Immune lysis of reconstituted myelin basic protein--lipid vesicles and myelin vesicles

Complement-mediated lysis of reconstituted lipid-myelin basic protein (BP) vesicles and myelin vesicles due to antibody raised against BP and isolated myelin is measured by determination of the amount of a water-soluble spin label, tempocholine chloride, released from the vesicles. The response is shown to be antigen-specific, antibody-dependent, and complement mediated. The relative response to different anti-BP antibody samples is similar to that determined by radioimmunoassay procedures. In contrast to immunoassays with BP in aqueous solution, this method measures immune recognition of the protein in either a synthetic or a natural membranous environment. This is important because this protein has been shown to have a different conformation when bound to lipid bilayers than in aqueous solution and its conformation depends on lipid composition. It is also a more rapid method because no separation of spin label still trapped in the vesicles and that released due to immune lysis is required. In synthetic membranes consisting of sphingomyelin, cholesterol, and an acidic lipid, either phosphatidylglycerol, phosphatidic acid, or phosphatidylserine, the response was greatest when the acidic lipid was phosphatidic acid. The response did not depend significantly on the antigen concentration expressed as molar ratio of BP to sphingomyelin, over the range 0.15:600 to 2:600, although it decreased at molar ratios less than 0.15:600. The antigen density required for immune lysis of vesicles containing this protein antigen is similar to that reported elsewhere for lipid antigens, although the time required for maximal lysis was greater. Both anti-BP and anti-myelin antibodies caused a greater specific complement-mediated response with synthetic vesicles than with myelin vesicles, which may be due to the different lipid and/or protein composition of myelin. Response was also obtained with the myelin vesicles, however, indicating that some determinants of BP can be recognized on the surface of the bilayer in isolated myelin by anti-BP.     

Conformation-dependent antigenic determinants in the toxic lectin ricin.

The major part of the ricin-precipitable antibodies in sera produced by immunizing rabbits with formaldehyde-treated ricin is precipitated also by the isolated ricin A and B chains. In contrast, in antisera produced by immunizing with formaldehyde-treated ricinus agglutinin only a small part of the antibodies cross-reacting with ricin can be precipitated by the isolated A and B chains, or bound to immunoabsorbents containing the isolated ricin chains. In immunodiffusion studies with anti-ricinus agglutinin sera, a star-shaped precipitate was formed when isolated A and B chains recombined to form intact ricin. Both anti-ricin and anti-ricinus agglutinin sera neutralized effectively the ability of ricin to inhibit protein synthesis in HeLa cells. Anti-ricin serum also neutralized the inhibitory effect of the isolated A chain on protein synthesis in a cell-free system and the ability of the isolated B chain to induce indirect hemagglutination. In contrast, antiricinus agglutinin serum did not neutralize the biologic activities of the isolated ricin A and B chains. Anti-ricinus agglutinin serum formed a precipitate with the hybrid ricin A chain/abrin B chain, and protected against the toxic effect on HeLa cells of this hybrid, indicating conformational changes of ricin A chain upon binding to the B chain. It is concluded that the anti-ricinus agglutinin serum contains antibodies directed against conformational determinants present on intact ricin, but not present or exposed in the isolated A and B chains. At least part of these conformational determinants appears to be carried by the A chain.     

Peptide:lipid ratio and membrane surface charge determine the mechanism of action of the antimicrobial peptide BP100. Conformational and functional studies

The cecropin-melittin hybrid antimicrobial peptide BP100 (H-KKLFKKILKYL-NH2) is selective for Gram-negative bacteria, negatively charged membranes, and weakly hemolytic. We studied BP100 conformational and functional properties upon interaction with large unilamellar vesicles, LUVs, and giant unilamellar vesicles, GUVs, containing variable proportions of phosphatidylcholine (PC) and negatively charged phosphatidylglycerol (PG). CD and NMR spectra showed that upon binding to PG-containing LUVs BP100 acquires α-helical conformation, the helix spanning residues 3–11. Theoretical analyses indicated that the helix is amphipathic and surface-seeking. CD and dynamic light scattering data evinced peptide and/or vesicle aggregation, modulated by peptide:lipid ratio and PG content. BP100 decreased the absolute value of the zeta potential (ζ) of LUVs with low PG contents; for higher PG, binding was analyzed as an ion-exchange process. At high salt, BP100-induced LUVS leakage requires higher peptide concentration, indicating that both electrostatic and hydrophobic interactions contribute to peptide binding. While a gradual release took place at low peptide:lipid ratios, instantaneous loss occurred at high ratios, suggesting vesicle disruption. Optical microscopy of GUVs confirmed BP100-promoted disruption of negatively charged membranes. The mechanism of action of BP100 is determined by both peptide:lipid ratio and negatively charged lipid content. While gradual release results from membrane perturbation by a small number of peptide molecules giving rise to changes in acyl chain packing, lipid clustering (leading to membrane defects), and/or membrane thinning, membrane disruption results from a sequence of events – large-scale peptide and lipid clustering, giving rise to peptide-lipid patches that eventually would leave the membrane in a carpet-like mechanism.     

