Initial Surface Pressure Research Articles

Phenylalanine interaction with lipid monolayers at different pHs

The influence of Phe on the surface pressure of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers at the air–water interface was studied at different initial surface pressures (26 and 40mN/m) and two pHs (5.0 and 7.3) at constant temperature (20°C). Changes produced by the aminoacid added to the subphase on the surface pressure and on the dipole potential of lipid monolayers were measured at a fixed area. Compressibility properties of the monolayers at different pHs were studied by (π–A) isotherms. The results suggest that Phe intercalates into a DPPC film at the air–water interface at pH 5 and forms a different arrangement at pH 7.3.The possible relevance of these results of the effect of Phe in physiological conditions is discussed.

General circulation models of the dynamics of Pluto’s volatile transport on the eve of the New Horizons encounter

Pluto’s atmospheric dynamics occupy an interesting regime in which the radiative time constant is quite long, the combined effects of high obliquity and a highly eccentric orbit can produce strong seasonal variations in atmospheric pressure, and the strong coupling between the atmosphere and volatile transport on the surface results in atmospheric flows that are quite sensitive to surface and subsurface properties that at present are poorly constrained by direct observations. In anticipation of the New Horizons encounter with the Pluto system in July 2015, we present a Pluto-specific three-dimensional general circulation model (GCM), PlutoWRF, incorporating the most accurate current radiative transfer models of Pluto’s atmosphere, a physically robust treatment of nitrogen volatile transport, and the flexibility to accommodate richly detailed information about the surface and subsurface conditions as new data become available. We solve for a physically self-consistent, equilibrated combination of surface, subsurface, and atmospheric conditions to specify the boundary conditions and initial state values for each GCM run. This is accomplished using two reduced versions of PlutoWRF: a two-dimensional surface volatile exchange model to specify the properties of surface nitrogen ice and the initial atmospheric surface pressure, and a one-dimensional radiative–conductive–convective model that uses the two-dimensional model predictions to determine the corresponding global-mean atmospheric thermal profile. We illustrate the capabilities of PlutoWRF in predicting Pluto’s general circulation, thermal state, and volatile transport of nitrogen by calculating the dynamical response of Pluto’s atmosphere, based on four different idealized models of Pluto’s surface ice distribution from Young (Young, L.A. [2013]. Astrophys. J. 766, L22) and Hansen et al. (Hansen, C.J., Paige, D.A., Young, L.A. [2015]. Icarus 246, 183). Our GCM runs typically span 30years, from 1985 to 2015, covering the period from the discovery of Pluto’s atmosphere to present. For most periods simulated, zonal winds are strongly forced by a gradient wind balance, relaxing in later (recent) years to an angular momentum conservation balance of the seasonal polar cap sublimation flow. Near-surface winds generally follow a sublimation flow from the sunlit polar cap to the polar night cap, with a Coriolis turning of the wind as the air travels from pole to pole. We demonstrate the strong contribution of nitrogen sublimation and deposition to Pluto’s atmospheric circulation. As New Horizons data become available, PlutoWRF can be used to construct models of Pluto’s atmospheric dynamics and surface wind regimes more constrained by physical observations.

Interaction of biofunctionalized gold nanoparticles with model phospholipid membranes

The understanding of the interaction of nanoparticles with cell membranes and the penetration of these nano-objects through cell wall is highly required for their biomedical application. In this work were aimed at the study of the interaction of gold nanoparticles with model phospholipid membranes prepared at the air/water interface in a Langmuir trough. Spherical (10 and 15 nm mean diameter) and rod-like gold (aspect ratio: 2.8) nanoparticles were synthesized and biofunctionalized with l-cysteine and l-glutathione. The gold nanoparticles were characterized by TEM images and UV–Vis absorbance measurements. The interaction of the biofunctionalized gold nanoparticles with the model monolayer membrane was studied by surface pressure versus surface area compressional isotherms and by the measurement of the change in surface pressure of a preformed model membrane. The effect of the initial surface pressure of the preformed membrane was evaluated to determine the maximum insertion pressure and synergy. We have found that the driving forces of the bioconjugated Au nanoparticle (NP) or Au nanorod (NR) penetration into the monolayer membrane is mostly determined by electrostatic interaction and orientational van der Waals forces. Monolayer films were transferred with Langmuir–Blodgett technique onto solid substrates and the nanoparticles were visualized with AFM technique.

