Myelin‐associated glycoprotein is phosphorylated by protein kinase C

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Abstract
The myelin‐associated glycoprotein (MAG) is a neural recognition molecule involved in heterophilic interactions between myelin‐forming cells and neurons. To characterize the molecular mechanisms underlying post‐translational modifications which may be instrumental in signal transduction following the recognition event, we have studied the stimuli leading to modification of 32P‐orthophosphate incorporation into MAG in cultures of oligodendrocytes or transformed differentiated Schwann cells. Here we show that in oligodendrocytes both the 67 and 72 kD isoforms of MAG were phosphorylated exclusively on serine, while in the transformed Schwann cells only the 67 kD isoform was found to be present and phosphorylated. The phorbol‐12‐myristoyl‐13‐acetate (PMA) did not affect biosynthesis of the protein backbone, but enhanced incorporation of phosphate by a factor of 2–3, indicating the involvement of protein kinase C. Exclusive phosphorylation of serine residues was also observed, when purified MAG was incubated with protein kinase C in the presence of [γ‐32P] ATP. In searching for the physiological stimuli which may trigger phosphorylation of MAG, cultures of oligodendrocytes were exposed to extracellular signals, such as coculture with dorsal root ganglion and spinal cord neurons carrying the MAG receptor, to membrane fractions of these neurons, monoclonal MAG antibody 513 binding to the recognition site of MAG, or platelet‐derived growth factor. None of these additives modified the phosphorylation of MAG. These observations point to the possibility that phosphorylation of MAG is controlled by yet unknown intracellular cues rather than by extracellular signals interacting with cell surface receptors of oligodendrocytes.

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