Conformational properties of alpha-synuclein in its free and lipid-associated states

alpha-Synuclein (alphaS) is a presynaptic terminal protein that is believed to play an important role in the pathogenesis of Parkinson's disease (PD). We have used NMR spectroscopy to characterize the conformational properties of alphaS in solution as a free monomer and when bound to lipid vesicles and lipid-mimetic detergent micelles. Free wild-type alphaS is largely unfolded in solution, but exhibits a region with a preference for helical conformations that may be important in the aggregation of alphaS into fibrils. The N-terminal region of alphaS binds to synthetic lipid vesicles and detergent micelles in vitro and adopts a highly helical conformation, consistent with predictions based on sequence analysis. The C-terminal part of the protein does not associate with either vesicles or micelles, remaining free and unfolded. These results suggest that one function of alphaS may be to tether as of yet unidentified partners to lipid surfaces via interactions with its C-terminal tail.     

Conformational changes of β2-human glycoprotein I and lipid order in lipid-protein complexes

We studied the conformation of β2-human glycoprotein (β2GPI) in solution and bound to the anionic lipids palmitoyl oleoyl phosphatidylglycerol (POPG), dimiristoyl phosphatidylglycerol (DMPG) and dipalmitoyl phosphatidylglycerol (DPPG) as a function of the temperature. We used the infrared amide I' band to study the protein conformation, and the position of the antisymmetric stretching band of the methylene groups in the lipid hydrocarbon chains to study the lipid order. Lipid-protein complexes were studied in media of low and high ionic strengths. In solution, β2GPI displayed a conformational pre-transition in the range 47-50°C, characterized by a shift in the band of β secondary structure, previous to the main unfolding at 64°C. When the protein was bound to the anionic lipid membranes at 25°C, a similar shift as in the pre-transition in solution was observed, together with an increase in the band corresponding to α-helix secondary structure. Lipid-protein complexes formed large aggregates within the temperature range 10?60°C. At temperatures above the protein unfolding, the complexes were disrupted to yield vesicles with bound protein. This finding indicated that the native fold was required for the formation of the lipid-protein aggregates. Cycles of heating and cooling showed hysteresis in the formation of aggregates.     

Delineation and conformational analysis of two synthetic peptide models of antigenic sites on rodent cytochrome c

Two regions of rodent cytochrome c, one within the first four residues of the molecule, which is N-acetylated, and one at a beta bend around residue 44, are known to be immunogenic and antigenic in rabbits. Using sequential peptide synthesis, we have determined the residues required for linear synthetic peptides within these sequences to bind to antibody raised in rabbits to intact rat cytochrome c. The residues that were important in binding the N-terminal peptides were N-acetylglycine at position 1 and valine at position 3. The smallest peptide sequence around residue 44 that would bind to antibodies was Gln-Ala-Ala-Gly-Phe. A theoretical conformational analysis of these peptides showed that the amino-terminal tetrapeptide adopts a wide statistical ensemble of conformational states and that the addition of residues beyond 41 and 45 in the other sequence does not appear to stabilize longer peptides in the native beta-bend conformation. Thus, the antigenicity conferred by Phe-46 and Gln-42 in this peptide is most likely due to the direct interaction of the side chains of these residues with the antibody binding site. The demonstration here that native conformation is not essential for antigenic peptides to bind to antibodies raised against the whole protein indicates that the association energy between antigen and antibody can be sufficient to induce conformation in conformationally flexible peptides. This supports the concept that anti-protein and anti-peptide antibodies may invoke conformational changes in cross-reactive protein antigens and may explain why longer peptides, which may adopt stable nonnative secondary structure, often do not bind to antibodies raised to the whole molecule.     

