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ABF1 is a multifunctional phosphoprotein that binds specifically
to yeast origins of replication and to transcriptional regulatory sites
of a variety of genes. We isolated a protein kinase from extracts of Saccharomyces cerevisiae on the basis of its ability to
specifically phosphorylate the ABF1 protein. Physical and biochemical
properties of this kinase identify it as casein kinase II (CKII). The
purified kinase has a high affinity for the ABF1 substrate as indicated
by a relatively low K
Biochemical and
genetic mapping localized a major site for phosphorylation at Ser-720
near the C terminus of the ABF1 protein. This serine is embedded within
a domain enriched for acidic amino acid residues. A Ser-720 to Ala
mutation abolishes phosphorylation by CKII in vitro. The same
mutation also abolishes phosphorylation of this site in vivo,
suggesting that CKII phosphorylates Ser-720 in vivo as well.
Although three CKII enzymes, yeast, sea star, and recombinant human,
utilize casein as a substrate with similar efficiencies, only the yeast
enzyme efficiently phosphorylates the ABF1 protein. This suggests that
ABF1 is a specific substrate of the yeast CKII and that this
specificity may reside within one of the
Volume 270,
Number 27,
Issue of July 07, pp. 16153-16159, 1995
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
value. Furthermore,
when incubated with ABF1 and anti-ABF1 antibodies, the kinase forms an
immunocomplex active in the phosphorylation of ABF1.
regulatory subunits of
the enzyme. Thus, phosphorylation of ABF1 by yeast CKII may prove to be
a useful system for studying targeting mechanisms of CKII to a
physiological substrate.
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