Adsorption of Bile Salts to Milk Phospholipid and Phospholipid–Protein Monolayers

The adsorption of bile salts to milk phospholipid and phospholipid-protein monolayers at the air-water interface was studied under simulated intestinal conditions using a Langmuir trough, epifluorescence microscopy, and atomic force microscopy. Surface pressure changes were affected by temperature, initial surface pressure, and bile composition. The rate of addition of bile salts and the initial surface pressure of the monolayers had an impact on the microstructure of the mixed monolayers. The presence of proteins in monolayers at different ratios did not affect the surface pressure change upon addition of bile. However, at 20 °C, the addition of bile to phospholipid and phospholipid-protein monolayers led to different features with branching and clustering of liquid-ordered domains and possible formation of bile salt-rich areas within liquid-ordered domains. This study provides a basic understanding of the interfacial changes occurring at the surface of milk fat globules and milk phospholipid liposomes during their passage in the duodenum.

Corrigendum: Improved Conformational Stability of the Visual G Protein‐Coupled Receptor Rhodopsin by Specific Interaction with Docosahexaenoic Acid Phospholipid

In this paper, the units for pressure were incorrectly given, and should have been mN m−1 as they refer to surface pressure. This affects Figures 1, 2 and 3, as well as two parts of the text on page 640 (in the penultimate paragraph on the left and first paragraph on the right). The corrected figures are given below. Kinetic adsorption process for the incorporation of rhodopsin to the air/water interface at different DDHA-PC initial pressures: 1 mN m−1 (curve 4), 6.1 mN m−1 (3), 10.4 mN m−1 (2) or absence of DDHA-PC (1). DDHA-PC stock solution was prepared and added drop wise to the subphase (PBS, pH 7.4) until the desired lipid pressure was achieved. The final concentration of rhodopsin was 1.4 nM. Determination of rhodopsin MIP. Plot of the surface pressure increase (Δπ) caused by rhodopsin against lipid initial surface pressure (πi) to determine the MIP of rhodopsin with DDHA-PC monolayers, by extrapolating the curve to the x-axis. The subphase was PBS (pH 7.4), and the final concentration of rhodopsin was 1.4 nM. Plot of the equilibrium adsorption pressure (πe) of rhodopsin against initial monolayer surface pressure (πi) of DDHA-PC. The subphase was PBS, pH 7.4, and the final concentration of rhodopsin was 1.4 nM. The editorial office apologises for this error.

An investigation of a super-Earth exoplanet with a greenhouse-gas atmosphere using a general circulation model

We use the Massachusetts Institute of Technology general circulation model (GCM) dynamical core, in conjunction with a Newtonian relaxation scheme that relaxes to a gray, analytical solution of the radiative transfer equation, to simulate a tidally locked, synchronously orbiting super-Earth exoplanet. This hypothetical exoplanet is simulated under the following main assumptions: (1) the size, mass, and orbital characteristics of GJ 1214b (Charbonneau, D. [2009]. Nature 462, 891–894), (2) a greenhouse-gas dominated atmosphere, (3), the gas properties of water vapor, and (4) a surface. We have performed a parameter sweep over global mean surface pressure (0.1, 1, 10, and 100bar) and global mean surface albedo (0.1, 0.4, and 0.7). Given assumption (1) above, the period of rotation of this exoplanet is 1.58 Earth-days, which we classify as the rapidly rotating regime. Our parameter sweep differs from Heng and Vogt (Heng, K., Vogt, S.S. [2011]. Mon. Not. R. Astron. Soc. 415, 2145–2157), who performed their study in the slowly rotating regime and using Held and Suarez (Held, I.M., Suarez, M.J. [1994]. Bull. Am. Meteorol. Soc. 75 (10), 1825–1830) thermal forcing. This type of thermal forcing is a prescribed function, not related to any radiative transfer, used to benchmark Earth’s atmosphere. An equatorial, westerly, superrotating jet is a robust feature in our GCM results. This equatorial jet is westerly at all longitudes. At high latitudes, the flow is easterly. The zonal winds do show a change with global mean surface pressure. As global mean surface pressure increases, the speed of the equatorial jet decreases between 9 and 15h local time (substellar point is located at 12h local time). The latitudinal extent of the equatorial jet increases on the nightside. For the two greatest initial surface pressure cases, an increasingly westerly component of flow develops at middle to high latitudes between 11 and 18h local time. On the nightside, the easterly flow in the midlatitudes also increases in speed as global mean surface pressure increases. Furthermore, the zonal wind speed in the equatorial and midlatitude jets decreases with increasing surface albedo. Also, the latitudinal width of the equatorial jet decreases as surface albedo increases.