Autoimmune encephalomyelitis (EAE) mediated or prevented by T lymphocyte lines directed against diverse antigenic determinants of myelin basic protein. Vaccination is determinant specific

Lines of T lymphocytes reactive against the basic protein of myelin (BP) were found in previous studies to mediate experimental autoimmune encephalomyelitis (EAE) in rats. Moreover, inoculation of rats with attenuated anti-BP line cells vaccinated them against subsequent attempts to induce active EAE by injection of BP in adjuvant. In the present study, we investigated the effects of T lymphocyte lines reactive to different antigenic determinants on the BP molecule, they are the major encephalitogenic peptide (EP) determinant present on guinea pig BP (G-BP), and minor, non-EP determinants present on bovine BP (B-BP). We found that both lines of T lymphocytes could mediate EAE. Resistance to active EAE acquired by spontaneous recovery from line mediated EAE or by vaccination with attenuated cells, however, was found to be specific for the particular BP determinant. Thus, EAE may be mediated by lines of T lymphocytes reactive to different determinants on the BP molecule, but the resistance to EAE acquired by exposure to line cells is determinant specific. This suggests that acquired resistance to EAE is directed by the receptor specificity of the autoimmune anti-BP T cells.     

Irreversible binding and activity control of the 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii at an anionic lipid bilayer surface

1,2-Diacylglycerol 3-glucosyltransferase is associated with the membrane surface catalyzing the synthesis of the major nonbilayer-prone lipid alpha-monoglucosyl diacylglycerol (MGlcDAG) from 1,2-DAG in the cell wall-less Acholeplasma laidlawii. Phosphatidylglycerol (PG), but not neutral or zwitterionic lipids, seems to be essential for an active conformation and function of the enzyme. Surface plasmon resonance analysis was employed to study association of the enzyme with lipid bilayers. Binding kinetics could be well fitted only to a two-state model, implying also a (second) conformational step. The enzyme bound less efficiently to liposomes containing only zwitterionic lipids, whereas increasing molar fractions of the anionic PG or cardiolipin (CL) strongly promoted binding by improved association (k(a1)), and especially a decreased rate of return (k(d2)) from the second state. This yielded a very low overall dissociation constant (K(D)), corresponding to an essentially irreversible membrane association. Both liposome binding and consecutive activity of the enzyme correlated with the PG concentration. The importance of the electrostatic interactions with anionic lipids was shown by quenching of both binding and activity with increasing NaCl concentrations, and corroborated in vivo for an active enzyme-green fluorescent protein hybrid in Escherichia coli. Nonbilayer-prone lipids substantially enhanced enzyme-liposome binding by promoting a changed conformation (decreasing k(d2)), similar to the anionic lipids, indicating the importance of hydrophobic interactions and a curvature packing stress. For CL and the nonbilayer lipids, effects on enzyme binding and consecutive activity were not correlated, suggesting a separate lipid control of activity. Similar features were recorded with polylysine (cationic) and polyglutamate (anionic) peptides present, but here probably dependent on the selective charge interactions with the enzyme N- and C-domains, respectively. A lipid-dependent conformational change and PG association of the enzyme were verified by circular dichroism, intrinsic tryptophan, and pyrene-probe fluorescence analyses, respectively. It is concluded that an electrostatic association of the enzyme with the membrane surface is accompanied by hydrophobic interactions and a conformational change. However, specific lipids, the curvature packing stress, and proteins or small molecules bound to the enzyme can modulate the activity of the bound A. laidlawii MGlcDAG synthase.     

Binding of anticoagulant vitamin K-dependent protein S to platelet-derived microparticles.

