Reconstitution of normal and hyperactivated forms of casein kinase-2 by variably mutated .beta.-subunits

Twenty-one mutants of the noncatalytic beta-subunit of human casein kinase-2 have been created, expressed in Escherichia coli, and purified to homogeneity. They are either modified at the autophosphorylation site (mutants beta delta 1-4 and beta A 5,6) or bear variable deletions in their C-terminal part (mutants beta delta 209-215, beta delta 194-215, beta delta 181-215, beta delta 171-215, beta delta 150-215) or have undergone Ala substitutions for the acidic and basic residues which are concentrated in the sequences 55-70 and 171-180, respectively. All these mutants have been examined for their ability to functionally replace the wild type beta-subunit. All substitutions and the deletions delta 1-4, delta 194-215, and delta 209-215 are compatible with effective binding of the catalytic alpha-subunit, as judged by sucrose density gradient analysis, stimulation of catalytic activity, and protection against thermal denaturation. Deletions delta 171-215 and delta 150-215, however, give rise to truncated molecules which are unable to associate with the alpha-subunit. The intermediate deletion delta 181-215 is still compatible with association, albeit the reconstituted holoenzyme exhibits an altered sedimentation coefficient. The holoenzymes reconstituted with substituted mutants beta A 55,57, beta A55-57, and, to a lesser extent, beta A 59-61, beta A63,64, and beta A5,6 display a basal activity which is higher (up to 4-fold) than that of the wild type holoenzyme.