Membrane lipid-protein interaction model

Baumgartner and coworkers [145,146] study lipid-protein interactions in lipid bilayers. The lipids are modeled as chains of hard spheres with heads tethered to two virtual surfaces, representing the two sides of the bilayer. Within this model, Baumgartner [145] has investigated the influence of membrane curvature on the conformations of a long embedded chain (a protein ). He predicts that the protein spontaneously localizes on the inner side of the membrane, due to the larger fluctuations of lipid density there. Sintes and Baumgartner [146] have calculated the lipid-mediated interactions between cylindrical inclusions ( proteins ). Apart from the... 

The present knowledge about molecular organization in lyotropic liquid crystalline phases is summarized. Particular attention is given to biologicaly relevant structures in lipid-water systems and to lipid-protein interactions. "New findings are presented on stable phases (gel type) that have ordered lipid layers and high water content. Furthermore, electrical properties of various lipid structures are discussed. A simple model of l/l noise in nerve membranes is presented as an example of interaction between structural and electrical properties of lipids and lipidr-protein complexes. 

Although numerous models for the structure of membranes have been proposed, the structural features which are generally accepted at present are rather similar to the original Danielli-Davson model. There is convincing evidence that the structure is dominated by lipid bilayers. The state of order of the hydrocarbon chains is now being studied extensively by many groups (see below). Less is known about the proteins. Besides the proteins that are located on the outside according to the Danielli-Davson model, there are also proteins that are partly buried in the hydro-phobic interior of the lipid layer however, little is known about the lipid-protein interaction. 

McElhaney, R. N. Differential scanning calorimetric studies of lipid-protein interactions in model membrane systems. Biochimica et Biophysica acta 564 361-421, 1986. 

Mouritsen, O. G. and Bloom, M., Models of lipid-protein interactions in membranes. Annual Review of Biophysics and Biomolecular Structure 22 145-171, 1993. 

Gonen T, Cheng Y, Sliz P, Hiroaki Y, Fujiyoshi Y, Harrison SC, Walz T. Lipid-protein interactions in double-layered two dimensional crystals of aquaporin-0. Nature 2005 438 633-638. Henderson R, Unwin PN. Three dimensional model of purple membrane obtained by electron microscopy. Nature 1975 257 28-32. 

Lipid—Lipid and Lipid—Protein Interactions. The DPL—cholesterol and the protein—DPL systems are particularly amenable to interpretation using our membrane model. The high viscosity lattice of DPL can be broken by cholesterol (Figure 9), and the lattice of BSA can be broken by a lipid (e.g., DPL, Figure 10), with a marked loss of surface viscosity. This lattice collapse means formation of independent membrane subunits whose lateral valences are saturated within the subunit, thereby producing a fluid system (Figure 1A) the subunit could be a lipid-lipid system, as with DPL and cholesterol, or a lipid-protein system. The phenomenon of lattice collapse with loss of surface viscosity is impressive in the DPL-albumin system since individually both components have a high surface viscosity. 

Net transfer of lipid occurs from the plasma to the erythrocyte membrane, presumably because of a shift in the equihbrium as the plasma lipoproteins become saturated with the excess cholesterol and phosphatidylcholine. This leads to membrane abnormalities and cholesterol-phospholipid ratios of up to 2 1. Changes in cellular physiology of the type referred to in section IV have also been reported [94,96,161]. These must reflect an alteration in lipid-protein interactions within the membranes. The molecular arrangement of the excessive amounts of cholesterol present in the cell membranes in diseased liver cells is not known. In model systems cholesterol is not present in molar amounts greater than 1 1. In liver disease a major change is in cellular morphology with the formation of abnormally shaped erythrocytes, as discussed earlier. 

