Identification in rat brain of a 19-kDa protein that comigrates on two-dimensional electrophoresis with p19, a hormonally regulated phosphoprotein of insulinoma cells.

We have previously shown that a set of 19-kDa cytosolic proteins, p19, undergoes hormone-dependent phosphorylation in several peptide hormone-producing tumor cells. Here we show, using comigration on two-dimensional electrophoresis with RIN-1122 rat insulinoma cell p19, that an identical set of 19-kDa proteins is present in rat brain but not in liver or skeletal muscle. We have partially purified p19 from rat brain and have compared the apparent isoelectric variants by tryptic peptide mapping. The data suggest that p19 is a novel phosphoprotein consisting of an unphosphorylated form and of three phosphoforms.