In vivo phosphorylation of myelin basic proteins: age-related differences in 32P incorporation.

Myelin basic proteins (MBPs) are phosphoproteins of central and peripheral nervous system myelin. We studied the phosphorylation of mouse MBPs in vivo at three different stages of development (12, 30, and 50 days) and found age-related differences in the incorporation of 32P into MBPs. At all ages studied, significant amounts of 32P were found in the MBPs as early as 1 min after intracranial injection of isotope. Incorporation of radioactive phosphate into MBPs proceeded rapidly and the resultant specific radioactivity (SA) of 32P-labeled MBPs appeared to be related to the SA of the acid-soluble phosphate pool of myelin. Changes in the SA of the myelin acid-soluble phosphate pool were observed in a 30 min time course of labeling in vivo in 50-day mice. Coincident changes were observed in the SA of the MBPs. Similar but less pronounced changes were seen in the SA of the polyphosphoinositides (PPIs) indicating that the turnover of the PPI phosphate groups is slower than the MBP phosphates or that the PPI phosphates are drawn from additional or different pools than the MBP phosphates. The phosphorylation of MBPs in developmentally related myelin fractions is investigated in a comparison paper (J. B. Ulmer and P. E. Braun (1986) Dev. Biol. 117, 502-510).