QUATERNARY STRUCTURE OF CASEIN KINASE-2 - CHARACTERIZATION OF MULTIPLE OLIGOMERIC STATES AND RELATION WITH ITS CATALYTIC ACTIVITY

The structure-activity relationship of casein kinase 2 (CK2) was exami ned with regard to its previously reported property to self-aggregate in vitro. Sedimentation velocity and electron microscopy studies showe d that the purified kinase exhibited four major, different oligomeric forms in aqueous solution. This self-polymerization was a reproducible and fully reversible process, highly dependent upon the ionic strengt h of the medium, suggesting that electrostatic interactions are mostly involved. At high salt concentrations (e.g. 0.5 M NaCl), CK2 appears as spherical moieties with a 18.7 +/- 1.6 nm average diameter, roughly corresponding to the alpha(2) beta(2) protomer, as deduced by measure ments of the Stokes radius and by light scattering studies. At lower i onic strength (e.g. 0.2 M NaCl), the protomers associate to form ring- like structures with a diameter (averaging 36.6 +/- 2.1 nm) and Stokes radius indicating that they are most likely made of four circularly a ssociated alpha(2) beta(2) protomers. At 0.1 M NaCl, two additional po lymeric structures were visualized: thin filaments (16.4 +/- 1.4 nm av erage), as long as 1 to 5 mu m, and thick and shorter filaments (28.5 +/- 1.6 nm average). Examination of the molecular organization of CK2 under different catalytic conditions revealed that the ring-like struc ture is the favored conformation adopted by the enzyme in the presence of saturating concentrations of substrates and cofactors. During cata lysis, well-known cofactors like MgCl2 or spermine are the main factor s governing the stabilization of the active ring-like structure, On th e other hand, inhibitory high salt concentrations promote the dissocia tion of the active ring-like structure into protomers. Such observatio ns suggest a strong correlation between the ring-like conformation of the enzyme and optimal specific activity. Thus, CK2 may be considered as an associating-dissociating enzyme, and this remarkable property su pports the hypothesis of a cooperative and allosteric regulation of th e kinase in response to appropriate regulatory ligands possibly taking place in intact cells.