The physical properties of lipid monolayers and bilayers containing calixarenes sensitive to cytochrome c

We studied physical properties of the monolayers and bilayer lipid membranes (BLM) formed by calix[6]arene carboxylic acid derivative (CX) and its mixtures with diphytanoylphosphatidylcholine (DPhPC) by means of measurement surface pressure, surface dipole potential and electrostriction. CX forms stable monolayers at an air-water interface and complexes in mixed monolayers contained DPhPC. Calixarenes increase the elastic moduli of lipid monolayers as well as BLM. Cytochrome c (cyt c) specifically binds to CX by incorporation of the amino groups of lysine residues at the protein surface. This binding affected the physical properties of CX monolayers depending on their initial surface pressure. Addition of cyt c into the water subphase induced increase of surface pressure of CX monolayers at relatively low initial pressure (15 mN/m) when monolayer was in liquid expanded state (LE). This may be due interaction of positively charged cyt c with negatively charged carboxylic groups of CX and also by its penetration into the air-water interface. However, much subtle changes were observed for higher initial surface pressure (20 and 35 mN/m) when monolayer is in liquid condensed (LC) and solid (S) state, respectively. Lysine induced substantially lower changes in surface pressure in comparison with that of cyt c.

A multi-headed surfactant as an efficient tool in solubilization of dimyristoylphosphatidycholine (DMPC) vesicles

In this study, the solubilization of DMPC (dimyristoylphosphatidycholine) vesicles by a novel multi-headed surfactant, C18N3, is reported. The headgroup of the surfactant contains two types of amine groups, which are protonated at different pH values. The protonation of the headgroup has significant effect on the solubilization process. The penetration of the surfactant into a DMPC monolayer was measured at different initial surface pressures and at different pH values. A linear relationship was found between the initial surfactant pressure (π) and the increase of the surfactant pressure (Δπ). Furthermore, turbidity titration shows that at low pH, the phase behavior with increasing surfactant concentration in the DMPC solution is similar to conventional ionic surfactants. This can be explained by the three-stage model. TEM images at different C18N3 concentrations confirmed the process of solubilization of DMPC vesicles, and small aggregates were observed at 1mmol/L C18N3. However, at higher pH, where the headgroup is deprotonated, the surfactant was not able to solubilize the DMPC vesicles. TEM images shows a coexistence of vesicles and a lamellar phase. The surfactant/lipid ratio of the multi-headed surfactant at pH 4 and 7 was compared to conventional nonionic surfactants with the same cross-sectional area of the headgroup, and a much lower ratio was observed for the new surfactant.

Atmospheric erosion and replenishment induced by impacts upon the Earth and Mars during a heavy bombardment

Consequences of a heavy bombardment for the atmospheres of Earth and Mars are investigated with a stochastic model. The main result is the dominance of the accumulation. The atmospheric pressure is strongly increasing both for Earth and Mars in the course of an enhanced bombardment. The effect of atmospheric erosion is found to be minor, regarding escape during meteorite entry, in the expanding vapor plume, and ejection due to free-surface motion. The initial atmospheric surface pressure if comparable to the modern value turns out as a less important additive constant of the final pressure. Impactor retention and atmospheric erosion are parametrized in terms of scaling laws, compatible with recent numerical simulations. The dependence on impactor size, atmospheric and planetary parameters is analyzed among alternative models and numerical results. The stochastic model is fed with the net replenishment originating from impactor material and the loss of preexisting atmospheric gas. Major input parameters are the total cumulative impactor mass and the relative mass of atmophile molecules in comets and asteroids. Input size distributions of the impactor ensemble correspond to presently observed main belt asteroids and KBOs. Velocity distributions are taken from dynamical simulations for the Nice model. Depending on the composition of large cometary impactors, the Earth could acquire a more massive atmosphere, a few bars in terms of surface pressure, mostly as CO and CO2. For Mars accumulation of 1–4 bars of CO and CO2 requires an asteroidal ‘late veneer’ of the order of 1024g containing 2% atmophiles.

