Membrane-promoted unfolding of acetylcholinesterase: a possible mechanism for insertion into the lipid bilayer

Tertiary Plasticity Drives the Efficiency of Enterocin 7B Interactions with Lipid Membranes

The ability of antimicrobial peptides to efficiently kill their bacterial targets depends on the efficiency of their binding to the microbial membrane. In the case of enterocins, there is a three-part interaction: initial binding, unpacking of helices on the membrane surface, and permeation of the lipid bilayer. Helical unpacking is driven by disruption of the peptide hydrophobic core when in contact with membranes. Enterocin 7B is a leaderless enterocin antimicrobial peptide produced from Enterococcus faecalis that functions alone, or with its cognate partner enterocin 7A, to efficiently kill a wide variety of Gram-stain positive bacteria. To better characterize the role that tertiary structural plasticity plays in the ability of enterocin 7B to interact with the membranes, a series of arginine single-site mutants were constructed that destabilize the hydrophobic core to varying degrees. A series of experimental measures of structure, stability, and function, including CD spectra, far UV CD melting profiles, minimal inhibitory concentrations analysis, and release kinetics of calcein, show that decreased stabilization of the hydrophobic core is correlated with increased efficiency of a peptide to permeate membranes and in killing bacteria. Finally, using the computational technique of adaptive steered molecular dynamics, we found that the atomistic/energetic landscape of peptide mechanical unfolding leads to free energy differences between the wild type and its mutants, whose trends correlate well with our experiment.

Aberrant environment and PS-binding to calnuc C-terminal tail drives exosomal packaging and its metastatic ability

The characteristic features of cancer cells are aberrant (acidic) intracellular pH and elevated levels of phosphatidylserine. The primary focus of cancer research is concentrated on the discovery of biomarkers directed towards early diagnosis and therapy. It has been observed that azoxymethane-treated mice demonstrate an increased expression of calnuc (a multi-domain, Ca2+- and DNA-binding protein) in their colon, suggesting it to be a good biomarker of carcinogenesis. We show that culture supernatants from tumor cells have significantly higher amounts of secreted calnuc compared to non-tumor cells, selectively packaged into exosomes. Exosomal calnuc is causal for epithelial-mesenchymal transition and atypical migration in non-tumor cells, which are key events in tumorigenesis and metastasis. In vitro studies reveal a significant affinity for calnuc towards phosphatidylserine, specifically to its C-terminal region, leading to the formation of 'molten globule' conformation. Similar structural changes are observed at acidic pH (pH 4), which demonstrates the role of the acidic microenvironment in causing the molten globule conformation and membrane interaction. On a precise note, we propose that the molten globule structure of calnuc caused by aberrant conditions in cancer cells to be the causative mechanism underlying its exosome-mediated secretion, thereby driving metastasis.

Generation of Reactive Oxygen Species by Photosensitizers and their Modes of Action on Proteins

In this review, we first survey the mechanisms underlying the chemical modification of amino acid residues in proteins by singlet oxygen elicited by photosensitizers. Singlet oxygen has the capacity to cause widespread chemical damage to cellular proteins. Its use in photodynamic therapy of tumors thus requires the development of methodologies for specific addressing of the photosensitizer to malignant cells while sparing normal tissue. We describe three targeting paradigms for achieving this objective. The first involves the use of a photosensitizer with a high affinity for its target protein; in this case, the photosensitizer is methylene blue for acetylcholinesterase. The second paradigm involves the use of the hydrophobic photosensitizer hypericin, which has the capacity to interact selectively with partially unfolded forms of proteins, including nascent species in rapidly dividing or virus-infected and cancer cells, acting preferentially at membrane interfaces. In this case, partially unfolded molten globule species of acetylcholinesterase serve as the model system. In the third paradigm, the photodynamic approach takes advantage of a general approach in 'state-of-the-art' chemotherapy, by coupling the photosensitizer emodin to a specific peptide hormone, GnRH, which recognizes malignant cells via specific GnRH receptors on their surface.

