Direct enthalpy measurements of factor X and prothrombin association with small and large unilamellar vesicles

Membrane partitioning: "classical" and "nonclassical" hydrophobic effects

The free energy of transfer of nonpolar solutes from water to lipid bilayers is often dominated by a large negative enthalpy rather than the large positive entropy expected from the hydrophobic effect. This common observation has led to the idea that membrane partitioning is driven by the “nonclassical” hydrophobic effect. We examined this phenomenon by characterizing the partitioning of the well-studied peptide melittin using isothermal titration calorimetry (ITC) and circular dichroism (CD). We studied the temperature dependence of the entropic (−TΔS) and enthalpic (ΔH) components of free energy (ΔG) of partitioning of melittin into lipid membranes made of various mixtures of zwitterionic and anionic lipids. We found significant variations of the entropic and enthalpic components with temperature, lipid composition and vesicle size but only small changes in ΔG (entropy–enthalpy compensation). The heat capacity associated with partitioning had a large negative value of about −0.5 kcal mol⁻¹ K⁻¹. This hallmark of the hydrophobic effect was found to be independent of lipid composition. The measured heat capacity values were used to calculate the hydrophobic-effect free energy ΔG_hΦ, which we found to dominate melittin partitioning regardless of lipid composition. In the case of anionic membranes, additional free energy comes from coulombic attraction, which is characterized by a small effective peptide charge due to the lack of additivity of hydrophobic and electrostatic interactions in membrane interfaces [Ladokhin and White J Mol Biol 309:543–552, 2001]. Our results suggest that there is no need for a special effect—the nonclassical hydrophobic effect—to describe partitioning into lipid bilayers.

CD spectroscopy of peptides and proteins bound to large unilamellar vesicles

Circular dichroism (CD) spectroscopy is an essential tool for determining the conformation of proteins and peptides in membranes. It can be particularly useful for measuring the free energy of partitioning of peptides into lipid vesicles. The belief is broadly held that such CD measurements can only be made using sonicated small unilamellar vesicles (SUVs) because light scattering associated with extruded large unilamellar vesicles (LUVs) is unacceptably high. We have examined this issue using several experimental approaches in which a chiral object (i.e., peptide or protein) is placed both on the membrane and outside the membrane. We show that accurate CD spectra can be collected in the presence of LUVs. This is important because SUVs, unlike LUVs, are metastable and consequently unsuitable for equilibrium thermodynamic measurements. Our data reveal that undistorted CD spectra of peptides can be measured at wavelengths above 200 nm in the presence of up to 3 mM LUVs and above 215 nm in the presence of up to 7 mM LUVs. We introduce a simple way of characterizing the effect on CD spectra of light scattering and absorption arising from suspensions of vesicles of any diameter. Using melittin as an example, we show that CD spectroscopy can be used to determine the fractional helical content of peptides in LUVs and to measure their free energy of partitioning of into LUVs.

Anionic phospholipid interactions of the prion protein N terminus are minimally perturbing and not driven solely by the octapeptide repeat domain

Although the N terminus of the prion protein (PrP(C)) has been shown to directly associate with lipid membranes, the precise determinants, biophysical basis, and functional implications of such binding, particularly in relation to endogenously occurring fragments, are unresolved. To better understand these issues, we studied a range of synthetic peptides: specifically those equating to the N1 (residues 23-110) and N2 (23-89) fragments derived from constitutive processing of PrP(C) and including those representing arbitrarily defined component domains of the N terminus of mouse prion protein. Utilizing more physiologically relevant large unilamellar vesicles, fluorescence studies at synaptosomal pH (7.4) showed absent binding of all peptides to lipids containing the zwitterionic headgroup phosphatidylcholine and mixtures containing the anionic headgroups phosphatidylglycerol or phosphatidylserine. At pH 5, typical of early endosomes, quartz crystal microbalance with dissipation showed the highest affinity binding occurred with N1 and N2, selective for anionic lipid species. Of particular note, the absence of binding by individual peptides representing component domains underscored the importance of the combination of the octapeptide repeat and the N-terminal polybasic regions for effective membrane interaction. In addition, using quartz crystal microbalance with dissipation and solid-state NMR, we characterized for the first time that both N1 and N2 deeply insert into the lipid bilayer with minimal disruption. Potential functional implications related to cellular stress responses are discussed.