These expressions however are essentially, mean-field approaches as little account was taken of the molecular diversity that exists in living cell membranes. This is in part due to the evident complexity of such membranes but some modelling simulations have now begun to incorporate such diversity (eg. lipid-protein interactions are emphasised within the context of computational modelling). These are important tasks as there is clearly much local variation in the magnitude of the membrane surface potentials. We have engaged this problem for some years and are able to visualise the spatial variation of the potential about the membrane and cell surface with high molecular and spatial resolution (see below). 

P. Bothorel and C. Lussan, On a biological membrane model based on lipid-protein interactions, C. R. Hebd. Seances Acad. Sci. 266C(26), 2492-2494,... 

Mouritsen OG, Bloom M (1984) Mattress model of lipid-protein interactions in membranes. Biophys J 36 141-153... 

John R. Silvius, Thermotropic Phase Transitions of Pure Lipids in Model Membranes and Their Modifications by Membrane Proteins , in Lipid-Protein Interactions, John Wiley Sons, Inc. New York, 1982. 

Silvius, J.R. Thermotropic phase transitions of pure lipids in model membranes and their modification by membrane proteins, in P.C. lost and O.H. Griffith (eds.), Lipid-Protein Interactions. Vol. 2, Wiley, New York, (1982), pp. 239-281. Somerville, C. Direct tests of the role of membrane lipid composition in low-temperature-induced photoinhibition and chilling sensitivity in plants and cyanobacteria. Proc. Natl. Acad. Sci. U.S.A. 92 (1995), 6215-6218. 

Figure 6 Artist s view of lipid/protein interaction derived from the electron density model from XR measurements and available structural data. The lipid monolayer is dipalmitoyl phosphatidyl ethanolamine (DPPE) and the protein is a membrane surface layer protein from B. sphaericus (from ref. [28]). The structural model sketched was based [28, 29] on the electron density profile (black line) inverted from measured reflectivity data as described in the text.

Gennis RB (1989) Biomembranes molecular structure and function. Springer, New York Yeagle P (1992) The structure of biological membranes. CRC Press, Boca Raton McElhaney RN (1986) Differential scanning calorimetric studies of lipid-protein interactions in model membrane systems. Biochim Biophys Acta 864(3-4) 361-42l HianikT, Passechnik VI (1995) Bilayer lipid membranes structure and mechanical properties. Kluwer, Netherlands... 

G. Mouritsen, M. Bloom, Models of lipid-protein interactions in membranes. In D. M. Engelman, C.R. Cantor, T. D. Pollard (eds.). Annual review of biophysics and biomolecular structure 22, Palo Alto Annual Reviews Inc. (1993) 145. 

Biological membranes arc composed of lipids and proteins, the lipids building up the bimolecular leaflet, which is the major permeability barrier, and the proteins providing the essential biological functions, The proteins can be either intrinsic proteins embedded in the lipid biiayers and having accessible polar surfaces on both sides of the bilayer, or they are mainly bound to the bilayer surface by electrostatic interactions. When proteins are incorporated into lipid bilayers or bound to the surface of the lamellae, the lipid packing in the gel phase is perturbed and the transition profiles obtained by DSC are changed in a characteristic way. Numerous studies of lipid-protein interactions have been performed in the past and the effects on the DSC transition curves have been modeled [99-104]. 

The interest in vesicles as models for cell biomembranes has led to much work on the interactions within and between lipid layers. The primary contributions to vesicle stability and curvature include those familiar to us already, the electrostatic interactions between charged head groups (Chapter V) and the van der Waals interaction between layers (Chapter VI). An additional force due to thermal fluctuations in membranes produces a steric repulsion between membranes known as the Helfrich or undulation interaction. This force has been quantified by Sackmann and co-workers using reflection interference contrast microscopy to monitor vesicles weakly adhering to a solid substrate [78]. Membrane fluctuation forces may influence the interactions between proteins embedded in them [79]. Finally, in balance with these forces, bending elasticity helps determine shape transitions [80], interactions between inclusions [81], aggregation of membrane junctions [82], and unbinding of pinched membranes [83]. Specific interactions between membrane embedded receptors add an additional complication to biomembrane behavior. These have been stud-... 

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