Conformational states and thermodynamics of alpha-lactalbumin bound to membranes: a case study of the effects of pH, calcium, lipid membrane curvature and charge

B2LiVe, a label-free 1D-NMR method to quantify the binding of amphitropic peptides or proteins to membrane vesicles

Amphitropic proteins and peptides reversibly partition from solution to membrane, a key process that regulates their functions. Experimental approaches classically used to measure protein partitioning into lipid bilayers, such as fluorescence and circular dichroism, are hardly usable when the peptides or proteins do not exhibit significant polarity and/or conformational changes upon membrane binding. Here, we describe binding to lipid vesicles (B2LiVe), a simple, robust, and widely applicable nuclear magnetic resonance (NMR) method to determine the solution-to-membrane partitioning of unlabeled proteins or peptides. B2LiVe relies on previously described proton 1D-NMR fast-pulsing techniques. Membrane partitioning induces a large line broadening, leading to a loss of protein signals; therefore, the decrease of the NMR signal directly measures the fraction of membrane-bound protein. The method uses low polypeptide concentrations and has been validated on several membrane-interacting polypeptides, ranging from 3 to 54 kDa, with membrane vesicles of different sizes and various lipid compositions.

α-casein micelles-membranes interaction: Flower-like lipid protein coaggregates formation

BACKGROUND

Environmental conditions regulate the association/aggregation states of proteins and their action in cellular compartments. Analysing protein behaviour in presence of lipid membranes is fundamental for the comprehension of many functional and dysfunctional processes. Here, we present an experimental study on the interaction between model membranes and α-casein. α-casein is the major component of milk proteins and it is recognised to play a key role in performing biological functions. The conformational properties of this protein and its capability to form supramolecular structures, like micelles or irreversible aggregates, are key effectors in functional and pathological effects.

METHODS

By means of quantitative fluorescence imaging and complementary spectroscopic methods, we were able to characterise α-casein association state and the course of events induced by pH changes, which regulate the interaction of this molecule with membranes.

RESULTS

The study of these complex dynamic events revealed that the initial conformation of the protein critically regulates the fate of α-casein, size and structure of the newly formed aggregates and their effect on membrane structures. Disassembly of micelles due to modification in electrostatic interactions results in increased membrane structure rigidity which accompanies the formation of protein lipid flower-like co-aggregates with protein molecules localised in the external part.

GENERAL SIGNIFICANCE

These results may contribute to the comprehension of how the initial state of a protein establishes the course of events that occur upon changes in the molecular environment. These events which may occur in cells may be essential to functional, pathological or therapeutical properties specifically associated to casein proteins.

A new variant of the human α-lactalbumin-oleic acid complex as an anticancer agent for chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder of hematopoietic stem cells. Although there have been advancements in treatment, there is still a need to develop a biotherapeutic agent. A new variant of the human alpha-lactalbumin-oleic acid (HALOA) complex has been synthesized, which showed similarities with SNARE. The native α-LA was treated with EDTA to remove Ca²⁺ ions confirmed by ICP-OES and Arsenazo III to unfold and attain apo structure. The apo LA was mixed with OA in a specific ratio, leading to HALOA complex formation. The conformational state from native to complex was elucidated by circular dichroism (far; 190–260 nm and near; 260–340 nm UV-CD), which confirmed that the complex consists of a majority of turns and β-sheet structure. SDS-PAGE result showed the masking effect of OA on apo α-LA. In the lane of the complex, there was no band detected. However, 1-anilino-8-naphthalene sulfonate (ANS) dye has shown maximum fluorescence intensity with the complex because of the availability of hydrophobic patches, which was further validated by NMR spectroscopy indicating the masking effect of OA on the apo α-LA. The SNARE behavior of the complex (500 nm) has been confirmed by TEM. This new structural variant complex shows anti-tumor activity on chronic myeloid leukemia by targeting the IL-8, survivin, and induces apoptosis through DNA fragmentation, but not against normal cells. Overall, the formulated complex shows that SNARE-like behavior can be used as a promising anti-tumor agent with lower toxicity and maximum bioavailability.

Investigating the Disordered and Membrane-Active Peptide A-Cage-C Using Conformational Ensembles

The driving forces and conformational pathways leading to amphitropic protein-membrane binding and in some cases also to protein misfolding and aggregation is the subject of intensive research. In this study, a chimeric polypeptide, A-Cage-C, derived from α-Lactalbumin is investigated with the aim of elucidating conformational changes promoting interaction with bilayers. From previous studies, it is known that A-Cage-C causes membrane leakages associated with the sporadic formation of amorphous aggregates on solid-supported bilayers. Here we express and purify double-labelled A-Cage-C and prepare partially deuterated bicelles as a membrane mimicking system. We investigate A-Cage-C in the presence and absence of these bicelles at non-binding (pH 7.0) and binding (pH 4.5) conditions. Using in silico analyses, NMR, conformational clustering, and Molecular Dynamics, we provide tentative insights into the conformations of bound and unbound A-Cage-C. The conformation of each state is dynamic and samples a large amount of overlapping conformational space. We identify one of the clusters as likely representing the binding conformation and conclude tentatively that the unfolding around the central W23 segment and its reorientation may be necessary for full intercalation at binding conditions (pH 4.5). We also see evidence for an overall elongation of A-Cage-C in the presence of model bilayers.

