Multilamellar packing of myelin modeled by lipid-bound MBP

Membrane compaction and adhesion at the major dense line (cytoplasmic appos ition) of myelin, particularly in the central nervous system (CNS), is typi cally attributed to myelin basic protein (MBP). To explore the role of MBP in myelin membrane adhesion, we attempted to reconstitute the major dense l ine of myelin from purified lipid-bound MBP, which is a detergent-soluble f orm of MBP that retains the binding of all the myelin lipids. Removal of de tergent by long-term dialysis yielded a precipitate, which, when analyzed b y sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) and t hin-layer chromatography, contained MBP that was still associated with myel in lipids, but in different proportions than in the native membrane. Compar ison of lipid composition among isolated myelin, MBP-free myelin lipids, an d lipid-bound MBP aggregates showed that the lipid-bound form of the protei n was specifically enriched in phosphatidylethanolamine, phosphatidylcholin e, sphingomyelin, phosphatidylinositol, and phosphatidylserine. Electron mi croscopy and x-ray diffraction demonstrated that the lipid-MBP complexes fo rmed multilayers having periods of 70-85 Angstrom, which correspond in widt h to individual myelin membranes. By contrast, the lipids alone assembled a s multilayers having a period of similar to 40 Angstrom. Thus, the detergen t-soluble form of MBP, which is bound to lipids, might serve as a simple mo del for the cytoplasmic apposition of myelin. (C) 2000 Wiley-Liss, Inc.