CKIP-1 is a recently identified interaction partner of protein kinase CK2 with a number of protein-protein interaction motifs, including an N-terminal pleckstrin homology domain. To test the hypothesis that CKIP-1 has a role in targeting CK2 to specific locations, we examined the effects of CKIP-1 on the localization of CK2. These studies demonstrated that CKIP-1 can recruit CK2 to the plasma membrane. Furthermore, the pleckstrin homology domain of CKIP-1 was found to be required for interactions with CK2 and for the recruitment of CK2 to the plasma membrane. In this regard, point mutations in this domain abolish membrane localization and compromise interactions with CK2. In addition, replacement of the pleckstrin homology domain with a myristoylation signal was insufficient to elicit any interaction with CK2. An investigation of the lipid binding of CKIP-1 reveals that it has broad specificity. A comparison with other pleckstrin homology domains revealed that the pleckstrin homology domain of CKIP-1 is distinct from other defined classes of pleckstrin homology domains. Finally, examination of CK2␣ for a region that mediates interactions with CKIP-1 revealed a putative HIKE domain, a complex motif found exclusively in proteins that bind pleckstrin homology domains. However, mutations within this motif were not able to abolish CKIP-1-CK2 interactions suggesting that this motif by itself may not be sufficient to mediate interactions. Overall, these results provide novel insights into how CK2, a predominantly nuclear enzyme, is targeted to the plasma membrane, and perhaps more importantly how it may be regulated.
CK21 (formerly casein kinase II) is a ubiquitously expressed extraordinary conserved Ser/Thr kinase found in all eukaryotic cells (1-3). CK2 has been shown to be essential for viability in a variety of models ranging from yeast to mammalian cells (4 -6). An elevated CK2 activity has been detected in leukemic cells, healthy tissues with high mitotic index, and in a variety of human cancers (7-9). In addition, CK2 exhibits oncogenic activity when overexpressed in transgenic mice (10). The majority of CK2 is found in the nucleus of logarithmically growing cells (11-13); however, there have been indications that the nuclear/cytoplasmic distribution of CK2 is regulated in a cell cycle-dependent manner (14). Antibodies that interfere with the nuclear uptake of CK2 inhibit mitogenic stimulation upon micro-injection into cells (15), indicating the importance of the nature of CK2 localization for its biological function. To date, CK2 has been shown to phosphorylate and/or interact with a broad range of proteins located in a variety of cellular compartments, including nuclear proteins (16 -20), cytoplasmic proteins (21-23), and proteins localized at the plasma membrane (24 -26).This tetrameric enzyme is composed of two catalytic (␣ and ␣Ј) and two regulatory subunits (). The two catalytic subunits are products of separate genes and show greater than 90% sequence identity over their N-terminal 330 amino acids (27). ...