Casein kinase 2 (CK2) was one of the first protein kinases to be discovered and has been suggested to be responsible for as much as one-fifth of the eukaryotic phosphoproteome. Despite being responsible for the phosphorylation of a vast array of proteins central to numerous dynamic cellular processes, the activity of CK2 appears to be unregulated. In the current study, we identified a protein kinase activity in rat liver supernatant that is up-regulated by inositol 1,3,4,5-tetrakisphosphate (IP 4 ) and inositol hexakisphosphate (IP 6 ). The substrate for the inositol phosphate-regulated protein kinase was identified as a phosphatidylcholine transfer protein-like protein. Using the phosphorylation of this substrate in an assay, we purified the inositol phosphate-regulated protein kinase and determined it to be CK2. Bacterially expressed recombinant CK2, however, showed very high basal activity and was only modestly activated by IP 6 and not regulated by IP 4 . We found that an endogenous component present in rat liver supernatant was able to inhibit both recombinant and liverpurified CK2 basal activity. Under these conditions, recombinant CK2 catalytic activity could be increased substantially by IP 4 , inositol 1,3,4,5,6-pentakisphosphate (IP 5 ), and IP 6 . We concluded that, contrary to the previously held view, CK2 can exist in a state of low constitutive activity allowing for its regulation by inositol phosphates. The ability of the higher inositol phosphates to directly stimulate CK2 catalytic activity provides the first evidence that these signaling molecules can operate via a direct control of protein phosphorylation.Numerous studies over the last 20 years have established inositol 1,4,5-trisphosphate (IP 3 ) 1 as a classical second messenger molecule (1). However, IP 3 is only one of a vast array (numbering potentially as high as 60) of inositol phosphate isomers found within cells (2). In contrast to IP 3 , the biological function of the majority of these inositol phosphates is either unknown or poorly understood. Of particular interest is the phosphorylated product of IP 3 , namely inositol 1,3,4,5-tetrakisphosphate (IP 4 ), which shows many of the hallmarks of a second messenger molecule. Early studies focused on a role for IP 4 in calcium mobilization; however, the cloning of GAP1 IP4BP , which shows high affinity binding to IP 4 (3), and the recent demonstration that IP 4 is involved in chromatin remodeling in yeast (4, 5) suggest that IP 4 function may extend beyond calcium signaling.Of the higher inositol phosphates, possibly the most enigmatic is inositol hexakisphosphate (IP 6 ). Found in all mammalian cells in micromolar concentrations (6, 7), the biological function of this molecule has remained elusive. An involvement in endo-and/or exocytosis (8, 9) has been indicated by the discovery that a number of proteins involved in this process have a high affinity for IP 6 (10 -12) and that microinjection of IP 6 can regulate synaptic vesicle fusion (13). Recently, the potential biological processes...
