Efficient autophosphorylation and phosphorylation of the beta-subunit by casein kinase-2 require the integrity of an acidic cluster 50 residues downstream from the phosphoacceptor site.

Various beta-mutants were investigated either as subunits or as substrates for casein kinase 2 (CK-2), in the absence of presence of polylysine. A total of 21 beta-mutants were characterized for their susceptibility to autophosphorylation, by combining them in equimolar amounts with the recombinant alpha-subunit. Six mutants, i.e. beta A5,6, beta A59-61,63,64, beta A55,57, beta 55-57, beta delta 171-215, and beta delta 150-215 exhibited a > 70% reduction in autophosphorylation. This strongly suggests that in addition to amino acid residues 5,6, distant amino acid residues within the sequence 55-64 are also involved in the process of autophosphorylation, possibly by means of a loop formation. The results obtained with the COOH-terminal-deleted mutants support the view that reconstitution of a functional holoenzyme must occur to allow efficient autophosphorylation. Polylysine prevents the autophosphorylation of beta wt (86% inhibition) inducing a parallel increase of the alpha-subunit autophosphorylation. The autophosphorylation of all mutants, with the exception of beta A55-57 and beta A59-61,63,64, is also inhibited by polylysine (>64%). The alpha-subunit autophosphorylation is increased with all mutants reconstituting a tetrameric holoenzyme. Only with the three largest COOH-terminal deletion mutants beta delta 150-215, beta delta 171-215, and beta delta 181-215 is no significant alpha-subunit autophosphorylation observed. The phosphorylation of the beta-subunit mutants added in large molar excess to CK-2 holoenzyme (either native or recombinant) is also severely impaired by Ala for Glu/Asp substitutions at position 5,6 and in the 55-64 region and by the deletion of the COOH-terminal segments 150-215 and 171-215. Such a phosphorylation is inhibited by polylysine, with the exception of mutants beta delta 171-215 and beta delta 150-215, whose phosphorylation is conversely stimulated by polylysine. The decreased phosphorylation efficiency of those mutants that are poor substrates is invariably accounted for by lower Vmax values, whereas the affinity for CK-2 is actually increased (Km values lower than that of beta wt).