Raft model

The most likely way for pardaxin molecules to insert across the membrane in an antiparallel manner is for them to form antiparallel aggregates on the membrane surface that then insert across the membrane. We developed a "raft"model (data not shown) that is similar to the channel model except that adjacent dimers are related to each other by a linear translation instead of a 60 rotation about a channel axis. All of the large hydrophobic side chains of the C-helices are on one side of the "raft" and all hydrophilic side chains are on the other side. We postulate that these "rafts" displace the lipid molecules on one side of the bilayer. When two or more "rafts" meet they can insert across the membrane to form a channel in a way that never exposes the hydrophilic side chains to the lipid alkyl chains. The conformational change from the "raft" to the channel structure primarily involves a pivoting motion about the "ridge" of side chains formed by Thr-17, Ala-21, Ala-25, and Ser-29. These small side chains present few steric barriers for the postulated conformational change. 

VTi. Keenan E. Dungey, George MJII Lisensky, and S. Michael Condren, "Kixium Monolayers A Simple Alternative to the Bubble Raft Model for Close-Packed Spheres," /. Chem. Educ., Vol. 76, 1999, 618-619. 

The lipid membrane provides shelter for membrane proteins to do their functions. However, instead of working alone, membrane proteins such as ion channels work together with the membrane, such that the lipid composition around the protein actually affects the activation and the functioning of the protein. This idea is largely the essence of the lipid raft model (8), which highlights the importance of lipids in a variety of cellular functions. It has been observed, for example, that rhodopsin, which is the light sensitive membrane protein, favors interactions with polyunsaturated lipids (9). 

Fig. 6.39. Images of dislocation nucleation in a bubble raft model of a single crystal subjected to surface indentation. In the case of the smallest surface roughness (a), dislocation nucleation occurs at the peaks of the asperities where they contact the indenter. At the intermediate scale (b), dislocation nucleation occurs at the reentrant corners at the bases of the asparities. Finally, for the largest scale asperity (c), the stress level near the stress concentrations has been reduced geometrically, and dislocations nucleated homogeneously at an interior point where the shear stress is the largest. Each bubble in this raft is 1 mm in diameter and it represents an atom which is approximately 0.3 nm in diameter. Reproduced with permission from Gouldstone et al. (2001).

There are two views of the motion of integral proteins in the bilayer. In the fluid mosaic model shown in Fig. 11.53, the proteins are mobile, but their diffusion coefficients are much smaller than those of the lipids. In the lipid raft model, a number of lipid and cholesterol molecules form ordered structures, or rafts, that envelop proteins and help carry them to specific parts of the cell. 

Distinguish between the fluid mosaic and lipid raft models for motion of integral proteins in a biological membrane. 

Lipopolysaccharide (LPS) is a major component of the external leaflet of bacterial outer membranes with many other functions. Ciesielski et al. use solid state and MAS NMR to investigate the interactions of LPS from three bacterial species with mixed lipid membranes, raft models. The dynamics and structure were investigated by ssNMR and the role of lipid rafts on these dynamics was also examined. ... 

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