α-Lactalbumin, Amazing Calcium-Binding Protein

α-Lactalbumin (α-LA) is a small (Mr 14,200), acidic (pI 4–5), Ca²⁺-binding protein. α-LA is a regulatory component of lactose synthase enzyme system functioning in the lactating mammary gland. The protein possesses a single strong Ca²⁺-binding site, which can also bind Mg²⁺, Mn²⁺, Na⁺, K⁺, and some other metal cations. It contains several distinct Zn²⁺-binding sites. Physical properties of α-LA strongly depend on the occupation of its metal binding sites by metal ions. In the absence of bound metal ions, α-LA is in the molten globule-like state. The binding of metal ions, and especially of Ca²⁺, increases stability of α-LA against the action of heat, various denaturing agents and proteases, while the binding of Zn²⁺ to the Ca²⁺-loaded protein decreases its stability and causes its aggregation. At pH 2, the protein is in the classical molten globule state. α-LA can associate with membranes at neutral or slightly acidic pH at physiological temperatures. Depending on external conditions, α-LA can form amyloid fibrils, amorphous aggregates, nanoparticles, and nanotubes. Some of these aggregated states of α-LA can be used in practical applications such as drug delivery to tissues and organs. α-LA and some of its fragments possess bactericidal and antiviral activities. Complexes of partially unfolded α-LA with oleic acid are cytotoxic to various tumor and bacterial cells. α-LA in the cytotoxic complexes plays a role of a delivery carrier of cytotoxic fatty acid molecules into tumor and bacterial cells across the cell membrane. Perhaps in the future the complexes of α-LA with oleic acid will be used for development of new anti-cancer drugs.

Direct observation of alpha-lactalbumin, adsorption and incorporation into lipid membrane and formation of lipid/protein hybrid structures

The interaction between proteins and membranes is of great interest in biomedical and biotechnological research for its implication in many functional and dysfunctional processes. We present an experimental study on the interaction between model membranes and alpha-lactalbumin (α-La). α-La is widely studied for both its biological function and its anti-tumoral properties. We use advanced fluorescence microscopy and spectroscopy techniques to characterize α-La-membrane mechanisms of interaction and α-La-induced modifications of membranes when insertion of partially disordered regions of protein chains in the lipid bilayer is favored. Moreover, using fluorescence lifetime imaging, we are able to distinguish between protein adsorption and insertion in the membranes. Our results indicate that, upon addition of α-La to giant vesicles samples, protein is inserted into the lipid bilayer with rates that are concentration-dependent. The formation of heterogeneous hybrid protein-lipid co-aggregates, paralleled with protein conformational and structural changes, alters the membrane structure and morphology, leading to an increase in membrane fluidity.

Membrane-Active Properties of an Amphitropic Peptide from the CyaA Toxin Translocation Region

The adenylate cyclase toxin CyaA is involved in the early stages of infection by Bordetella pertussis, the causative agent of whooping cough. CyaA intoxicates target cells by a direct translocation of its catalytic domain (AC) across the plasma membrane and produces supraphysiological levels of cAMP, leading to cell death. The molecular process of AC translocation remains largely unknown, however. We have previously shown that deletion of residues 375–485 of CyaA selectively abrogates AC translocation into eukaryotic cells. We further identified within this “translocation region” (TR), P454 (residues 454–484), a peptide that exhibits membrane-active properties, i.e., is able to bind and permeabilize lipid vesicles. Here, we analyze various sequences from CyaA predicted to be amphipatic and show that although several of these peptides can bind membranes and adopt a helical conformation, only the P454 peptide is able to permeabilize membranes. We further characterize the contributions of the two arginine residues of P454 to membrane partitioning and permeabilization by analyzing the peptide variants in which these residues are substituted by different amino acids (e.g., A, K, Q, and E). Our data shows that both arginine residues significantly contribute, although diversely, to the membrane-active properties of P454, i.e., interactions with both neutral and anionic lipids, helix formation in membranes, and disruption of lipid bilayer integrity. These results are discussed in the context of the translocation process of the full-length CyaA toxin.

