM structures of SecA-SecYEG complexes, suggesting that SecA can switch between the membrane-associated and the translocon-associated states without significant changes in conformation.

Bio-nano scale modifications of melittin for improving therapeutic efficacy

INTRODUCTION

Melittin (MLT), a natural membrane-active component, is the most prominent cytolytic peptide from bee venom. Remarkable biological properties of MLT, including anti-inflammatory, antimicrobial, anticancer, anti-protozoan, and antiarthritic activities, make it an up-and-coming therapeutic candidate for a wide variety of human diseases. Therapeutic applications of MLT may be hindered due to low stability, high toxicity, and weak tissue penetration. Different bio-nano scale modifications hold promise for improving its functionality and therapeutic efficacy.

AREAS COVERED

In the current review, we aimed to provide a comprehensive insight into strategies used for MLT conjugations and modifications, cellular delivery of modified forms, and their clinical perspectives by reviewing the published literature on PubMed, Scopus, and Google Scholar databases. We also emphasized the MLT structure modifications, mechanism of action, and cellular toxicity.

EXPERT OPINION

Developing new analogs and conjugates of MLT as a natural drug with improved functions and fewer side effects is crucial for the clinical translation of this approach worldwide, especially where the chemicals and synthetic drugs are more expensive or unavailable in the healthcare system. MLT-nanoconjugation may be one of the best-optimized strategies for improving peptide delivery, increasing its therapeutic efficacy, and providing minimal nonspecific cellular lytic activity. [Figure: see text].

Measuring Membrane Penetration Depths and Conformational Changes in Membrane Peptides and Proteins

The structural organization and dynamic nature of the biomembrane components are important determinants for numerous cellular functions. Particularly, membrane proteins are critically important for various physiological functions and are important drug targets. The mechanistic insights on the complex functionality of membrane lipids and proteins can be elucidated by understanding the interplay between structure and dynamics. In this regard, membrane penetration depth represents an important parameter to obtain the precise depth of membrane-embedded molecules that often define the conformation and topology of membrane probes and proteins. In this review, we discuss about the widely used fluorescence quenching-based methods (parallax method, distribution analysis, and dual-quencher analysis) to accurately determine the membrane penetration depths of fluorescent probes that are either membrane-embedded or attached to lipids and proteins. Further, we also discuss a relatively novel fluorescence quenching method that utilizes tryptophan residue as the quencher, namely the tryptophan-induced quenching, which is sensitive to monitor small-scale conformational changes (short distances of < 15 Å) and useful in mapping distances in proteins. We have provided numerous examples for the benefit of readers to appreciate the importance and applicability of these simple yet powerful methods to study membrane proteins.

Site-directed fluorescence approaches to monitor the structural dynamics of proteins using intrinsic Trp and labeled with extrinsic fluorophores

Comprehensive understanding of a protein’s function depends on having reliable, sophisticated tools to study protein structural dynamics in physiologically-relevant conditions. Here, we present an effective, robust step-by-step protocol to monitor the structural dynamics (including hydration dynamics) of a protein utilizing various site-directed fluorescence (SDFL) approaches. This protocol should be widely applicable for studying soluble proteins, intrinsically-disordered proteins, and membrane proteins.

For complete details on the use and execution of this protocol, please refer to Das et al. (2020), Das and Raghuraman (2021), and Chatterjee et al. (2021).

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A step-by-step protocol to monitor the structural dynamics of proteins using SDFL

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Applicable to proteins with intrinsic Trp and labeled with extrinsic fluorophores

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This protocol should be widely applicable for soluble and membrane proteins

Effectiveness of dual-detergent strategy using Triton X-100 in membrane protein purification

Membrane solubilization by detergents is a critical step for successful membrane protein purification. Alkyl maltoside detergents such as DDM and DM are very expensive and are commonly used to produce most of the high-quality proteins in stable and functional form. Recently, dual-detergent strategy using inexpensive detergents for membrane solubilization step has been shown to be highly effective in purifying different classes of membrane proteins in a cost-effective manner. In this work, we have monitored the effectiveness of 'dual-detergent strategy' towards successful purification of the isolated voltage sensing domain (VSD) of KvAP and the inward rectifying K⁺ channel, KirBac1.1. We demonstrate that the inexpensive detergent Triton X-100 extracts the activated conformation of the KvAP-VSD well without compromising the structural integrity of the sensor, and also retains its proper structural dynamics. Importantly, the cost associated with solubilizing the KvAP sensor can be reduced by ∼2000 fold. To the best of our knowledge, our results constitute the first report characterizing the purification of KvAP voltage sensor using an inexpensive detergent. However, the dual-detergent strategy using Triton X-100 for membrane solubilization is not effective for the purification of inward rectifying K⁺ channel, KirBac1.1 even in presence of high salt concentration during solubilization. We propose that the dual-detergent strategy will be useful for extracting stable and functional proteins that are both DDM- and DM-extractable, but will be ineffective if the protein is only DM-extractable. The relevance of the effectiveness of dual-detergent strategy with respect to the hydrophobic thickness of proteins is discussed.