Mixed layers of DPPC and a linear poly(ethylene glycol)-b-hyperbranched poly(glycerol) block copolymer having a cholesteryl end group

Interactions of the phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) with the amphiphilic diblock copolymer Ch-lPEG30-b-hbPG24 (ChP) are studied at the air–water interface by surface pressure–mean molecular area (π–mmA) measurements of mixed Langmuir films and adsorption measurements of ChP to the air–water interface covered with DPPC monolayers at different initial surface pressure values π0. ChP is composed of a single hydrophobic cholesteryl (Ch) moiety covalently bound to a diblock copolymer consisting of a hydrophilic linear poly(ethylene glycol) (lPEG) block and a hydrophilic hyperbranched poly(glycerol) (hbPG) block. Langmuir isotherms and compression moduli of the mixed Langmuir films of different molar ratios reveal distinct interactions between DPPC and ChP during compression. It is demonstrated that the behavior of the DPPC/ChP mixtures is dominated by DPPC up to a molar ratio of 10:1, whereas the behavior is predominantly governed by ChP in mixtures with lower DPPC content (molar ratios of 5:1, 2:1, and 1:1). In adsorption measurements, a strong affinity of ChP to DPPC is observed after injection into the water subphase. The surface pressure value πin up to which ChP is able to penetrate into DPPC monolayers is determined to the remarkably high value of 48.2 mN/m which attests the favorable interactions between DPPC and the Ch moiety of ChP. Atomic force microscopy on LB films of DPPC/ChP mixtures of different molar ratios transferred onto hydrophilic substrates confirms the presence of two different phases, a DPPC-rich phase and a ChP-rich phase.

Interactions of the antifungal mycosubtilin with ergosterol-containing interfacial monolayers

Mycosubtilin, an antimicrobial lipopeptide produced by Bacillus subtilis, is characterized by strong antifungal activities. The molecular mechanisms of its biological activities on the membranes of the sensitive yeasts or fungi have not yet been clearly elucidated. Our purpose was to mimic the mycosubtilin interactions with these membranes using various Langmuir monolayers. Since the major sterol of yeasts or fungi is ergosterol, the interactions of mycosubtilin with monolayers constituted by ergosterol, DPPC/ergosterol or DPPC/sphingomyelin/ergosterol were examined at different initial surface pressures (Πi). Plotting the mycosubtilin-induced surface pressure increases versus Πi allowed to determine that the exclusion pressures of mycosubtilin from these different monolayers is higher than the surface prevailing within the biological membranes. However, this behavior was lost when mycosubtilin was interacting with ergosteryl acetate-containing monolayers. This suggests the involvement of the sterol alcohol group in the mycosubtilin interactions within membranes. Furthermore, the behavior of mycosubtilin with stigmasterol, similar to that observed with ergosterol, differs from that previously observed with cholesterol, suggesting a role of the alkyl side chain of the sterols. The adsorption of mycosubtilin to ergosterol monolayers induced changes in the lipopeptide orientation at the air–water interface as revealed by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Moreover, imaging the air–water interface by Brewster angle microscopy (BAM) indicates that mycosubtilin induced changes in the organization and morphology of monolayers containing pure ergosterol with the appearance of small condensed dots, suggesting again that the target of mycosubtilin might be the ergosterol present in the membranes of the sensitive yeasts or fungi.