Stability, structural and functional properties of a monomeric, calcium-loaded adenylate cyclase toxin, CyaA, from Bordetella pertussis

Bordetella pertussis, the causative agent of whooping cough, secretes an adenylate cyclase toxin, CyaA, which invades eukaryotic cells and alters their physiology by cAMP overproduction. Calcium is an essential cofactor of CyaA, as it is the case for most members of the Repeat-in-ToXins (RTX) family. We show that the calcium-bound, monomeric form of CyaA, hCyaAm, conserves its permeabilization and haemolytic activities, even in a fully calcium-free environment. In contrast, hCyaAm requires sub-millimolar calcium in solution for cell invasion, indicating that free calcium in solution is involved in the CyaA toxin translocation process. We further report the first in solution structural characterization of hCyaAm, as deduced from SAXS, mass spectrometry and hydrodynamic studies. We show that hCyaAm adopts a compact and stable state that can transiently conserve its conformation even in a fully calcium-free environment. Our results therefore suggest that in hCyaAm, the C-terminal RTX-domain is stabilized in a high-affinity calcium-binding state by the N-terminal domains while, conversely, calcium binding to the C-terminal RTX-domain strongly stabilizes the N-terminal regions. Hence, the different regions of hCyaAm appear tightly connected, leading to stabilization effects between domains. The hysteretic behaviour of CyaA in response to calcium is likely shared by other RTX cytolysins.

Molecular Basis of Membrane Association by the Phosphatidylinositol Mannosyltransferase PimA Enzyme from Mycobacteria

Phosphatidyl-myo-inositol mannosyltransferase A (PimA) is an essential glycosyltransferase that initiates the biosynthetic pathway of phosphatidyl-myo-inositol mannoside, lipomannan, and lipoarabinomannan, which are key glycolipids/lipoglycans of the mycobacterial cell envelope. PimA belongs to a large family of membrane-associated glycosyltransferases for which the understanding of the molecular mechanism and conformational changes that govern substrate/membrane recognition and catalysis remains a major challenge. Here, we determined that PimA preferentially binds to negatively charged phosphatidyl-myo-inositol substrate and non-substrate membrane model systems (small unilamellar vesicle) through its N-terminal domain, inducing an important structural reorganization of anionic phospholipids. By using a combination of single-point mutagenesis, circular dichroism, and a variety of fluorescence spectroscopy techniques, we determined that this interaction is mainly mediated by an amphipathic α-helix (α2), which undergoes a substantial conformational change and localizes in the vicinity of the negatively charged lipid headgroups and the very first carbon atoms of the acyl chains, at the PimA-phospholipid interface. Interestingly, a flexible region within the N-terminal domain, which undergoes β-strand-to-α-helix and α-helix-to-β-strand transitions during catalysis, interacts with anionic phospholipids; however, the effect is markedly less pronounced to that observed for the amphipathic α2, likely reflecting structural plasticity/variability. Altogether, we propose a model in which conformational transitions observed in PimA might reflect a molten globule state that confers to PimA, a higher affinity toward the dynamic and highly fluctuating lipid bilayer.

A simple interfacial pH detection method for cationic amphiphilic self-assemblies utilizing a Schiff-base molecule

A simple pH-sensing method to monitor interfacial pH deviation from the bulk pH for cationic micelle and vesicle is introduced by estimating the change in the Schiff-base molecule (AH) proton dissociation between interface and bulk.

Characterization of a membrane-active peptide from the Bordetella pertussis CyaA toxin

Bordetella pertussis, the pathogenic bacteria responsible for whooping cough, secretes several virulence factors, among which is the adenylate cyclase toxin (CyaA) that plays a crucial role in the early stages of human respiratory tract colonization. CyaA invades target cells by translocating its catalytic domain directly across the plasma membrane and overproduces cAMP, leading to cell death. The molecular process leading to the translocation of the catalytic domain remains largely unknown. We have previously shown that the catalytic domain per se, AC384, encompassing residues 1-384 of CyaA, did not interact with lipid bilayer, whereas a longer polypeptide, AC489, spanning residues 1-489, binds to membranes and permeabilizes vesicles. Moreover, deletion of residues 375-485 within CyaA abrogated the translocation of the catalytic domain into target cells. Here, we further identified within this region a peptidic segment that exhibits membrane interaction properties. A synthetic peptide, P454, corresponding to this sequence (residues 454-485 of CyaA) was characterized by various biophysical approaches. We found that P454 (i) binds to membranes containing anionic lipids, (ii) adopts an α-helical structure oriented in plane with respect to the lipid bilayer, and (iii) permeabilizes vesicles. We propose that the region encompassing the helix 454-485 of CyaA may insert into target cell membrane and induce a local destabilization of the lipid bilayer, thus favoring the translocation of the catalytic domain across the plasma membrane.

