Glial Fibrillary Acidic Protein: Norepinephrine Stimulated Phosphorylation in Intact C-6 Glioma Cells

Abstract: Coelectrophoresis in two-dimensional gels of rat glial fibrillary acidic protein (GFA) and ³²P-labeled whole cell extracts of rat C-6 glioma cells showed that the GFA migrated in close proximity to a previously noted phosphoprotein, 50K-6.1, of these cells. GFA electrophoresed as a 50K polypeptide with at least four charge variants, the most acidic of which coelectrophoresed with 50K-6.1. Exposure of the C-6 cultures to dibutyryl cyclic AMP (dbcAMP) for 48 h increased the relative abundance of the endogenous polypeptide associated with 50K-6.1 by threefold, consistent with the hypothesis that 50K-6.1 was GFA. Norepinephrine stimulated 50K-6.1 phosphorylation 3.2-fold in dbcAMP-induced cultures. Peptide mapping with V8 protease and subtilisin was used to test the hypothesis that GFA and 50K-6.1 were identical polypeptides. With V8 protease, the peptides generated from the [³⁵S]methionine labeled putative GFA spot of the C-6 cells were indistinguishable from the stained bands derived from authentic GFA in mixed samples of the two proteins. Likewise, the ³⁵S-labeled acidic satellite to the putative GFA spot also yielded a peptide map that matched that of the authentic GFA. ³²P-labeled peptides derived from the 50K-6.1 protein were a subset of those from authentic GFA. With three subtilisin concentrations, ³²P-labeled 50K-6.1 was degraded to peptides which were again a subset of the stained GFA peptides. A cytoskeletal fraction from ³²P-labeled C-6 cells contained a 50K phosphoprotein. Pep-, tide mapping with V8 protease produced a ³²P-peptide pattern which was a subset of that from authentic GFA. The pattern closely resembled the ³²P-peptide pattern for the 50K-6.1 protein from 2-dimensional gels of whole cell extract. It was concluded that the protein 50K-6.1 is a phosphorylated form of GFA and that GFA is a phosphoprotein whose phosphòrylation is stimulated by norepinephrine in C-6 glioma cells.