Basal and hydrogen peroxide stimulated sites of phosphorylation in heterogeneous nuclear ribonucleoprotein C1/C2

Hydrogen peroxide (H2O2) is a recently recognized second messenger, which regulates mammalian cell proliferation and migration. The biochemical mechanisms by which mammalian cells sense and respond to low concentrations of H2O2 are poorly understood. Recently, heterogeneous nuclear ribonucleoprotein C1/C2 (hnRNP-C1/C2) was found to be rapidly phosphorylated in response to the application of low concentrations of H2O2 to human endothelial cells. Here, using tandem mass spectrometry, four sites of phosphorylation are identified in hnRNP-C1/C2, all of which are in the acidic C-terminal domain of the protein. Under resting conditions, the protein is phosphorylated at S247 and S286. In response to low concentrations of H2O2, there is increased phosphorylation at S240 and at one of the four contiguous serine residues from S225-S228. Studies using a recombinant acidic C-terminal domain of hnRNP-C overexpressed in Escherichia coli demonstrate that protein kinase CK2 phosphorylates hnRNP-C1/C2 at S247, while protein kinase A and several protein kinase C isoforms fail to phosphorylate the isolated domain. These findings demonstrate that the acidic C-terminal domain of hnRNP-C1/C2 serves as the site for both basal and stimulated phosphorylation, indicating that this domain may play an important role in the regulation of mRNA binding by hnRNP-C1/C2.