Vitamin K-dependent protein S is an anticoagulant plasma protein serving as cofactor to activated protein C in degradation of coagulation factors Va and VIIIa on membrane surfaces. In addition, it forms a noncovalent complex with complement regulatory protein C4b-binding protein (C4BP), a reaction which inhibits its anticoagulant function. Both forms of protein S have affinity for negatively charged phospholipids, and the purpose of the present study was to elucidate whether they bind to the surface of activated platelets or to platelet-derived microparticles. Binding of protein S to human platelets stimulated with various agonists was examined with FITC-labeled monoclonal antibodies and fluorescence-gated flow cytometry. Protein S was found to bind to membrane microparticles which formed during platelet activation but not to the remnant activated platelets. Binding to microparticles was saturable and maximum binding was seen at approximately 0.4 microM protein S. It was calcium-dependent and reversed after the addition of EDTA. Inhibition experiments with monoclonal antibodies suggested the gamma-carboxyglutamic acid containing module of protein S to be involved in the binding reaction. An intact thrombin-sensitive region of protein S was not required for binding. The protein S-C4BP complex did not bind to microparticles or activated platelets even though it bound to negatively charged phospholipid vesicles. Intact protein S supported binding of both protein C and activated protein C to microparticles. Protein S-dependent binding of protein C/activated protein C was blocked by those monoclonal antibodies against protein S that inhibited its cofactor function. In conclusion, we have found that free protein S binds to platelet-derived microparticles and stimulates binding of protein C/activated protein C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Photolabeling of myelin basic protein in lipid vesicles with the hydrophobic reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine

The hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine([125I]TID) was used to label myelin basic protein or polylysine in aqueous solution and bound to lipid vesicles of different composition. Although myelin basic protein is a water soluble protein which binds electrostatically only to acidic lipids, unlike polylysine it has several short hydrophobic regions. Myelin basic protein was labeled to a significant extent by TID when in aqueous solution indicating that it has a hydrophobic site which can bind the reagent. However, myelin basic protein was labeled 2-4-times more when bound to the acidic lipids phosphatidylglycerol, phosphatidylserine, phosphatidic acid, and cerebroside sulfate than when bound to phosphatidylethanolamine, or when in solution in the presence of phosphatidylcholine vesicles. It was labeled 5-7-times more than polylysine bound to acidic lipids. These results suggest that when myelin basic protein is bound to acidic lipids, it is labeled from the lipid bilayer rather than from the aqueous phase. However, this conclusion is not unequivocal because of the possibility of changes in the protein conformation or degree of aggregation upon binding to lipid. Within this limitation the results are consistent with, but do not prove, the concept that some of its hydrophobic residues penetrate partway into the lipid bilayer. However, it is likely that most of the protein is on the surface of the bilayer with its basic residues bound electrostatically to the lipid head groups.     

Specificity and neutralizing capacity of antibodies elicited by a synthetic peptide of scorpion toxin

Polyclonal antibodies raised against a synthetic peptide (sequence 50-59) of Androctonus australis Hector toxin II can neutralize the effects of toxin II in vivo. The antigenic specificities of anti-peptide and anti-toxin antibodies were compared by competitive aqueous phase radioimmunoassay by using 125I-toxin II, chemically modified or homologous toxins, and the synthetic peptide 50-59, either free or bound to bovine serum albumin (BSA). The antipeptide and anti-toxin antibodies had a comparable high affinity for the native toxin, but anti-peptide antibodies exhibited a lower binding capacity. Anti-peptide antibodies had a higher affinity for native toxin than for the peptide 50-59 bound to BSA, used as immunogen, and were unable to recognize the free peptide. These results suggest that it is necessary to restrict the conformational freedom of the immunizing peptide in order to obtain anti-peptide antibodies with a high affinity for the toxin. The lysine residue at position 58 of toxin II, essential for toxicity, appears to be immunogenic when immunization is with peptide 50-59 bound to BSA and not with the native toxin. This residue is antigenic in the native toxin, however, as shown by the anti-peptide antibodies.     

The interaction of peripheral proteins and membranes studied with alpha-lactalbumin and phospholipid bilayers of various compositions

To characterize the interaction of peripheral proteins and membranes at the molecular level, we studied the reversible association of bovine alpha-lactalbumin (BLA) with lipid bilayers composed of different molecular forms of phosphatidylserine or equimolar mixtures of these phosphatidylserine forms and egg yolk phosphatidylcholine. At pH 4.5, almost all BLA (>90%) associates to negatively charged small unilamellar vesicles. The conformational changes that binding to these bilayers induced on the protein were characterized by circular dichroism and fluorescence spectroscopy. Because binding of BLA to negatively charged vesicles is reverted by adjusting the pH back to >6.0, we also investigated the conformation of the membrane-bound protein by NMR-monitored H-D exchange of the backbone amide protons. The conformation adopted by BLA bound to these bilayers resembles a molten globule-like state but the negative ellipticity at 222 nm and the apparent alpha-helix content of the bound protein senses the changes in the physical properties of the membrane. Binding to bilayers in the gel state appears to correlate with an increased amount of alpha-helical structure and with a lower extent of integration into the membrane, corresponding to the adsorbed protein, while the opposite is found for BLA bound to vesicles in the liquid-crystalline phase, corresponding to the embedded conformation. A common feature for the membrane-bound conformations of BLA is that the amphipathic helix C (residues 86 to 99) is an important determinant for the adsorption and further integration of the protein into the membrane.     

