THE PHYSICAL ASSOCIATION OF CASEIN KINASE-2 WITH NUCLEOLIN

CK2 (formerly called casein kinase 2) is a ubiquitous messenger-indepe ndent serine/threonine protein kinase implicated in cell growth and pr oliferation. To investigate the regulation and functions of this enzym e, experiments were carried out to search for CK2-interacting proteins . The methods employed included an overlay technique, co-purification, co-immunoprecipitation, and the use of glutathione S-transferase (GST ) CK2 fusion proteins, By the CK2 overlay technique, one protein of 11 0 kDa was found to bind to CK2 with very high affinity. The binding wa s inhibited by CK2 effectors such as heparin, polyarginine, and histon e H1, but was not affected by the CK2 substrate, casein. Protein p110 was also detected by co-immunoprecipitation using anti-CK2 antiserum, suggesting an in vivo association of this protein with CK2. Go-purific ation of p110 with CK2 from Sf-9 cells that overexpressed CK2 was also observed through sequential chromatographic steps. Using GST fusion p roteins of CK2, the CK2-p110 interaction was investigated further and was found to occur primarily through CK2 alpha or alpha' subunits, but not the beta subunit. Protein p110 was purified from 3T3 LI mouse fib roblast cell lines using a GST-CK2 affinity resin. Amino acid sequence analysis of peptides obtained from the protein indicated that it is t he nuclear protein, nucleolin. Furthermore, p110 was recognized by ant i-nucleolin antiserum. At present, the physiological significance of t he strong interaction between CK2 and nucleolin, an excellent substrat e for the enzyme, is not clear. However, this association may be impor tant for regulating rDNA transcription.