Identification of a region that assists membrane insertion and translocation of the catalytic domain of Bordetella pertussis CyaA toxin

The adenylate cyclase (CyaA) toxin, one of the virulence factors secreted by Bordetella pertussis, the pathogenic bacteria responsible for whooping cough, plays a critical role in the early stages of respiratory tract colonization by this bacterium. The CyaA toxin is able to invade eukaryotic cells by translocating its N-terminal catalytic domain directly across the plasma membrane of the target cells, where, activated by endogenous calmodulin, it produces supraphysiological levels of cAMP. How the catalytic domain is transferred from the hydrophilic extracellular medium into the hydrophobic environment of the membrane and then to the cell cytoplasm remains an unsolved question. In this report, we have characterized the membrane-interacting properties of the CyaA catalytic domain. We showed that a protein covering the catalytic domain (AC384, encompassing residues 1-384 of CyaA) displayed no membrane association propensity. However, a longer polypeptide (AC489), encompassing residues 1-489 of CyaA, exhibited the intrinsic property to bind to membranes and to induce lipid bilayer destabilization. We further showed that deletion of residues 375-485 within CyaA totally abrogated the toxin's ability to increase intracellular cAMP in target cells. These results indicate that, whereas the calmodulin dependent enzymatic domain is restricted to the amino-terminal residues 1-384 of CyaA, the membrane-interacting, translocation-competent domain extends up to residue 489. This thus suggests an important role of the region adjacent to the catalytic domain of CyaA in promoting its interaction with and its translocation across the plasma membrane of target cells.

NMR-NOE and MD simulation study on phospholipid membranes: dependence on membrane diameter and multiple time scale dynamics

Motional correlation times between the hydrophilic and hydrophobic terminal groups in lipid membranes are studied over a wide range of curvatures using the solution-state (1)H NMR-nuclear Overhauser effect (NOE) and molecular dynamics (MD) simulation. To enable (1)H NMR-NOE measurements for large vesicles, the transient NOE method is combined with the spin-echo method, and is successfully applied to a micelle of 1-palmitoyl-lysophosphatidylcholine (PaLPC) with diameter of 5 nm and to vesicles of dipalmitoylphosphatidylcholine (DPPC) with diameters ranging from 30 to 800 nm. It is found that the NOE intensity increases with the diameter up to ∼100 nm, and the model membrane is considered planar on the molecular level beyond ∼100 nm. While the NOE between the hydrophilic terminal and hydrophobic terminal methyl groups is absent for the micelle, its intensity is comparable to that for the neighboring group for vesicles with larger diameters. The origin of NOE signals between distant sites is analyzed by MD simulations of PaLPC micelles and DPPC planar bilayers. The slow relaxation is shown to yield an observable NOE signal even for the hydrophilic and hydrophobic terminal sites. Since the information on distance and dynamics cannot be separated in the experimental NOE alone, the correlation time in large vesicles is determined by combining the experimental NOE intensity and MD-based distance distribution. For large vesicles, the correlation time is found to vary by 2 orders of magnitude over the proton sites. This study shows that NOE provides dynamic information on large vesicles when combined with MD, which provides structural information.

Conformational dynamics of the bovine mitochondrial ADP/ATP carrier isoform 1 revealed by hydrogen/deuterium exchange coupled to mass spectrometry

The mitochondrial adenine nucleotide carrier (Ancp) catalyzes the transport of ADP and ATP across the mitochondrial inner membrane, thus playing an essential role in cellular energy metabolism. During the transport mechanism the carrier switches between two different conformations that can be blocked by two toxins: carboxyatractyloside (CATR) and bongkrekic acid. Therefore, our understanding of the nucleotide transport mechanism can be improved by analyzing structural differences of the individual inhibited states. We have solved the three-dimensional structure of bovine carrier isoform 1 (bAnc1p) in a complex with CATR, but the structure of the carrier-bongkrekic acid complex, and thus, the detailed mechanism of transport remains unknown. Improvements in sample processing in the hydrogen/deuterium exchange technique coupled to mass spectrometry (HDX-MS) have allowed us to gain novel insights into the conformational changes undergone by bAnc1p. This paper describes the first study of bAnc1p using HDX-MS. Results obtained with the CATR-bAnc1p complex were fully in agreement with published results, thus, validating our approach. On the other hand, the HDX kinetics of the two complexes displays marked differences. The bongkrekic acid-bAnc1p complex exhibits greater accessibility to the solvent on the matrix side, whereas the CATR-bAnc1p complex is more accessible on the intermembrane side. These results are discussed with respect to the structural and biochemical data available on Ancp.