Lipid Headgroup Charge Controls Melittin Oligomerization in Membranes: Implications in Membrane Lysis

Melittin, a hemolytic peptide present in bee venom, represents one of the most well-studied amphipathic antimicrobial peptides, particularly in terms of its membrane interaction and activity. Nevertheless, no consensus exists on the oligomeric state of membrane-bound melittin. We previously reported on the differential microenvironments experienced by melittin in zwitterionic and negatively charged phospholipid membranes. In this work, we explore the role of negatively charged lipids in the oligomerization of membrane-bound melittin (labeled with 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)) utilizing a quantitative photobleaching homo-FRET assay. Our results show that the presence of negatively charged lipids decreases melittin oligomeric size to ∼50% of that observed in zwitterionic membranes. This is possibly due to differential energetics of binding of the peptide monomer to membranes of different compositions and could explain the reduced lytic activity yet tighter binding of melittin in negatively charged membranes. These results constitute one of the first experimental observations on the role of phospholipid headgroup charge in the oligomerization of melittin in membranes and is relevant in light of previous apparently contradictory reports on oligomerization of membrane-bound melittin. Our results highlight the synergistic interplay of peptide-membrane binding events and peptide oligomerization in modulating the organization, dynamics, and function of amphipathic α-helical peptides.

Environment-Sensitive Fluorescence of 7-Nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-Labeled Ligands for Serotonin Receptors

Serotonin is a neurotransmitter that plays a crucial role in the regulation of several behavioral and cognitive functions by binding to a number of different serotonin receptors present on the cell surface. We report here the synthesis and characterization of several novel fluorescent analogs of serotonin in which the fluorescent NBD (7-nitrobenz-2-oxa-1,3-diazol-4-yl) group is covalently attached to serotonin. The fluorescent ligands compete with the serotonin_1A receptor specific radiolabeled agonist for binding to the receptor. Interestingly, these fluorescent ligands display a high environmental sensitivity of their fluorescence. Importantly, the human serotonin_1A receptor stably expressed in CHO-K1 cells could be specifically labeled with one of the fluorescent ligands with minimal nonspecific labeling. Interestingly, we show by spectral imaging that the NBD-labeled ligand exhibits a red edge excitation shift (REES) of 29 nm when bound to the receptor, implying that it is localized in a restricted microenvironment. Taken together, our results show that NBD-labeled serotonin analogs offer an attractive fluorescent approach for elucidating the molecular environment of the serotonin binding site in serotonin receptors. In view of the multiple roles played by the serotonergic systems in the central and peripheral nervous systems, these fluorescent ligands would be useful in future studies involving serotonin receptors.

Conformational heterogeneity of the voltage sensor loop of KvAP in micelles and membranes: A fluorescence approach

KvAP is a tetrameric voltage-gated potassium channel that is composed of a pore domain and a voltage-sensing domain (VSD). The VSD is crucial for sensing transmembrane potential and gating. At 0 mV, the VSD adopts an activated conformation in both n-octylglucoside (OG) micelles and phospholipid membranes. Importantly, gating-modifier toxins that bind at S3b-S4 loop of KvAP-VSD exhibit pronounced differences in binding affinity in these membrane-mimetic systems. However, the conformational heterogeneity of this functionally-important sensor loop in membrane mimetics is poorly understood, and is the focus of this work. In this paper, we establish, using intrinsic fluorescence of the uniquely positioned W70 in KvAP-VSD and environment-sensitive NBD (7-nitrobenz-2-oxa-1,3-diazol-4-yl-ethylenediamine) fluorescence of the labelled S3b-S4 loop, that the surface charge of the membrane does not significantly affect the topology and structural dynamics of the sensor loop in membranes. Importantly, the dynamic variability of the sensor loop is preserved in both zwitterionic (POPC) and anionic (POPC/POPG) membranes. Further, the lifetime distribution analysis for the NBD-labelled residues by maximum entropy method (MEM) demonstrates that, in contrast to micelles, the membrane environment not only reduces the relative discrete population of sensor loop conformations, but also broadens the lifetime distribution peaks. Overall, our results strongly suggest that the conformational heterogeneity of the sensor loop is significantly altered in membranes and this correlates well with its environmental heterogeneity. This constitutes the first report demonstrating that MEM-lifetime distribution could be a powerful tool to distinguish changes in conformational heterogeneity in potassium channels with similar architecture and topology.

Novel insights in linking solvent relaxation dynamics and protein conformations utilizing red edge excitation shift approach

Protein hydration dynamics plays an important role in many physiological processes since protein fluctuations, slow solvation, and the dynamics of hydrating water are all intrinsically related. Red edge excitation shift (REES) is a unique and powerful wavelength-selective (i.e. excitation-energy dependent) fluorescence approach that can be used to directly monitor the environment-induced restriction and dynamics around a polar fluorophore in a complex biological system. This review is mainly focused on recent applications of REES and a novel analysis of REES data to monitor the structural dynamics, functionally relevant conformational transitions and to unmask the structural ensembles in proteins. In addition, the novel utility of REES in imaging protein aggregates in a cellular context is discussed. We believe that the enormous potential of REES approach showcased in this review will engage more researchers, particularly from life sciences.

