Comparison of the phosphorylation of myelin-associated glycoprotein in cultured oligodendrocytes and Schwann cells.
Phosphorylation of the large and small isoforms of myelin-associated glycoprotein (L- and S-MAG) was investigated in primary oligodendrocyte cultures and in immortalized Schwann cells by incubating the cells with inorganic [32P]phosphate and immunoprecipitating MAG. In oligodendrocytes, both L- and S-MAG were phosphorylated, but L-MAG was much more heavily labeled. In Schwann cells, most of the phosphorylation was in S-MAG, which is the predominant isoform expressed by these cells. In both types of cells, the principal phosphorylated amino acid in MAG was serine. Radioactive phosphothreonine and phosphotyrosine were also detected in the MAG from oligodendrocytes. In Schwann cells, there was less phosphorylation of threonine and labeled phosphotyrosine was not detected. In both oligodendrocytes and Schwann cells, the phosphorylation of MAG was stimulated by phorbol ester and a calcium ionophore, but not by forskolin. The results indicate that the phosphorylation of MAG is catalyzed by protein kinase C and possibly other calcium-activated kinases in both types of myelinating cells, but not by cAMP-activated kinase. An inhibitor of tyrosine phosphatase, ammonium vanadate, increased the amount of radioactive phosphate in MAG several fold in both oligodendrocytes and Schwann cells. However, even in the presence of vanadate, the great majority of radioactivity in MAG was in phosphoserine and only a small amount was in phosphotyrosine, suggesting that tyrosine phosphorylation of other proteins may indirectly increase the phosphorylation of MAG. The current status of our understanding of MAG phosphorylation is reviewed in the context of similarities and differences between our results and other reports in the literature.
Yim
S H
SH
Myelin and Brain Development Section, Laboratory of Molecular and Cellular Neurobiology, NINDS, NIH, Bethesda, MD, USA.
Toda
K
K
Goda
S
S
Quarles
R H
RH
D000818:Animals; D000831:Animals, Newborn; D002478:Cells, Cultured / Q000378:metabolism; D002490:Central Nervous System / Q000166:cytology; D003599:Cytoskeleton / Q000378:metabolism; D018992:Myelin-Associated Glycoprotein / Q000378:metabolism*; D009836:Oligodendroglia / Q000378:metabolism*; D017933:Peripheral Nervous System / Q000166:cytology; D010766:Phosphorylation; D051381:Rats; D012583:Schwann Cells / Q000378:metabolism*
D018992:Myelin-Associated Glycoprotein