Accessibility changes within diphtheria toxin T domain when in the functional molten globule state, as determined using hydrogen/deuterium exchange measurements

The translocation domain (T domain) of diphtheria toxin adopts a partially folded state, the so-called molten globule state, to become functional at acidic pH. We compared, using hydrogen/deuterium exchange experiments associated with MS, the structures of the T domain in its soluble folded state at neutral pH and in its functional molten globule state at acidic pH. In the native state, the alpha-helices TH5 and TH8 are identified as the core of the domain. Based on the high-resolution structure of the T domain, we propose that TH8 is highly protected because it is buried within the native structure. According to the same structure, TH5 is partly accessible at the surface of the T domain. We propose that its high protection is caused by the formation of dimers. Within the molten globule state, high protection is still observed within the helical hairpin TH8-TH9, which is responsible for the insertion of the T domain into the membrane. In the absence of the lipid bilayer, this hydrophobic part of the domain self-assembles, leading to the formation of oligomers. Overall, hydrogen/deuterium-exchange measurements allow the analysis of interaction contacts within small oligomers made of partially folded proteins. Such information, together with crystal structure data, are particularly valuable for using to analyze the self-assembly of proteins.

The immediate-early oncoproteins Fra-1, c-Fos, and c-Jun have distinguishable surface behavior and interactions with phospholipids

This work explores the surface properties of the transcription factor Fra-1 and compares them with those of two other immediate early proteins, c-Fos and c-Jun, to establish generalities and differences in the surface behavior and interaction with phospholipids of this type of proteins. We present several experimental clues of the flexible nature of Fra-1, c-Fos, and c-Jun that support sequence-based predictions of their intrinsical disorder. The values of surface parameters for Fra-1 are similar in general to those of c-Fos and c-Jun. However, we find differences in the interactions of the three proteins with phospholipids. The closely related Fra-1 and c-Fos share affinity for anionic lipids but the former has more affinity for a condensed phase and senses a change in DPPC phase, while the latter has more affinity for an expanded phase. These features are in contrast with our previous finding that c-Jun is not selective for phospholipid polar head group or charge. We show here that at least some immediate early transcription factors can interact with membrane phospholipids in a distinguishable manner, and this shall provide a basis for their potential capacity to regulate membrane-mediated cellular processes.

Structural dynamics of soluble chloride intracellular channel protein CLIC1 examined by amide hydrogen-deuterium exchange mass spectrometry

Chloride intracellular channel protein 1 (CLIC1) functions as an anion channel in plasma and nuclear membranes when its soluble monomeric form converts to an integral-membrane form. The transmembrane region of CLIC1 is located in its thioredoxin-like domain 1, but the mechanism whereby the protein converts to its membrane conformation has yet to be determined. Since channel formation in membranes is enhanced at low pH (5 to 5.5), a condition that is found at the surface of membranes, the structural dynamics of soluble CLIC1 was studied at pH 7 and at pH 5.5 in the absence of membranes by amide hydrogen-deuterium exchange mass spectrometry (DXMS). Rapid hydrogen exchange data indicate that CLIC1 displays a similar core structure at these pH values. Domain 1 is less stable than the all-helical domain 2, and, while the structure of domain 1 remains intact, its conformational flexibility is further increased in an acidic environment (pH 5.5). In the absence of membrane, an acidic environment appears to prime the solution structure of CLIC1 by destabilizing domain 1 in order to lower the activation energy barrier for its conversion to the membrane-insertion conformation. The significantly enhanced H/D-exchange rates at pH 5.5 displayed by two segments (peptides 11-31 and 68-82) could be due to the protonation of acidic residues in salt bridges. One of these segments (peptide 11-31) includes part of the transmembrane region which, in the solution structure, consists of helix alpha1. This helix is intrinsically stable and is most likely retained in the membrane conformation. Strand beta2, another element of the transmembrane region, displays a propensity to form a helical structure and has putative N- and C-capping motifs, suggesting that it too most likely forms a helix in a lipid bilayer.

alpha-lactalbumin-AOT charge interactions tune phase structures in isooctane/brine mixtures

Self-assembly of the anionic surfactant AOT with the protein alpha-lactalbumin in isooctane/brine mixtures results in phase structures whose type, size, and shape differ considerably from those formed by the surfactant alone. Small-angle X-ray scattering was used to determine the size and shape of these structures for 5.4 < pH < 11.2 and 0.25, 0.33, and 0.4 wt % NaCl. All pH values were above the reported isoelectric point for the protein. The composition of the system (except for salt) was fixed, with 2.5 wt % surfactant in equivolume mixtures of oil and water and either 0 or 0.4 wt % protein. Under these conditions, AOT in the absence of protein always formed spherical, water-in-oil (w/o) microemulsion droplets in the organic phase with no self-assembly in the aqueous phase. In the presence of alpha-lactalbumin, self-assembled structures were formed in both aqueous and organic phases, and the size and shape of these was tuned by both pH and [NaCl]. Protein-surfactant interaction was weakest at the most alkaline pH, with protein-free, spherical droplets forming in the organic phase and surfactant-decorated soluble protein clusters forming in the aqueous phase. As pH was decreased, protein increasingly partitioned to the organic phase and droplets became ellipsoidal and much larger in volume, with these effects enhanced at lower salt concentration. Aqueous structures were also strongly affected by pH, shifting from prolate protein/surfactant aggregates at alkaline pH to oil-in-water, oblate microemulsion droplets at neutral pH. At acidic pH and higher salt concentration, self-assembly shifted toward a third, anisotropic aqueous phase, which contained discoid bilayer structures. It is proposed that hydrophobic attraction causes association of the protein with the surfactant monolayer, and pH and [salt] tune the system via the protein by modifying electrostatic repulsion and monolayer curvature.