Gating-related Structural Dynamics of the MgtE Magnesium Channel in Membrane-Mimetics Utilizing Site-Directed Tryptophan Fluorescence

Magnesium is the most abundant divalent cation present in the cell, and an abnormal Mg²⁺ homeostasis is associated with several diseases in humans. However, among ion channels, the mechanisms of intracellular regulation and transport of Mg²⁺ are poorly understood. MgtE is a homodimeric Mg²⁺-selective channel and is negatively regulated by high intracellular Mg²⁺ concentration where the cytoplasmic domain of MgtE acts as a Mg²⁺ sensor. Most of the previous biophysical studies on MgtE have been carried out in detergent micelles and the information regarding gating-related structural dynamics of MgtE in physiologically-relevant membrane environment is scarce. In this work, we monitored the changes in gating-related structural dynamics, hydration dynamics and conformational heterogeneity of MgtE in micelles and membranes using the intrinsic site-directed Trp fluorescence. For this purpose, we have engineered six single-Trp mutants in the functional Trp-less background of MgtE to obtain site-specific information on the gating-related structural dynamics of MgtE in membrane-mimetic systems. Our results indicate that Mg²⁺-induced gating might involve the possibility of a 'conformational wave' from the cytosolic N-domain to transmembrane domain of MgtE. Although MgtE is responsive to Mg²⁺-induced gating in both micelles and membranes, the organization and dynamics of MgtE is substantially altered in physiologically important phospholipid membranes compared to micelles. This is accompanied by significant changes in hydration dynamics and conformational heterogeneity. Overall, our results highlight the importance of lipid-protein interactions and are relevant for understanding gating mechanism of magnesium channels in general, and MgtE in particular.

Site-Directed Fluorescence Approaches for Dynamic Structural Biology of Membrane Peptides and Proteins

Membrane proteins mediate a number of cellular functions and are associated with several diseases and also play a crucial role in pathogenicity. Due to their importance in cellular structure and function, they are important drug targets for ~60% of drugs available in the market. Despite the technological advancement and recent successful outcomes in determining the high-resolution structural snapshot of membrane proteins, the mechanistic details underlining the complex functionalities of membrane proteins is least understood. This is largely due to lack of structural dynamics information pertaining to different functional states of membrane proteins in a membrane environment. Fluorescence spectroscopy is a widely used technique in the analysis of functionally-relevant structure and dynamics of membrane protein. This review is focused on various site-directed fluorescence (SDFL) approaches and their applications to explore structural information, conformational changes, hydration dynamics, and lipid-protein interactions of important classes of membrane proteins that include the pore-forming peptides/proteins, ion channels/transporters and G-protein coupled receptors.

Effects of Peptide Charge, Orientation, and Concentration on Melittin Transmembrane Pores

Melittin is a short cationic peptide that exerts cytolytic effects on bacterial and eukaryotic cells. Experiments suggest that in zwitterionic membranes, melittin forms transmembrane toroidal pores supported by four to eight peptides. A recently constructed melittin variant with a reduced cationic charge, MelP5, is active at 10-fold lower concentrations. In previous work, we performed molecular dynamics simulations on the microsecond timescale to examine the supramolecular pore structure of a melittin tetramer in zwitterionic and partially anionic membranes. We now extend that study to include the effects of peptide charge, initial orientation, and number of monomers on the pore formation and stabilization processes. Our results show that parallel transmembrane orientations of melittin and MelP5 are more consistent with experimental data. Whereas a MelP5 parallel hexamer forms a large stable pore during the 5-μs simulation time, a melittin hexamer and an octamer are not fully stable, with several monomers dissociating during the simulation time. Interaction-energy analysis shows that this difference in behavior between melittin and MelP5 is not due to stronger electrostatic repulsion between neighboring melittin peptides but to peptide-lipid interactions that disfavor the isolated MelP5 transmembrane monomer. The ability of melittin monomers to diffuse freely in the 1,2-dimyristoyl-SN-glycero-3-phosphocholine membrane leads to dynamic pores with varying molecularity.

Structural Dynamics of the Paddle Motif Loop in the Activated Conformation of KvAP Voltage Sensor

Voltage-dependent potassium (K_v) channels play a fundamental role in neuronal and cardiac excitability and are potential therapeutic targets. They assemble as tetramers with a centrally located pore domain surrounded by a voltage-sensing domain (VSD), which is critical for sensing transmembrane potential and subsequent gating. Although the sensor is supposed to be in "Up" conformation in both n-octylglucoside (OG) micelles and phospholipid membranes in the absence of membrane potential, toxins that bind VSD and modulate the gating behavior of K_v channels exhibit dramatic affinity differences in these membrane-mimetic systems. In this study, we have monitored the structural dynamics of the S3b-S4 loop of the paddle motif in activated conformation of KvAP-VSD by site-directed fluorescence approaches, using the environment-sensitive fluorescent probe 7-nitrobenz-2-oxa-1,3-diazol-4-yl-ethylenediamine (NBD). Emission maximum of NBD-labeled loop region of KvAP-VSD (residues 110-117) suggests a significant change in the polarity of local environment in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) membranes compared to OG micelles. This indicates that S3b-S4 loop residues might be partitioning to membrane interface, which is supported by an overall increased mean fluorescence lifetimes and significantly reduced water accessibility in membranes. Further, the magnitude of red edge excitation shift (REES) supports the presence of restricted/bound water molecules in the loop region of the VSD in micelles and membranes. Quantitative analysis of REES data using Gaussian probability distribution function clearly indicates that the sensor loop has fewer discrete equilibrium conformational states when reconstituted in membranes. Interestingly, this reduced molecular heterogeneity is consistent with the site-specific NBD polarization results, which suggest that the membrane environment offers a relaxed/dynamic organization for most of the S3b-S4 loop residues of the sensor. Overall, our results are relevant for understanding toxin-VSD interaction and gating mechanisms of K_v channels in membranes.