Cation modulation of hemoglobin interaction with sodium n-dodecyl sulfate (SDS). III: Calcium interaction with R- and mixed spin states of hemoglobin S at pH 5.0: the musical chair paradox

We investigate the interaction of Ca(2+) (0-500 µM) and a membrane mimic (0.60 mM SDS) with both the R- and mixed spin states hemoglobin S (HbS) as a function of time. These interactions were carried out at pH 5.0. We aim at ascertaining if there is or are differences in the UV-Visible spectra of such interactions to account for the dynamics of calcium ion concentrations [Ca(2+)] in initiating structures which may ultimately suggest HbS polymerization and or resistance to Plasmodium attack. From our results, we conclude that (a) simultaneous interaction of 40 µM Ca(2+) and 0.60 mM SDS with the R state protein would promote structural formations that can "lock up" the protein for nucleation on the membranes and or become cytotoxic to the parasite; (b) simultaneous R state HbS-SDS or R state HbS-Ca(2+) would lead to enhanced hemin formation and less deoxyHb species. This condition is unlikely to precipitate polymerization in the HbS but the resulting hemin would poison the parasite; (c) the mixed spin state HbS-SDS and 40 µM Ca(2+) interaction yields more toxic products to that of the interaction of the mixed spin HbS-SDS with 500 µM Ca(2+) thus suggesting why the 40 µM Ca(2+) is important in parasite Hb proteolysis; and (d) pronounced structural changes on interaction with SDS and Ca(2+) are more in the R state to the mixed spin state.

Origin of the conformational heterogeneity of cardiolipin-bound cytochrome C

Interactions of cytochrome c (cyt c) with cardiolipin (CL) partially unfold the protein, activating its peroxidase function, a critical event in the execution of apoptosis. However, structural features of the altered protein species in the heterogeneous ensemble are difficult to probe with ensemble averaging. Analyses of the dye-to-heme distance distributions P(r) from time-resolved FRET (TR-FRET) have uncovered two distinct types of CL-bound cyt c conformations, extended and compact. We have combined TR-FRET, fluorescence correlation spectroscopy (FCS), and biolayer interferometry to develop a systematic understanding of the functional partitioning between the two conformations. The two subpopulations are in equilibrium with each other, with a submillisecond rate of conformational exchange reflecting the protein folding into a compact non-native state, as well as protein interactions with the lipid surface. Electrostatic interactions with the negatively charged lipid surface that correlate with physiologically relevant changes in CL concentrations strongly affect the kinetics of cyt c binding and conformational exchange. A predominantly peripheral binding mechanism, rather than deep protein insertion into the membrane, provides a rationale for the general denaturing effect of the CL surface and the large-scale protein unfolding. These findings closely relate to cyt c folding dynamics and suggest a general strategy for extending the time window in monitoring the kinetics of folding.

Selective and irreversible inhibitors of mosquito acetylcholinesterases for controlling malaria and other mosquito-borne diseases

New insecticides are urgently needed because resistance to current insecticides allows resurgence of disease-transmitting mosquitoes while concerns for human toxicity from current compounds are growing. We previously reported the finding of a free cysteine (Cys) residue at the entrance of the active site of acetylcholinesterase (AChE) in some insects but not in mammals, birds, and fish. These insects have two AChE genes (AP and AO), and only AP-AChE carries the Cys residue. Most of these insects are disease vectors such as the African malaria mosquito (Anopheles gambiae sensu stricto) or crop pests such as aphids. Recently we reported a Cys-targeting small molecule that irreversibly inhibited all AChE activity extracted from aphids while an identical exposure caused no effect on the human AChE. Full inhibition of AChE in aphids indicates that AP-AChE contributes most of the enzymatic activity and suggests that the Cys residue might serve as a target for developing better aphicides. It is therefore worth investigating whether the Cys-targeting strategy is applicable to mosquitocides. Herein, we report that, under conditions that spare the human AChE, a methanethiosulfonate-containing molecule at 6 microM irreversibly inhibited 95% of the AChE activity extracted from An. gambiae s. str. and >80% of the activity from the yellow fever mosquito (Aedes aegypti L.) or the northern house mosquito (Culex pipiens L.) that is a vector of St. Louis encephalitis. This type of inhibition is fast ( approximately 30 min) and due to conjugation of the inhibitor to the active-site Cys of mosquito AP-AChE, according to our observed reactivation of the methanethiosulfonate-inhibited AChE by 2-mercaptoethanol. We also note that our sulfhydryl agents partially and irreversibly inhibited the human AChE after prolonged exposure (>4 hr). This slow inhibition is due to partial enzyme denaturation by the inhibitor and/or micelles of the inhibitor, according to our studies using atomic force microscopy, circular dichroism spectroscopy, X-ray crystallography, time-resolved fluorescence spectroscopy, and liquid chromatography triple quadrupole mass spectrometry. These results support our view that the mosquito-specific Cys is a viable target for developing new mosquitocides to control disease vectors and to alleviate resistance problems with reduced toxicity toward non-target species.

