Phosphorylation of myelin basic protein

Myelin membranes prepared from rat brain possess both the enzyme and substrates to incorporate ³²P from [γ ³²P]ATP into membrane protein constituents. Of the myelin polypeptides, only the 2 basic proteins were phosphorylated; and both components appeared to be equally good substrates for endogenous or added protein kinases. The ³²P was transferred primarily to serine residues of the basic protein. With myelin membranes in vitro, the phosphorylation reaction was linear for less than 1 min at 24°, and was not stimulated by cyclic adenosine 3':5' monophosphate. Myelin basic protein was also labeled following intracranial injection of ³²P. Basic protein of myelin prepared from adult rats was more readily phosphorylated than the basic protein of myelin prepared from 14 day old rats. However, when differences in myelin protein kinase activity were minimized by the addition of rabbit muscle protein kinase to heated or native myelin preparations, the basic protein of 14 day old myelin was phosphorylated more readily and to a greater extent than adult myelin. These studies suggest that myelin basic protein is phosphorylated in vitro and in vivo and such phosphorylation may have potentially profound effects on myelin structure and function.