First Inactive Conformation of CK2α, the Catalytic Subunit of Protein Kinase CK2

The Ser/Thr kinase casein kinase 2 (CK2) is a heterotetrameric enzyme composed of two catalytic chains (CK2 alpha, catalytic subunit of CK2) attached to a dimer of two noncatalytic subunits (CK2 beta, noncatalytic subunit of CK2). CK2 alpha belongs to the superfamily of eukaryotic protein kinases (EPKs). To function as regulatory key components, EPKs normally exist in inactive ground states and are activated only upon specific signals. Typically, this activation is accompanied by large conformational changes in helix alpha C and in the activation segment, leading to a characteristic arrangement of catalytic key elements. For CK2 alpha, however, no strict physiological control of activity is known. Accordingly, CK2 alpha was found so far exclusively in the characteristic conformation of active EPKs, which is, in this case, additionally stabilized by a unique intramolecular contact between the N-terminal segment on one side, and helix aC and the activation segment on the other side. We report here the structure of a C-terminally truncated variant of human Mu in which the enzyme adopts a decidedly inactive conformation for the first time. In this CK2 alpha. structure, those regulatory key regions still are in their active positions. Yet the glycine-rich ATP-binding loop, which is normally part of the canonical anti-parallel beta-sheet, has collapsed into the ATP-binding site so that ATP is excluded from binding; specifically, the side chain of Arg47 occupies the ribose region of the ATP site and Tyr50, the space required by the triphospho moiety. We discuss some factors that may support or disfavor this inactive conformation, among them coordination of small molecules at a remote cavity at the CK2 alpha/CK2 beta interaction region and binding of a CK2 beta dimer. The latter stabilizes the glycine-rich loop in the extended active conformation known from the majority of CK2 alpha structures. Thus, the novel inactive conformation for the first time provides a structural basis for the stimulatory impact of CK2 beta on CK2 alpha. (c) 2009 Elsevier Ltd. All rights reserved.