Ability of CK2beta to selectively regulate cellular protein kinases.

The Wee1 protein kinase plays a prominent role in keeping cyclin dependent kinase 1 (CDK1) inactive during the G2 phase of the cell cycle. At the onset of mitosis, Wee1 is ubiquitinated by the E3 ubiquitin ligase SCF(beta-TrCP) and subsequently degraded by the proteasome machinery. Previously, it has been reported that although Wee1 lacks the conserved binding motif recognised by beta-TrCP, the CDK-catalysed phosphorylation of Wee1 at Ser123 creates a phosphodegron and primes phosphorylation of two other protein kinases, polo-like kinase 1 (PLK1) and protein kinase CK2, which create two additional phosphodegrons recognised by beta-TrCP. These events contribute to destabilise Wee1 at the onset of mitosis (Watanabe et al. Proc Natl Acad Sci USA 101:4419-4424, 2004). We show here that in addition to the ability of CK2 to phosphorylate Wee1 as reported earlier, the regulatory beta-subunit of protein kinase CK2 can interact with Wee1 in high molecular mass complexes. Indirect immunofluorescence microscopy revealled subcellular co-localisation of CK2beta and Wee1 in the nucleus. Moreover, in vitro phosphorylation assays showed that CK2beta indirectly up-regulates the activity of CDK1 with respect to histone H1 phosphorylation by inhibiting Wee1 kinase. These findings support the view that CK2beta regulates various intracellular processes by modulating the activity of protein kinases that are distinct from CK2 and that protein kinase CK2 plays an important role in events related to the regulation of cell cycle progression as a tetrameric enzyme but also through the individual subunits.