Thermal denaturation of wild type and mutant recombinant acetylcholinesterase from amphioxus: effects of the temperature of in vitro expression and of reversible inhibitors

We have studied the thermal inactivation at 37 degrees C of wild type and mutant ChE2 (C310A, F312I, C466A, C310A/F312I, and C310A/C466A) from amphioxus (Branchiostoma floridae) expressed in vitro in COS-7 monkey cells under three sets of conditions: 30 degrees C for 48 h, 30 degrees C for 24 h and 37 degrees C for 24 h, and 37 degrees C for 48 h. We found biphasic denaturation curves for all enzymes and conditions, except wild type and C310A ChE2 expressed at 30 degrees C for 48 h. Generally, single mutants are more unstable than wild type, and the double mutants are even more unstable. We propose a model involving stable and unstable conformations of the enzymes to explain these results, and we discuss the implications of the model. We also found a correlation between the melting temperature of the ChEs and the rates at which they denature at 37 degrees C, with the denaturation of the unstable conformation dominating the relationship. Reversible cholinergic inhibitors protect the ChEs from thermal denaturation, and in some cases produce monophasic denaturation curves; we also propose a model to explain this stabilization.

Binding of lysozyme to phospholipid bilayers: evidence for protein aggregation upon membrane association

Biological functions of lysozyme, including its antimicrobial, antitumor, and immune-modulatory activities have been suggested to be largely determined by the lipid binding properties of this protein. To gain further insight into these interactions on a molecular level the association of lysozyme to liposomes composed of either 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine or its mixtures with 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-phosphatidylserine, or bovine heart cardiolipin was studied by a combination of fluorescence techniques. The characteristics of the adsorption of lysozyme to lipid bilayers were investigated using fluorescein 5'-isothiocyanate labeled protein, responding to membrane association by a decrease in fluorescence. Upon increasing the content of anionic phospholipids in lipid vesicles, the binding isotherms changed from Langmuir-like to sigmoidal. Using adsorption models based on scaled particle and double-layer theories, this finding was rationalized in terms of self-association of the membrane-bound protein. The extent of quenching of lysozyme tryptophan fluorescence by acrylamide decreased upon membrane binding, revealing a conformational transition for the protein upon its surface association, resulting in a diminished access of the fluorophore to the aqueous phase. Steady-state fluorescence anisotropy of bilayer-incorporated probe 1,6-diphenyl-1,3,5-hexatriene was measured at varying lipid-to-protein molar ratios. Lysozyme was found to increase acyl-chain order for liposomes with the content of acidic phospholipid exceeding 10 mol %. Both electrostatic and hydrophobic protein-lipid interactions can be concluded to modulate the aggregation behavior of lysozyme when bound to lipid bilayers. Modulation of lysozyme aggregation propensity by membrane binding may have important implications for protein fibrillogenesis in vivo. Disruption of membrane integrity by the aggregated protein species is likely to be the mechanism responsible for the cytotoxicity of lysozyme.

The role of lipid–protein interactions in amyloid-type protein fibril formation

Structural transition of polypeptide chains into the beta-sheet state followed by amyloid fibril formation is the key characteristic of a number of the so-called conformational diseases. The multistep process of protein fibrillization can be modulated by a variety of factors, in particular by lipid-protein interactions. A wealth of experimental evidence provides support to the notion that amyloid fibril assembly and the toxicity of pre-fibrillar aggregates are closely related and are both intimately membrane associated phenomena. The present review summarizes the principal factors responsible for the enhancement of fibril formation in a membrane environment, viz. (i) structural transformation of polypeptide chain into a partially folded conformation, (ii) increase of the local concentration of a protein upon its membrane binding, (iii) aggregation-favoring orientation of the bound protein, and (iv) variation in the depth of bilayer penetration affecting the nucleation propensity of the membrane associated protein. The molecular mechanisms of membrane-mediated protein fibrillization are discussed. Importantly, the toxicity of lipid-induced pre-fibrillar aggregates is likely to have presented a very strong negative selection pressure in the evolution of amino acid sequences.

Disease-related misassembly of membrane proteins

Medical genetics so far has identified approximately 16,000 missense mutations leading to single amino acid changes in protein sequences that are linked to human disease. A majority of these mutations affect folding or trafficking, rather than specifically affecting protein function. Many disease-linked mutations occur in integral membrane proteins, a class of proteins about whose folding we know very little. We examine the phenomenon of disease-linked misassembly of membrane proteins and describe model systems currently being used to study the delicate balance between proper folding and misassembly. We review a mechanism by which cells recognize membrane proteins with a high potential to misfold before they actually do, and which targets these culprits for degradation. Serious disease phenotypes can result from loss of protein function and from misfolded proteins that the cells cannot degrade, leading to accumulation of toxic aggregates. Misassembly may be averted by small-molecule drugs that bind and stabilize the native state.

