SPECIFIC INTERACTION BETWEEN CASEIN KINASE-2 AND THE NUCLEOLAR PROTEIN NOPP140

Casein kinase 2 (CK2) is a multifunctional second messenger-independen t protein serine/threonine kinase that phosphorylates many different p roteins. To understand the function and regulation of this enzyme, bio chemical methods were used to search for CK2-interacting proteins. Usi ng immobilized glutathione S-transferase fusion proteins of CK2, the n ucleolar protein Nopp140 was identified as a CK2-associated protein. I t was found that Nopp140 binds primarily to the CK2 regulatory subunit , beta. The possible in vivo association of Nopp140 with CK2 was also suggested from a coimmunoprecipitation experiment in which Nopp140 was detected in immunoprecipitates of CK2 prepared from cell extracts, Fu rther studies using an overlay technique with radiolabeled CK2 as a pr obe revealed a direct CK2-Nopp140 interaction. Using deletion mutants of CK2 beta subunits, the binding region of the CK2 beta subunit to No pp140 has been mapped, It was found that the NH2-terminal 20 amino aci ds of CK2 beta are involved. Since Nopp140 has been identified as a nu clear localization sequence-binding protein and has been shown to shut tle between the cytoplasm and the nucleus, the finding of a CK2-Nopp14 0 interaction could shed light on our understanding of the function an d regulation of CK2 and Nopp140.