The influence of cholesterol on melittin lipidation in neutral membranes

Cholesterol inclusion in membranes influences the rate and selectivity of acyl transfer from lipids to a membrane-embedded peptide.

Potential of mean force for insertion of antimicrobial peptide melittin into a pore in mixed DOPC/DOPG lipid bilayer by molecular dynamics simulation

Antimicrobial peptides (AMPs) inactivate microorganisms by forming transmembrane pores in a cell membrane through adsorption and aggregation. Energetics of addition of an AMP to a transmembrane pore is important for evaluation of its formation and growth. Such information is essential for the characterization of pore forming ability of peptides in cell membranes. This study quantifies the potential of mean force through molecular dynamics (MD) simulation for the addition of melittin, a naturally occurring AMP, into a DOPC/DOPG mixed bilayer, a mimic of bacterial membrane, for different extents of insertion into either a bilayer or a pore consisting of three to six transmembrane peptides. The energy barrier for insertion of a melittin molecule into the bilayer was highest in the absence of transmembrane peptides and decreased for the number of transmembrane peptides from three to six, eventually approaching zero. The decrease in free energy for complete insertion of peptide was found to be higher for larger pore size. Water channel formation occurred only for insertion into pores consisting of three or more transmembrane peptides with the radius of water channel being larger for a larger number of transmembrane peptides. The structure of the pore was found to be paraboloid. The estimated free energy barrier for insertion of melittin into an ideal paraboloid pore accounting for different intermolecular interactions was consistent with MD simulation results. The results reported in this manuscript will be useful for the development of a model for nucleation of pores and a rational methodology for selection of synthetic antimicrobial peptides.

Host defense cathelicidins in cattle: types, production, bioactive functions and potential therapeutic and diagnostic applications

Cathelicidins are a primitive class of host defense peptides and are known for their broad-spectrum antimicrobial activity against bacteria, fungi, and enveloped viruses. These small, cationic, proteolytically-activated peptides are diverse in structure, encompassing a wide range of activities on host immune and inflammatory cell responses. The dual capacity of cathelicidins to directly control infection and regulate host defenses highlights the potential use of these peptides as alternatives to antibiotics and immunomodulators. Cathelicidins are found in many mammalian species; this review focuses on bovine cathelicidins. Eight naturally and two synthetically occurring bovine cathelicidins are described in detail, with a focus on recent advances in their expression, location and biological roles. This review also presents an overview of the bioactive functions of cathelicidins in bovine mastitis, a disease causing economic losses in cattle dairy production. Comparison of the structural, antimicrobial, cytotoxic and mechanistic properties of bovine cathelicidins advances the knowledge needed for the development of these peptides as potential identifiers of infectious diseases (e.g., bovine mastitis) and as novel therapeutic alternatives to antibiotics.

Imaging Cellular Dynamics with Spectral Relaxation Imaging Microscopy: Distinct Spectral Dynamics in Golgi Membranes of Living Cells

Spectral relaxation from fluorescent probes is a useful technique for determining the dynamics of condensed phases. To this end, we have developed a method based on wide-field spectral fluorescence lifetime imaging microscopy to extract spectral relaxation correlation times of fluorescent probes in living cells. We show that measurement of the phase and modulation of fluorescence from two wavelengths permit the identification and determination of excited state lifetimes and spectral relaxation correlation times at a single modulation frequency. For NBD fluorescence in glycerol/water mixtures, the spectral relaxation correlation time determined by our approach exhibited good agreement with published dielectric relaxation measurements. We applied this method to determine the spectral relaxation dynamics in membranes of living cells. Measurements of the Golgi-specific C₆-NBD-ceramide probe in living HeLa cells revealed sub-nanosecond spectral dynamics in the intracellular Golgi membrane and slower nanosecond spectral dynamics in the extracellular plasma membrane. We interpret the distinct spectral dynamics as a result of structural plasticity of the Golgi membrane relative to more rigid plasma membranes. To the best of our knowledge, these results constitute one of the first measurements of Golgi rotational dynamics.