Penetration behaviour of alkylbetainate chlorides into lipid monolayers

In this paper, the penetration behaviour of the alkylbetainate chloride surfactants (C(n)BC, n=10-16) into lipid monolayers of dipalmitoylphosphatidylserine (DPPS), dipalmitoylphosphatidic acid (DPPA), dipalmitoylphosphatidylethanolamine (DPPE), palmitoyoleoylphosphatidylcholine (POPC) and cholesterol (CHOL) is investigated using the Langmuir trough technique. The penetration of C(n)BC is followed by measurement of the surface pressure increase (Δπ) at a constant surface area after the injection of C(n)BC into the aqueous phase, underneath the lipid monolayer previously spread at the air-water interface at 25°C and at different initial surface pressures (π(i)). The influence of both the lipid head group and the surfactant hydrocarbon chain length on the effectiveness of C(n)BC penetration into these monolayers is discussed. The results have shown that C(n)BC adsorb at the air-water interface giving evidence of their surface-active properties. The adsorption kinetics of C16BC into different lipid monolayers are lipid head charge and lipid head volume-dependent. The magnitude of the surface pressure increase (Δπ) arises in the following order: DPPA>DPPS≫CHOL≈DPPE>POPC. C(n)BC penetration into negatively-charged (DPPS and DPPA) monolayers does not seem to depend on surfactant alkyl-chain length compared to uncharged (CHOL) and zwitterionic (DPPE and POPC) monolayers for which Δπ increases with a larger alkyl-chain length. Electrostatic interactions are mainly involved in the affinity of C(n)BC with monolayers but the hydrophobic effect plays also a role.

Effects of impacts on the atmospheric evolution: Comparison between Mars, Earth, and Venus

Classified as a terrestrial planet, Venus, Mars, and Earth are similar in several aspects such as bulk composition and density. Their atmospheres on the other hand have significant differences. Venus has the densest atmosphere, composed of CO 2 mainly, with atmospheric pressure at the planet's surface 92 times that of the Earth, while Mars has the thinnest atmosphere, composed also essentially of CO 2, with only several millibars of atmospheric surface pressure. In the past, both Mars and Venus could have possessed Earth-like climate permitting the presence of surface liquid water reservoirs. Impacts by asteroids and comets could have played a significant role in the evolution of the early atmospheres of the Earth, Mars, and Venus, not only by causing atmospheric erosion but also by delivering material and volatiles to the planets. Here we investigate the atmospheric loss and the delivery of volatiles for the three terrestrial planets using a parameterized model that takes into account the impact simulation results and the flux of impactors given in the literature. We show that the dimensions of the planets, the initial atmospheric surface pressures and the volatiles contents of the impactors are of high importance for the impact delivery and erosion, and that they might be responsible for the differences in the atmospheric evolution of Mars, Earth and Venus.

Relaxation Processes in the Adsorption of Surface Layer Proteins to Lipid Membranes

The present work evaluates the kinetics of the interaction of S-layer protein from Lactobacillus brevis with lipid monolayers by measuring the changes in the surface pressure as a function of time for different lipid compositions and at different lateral compressions. At high surface pressures, or at high cholesterol ratios, in which membrane rigidity and surface polarity are increased, the kinetics can be described by a pure diffusional process. At low pressures or in the absence of cholesterol, the kinetics of protein interaction can be interpreted as a consequence of a relaxation process of the membrane structure coupled to diffusion. As the less packed monolayers are more hydrated, the relaxation processes at low initial surface pressures could be ascribed to changes in water organization in the membrane. These observations denote that kinetic insertion of proteins can be modulated by components that modify the hydration state of the interface.

Interaction of chromium(III) complexes with model lipid bilayers: Implications on cellular uptake

To understand molecular cytotoxicity of chromium(III) and how it affects the stability of biological membranes, studies on the interaction of chromium(III) complexes aquapentaminechromium complex (complex I) and trans- [Cr(5-methoxysalcyclohex) (H2O) 2] ClO4 (complex II) with model biomembranes have been carried out. Langmuir films of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidic acid (DPPA), dioctadecyldimethylammoniumbromide (DOMA) at air/water interface interacting with the chromium(III) complexes have been characterized using the surface pressure-molecular area (π-A) isotherms. Initial surface pressures changes for the two complexes show that the chromium(III) complexes inserted in the Langmuir films and complex I interacted strongly compared to complex II. Supported bilayers (SB) of the lipids on solid substrates formed by hydrating their Langmuir-Blodgett films (LB films) have been characterized using linear dichroic spectra, low angle X-ray diffraction and steady state fluorescence anisotropy. Depending on the geometry of the ligands and concentration, the complexes either insert in the alkyl or in the head group region of the SB and sometimes in both regions. The Supported lipid bilayers are well-layered and at low concentration, the metal complexes are incorporated near the head group region. Order and increase in lamellar spacing show stronger interaction of complex I with the lipids compared with complex II. This study provides some insights into the mechanism of chromium(III) toxicity and uptake of chromium(III) by the cells.

