Selective changes in the phosphorylation of endogenous proteins in subcellular fractions from cyclic AMP-induced...

The endogenous phosphorylation of specific proteins was studied in subcellular fractions from proliferating and cAMP-induced differentiated neuroblastoma cells. Fractions containing nuclear, membrane-bound, and cytosolic proteins were incubated with [gamma-32P]ATP, in the presence and absence of added cyclic nucleotides. Phosphate incorporation into specific proteins was determined by slab-gel electrophoresis of sodium dodecyl sulfate-solubilized reaction products. Cytosol fractions from differentiated cells demonstrated a twofold increase in cAMP-dependent phosphorylation of a specific protein with apparent mol wt of 59,000 daltons and a comparable decrease in cAMP-independent phosphorylation of another protein (97,000). The nuclear fraction of differentiated cells showed an increase in the cAMP-independent phosphorylation of two nonhistone proteins (110,000 and 102,000). Membrane fractions from differentiated cells exhibited a differential decrease in endogenous phosphorylation of specific proteins. Selective alterations in the phosphorylation of specific proteins in various subcellular components may be important biochemical events associated with the increased levels of differentiated functions in neuroblastoma cells in culture.