Toward the de novo design of antimicrobial peptides: Lack of correlation between peptide permeabilization of lipid vesicles and antimicrobial, cytolytic, or cytotoxic activity in living cells

We previously performed a lipid vesicle-based, high-throughput screen on a 26-residue combinatorial peptide library that was designed de novo to yield membrane-permeabilizing peptides that fold into β-sheets. The most active and soluble library members that were identified permeabilized lipid vesicles detectably, but not with high potency. Nonetheless, they were broad-spectrum, membrane-permeabilizing antibiotics with minimum sterilizing activity at low µM concentrations. In an expansion of that work, we recently performed an iterative screen in which an active consensus sequence from that first-generation library was used as a template to design a second-generation library which was then screened against lipid vesicles at very high stringency. Compared to the consensus sequence from the first library, the most active second-generation peptides are highly potent, equilibrium pore-formers in synthetic lipid vesicles. Here, we use these first- and second-generation families of peptides to test the hypothesis that a large increase in potency in bacteria-like lipid vesicles will correlate with a large improvement in antimicrobial activity. The results do not support the hypothesis. Despite a 20-fold increase in potency against bacteria-like lipid vesicles, the second-generation peptides are only slightly more active against bacteria, and at the same time, are also more toxic against mammalian cells. The results suggest that a "pipeline" strategy toward the optimization of antimicrobial peptides could begin with a vesicle-based screen for identifying families with broad-spectrum activity, but will also need to include screening or optimization steps that are done under conditions that are more directly relevant to possible therapeutic applications.

Characterization of antimicrobial activity against Listeria and cytotoxicity of native melittin and its mutant variants

Antimicrobial peptides (AMPs) are relatively short peptides that have the ability to penetrate the cell membrane, form pores leading to cell death. This study compares both antimicrobial activity and cytotoxicity of native melittin and its two mutants, namely, melittin I17K (GIGAVLKVLTTGLPALKSWIKRKRQQ) with a higher charge and lower hydrophobicity and mutant G1I (IIGAVLKVLTTGLPALISWIKRKRQQ) of higher hydrophobicity. The antimicrobial activity against different strains of Listeria was investigated by bioassay, viability studies, fluorescence and transmission electron microscopy. Cytotoxicity was examined by lactate dehydrogenase (LDH) assay on mammalian Caco-2 cells. The minimum inhibitory concentration of native, mutant I17K, mutant G1I against Listeria monocytogenes F4244 was 0.315±0.008, 0.814±0.006 and 0.494±0.037μg/ml respectively, whereas the minimum bactericidal concentration values were 3.263±0.0034, 7.412±0.017 and 5.366±0.019μg/ml respectively. Lag time for inactivation of L. monocytogenes F4244 was observed at concentrations below 0.20 and 0.78μg/ml for native and mutant melittin I17K respectively. The antimicrobial activity against L. monocytogenes F4244 was in the order native>G1I>I17K. Native melittin was cytotoxic to mammalian Caco-2 cells above concentration of 2μg/ml, whereas the two mutants exhibited negligible cytotoxicity up to a concentration of 8μg/ml. Pore formation in cell wall/membrane was observed by transmission electron microscopy. Molecular dynamics (MD) simulation of native and its mutants indicated that (i) surface native melittin and G1I exhibited higher tendency to penetrate a mimic of bacterial cell membrane and (ii) transmembrane native and I17K formed water channel in mimics of bacterial and mammalian cell membranes.

Effects of N-terminus modifications on the conformation and permeation activities of the synthetic peptide L1A

We investigate the effect of the N-terminus modification of the L1A, a synthetic octadecapeptide, on its helical content, affinity and lytic action in model membranes and on its hemolytic and antibacterial activities. L1A and its acetylated analog displayed a selective antibacterial activity to Gram-negative bacteria without being hemolytic. The covalently linked 2-aminobezoic acid to the N-terminus impaired the antibacterial efficacy and increased hemolysis. Despite their lower net charge (+2), N-terminus modifications resulted in enhanced affinity and improved lytic efficiency in anionic vesicles. The analogs also showed higher helical content and consequently higher amphipathicity in these vesicles. The conformational analysis by molecular dynamics simulations in 30 % of TFE/water showed that the hydrophobic faces of the peptides are in close contact with CF3 groups of TFE while the hydrophilic faces with water molecules. Due to the loss of the amino charge, the N-termini of the analogs are buried in TFE molecules. The analysis of the pair distribution functions, obtained for the center of mass of the charged groups, has evidenced that the state of the N-terminus has influenced the possibility of different ion-pairing. The higher complexity of the bacterial cells compared with anionic vesicles hampers to establish correlations structure-function for the analogs.

Constitutive expression of a novel antimicrobial protein, Hcm1, confers resistance to both Verticillium and Fusarium wilts in cotton

Fusarium and Verticillium wilts, two of the most important diseases in cotton, pose serious threats to cotton production. Here we introduced a novel antimicrobial protein Hcm1, which comprised harpin protein from Xanthomonas oryzae pv. oryzicola (Xoc), and the chimeric protein, cecropin A-melittin, into cotton. The transgenic cotton lines with stable Hcm1 expression showed a higher resistance to Verticillium and Fusarium wilts both in greenhouse and field trials compared to controls. Hcm1 enabled the transgenic cotton to produced a microscopic hypersensitive response (micro-HR), reactive oxygen species (ROS) burst, and caused the activation of pathogenesis-related (PR) genes in response to biotic stress, indicating that the transgenic cotton was in a primed state and ready to protect the host from pathogenic infection. Simultaneously, Hcm1 protein inhibited the growth of Verticillium dahliae (V. dahliae) and Fusarium oxysporum (F. oxysporum) in vitro. The spread of fungal biomass was also inhibited in vivo since the V. dahliae biomass was decreased dramatically in transgenic cotton plants after inoculation with V. dahliae. Together, these results demonstrate that Hcm1 could activate innate immunity and inhibit the growth of V. dahliae and F. oxysporum to protect cotton against Verticillium and Fusarium wilts.

