Purification and characterization of protein IIIb, a mammalian brain phosphoprotein

The phosphorylation of a 55,000-dalton protein (Protein IIIb) present in mammalian brain was previously shown to be increased by depolarizing agents in the presence of calcium, by cyclic nucleotides, and by appropriate neurotransmitters. We now report that Protein IIIb has been purified 660-fold to near homogeneity and partially characterized. The hydrodynamic properties of the purified protein indicate that it exists as an elongated monomer. cAMP-dependent protein kinase catalyzes the incorporation of 0.82 mol of phosphate into serine/mol of protein. The protein is heterogeneous in isoelectric focusing, exhibiting multiple forms with isoelectric points ranging in pH from 6.6 to 7.3.