Structure of the regulatory subunit of CK2 in the presence of a p21WAF1 peptide demonstrates flexibility of the acidic loop

A truncated form of the regulatory subunit of the protein kinase CK2beta ( residues 1 - 178) has been crystallized in the presence of a fragment of the cyclin-dependent kinase inhibitor p21(WAF1) ( residues 46 - 65) and the structure solved at 2.9 Angstrom resolution by molecular replacement. The core of the CK2beta dimer shows a high structural similarity with that identified in previous structural analyses of the dimer and the holoenzyme. However, the electron density corresponding to the substrate-binding acidic loop ( residues 55 - 64) indicates two conformations that differ from that of the holoenzyme structure [ Niefind et al. ( 2001), EMBO J. 20, 5320 - 5331]. Difference electron density near the dimerization region in each of the eight protomers in the asymmetric unit is attributed to between one and eight amino-acid residues of a complexed fragment of p21(WAF1). This binding site corresponds to the solvent-accessible part of the conserved zinc-finger motif.