Small basic proteins of myelin from central and peripheral nervous systems are encoded by the same gene

Peripheral nervous system (PNS) and central nervous system (CNS) rodent myelins, which are produced by different cell types, share common morphological and functional characteristics although their major integral membrane proteins are completely different. Both types of myelin however, contain sets of four myelin basic proteins (MBPs), which share similar immunochemical and electrophoretic properties. We have isolated and characterized cDNA clones corresponding to the rat mRNAs encoding the small MBPs (SMBPs) found in both CNS and PNS myelin. Sequence analysis of these clones indicate that SMBPs in both divisions of the nervous system are encoded by the same nucleotide sequences, which suggests that they are the products of the same gene expressed in both oligodendrocyte and Schwann cells. In dot-blot hybridization experiments with the CNS SMBP cDNA as a probe, it was shown that there is a 20-fold higher level of MBP mRNA in a CNS myelin fraction than in total brainstem mRNA. It also was found that in optic and sciatic nerves, which contain oligodendrocytes and Schwann cells respectively, there are higher levels (4-fold and 2-fold, respectively) of MBP mRNA than in brainstem. Blot-hybridization experiments showed that a probe derived from the coding region of the rat SMBP cDNA hybridizes to an homologous mRNA (approximately equal to 2.6 kilobases) present in human optic nerve, which is not detectable with a probe derived from the 3' untranslated region. This conservation of coding-region sequences is in accord with the highly homologous amino acid sequences reported for the MBPs in the two species.