Large-scale modulation of thermodynamic protein folding barriers linked to electrostatics

Protein folding barriers, which range from zero to the tens of RT that result in classical two-state kinetics, are primarily determined by protein size and structural topology [Plaxco KW, Simons KT, Baker D (1998) J Mol Biol 277:985-994]. Here, we investigate the thermodynamic folding barriers of two relatively large proteins of the same size and topology: bovine alpha-lactalbumin (BLA) and hen-egg-white lysozyme (HEWL). From the analysis of differential scanning calorimetry experiments with the variable-barrier model [Muñoz V, Sanchez-Ruiz JM (2004) Proc Natl Acad Sci USA 101:17646-17651] we obtain a high barrier for HEWL and a marginal folding barrier for BLA. These results demonstrate a remarkable tuning range of at least 30 kJ/mol (i.e., five to six orders of magnitude in population) within a unique protein scaffold. Experimental and theoretical analyses on these proteins indicate that the surprisingly small thermodynamic folding barrier of BLA arises from the stabilization of partially unfolded conformations by electrostatic interactions. Interestingly, there is clear reciprocity between the barrier height and the biological function of the two proteins, suggesting that the marginal barrier of BLA is a product of natural selection. Electrostatic surface interactions thus emerge as a mechanism for the modulation of folding barriers in response to special functional requirements within a given structural fold.

Membrane-bound structure and energetics of alpha-synuclein

Aggregation and fibrillation of alpha-synuclein bound to membranes are believed to be involved in Parkinson's and other neurodegenerative diseases. On SDS micelles, the N-terminus of alpha-synuclein forms two curved helices linked by a short loop. However, its structure on lipid bilayers has not been experimentally resolved. Using MD simulations with an implicit membrane model we show here that, on a planar mixed membrane, the truncated alpha-synuclein (residues 1-95) forms a bent helix. Bending of the helix is not due to the protein sequence or membrane binding, but to collective motions of the long helix. The backbone of the helix is approximately 2.5 A above the membrane surface, with some residues partially inserted in the membrane core. The helix periodicity is 11/3 (11 residues complete three full turns) as opposed to 18/5 periodicity of an ideal alpha-helix, with hydrophobic residues towards the membrane, negatively charged residues towards the solvent and lysines on the polar/nonpolar interface. A series of threonines, which are characteristic for alpha-synuclein and perhaps a phosphorylation site, is also located at the hydrophobic/hydrophilic interface with their side chain often hydrogen bonded to the main-chain atom. The calculations show that the energy penalty for change in periodicity from the 18/5 to 11/3 on the anionic membrane is overcome by favorable solvation energy. The binding of truncated alpha-synuclein to membranes is weak. It prefers anionic membranes but it also binds marginally to a neutral membrane, via its C-terminus. Dimerization of helical monomers on the mixed membrane is energetically favorable. However, it slightly interferes with membrane binding. This might promote lateral diffusion of the protein on the membrane surface and facilitate assembly of oligomers that precede fibrillation.

Heat Treatment of Bovine α-Lactalbumin Results in Partially Folded, Disulfide Bond Shuffled States with Enhanced Surface Activity

Prolonged heating of holo bovine alpha-lactalbumin (BLA) at 80 degrees C in pH 7 phosphate buffer in the absence of a thiol initiator improves the surface activity of the protein at the air:water interface, as determined by surface tension measurements. Samples after 30, 60, and 120 min of heating were analyzed on cooling to room temperature. Size-exclusion chromatography shows sample heterogeneity that increases with the length of heating. After 120 min of heating monomeric, dimeric, and oligomeric forms of BLA are present, with aggregates formed from disulfide bond linked hydrolyzed protein fragments. NMR characterization at pH 7 in the presence of Ca2+ of the monomer species isolated from the sample heated for 120 min showed that it consisted of a mixture of refolded native protein and partially folded protein and that the partially folded protein species had spectral characteristics similar to those of the pH 2 molten globule state of the protein. Circular dichroism spectroscopy showed that the non-native species had approximately 40% of the alpha-helical content of the native state, but lacked persistent tertiary interactions. Proteomic analysis using thermolysin digestion of three predominant non-native monomeric forms isolated by high-pressure liquid chromatography indicated the presence of disulfide shuffled isomers, containing the non-native 61-73 disulfide bond. These partially folded, disulfide shuffled species are largely responsible for the pronounced improvement in surface activity of the protein on heating.