Preparation and Properties of an Immunosorbent Column Specific for the Myelin Basic Protein

Abstract: An immunosorbent column specific for the myelin basic protein (BP) was prepared by coupling purified anti-BP antibodies to cyanogen bromide (BrCN)-activated Sepharose 4B. The BP-immunosorbent column bound BP between pH 4.5 and pH 6.8. In its working range the column bound approximately 400-475 μg of BP at pH 6.8 and 250 μg at pH 4.5 with recoveries of 72-77%. The BP-immunosorbent column could effectively separate BP from simple mixtures of BP and proteins of similar size and charge and from acid extracts of bovine brain. The results indicate that the BP-immunosorbent column can be used to isolate BP from a mixture of proteins and may be adapted for use in the small-scale purification of the myelin basic proteins involving a minimum number of steps.     

α-Synuclein Oligomers with Broken Helical Conformation Form Lipoprotein Nanoparticles

α-Synuclein (αS) is a membrane-binding protein with sequence similarity to apolipoproteins and other lipid-carrying proteins, which are capable of forming lipid-containing nanoparticles, sometimes referred to as “discs.” Previously, it has been unclear whether αS also possesses this property. Using cryo-electron microscopy and light scattering, we found that αS can remodel phosphatidylglycerol vesicles into nanoparticles whose shape (ellipsoidal) and dimensions (in the 7–10-nm range) resemble those formed by apolipoproteins. The molar ratio of αS to lipid in nanoparticles is ∼1:20, and αS is oligomeric (including trimers and tetramers). Similar nanoparticles form when αS is added to vesicles of mitochondrial lipids. This observation suggests a mechanism for the previously reported disruption of mitochondrial membranes by αS. Circular dichroism and four-pulse double electron electron resonance experiments revealed that in nanoparticles αS assumes a broken helical conformation distinct from the extended helical conformation adopted when αS is bound to intact vesicles or membrane tubules. We also observed αS-dependent tubule and nanoparticle formation in the presence of oleic acid, implying that αS can interact with fatty acids and lipids in a similar manner. αS-related nanoparticles might play a role in lipid and fatty acid transport functions previously attributed to this protein.     

Lipid-induced conformational transition of the amyloid core fragment Abeta(28-35) and its A30G and A30I mutants

The interaction of the beta-amyloid peptide (Abeta) with neuronal membranes could play a key role in the pathogenesis of Alzheimer's disease. Recent studies have focused on the interactions of Abeta oligomers to explain the neuronal toxicity accompanying Alzheimer's disease. In our study, we have investigated the role of lipid interactions with soluble Abeta(28-35) (wild-type) and its mutants A30G and A30I in their aggregation and conformational preferences. CD and Trp fluorescence spectroscopic studies indicated that, immediately on dissolution, these peptides adopted a random coil structure. Upon addition of negatively charged 1,2-dipalmitoyl-syn-glycero-3-phospho-rac-(glycerol) sodium salt (PG) lipid, the wild-type and A30I mutant underwent reorganization into a predominant beta-sheet structure. However, no conformational changes were observed in the A30G mutant on interaction with PG. In contrast, the presence of zwitterionic 1,2-dipalmitoyl-syn-glycero-3-phosphatidylcholine (PC) lipid had no effect on the conformation of these three peptides. These observations were also confirmed with atomic force microscopy and the thioflavin-T assay. In the presence of PG vesicles, both the wild-type and A30I mutant formed fibrillar structures within 2 days of incubation in NaCl/P(i), but not in their absence. Again, no oligomerization was observed with PC vesicles. The Trp studies also revealed that both ends of the three peptides are not buried deep in the vesicle membrane. Furthermore, fluorescence spectroscopy using the environment-sensitive probe 1,6-diphenyl-1,3,5-hexatriene showed an increase in the membrane fluidity upon exposure of the vesicles to the peptides. The latter effect may result from the lipid head group interactions with the peptides. Fluorescence resonance energy transfer experiments revealed that these peptides undergo a random coil-to-sheet conversion in solution on aging and that this process is accelerated by negatively charged lipid vesicles. These results indicate that aggregation depends on hydrophobicity and propensity to form beta-sheets of the amyloid peptide, and thus offer new insights into the mechanism of amyloid neurodegenerative disease.     