Interaction of Poly(l-arginine) with Negatively Charged DPPG Membranes: Calorimetric and Monolayer Studies

The interaction of poly(L-arginine) (PLA) with dipalmitoyl-phosphatidylglycerol (DPPG) bilayer membranes and monolayers was studied by differential scanning calorimetry (DSC), isothermal titration calorimetry (ITC), and monolayer experiments. The binding of PLA affected the main transition temperature of lipid bilayers (T(m)) only marginally. Depending on the PLA chain length, T(m) was slightly increased or decreased. This finding was attributed to the superposition of two counteracting effects on the transition after PLA binding. The main transition enthalpy (DeltaH(m)) was decreased upon PLA binding and the formation of a ripple phase (P(beta)') was suppressed. ITC experiments showed that two distinct processes are involved in binding of PLA to gel phase (L(beta)') membranes. At low peptide content the binding reaction is endothermic, and at high peptide concentration the binding becomes exothermic. However, the enthalpy of binding to fluid (L(alpha)) membranes was exothermic for all peptide-to-lipid ratios. The temperature dependence of PLA binding to fluid palmitoyl-oleoyl-phosphatidylglycerol (POPG) membranes showed a decrease in binding enthalpy with increasing temperature (Delta(R)C(p) < 0), indicating hydrophobic contributions to the free energy of binding. For longer PLA chains, the binding enthalpy for L(alpha) membranes was more exothermic than for shorter chains. Monolayer adsorption experiments showed two consecutive binding processes. At low initial surface pressures (pi(0)) a condensation of the lipid film (Deltapi < 0) is first observed after PLA injection into the subphase, followed by an increase in film pressure (Deltapi > 0) due to insertion of peptide side chains into the monolayer. At higher pi(0) only an increase in film pressure can be observed due to the insertion of the side chains. Deltapi increases with increasing pi(0). The insertion of the peptide into the monolayer is corroborated by the observed shift of pi-A isotherms to higher molecular areas. All presented experiments show that the binding of PLA to DPPG membranes has not only electrostatic but also nonelectrostatic contributions.

Topography of dipalmitoyl-phosphatidyl-choline monolayers penetrated by β-casein

In this work we have analyzed the topography by atomic force microscopy (AFM) of dipalmitoyl-phosphatidyl-choline (DPPC) monolayers previously spread at the air–water interface and penetrated by β-casein. AFM images of β-casein–DPPC monolayers were taken from Langmuir–Blodgett films deposited onto hydrophilic mica substrates at different initial surface pressures ( π i) and after the compression of the mixed films. The monolayer topography depends on the initial structure of the phospholipid:liquid expanded (LE) at 3 mN/m, coexistence between LE and liquid condensed (LC) structures at 7 mN/m, at the end of the LE–LC transition at 10 mN/m, and with a LC structure at 15 mN/m. The area occupied by DPPC domains in the mixed film increases with the π i value, especially for DPPC with a LC structure at 15 mN/m. At this surface pressure the thickness of the film is at a maximum. After the film compression at 25 mN/m, which is above the equilibrium spreading pressure of β-casein ( π e β -casein ), this protein is displaced from the interface by DPPC and the topography of the mixed monolayer depends on the initial structure of the DPPC monolayer. A notable feature of the topography of these mixed monolayers is the presence of multilayers of β-casein and DPPC of high thickness (50–70 nm) at the lower π i values. Although the film is dominated by DPPC at the highest surface pressures (at 25 mN/m), β-casein is not displaced totally from the interface and coexists as β-casein collapsed domains within the network of the DPPC structure.

Dynamics of penetration of dipalmitoyl-phosphatidyl-choline (DPPC) monolayers by β-casein

