There are indications from genetic, biochemical and cell biological studies
that protein kinase CK2 (formerly casein kinase II) has a variety of funct
ions at different stages in the cell cycle. To further characterize CK2 and
its potential roles during cell cycle progression, one of the objectives o
f this study was to systematically examine the expression of all three subu
nits of CK2 at different stages in the cell cycle. To achieve this objectiv
e, we examined levels of CK2 alpha, CK2 alpha' and CK2 beta on immunoblots
as well as CK2 activity in samples prepared from: (i) elutriated population
s of MANCA (Burkitt lymphoma) cells, (ii) serum-stimulated GL30-92/R (prima
ry human fibroblasts) cells and (iii) drug-arrested chicken bursal lymphoma
BK3A cells. On immunoblots, we observed a significant and co-ordinate incr
ease in the expression of CK2 alpha and CK2 alpha' following serum stimulat
ion of quiescent human fibroblasts. By comparison, no major fluctuations in
CK2 activity were detected during any other stages during the cell cycle.
Furthermore, we did not observe any dramatic differences between the relati
ve levels of CK2 alpha to CK2 alpha' during different stages in the cell cy
cle. However, we observed a significant increase in the amount of CK2 beta
relative to CK2 alpha in cells arrested with nocodazole. We also examined t
he activity of CK2 in extracts or in immunoprecipitates prepared from drug-
arrested cells. Of particular interest is the observation that the activity
of CK2 is not changed in nocodazole-arrested cells. Since CK2 is maximally
phosphorylated in these cells, this result suggests that the phosphorylati
on of CK2 by p34(cdc2) does not affect the catalytic activity of CK2. Howev
er, the activity of CK2 was increased by incubation with p34(cdc2) in vitro
. Since this activation was independent of ATP we speculate that p39(cdc2)
may have an associated factor that stimulates CK2 activity. Collectively, t
he observations that relative levels of CK2 beta increase in mitotic cells,
that CK2 alpha: and CK2 beta are phosphorylated in mitotic cells and that
p34(cdc2) affects CK2 activity in vitro suggest that CK2 does have regulato
ry functions associated with cell division.