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J. Biol. Chem., Vol. 269, Issue 7, 4827-4831, 02, 1994
B Boldyreff, F Meggio, LA Pinna and OG Issinger
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).
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
Institute for Human Genetics, University of Saarland, Homburg/Saar, Germany.
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