In this work we have analyzed the dynamics of the penetration of β-casein into dipalmitoyl-phosphatidyl-choline (DPPC) monolayers and the surface rheological characteristics (under dilatation and shear conditions) of mixed films formed by DPPC penetrated by β-casein. The penetration of β-casein into a DPPC monolayer was followed by measurements of the surface pressure ( π) increase at a constant surface area ( A) after the injection of β-casein into the aqueous phase, underneath the DPPC monolayer previously spread at the air–water interface, at different initial surface pressures ( π i) and at pH 5, 7, and 9. The evolution of surface pressure with time corresponds to an adsorption mechanism with three kinetic steps: (i) penetration of β-casein from underneath the DPPC monolayer, (ii) accumulation of the protein at the interface, and (iii) progressive penetration of β-casein into the DPPC monolayer. The results reflect the fact that β-casein–DPPC electrostatic interactions and the DPPC monolayer structure have an effect on the incorporation of β-casein into DPPC monolayers at the air–water interface. The penetration of β-casein is facilitated at acidic or neutral pH, but is hindered at basic pH. The surface viscoelastic properties under dilatational (using a ring trough with an isotropic area deformation) and shear (using an automatic Du Noüy ring oscillatory apparatus) conditions of a DPPC monolayer penetrated by β-casein are complementary and also demonstrated the complexity of the penetration mechanisms, due to the effect of electrostatic interactions between DPPC and β-casein as a function of the aqueous phase pH.

Enzymatic Activity and Monolayer Binding of a Truncated Form of Lecithin Retinol Acyltransferase

Lecithin retinol acyltransferase (LRAT) is a 230 amino acids membrane-associated protein. It has two enzymatic activities: first, it catalyzes hydrolysis of the sn-1 acyl chain of phospholipids and then transfers this acyl group to all-trans retinol to generate all-trans retinyl esters. This reaction is essential in the vertebrate visual cycle. The present study was performed to study the enzymatic activity of a truncated form of LRAT (tLRAT), where transmembrane domains have been removed. tLRAT extends from residues 31 to 196. It has been previously determined that the deleted domains of tLRAT do not contain residues known to be required for catalysis. tLRAT has been produced in E. coli and purified using affinity chromatography. Its enzymatic activity was studied using the short-chain diheptanoyl phosphatidylcholine (DHPC), which behaves like a mild detergent. The low critical micellar concentrations of DHPC allows to solubilize tLRAT and retinol. The maximal enzymatic activity of tLRAT is approximately 900 mol of ester/min • mol of protein. This value is more than 20 000 times higher than the largest enzyme activity reported in the literature. This huge difference can be explained by the use of a solution where DHPC serves both as a substrate as well as to solubililize the second substrate which highly favors the hydrolytic activity of tLRAT. Moreover, the injection tLRAT into the subphase of a phospholipid monolayer at different initial surface pressures allowed to determine the maximum insertion pressure (MIP) of tLRAT. A similar MIP of 38 mN/m has been obtained for dioleoyl phosphatidylcholine, ethanolamine and serine which is much higher than the lateral pressure of membranes. It can thus be postulated that tLRAT strongly binds membranes in the absence of its putative N- and C-terminal transmembrane domains.

Endophilin B1/Bif-1 Stimulates BAX Activation Independently from Its Capacity to Produce Large Scale Membrane Morphological Rearrangements

Endophilin B1/BAX-interacting factor 1 (Bif-1) is a protein that cooperates with dynamin-like protein 1 (DLP1/Drp1) to maintain normal mitochondrial outer membrane (MOM) dynamics in healthy cells and also contributes to BAX-driven MOM permeabilization (MOMP), the irreversible commitment point to cell death for the majority of apoptotic stimuli. However, despite its importance, exactly how Bif-1 fulfils its proapoptotic role is unknown. Here, we demonstrate that the stimulatory effect of Bif-1 on BAX-driven MOMP and on BAX conformational activation observed in intact cells during apoptosis can be recapitulated in a simplified system consisting of purified proteins and MOM-like liposomes. In this reconstituted model system the N-BAR domain of Bif-1 reproduced the stimulatory effect of Bif-1 on functional BAX activation. This process was dependent on physical interaction between Bif-1 N-BAR and BAX as well as on the presence of the mitochondrion-specific lipid cardiolipin. Despite that Bif-1 N-BAR produced large scale morphological rearrangements in MOM-like liposomes, this phenomenon could be separated from functional BAX activation. Furthermore, DLP1 also caused global morphological changes in MOM-like liposomes, but DLP1 did not stimulate BAX-permeabilizing function in the absence or presence of Bif-1. Taken together, our findings not only provide direct evidence for a functional interplay between Bif-1, BAX, and cardiolipin during MOMP but also add significantly to the growing body of evidence indicating that components of the mitochondrial morphogenesis machinery possess proapoptotic functions that are independent from their recognized roles in normal mitochondrial dynamics.