Abstract

Norepinephrine rapidly induced phosphorylation of 7 of the more than 200 neutral and acidic phosphoproteins which were resolved by two-dimensional gel electrophoresis of a whole cell extract of C-6 glioma cells. Cellular ATP pools were nearly optimally labeled by ³²P_i, after a 4-hr exposure, and norepinephrine treatment did not modify the specific radioactivity of the γ-phosphate of cellular ATP. That the ³²P-labeled spots of two-dimensional gels were indeed phosphopolypeptides was verified by comparing the migration of ³⁵S-methionine-labe1ed and ³²P_i-labeled samples and by extensive solvent extractions and chemical treatments. Subcellular fractionation resulted in assignment of a subcellular compartment to five of the seven norepinephrine-dependent phosphopolypeptides. These modified proteins were designated the "phosphoprotein domain" of norepinephrine for the C-6 glioma cell and each was provisionally named in terms of molecular weight, isoelectric point, and, where possible, subcellular compartment. Accordingly the proteins were named: 58K-5.7-nuclear; 50K-6.1, 48K-6.8-cytosolic; 38K-6.4-cytosolic; 20K-6.2, 19K-6.6-mitochondrial; and 16.5K-6.3-cytosolic. We have concluded that protein 58K-5.7-nuclear is the intermediate filament protein of the C-6 glioma cell based on similarities of molecular weight, isoelectric point, abundance, subcellular fractionation, nuclear binding, and elution as well as phosphorylation.