3D-Structure of the interior fusion peptide of HGV/GBV-C by 1H NMR, CD and molecular dynamics studies

In this work, we present a structural characterization of the putative fusion peptide E2(279-298) corresponding to the E2 envelope protein of the HGV/GBV-C virus by (1)H NMR, CD and MD studies performed in H(2)O/TFE and in lipid model membranes. The peptide is largely unstructured in water, whereas in H(2)O/TFE and in model membranes it adopts an helical structure (approximately 65-70%). The partitioning free energy DeltaG ranges from -6 to -7.5 kcal mol(-1). OCD measurements on peptide-containing hydrated and oriented lipid multilayers showed that the peptide adopts a predominantly surface orientation. The (1)H NMR data (observed NOEs, deuterium exchange rates, Halpha chemical shift index and vicinal coupling constants) and the molecular dynamics calculations support the conclusions that the peptide adopts a stable helix in the C-terminal 9-18 residues slightly inserted into the lipid bilayer and a major mobility in the amino terminus of the sequence (1-8 residues).

Probing the interaction of coagulation factors with phospholipid vesicle surfaces by surface plasma resonance

The dynamics of the binding of human coagulation factor Xa (FXa) and prothrombin to small unilamellar vesicles (25% phosphatidylserine, 75% phosphatidylcholine) were compared and quantified by Biacore, using two immobilization techniques. The vesicles were either tagged with different molar ratios of cholesterol-DNA and attached on Au chips or fused directly on L1 chips. The diameter in solution was 145 nm, but the more DNA tags/vesicle the more compressed the immobilized vesicles became; with 30 DNA tags the calculated thickness was 88 nm and with 1 DNA tag it was 138 nm. In both models the affinity for the vesicles was higher for the activated coagulation factors than for the corresponding zymogens. FXa and prothrombin had the highest affinities. The affinity was dependent on the vesicle preparation since overall K(D) values were up to 10 times lower for N(2)-dried than for vacuum-dried phospholipids, although with apparently fewer binding sites. However, compression of the vesicles had no effect on the K(D). In contrast, the rate constants were dependent on the number of DNA tags; thus deformation of the vesicles was observed. The k(a) and k(d) for FXa were similar for vesicles attached with 30 DNA tags or fused on the L1 chip but higher with fewer tags and approximately 10 times higher if attached with 1 tag. Thus for controlled kinetic studies immobilized DNA-tagged vesicles should be used.

Effects of phosphatidylserine on membrane incorporation and surface protection properties of exchangeable poly(ethylene glycol)-conjugated lipids

Liposomes containing the acidic phospholipid phosphatidylserine (PS) have been shown to avidly interact with proteins involved in blood coagulation and complement activation. Membranes with PS were therefore used to assess the shielding properties of poly(ethylene glycol 2000)-derivatized phosphatidylethanolamine (PE-PEG(2000)) with various acyl chain lengths on membranes containing reactive lipids. The desorption of PE-PEG(2000) from PS containing liposomes was studied using an in vitro assay which involved the transfer of PE-PEG(2000) into multilamellar vesicles, and the reactivity of PS containing liposomes was monitored by quantifying interactions with blood coagulation proteins. The percent inhibition of clotting activity of PS liposomes was dependent on the PE-PEG(2000) content. 1,2-Distearoyl-sn-glycero-3-phosphoethanolamine (DSPE)-PEG(2000) which transferred out slowly from PS liposomes was able to abolish >80% of clotting activity of PS liposomes at 15 mol%. This level of DSPE-PEG(2000) was also able to extend the mean residence time of PS liposomes from 0.2 h to 14 h. However, PE-PEG(2000) with shorter acyl chains such as 1,2-dimyristyl-sn-glycero-3-phosphoethanolamine-PEG(2000) were rapidly transferred out from PS liposomes, which resulted in a 73% decrease in clotting activity inhibition and 45% of administered intravenously liposomes were removed from the blood within 15 min after injection. Thus, PS facilitates the desorption of PE-PEG(2000) from PS containing liposomes, thereby providing additional control of PEG release rates from membrane surfaces. These results suggest that membrane reactivity can be selectively regulated by surface grafted PEGs coupled to phosphatidylethanolamine of an appropriate acyl chain length.