Site of fluorescent label modifies interaction of melittin with live cells and model membranes

The mechanism of membrane disruption by melittin (MLT) of giant unilamellar vesicles (GUVs) and live cells was studied using fluorescence microscopy and two fluorescent synthetic analogues of MLT. The N-terminus of one of these was acylated with thiopropionic acid to enable labeling with maleimido-AlexaFluor 430 to study the interaction of MLT with live cells. It was compared with a second analogue labeled at P14C. The results indicated that the fluorescent peptides adhered to the membrane bilayer of phosphatidylcholine GUVs and inserted into the plasma membrane of HeLa cells. Fluorescence and light microscopy revealed changes in cell morphology after exposure to MLT peptides and showed bleb formation in the plasma membrane of HeLa cells. However, the membrane disruptive effect was dependent upon the location of the fluorescent label on the peptide and was greater when MLT was labeled at the N-terminus. Proline at position 14 appeared to be important for antimicrobial activity, hemolysis and cytotoxicity, but not essential for cell membrane disruption.

Antibacterial and anti-inflammatory activity of a temporin B peptide analogue on an in vitro model of cystic fibrosis

Natural peptides with antimicrobial properties are deeply investigated as tools to fight bacteria resistant to common antibiotics. Small peptides, as those belonging to the temporin family, are very attractive because their activity can easily be tuned after small modification to their primary sequence. Structure-activity studies previously reported by us allowed the identification of one peptide, analogue of temporin B, TB_KKG6A, showing, unlike temporin B, antimicrobial activity against both Gram-positive and Gram-negative bacteria. In this paper, we investigated the antimicrobial and anti-inflammatory activity of the peptide TB_KKG6A against Pseudomonas aeruginosa. Interestingly, we found that the peptide exhibits antimicrobial activity at low concentrations, being able to downregulate the pro-inflammatory chemokines and cytokines interleukin (IL)-8, IL-1β, IL-6 and tumor necrosis factor-α produced downstream infected human bronchial epithelial cells. Experiments were carried out also with temporin B, which was found to show pro-inflammatory activity. Details on the interaction between TB_KKG6A and the P. aeruginosa LPS were obtained by circular dichroism and fluorescence studies.

Protective effects of melittin on tumor necrosis factor-α induced hepatic damage through suppression of apoptotic pathway and nuclear factor-kappa B activation

Melittin, a major polypeptide in honeybee venom, have been used to treat inflammatory disease. Various studies have demonstrated the anti-bacterial, anti-viral, anti-inflammatory and anticancer effects of bee venom and melittin. However, the precise mechanism of melittin in liver disease is not yet known. Apoptosis contributes to liver inflammation and fibrosis. Knowledge of the apoptotic mechanisms is important to develop new and effective therapies for treatment of cirrhosis. In the present study, we investigated the anti-apoptotic effect of melittin on tumor necrosis factor (TNF)-α/actinomycin (Act) D-induced apoptosis in hepatocytes. Our results show significant protection from DNA damage by melittin treatment compared with corresponding TNF-α/Act D-treated hepatocytes without melittin. Melittin inhibited TNF-α/Act D-induced activation of the caspase, bcl-2 family of proteins and poly ADP-ribose polymerase (PARP)-1. Our results also indicate that melittin decreased nuclear factor-kappa B (NF-κB) by degradation of phosphorylation of IκB kinase (p-IKK) and NF-κB DNA binding activity in TNF-α/Act D-treated hepatocytes. These results suggest that melittin possesses a potent suppressive effect on apoptotic responses in TNF-α/Act D-treated hepatocytes via the NF-κB pathway.

Dynamics transitions at the outer vestibule of the KcsA potassium channel during gating

In K(+) channels, the selectivity filter, pore helix, and outer vestibule play a crucial role in gating mechanisms. The outer vestibule is an important structurally extended region of KcsA in which toxins, blockers, and metal ions bind and modulate the gating behavior of K(+) channels. Despite its functional significance, the gating-related structural dynamics at the outer vestibule are not well understood. Under steady-state conditions, inactivating WT and noninactivating E71A KcsA stabilize the nonconductive and conductive filter conformations upon opening the activation gate. Site-directed fluorescence polarization of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled outer vestibule residues shows that the outer vestibule of open/conductive conformation is highly dynamic compared with the motional restriction experienced by the outer vestibule during inactivation gating. A wavelength-selective fluorescence approach shows a change in hydration dynamics in inactivated and noninactivated conformations, and supports a possible role of restricted/bound water molecules in C-type inactivation gating. Using a unique restrained ensemble simulation method, along with distance measurements by EPR, we show that, on average, the outer vestibule undergoes a modest backbone conformational change during its transition to various functional states, although the structural dynamics of the outer vestibule are significantly altered during activation and inactivation gating. Taken together, our results support the role of a hydrogen bond network behind the selectivity filter, side-chain conformational dynamics, and water molecules in the gating mechanisms of K(+) channels.

