Evolved to be active:: Sulfate ions define substrate recognition sites of CK2α and emphasise its exceptional role within the CMGC family of eukaryotic protein kinases

CK2 alpha is the catalytic subunit of protein kinase CK2 and a member of the CMGC family of eukaryotic protein kinases like the cyclin-dependent kinases, the MAP kinases and glycogen-synthase kinase 3. We present here a 1.6 angstrom resolution crystal structure of a fully active C-terminal deletion mutant of human CK2 alpha liganded by two sulfate ions, and we compare this structure systematically with representative structures of related CMGC kinases. The two sulfate anions occupy binding pockets at the activation segment and provide the structural basis of the acidic consensus sequence S/T-D/E-X-D/E that governs substrate recognition by CK2. The anion are conserved are neutralized by phosphorylation of a neighbouring threonine or tyrosine side-chain, which triggers conformational changes for regulatory purposes. CK2 alpha, however, lacks both phosphorylation sites at the activation segment and structural plasticity. Here the anion binding sites are functionally changed from regulation to substrate recognition. These findings underline the exceptional role of CK2 alpha as a constitutively active enzyme within a family of strictly controlled protein kinases. (c) 2007 Elsevier Ltd. All rights reserved.