Four major phosphoproteins and active phosphatidylinositol turnover in oligodendroglial plasma membranes

Phosphorylation is an important mechanism used by cells for the posttranslational regulation of protein function. While it has been shown that many of the phosphorylation systems are concentrated in the synaptic junction, we have found that oligodendroglial plasma membranes also have active protein kinase and phosphatidylinositol kinase systems as well. Four major phosphoproteins were identified in the plasma proteins with molecular weights of 67,000, 61,000, 53,000 and 44,000 Da. The 44,000 Da protein was phosphorylated by multiple protein kinases, while the phosphorylation of the three other proteins was affected by the lot of fetal calf serum used during the culture of the oligodendroglia. This suggests that these phosphoproteins are affected by the presence of agents such as growth factors or hormones which are present in serum. When the phosphorylation of the proteins was compared with that of the lipids, phosphatidylinositol 4-monophosphate exhibited a 20-fold increase in radioactivity. Thus active phosphatidylinositol kinase and phospholipase enzymes are also present in these plasma membranes.