Stabilization of a metastable state of Torpedo californica acetylcholinesterase by chemical chaperones

Chemical modification of Torpedo californica acetylcholinesterase by the natural thiosulfinate allicin produces an inactive enzyme through reaction with the buried cysteine Cys 231. Optical spectroscopy shows that the modified enzyme is "native-like," and inactivation can be reversed by exposure to reduced glutathione. The allicin-modified enzyme is, however, metastable, and is converted spontaneously and irreversibly, at room temperature, with t(1/2) approximately 100 min, to a stable, partially unfolded state with the physicochemical characteristics of a molten globule. Osmolytes, including trimethylamine-N-oxide, glycerol, and sucrose, and the divalent cations, Ca(2+), Mg(2+), and Mn(2+) can prevent this transition of the native-like state for >24 h at room temperature. Trimethylamine-N-oxide and Mg(2+) can also stabilize the native enzyme, with only slight inactivation being observed over several hours at 39 degrees C, whereas in their absence it is totally inactivated within 5 min. The stabilizing effects of the osmolytes can be explained by their differential interaction with the native and native-like states, resulting in a shift of equilibrium toward the native state. The stabilizing effects of the divalent cations can be ascribed to direct stabilization of the native state, as supported by differential scanning calorimetry.

Nonclassical transport proteins and peptides: an alternative to classical macromolecule delivery systems

The number of peptides and proteins known to exhibit nonclassical transport activity has increased significantly in recent years. In most cases, these entities have been studied in relation to their ability to deliver high molecular weight compounds, including proteins and DNA, for the ultimate purpose of developing new drug delivery strategies. In this review, an overview of the various types of vectors is presented. The in vitro and in vivo delivery successes of this technology, as well as preliminary therapeutic efforts, are described. Although a comprehensive mechanism of nonclassical transport has not yet been clearly established, we propose a straightforward model based on the cationic nature of the vectors and the need for lack of highly organized structure. In this hypothesis we suggest that the movement of polycations is mediated by a network of extra- and intracellular polyanions while transport across the bilayer is facilitated by cation-pi interactions between the vectors' basic groups and aromatic amino acid side chains in the bilayer spanning helices of membrane proteins.

Thermal denaturation of Bungarus fasciatus acetylcholinesterase: Is aggregation a driving force in protein unfolding?

A monomeric form of acetylcholinesterase from the venom of Bungarus fasciatus is converted to a partially unfolded molten globule species by thermal inactivation, and subsequently aggregates rapidly. To separate the kinetics of unfolding from those of aggregation, single molecules of the monomeric enzyme were encapsulated in reverse micelles of Brij 30 in 2,2,4-trimethylpentane, or in large unilamellar vesicles of egg lecithin/cholesterol at various protein/micelle (vesicle) ratios. The first-order rate constant for thermal inactivation at 45 degrees C, of single molecules entrapped within the reverse micelles (0.031 min(-1)), was higher than in aqueous solution (0.007 min(-1)) or in the presence of normal micelles (0.020 min(-1)). This clearly shows that aggregation does not provide the driving force for thermal inactivation of BfAChE. Within the large unilamellar vesicles, at average protein/vesicle ratios of 1:1 and 10:1, the first-order rate constants for thermal inactivation of the encapsulated monomeric acetylcholinesterase, at 53 degrees C, were 0.317 and 0.342 min(-1), respectively. A crosslinking technique, utilizing the photosensitive probe, hypericin, showed that thermal denaturation produces a distribution of species ranging from dimers through to large aggregates. Consequently, at a protein/vesicle ratio of 10:1, aggregation can occur upon thermal denaturation. Thus, these experiments also demonstrate that aggregation does not drive the thermal unfolding of Bungarus fasciatus acetylcholinesterase. Our experimental approach also permitted monitoring of recovery of enzymic activity after thermal denaturation in the absence of a competing aggregation process. Whereas no detectable recovery of enzymic activity could be observed in aqueous solution, up to 23% activity could be obtained for enzyme sequestered in the reverse micelles.

Chemical modification of bacterial alpha-amylases: changes in tertiary structures and the effect of additional calcium

A comparative study was performed on the effect of calcium on native and chemically modified forms of mesophilic and thermophilic alpha-amylases. Circular dichroism (CD) and irreversible thermoinactivation studies were carried out in the absence and presence of 10 mM calcium. From the CD experiments, changes in the tertiary structure of these enzymes, brought about by modification, were concluded. Furthermore, these changes were found to be influenced by the presence of calcium. Sorbitol was very effective in affording protection against irreversible thermoinactivation of native and modified forms of the enzymes, both in the absence and presence of calcium. Results are discussed in terms of the usefulness of this new approach involving a combination of medium and chemical modification for protein stabilization and enhancement of catalytic potential.

