Phylogenetic Aspects of Myelin Structure

Abstract

Proteins are the major determinants of the spacing between membranes in compact, internodal myelin. This conclusion derives from X-ray diffraction and biochemical analyses (Kirschner et al., 1984; Inouye and Kirschner, 1988a,b; Lemke, 1988). For example, the idea that proteins protrude into the spaces between the membrane bilayers and act as struts to maintain their separation is based on the finding that the distance between them is at least twice as wide as that measured for hydrated myelin lipid multilayers. That P0-glycoprotein has a more extended conformation than PLP derives from the observation that inter-membrane spaces in PNS myelin are greater than those in CNS myelin; and this difference in extension may be consistent with the relative sizes of their putative cytoplasmic and extracellular domains as deduced from primary sequence data (Laursen et al., 1984; Stoffel et al., 1984; Lemke & Axel, 1985). Non-specific forces (such as electrostatic and hydration repulsion, and van der Waals attraction) account for many of the changes in membrane packing at the extracellular apposition when pH or ionic strength are altered; however, specific and short-range molecular interactions presumably involving proteins appear to define the native separation and to underlie the invariance of the cytoplasmic apposition.