Phosphorylation of the glial fibrillary acidic protein

The glial fibrillary acidic protein (GFAP) was found to be phosphorylated in vivo after intracerebral injection of [32P]-orthophosphate, in brain slices, and in a cell free system. The phosphorylated proteins were separated by two-dimensional gel electrophoresis and then transferred to nitrocellulose sheets. Two isoelectric variants of GFAP were immunochemically identified by monoclonal antibodies. Autoradiography demonstrated that only the more acidic isoelectric variant of GFAP was phosphorylated. Phosphoamino acid analysis revealed that under all conditions GFAP was phosphorylated at serine and threonine residues. Incubation of brain slices with [32P]-orthophosphate and the protein kinase C activator phorbol 12-myristate 13-acetate or forskolin, an activator of cyclic AMP-dependent protein kinase, stimulated phosphorylation of GFAP. Likewise phosphorylation of GFAP was also accentuated by calcium/phosphatidylserine/diolein and by exogenous cyclic AMP-dependent kinase in a cell free system. These findings announce that protein kinase C and cyclic-AMP dependent kinase may play physiologic roles in the in situ phosphorylation of GFAP. When isolated cytoskeletal preparations were incubated with [gamma-32P] ATP, GFAP was phosphorylated in vitro by two additional protein kinases, a Ca2++/calmodulin-dependent kinase and an effector-independent kinase. The results of these investigations strongly suggest that phosphorylation of GFAP appears to be regulated by multiple second messenger pathways.