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 (CK2β) binding two catalytic subunits. In humans, there are two isoforms of the catalytic subunit (and an additional splicing variant), one of which (CK2α) is well characterized. To supplement the limited biochemical knowledge about the second paralog (CK2α'), we developed a well-soluble catalytically active full-length mutant of human CK2α', characterized it by Michaelis-Menten kinetics and isothermal titration calorimetry, and determined its crystal structure to a resolution of 2 Å. The affinity of CK2α' for CK2β is about 12 times lower than that of CK2α and is less driven by enthalpy. This result fits the observation that the β4/β5 loop, a key element of the CK2α/CK2β interface, adopts an open conformation in CK2α', while in CK2α, it opens only after assembly with CK2β. The open β4/β5 loop in CK2α' is stabilized by two elements that are absent in CK2α: (1) the extension of the N-terminal β-sheet by an additional β-strand, and (2) the filling of a conserved hydrophobic cavity between the β4/β5 loop and helix αC by a tryptophan residue. Moreover, the interdomain hinge region of CK2α' adopts a fully functional conformation, while unbound CK2α is often found with a nonproductive hinge conformation that is overcome only by CK2β binding. Taken together, CK2α' exhibits a significantly lower affinity for CK2β than CK2α; moreover, in functionally critical regions, it is less dependent on CK2β to obtain a fully functional conformation.
Department für Chemie, Institut für Biochemie, Universität zu Köln, Zülpicher Straße 47, D-50674 Köln, Germany.
Nils
N
Bischoff
Birgitte
B
Olsen
Jennifer
J
Raaf
Maria
M
Bretner
Olaf-Georg
OG
Issinger
Karsten
K
Niefind
D000595:Amino Acid Sequence; D047390:Casein Kinase II / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D006801:Humans; D007527:Isoenzymes; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D011485:Protein Binding; D017433:Protein Structure, Secondary; D013816:Thermodynamics
D007527:Isoenzymes; D047390:Casein Kinase II