Protein and peptide interactions with lipids: Structure, membrane function and new methods

Particulate and Bulk Bicontinuous Cubic Phases Obtained from Mixtures of Glyceryl Monooleate and Copolymers Bearing Blocks of Lipid−Mimetic Anchors in Water

Copolymers based on poly(ethylene glycol) bearing one or more lipid-mimetic anchors were mixed with glycerylmonooleate (GMO)-a lipid with nonlamellar propensity-to form bulk and particulate bicontinuous cubic phases in water. The particulate phase was obtained via a liquid precursor method. Three forms of copolymer/GMO mixtures were investigated-precursor dispersions in glycerol and bulk and particulate phases in water-by visual observations, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). The bulk phases were found to very slowly develop a macroscopic appearance that can be associated with the bicontinuous cubic phase. They were prepared in a slight excess of water, which became opalescent in some of the preparations. Cryo-TEM investigation of the excess showed that vesicles and particles with a dense interior coexisted. The precursors were prepared as solutions in glycerol. The viscous liquid material was investigated by DLS. Diffusion coefficients and the corresponding hydrodynamic radii, ranging from about 10 to 30 nm, were calculated. The particles are presumably of a structure similar to that of conventional emulsion droplets with GMO in the interior and copolymer molecules in the outer regions. The particulate phase in water was obtained upon hydration of the liquid precursors. The dispersions were investigated by DLS and cryo-TEM. DLS revealed the formation of nanosized particles. The size was found to increase with increasing copolymer content for copolymers with only one lipid-mimetic anchor, whereas the opposite trend was observed for the formulations with copolymers bearing more than one lipid-mimetic anchor. The shape and interior of the particles were studied by cryo-TEM. It was found that most particles were globular. For some of the compositions, particles with a dense internal structure dominated. The texture of the internal structures was assigned to dispersed bicontinuous cubic or L3 phases. In other compositions, the interior seemingly consists of arrays of interlamellar attachments.

Protein structural changes induced by their uptake at interfaces

For insertion into lipidic media, most hydrosoluble proteins must cross the lipid-water interface and thus undergo conformational transitions. According to their chemical sequences these transitions may be restricted to changes involving only the tertiary structure, while for other proteins this environment modification will induce drastic changes such as the unfolding of large domains. The structural transitions are mainly governed by the presence of hydrophobic domains and/or by the existence of induced amphipathic properties.

Influence of a lipid interface on protein dynamics in a fungal lipase

Lipases catalyze lipolytic reactions and for optimal activity they require a lipid interface. To study the effect of a lipid aggregate on the behavior of the enzyme at the interfacial plane and how the aggregate influences an attached substrate or product molecule in time and space, we have performed molecular dynamics simulations. The simulations were performed over 1 to 2 ns using explicit SPC water. The interaction energies between protein and lipid are mainly due to van der Waals contributions reflecting the hydrophobic nature of the lipid molecules. Estimations of the protonation state of titratable residues indicated that the negative charge on the fatty acid is stabilized by interactions with the titratable residues Tyr-28, His-143, and His-257. In the presence of a lipid patch, the active site lid opens wider than observed in the corresponding simulations in an aqueous environment. In that lid conformation, the hydrophobic residues Ile-85, Ile-89, and Leu-92 are embedded in the lipid patch. The behavior of the substrate or product molecule is sensitive to the environment. Entering and leaving of substrate molecules could be observed in presence of the lipid patch, whereas the product forms strong hydrogen bonds with Ser-82, Ser-144, and Trp-88, suggesting that the formation of hydrogen bonds may be an important contribution to the mechanism by which product inhibition might take place.