Partly folded states of bovine carbonic anhydrase interact with zwitterionic and anionic lipid membranes

The acidic, partly folded states of bovine carbonic anhydrase II (BCAII) were used as an experimental system to study the interactions of partly denatured proteins with lipid membranes. The pH dependence of their interactions with palmitoyloleoyl phosphatidylcholine (POPC) and palmitoyloleoyl phosphatidylglycerol (POPG) membranes was studied. A filtration binding assay shows that acidic partly folded states of BCAII bind to POPC membranes. Fluorescence emission spectra from Trp residues of the bound protein are slightly shifted to shorter wavelength and can be quenched by a water-soluble quencher of fluorescence, indicating that the binding occurs without deep penetration of Trp residues into the membrane. The content of beta-structures of the protein in solution, as revealed by FT-IR spectroscopy, decreases in the partly folded states and the binding to POPC membrane occurs without further changes of secondary structure. In the presence of 0.1 M NaCl, a partly folded state self-aggregates and does not bind to POPC membrane. At acidic pH, BCAII binds to POPG membranes both at high and low ionic strength. The binding to the anionic lipid occurs with protein self-aggregation within the lipid-protein complexes and with changes in the secondary structure; large blue shifts in the fluorescence emission spectra and the decrease in the exposure to water-soluble acrylamide quencher of Trp fluorescence strongly suggest that BCAII penetrates the hydrocarbon domain in the POPG-protein complexes.

Misfolding of membrane proteins in health and disease: the lady or the tiger?

Protein misfolding is increasingly recognized as a factor in many diseases, including cystic fibrosis, Parkinson's, Alzheimer's and atherosclerosis. Many proteins involved in misfolding-based pathologies are membrane-associated, such that the bilayer may play roles in normal and aberrant folding. It can be argued that the in vivo partitioning of eukaryotic membrane proteins between folding and misfolding pathways is under kinetic control. Moreover, the balance between these pathways can be surprisingly delicate.

Membrane environment alters the conformational structure of the recombinant human prion protein

The prion protein (PrP) in a living cell is associated with cellular membranes. However, all previous biophysical studies with the recombinant prion protein have been performed in an aqueous solution. To determine the effect of a membrane environment on the conformational structure of PrP, we studied the interaction of the recombinant human prion protein with model lipid membranes. The protein was found to bind to acidic lipid-containing membrane vesicles. This interaction is pH-dependent and becomes particularly strong under acidic conditions. Spectroscopic data show that membrane binding of PrP results in a significant ordering of the N-terminal part of the molecule. The folded C-terminal domain, on the other hand, becomes destabilized upon binding to the membrane surface, especially at low pH. Overall, these results show that the conformational structure and stability of the recombinant human PrP in a membrane environment are substantially different from those of the free protein in solution. These observations have important implications for understanding the mechanism of the conversion between the normal (PrP(C)) and pathogenic (PrP(Sc)) forms of prion protein.

Conformational change in the pheromone-binding protein from Bombyx mori induced by pH and by interaction with membranes

The pheromone-binding protein (PBP) from Bombyx mori was expressed in Escherichia coli periplasm. It specifically bound radiolabeled bombykol, the natural pheromone for this species. It appeared as a single band both in native and SDS-polyacrylamide gel electrophoresis and was also homogeneous in most chromatographic systems. However, in ion-exchange chromatography, multiple forms sometimes appeared. Attempts to separate them revealed that they could be converted into one another. Analysis of the protein by circular dichroism and fluorescence spectroscopy demonstrated that its tertiary structure was sensitive to pH changes and that a dramatic conformational transition occurred between pH 6.0 and 5.0. This high sensitivity to pH contrasted markedly with its thermal stability and resistance to denaturation by urea. There was also no significant change in CD spectra in the presence of the pheromone. The native protein isolated from male antennae displayed the same changes in its spectroscopic properties as the recombinant material, demonstrating that this phenomenon is not an artifact arising from the expression system. This conformational transition was reproduced by interaction of the protein with anionic (but not neutral) phospholipid vesicles. Unfolding of the PBP structure triggered by membranes suggests a plausible mechanism for ligand release upon interaction of the PBP-pheromone complex with the surface of olfactory neurons. This pH-linked structural flexibility also explains the heterogeneity reported previously for B. mori PBP and other members of this class of proteins.