The microtubule-associated protein MAP1B is believed to play an important role in the outgrowth of neurites from neurons (Tucker and Matus, Dev Biol 130: 423-434, 1988). We have investigated the possibility that MAP1B might participate in the formation of processes in cultured oligodendrocytes by an analysis of the expression of MAP1B during oligodendrocyte progenitor development. The appearance of the antigens recognized by the monoclonal antibodies A2B5, O4, and O1 which define distinct stages in the maturation of progenitors, was compared with the developmental expression of MAP1B. MAP1B is first detectable in O4+ preoligodendrocytes prior to the acquisition of galactocerebroside and immediately before they develop the complex process-bearing morphology characteristic of terminally differentiated myelin-forming oligodendrocytes in the CNS. In contrast, astrocytes have negligible amounts of MAP1B. These results demonstrate that the expression of MAP1B precedes the development of the mature oligodendrocyte phenotype and suggest that interactions between microtubules and MAP1B might have a role in the formation and stabilization of myelin-forming processes.

No abstract

The myelin basic protein (MBP) from bovine spinal cord was cleaved at the single tryptophan residue to produce an N-terminal fragment ( F l ) of molecular weight 12.6K and a C-terminal fragment (F2) of molecular weight 5.8K. The interactions of the two fragments with bilayers of the acidic lipid dimyristoylphosphatidylglycerol (DMPG) were compared with those of the intact protein, by using both chemical binding assays and spin-label electron spin resonance spectroscopy. The saturation binding stoichiometries of the two fragments were found to sum to that of the MBP, having values of 1 1, 24, and 36 mol of DMPG/mol of protein for F2, F1, and the MBP, respectively. The strength of binding was found to increase in the order F2 < F1 < MBP, which follows that of the net charges on the different fragments.The ionic strength dependence of the protein binding indicated that the interaction is primarily of electrostatic origin. The efficiency of displacement of the proteins by salt was in the order F2 > F1 > MBP, which correlates with both the strength of binding and the net charge on the different protein fragments. Nitroxide derivatives of phosphatidylglycerol (PG) labeled on the sn-2 chain were used to probe the protein-induced changes in the acyl chain dynamics. Both the fragments and the MBP decreased the lipid chain mobility as recorded by the C-5 atom and (2-12 atom position nitroxide-PG spin-labels, in a manner which followed the protein binding curves. At saturation binding, the reduction in mobility recorded by the C-5 atom label was in the order M B P > F1 > F2. An additional population of lipids, whose chain motion was restricted relative to that of the bulk population of fluid lipids, was resolved in the case of the F1 fragment and the MBP, when using the C-12 atom position labeled PG. Approximately nine DMPG molecules per F1 fragment were found in this motionally restricted population, which was assigned to lipids in direct contact with partially penetrant sections of the protein, as opposed to a 18: 1 mol/mol lipid/protein stoichiometry found for the restricted component with the intact MBP. These results suggest that the principal sites of hydrophobic interaction of the protein with lipid bilayers are at least partly located in the 12.6-kDa fragment. The tryptophan residue at position 116 appears also to be important for the structural and functional properties of the intact bovine myelin basic protein.x e water-soluble peripheral myelin basic protein (MBP)' plays an important role in the structural organization of the myelin sheath [see Boggs and Moscarello (1978a,b) and Boggs et al. (1982)l. Induction and maintenance of the multilamellar structure of myelin are thought to be brought about by a bridging of apposing cytoplasmic surfaces. A hairpin structure of the protein caused by a tri-proline sequence approximately in the middle of the molecule is considered to cause interlamellar interactions (Eylar et al., 1971).