Protein interactions, phospholipid membranes/surfaces

New developments in immobilization surfaces have lead to the use of SPR biosensors to monitor protein interactions with lipid surfaces and membrane-associated proteins. Commercially available (BIACORE) hydrophobic and lipophilic sensor surfaces have been designed to create stable membrane surfaces. It has been shown that the hydrophobic sensor surface can be used to form a lipid monolayer (Evans and MacKenzie, 1999). This monolayer surface can be used to monitor protein-lipid interactions. For example, a biosensor was used to examine binding of Src homology 2 domain to phosphoinositides within phospholipid bilayers (Surdo et al., 1999). In addition, a lipophilic sensor surface can be used to capture liposomes and form a lipid bilayer resembling a biological membrane. 

Membranes of plant and animal cells are typically composed of 40-50 % lipids and 50-60% proteins. There are wide variations in the types of lipids and proteins as well as in their ratios. Arrangements of lipids and proteins in membranes are best considered in terms of the fluid-mosaic model, proposed by Singer and Nicolson % According to this model, the matrix of the membrane (a lipid bilayer composed of phospholipids and glycolipids) incorporates proteins, either on the surface or in the interior, and acts as permeability barrier (Fig. 2). Furthermore, other cellular functions such as recognition, fusion, endocytosis, intercellular interaction, transport, and osmosis are all membrane mediated processes. 

These reconstitution experiments supported the model for electron transfer shown in figure 14.8. In this model the complexes do not bind to each other directly. Instead, movement of electrons from complexes I and II to complex III is mediated by diffusion of UQH2 from one complex to the other within the phospholipid bilayer. Similarly, electrons move from complex III to complex IV by the diffusion of reduced cytochrome c along the surface of the membrane. Remember that cytochrome c differs from the other cytochromes in being a water-soluble protein. It is attached loosely to the membrane surface by electrostatic interactions. 

The arrangement of the proteins within the membrane seems to depend to some extent on the electrostatic surface potential and interface permittivity. It is influenced by electrostatic interaction between the proteins, polar head groups of the phospholipid and ions within the aqueous medium of the membrane surface. This can be affected by exogenous molecules such as drugs. Phospholipid-induced conformational change in intestinal calcium-binding protein in the absence and presence of Ca2+ has been described [37]. There is, however, no doubt that hydrophobic interactions between peptides and membrane interfaces play an important role. A general frame-... 

Heid et al., 1996), so it is likely that this protein originates from the surface of intracellular lipid droplets and interacts with BTN, XDH, or perhaps other proteins or protein complexes on the inner face of the MFGM. Phospholipids and glycosphingolipids are known to be asymmetrically organized in cellular membranes but we have no specific information as to how these constituents are oriented in the MFGM. 

Thin liquid films (especially foam films) stabilised with phospholipids, proteins, etc., prove to be very suitable in the study of surface forces, since they could model the interacting biological membranes in aqueous medium. 

These results show that the ability of these signal peptides to interact with phospholipid monolayers indeed correlates with their in vivo activity. The pressure increases due to the functional signal peptides (8—11 dyn/cm) are in the same range as those caused by proteins known to insert into monolayers (Bougis et al., 1981). In contrast, prothrombin, which binds only to the membrane surface, causes a pressure increase in a phospholipid monolayer of 1.9-2.3 dyn/cm (Mayer et al., 1983). These values are almost identical to those obtained for perturbation of the monolayer by the deletion-mutant signal peptide. 

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