The self-organization of lipids and proteins of myelin at the membrane interface. Molecular factors underlying the microheterogeneity of domain segregation

The advances over the last 10 years on the understanding of myelin heterogeneity are reviewed. The main focus is on the applicability of Langmuir monolayers, Langmuir–Blodgett films and some associated techniques to unravelling the behaviour of interfaces formed with all the components of a natural membrane. Lipid–protein lateral segregation appears as a major driving force to determine surface patterns that can change under compression from circular domains to two-dimensional fractal structures. The major proteins of the myelin membrane induce lateral segregation in an otherwise homogeneous surface formed by the mixture of total myelin lipids. The lipid and protein components appear to distribute in the surface domains according to their charge, compressibility and relative molecular weight: myelin proteins, ganglioside GM1 and fluorescent lipid probes partition into liquid-expanded phase domains; other components such as phosphatidylserine and galactocerebroside partition into another liquid phase enriched in cholesterol. Simplified protein–lipid mixtures allow assessment of the participation of the major proteins in the two dimensional pattern development. One of the major myelin proteins, the Folch–Lees proteolipid, self-segregates into, and determines formation of, fractal-like patterns. The presence of the second major protein, myelin basic protein, leads to round liquid-expanded domains in the absence of Folch–Lees proteolipid and softens the boundaries of the fractal structures in its presence. The location of myelin basic protein in the interface is surface pressure sensitive, being slightly squeezed out at high surface pressure, allowing the fractal domains enriched in Folch–Lees proteolipid to evolve.