Influence of transmembrane peptides on bilayers of phosphatidylcholines with different acyl chain lengths studied by solid-state NMR

The molecular orientation in a lipid membrane of the peptide fragment VEYAGIALFFVAAVLTLWSMLQYLSAAR (phosphatidylglycerophosphate synthase (Pgs) peptide E) of an integral membrane protein, Pgs, in Escherichia coli has been investigated by solid-state 15N nuclear magnetic resonance (NMR) on macroscopically aligned lipid bilayers. The secondary structure of the peptide in lipid vesicles was determined by circular dichroism spectroscopy. Furthermore, the phase behaviour of the Pgs peptide E/dierucoylphosphatidylcholine (DEruPC)/water system was determined by (2)H, (31)P and 15N solid-state NMR spectroscopy. The phase behaviour obtained was then compared to that of the Pgs peptide E solubilised in dioleoylphosphatidylcholine and water that was previously studied by Morein et al. [Biophys. J. 73 (1997) 3078-3088]. This was aimed to answer the question whether a difference in the length of the hydrophobic part of this peptide and the hydrophobic thickness of the lipid bilayer (hydrophobic mismatch) will affect the phase behaviour. The peptide mostly has a transmembrane orientation and is in an alpha-helical conformation. An isotropic phase is formed in DEruPC with high peptide content (peptide/lipid molar ratio (p/l) > or =1:15) and high water content (> or =50%, w/w) at 35 degrees C. At 55 and 65 degrees C an isotropic phase is induced at high water content (> or =50%, w/w) at all peptide contents studied (no isotropic phase forms in the lipid/water system under the conditions in this study). At high peptide contents (p/l> or =1:15) an isotropic phase forms at 20 and 40% (w/w) of water at 55 and 65 degrees C. A comparison of the phase behaviour of the two homologous lipid systems reveals striking similarities, although the thicknesses of the two lipid bilayers differ by 7 A. This suggests that the rationalisation of the phase behaviour in terms of the hydrophobic mismatch is not applicable to these systems. The C-terminus of Pgs peptide E is amphiphilic and a considerable part of the peptide is situated outside the hydrophobic part of the bilayer, a property of the peptide that to a large extent will affect the lipid/peptide phase behaviour.

Preparation and Characterization of a Phospholipid Membrane-Bound Tetrapeptide That Corresponds to the C-Terminus of the Gastrin/Cholecystokinin Hormone Family

The present work deals with the synthesis of a hydrophobized peptide and its localization at the membrane surface, after its incorporation into phospholipid vesicles. The tetrapeptide, Trp-Met-Asp-Phe-NH(2), which corresponds to the C-terminus of the cholecystokinin/gastrin hormone family, is conjugated to N-glutaryldioleoylphosphatidylethanolamine using a carbodiimide-catalyzed reaction method. Sonication of the lipophilized hormone in the presence of dimyristoylphosphatidylcholine results in a strong sequestration of the conjugate in the artificial membrane structures that are formed. More detailed information on the localization of the peptide moiety with respect to the membrane surface is gathered from fluorescence measurements. Both the observed blue shift in the fluorescence spectra and the quenching of Trp emission in the presence of potassium iodide point to a partial screening of the hormone moiety from the surrounding aqueous phase. The different parameters that may influence the physicochemical behavior of a hydrophobized peptide in a membrane structure are briefly discussed Copyright 2000 Academic Press.

Protein-induced fusion can be modulated by target membrane lipids through a structural switch at the level of the fusion peptide

Regulatory features of protein-induced membrane fusion are largely unclear, particularly at the level of the fusion peptide. Fusion peptides being part of larger protein complexes, such investigations are met with technical limitations. Here, we show that the fusion activity of influenza virus or Golgi membranes is strongly inhibited by minor amounts of (lyso)lipids when present in the target membrane but not when inserted into the viral or Golgi membrane itself. To investigate the underlying mechanism, we employ a membrane-anchored peptide system and show that fusion is similarly regulated by these lipids when inserted into the target but not when present in the peptide-containing membrane. Peptide-induced fusion is regulated by a reversible switch of secondary structure from a fusion-permissive alpha-helix to a nonfusogenic beta-sheet. The "on/off" activation of this switch is governed by minor amounts of (lyso)-phospholipids in targets, causing a drop in alpha-helix and a dramatic increase in beta-sheet contents. Concomitantly, fusion is inhibited, due to impaired peptide insertion into the target membrane. Our observations in biological fusion systems together with the model studies suggest that distinct lipids in target membranes provide a means for regulating membrane fusion by causing a reversible secondary structure switch of the fusion peptides.