Dual-fluorescence L-amino acid reports insertion and orientation of melittin peptide in cell membranes

Monitoring insertion and orientation of peptides in situ on cell membranes remains a challenge. To this end, we synthesized an l-amino acid (AFaa) containing a dual-fluorescence dye of the 3-hydroxyflavone family, as a side chain. In contrast to other labeling approaches using a flexible linker, the AFaa fluorophore, introduced by solid phase synthesis into desired position of a peptide, is attached closely to its backbone with well-defined orientation, and, therefore, could reflect its localization in the membrane. This concept was validated by replacing the leucine-9 (L9) and tryptophan-19 (W19) residues by AFaa in melittin, a well-studied membrane-active peptide. Due to high sensitivity of AFaa dual emission to the environment polarity, we detected a much deeper insertion of L9 peptide position into the bilayer, compared to the W19 position. Moreover, using fluorescence microscopy with a polarized light excitation, we found different orientation of AFaa at L9 and W19 positions of melittin in the bilayers of giant vesicles and cellular membranes. These results suggested that in the natural membranes, similarly to the model lipid bilayers, melittin is preferentially oriented parallel to the membrane surface. The developed amino acid and the proposed methodology will be of interest to study other membrane peptides.

Toxicity of bovicin HC5 against mammalian cell lines and the role of cholesterol in bacteriocin activity

Bacteriocins are ribosomally synthesized antimicrobial peptides produced by Bacteria and some Archaea. The assessment of the toxic potential of antimicrobial peptides is important in order to apply these peptides on an industrial scale. The aim of the present study was to investigate the in vitro cytotoxic and haemolytic potential of bovicin HC5, as well as to determine whether cholesterol influences bacteriocin activity on model membranes. Nisin, for which the mechanism of action is well described, was used as a reference peptide in our assays. The viability of three distinct eukaryotic cell lines treated with bovicin HC5 or nisin was analysed by using the MTT assay and cellular morphological changes were determined by light microscopy. The haemolytic potential was evaluated by using the haemoglobin liberation assay and the role of cholesterol on bacteriocin activity was examined by using model membranes composed of DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) and DPoPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine). The IC(50) of bovicin HC5 and nisin against Vero cells was 65.42 and 13.48 µM, respectively. When the MTT assay was performed with MCF-7 and HepG2 cells, the IC(50) obtained for bovicin HC5 was 279.39 and 289.30 µM, respectively, while for nisin these values were 105.46 and 112.25 µM. The haemolytic activity of bovicin HC5 against eukaryotic cells was always lower than that determined for nisin. The presence of cholesterol did not influence the activity of either bacteriocin on DOPC model membranes, but nisin showed reduced carboxyfluorescein leakage in DPoPC membranes containing cholesterol. In conclusion, bovicin HC5 only exerted cytotoxic effects at concentrations that were greater than the concentration needed for its biological activity, and the presence of cholesterol did not affect its interaction with model membranes.

Molecular response and cooperative behavior during the interactions of melittin with a membrane: dissipative quartz crystal microbalance experiments and simulations

The molecular-level interactions of an antimicrobial peptide melittin with supported membrane were studied by the combination of dissipative quartz crystal microbalance (QCM-D) experiments and computer simulations. We found the response behavior of lipids upon peptide adsorption greatly influence their interactions. The perturbance and reorientation of the lipid in liquid phase facilitate the insertion of melittin in a trans-membrane way, but in solid phase, asymmetrical membrane disruption happens. Apart from the lipid state, the local peptide-to-lipid ratio also affects the insertion capacity of melittin. When the local peptide number density is high, adjacent peptides can cooperatively penetrate into the membrane. This observation explains the occurrence of the conventional "carpet" mechanism.

Gain-of-function analogues of the pore-forming peptide melittin selected by orthogonal high-throughput screening

We recently developed an orthogonal, high-throughput assay to identify peptides that self-assemble into potent, equilibrium pores in synthetic lipid bilayers. Here, we use this assay as a high-throughput screen to select highly potent pore-forming peptides from a 7776-member rational combinatorial peptide library based on the sequence of the natural pore-forming peptide toxin melittin. In the library we varied ten critical residues in the melittin sequence, chosen to test specific structural hypotheses about the mechanism of pore formation. Using the new high-throughput assay, we screened the library for gain-of-function sequences at a peptide to lipid ratio of 1:1000 where native melittin is not active. More than 99% of the library sequences were also inactive under these conditions. A small number of library members (0.1%) were highly active. From these we identified 14 potent, gain-of-function, pore-forming sequences. These sequences differed from melittin in only 2-6 amino acids out of 26. Some native residues were highly conserved and others were consistently changed. The two factors that were essential for gain-of-function were the preservation of melittin's proline-dependent break in the middle of the helix and the improvement and extension the amphipathic nature of the α-helix. In particular the highly cationic carboxyl-terminal sequence of melittin, is consistently changed in the gain-of-function variants to a sequence that it is capable of participating in an extended amphipathic α-helix. The most potent variants reside in a membrane-spanning orientation, in contrast to the parent melittin, which is predominantly surface bound. This structural information, taken together with the high-throughput tools developed for this work, enable the identification, refinement and optimization of pore-forming peptides for many potential applications.

