Fluorescence properties of anthrylvinyl lipid probes

Membrane curvature effects on glycolipid transfer protein activity

The glycolipid transfer protein (GLTP) is monomeric in aqueous solutions, and it binds weakly to membrane interfaces with or without glycolipids. GLTP is a surface-active protein and adsorbs to exert a maximal surface pressure value of 19 mN/m. The change in surface pressure following GLTP adsorption decreased linearly with initial surface pressure. The exclusion pressure for different phospholipids and sphingolipids was between 23 and 31 mN/m, being clearly highest for the negatively charged dipalmitoyl-phosphatidylserine. This can be explained by electrostatic forces when GLTP is positively charged at neutral pH (isoelectric point = 9.0) and by phosphatidylserine being negatively charged. If GLTP is injected under a palmitoyl-galactosylceramide monolayer above 30 mN/m, the presence of GLTP leads to a decrease in the surface pressure as a function of time. This suggests that GLTP is able to remove glycolipids from the monolayer without penetrating the monolayer. On the other hand, if phospholipid vesicles with or without glycolipids are also present in the subphase, no change in the surface pressure takes place. This suggests that GLTP in the presence of curved membranes is not able to transfer from or to planar membranes. We also show that transfer of fluorescently labeled galactosylceramide is faster from small highly curved palmitoyl-oleoyl-phosphatidylcholine and dipalmitoyl-phosphatidylcholine bilayer vesicles but not from palmitoyl-sphingomyelin vesicles regardless of the size.

Cytochrome C interaction with cardiolipin/phosphatidylcholine model membranes: effect of cardiolipin protonation

Resonance energy transfer between anthrylvinyl-labeled phosphatidylcholine as a donor and heme moiety of cytochrome c (cyt c) as an acceptor has been employed to explore the protein binding to model membranes, composed of phosphatidylcholine and cardiolipin (CL). The existence of two types of protein-lipid complexes has been hypothesized where either deprotonated or partially protonated CL molecules are responsible for cyt c attachment to bilayer surface. To quantitatively describe cyt c membrane binding, the adsorption model based on scaled particle and double layer theories has been employed, with potential-dependent association constants being treated as a function of acidic phospholipid mole fraction, degree of CL protonation, ionic strength, and surface coverage. Multiple arrays of resonance energy transfer data obtained under conditions of varying pH, ionic strength, CL content, and protein/lipid molar ratio have been analyzed in terms of the model of energy transfer in two-dimensional systems combined with the adsorption model allowing for area exclusion and electrostatic effects. The set of recovered model parameters included effective protein charge, intrinsic association constants, and heme distance from the bilayer midplane for both types of protein-lipid complexes. Upon increasing CL mole fraction from 10 to 20 mol % (the value close to that characteristic of the inner mitochondrial membrane), the binding equilibrium dramatically shifted toward cyt c association with partially protonated CL species. The estimates of heme distance from bilayer center suggest shallow bilayer location of cyt c at physiological pH, whereas at pH below 6.0, the protein tends to insert into membrane core.

Coverage-dependent changes of cytochrome c transverse location in phospholipid membranes revealed by FRET

The method of fluorescence resonance energy transfer (FRET) has been employed to monitor cytochrome c interaction with bilayer phospholipid membranes. Liposomes composed of phosphatidylcholine and varying amounts of anionic lipid cardiolipin (CL) were used as model membranes. Trace amount of fluorescent lipid derivative, anthrylvinyl-phosphatidylcholine was incorporated into the membranes to serve energy donor for heme moiety of cytochrome c. Energy transfer efficiency was measured at different lipid and protein concentrations to obtain extensive set of data, which were further analyzed globally in terms of adequate models of protein adsorption and energy transfer on the membrane surface. It has been found that the cytochrome c association with membranes containing 10 mol% CL can be described in terms of equilibrium binding model (yielding dissociation constant Kd = 0.2-0.4 microM and stoichiometry n = 11-13 lipid molecules per protein binding site) combined with FRET model assuming uniform acceptor distribution with the distance of 3.5-3.6 nm between the bilayer midplane and heme moiety of cytochrome c. However, increasing the CL content to 20 or 40 mol% (at low ionic strength) resulted in a different behavior of FRET profiles, inconsistent with the concepts of equilibrium adsorption of cytochrome c at the membrane surface and/or uniform acceptor distribution. To explain this fact, several possibilities are analyzed, including cytochrome c-induced formation of non-bilayer structures and clusters of charged lipids, or changes in the depth of cytochrome c penetration into the bilayer depending on the protein surface density. Additional control experiments have shown that only the latter process can explain the peculiar concentration dependences of FRET at high CL content.