How to measure and analyze tryptophan fluorescence in membranes properly, and why bother?

Tryptophan fluorescence is a powerful tool for studying protein structure and function, especially membrane-active proteins and peptides. It is arguably the most frequently used tool for examining the interactions of proteins and peptides with vesicular unilamellar model membranes. However, high light scattering associated with vesicular membrane systems presents special challenges. Because of their reduced light scattering compared to large unilamellar vesicles (LUV), small unilamellar vesicles (SUV) produced by sonication are widely used membrane models. Unfortunately, SUV, unlike LUV, are metastable and consequently unsuitable for equilibrium thermodynamic measurements. We present simple and easily implemented experimental procedures for the accurate determination of tryptophan (Trp) fluorescence in either LUV or SUV. Specifically, we show that Trp spectra can be obtained in the presence of up to 6 mM LUV that are virtually identical to spectra obtained in buffer alone, which obviates the use of SUV. We show how the widths and peak positions of such spectra can be used to evaluate the heterogeneity of the membrane conformation and penetration of peptides. Finally, we show how to use a reference fluorophore for the correction of intensity measurements so that the energetics of peptide partitioning into membranes can be accurately determined.

Enhancement of phototransduction protein interactions by lipid surfaces

The G protein cascade of vision depends on two peripheral membrane proteins: the G protein, transducin (G(t)), and cGMP phosphodiesterase (PDE). Each has covalently attached lipids, and interacts with transduction components on the membrane surface. We have found that their surface interactions are critically dependent on the nature of the lipid. Membranes enhance their protein-protein interactions, especially if electrostatic attraction is introduced with positively charged lipids. These interactions are less enhanced on highly curved surfaces, but are most enhanced by unsaturated or bulky acyl chains. On positively charged membranes, G(t) assembles at a high enough density to form two-dimensional arrays with short-range crystalline order. Cationic membranes also support extremely efficient activation of PDE by the GTPgammaS (guanosine 5'-O-(thiotriphosphate)) form of Galpha(t) (Galpha(t)-GTPgammaS), minimizing functional heterogeneity of transducin and allowing activation with nanomolar Galpha(t)-GTPgammaS. Quantification of PDE activation and of the amount of Galpha(t)-GTPgammaS bound to PDE indicated that G(t) activates PDE maximally when bound in a 1:1 molar ratio. No cooperativity was observed, even at nanomolar concentrations. Thus, under these conditions, the one binding site for Galpha(t)-GTPgammaS on PDE that stimulates catalysis must be of higher affinity than one or more additional sites which are silent with respect to activation of PDE.

Lupus Anticoagulants Form Immune Complexes With Prothrombin and Phospholipid That Can Augment Thrombin Production in Flow

Lupus anticoagulants (LA) are a family of autoantibodies that are associated with in vitro anticoagulant activity but a strong predisposition to in vivo thrombosis. They are directed against plasma phospholipid binding proteins, including prothrombin. We found that a murine monoclonal antiprothrombin antibody and 7 of 7 LA IgGs tested enhanced binding of prothrombin to 25:75 phosphatidyl serine:phosphatidyl choline vesicles in a concentration-dependent manner. We hypothesized that enhanced binding of prothrombin to phospholipid in the presence of LA IgG might result in increased thrombin production when reactions are performed in flow. Thrombin production by purified prothrombinase components was measured in a phospholipid-coated flow reactor. The flow reactor was incubated with prothrombin, calcium ions, and the IgGs and then perfused with prothrombin, calcium ions, the IgGs, factor Va, and factor Xa. A murine monoclonal antiprothrombin antibody and 4 of 6 LA IgGs from patients with a history of thrombosis increased thrombin production up to 100% over control in the first 15 minutes. In summary, LA IgGs concentrate prothrombin on a phospholipid surface that can augment thrombin production by prothrombinase in flow. These observations suggest that LA might propagate coagulation in flowing blood by facilitating prothrombin interaction with the damaged blood vessel wall.

