Model for raft creation as a result of incomplete wetting of membrane protein
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We propose a model explaining a creation of specific membrane liquid-ordered phase, rich in sphingomyelin and cholesterol, in the vicinity of integral membrane proteins. We assume that global phase transition does not take place, i.e. membrane is undersaturated. We hypothesize a fort-nation of a thin 2D film of liquid-ordered phase near a protein by the mechanism of incomplete wetting, and determine the conditions for stability of this film. This mechanism is likely to account for the raft assembly in cell membranes. The work consists of three Sections. In the first Section, using the Landau technique, we calculate the film equilibrium width on condition that one knows effective line tension of the protein/surrounding membrane boundary. In the second Section we calculate the line tension assuming that it is defined by hydrophobic mismatch of the protein transmembrane region, thickness of the film and thickness of the surrounding membrane. In the calculations membrane is considered as continuous elastic medium. The line tension is estimated from splay and tilt. In the third Section the results of our calculations are illustrated graphically. We show that if the length of the transmembrane domain of the protein is not less than the liquid-ordered bilayer thickness, the equilibrium width of the film is about 4-6 nm. Moreover, at a slight undersaturation, a formation of metastable films of 10-14 nm width is possible. These results are practically not sensitive to variations in spontaneous curvature of rafts and surrounding membrane.