Molecular Organisation in Central Nerve Myelin

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Abstract
Pertinent data from the literature and in press is summarised and used to construct a model for the molecular arrangement of lipid and protein in the lamellae of compact central nerve myelin. For the lipid phase of myelin the available data is best interpreted in terms of a bilayer arrangement while physical studies suggest that the lipids are in an intermediate fluid state maintained by the presence of cholesterol and water in the system. Lipids will interact to maintain this condition. The proteins of myelin differ in their membrane locations. The high molecular weight proteins are considered to be intrinsic components with at least part of their polypeptide chains in the lipid phase. The proteolipid protein is also intrinsic and may be completely buried in the lipid phase. The basic protein of myelin is an extrinsic component and must be localised at the surface of the lipid phase at either the external or cytoplasmic face of the lamellae. Present results suggest an elusive location at the cytoplasmic apposition region. The lipid-interacting properties of the basic protein are segregated on the polypeptide chain of the molecule and this may be important for the possible role of the basic protein in bridging adjacent lamellae at the cytoplasmic apposition. It is speculated that association of the proteolipid protein with the basic protein in a 1:1 molar ratio would form an effective lipid-complexing nucleus in the lipid rich myelin lamellae but experimental data to support this idea is lacking at present.