Potent inhibition of terminal complement assembly by clusterin: characterization of its impact on C9 polymerization

The interactions of the heterodimeric apolipoprotein and complement inhibitor, clusterin (CL, 80 kDa), with actively assembling terminal complement proteins were characterized. Clusterin inhibited at three sites and by two modes of action. Clusterin inhibited C9 assembly on C5b-8 and C5b-9 and also bound to C5b-7 to prevent membrane attachment. The impact on C5b-9 assembly was the most potent. C9 assembly was monitored by assembly-induced fluorescence changes of C9 labeled with fluorescein isothiocyanate (FITC-C9). Assembly of monomeric FITC-C9 with C5b-8 or C5b-9(1) produced a substantial decrease in fluorescence intensity due to changes in the environment of the probe. Addition of the next subunit of unlabeled C9 produced a further small change. One equivalent of FITC-C9 bound to C5b-8 at low temperatures, but the fluorescence change and addition of more C9 did not occur until the temperaure was increased. Kinetic analysis of the fluorescence change suggested an irreversible, first-order process with an activation energy of 29 kcal/mol (k = 0.12 s(-1) at 25 degrees C). The kinetic properties differed for C9 addition to C5b-9(1) (0.27 s(-1) at 25 degrees C, 21 kcal/mol), indicating that C9 activation occurred at a different or altered site. Clusterin binding to C5b-8-(FITC-C9)1 caused fluorescence quenching similar to that of unlabeled C9, indicating that it bound to the C9 binding site. Clusterin binding to C5b-8 and C5b-9(1) was reversible with affinities that were 2 and 15 times that of C9 for the C5b-8 and C5b-9(1) complexes, respectively. The results suggested that the presence of <10% of the circulating clusterin in its heterodimeric, active form could reduce the rate of complement cytolysis of nucleated cells by 10-fold, and under some conditions by 100-fold or more. This would provide a high level of protection for certain cells and may allow time for action by other inhibitors of complement.

Dynamic features of prothrombin interaction with phospholipid vesicles of different size and composition: implications for protein--membrane contact

The dynamics of prothrombin interaction with membrane vesicles of different size and composition was investigated to ascertain the impact of membrane surface characteristics and particle size on this interaction. Dissociation rates were highly sensitive to membrane composition and varied from about 20/s for membranes of 10% PS to 0.1/s for membranes of 50% PS. Overall affinity also varied by more than two orders of magnitude. Very small differences between prothrombin binding to SUV versus LUV were found. Association with large unilamellar vesicles (LUV of 115 nm diameter) was about 4-fold slower, when expressed on the basis of binding sites, than association with small unilamellar vesicles (SUV, 30 nm diameter) of the same composition. Both reactions proceeded at less than 25% of the collisional limit so that the differences were largely due to intrinsic binding properties. Vesicles of 325 nm diameter showed even slower association velocities. Dissociation rates from LUV were about 2-fold slower than from SUV. Again, these differences arose primarily from intrinsic binding properties. Dissociation conformed to a single first order reaction over a wide range of protein occupancy on the membrane. At very high packing density, the dissociation rate increased by about 2-fold. At equilibrium, prothrombin preferred binding to SUV over LUV by about 2-fold. This very small difference, despite substantial differences in phospholipid headgroup packing and hydrocarbon exposure, appeared inconsistent with an important role for protein insertion into the hydrocarbon region of the membrane. However, prothrombin-membrane interaction may arise from a series of interaction forces that have compensating features at equilibrium. The small differences in prothrombin binding to SUV versus LUV, together with differences in the number of protein binding sites per vesicle, were important to identify mechanisms of substrate delivery to the active site of the prothrombinase enzyme [Lu, Y., & Nelsestuen, G. L. (1996) Biochemistry 35, 8201-8209].