Protein kinase CK2 is inhibited by human nucleolar phosphoprotein p140 in an inositol hexakisphosphate-dependent manner

Protein kinase CK2 is a ubiquitous protein kinase that can phosphorylate various proteins involved in central cellular processes, such as signal transduction, cell division, and proliferation. We have shown that the human nucleolar phosphoprotein p140 (hNopp140) is able to regulate the catalytic activity of CK2. Unphosphorylated hNopp140 and phospho-hNopp140 bind to the regulatory and catalytic subunits of CK2, respectively, and the interaction between hNopp140 and CK2 was prevented by inositol hexakisphosphate (InsP6). Phosphorylation of alpha-casein, genimin, or human phosphatidylcholine transfer protein-like protein by CK2 was inhibited by hNopp140, and InsP6 recovered the suppressed activity of CK2 by hNopp140. These observations indicated that hNopp140 serves as a negative regulator of CK2 and that InsP6 stimulates the activity of CK2 by blocking the interaction between hNopp140 and CK2.