Calculations of pH-dependent binding of proteins to biological membranes

Binding of proteins to membranes is often accompanied by titration of ionizable residues and is, therefore, dependent on pH. We present a theoretical treatment and computational approach for predicting absolute, pH-dependent membrane binding free energies. The standard free energy of binding, DeltaG, is defined as -RTln(P(b)/P(f)), where P(b) and P(f) are the amounts of bound and free protein. The apparent pK(a) of binding is the pH value at which P(b) and P(f) are equal. Proteins bind to the membrane in the pH range where DeltaG is negative. The components of the binding free energy are (a) the free energy cost of ionization state changes (DeltaG(ion)), (b) the effective energy of transfer from solvent to the membrane surface, (c) the translational/rotational entropy cost of binding, and (d) an ideal entropy term that depends on the relative volume of the bound and free state and therefore depends on lipid concentration. Calculation of the first term requires determination of pK(a) values in solvent and on the membrane surface. All energies required by the method are obtained from molecular dynamics trajectories on an implicit membrane (IMM1-GC). The method is tested on pentalysine and the helical peptide VEEKS, derived from the membrane-binding domain of phosphocholine cytidylyltransferase. The agreement between the measured and the calculated free energies of binding of pentalysine is good. The extent of membrane binding of VEEKS is, however, underestimated compared to experiment. Calculations of the interaction energy between two VEEKS helices on the membrane suggest that the discrepancy is mainly due to the neglect of protein-protein interactions on the membrane surface.

Defining the interacting regions between apomyoglobin and lipid membrane by hydrogen/deuterium exchange coupled to mass spectrometry

Sperm whale myoglobin can be considered as the model protein of the globin family. The pH-dependence of the interactions of apomyoglobin with lipid bilayers shares some similarities with the behavior of pore-forming domains of bacterial toxins belonging also to the globin family. Two different states of apomyoglobin bound to a lipid bilayer have been characterized by using hydrogen/deuterium exchange experiments and mass spectrometry. When bound to the membrane at pH 5.5, apomyoglobin remains mostly native-like and interacts through alpha-helix A. At pH 4, the binding is related to the stabilization of a partially folded state. In that case, alpha-helices A and G are involved in the interaction. At this pH, alpha-helix G, which is the most hydrophobic region of apomyoglobin, is available for interaction with the lipid bilayer because of the loss of the tertiary structure. Our results show the feasibility of such experiments and their potential for the characterization of various membrane-bound states of amphitropic proteins such as pore-forming domains of bacterial toxins. This is not possible with other high-resolution methods, because these proteins are usually in partially folded states when interacting with membranes.

Behavior of the N-Terminal Helices of the Diphtheria Toxin T Domain during the Successive Steps of Membrane Interaction

During intoxication of a cell, the translocation (T) domain of the diphtheria toxin helps the passage of the catalytic domain across the membrane of the endosome into the cytoplasm. We have investigated the behavior of the N-terminal region of the T domain during the successive steps of its interaction with membranes at acidic pH using tryptophan fluorescence, its quenching by brominated lipids, and trypsin digestion. The change in the environment of this region was monitored using mutant W281F carrying a single native tryptophan at position 206 at the tip of helix TH1. The intrinsic propensity to interact with the membrane of each helix of the N-terminus of the T domain, TH1, TH2, TH3, and TH4, was also studied using synthetic peptides. We showed the N-terminal region of the T domain was not involved in the binding of the domain to the membrane, which occurred at pH 6 mainly through hydrophobic effects. At that stage of the interaction, the N-terminal region remained strongly solvated. Further acidification eliminated repulsive electrostatic interactions between this region and the membrane, allowing its penetration into the membrane by attractive electrostatic interactions and hydrophobic effects. The peptide study indicated the nature of forces contributing to membrane penetration. Overall, the data suggested that the acidic pH found in the endosome not only triggers the formation of the molten globule state of the T domain required for membrane interaction but also governs a progressive penetration of the N-terminal part of the T domain in the membrane. We propose that these physicochemical properties are necessary for the translocation of the catalytic domain.