A novel antimicrobial protein for plant protection consisting of a Xanthomonas oryzae harpin and active domains of cecropin A and melittin

Discoveries about antimicrobial peptides and plant defence activators have made possible the de novo and rational design of novel peptides for use in crop protection. Here we report a novel chimeric protein, Hcm1, which was made by linking the active domains of cecropin A and melittin to the hypersensitive response (HR)‐elicitor Hpa1 of Xanthomonas oryzae pv. oryzicola, the causal agent of rice bacterial leaf streak. The resulting chimeric protein maintained not only the HR‐inducing property of the harpin, but also the antimicrobial activity of the cecropin A‐melittin hybrid. Hcm1 was purified from engineered Escherichia coli and evaluated in terms of the minimal inhibitory concentration (MIC) and the 50% effective dose (ED₅₀) against important plant pathogenic bacteria and fungi. Importantly, the protein acted as a potential pesticide by inducing disease resistance for viral, bacterial and fungal pathogens. This designed drug can be considered as a lead compound for use in plant protection, either for the development of new broad‐spectrum pesticides or for expression in transgenic plants.

The role of electrostatic interactions in the membrane binding of melittin

The binding of melittin and the C-terminally truncated analogue of melittin (21Q) to a range of phospholipid bilayers was studied using surface plasmon resonance (SPR). The phospholipid model membranes included zwitterionic dimyristylphosphatidylcholine (DMPC) and dimyristylphosphatidylethanolamine (DMPE), together with mixtures DMPC/dimyristylphosphatidylglycerol (DMPG), DMPC/DMPG/cholesterol and DMPE/DMPG. Melittin bound rapidly to all membrane mixtures, whereas 21Q, which has a reduced charge, bound much more slowly on the DMPC and DMPC/DMPG mixtures reflecting the role of the initial electrostatic interaction. The loss of the cationic residues also significantly decreased the binding of 21Q with DMPC/DMPG/Cholesterol, DMPE and DMPE/DMPG. The role of electrostatics was also highlighted with NaCl in the buffer, which affected the way melittin bound to the different membranes, causing a more uniform, concentration dependant increase in response. The biosensor results were correlated with the conformation of the peptides determined by circular dichroism analysis, which indicated that high α-helicity was associated with high binding affinity. Overall, the results demonstrate that the positively charged residues at the C-terminus of melittin play an essential role in membrane binding, that modulation of peptide charge influences selectivity of binding to different phospholipids and that manipulation of the cationic regions of antimicrobial peptides can be used to modulate membrane selectivity.

Structural dynamics of a lytic peptide interacting with a supported lipid bilayer

The interaction of a melittin mutant with a 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC)-supported lipid bilayer was studied with the use of time-resolved evanescent wave-induced fluorescence spectroscopy (TREWIFS) and evanescent wave-induced time-resolved fluorescence anisotropy measurements (EW-TRAMs). The mutant peptide was labeled at position K14 with AlexaFluor 430 and retained the lytic activity characteristic of native melittin. The fluorescence decay kinetics of the conjugate was found to be biexponential with a short-lived component, τ(1), due to photoinduced electron transfer between AlexaFluor 430 and proximal side chains within or between the peptides. The longer-lived component, τ(2), was sensitive to the polarity of the microenvironment at or near the K14 position of the peptide. Upon interaction with a DPPC-supported bilayer, the proportional contribution of τ(1) increased, indicating a conformational change of the peptide. The values of τ(1) and τ(2) indicate that the AlexaFluor 430 probe experienced an environment with an equivalent polarity no less than that of methanol. EW-TRAMs data from the melittin mutant revealed hindered rotational motions of the AlexaFluor 430 probe both in the plane and perpendicular to the plane of the supported lipid bilayer. The data indicate a highly ordered and polar environment near the center of the melittin helix consistent with the formation of a toroidal pore.

Organization and dynamics of membrane probes and proteins utilizing the red edge excitation shift

Dynamics of confined water has interesting implications in the organization and function of molecular assemblies such as membranes. A direct consequence of this type of organization is the restriction imposed on the mobility of the constituent structural units. Interestingly, this restriction (confinement) of mobility couples the motion of solvent (water) molecules with the slow moving molecules in the assembly. It is in this context that the red edge excitation shift (REES) represents a sensitive approach to monitor the environment and dynamics around a fluorophore in such organized assemblies. A shift in the wavelength of maximum fluorescence emission toward higher wavelengths, caused by a shift in the excitation wavelength toward the red edge of the absorption band, is termed REES. REES relies on slow solvent reorientation in the excited state of a fluorophore that can be used to monitor the environment and dynamics around a fluorophore in a host assembly. In this article, we focus on the application of REES to monitor organization and dynamics of membrane probes and proteins.