Specific Interaction between Casein Kinase 2 and the Nucleolar Protein Nopp140

Abstract

Casein kinase 2 (CK2) is a multifunctional second messenger-independent protein serine/threonine kinase that phosphorylates many different proteins. To understand the function and regulation of this enzyme, biochemical methods were used to search for CK2-interacting proteins. Using immobilized glutathione S-transferase fusion proteins of CK2, the nucleolar protein Nopp140 was identified as a CK2-associated protein. It was found that Nopp140 binds primarily to the CK2 regulatory subunit, ␤. 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. Further studies using an overlay technique with radiolabeled CK2 as a probe revealed a direct CK2-Nopp140 interaction. Using deletion mutants of CK2␤ subunits, the binding region of the CK2␤ subunit to Nopp140 has been mapped. It was found that the NH 2 -terminal 20 amino acids of CK2␤ are involved. Since Nopp140 has been identified as a nuclear localization sequence-binding protein and has been shown to shuttle between the cytoplasm and the nucleus, the finding of a CK2-Nopp140 interaction could shed light on our understanding of the function and regulation of CK2 and Nopp140.Protein phosphorylation is known to be a very important means of cellular regulation, and in recent years much information about protein kinases and phosphatases, especially those involved in the mitogen-activated protein kinase pathway, has been obtained (for review, see Refs. 1-3). Little is known, however, about the function and regulation of one particular protein kinase, casein kinase 2 (CK2), 1 although increasing data suggest that it may be an important mediator in mitogenic signal transduction (for review, see Ref. 4).CK2 is a multifunctional, second messenger-independent eukaryotic protein serine/threonine kinase present in the nucleus and cytoplasm of all eukaryotic cells. The holoenzyme form is generally composed of catalytic subunits ␣, ␣Ј, and a regulatory subunit ␤ combined so as to form ␣ 2 ␤ 2 , ␣␣Ј␤ 2 , or ␣Ј 2 ␤ 2 heterotetramers. The ␣ and ␣Ј subunits are catalytically active, whereas the ␤ subunit is inactive but can stimulate the catalytic activity of ␣ and ␣Ј under certain circumstances. Furthermore, the ␤ subunit stabilizes the ␣ and ␣Ј subunits and can facilitate substrate recognition (4). CK2 is widely expressed, and its sequence is highly conserved throughout evolution, indicating that the enzyme may have a critical role in cell function. In fact, it has been shown in Saccharomyces cerevisiae that the simultaneous disruption of genes encoding CK2 ␣ and ␣Ј subunits is lethal (5). CK2 phosphorylates a large number of proteins including enzymes involved in nucleic acid synthesis, transcription and protein synthesis factors, structural proteins, and signal transduction proteins, suggesting a global role in the regulation of cellular processes (6). Several recent studies demonstrate that CK2 is important for cell growth and division. Fir...

Protein kinase CK2 regulates CDC25B phosphatase activity

“…This interaction is mediated by the b regulatory subunit of CK2. Similar interactions with the b subunit have been reported for CK2-specific substrates: p53 (Filhol et al, 1992), FGF2 (Bonnet et al, 1996), DNA Topoisomerase IIa , CD5 (Raman et al, 1998), Nopp 140 (Li et al, 1997a) and the p21 protein (a CDK inhibitor) (Gotz et al, 2000;Romero-Oliva and Allende, 2001). It is clearly established that on binding to CK2b, CK2a changes activity and substrate specificity.…”

The nucleolus: reviewing oldies to have new understandings

“…They vary in the length of amino acids but share a similar organization (Figure 3 npg binding motif/glycine/arginine-rich stretches. The acidic regions contain exclusively aspartic acid, glutamic acid, and serine, in which the serine residues are phosphorylated by casein kinase type II [51,52]. The interspersed basic regions are rich in lysine, alanine, and proline.…”

Identification and Characterization of CKIP-1, a Novel Pleckstrin Homology Domain-containing Protein That Interacts with Protein Kinase CK2

“…In this regard, it may be notable that there is mounting evidence demonstrating that CK2 interacts with a variety of cellular proteins. For example, CK2␤ has been reported to interact with FGF-2 (24), A-Raf (25,26), Nopp140 (27), p53 (28 -30), p21 WAF1/CIP1 (31), c-MOS (32), and CD5 (33). CK2␣ and CK2␣Ј have also been observed to interact with specific protein partners.…”