Resonance energy transfer study of peptide-lipid complexes

Resonance energy transfer involving tryptophan as a donor and anthrylvinyl-labeled phosphatidylcholine (AV-PC), 3-methoxybenzanthrone (MBA) and 8-anilino-1-naphthalene sulfonic acid (ANS) as acceptors has been examined to obtain information on the structure of peptide-lipid systems consisting of 18A or Ac-18A-NH(2) peptides and large unilamellar phosphatidylcholine vesicles. The lower and upper limits for the tryptophan distance from the bilayer midplane have been assessed in terms of the models of energy transfer in two-dimensional systems, taking into account orientational effects. Evidence for the existence of preferential orientations of Ac-18A-NH(2) with respect to the lipid-water interface has been obtained.

New fluorescent cholesterol analogs as membrane probes

New fluorescent cholesterol analogs, (22E, 20R)-3beta-hydroxy-23-(9-anthryl)-24-norchola-5,22-die ne (R-AV-Ch), and the 20S-isomer (S-AV-Ch) were synthesized, their spectral and membrane properties were characterized. The probes bear a 9-anthrylvinyl (AV) group instead of C22-C27 segment of the cholesterol alkyl chain. Computer simulations show that both of the probes have bulkier tail regions than cholesterol and predict some perturbation in the packing of membranes, particularly for R-AV-Ch. In monolayer experiments, the force-area behavior of the probes was compared with that of cholesterol, pure and in mixtures with palmitoyloleoyl phosphatidylcholine (POPC) and N-stearoyl sphingomyelin (SSM). The results show that pure R-AV-Ch occupies 35-40% more cross-sectional area than cholesterol at surface pressures below film collapse (0-22 mN/m); whereas S-AV-Ch occupies nearly the same molecular area as cholesterol. Isotherms of POPC or SSM mixed with 0.1 mol fraction of either probe are similar to isotherms of the corresponding mixtures of POPC or SSM with cholesterol. The probes show typical AV absorption (lambda 386, 368, 350 and 256 nm) and fluorescence (lambda 412-435 nm) spectra. Steady-state anisotropies of R-AV-Ch and S-AV-Ch in isotropic medium or liquid-crystalline bilayers are higher than the values obtained for other AV probes reflecting hindered intramolecular mobility of the fluorophore and decreased overall rotational rate of the rigid cholesterol derivatives. This suggestion is confirmed by time-resolved fluorescence experiments which show also, in accordance with monolayer data, that S-AV-Ch is better accommodated in POPC-cholesterol bilayers than R-AV-Ch. Model and natural membranes can be labeled by either injecting the probes via a water-soluble organic solvent or by co-lyophilizing probe and phospholipid prior to vesicle production. Detergent-solubilization studies involving 'raft' lipids showed that S-AV-Ch almost identically mimicked the behavior of cholesterol and that of R-AV-Ch was only slightly inferior. Overall, the data suggest that the AV-labeled cholesterol analogs mimic cholesterol behavior in membrane systems and will be useful in related studies.