Binding of antibodies to acetylcholine receptors in Electrophorus and Torpedo electroplax membranes

Antisera against purified acetylcholine receptors from the electric tissues of Torpedo californica and of Electrophorus electricus were raised in rabbits. The antisera contain antibodies which bind to both autologous and heterologous receptors in solution as shown by an immunoprecipitation assay. Antibodies in both types of antisera bind specifically to the postjunctional membrane on the innervated surface of the intact electroplax from Electrophorus electric tissue as demonstrated by an indirect immunohistochemical procedure using horseradish peroxidase conjugated to anti-rabbit IgG. Only anti-Electrophorus receptor antisera, however, cause inhibition of the receptor-mediated depolarization of the intact Electrophorus electroplax. The lack of inhibition by anti-Torpedo receptor antibodies, which do bind, suggests that the receptor does not undergo extensive movement during activity. The binding of anti-Torpedo antibodies to receptor-rich vesicles prepared by subcellular fractionation of Torpedo electric tissue was demonstrated by both direct and indirect immunohistochemical methods using ferritin conjugates. These vesicles can be conveniently collected and prepared for electron microscopy on Millipore filters, a procedure requiring only 25 micrograms of membrane protein per filter. In addition, it was possible to visualize the binding of anti-Torpedo receptor antibodies directly, without ferritin. These anti-Torpedo receptor antibodies, however, do not inhibit the binding of acetylcholine or of alpha-neurotoxin to receptor in Torpedo microsacs but do inhibit binding of alpha-neurotoxin to Torpedo receptor in Triton X-100 solution. It is likely that the principal antigenic determinants on receptor are at sites other than the acetylcholine-binding sites and that inhibition of receptor function, when it occurs, may be due to a stabilization by antibody binding of an inactive conformational state.     

Passive transfer of collagen arthritis: heterogeneity of anti-collagen IgG

Using a combination of affinity chromatography procedures, anticollagen IgG was fractionated into three distinct populations. One population reacted only to conformational determinants, another population reacted only to structural determinants, and the third population reacted to both conformation and structural determinants. When these populations were tested for their arthritogenicity, only those fractions that reacted to conformational and to conformational and structural determinants were active in inducing clinical arthritis. Immunofluorescence analysis of the hind paw of recipient rats indicated that antibodies directed only to conformational and to both conformational and structural determinants bound to articular cartilage and activated the complement system. Antibodies directed strictly to structural determinants did not bind to articular cartilage and were nonarthritogenic.     

Evidence that membrane insertion of the cytosolic domain of Bcl-xL is governed by an electrostatic mechanism

Signals from different cellular networks are integrated at the mitochondria in the regulation of apoptosis. This integration is controlled by the Bcl-2 proteins, many of which change localization from the cytosol to the mitochondrial outer membrane in this regulation. For Bcl-xL, this change in localization reflects the ability to undergo a conformational change from a solution to integral membrane conformation. To characterize this conformational change, structural and thermodynamic measurements were performed in the absence and presence of lipid vesicles with Bcl-xL. A pH-dependent model is proposed for the solution to membrane conformational change that consists of three stable conformations: a solution conformation, a conformation similar to the solution conformation but anchored to the membrane by its C-terminal transmembrane domain, and a membrane conformation that is fully associated with the membrane. This model predicts that the solution to membrane conformational change is independent of the C-terminal transmembrane domain, which is experimentally demonstrated. The conformational change is associated with changes in secondary and, especially, tertiary structure of the protein, as measured by far and near-UV circular dichroism spectroscopy, respectively. Membrane insertion was distinguished from peripheral association with the membrane by quenching of intrinsic tryptophan fluorescence by acrylamide and brominated lipids. For the cytosolic domain, the free energy of insertion (DeltaG degrees x) into lipid vesicles was determined to be -6.5 kcal mol(-1) at pH 4.9 by vesicle binding experiments. To test whether electrostatic interactions were significant to this process, the salt dependence of this conformational change was measured and analyzed in terms of Gouy-Chapman theory to estimate an electrostatic contribution of DeltaG degrees el approximately -2.5 kcal mol(-1) and a non-electrostatic contribution of DeltaG degrees nel approximately -4.0 kcal mol(-1) to the free energy of insertion, DeltaG degrees x. Calcium, which blocks ion channel activity of Bcl-xL, did not affect the solution to membrane conformational change more than predicted by these electrostatic considerations. The lipid cardiolipin, that is enriched at mitochondrial contact sites and reported to be important for the localization of Bcl-2 proteins, did not affect the solution to membrane conformational change of the cytosolic domain, suggesting that this lipid is not involved in the localization of Bcl-xL in vivo. Collectively, these data suggest the solution to membrane conformational change is controlled by an electrostatic mechanism. Given the distinct biological activities of these conformations, the possibility that this conformational change might be a regulatory checkpoint for apoptosis is discussed.     

