In vivo phosphorylation of myelin basic proteins in developing mouse brain: evidence that phosphorylation is an early event in myelin formation.

Myelin basic proteins (MBPs) can be detected in 5-day mouse brain, even though there is very little myelin present. In addition to the 4 MBP components (with molecular weights of 14, 17, 18.5 and 21.5 kilodaltons), that are commonly observed, other larger MBP-related polypeptides are also present. We have investigated the possibility of MBP phosphorylation at early stages of development in order to establish the point at which this modification occurs in the assembly of myelin. In vivo isotope studies show that in the 5-day mouse brain 14-, 17-, 18.5- and 21.5- kilodalton MBPs are phosphorylated and that they may be present in an immature form of myelin. Larger MBP-related proteins with a molecular weight of 35 and 42 kilodaltons are also phosphorylated but appear to be present mostly in nonmyelin structures or compartments.