Interactions of apomyoglobin with membranes: mechanisms and effects on heme uptake

The last step of the folding reaction of myoglobin is the incorporation of a prosthetic group. In cells, myoglobin is soluble, while heme resides in the mitochondrial membrane. We report here an exhaustive study of the interactions of apomyoglobin with lipid vesicles. We show that apomyoglobin interacts with large unilamellar vesicles under acidic conditions, and that this requires the presence of negatively charged phospholipids. The pH dependence of apomyoglobin interactions with membranes is a two-step process, and involves a partially folded state stabilized at acidic pH. An evident role for the interaction of apomyoglobin with lipid bilayers would be to facilitate the uptake of heme from the outer mitochondrial membrane. However, heme binding to apomyoglobin is observed at neutral pH when the protein remains in solution, and slows down as the pH becomes more favorable to membrane interactions. The effective incorporation of soluble heme into apomyoglobin at neutral pH suggests that the interaction of apomyoglobin with membranes is not necessary for the heme uptake from the lipid bilayer. In vivo, however, the ability of apomyoglobin to interact with membrane may facilitate its localization in the vicinity of the mitochondrial membranes, and so may increase the yield of heme uptake. Moreover, the behavior of apomyoglobin in the presence of membranes shows striking similarities with that of other proteins with a globin fold. This suggests that the globin fold is well adapted for soluble proteins whose functions require interactions with membranes.

The N-terminal domain of Bcl-xL reversibly binds membranes in a pH-dependent manner

Bcl-xL regulates apoptosis by maintaining the integrity of the mitochondrial outer membrane by adopting both soluble and membrane-associated forms. The membrane-associated conformation does not require a conserved, C-terminal transmembrane domain and appears to be inserted into the bilayer of synthetic membranes as assessed by membrane permeabilization and critical surface pressure measurements. Membrane association is reversible and is regulated by the cooperative binding of approximately two protons to the protein. Two acidic residues, Glu153 and Asp156, that lie in a conserved hairpin of Bcl-xLDeltaTM appear to be important in this process on the basis of a 16% increase in the level of membrane association of the double mutant E153Q/D156N. Contrary to that for the wild type, membrane permeabilization for the mutant is not correlated with membrane association. Monolayer surface pressure measurements suggest that this effect is primarily due to less membrane penetration. These results suggest that E153 and D156 are important for the Bcl-xLDeltaTM conformational change and that membrane binding can be distinct from membrane permeabilization. Taken together, these studies support a model in which Bcl-xL activity is controlled by reversible insertion of its N-terminal domain into the mitochondrial outer membrane. Future studies with Bcl-xL mutants such as E153Q/D156N should allow determination of the relative contributions of membrane binding, insertion, and permeabilization to the regulation of apoptosis.

Natively folded HypF-N and its early amyloid aggregates interact with phospholipid monolayers and destabilize supported phospholipid bilayers

Recent data depict membranes as the main sites where proteins/peptides are recruited and concentrated, misfold, and nucleate amyloids; at the same time, membranes are considered key triggers of amyloid toxicity. The N-terminal domain of the prokaryotic hydrogenase maturation factor HypF (HypF-N) in 30% trifluoroethanol undergoes a complex path of fibrillation starting with initial 2-3-nm oligomers and culminating with the appearance of mature fibrils. Oligomers are highly cytotoxic and permeabilize lipid membranes, both biological and synthetic. In this article, we report an in-depth study aimed at providing information on the surface activity of HypF-N and its interaction with synthetic membranes of different lipid composition, either in the native conformation or as amyloid oligomers or fibrils. Like other amyloidogenic peptides, the natively folded HypF-N forms stable films at the air/water interface and inserts into synthetic phospholipid bilayers with efficiencies depending on the type of phospholipid. In addition, HypF-N prefibrillar aggregates interact with, insert into, and disassemble supported phospholipid bilayers similarly to other amyloidogenic peptides. These results support the idea that, at least in most cases, early amyloid aggregates of different peptides and proteins produce similar effects on the integrity of membrane assembly and hence on cell viability.

alpha-Lactalbumin binding and membrane integrity--effect of charge and degree of unsaturation of glycerophospholipids

Several studies have shown that the physical state of the phospholipid membrane has an important role in protein-membrane interactions, involving both electrostatic and hydrophobic forces. We have investigated the influence of the interaction of the calcium-depleted, (apo)-conformation of bovine alpha-lactalbumin (BLA) on the integrity of anionic glycerophospholipid vesicles by leakage experiments using fluorescence spectroscopy. The stability of the membranes was also studied by measuring surface tension/molecular area relationships with phospholipid monolayers. We show that the degree of unsaturation of the acyl chains and the proportion of charged phospholipid species in the membranes made of neutral and acidic glycerophospholipids are determinants for the association of BLA with liposomes and for the impermeability of the bilayer. Particularly, tighter packing counteracted interaction with BLA, while unsaturation-leading to looser packing-promoted interaction and leakage of contents. Equimolar mixtures of neutral and acidic glycerophospholipids were more permeable upon protein binding than pure acidic lipids. The effect of lipid structure on BLA-membrane interaction and bilayer integrity may throw new light on the membrane disrupting mechanism of a conformer of human alpha-lactalbumin (HAMLET) that induces death of tumour cells but not of normal cells.