Human thyroid peroxidase: mapping of autoantibodies, conformational epitopes to the enzyme surface

The enzyme, thyroid peroxidase (TPO), is a dominant antigen in thyroid autoimmune diseases. Autoantibodies recognised two major dominant conformational epitopes termed A and B. The epitopes have been defined by mAbs, but the amino acid residues which constitute these determinants remain unknown. Using a model of TPO, built from the structure of myeloperoxidase (MPO), we have synthesised peptides corresponding to exposed loops and generated rabbit antibodies to the peptides. Antisera to peptide sequence 599-617 (peptide 14) representing a highly protrusive loop on the TPO, showed the highest inhibition in 65 sera from patients positive with anti-TPO antibodies. The inhibition was by 15-80% (mean 41%), and no other antibody showed any inhibition. Binding of hFabs to the B determinant on TPO was inhibited by anti-peptide 14 antibodies more then 85%, but not Fabs to the A determinant. In conclusion, the peptide 14 defines a sequence taking part in building up the B major conformational epitope. None of generated anti-peptide antibodies alone inhibited the binding of human Fabs to the A epitope, however a combination of four anti-peptide antibodies (P1, P12, P14 and P18) inhibits Fabs binding to the A determinant by more then 60% and autoantibodies binding from 65% to 94%. Combination of antibodies reacting with peptides outside the surface defined by those four antipeptide antibodies did not give any inhibition of Fabs to TPO. The inhibition of Fabs and auto Abs to TPO by this combination of anti-peptide Abs is the result of steric hindrance as none of these Abs individually inhibited auto Abs' or Fabs' binding to TPO. The four peptides define an area on the enzyme surface where the A and B major conformational epitopes are localised.     

Lipid interaction converts prion protein to a PrPSc-like proteinase K-resistant conformation under physiological conditions

The conversion of prion protein (PrP) to the pathogenic PrPSc conformation is central to prion disease. Previous studies revealed that PrP interacts with lipids and the interaction induces PrP conformational changes, yet it remains unclear whether in the absence of any denaturing treatment, PrP-lipid interaction is sufficient to convert PrP to the classic proteinase K-resistant conformation. Using recombinant mouse PrP, we analyzed PrP-lipid interaction under physiological conditions and followed lipid-induced PrP conformational change with proteinase K (PK) digestion. We found that the PrP-lipid interaction was initiated by electrostatic contact and followed by hydrophobic interaction. The PrP-lipid interaction converted full-length alpha-helix-rich recombinant PrP to different forms. A significant portion of PrP gained a conformation reminiscent of PrPSc, with a PrPSc-like PK-resistant core and increased beta-sheet content. The efficiency for lipid-induced PrP conversion depended on lipid headgroup structure and/or the arrangement of lipids on the surface of vesicles. When lipid vesicles were disrupted by Triton X-100, PrP aggregation was necessary to maintain the lipid-induced PrPSc-like conformation. However, the PK resistance of lipid-induced PrPSc-like conformation does not depend on amyloid fiber formation. Our results clearly revealed that the lipid interaction can overcome the energy barrier and convert full-length alpha-helix-rich PrP to a PrPSc-like conformation under physiological conditions, supporting the relevance of lipid-induced PrP conformational change to in vivo PrP conversion.