Structure of the Human Protein Kinase CK2 Catalytic Subunit CK2α′ and Interaction Thermodynamics with the Regulatory Subunit CK2β

Protein kinase CK2 (formerly casein kinase 2) is composed of a central dimer of noncatalytic subunits (CK2 beta) binding two catalytic subunits. In humans, there are two isoforms of the catalytic subunit (and an additional splicing variant), one of which (CK2 alpha) is well characterized. To supplement the limited biochemical knowledge about the second paralog (CK2 alpha'), we developed a well-soluble catalytically active full-length mutant of human CK2 alpha', characterized it by Michaelis-Menten kinetics and isothermal titration calorimetry, and determined its crystal structure to a resolution of 2 angstrom. The affinity of CK2 alpha' for CK2 beta is about 12 times lower than that of CK2 alpha and is less driven by enthalpy. This result fits the observation that the beta 4/beta 5 loop, a key element of the CK2 alpha/CK2 beta interface, adopts an open conformation in CK2 alpha', while in CK2 alpha, it opens only after assembly with CK2 beta. The open beta 4/beta 5 loop in CK2 alpha' is stabilized by two elements that are absent in CK2 alpha: (1) the extension of the N-terminal beta-sheet by an additional beta-strand, and (2) the filling of a conserved hydrophobic cavity between the beta 4/beta 5 loop and helix alpha C by a tryptophan residue. Moreover, the interdomain hinge region of CK2 alpha' adopts a fully functional conformation, while unbound CK2 alpha is often found with a nonproductive hinge conformation that is overcome only by CK2 beta binding. Taken together, CK2 alpha' exhibits a significantly lower affinity for CK2 beta than CK2 alpha; moreover, in functionally critical regions, it is less dependent on CK2 beta to obtain a fully functional conformation. (C) 2011 Elsevier Ltd. All rights reserved.