Identification of Phosphorylation Sites on Glial Fibrillary Acidic Protein for cdc2 Kinase and Ca2+-Calmodulin-Dependent Protein Kinase II¹

We identified the phosphorylation sites of glial fibrillary acidic protein (GFAP) for cdc2 kinase and Ca²⁺-calmodulin (CaM)-dependent protein kinase II. GFAP was phosphorylated to ∼0.2 mol of phosphate/mol of GFAP by cdc2 kinase, and this phosphorylation did not induce disassembly of the filament structure. On the other hand, GFAP was phosphorylated to ∼1.9 mol of phosphate/mol of GFAP by Ca²⁺-CaM-dependent protein kinase II, and this phosphorylation did induce disassembly of the filament. Sequential analysis of the purified phosphopeptides revealed that only Ser8 on GFAP was phosphorylated by cdc2 kinase, whereas Serl3, Serl7, Ser34, and Ser389 on GFAP were phosphorylated by Ca²⁺-CaM-dependent protein kinase II.