Amphipathic peptide affects the lateral domain organization of lipid bilayers

Using lipid-specific fluorescent probes, we studied the effects of amphipathic helical, membrane active peptides of the A- and L-type on membrane domain organization. In zwitterionic binary systems composed of mixtures of phosphatidylcholine and phosphatidylethanolamine, both types of peptides associated with the fluid phase. While binding with high affinity to fluid membranes, peptides were unable to penetrate into the lipid membrane in the gel state. If trapped kinetically by cooling from the fluid phase, peptides dissociated from the gel membrane on the time scale of several hours. While the geometrical shape of the alpha-helical peptides determines their interactions with membranes with non-bilayer phase propensity, the shape complementarity mechanism by itself is unable to induce lateral phase separation in a fluid membrane. Charge-charge interactions are capable of inducing lateral domain formation in fluid membranes. Both peptides had affinity for anionic lipids which resulted in about 30% enrichment of acidic lipids within several nanometers of the peptide's tryptophan, but there was no long-range order in peptide-induced lipid demixing. Peptide insertion in fluid acidic membranes was accompanied by only a small increase in bilayer surface and a decrease in polarity in the membrane core. Peptide-lipid charge-charge interactions were also capable of modulating existing domain composition in the course of the main phase transition in mixtures of anionic phosphatidylglycerol with zwitterionic phosphatidylcholine.

Dynamic lipid heterogeneity and receptor events

Receptor occupation by specific ligands induces changes in the dynamic domain organization of surrounding lipids. Such changes were observed by measuring changes in the fluorescence parameters of fluorescent-labelled lipids incorporated into plasma membranes of intact cells, membrane vesicles or lipoprotein particles in response to specific binding of a broad range of biologically active agents, including drugs, prostaglandins, neuropeptides, antibodies and viruses. The high sensitivity of the fluorescence response allowed us to register changes in lipid heterogeneity induced in a multitude of discrete targets by transient weak binding of a single rapidly translocating molecule. To explain these observations a non-equilibrium model of ligand-receptor interaction based on low relaxation phenomena in heterogeneous lipid matrixes is proposed.

Anthrylvinyl-labeled phospholipids as membrane probes: the phosphatidylcholine-phosphatidylethanolamine system

The phase behavior of mixtures of phosphatidylcholine (PC) with phosphatidylethanolamine (PE) identical or differing in their fatty acid composition has been investigated by using the steady-state fluorescence anisotropy of anthrylvinyl-labeled PC and PE (APC and APE) as well as of the non-lipid probe 1,6-diphenyl-1,3,5-hexatriene (DPH) to detect temperature-dependent changes in multilayer liposomes. APC, but not APE, was able to detect the pretransition of dimyristoyl-PC. The phospholipid probes APC and APE showed the main phase transition of their unlabeled disaturated analogues at temperatures almost identical with those revealed by differential scanning calorimetry, whereas the onset of the PE phase transition recorded by DPH was several degrees higher. In PC-PE mixtures with high content of PE the phase transitions shown by APC and APE were broader than those recorded by DPH. Comparison of phase diagrams constructed on the basis of fluorescence anisotropy and calorimetric data led to the conclusion that in biphasic PE and PC-PE systems DPH tends to partition into solid regions, whereas the anthrylvinyl-labeled phospholipids distribute more evenly between coexisting phases or prefer fluid domains. The use of anthrylvinyl phospholipid probes made it possible to demonstrate that PEs and PCs identical in their fatty acids are not miscible completely, not only below but also well above Tm of the higher melting component. Generally, APC and APE fluorescence anisotropy measurements correctly reflect headgroup-dependent phase segregations in mixtures of PC with PE, but may lead to ambiguous conclusions if demixing is caused by differences in the hydrocarbon chains.

Anthrylvinyl-labeled phospholipids as fluorescent membrane probes. The action of melittin on multilipid systems

The interaction of melittin with multicomponent lipid mixtures composed of phosphatidylcholine, sphingomyelin and phosphatidylserine or phosphatidylglycerol was investigated by measuring the intrinsic fluorescence of the peptide, steady state fluorescence anisotropy of, and Trp-fluorescence energy transfer to fluorescent analogs of the same phospholipids bearing the anthrylvinyl fluorophore in one of the aliphatic chains at various distances from the polar head group. Based on the finding that at high lipid/peptide ratio the peptide induces unequal changes in the fluorescence parameters of phospholipid probes differing structurally only in their polar head groups, it is concluded that melittin induces lipid demixing in its nearest environment. Comparison of the fluorescence energy transfer from Trp to different lipid probes indicates that the depth of penetration of melittin into the bilayer depends on the polar head group composition of the phospholipid matrix and that certain segments of the melittin chain display a specific affinity for a given lipid head group.